Shearing Interferometers

Test Collimation of Coherent Sources
Options for Beam Diameters from 1 to 50 mm
Additional Shear Plates Available

SI254

Shearing Interferometer with a 10 - 25.4 mm Beam Diameter Shear Plate

SI035P

Shear Plate,
1 - 3 mm Beam Diameter

Application Idea

SI050 Interferometer with SICP Cage System Adapter, 30 mm Cage Components, and SM1 Lens Tube

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OVERVIEW

This Video Insight demonstrates the use of the shearing interferometers offered on this page, shares some troubleshooting tips, and provides guidance for determining whether these devices are a good option for testing the collimation of a particular beam.

Converging or Diverging

Collimated

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Cross-Section of a Shearing Interferometer with Optional SICP 30 mm Cage System Adapter

Features

Qualitative Collimation Testers for Coherent Beams
Options for Beam Diameters from 1 mm to 50 mm
<10% Beam Pick-Off
Magnetically Coupled Kinematic Design to Quickly Interchange Shear Plates
Imperial and Metric Threaded Mounting Holes
30 mm Cage System Compatible at Exit Port (Except Item # SI500)
Replacement Shear Plates Offered Below
Accessories Offered Separately Below (Not Compatible with Item # SI500)
- SIVS Magnified Viewing Screen for Ø1 mm - Ø10 mm Beams
- SITST Mounting Plate with SM1 (1.035"-40) Internal Threads
- SICP and SICPSM1 Adapters for Input 30 mm Cage System Compatibility

Thorlabs' Shearing Interferometers are designed to qualitatively test the collimation of a coherent beam of light. They provide a quick method to aid in the alignment of a collimating lens. The shearing interferometers consist of a wedged optical flat mounted at 45° to the incident beam. The reflection from both the front and the back surfaces of the optical flat overlap on a diffuser plate on the top of the device. A ruled reference on the diffuser plate indicates whether the incident beam is collimated. As shown in the image to the middle right, if the fringes are parallel to the line, the beam is collimated, and if they are rotated with respect to the ruled line, the beam is either diverging of converging.

The wedged optical flat is made of uncoated UV fused silica, which has a transmission range from 185 nm - 2.1 µm. For use with IR light, an IR viewer is necessary to view the fringes on the diffuser plate. The wedge angle for each plate size is optimized for the range of the acceptable beam sizes indicated on the shear plate; see the Specs tab for details. Since the light is incident on the plate at a 45° angle, the intensity of the fringe pattern is dependent on the polarization of the light. Maximum intensity will occur when the polarization is perpendicular to the plane of incidence.

Usage Guidelines
This technique requires light with sufficient coherence, low wavefront aberrations, and a shear plate thickness compatible with the beam diameter. The interferometer also needs to be used within the Rayleigh range of the collimated beam. Please see the video at the top right for more details on these limitations.

To observe interference fringes, the coherence length of the incident light must be longer than the change in optical path length caused by the shear plate being used. See the Specs tab for the approximate change in optical path length, ΔOPL, for each shear plate offered here. Broadband sources, such as Superluminescent Diodes (SLDs) and LEDs, will not create interference fringes due to their low coherence lengths.

In addition to the degree of collimation, the fringes will also be sensitive to spherical aberration, coma, and astigmatism. Aberrations will result in fringes that are no longer straight, which makes it difficult to tell whether the fringes are aligned with the reference line. For quantitative information on wavefront errors, we recommend a Shack-Hartmann Wavefront Sensor or similar device.

For fringes to be clearly visible, the incident collimated beam diameter must be matched to the shear plate used. The shear plates sold on this page support incident beam diameters from 1 mm to 50 mm. We recommend the additional SIVS Magnified Viewing Screen for beam diameters from 1 mm to 10 mm.

The distance over which a beam is collimated is never infinite, and so the device should be located within approximately one Rayleigh range of the collimated beam's waist. For more information on collimation and calculating the Rayleigh range for your application, please see our Collimated Light Insight.

Construction
Each shearing interferometer comes in a case as a set of three pieces: a base, a plate with the wedged optical flat, and a plate with the diffuser viewing screen. To set up the interferometer, attach the viewing screen plate to the base using the included cap screws and hex key. The shear plate is held in place magnetically, allowing it to be easily swapped out for a plate with a different wedged optical flat.

Behind the wedged optical flat, a hole is bored through the base to allow the transmitted beam to pass through the optical flat unimpeded. Please note, the output angle and position will be affected by the wedge angle and thickness of the shear plate used. For all models except the SI500 interferometer, this output hole has internal SM1 (1.035"-40) threads and has four 4-40 tapped holes around it for integration into our 30 mm Cage System. The base is constructed from anodized aluminum and features tapped holes for mounting to Ø1/2" posts. The cross-sectional view to the right illustrates how the shearing interferometer is constructed as well as the laser beam's propagation path. This view includes an optional SICP adapter for mounting to a 30 mm cage system from the front of the interferometer.

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SPECS

Item #	Wedge Angle	Diameter	Diameter Tolerance	Thickness	Thickness Tolerance	Approximate ΔOPL^a
SI035P	117 arcsec	7.94 mm	+0/-0.25 mm	0.75 mm	±0.25 mm	1.9 mm
SI050P	83 arcsec	12.7 mm	+0/-0.25 mm	1.30 mm	±0.25 mm	3.3 mm
SI100P	40 arcsec	15.60 mm	+0/-0.25 mm	2.60 mm^b	±0.25 mm	6.6 mm
SI254P	18 arcsec	38.00 mm	+0/-0.25 mm	6.35 mm^b	±0.25 mm	16.2 mm
SI500P	10 arcsec	78.00 mm	+0/-0.25 mm	13.00 mm^b	±0.25 mm	33.1 mm

In order to ensure interference takes place, check to see that the effective coherence length of your source is longer than the approximate change in optical path length (ΔOPL). As the coherence length of your source approaches a few multiples of ΔOPL, the contrast of the interference fringes will decrease. Once the coherence length reaches the ΔOPL, you are effectively imaging incoherent light. This ΔOPL was calculated for UVFS (n=1.457 @ 632.8 nm) with the assumption of parallel reflective surfaces separated by the thickness listed above.
Due to the thickness of these optics, the back surface of the shear plate protrudes from the mounting plate. Therefore, care should be taken when inserting or removing these plates from the base unit, especially if the SICP or SICPSM1 cage adapter plates are installed.

Item #	Included Plate^a	Beam Diameter for Included Plate^b	Compatible Plates	Compatible Accessories	Threaded Mounting Holes	Dimensions (L x W x H)
SI035	SI035P	1 - 3 mm	SI035P, SI050P, SI100P, and SI254P	SIVS, SITST, SICP, SICPSM1	8-32 and M4	50.8 mm x 48.3 mm x 55.9 mm
SI050	SI050P	2.5 - 5 mm
SI100	SI100P	5 - 10 mm
SI254	SI254P	10 - 25.4 mm		SIVS^c, SICP, SICPSM1
SI500	SI500P	25.4 - 50 mm	SI500P	None Available	1/4"-20 and M6	116.8 mm x 133.4 mm x 134.6 mm

The wedged plates are made from UVFS
Different plates with differing wedge angles are offered for different sized beams. This is to optimize the separation between the plates for particular beam diameters, and thus produce an interference pattern.
The SIVS is designed for viewing the small fringes created by small-diameter beams. It is compatible with the SI254 but is not intended or necessary for viewing the fringes of the larger diameter beams that the SI254 is intended for use with.

Click to Enlarge
Above is the transmission curve for UVFS, with light incident normal to the surface. The data shown is for a 1 mm thick, uncoated sample and includes surface reflections.

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Shearing Interferometers

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30 mm Cage Rods Secured to the Output of an SI254 Shearing Interferometer

5 Models Available for Beam Diameters Ranging from 1 - 50 mm
Qualitatively Determines Light Collimation
Magnetic Steel Balls Secure the Shear Plate to the Base
Applicable Beam Diameters Engraved on Shear Plate

These Shearing Interferometers can be used with incident beams as small as Ø1 mm and as large as Ø50 mm depending on which shear plate is used. The interference pattern is sensitive to the beam divergence and can be used to qualitatively determine whether the beam is collimated. The shear plate has three steel balls fixed to the bottom of the plate that attach magnetically to the base. This magnetic retention mechanism enables easy swapping of plates when different beam diameters are used. Please see the Specs tab for compatible plates and bases.

In addition, there is a hole drilled into the back plate of the base, enabling the light that is transmitted through the wedged optical flat to continue unimpeded. Please note, the output angle and position will be affected by the wedge angle and thickness of the shear plate used. For all models except the SI500 interferometer, this output hole has internal SM1 (1.035"-40) threads and has four 4-40 tapped holes around it for integration into our 30 mm Cage System. The SI500 output is a Ø2.95" (Ø75 mm) smooth bore. The base, diffuser plate, and shear plate come packaged in a carrying case.

Please note that due to the thickness of the shear plate optic in the SI100, SI254, and SI500 inteferometers, the back surface of the shear plate protrudes from the mounting plate as can be seen in the image to the right. Therefore, care should be taken when inserting or removing these plates from the base unit, especially if the SICP or SICPSM1 cage adapter plates are installed.

Part Number	Description	Price	Availability
SI035	Shearing Interferometer with a 1-3 mm Beam Diameter Shear Plate	$475.17	Today
SI050	Shearing Interferometer with a 2.5-5 mm Beam Diameter Shear Plate	$489.44	Today
SI100	Shearing Interferometer with a 5-10 mm Beam Diameter Shear Plate	$509.62	Today
SI254	Shearing Interferometer with a 10-25.4 mm Beam Diameter Shear Plate	$544.07	Today
SI500	Shearing Interferometer with a 25.4-50 mm Beam Diameter Shear Plate	$930.14	Today

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Replacement Shear Plates

Shear-Plate with Shearing Interferometer Base

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The shear plate is held in place with three magnets.

5 Models Available for Beam Diameters Ranging from 1 mm to 50 mm
Compatible with Multiple Shearing Interferometer Bodies
Magnetic Steel Balls Secure the Plate to the Base
Applicable Beam Diameters Engraved on Plate

Thorlabs offers five interchangeable shear plates, which together are designed to be used with incident beam diameters ranging from 1 mm to 50 mm. As seen in the image at the right, the shear plates each have three steel balls fixed to the bottom of the plates that attach magnetically to the base. The magnetically coupled kinematic design of these plates allows them to be precisely positioned on the shearing interferometer. Multiple plates are compatible with the same base (see the table below for compatibility), thus enabling the plate to be interchanged easily if the beam diameter being used for a particular experiment changes. Different shear plates are offered for different diameter beams due to the optimization of the wedge angle for the beam diameter.

Item #	Beam Diameter for Included Plate	Compatible Shearing Interferometer Base Item #
SI035P	1 - 3 mm	SI035, SI050, SI100, and SI254
SI050P	2.5 - 5 mm	SI035, SI050, SI100, and SI254
SI100P	5 - 10 mm	SI035, SI050, SI100, and SI254
SI254P	10 - 25.4 mm	SI035, SI050, SI100, and SI254
SI500P	25.4 - 50 mm	SI500

Part Number	Description	Price	Availability
SI035P	Shear Plate, 1-3 mm Beam Diameter	$172.25	Today
SI050P	Shear Plate, 2.5-5 mm Beam Diameter	$186.51	Today
SI100P	Shear Plate, 5-10 mm Beam Diameter	$205.50	Today
SI254P	Shear Plate, 10-25.4 mm Beam Diameter	$241.15	Today
SI500P	Shear Plate, 25.4-50 mm Beam Diameter	$447.85	Today

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Beam Viewing Accessories

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The SITST Mounting Plate has internal SM1 threads and comes with two 4-40 cap screws and a 1/16" hex key.

Recommended for Beam Diameters from 1 mm - 10 mm
Increases the Size of Fringes on the Diffuser Plate

The SIVS Magnified Viewing Screen is helpful for viewing the fringe pattern created by a small diameter beam. By replacing the standard diffuser plate viewing screen on the SI035, SI050, SI100, or SI254^a interferometers with this accessory, the size of the fringes will increase, thereby making it easier to determine if the light is collimated. Magnification of the fringe pattern is achieved by using a LC2265 N-SF11 Plano-Concave Lens at the base of the assembly.

If desired, the mounting plate for the SIVS viewing screen can be purchased separately (Item # SITST). It features a standard Thorlabs internal SM1 (1.035"-40) thread, which is compatible with any of our SM1-threaded components.

Both the SIVS viewing screen and the SITST mounting plate come with two 4-40 cap screws and one 1/16" hex key so they can be secured to the shearing interferometer.

Item #	Description	Compatible Shearing Interferometer Base Item #
SIVS	Magnified Viewing Screen System	SI035, SI050, SI100, and SI254^a
SITST	SM1-Threaded Mounting Plate	SI035, SI050, SI100, and SI254

The SIVS magnified viewing screen is designed for viewing fringes created by small-diameter beams. Although it is compatible with the SI254 interferometer, it should not be necessary for viewing the fringes of the larger-diameter beams for which the SI254 interferometer is intended.

Part Number	Description	Price	Availability
SIVS	Magnified Viewing Screen System for Ø1-10 mm Beams	$344.49	Today
SITST	Customer Inspired! SM1-Threaded Mounting Plate for use with Shearing Interferometers	$68.90	Today

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30 mm Cage Adapter Plates

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The SICP Cage System Adapter replaces the diffuser plate included with the shearing interferometer and can be secured using the two included 4-40 screws.

Integrate a Shearing Interferometer into a 30 mm Cage System
Replaces the Standard Diffuser Plate Viewing Screen Provided with the Shearing Interferometers
Two Versions Available:
- Integrated Diffuser Plate
- Integrated Viewing Plate with Internal SM1 (1.035"-40) Threads

These two adapter plates are designed to replace the standard diffuser plate viewing screen on a SI035, SI050, SI100, or SI254 interferometer and enable the input side of the shearing interferometer to be attached to a 30 mm cage or SM1 lens tube system. The SICP Adapter offers a diffuser for viewing the fringes, while the SICPSM1 Adapter has internal SM1 (1.035"-40) threads, allowing a Ø1" lens tube to be attached where the fringes would normally be viewed.

Since these plates feature tapped 4-40 holes for attaching the cage system, the plate cannot be attached to an existing cage system. The ERSCB cage rod adapters will allow this plate to be inserted and removed without having to disassemble the cage system.

Item #	Description	Compatible Shearing Interferometer Base Item #
SICP	30 mm Cage System Adapter with Viewing Screen	SI035, SI050, SI100, and SI254
SICPSM1	30 mm Cage System Adapter with SM1-Threaded Viewing Port	SI035, SI050, SI100, and SI254

Part Number	Description	Price	Availability
SICP	30 mm Cage System Adapter for Shearing Interferometers with Diffuser Viewing Screen	$85.24	Today
SICPSM1	30 mm Cage System Adapter for Shearing Interferometers with SM1-Threaded Viewing Port	$78.70	Today