Photonic crystal waveguide arrays

A waveguide device for optical signals in an optical communication system relies upon photonic bandgap material to guide the optical signal via a plurality of pathways. A two-dimensional photonic bandgap material is formed in a planar slab of dielectric medium by a two-dimensional lattice in which discontinuities in the lattice region define waveguides. A waveguide array formed in the device allows multiple Mach-Zehnder interferometer devices to be constructed including small radius turns without significant losses. An optical signal equalizer formed of such a device relies upon the transfer function defined by a multiple arm Mach-Zehnder having arms of different length. By applying controlled modulation to propagation through the arms, an adaptive equalizer is formed. The adaptive equalizer has application in correcting gain tilt in line amplifiers of optical communications systems. The method of applying equalization may include measurement of the spectrum of the output optical signal and providing feedback to control the transfer function applied by the adaptive equalizer.

PHOTONIC CRYSTAL WAVEGUIDE ARRAYS
 This invention relates to devices and systems incorporating waveguide
 arrays, methods of use of such devices and systems, and methods of
 manufacture thereof, and in particular but not exclusively to such devices
 for use in the equalization of optical signals in an optical
 communications system.
 BACKGROUND OF THE INVENTION
 Planar structures comprising waveguide arrays such as multiple arm Mach
 Zehnder interferometers and simple two-arm interferometers have been used
 in various devices for processing optical signals. Waveguide arrays in
 such devices may for example be constructed using silica on silicon
 technology in which waveguides are defined by doped silica regions which
 have a higher refractive index than a surrounding cladding layer of
 germanium doped silica.
 Arrays with large numbers of waveguides of different length formed using
 such structures have been proposed to provide filtering of optical
 signals, as for example disclosed by Dragone, IEEE Photon Technology
 Letters, September 1991, pp 812-815.
 The inventor of the present invention has also disclosed in a co-pending
 application entitled "Optical Equalizer", the contents of which are
 incorporated herein by reference, an adaptive optical equalizer using an
 array of controlled waveguides to provide a transfer function which is
 synthesized by a Fourier analysis technique in order to achieve a desired
 characteristic of equalization between components in a wavelength division
 multiplexed optical signal.
 The feasibility of such a waveguide array device to achieve adaptive
 equalization presents difficulties of manufacture using silica on silicon
 technology since, for example, reliance upon total internal reflection as
 the method of guidance means that the bend radius of waveguides must be
 sufficiently large to avoid excessive losses whereas the overall size of
 the planar structure is limited. A further difficulty is that of providing
 adequate means of controlling transmission parameters within waveguides.
 It has been proposed to use heating strips to locally vary the temperature
 of the waveguide material, thereby providing controlled adjustment of
 optical path length. The response time of such temperature control may not
 be adequate in some applications. There remains a need to provide an
 improved waveguide structure in such devices to facilitate manufacture and
 improve control.
 A particular area of interest is the control of optical power levels in
 optical communications system to obtain optimum performance, the power
 level being required to be sufficient to establish a signal to noise ratio
 which will provide an acceptable bit error rate but without the power
 level exceeding a level at which limiting factors such as the onset of
 non-linear effects result in degradation of the signal. In wavelength
 division multiplexed transmission it is desirable to maintain each of the
 power levels of the individual wavelengths components at substantially the
 same level.
 The inventor has disclosed in U.S. Pat. No. 5,513,029 a method of
 monitoring component power levels in WDM transmission using orthogonal low
 frequency dither signals and controlling component signal power to
 maintain optical performance.
 It is also known from GB 2314714A that an imbalance of component signal
 powers in a WDM transmission is likely to occur at an optical amplifier
 stage, as used to boost signal power at stages in a long distance
 transmission utilising optical amplifiers such as erbium doped fibre
 amplifiers. Such amplifiers have a non-uniform gain characteristic as a
 function of wavelength which is variable in dependence upon the amplifier
 gain, this change in gain characteristic consequent on change of gain
 being commonly referred to as dynamic gain tilt.
 There is therefore a need to provide optical filtering which is adaptive
 and which can be used in conjunction with optical amplifiers in order to
 maintain a preferred spectral profile of an optical signal.
 It is known from J. D. Joannopoulous et al, Photonic Crystals: Moulding the
 Flow of Light, published 1995 by Princeton University Press, in Chapter 5,
 to provide two-dimensional photonic crystals within a planar slab of
 dielectric medium which exhibits a photonic band gap whereby the medium is
 non-transmissive to an optical signal within a defined frequency range for
 directions of propagation co-planar with the slab of dielectric medium.
 The slab is sandwiched between parallel slabs of material having lower
 refractive index to contain the optical signal by internal reflection. The
 photonic crystal is formed by providing in the planar slab of dielectric
 medium a lattice formed by lattice sites at which the dielectric
 properties of the medium are varied relative to the bulk properties of the
 dielectric medium such that a latticed region is formed which is
 essentially opaque to the optical signal and a waveguide region may then
 be formed by discontinuities in the periodic lattice, for example by
 omitting a contiguous set of lattice sites. The optical signal is
 therefore constrained to propagate through the waveguide region.
 An advantage of such structures is that the waveguides may have a very
 small turn radius of the order of several wavelengths of the optical
 signal which compares favourably with a typical turn radius of the order
 of several centimeters which would be required for traditional
 core-cladding waveguides describes above which rely upon total internal
 reflection.
 U.S. Pat. No. 5,651,818, Milstein et al, discusses in the introduction
 thereby a number of available techniques of manufacturing photonic band
 gap materials.
 U.S. Pat. No. 5,784,400, Joannopoulous et al, proposes to utilise
 two-dimensional photonic band gap materials in an optical device in the
 form of a resonant cavity.
 It is known from U.S. Pat. No. 5,389,943, Brommer et al, to utilise the
 frequency selective transmission properties of such two-dimensional
 photonic band gap materials in a filter in which transmitted light is
 modified in frequency response by the optical transmission characteristics
 of the bulk properties of the material. Further disclosed is the active
 control of material forming the lattice sites, such as by the application
 of an external field, in order to modify the refractive index of material
 at the sites and thereby actively control the transmissive properties of
 the filter.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide an improved device
 having an array of waveguides for an optical signal.
 It is a further object of the present invention to provide an improved
 method of manufacture of such devices for providing multiple waveguide
 structures.
 It is a further object of the present invention to provide an improved
 optical equalization device for use in optical communications systems.
 According to the present invention there is disclosed a waveguide device
 for an optical signal comprising;
 a slab of dielectric medium;
 a latticed region formed in the dielectric medium by a periodic array of
 lattice sites comprising localised structures having dielectric properties
 which are different from those of the surrounding medium, the periodic
 array being dimensioned such that a photonic bandgap exists in the
 latticed region inhibiting propagation of the optical signal therethrough;
 and
 a waveguide region formed in the dielectric medium by discontinuities in
 the periodic array of lattice sites allowing propagation of the signal
 therethrough;
 wherein the waveguide region defines an input region for the input of the
 optical signal, an output region for the output of the optical signal and
 a plurality of optical pathways for conducting respective components of
 the optical signal between the input and output regions.
 According to a further aspect of the present invention there is disclosed a
 method of forming a waveguide device for an optical signal comprising;
 forming a slab of dielectric medium;
 forming a latticed region in the dielectric medium by a periodic array of
 lattice sites comprising localised structures having dielectric properties
 which are different from those of the surrounding medium, the periodic
 array being dimensioned such that a photonic bandgap exists in the
 latticed region inhibiting propagation of the optical signal therethrough;
 and
 forming a waveguide region in the dielectric medium by discontinuities in
 the periodic array of lattice sites allowing propagation of the signal
 therethrough;
 wherein the waveguide region defines an input region for the input of the
 optical signal, an output region for the output of the optical signal and
 a plurality of optical pathways for conducting respective components of
 the optical signal between the input and output regions.
 According to a further aspect of the present invention there is disclosed a
 method of applying equalization to an optical signal comprising the steps
 of;
 inputting the optical signal to an input region of a waveguide device;
 conducting components of the optical signal to an output region of the
 waveguide device via a plurality of respective optical pathways defined by
 the waveguide device; and
 outputting the optical signal from the output region;
 wherein the step of conducting the components of the optical signal
 comprises inhibiting propagation of the optical signal through a latticed
 region formed by a periodic array of lattice sites defining a photonic
 bandgap and propagating the components of the optical signal via a
 waveguide region formed by discontinuities in the periodic array of
 lattice sites.
 According to a further aspect of the present invention there is disclosed a
 method of dividing an optical signal into a plurality of components for
 conduction via respective waveguides comprising the steps of:
 inputting the optical signal into a waveguide region defined by
 discontinuities in a latticed region in which propagation is inhibited by
 a photonic band gap;
 directing the optical signal into grazing incidence with a linearly
 extending surface defined by a transition between the latticed region and
 the waveguide region so that surface wave propagation of the optical
 signal occurs in proximity with the linearly extending surface;
 deflecting portions of the surface wave by scattering from the linearly
 extending surface; and
 conducting the deflected portions into said waveguides.

DETAILED DESCRIPTION
 In FIG. 1, a photonic crystal 1 is formed in a slab 2 of dielectric
 material in which a lattice region 3 is defined by a regular array in two
 dimensions of lattice sites 4. The lattice sites 4 are arranged in a
 square lattice configuration in which each lattice site 4 comprises a
 cylindrical column of a second dielectric material having a lower
 refractive index than that of the bulk properties of the slab 2.
 As shown in FIG. 2, the slab 2 is sandwiched between a substrate 5 and a
 cover layer 6 and an optical signal 7 is launched into the slab 2 in a
 direction which is parallel to the planar extent of the slab.
 FIG. 3 illustrates the formation of the lattice sites 4 as regularly spaced
 columns 8 of dielectric material. In the lattice region 3, the array of
 lattice sites 4 is dimensioned such that a photonic band gap exists for
 propagation of the optical signal in directions parallel to the planar
 extent of the slab 2. As seen in FIG. 1, rows and columns of the lattice
 sites define axes which are parallel to the plane of the slab 2 and a
 waveguide region shown as waveguides 11, 12 and 13 in FIG. 1 is formed by
 an area of the slab 2 in which there are no lattice sites. Transmission of
 the optical signal is thereby facilitated within the waveguide region and
 penetrates only into the latticed region to a minimal extent as evanescent
 mode propagation which decays exponentially, permitted propagation modes
 being confined to the waveguide region.
 The optical signal is confined in directions normal to the plane of the
 slab 2 by guidance in the form of internal reflection at surfaces 77 and
 78 shown in FIG. 2 between the slab 2, the substrate 5 and the cover layer
 6. The waveguides may thereby be regarded as channels through which the
 optical signal is confined by the presence of the surrounding band gap
 material.
 FIG. 1 illustrates a small rectangular portion 9 of the slab 2 shown in
 FIG. 4 and which defines waveguides forming a simple two-arm Mach Zehnder
 filter. As illustrated in FIG. 4 in which, for convenience, waveguides are
 represented by single lines, the portion 9 of FIG. 1 corresponds to a Y
 junction 10 at which a first waveguide 11 branches into second and third
 waveguides 12 and 13. The waveguides 12 and 13 are brought into
 sufficiently close proximity to provide coupling at a 3 db coupler 26 and
 thereafter diverge. The waveguides 12 and 13 define optical path lengths
 of L and L+dL respectively between the junction 10 and the coupling 26.
 An input optical fibre 14 is connected to the slab 2 to launch the optical
 signal 7 into an input region 15 defined by the first waveguide 11. Output
 optical fibres 16 and 17 are connected to the slab 2 to receive components
 of the optical signal from output regions 18 and 19 of the slab defined by
 the second and third waveguides respectively.
 The difference in path length dL between the first and second waveguides 12
 and 13 results in there being a modification to the frequency
 characteristics in an output optical signal 20 as illustrated in FIG. 5.
 The depth of modulation of signal power as a function of frequency is
 dependent upon the relative powers input into waveguides 12 and 13 from
 the Y junction 10. The location of the frequency peaks f.sub.x, f.sub.y,
 f.sub.z relative to the frequency scale, i.e. the "phase" in frequency
 space of the modulation, is dependent upon the value of the difference in
 path length dL, as is the separation between peaks in the envelope of the
 spectrum.
 The filter provided by the Mach-Zehnder interferometer of FIG. 4 has a
 fixed frequency characteristic. The filter may be made adaptive by
 modulating transmission in the waveguides of the interferometer as
 illustrated in the example of FIG. 6.
 FIG. 6 illustrates a Mach-Zehnder interferometer similar to that of FIG. 4
 and will be described using corresponding reference numerals for
 corresponding elements where appropriate. A first modulator 21 is
 externally controllable to vary the ratio of the input optical signal
 diverted into each one of the second and third waveguides 12 and 13. FIG.
 7 illustrates an example of the manner in which the first modulator 21 may
 be implemented to vary the proportion of the optical signal diverted into
 arms of the Y junction 10. The third waveguide 13 is modified by the
 presence of controllable lattice sites 22 located in the waveguide region
 in areas 38 and 39 and comprising cylindrical columns formed of a ferrite
 material to which a variably controllable external field is applied. The
 locations of the controllable lattice sites conform to the column and row
 positions of the surrounding lattice region and in effect form an
 extension of the lattice. The presence of the controllable lattice sites
 22 can therefore in effect be turned on or off in variable number to
 thereby variably control the effective apertures of the second and third
 waveguides 12 and 13. This is represented in FIG. 7 by showing only those
 controlled lattice sites 22 which are turned "on" and which in this
 example are located only in the third waveguide 13. The amount of optical
 signal coupled into the third waveguide is thereby controllable by setting
 the number of sites which are turned on, the remainder of the optical
 signal being diverted into the second waveguide 12.
 Control of the controllable lattice sites 22 is effected such as to vary
 the dielectric constant of the material forming the cylindrical columns 8.
 This variation may be continuous or discrete, i.e. variable between one of
 a number of stepped levels.
 In FIG. 6, a second modulator 23 is provided for varying the optical path
 length dL and is shown in greater detail in FIG. 8. A dielectric region 24
 within the waveguide region defining the third waveguide 13 is subject to
 an externally applied control to vary the dielectric properties of the
 region. The externally applied control may be one of a number of available
 options including the application of local heating, the injection of
 electrical current into the semi-conductor material forming the
 dielectric, or other suitable optically, electromagnetically or
 electro-mechanically induced effects. The surrounding lattice region is
 substantially unaffected by this control and continues to serve as a means
 of confining the optical signal 7 within the waveguide so as to pass
 through the controlled dielectric region 24.
 FIG. 9 illustrates in greater detail the construction of the 3dB coupler
 26. In FIG. 9, the second and third waveguides 12 and 13 are represented
 in each case by pairs of parallel lines which denote the boundary between
 the lattice region 3 and the waveguide regions defining the waveguides and
 which are free of lattice sites. On each side of the waveguides 12 and 13,
 the extent of penetration of evanescent wave components of the optical
 signal in the lattice region 3 are represented as boundaries 40. The
 evanescent wave components decay exponentially as a function of
 penetration distance into the lattice region 3 and the indication of
 boundaries 40 should therefore be regarded only as schematic.
 In FIG. 9, the boundaries 40 serve to indicate a merging of the region of
 lattice region 3 within which the evanescent field components of the
 respective waveguides 12 and 13 penetrate so that a coupling region 41
 within the lattice region 3 is defined and serves as a mechanism for phase
 dependent transfer of energy between modes propagated through the
 respective waveguides.
 A third modulator 25 in FIG. 6 is used to control the recombining and
 splitting characteristics of the coupler 26. The third modulator 25 as
 shown in greater detail in FIG. 10 comprises a portion 27 of the lattice
 extending into the waveguide region and comprised of controllable lattice
 sites 28 of the type referred to above with reference to FIG. 7.
 By control of the lattice sites 28, the amount of optical signal output via
 the output optical fibre 17 is variably controllable. Light which cannot
 be transmitted because of the presence of the controlled lattice sites 28
 is redirected to emerge from the output optical fibre 16.
 The frequency characteristics of the filter constituted by the Mach-Zehnder
 interferometer of FIG. 6 may thereby be variably controlled using any one
 of, or a combination of, the first, second and third modulators 21, 23 and
 25.
 A more complex equalizer may be constructed from multiple waveguides as
 shown for example in FIG. 11 in which waveguides are, for simplicity,
 represented by single lines, such as waveguides 30 and 31 of substantially
 equal length which extend between a first star coupler 32 and second star
 coupler 33. The equalizer of FIG. 11 provides a multiple arm Mach-Zehnder
 interferometer for frequency response synthesis in which the additive
 effect of transmission through the arms of the waveguide array is
 represented in the block representation of FIG. 12. The equalizer transfer
 function T is synthesized by the effect of a multiple arm Mach-Zehnder
 interferometer to provide the transfer function shown in FIG. 13. As
 represented in FIG. 12, a series of delays L, L+dL, L+2dL, L+KdL are
 provided by delay waveguides 53 to 55 and the contribution made by each
 delay waveguide to the output combined by a combiner 50 is adaptively
 controlled by means of a set of modulators 56 where each of one the delays
 53 to 55 is provided with a respective modulator. Each modulator 56 is
 capable of independently setting an amplitude and phase modulation to the
 component transmitted through the corresponding delay 53 to 55, the values
 of the amplitude and delay being characterized by complex coupling
 coefficients C.sub.r, r=0 to K where there are K+1 waveguides.
 The term "phase modulation" here implies a variation in optical path length
 resulting in a corresponding phase variation at the point of combination
 in combiner 50.
 The result of the summation of the outputs from the modulated delay
 waveguides is illustrated schematically in FIG. 13 and has a form which is
 related to the values of C.sub.r by a discrete Fourier transform. The
 values of C.sub.r for a required transfer function may be calculated and,
 in the case of a wavelength division multiplexed optical signal, it is
 preferable to define the desired transfer function in terms of
 coefficients T.sub.n for N frequencies corresponding in both number and
 frequency value to the N wavelength channels of the input optical signal
 to be processed.
 The arrangement of FIG. 12 may be implemented using a photonic crystal
 provided that the modulators 56 are capable of providing the required
 amplitude and phase variation. The example of FIG. 11 however makes use of
 a simpler form of modulation for which each one of the waveguides of FIG.
 13 is replaced by a respective pair of waveguides in the array of FIG. 11.
 For example, in FIG. 11 the pair of waveguides 30 and 31 have respective
 modulators 37 and 36, each of which has the form of modulator described
 above with reference to FIG. 8 in which a controlled dielectric region 24
 has variable dielectric properties controlled thermally or otherwise. This
 form of control provides adjustment to the optical path length of each of
 the waveguides in the array of FIG. 12, thereby effectively controlling
 the phase of each component. For each pair of waveguides such as 30 and
 31, the phase values implemented in the modulators 36 and 37 are selected
 such that the combined output of these two waveguides has the required
 amplitude and phase corresponding to the respective one of the complex
 coupling coefficients C.sub.r.
 Alternatively, each one of the waveguides represented in FIG. 12 may be
 implemented as a respective group of 3 waveguides of equal lengths in the
 equalizer of FIG. 11, each of the group of waveguides having respective
 phase control modulators.
 FIG. 14 illustrates the manner in which the equalizer of FIG. 11 may be
 used to provide equalization for a line amplifier in an optical
 communications system, as for example where it is necessary to compensate
 for the effects of gain tilt. The line amplifier consists of first and
 second fibre amplifiers 64 and 65, each being formed by erbium doped
 optical fibre amplifiers, the first fibre amplifier receiving the input
 optical signal 66 in the form of a wavelength division multiplexed signal
 having N channels separated by 100 GHz. Typically N=32. The optical signal
 output from the first fibre amplifier 64 is input to the adaptive
 equalizer 67, the output of which is input to the second fibre amplifier
 65 and amplified before onward transmission in the optical system.
 An optical tap 68 provides an optical sample of the optical signal input to
 the equalizer 67 which is detected and measured by a first detector 69 to
 provide measurements in the form of input channel levels V.sub.n where n=0
 to 31. Similarly, a second optical tap 70 provides an optical sample of
 the optical signal output from the equalizer 67 which is detected and
 measured by a second detector 71 producing measured output channel levels
 X.sub.n.
 The equalizer 67 is adaptive in the sense of being operable to
 independently control transmission amplitudes of each of the wavelength
 division multiplexed channels, as represented by transfer function T
 having complex coefficients T.sub.n relating the amplitude and phase of
 each component output to its respective input.
 These coefficients T.sub.n are target values representative of E field
 values arrived at by calculation and applied by calculating corresponding
 settings S.sub.m of variable components of the equalizer 67. An equalizer
 controller 72 controls the value of the equalizer settings S.sub.m applied
 to the variable components of the equalizer 67. The number of settings
 S.sub.m may be typically greater than the number N of channels and will
 depend upon the manner in which the equalizer is implemented. In the
 example of FIG. 11, each S.sub.m is the phase control of one of the
 waveguides and therefore the number of S.sub.m to be calculated is equal
 to 2.times.N.
 The required values of the equalizer settings S.sub.m are determined by
 calculator 73 to enable a user to input via an input device 76 a
 referenced spectral characteristic W.sub.n which serves as a target value
 to which output channel levels X.sub.n are driven to correspond under
 ideal operating conditions.
 The timing of operation of the first and second detectors 69 and 71 and of
 the equalizer controller 72 is determined by timing controller 75 which
 periodically outputs control signals to the first and second detectors to
 determine the sampling times at which V.sub.n and X.sub.n are calculated
 and correspondingly controls the timing at which the settings S.sub.m of
 the equalizer 67 are updated.
 The arrangement of FIG. 14 allows the output to be monitored and its
 frequency characteristics measured and compared with target settings. This
 feedback allows a desired frequency characteristic to be achieved
 iteratively over a number of applications of the values of S.sub.m.
 FIG. 15 illustrates an alternative device for providing a number of
 distinct optical pathways for conducting components of the optical signal
 between an input region 87 and output region 88 where the number of
 pathways is greater than two. A latticed region 80 of photonic band gap
 material is formed with a waveguide region 81 which defines a cavity 82,
 the term cavity being used here to indicate an enclosed region of the
 dielectric material of the slab 2 which is free of lattice sites and
 therefore transmissive to the optical signal. A boundary 83 defined by the
 transition between latticed region 80 and waveguide region 81 is shaped to
 provide first and second sidewalls 84 and 85, each having a stepped
 profile when viewed orthogonally to the plane of the slab 2. The stepped
 sidewalls 84 and 85 each include a series of concave reflectors 86,
 defined by portions of the boundary 83, which are oriented to reflect
 components of the optical signal. The input region 87 of the waveguide
 region 81 is coupled to an optical fibre 89 via which the optical signal
 is input and a further optical fibre 90 is coupled to the output region 88
 of the waveguide region.
 The optical signal on entering the input region 87 is spread out in fan
 formation to illuminate the reflectors 86 of the first sidewall 84 and
 individual components such as 91 and 92 illustrated in FIG. 15 of the
 optical signal are separately reflected by respective reflectors 86 and
 directed onto corresponding reflectors of the second sidewall 85.
 Reflection of the components from the sidewall 85 converges the optical
 signal to be collected and output via the output region 88 and optical
 fibre 90.
 Each of the reflectors 86 is concave to direct and concentrate the
 respective component onto the corresponding reflector of the second
 sidewall 85 and achieve convergence at the output region 88.
 The device of FIG. 15 thereby provides for the input signal to be divided
 into a number of distinct components 91, 92 which traverse different
 optical pathlengths before being recombined in the output region 88. The
 respective path lengths remain constant and the effects of interference in
 the output region produce a predetermined transfer function, or in other
 words a set profile of equalization applied to the frequency components of
 the optical signal.
 FIG. 16 shows an alternative device which is equivalent to the device of
 FIG. 15 but with the addition of an array 93 of modulators 94 located
 within the cavity 82 such that each modulating element 94 lies in the path
 of a respective component 91 of the optical signal. Controlled actuation
 of each of the modulator elements 94 in the array 93 provides means for
 modifying the transfer function of the device, for example by controlling
 the amplitude of each of the components. Alternatively, the modulator
 elements 94 may introduce controlled amounts of change in optical
 pathlength, thereby effectively changing the relative phase of the
 components before recombination, or alternatively both amplitude and phase
 may be variably controlled by appropriate modulator elements.
 The modulator elements of FIG. 16 may be controlled in a similar manner to
 the method of control described above with reference to FIGS. 13 and 14 in
 order to achieve adaptive equalization.
 The modulator elements 94 may conveniently be formed by providing
 controlled lattice sites of the type referred to above with reference to
 FIG. 7 as a means of providing amplitude modulation. Phase modulation may
 be provided by the external application of thermal or electromagnetic
 fields to the bulk material forming the slab 2 in the locality of the
 modulator element 94.
 FIG. 17 illustrates an alternative device for achieving adaptive
 equalization of an optical signal and comprises a latticed region 100
 formed by a regular array of lattice sites providing a photonic bandgap at
 optical frequencies including the frequency range of the optical signal.
 In FIG. 17, waveguides are represented by single lines which are bold to
 distinguish waveguides from other boundary features. A waveguide region
 formed by the omission of latticed sites is provided with an input region
 101, an output region 102 and a plurality of channels defining waveguides
 103 for conducting respective components of the optical signal by
 respectively different optical path lengths extending between the input
 and output regions.
 The input region 101 communicates with a first coupling region 104 defined
 by a fan shaped portion of waveguide region which is free of lattice
 sites, the first coupling region having a side wall 105 extending linearly
 and oriented relative to the input region such that the input optical
 signal is incident upon the sidewall at a shallow angle of grazing
 incidence. A Bloch surface wave thereafter propagates along the sidewall
 105 in the manner described for example by Joannopoulous, "Photonic
 Crystals Moulding the Flow of Light", Princeton University, 1995, at page
 73 to 77. Such surface modes in the case of a perfectly formed sidewall
 105 of regular lattice sites will guide the optical signal close to the
 sidewall to enter a waveguide 106 located at the end of the sidewall.
 An array of controlled lattice sites 107 is distributed along the sidewall
 105 and includes lattice sites which are selectively activated to perturb
 the surface wave and cause deflection of components of the optical signal
 away from the sidewall into the first coupling region 104 so as to enter
 selectively ones of the waveguides 103. In this manner, the proportion of
 the optical signal diverted into each of the waveguides 103 is controlled.
 Similarly, the outputs of the waveguides 103 communicate with a second
 coupling region 108 in which the waveguide 106 enters relative to a second
 sidewall 109 at a shallow angle of grazing incidence, thereby confining
 the component conducted by waveguide 103 to a surface wave communicating
 with the output region 102.
 A further array of controlled lattice sites 110 is provided along the
 boundary between latticed region 100 and waveguide region at the sidewall
 109 to selectively divert a proportion of the optical signal component
 received from the waveguide 106. The outputs of the remaining waveguides
 103 enter the second coupling region 108 and converge on the output region
 102 so as to be collected and emerge via an output optical fibre 111.
 Equalization may thereby be adaptively applied to an optical signal
 entering the device via input optical fibre 99 and exiting via output
 optical fibre ill, the control mechanism for determining the control
 parameters applied to the arrays 107 and 110 of controlled latticed sites
 corresponding generally to the arrangement described above with reference
 to FIGS. 13 and 14.
 FIG. 18 shows schematically the propagation of a surface wave 112 parallel
 to the boundary between lattice region 100 and the coupling region 108. A
 surface layer 113 of controlled lattice sites 114 allows selected sites to
 be varied in dielectric properties so as to create discontinuities in the
 surface layer, thereby achieving selectively a controlled amount of
 scattering of the optical signals in directions indicated by arrows 115.
 By appropriate control, the amount of light scattered into each of the
 waveguides 103 may thereby be variably controlled.
 In the above described embodiments, the band gap is formed by a two
 dimensional array of lattice sites in which the array is of square
 configuration. Other forms of array may be utilised such as a triangular
 array or honeycomb shaped array. Alternatively, the array may be three
 dimensional.
 In the above described embodiments, multiple waveguides have been disclosed
 in arrays which have discrete non-overlapping paths. More complex devices
 are envisaged in which overlapping meshed paths are utilised in order to
 achieve more complex relationships between division and recombination of
 components of the optical signal. The path lengths described in the above
 embodiments include linearly increasing length differentials, i.e.
 multiples of dL. More generally, embodiments are envisaged in which
 non-linear progressions of path lengths are utilised, such as chirped
 systems where the differential lengths follow a squared, cubic or
 exponential progression. Gapped or staggered sequences of increasing path
 length may also be utilised.
 In the above described embodiments, the input region and output region are
 physically separated. It is however envisaged that devices and methods in
 accordance with the present invention may utilise input and output of
 optical signals from the same region, as in the case of a Michelson
 interferometer configuration, and in such instances it is intended that
 the terms input region and output region may simultaneously apply to the
 same region.
 It is also noted that in many embodiments of the present invention the path
 taken by optical signals may be reversed, or simultaneously propagated in
 opposite directions, so that the terms input and output are
 interchangeable in this context.