A Spatial light modulator (SLM) is a special type of liquid crystal display, which is often used in research on diffraction. A common SLM is build up as a two-dimensional array of reflective or transmissive pixels. Each of these pixels can modulate the phase and/or the intensity of the light that propagates through it. Often the image on a SLM is not regarded directly as one does with for instance a computer monitor, but because of the small size of the pixels and the use of lenses after the display the light one looks at the diffracted image.
A case where SLMs are very useful is when a two-dimensional Fourier-transform of an image has to be calculated. If the image has a large number of pixels and the calculation needs to be done many times, this requires a lot of time. Using an SLM, this can be done instantaneously. The calculation time per image is then limited by the switching of the display or and the speed of the detector. The SLM is illuminated with a plane wave, the pixels are switched so that the intensity and phase of the light at each pixel correspond with that of the input image. Two lenses are positioned at the correct distance at the back of the SLM. The image obtained at the other side is the two-dimensional Fourier-transform of the original image.
An application where this kind of calculation is done frequently and where this proves it's use is in recognition of text, finger prints or pictures (e.g. of the face or eye). This is used in the Joint Transform Correlator (JTC) of which the principle is shown on the picture below. Both the recorded image (e.g. a finger print from the suspect) and the reference image (e.g. a finger print found at the scene of the crime) are applied on the first SLM. The image at the other side of the lens is recorded with a CCD camera and is sent to a second SLM. By looking at the intensity and place of the spots in the output diffracted image one can see if the two input images were the same or not.
A second possibility to use SLMs is as a switchable lens. Light incident on a lens will undergo a phase shift depending on the place of incidence. The same 2D phase shift can be applied using a phase SLM. In this manner a switchable lens can be build. This is very useful when thinking of a way to build a pure optical switch for optical fibers. The light can be switched to the correct output fiber without the need of first converting it back to an electrical signal. An optical switch build in this way will be extremely fast.
More detailed information about the results we have obtained in our research can be found in our publications.