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Scanning Fabry-Perot Interferometers


  • Operating Wavelength Ranges from UV to MIR
  • Confocal Fabry-Perot Cavity for Simplified Beam Alignment
  • Controller for Scanning Fabry-Perot Interferometers

SA201

Voltage Controller
(Sold Separately)

 

SA210-8B

10 GHz FSR, 820 - 1275 nm

SA200-2B

1.5 GHz FSR, 290 - 355 nm & 520 - 545 nm

Application Idea

Mounting components on an optical rail system reduces the degrees of freedom while aligning the beam to the cavity. The SA200, focusing lens, and laser are each mounted to an XT66C4 clamping platform which centers the optical axis over the XY66-500 rail axis. 

Related Items


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Diagram of Fabry Perot Interferometer
Schematic Representation of a Confocal Fabry-Perot Interferometer
Diagram of Fabry Perot Interferometer
Each Fabry-Perot interferometer features a thermally stable Invar® cavity.
SA200-18C with Included Photodiode Removed
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View Imperial Product List
Item #QtyDescription
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR31Ø1/2" Optical Post, SS, 8-32 Setscrew, 1/4"-20 Tap, L = 3"
PH21Ø1/2" Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L = 2"
BA21Mounting Base, 2" x 3" x 3/8"
View Metric Product List
Item #QtyDescription
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR75/M1Ø12.7 mm Optical Post, SS, M4 Setscrew, M6 Tap, L = 75 mm
PH50/M1Ø12.7 mm Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L=50 mm
BA2/M1Mounting Base, 50 mm x 75 mm x 10 mm
SA200-18C with Included Photodiode Removed
SA200-18C Mounted on KS2 Kinematic Mount
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View Imperial Product List
Item #QtyDescription
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR31Ø1/2" Optical Post, SS, 8-32 Setscrew, 1/4"-20 Tap, L = 3"
PH21Ø1/2" Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L = 2"
BA21Mounting Base, 2" x 3" x 3/8"
View Metric Product List
Item #QtyDescription
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR75/M1Ø12.7 mm Optical Post, SS, M4 Setscrew, M6 Tap, L = 75 mm
PH50/M1Ø12.7 mm Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L=50 mm
BA2/M1Mounting Base, 50 mm x 75 mm x 10 mm
SA200-18C Mounted on KS2 Kinematic Mount

Features

  • Analyze Fine Spectral Features of CW Lasers
  • Eight Optical Coatings for Wavelengths from 290 nm to 4400 nm (See the Graphs Tab for More Details)
  • Free Spectral Range of 1.5 or 10 GHz
  • Minimum Finesse Values of 150, 200, or 1500 Available
  • Factory-Calibrated Finesse 
  • Ultrastable Athermal Invar® Cavity
  • SMA- or BNC-Coupled for Connection to an Oscilloscope
  • SA201 Controller (Sold Separately) Provides Triangle or Sawtooth Scan Voltage for Piezoelectric Transducer
  • Custom Mirror Coatings from UV to Mid-IR (Contact Tech Support)

Thorlabs' Scanning Fabry-Perot Interferometers are spectrum analyzers that are ideal for examining fine spectral characteristics of CW lasers. Interferometers are available with a Free Spectral Range (FSR) of 1.5 GHz or 10 GHz. The resolution, which varies with the FSR and the finesse, ranges from <1 MHz to 67 MHz. For information on these quantities and their applications in Fabry-Perot interferometry, see the Fabry-Perot Tutorial tab.

The confocal Fabry-Perot cavity transmits only very specific frequencies. These transmission frequencies are tuned by adjusting the length of the cavity using piezoelectric transducers, as shown in the diagram to the right. The transmitted light intensity is measured using a photodiode, amplified by the transimpedence amplifier in the SA201 controller (or equivalent amplifier), and then displayed or recorded by an oscilloscope or data acquisition card. Each Fabry-Perot interferometer has a cable ending in a BNC connector for controlling the piezo.

The mirrors in the SA200-18C and SA210-18C are made of IR-grade fused silica (Infrasil®), the mirrors in the SA200-30C are made of yttrium aluminum garnet (YAG), and the mirrors on all other models are made of UV fused silica. Each Fabry-Perot interferometer also features an internal housing made of thermally stable invar to eliminate misalignment due to temperature fluctuations.

The SA200 and SA210 models feature SM1 (1.035"-40) and SM05 (0.535"-40) threading, respectively, on the back of the instrument for detector mounting. Except for the SA200-30C, each Fabry-Perot scanning interferometer comes with a photodiode detector included, as well as an SMA-to-BNC cable to connect the detector to an amplifier. This photodiode can be removed for alignment purposes or for replacement with another detector.

Alignment
The confocal design of the Fabry-Perot interferometer cavity allows for easy alignment of the input beam. The optical axis of the Fabry-Perot interferometer can be aligned with sufficient accuracy to the input beam by mounting the interferometer on a standard kinematic mirror mount and following the steps on the Alignment Guide tab. An example of our SA200 Fabry-Perot interferometer being mounted in a KS2 kinematic mount is shown to the right.

For the SA30-52, which has a finesse of >1500, additional steps are necessary to ensure proper alignment of the interferometer. The system will need to be fine-tuned while observing a transmission mode in order to suppress higher order modes. For more details, see the Alignment Guide tab.

Controller
The SA201 controller (sold separately) generates a sawtooth or triangle wave voltage required to repetitively scan the length of the Fabry-Perot cavity in order to sweep through one FSR of the interferometer. The SA201 controller also houses a transimpedance amplifier that can be used to amplify the output of the photodiode detector in the Fabry-Perot interferometer; this detector measures the intensity of the light transmitted through the confocal Fabry-Perot cavity. The controller also provides a trigger signal to the oscilloscope, which allows the oscilloscope to easily trigger at the beginning or the middle of the scan. The time axis of the oscilloscope can be precisely calibrated by observing two instances of a given spectral feature separated by one FSR (see the Calibration tab for more details). For more information on connecting the Fabry-Perot to the controller and an oscilloscope, see the Pin Diagrams tab.


Reflectance per surface for all of our Fabry-Perot interferometers are plotted here. See the Graphs tab for more information. Custom mirror coatings for wavelengths from the UV to the mid-IR
(200 nm to 4700 nm) are available upon request. If the coatings represented in the graphs below do not suit the needs of your application, please contact Tech Support

The plots below show the reflectance of the mirrors in our Fabry-Perot interferometers. "Mirror Finesse" refers to the contribution to the total finesse value due to the reflectance of the mirrors. In a system with near-perfect alignment, the finesse of the Fabry-Perot cavity will be limited by the reflectance of the mirrors, and the finesse values will approach those shown on the plots below. For more information on finesse, please see the Fabry-Perot Tutorial tab.

Please remember that the actual reflectance of the mirror will vary slightly from coating run to coating run within the specified region and can vary significantly from coating run to coating run outside of the specified region. The total cavity finesse depends on additional factors. Please see the Fabry-Perot Tutorial tab for more information.

Custom mirror coatings for wavelengths from the UV to the mid-IR (200 nm to 4700 nm) are available upon request. If the coatings represented in the graphs below do not suit the needs of your application, please contact Tech Support.

Interferometer with Finesse ≥1500

FP Interferometer reflectance plot for high finesse interferometer
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Measured Reflectance Data and Calculated Mirror Finesse

Interferometers with Finesse >150 or >200


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Theoretically Calculated Data
FP Interferometer reflectance coating for 3B mirrors
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Theoretically Calculated Data
fp interferometer mirror reflectance plot for 5b coating
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Theoretically Calculated Data
FP Interferometer mirror reflectance plot for 8B coating
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Theoretically Calculated Data
fp interferometer mirror reflectance plot for 12b coating
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Theoretically Calculated Data
FP Interferometer reflectance plot for 18B coating
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Click Here for Raw Data
Measured Reflectance Data and Calculated Mirror Finesse
This data has been smoothed in the wavelength region from 2.65 - 2.7 µm due to measurement inaccuracies introduced by absorption by water vapor in the air. For details on operation from 2.6 - 2.8 µm, see the -18C/30C Detectors tab.
fp interferometer mirror reflectance plot for 12b coating
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Click Here for Raw Data
Theoretically Calculated Data
alignment setup
Figure 2: Simple Diagram Showing Laser Focus Inside FP Cavity
confocal cavity ray trace
Figure 1: Ray Trace of an Off-Center Input Beam

General Instructions

Our Scanning Fabry-Perot (FP) Interferometers have confocal FP cavities. Since the transverse modes of a confocal cavity are degenerate, the cavity is fairly insensitive to the alignment of the input beam. As seen in the ray trace in Figure 1, even an off-axis input beam that is not parallel to the optical axis of the FP cavity will make one round trip through the cavity with an approximate path length of 4R-H4/R3, where R is the distance between the mirrors and H is the distance that the input beam is from the optical axis when the beam enters the cavity. As long as the second term in the path length expression is much less than the wavelength of the light, then the off-axis input beam will be degenerate with the on-axis input beam. The second term in the path length expression also limits the diameter of the input beam.

In practice, the cavity can be aligned by mounting the confocal FP interferometer in a standard kinematic mirror mount (KS2 for SA200 and SA30, KS1 for SA210), which is then placed in a free-space beam after a fold mirror. While the cavity is being scanned, iteratively adjust the position of the mirror and FP interferometer until the cavity is aligned with the input beam. After the cavity is aligned to the beam, a lens should be placed in the beam so that a beam waist with the specified diameter is formed in the center of the cavity, which is marked by a groove in the outer housing of the instrument (See Figures 3 and 4).

Recommended Beam Sizes and Lenses for FP Interferometers
Item # Prefix EFLa Waist Diameterb
SA200 250 mm 600 µm
SA210 100 mm 150 µm
SA30 250 mm 600 µm
  • Effective Focal Length of the Lens
  • Waist Diameter Inside the FP Cavity
CP360R Mounting a Mirror
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Figure 4: SA210 Interferometer
CP360R Mounting a Mirror
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Figure 3: SA200 Interferometer

Coupling a Free-Space Beam into a Scanning Fabry-Perot Interferometer

fabry perot alignment application photo
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Figure 5: SA210 FP Interferometer Free-Space System
fabry perot alignment application photo
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Figure 6: SA200 FP Interferometer Free-Space System
Fabry-Perot Alignment Setup Parts List
Callout # Item # Qty. Description Callout # Item # Qty. Description
 1 UPH2 3 Universal Post Holder, L = 2.00"  6 LMR1 1 Lens Mount for Ø1" Optics
 2 TR3 3 Ø1/2" × 3" Stainless Steel Optical Post  7 LA1461-A-MLb
LA1509-A-MLb
1 f = 250 mm Mounted, Visible Plano-Convex Lens (SA200)
f = 100 mm Mounted, Visible Plano-Convex Lens (SA210)
 3 FM90 1 Flip Mount  8 KC2
KC1
1 Kinematic Cage Mount for 2" Components (SA200)
Kinematic Cage Mount for 1" Components (SA210)
 4 KM100 1 Kinematic Mount for Ø1" Optics  9 SA200
SA30
SA210
1 Fabry-Perot Interferometer
 5 BB1-E02a 1 Ø1" Broadband Dielectric Mirror
View Metric Product List
Item #QtyDescription
UPH50/M3Ø12.7 mm Universal Post Holder, Spring-Loaded Locking Thumbscrew, L = 50 mm
TR75/M3Ø12.7 mm Optical Post, SS, M4 Setscrew, M6 Tap, L = 75 mm
FM90/M1Flip Mount Adapter, Metric
KM1001Kinematic Mirror Mount for Ø1" Optics
BB1-E021Ø1" Broadband Dielectric Mirror, 400 - 750 nm
LMR1/M1Lens Mount with Retaining Ring for Ø1" Optics, M4 Tap
LA1461-A-ML1Ø1" N-BK7 Plano-Convex Lens, SM1-Threaded Mount, f = 250.0 mm, ARC: 350-700 nm
LA1509-A-ML1Ø1" N-BK7 Plano-Convex Lens, SM1-Threaded Mount, f = 100.0 mm, ARC: 350-700 nm
KC2/M1Locking Kinematic Mirror Mount for Ø2" Optics, M4 Taps
KC1/M1Kinematic 30 mm-Cage-Compatible Mount for Ø1" Optic, Metric
  • In addition to the visible (350-700 nm) spectral range broadband dielectric mirror listed here, Thorlabs also sells Broadband Dielectric Mirrors suitable for other spectral ranges. Alternatively, a protected metallic mirror made from silver or aluminum could also be used.
  • In addition to the visible (400-700 nm) plano-convex lens listed here, Thorlabs also sells mounted and unmounted plano-convex lenses suitable for other spectral ranges.
Flip Mirror Mount
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Figure 7: Flipper Mirror in the Up Position (left) Intercepts Beam and Directs it to Interferometer Setup. Flipper Mirror in the Down Position (right) Allows Beam to Downstream Optics.

Figure 5 above depicts a setup to integrate an SA210 FP Interferometer into a free-space system. Figure 6 shows a similar setup for the SA200. The easiest method for integration is through the use of a flipper mirror and kinematic interferometer mount. The advantage of the flipper mirror is that it allows normal system operation with the ability to measure the source spectrum when necessary, without repositioning optics and equipment. During normal operation, the mirror can simply be flipped out of the way of the beam, allowing the beam unobstructed propagation downstream. When necessary, the mirror can be flipped upright to intercept the beam and pass it down to the interferometer (see Figure 7). The system shown in Figure 5 is a single mirror system; here, the flipper mirror intercepts the beam, which is then directed toward the focusing lens and the interferometer in a kinematic mount. This minimizes space and components required, useful in a compact setup.

The SA210 FP Interferometer is shown mounted in a KC1(/M) (the KS1 would work equally as well) kinematic mount; the SA200 FP Interferometer is shown mounted in a KC2(/M) (the KS2 would work equally as well) kinematic mount. Figures 5 and 6 show a mounted lens threaded into the LMR1(/M) lens mount (an unmounted lens will work just as well). The specific optics (mirror and lens) will depend on the wavelength of the system.

Alignment Guide:

Measure the height of the beam from the table surface; in general, it is good practice to start with your optics centered at the height of your beam. Install the flipper mirror assembly (mirror mount with mirror, flip mount, post, and post holder) with the mirror in the up position at a 45° angle to the beam. 90° bounces make the initial alignment and walking the beam for fine-alignment much easier. The tapped holes of the optical table make an excellent guide for the initial setup.

With the flipper mirror installed and correctly deflecting the beam by 90°, secure the flipper mirror assembly to the table with a 1/4"-20 (M6) screw. Then mount the interferometer so that the beam enters the center of the aperture; the iris may be used to guide the alignment. While not necessary, if the vertical centerline of interferometer is set at the height of the beam, the initial setup should line up nicely and produce enough of a signal to simply use the kinematic mount of the flipper mirror and the kinematic interferometer mount to guide the beam into its optimal alignment.

Turn on the Fabry-Perot controller box, and start scanning the length of the cavity (set the amplitude at >10 V to ensure that more than 1 peak is displayed) since light will only be transmitted when the cavity length is resonant with the wavelength of the light beam. Connect the detector output and the trigger or ramp signal to an oscilloscope. If no signal is detected at this point, it might be necessary to remove the detector from the back of the Fabry-Perot cavity in order to coarsely align the cavity. The iris located in the back of the interferometer can also be used to guide the alignment. However, this may be unnecessary if care is taken in the initial placement of the optics. Use the kinematic mount holding the interferometer and the flipper mirror to walk the beam until the Fabry-Perot cavity is correctly aligned.

Insert the lens (according to the table above) in the path at the specified distance so that the beam waist is centered in the Fabry-Perot cavity (marked by a groove on the FP housing, see Figures 3 and 4). Adjust the height and position of the lens to center the beam on the entrance aperture. The mirror and interferometer mount can be used to tweak the signal back into its optimal levels.

SA30 Series
Compared to the SA200 and SA210 series interferometers, the SA30 series requires several additional steps in order to properly align the system. After following the alignment steps above, zoom in on a single transmission mode and continue tweaking the alignment for maximum suppression of higher order modes; these higher order modes typically occur within a few MHz of the fundamental transmission mode. To do this, it is convenient to set the oscilloscope to one of the larger transmission modes in the spectrum. The lens position may also need to be adjusted slightly in order to achieve proper alignment and suppress the higher order modes. The images below show examples of transmission spectra before and after fine tuning the alignment for higher order mode suppression.


Here, the higher order transverse modes are still visible. This setup requires further tweaking to optimize alignment.

Here, the higher order modes are suppressed. This setup is properly aligned.

Connections

This section describes the electrical connections between a Fabry-Perot (FP) interferometer, an SA201 control box, and an oscilloscope. The SA201 provides a voltage ramp to the piezoelectric transducer inside the FP cavity, which controls the cavity length. The oscilloscope is used to view the output from the scanning FP and the controller.


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Recommended Setup for SA200 Series Fabry-Perot Interferometers (Except SA200-30C; See Diagram to the Right)

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Recommended Setup for SA200-30C Fabry-Perot Interferometer
Connection Description
1 Controller (BNC) to Piezo (Cable is Attached to FP Interferometer)
2a Photodiode (SMA) to Controller (BNC) (Included with FP Interferometer)
3a Amplified Photodiode Output (BNC) to Oscilloscope (Not Included)
4 Trigger Output of Controller (BNC) to Oscilloscope (Not Included)
5 Optional Connection that Allows the User to Monitor the Signal used to Drive the Piezoelectric Transducers (Not Included)
6b PDAVJ5 Output (BNC) to Oscilloscope (Detector and Cable Not Included)
  • This connection is not part of the setup for the SA200-30C.
  • This connection is part of the setup only for the SA200-30C.


SA30, SA200, & SA210 Series Scanning Fabry-Perot Interferometers

Piezo (Ramp In) - BNC Male

BNC Male

150 V Maximum

Photodiode Out - SMA Female


SMA Female

 


The SMA to BNC cable is included except for the SA200-30C, which does not include a detector. See the -18C/30C Detectors tab for more details.

SA201 Control Box for Scanning Fabry-Perot Interferometers

Trigger Output BNC Female

BNC Female

This trigger output signal may be used to externally trigger the oscilloscope. The trigger is capable of driving 50 Ω terminated cables, as well as Hi Z loads such as oscilloscopes. The trigger will provide an edge on the beginning and middle of the scanning ramp.

Output BNC Female

BNC Female

The output BNC is used to drive the SA200 scanning piezos from 1 to 45 V. The output is capable of driving 0.6 μF piezo loads at a ramp rate of 1 ms over the full voltage range. The output current is internally limited to prevent damage to the output drive.

PD Amplifier Input BNC

BNC Female

This input BNC is used to interface the photodetector, provided with the SA200 scanning heads, to the amplifier circuit. The photodiode amplifier is configured to operate with the Thorlabs supplied photo detectors; however, it is possible to operate user supplied photodetectors. To do so, the BNC center contact must be connected to the photo detector cathode and the BNC shell must be connected to the photodiode anode (unbiased operation). If a biased detector is to be used, the BNC shell must be connected to the bias ground and the bias voltage must be negative for the circuit to operate properly.

PD Amplifier Output BNC

BNC Female

This BNC is the amplifier output and may be connected directly to an oscilloscope to view the cavity spectrum. The amplifier gain will be set using the front panel 'DETECTOR' control knob. The amplifier output includes a 50 Ω series resistor to minimize noise when operating with a 50 Ω coax cable. For best results, a 50 Ω load resistor is recommended at the oscilloscope. Note, the amplifier gain will be halved with a 50 Ω load connected.

Calibrating the Oscilloscope Time Scale

iCyte CytometerFigure 1: An FSR Plot, Using a 1550 nm DFB Laser (PRO8000 Series). Using the SA200-12B 1.5 GHz interferometer, this plot is used to calibrate the time-base of the oscilloscope. Knowing that the FSR of the interferometer is 1.5 GHz, the calibration factor is found by setting 1.5 GHz = 3.2 ms between the two peaks.

Light transmitted through the Fabry-Perot interferometer can be displayed on an oscilloscope screen by following the setup shown on the Connections tab. Before taking a quantitative measurement for the laser or resonator mode widths, the time scale of the oscilloscope has to be calibrated so results can be determined in terms of optical frequency.

Figures 1 and 2 show the process for manual calibration of the time scale using the SA200-12B 1.5 GHz interferometer. Figure 1 shows the full free spectral range (FSR) of the interferometer (blue), with the two peaks separated by 1.5 GHz, and the linear yellow line indicates the voltage ramp. By measuring the time between the peaks (3.2 ms in this example), the proper calibration can be calculated; 468.8 MHz/ms for our example. Once the time scale calibration is known, we can zoom in on one of the peaks to measure the FWHM in time (shown in Figure 2). The measured FWHM in this example is 10 µs (0.010 ms) and yields a linewidth of 4.7 MHz.

For some of the more advanced oscilloscopes, linewidth and period analysis are automatic. These oscilloscopes will typically display the information somewhere on the screen, as shown in Figure 3 (in this case, the values are displayed at the bottom).

It may be beneficial to express either the FSR or the linewidth in terms of wavelength. The conversion is given by

where δλ is the FSR or linewidth in space, δν is the FSR or linewidth in frequency, λ is the wavelength of the laser, and c is the speed of light. For example, in Figure 2 we have a FSR of 1.5 GHz for a 1550 nm laser. Converting to wavelength, we find that the FSR is 0.0121 nm. Likewise, the linewidth from Figure 3 is 5.1 MHz, which yields a linewidth of 0.000038 nm.

Please note that Thorlabs factory-calibrates the finesse, and a calibration sheet is included with each unit.

iCyte Cytometer
Figure 2: This plot shows a close-up of the actual signal of the laser, which results from the convolution of the laser linewidth and finesse of the cavity; with the oscilloscope timebase calibrated from Figure 1, at 468.8 MHz/ms, we determine the FWHM for the interferometer to be 0.010 ms x 468.8 MHz/ms for a FWHM of 4.7 MHz.
iCyte Cytometer
Figure 3: This plot shows an FSR plot on a scope that automatically analyzes pulse width and period. The linear yellow line shows the voltage ramp, the blue line shows the FSR trace.

Scanning Fabry-Pérot Interferometers

Fabry-Pérot interferometers are optical resonators used for high-resolution spectroscopy. With the ability to detect and resolve the fine features of a transmission spectrum with high precision, these devices are commonly used to determine the resonant modes of a laser cavity, which often feature closely-spaced spectral peaks with narrow line widths.

Spatial Mode Structure


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Figure 1: Spatial mode structure for lowest-order TEM modes. Figure reproduced from Further Development of NICE-OHMs.2

The most common configuration of a Fabry-Pérot interferometer is a resonator consisting of two highly reflective, but partially transmitting, spherical mirrors that are facing one another. This type of resonator can be fully characterized by the following set of parameters:

  • the resonator length or mirror spacing, L
  • the radii of curvature of the input and output mirror, R1 and R2, respectively
  • the transmission, reflection, and loss coefficients of the input and output mirror, t1,2, r1,2, and l1,2, respectively, related to each other in such a way that t1,2 + r1,2 + l1,2 = 1 is fulfilled

Light waves entering the resonator through the input mirror will, depending on the mirror reflectivity, travel a large number of round-trips between the two mirrors. During this time, the waves also undergo either constructive or destructive interference. Constructive interference reinforces the wave and builds up an intracavity electric field. This corresponds to the case where a standing wave pattern is formed between the resonator mirrors, which occurs when the resonator length L is equal to an integer multiple of half the wavelength, qλ/2. All other wavelengths that do not fit this criteria are not supported by the resonator and destructively interfere.

By assuming a general Gaussian beam solution to the paraxial wave equation, it can be shown1 that only the following frequencies, νqmn, can exist inside the resonator:

where q, m, and n are mode numbers that can take on positive integers or zero and c is the speed of light. The g-parameters of the resonator g1,2 are given by

where R1,2 > 0 for concave mirrors and R1,2 < 0 for convex mirrors. These frequencies are referred to as Gaussian transverse electromagnetic (TEM) modes of order (m,n), or the Hermite-Gaussian modes, and are typically denoted as TEMm,n. The mode numbers m and n are associated with transverse modes, which describe the intensity pattern perpendicular to the optical axis, while q corresponds to the longitudinal mode. The TEM00 modes with m = n = 0 are called the fundamental TEM modes, or longitudinal modes, while TEM modes with m,n > 0 are called higher-order TEM modes. Figure 1 shows the spatial intensity pattern for a number of modes. Indices m and n correspond to the number of nodes in the vertical and horizontal directions respectively. For the case of light in the near infrared region, the parameter q is on the order of 106. The g-parameters of a resonator are often found in the so-called stability criterion. A resonator is called stable when its g-parameters fulfill the condition 0 ≤ g1g2 ≤ 1.1 


Transmission Spectrum of a Fabry-Pérot Interferometer


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Figure 2: Mode spectrum of a Fabry-Pérot interferometer for mirror reflectances of 99.7%, 80%, and 4%, illustrated by a blue, red, and green curve, respectively. 99.7% reflectance corresponds to the case for the SA200 series, which has a free spectral range of 1.5 GHz. The 4% reflectance corresponds to a typical "fringing effect" arising from reflections between parallel surfaces on glass plates.

For the case where light is spatially mode matched to the fundamental TEM00 mode, i.e., when the wave fronts of the Gaussian beam perfectly match with the mirror surfaces and the incoming beam is aligned to the optical axis of the resonator, no higher order modes (m,n > 0) are evoked. The transmission spectrum of the resonator consists only of TEM00 modes that differ from each other by different values of the parameter q. The distance between two consecutive TEM00 modes νq00 and νq+1 00 is called the free spectral range (FSR) of the resonator and is given by

This equation holds for all linear resonators comprised of two mirrors. The transmission intensity of a resonator, It, as a function of the frequency detuning from mode q, Δq = ν-νq, is given by the well-known Airy-formula3

where I0 is the intensity of the light incident on the instrument and all other variables are as described above. Figure 2 to the right shows the typical transmission spectrum of a Fabry-Pérot interferometer. From the above equation it can be seen that the on-resonance transmission, Tcres, is given by

which also makes clear that the on-resonance transmission not only depends on the transmission of a single mirror, but also on the mirror's reflection and loss coefficients, since all mirror coefficients are connected through t1,2 + r1,2 + l1,2 = 1 by definition. One always strives to keep absorption losses as small as possible to obtain maximum transmission for a given set of r1,2.

It can be seen from Equations (1) and (2) that the position of higher order transverse modes strongly depends on mirror spacing, L, and the radii of curvature of the mirrors, R1,2. For the special case when the two mirrors have the same radius, i.e. R1 = R2 = R, and the distance between the mirrors is equal to the mirror radius, i.e. L= R, the resonator is called a confocal resonator. Figure 3 shows a typical ray trace for a beam entering the resonator parallel to the optical axis at a distance H. All Thorlabs Fabry-Pérot interferometer models are based on such a confocal resonator design. For such a configuration, Equation (1) above simplifies to

Two important conclusions can be drawn from this equation. First, all modes are degenerate, i.e., there exist higher order TEM modes that share the same frequency as the fundamental TEM00 (e.g. TEMq' 00, TEMq'-1 02, TEMq'-1 11, TEMq'-1 20, TEMq'-2 40, TEMq'-2 31, TEMq'-2 22, … all share the same frequency). Second, the spectrum shows a regular, equidistant mode structure and the spacing between two consecutive modes is given by c/4L. If special care is not taken to obtain spatial mode matching, it is highly unlikely that higher order modes are suppressed. As a result, several higher order modes exist between two consecutive fundamental modes (TEMq00 and TEMq+1 00) and the equidistant spacing between modes makes it appear that this FSR is equal to c/4L. To account for the existence of higher order modes, all values given for the FSR of the Thorlabs Fabry-Pérot interferometers are referring to the so-called confocal free spectral range, νFSR,conf = c/4L. The arrows in Figure 3 highlight the difference between νFSR and νFSR,conf. With careful alignment along the resonator's optical axis and near-perfect spatial mode matching of the incident light, it is possible to extinguish every other mode in the spectrum. Figure 4 below shows a configuration with near perfect mode matching. Higher order modes still exist, but they are smaller than the fundamental modes. Further tweaking of the alignment will eventually distinguish every other mode in the spectrum.



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Figure 3: Schematic of a confocal Fabry-Pérot resonator. Mirrors with radius R= R2 (brown arrows) are separated by a distance L that is equal to the mirror radius. The solid green lines show a ray-trace of an off-axis input beam entering the resonator at a height H. The dashed light green lines represent the beam transmitted through the second mirror; light transmitted through the first mirror is not pictured.

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Figure 4: Spectrum of a confocal resonator with near perfect spatial mode matching, where every other mode is extinguished as only the fundamental mode is excited. The TEMqmn labels represent only one mode contained at that particular frequency; all modes are degenerate and, as described in the text, there are other modes sharing the same frequency.

Finesse and Mode Width (Resolution)


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Figure 5: When two Lorentzian lineshapes are separated by their FWHM, they meet the Rayleigh criteria for being resolvable.

A Fabry-Pérot interferometer's performance, to a large extenta, depends on the mirror reflectivity. Low-reflectivity mirrors will yield broader transmission peaks, while high-reflectivity mirrors will yield narrower transmission peaks. The mirror reflectivity plays a significant role in how well the interferometer can distinguish features of the transmission spectrum. Beyond the free spectral range, two more important quantities of a Fabry-Pérot interferometer are its finesse and mode width. The finesse, F, for mirrors having identical reflection coefficients, r, is given by

For the case of a confocal interferometer, it can sometimes be convenient to express the finesse as

An interferometer with a higher finesse will produce narrower transmission peaks than one with a lower finesse. Thus, a higher finesse increases the resolution of the interferometer, allowing it to more easily distinguish closely spaced transmission peaks from one another. According to the Rayleigh criterion (see Figure 5), two identical Lorentzian line shapes are resolvable when the peaks are separated by the full width at half maximum (FWHM) of each peak, denoted as ΓFWHM. The FWHM mode width, also called resolution, is related to the finesse and the FSR of a confocal resonator by

which is a measure of the minimum allowed spacing between two peaks to be resolved. For example, an interferometer with an FSR of 1.5 GHz and a finesse of 250 will have a FWHM of 6 MHz, and thus it will be able to distinguish features of the transmission spectrum as long as the peak values are at least 6 MHz apart. For visible light, this corresponds to a wavelength resolution of about 10 fm (10-14 m).

Further Considerations on the Finesse

In fact, a measured finesse has a number of contributing factors: the mirror reflectivity finesse, simply denoted F above, the mirror surface quality finesse FQ, and the finesse due to the illumination conditions (beam alignment and diameter) of the mirrors Fi. The overall finesse of a system, Ft, is given by the relation4

Often, the reflectivity finesse, Equation (8), is presented as an effective finesse, which is true for the case when the other contributing factors are negligible. For Thorlabs' interferometers, the reflectivity finesse dominates when operating with proper illumination.b

The second term in Equation (10) involves, FQ, which accounts for mirror irregularities that cause a symmetric broadening of the lineshape. The effect of these irregularities is a random position-dependent path length difference that blurs the lineshape. The manufacturing process that is used to produce the cavity mirror substrates always has to ensure that the contribution from FQ is negligible in comparison to the specified total finesse of a resonator; in other words, the substrates surface figure should never be the limiting factor for the finesse.

The final term in Equation (10), which deals with the illumination finesse, Fi, will reduce the resolution as the beam diameter is increased or as the input beam is offset. When the finesse is limited by the Fi term, the measured lineshape will appear asymmetric. The asymmetry is due to the path length difference between an on-axis beam and an off-axis beam, resulting in different mirror spacings to satisfy the maximum transmission criteria.

To quantify the effects of the variable path length on Fi, consider an ideal monochromatic input, a delta function in wavelength with unit amplitude, entering the Fabry-Pérot cavity coaxial to the optic axis and having a beam radius a. The light entering the interferometer at H = +e, where e is infinitesimally small but not zero, will negligibly contribute to a deviation in the transmitted spectrum. Light entering the cavity at H = +a will cause a shift in the transmitted output spectrum, since the optical path length of the cavity will be less by an approximate distance of a4/R3. Assuming the input beam has a uniform intensity distribution, the transmitted spectrum will appear uniform in intensity and broader due to the shifts in the optical path length. As a result, the wavelength input delta function will produce an output peak with a FWHM of H4/R3 (Ref. 6).


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Figure 6: The total finesse Ft as a function of beam diameter, 2H, for the SA200 and SA210 Fabry-Pérot Interferometers using Equation (12). The finesse is calculated for a wavelength λ of 633 nm.

Assuming that only Fi contributes significantly to the total finesse, then Eq. (9) can be used to calculate Fi for the idealized input beam. Substituting λ/4 for the FSR, and (H4/R3) for FWHM, yields:

The λ/4 substitution for the FSR is understood by considering that the cavity expands by λ/4 to change from one longitudinal mode to the next. For an input beam with a real spectral distribution, the effect of the shift will be a continuous series of shifted lineshapes. It should be noted that the shift is always in one direction, leading to a broadened or assymmetric lineshape due to the over-sized or misaligned beam.

Now, the total finesse for the case of high reflectivity mirrors (r ≈ 1) can be found using Equation (10), which includes significant contributions from both and Fi (Note: Fq is still considered to have a negligible effect on Ft):

Equation (12) is used to provide an estimate, in general an overestimate, of beam diameter effects on the total finesse of a Fabry-Pérot Interferometer, and several assumptions have been made. The first assumption is that the diameter of the beam is the same as the diameter of the mirror. In practice, the diameter of the beam is typically significantly smaller than that of the mirror, which also helps to reduce spherical aberration.5 A second assumption is that the light is focused down to an infinitesimally small waist size. Even for monochromatic light, the minimum waist size is limited by diffraction, and in multimode applications the waist size can be quite large at the focus. Figure 6 provides a plot of Equation (12) evaluated at 633 nm for the SA200 and SA210 Fabry-Pérot Interferometers, which have cavity lengths of 50 mm and 7.5 mm respectively. The traces in the plot assume that the reflectivity finesse is equal to 250 for the SA200 and 180 for the SA210, which are typical values obtained for mirrors used in our interferometers.

Cavity Ring-Down Time and Intracavity Power Build-Up

As a light wave travels many round-trips inside the resonator, the light is stored inside for a certain amount of time, and only a small portion of its energy leaks out as it impinges on either the input or the output mirror. In other words, the light wave has a certain life-time inside the resonator. This time is called the cavity ring-down time or cavity storage time, τcav, and is given by

This relation can show that τcav increases with the cavity finesse, i.e., the higher the finesse and the mirror reflectivity, the longer light is stored inside the resonator. In-line with this, another important quantity is the so-called intracavity power build-up, defined by the ratio of the intracavity intensity, Ic, and the incident intensity according to

which is given by F/π for a impedance-matched cavity (i.e., with a vanishing on-resonance reflection). This relates the intracavity intensity to the finesse by

The fact that the power stored inside the cavity increases with finesse has to be kept in mind, when beams with high incident power are evaluated with a Fabry-Pérot interferometer.

Spectral Resolving Power and Étendue

The spectral resolving power of an interferometer is a metric to quantify the spectral resolution of an interferometer, and is an extension of the Rayleigh criterion. The spectral resolving power, SR, is defined as:

where ν is the frequency of light and λ is its wavelength. It can be shown that for a confocal Fabry-Pérot interferometer, the SR is given by:

where F is the finesse of the interferometer, R is the radius of curvature of the mirrors, and λ is the wavelength. However, to achieve this maximum instrumental profile while the interferometer is in scanning mode, the aperture of the detector would need to be infinitesimally small; as the size of the aperture is increased, the spectral resolving power begins to decrease. The spectral resolving power must be balanced with the étendue of the interferometer. The étendue (U) is the metric for the net light-gathering power of the interferometer. When the light source is a laser beam, the étendue provides a measure of the alignment tolerance between the interferometer and the laser beam. The étendue is defined as the product of the maximum allowed solid angle divergance (Ω) and the maximum allowed aperture area (A). For the confocal system, the étendue is given by:

where F is the finesse of the interferometer, λ is the wavelength, and L is the mirror spacing. The spectral resolving power and étendue need to be balanced for the interferometer to work correctly. The accepted compromise for this balance is to increase the mirror aperture until the the spectral resolving power is decreased by 70% (0.7*SR) (Ref. 4). Under this condition the "ideal" étendue becomes π2λR/F, where R is the mirror's radius.


References

  1. P. W. Milonni and J. H. Eberly, Lasers (John Wiley & Sons, Inc., 1988) p. 302.
  2. P. Ehlers, Further Development of NICE-OHMS, Ph.D. thesis, Umeå University, Sweden, 2014.
  3. D. Romanini et al., in Cavity-Enhanced Spectroscopy and Sensing, edited by G. Gagliardi and H.P. Loock (Springer, 2014), Vol. 179, Chap. 1, pp. 1 - 60.
  4. M. Hercher, "The Spherical Mirror Fabry-Perot Interferometer," Applied Optics, vol. 7, no. 5, pp. 951 - 966, 1968.
  5. J. Johnson, "A High Resolution Scanning Confocal Interferometer," Applied Optics, vol. 7, no. 6, pp. 1061 - 1072, 1968.
  6. W. Demtröder, Laser Spectroscopy, Vol. 1: Basic Principles, (Springer, 2008) p. 152.

Footnotes

  1. It can be seen from Equation (1) above, that the optical path length and subsequently the free spectral range, finesse, and mode width depend on the beam offset, h, from the optical axis of the interferometer. Hence, best performance is achieved for the beam being well aligned to the optical axis. Also, the radius of curvature of the Gaussian beam impinging on the interferometer should equal the radius of curvature of the mirrors on the corresponding reflective surface. Provided a well collimated beam, this can be achieved sufficiently well by following the lens recommendations in the Alignment Guide Tab.
  2. The reflective coatings in all Thorlabs Fabry-Pérot interferometers have been designed so that the minimum F is better than 1.5 times the minimum specified finesse across their entire operating wavelength range for each model.

Thorlabs Scanning Fabry-Pérot Interferometers can be used in a wide range of applications, of which three examples are presented below. The linewidth measurement examples are valid for a laser emitting a single mode spectrum or an individual mode from a multimode laser with nonoverlapping modes.

Application 1: Determining the Laser Mode Linewidth Through Scanning

As described in the Calibration tab, laser mode linewidth can be measured using an oscilloscope by calibrating the time axis, zooming into the mode, and measuring the full width at half max (FWHM). However, there are three regimes that need to be considered when making this measurement, which are defined by the ratio of the laser line width ΓFWHMlaser to the Fabry-Pérot resonator mode width ΓFWHM.

Regime Comments
ΓFWHMlaser >> ΓFWHM This is the preferred mode of operation. The measured FWHM is the laser mode width since the line shape is dominated by the laser linewidth; contributions from the resonator mode width are negligible.
ΓFWHMlaser ≈ ΓFWHM The FWHM extracted from the oscilloscope is a convolution of the laser line shape and the resonator mode. A deconvolution procedure is needed to determine the true laser linewidth.
ΓFWHMlaser << ΓFWHM The line shape is dominated by the resonator mode width; therefore the measured FWHM is the resonator mode width. In this case, for an assessment of the laser linewidth, a Fabry-Pérot resonator with a higher finesse must be chosen. Another possibility is to make an estimate of the linewidth with the so called side-of-mode technique described in Application 2. 

Application 2: Determining the Laser Mode Linewidth Through Side-of-Mode Locking

An estimate for the laser linewidth can be determined by knowing the slope of the Fabry-Pérot resonator mode and measuring the intensity fluctuations that occur when the laser and resonator modes are offset from one another. This technique should be used in cases where the laser linewidth is much less than the Fabry-Pérot resonator mode width. For example, if a laser has a 500 kHz linewidth, it will be difficult to determine the linewidth using an SA200, which has a 7.5 MHz mode width. Note that the measurement described in this section cannot be done with the SA201 controller, which does not provide a pure DC output. Instead, a low-noise PZT driver that is able to provide a constant and stable DC output, e.g. the MDT694B Single-Channel, Open-Loop Piezo Controller, should be used.

The illustrations below show the origin of the intensity noise used in this technique. In Figure 1, the laser (red) is tuned to the center of the Fabry-Pérot resonator mode (gray). The laser inherently has frequency noise Δν, defined by the FWHM, and as a result the intensity of the light transmitted through the interferometer fluctuates. The black dotted lines drawn from the FWHM of the laser up through the Fabry-Pérot cavity mode profile show that fluctuations in the frequency (blue arrows) lead to small amounts of noise in the intensity (red arrow). For the same amount of frequency noise Δν, a laser tuned to the side of the resonator mode will produce a larger amount of intensity noise ΔV, as shown in Figure 2.


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Figure 1: Illustration of frequency-to-noise amplitude conversion. Frequency noise Δν (blue arrows) creates a small amount of intensity noise ΔV (red arrow) when the laser is tuned to the center of the resonator mode. The gray and red peaks illustrate the Fabry-Pérot resonator and laser modes respectively. 

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Figure 2: Illustration of frequency-to-noise amplitude conversion. The same frequency noise Δν (blue arrows) from Figure 1 creates larger intensity noise ΔV (red arrow) when the laser is tuned to the side of the resonator mode. The gray and red peaks illustrate the Fabry-Pérot resonator and laser modes respectively.


Because the laser is tuned to the side of the Fabry-Pérot cavity mode, it is possible to estimate the linewidth by relating the voltage fluctuations to the slope of the cavity mode. Figure 3 shows a visual representation of this relationship; the green lines indicate the two slopes that are assumed to be equal for this calculation. The Fabry-Pérot resonator mode has a Lorentzian line shape1


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Figure 3: Illustration showing the relationship between the slope of the Lorentzian Fabry-Pérot resonator mode profile and the intensity noise. The gray, red, and green lines represent the Fabry-Pérot resonator mode, laser mode, and slope respectively. The blue arrows indicate the frequency noise  ΓFWHMlaser (Δν from Figure 2), while red arrows highlight the intensity noise ΔV.  


where ΓL is the half-width-at-half-maximum (HWHM) of the mode and Δν is ν-νq. By taking the first derivative of the lineshape, the slope γ’q,L of the resonator mode at the HWHM point (point in Figure 3) can be found to be

The  ΓL of the Fabry-Pérot resonator can be calculated by ΓFWHM/2, which refers to equation (8) on the Tutorial tab and is also given in the specifications for all Thorlabs Fabry-Pérot interferometers. The linewidth (FWHM) of the laser, ΓFWHM laser, can be found by taking

where ΔV is the intensity variation that can be determined from the oscilloscope reading, γ'q,L(-ΓL) is the cavity mode slope a the HWHM point, and the factor C accounts for an estimate of the relationship between the peak-to-peak value and the root-mean-square of the noise type present in the measurement. For white noise, the C factor is found to be sqrt(2). It should be noted that the C factor results in an overestimate of the linewidth of the laser, so the calculated linewidth will never be smaller than the actual physical value.

Application 3: Measuring Mode Spectra

As a high-resolution spectrum analyzer, the scanning Fabry-Pérot interferometer is a useful tool for monitoring the performance of a laser. In a manufacturing environment, it could be used during production to ensure side modes are sufficiently suppressed or desired modulation depths are obtained. The Fabry-Pérot interferometers can also be used in an educational setting, specifically as a method to observe HeNe resonator modes while a HeNe laser is warming up, as well as to determine properties such as the free spectral range (FSR) and the actual resonator length inside the HeNe. 

To demonstrate the importance of choosing an interferometer with an FSR larger than the laser mode spectrum, Figures 4 and 5 below show simulated HeNe spectra of an unpolarized HNL100RB laser and how they would be detected by SA210-5B and SA200-5B interferometers respectively. With a ~1.3 GHz gain profile, the HeNe laser spectrum is better viewed on the SA210-5B, where the spectrum is sensed by two consecutive modes of the Fabry-Pérot resonator that are spaced out by 10 GHz. The individual modes are shown in gray, while the red dotted line in the figure indicates the laser gain profile. Please note that the red dotted line has been included to highlight the envelope of the gain profile but will not be visible on the scope. In contrast, when viewed on the SA200-5B that has a 1.5 GHz free spectral range, the laser mode profiles overlap. Since the line indicating the gain profile will not be visible on the oscilloscope, it is difficult to distinguish an individual laser spectrum from the neighboring spectra. This is a result of the gain profile being larger than the free spectral range of the scanning Fabry-Pérot Interferometer. Note that the dashed black line in Figure 4 shows a possible gain threshold, and only modes above this threshold will lase and appear in the spectrum. When the laser is warming up, the HeNe resonator expands due to thermal expansion, changing the resonator's resonance frequencies. As a result, the modes under the gain profile will move, becoming stronger towards the center and vanishing when they fall below the threshold.


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Figure 4: Simulation showing two consecutive modes of the SA210-5B detecting the mode spectrum of a HeNe laser with a ~1.3 GHz gain profile. With a 10 GHz FSR that is much larger than the gain profile, the laser spectra do not overlap. The dotted red, solid gray, and dotted black lines correspond to the gain profile, longitudinal modes, and gain threshold respectively.

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Figure 5: Simulation showing consecutive modes of the SA200-5B detecting the mode spectrum of a HeNe laser with a ~1.3 GHz gain profile. With a 1.5 GHz FSR, the observed laser spectra overlap with one another, making it difficult to extract information. The dotted red, solid gray, and dotted black lines correspond to the gain profile, longitudinal modes, and gain threshold respectively.



References

  1. N.Ismail, C.C. Kores, D. Geskus, and M. Pollnau,"Fabry-Pérot resonator:spectral line shapes, generic and related Airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity," Optics Express, vol. 24, no. 15, 2016. 

Posted Comments:
Vladimir Zenin  (posted 2019-04-15 07:27:50.41)
Hi, I am wondering, can I use Fabry-Perot Interferometer as a spectral filter? I mean, for example as an input I have a nice fiber-coupled beam with spectral width of 10 nm (for example, 1440-1450 nm). With collimator it can be made nearly perfect Gaussian. At the output I need a Gaussian or fiber-coupled light with narrower spectra. I have spectrometer to measure precisely the wavelength, but need a filter. Can Fabry-Perot Interferometer be such a tunable filter? Or are there any better product in Thorlabs? You might be interested in my source - it is broadband source SuperK Extreme, from NKT Photonics, equipped with SELECT filter (https://www.nktphotonics.com/lasers-fibers/product/superk-extreme-fianium-supercontinuum-lasers/, https://www.nktphotonics.com/lasers-fibers/product/superk-select-multi-line-tunable-filter/). At output I can get as narrow as 5-10 nm spectrum, and its central wavelength is tunable. Thank you in advance, and happy Easter! Best, Vladimir
YLohia  (posted 2019-04-17 11:07:22.0)
Hi Vladimir, thank you for contacting Thorlabs. In principle, using an FPI as a spectral filter is possible. However, the Free Spectral Range of our high resolution Scanning Fabry-Perot Interferometers is as short as 10 GHz or 1.5 GHz depending on model (SA210 or SA200 series). In the wavelength plane, this corresponds to narrow wavelength bands 75pm or 11.25pm at 1500nm absolute wavelength. And filter bandwidth is as narrow as in the 7-70 MHz range (corresponding to 0.05-0.5pm). This means that the spectrum obtained by the Scanning Fabry-Perot will repeat itself periodically in either 75pm or 11.25pm wavelength bands and filter function is in the 0.05-0.5pm range due to the high resolving power of these instruments. So, filtering from these instruments would only be meaningful if your input signal is narrower than 75pm (or 11.25pm). Commercially available tunable Fabry-Perot filters typically have a Free Spectral Range in the 10 THz range (corresponding to ~100nm) and a bandwidth of a few hundred pm.
julian.robertz  (posted 2018-12-11 14:12:51.273)
Dear Support-Team Tank you for the information in the alignment guide. I have one question regarding the recommended beam size and lens for the FP interferometers you are mentioning in the table. The beam waist behind the lens is highly dependent on the beam waist in front of the lense. Which beam waist have you assumed to give these values?
swick  (posted 2019-01-31 09:57:50.0)
Thank you for your feedback. For example lenses with focal length 250 mm like LA1461-B or mounted version LA1461-B-ML will provide small enough waist well below recommended 600 µm for 2 mm diameter beam @ 812 nm on input.
user  (posted 2018-11-01 15:23:01.95)
Can you also provide the high finesse FP cavities (SA30-52) at other wavelengths?
YLohia  (posted 2018-11-02 09:17:07.0)
Hello, thank you for contacting Thorlabs. We may be able to provide this, depending on your requirements. Please email us at techsupport@thorlabs.com to request a quote for custom items.
abc124771  (posted 2018-08-29 22:14:15.52)
Can SA201 or SA201-EC be used to drive PC4FL?
nbayconich  (posted 2018-09-11 01:17:30.0)
Thank you for contacting Thorlabs. The SA201 is designed specifically for driving the piezos of our fabry perot devices. It is not intended to be a general purpose piezo driver. In order to drive the pc4fl actuator to it's full displacement you will need a driver that can supply 150 volts where as the SA201 provide 45 volts max output voltage at 15mA. I would recommend looking into the BPC301 or MDT694 piezo controllers which can provide a much higher operating voltage and current.
jean-michel.melkonian  (posted 2018-07-20 09:03:55.237)
HI, I have exactly the same question as geoffroy.aubry, about the piezo hysteresis. Not only that, but I also observe it on my setup. We use the FPI to measure the frequency spacing between two modes of our laser source. We calculate the spacing as: Dvu = FSR_FP*(DT_laser/DT_FP), where FSR_FP is the Fabry-Perot FSR (10 GHz nominaly), DT_FP the time spacing between two resonance peaks of the FP (i.e. one FSR in time), and DT_laser the time spacing between the 2 resonance peaks due to the two longitudinal modes of our laser. The problem is that DT_laser is not the same whether we are looking at the begining of the ramp or at the end, ie there is some dilatation. Could you contact me to help me calibrate this effect? Thank you.
YLohia  (posted 2018-07-23 09:36:52.0)
Hello, while the voltage is a linear triangle or sawtooth curve, the cavity length is changed by piezoelectric actuator which will have some hysteresis. This is most exaggerated at the edges of travel, particularly when the direction of travel changes. The response between voltage and position will be most linear in the center of the movement. Due to this PZT hysteresis, the conversion between frequency space and time space varies over the scan of the PZT elements. In order to get the best possible accuracy, we recommend calibrating the time axis in the vicinity of the mode to be measured. In other words, when evaluating the width of a particular transmission peak, calibrate the time axis with the two immediate neighbors of the mode under measurement.
abc124771  (posted 2018-05-25 03:13:21.513)
Could you please give detailed specifications of the mirrors used for the cavity? Are those mirrors separately available? What are the reflectances and transmissions for the back side and front side of the mirrors?
nbayconich  (posted 2018-05-29 09:01:06.0)
Thank you for contacting Thorlabs. Yes, we can provide the mirrors of our fabry perot interferometers separately as a custom order item. Any custom or specials orders can be requested through techsupport@thorlabs.com. The reflectivity of the mirror coating material is above 99% for the coating design range however the typical transmission through the fabry perot interferometer is about 10-20%, the loss is do from absorption in the mirror substrate and coating material after multiple reflections through the cavity. In theory with no absorption loss present the transmission would be significantly higher. I will reach out to you directly with more information about ordering our fabry perot mirrors separately.
abc124771  (posted 2018-05-23 04:07:57.34)
Can the 7.5mm FP be used to obtain fringes with a white light source?
mmcclure  (posted 2018-08-13 10:59:52.0)
Yes, it is possible to see fringes without using the drive electronics. You will have to open up the second iris all the way and remove the photodiode connected to the female SMA. The will allow the beam to leave the FPI housing and be projected on to your screen. In order to have good alignment, you will need to first set up your two alignment mirrors before the unit and make adjustments till you see fringes on the output. Once you see the fringes, adjust the z-position of your lens till you obtain optimal fringe contrast.
abc124771  (posted 2018-05-14 23:29:51.337)
In the 'Calibration' tab, in the example given, linewidth is found to be 4.7MHz, but the resolution for that paticular FPI is just 7.5MHz. If I'm not wrong, resolution is the least we can resolve any spectral line... then how can we measure a line of FWHM 4.7MHz using that particular FPI?
YLohia  (posted 2018-05-16 08:39:42.0)
We state that the SA 200 resolution is 7.5 MHz corresponding to Finesse = 200. The F=200 is a minimum spec, while the typical spec is F=250 corresponding to resolution = 6 MHz. The relation between finesse and resolution is defined F = FSR/FWHM, where F = Finesse, FSR = Free Spectral Range and FWHM = resolution. Our finesse spec is set quite conservatively, so it's not unusual that we ship units with finesse in the 300-350 range. In the example on the web, a F=320 cavity is used (a random unit tested from stock) corresponding to 4.7 MHz resolution.
abc124771  (posted 2018-05-14 02:39:32.15)
The 'd' (50mm) is given as 'nominal distance'. So is this 'd' exactly the minimum distance between the two mirrors or the maximum? I mean when we apply voltage, does the distance between the mirrors increase from 50mm or does it decrease to an even lower value than 50mm? Or is it the average? If so, then what is the minimum and maximum 'd' achievable by applying voltage? And also could you please specify the sensitivity of the piezo, as in what is its V/mm rating?
YLohia  (posted 2018-05-15 10:00:00.0)
Hello, thank you for contacting Thorlabs. The nominal mirror separation d is 50mm with a ±0.1mm tolerance. The piezos used in the SA200 series have a displacement of 9µm for 150V. The SA201 Control box has a max voltage swing of 30V, so the maximum displacement is typically 1.8µm and, in most cases, the full 30V swing is not really necessary. Thus, the distance variation due to applied piezo voltage is typically on the ~1µm scale.
unnati_a  (posted 2018-05-07 05:08:31.63)
Is it possible to get just fringes with this instrument without scanning and all? I mean if I just want to display the fringe pattern on a screen for some purpose apart from scanning? What alignment precautions will have to be taken in that case, since we won't be using the controller and scope for alignment?
YLohia  (posted 2018-05-31 08:59:02.0)
Yes, it is possible to see fringes without using the drive electronics. You will have to open up the second iris all the way and remove the photodiode connected to the female SMA. The will allow the beam to leave the FPI housing and be projected on to your screen. In order to have good alignment, you will need to first set up your two alignment mirrors before the unit and make adjustments till you see fringes on the output. Once you see the fringes, adjust the z-position of your lens till you obtain optimal fringe contrast.
aspindler  (posted 2018-04-11 12:46:49.307)
Hello, I am wondering if your products are suitable for use in a vacuum. My company is interested in developing a space based system using an interferometer. Are your FPIs air spaced? I would assume they might just pop in a vacuum if they don't have a way to outgas.
YLohia  (posted 2018-04-12 11:15:11.0)
Response from Yashasvi at Thorlabs USA: Hello, thank you for contacting Thorlabs. The stock FPI's you see on our website are not suitable for vacuum use. What is the pressure/vacuum level you intend to use this at? The mirrors used in these are air-spaced, however. We could look into offering just the stripped invar cavity module without the outer housing. We would also have to use vacuum grease and vacuum compatible glue. I have reached out to you directly to discuss this further.
user  (posted 2018-03-19 12:48:34.75)
Hi guys, Can i use a sawtooth voltage from an external function generator to drive the piezo of the FPI? I´m planning a servo-loop feedback to lock a laser to a peak of the FPI, but i need to close the scan. So, can i feed the piezo with a near DC cero voltage from this external generator?
nbayconich  (posted 2018-04-05 08:10:44.0)
Thank you for contacting Thorlabs. Yes you can use an external function generator to drive the piezo actuators in these fabry perot interferometers. As long as your operating voltage is within our specified range of 0-150V this will be fine.
f.loignon.houle  (posted 2018-01-17 06:56:39.62)
Hi, If there is a refractive index variation in a material that is probed by a FPI, the phase delay would shift and the transmitted intensity would thus vary. What would be the required sensitivity to measure a varying intensity under very small refractive index modulation? Would your products be able to detect a RI variation of 10^-6 or 10^-5 ?
tfrisch  (posted 2018-01-19 09:44:14.0)
Hello, thank you for contacting Thorlabs. These FPIs are not for measuring absolute or relative changes in refractive index. They could however measure sidebands if that refractive index change is periodic and if it fast enough that the sideband spacing is greater than the resolution of the FPI and if the depth of modulation (change of index times the path length) is great enough to transfer significant power into the sidebands. I will reach out to you directly to discuss this application.
albert.romann  (posted 2017-12-13 16:48:11.27)
Can you provide a coating for 260/266nm? (for the 50mm long linear cavity)
nbayconich  (posted 2018-01-03 11:32:09.0)
Thank you for contacting Thorlabs. We can provide custom coatings for our scanning fabry perots for this wavelength range. I will contact you directly to discuss our custom capabilities.
geoffroy.aubry  (posted 2017-10-30 15:50:46.253)
Dear Support, How comes that the distance between first peak and second peak is not the same as between second peak and third peak on figure 3 in the "calibration" tab? Best, Geoffroy
tfrisch  (posted 2017-11-15 11:57:55.0)
Hello, thank you for contacting Thorlabs. The cavity length is controlled by piezo-electric actuators which have some hysteresis. This leads to small non-linearity between voltage and position, particularly at the edges of travel where the direction switches. I will reach out to you directly to discuss this effect.
taloh  (posted 2017-10-18 16:17:35.643)
I am working on 2um. For SA200-18C and SA210-18C, which fiber collimator is suitable? Besides, among the two interferometer models (1.5 GHz and 10 GHz), which FSR is more suitable for optimizing the mode hopping of TLK-L1950R?
tfrisch  (posted 2018-01-02 10:49:25.0)
Hello, thank you for contacting Thorlabs. I am unsure whether you are looking at the 10dB tuning range of the TLK (about 9THz) or the linewidth of a single mode (about 100kHz). In the case of the whole range, the bandwidth would be much larger than the free spectral range of SA200-18C or SA210-18C. In the second case, the linewidth would be smaller than the resolution of SA200-18C or SA210-18C, so you wouldn't be able to look at structure. I will reach out to you to clarify your application, but I'm not sure either of these interferometers would be suitable for your application. For the collimation, a reflective collimator would be convenient for avoiding chromatic focal shift.
al  (posted 2017-10-03 19:57:51.537)
Hi, Could this product be fit with free-space to SM-fibre-couplers on both the front and back faces of the cavity? Thanks.
tfrisch  (posted 2017-10-11 04:37:20.0)
Hello, thank you for contacting Thorlabs. While we do not currently offer a fiber pigtailed version, I will post this in our internal engineering forum. I will reach out to you directly to discuss your application.
greenjjag  (posted 2017-07-10 11:25:24.217)
Could you please tell me the transmittance percentage of 1064nm laser light (or similar) through the back side of the input mirror? For Model SA200-8B.
tfrisch  (posted 2017-10-09 03:50:45.0)
Hello, thank you for contacting Thorlabs. The peak transmission will be about 10-20%. After many passes through the cavity, the normally negligible absorptivity of the mirror coatings becomes relevant. I will reach out to you directly about calculating this.
pevere  (posted 2017-06-25 22:03:00.91)
1) In the tutorial you claim there are no lensing effects due to equal curvature of the inner and outer surfaces of the cavity mirrors while we have measured a diverging lensing effect of the cavity (though not very strong). Can you confirm this? 2) Why is the transmissivity at resonance so low (10-20% instead of a theoretical 50% given the AR coatings) for this device? Is there a way to improve it? Thanks a lot.
tfrisch  (posted 2017-10-09 03:24:36.0)
Hello, thank you for contacting Thorlabs. Because light light is typically focused into the cavity, I'd expect it to diverge out the opposite end into the detecting photodiode. I have contacted you directly to discuss the lensing you mentioned. As for the transmission, it will depend primarily on the reflectivity and absorptivity. Though the absorptivity would normally be considered negligible, in a Fabry-Perot cavity, the light may reflect many times off of each surface, and the small components of absorption because relevant. In general, out Fabry-Perot Interferometers would be expected to have a maximum transmission of about 10-20%. I've also contacted you to discuss this.
yao_chin  (posted 2017-05-15 21:44:07.703)
Do your company sell a pair confocal mirrors of SA200-5B, respectively?
nbayconich  (posted 2017-05-18 11:04:04.0)
Thank you for contacting Thorlabs. We can provide the mirror components. A Tech Support representative will contact you directly about information for this quote.
p.k.molony  (posted 2017-04-21 12:07:05.193)
When the voltage to the piezoelectric is increased, does this increase or decrease the length of the cavity? The diagrams suggest an increase, but I would like to know for sure. I need to know this as I would like to know which direction corresponds to increasing optical frequency, to distinguish +ve and -ve sidebands applied to a laser.
jlow  (posted 2017-04-25 04:04:30.0)
Response from Jeremy at Thorlabs: The cavity length is increased with increase voltage.
michael.clarage  (posted 2017-01-28 10:10:01.86)
I suggest considering a "FPI for Dummies" tab. We are trying to decide if we need a Fabry-Perot interferometer. We are using a good spectrometer, but need more resolution. I knew nothing about FPI before coming to your site. After several times through your pages, I still am asking myself, "how does all this work, and will it give us the resolution we need." Leo in your Engineering dept has been helping, so thanks to him. I am a physicist, educator, and writer, so I know the difficulties of conveying complex ideas to someone who does not already know the subject. But I find your web pages are talking to someone who already knows the subject.
tcampbell  (posted 2017-02-06 09:06:29.0)
Response from Tim at Thorlabs: thank you very much for your feedback. We will look into improving our tutorial on Fabry-Perot interferometry in the near future. Please continue to contact Tech Support with any questions you may have.
jabez  (posted 2016-12-16 13:36:02.58)
I'm looking for something like the SA-200-3B Fabry-Perot, but I need it to work at 323 nm. Is it possible to get a custom coating?
tfrisch  (posted 2016-12-19 03:51:41.0)
Hello, thank you for contacting Thorlabs. I will reach out to you directly about the availability of custom coatings.
lw493  (posted 2016-11-15 05:37:09.717)
Would it be possible to purchase the two concave mirrors (center wavelength 532nm) and what would be their reflectivity? Also do you have a range of curvatures or is it just 50mm?
tfrisch  (posted 2016-11-16 01:43:32.0)
Hello. Thank you for contacting Thorlabs. I have reached out to you directly with information on our stock concave mirrors as well as information on how the coatings differ. Whether these will be suitable or whether you need the mirrors from our FPIs will depend on the application.
chantianseng  (posted 2016-10-12 18:29:02.007)
do the mirrors used in thorlab fpi are wedged mirror? could you pls provide the links for the dielectric coated mirrors that could use in fpi for laser wavelength 660nm?
jlow  (posted 2016-10-13 02:53:24.0)
Response from Jeremy at Thorlabs: The mirrors used in the FPI are two concave mirrors. We do not have these as stock items on the web but we can provide them separately. I will contact you directly about this.
vijay  (posted 2016-04-08 11:17:49.63)
Hi, I am looking for a solid etalon. I could only find the Fabry-Perot interferometer on Thorlabs but not a solid etalon. By a solid etalon, I mean a parallel glass with coatings on both sides. Here is my requirement: I have an optical beam (632nm or 66nm) which I want to split into multiple parallel beams. These are normally created by internal reflection and transmission. I'd really appreciate it if you can help me locate this on ThorLabs catalog - I searched a lot but could not find one. Many thanks, Vijay.
besembeson  (posted 2016-04-13 08:43:00.0)
Response from Bweh at Thorlabs USA: Thanks for contacting Thorlabs. We don't offer such solid etalons at this time. We will however look into this based on your feedback. I will contact you with recommendations on where to get these.
leemireu  (posted 2016-03-22 05:30:24.083)
and whats the transmission of this interferometer itself?
besembeson  (posted 2016-03-25 08:57:28.0)
Response from Bweh at Thorlabs USA: The transmission at resonance will be in the 10-20% range. This will vary between individual units and different coating runs. Outside resonance, transmission will be in the 10^-4 range through the cavity due to the extremely high mirror reflectivity of >99%.
leemireu  (posted 2016-03-22 05:16:26.403)
Could you let me know whats the radius of curvature of the mirror? is it same with cavity length?
besembeson  (posted 2016-03-25 08:48:13.0)
Response from Bweh at Thorlabs USA: Due to the confocal cavity, yes it will be equivalent to the cavity length.
sbayliss30  (posted 2016-02-29 12:33:47.397)
Hello, could anyone tell me which photodiode is included with the SA200-12B? Thank you
jlow  (posted 2016-02-29 11:50:40.0)
Response from Jeremy at Thorlabs: The SA200-12B includes the SM05PD4B photodiode.
rssi_2nava  (posted 2015-02-24 02:02:25.563)
Hey guys, My question is about the finesse of the SA200-8B FPI. In the .XLS of the reflectance and Finesse you have values above 200, and you say that the values may vary slightly, but i got values beneath 200 in wavelengths between 790-900. The equation for the finesse is F=FSR/FWHM, and i'd measured the distance of adjacent transmission peaks to be the FSR and with mathematica computed the FWHM=gamma with a lorentzian function. Still i got very low values, What am i doing wrong? its laser power dependent? or the alignment its wrong? Hope to hear from you soon, thanks.
jlow  (posted 2015-02-25 03:36:25.0)
Response from Jeremy at Thorlabs: This is sensitive to your input alignment and beam size. I will contact you directly to troubleshoot about your application. You can also contact us at techsupport@thorlabs.com in the future about similar requests.
vittorgd  (posted 2015-02-23 17:36:24.757)
Is the internal Invar cavity hermetically sealed?
jlow  (posted 2015-02-25 09:08:58.0)
Response from Jeremy at Thorlabs: The Invar cavity is not hermetically sealed.
philippe.velha  (posted 2015-02-19 04:17:47.433)
Hi, I was wondering if fibers can be adapted, somehow, on both sides of the interferometer to include it in a larger set-up. That would be useful for us to have this possibility. Thanks in advance.
jlow  (posted 2015-02-25 03:21:17.0)
Response from Jeremy at Thorlabs: We will contact you directly about customizing this.
lmarin  (posted 2014-10-03 12:43:54.96)
Which FSR is better for measuring HeNeLaser and laser diodes (around) 650 nm both
jlow  (posted 2014-10-03 02:19:31.0)
Response from Jeremy at Thorlabs: You would generally want all the modes to be captured within 1 FSR. For regular HeNe, the 10GHz FSR version would be better. Typical Fabry-Perot LD would have modes much wider than 10GHz so this will probably not work well. I will contact you directly to discuss more about this.
ilia.sergachev  (posted 2014-08-08 10:48:02.863)
Hello, can you offer us a version of this for 4400-4600nm range or sell a version without mirrors/detector? Thank you.
jlow  (posted 2014-08-12 08:58:08.0)
Response from Jeremy at Thorlabs: We will contact you directly about this special.
foggy  (posted 2014-04-22 02:56:00.61)
Hello ,I want a Fabry-Perot with 300~600GHz FSR. Do you have this product?? Please contact me as soon as possible.
jlow  (posted 2014-04-23 02:20:02.0)
Response from Jeremy at Thorlabs: We do not have a version with such a large FSR at the moment.
laura.wollny  (posted 2014-04-15 15:19:23.453)
Do you also offer custom made versions that are usable in the range of 0.3 to 4 THz?
jlow  (posted 2014-04-17 10:34:04.0)
Response from Jeremy at Thorlabs: We do not currently have this for THz application. I will contact you directly to provide more details.
Rssi_2nava  (posted 2014-03-28 13:04:42.027)
Maybe i wasn't clear enough. I'm characterizing the SA200-8B. I want to know with accuracy the wavelength of a laser in order to tune in the correct frecuency for a rubidium transition. So, i was working with the SA200-8B with two lasers,(i know the wavelength of one of them) when i realize that the diference between the consecutive lineshapes or peaks(of one laser) decreased, i mean it's not the same diference of voltage if i measure the 2nd and 3rd or the 1st and the 2nd peak or the 3rd and the 4th. Why is that? Moreover, i was thinking in this relation for obtaining the wavelength: V2-V1=lambda/4, but it seems that if i pick the 3rd and the 4th peak, the relation is not true
besembeson  (posted 2014-04-08 08:29:30.0)
Response from Bweh E at Thorlabs: The SA200-8B Scanning Fabry Perot Interferometer has a free spectral range FSR of 1.5GHz, i.e the spectrum will repeat itself with 1.5 GHz mode spacing. Remember that 1.5GHz optical frequency shift at 780nm (for example) corresponds to only about 3pm wavelength shift. Given that the wavelengths of the two laser sources are separated by less than 1.5 GHz (or 3 pm at about 780nm) they can be compared and you are able to determine the frequency difference between them very accurately. If the frequency spacing between the two lasers is larger than 1.5GHz, then you are not really able to tell what the total frequency spacing is other than measured value ( distance between peaks) plus or minus an integer number of FSR’s. So, to be able to tune the unknown laser frequency to the known one, you would first have to know that the frequency separation between the two is less than 1.5GHz to be able to make a useful measurement using the SA200-8B.
Rssi_2nava  (posted 2014-03-24 02:26:10.113)
I hope you can help me guys, i have two lasers impacting on the FP. I know the wavelength of one of them, but i need a reliable way of obtaining the wavelength of the other one. Can you tell me how can i get that wavelength? I know the FSR of my FP(SA200-8B), and the linewidth of the lineshape in frequency, but i can't come up with a way of obtaining lambda.
david.n.hutch  (posted 2013-12-31 15:16:48.507)
What voltage output does the SA201 apply to the SA200? (For full-scale triangle wave sweep.)
jlow  (posted 2014-01-02 04:44:29.0)
Response from Jeremy at Thorlabs: The output voltage range is 1-45V. The scan voltage is adjustable from 1-30V and you can put in an offset voltage of 0-15V.
nnichen  (posted 2013-11-16 18:02:48.04)
Can this FP cavity reconstruct for those: 1. replace the mirror with our own mirror. 2. insert one piece of glass in between the cavity. 3. control with our exist function generator.
jlow  (posted 2013-11-18 04:59:09.0)
Response from Jeremy at Thorlabs: The components of the cavity are epoxied in place and we do not recommend replacing this yourself. We will get in touch with you directly to discuss about your application.
zimmicha  (posted 2013-10-10 23:38:38.39)
I have n SAPlus Burleigh confocal spectrum analyzer for the 1500nm spectral range. Can you provide replacement mirrors for the 808nm region?
jlow  (posted 2013-10-14 11:34:00.0)
Response from Jeremy at Thorlabs: We will contact you directly regarding the mirrors.
gerhard.pfeifer  (posted 2013-10-10 09:22:07.707)
Can you specify what you mean with "Ultra-Stable Athermal Invar Cavity"? How large ist the drift in Mhz/K?
rscholl  (posted 2013-10-10 16:28:00.0)
Using an external heater with SA200-14A, we measured a temperature coefficient of frequency of approximately -0.097 GHz/K around 297K. Using an external heater with SA210-14A, we measured a temperature coefficient of frequency of approximately +1.56 GHz/K around 297K. The measurement was made using an older model, but the drift values should be comparable as the mirror dimensions and substrate are identical. The values should also be comparable within the respective SA200 and SA210 families with different mirror coatings. I will contact you directly.
darmstr  (posted 2013-08-23 12:39:08.213)
I have a SA210-8B that has damaged coatings. I calculate the peak on-axis irradiance inside the cavity, with enhancement from finesse, to be about 17W/cm^2 CW at 1053 nm. That is the highest power this etalon has ever been exposed to. Whatever the damage threshold is for these coatings, it's pretty low. I purchased a replacement SA210-8B and will greatly reduce the input power, however it will probably be insufficient for my application. So here's my question: If I return the etalon with the damaged coatings, can I have replacement mirrors installed that have higher damage threshold V-coatings at 1053 nm, or with V-coatings centered at 1064 nm, if that is more convenient? If yes, can you provide a quote for cost and lead time? Thanks, Darrell Armstrong.
tcohen  (posted 2013-08-29 16:09:00.0)
Response from Tim at Thorlabs to Darrell: Thank you for your feedback. We will contact you to go over your beam parameters and to discuss a quote.
user  (posted 2013-08-09 12:03:35.637)
Does the controller provide FP with a DC output instead of sawtooth/triangle?
jlow  (posted 2013-08-09 08:11:00.0)
Response from Jeremy at Thorlabs: The SA201 only provide sawtooth or triangle waveform and no pure DC output. There's however an adjustable DC offset feature.
jlow  (posted 2012-12-27 11:43:00.0)
Response from Jeremy at Thorlabs: We are currently revising the FP interferometer tutorial online and we will make it available on the website shortly. I will get in contact with you directly regarding some references to the working principle of this device.
aadhi.prl.res.in  (posted 2012-12-27 05:50:19.493)
We purchased two Scanning Fabry-Perot interferometer and we are using in our laboratory. We want to know the working principles of this instrument but it is not available from the thorlab website. So can you send some papers or tutorial about the SFB interferometer.
rscholl  (posted 2012-12-10 11:25:00.0)
Response from Ryan at Thorlabs: Thank you for contacting us. A confocal cavity, such as is used here, works best with a TEM00 (Gaussian, spatially coherent) beam, as light is continually focused at the center by the cavity mirrors. My feeling is that it wouldn’t be suitable for your application if the emitter is larger than a couple tens of microns because rays will traverse different paths (and path lengths) through the cavity. This would result in multiple lines apparent on the scope or lower resolution. We are not aware of an appropriate solution for collimating light that does not emanate from a point source. Using a small pinhole and aspheric lens may be effective to some extent, but the power will be quite limited. The minimum power required depends on the quality of the beam, spectral distribution, wavelength, amplification of the detector and the skill of the user. Using the included detector and recommended setup at 800nm, ~50uW may be used as a guideline for the lower bound of a practical value of TEM00 light that can be detected. However, it cannot be said for certain in the case of this application.
shoryerland  (posted 2012-11-14 03:08:21.84)
Can SA210-5B be used to detecte the radiation from an ordinary rubidium hollow-cathode lamp?And how can I collimate the light into the interferometer?What is the minmum allowed input power for the SA210-5B ?
sharrell  (posted 2012-10-09 10:33:00.0)
Response from Sean at Thorlabs: Thank you for your feedback. We have been investigating the text issues on this page. There appears to be several formatting issues on the tutorial tab which are causing some text to display unusually in several browsers, and we will fix that problem today. We will also revisit the text on this page within the next several weeks to improve it. I am contacting you directly to see if you have any specific recommendations.
cdurfee  (posted 2012-09-27 06:01:14.0)
You should proofread the overview section of the fabry-perot description. There are some missing words that make it hard to read.
jlow  (posted 2012-08-27 17:04:00.0)
Response from Jeremy at Thorlabs: The photodiode used in the SA200-18B is the FGA20. You can see the specs you are looking for in the spec sheet found at http://www.thorlabs.com/Thorcat/12100/FGA20-SpecSheet.PDF.
linqian  (posted 2012-08-24 23:42:13.0)
Do you have the specification for the photodiode that comes with SA200-8B? More specifically, I want to know the photo sensitivity, effective area, dark current, cutoff frequency and capacitance. Thanks.
bdada  (posted 2012-04-23 18:39:00.0)
Response from Buki at Thorlabs to littlefox121: Thank you for participating in our feedback forum. The scanning Fabry Perot is designed for CW light sources. I have contacted you to provide further assistance.
littlefox121  (posted 2012-04-20 09:41:21.0)
I'm wondering the product scanning Fabry Perot suits for pulsed-laser source measurement? (repetition rate:10Hz duration:~10ns)
jjurado  (posted 2011-07-05 14:22:00.0)
Response from Javier at Thorlabs to ghiyas1111: Thank you very much for contacting us. All of the documentation concerning the operation and setup instructions for our scanning Fabry-Perot interferometers is available on this page. I would suggest starting with the online tutorial so that you can familiarize with the theoretical background of these devices. You can find it in the Overview tab and here: http://www.thorlabs.com/tutorials.cfm?tabid=21118. Now, for optomechanical alignment instructions of the interferometer, you can refer to the manual. This document is available on the Documents and Drawings tab and also here: http://www.thorlabs.com/Thorcat/19500/19501-D02.pdf. I will also contact you directly for further support.
ghiyas1111  (posted 2011-07-02 15:10:28.0)
i am from pakistan i had purchased it please tell me how to use it or if u have any video please send me or tell me ur phone number i will call u
Thorlabs  (posted 2010-08-16 16:05:08.0)
Response from Javier at Thorlabs to Lars Sandström: we do not have test data for the damage threshold of these Fabry-Perot interferemometers. However, the limiting factor is the power handling capability of the detector. As a guideline, it is recommended to limit the input to below 100 mW/cm^2.
lsandstrom  (posted 2010-08-16 12:40:47.0)
What is the maximum allowed input power for the SA210-5B to avoid dammage to the device?
Adam  (posted 2010-04-28 10:00:10.0)
A response from Adam at Thorlabs to jdonnelly: We recommend using as much force as possible. It is possible for these to get stuck and become impossible to remove. If that is the case, we can take it back for repair.
jdonnelly  (posted 2010-04-27 20:49:21.0)
It is not clear to me how to remove the detector unit, as specified in your alignment directions. The threaded piece at the back looks like it should turn but it doesnt, at least by hand. We are reluctant to use more force unless you so recommend.
Tyler  (posted 2009-11-30 18:10:59.0)
A response from Tyler at Thorlabs: In addition to a plot of the mirrors reflectance, an Excel file has been added to the Graphs tab that contains the data used to make the plot.
Tyler  (posted 2009-11-20 08:54:33.0)
A response from Tyler at Thorlabs: Thank you for suggesting the need to have the mirror performance data on this webpage. I added the Graphs tab to the presentation with this data on it. Please let us know if you need any additional information.
klee  (posted 2009-11-19 10:55:27.0)
A response from Ken at Thorlabs: We can send you the coating curves if you can provide your email address.
user  (posted 2009-11-18 20:54:27.0)
would be useful to have the coating curves
Greg  (posted 2009-03-12 10:09:53.0)
A response from Greg at Thorlabs to dergachev: Thank you for your interest in our SA200 series of Fabry-Perot Interferometers. We should be able to make a custom SA200 that fits your needs. A member of our Technical Support team has e-mailed you to discuss this possibility.
dergachev  (posted 2009-03-11 14:13:08.0)
Do you have a SA-200 version suitable for operation at 2 um? Please advise. Thanks. Alex Dergachev
Tyler  (posted 2009-02-12 09:45:01.0)
A response from Tyler to melsscal:For the SA200-14A, which has a wavelength range from 1450 nm to 1625 nm, the following parts are required to replicate the setup shown in the Alignment Guide tab. PH4E (Qty. 2), TR4 (Qty. 2), ER6 (Qty. 8), ER4 (Qty. 8), LCP02 (Qty. 1), CP02 (Qty. 1), SM1V10 (Qty. 1), AC254-250-C (Qty. 1), SM1L03 (Qty. 1), KCB1 (Qty. 1), BB1-E04 (Qty. 1), CP02FP (Qty. 1), and PAF-X-18-PC-C (Qty. 1). The optional alignment guides, which I strongly recommend, are the LCPA1 and CPA1 (1 Each). There are definitely other ways to do this so I recommend talking to a member of our tech support department. I will email you this list, which can then be turned into a quote if you desire. Thank you for using this feedback forum to post your question, as I think that this information may be useful to other customers.
melsscal  (posted 2009-02-12 05:52:05.0)
Can you please confirm the parts list for the fiber coupled setup with SA200-14A.
jwk  (posted 2008-06-17 11:10:31.0)
Hello, I used your FP interferometer well. Could you tell me whether you can supply the FP for 1800-2100 nm? I look forward to your reply. Best regards, Ji Won
Tyler  (posted 2008-03-19 08:40:39.0)
Response from Tyler at Thorlabs to melsscal: Thank you for your interest in our product. The output connector on the photodiode in the Fabry-Perot Interferometer is SMA. Included with the interferometer is a cable with an SMA connector on one end and a BNC connector on the other end. This information will soon be part of our product presentation. Thank you once again for taking the time to help us make our product presentation better.
melsscal  (posted 2008-03-19 06:24:39.0)
How we will get the output of this etalon?
kbuffington  (posted 2007-10-18 11:45:57.0)
Consider adding the following: If launching a free space beam into the interferometer, the beam size should be approximately 4mm. For optimum performance, the beam should then be focused to a waist with less than 600um diameter using a lens with approximately 250mm focal length. The center of the Fabry-Perot cavity should be located near the beam waist. To achieve this, use an FC/PC collimator (Thorlabs item# F260FC-*, page 1010 Thorlabs catalog, Vol 19) to collimate the beam from your fiber coupled laser. Then a lens with 250mm focal length should be inserted into the collimated beam to produce the desired spot size. We recommend the Thorlabs item# LB1056-C found on page 708 in the Thorlabs catalog, Vol 19.

Scanning Fabry-Perot Interferometer: 1.5 GHz FSR, Finesse > 1500

Item # SA30-52
Free Spectral Range (FSR) 1.5 GHz
Total Finesse 1500 (Minimum)
Resolution <1 MHz
Cavity Lengtha 50 mm
Wavelength Range 488 - 545 nm
Mirror Substrate UV Fused Silica
Detector Yes
  • Nominal Distance Between Mirrors
  • Confocal Fabry-Perot Design with Sub-MHz Resolution
  • High Finesse ≥ 1500
  • Ultrastable Athermal Invar Cavity
  • Low Scan Voltage (2.5 V per FSR @ 633 nm)
  • Ø2" Flange for Mounting in Thorlabs' KS2 or KC2 (KC2/M) Mounts
  • SMA-to-BNC Cable Included

The SA30-52 is a high finesse version of our 1.5 GHz FSR Fabry-Perot interferometers. The resolution of this interferometer is 1.0 MHz. The mirrors are designed for use in the 488 - 545 nm range (see the Graphs tab above).

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
SA30-52 Support Documentation
SA30-52Scanning Fabry-Perot Interferometer, Finesse ≥ 1500, 488 - 545 nm, 1.5 GHz FSR
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Scanning Fabry-Perot Interferometers: 1.5 GHz FSR, Finesse > 200

SA200-30C with PDAVJ5 Detector Installed
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PDAVJ51HgCdTe Amplified Photodetector, 2.7 - 5.0 µm, DC - 1 MHz BW, 1 mm2, 100 - 120 or 220 - 240 VAC
SA200-30C1Scanning Fabry-Perot Interferometer, 3000-4400 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR31Ø1/2" Optical Post, SS, 8-32 Setscrew, 1/4"-20 Tap, L = 3"
PH31Ø1/2" Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L = 3"
BA21Mounting Base, 2" x 3" x 3/8"
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Item #QtyDescription
PDAVJ51HgCdTe Amplified Photodetector, 2.7 - 5.0 µm, DC - 1 MHz BW, 1 mm2, 100 - 120 or 220 - 240 VAC
SA200-30C1Scanning Fabry-Perot Interferometer, 3000-4400 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR75/M1Ø12.7 mm Optical Post, SS, M4 Setscrew, M6 Tap, L = 75 mm
PH50/M1Ø12.7 mm Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L=50 mm
BA2/M1Mounting Base, 50 mm x 75 mm x 10 mm
SA200-30C with PDAVJ5 Detector Installed
SA200-18C with PDA10PT Detector Installed
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Item #QtyDescription
PDA10PT1InAsSb Amplified Detector with TEC, 1.0 - 5.8 µm, AC-Coupled Amplifier, Ø1 mm, 100 - 240 VAC
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR31Ø1/2" Optical Post, SS, 8-32 Setscrew, 1/4"-20 Tap, L = 3"
PH21Ø1/2" Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L = 2"
BA21Mounting Base, 2" x 3" x 3/8"
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Item #QtyDescription
PDA10PT-EC1InAsSb Amplified Detector with TEC, 1.0 - 5.8 µm, AC-Coupled Amplifier, Ø1 mm, 220 - 240 VAC
SA200-18C1Scanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
KS21Ø2" Precision Kinematic Mirror Mount, 3 Adjusters
TR75/M1Ø12.7 mm Optical Post, SS, M4 Setscrew, M6 Tap, L = 75 mm
PH50/M1Ø12.7 mm Post Holder, Spring-Loaded Hex-Locking Thumbscrew, L=50 mm
BA2/M1Mounting Base, 50 mm x 75 mm x 10 mm
SA200-18C with PDA10PT Detector Installed
  • Confocal Fabry-Perot Design
  • Wavelength Ranges from UV to MIR
  • Ultrastable Athermal Invar Cavity
  • Low Scan Voltage (2.5 V per FSR @ 633 nm)
  • Ø2" Flange for Mounting in Thorlabs' KS2 or KC2 (KC2/M) Mounts
  • SMA-to-BNC Cable Included (Except for SA200-30C)

The SA200 series of Fabry-Perot interferometers have a free spectral range of 1.5 GHz. With a minimum finesse of 200, the resolution of these interferometers is 7.5 MHz. Seven optical coatings are available with operating ranges from 290 to 4400 nm, including one dual-wavelength coating (SA200-2B). For details on these wavelength ranges, see the Graphs tab above.

For the SA200-18C, the included photodiode detector is sensitive to wavelengths from 1.8 - 2.6 µm. Since the reflectance of the mirrors inside this device remains over 99.0% in the 2.6 - 2.8 µm range, an alternative detector can be installed to use the instrument over an extended wavelength range. For this purpose, we recommend Thorlabs' PDA10PT (PDA10PT-EC) detector, which can be attached to the Fabry-Perot interferometer using an SM1T2 lens tube coupler.

The SA200-30C has a coating designed for 3.0 - 4.4 µm and does not include a detector. We recommend using the PDAVJ5 HgCdTe Amplified Photodetector, which is sensitive to wavelengths in the 2.7 - 5.0 µm range. The PDAVJ5 and the SA200-30C, which have external and internal SM1-threading respectively, can be directly mounted together without using additional threaded adapters. Please note that the PDAVJ5 includes a transimpedance amplifier, so the detector should not be connected to the transimpedance amplifier of the SA201 controller when operating the interferometer. Additionally, due to saturation effects of the diode inside the PDAVJ5 detector, the optical power entering the SA200-30C should be kept below 200 µW in order to avoid saturation. 

Item # Wavelength Range Free Spectral Range Total Finesse Resolution Cavity Lengtha Mirror Substrate Detector
SA200-2B 290 - 355 nm; 520 - 545 nm 1.5 GHz >200 (Minimum)
250 (Typical)
7.5 MHz 50 mm UV Fused Silica Yes
SA200-3B 350 - 535 nm Yes
SA200-5B 535 - 820 nm Yes
SA200-8B 820 - 1275 nm Yes
SA200-12B 1275 - 2000 nm Yes
SA200-18C 1800 - 2600 nm IR-Grade Fused Silica (Infrasil®) Yes
SA200-30C 3000 - 4400 nm Yttrium Aluminum Garnet (YAG) No
  • Nominal Distance Between Mirrors
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
SA200-2B Support Documentation
SA200-2BScanning Fabry-Perot Interferometer, 290-355 nm & 520-545 nm, 1.5 GHz FSR
$3,076.61
Today
SA200-3B Support Documentation
SA200-3BScanning Fabry-Perot Interferometer, 350-535 nm, 1.5 GHz FSR
$2,899.00
Today
SA200-5B Support Documentation
SA200-5BScanning Fabry Perot Interferometer, 535-820 nm, 1.5 GHz FSR
$3,087.28
Today
SA200-8B Support Documentation
SA200-8BScanning Fabry Perot Interferometer, 820-1275 nm, 1.5 GHz FSR
$3,275.58
Today
SA200-12B Support Documentation
SA200-12BScanning Fabry Perot Interferometer, 1275-2000 nm, 1.5 GHz FSR
$3,463.86
Today
SA200-18C Support Documentation
SA200-18CScanning Fabry-Perot Interferometer, 1800-2600 nm, 1.5 GHz FSR
$3,718.16
Today
SA200-30C Support Documentation
SA200-30CScanning Fabry-Perot Interferometer, 3000-4400 nm, 1.5 GHz FSR, No Detector
$3,704.66
Today

Scanning Fabry-Perot Interferometers: 10 GHz FSR, Finesse > 150

  • Confocal Fabry-Perot Design
  • Ultrastable Athermal Invar Cavity
  • Low Scan Voltage (2.5 V per FSR @ 633 nm)
  • Ø1" Flange for Mounting in Thorlabs' KS1 or KC1 (KC1/M) Mounts
  • SMA-to-BNC-Cable Included

The SA210 series of Fabry-Perot interferometers have a free spectral range of 10 GHz. With a minimum finesse of 150, the resolution of these interferometers is 67 MHz. Five wavelength ranges are available from 350 nm to 2600 nm. For details on these wavelength ranges, see the Graphs tab above.

For the SA210-18C, the included photodiode detector is sensitive to wavelengths from 1.8 - 2.6 µm. Since the reflectance of the mirrors inside this device remains over 99.0% in the 2.6 - 2.8 µm range, an alternative detector can be installed to use the instrument over an extended wavelength range. For this purpose, we recommend Thorlabs' PDA10PT (PDA10PT-EC) detector, which can be attached to the Fabry-Perot interferometer using an SM1T2 lens tube coupler.

Item # Wavelength Range Free Spectral Range Total Finesse Resolution Cavity Lengtha Mirror Substrate Detector
SA210-3B 350 - 535 nm 10 GHz >150 (Minimum)
180 (Typical)
67 MHz 7.5 mm UV Fused Silica Yes
SA210-5B 535 - 820 nm Yes
SA210-8B 820 - 1275 nm Yes
SA210-12B 1275 - 2000 nm Yes
SA210-18C 1800 - 2600 nm Yes
  • Nominal Distance Between Mirrors
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
SA210-3B Support Documentation
SA210-3BScanning Fabry-Perot Interferometer, 350-535 nm, 10 GHz FSR
$2,711.78
Today
SA210-5B Support Documentation
SA210-5BScanning Fabry-Perot Interferometer, 535-820 nm, 10 GHz FSR
$2,899.00
Today
SA210-8B Support Documentation
SA210-8BScanning Fabry-Perot Interferometer, 820-1275 nm, 10 GHz FSR
$3,087.28
Today
SA210-12B Support Documentation
SA210-12BScanning Fabry-Perot Interferometer, 1275-2000 nm, 10 GHz FSR
$3,275.58
Today
SA210-18C Support Documentation
SA210-18CScanning Fabry-Perot Interferometer, 1800-2600 nm, 10 GHz FSR
$3,422.74
Today

Control Box for Scanning Fabry-Perot Interferometers

Ramp Specifications
Waveform Sawtooth or Triangle
Output Voltage Range 1 - 45 V (Offset + Amplitude)
Offset Adjustment Range 0 - 15 VDC
Amplitude Adjustment Range 1 - 30 V
Rise Time Adjustment Range 1X Sweep Expansion: 0.01 - 0.1 s
100X Sweep Expansion: 1 - 10 s
Sweep Expansion 1X, 2X, 5X, 10X, 20X, 50X, 100X
Sweep Scale Error ±0.5%
Output Noise 1 mV (RMS)
~6.6 mV (Peak to Peak)
Trigger Ramp Start or Midpoint
Photoamplifier Specifications
Gain Steps 0, 10, 20 dB
Transimpedance Gain (Hi-Z) 10, 100, or 1000 kV/A
Transimpedance Gain (50 Ohms) 5, 50, or 500 kV/A
Output Voltage (Hi-Z) 0 - 10 V (Minimum Range)
Output Voltage (50 Ohms) 0 - 5 V (Minimum Range)
Bandwidth 250 kHz
Noise (RMS) <0.1 mV @ 10 kV/A
0.2 mV @ 100 kV/A
1.5 mV @ 1000 kV/A
  • TTL Trigger Output
    • Trigger on Rise for Start of Scan
    • Trigger on Fall for Midpoint of Scan
  • Adjustable DC Offset of Scan Voltage (Center Signal on Scan Midpoint)
  • Adjustable (0.01 - 10 s) Scan Time
  • Triangle or Sawtooth Scan Voltage
  • Transimpedance Gain Amplifier for Photodiode Output
  • Switch-Selectable Input: 100, 115, or 230 VAC

The SA201 is specifically designed to control Thorlabs' Fabry-Perot interferometers by generating a highly stable, low-noise voltage ramp. This ramp signal is used to scan the separation between the two cavity mirrors.

The controller, which features a power supply with a 100, 115, or 230 VAC switch-selectable input, provides adjustment of the ramp voltage and scan time, allowing the user to choose the scan range and speed, while an offset control is provided to allow the spectrum displayed on the oscilloscope to be shifted right or left.

The output trigger allows the user to externally trigger an oscilloscope on either the beginning or midpoint of the ramp waveform. The ability to trigger the oscilloscope from the midpoint makes zooming in on a line shape more convenient; just place the spectral component of interest on the center of the screen and increase the timebase of the scope. There is no need to use the offset to re-center the signal; the scope expands about the point of interest. A calibrated zoom capability provides a 1X, 2X, 5X, 10X, 20X, 50X, or 100X increase in the length of the ramp signal, thus allowing an extremely wide range of scan times.

The SA201 also includes a high precision photodetector amplifier circuit used to monitor the transmission of the cavity. The amplifier provides an adjustable transimpedance gain of 10 kV/A, 100 kV/A, or 1000 kV/A when driving a high impedance load, such as an oscilloscope. Using the output sync signal from the controller, an oscilloscope can be used to display the spectrum of the input laser. The detector circuitry incorporates a blanking circuit, which disables the photodiode response during the falling edge of the sawtooth waveform.

The SA201 is shipped with a 120 VAC power supply line cord for use in the US, while the SA201-EC is shipped with a 230 VAC power supply line cord for use in Europe.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
SA201 Support Documentation
SA201Control Box for Scanning Fabry-Perot Interferometers, 120 VAC Power Cord
$966.34
Today
+1 Qty Docs Part Number - Metric Price Available
SA201-EC Support Documentation
SA201-ECControl Box for Scanning Fabry-Perot Interferometers, 230 VAC Power Cord
$966.34
Today
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