A spatial light modulator [SLM] is an optical device which can modify the amplitude, phase, and/or polarization pattern of a
coherent light beam under computer control.
SLMs can readily produce structured beams with specialized "designer" wavefronts, such as optical vortices, and have been applied to
many fields of research and technology. They can also be used to emulate optical elements such as lenses and diffraction gratings in
educational laboratories.
Most modern SLMs use liquid crystals as the modulating material and hence are very similar to the LC displays found in many commercial
products.
Only a few companies - including Hamamatsu in Japan and Holoeye in Germany - make liquid crystal SLMs, and nearly all of these are
specialized research grade devices that cost $15,000 or more.
Recently, however a small company in the UK, Cambridge Correlators, has offered a relatively inexpensive SLM (Model SDE1024) for less
than one-tenth this cost. It is intended to be used primarily for educational purposes and has some performance limitations.
The Laser Teaching Center [LTC] recently purchased two of these SDE1024 units [2] for future use in a variety of
optics projects by undergraduates and high school students.
The SDE1024 utilizes a reflective twisted-nematic liquid-crystal-on-silicon technology. It offers XGA resolution with
1024×768 9×9 μm pixels.
The liquid crystal element (similar in size to a postage stamp) is mounted on the side of a small metal box which contains the control
circuit.
The bit-depth is 8 bits, which corresponds to 256 possible phase shift levels. However, the maximum phase shift achievable is just
0.8π for red light versus the desired 2π or more.
This project is concerned with putting the SLMs into operation and investigating and evaluating their capabilities. The SLM is
programmed by sending it a video signal from an auxiliary display port on the control computer. The transmitted XGA image can be
calculated in MATLAB or drawn in a graphics program.
We will initially be illuminating the SLM with a broad beam of 635 nm light obtained by collimating the output of a fiber-coupled
diode laser (Cambridge Correlators Model LM635 Laser Module). Later experiments could use a shorter wavelength, which will improve
the achievable phase range. Resulting patterns will be viewed and recorded with a Thorlabs DCC1545M high-resolution CMOS camera.
Once the SLM is operational, we will perform a series of simple experiments to confirm our understanding of twisted-nematic displays.
In particular we will study the phase and polarization shifts as a function of the control signal.
Lastly, time-permitting, we hope to study how to mitigate diffraction efficiency limits imposed by the limited phase range of our SLM
[3]. One possible way to do this might be to combine our two SLMs in such a way that phase shifted light from the first unit is
further phase shifted in the second one.
We thank Catherine Herne (Colgate) for assistance with the MATLAB programs, Hui Cao (Yale) for invaluable conversations, and Melia
Bonomo for researching and facilitating the purchase of our SLMs and diode laser module.
References:
[1]
A. Hansen, "Twisting Light with Twisty Molecules," unpublished report (2004).
[2]
Cambridge Correlators, "SDE1024 Spatial Light Modulator Kit", datasheet.
[3] Bowman et al "Optimisation of a
low-cost SLM for diffraction efficiency and ghost order suppression." Eur. Phys. J. Special Topics 199, 149-158 (2011).