Shear Interferometer to test collimation

[Woland]

Hello everyone,

I am trying to test the collimation of light emerging from an off-axis parabolic mirror. My mirror has a diameter of 0.5m or so. The focal length is about 5m. The light source I am using is a red laser focused at a pinhole. To test the collimation of this point source, I am using a shear plate. Basically light comes in, reflects from both sides of the thick piece of glass, and produces an interference pattern. The laser is good enough to produce fringe patterns. The angle of the fringe patterns indicates the radius of curvature of the wavefront, and therefore the collimation.

What I am finding is that this method is not sensitive enough for me, because I can move the point source by several mm before I notice a change in the pattern angle. Also, my patterns go wavy once in a while, so that doesn't help.

Is anyone familiar with this technique, and can give some advice on how to improve this set up. Maybe a different approach all together?

 

[Andy Resnick]

What is the diameter of the pinhole? What is the flatness of your shear plate? When you say you move the point source several mm, is that along the optical axis, transverse, etc.? How well are you filling the mirror?

 

[Woland]

The pinhole diameter is 100um. I am not sure about the flatness of the plate, but here is a link to the model I am using:
http://www.thorlabs.com/thorProduct.cfm?partNumber=SI750

When I mention moving the point, I mean along the optical axis. I start with a position which provides a diverging beam, move the source through the focal point, and, in theory, stop once it is at the focal point of the mirror.

The mirror is filled fully by the point source, in fact the point source overfills the mirror.

 

[Andy Resnick]

The pinhole diameter is too big. It should be closer to 5 or 10 microns (using the proper spatial filtering optics) in order to produce a good spatially coherent beam.

Try that and see what happens.