Hello, Community,
I knew that this community is awesome, but now I like it more 
I have another quick question…
How do we measure and evaluate grating / CGH is made well or not? Are we measure the wavefront of CGH and diffracted pattern from grating? Or is there any other direct testing method?
Thank you!!
That is a great question! I started to write out an answer but … realized I just was confusing myself. The CGH as you know is made to be the ‘perfect’ standard, so it really has to have the pattern printed to a high quality. How do you test a ‘perfect’ shape? I think your suggestion of measuring the wavefront makes sense, and I could also imagine that you could take sub-aperture measurements directly to measure the periodicity and shape of the pattern. But, all of that is to say I don’t know! Fortunately, we have some great experts on this forum so I hope one of them can chime in to answer this.
Hello!
I am a senior optical engineer at Arizona Optical Metrology, a company that manufactures CGHs.
This is a great question. There are a number of things that can be tested, depending on what you need to measure.
When you design a CGH for a specific test optic, it becomes hard to separate errors in the CGH from errors in the test optic. One thing you can do is validate the manufacturing process by designing a CGH for a “perfect” test optic. This perfect optic could simply be a spherical Caliball that gets measured in random orientations to average out imperfections. That would be an example of a functional test.
The other thing you could test is the accuracy of the CGH pattern itself. This can be done by measuring regularly spaced marks that are printed across the pattern in known lateral locations.
Other aspects of a CGH/grating that can be tested are depth of features, and duty cycle of features, both of which affect diffraction efficiency into the different orders. These aspects could be measured directly (i.e. with a probe or microscope), or they could be indirectly assessed by measuring intensity of light diffracted into each order.
I hope that helps!
Wow! Thank you, @ShelbyAment ! It really helped me a lot. Especially ‘validate the manufacturing process’ is very impressive for me. Fabricating / calibrating of ‘reference’ sounds always tough.