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Does anybody knows how to estimate the angular acceptance of the SLM? What are the problems if I use it at 10 deg? and at 30 deg?

This is a well-known issue but I would like to quantify the effect of oblique incidence.

Thanks

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Hi Ricardo,

I think the answer depends on the type of SLM you use (I assume that it is a liquid crystal SLM).

For a modulator that is supposed to give a phase only modulator for a given input polarization, they usually give a maximum angle you can use. For instance, I just checked in the technical notes for the Hamamatsu X10468 serie, they say to use an angle "less that 10 degrees". Because the system is calibrated for a normal incidence, the tolerance is quite small. If you increase that angle, you have a mix of amplitude and phase modulation and the phase modulation would not be linear anymore.

In the other hand, I think things will be different with older models (for instance the Holoeye LC-R 2500 that I used few years ago). For these models, the SLM does not give right away a phase only modulation, you have to use a polarizer before and after the SLM and find the best configuration (combination of polarizations) that give the best phase modulation with the lowest amplitude modulation. My guess is that with these devices, if you work at a bigger angle, you may find a suitable combination of polarizations for a good phase modulation, even if this combination is not the same one as for a zero angle.

When the modulation is not anymore a phase only modulation, you may still use a technique as the one presented in that Vellekoop et al. paper (dx.doi.org/10.1364/AO.47.002076) in which they use a combination of four pixels to achieve a phase only modulation.

I never tried to quantify the effect of the angle. It is possible to do the calculation for a given configuration of a the liquid crystals, I never did that. But again, it will depend on the SLM you are using.

Hope it helps,

Sebastien