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Liquid crystal on silicon (LCoS or LCOS) is a miniaturized reflective active-matrix liquid-crystal display or "microdisplay" using a
liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...
layer on top of a silicon backplane. It is also known as a
spatial light modulator A spatial light modulator (SLM) is a device that can control the intensity, phase, or polarization of light in a spatially varying manner. A simple example is an overhead projector transparency. Usually when the term SLM is used, it means that ...
. LCoS initially was developed for projection televisions, but has since found additional uses in wavelength selective switching, structured illumination, near-eye displays and optical pulse shaping. LCoS is distinct from other
LCD projector An LCD projector is a type of video projector for displaying video, images or computer data on a screen or other flat surface. It is a modern equivalent of the slide projector or overhead projector. To display images, LCD (liquid crystal display, ...
technologies which use transmissive
LCD A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liquid crystals do not em ...
, allowing light to pass through the light processing unit (s). LCoS is more similar to DLP micro-mirror displays.


Technology

The Hughes liquid crystal light valve (LCLV) was designed to modulate a high-intensity light beam using a weaker light source, conceptually similar to how an
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
increases the amplitude of an electrical signal; LCLV was named after the common name for the
triode A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
. A high-resolution, low-intensity light source (typically a
CRT CRT or Crt most commonly refers to: * Cathode-ray tube, a display * Critical race theory, an academic framework of analysis CRT may also refer to: Law * Charitable remainder trust, United States * Civil Resolution Tribunal, Canada * Columbia ...
) was used to "write" an image in the
photosensor Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical c ...
layer, which is energized by a transparent
indium tin oxide Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturate ...
electrode, driven by an alternating current source at approximately 10 mV. A light-blocking layer prevents the low-intensity writing light from shining through the device; the photosensor and light-blocking layer together form a
rectifying A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as ''rectification'', since it "straightens" t ...
junction, producing a DC voltage bias across the
liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...
layer, transferring the image to the reflecting side by changing the rotation of polarization in the
twisted nematic The twisted nematic effect (''TN effect'') was a major technological breakthrough that made the manufacture of large, thin liquid crystal displays practical and cost competitive. Unlike earlier flat-panel displays, TN cells did not require a curre ...
liquid crystal. On the reflecting side, a high-intensity, polarized projection light source reflects selectively from the
dielectric mirror A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin film, thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type a ...
based on the polarization within the liquid crystal being controlled by the photosensor. The dielectric mirror is formed by
sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
alternating layers of and , with the final layer etched to align the liquid crystal material. Later development of the LCLV used similar semiconductor materials arranged in the same basic structures. The LCLV principle is carried forward in a digital LCoS display device, which features an array of
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
s, each equivalent to the reflecting side of a single LCLV. These pixels on the LCoS device are driven directly by signals to modulate the intensity of reflected light, rather than a low intensity "writing light" source in the LCLV. For example, a chip with XGA resolution has an array of 1024×768 pixels, each with an independently addressable transistor. In the LCoS device, a complementary metal–oxide–semiconductor (CMOS) chip controls the voltage on square reflective aluminium electrodes buried just below the chip surface, each controlling one pixel. Typical chips are approximately square and approximately thick, with
pixel pitch Dot pitch (sometimes called line pitch, stripe pitch, or phosphor pitch) is a specification for a computer display, computer printer, image scanner, or other pixel-based devices that describe the distance, for example, between dots ( sub-pixels) ...
as small as . A common voltage for all the pixels is supplied by a transparent conductive layer made of indium tin oxide on the cover glass.


Displays


History

The history of LCoS projectors dates back to June 1972, when LCLV technology was first developed by scientists at Hughes Research Laboratories working on an internal research and development project. General Electric demonstrated a low-resolution LCoS display in the late 1970s. LCLV projectors were used primarily for military
flight simulator A flight simulator is a device that artificially re-creates aircraft flight and the environment in which it flies, for pilot training, design, or other purposes. It includes replicating the equations that govern how aircraft fly, how they rea ...
s due to their large and bulky size. A joint venture between
Hughes Electronics Hughes Electronics Corporation was formed in 1985, when Hughes Aircraft was sold by the Howard Hughes Medical Institute to General Motors for $5.2 billion. Surviving parts of Hughes Electronics are today known as DirecTV Group, while the automoti ...
and
JVC JVC (short for Japan Victor Company) is a Japanese brand owned by JVCKenwood. Founded in 1927 as the Victor Talking Machine Company of Japan and later as , the company was best known for introducing Japan's first televisions and for developin ...
(Hughes-JVC) was founded in 1992 to develop LCLV technology for commercial movie theaters under the branding ILA (Image Light Amplifer). One example was tall and weighed , using a 7 kW
Xenon arc lamp A xenon arc lamp is a highly specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light to simulate sunlight, with applications ...
. In 1997, engineers at JVC developed the D-ILA (Direct-Drive Image Light Amplifier) from the Hughes LCLV, which led to smaller and more affordable digital LCoS projectors, using three-chip D-ILA devices. Although these were not as bright and had less resolution than the cinema ILA projectors, they were more portable, starting at . The early LCoS projectors had their challenges. They suffered from a phenomenon called "image sticking," where the image would remain on the screen after it was supposed to be gone. This was due to the mirrors sticking in their positions, which resulted in ghosting on the screen. However, manufacturers continued to refine the technology, and today's LCoS projectors have largely overcome this issue. Sony introduced its SXRD (Silicon X-tal Reflective Display) technology in 2004. SXRD was an evolution of LCoS technology that used even smaller pixels and a higher resolution, resulting in an even more accurate image. The SXRD technology was used in Sony's high-end home theater projectors, and it quickly gained a reputation for its exceptional picture quality. JVC introduced an updated D-ILA technology in 2006, which eliminated the need for a polarizing filter, resulting in a brighter and more vibrant image. The D-ILA technology has since become a popular choice for home theater enthusiasts. LCoS projectors have continued to evolve, with manufacturers introducing features like
4K resolution 4K resolution refers to a horizontal display resolution of approximately 4,000 pixels. Digital television and digital cinematography commonly use several different 4K resolutions. In television and consumer media, 38402160 (4K UHD) with a 16:9 asp ...
and HDR (
High Dynamic Range High dynamic range (HDR), also known as wide dynamic range, extended dynamic range, or expanded dynamic range, is a signal with a higher dynamic range than usual. The term is often used in discussing the dynamic ranges of images, videos, audio o ...
) support. LCoS projectors are now available at a range of price points, from affordable models for home theater use to high-end professional models used in commercial installations.


Display system architectures

LCoS display technology is a type of microdisplay that has gained popularity due to its high image quality and ability to display high-resolution images. LCos display systems typically consist of three main components: the LCos panel, the light source, and the optical system. The LCos panel is the heart of the display system. It consists of an array of pixels that are arranged in a grid pattern. Each pixel is made up of a liquid crystal layer, a reflective layer, and a silicon substrate. The liquid crystal layer controls the polarization of light that passes through it, while the reflective layer reflects the light back towards the optical system. The silicon substrate is used to control the individual pixels and provides the necessary electronics to drive the LCos panel. The light source is used to provide the necessary illumination for the LCos panel. The most common light source used in LCos display systems is a high-intensity lamp. This lamp emits a broad spectrum of light that is filtered through a color wheel or other optical components to provide the necessary color gamut for the display system. The optical system is responsible for directing the light from the light source onto the LCos panel and projecting the resulting image onto a screen or other surface. The optical system consists of a number of lenses, mirrors, and other optical components that are carefully designed and calibrated to provide the necessary magnification, focus, and color correction for the display system.


Three-panel designs

The white light is separated into three components (red, green and blue) and then combined back after modulation by the 3 LCoS devices. The light is additionally polarized by
beam splitter A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...
s.


One-panel designs

Both Toshiba's and Intel's single-panel LCOS display program were discontinued in 2004 before any units reached final-stage prototype. There were single-panel LCoS displays in production: One by
Philips Koninklijke Philips N.V. (), simply branded Philips, is a Dutch multinational health technology company that was founded in Eindhoven in 1891. Since 1997, its world headquarters have been situated in Amsterdam, though the Benelux headquarter ...
and one by Microdisplay Corporation. Forth Dimension Displays continues to offer a
Ferroelectric In physics and materials science, ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. All ferroelectrics are also piezoel ...
LCoS display technology (known as Time Domain Imaging) available in
QXGA A display resolution standard is a commonly used width and height dimension (display resolution) of an electronic visual display device, measured in pixels. This information is used for electronic devices such as a computer monitor. Certain comb ...
,
SXGA A display resolution standard is a commonly used width and height dimension (display resolution) of an electronic visual display device, measured in pixels. This information is used for electronic devices such as a computer monitor. Certain comb ...
and
WXGA WXGA may refer to: * Wide Extended Graphics Array, a computer graphics display resolution * WXGA-TV, a television station in the U.S. state of Georgia {{Disambiguation, callsign ...
resolutions which today is used for high resolution near-eye applications such as Training & Simulation, structured light pattern projection for AOI. Citizen Finedevice (CFD) also continues to manufacturer single panel RGB displays using FLCoS technology (Ferroelectric Liquid Crystals). They manufacture displays in multiple resolutions and sizes that are currently used in pico-projectors,
electronic viewfinder An electronic viewfinder (EVF) is a camera viewfinder where the image captured by the lens is displayed on a small screen (usually LCD or OLED) which the photographer can look through when composing their shot. It differs from a live preview sc ...
s for high end digital cameras, and
head-mounted display A head-mounted display (HMD) is a display device, worn on the head or as part of a helmet (see helmet-mounted display for aviation applications), that has a small display optic in front of one (monocular HMD) or each eye (binocular vision, bi ...
s.


Pico projectors, near-eye and head-mounted displays

Whilst initially developed for large-screen projectors, LCoS displays have found a consumer niche in the area of pico-projectors, where their small size and low power consumption are well-matched to the constraints of such devices. LCoS devices are also used in near-eye applications such as
electronic viewfinder An electronic viewfinder (EVF) is a camera viewfinder where the image captured by the lens is displayed on a small screen (usually LCD or OLED) which the photographer can look through when composing their shot. It differs from a live preview sc ...
s for digital cameras, film cameras, and head-mounted displays (HMDs). These devices are made using ferroelectric liquid crystals (so the technology is named FLCoS) which are inherently faster than other types of liquid crystals to produce high quality images. Google's initial foray into wearable computing, Google glass, also uses a near-eye LCoS display. At CES 2018, Hong Kong Applied Science and Technology Research Institute Company Limited ( ASTRI) and
OmniVision OmniVision Technologies Inc. is an American subsidiary of Chinese semiconductor device and mixed-signal integrated circuit design house Will Semiconductor. The company designs and develops digital imaging products for use in mobile phones, lap ...
showcased a reference design for a wireless augmented reality headset that could achieve 60 degree
field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
(FoV). It combined a single-chip 1080p LCOS display and image sensor from OmniVision with ASTRI's optics and electronics. The headset is said to be smaller and lighter than others because of its single-chip design with integrated driver and memory buffer.


Wavelength-selective switches

LCoS is particularly attractive as a switching mechanism in a wavelength-selective switch (WSS). LCoS-based WSS were initially developed by Australian company Engana, now part of Finisar. The LCoS can be employed to control the phase of light at each pixel to produce beam-steering where the large number of pixels allow a near continuous addressing capability. Typically, a large number of phase steps are used to create a highly efficient, low-insertion loss switch shown. This simple optical design incorporates polarisation diversity, control of mode size and a 4-f wavelength optical imaging in the dispersive axis of the LCoS providing integrated switching and optical power control. In operation, the light passes from a fibre array through the polarisation imaging optics which separates physically and aligns orthogonal polarisation states to be in the high efficiency s-polarisation state of the diffraction grating. The input light from a chosen fibre of the array is reflected from the imaging mirror and then angularly dispersed by the grating which is at near Littrow incidence, reflecting the light back to the imaging optics which directs each channel to a different portion of the LCoS. The path for each wavelength is then retraced upon reflection from the LCoS, with the beam-steering image applied on the LCOS directing the light to a particular port of the fibre array. As the wavelength channels are separated on the LCoS the switching of each wavelength is independent of all others and can be switched without interfering with the light on other channels. There are many different algorithms that can be implemented to achieve a given coupling between ports including less efficient "images" for attenuation or power splitting. WSS based on
MEMS MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...
and/or liquid crystal technologies allocate a single switching element (pixel) to each channel which means the bandwidth and centre frequency of each channel are fixed at the time of manufacture and cannot be changed in service. In addition, many designs of first-generation WSS (particularly those based on MEMs technology) show pronounced dips in the transmission spectrum between each channel due to the limited spectral ‘fill factor’ inherent in these designs. This prevents the simple concatenation of adjacent channels to create a single broader channel. LCoS-based WSS, however, permit dynamic control of channel centre frequency and bandwidth through on-the-fly modification of the pixel arrays via embedded software. The degree of control of channel parameters can be very fine-grained, with independent control of the centre frequency and either upper- or lower-band-edge of a channel with better than 1 GHz resolution possible. This is advantageous from a manufacturability perspective, with different channel plans being able to be created from a single platform and even different operating bands (such as C and L) being able to use an identical switch matrix. Additionally, it is possible to take advantage of this ability to reconfigure channels while the device is operating. Products have been introduced allowing switching between 50 GHz channels and 100 GHz channels, or a mix of channels, without introducing any errors or "hits" to the existing traffic. More recently, this has been extended to support the whole concept of Flexible or Elastic networks under ITU G.654.2 through products such as Finisar's ''Flexgrid™'' WSS.


Other LCoS applications


Optical pulse shaping

The ability of an LCoS-based WSS to independently control both the amplitude and phase of the transmitted signal leads to the more general ability to manipulate the amplitude and/or phase of an optical pulse through a process known as Fourier-domain pulse shaping. This process requires full characterisation of the input pulse in both the time and spectral domains. As an example, an LCoS-based Programmable Optical Processor (POP) has been used to broaden a mode-locked laser output into a 20 nm supercontinuum source whilst a second such device was used to compress the output to 400 fs, transform-limited pulses. Passive mode-locking of fiber lasers has been demonstrated at high repetition rates, but inclusion of an LCoS-based POP allowed the phase content of the spectrum to be changed to flip the pulse train of a passively mode-locked laser from bright to dark pulses. A similar approach uses spectral shaping of optical frequency combs to create multiple pulse trains. For example, a 10 GHz optical frequency comb was shaped by the POP to generate dark parabolic pulses and Gaussian pulses, at 1540 nm and 1560 nm, respectively.


Light structuring

Structured light using a fast ferroelectric LCoS is used in 3D-
superresolution Super-resolution imaging (SR) is a class of techniques that improve the image resolution, resolution of an digital imaging, imaging system. In optical SR the diffraction-limited, diffraction limit of systems is transcended, while in geometrical SR ...
microscopy techniques and in fringe projection for 3D-
automated optical inspection Automated optical inspection (AOI) is an automated visual inspection of printed circuit board (PCB) (or LCD, transistor) manufacture where a camera machine vision, autonomously scans the device under test for both catastrophic failure (e.g. missin ...
.


Modal switching in space division multiplexed optical communications systems

One of the interesting applications of LCoS is the ability to transform between modes of few-moded optical fibers which have been proposed as the basis of higher capacity transmission systems in the future. Similarly LCoS has been used to steer light into selected cores of multicore fiber transmission systems, again as a type of Space Division Multiplexing.


Tunable lasers

LCoS has been used as a filtering technique, and hence a tuning mechanism, for both semiconductor diode and fiber lasers.


See also

*


References


External links

* Biever, Celeste
'Intel inside' comes to flat panel TVs
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