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Hi all,

I am new to SLM so got a Santec SLM-100 with dual windows (visible and 1550 nm). A graduate student has used the visible windows for a while and it seems to work ok. I am now trying to use this for some experiment in the 1550 nm region and want to test to make sure that the SLM is working fine for that wavelength? Atm I made up a simple line grating with a pitch of 60 um (6 pixel) and diffract the zero-order to the 1st order when the grating is On. I then changed the level of phase modulation at all lines (increase the value in the csv file) and observe increasing power at the 1st spot.

My expectation is that the power would gradually increase to a peak at Pi (something between 0 and 1024 values of the pixel), then gradually reduce to near zero when the phase change reaches 2pi (1024). However I have power fluctuating and increasing all the way to nearly 2Pi (1000 value). What did I do wrong here?

Your comments are highly appreciated.

A

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Hi,

I do not know that brand at all, I will just extrapolate from what I already encountered, probably not everything is relevant in your case.

I made up a simple line grating with a pitch of 60 um (6 pixels) and diffract the zero-order to the 1st order when the grating is On. I then changed the level of phase modulation at all lines (increase the value in the csv file) and observe increasing power at the 1st spot.

That is not a bad idea, however, I would do it a bit differently. By putting a grating, you end up with a spatial square signal, that has many harmonic components in the Fourier domain, hence multiple order of diffractions.
As you have phase SLM, I would do a phase ramp. Over N pixels, I would linearly increase the pixel value between 0 and V_max and I will then increase V_max gradually. I will repeat this ramp to have it periodic of course.
If V_max corresponds to 2pi, you have a tilted plane wave with ideally (if everything was perfect and pixels infinitely small) one only spot (+1 order) in the Fourier plane.
Well, you will always have intensity in the 0-order due to the limited diffraction efficiency and filling fraction, but the other orders should be very small.

That being said, your experiment should work too, It is just not as easy (visually) to find the maximum of intensity in the +1 compared to a minimum elsewhere.

So, what can go wrong?

First, the calibration is usually given for one wavelength, meaning that increasing linearly the pixel value increase proportionally the phase for this wavelength, but for another one, the relation is not linear anymore. That is probably not the cause, but it is worth keeping in mind.

Another thing is that SLMs do not have a plane response, meaning that if you send a constant value, you do not have a plane wave. Companies usually provide a correction mask, that also depends on the wavelength.

Moreover, the value for pi depends also on the wavelength. The phase shift is proportional to 2pi/lambda*delta_n, with delta_n the birefringence induced by a given voltage. So for the same voltage (pixel value), you have much less phase modulation at 1550 nm compared to visible. So if the pi value is 500 at 775nm, it will be 1000 at 1550nm.
So most likely, your experiment worked and you find the pi value around 1000.

I may be wrong, but that would be my first guess.

Best,

Sebastien

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Thanks Sebastien.

I also applied a Sine profile instead of the rectangular one on the SLM, results were the same. Changed the incident angle, did not help. As the SLM came with dual windows AR coating I made sure to change the wavelength to 1550 nm before experiment.

Perhaps I would contact the vendor with my data and see if I did something wrong with the setup, or make up a Michelson interferometer or a reflection setup with polarizer and analyzer to check the phase directly.

Cheers,

A

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My problem turned out to be with the incident angle. With the angle less than 10 deg the issue was solved. Before I noticed extra brighter spot jump on to the position of the 1st order at phase value above Pi and increase the power at the spot (filtered through a pinhole).

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My problem turned out to be with the incident angle. With the angle less than 10 deg the issue was solved.

I did not think about that one. It is usually stated in the data-sheet to keep an incident angle below a given value, typically around 15 degrees.

Before I noticed extra brighter spot jump on to the position of the 1st order at phase value above Pi and increase the power at the spot (filtered through a pinhole).

Not sure I understand that part.

Coming back to your initial issue:

I have power fluctuating and increasing all the way to nearly 2Pi (1000 value).

I guess you got rid of the fluctuations but I would expect the actual correct Pi value to be around the maximum pixel value, so around 1000 seems legit, right?
That makes sense that around the maximum wavelength the SLM is supposed to work, you approach the limit of the phase modulation.