This document discusses spatial light modulators (SLMs), which are devices that spatially modulate light beams. It describes different types of SLMs including electrically-addressed SLMs (EASLMs) and optically-addressed SLMs (OASLMs). EASLMs are controlled electronically via computer signals, while OASLMs are controlled optically by light patterns. The document focuses on liquid crystal EASLMs, which modulate light by altering the orientation of liquid crystal molecules with electric fields. Common applications of SLMs include optical processing, image projection, and adaptive optics.

1. IDL737
Term Paper
On
Spatial Light Modulators
Submitted by :
Ajay Singh
2014JOP2558

2. 1. Introduction:
A spatial light modulator (SLM) is an object that imposes some form of spatially
varying modulation on a beam of light. Usually, an SLM modulates the intensity of the
light beam. However, it is also possible to produce devices that modulate the phase of
the beam or both the intensity and the phase simultaneously.
Fig.1 Use of SLM in a projector to project image
2. The Addressing Mode
The addressing mode refers to the type of input signal that controls the optical
properties of the SLM. It contains information regarding how the incident light beam
should be modified.
Optically-addressed – one light beam (the optical control beam) is used to change a
variable associated with another light beam (the incident beam); the optical control
beam is often called the “write beam” and the incident beam is the “read beam”.
Electrically-addressed – an electric signal is used to change a variable associated with
the incident light beam; this will often be computer generated
3. The Modulation Mechanism
The modulation mechanism refers to the intermediate step between addressing the
SLM with an input signal and actually altering the incident light beam. There are various
methods by which the information from the input signal is passed on to a modulation
material, which then interacts with incident light.

3. Mechanical – the modulating material is macroscopically deformed by the write signal
to physically block or alter the read light beam. i.e. membrane
Electrooptical – the write signal causes an electric field, which affects the modulation
material. For example microchannels, liquid crystal
Thermooptical – the optical properties of the modulating material change because
certain characteristics of the material are temperature dependent. For example liquid
crystal and thermoplastics.
Magnetooptical – a modulation material with a permanent magnetic dipole moment is
altered from the write signal creating a magnetic field. i.e. bismuth doped film of
yittrium iron garnet.
4. Liquid crystals and polarization rotation
Why do the liquid crystal molecules in a twisted nematic alignment rotate the
polarization of light (i.e. how a liquid crystal molecule’s “tilt” changes the phase of an
incident beam and it’s “helical alignment” rotates the polarization of an incident beam)?
If light enters a liquid crystal molecule with its polarization axis parallel to the slow axis
of the molecule this extraordinary ray will be slowed down; the rays will travel faster
when the polarization axis is perpendicular to the slow axis. Therefore, phase
modulation occurs as the liquid crystal molecule is tilted from having the polarization
axis of incoming light parallel to the slow axis to perpendicular to the slow axis.
Fig. 2 Rotation of liquid crystal molecules while applying electric field [7]
This can be seen in the parallel-aligned nematic liquid crystal, where the molecules
begin in an upright position and then tilt towards the direction of the electric field at an
angle θ in the longitudinal (y-z) plane. The stronger the electric field, the more the
tilting angle increases in the direction of the axis of propagation, and the more the phase
is modulated.

4. On a much smaller scale, Rayleigh scattering occurs between the electric field of the
incident light beam and the electrons of the liquid crystal rod-like molecule. This is a
type of elastic light scattering, in which negligible energy is transferred and therefore
the wavelength of the incident photon is conserved – only the direction changes. Since
these electrons are bound to the crystal, scattering occurs along the axis of the rod.
Fig. 3 Rotation of TN liquid crystal molecules while applying electric field [7]
In a twisted nematic liquid crystal, where the molecules are aligned in a helical fashion,
the scattering causes light’s polarization axis to follow the helix. Since each molecule is
tilted away from the longitudinal axis (in the transverse x-y plane) by a certain angle α,
the polarization of incident light will be rotated by an angle α as it exits the molecule.
5. Electrically addressed spatial light modulator (EASLM)
One of the most commonly used modulation mechanisms today is the electrooptical
spatial light modulator containing liquid crystals as the modulation material. The optical
properties of the liquid crystals are modified by means of an electric field.
As its name implies, the image on an electrically addressed spatial light modulator is
created and changed electronically, as in most electronic displays. EASLMs usually
receive input via a conventional interface such as VGA or DVI input. They are available
at resolutions up to QXGA (2048 × 1536). Unlike ordinary displays, they are usually
much smaller (having an active area of about 2 cm²) as they are not normally meant to
be viewed directly. An example of an EASLM is the Digital Micromirror Device at the
heart of DLP displays or LCoS Displays using ferroelectric liquid crystals (FLCoS) or
nematic liquid crystals (Electrically Controlled Birefringence effect).
5.1 ESLM Technologies : there are various technologies available for such modulation.
The most promising are :
Liquid Crystal: LC modulators switched by either thin-film transistors (transmissive
displays), or silicon backplanes (reflective devices). Usable in all applications, but rather
“slow”.

5. Magneto-Optic: Pixelated crystal of Aluminum Garnet switched by array of magnetic
coils using magneto-optic effect. High powered drive circuits, and low efficiency, but are
commercially available.
Deformable Mirror: Array of “sprung” mirrorsmake by nano-technology techniques.
Very expensive to make, rather slow, and not flat. (Excellent for incoherent light).
Multiple Quantum Well: non-linear optical effect, Quantum Stark Effect in stack of
very thin layer (_ 100rA). Extremely fast (quantum limited), but poor contrast, difficult
to make in large arrays, and difficult to drive. Future of fast optical switching.
Here we are mainly focused on Liquid crystal ESLMs. The technique is based on
change in birefringence of a birefringent material while applying electric field .
5.2 Liquid Crystal ESLMs
Transmissive LC panels: Liquid crystal is placed between two glass sheets,
with control circuitry added with thin film transistors. the “pixels” are addressed to
change the local electric field across the liquid layer and hence switch pixels on or off.
Fig. 4 Typical Structure of LCSLM [6]
Add “grey-level” by altering the time each pixel is on for and colour by placing an array
of colour filters on top of the display to group pixels in threes. Typical displays are very
large (up to 30 cm) for laptop computers, but small displays are also available for
projection TVs and head-up displays.
One of the biggest distinguishing qualities of liquid crystal spatial light modulators is the
type of microdisplay that is used to collect and modulate the incident light – either
transmissive (LCD) or reflective (LCOS). A second distinguishing characteristic is the
alignment of the liquid crystal molecules, which is typically either a parallel, vertical, or
twisted formation. This determines which variable(s) of the incident light beam can be
altered – either phase only, or amplitude and phase

6. Typical Uses of EASLM:
 Optical Processing (input and/or Fourier filter).
 Optical Switching.
 Optical neural systems.
 Real-time optical beam steering.
 Image projection, (projection TV, computer projection, VR projection).
The main problems in EASLMs are :
 Large pixels (TFT are always big)
 Small “fill-factor” (large dead areas due to TFTs)
 Not very flat, problem in coherent optics.
 Cost still rather high, mainly due to yield problems.
 All ESLMs are pixelated. Leads to some diffraction problems when used in
coherent optical systems.
6. Optically addressed spatial light modulator (OASLM)
The image on an optically addressed spatial light modulator, also known as a light valve,
is created and changed by shining light encoded with an image on its front or back
surface. A photosensor allows the OASLM to sense the brightness of each pixel and
replicate the image using liquid crystals. As long as the OASLM is powered, the image is
retained even after the light is extinguished. An electrical signal is used to clear the
whole OASLM at once.
They are often used as the second stage of a very-high-resolution display, such as one
for a computer-generated holographic display. In a process called active tiling, images
displayed on an EASLM are sequentially transferred to different parts on an OASLM,
before the whole image on the OASLM is presented to the viewer. As EASLMs can run as
fast as 2500 frames per second, it is possible to tile around 100 copies of the image on
the EASLM onto an OASLM while still displaying full-motion video on the OASLM. This
potentially gives images with resolutions of above 100 megapixels.
6.1. The basic System :
Fig.5 Working principle of OASLM [6]

7. The “incoherent” light is detected (as intensity), by a photo-detector (as an electrical
change distribution). This charge distribution affects the modulator, and so changes the
Amplitude or Phase of the reflected coherent light. Vast range of technologies for both
photo-detector and modulator are available. Most common (and only commercially
available) Photo-conductors are Amorphous Silicon, (low light levels) or thin film
Photo-transistor (high light levels) and Most common
Modulator is Liquid Crystal. The basic structure of such a device is given below :
Fig. 6 Typical Structure of OASLM
with the LC is filled in a thin cell with surface groves that align the molecules. Need a
apply electric field, so need transparent conductor, (Indium-Tin Oxide).
Operation of SLM
Fig. 7 Operation of OASLM [6]

8.  No applied voltage: the molecules are “aligned” by the surface groves.
 SquareWave applied: “induced” dipole on molecule that is then “twisted”
by the electric field.
 Square Wave plus light: photo-conductor is locally discharged by the light, so
molecules in these regions not effected by electric field, so do not twist round.
LC has a different refractive index in “aligned” and “twisted” state, so changes phase of
reflected light. Crystal is also bi-refringent, so if illuminated with polarsied light it can
be used to rotate axis of polarisation, and hence change Amplitude (with analyser).
Problems:
_ Variable contrast and sensitivity across device.
_ Relatively insensitive to light.
_ Tends to retain image.
_ Liquid crystal degrades
_ Very low yield during manufacture.
7. Application research of SLM
*Optical correlator - Optical pattern matching (Real-time input to Optical Correlator,
Joint Transform Correlator etc )
*Optical associatron - Optical neural network
*Lock-in double-exposure optical measurement - High-speed image measurement
*Adaptive control of femtosecond pulsed laser (RECAPS)
*Phase contrast filter - Phase image visualization
*Wavefront control, adaptive optics - Laser processing/machining, advanced
microscopic observation
*Reference wavefront forming, lens inspection
*Singular optics, higher-order spatial mode light (LG beam, etc.)
Potential medical and physiological applications
Nerve fiber layer image: Early diagnosis of age-related eyedisease
Micro blood vessel image: Early diagnosis of circulatory disease
Visual cell image: Early diagnosis of eye disease, visual physiology
8. Conclusion
Spatial light modulators are used to spatially modify an optical wavefront in two
dimensions. The most commonly used models are electrooptical with liquid crystal
molecules used as the modulation material; the alignment of these molecules and their
birefringence make it possible to modulate the amplitude and/or phase of an incident
light beam. Reflective SLMs are high quality devices, however they can be quite
expensive; the alternative is transmissive SLMs, which tend to have extra diffraction

9. issues but are much more affordable for a teaching lab. Spatial light modulators in
general have a variety of interesting applications in industry and research.
9. References:
[1] Coomber, Stuart D.; Cameron, Colin D.; Hughes, Jonathon R.; Sheerin, David
T.; Slinger, Christopher W.; Smith, Mark A.; Stanley, Maurice (QinetiQ), "Optically
addressed spatial light modulators for replaying computer-generated
holograms", Proc. SPIE Vol. '4457', p. 9-19 (2001)
[2] Slinger, C.; Cameron, C.; Stanley, M.; "Computer-Generated Holography as a Generic
Display Technology", IEEE Computer, Volume 38, Issue 8, Aug. 2005, pp 46–53
[3] A.M. Weiner. "Femtosecond pulse shaping using spatial light modulators". REVIEW
OF SCIENTIFIC INSTRUMENTS VOLUME 71, NUMBER 5 MAY 2000. Retrieved 2010-07-06.
[4] http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5069618&tag=1
[5] http://www2.ph.ed.ac.uk/~wjh/teaching/mo/slides/slms/slm.pdf
[6] http://en.wikipedia.org/wiki/Spatial_light_modulator
[7] http://laser.physics.sunysb.edu/~melia/SLM_intro.html#4