111111
`
`(19) United States
`(12) Patent Application Publication
`Crossland et al.
`
`11111111111111111111111111111111111111111111111111111111111111
`US 20010050787 Al
`
`(10) Pub. No.: US 2001/0050787 A1
`Dec. 13, 2001
`(43) Pub. Date:
`
`(54) ELECTRO-OPTICAL COMPONENT HAVING
`A RECONFIGURABLE PHASE STATE
`
`(75)
`
`Inventors: William Alden Crossland, Essex (GB);
`Timothy David Wilkinson, Cambridge
`(GB); Kamran Eshraghian, Mindarie
`(AU)
`
`Correspondence Address:
`Charles N.J. Ruggiero, Esq.
`Ohlandt, Greeley, Ruggiero & Perle, L.L.P.
`One Landmark Square, lOth Floor
`Stamford, CT 06901-2682 (US)
`
`(73) Assignee: Intelligent Pixels, Inc.
`
`(21) Appl. No.:
`
`09/860,289
`
`(22) Filed:
`
`May 18,2001
`
`10()
`
`Related U.S. Application Data
`
`(63)
`
`Non-provisional of provisional application No.
`60/206,074, filed on May 22, 2000.
`
`Publication Classification
`
`(51)
`
`Int. Cl? ............................. G03H l/02; G03H 1!08;
`G02B 5!32; H04J 14/02
`(52) U.S. Cl. .................... 359/15; 359/3; 359!9; 359/128
`(57)
`ABSTRACT
`There is provided an electro-optical component comprising
`(a) a substrate, (b) a phase-variable element carried on the
`substrate, (c) a memory carried on the substrate for storing
`data representative of a phase state for the phase-variable
`element; and (d) a controller carried on the substrate, for
`utilizing the data and setting the phase state for the element.
`There is also provided an electro-optical component com(cid:173)
`prising (a) a substrate, (b) a phase-variable element carried
`on the substrate, and (c) a circuit carried on the substrate for
`computing and applying a phase state for the phase-variable
`element.
`
`tro
`
`/
`
`FNC 1010
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 1 of 8
`
`US 2001!0050787 A1
`
`tao
`
`110
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 2 of 8
`
`US 2001/0050787 A1
`
`200
`~
`
`220
`
`225
`
`205
`
`FIG. 2A
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 3 of 8
`
`US 2001!0050787 A1
`
`230
`
`232
`
`235
`
`240
`
`250 ~
`
`··o
`. .
`.·
`0
`··o
`
`!
`
`:
`
`d
`
`234
`
`I~·-- 1 ------~~---- f
`
`FIG. 2B
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 4 of 8
`
`US 2001!0050787 A1
`
`305
`
`/
`
`Fourier Transform
`
`150
`
`Hologram
`
`Replay Field
`
`FIG. 3
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 5 of 8
`
`US 2001!0050787 A1
`
`400 <.
`
`410
`
`407
`
`415
`
`405
`
`406
`
`FIG. 4
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 6 of 8
`
`US 2001!0050787 A1
`
`515A 505
`
`510
`
`515
`
`515B
`
`519
`
`520A
`
`525A
`
`520B
`
`525B
`
`516A
`
`517A
`
`518A
`
`515C
`
`525C
`
`525D
`
`FIG. SA
`
`FIG. SB
`
`FIG. SC
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 7 of 8
`
`US 2001!0050787 A1
`
`Hologram Plane
`
`625
`
`Replay Field
`
`630
`
`600
`
`f' = exp (;':P)
`
`r
`
`Quantize () to
`obtain 'P
`
`i
`
`f=l~exp(j9)
`
`FFT
`
`/620
`.;
`
`615
`
`IFFT
`
`635
`
`./
`
`640
`/
`./
`
`Constraints
`
`F' = IF'I· exp(j¢)
`1
`JF(u, v)l-b(u, v) - END
`l
`
`610
`
`Start
`
`/
`F=b exp(j$) +--
`
`Input b, g)
`
`60
`5
`/
`
`1
`
`FIG. 6
`
`

`

`Patent Application Publication Dec. 13, 2001 Sheet 8 of 8
`
`US 2001!0050787 A1
`
`700 <
`
`705
`I
`~:::::··_:::_··::::
`
`710
`
`720
`
`715
`
`725A
`
`I
`- - --
`__________ ... -----~
`~rna
`
`72SC
`
`740
`
`730
`
`735
`
`FIG. 7
`
`

`

`US 2001/0050787 Al
`
`Dec. 13,2001
`
`1
`
`ELECTRO-OPTICAL COMPONENT HAVING A
`RECONFIGURABLE PHASE STATE
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] The present application is claiming priority of U.S.
`Provisional Patent Application Serial No. 60/206,074 filed
`on May 22, 2000.
`
`BACKGROUND OF THE INVENTION 1. Field
`of the Invention
`[0002] The present invention relates to an electro-optical
`component having a reconfigurable phase state. The com(cid:173)
`ponent is particularly suitable for steering an optical beam.
`Such a component can be used in applications such as
`metropolitan area network (MAN) optical terabit switching/
`routing, all-optical cross-connect systems for dense wave
`division multiplexing (DWDM) networks, photonics signal
`processing, and free space laser communication.
`
`[0003] 2. Description of the Prior Art
`
`[0004] One of the most critical elements within the frame(cid:173)
`work of optical transport networks based on wavelength(cid:173)
`division multiplexing is an optical cross-connect (OXC).
`This optical routing device provides network management in
`the optical layer, with potential throughputs of terabits per
`second. An optical cross connection may be accomplished
`by either a hybrid approach or by an all-optical approach.
`
`[0005] The hybrid approach converts an optical data
`stream into an electronic data stream. It uses an electronic
`cross connection, and then performs an electrical-optical
`conversion. There is an inherent problem with the hybrid
`approach when used in a networked environment. Histori(cid:173)
`cally, microprocessor speed has doubled almost every 18
`months, but demand for network capacity has increased at a
`much faster rate, thus causing a widening gap between the
`microprocessor speed and the volume of network traffic. The
`effect of this gap places a great burden on the electronic
`cross connections for optical links that are implemented in
`metropolitan and long-haul networks. Optical carrier 48
`(OC-48) is one of the layers of hierarchy in a conventional
`synchronous optical network (SONE1). The procedure of
`optical-electrical-optical (OEO) conversion becomes more
`difficult as the speed of the link reaches OC-48 (2.5 Gbps),
`and is even more difficult at higher speeds. At such speeds,
`the electronic circuitry of the OEO causes a network bottle(cid:173)
`neck.
`
`[0006] The all optical approach performs the cross con(cid:173)
`nection entirely in the optical domain. The all optical
`approach does not have the same speed limitations as the
`hybrid approach. It is normally used for fiber channel, high
`bandwidth cross connections. Taking NxN to represent the
`dimension of the OXC, i.e. the number of input and output
`ports, then N is typically between 2 and 32 for an all optical
`OXC. However, larger dimension OXCs, with N up to
`several hundreds or even a thousand are contemplated.
`Many proposed optical cross-connect architectures include a
`set of optical space switches capable of switching a large
`number of input and output fibers. However, despite a
`significant investment for development of an all photonics
`OXC, it is presently a major challenge to design a reliable
`all photonics, non-blocking, low loss, scalable and recon(cid:173)
`figurable optical switch, even for N in the order of 32-40.
`
`[0007] Several different technologies have been tried for
`optical interconnects, but none is yet regarded as a technol(cid:173)
`ogy or market place leader. This is due, in part, to an
`impracticality of the switching media or to a lack of scal(cid:173)
`ability in cross-connecting a suitable number of input and
`output ports.
`
`[0008] For example, guided wave systems use nonlinear
`electro-optic components, sometimes with diffraction
`effects, to couple optical signals from one fiber wave-guide
`to another. Prominent attention in this class of devices has
`been given to fiber Bragg switches and other fiber proximity
`coupling schemes such as devices using electro-optic effects
`in lithium niobite, silica or polymer based materials. A
`limitation of these switch mechanisms is scalability. It is
`difficult to construct guided wave switches greater than an
`8x8 size because they use substrates of limited size. The
`interconnection of several small switches to construct a large
`switch is also impractical because of the bulkiness of optical
`fiber harnesses.
`
`[0009] An advantage of a free-space optical switching
`system is that it can exploit the non-interference property of
`optical signals to switch a large number of optical ports. The
`two most common mechanisms for beam steering in this
`class of devices are diffraction and mechanical steering.
`
`[0010] For mechanical beam steering devices, a good deal
`of development effort appears to be concentrated on mirrors
`using micro electro mechanical systems (MEMS). Several
`devices being manufactured commercially, such as the
`Lambda Router™ from Lucent Technologies, Inc.
`
`[0011] Another mechanical approach that has received
`considerable attention is the use of micro-"bubbles", such as
`in the N3565A "32x32 Photonic Switch", offered by Agilent
`Technologies. In a micro-bubble system, the index of refrac(cid:173)
`tion of a transmission media is modified by mechanically
`moving a microscopic bubble in the media.
`
`[0012] Disadvantages of a MEMS-based switch include
`limitations relating to mechanical, thermal and electrostatic
`stability. A MEMS-based switch typically requires continu(cid:173)
`ous adaptive alignment to maintain a connection and its
`reliability is a function of that adaptive alignment. Another
`disadvantage of the MEMS-based switch is its optics, which
`typically require highly collimated optical paths, usually
`employing microlenses that cannot significantly diffract the
`light beam.
`
`[0013]
`In diffractive steering, an optical signal is redi(cid:173)
`rected using a phase hologram, also known as a grating or
`a diffraction pattern, recorded on a spatial light modulator.
`Several materials have been proposed for use in such
`systems, including III-V semiconductors such as InGaAs/
`InP, and liquid crystal on silicon systems (LCOS). One
`advantage of using direct-gap semiconductors is the ease
`with which active optical components, such as lasers and
`optical amplifiers, can be incorporated into a circuit, thus
`allowing the possibility of signal boosting at the switching
`stage. A disadvantage of such materials is the cost and
`difficulty of large-scale manufacturing.
`
`SUMMARY OF THE INVENTION
`
`[0014]
`It is an object of the present invention to provide an
`improved optical component having a variable phase state.
`
`

`

`US 2001/0050787 Al
`
`Dec. 13,2001
`
`2
`
`It is another object of the present invention to
`[0015]
`employ such a component in an optical switch in which a
`plurality of the components are configured in an array for
`phase modulating light in order to steer the light from an
`input port to an output port by diffraction.
`
`It is yet another object of the present invention to
`[0016]
`provide such a switch in which the array of phase modulat(cid:173)
`ing components and the parallel processing capability are
`both carried on the same substrate.
`
`It is a further object of the present invention to
`[0017]
`provide such a switch in which the circuit computes a
`reconfigurable phase pattern or hologram to optimize the
`performance of the switch by reducing optical losses, and to
`minimize the quanta of optical signal falling into adjacent
`channels, i.e. crosstalk.
`
`It is yet a further object of the present invention to
`[0018]
`provide such a switch in which a hologram routes light from
`a single input port to a single output port, or from a single
`input port to multiple output ports, i.e., multicasting, or from
`multiple input ports to a single output port, i.e., inverse(cid:173)
`multicasting.
`
`[0019] These and other objects of the present invention are
`provided by an electro-optical component in accordance
`with the present invention. One embodiment provides an
`electro-optical component comprising (a) a substrate, (b) a
`phase-variable element carried on the substrate, (c) a
`memory carried on the substrate for storing data represen(cid:173)
`tative of a phase state for the phase-variable element; and (d)
`a controller carried on the substrate, for utilizing the data and
`setting the phase state for the element. Another embodiment
`provides an electro-optical component comprising (a) a
`substrate, (b) a phase-variable element carried on the sub(cid:173)
`strate, and (c) a circuit carried on the substrate for computing
`a phase state for the phase-variable element.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0020] FIG. 1 is a schematic representation of an optical
`switch in accordance with the present invention.
`[0021] FIGS. 2A and 2B are schematic representations of
`alternate embodiments of optical switches in accordance
`with the present invention.
`[0022] FIG. 3 is an illustration showing a relationship
`between a hologram and its replay field.
`[0023] FIG. 4 is a side-section view of a spatial light
`modulator, as used in an optical switch in accordance with
`the present invention.
`
`[0024] FIGS. 5A-5C are illustrations of various arrange(cid:173)
`ments of one or more phase-variable elements and circuitry
`on a substrate.
`[0025] FIG. 6 is a flowchart of an algorithm for generating
`a hologram by projection of constraints.
`[0026] FIG. 7 is a schematic representation of an optical
`switch in accordance with the present invention.
`
`DESCRIPTION OF THE INVENTION
`
`[0027] An embodiment of the present invention provides
`for an electro-optical component comprising (a) a substrate,
`(b) a phase-variable element carried on the substrate, (c) a
`
`memory carried on the substrate for storing data represen(cid:173)
`tative of a phase state for the phase-variable element; and (d)
`a controller carried on the substrate, for utilizing the data and
`setting the phase state for the phase-variable element. The
`component can be employed in an optical switch to direct
`light from a first port to a second port.
`
`[0028] Another embodiment of the present invention pro(cid:173)
`vides for an electro-optical component comprising (a) a
`substrate, (b) a phase-variable element carried on the sub(cid:173)
`strate, and (c) a circuit carried on the substrate for computing
`a phase state for the phase-variable element. This component
`can also be employed in an optical switch to direct light from
`a first port to a second port.
`
`In another embodiment, the optical switch includes
`[0029]
`(a) a substrate, (b) a liquid crystal carried on the substrate;
`and (c) a circuit carried on the substrate for computing a
`hologram and controlling the liquid crystal to produce the
`hologram to direct light from a first port to a second port.
`
`[0030] The optical switch uses a dynamic beam steering
`phase hologram written onto a liquid crystal over silicon
`(LCOS) spatial light modulator (SLM). A phase hologram is
`a transmissive or reflective element that changes the phase
`of light transmitted through, or reflected by, the element. A
`replay field is the result of the phase hologram. The LCOS
`SLM produces a hologram, i.e., a pattern of phases, that
`steers light by diffraction in order to route the light from one
`or more input fibers to one or more output fibers.
`
`[0031] The holograms produced on the SLM may appear
`as a one-dimensional or two-dimensional image. Accord(cid:173)
`ingly, the image elements, whether transmissive or reflec(cid:173)
`tive, are sometimes referred to as "pixels", i.e., picture
`elements.
`[0032] FIG. 7 is a schematic representation of an optical
`switch 700 in accordance with the present invention. The
`principal elements of switch 700 include an input port 705,
`a spatial light modulator (SLM) 715, and a plurality of
`output ports 725A, 725B and 725C. A first lens 710 is
`interposed between input port 705 and SLM 715, and a
`second lens 720 is interposed between SLM 715 and output
`ports 725A, 725B and 725C.
`
`[0033] Light from input port 705 is cast upon lens 710,
`which collimates the light and projects it onto SLM 715. The
`light travels through SLM 715 and onto lens 720, which
`focuses the light onto one or more of output ports 725A,
`725B and 725C. A hologram produced on SLM 715 directs
`the light to one or more of output ports 725A, 725B and
`725C. In FIG. 7, the light is shown as being directed to
`output port 725A.
`
`[0034] SLM 715 is an electro-optical component that
`includes a substrate 730 upon which is carried (a) an
`element, shown in FIG. 7 as one of an array of elements 735
`and (b) a circuit 740. Elements 735 have a variable phase.
`That is, the phase, i.e., time delay, of light propagating
`through elements 735 can be varied. When the phase of light
`propagating through an element 735 is varied relative to the
`phase of another element 735, the light forms an interference
`pattern that influences the direction in which the light
`travels, as is well known in the field of optics. Thus, by
`controlling the relative phasing, the light can be directed to
`a desired target. Liquid crystal is a suitable material for
`elements 735. Liquid crystal is conventionally provided in a
`
`

`

`US 2001/0050787 Al
`
`Dec. 13,2001
`
`3
`
`thin film sheet, and as such, individuals of elements 735
`would correspond to regions of the liquid crystal rather than
`being discrete, separate, liquid crystal elements.
`
`[0035] Circuit 740 sets the phase states for elements 735
`for directing the light from input port 705 to output port
`725A. That is, a hologram is produced by elements 735.
`Optionally, circuit 740 also computes the hologram. In FIG.
`7, the result of the hologram causes a point of light intensity
`at output port 725A.
`
`[0036] FIG. 1 is a schematic representation of an optical
`switch 100 in accordance with the present invention. The
`principal components of switch 100 include a fiber array
`115, an SLM 105, and a lens 110 interposed between fiber
`array 115 and SLM 105.
`
`[0037] Fiber array 115 has a first port 120 and a second
`port 125. Light enters switch 100 via first port 120 and
`proceeds to lens 110, which is, for example, a Fourier lens
`having a positive focal length. Lens 110 collimates the light.
`From lens 110, the light is projected onto SLM 105. The
`light is reflected by SLM 105, and travels via lens 110 to
`second port 125. As explained below, a hologram produced
`on SLM 105 steers the light from first port 120 to second
`port 125.
`
`[0038] SLM 105 is an electro-optical component that
`includes a substrate 130 upon which is disposed (a) a
`reflective element, shown in FIG. 1 as one of an array of
`reflective elements 135, and (b) a circuit 140 underneath and
`around reflective elements 135. Reflective elements 135
`have a variable phase state That is, the phase, i.e., time delay,
`of light reflected by reflective elements 135 can be varied.
`When the light is reflected by two or more of reflective
`elements 135, the light forms an interference pattern that
`influences the direction in which the light is reflected. As the
`phase state of an individual reflective element 135 is vari(cid:173)
`able, it can be altered relative to the phase state of other
`reflective elements 135 to control the direction in which the
`light is reflected. A practical embodiment of array reflective
`elements 135 can be realized by employing a liquid crystal
`over an array of mirrors.
`
`[0039] Circuit 140 controls the phase state, i.e., hologram,
`for reflective elements 135 to control the direction in which
`the light is reflected. Optionally, circuit 140 also computes
`the hologram. The result of the hologram is projected on
`fiber array 115, with points of intensity at one or more ports
`in fiber array 115. In FIG. 1 the light is shown as being
`directed from first port 120 to second port 125, however, in
`terms of functionality, first port 120 and second port 125 are
`preferably each a bi-directional input/output port.
`
`[0040] FIG. 2A is a schematic representation of an optical
`switch 200 configured with two SLMs 210 and 215, to
`provide a greater number of ports than that of the configu(cid:173)
`ration in FIG. 1. Switch 200 also includes a first fiber array
`205, a second fiber array 220, and a lens array 225.
`
`[0041] First fiber array 205 and second fiber array 220 are
`each an array of bi-directional fiber ports. Lens array 225 is
`a series of refractive optical elements that transfer one or
`more optical beams through switch 200.
`
`[0042]
`Input signals in the form of light beams or pulses
`are projected from one or more ports in first fiber array 205,
`through one or more lenses of lens array 225 onto SLM 210.
`
`SLM 210 produces a first routing hologram that directs the
`light through one or more lenses of lens array 225 onto SLM
`215. SLM 215 produces a second routing hologram that
`directs the light through one or more lenses of lens array 225
`onto one or more ports in second fiber array 220.
`
`[0043] SLMs 210 and 215 each have an array of phase(cid:173)
`variable reflective elements on its surface to produce recon(cid:173)
`figurable phase holograms that control the deflection angle
`of a beam of light. Thus, light from any port of first fiber
`array 205 can be selectively routed to any port of second
`fiber array 220, and vice versa.
`
`[0044] Switch 200 accommodates fiber arrays of a sub(cid:173)
`stantially greater dimension than that of typical prior art
`switches. For example, first fiber array 205 and second fiber
`array 220 may each have 1000 ports.
`
`[0045] The optical configuration of switch 200 influences
`the distribution of the pixels on each of SLMs 210 and 215,
`and the manner in which a hologram is generated thereon.
`For example, the optical configuration influences the size of
`the region on each of SLMs 210 and 215 onto which the light
`is projected.
`
`[0046] FIG. 2B is a schematic representation of an optical
`switch 250 in another embodiment of the present invention.
`Switch 250 includes an input/output fiber array 230, a lens
`235, e.g., a Fourier transform lens, an SLM 240 and a
`reflector 245.
`
`[0047] Light from a first port 232 of input/output fiber
`array 230 is projected through lens 235 onto a first region
`242 of SLM 240. SLM 240 produces a first hologram in first
`region 242 that directs the light to reflector 245, which, in
`turn, directs the light to a second region 247 of SLM 240.
`SLM 240 produces a second hologram in second region 247
`to direct the light through lens 235 and onto a selected
`second port 234 of input/output fiber array 230.
`
`[0048] Referring again to FIG. 2A, the architecture in
`FIG. 2A can be made to mimic that of FIG. 2B by "folding"
`switch 200 about a central point. That is, the architecture of
`FIG. 2A approaches that of FIG. 2B by placing a mirror at
`the central point so that first fiber array 205 and second fiber
`array 220 are side by side, and SLM 210 and SLM 215 are
`side by side.
`
`[0049] FIG. 3 is an illustration showing a relationship
`between a hologram and its replay field as can be provided
`by the optical switch of the present invention. Referring
`again to FIG. 2A for example, a reconfigurable phase
`hologram 305 is situated at a Fourier plane, e.g., on the array
`of phase-variable reflective elements at the surface of SLMs
`210 and 215. In the preferred embodiment, phase hologram
`305 is written into, that is, programmed into, the reflective
`elements to provide phase-only modulation of the incident
`light. The reflective elements diffract the light from first fiber
`array 205 to produce phase hologram 305. After a Fourier
`transform of the hologram, a resulting diffracted pattern,
`also known as a replay field 310, is produced at second fiber
`array 220.
`
`[0050] Note that replay field 310 shows 16 points of light.
`FIG. 3 illustrates a feature of the present invention called
`multicasting. In a multicast, one input port is coupled to two
`or more output ports, i.e., simultaneous routing of light from
`one input port to a plurality of output ports. This can be done
`
`

`

`US 2001/0050787 Al
`
`Dec. 13,2001
`
`4
`
`with a hologram that generates multiple peaks, as shown in
`replay field 310, rather than a single peak. FIG. 3 shows an
`example of a 1 to 16 multicast hologram. In a similar
`fashion, the same set of holograms can also be used to route
`multiple input ports to a single output port, referred to as
`multiplexing, provided that the inputs have different wave(cid:173)
`lengths. This can be used for wavelength division multi(cid:173)
`plexing (WDM).
`
`[OOS1] FIG. 4 shows a cross section of an exemplary SLM
`400 in accordance with the present invention. The principal
`features of SLM 400 are a substrate 410 that carries (a) a
`silicon die 40S containing a circuit 406, (b) an array of
`mirrors 407, and (c) a liquid crystal element 41S, which has
`a variable phase state. In FIG. 4, SLM 400 is configured to
`show liquid crystal element 41S positioned upon array of
`mirrors 407, which is positioned upon circuit 406. However,
`any convenient arrangement of these components is con(cid:173)
`templated as being within the scope of the present invention.
`
`[OOS2] The phase shift of light through liquid crystal
`element 41S is varied, or set for a specific value, by applying
`an electric field across liquid crystal element 41S. Circuit
`406 controls the phase state of liquid crystal element 41S by
`applying voltages to the array of mirrors 407 and thus
`developing the electric field across liquid crystal element
`41S. In practice each mirror 407 influences the phase state
`of a region of liquid crystal element 41S to which the mirror
`is adjacent. Thus, circuit 406 controls the phase state of a
`plurality of regions of liquid crystal element 41S by con(cid:173)
`trolling the individual voltages applied to each of mirrors
`407.
`
`[OOS3] Circuit 406 executes the processes described
`herein, and it may include one or more subordinate circuits
`for executing portions of the processes or ancillary func(cid:173)
`tions. In one embodiment of the present invention, circuit
`406 includes a memory for storing data representative of a
`plurality of configurations of phase state for liquid crystal
`element 41S, and a controller for utilizing the data and
`setting the phase states by applying signals to mirrors 407.
`Such data can be determined by an external system in a
`calibration procedure during manufacturing of SLM 400, or
`during manufacturing of an assembly in which SLM 400 is
`a component. The external system computes the phase
`states, and thereafter, the data is written into the memory of
`circuit 406. In another embodiment, circuit 406 includes a
`processor and associated memory for storing data in order to
`compute the phase states locally, and a controller to set the
`phases states by applying signals to mirrors 407.
`
`[OOS4] SLM 400 also includes, on top of liquid crystal
`elements 41S, a glass cover 420. Glass cover 420 has a layer
`43S of Indium Tin Oxide (ITO) to provide a return path
`conductor for signals from circuit 406 via bond wires 42S.
`
`[OOSS] On the optical side of SLM 400, the array of
`mirrors 407 allows for steering of a light beam by producing
`a hologram using variable phase liquid crystal elements 41S.
`In circuit 406, the following functionalities can be imple(cid:173)
`mented:
`
`[OOS6] DC balance schemes including shifting and
`scrolling;
`
`[OOS7] Algorithms for reconfigurable beam steering
`and hologram generation;
`
`[OOS8] Generation of multicast hologram patterns;
`
`[OOS9] Hologram tuning for crosstalk optimization;
`
`[0060] Hologram tuning for adaptive port alignment;
`
`[0061] Phase aberration correction; and
`
`[0062] Additional processing of various network traf(cid:173)
`fic parameters.
`
`[0063] FIGS. SA-SC illustrate several viable arrangements
`of phase-variable elements and circuitry on the SLM of the
`present invention. FIG. SA illustrates a die-based arrange(cid:173)
`ment with a substrate SOS carrying circuitry S10 around
`and/or underneath an array of phase-variable elements S1S.
`The array of phase-variable elements S1S is partitioned into
`several subsets of phase-variable elements (S1SA, S1SB,
`S1SC and S1SD), each operating as an independent SLM.
`FIG. SB shows a substrate S19 carrying several groups of
`components, namely, circuitry S20A, S20B, S20C and S20D,
`and an array of phase-variable elements S2SA, S2SB, S2SC
`and S2SD, respectively. FIG. SC shows an individual phase(cid:173)
`variable element S3S and circuitry S30 for controlling phase(cid:173)
`variable element S3S.
`
`In FIG. SA, circuit S10 controls the operation of
`[0064]
`the full array of phase-variable elements, that is, each of
`S1SA, S1SB, S1SC and S1SD. FIG. SA illustrates an
`arrangement in which four holograms can be simultaneously
`produced, i.e., one for each of subsets S1SA, S1SB, S1SC
`and S1SD. Circuit S10 computes a first phase state for subset
`S1SA to direct a first light beam from a first port to a second
`port, and computes a second phase state for subset S1SB to
`direct a second light beam from a third port to a fourth port.
`Similarly for subsets S1SC and S1SD, circuit computes
`respective phase states for routing of a third light beam and
`a fourth light beam. Because the phase states of the indi(cid:173)
`viduals in the array phase-variable elements S1S are indi(cid:173)
`vidually reconfigurable, circuit S10 can determine which of
`phase-variable elements S1S are members of the first subset
`S1SA, which of phase-variable elements S1S are members of
`the second subset S1SB, and likewise, which of phase(cid:173)
`variable elements S1S are members of the subsets S1SC and
`S1SD. In FIG. SA, subsets S1SA, S1SB, S1SC and S1SD can
`be located adjacent to one another, or alternatively they can
`be spaced apart from one another by a region of substrate
`SOS that does not include any phase-variable elements.
`
`[006S] An appropriate dimension for an array of phase(cid:173)
`variable elements, i.e., pixels, per hologram, is about lOOx
`100 pixels for good Gaussian beam performance. Accord(cid:173)
`ingly, an array of 600x600 pixels provides for 36 holograms.
`However, the present invention is not limited to any par(cid:173)
`ticular dimension for the array, nor is it limited to any
`particular number of phase-variable elements or any
`arrangement of phase-variable elements. Theoretically,
`some beam steering functionality can be achieved with as
`few as two phase-altering elements, only one of which needs
`to have a variable phase. Furthermore, the phase-variable
`elements do not need to be arranged in an array, per se, as
`any suitable arrangement is contemplated as being within
`the scope of the present invention.
`
`[0066] Referring again to FIG. SB, a gap S26 is a region
`of substrate S19 that does not include any phase-variable
`elements. Gap S26 is located between phase-variable ele-
`
`

`

`US 2001/0050787 Al
`
`Dec. 13,2001
`
`5
`
`ments S2SB and S2SD, and thus prevents crosstalk between
`the holograms of phase-variable elements S2SB and S2SD.
`
`[0067] The arrangement shown in FIG. SA can deal with
`crosstalk in a manner different from that of FIG. SB. In FIG.
`SA, pixel subset S1SAincludes a region of pixels S16A upon
`which a hologram is produced. Pixel subset S1SA also
`includes a subset of pixels S17 A positioned along a periph(cid:173)
`eral edge of subset S16A. Subset S17 A is thus a buffer region
`for preventing crosstalk between the hologram of subset
`S1SA, and the holograms of subsets S1SB and S1SC.
`
`[0068] Also, as those skilled in the art will appreciate, a
`hologram is shift invariant, that is the same replay field is
`generated for any shifted position of the hologram. Thus, as
`a further improvement, the phases of the pixels in subset
`S17 A are set by circuit S10 to take advantage of the shift
`invariant property of the hologram such that a misalignment
`of the light beam incident on subset S1SA will nevertheless
`produce the desired hologram. Therefore, provided that the
`misalignment is within a predetermined tolerance, i.e., such
`that the incident light falls within the bounds of subset S1SA,
`the hologram is produced notwithstanding a misalignment
`of the light from an input port.
`
`[0069] To take further advantage of the reconfigurable
`capability of the optical switch, circuit S10 receives a signal
`that represents whether light is being directed to a particular
`port. This feature enables circuit S10 to perform an adaptive
`optical alignment, where circuit S10 receives an input signal
`indicating that the light is to be directed from a first port to
`a second port. Circuit S10 locates the second port and then
`optimizes the hologram to minimize switch loss and
`crosstalk. For example, assume that pixel subset S1SA is
`selected to direct light from the first port to the second port.
`Circuit S10 determines a position of the second port by
`successively recomputing the phase state for pixel subset
`S1SA to successively redirect the light, and by successively
`evaluating the signal to determine whether the light is
`aligned with the second port.
`
`[0070] A hologram can be calculated to route light to any
`position in the replay field. Hence, there are more positions
`to which the light can be routed in the replay field than there
`are pixels in the hologram. A hologram can be designed with
`a higher resolution replay field than the original hol