Spatial Light Modulators
Introduction
A spatial light modulator (SLM) is a device that can modulate the phase,
amplitude, or polarization of light waves in space and time. [MORE TO
COME]
Addressing Mode: Where is the information coming from?
[MORE FOR THIS SECTION]
The addressing mode refers to the type of input signal that is used to
modulate the readout beam. The input signal is carrying the information
that we want to transmit.
[ADDING IN A FIGURE1]
Optically-Addressed: uses an optical control beam to change
something about the readout (input) light beam. A detection mechanism
couples the input signal into a modulating material.
Electrically-Addressed: uses an electrical control signal to change
something about the readout light beam. In other words, this type of SLM
converts an electrical input to an optical output.
Modulation Mechanism: The Intermediate Step
The modulation mechanism is what obtains information from the input signal
and modifies the readout optical wavefront accordingly. There are various
ways in which this modulation material can be altered to represent the
information being transmitted. The response time, required activation
energy, and spatial scale for each of these methods affect the speed,
sensitivity, and spatial resolution of the SLM respectively.
mechanically: the modulation material is macroscopically deformed
by the input signal, causing a physical force field
electrically: the modulation material is affected from interactions
with the input signal that causes an electric field. The electric field
either:
The Modulation Material: Liquid Crystals
(MORE TO BE ADDED TO THIS SECTION)
The spatial light modulators that we’re interested in from Holoeye have
nematic liquid crystal microdisplays. Nematic means that the molecules
are oriented in parallel but not in well defined planes (in comparison to
smectic, in which the liquid crystal molecules are oriented in parallel
and arranged in well defined planes)
1. LCOS (liquid crystal on silicon): reflective
a. PAN (parallel aligned nematic)
b. VAN (vertical aligned nematic)
2. LCD (liquid crystal display): translucent
c. TN (twisted nematic)
Modulation Variables: How are we encoding information?
The modulation variable is the “thing” about a particular optical
wavefront that is being modified, as a function of space and time, to
carry information from the input signal.
Amplitude (Intensity): achieved by changing the absorption
characteristics of the modulating material
Analog multiplication: modification of the amplitude of th optical
wavefront according to the reflectivity or transmissivity of the
propagation medium.
When a uniform plane wave passes through a thin film, we need to multiply
the wave’s amplitude by the transmissivity of that film. This was the
original method used to encode information into optical processors; now,
instead of having to mechanically advance the film, the SLM can rapidly
change the reflectivity or transmissivity of the propoagation medium.
Optical Correlator: a device used to compare two signals by means
of the Fourier transforming properties of a lens.
The Fourier transform of the input beam is multiplied by a stored
Fourier transform in the Fourier plane. If the input and reference
patterns match exactly (i.e. the stored transform is an exact complex
conjugate of the transform of the input), a second Fourier transform will
lead to an autocorrelation function containing a sharp peak (i.e. a well
focused spot of light in the output plane). If the input and reference
patterns don’t match, the second Fourier transform results in a
cross-correlation function, which has no sharp peaks.
An optical correlator can also pick out matches from cluttered or
partial data, with an appropriately sized peak according to the strength
of the match. Read more about optical correlators and their uses
here. The SLM
could
be
used as an input beam or in the Fourier plane as the reference, either way
searching through thousands of 2D data patterns each second.
[ADDING IN A FIGURE3]
Displays: SLM-based projection system can overcome the limitations
of using three cathode ray tubes (which are costly, limited in brightness,
and require precise adjustments). Three separate SLMs modulate the
amplitudes of the readout beams (red, green, and blue inputs), which are
then combined to produce and project the image.
Binary thresholding: creation of a binary image from a grayscale
analog image by allowing light to pass through a single active resolution
element (pixel) only if the signal intensity is greater than an
established threshold.
Binary Optical Matrix Processor: source input points are either
blocked or transmitted in each element of an NxN matrix mask.
The SLM can be used an optical crossbar switch, which is a device that can
rearrange interconnects between N transmitting ports and N receiving ports
rapidly and in a manner that doesn’t affect the other interconnects. The
source/detector configuration ensures that any input can be channeled to
any output. Read more about optical crossbar switches and their
applications
here.
[ADDING IN A FIGURE5]
Phase: achieved by changing the refractive index of the modulating
material or the physical path length the light must travel
Polarization: achieved by changing the birefringence of the
modulation material. Birefringence is a property in which the refractive
index depends on the state of polarization and direction of light
propagation.
How to choose between performance parameters
[MORE TO COME]
(type of LC microdisplay, resolution of pixels, input image frame rate,
wavelength limits, size of active area, etc)
|