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Hollow Retroreflector Mirrors


  • Reflective Design Eliminates Dispersion, Chromatic Aberrations, and Material Absorption
  • Protected Silver or Gold Coating
  • Beam Deviation: <20 arcsec
  • Ø0.6", Ø0.9", and Ø1.8" Clear Aperture Mirrors

HRR201-M01

Protected Gold Coating,
SM05-Threaded Mount

Application Idea

HRR201-M01 Mirror in
LMR05 Mount on
XRN25P Linear Stage

Mirror Reflects an Inverted and
Reversed Image Back to Source

HRR203-P01

Protected Silver Coating,
SM1-Threaded Mount

Related Items


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Mirror in Protective Case
Click to Enlarge

SM05-threaded retroreflector mirrors are shipped inside a storage case composed of a modified OC22 Canister and OC2SM05 Lid. Note that the canister has had its top magnet removed.

Features

  • Mirror Reflects an Inverted and Reversed Image Back to Source
  • Two Metallic Coating Options
    • Protected Silver Coating (N-BK7 or UV Fused Silica Substrate)
    • Protected Gold Coating (UV Fused Silica Substrate)
  • Three Entrance Aperture Options: Ø0.60'', Ø0.90'', and Ø1.80''
  • SM05- or SM1-Threaded Mounts
  • Custom Coatings Available (Contact Tech Support)

Thorlabs' Hollow Retroreflector Mirrors use three orthogonal surfaces to reflect light back to the source at the same angle as the incident beam, regardless of changes in position or alignment, creating an inverted image. This insensitivity to mirror alignment and lack of refraction through glass make these hollow retroreflectors ideal for applications where dispersion, chromatic aberrations, material absorption, or front surface reflections from glass prisms are undesirable. Examples of such applications include interferometry and optical delay lines.

These hollow retroreflector mirrors are constructed using three substrates of either N-BK7 or UV Fused Silica. A protected silver or gold coating is applied on the polished faces. Please note that only UV fused silica retroreflectors are available with a gold coating. Each mirror is mounted in an engraved adapter that has either external SM05 (0.535"-40) or SM1 (1.035"-40) threads for compatibility with SM05- or SM1-threaded mounts. The robust corner cube construction, rotational symmetry, and stability result in low beam deviation of <20 arcsec. Each mirror with an SM05-threaded mount is shipped inside one of our objective containers for protection and storage; please refer to the photo to the right for details. The retroreflector with a SM1-threaded mount is shipped in a unique container to accomodate the mount size; please refer to the HRR203-P01 product photo for details.

Note that unless the incident and reflected beams strike the exact center of the hollow retroreflector, they will not overlap but rather be shifted with respect to each other, as illustrated in the animation below. For example, if the incident beam strikes the optic 3 mm to the right of center, the retroreflected beam will emerge 3 mm to the left of center. 

The video above shows the beampath when using a retroreflector mirror.
Note that the coating has been rendered translucent for illustration purposes.
Optical Coatings and Substrates
Optic Cleaning Tutorial
Laser Diode Module
Click to Enlarge

Retroreflector Dimensions

Item # HRR202-P01 HRR201-P01 HRR203-P01 HRR201-M01
Clear Aperture Entrance: Ø0.60" (15.2 mm)
Single Leg: Ø0.28" (7.1 mm)
Entrance: Ø0.90" (22.9 mm)
Single Leg: Ø0.41" (10.4 mm)
Entrance: Ø1.80" (45.7 mm)
Single Leg: Ø0.84" (21.2 mm)
Entrance: Ø0.90" (22.9 mm)
Single Leg: Ø0.41" (10.4 mm)
Coating Protected Silver Protected Gold
Reflectance per
Surface (Average)
>97.5% for 450 nm - 2 µm
>96% for 2 - 20 µm
>96% for 800 nm - 20 μm
Reflectance Curvesa
(Click for Plot)

12° AOI

45° AOI

12° AOI

45° AOI
Substrate Thickness 4 mm 6 mm 5 mm 6 mm
Substrateb N-BK7 UV Fused Silica N-BK7 UV Fused Silica
Surface Flatness λ/4 at 633 nm λ/8 at 633 nm λ/4 at 633 nm λ/8 at 633 nm
Surface Quality 40-20 Scratch-Dig
Beam Deviation
(Parallelism)
<20 arcsec
Damage
Threshold
Pulsed - 0.225 J/cm2 (800 nm, 99 fs, 1 kHz, Ø0.167 mm)
3 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)
- 2 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)
CWc - 500 W/cm (1.07 µm, Ø0.974 mm)
1500 W/cm (10.6 µm, Ø0.339 mm)
- 500 W/cm (1.07 µm, Ø0.089 mm)
750 W/cm (10.6 µm, Ø0.339 mm)
Threading SM05 (0.535"-40) SM1 (1.035"-40) SM05 (0.535"-40)
Dimensions (H x W) 0.99" x 0.93"
(25.1 mm x 23.7 mm)
1.41" x 1.52"
(35.8 mm x 38.7 mm)
2.33" x 2.52"
(59.1 mm x 64.0 mm)
1.41" x 1.52"
(35.8 mm x 38.7 mm)
  • The angle of incidence (AOI) is given relative to the individual surfaces.
  • Click Link for Detailed Specifications on the Substrate Glass
  • The power density of your beam should be calculated in terms of W/cm. For an explanation of why the linear power density provides the best metric for long pulse and CW sources, please see the Damage Thresholds tab.

The shaded regions in the graphs denote the ranges over which we guarantee the specified reflectance. Please note that the reflectance outside of these bands is typical and can vary from lot to lot, especially in out-of-band regions where the reflectance is fluctuating or sloped. The reflectance is specified per surface and the angle of incidence (AOI) is given relative to the individual surfaces.

-P01 Protected Silver at Near-Normal Incident Angle
Click to Enlarge

Excel Spreadsheet with Raw Data for -P01 Protected Silver Coating
-P01 Protected Silver at 45 Degree Incident Angle
Click to Enlarge

Excel Spreadsheet with Raw Data for -P01 Protected Silver Coating
Protected Gold at Near-Normal Incident Angle
Click to Enlarge

Excel Spreadsheet with Raw Data for Protected Gold
Protected Gold at 45 Degree Incident Angle
Click to Enlarge

Excel Spreadsheet with Raw Data for Protected Gold
Damage Threshold Specifications
Item # Type Damage Threshold
HRR201-P01 Pulsed 0.225 J/cm2 (800 nm, 99 fs, 1 kHz, Ø0.167 mm)
3 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)
CWa 500 W/cm (1.07 µm, Ø0.974 mm)
1500 W/cm (10.6 µm, Ø0.339 mm)
HRR201-M01 Pulsed 2 J/cm2 (1064 nm, 10 ns, 10 Hz, Ø1.000 mm)
CWa 500 W/cm (1.07 µm, Ø0.089 mm)
750 W/cm (10.6 µm, Ø0.339 mm)
  • The power density of your beam should be calculated in terms of W/cm. For an explanation of why the linear power density provides the best metric for long pulse and CW sources, please see the "Continuous Wave and Long-Pulse Lasers" section below.
  • For ultrafast optics, the laser-induced damage threshold (LIDT) is defined as the fluence (per pulse) that produces visible damage after a given number of pulses. LIDT values are not guaranteed in the ultrashort pulse regime. As such, they are provided as a service to customers. See the Graphs tab for a plot of LIDT as a function of the number of incident pulses.

Damage Threshold Data for Thorlabs' Hollow Retroreflector Mirrors

The specifications to the right are measured data for Thorlabs' Hollow Retroreflector Mirrors.

 

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.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

LIDT metallic mirror
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.
LIDT BB1-E02
Example Test Data
Fluence # of Tested Locations Locations with Damage Locations Without Damage
1.50 J/cm2 10 0 10
1.75 J/cm2 10 0 10
2.00 J/cm2 10 0 10
2.25 J/cm2 10 1 9
3.00 J/cm2 10 1 9
5.00 J/cm2 10 9 1

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) [1]. 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.

Linear Power Density Scaling

LIDT in linear power density vs. pulse length and spot size. For long pulses to CW, linear power density becomes a constant with spot size. This graph was obtained from [1].

Intensity Distribution

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:

  1. Wavelength of your laser
  2. Beam diameter of your beam (1/e2)
  3. Approximate intensity profile of your beam (e.g., Gaussian)
  4. Linear power density of your beam (total power divided by 1/e2 beam diameter)

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):

CW Wavelength Scaling

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.

Pulsed Lasers

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 [2]. 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.

Pulse Duration t < 10-9 s 10-9 < t < 10-7 s 10-7 < t < 10-4 s t > 10-4 s
Damage Mechanism Avalanche Ionization Dielectric Breakdown Dielectric Breakdown or Thermal Thermal
Relevant Damage Specification No Comparison (See Above) Pulsed Pulsed and CW CW

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

Energy Density Scaling

LIDT in energy density vs. pulse length and spot size. For short pulses, energy density becomes a constant with spot size. This graph was obtained from [1].

  1. Wavelength of your laser
  2. Energy density of your beam (total energy divided by 1/e2 area)
  3. Pulse length of your laser
  4. Pulse repetition frequency (prf) of your laser
  5. Beam diameter of your laser (1/e2 )
  6. Approximate intensity profile of your beam (e.g., Gaussian)

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 [3]. 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):

Pulse Wavelength Scaling

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 [4]. 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:

Pulse Length Scaling

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.


[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).

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.

Intensity Distribution
A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile.

CW Laser Example
Suppose that a CW laser system at 1319 nm produces a 0.5 W Gaussian beam that has a 1/e2 diameter of 10 mm. A naive calculation of the average linear power density of this beam would yield a value of 0.5 W/cm, given by the total power divided by the beam diameter:

CW Wavelength Scaling

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:

CW Wavelength Scaling

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
Suppose that a pulsed Nd:YAG laser system is frequency tripled to produce a 10 Hz output, consisting of 2 ns output pulses at 355 nm, each with 1 J of energy, in a Gaussian beam with a 1.9 cm beam diameter (1/e2). The average energy density of each pulse is found by dividing the pulse energy by the beam area:

Pulse Energy Density

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:

Pulse Length Scaling

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
Suppose that a pulsed laser system emits 10 ns pulses at 2.5 Hz, each with 100 mJ of energy at 1064 nm in a 16 mm diameter beam (1/e2) that must be attenuated with a neutral density filter. For a Gaussian output, these specifications result in a maximum energy density of 0.1 J/cm2. The damage threshold of an NDUV10A Ø25 mm, OD 1.0, reflective neutral density filter is 0.05 J/cm2 for 10 ns pulses at 355 nm, while the damage threshold of the similar NE10A absorptive filter is 10 J/cm2 for 10 ns pulses at 532 nm. As described on the previous tab, the LIDT value of an optic scales with the square root of the wavelength in the nanosecond pulse regime:

Pulse Wavelength Scaling

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
Consider a laser system that produces 1 µs pulses, each containing 150 µJ of energy at a repetition rate of 50 kHz, resulting in a relatively high duty cycle of 5%. This system falls somewhere between the regimes of CW and pulsed laser induced damage, and could potentially damage an optic by mechanisms associated with either regime. As a result, both CW and pulsed LIDT values must be compared to the properties of the laser system to ensure safe operation.

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.

Insights into Retroreflectors

Scroll down to read about interactions between light and corner-cube retroreflectors.

  • How does alignment affect the beam path through a retroreflector?
  • Why coat the backsides of solid prism retroreflectors with metal?
  • Does the angle of incidence affect the output beam power from a corner-cube retroreflector?

Click here for more insights into lab practices and equipment.

 

How does alignment affect the beam path through a retroreflector?

Labeled sextants of a retroreflector illustrated in 2D
Click to Enlarge

Figure 2: There are six possible sequences of reflections for a beam. The zone in which the first reflection occurs determines the sequence. These maps apply to beams approximately parallel with the retroreflector's normal axis. The beam paths are indcated by arrows, and dots mark reflections.
Labeled Sextants of a cube retroreflector
Click to Enlarge

Figure 1: The three reflective faces of a corner-cube retroreflector are shown in false color and with numerical labels assigned to each half. Retroreflectors are designed to reflect an incident beam once from each face and provide an output beam parallel to the input.
Example beam path through a hollow retroreflector (In 5 Out 2)
Click to Enlarge

Figure 4: Shifting the position of the first reflection to below the diagonal of the red face causes the next reflection to occur from the yellow face. After the third reflection, from the blue face, the beam exits the retroreflector travelling parallel to but shifted from the output beam in Figure 3.
Example beam path through a hollow retroreflector (In 4 Out 1)
Click to Enlarge

Figure 3: When the first reflection occurs above the diagonal of the red face, and the beam is parallel to the retroreflector's normal axis, the second reflection occurs from the blue face. The beam then reflects from the yellow face before exiting the retroreflector.

Beams output from corner-cube retroreflectors travel parallel to the input beam, but in the opposite direction. The input beam can be aligned to the vertex or to a point on one of the three faces. The input and output beams are colinear if the input beam is aligned to the vertex. The two beams will be separated if the input beam spot does not overlap the vertex.

Input beams aligned to one of the retroreflector's faces will reflect from that face and then the other two before exiting the retroreflector. For a range of incident angles, there are six possibilities for the order in which the beam will reflect from the three different faces. lt can be useful to select the path through the retroreflector for reasons that include optimal beam positioning and minimizing polarization effects.

For a beam to follow a particular sequence of reflections, it is not sufficient to align the beam so that it is incident on a specific face. The beam must also be incident on the proper half of that face.

Tracing the Beam Path
When looking into the vertex of the retroreflector, reflective effects make it possible to see the six halves of the three faces. Here, they are identified using dashed diagonal lines (Figure 1). In addition, the three faces of the retroreflector are shaded with false color for illustrative purposes. The normal axis is not shown, but it passes through the vertex and is equidistant from all three faces.

The six different possible reflection sequences can vary with angle of incidence. The maps in Figure 2 apply to beams nearly parallel with the normal axis. While a hollow retroreflector is used for these illustrations, these sequences of reflections also apply to prism retroreflecting mirrors.

The position of the first reflection determines which sequence of reflections the beam will follow through the retroreflector. The beam always exits from a different face than it entered.

Example
Figures 3 and 4 illustrate the two orders of reflections that can occur when the first reflection occurs from the left-most vertical face. The incident beam is parallel to the retroreflector's normal axis.

When the first reflection occurs above the diagonal, as shown in Figure 3, the last reflection occurs from the horizontal (yellow) mirror. However, locating the first reflection below the diagonal results in a last reflection from the other vertical (blue) mirror. The output beams of these two cases are parallel to, but shifted from, one another.  

Date of Last Edit: July 8, 2020

 

Why coat the backsides of solid prism retroreflectors with metal?

Data plot and graphical Representation of Psi and Chi parameters of polarization ellipses associated with each retroreflector output sextant when input light was polariized vertical
Click to Enlarge

Figure 6: Vertically polarized beams were input to a TIR solid prism retroreflector (PS975M) and a backside-gold-coated solid prism retroreflector (PS975M-M01B). The polarization ellipse of each output beam is shown in the zone that provided the beam's third reflection. For a plot of the ellipticity angle ( χ ) and orientation angles ( ψ ) with respect to the horizontal axis, click here.
Data plot and graphical Representation of Psi and Chi parameters of polarization ellipses associated with each retroreflector output sextant when input light was polariized horizontal
Click to Enlarge

Figure 5: Horizontally polarized beams were input to a TIR solid prism retroreflector (PS975M) and a backside-gold-coated solid prism retroreflector (PS975M-M01B). The polarization ellipse of each output beam is shown in the zone that provided the beam's third reflection. For a plot of the ellipticity angles ( ψ ) with respect to the horizontal axis, click here.
The percent of light output by each sextant of a retroreflector that is polarized parallel to the input vertically or horizontally polarized light.
Click to Enlarge

Figure 8: Retroreflectors convert some of the input light to the orthogonal polarization. Over 90% of the light output from the backside-gold-coated solid prism retroreflector (PS975M-M01B) remained polarized in the input state. In the case of the TIR solid prism retroreflector (PS975M), that percentage strongly depended on beam path and did not exceed 80%.
Relationships between input and output sextants of corner-cube retroreflectors
Click to Enlarge

Figure 7: A retroreflector is designed to reflect an input beam once off of each face. When the beam is approximately normal to the viewing plane illustrated in Figures 5 and 6, the beam will follow one of six beam paths.

When the backsides of solid prism retroreflectors are coated with metal, polarization changes induced in the output beam are significantly reduced.

This is due to the difference between specular reflections, which occur from interfaces between glass and the higher refractive index metal, and reflections that occur due to total internal reflection (TIR), which require the backside material, like air, to have a lower refractive index.

Compared with TIR, a specular reflection from a glass-metal interface better preserves the input beam's polarization ellipticity.

Polarization and Beam Path Diagrams
Beam paths through a retroreflector can be described by dividing its three reflective faces into six wedge-shaped zones (Figures 5, 6 and 7). Solid gray boundary lines mark physical lines of contact between reflective faces. Dotted gray lines indicate boundaries between the halves of each face.

The retroreflectors in these figures are oriented with one face-to-face interface aligned with the vertical axis. When the input beam is normal to these figures' viewing planes, Figure 7 describes the order in which the input beam reflects from the three faces before being output.

Output Polarization State
Two sets of six measurements were made for both a PS975M TIR solid prism retroreflector and a PS975M-M01B backside-gold-coated solid prism retroreflector. Input light was linearly polarized, vertically for one set of measurements and horizontally for the other. In a set, each measurement was taken with the beam aligned to a different zone. At all three reflections, the beam was confined within a single zone.

In Figures 5 and 6, the polarization states of the output beam are represented using polarization ellipses. Each output beam's polarization ellipse is shown in the zone that provided the third reflection. 

Ideally, the output beam would have the same polarization state as the input beam. However, these measurements indicate the retroreflectors converted some of the incident light to the orthogonal polarization. The plot in Figure 8 is a measure of the fraction of light in the output beam that was polarized parallel to the input. 

The backside-gold-coated solid prism retroreflector was significantly more successful in maintaining the polarization state of these linearly polarized input beams.

Date of Last Edit: July 7, 2020

 

Does the angle of incidence affect the output beam power from a corner-cube retroreflector?

Reflections cause multiple beam interference at the output of a retroreflector.
Click to Enlarge

Figure 10: Since the refractive indices of glass and air are different, the beam reflects at the front face. Reflected light can make multiple passes through the retroreflector before being output. Coherent overlapping beams produce interference effects.
The primary beam path through a solid prism retroreflector.
Click to Enlarge

Figure 9: The beam path through a corner-cube retroreflector includes a reflection from each of the three back faces, in an order determined by the position of the incident beam. The incident beam shown above has a 0° AOI and is displaced from the vertex.
Oscillations of the power output by solid prism retroreflectors with uncoated and AR-coated front faces as well as a hollow retroreflector, all as a function of small AOI.
Click to Enlarge

Figure 12: Output power as a function of AOI differed depending on the type of corner-cube retroreflector. Data from measurements, made as described in Figure 11, were normalized to the same scale, and traces were vertically shifted as a visual aid. Oscillation amplitude was strongly suppressed when the front face was AR-coated (PS975-C). Oscillations were not observed for the hollow retroreflector (HRR201-M01).
Oscillations of the power output by a solid prism retroreflector, as a function of small AOI.
Click to Enlarge

Figure 11: The power output by a TIR solid prism retroreflector (PS975M) was measured as a function of AOI. The incident beam was provided by a DBR1064S 1064 nm laser source, whose coherence length was several meters. The largest-amplitude oscillations resulted around 0° AOI, where the first- and third-pass beams overlapped. The 1/e2 beam diameters did not overlap for AOIs larger than ±1° at a distance of 30 cm from the front face of the retroreflector.

The beam power output by solid prism retroreflectors may oscillate around an average value as the angle of incidence (AOI) varies. This is due to a multiple-beam interference effect that can occur when the coherence length of the light source is at least twice the optical path length through the retroreflector.

When the front face of a solid retroreflector has an anti-reflective coating, oscillation amplitudes for all AOIs are substantially reduced. Hollow metal-coated retroreflectors provide output beams whose power is approximately independent of AOI.

Beam Path
These corner-cube retroreflectors provide an output beam that travels in a direction parallel and opposite to the incident beam. Figure 9 shows one beam path.

The AOI is determined using a reference axis normal to the front face of the retroreflector. This axis passes through the vertex and is equidistant from the three back faces.

Reflections from the Front Face
As illustrated in Figure 10, light can make multiple passes through a solid prism retroreflector, depending on whether the light reflects from or is transmitted through interfaces between the front face and the surrounding medium.

When a glass retroreflector is surrounded by air, ~96% of the light is in the primary output beam, which makes a single pass through the retroreflector, and ~0.16% is in the beam that completes an additional round trip. In this work, light making additional round trips had negligible intensity.

Conditions for Interference
Since the output of solid prism retroreflectors consists of beams that have travelled different optical path lengths, they will interfere if:

  • The beams overlap, which is more likely when the AOI of the incident beam is near 0° and the output is measured closer to the retroreflector. At larger distances, the beam deviation specified for the retroreflector and the AOI will more widely separate the first- and third-pass beams. 
  • The coherence length of the source is longer than the difference in path length between the primary beam and the overlapping beam that has made more than one pass through the retroreflector.

Corner-Cube Retroreflectors Compared
The variation of output power with small AOI was compared for four different types of corner-cube retroreflectors: a PS975M TIR solid prism retroreflector, a PS975M-M01B backside-gold-coated solid prism retroreflector, a PS975M-C TIR solid prism retroreflector with an antireflective-coated front face, and a HRR201-M01 that has a hollow construction. The input source was a DBR1064S 1064 nm laser diode with a coherence length of several meters, and the power detector was placed 30 cm from the front face of the retroreflectors. The beam size was small enough to ensure that each reflection occurred from a single face.

Figure 11 plots the normalized measurements made for the TIR solid prism retroreflector. As the AOI increased, the centers of the first- and third-pass beams shifted away from one another. At AOIs greater than around ±1°, the beams' 1/e2 diameters no longer overlapped. This resulted in the oscillation amplitude decreasing with AOI. The range of AOIs over which oscillations were significant would increase if the detector were located closer to the front face.

Figure 12 plots the trace from Figure 11, as well as traces measured for the other three retroreflectors, on the same scale but vertically shifted as a visual aid. These results indicate that an antireflective-coated front face suppresses power oscillations in beams output by solid prism retroreflectors. The power output by hollow retroreflectors does not oscillate, since there is no material boundary at the front face.

Date of Last Edit: July 7, 2020


Posted Comments:
user  (posted 2020-09-30 16:08:12.103)
Dear Sir/Madam, Are You able to make Hollow Retroreflector Mirrors with 1.) coating same as for UM10-Y4HP Picosecond Yb Laser Line Mirror, Fourth Harmonic, 250 - 272 nm? 2.) bigger optical aperture like 1.8inch diameter? Best Regards, Pawel
YLohia  (posted 2020-09-30 04:40:16.0)
Hello Pawel, thank you for contacting Thorlabs. Custom optics can be requested by emailing your local Thorlabs Tech Support team (in your case techsupport.se@thorlabs.com) or by clicking the red "Request Quote" button above. We will discuss the possibility of offering this directly.
Markus Greiner  (posted 2020-01-03 14:19:43.787)
POLARIZATION: A common challenge for corner cubes is the polarization performance. While angled bare metal coatings preserve polarizations quite well, protected or enhanced metal coatings can change the polarization state quite a bit. How does this part affect polarization?
YLohia  (posted 2020-01-06 03:42:00.0)
Thank you for contacting Thorlabs. We are currently in the process of gathering this data. I will reach out to you directly to discuss this.

Protected-Silver-Coated Hollow Retroreflectors

  • SM05- or SM1-Threaded Mounts (See Table Below)
  • N-BK7 or UV Fused Silica Substrates
  • Protected Silver Coating
    • Ravg > 97.5% per Surface from 450 nm to 2 µm
    • Ravg > 96% per Surface from 2 µm to 20 µm

These hollow retroreflectors have either SM05- or SM1-threaded mounts and come with a -P01 protected silver coating for the 450 nm - 20 µm wavelength range. The UV fused silica (UVFS) substrate on the HRR201-P01 mount is 6 mm thick, while the N-BK7 substrates on the HRR202-P01 and HRR203-P01 mounts are 4 mm and 5 mm thick, respectively. The mirrors are mounted in an engraved SM05P05 or SM1P1 adapter that has either external SM05 (0.535"-40) or SM1 (1.035"-40) threads for compatibility with our SM05-threaded or SM1-threaded mounts.

Item # HRR202-P01 HRR201-P01 HRR203-P01
Clear Aperture Entrance: Ø0.60" (15.2 mm)
Single Leg: Ø0.28" (7.1 mm)
Entrance: Ø0.90" (22.9 mm)
Single Leg: Ø0.41" (10.4 mm)
Entrance: Ø1.80" (45.7 mm)
Single Leg: Ø0.84" (21.2 mm)
Coating Protected Silver
Reflectance per Surface (Average) >97.5% for 450 nm - 2 µm
>96% for 2 - 20 µm
Substrateb N-BK7 UV Fused Silica N-BK7
Substrate Thickness 4 mm 6 mm 5 mm
Surface Flatness λ/4 at 633 nm λ/8 at 633 nm λ/4 at 633 nm
Dimensionsa 0.99" x 0.93"
(25.1 mm x 23.7 mm)
1.41" x 1.52"
(35.8 mm x 38.7 mm)
2.33" x 2.52"
(59.1 mm x 64.0 mm)
Mount Threading SM05 (0.535"-40) SM1 (1.035"-40)
  • Height including mirror and mount x mirror width.
  • Click Link for Detailed Specifications on the Substrate Glass
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
HRR202-P01 Support Documentation
HRR202-P01NEW!N-BK7 Hollow Retroreflector, Ø0.6" Clear Aperture, Protected Silver Coating
$490.00
Today
HRR201-P01 Support Documentation
HRR201-P01Customer Inspired! UVFS Hollow Retroreflector, Ø0.9" Clear Aperture, Protected Silver Coating
$477.41
5-8 Days
HRR203-P01 Support Documentation
HRR203-P01NEW!N-BK7 Hollow Retroreflector, Ø1.8" Clear Aperture, Protected Silver Coating
$690.00
Today

Protected-Gold-Coated Hollow Retroreflectors

Item # HRR201-M01
Clear Aperture Entrance: Ø0.90" (22.9 mm)
Single Leg: Ø0.41" (10.4 mm)
Coating Protected Gold
Reflectance per Surface (Average) >96% for 800 nm - 20 µm
Substrateb UV Fused Silica
Substrate Thickness 6 mm
Surface Flatness λ/8 at 633 nm
Dimensionsa 1.41" x 1.52"
(35.8 mm x 38.7 mm)
Mount Threading SM05 (0.535"-40)
  • Height including mirror and mount x mirror width.
  • Click Link for Detailed Specifications on the Substrate Glass
  • SM05-Threaded Mount
  • UV Fused Silica Substrate
  • Protected Gold Coating
    • Ravg > 96% from 800 nm to 20 µm

This UVFS hollow retroreflector has an SM05-threaded mount and comes with a -M01 protected gold coating for the 800 - 20 µm wavelength range. The substrate the mount is 6 mm thick. The base the UV fused silica mirror is mounted in an engraved SM05P05 adapter that has external SM05 (0.535"-40) threads for compatibility with our wide range of SM05-threaded mounts.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
HRR201-M01 Support Documentation
HRR201-M01Customer Inspired! UVFS Hollow Retroreflector, Ø0.9" Clear Aperture, Protected Gold Coating
$477.41
Lead Time
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