Patent Publication Number: US-11656515-B2

Title: WSS utilizing LCOS arrays comprising rectangular pixels

Description:
FIELD OF THE DISCLOSURE 
     The present application relates to liquid crystal devices and in particular to a liquid crystal on silicon array having a pixel pitch that is different in orthogonal axes. 
     Embodiments of the present disclosure are particularly adapted for use in optical switches such as a wavelength selective switch. However, it will be appreciated that the disclosure is applicable in broader contexts and other applications. 
     DESCRIPTION OF RELATED ART 
     Liquid crystal on silicon (LCOS) devices are one of the most commonly used switching engines for wavelength selective switches (WSS). In WSS devices, the optical fibers supporting the channels to be switched are separated along one axis, and the optical system is designed so that the LCOS can steer to any one of the fibers. This is called port switching. In an orthogonal axis, the light is dispersed in frequency so that small regions of the spectrum can be addressed independently by the LCOS. This is called wavelength selectivity. 
     To achieve the port switching and wavelength selectivity, one axis of the LCOS is used for separately manipulating individual wavelength channels while the other axis of pixels is used for switching the wavelength channels between the different optical fibers. 
     The performance of the device is generally proportional to the number of pixels contained in the pixel array. For example, an LCOS device with a higher number of pixels allows for switching between a greater number of optical fibers. Having a greater number of pixels available may also provide additional pixels to perform improved calibration processes and improve the granularity of the wavelength axis switching. 
     As the amount of global network traffic increases, there is a growing need for WSS devices to perform better and to switch between higher numbers of fibers. In addition, modules are being developed which incorporate multiple WSS devices that leverage a single common LCOS device to perform switching. In these higher performance devices, a greater number of pixels are required in the switching axis. In addition, wider frequency ranges are being pushed to increase fiber capacity (e.g. into both the C+L wavelength bands in a single WSS), which may require increased pixels in the wavelength axis. 
     Unfortunately, an increase in the number of pixels on an LCOS device increases the complexity of the driving circuitry, increases the overall power consumption of the device and places limitations on the available opto-mechanical design space. This, in turn, increases the manufacturing cost and ongoing operational cost of the devices. 
     Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field. 
     SUMMARY OF THE DISCLOSURE 
     In one configuration, a liquid crystal on silicon (LCOS) device disclosed herein comprises a silicon substrate, a pair of electrodes, and a liquid crystal layer. The pair of electrodes includes upper and lower electrodes. The lower electrode is mounted to the silicon substrate and includes a two-dimensional array of pixels, which extend in both first and second dimensions. The liquid crystal layer is disposed between the upper and lower electrodes. The liquid crystal layer is configured to be driveable into a plurality of electrical states by drive signals provided to the pixels of the lower electrode. The pixels are rectangular in profile having longer sides in the first dimension than in the second dimension. The two-dimensional array includes a pixel pitch that is greater in the first dimension than in the second dimension. 
     In another arrangement, a liquid crystal on silicon (LCOS) device comprises a two-dimensional array of independently driveable pixels disposed on a silicon substrate. The pitch of the pixels in a first dimension across the array is greater than a pitch of the pixels in a second dimension across the array. 
     In yet another arrangement, a liquid crystal on silicon (LCOS) device is used in switching optical channels between a plurality of optical ports of one or more wavelength selective switches (WSS). The LCOS device comprises a silicon substrate, a pair of electrodes, and a liquid crystal layer. The pair of electrodes includes upper and lower electrodes. The lower electrode is mounted to the silicon substrate and includes a two-dimensional array of pixels extending in both a first and second dimension. The liquid crystal layer is disposed between the upper and lower electrodes and is configured to be driveable into a plurality of electrical states by drive signals provided to the pixels of the lower electrode. The pitch of the pixels in one or two dimensions across the array is defined based on one or more characteristics of the one or more WSS devices. 
     A wavelength selective switch according to the present disclosure can incorporate the LCOS device disclosed herein and above. 
     A method of determining pixel specifications of a liquid crystal on silicon (LCOS) device is disclosed for use in one or more wavelength selective switches (WSS). The method includes: determining a number and/or a position of optical ports for the wavelength selective switch; calculating a range of switching angles required to switch the optical channels between the optical ports; and based on the calculated range of switching angles, determining a pixel size and a number of pixels for a switching axis of the LCOS device, wherein the pixels of the LCOS device are rectangular and have a degree of rectangularity that is based on the pixel size and number of pixels for the switching axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG.  1    is an exploded perspective sectional view of an LCOS device; 
         FIG.  2    is illustrated a plan view of a pixelated electrode of an LCOS device, showing a two-dimensional array of pixels; 
         FIG.  3    is illustrated a sectional view of the pixelated electrode of  FIG.  2    illustrating six rectangular pixels; 
         FIG.  4    is a schematic illustration of a wavelength selective switch showing optical channels incident onto the pixelated electrode of an LCOS device; 
         FIG.  5    is a flow chart of the primary steps in a method of determining pixel specifications for an LCOS device; and 
         FIG.  6    is a flow chart of the primary steps in an alternate method of determining pixel specifications for an LCOS device. 
     
    
    
     DETAILED DESCRIPTION 
     Looking initially at a device overview,  FIG.  1    illustrates a liquid crystal on silicon (LCOS) device  100 . Device  100  may also be referred to as an LCOS optical phase modulator as it modulates the phase of an incident optical signal propagating in a propagation dimension (z dimension). Device  100  comprises a silicon substrate  102  and a liquid crystal material layer  104  disposed between a pair of opposing electrodes  106  and  108 . A first or “upper” electrode  106  is disposed above liquid crystal layer  104 . A second or “lower” electrode  108  is mounted to the silicon substrate  102  and includes a two dimensional array  110  of pixels extending in both a first (x) and second (y) lateral dimension across the device  100 . Liquid crystal layer  104  is configured to be driveable into a plurality of electrical states by voltage drive signals provided to lower electrode  108  by an electrical controller  112 . 
     Upper electrode  106  is transparent or partially transparent indium-tin-oxide and allows the transmission of the optical signal into and out of device  100 . Lower electrode  108  is reflective and includes array  110  of individually controllable aluminum pixels (e.g.  114 ). The pixels of lower electrode  108  are electrically driven for supplying an electric potential V across the liquid crystal layer  104  between upper and lower electrodes  106  and  108  to drive the liquid crystals within layer  104  in a predetermined configuration. Each pixel  114  in array  110  is individually drivable by electrical controller  112  at one of a number of predetermined voltage levels to provide a local phase modulation to an incident optical signal. Electrical control of the pixels  114  is provided by interconnections to electrical controller  112  through silicon substrate  102 . 
     Pre-alignment of the liquid crystal materials within layer  104  may be provided by alignment layers  118  and  120 . Layers  118  and  120  include a plurality of small grooves aligned along a predetermined direction to define the slow axis of the liquid crystal material. 
     Turning now to the rectangular pixel structure of the present disclosure,  FIG.  2    illustrates a plan view of lower electrode  108  showing two-dimensional array  110  of pixels. As illustrated, the pixels are rectangular in profile having longer sides in the x dimension than in the y dimension. In some embodiments, the pixels have a length in the y dimension that is between 5 μm to 7 μm. In some embodiments, the pixels have a length in the x dimension that is between 8 μm to 12 μm. However, it will be appreciated that pixels having other rectangular dimensions may be implemented depending on the particular application. 
     A central pixel region  124  is used for active switching of optical channels. Around the central pixel region  124  is an apron  126  and a gasket region  128 . These outer regions are generally not pixelated and/or not individually addressable, as such, two dimensional array  110  is wholly defined by central pixel region  124 . The apron  126  is the area that immediately surrounds the central active pixel region, and does not contain a two-dimensional array of addressable pixels. The gasket region  128  at least partially surrounds the central pixel region  124  and apron  126 , and connects the silicon substrate  102  to upper electrode  106 . It may physically constrain the liquid crystal material. In some embodiments, more than one apron region may be included to perform different functions. In some embodiments, LCOS device  100  includes other regions or components not shown here. 
     Furthermore, the two dimensional array  110  includes a pixel pitch that is greater in the x dimension than in the y dimension. In some embodiments, the pixel pitch in the x dimension is 1.3 to 2.0 times greater than the pixel pitch in the y dimension. Pixel pitch, which is also referred to as dot pitch, is the distance between pixels in array  110  along one dimension. This is illustrated schematically in  FIG.  3   , which illustrates six pixels across two x rows an three y columns of array  110 . The pixel pitch is measured from the centers of each pixel and includes the width of the pixels plus a gap between pixels. This gap is defined to isolate adjacent pixels to reduce cross-talk. The gap is usually made as small as possible to maximize fill factor. The size of the gap is limited by factors such as a permissible level of electrical cross-talk between pixels and the integrated circuit layout. By way of example, the gap between pixels may be 0.2 μm. 
     This rectangular pixel structure allows for an increased number of pixels to be implemented in the y dimension than in the x dimension for a given LCOS device size. However, it will be appreciated that, in other embodiments and applications, an increased number of pixels may be implemented in the x dimension than in the y dimension for a given LCOS device size. In display applications, such as monitors, screens, televisions and projectors, the array  110  is typically required to follow standard display formats (aspect ratios) to correctly display content. Thus, the ratio of pixels in the x and y dimensions are constrained to predefined values set by industry. However, a primary application for the LCOS device  100  with rectangular pixel array is for a wavelength selective switch (WSS) device or multiple WSS devices operated concurrently. WSS devices have inherently different optical requirements in the two axes of the system and the inventors have recognized that this property can be used to design an optimized LCOS device. 
     In a WSS, the optical fibers representing the input and output ports are separated in one axis, and the optical system is designed so that the LCOS can steer to any one of the fibers. In the orthogonal axis, individual optical channels can be dispersed into different wavelengths/frequencies such that the channels are incident onto different pixel regions of device  100  in the x dimension. The channel separation allows each channel to fall on a separate area of the LCOS and be independently switched by the LCOS device. This is illustrated schematically in  FIG.  4   , which schematically illustrates a WSS device  400  having two input common ports  402  and  404  and eight add/drop ports  406 - 413 . In most practical applications, a WSS will have a much higher number of ports. In  FIG.  4   , two rows  121  and  123  of optical channels (e.g.  125 ) are dispersed by a dispersive element and spread across the x dimension of array  110 . Each channel represents a different wavelength band used to carry optically encoded information. 
     Within the central pixel region  124 , the y dimension of array  110  is used for switching the different wavelength channels between different ports in the wavelength selective switch. The beams of the optical channels are elongated in the y dimension by optics (not shown) so that they are incident onto a number of pixels in that dimension. Switching is performed by controlling the pixels along the y dimension of a channel region with different voltage signals so as to impose a phase change to the wavefront that reflects it at a given angle relative to the plane of device  100 . Different predefined voltage signals provide different switching angles to the various wavelength channels to switch the channels between input and output ports of the WSS device. 
     The two rows  121  and  123  of channels represent different sets of optical ports or, equivalently, two different WSS devices which share common device  110  as the switching element. Some WSS devices only include a single row of wavelength channels, while others may include more than two rows of wavelength channels. 
     The inventors have identified that the shape and number of pixels of an LCOS device can be tailored to be optimized for a specific application having a number and arrangement of optical ports and number of channels to be switched. Firstly, the shape of the pixels can be designed to be rectangular to suit specific requirements in each dimension. Second, the number of pixels in each dimension can be determined to optimize for switching and channel manipulation. 
     In some embodiments, array  110  of pixels includes 1500 to 1900 pixels in the x dimension. In some embodiments, array  110  of pixels includes 2500 to 2700 pixels in the y dimension. In some embodiments, the two dimensional array of pixels is defined such that it has an aspect ratio that is non-standard display format (e.g. non-standard aspect ratio). A standard display format relates to a standard X:Y aspect ratio of a display such as a television or computer screen. Example, standard display formats include 16:9 (e.g. 1366×768 or 1920×1080 pixels), 3:2 (e.g. 2160×1440 or 2560×1700 pixels) and 16:10 (e.g. 1280×800 or 1920×1200 pixels). A non-standard display format refers to aspect or pixel ratios that are outside those that are known and used in the industry. 
     As the LCOS device  100  represents the primary switching element of a WSS, the geometry of the WSS and the LCOS characteristics (size, number of pixels etc.) are designed in conjunction with each other. An exemplary process of designing a tailored LCOS device for specific WSS applications is described below. A WSS incorporating an LCOS device  100  described above is another aspect of the present disclosure. In some embodiments, a single LCOS device  100  can be used as a common switching element for multiple WSS devices where the different WSS devices use different regions of pixels of the LCSO device. 
     Although described with reference to WSS applications, it will be appreciated that the LCOS device  100  may also have applications in other types of optical switching devices and display devices such as screens, monitors, televisions and projectors. 
     Method of Designing a LCOS Device 
     Referring now to  FIG.  5   , there is illustrated a method  500  of determining specifications for a liquid crystal on silicon (LCOS) device  100  for a WSS (or multiple WSS devices used concurrently). Method  500  includes, at step  501 , determining a number and/or position of optical ports for the WSS. More broadly, this step may include determining what are the constraints required to achieve the specifications of the WSS device the LCOS will be used in. By way of example, one type of device includes a dual WSS having 60 (2×1×30) optical fiber ports disposed at no greater than 127 μm pitch to meet a device size restriction of 8 mm in the switching axis. 
     Method  500  includes, at step  501 , defining a fiber map and/or calibration concept for the WSS device. This is a visual representation defining the geometry of the WSS including where the ports will be located in the WSS. Initially the fiber map may be dimensionless but knowledge of the optical and/or mechanical requirements of the device, such as port isolation and size restriction, will add dimensions to the map. 
     At step  502 , a range of switching angles required to switch the optical channels between the optical ports is calculated. This includes determining the highest switching angles required to switch between the most angularly separated ports in the WSS. As illustrated in  FIG.  4   , different switching paths require different switching angles. To achieve greater switching angles requires a reduced distance between phase resets. To prevent increased optical loss, the number of pixels per phase reset must be maintained, and therefore more pixels in the y dimension are required. Therefore, the minimum number of pixels in the y dimension is proportional to the largest switching angles of the WSS. At this stage knowledge of the WSS needs to be incorporated; mechanical considerations such as size restriction will lead to a maximum size of the LCOS and optical considerations such as port isolation will lead to system focal length; which, when considered with step  501 , will lead to the range of switching angles required for the WSS. 
     In some embodiments, the strictest constraint is not the largest overall switching angle within the device but the largest switching angle between a common port and an add/drop port. Common ports are considered the most important ports as they carry all of the optical channels. 
     Furthermore, if the WSS device is a dual type switching device as illustrated in  FIG.  4   , two rows of wavelength channels are required. This doubles the number of pixels required in the y dimension. Thus, the minimum number of pixels required in the y dimension is also dependent on the number of switching devices utilizing the LCOS device. In addition to dual type devices, quad type devices are also possible in which four WSS devices share a common LCOS device. 
     At step  503 , more optical &amp; mechanical requirements of the WSS should be incorporated; optical requirements such as the frequency range of the WSS, and the bandwidth of each channel. Also, mechanical constraints such as maximum size of LCOS need to be considered. In some embodiments, this includes determining a bandwidth of the collective optical channels to be switched by the WSS. By way of example, the WSS may be configured to switch channels across the span of the C-Band, which covers a bandwidth of about 5.0 THz between the 1530 nm and 1565 nm range. Mechanical constraints will define a maximum LCOS size in the wavelength axis, which for this example may be 20 mm. 
     Step  503  may also comprise determining any channel shaping to be applied to the optical channels (e.g. width of the optical beam for each channel), and what tuning and calibration is required per channel. By way of example, the granularity in wavelength axis (i.e. pixels/frequency range) will impact the frequency accuracy of channel centering and width. 
     At step  504 , based on the calculated range of switching angles, a minimum pixel size and number of pixels for a switching (y) axis of the LCOS device  100  is determined to achieve that switching. In some embodiments, step  504  includes determining an overall size constraint of the LCOS device or WSS device. Step  504  may also include applying a target power consumption constraint of the LCOS device or WSS device. In some embodiments, this target power consumption may be based on desired frame rates, data rates and pixel resolution of the WSS device or devices. 
     Finally, at step  505 , a minimum pixel size and minimum number of the pixels in a dispersive (x) axis perpendicular to the switching axis is determined. This calculation is based on the minimum required pixels in the switching axis and takes into account the total number of pixels and/or power consumption available. 
     Additional constraints may also be applied to the design process. These include a maximum electrical power consumption by the device, e.g. 1 W power consumption. Power consumption is proportional to the number of pixels in the LCOS device where a device with a higher number of pixels consumes more power. Other constraints include a desired frame rate, data rate or pixel level resolution. 
     Another constraint applied may be the overall physical dimensions of the WSS device. Optical devices like a WSS are typically required to fit into standardized optical rack units. This size constraint is typically requested by a customer. These requirements can be used to define what is termed an overall ‘aperture’ of the optical device, defining a filtered version of the physical constraints. 
     An example calculation of LCOS pixel specifications is included below. 
     Inputs: 
     WSS Requirements
         2×1×30 ports   8 mm Mechanical height allowance for optics, driving a 127 μm fiber pitch   5 dB Insertion loss   30 dB Port isolation   Frequency window in C-Band (5.0 THz)       

     Max LCOS y dimension of 8 mm. 
     Max LCOS x dimension of 20 mm. 
     0.5 pixels/GHz to meet WSS optical bandwidth setting requirements. 
     Maximum electrical power consumption of 1 W. 
     These factors combine to define the maximum switching angle and the size of the LCOS. To maintain a minimum of 6 pixels per phase reset (as an example minimum number of pixels per phase reset) on outer ports over this propagation length:
         Pixel size in switching axis&lt;6.2 μm (or 6 um for this).       

     There may be a limit to the pixel area that is imposed by the required size of the pixel circuitry. By way of example, this pixel area may be constrained to 50 μm 2  or greater. To maintain&gt;50 μm 2  pixel area:
         Pixel size in dispersive axis must be &gt;8.4 μm.       

     To meet desired channel performance across the 5.0 THz C-Band, channels should be dispersed on a 0.3 pixels per GHz basis. These numbers can be determined experimentally from various WSS/LCOS combinations. 
     In the switching axis:
         8 mm @ 6 μm per pixel provides 1,333 pixels in the switching (y) dimension.       

     For power constraints, the total desired pixels is 2.4 megapixels. Therefore, the maximum number of pixels in the dispersive axis can be: 
     
       
         
           
             
               
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     This meets the 0.3 pixels per GHz requirement above. 
     The maximum dispersive axis pixel size can then be:
 
20 mm/1800 pixels=11.1 μm.
 
Which is greater than the minimum of 8.4 μm to maintain pixel area.
 
     The final LCOS pixel size and number was determined to be: 
     Py=6.0 μm 
     Px=10.0 μm 
     Ny=1,340 
     Nx=1,600 
     Number of pixels=2.144 MPx 
     Pixel Area=60 μm 2    
     Note that alternate designs that would use square pixel pitch would have issues for this design. Either the pixel area would not allow adequate space for pixel circuitry (in a 6 μm×6 μm pixel) or there would not be enough space for the number of pixels required in the switching axis (in a 10 μm×10 μm pixel). 
     In some embodiments, method  500  may be performed only for the switching (y) axis with the pixels in the dispersive (x) axis unchanged. In these embodiments, only steps  501 ,  502  and  504  are performed. 
     In some embodiments, the required minimum pixels for the two different axes can be determined independently and combined to determine the required pixel size and pitch. This alternate method  600  for determining LCOS pixel specifications is illustrated in  FIG.  6   . A first design process  601  is performed in relation to the switching (y) axis only. This involves, at step  602 , defining WSS products to be used with the LCOS and, at step  603 , determining a fiber map and calibration concept for the type of WSS product. These steps are equivalent to step  501  of method  500 . At step  604 , worst cast switching angles are determined. This step is equivalent to step  502  of method  500 . At step  605 , a minimum number of pixels is calculated for the switching axis. This is performed in step  504  of method  500 . 
     The output of design process  601  is a minimum number of pixels that are required in the switching axis of the LCOS. Separately, a second design process  606  is performed in relation to the dispersive (x) axis. At step  607 , the WSS product to be used with the LCOS is defined. This step may be the same as step  602  or be performed in conjunction with step  602 . At step  608 , worst case channel shape specifications are determined. These are the minimum requirements to achieve efficient switching and considers factors such as channel cross talk, number of channels and the transmission bandwidth (e.g. C-Band). At step  609 , a minimum number of pixels in the dispersive axis is determined. 
     The outputs from both design processes  601  and  606  are combined at step  710  and used in a pixel dimension calculation at step  711 . This pixel dimension calculation step calculates the minimum required pixel dimensions in both the switching and dispersive axes and takes in other constraints. These other constraints include mechanical constraints such as a maximum allowable switching aperture, determined at step  712 , and a maximum allowable dispersive aperture, determined at step  713 . These steps are combined at step  714  to determine an overall LCOS size in both the x and y dimensions. These dimensions are fed to the pixel dimension calculation step  711 . Also, at step  715 , pixel area requirements (typically specified by the LCOS manufacturer) are fed to the pixel dimension calculation step  711 . Finally, at step  716 , LCOS electrical power constraints are input to the pixel dimension calculation step  711 . 
     With all of these constraints applied, an appropriate pixel shape (dimensions in the x and y axes) and pitch in both axes can be calculated. The optimum result in most applications is a rectangular pixel design in which the pixel width in the dispersive axis is longer than the pixel height in the switching axis. Also, the pitch in the switching axis is higher than in the dispersive axis. These design parameters are provided to an LCOS manufacturer for manufacture of the desired optimized device. 
     The methods  500  and  600  provide for defining an LCOS device  100  in which the pitch of the pixels in one or two dimensions across the array is defined based on one or more characteristics of the WSS device. The shape of the pixels may also be defined based on one or more characteristics of the WSS device. The characteristics of the WSS device include: 
     the number and/or position of optical ports in the WSS; 
     an overall size of the WSS device; 
     a target power consumption value of the WSS device; 
     a number of optical channels; and/or 
     a bandwidth of the collective optical channels to be switched by the WSS. 
     Allowing the pixels to move significantly away from ‘square’ allows for balancing pixel and thermal constraints, while still being able to meet optical performance. 
     Embodiments described herein are intended to cover any adaptations or variations of the present disclosure. Although the present disclosure has been described and explained in terms of particular exemplary embodiments, one skilled in the art will realize that additional embodiments can be readily envisioned that are within the scope of the present disclosure.