Small Beam Shear Interferometer

J. Kevin Erwin

The small beam shear interferometer allows adjustment of focus, coma, astigmatism and spherical aberration in optical systems for laser beam sizes ranging from 2.5 to 8 mm. The beam is sheared using an air gap between two glass plates, magnified, then projected on a ground glass viewing screen. Interference between the sheared beams produces interference fringes indicating the wave front quality of the beam under test. A reference line on the screen indicates the perfect focus or collimation angle for the fringes. Focus alignment sensitivity is approximately 170 micro radians of beam divergence, corresponding to a 3o rotation of the fringes.

In the model shown in the photo on this page, a beam can be checked in two directions using a rotatable beam-splitter cube mounted on the front. The cube is inserted into the beam path of the optics system, and the wave-front quality can be checked directly or in reflection back from a mirror element or optical disk surface. This is a popular design because the beam splitter cube can be inserted into a small space between optical elements to check the beam. Special designs can include a CCD camera to capture images.

This instrument is well suited for fast alignment of optical elements in systems using laser diodes in research and manufacturing. Until now, there have been no compact, inexpensive interferometers commercially available that can do this. For many applications, this instrument can replace expensive phase-shifting laser diode testers with far less setup time. It operates at all visible wavelengths as well as near IR. Beams from laser diodes at 830 and 780 nm can be easily tested by viewing the instrument's ground glass screen with an IR viewer.

The following images were made with DIFFRACT software. They simulate the Small Beam Shear Interferometer using a 5 mm beam at 650 nm. The amount of tilt in the shear plate is 1.75 waves at 90 degrees, with a shear of 1.415 mm. The astigmatism images will show that there is a small change in the number of fringes as the image is rotated, however the fringe tilt at different rotational angles is more apparent. Be sure to scroll down to see all the images—there are 23 of them, and the last one is a color image.

  No aberrations, perfect focus.

  .25 waves of defocus, (diverging beam).

  -.25 waves of defocus, (converging beam).

  .25 waves of spherical.

  -.25 waves of spherical.

  .25 waves of coma at 0 degrees.

  -.25 waves of coma at 0 degrees.

  .25 waves of coma at 90 degrees.

  -.25 waves of coma at 90 degrees.

  .25 waves of coma at –90 degrees.

  -.25 waves of coma at –90 degrees.

  .25 degrees of astigmatism at –90 degrees.

  .25 waves of astigmatism at –45 degrees.

  .25 waves of astigmatism at 0 degrees.

  .25 waves of astigmatism at 22.5 degrees.

  .25 waves of astigmatism at 45 degrees.

  .25 waves of astigmatism at 90 degrees.

  .25 waves coma and .25 waves spherical at 0 degrees.

  .5 waves coma and .5 waves spherical at 0 degrees.

  1 wave of spherical.

  3 waves of spherical.

  5 waves of spherical.

3 waves of spherical using color DIFFRACT intensity graph.