For fast collimation testing of laser beams under laboratory conditions, Qioptiq offers a new, compact Shear Plate Interferometer that delivers outstanding value for money. Even under unfavorable ambient conditions – whether vibration, dirt, or temperature fluctuations – it provides fast, reliable results with certified, consistent quality.

When expanded laser beams are used in the laboratory, the quality of the laser beam collimation is crucial for the application; in particular when collimated laser light serves as a solid measure. Collimation can be checked using applicable metrology devices, such as a Shack-Hartmann sensor (see the article beginning on page 22 in this issue of Optolines). Shear Plate Interferometers offer an interesting and, moreover, economical alternative.

Principles of Shear Plate Interferometry
Shear Plate Interferometers are based on the principle of superimposing a wave front on itself. The light beam being tested is split into two waves which are shifted laterally to one another, producing the shear. The division and shifting of the wave fronts is a result of the reflectivity on the two surfaces of an extremely flat plate, called the shear plate. The two partial waves interfere in the overlap area. The original wave front can be derived from the resulting interferogram. Wedge-shaped shear plates are used to check the collimation of a laser light beam. In the case of an ideally collimated light beam, a smooth wave front is incident on the wedge plate and parallel interference fringes perpendicular to the wedge are formed. This is indicated by a reference band on the observation plane. With this technique, for example, expansion systems can be adjusted for reproducible collimation.

Evaluating the tilt angle
If the expansion system is not aligned properly – it shows either a positive or negative focusing effect – the interference fringes tilt clockwise (negative) or counter-clockwise (positive). This effect can be evaluated quantitatively. Using the tilt angle of the interference fringes, the radius of curvature of the incident wave front, R, can be derived.

Many assembly options
The interference fringes are easy to evaluate visually on the integrated dispersion plate. A reference band is embedded in the plate to detect the tilt angle of the fringes. Qioptiq Shear Plate Interferometers are available in three different models, each optimized for a different beam-diameter range between 1 mm and 25 mm:

• 1 mm to 3 mm
• 3 mm to 8 mm
• 8 mm to 25 mm

Moreover, these Shear Plate Interferometers are designed with flexible mounting options for trouble-free integration in highly individual optical assemblies. They are compatible out-of-the-box with Qioptiq Rail and Column systems, and adapter plates can be ordered separately for mounting the Shear Plate Interferometer in other systems. Bore holes for versatile mounting options are a standard feature of the
interferometer housing.

Applications and typical accuracies
Shear Plate Interferometers are primarily used for testing the collimation of laser beams. With a beam diameter of 25 mm, collimation can be determined with a precision of 50 ?rad. Because it can determine the radius of curvature of the wave front, the Shear Plate Interferometer can also be used to determine the focal length of weakly focused optical systems, or to determine the radius of curvature of weakly focused convex and concave mirrors in the range above 8 m. For example, using a Shear Plate Interferometer and an expanded 25 mm laser beam, the radius of curvature of a 45 m mirror can be determined with a precision of +/- 3 % by reading off the relevant parameters, given in the equation above.

Other applications and typical accuracies

• Refractive index determination of 10 mm thick plane-parallel plates to +/- 0.001 
• Focal length determination of lenses/mirrors to +/- 0.03 %

Tested by MPI in Göttingen
A prototype of the Shear Plate Interferometer has already been used at the Max Planck Institute, where it was employed in the Nano-Biophotonics Department for measurement tasks in connection with 4Pi microscopy.

Focus on the Qioptiq Shear Plate Interferometer

• Optimum collimation determination; three models to provide immediate results for beam diameters of 1 mm to 25 mm in Vis-NIR spectra
• Determination of wave front radius of curvature opens up a wide range of application areas
• Adapter plates available for a broad variety of assembly options
• Robust design thanks to common path interferometry and an operating principle without electronic parts

For more information, visit www.qioptiq.com