View source for Liquid crystal on silicon

{{Short description|Type of display technology}}
{{redirect|LCOS|the energy economics metric|levelized cost of storage}}
{{Use mdy dates|date=October 2013}}

{{jargon|date=December 2022}}

'''Liquid crystal on silicon''' ('''LCoS''' or '''LCOS''') is a miniaturized reflective [[active-matrix liquid-crystal display]] or "microdisplay" using a [[liquid crystal]] layer on top of a silicon backplane. It is also known as a [[spatial light modulator]]. LCoS initially was developed for [[projection television]]s, 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]] technologies which use transmissive [[LCD]], allowing light to pass through the light processing unit (s). LCoS is more similar to [[Digital Light Processing|DLP]] micro-mirror displays.

==Technology==
[[File:Hughes LCLV.svg|thumb|right|upright=1.5|Hughes LCLV schematic]]
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]] increases the amplitude of an electrical signal; LCLV was named after the common name for the [[triode]] [[vacuum tube]].<ref>{{cite journal |doi=10.1063/1.1654574 |title=AC Liquid-Crystal Light Valve |author1=Beard, T.D. |author2=Bleha, W.P. |author3=Wong, S-Y |date=1 February 1973 |volume=22 |issue=3 |journal=Applied Physics Letters|pages=90–92 |bibcode=1973ApPhL..22...90B }}</ref> A high-resolution, low-intensity light source (typically a [[cathode-ray tube|CRT]]) was used to "write" an image in the {{chem|Cd|S|link=Cadmium sulfide}} [[photosensor]] layer, which is energized by a transparent [[indium tin oxide]] electrode, driven by an alternating current source at approximately 10&nbsp;mV. A {{chem|Cd|Te|link=Cadmium telluride}} light-blocking layer prevents the low-intensity writing light from shining through the device; the photosensor and light-blocking layer together form a [[Rectifier|rectifying]] junction, producing a DC voltage bias across the [[liquid crystal]] layer, transferring the image to the reflecting side<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA060666 |title=Image Processing Applications of the Liquid Crystal Light Valve |author1=Smith, J. Lynn |publisher=Technology Laboratory, Redstone Arsenal |date=June 12, 1978 |access-date=16 January 2024}}</ref>{{rp|5}} by changing the rotation of [[Polarization (waves)|polarization]] in the [[twisted nematic]] liquid crystal. On the reflecting side, a high-intensity, polarized projection light source reflects selectively from the [[dielectric mirror]] based on the polarization within the liquid crystal being controlled by the photosensor. The dielectric mirror is formed by [[sputtering]] alternating layers of {{chem|Ti|O|2|link=Titanium dioxide}} and {{chem|Si|O|2|link=Silicon dioxide}}, with the final {{chem|Si|O|2}} layer etched to align the liquid crystal material.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA039118 |title=Development of a Color Symbology AC Liquid Crystal Light Valve |author1=Jacobson, A.D. |author2=Bleha, W.P. |publisher=Hughes Research Labs |date=April 1977 |access-date=16 January 2024}}</ref>{{rp|10–12}} Later development of the LCLV used similar semiconductor materials arranged in the same basic structures.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA123887 |title=Development of a Silicon Liquid-Crystal Light Valve for Multimode Operation |author1=Efron, U. |author2=Wiener-Avnear, E. |author3=Grinberg, J. |author4=Braatz, P.O. |author5=Little, M.J. |author6=Schwartz, R.N. |publisher=Hughes Research Labs |date=July 1982 |access-date=16 January 2024}}</ref>{{rp|10}}

[[File:JVC D-ILA.svg|thumb|left|upright=1.5|Hughes-JVC D-ILA schematic]]
The LCLV principle is carried forward in a digital LCoS display device, which features an array of [[pixel]]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.<ref>{{cite web |url=http://pro.jvc.com/pro/hjt/technology/download/dilaguide.pdf |title=D-ILA: The Technology for images of perfection |publisher=JVC |date=June 2000 |access-date=16 January 2024}}</ref> 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 {{cvt|1–3|cm}} square and approximately {{cvt|2|mm}} thick, with [[pixel pitch]] as small as {{cvt|2.79|μm|mil}}.<ref>{{cite web|author=Compound Photonics |title=Products Compound Photonics |url=http://www.compoundphotonics.com/products/light-engines |access-date=2014-10-13 |url-status=dead |archive-url=https://web.archive.org/web/20141018065638/http://www.compoundphotonics.com/products/light-engines |archive-date=October 18, 2014 }}</ref> 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.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA010553 |title=Development of a Reflective Mode Liquid Crystal Light Valve |author1=Jacobson, A.D. |publisher=Hughes Research Labs |date=May 1975 |access-date=16 January 2024}}</ref> General Electric demonstrated a low-resolution LCoS display in the late 1970s.<ref>{{cite book |author1=Armitage, David |author2=Underwood, Ian |author3=Wu, Shin-Tson |date=2006 |title=Introduction to Microdisplays |publisher=Wiley |isbn=978-0-470-85281-1}}</ref> LCLV projectors were used primarily for military [[flight simulator]]s due to their large and bulky size.<ref>{{cite report |url=https://apps.dtic.mil/sti/citations/ADA209580 |title=Display Characteristics of Example Light-Valve Projectors |author=Howard, Celeste M. |date=June 1989 |publisher=University of Dayton Research Institute |access-date=17 January 2024}}</ref> A joint venture between [[Hughes Electronics]] and [[Japan Victor Corporation|JVC]] (Hughes-JVC) was founded in 1992<ref name=JVCPro-pr>{{cite press release |url=http://pro.jvc.com/pro/vsd/jvchjt.htm |title=JVC consolidates projector operations with absorption of Hughes-JVC |date=December 16, 1999 |publisher=JVC Professional |access-date=16 January 2024}}</ref> to develop LCLV technology for commercial movie theaters under the branding ILA (Image Light Amplifer).<ref>{{cite web |url=http://pro.jvc.com/pro/hjt/technology/download/sid99.pdf |title=Electronic Cinema Using ILA Projector Technology |author1=Sterling, R.D. |author2=Bleha, W.P. |publisher=Hughes-JVC Technology Corporation |access-date=16 January 2024}}</ref> One example was {{cvt|72.5|in}} tall and weighed {{cvt|1670|lb}}, using a 7&nbsp;kW [[Xenon arc lamp]].<ref>{{cite web |url=http://pro.jvc.com/pro/hjt/products/ila12k.html |title=ILA-12K Projector |website=JVC Professional |access-date=16 January 2024}}</ref>

[[File:Lcos.svg|thumb|right|upright=1.5|Conceptual diagram of an LCoS projector]]
In 1997, engineers at JVC developed the D-ILA (Direct-Drive Image Light Amplifier) from the Hughes LCLV,<ref name=JVCPro-pr/><ref>{{cite conference |doi=10.1117/12.305518 |title=Reflective active-matrix LCD: D-ILA |author1=Nakano, Atsushi |author2=Honma, Akira |author3=Nakagaki, Shintaro |author4=Doi, Keiichiro |date=1998 |conference=Photonics West / Electronic Imaging |location=San Jose, California |publisher=Society of Photo-Optical Instrumentation Engineers}}</ref> which led to smaller and more affordable digital LCoS projectors, using three-chip D-ILA devices.<ref>{{cite conference |doi=10.1117/12.497532 |title=D-ILA technology for high-resolution projection displays |author1=Bleha, William P. |author2=Sterling, Rodney D. |publisher=Society of Photo-Optical Instrumentation Engineers |conference=AeroSense |date=2003 |location=Orlando, Florida}}</ref> Although these were not as bright and had less resolution than the cinema ILA projectors, they were more portable, starting at {{cvt|33|lb}}.<ref>{{cite web |url=http://pro.jvc.com/pro/special/dila/pdf_u/DLA_G11_U.pdf |title=D-ILA Projector: DLA-G11 |publisher=JVC Professional |date=November 1999 |access-date=16 January 2024}}</ref>

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.

[[File:JVC D-ILA projector.jpg|thumb|JVC projector "D-ILA" LCoS]]
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]] and HDR ([[High Dynamic Range]]) 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 [[Polarization (waves)|polarized]] by [[beam splitter]]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.<ref>{{cite web|last=Hachman|first=Mark|title=Update: Intel Cancels LCOS Chip Plans|url=http://www.extremetech.com/extreme/73648-update-intel-cancels-lcos-chip-plans|work=415.992.5910|publisher=Extreme Tech|access-date=June 17, 2011}}</ref> There were single-panel LCoS displays in production: One by [[Philips]] and one by Microdisplay Corporation. [[Forth Dimension Displays]] continues to offer a [[Ferroelectricity|Ferroelectric]] LCoS display technology (known as Time Domain Imaging) available in [[graphic display resolutions#Extended Graphics Array|QXGA]], [[graphic display resolutions#Extended Graphics Array|SXGA]] and [[graphic display resolutions#Extended Graphics Array|WXGA]] resolutions which today is used for high resolution near-eye applications such as Training & Simulation, structured light pattern projection for [[Automated optical inspection|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 [[Handheld projectors|pico-projectors]], [[electronic viewfinder]]s for high end digital cameras, and [[head-mounted display]]s.<ref>[https://www.miyotadca.com/ Homepage for MDCA a subsidiary of Citizen Finedevice]</ref>

===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 [[Handheld projectors|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]]s for digital cameras, film cameras, and [[Head-mounted display|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.<ref>{{cite journal|author=Collings, N.|title=The Applications and Technology of Phase-Only Liquid Crystal on Silicon Devices|doi=10.1109/JDT.2010.2049337|journal=IEEE Journal of Display Technology|volume= 7|issue= 3|pages=112–119|year=2011|bibcode=2011JDisT...7..112C |s2cid=34118772 }}</ref> Google's initial foray into wearable computing, Google glass,<ref>[https://www.google.com/glass/start/ Google glass]. google.com</ref> also uses a near-eye LCoS display.

At [[Consumer Electronics Show|CES]] 2018, Hong Kong Applied Science and Technology Research Institute Company Limited ([[Hong Kong Applied Science and Technology Research Institute|ASTRI]]) and [[OmniVision Technologies|OmniVision]] showcased a [[reference design]] for a wireless augmented reality headset that could achieve 60 degree [[field of view]] (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.<ref>{{Cite web|title=This AR Headset Surpasses the Field of View of HoloLens, but You Still Won't Wear It in Public|url=https://augmented.reality.news/news/ar-headset-surpasses-field-view-hololens-but-you-still-wont-wear-public-0182110/|access-date=2020-06-23|website=Next Reality|date=January 11, 2018 |language=en}}</ref>

==Wavelength-selective switches==

LCoS is particularly attractive as a switching mechanism in a [[Wavelength selective switching|wavelength-selective switch]] (WSS). LCoS-based WSS were initially developed by Australian company Engana,<ref>Baxter, G. et al. (2006) "Highly Programmable Wavelength Selective Switch Based on Liquid Crystal on ," in Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference.</ref> now part of Finisar.<ref>[http://www.finisar.com/products/wss-roadms ROADMs & Wavelength Management]. finisar.com</ref> The LCoS can be employed to control the phase of light at each pixel to produce beam-steering<ref>{{cite journal|author=Johnson, K. M. |title=Smart spatial light modulators using liquid crystals on silicon|journal= IEEE J. Quantum Electron.|volume= 29|issue=2|pages= 699–714 |year=1993|doi=10.1109/3.199323|bibcode=1993IJQE...29..699J }}</ref> 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.<ref>{{cite book|chapter=Ch. 16 |title=Optical Fiber Telecommunications VIA| editor =Kaminov, Li and Wilner|publisher= Academic Press |isbn=978-0-12-396958-3}}
</ref>

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 [[Blazed grating#Littrow configuration|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]]<ref>Marom, D. M. et al. (2002) "Wavelength-selective 1×4 switch for 128 WDM channels at 50&nbsp;GHz spacing," in Proc. Optical Fiber Communications), Anaheim, CA, Postdeadline Paper FB7, pp. FB7-1–FB7-3</ref> and/or liquid crystal<ref>Kondis, J. et al. (2001) "Liquid crystals in bulk optics-based DWDM optical switches and spectral equalizers," pp. 292–293 in Proc. LEOS 2001, Piscataway, NJ.</ref> 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&nbsp;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&nbsp;GHz channels and 100&nbsp;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.<ref>{{cite journal|author= Weiner, A.M.|url=https://engineering.purdue.edu/~fsoptics/articles/Femtosecond_pulse_shaping-Weiner.pdf|doi=10.1063/1.1150614|title=Femtosecond pulse shaping using spatial light modulators|journal= Rev. Sci. Instrum. |volume=71|issue=5|pages= 1929–1960 |year=2000|bibcode=2000RScI...71.1929W }}</ref> 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&nbsp;nm supercontinuum source whilst a second such device was used to compress the output to 400 fs, transform-limited pulses.<ref>A. M. Clarke, D. G. Williams, M. A. F. Roelens, M. R. E. Lamont, and B. J. Eggleton, "Parabolic pulse shaping for enhanced continuum generation using an LCoS-based wavelength selective switch," in 14th OptoElectronics and Communications Conference (OECC) 2009.</ref> 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.<ref>{{cite journal|author=Schroeder, Jochen B. |title=Dark and Bright Pulse Passive Mode-locked Laser with In-cavity Pulse-shaper|journal=Optics Express |volume=18|issue= 22 |year= 2010|pages=22715–22721|pmid=21164610|doi=10.1364/OE.18.022715|bibcode=2010OExpr..1822715S |doi-access=free}}</ref> A similar approach uses spectral shaping of optical frequency combs to create multiple pulse trains. For example, a 10&nbsp;GHz optical frequency comb was shaped by the POP to generate dark parabolic pulses and Gaussian pulses, at 1540&nbsp;nm and 1560&nbsp;nm, respectively.<ref>Ng, T. T. et al. (2009) "Complete Temporal Optical Fourier Transformations Using Dark Parabolic Pulses," in 35th European Conference on Optical Communication.</ref>

=== Light structuring===
[[Structured light]] using a fast ferroelectric LCoS is used in 3D-[[superresolution]] microscopy techniques and in [[fringe projection]] for 3D-[[automated optical inspection]].

===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<ref>{{cite journal|title=Mode-Division Multiplexing of 2×100 Gb/s Channels Using an LCOS-Based Spatial Modulator|journal=Journal of Lightwave Technology|volume=30|issue=4|pages=618|doi=10.1109/JLT.2011.2178394|year=2012|last1=Salsi|first1=Massimiliano|last2=Koebele|first2=Clemens|last3=Sperti|first3=Donato|last4=Tran|first4=Patrice|last5=Mardoyan|first5=Haik|last6=Brindel|first6=Patrick|last7=Bigo|first7=Sébastien|last8=Boutin|first8=Aurélien|last9=Verluise|first9=Frédéric|last10=Sillard|first10=Pierre|last11=Astruc|first11=Marianne|last12=Provost|first12=Lionel|last13=Charlet|first13=Gabriel|bibcode=2012JLwT...30..618S |s2cid=38004325 }}</ref> 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.<ref>{{cite journal|author=Xiao, Feng |title=Opto-VLSI-based Tunable Single-mode Fiber Laser|journal= Optics Express |volume=17|issue= 21 |year= 2009|pages= 18676–18680|pmid=20372600|doi=10.1364/OE.17.018676|bibcode=2009OExpr..1718676X |url=https://ro.ecu.edu.au/cgi/viewcontent.cgi?article=1565&context=ecuworks|doi-access=free}}</ref>

== See also ==

* {{Annotated link|Digital Light Processing}}

==References==
{{reflist|colwidth=30em}}

==External links==
* Biever, Celeste. [https://www.newscientist.com/news/news.jsp?id=ns99994550 'Intel inside' comes to flat panel TVs] (January 9, 2004 – No longer planned for development) ''[[New Scientist]]''
* [http://www.hardwaresecrets.com/article/544 Everything You Need to Know About TV Technologies] from [[Hardware Secrets]]
* [https://projectorspick.com/lcos-projectors-the-ultimate-guide/ LCoS projectors] at Projectors Pick

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