Spatial light modulators (SLMs) are devices that spatially modulate light beams. They use liquid crystals to vary the intensity, phase, or both of incident light. SLMs can be electrically or optically addressed. Electrically addressed SLMs use thin-film transistors to apply electrical signals that change the liquid crystals and modulate light. Optically addressed SLMs use a photoconductor layer to sense an optical input which then controls the liquid crystals to modulate a read light beam. Common applications of SLMs include high resolution displays, 3D holographic displays, and optical processing systems like correlators.

1. Liquid crystal
Spatial Light Modulators
(LCSLMs)
Ajay Singh
Engineering Physics Department
Indian Institute of Technology Delhi

2. What is SLM
 Device that “spatially modulates” a coherent beam of light
 Briefly , a liquid crystal device that acts as a variable
 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
 Addressed Electrically or Optically
http://rumalis.com/40875-liquid-crystal-display
Ajay Singh , Department of Physics IIT Delhi
http://imgarcade.com/1/computer-screen-clipart/

3. 3
Spatial Light
Modulators
Transmittance T(x,y)
x
y
Incident
light
Modulated
light
+
+
+
-
-
Modulated
light
Transparent
electrodes
Mirror
Electro-optic
material
x
y
Write iamge
IW(x,y)
Incident
light
Photoconductive
material
Electrically addressed
spatial light modulator Photo-addressed spatial light modulator
Ajay Singh , Department of Physics IIT Delhi

4. Electrically addressed spatial light modulator (EASLM)
 Electrical signals to coherent (or incoherent) modulation.
 Fundamental optical “input device” so link between imaging optics
Ajay Singh , Department of Physics IIT Delhi
and electronics.
 Transmissive LC panels: Liquid crystal between two glass sheets,
with control circuitry added with thin film transistors.

5. Electrically addressed spatial light modulator (EASLM)
 Electrical signals to coherent (or incoherent) modulation.
 Fundamental optical “input device” so link between imaging optics
Common Modulation Techniques:
Ajay Singh , Department of Physics IIT Delhi
and electronics.
 Transmissive LC panels: Liquid crystal between two glass sheets,
with control circuitry added with thin film transistors.
Liquid Crystal: LC modulators switched by either thin-film transistors (transmissive
displays), or silicon backplanes (reflective devices). Usable in all applications, but
rather “slow”.
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.

6. Electrically addressed spatial light modulator (EASLM)
Address the “pixels” to change the local electric field across the liquid layer
and hence switch pixels on or off. 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 also
small displays for projection TVs and head-up displays.
Best “optics” device is 320320 pixels, in 44 cm display.
Ajay Singh , Department of Physics IIT Delhi

7. Electrically addressed spatial light modulator (EASLM)
 Excellent for image display and projection system
 This technology is set to take over from CRT monitors for
computer displays and televisions, (replacement for 17”
colour monitor available).
 Big commercial growth area, Many commercial system
from Thorn-EMI, Philips, Sony, Casio, Sharp.
Problems:
 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.
Ajay Singh , Department of Physics IIT Delhi

8. Optically addressed spatial light modulator (OASLM)
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.
Most common (and only commercially
available) are:
Photo-conductor: Amorphous Silicon, (low
light levels) or thin film
Photo-transistor (high light levels).
Modulator: Liquid Crystal.
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”
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
Ajay Singh , Department of Physics IIT Delhi

9. LC Spatial Light Modulator
Chang-Kui Duan, Institute of Modern Physics, CUPT
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)
Ajay Singh , Department of Physics IIT Delhi
A typical optically
addressed liquid crystal
spatial light modulator
(OALCSLM) operating in
the reflective mode
Photo-conductor: Amorphous
Silicon, (low light levels) or thin film
Photo-transistor (high light levels).
Modulator: Liquid Crystal.

10. Ajay Singh , Department of Physics IIT Delhi
Operation of SLM
 No applied voltage the molecules are “aligned” by the surface groves
(Alignment layers)

11. Ajay Singh , Department of Physics IIT Delhi
Operation of SLM
 No applied voltage the molecules are “aligned” by the surface groves
(Alignment layers)
 SquareWave applied, “induced” dipole on molecule that is then “twisted”
by the electric field

12. Ajay Singh , Department of Physics IIT Delhi
Operation of SLM
 No applied voltage the molecules are “aligned” by the surface groves
(Alignment layers)
 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

13. Practical Uses of AOSLMs
 very-high-resolution display
 such as one for a computer-generated holographic display
 3D holographic display
 Real-time input to Optical Correlator
 Joint Transform Correlator
Problems:
• Variable contrast and sensitivity across device.
• Relatively insensitive to light.
• Tends to retain image.
• Liquid crystal degrades
• Very low yield during manufacture
Ajay Singh , Department of Physics IIT Delhi

14. Ajay Singh , Department of Physics IIT Delhi
References:
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)
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
A.M. Weiner. "Femtosecond pulse shaping using spatial light modulators". REVIEW
OF SCIENTIFIC INSTRUMENTS VOLUME 71, NUMBER 5 MAY 2000. Retrieved 2010-
07-06.
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5069618&tag=1
http://www2.ph.ed.ac.uk/~wjh/teaching/mo/slides/slms/slm.pdf
http://en.wikipedia.org/wiki/Spatial_light_modulator
http://laser.physics.sunysb.edu/~melia/SLM_intro.html#4

15. Ajay Singh , Department of Physics IIT Delhi

16. Ajay Singh , Department of Physics IIT Delhi
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