Converging or Diverging
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Cross-Section of Shearing Interferometer with Optional Cage System Adapter Plate
Thorlabs' Shearing Interferometers can be used to determine if a coherent beam of light is collimated. The design consists of a wedged optical flat mounted at 45° and a diffuser plate with a ruled reference line down the middle. These interferometers are designed to provide qualitative analysis of a beam's collimation.
The diffuser plate is used to view the interference fringes created by Fresnel reflections from the front and back surfaces of the optical flat. If the beam is collimated, the resulting fringe pattern will be parallel to the ruled reference line. In addition to the degree of collimation, the fringes will also be sensitive to spherical aberration, coma, and astigmatism.
The wedged optical flat is made of uncoated UV fused silica. The wedge angle for each plate size is optimized for the range of the acceptable beam sizes; 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.
In order 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. See the table footnote on the Specs tab for more information.
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30 mm Cage Plate secured to the rear of a SI254 Shearing Interferometer
The base is constructed from anodized aluminum and features tapped holes for mounting to Ø1/2" posts. The base of all interferometers except the SI500 features four 4-40 tapped holes on the back side, allowing them to be mounted in our 30 mm Cage Systems (see the photo to the left). Please see the Accessories section below for information about mounting the unit into a 30 mm cage system from the front.
Behind the wedged optical flat, a hole is bored through the base to allow the transmitted beam to pass through the optical flat unimpeded. For all models except the SI500, this hole is SM1 threaded (1.035"-40). The cross-sectional view to the right illustrates how the shearing interferometer is constructed as well as the laser beam's propagation path.
We also offer two optional adapter plates for attaching a 30 mm cage system or SM1 lens tubes to the input of the shearing interferometer. Both plates integrate the viewing screen and offer 4-40 tapped holes for attaching a 30 mm cage system around a SM1-threaded input aperture. The SICP includes a diffuser for viewing fringes similar to the one provided with our shearing interferometers. Alternatively, the SICPSM1 has SM1 threading in place of the diffuser, which allows lens tubes and other accessories to be mounted at the top of the interferometer where the fringes are viewed.
Please note that none of the accessory plates are compatible with the SI500, which uses a larger base.
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The Wedged Optical Flat is Held in Place with Three Magnets.
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. As shown in the figure to the right, the plate has three steel balls fixated to the bottom of the plate, which 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 of the base plate, thus enabling the light that is transmitted through the wedged optical flat to continue unimpeded. For all models except the SI500, this hole is SM1 threaded (1.035"-40). The SI500 output is a Ø2.95" (Ø75 mm) smooth bore. The base plate, 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, 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.
Thorlabs offers five interchangeable shear plates, which together are designed to be used with incident beam diameters ranging from 1 mm to 50 mm. The magnetically coupled kinematic design of these plates allows them to be precisely located 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.
Please note that due to the thickness of the shear plate optic in the SI100P, SI254P, and SI500P, 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.
The SIVS accessory 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 SI254a with this accessory, the size of the fringes will increase, thereby making it easier to determine if the light is collimated. The SIVS comes packaged with two 1/16" screws so that it can be secured to the shearing interferometer.
If desired, the mounting plate for the SIVS can be purchased separately (SITST). It features a standard Thorlabs SM1 (1.035"-40) thread, which is compatible with any of our SM1-threaded components.
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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 4-40 screws.
These two adapter plates are designed to replace the standard diffuser plate viewing screen on the SI035, SI050, SI100, or SI254 and enable the input side of the shearing interferometer to be attached to a 30 mm cage or SM1 lens tube system. The SICP offers a diffuser for viewing the fringes. The SICPSM1 has an SM1-threaded bore, allowing a Ø1" lens tube such as the one included with the SIVS magnified viewing screen (above) 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 ERSCA cage rod adapters will allow this plate to be inserted and removed without having to disassemble the cage system.