Abstract:
An electro-optic arrayed grating comprises an array of waveguides which provide a plurality of optical paths. The array includes a plurality of electro-optic elements disposed along the optical paths. The electro-optic elements control the optical path lengths of the optical paths to multiplex or demultiplex an optical signal.

Description:
PRIORITY APPLICATION  
       [0001]    This application claims priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/289,207, filed May 7, 2001 and entitled “Electro-Optic Grating”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to optical network technology and, in particular, to arrayed waveguide gratings utilizing electro-optic material.  
           [0004]    2. Description of the Related Art  
           [0005]    Demands for transmitting signals optically is growing at a rapid pace. Optically transmitted signals are typically in digital format, and may be carried through some form of a waveguide such as an optical fiber.  
           [0006]    The optical waveguide&#39;s capacity to carry data can be increased by coupling a plurality of optical signals into the waveguide. This process is known in the art as multiplexing (MUX). A multiplexed signal can be separated into its constituent signals in a reverse process known as demultiplexing (DEMUX). The process of multiplexing and demultiplexing is typically performed in a system known in the art as dense wavelength division multiplexing (DWDM).  
           [0007]    Typically, each of the plurality of optical signals that are multiplexed have a different wavelength relative to the remaining plurality of signals. Multiplexing and demultiplexing rely on a physical principle known as the superposition principle, which essentially states that waves can be combined and separated without loss of information. In the DWDM system, multiplexing is achieved by combining signals of different wavelengths into a single optical fiber. The multiplexed signal is then typically carried over a long distance to a receiving end, where demultiplexing is performed.  
           [0008]    Demultiplexing devices in use today commonly rely on an arrayed waveguide grating (AWG) to demultiplex the optical signal. The AWG comprises a first coupler that optically couples the multiplexed signal to an array of waveguides such that each waveguide receives a fraction of the multiplexed signal. As the multiplexed signal travels through each of the waveguides, the fractions travel over path lengths which are different for each waveguide, and therefore the phases of the fractions are different when each fraction arrives at the input of a second coupler. The fractions received by the second coupler are combined inside the second coupler, and interfere constructively and destructively in the same manner as that which occurs in conventional gratings. The effect of such interference is to separate the wavelengths contained in such portions for output on separate outputs of the second coupler.  
           [0009]    In order to achieve optimum efficiency of the AWG, the length and position of each waveguide in the array of waveguides must be established with accuracy during fabrication. Furthermore, once fabricated, the optical path length geometry of the array of waveguides is fixed and cannot be altered. Variations in dimension, geometry and refractive index of waveguide material of the AWG, whether from manufacturing tolerances or environmental conditions, may adversely affect the performance of the AWG.  
           [0010]    Therefore, there is a need for a method of controlling the transmission of light in the array of waveguides so as to allow a user to tune or otherwise alter the propagation characteristics of the AWG.  
         SUMMARY OF THE INVENTION  
         [0011]    The aforementioned needs are satisfied by an electro-optic arrayed grating. According to one aspect of the invention, the arrayed grating comprises a first coupler, a second coupler, and an array which provides a plurality of optical paths between the first and the second couplers. The array comprises a plurality of electro-optic elements along the optical paths. The array controls the optical path lengths of the optical paths to permit multiplexing or demultiplexing of an optical signal.  
           [0012]    In another aspect of the invention, the electro-optic element comprises an electro-optic material interposed between a pair of electrodes. Preferably, the electro-optic material is a polycrystalline lanthanum-modified lead titanate zirconate (PLZT), and the electrode is an indium tin oxide layer. At least one of the optical paths extends through the electrodes such that an electric field between the electrodes is generally parallel to one of the optical paths.  
           [0013]    In still another aspect of the invention, each of the electro-optic elements comprises an electro-optic material interposed between a first and a second set of electrodes that are generally symmetric with respect to each other such that an electric field between the first and second sets of electrodes is generally parallel to one of the optical paths.  
           [0014]    In yet another aspect of the invention, each of the electro-optic elements comprises an electro-optic material interposed between a first and a second set of electrodes that are generally symmetric with respect to each other such that an electric field between the first and second sets of electrodes is generally perpendicular to one of the optical paths.  
           [0015]    In one aspect of the invention, the array comprises a plurality of waveguides that provide the plurality of optical paths. Each of the plurality of optical paths includes one or more electro-optic elements in line with the waveguide. The electro-optic element provides a variable or a fixed phase delay of the optical signal. In one implementation, the electro-optic element comprising an electro-optic material without electrodes provides the fixed phase delay.  
           [0016]    Another aspect of the invention comprises a method of demultiplexing an optical signal comprised of a plurality of wavelengths using an electro-optic arrayed grating. The method comprises distributing the optical signal into a plurality of optical signals, each of which includes the plurality of wavelengths. The method further comprises delaying the plurality of optical signals by propagating the plurality of signals along respective optical paths, wherein at least some of the paths have an optical path length different than other of the paths. The electro-optic material causes each of plurality of signals to be delayed. The method further comprises combining the plurality of delayed signals, such that the delayed signals spatially separate plurality of wavelengths.  
           [0017]    In still another aspect of the invention, the physical lengths of the optical paths are substantially equal. Preferably, the signals are delayed by exposing the electro-optic material to an electric field in a direction that is substantially parallel to the direction of propagation of the optical signal through the electro-optic material. Such orientation of the direction of propagation relative to the electric field may be achieved by passing the optical signals through respective electrodes disposed on opposite sides of the electro-optic material. Each of the delays of the delayed signals is adjustable by adjusting a voltage applied between the electrodes. Delaying of the signals includes altering the spatial separation of wavelengths by altering the voltages applied to the electrodes.  
           [0018]    In another aspect of the invention, delaying comprises providing an electric field in the electro-optic material, where the electric field is oriented in a direction that is substantially perpendicular to the direction of propagation of the optical signal. Such implementation may be achieved by applying a voltage to electrodes disposed on opposite lateral sides of the electro-optic material.  
           [0019]    Another aspect of the invention comprises a phase delay device for introducing phase delay into an optical signal. This device comprises a plurality of electro-optic elements along a path and one or more optical waveguides optically connecting the electro-optic elements together. The electro-optic elements comprise electro-optic material interposed between a pair of electrodes. The electro-optic elements have dimensions such that the optical signal propagating therethrough is unguided within the electro-optic element. The optical waveguide(s) and electro-optical elements together form an optical path for the optical signal. The electro-optic elements control the optical path lengths of the optical paths and the waveguide(s) limit divergence of the optical signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic illustration of a dense wavelength division multiplexing (DWDM) system;  
         [0021]    [0021]FIG. 2 is a schematic illustration of a demultiplexer;  
         [0022]    [0022]FIG. 3 is a schematic illustration of an arrayed waveguide grating (AWG) multiplexer, specifically illustrating the various lengths of the waveguides;  
         [0023]    [0023]FIG. 4A is a schematic illustration of the AWG employing a plurality of electro-optic elements to alter optical path lengths of signals;  
         [0024]    [0024]FIG. 4B illustrates an embodiment of a delay module comprising two or more electro-optic elements;  
         [0025]    [0025]FIG. 4C illustrates an alternate embodiment of the AWG employing a combination of fixed and variable delays to alter optical path lengths of signals;  
         [0026]    [0026]FIG. 5A is a schematic illustration of a single electro-optic element showing a path of the signal and an electric field in the electro-optic element that alters refractive index;  
         [0027]    [0027]FIG. 5B illustrates another embodiment of the electro-optic element that uses symmetric electrodes disposed at corners of electro-optic material to generate an electric field generally parallel to the direction of propagation of the optical signal;  
         [0028]    [0028]FIG. 5C illustrates yet another embodiment of the electro-optic element that applies an electrical field generally perpendicular to the direction of propagation of the optical signal, thereby allowing different polarized optical signals to be affected differently; and  
         [0029]    [0029]FIG. 6 is a side view of the electro-optic element of FIG. 5A. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Reference will now be made to the drawings wherein like numerals refer to like parts throughout. FIG. 1 schematically illustrates a dense wavelength division multiplexing (DWDM) system  100  comprising an array  102  of inputs which receive N input signals  103 , a multiplexer  104  which outputs a single multiplexed signal  106 , a demultiplexer  110  which receives the multiplexed signal  106 , and an array  112  of outputs which output N output signals  113 . The N output signals  113  that comprise the array of output signals  112  have different wavelengths corresponding to those of the N input signals  103 , respectively. The multiplexed signal  106  comprises a combination of all of the N input signals  103 , and is carried through a single waveguide so as to effectively multiply the signal carrying capacity by a factor of N.  
         [0031]    The demultiplexer  110  demultiplexes or separates the multiplexed signal  106  into the output signals in a manner described below. It will be appreciated that the DWDM system  100  is symmetric such that the direction of the signals can be reversed. That is, the demultiplexer can be used as a multiplexer, and the multiplexer can be used as a demultiplexer.  
         [0032]    As shown in FIG. 2, the demultiplexer  110  receives the multiplexed signal  106  as an input, passes the signal through an arrayed waveguide grating (AWG)  120 , and outputs signals on the output signal array  112 . The AWG comprises a first coupler  122 , and a waveguide array  124  comprising M individual waveguides  126  of varying optical lengths, where M typically equals N. The first coupler  122  receives the input multiplexed signal  106  and splits its optical energy among the M waveguides  126  in the waveguide array  124  to provide Mmultiplexed signals.  
         [0033]    The AWG also comprises a second coupler  132  which receives light from the waveguide array  124 . Each of the individual waveguides  126  in the array  124  has a different optical length that provides a correspondingly different optical path. The optical path differences are selected to cause the wavelengths to spatially separate at the second coupler  132  due to interference effects. The second coupler  132  outputs the spatially separated wavelengths to their assigned output signal waveguides  113 .  
         [0034]    [0034]FIG. 3 illustrates, in particular, the variations in lengths of the waveguides  126  used in the AWG  120 . The M waveguides  126  shown are arranged in a coplanar manner such that the length of the first waveguide L 1  is larger than the length of the second waveguide L 2  by an amount ΔL. The L 2  is in turn longer than L 3  by ΔL, and so on. The multiplexed signal  106  is distributed in the first coupler  122  by a first coupler assembly  123  to the waveguide array  124 .  
         [0035]    The M multiplexed signals all share a common degree of incoherence due to the multiplexed signal  106  having gone through the same optical path up to that point. As the signals in the waveguides  126  arrive at the second coupler  132 , however, relative phases of each of the M multiplexed signals of each waveguide  126  will be different due to the differences in length of the waveguides  126 . The difference in phase between any two of the signals is expressed as:  
             δ   =       2      πΛ     λ             (   1   )                               
 
         [0036]    where λ is the wavelength of light in vacuum and Λ is an optical path difference. In general, an optical path is defined as nL, where n is the refractive index, and L is the physical path length. The difference in optical path length is:  
         Λ=Δ nL+nΔL   (2)  
         [0037]    In AWG  120  illustrated in FIG. 3, the waveguides of the array  124  are all fabricated from same dielectric material, such that n is constant. As such, Δn=0, and the optical path difference Λ is equal to nΔL.  
         [0038]    As an example, an AWG component manufactured by DERA Electronics of United Kingdom has  101  waveguides  126  packaged into a single unit, with ΔL of 10 micrometers (μm). At such ΔL, and for a dielectric medium with refractive index n=1.5, the difference in optical path length Λ is nΔL=1.5×10=15 μm between two adjacent waveguides  126 .  
         [0039]    The waveguide array  124  is typically fabricated by creating glass (SiO 2 ) waveguides  126  directly on a silicon substrate in a manner well known in the art. The difference in the optical path lengths of the waveguides  126  described above introduces change in relative phases such that when the signals enter the second coupler  132 , a second coupler assembly  133  allows constructive and destructive interference to occur between the fractions from the waveguide array  124 , such that each wavelength is separated from adjacent wavelengths and is placed at its assigned location such that the demultiplexed signal can be transmitted as the array of output signals  112 . The couplers  122 ,  132  act as splitters and combiners and may be symmetric, i.e., the same design can be used as a multiplexer and an demultiplexer, however, the light is propagated through the device in opposite directions to achieve the different functionalities.  
         [0040]    [0040]FIG. 4A schematically illustrates one preferred AWG  121  comprising a waveguide array  125  in which the physical lengths of the waveguides  126  of the AWG are all substantially equal to each other. The variations in optical path length through the waveguides  126 , A, are introduced by respective electro-optic delay modules  140 . This electro-optic delay module  140  may comprise a portion of electro-optic material surrounded on opposite sides by electrodes for inducing an electric field therein. Light propagating through this element  140  is delayed by varying the index of refraction of the electro-optic material with application voltage across the electrodes.  
         [0041]    [0041]FIG. 4B illustrates another embodiment of a delay module  210  that comprises a two or more electro-optic elements  212   a ,  212   b , etc. (two shown) serially interconnected by waveguides  213 . The electro-optic elements  212  of the delay module  210  may have substantially same delaying properties or each of the electro-optic elements  212  may have a different delaying property. While increased delaying of optical signals can be achieved by use of a larger dimensioned electro-optic element, divergence losses increase in such an electro-optic element in the case where the element is a free-space device. Preferably, the electro-optic element  212  has dimensions sufficiently large that the light propagating therethrough is unguided and propagates as if in free space. The boundaries of the device  212  do not limit the propagation of the light therein, which travels in a free space region and is not guided as if in a waveguide that confines the beam therein. Additional details regarding free space devices are included in copending U.S. Patent Application No. ______ (TOPTICS.004CP4) entitled “Optical Switching Network and Network Node and Method of Optical Switching”, filed Romanovsky on May 6, 2006, now U.S. Pat. No. ______, which is incorporated herein by reference in its entirety. Accordingly, within this free space optical device, the beam of light passing therethrough will diverge. To limit this divergence within the electro-optic materal, the electro-optic elements  212  are optically coupled together via waveguides which include reflecting boundaries for containing the light therein. Hence, use of waveguides to interconnect two or more electro-optics modules in series allow increased delaying of optical signals with lower divergence loss. This delay module  210  may be included one or more or each of the M waveguides  126  of the AWG described above in reference to FIG. 4A.  
         [0042]    [0042]FIG. 4C illustrates another embodiment of an AWG  160  that comprises a waveguide array  162  interconnecting the first and second couplers  122 ,  132 . The waveguide array  162  comprises a plurality of waveguides  126 , wherein optical path length through each of the waveguides  126  is determined in part by a fixed delay module  166  in series with a variable delay module  164 . The variable delay module  164  may be the single-element delay module  140  of FIG. 4A, or the multi-element delay module  210  of FIG. 4B. The fixed delay module  166  may be formed from, by way of example, a PLZT element without electrodes.  
         [0043]    In one embodiment, the fixed delay modules  166  may be substantially the same for all of the waveguides  126 . Alternatively, as shown in FIG. 4C, the fixed delay module  166  may differ for each waveguide. The delaying property of a given fixed delay  166  module is determined in part by the material, i.e., its index of refraction, and the length of the optical element, which together determine the optical path length of the device  166 . Thus, the different fixed delay modules  166  depicted in FIG. 4C may be formed using different materials and/or different lengths. In one preferred embodiment, the fixed delay element  166  has a length that is different for adjacent waveguides  126 . The amount of delay may, for example, be increased sequentially for a sequence of spatially separated outputs. FIG. 4C depicts the fixed delay increasing sequentially for a sequence of optical waveguides having outputs that feed into the second coupler  132 . This sequentially increasing fixed delay may be provided by fixed delay elements with increasing length.  
         [0044]    In one preferred embodiment of the invention, as illustrated in FIG. 5A, the electro-optic delay module  140  is interposed between first and second waveguide segments  127   a ,  127   b , respectively, such that light propagating in one of the segments passes through the module  140  and into the other segment. The modules  140  comprise an electro-optic material  144  interposed between a first transparent electrode  142   a  and a second transparent electrode  142   b . The first transparent electrode  142   a  is between the electro-optic material  144  and an end of the first waveguide segment  127   a . The second transparent electrode  142   b  is between the electro-optic material  144  and an end of the second waveguide segment  127   b . A light beam  150  propagating through the first waveguide  127   a  passes through the first transparent electrode  142   a , then the electro-optic material  144 , and then the second transparent electrode  142   b  before entering the second waveguide  127   b  as a phase-delayed light beam  152 . The phase delay introduced by the electro-optic material  144  may be selected to allow demultiplexing of the multiplexed signal in a manner substantially similar to that described above using the conventional AWG device.  
         [0045]    As illustrated in FIG. 5A, the phase delay is produced by an electrical field  146  in the electro-optic material  144 . The field  146  is generated by applying a bias voltage ΔV between the transparent electrodes  142   a  and  142   b . In one preferred embodiment of the invention, the transparent electrodes  142   a  and  142   b  are comprised of an indium tin oxide layer. Additionally, the two transparent electrodes  142   a  and  142   b  are arranged in a substantially parallel configuration to provide a substantially uniform electric field parallel to the direction of propagation of the light beams  150 ,  152  in the central region of the electro-optic delay module  140 . This parallel arrangement allows the electro-optic material  144  to delay the light beam  150  independent of polarization.  
         [0046]    [0046]FIG. 5B illustrates another design of an electro-optic delay module  170  that also delays the optical signal by applying an electric field substantially along a direction of propagation  176 . The delay module  170  comprises a portion of electro-optic material  172  having proximal and distal ends  173   a ,  173   b  and preferably having a top and bottom  175   a ,  175   b . An input waveguide  174   a  is attached to the proximal end  173   a  and an output waveguide  174   b  is attached to the distal end  173   b . A direction of propagation  176  is defined in part by the placement of the input and output waveguides  174   a  and  174   b . Light coupled into the input waveguide  174   a  propagates through the portion of electro-optic material  172  largely in the direction of propagation  176  to the output waveguide  174   b.    
         [0047]    The delay module  170  further comprises electrodes disposed at each of the proximal and distal ends  173   a  and  173   b  so as to enable an electric field to be produced that is substantially parallel to the direction of propagation  176  of the light through the electro-optic delay element  170 . In one preferred embodiment, these electrodes are located on corners or edges of the portion the electro-optic material  172  on the top and bottom  175   a ,  175   b  of the delay element  170  near the proximal and distal ends  173   a  and  173   b . In an alternative preferred embodiment, these electrodes may be placed on sides of the electro-optic portion  172  adjacent or near the proximal and distal ends  173   a  and  173   b . In the cross-sectional view shown in FIG. 5B, for example, a positive terminal is connected to an electrode  180   a  at the top left corner of the electro-optic material  172  and to an electrode  182   a  at the bottom left corner. A negative terminal is connected to an electrode  180   b  at the top right corner and to an electrode  182   b  at the bottom right corner.  
         [0048]    A bias voltage ΔV is applied between the positive and negative terminals thereby generating an electric field extending between the proximal and the distal ends  173   a  and  173   b  of the electro-optic portion  172 . Because the electrodes  180   a ,  180   b ,  182   a ,  182   b  are placed on opposite ends  173   a  and  173   b  of the electro-optic portion  172 , as defined by the input and output waveguides  174   a  and  174   b  and the propagation of the light within the delay module  170 , the resulting electric fields  184  and  186  have a substantial component aligned with the propagation direction  176 .  
         [0049]    Some electro-optic materials, such as PLZT, are birefringent; the index of refraction varies differently depending on the direction of the applied electric field. In PLZT, for example, the index of refraction decreases for light polarized parallel to the applied electric field and increases for light polarized perpendicular to the electric field. The magnitude of this increase for perpendicular polarization states is also about one-third as large as the decrease for parallel polarizations. See, for example, copending U.S. Patent Application No. ______ (TOPTICS.004CP4) entitled “Optical Switching Network and Network Node and Method of Optical Switching”, filed Romanovsky on May 6, 2006, now U.S. Pat. No. ______, which is incorporated herein by reference in its entirety. By providing an electric field substantially aligned with the propagation direction of the light, the electric field will be substantially perpendicular to the polarization of the light regardless of its polarization state. Light will therefore not experience a different polarization depending on its polarization, and thus, a polarization-independent delay can be provided.  
         [0050]    [0050]FIG. 5C illustrates another embodiment of an electro-optic delay module  190  that is polarization-dependent, i.e., the amount of delay depends on the polarization of the optical signal passing through the device  190 . The module  190  comprises a portion of electro-optic material  192  having proximal and distal ends  193   a ,  193   b  and preferably having a top and bottom  195   a ,  195   b . An input waveguide  194   a  is attached to the proximal end  193   a  and an output waveguide  194   b  is attached to the distal end  193   b . A direction of propagation  196  is defined in part by the placement of the input and output waveguides  194   a  and  194   b . Light coupled into the input waveguide  194   a  propagates through the portion of electro-optic material  192  largely in the direction of propagation  196  to the output waveguide  194   b . In the embodiment depicted in FIG. 5C, first and second substantially planar electrodes  200   a , and  200   b  are on the top and bottom  195   a ,  195   b , respectively, of the portion of electro-optic material  192 . The first and second electrodes  200   a ,  200   b  are preferably parallel to each other, and with respect to the top and bottom faces  195   a ,  195   b  of the electro-optic material  192 . In alternative embodiments, these electrodes may be on opposite sides or sidewalls of the portion of electro-optic material  192 .  
         [0051]    When a bias voltage ΔV is applied between the first and second electrodes  200   a ,  200   b , an electric field  202  that is transverse to the direction of propagation  196  is induced within the electro-optic portion  192 . Accordingly, light comprising transverse electromagnetic waves having an arbitrary polarization may include polarization components parallel and/or perpendicular to the applied electric field. Because of the birefringent behavior of some electro-optic material  192  (such as PLZT) as described above, polarization components parallel to the electric field will experience a different index of refraction and a different amount of phase delay than perpendicular polarization components when propagating through the delay module  190 . Accordingly, the delay introduced by the module  190  depends on the polarization of the optical signal. Such features may be utilized advantageously if the incoming optical signal has a given polarization.  
         [0052]    In one preferred embodiment of the invention, the electro-optic material  144  is a polycrystalline lanthanum-modified lead titanate zirconate (PLZT). FIG. 6 illustrates the electro-optic module  140  fabricated on a silicon substrate  156 . Using silicon fabrication methods well known in the art, a bottom SiO2 layer  154   b  and a top SiO2 layer  154   a  are formed with the first waveguide segment  127   a  and the second waveguide segment  127   b  interposed therebetween. The first transparent electrode  142   a , the electro-optic material  144 , and the second transparent electrode  142   b  are formed by filling a groove as shown in FIG. 6.  
         [0053]    PLZT is an electro-optic material which has a refractive index n that depends on the electric field  146 . The velocity of propagation of light in a dielectric medium is given by v=c/n, where c is a constant velocity of light in vacuum. By changing the electric field strength in the electro-optic material  144 , the velocity of propagation v can be controlled, thus allowing controllable variations in delays of the light signals in the waveguide array  125 . Preferably, the electro-optic delay module  140  can delay the phase of the light beam  150  by at least 2π radians, which is one full wavelength. As indicated above, 15 μm is a typical difference in the optical path length between the adjacent waveguides  126  of the example conventional AWG  120 . For a light with wavelength of 1550 nM, the corresponding phase delay δ, according to Eq. 1, is about 10×(2π). In the AWG  121  of the preferred embodiment described above, the waveguides comprised of the waveguide array  125  have substantially same physical length. As such, the difference in optical path length Λ is given by Δn L′ from Eq. 2, where L′ in this case is the physical length of the electro-optic material  144  through which the light travels. If a user selects a Λ of 15 μm, as in the example, then L′=(15/Δn) μm. As is well known in the art relating to PLZT, Δn of 0.010 is an easily obtainable value. Using this number for Δn, L′ is calculated to be 1500 μm, or 1.5 mm. This dimension for L′ corresponds to phase delay of about 10 cycles when using light with wavelength of 1550 nm. The dimension for L′ can be as small as 150 μm, which corresponds to phase difference of 2π radians, or one full wavelength. This length, L′, can be that of a single delay element or of multiple (fixed and/or variable) delay elements optically coupled together as described above.  
         [0054]    The use of electro-optic material  144  thus allows delaying of optical signals, with the amount of delay comparable to that in the conventional AWG devices. Furthermore, the use of electro-optic material  144  allows the amount of delay to be tuned by changing the bias voltage applied to the transparent electrodes  142   a  and  142   b . The tuning feature allows the user to adjust the AWG  121  to compensate for variations due to manufacturing tolerances and environmental factors. Furthermore, since each of the electro-optic delay modules  140  can be tuned independently, the AWG  121  as a whole can be configured in a variety of ways to suit the needs of the user. It will also be appreciated that the AWG  121  can be used in reverse as a multiplexer, as referred to above. Similarly, the delay modules or -elements themselves are bidirectional, imparting adjustable amounts of phase delay unto light propagating in either direction therethrough. It will also be understood that, although the AWG  121  preferably utilizes waveguides of equal length (so that all of the delay differentials are due to the electro-optic material), in an alternative embodiment, the delay may be provided in part by waveguides of unequal length and in part by electro-optic material.  
         [0055]    Although the foregoing description of the preferred embodiment of the present invention has shown, described and pointed out the fundamental novel features of the invention, it will be understood that various omissions, substitutions and changes in the form of the detail of the apparatus as illustrated, as well as the uses thereof, may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing discussion, but should be defined by the appended claims.