111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US007145710B2
`
`c12) United States Patent
`Holmes
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,145,710 B2
`Dec. 5, 2006
`
`(54) OPTICAL PROCESSING
`
`(56)
`
`References Cited
`
`(75)
`
`Inventor: Melanie Holmes, Ipswich (GB)
`
`(73) Assignee: Thomas Swan & Co Ltd. (GB)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 122 days.
`
`(21) Appl. No.:
`
`10/487,810
`
`(22) PCT Filed:
`
`Sep. 2, 2002
`
`(86) PCTNo.:
`
`PCT /GB02/04011
`
`§ 371 (c)(l),
`(2), ( 4) Date:
`
`Sep. 10, 2004
`
`(87) PCT Pub. No.: W003/021341
`
`PCT Pub. Date: Mar. 13, 2003
`
`(65)
`
`Prior Publication Data
`
`US 2005/0018259 Al
`
`Jan. 27, 2005
`
`(30)
`
`Foreign Application Priority Data
`
`................ 359/11
`
`U.S. PATENT DOCUMENTS
`4,952,010 A *
`5,107,359 A
`5,428,466 A *
`5,539,543 A
`5,589,955 A *
`5,959,747 A *
`5,995,251 A *
`6,072,608 A *
`
`8/1990 Healey et a!.
`4/1992 Ohuchida
`6/1995 Rejman-Greene et al ..... 359/15
`7/1996 Liu eta!.
`12/1996 Amako et a!. ................. 359/9
`9/1999 Psaltis et a!.
`................. 359/22
`11/1999 Hesselink et a!.
`............ 359/30
`6/2000 Psaltis et a!.
`................. 359/22
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`1 050 77 5 A1
`
`1112000
`
`(Continued)
`
`OTHER PUBLICATIONS
`
`Marom, D.M., et a!., "Wavelength-Selective 1x4 Switch for 128
`WDM Channels at 50 Ghz Spacing," OFC Postdeadline Paper, pp.
`FB7-1-FB7-3 (2002).
`
`(Continued)
`
`Primary Examiner-Loha Ben
`(74) Attorney, Agent, or Firm-Hamilton, Brook, Smith &
`Reynolds, P.C.
`
`Sep. 3, 2001
`
`(GB)
`
`................................. 0121308.1
`
`(57)
`
`ABSTRACT
`
`(51)
`
`Int. Cl.
`G02F 1101
`(2006.01)
`G02F 1129
`(2006.01)
`G03H 1112
`(2006.01)
`G02B 6126
`(2006.01)
`B23K 26106
`(2006.01)
`(52) U.S. Cl. ............................ 359/279; 359/9; 359111;
`359/15; 359/298; 385/15; 385/16; 385/17;
`219/121.73; 219/121.75
`(58) Field of Classification Search .............. 359/9-11,
`359/15,24,22,30, 559,298,279; 385/15-18;
`219/121.73, 121.75; 398/79, 87; 356/457
`See application file for complete search history.
`
`To operate an optical device comprising an SLM with a
`two-dimensional array of controllable phase-modulating
`elements groups of individual phase-modulating elements
`are delineated, and control data selected from a store for
`each delineated group of phase-modulating elements. The
`selected control data are used to generate holograms at each
`group and one or both of the delineation of the groups and
`the selection of control data is/are varied. In this way upon
`illumination of the groups by light beams, light beams
`emergent from the groups are controllable independently of
`each other.
`
`14 Claims, 36 Drawing Sheets
`
`/'
`"'"'
`
`FINISAR 1001
`
`

`

`US 7,145,710 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`6,115,123 A * 9/2000 Stappaerts eta!. .......... 356/457
`6,710,292 B1 * 3/2004 Fukuchi eta!. ........ 219/121.73
`6,975,786 B1 * 12/2005 Warr et a!.
`................... 385/17
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`wo
`wo
`wo
`wo
`wo
`
`1 053 501 B1
`WO 01 25840 A1
`WO 01 25848 A2
`WO 01 90823 A1
`WO 02 079870 A2
`WO 02 101451 A1
`
`7/2003
`4/2001
`4/2001
`1112001
`10/2002
`12/2002
`
`OTHER PUBLICATIONS
`
`Mears, R. J., et a!., "Telecommunications Applications of Fer(cid:173)
`roelectric Liquid-Crystal Smart Pixels," IEEE Journal of Selected
`Topics in Quantum Electronics, vol. 2, No. 1, Apr. 1996, pp. 35-46.
`Mears, R. J., et al., "WDM Channel Management Using Program(cid:173)
`mable Holographic Elements," lEE Colloquim on Multi wavelength
`Optical Networks: Devices, Systems and Network Implementa(cid:173)
`tions, lEE, London, GB, Jun. 18, 1998, pp. 11-1-11-6.
`Pan, Ci-Ling, et a!., "Tunable Semiconductor Laser with Liquid
`Crystal Pixel Mirror in Grating-Loaded External Cavity," Electron(cid:173)
`ics Letters, lEE Stevenage, GB, vol. 35, No. 17, Aug. 19, 1999, pp.
`1472-1473.
`* cited by examiner
`
`

`

`U.S. Patent
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`Dec. 5, 2006
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`US 7,145,710 B2
`
`1
`OPTICAL PROCESSING
`
`RELATED APPLICATIONS
`
`This application is the U.S. National Stage of Interna(cid:173)
`tional Application No. PCT/GB02/04011, filed 2 Sep. 2002,
`published in English, which application claims priority
`under 35 U.S.C. § 119 or 365 to Great Britain Application
`No. 0121308.1, filed 3 Sep. 2001, the entire teachings of the
`above referenced applications are incorporated herein by
`reference.
`
`FIELD OF THE INVENTION
`
`The present invention relates to an optical device and to
`a method of controlling an optical device.
`More particularly but not exclusively the invention relates
`to the general field of controlling one or more light beams by
`the use of electronically controlled devices. The field of
`application is mainly envisaged as being to fields in which
`reconfiguration between inputs and outputs is likely, and
`stability of performance is a significant requirement.
`
`BACKGROUND OF THE INVENTION
`
`35
`
`2
`The prior art does not adequately address this situation.
`Other optical systems are static in terms of input/output
`configuration. In such systems, effects such as assembly
`errors, manufacturing tolerances in the optics and also
`changes in the system behaviour due to temperature and
`ageing, create the desirability for dynamic direction control,
`aberration correction, phase distortion compensation or mis(cid:173)
`alignment compensation.
`It should be noted that the features of dynamic direction
`10 control, phase distortion compensation and misalignment
`control are not restricted to systems using input beams
`coming from optical fibres. Such features may also be
`advantageous in a reconfigurable optical system. Another
`static system in which dynamic control of phase distortion,
`15 direction and (relative) misalignment would be advanta(cid:173)
`geous is one in which the quality and/or position of the input
`beams is time-varying.
`Often the input and output beams for optical systems
`contain a multiplex of many optical signals at different
`20 wavelengths, and these signals may need to be separated and
`adaptively and individually processed inside the system.
`Sometimes, although the net aim of a system is not to
`separate optical signals according to their wavelength and
`then treat them separately, to do so increases the wavelength
`25 range of the system as a whole. Where this separation is
`effected, it is often advantageous for the device used to route
`each channel to have a low insertion loss and to operate
`quickly.
`It is an aim of some aspects of the present invention at
`least partly to mitigate difficulties of the prior art.
`It is desirable for certain applications that a method or
`device for addressing these issues should be polarisation(cid:173)
`independent, or have low polarisation-dependence.
`SLMs have been proposed for use as adaptive optical
`components in the field of astronomical devices, for
`example as wavefront correctors. In this field of activity, the
`constraints are different to the present field-for example in
`communication and like devices, the need for consistent
`performance is paramount if data is to be passed without
`errors. Communication and like devices are desirably inex(cid:173)
`pensive, and desirably inhabit and successfully operate in
`environments that are not closely controlled. By contrast,
`astronomical devices may be used in conditions more akin
`to laboratory conditions, and cost constraints are less press(cid:173)
`ing. Astronomical devices are unlikely to need to select
`successive routings of light within a system, and variations
`in performance may be acceptable.
`
`It has previously been proposed to use so-called spatial
`light modulators to control the routing oflight beams within
`an optical system, for instance from selected ones of a
`number of input optical fibres to selected ones of output 30
`fibres.
`Optical systems are subject to performance impairments
`resulting from aberrations, phase distortions and component
`misaligmnent. An example is a multiway fibre connector,
`which although conceptually simple can often be a critical
`source of system failure or insertion loss due to the very tight
`aligmnent tolerances for optical fibres, especially for single(cid:173)
`mode optical fibres. Every time a fibre connector is con(cid:173)
`nected, it may provide a different alignment error. Another
`example is an optical switch in which aberrations, phase
`distortions and component misalignments result in poor
`optical coupling eff1ciency into the intended output optical
`fibres. This in turn may lead to high insertion loss. The
`aberrated propagating waves may diffract into intensity
`fluctuations creating significant unwanted coupling of light
`into other output optical fibres, leading to levels of crosstalk
`that impede operation. In some cases, particularly where
`long path lengths are involved, the component misalignment
`may occur due to ageing or temperature effects.
`Some prior systems seek to meet such problems by use of
`expensive components. For example in a communications
`context, known free-space wavelength multiplexers and
`demultiplexers use expensive thermally stable opto-mechan-
`ics to cope with the problems associated with long path 55
`lengths.
`Certain optical systems have a requirement for reconfig(cid:173)
`include optical
`urability. Such reconfigurable systems
`switches, add/drop multiplexers and other optical routing
`systems where the mapping of signals from input ports to 60
`output ports is dynamic. In such systems the path-dependent
`losses, aberrations and phase distortions encountered by
`optical beams may vary from beam to beam according to the
`route taken by the beam through the system. Therefore the
`path-dependent loss, aberrations and phase distortions may
`vary for each input beam or as a function of the required
`output port.
`
`40
`
`45
`
`50
`
`SUMMARY OF THE INVENTION
`
`According to a first aspect of the invention, there is
`provided a method of operating an optical device comprising
`an SLM having a two-dimensional array of controllable
`phase-modulating elements, the method comprising
`delineating groups of individual phase-modulating ele(cid:173)
`ments;
`selecting, from stored control data, control data for each
`group of phase-modulating elements;
`generating from the respective selected control data a
`respective hologram at each group of phase-modulating
`elements; and
`varying the delineation of the groups and/or the selection
`of control data whereby upon illumination of said groups by
`65 respective light beams, respective emergent light beams
`from the groups are controllable independently of each
`other.
`
`

`

`US 7,145,710 B2
`
`3
`In some embodiments, the variation of the delineation
`and/or control data selection is in response to a signal or
`signals indicating a non-optimal performance of the device.
`In other embodiments, the variation is performed during a
`set up or training phase of the device. In yet other embodi(cid:173)
`ments, the variation is in response to an operating signal, for
`example a signal giving the result of sensing non-perfor(cid:173)
`mance system parameters such as temperature.
`An advantage of the method of this aspect of the invention
`is that stable operation can be achieved in the presence of 10
`effects such as ageing, temperature, component, change of
`path through the system and assembly tolerances.
`Preferably, control of said light beams is selected from the
`group comprising: control of direction, control of power,
`focussing, aberration compensation, sampling and beam 15
`shaping.
`Clearly in most situations more than one of these control
`types will be needed-for example in a routing device (such
`as a switch, filter or add/drop multiplexer) primary changes
`of direction are likely to be needed to cope with changes of 20
`routing as part of the main system but secondary correction
`will be needed to cope with effects such as temperature and
`ageing. Additionally such systems may also need to control
`power, and to allow sampling (both of which may in some
`cases be achieved by direction changes).
`Advantageously, each phase modulating element is
`responsive to a respective applied voltage to provide a
`corresponding phase shift to emergent light, and the method
`further comprises;
`controlling said phase-modulating elements of the spatial 30
`light modulator to provide respective actual holograms
`derived from the respective generated holograms, wherein
`the controlling step comprises;
`resolving the respective generated holograms modulo 2pi.
`The preferred SLM uses a liquid crystal material to 35
`provide phase shift and the liquid crystal material is not
`capable of large phase shifts beyond plus or minus 2 pi.
`Some liquid crystal materials can only provide a smaller
`range of phase shifts, and if such materials are used, the
`resolution of the generated hologram is correspondingly 40
`smaller.
`Preferably the method comprises:
`providing a discrete number of voltages available for
`application to each phase modulating element;
`on the basis of the respective generated holograms, deter- 45
`mining the desired level of phase modulation at a predeter(cid:173)
`mined point on each phase modulating element and choos(cid:173)
`ing for each phase modulating element the available voltage
`which corresponds most closely to the desired level.
`Where a digital control device is used, the resolution of 50
`the digital signal does not provide a continuous spectrum of
`available voltages. One way of coping with this is to
`determine the desired modulation for each pixel and to
`choose the individual voltage which will provide the closest
`modulation to the desired level.
`In another embodiment, the method comprises:
`providing a discrete number of voltages available for
`application to each phase modulating element;
`determining a subset of the available voltages which
`provides the best fit to the generated hologram.
`Another technique is to look at the pixels of the group as
`a whole and to select from the available voltages those that
`give rise to the nearest phase modulation across the whole
`group.
`Advantageously, the method further comprises the step of
`storing said control data wherein the step of storing said
`control data comprises calculating an initial hologram using
`
`4
`a desired direction change of a beam of light, applying said
`initial hologram to a group of phase modulating elements,
`and correcting the initial hologram to obtain an improved
`result.
`The method may further comprise the step of providing
`sensors for detecting temperature change, and performing
`said varying step in response to the outputs of those sensors.
`The SLM may be integrated on a substrate and have an
`integral quarter-wave plate whereby it is substantially
`polarisation insensitive.
`Preferably the phase-modulating elements are substan(cid:173)
`tially reflective, whereby emergent beams are deflected from
`the specular reflection direction.
`In some aspects, for at least one said group of pixels, the
`method comprises providing control data indicative of two
`holograms to be displayed by said group and generating a
`combined hologram before said resolving step.
`According to a second aspect of the invention there is
`provided an optical device comprising an SLM and a control
`circuit, the SLM having a two-dimensional array of control(cid:173)
`lable phase-modulating elements and the control circuit
`having a store constructed and arranged to hold plural items
`of control data, the control circuit being constructed and
`25 arranged to delineate groups of individual phase-modulating
`elements, to select, from stored control data, control data for
`each group of phase-modulating elements, and to generate
`from the respective selected control data a respective holo-
`gram at each group of phase-modulating elements,
`wherein the control circuit is further constructed and
`arranged, to vary the delineation of the groups and/or the
`selection of control data
`whereby upon illumination of said groups by respective
`light beams, respective emergent light beams from the
`groups are controllable independently of each other.
`An advantage of the device of this aspect of the invention
`is that stable operation can be achieved in the presence of
`effects such as ageing, temperature, component and assem(cid:173)
`bly tolerances. Embodiments of the device can handle many
`light beams simultaneously. Embodiments can be wholly
`reconfigurable, for example compensating differently for a
`number of routing configurations.
`Preferably, the optical device has sensor devices arranged
`to detect light emergent from the SLM, the control circuit
`being responsive to signals from the sensors to vary said
`delineation and/or said selection.
`In some embodiments, the optical device has temperature
`responsive devices constructed and arranged to feed signals
`indicative of device temperature to said control circuit,
`whereby said delineation and/or selection is varied.
`In another aspect, the invention provides an optical rout(cid:173)
`ing device having at least first and second SLMs and a
`control circuit, the first SLM being disposed to receive
`55 respective light beams from an input fibre array, and the
`second SLM being disposed to receive emergent light from
`the first SLM and to provide light to an output fibre array, the
`first and second SLMs each having a respective two-dimen(cid:173)
`sional array of controllable phase-modulating elements and
`60 the control circuit having a store constructed and arranged to
`hold plural items of control data, the control circuit being
`constructed and arranged to delineate groups of individual
`phase-modulating elements, to select, from stored control
`data, control data for each group of phase-modulating ele-
`65 ments, and to generate from the respective selected control
`data a respective hologram at each group of phase-modu(cid:173)
`lating elements,
`
`

`

`US 7,145,710 B2
`
`5
`wherein the control circuit is further constructed and
`arranged, to vary the delineation of the groups and/or the
`selection of control data
`whereby upon illumination of said groups by respective
`light beams, respective emergent light beams from the
`groups are controllable independently of each other.
`In a further aspect, the invention provides a device for
`shaping one or more light beams in which the or each light
`beam is incident upon a respective group of pixels of a
`two-dimensional SLM, and the pixels of the or each respec(cid:173)
`tive group are controlled so that the corresponding beams
`emerging from the SLM are shaped as required.
`According to a further aspect of the invention there is
`provided an optical device comprising one or more optical
`inputs at respective locations, a diffraction grating con(cid:173)
`structed and arranged to receive light from the or each
`optical input, a focussing device and a continuous array of
`phase modulating elements, the diffraction grating and the
`array of phase modulating elements being disposed in the
`focal plane of the focussing device whereby diverging light 20
`from a single point on the diffraction grating passes via the
`focussing device to form beams at the array of phase
`modulating elements, the device further comprising one or
`more optical output at respective locations spatially separate
`from the or each optical input, whereby the diffraction
`grating is constructed and arranged to output light to the or
`each optical output.
`This device allows multiwavelength input light to be
`distributed in wavelength terms across different groups of
`phase-modulating elements. This allows different processing
`effects to be applied to any desired part or parts of the
`spectrum.
`According to a still further aspect of the invention there is
`provided a method of filtering light comprising applying a
`beam of said light to a diffraction grating whereby emerging 35
`light from the grating is angularly dispersed by wavelength,
`forming respective beams from said emerging light by
`passing the emerging light to a focussing device having the
`grating at its focal plane, passing the respective beams to an
`SLM at the focal plane of the focussing device, the SLM 40
`having a two-dimensional array of controllable phase-modu(cid:173)
`lating elements, selectively reflecting light from different
`locations of said SLM and passing said reflected light to said
`focussing element and then to said grating.
`Preferably the method comprises delineating groups of 45
`individual phase-modulating elements to receive beams of
`light of differing wavelength;
`selecting, from stored control data, control data for each
`group of phase-modulating elements;
`generating from the respective selected control data a 50
`respective hologram at each group of phase-modulating
`elements; and
`varying the delineation of the groups and/or the selection
`of control data.
`According to a still further aspect of the invention there is 55
`provided an optical add/drop multiplexer having a reflective
`SLM having a two-dimensional array of controllable phase(cid:173)
`modulating elements, a diffraction device and a focussing
`device wherein light beams from a common point on the
`diffraction device are mutually parallel when incident upon 60
`the SLM, and wherein the SLM displays respective holo(cid:173)
`grams at locations of incidence of light to provide emergent
`beams whose direction deviates from the direction of specu-
`lar reflection.
`In a yet further aspect, the invention provides a test or
`monitoring device comprising an SLM having a two-dimen(cid:173)
`sional array of pixels, and operable to cause incident light to
`
`6
`emerge in a direction deviating from the specular direction,
`the device having light sensors at predetermined locations
`arranged to provide signals indicative of said emerging light.
`The test or monitoring device may further comprise
`further sensors arranged to provide signals indicative oflight
`emerging in the specular directions.
`Yet a further aspect of the invention relates to a power
`control device for one or more beams of lights in which the
`said beams are incident on respective groups of pixels of a
`10 two-dimensional SLM, and holograms are applied to the
`respective group so that the emergent beams have power
`reduced by comparison to the respective incident beams.
`The invention further relates to an optical routing module
`having at least one input and at least two outputs and
`15 operable to select between the outputs, the module compris(cid:173)
`ing a two dimensional SLM having an array of pixels, with
`circuitry constructed and arranged to display holograms on
`the pixels to route beams of different frequency to respective
`outputs.
`According to a later aspect of the invention there is
`provided an optoelectronic device comprising an integrated
`multiple phase spatial light modulator (SLM) having a
`plurality of pixels, wherein each pixel can phase modulate
`light by a phase shift having an upper and a lower limit, and
`25 wherein each pixel has an input and is responsive to a value
`at said input to provide a phase modulation determined by
`said value, and a controller for the SLM, wherein the
`controller has a control input receiving data indicative of a
`desired phase modulation characteristic across an array