Editing Liquid crystal on silicon (section)

==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}}
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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.