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Triplet Fiber Optic Collimators/Couplers
Our Triplet Collimators Use
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Thorlabs' Triplet Fiber Collimators use air-spaced triplet lenses that produce beam quality superior to aspheric lens collimators. The benefits of the low-aberration triplet design include an M2 term closer to 1 (Gaussian), less divergence, and less wavefront error. Detailed performance testing results comparing these triplet collimators to our fixed aspheric collimators are presented on the Performance tab.
Our triplet fiber collimators are available from stock with alignment wavelengths ranging from 405 nm to 2 μm, effective focal lengths (EFL) of approximately 6 mm, 12 mm, 18 mm, or 25 mm, and either an FC/PC or FC/APC connector. The exact focal length at the specified wavelength for each collimator is given in the tables below. Each lens in the collimator has a broadband antireflection coating (see the Coatings tab) in order to minimize losses caused by surface reflections.
In order to take full advantage of the superior beam quality, we recommend using our triplet collimators with our AR-coated single mode or polarization-maintaining fiber optic patch cables. These cables, which are available with either an FC/PC or FC/APC 2.0 mm narrow key connector, have an antireflective coating on one fiber end for increased transmission and improved return loss at the fiber-to-free-space interface.
These triplet collimator packages use high-precision 2.2 mm wide key connectors with tightly toleranced ceramic sleeves that provide excellent pointing repeatability, allowing the user to easily remove and replace the fiber. Please note that careful alignment is needed when mating a narrow key PM fiber with the collimator's wide key receptacle. The receptacles for the APC versions are angled so that light exiting the fiber enters the collimator perpendicular to the focal plane. All of our triplet collimators are compatible with the AD12BA, AD12F, AD12NT, KAD12F, or KAD12NT collimator mounting adapters. The longer collimators have a housing design that makes them compatible with additional adapters; triplet collimators with an EFL of approximately 18 mm are also compatible with the AD15F and AD15NT adapters, while those with an EFL of approximately 25 mm are also compatible with the AD16F and AD16NT adapters. The collimation packages with external M12 x 0.5 threading are compatible without an adapter with our cage plate CP1TM12 (CP1TM12/M) to be integrated into our 30 mm cage system.
For triplet collimators aligned to a wavelength other than what is available from stock, please contact Tech Support for additional information. Performance of each collimator over an extended wavelength range can be obtained by the using the black box Zemax files. Please see the Zemax Files tab for more information on these files.
When using these collimators as a free-space coupler, precise alignment is needed for good coupling efficiency. We recommend using a kinematic tip and tilt mount paired with an XYZ adjustable platform (such as our KM100V and MT3 (MT3/M), or our 6-axis kinematic mount paired with a lens tube coupler (K6XS with AD12F or AD16F). Additional collimator mounting adapters are also available, including Ø1/2" and Ø1" unthreaded versions and externally threaded SM05 (0.535"-40), RMS (0.800"-36), or SM1 (1.035"-40) versions. Additionally, these couplers may be used in pairs with a free-space beam between the lenses. This free-space beam can be manipulated with many types of optics prior to entering the second lens. See the Performance Tab for back-coupling efficiency graphs for these collimators.
We also offer a line of aspheric fiber collimators, including our fixed collimators and our FiberPort adjustable collimation packages, that are well suited for use with a wide range of wavelengths. For our complete line of collimation and coupling options, please see the Collimator Guide tab.
Triplet Collimator Performance
The graph above compares the pointing repeatability of an aspheric lens collimator to that of a TC12 triplet collimator. Beam deviation is graphed in X and Y components, each measured in microradians. Nineteen data points were taken with each collimator type, and the beam's position was measured on a beam profiler. This graph shows beam deviation when a fiber is inserted into and removed from the receptacle many times, and is not necessarily a measure of the beam pointing error between the centerline of mechanical reference and the optical axis.
Our triplet collimators use 2.2 mm wide key fiber connectors with tightly toleranced ceramic sleeves, leading to a pointing accuracy that is an order of magnitude better than that of a similar aspheric lens collimator without a high-precision fiber receptacle. Features such as this make our triplet collimators excellent choices for demanding applications.
The graph above plots the beam quality, M2, of 68 TC12 triplet collimators and 68 aspheric collimators. The measured beam qualities have been binned into increments of 0.02. For example, the two bars between 1.00 and 1.02 represent the number of units tested that had M2 values between 1.00 and 1.019.
This data shows that beam quality when using a triplet collimator is typically closer to 1, making it more Gaussian than when using an aspheric collimator. It also shows that beam quality achieved with a triplet collimator is more consistent from unit to unit.
Another measure of a beam's quality is the flatness of the wavefront at an image plane conjugate to the fiber tip. Using our previous generation WFS150-5C CCD wavefront sensor, we measured the wavefront of a 633 nm beam collimated with a triplet collimator. The result was less than λ/8 deviation from a flat wavefront.
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Triplet fiber collimation packages can be used for collimator-to-collimator coupling as shown in the above setup. The separation distance is the distance between the front lens' surfaces.
Light exiting one triplet collimator can be fed back into a fiber through another triplet collimator, as seen in the photo to the right. The coupling efficiencies for a subset of pairs of triplet collimators sold on this page are plotted below as a function of the separation distance, which is the distance between the front lens surfaces. This data was experimentally measured using the two-collimator setup shown in the photo to the right or by using a mirror (not shown below) to reflect the light back into the same collimator. Back-coupling efficiency is the same for FC/PC- and FC/APC-connectorized collimators.
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The divergence angle (in Degrees)
where D and f must be in the same units.
Theoretical Approximation of the Divergence Angle
The divergence angle listed in the specifications tables below is the theoretical full-angle divergence when using the fiber collimator at its design wavelength with the listed fiber. This divergence angle is calculated using the formula shown to the right, given that the light emerging from the fiber has a Gaussian intensity profile. This formula works well for single mode fibers but will underestimate the divergence angle for multimode fibers where the light emerging from the fiber has a non-Gaussian intensity profile. The graphs below illustrate the theoretical beam diameter as a function of propagation distance for each of our collimators.
The graphs below show the reflectance with respect to wavelength of the AR coatings used on the 6 lens surfaces in our triplet fiber optic collimators/couplers. The blue shaded region indicates the wavelength range specified for each coating. The table below details which AR coating is used with each item.
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This applies to item numbers with the suffixes -405, -543, and -633.
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This applies to item numbers with the suffixes -780, -850, and -980.
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This applies to item numbers with the suffixes -1064, -1310, and -1550.
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This applies to item numbers with the suffix -2000.
Performance over an extended wavelength range can be obtained by using the black box Zemax files. Below are screen shots of what you will see when you open the file as well as descriptions on how to change the operating wavelength. For this example, the TC06FC-633 black box file was used. Using these black box files allows you to see how the beam profile will change as you move away from the alignment wavelength.
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The black box Zemax file should display this screen upon opening the file.
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Click on "System" in the program header bar and then select "Wavelengths".
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Change the current alignment wavelength, indicated by the first checkmarked row, to your operating wavelength. Click "Update" in the graph window from the previous screen.
Damage Threshold Data for Triplet Collimator/Coupler AR Coatings
The specifications to the right are measured data for the antireflective (AR) coatings deposited onto the optical surface of our triplet fiber optic collimators and couplers. Damage threshold specifications are constant for a given coating type, regardless of the focal length and connector type.
Laser Induced Damage Threshold Tutorial
The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.
Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.
The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm2 (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
According to the test, the damage threshold of the mirror was 2.00 J/cm2 (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.
Continuous Wave and Long-Pulse Lasers
When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) . Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.
When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.
Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.
In order to use the specified CW damage threshold of an optic, it is necessary to know the following:
Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below.
The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).
Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):
While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.
As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.
Pulses shorter than 10-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism . In contrast, pulses between 10-7 s and 10-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
The energy density of your beam should be calculated in terms of J/cm2. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e2 beam.
Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately . A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm2 at 1064 nm scales to 0.7 J/cm2 at 532 nm):
You now have a wavelength-adjusted energy density, which you will use in the following step.
Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT . For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.
The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:
Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10-9 s and 10-7 s. For pulses between 10-7 s and 10-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.
Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.
 R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration.
A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.
CW Laser Example
However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm.
An AC127-030-C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value:
The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application.
Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations
As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm2.
The energy density of the beam can be compared to the LIDT values of 1 J/cm2 and 3.5 J/cm2 for a BB1-E01 broadband dielectric mirror and an NB1-K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration:
This adjustment factor results in LIDT values of 0.45 J/cm2 for the BB1-E01 broadband mirror and 1.6 J/cm2 for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm2 maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system.
Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths
This scaling gives adjusted LIDT values of 0.08 J/cm2 for the reflective filter and 14 J/cm2 for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage.
Pulsed Microsecond Laser Example
If this relatively long-pulse laser emits a Gaussian 12.7 mm diameter beam (1/e2) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10-4 J/cm2 per pulse. This can be compared to the LIDT values for a WPQ10E-980 polymer zero-order quarter-wave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm2 for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm2 for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a high-power CW beam.
Fiber Collimator Selection Guide
Click on the collimator type or photo to view more information about each type of collimator.
*See the table below for the exact focal length at the specified wavelength for each TC06FC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC06APC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC12FC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC12APC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC18FC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC18APC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC25FC Triplet Collimator.
*See the table below for the exact focal length at the specified wavelength for each TC25APC Triplet Collimator.