The Applications and Technology of Phase-Only Liquid Crystal on Silicon Devices

Abstract

An introduction to the technology of liquid crystal on silicon (LCOS)
devices leads on to a discussion of the application areas which have been
and are being opened up by the development of phase-only devices.

Full text

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 *Corresponding author: nc229@cam.ac.uk
Abstract—An introduction to the technology of liquid crystal
on silicon devices leads on to a discussion of the application areas
which have been and are being opened up by the development of
phase only devices.
Index Terms—Holography, Optical interconnections, Optical
switches, Optical device fabrication.
I. INTRODUCTION
Liquid crystals on silicon (LCOS) devices were conceived in
an attempt to marry the complexity of CMOS integrated circuit
technology (Fig. 1) with an electrooptic material which could
be switched by the low voltages available from this
technology.
Fig. 1 Processed 4” silicon wafer used for constructing liquid
crystal on silicon video display [3]
Fig. 2 Cross section of a liquid crystal on silicon device
The construction of the device is along the lines of the familiar
liquid crystal cell except that the silicon die forms one of the
Manuscript received February 28, 2010. This work was supported by the
Liquid Crystal Photonics Platform Grant (EP/F00897X/1) from the EPSRC.
N. Collings, Tony Davey, and Bill Crossland are with the Engineering Dept,
Cambridge University, Cambridge CB3 0FA, UK (phone: +44.1223.748295;
fax: +44.1223.748348; e-mail: nc229@ cam.ac.uk).
J. Christmas is with Two Trees Photonics, Milton Keynes MK8 8LW, UK
bounding surfaces of the cell in place of the traditional glass
(Fig. 2). The advantages of reflective LCOS include:
capitalizing on standard silicon CMOS technology; integration
of high performance drive circuitry on the silicon chip; high
pixel fill factor; high quality process technology for excellent
pixel mirror reflectivity; and scalability to smaller feature
sizes/ higher pixel number. However, in the early days, it was
low complexity displays in aircraft instrumentation and low-
power terminals which were the first target products of this
technology [1,2]. During the late 80s/early 90s, research
workers started to contemplate their use as amplitude screens
in Optical Computing [4], and as spatial filters in Optical
Correlators [5]. It was the latter field which mediated the
transition to phase-only liquid crystal spatial light modulators.
Initially, the binary phase modulator configuration was used
employing a magnetooptic spatial light modulator [6]. The
analogue liquid crystal transmissive television screens could
be used to advantage if the phase/amplitude nature of the
modulation was taken into account [7]. Later, projector screens
which gave sufficient phase modulation with negligible
amplitude modulation were used, eg as the programmable
matched filter in the Brite-Euram optical correlator [8]. The
initial motivation was the increase in light throughput
compared to the binary device. As the numerous other
advantages of using phase coding technology became
appreciated, the number of application areas has grown.
During the late 80‟s and early 90‟s, in response to the US
Defense DARPA initiative on Optical Computing, the UK
government instigated an important research program under
the JOERS Alvey scheme, called DOACC. The acronym stood
for Digital Optics and its Application to Computing and
Communications. The purpose of DOACC was to define
applications which would benefit from Digital Optics, and to
develop follow-on programs which would address these
applications. One of the follow-on research programs which
developed from DOACC was HPSLM, or High Performance
Spatial Light Modulators. This program laid the foundations
for high complexity LCOS spatial light modulators. One of the
device outcomes of HPSLM was the 176x176 LCOS device -
the first ferroelectric liquid crystal (FLC) LCOS device [4].
Applications were envisaged in artificial neural networks;
image processing; fuzzy logic; optical correlators; and
switching networks for computers and telecommunications.
The applications and technology of phase-only
liquid crystal on silicon devices
Neil Collings* a, Tony Daveya, Jamie Christmasb, Bill Crosslanda
aDepartment of Electrical Engineering, University of Cambridge, CB3 0FA, UK
bTwo Trees Photonics Ltd, 8 Garamonde Drive, Wymbush, Milton Keynes MK8 8LW, UK
Transparent conductor
(Front electrode)
Liquid crystal
Glue seal
Silicon die
Substrate
Flex
Front glass

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Eventually, a UK based LCOS industry, which is now known
as Forth Dimension Display, resulted from this program.
Nevertheless, current impediments to the adoption of this
technology are the lack of good phase modulating liquid
crystal spatial light modulators (LCSLMs). High-end devices
are now available commercially from Holoeye (Table 1) and
sub-systems from Hamamatsu (Table 2), and also some
university groups, such as the Photonics and Sensors Group at
Cambridge University Engineering Department, have started
to make their own.
The application areas which will be discussed have their
roots in the past, and these roots will be introduced for each
area. Prior to this discussion, however, the technology itself
will be presented in Section II.
II. LCOS TECHNOLOGY
A. Historical background
The liquid crystal electrooptic effects employed in LCOS
devices have been emblematic of the period in which the
device was developed. For example, in the 70s, the first
published LCOS device was based on a dynamic scattering
nematic liquid crystal [1]. In the 80s, a dyed phase change
nematic was used [2]. The size of the silicon die used in LCOS
was much larger at that time than the size used today (Fig. 1).
I. Underwood‟s thesis, in the 90s, was based on twisted
nematics [9]. The development of LCOS devices, with an
emphasis on fast binary devices using ferroelectric liquid
crystal (FLC), proceeded in both the UK [2, 4] and the US
[10-13] during the early 90‟s. The strong motivation was fast
binary phase only filtering in an optical correlator.
The FLC devices give binary amplitude (black/white)
modulation when the polarizer is oriented parallel to one of the
director states and the analyser is oriented perpendicular to the
polarizer. The optic axis of the liquid crystal switches in the
plane of the LC layer and that the switch angle, which depends
on the liquid crystal used, is close to 45 degrees. In order to
convert the device from amplitude modulating to phase
modulating, the polarisation of the incident beam should be
rotated by 22.5 degrees. The analyser remains oriented
orthogonal to the polariser. This will give binary phase
modulation. The limited switching angle of the FLC introduces
loss. For high efficiency a switching angle of 90 degrees is
preferred. However, FLCs with a switching angle approaching
90 deg are not readily available and are not easy to use.
Nematic LCOS devices give analogue (grey scale) modulation.
There have been several projects aimed to develop high-end
devices. IBM worked on a 2048 x 2048 pixel device [14].
ESPRIT and IST research projects MOSAREL [15]
and LCOS4LCOS [16] targeted 2560 x 2048 pixel and 1920 x
1080 pixel devices, respectively. These projects have not
resulted in any commercially available devices. The
processing of large silicon die involves stitching the fields
during the lithography which is a step prone to human error
[17].
Of the devices which are now commercially available, the
Holoeye LC-R 2500 uses the Philips 1024 x 768 pixel LCOS
with a custom interface; the Holoeye HE 1080P is based on
the Aurora Systems 1920 x 1080 pixel LCOS. The liquid
crystal in the Philips device is configured is a 45 deg. twisted
nematic (TN) which provides reasonably high contrast with a
good cell gap tolerance [18]. The liquid crystal in the Holoeye
HE 1080P device is a parallel aligned nematic which provides
electrically controlled birefringence (ECB), ie phase-only
modulation. For phase-mostly modulation in the 45 deg TN,
the polariser and analyser orientations have to be optimised.
The optimal orientation is that which gives a 2pi phase
modulation with minimal amplitude variation. For phase only
operation in the ECB device, the thickness of the layer must be
doubled compared with an ECB device designed for amplitude
modulation. If the incident polarization is random, then a
further doubling of the layer thickness together with a quarter
wave plate on the reflecting mirror is required [19].
B. Cell assembly
A good overview of LCOS fabrication technology for
ferroelectric liquid crystal devices is given in [20]. A complete
wafer of cells is made simultaneously (wafer-level assembly).
Nematic LCOS wafer-level assembly is similar except for the
alignment layer. The cell assembly in our group is based on
die-level assembly where excellent results can be achieved if
cells are assembled individually with a robotic assembly tool
[21].
Figure 3: Die-level assembly Flow (taken from [21])
The LCOS backplane is not flat and can, typically, have 1
micron of peak to valley curvature. Die level assembly allows
the assembler to select plate glass for the cell which
complements the curvature of the backplane (Fig. 3). By this
means the uniformity of the LC layer thickness can be kept
within defined tolerances. The cell gap is defined by plastic
spacers. The spacer technology can be improved by defining
post spacers either at the stage of the silicon processing or
post-processing using photolithography [22]. The surface
treatment of the glass and LCOS in order to define the
orientation of the LC director is an issue. Some of the
treatments which are conventional in glass cells cannot be
applied to the LCOS surface. For example, some of the
polyimide treatments require high temperature curing which
can degrade the reflectivity of the pixel mirrors. The rubbing
of the alignment layer can also degrade the reflectivity.
Inorganic evaporated alignment layers [23] are the alignment
technology of choice for LCOS Since the nature of the
alignment depends on the angle of the evaporated beam, die-
level assembly is convenient because the small die size
reduces the dispersion of angles in the evaporation beam.
An engineering approach to the mechanical issues of cell
assembly has been presented in [24].

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TABLE 1
COMMERCIALLY AVAILABLE LCOS DEVICES (PHASE ONLY LCOS IN BOLD)
Manufacturer
Products
Mode
Array Size
Resolution
Pitch
FF
Speed
Waveband
Aurora Systems
ASI6010
ECB
0.7” diag
1920*1080
8
>87
420 - 700 nm
BNS
P512
7.68*7.68
512*512
15
83.4
142
Hz
515 – 585 nm1
Forth DD
SXGA-R3
FLC
17.43*13.95
1280*1024
13.6
93
510 Hz
420 - 700 nm
Micron
SVGA
FLC
0.463" diag
800*600
11.75
91.7
360 Hz
Hitachi
CP-SX5500
0.7" diag
1400*1050
92
Holoeye
HEO 1080 P
ECB
0.7” diag
1920* 1080
8
87
60 Hz
Up to 800 nm2
iMD
HX-7015
0.59" diag
800*600
JVC
D-ILA
VAN
1.27" diag
4096*2400
6.8
93
Sony
4K SXRD
VAN 5 ms
1.55"diag
4096*2160
8.5
72
72
Syndiant
SYL2061
0.44” width
1024*600
Varitronix
VMD6100
0.82" diag
1920*1280
9
90
1. Four other wavebands available up to 1650 nm, with reduced response times
2. Second version optimized for the near IR up to 1064 nm also available. Third version, PLUTO-TELCO, for
wavelengths up to 1550 nm
(Legend: Pitch – Interpixel spacing in microns; FF – Fill Factor or Percentage of pixel which forms a reflective mirror)
C. Switching speed
The frame rate of a particular ECB device has been measured
under a variety of drive conditions and is worse case limited to
1.7 Hz [25]. This is the major drawback of using ECB for
phase-only modulators and limits most of the applications to
be discussed. Various attempts have been made to improve the
speed. The use of voltages above the Freedericksz threshold
will allow higher frame rates. By reducing the thickness of the
LC layer and introducing a twist, the speed has been reduced
in the Microdisplay EHD. This device uses a proprietary
ultranematic mode [26, 27] which can switch at 5 V with a rise
time of 0.2 ms and a fall time of between 0.9 and 1.38 ms.
However, this device produces amplitude modulation. In order
to provide phase only modulation up to 2pi, the thickness of
the LC layer should be doubled and there should be no twist.
The thickness of the LC layer can be reduced by using high
birefringence liquid crystals such as those designed for
Polymer Dispersed Liquid Crystals (PDLC) by Merck [28].
The voltage available from the silicon backplane can be
increased by using proprietary high voltage processes [29],
and hence the switch-on speed can be improved. An additional
technique for increasing the speed is to use a pi cell [30]. In
this cell, the alignment pre-tilt provides opposite tilts on either
side of the cell so that the switching to the homeotropic state
does not couple to backflow. The switching of the liquid
crystal to the homeotropic state is promoted by a flow of the
liquid crystal [31]. A particular version of the pi-cell is given
the name of Optically Compensated Birefringence (OCB).
This has been used to produce a 720 Hz frame rate in LCOS
[32].
D. Backplane design
The LCOS backplane contains all necessary circuits, such as
demultiplexers, shift registers, timing controllers, row
scanners, digital-to-analogue convertors (DACs), and pixel
circuitry. A distinction is made between a DRAM-like and an
SRAM-like pixel circuitry [33]. This is also the differentiation
between analogue addressing and digital or Pulse Width
Modulated (PWM) addressing, which is described in [34].
Provided that a memory can be provided at the pixel, in the
form of an analogue storage capacitor (DRAM) or digital
memory circuit (SRAM), the array can be frame-addressed.
This has the advantage of reducing the low frequency flicker
of the line-addressed arrays. The advantage of PWM is that the
LC is driven between zero and saturation voltages in a time
multiplexed fashion [35], so that the slow response of
switching between adjacent voltages states is avoided. The
nematic liquid crystal responds to the RMS of the applied
waveform integrated over the switching time of the liquid
crystal. However, the time varying AC modulation of the drive
voltage is partially transferred to the LC and this temporally
modulates the reflected light [36, 108]. In addition, the
physical properties of the liquid crystal should be optimized
for PWM drive [37]. The DRAM pixel array requires DACs
which can increase the energy dissipation of the backplane.
Low power DACs for this application was suggested in [38].
The foundries which produce LCOS backplanes are: SMIC
(0.25 um process); Citizen (0.25um); Chartered (0.18 um); and
Fujitsu (0.18 um). The last three offer high voltage processes.
An alternative to the high voltage process is to design the pixel
circuitry according to the bootstrapped pixel method [39].
III. ADAPTIVE OPTICS
The first adaptive lenses using a LC layer were probably made
using plano-convex and plano-concave glass lenses [40]. The
LC is imbibed between the lens and a flat glass substrate in
each case. Both the flat glass and the curved surface of the lens
are treated with a planar alignment agent so that a parallel
aligned nematic LC cell is created. The problems with thick
liquid crystal cells, turbidity and slow speed, were rectified by
developments in two directions. On the one hand, simple
electrode structures were developed together with a heavy
reliance on modeling the field and director profiles in order to
make low cost devices [41,42]. On the other hand, multiple
electrode structures were explored in order to gain more
control over the resulting phase profile [43,44]. In the UK, it
was the work of the Durham PhD student, G. Williams [45],

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TABLE 2
COMMERCIAL LCOS SYSTEMS (PHASE-ONLY IN BOLD)
Manufacturer
Product(s)
Application area
Arryx
BioRyx 200
Holographic optical trapping
Aurora Systems
1 and 3-panel
optical engines
Low-end (laser based for mobile
phones) and high-end projectors
Barco
Sim 7
High quality front projection
Canon
REALiS SX7
Multimedia projector
Finisar
DWP 100
WSS ROADM for telecomms
Finisar
WS 400S
Multiport optical processor
ForthDD
SXGA-FBS
3D Metrology
Hamamatsu
X10468
Adaptive optics, pulse shaping
HDI 3D
1080p RGB
2D/3D front projection
Micron
(Displaytech)
VGA and
SVGA panels
Picoprojector, electronic
viewfinder, head mount display
Hitachi
CP-SX5500W
Rear projection TV
Holoeye Systems
HSI-K03
Near-to-eye projection module
Sony
SRX-T420
Front projection TV
and A. Purvis [46] which first showed the advantages of
driving the liquid crystal with patterned electrodes and devices
of display thickness. The work of the University of Durham
was continued in the excellent demonstration of the ability of a
LCSLM to correct a range of aberrations [47]. The
mathematical underpinning and detailed experimental results
for tunable lenses on LCSLMs was given by V. Laude,
working at Thomson CSF in Paris [48].
Novel simplified structures for the tunable lens employing
liquid crystals without multi-electrode address have been
pursued for applications in a zoom lens on the flash of a
camera [49], and dynamic optical interconnects [50].
However, the multi-electrode LC SLMs are also important in
ophthalmology [51], defense [52], free space communication
[53], and correction of atmospheric turbulence [54,55]. The
adaptive optics Phoropter can measure the higher order
aberration corrections required for supranormal vision [56]. A
Hamamatsu X7665 spatial light modulator with 480 x 480
pixels was employed. for testing the Phoropter. Closed loop
visualization of the retina allowing accurate diagnosis has been
achieved [57,58]. Corrections of ocular aberrations for both
eyes simultaneously under binocular vision has also been
achieved [59]. The field of adaptive optics would benefit from
the availability of a low-cost LCOS phase modulator with a
reasonable frame frequency for the above applications.
IV. HOLOGRAPHIC PROJECTION DISPLAY
The Eidophor display was based on phase modulation by an
e-beam addressed oil film coupled with Schlieren optics [60].
By replacing the oil film with a liquid crystal phase modulator,
it was calculated that a light output of 40% efficiency could be
achieved theoretically [61]. The development of projection
display with a liquid crystal phase modulator but without the
Schlieren optics at Cambridge University was a result of the
conjuncture of three significant developments. These were:
solid state light sources; iterative algorithms which give high
quality static DOEs; and fast Fourier transform processors,
because the Fourier transform was the basic operation of the
iterative algorithm. A schematic of a holographic projection
display is illustrated in Fig. 4. A laser beam is expanded using
a two lens beam expander. A 45 degree mirror with an
aperture is placed at the internal focal point of this expander.
The expanded beam is incident on a phase-only LCOS device
which is displaying a phase representation of the object. This
representation is computed by using an iterative Fourier
transform algorithm and is called a kinoform [62]. The beam
reflected from the LCOS device consists of two components,
one which is specularly reflected and one which is diffracted
by the kinoform. The specularly reflected beam passes back
through the hole in the mirror, whilst the diffracted beam is
reflected by the mirror and imaged by a lens on a screen.
Fig. 4 Optical layout of a holographic projection display
The use of the iterative Fourier transform approach to compute
the optimum phase only representation of a scene in the
Fourier plane was worked on in the early 70‟s [62, 63]. This
approach had been developed to the point where a good
solution can be found in around 20 iterations [64, 65].
In the Brite-Euram optical correlator [8], the FFT subsystem
was centered around the Sharp/Butterfly Model LH9124
digital signal processor. This device was capable of computing
a 512 x 512 FFT in 39.9 ms. Exploiting the symmetry of the
resulting output data permitted the FFT to be computed in 31
ms.
Holographic projectors are being developed in two
companies, Alps Electric UK and Light Blue Optics. The
former are developing the technology based on nematic LCOS
and have made a small full colour mini-projector [66]. The
technology has benefited from the commercial availability of
high power compact light sources and fast Fourier transform
processing boards. Improvements in the iterative algorithm
now facilitate real time processing of the image [67]. We have
been able to specify a suitable LCOS device for this
application [68].
There is a significant difference between analogue addressing
and PWM addressing for dynamic holography. The PWM of
the SRAM pixel, which can be integrated by the eye response
in an intensity modulated display, leads to noise in a display
which uses the interference between pixels to produce far-field
patterns. The time averaging of the interference has been
calculated and correlated with excess noise in the
reconstructed image [66]. The DRAM pixel generates less
noise because there is no temporal modulation of the pixel
voltage during the frame time.
LCOS
Laser
Screen
Mirror
Mirror
with hole
L
L
L

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Light Blue Optics are exploring potential products for the
FLC LCOS in the areas of: projection onto non-flat surfaces
[69]; automotive head-up display; and Light Touch™ which is
an interactive projector that instantly transforms any flat
surface into a touch screen.. The main problems with binary
phase modulation are: significant quantisation noise; reduced
diffraction efficiency due to the unavoidable symmetric order;
and further loss of efficiency because conventional and
available chiral smectic C liquid crystals do not have a 90
degree switch angle
Although the efficiency of the projector is limited by the
binary nature of the hologram, the higher frame rate of the
ferroelectric liquid crystal allows sub-frame averaging. This
reduces the speckle noise at the expense of overall contrast
ratio [70].
However, the fast switching speed of the FLC LCOS and
clever modulation schemes allow 24-bit colour display without
colour breakup. Colour breakup is perceived during saccadic
eye movements when the Field Sequential Colour display rate
is insufficient. Recent tests indicate that Field rates greater
than 1 kHz are required to avoid this artefact [71]. Alps
Electric in Sendai have measured blurring in gray scale display
for fast moving objects [72]. They developed target response
times for the LC material which depend on the gray level
difference between the two switching states. For a gray level
difference of 255 the response time should be better than 0.3
ms whilst for a difference of 3 it should be better than 10 ms.
Philips also use scrolling colour, where an optical system
incorporating rotating prisms shapes the beams into lines of
coloured light [73]. These are sequentially scanned from the
top to the bottom of the display. This also avoids colour
breakup.
Strong competition for high quality projection display comes
from the intensity modulating Vertically Aligned Nematic
(VAN) devices. When a negative dielectric anisotropy liquid
crystal is used with a homeotropic alignment, it is called a
VAN configuration. Application of a voltage rotates the liquid
crystal director parallel to the cell electrodes. The advantage of
this configuration is the good dark state which is obtained
using crossed polarizer/analyzer. This provides contrast ratios
above 1000:1. The concatenation of two modulation blocks
provided a dynamic range of over one million in one specialist
projector [74].The device format of 8192 x 4320 pixels also
provided very high resolution.
V. BEAM DEFLECTION HOLOGRAMS
Reconfigurable holographic interconnections have been
demonstrated in a variety of devices ranging from
photorefractive crystals [75], through magnetooptical SLMs
[76] to acoustooptical SLMs [77]. In all these cases phase-only
modulation was used, analogue in the cases of photorefractive
and acoustooptic crystals and binary in the case of the
magnetoptical device. Beam deflection using a liquid crystal
spatial light modulator was studied initially using the
birefringent phase grating of the variable grating mode device
[78]. Some of us studied gratings on liquid crystal SLMs under
the DOACC program mentioned previously [79]. A full
investigation of the LC phase grating was made by Barnes et al
[80].
However, the main focus of attention in the 90‟s as far as LC
SLMs were concerned was the faster ferroelectrics.
O‟Callaghan showed that FLC beam deflection gratings are
polarization insensitive [81]. The light beam issuing from an
optical fibre in a telecoms switch is in an arbitrary state of
polarization, so that a polarization insensitive switch is a
distinct advantage. Work continued on the FLC devices under
the UK ROSES project [82]. The main disadvantage of the
FLC SLM is that the phase grating is a binary one, so that the
efficiency of diffraction cannot exceed about 40%. In the
ROSES switch this efficiency (equivalent to 4 dB loss)
accounted for nearly half the overall loss of the switch Some
attempts to overcome this problem by using two cascaded FLC
SLMs have been experimentally tested. By coherently imaging
a 180° binary-phase FLC SLM onto a 90° FLC SLM, with
high precision, an effective four-level phase modulator was
realized experimentally. Beam steering was demonstrated in
the angular range ±10.9 mrad [83]. The complexity of imaging
one hologram on another in these experiments cannot be
underestimated.
As reviewed above, the telecoms applications of phase only
LCOS started with ferroelectric devices. In order to couple
between optical fibres at both the input and output ports it is
advantageous to use beam deflection holograms at both the
input and output stages [84]. The loss of 4 dB on one
deflection hologram is doubled in this case. The drive to lower
throughput losses moved the attention towards analogue
blazed gratings using nematic liquid crystals [85]. The
efficiency of the beam deflection using nematic liquid crystals
is improved when the switch is made polarization insensitive.
This is accomplished by fabricating a quarter wave plate on
the reflective mirrors of the LCOS [86]. Meanwhile, an
important advance in demonstrating the precision with which
the deflection angle can be tuned was demonstrated in a binary
ferroelectric device [87]. This should carry over to the more
efficient nematic devices.
The commercial interest in recent times has focused on
reconfigurable add drop multiplexers (ROADMs). These are
nodes at both the core and the edge of a wavelength division
multiplexed (WDM) network which manage the loading in the
network. The ROADM node contains a switch which can
selectively remove or add a given wavelength to the
wavelength multiplexed data stream. The removal can be to
any of a limited number of fibres, and vice versa for the
addition. Beam deflection gratings on LCOS is a particularly
efficient means of implementing the switch [88]. Alternative
approaches are: MEMS devices (Xtellus); Polarization
modulation using LC SLMs (Oclaro/Avanex, Xtellus, and
CoAdna Photonics). Nematic liquid crystal on silicon
technology has been developed by Optium for beam deflection
gratings [89]. Finisar, who merged with Optium in 2008, have
since started to market analogue beam deflection gratings
based on phase-only LCOS for reconfigurable add drop
multiplexers (ROADMs) [90].

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VI. LASER MATERIALS PROCESSING
Laser materials processing is a diverse field covering the
cutting, welding, surface treatment (including cleaning),
bending, cleaning, automation, and rapid prototyping of
materials using lasers. Rapid prototyping covers
stereolithography, sintering, lamination, and direct casting. In
those processes, where the speed of processing is a critical
factor, the process can commonly be improved by increasing
the number of focused beams.
Materials processing has had a long history back to the
pioneering work of Moran with a static diffractive optical
element (DOE) [91]. In several respects, the use of dynamic
SLMs rather than static DOEs for beam forming is preferred.
For example, a dynamic beam profile can be important for
some applications. Moreover, a fixed diffractive element can
be difficult to maintain in a pristine condition. Finally, the
noise from laser speckle can be reduced by the use of
replicated sub-frames [92].
An example of the kind of task which benefits from
parallelism and precise beam profiling and controllability is
the inscription of waveguides [93]. Here, holograms written
onto an SLM provide excellent control and the possibility of
high parallelism. Laser machining can be combined with
Adaptive optics to provide more precision. A feedback signal
from the part being machined allows closed loop adaptive
optical control [94]. SVG Optronics Co. Ltd. use a deformable
mirror SLM for pattern writing [95]. The deformable mirror
device is preferred over the LC SLM where the wavelength of
the light is close to the UV. The high intensity short
wavelengths can form free radicals, and thus degrade the
alignment, with the type of liquid crystals which are used in
SLMs [96]. Nevertheless, UV stereolithography has been
performed with an LC SLM [97].
VII. HOLOGRAPHIC OPTICAL TWEEZERS
The field of Optical trapping is as old as that of Laser
machining [98]. It was the work of D.G. Grier which extended
the work to multiple, dynamic traps based on SLM technology.
The application of LC SLM technology remains an active
research field [100, 101, 102] and there is currently
commercial exploitation of this technology from Arryx (Table
2).
VIII. OTHER APPLICATION AREAS AND CHARACTERIZATION
STUDIES
The preceding application areas have all resulted in some form
of commercial product. There are also a range of applications
which are still in research laboratories. Perhaps the longest
standing of these is Spatial Filtering. The use of a phase
structure in the back focal plane of the objective lens of a
microscope was the first use of phase filtering for enhancing
the contrast of objects [103]. This has been developed steadily
over the years and current research shows quite sophisticated
developments using phase-only LC SLMs [104, 105]. Phase
contrast methods have been generalized [106]. Other
application areas are: fast pulse shaping, metrology, and the
generation of speckles and turbulence. The possibility of
commercial exploitation is linked with the availability of high
performance, low-cost devices and research laboratories who
engage with the detailed characterization of LCOS devices.
Holoeye, Hamamatsu and our own laboratory are working to
provide the devices. Meanwhile, research groups have targeted
the polarimetric, diffractive, and temporal characterization of
devices [107, 108, 109, 110, 36].The ultimate goal is to
provide a low cost, versatile opto-ASIC device to photonic
system designers.
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Neil Collings was born in Stalybridge, Cheshire in 1949. He was educated at
Manchester Grammar School and Trinity Hall, Cambridge, from where he
graduated with a BA (Hons) degree in the Natural Sciences Tripos in 1971.
He received his PhD in Physics from the University of Salford in 1977.

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At the beginning of his career he worked in birefringence studies and
optical sensors. He began studying liquid crystal spatial light modulators and
their application to optical correlators during his work at the Standard
Telecommunications Laboratories between 1984 and 1989. He wrote a book
entitled “Optical pattern recognition using holographic techniques” (Addison-
Wesley, 1988; ISBN 0-201-14549-9), based on his research during this
period. He continued working in the field of liquid crystal devices and their
associated optical systems until he moved to the University of Cambridge in
1999. He is currently a Reader in Liquid Crystal Photonics in the Photonics &
Sensors group of the Department of Engineering..
Dr Collings became a Fellow of the Institute of Physics in 1995.