liquid crystal slms

Liquid crystal

Spatial Light Modulators

(LCSLMs)

Ajay Singh

Engineering Physics Department Indian Institute of Technology Delhi

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

Ajay Singh , Department of Physics IIT Delhi

http://rumalis.com/40875-liquid-crystal-display

http://imgarcade.com/1/computer-screen-clipart/

http://en.wikipedia.org/wiki/Intensity_(physics)

http://en.wikipedia.org/wiki/Phase_(waves)

3

Spatial Light Modulators

Electrically addressed

spatial light modulator Photo-addressed spatial light modulator

x

y

Incident

light

Modulated

light

Transmittance T(x,y)

+

+

+

-

-

Electro-optic

material

x

y

Write iamge

IW(x,y)

Incident

light

Modulated

light

Transparent

electrodes

Mirror

Photoconductive

material


Electrically addressed spatial light modulator (EASLM)


Electrical signals to coherent (or incoherent) modulation. Fundamental optical “input device” so link between imaging optics

and electronics. Transmissive LC panels: Liquid crystal between two glass sheets,

with control circuitry added with thin film transistors.



Electrical signals to coherent (or incoherent) modulation. Fundamental optical “input device” so link between imaging optics

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.

Common Modulation Techniques:



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.



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.

one light beam (the optical control beam) is used to change a variableassociated with another light beam (the incident beam); the optical control beamis 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 andreplicate the image using liquid crystals.

As long as the OASLM is powered, the image is retained even after the light isextinguished. An electrical signal is used to clear the whole OASLM at once


Optically addressed spatial light modulator (OASLM)

The “incoherent” light is detected (asintensity), by a photo-detector (as anelectrical change distribution).This charge distribution affects themodulator, and so changes the Amplitude orPhase of the reflected coherent light.Most common (and only commerciallyavailable) are:Photo-conductor: Amorphous Silicon, (lowlight levels) or thin filmPhoto-transistor (high light levels).Modulator: Liquid Crystal.

Chang-Kui Duan, Institute of Modern Physics, CUPT

LC Spatial Light Modulator


Crystal is also bi-refringent, so ifilluminated with polarsied light it canbe used to rotate axis ofpolarisation, and hence changeAmplitude (with analyser)

A typical opticallyaddressed liquid crystalspatial light modulator(OALCSLM) operating inthe reflective mode

Photo-conductor: AmorphousSilicon, (low light levels) or thin filmPhoto-transistor (high light levels).Modulator: Liquid Crystal.


Operation of SLM

No applied voltage the molecules are “aligned” by the surface groves (Alignment layers)


Operation of SLM


SquareWave applied, “induced” dipole on molecule that is then “twisted”by the electric field


Operation of SLM


SquareWave applied, “induced” dipole on molecule that is then “twisted”by the electric field

Square Wave plus light: photo-conductor is locally discharged by thelight, so molecules in these regions not effected by electric field, sodo not twist round


Problems:• Variable contrast and sensitivity across device.• Relatively insensitive to light.• Tends to retain image.• Liquid crystal degrades• Very low yield during manufacture

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


References:

Coomber, Stuart D.; Cameron, Colin D.; Hughes, Jonathon R.; Sheerin, David T.;Slinger, Christopher W.; Smith, Mark A.; Stanley, Maurice (QinetiQ), "Opticallyaddressed spatial light modulators for replaying computer-generatedholograms", Proc. SPIE Vol. '4457', p. 9-19 (2001)

Slinger, C.; Cameron, C.; Stanley, M.; "Computer-Generated Holography as aGeneric Display Technology", IEEE Computer, Volume 38, Issue 8, Aug. 2005, pp46–53

A.M. Weiner. "Femtosecond pulse shaping using spatial light modulators". REVIEWOF SCIENTIFIC INSTRUMENTS VOLUME 71, NUMBER 5 MAY 2000. Retrieved 2010-07-06.

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5069618&tag=1http://www2.ph.ed.ac.uk/~wjh/teaching/mo/slides/slms/slm.pdfhttp://en.wikipedia.org/wiki/Spatial_light_modulatorhttp://laser.physics.sunysb.edu/~melia/SLM_intro.html#4


!