Hello,

I’m working on loading fresnel lens pattern (quadratic phase) onto PLUTO phase only SLM to see whether the SLM can really functions as lens. The SLM is reflective and illuminated by uniform light. However, when placing a camera to capture the modulated light, I found that the light reflected from SLM has same fresnel lens pattern as the one I load on SLM. It’s supposed to see no pattern since the SLM only modulate the phase of light. Does anyone know the reason for that?

Thanks!

Hi
Did you make sure that the Laser is polarized in the working polarizing direction of the SLM ?

Yes, it's strange that only when Laser polarizes in the working polarization of SLM then you can see the pattern.

Due you mean to say that it works ?

When I load a quadratic phase pattern onto SLM and make sure that the Laser polarization is same as the working polarization direction of SLM, I get a bright spot (focus spot) at the center but surrounded by multiple rings. This pattern is quite similar as the quadratic phase pattern I load onto SLM. (If I load a 'Windows' logo onto SLM, the modulated light will have a 'Windows' logo pattern)

After that, I change the Laser polarization to not align with the working polarization direction of SLM, and the pattern disappears. No rings, no bright spot. At that time, the SLM functions like a mirror. It seems that when the SLM modulates the phase, the amplitude of input light also change. However, the SLM I used is called phase only modulation. I don't know where the pattern comes from.

Hi,

Sorry for the delay, I missed this post.

Well, one thing that can happen is the following:
You do have a phase only modulator, but due to the finite numerical aperture of every optical setup, you always end up with some amplitude modulation, at least for the high spatial frequencies.

Consider the following. You display a checkerboard on the SLM. I assume that the SLM is working in pure phase only modulation, that means that the squares are either with a 0 phase or a pi phase. If I use two lenses as a 4-f system, what should I see in the image plane? If everything was perfect, I should see nothing with a CCD camera as it should be a phase only image similar to the one in the SLM plane. But if you do the experiment, you will see black lines at the edges or the squares but the inside of the squares themselves are all white. What happens is that the lenses you use do not have a numerical aperture equal to 1. When you display something with sharp edges, the corresponding high spatial frequencies are diffracted quickly and do not enter your lenses (or whatever optical system you have). So in your object plane, you do not have exactly the same image as in the SLM plane as you filtered out (involuntarily) the high spatial frequencies.

The short story is, when you have high spatial frequencies in a phase only image, such as sharp edges, you will always have a bit of amplitude modulation.

Now, back to the Fresnel lenses. In such pattern, you should not have sharp edges ideally. The phase should gradually go from 0 to 2pi and then to 2pi to 4pi (so 0 to 2pi again) etc... BUT, if you did not calibrated the SLM correctly, this means that you do not know accurately the value corresponding to 2pi, you will have sharp edges. If for example the value you put for the 2 pi pixel value really corresponds to 1.8 pi, the value would go gradually from 0 to 1.8 pi then from 2 pi to 3.8 pi, so you will have a sharp jump from 1.8 pi to 2 pi. These sharp edges will give rise to amplitude modulation in a finite numerical aperture system.

Sebastien

Hi, Sebastien

Thanks for your detailed explanation!

Best
Liangqing Cui