Abstract:
A modular optical platform for selective wavelength switching that can be adapted to perform various other functions, such as dispersion compensation, dynamic gain equalization (DGE) and add/drop multiplexing (ADM) provides the versatility and modularity that will be essential to the future of the fiber optics industry. The basic platform includes a first lens for directing an optical signal, a diffraction grating for dispersing an optical signal into its component wavelength channels, a second lens for directing the component wavelength channels, and a modifying device for conducting one or more of a variety of functions including dispersion compensation, switching, DGE and COADM. The first and second lens are preferably replaced by a single concave reflective mirror having optical power. The modifying means for dispersion compensation according to the present invention includes a tunable etalon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application No. 09/729,270 filed May 12, 2000 and claiming priority from Provisional Appl. No. 60/326,844 filed on Oct. 4, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present application relates to a wavelength selective optical platform, and in particular to a wavelength selective optical device with dynamically-tunable dispersion compensation.  
         BACKGROUND OF THE INVENTION  
         [0003]    In high bit rate light-wave systems, tunable chromatic dispersion compensators are required to compensate for the various dispersions accumulated along the different paths taken by each of the individual signal channels.  
           [0004]    Known dispersion compensation techniques include dispersion compensation fibers, chirped Bragg grating, and cascaded Mach-Zehnder filters. Devices for dispersion compensation are known. U.S. Pat. No. 5,283,845 granted to J. W. Ip on Feb. 1, 1994 discloses a multi-port tunable fiber-optic filter. U.S. Pat. No. 6,141,130 granted to J. W. Ip on Oct. 31, 2000 discloses an amplitude-wavelength equalizer for a group of wavelength division multiplexed channels. U.S. Pat. No. 5,557,468 granted to J. W. Ip on Sep. 17, 1996 discloses a chromatic dispersion compensation device.  
           [0005]    A recent approach for dispersion compensation utilizes Gires-Tournois Interferometers (GTI), which potentially provide low-loss and polarization insensitivity, while offering high negative dispersion without exhibiting the nonlinear behavior found in fiber-based dispersion compensating devices. Moreover, GTI&#39;s can be made to be colorless for compensating multi-channel dispersion in a very compact device. U.S. Pat. No. 6,081,379 granted to R. R. Austin et al. discloses a monolithic multiple GTI device providing negative group delay dispersion. A GTI is basically an asymmetric Fabry-Perot etalon, providing a constant amplitude response over all frequencies and a phase response that varies with frequency. The key to using a GTI for dispersion compensation is that each frequency component of the signal remains trapped in the interferometer for a longer time as the frequency approaches the interferometer&#39;s resonant frequency. Therefore, negative or positive delays depend on the position of the signal spectrum with respect to the resonance peak, and the closer the signal frequency component is to the cavity resonance the greater the delay.  
           [0006]    A paper by C. K. Madsen, entitled “Tunable Dispersion Compensating MEMS All-Pass Filter”, IEEE Photonics Technology Letters, Vol. 12; No. 6, June 2000, which is incorporated herein by reference, discloses a tunable dispersion compensation technique with a microelectromechanical (MEM) actuated variable reflector and a thermally tuned cavity.  
           [0007]    U.S. Pat. No. 6,289,151 granted to Kazrinov et al. discloses an all-pass optical filter for reducing the dispersion of optical pulses by applying a desired phase response to optical pulses transmitted through the filter.  
           [0008]    Typically, gain equalizing and add/drop multiplexer devices involve some form of multiplexing and demultiplexing to modify each individual channel of the telecommunication signal. In particular, it is common to provide a first diffraction grating for demultiplexing the optical signal and a second spatially separated diffraction grating for multiplexing the optical signal after it has been modified. An example of the latter is disclosed in U.S. Pat. No. 5,414,540, incorporated herein by reference. However, in such instances it is necessary to provide and accurately align two matching diffraction gratings and at least two matching lenses. This is a significant limitation of prior art devices.  
           [0009]    To overcome this limitation, other prior art devices have opted to provide a single diffraction grating that is used to demultiplex an optical single in a first pass through the optics and multiplex the optical signal in a second pass through the optics. For example, U.S. Pat. Nos. 5,233,405; 5,526,155; 5,745,271; 5,936,752; and 5,960,133; which are incorporated herein by reference, disclose such devices.  
           [0010]    However, none of these prior art devices disclose an optical arrangement suitable for dynamic gain equalizer (DGE), configurable optical add/drop multiplexer (COADM), and dispersion compensation applications. In particular, none of these prior art devices recognize the advantages of providing a simple, symmetrical optical arrangement suitable for use with a dynamic dispersion compensator.  
           [0011]    For example, U.S. Pat. No. 5,414,540 to Patel et al. discloses a liquid crystal optical switch for switching an input optical signal to selected output channels. The switch includes a diffraction grating, a liquid crystal modulator, and a polarization dispersive element. In one embodiment, Patel et al. suggest extending the 1×2 switch to a 2×2 drop-add circuit and using a reflector. However, the disclosed device is limited in that the add/drop beams of light are angularly displaced relative to the input/output beams of light. This angular displacement is disadvantageous with respect to coupling the add/drop and/or input/output beams of light into parallel optical waveguides, in addition to the additional angular alignment required for the input beam of light.  
           [0012]    With respect to compactness, prior art devices have been limited to an excessively long and linear configurations, wherein the input beam of light passes through each optical component sequentially before being reflected in a substantially backwards direction.  
           [0013]    U.S. Pat. No. 6,081,331 discloses an optical device that uses a concave mirror for multiple reflections as an alternative to using two lenses or a double pass through one lens. However, the device disclosed therein only accommodates a single pass through the diffraction grating and does not realize the advantages of the instant invention.  
           [0014]    An object of the present invention is to provide an optical configuration for rerouting and modifying an optical signal that can be used as a dynamic dispersion compensator.  
         SUMMARY OF THE INVENTION  
         [0015]    Accordingly, the present invention relates to an optical device comprising:  
           [0016]    a first port for launching an input beam of light including a plurality of wavelength channels;  
           [0017]    a second port for receiving an output beam including at least a portion of one of the plurality of wavelength channels;  
           [0018]    first redirecting means for receiving the input beam of light, the first redirecting means having optical power;  
           [0019]    a dispersive element for receiving the input beam of light from the first redirecting means, and for dispersing the input beam of light into the plurality of wavelength channels;  
           [0020]    second redirecting means for receiving the dispersed wavelength channels, the second redirecting means having optical power; and  
           [0021]    a plurality of modifying means, each modifying means for receiving a corresponding one of the dispersed wavelength channels from the second redirecting means, and for reflecting at least a portion of the corresponding wavelength channel back to the second redirecting means;  
           [0022]    wherein each of said modifying means includes a tunable etalon for providing dispersion compensation to said corresponding wavelength channel; and  
           [0023]    wherein at least one of the wavelength channels travel back via the second redirecting means to the dispersive element for recombination into the output beam, which is output the second port via the first redirecting means.  
           [0024]    Another aspect of the present invention relates to a dispersion compensator comprising:  
           [0025]    a first port for launching an input beam of light including a plurality of wavelength channels;  
           [0026]    a second port for receiving an output beam including the plurality of wavelength channels;  
           [0027]    first redirecting means for receiving the input beam of light, the first redirecting means having optical power;  
           [0028]    a dispersive element for receiving the input beam of light from the first redirecting means, and for dispersing the input beam of light into the plurality of wavelength channels;  
           [0029]    second redirecting means for receiving the dispersed wavelength channels, the second redirecting means having optical power; and  
           [0030]    a plurality of tunable etalons, each tunable etalon for receiving a corresponding one of the dispersed wavelength channels from the second redirecting means, and for reflecting the corresponding wavelength channel back to the second redirecting means, each tunable etalon for providing dispersion compensation to said corresponding wavelength channel;  
           [0031]    wherein the plurality of wavelength channels travel back via the second redirecting means to the dispersive element for recombination into the output beam, which is output the second port via the first redirecting means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:  
         [0033]    [0033]FIG. 1 a  is a schematic diagram illustrating an embodiment of an optical configuration that can be used as a dynamic gain equalizer (DGE), add-drop multiplexer (COADM) or a dynamic dispersion compensator in accordance with the invention;  
         [0034]    [0034]FIG. 1 b  is a top view of the device of FIG. 1 a  modified to be a dynamic dispersion compensator;  
         [0035]    [0035]FIG. 2 a  is a detailed side view of a front-end module for use with the optical configuration shown in FIG. 1 having means for compensating for polarization mode dispersion (PMD);  
         [0036]    [0036]FIG. 2 b  is a detailed side view of an alternative front-end module having means for reducing or substantially eliminating PMD;  
         [0037]    [0037]FIG. 3 a  is a top view of one embodiment of modifying means comprising a liquid crystal array for use with the DGE/COADM shown in FIG. 1, wherein a liquid crystal element is switched to an ON state;  
         [0038]    [0038]FIG. 3 b  is a top view of the modifying means shown in FIG. 3 a,  wherein the liquid crystal element is switched to an OFF state;  
         [0039]    [0039]FIG. 3 c  is a top view of another embodiment of the modifying means for use with the DGE/COADM shown in FIG. 1, wherein the liquid crystal element is switched to an ON state;  
         [0040]    [0040]FIG. 3 d  is a top view of the modifying means shown in FIG. 3 c,  wherein the liquid crystal element is switched to an OFF state;  
         [0041]    [0041]FIG. 4 a  is a top view of another embodiment of the modifying means for use with the DGE/COADM shown in FIG. 1 having a birefringent crystal positioned before the liquid crystal array, wherein the liquid crystal element is switched to an OFF state;  
         [0042]    [0042]FIG. 4 b  is a top view of the modifying means shown in FIG. 4 a,  wherein the liquid crystal element is switched to an ON state;  
         [0043]    [0043]FIG. 4 c  is a top view of yet another embodiment of the modifying means for use with the DGE shown in FIG. 1 utilizing a MEMS device;  
         [0044]    [0044]FIG. 5 a  is a side view of another embodiment of the modifying means including a tunable etalon for use as a dynamic dispersion compensator;  
         [0045]    [0045]FIG. 5 b  is a side view of schematically illustrates another example of a tunable etalon for use in a dynamic dispersion compensator;  
         [0046]    [0046]FIGS. 6 a  and  6   b  are schematic diagrams of an embodiment of the invention that is preferred over the one shown in FIG. 1, wherein the focal plane of a single spherical reflector is used to locate the input/output ports, diffraction grating, and modifying means;  
         [0047]    [0047]FIG. 7 is a schematic diagram of an embodiment of the invention that is similar to that shown in FIGS. 6 a  and  6   b,  wherein the input/output ports are disposed between the modifying means and dispersive element;  
         [0048]    [0048]FIG. 8 is a schematic diagram of an optical platform having a configuration similar to that shown in FIGS. 6 a  and  6   b  including an optical circulator; and  
         [0049]    [0049]FIG. 9 is a schematic diagram of an optical platform in accordance with the instant invention including a lens having a single port for launching and receiving light from the spherical reflector;  
         [0050]    [0050]FIG. 9 a  is a top view showing a lens array coupling input/output optical waveguides to the lens in accordance with the instant invention;  
         [0051]    [0051]FIG. 9 b  is a top view showing a prior art polarization diversity arrangement coupling input/output optical waveguides to the lens in accordance with the instant invention;  
         [0052]    [0052]FIG. 9 c  is a side view of the prior art polarization diversity arrangement shown in FIG. 9 b;    
         [0053]    [0053]FIG. 9 d  is a top view showing an alternative arrangement to the optical components shown in FIG. 9 b;    
         [0054]    [0054]FIG. 9 e  is a side view of the alternate arrangement shown in FIG. 9 d;    
         [0055]    [0055]FIG. 9 f  is a top view showing an asymmetric offset of the input/output optical waveguides with respect to the optical axis of the lens, in accordance with the instant invention;  
         [0056]    [0056]FIG. 10 is a schematic diagram of another embodiment of an optical platform arrangement in accordance with the invention;  
         [0057]    [0057]FIG. 11 is a schematic diagram of the preferred embodiment of an optical platform with COADM functionality in accordance with the instant invention; and  
         [0058]    [0058]FIG. 12 is a schematic diagram of an optical platform with COADM functionality in accordance with the instant invention, wherein an asymmetric arrangement of the input/output optical waveguides complements the angular displacement provided by a MEMS element. 
     
    
     DETAILED DESCRIPTION  
       [0059]    Referring now to FIG. 1 a,  an optical device for rerouting and modifying an optical signal in accordance with the instant invention is capable of operating as a Dynamic Gain/Channel Equalizer (DGE), a Configurable Optical Add/Drop Multiplexer (COADM), and/or a dynamic dispersion compensator.  
         [0060]    The optical device of FIG. 1 a  includes a diffraction element  120  disposed between and at a focal plane of identical lens elements with optical power  110   a  and  110   b.  Two ports  102   a  and  102   b  are shown at an input/output end with bi-directional arrows indicating that light launched into port  102   a  can be transmitted through the optical device and can be reflected backward to the input port from which it was launched  102   a,  or alternatively, can be switched to port  102   b  or vice versa in a controlled manner. The input/output ports  102   a  and  102   b  are also disposed about one focal plane away from the lens element  110   a  to which they are optically coupled. Although only two input/output ports are shown to facilitate an understanding of this device, a plurality of such pairs of ports is optionally provided. At the other end of the device, a modifying means  150  is provided at the focal plane of the lens  110   b  for modifying at least a portion of the light incident thereon.  
         [0061]    [0061]FIG. 1 b  illustrates the path taken by an input beam of light  121  as it passes through a first port  122  to a second port  123  of a circulator  124 . The first lens  100   a  redirects the beam  121  at the diffraction grating  120 , which disperses the beam of light into component sub-beams  121   a  to  121   g.  The second lens  110   b  redirects the sub-beams  121   a  to  121   g  towards the modifying means  150 , which in this case is an array of dynamically tunable etalons with a partially reflective front surface  125 , a fully reflective rear surface  126  and a cavity  127 . The etalons are independently tunable to provide dispersion compensation for each individual sub-beam  121   a  to  121   g.  In this embodiment the sub-beams  121   a  to  121   g  are reflected directly back to the second lens  110   b,  which redirects the sub-beams back together for recombining by the diffraction grating  120 . The recombined output beam of light is redirected by the first lens  110   a  to the second port  123  of the circulator  124 , which subsequently directs the output beam out the third port  126 .  
         [0062]    Since the modifying means and/or dispersive element are generally dependent upon polarization of the incident light beam, light having a known polarization state is provided to obtain the selected switching and/or attenuation. FIGS. 2 a  and  2   b  illustrate two different embodiments of polarization diversity arrangements for providing light having a known polarization state, for use with the dispersion compensators, DGE, and COADM devices described herein. The polarization diversity arrangement, which is optionally an array, is optically coupled to the input and output ports.  
         [0063]    Referring to FIG. 2 a,  an embodiment of a front-end micro-optical component  105  for providing light having a known polarization includes a fibre tube  107 , a micro-lens  112 , and a birefringent crystal element  114  for separating an input beam into two orthogonally polarized sub-beams. At an output end, a half waveplate  116  is provided to rotate the polarization of one of the beams by 90° so as to ensure both beams have a same polarization state, e.g. horizontal. A glass plate or a second waveplate  118  is added to the fast axis path of the crystal  114  to lessen the effects of Polarization Mode Dispersion (PMD) induced by the difference in optical path length along the two diverging paths of crystal  114 .  
         [0064]    [0064]FIG. 2 b  illustrates an alternative embodiment to that of FIG. 2 a,  wherein two birefringent elements  114   a,    114   b  have a half waveplate  116   a  disposed therebetween; here an alternate scheme is used to make the path lengths through the birefringent materials substantially similar. Optionally, a third waveplate  119  is provided for further rotating the polarization state.  
         [0065]    Although, FIGS. 2 a  and  2   b  both illustrate a single input beam of light for ease of understanding, the front end unit  105  is capable of carrying many more beams of light therethrough, in accordance with the instant invention (i.e., can be designed as an array as described above).  
         [0066]    [0066]FIGS. 3 a - 3   b ,  3   c - 3   d ,  4   a - 4   b , and  5 , each illustrate a different embodiment of the modifying means for use with the DGE/COADM devices described herein. Each of these embodiments is described in more detail below. Note that the modifying means are generally discussed with reference to FIG. 1 a . Although reference is made to the dispersive element  120  and the lens elements  110   a  and  110   b , these optical components have been omitted from FIGS. 3 a - 3   b ,  3   c - 3   d,    4   a - 4   b , and  5  for clarity.  
         [0067]    Referring to FIGS. 3 a  and  3   b  a schematic diagram of the modifying means  150  is shown including a liquid crystal array  130  and a reflector  140 . The reflector includes first and second polarizing beam splitters  144  and  146 , and a reflective surface  142 .  
         [0068]    When the device operates as a COADM, each pixel of the liquid crystal array  130  is switchable between a first state, e.g. an “ON” state shown in FIG. 3 a,  wherein the polarization of a beam of light passing therethrough is unchanged, e.g. remains vertical, and a second state, e.g. an “OFF” state shown in FIG. 3 b , wherein the liquid crystal cell rotates the polarization of a beam of light passing therethrough 90°, e.g. is switched to horizontal. The reflector  140  is designed to pass light having a first polarization, e.g. vertical, such that a beam of light launched from the port  102   a  is reflected back to the same port, and designed to reflect light having another polarization, e.g. horizontal, such that a beam of light launched from the port  102   a  is switched to the port  102   b.    
         [0069]    When the device operates as a DGE, each liquid crystal cell is adjusted to provide phase retardations between 0° to 180°. For a beam of light launched and received from port  102   a,  0% attenuation is achieved when liquid crystal cell provides no phase retardation, and 100% attenuation is achieved when the liquid crystal cell provides 180° phase retardation. Intermediate attenuation is achieved when the liquid crystal cells provide a phase retardation greater than 0° and less than 180°. In some DGE applications, the reflector  140  includes only a reflective surface  142 , i.e. no beam splitter.  
         [0070]    Preferably, the liquid crystal array  130  has at least one row of liquid crystal cells or pixels. For example, arrays comprising 64 or 128 independently controlled pixels have been found particularly practical, but more or fewer pixels are also possible. Preferably, the liquid crystal cells are of the twisted nematic type cells, since they typically have a very small residual birefringence in the “ON” state, and consequently allow a very high contrast ratio (&gt;35 dB) to be obtained and maintained over the wavelength and temperature range of interest. It is possible that the inter-pixel areas of the liquid crystal array  130  are covered by a black grid.  
         [0071]    [0071]FIGS. 3 c  and  3   d  are schematic diagrams analogous to FIGS. 3 a  and  3   b  illustrating an alternate form of the modifying means  150  discussed above, wherein the reflector  140  includes a double Glan prism  148 . The arrangement shown in FIGS. 3 c  and  3   d  is preferred over that illustrated in FIGS. 3 a  and  3   b , since the respective positions of the two-sub beams emerging from the polarization diversity arrangement (not shown) does not change upon switching.  
         [0072]    Note that in FIGS. 3 a - 3   d  the dispersion direction is perpendicular to the plane of the paper. For exemplary purposes a single ray of light is shown passing through the modifying means  150 .  
         [0073]    [0073]FIGS. 4 a  and  4   b  are schematic diagrams showing another embodiment of the modifying means  150 , wherein a birefringent crystal  152  is disposed before the liquid crystal array  130 . A beam of light having a predetermined polarization state launched from port  102   a  is dispersed into sub-beams, which are passed through the birefringent crystal  152 . The sub-beams of light passing through the birefringent crystal  152  remain unchanged with respect to polarization. The sub-beams of light are transmitted through the liquid crystal array  130 , where they are selectively modified, and reflected back to the birefringent crystal  152  via reflective surface  142 . If a particular sub-beam of light passes through a liquid crystal cell in an “OFF” state, as shown in FIG. 4 a,  then the polarization thereof will be rotated by 90° and the sub-beam of light will be refracted as it propagates through the birefringent crystal  152  before being transmitted to port  102   b . If the sub-beam of light passes through a liquid crystal cell in an “ON” state, as shown in FIG. 4 b , then the polarization thereof will not be rotated and the sub-beam of light will be transmitted directly back to port  102   a.  A half wave plate  153  is provided to rotate the polarization of the refracted sub-beams of light by 90° to ensure that both reflected beams of light have a same polarization state.  
         [0074]    [0074]FIG. 4 c  is a schematic diagram of another embodiment of the modifying means  150  including a micro electro-mechanical switch (MEMS)  155 , which is particularly useful when the device is used as a DGE. A beam of light having a predetermined polarization state launched from port  102   a  is dispersed into sub-beams and is passed through a birefringent element  156  and quarter waveplate  157 . The birefringent element  156  is arranged not to affect the polarization of the sub-beam of light. After passing through the quarter waveplate  157 , the beam of light becomes circularly polarized and is incident on a predetermined reflector of the MEMS array  155 . The reflector reflects the sub-beam of light incident thereon back to the quarter waveplate. The degree of attenuation is based on the degree of deflection provided by the reflector (i.e. the angle of reflection). After passing through the quarter waveplate  157  for a second time, the attenuated sub-beam of light will have a polarization state that has been rotated 90° from the original polarization state. As a result the attenuated sub-beam is refracted in the birefringent element  156  and is directed out of the device to port  102   b . A half wave plate  158  is provided to rotate the polarization of the refracted sub-beams of light by 90°.  
         [0075]    Of course, other modifying means  150  including at least one optical element capable of modifying a property of at least a portion of a beam of light and reflecting the modified beam of light back in substantially the same direction from which it originated are possible.  
         [0076]    Advantageously, each of the modifying means discussed above utilizes an arrangement wherein each spatially dispersed beam of light is incident thereon and reflected therefrom at a 90° angle. The 90° angle is measured with respect to a plane encompassing the array of modifying elements (e.g. liquid crystal cells, MEMS reflectors). Accordingly, each sub-beam of light follows a first optical path to the modifying means where it is selectively switched such that it is reflected back along the same optical path, or alternatively, along a second optical path parallel to the first. The lateral displacement of the input and modified output beams of light (i.e., as opposed to angular displacement) allows for highly efficient coupling between a plurality of input/output waveguides. For example, the instant invention is particular useful when the input and output ports are located on a same multiple bore tube, ribbon, or block.  
         [0077]    In order to maintain the desired simplicity and symmetry, it is preferred that the element having optical power be rotationally symmetric, for example a rotationally symmetric lens or spherical reflector. Moreover, it is preferred that the diffraction element  120  be a high efficiency, high dispersion diffraction grating. Optionally, a circulator (not shown) is optically coupled to each of ports  102   a  and  102   b  for separating input/output and/or add/drop signals.  
         [0078]    Referring again to FIG. 1 a,  the operation of the optical device operating as a COADM is described by way of the following example. A collimated beam of light having a predetermined polarization and carrying wavelengths λ 1 , λ 2 , . . . λ 8  is launched through port  102   a  to a lower region of lens  110   a  and is redirected to the diffraction grating  120 . The beam of light is spatially dispersed (i.e. de-multiplexed) according to wavelength in a direction perpendicular to the plane of the paper. The spatially dispersed beam of light is transmitted as 8 sub-beams of light corresponding to 8 different spectral channels having central wavelengths λ 1 , λ 2 , . . . λ 8  through lens  110   b , where it is collimated and incident on the modifying means  150 , which for exemplary purposes is shown in FIGS. 3 a - 3   b . Each sub-beam of light is passed through an independently controlled pixel in the liquid crystal array  130 . In particular, the sub-beam of light having central wavelength λ 3  passes through a liquid crystal cell in an “OFF” state, and each of the other 7 channels having central wavelengths λ 1 -λ 2  and λ 4 -λ 8  pass through liquid crystal cells in an “ON” state. As the sub-beam of light having central wavelength λ 3  passes through the liquid crystal in the “OFF” state, the polarization thereof is rotated 90°, it is reflected by the polarization beam splitter  144  towards a second beam splitter  146 , and is reflected back to port  102   b , as shown in FIG. 3 b . As the other 7 channels having central wavelengths λ 1 -λ 2  and λ 4 -λ 8  pass through liquid crystal cells is in an “ON” state, the polarizations thereof remain unchanged, and they are transmitted through the polarization beam splitter  144  and are reflected off reflective surface  142  back to port  102   a.  In summary, the beam of light originally launched from port  102   a  will return thereto having dropped a channel (i.e. having central wavelength λ 3 ) and the sub-beam of light corresponding to the channel having central wavelength λ 3  will be switched to port  102   b.    
         [0079]    Simultaneously, a second beam of light having a predetermined polarization and carrying another optical signal having a central wavelength λ 3  is launched from port  102   b  to a lower region of lens  110   a.  It is reflected from the diffraction grating  120 , and is transmitted through lens  110   b , where it is collimated and incident on the modifying means  150 . The second beam of light passes through the liquid crystal cell in the “OFF” state, the polarization thereof is rotated 90°, it is reflected by the second polarization beam splitter  146  towards the first beam splitter  144 , and is reflected back to port  102   a,  as shown in FIG. 3 b . Notably, the 7 express channels and the added channel are multiplexed when they return via the dispersion grating  120 .  
         [0080]    Since every spectral channel is passed through an independently controlled pixel before being reflected back along one of the two possible optical paths, a fully re-configurable switch for a plurality of channels is obtained.  
         [0081]    Notably, the choice of eight channels is arbitrarily chosen for exemplary purposes. More or fewer channels are also within the scope of the instant invention.  
         [0082]    Referring again to FIG. 1 a,  the operation of the optical device operating as a DGE is described by way of the following example. A collimated beam of light having a predetermined polarization and carrying channels λ 1 , λ 2 , . . . λ 8  is launched from port  102   a  through lens  110   a,  where it is redirected to diffraction grating  120 . The beam of light is spatially dispersed according to wavelength in a direction perpendicular to the plane of the paper. The spatially dispersed beam of light is transmitted as 8 sub-beams of light corresponding to 8 different spectral channels having central wavelengths λ 1 , λ 2 , . . . λ 8  through lens  110   b , where it is collimated and incident on the modifying means  150  such that each sub-beam of light is passed through an independently controlled pixel in the liquid crystal array  130  wherein the polarization of each sub-beam of light is selectively adjusted. In particular, the sub-beam of light having central wavelength λ 3  is passed through a liquid crystal cell in an “ON” state, the polarization thereof is not adjusted, it passes through the beam splitter  144 , and is reflected back to port  102   a  with no attenuation, as illustrated in FIG. 3 a.  Simultaneously, a sub-beam of light having central wavelength λ 4  is passed through a liquid crystal cell in an “OFF” state, the polarization thereof is rotated by 90°, it is reflected from beam splitters  144  and  146  and is directed to port  102   b . 100% attenuation is achieved with respect to this sub-beam of light returning to port  102   a.  Simultaneously, a sub-beam of light having central wavelength λ 5  is passed through a liquid crystal cell that provides phase retardation between 0° and 180°, it is partially transmitted through from beam splitter  144  and returns to port  102   a  an attenuated signal. The degree of attenuation is dependent upon the phase retardation.  
         [0083]    Optionally, a second beam of light is simultaneously launched from port  102   b  into the optical device for appropriate attenuation. In fact, this optical arrangement provides a single optical system that is capable of providing simultaneous attenuation for a plurality of input ports (not shown).  
         [0084]    Alternatively, the attenuated light is received from port  102   b , hence obviating the need for a circulator. In this instance, when the polarization of a beam of light having central wavelength λ 3  is rotated by 90°, i.e. the liquid crystal array provides 180° phase retardation, it is reflected from the beam splitter  144  to the second beam splitter  146  (shown in FIG. 3 a ) and is directed to port  102   b  with no attenuation. Similarly, when the polarization of this beam of light is not adjusted, i.e. the liquid crystal array provides no phase retardation, it passes through the beam splitter  144  (shown in FIG. 3 a ) and is reflected back to port  102   a.  100% attenuation with respect to this sub-beam of light reaching port  102   b  is achieved. Variable attenuation is achieved when the liquid crystal cell selectively provides phase retardation between 0° and 180°.  
         [0085]    [0085]FIG. 5 a  illustrates a modifying means for dynamic dispersion compensation in the form of a tunable GT etalon 500 including a front partially reflective surface  525 , and a rear fully reflective surface  526  defining a cavity  527  therebetween. The front surface  525  is comprised of a dielectric coating on a glass block  528 . The rear surface  526  is coated onto a substrate  529 . The cavity  527  is comprised of the glass block  528  with an array of transparent indium-tin oxide (ITO) electrodes  531  connected thereto adjacent a liquid crystal fluid  532 . The liquid crystal axis of the fluid  532  is aligned with the polarization direction of the incoming light. Actuation of the individual electrodes  531  changes the index of refraction of the fluid  532  therebelow and thereby the optical path length of the cavity  527 . Accordingly, each sub-beam  221   a  to  221   g  have a corresponding tunable etalon for individual dispersion compensation.  
         [0086]    [0086]FIG. 5 b  illustrates an alternative arrangement for a tunable etalon  550  in which the front partially reflective surface  525  is in the form of a dielectric coating on a substrate  551 , and the rear fully reflective coating is in the form of an array of piston MEMS mirrors  552  formed in a substrate  553 . The cavity  527  is an air cavity  554  defined by spacers  556  constructed of low coefficient of thermal expansion (CTE) material, such as Invar®. In this case the cavity length of the individual tunable etalons is adjusted by physically moving the position of the back mirror, i.e. the piston mirror  552  to provide dispersion compensation to the individual sub-beams  221   a  to  221   e.  It is also possible to  
         [0087]    Turning now to FIG. 6 a  another embodiment of the optical platform for use as a dispersion compensator, a DGE and/or a COADM, which is preferred over the embodiment shown in FIG. 1, is shown. For clarity only one beam is shown exiting the front-end unit  605 , however at least one other beam (not shown) can be disposed behind this beam.  
         [0088]    In FIG. 6 a  a single element having optical power in the form of a spherical reflector  610  is used to receive a collimated beam of light from the front-end unit  605  and to receive and reflect beams of light to and from the diffraction grating  620  and the modifying means  650 . The front-end unit  605 , the diffraction grating  620 , and the modifying means  650 , are analogous to parts  105 ,  120 , and  150  described above. However, in this embodiment the front-end unit  605 , the diffraction grating  620 , and the modifying means are each disposed about the single focal plane of the spherical reflector  610 . Preferably, the diffraction grating is further disposed about the optical axis of the spherical reflector  610 . In general, two circulators (not shown) are optically coupled to the front-end unit  605  to separate input/out and add/drop signals in ports  102   a  and  102   b , as described above.  
         [0089]    Preferably, the diffraction grating  620 , the spherical reflector  610 , and the modifying means  650  are each made of fused silica and mounted together with a beam folding mirror or prism  660  to a supporting plate  670  made of the same material or a material with a low coefficient of thermal expansion, e.g. Invar, as illustrated in FIG. 6 b . The beam folding mirror or prism  660  is provided for space considerations. Advantageously, this design provides stability with respect to small temperature fluctuations. Moreover, this design is defocus free since the radius of curvature of the spherical reflector  610  changes in proportion to thermal expansion or contraction of any other linear dimensions. Advantageously, the spherical mirror  610  has substantially no chromatic aberrations.  
         [0090]    When the optical device operates as a DGE, a detector array  657  is optionally positioned behind the beam-folding mirror  660  to intercept part of the wavelength dispersed beam of light. This design allows the signal to be tapped while eliminating the need for external feedback.  
         [0091]    Preferably, the diffraction grating  620  and the modifying means  650  are disposed substantially one focal length away from the spherical mirror  610  or substantially at the focal plane of the spherical reflector  610 , as discussed above. For example, in COADM applications it is preferred that the modifying means  650  are substantially at the focal plane to within 10% of the focal length. For DGE applications, it is preferred that the modifying means  650  are substantially at the focal plane to within 10% of the focal length if a higher spectral resolution is required, however, the same accuracy is not necessary for lower resolution applications.  
         [0092]    In operation, a multiplexed beam of light is launched into the front-end unit  605 . The polarization diversity arrangement  105  provides two substantially collimated sub-beams of light having the same polarization, e.g. horizontal, as discussed above. The two beams of light are transmitted to the spherical reflector  610  and are reflected therefrom towards the diffraction grating  620 . The diffraction grating  620  separates each of the two sub-beams into a plurality of sub-beams of light having different central wavelengths. The plurality of sub-beams of light are transmitted to the spherical reflector  610  where they are collimated and transmitted to the modifying means  650  where they are incident thereon as spatially separated spots corresponding to individual spectral channels. Each sub-beam of light corresponding to an individual spectral channel is modified and reflected backwards either along the same optical path or another optical path according to its polarization state, as described above. The sub-beams of light are transmitted back to the spherical reflector  610  and are redirected to the dispersive element  620 , where they are recombined and transmitted back to the spherical reflector  610  to be transmitted to the predetermined input/output port.  
         [0093]    Optionally, second, third, forth, . . . etc. multiplexed beams of light are launched into the front-end unit  605 . In fact, this optical arrangement is particularly useful for applications requiring the manipulation of two bands, e.g. C and L bands, simultaneously, wherein each band has its own corresponding in/out/add/drop ports.  
         [0094]    Advantageously, the optical arrangement shown in FIGS. 6 a  and  6   b  provides a symmetrical 4-f optical system with fewer alignment problems and less loss than prior art systems. In fact, many of the advantages of this design versus a conventional 4f system using separate lenses is afforded due to the fact that the critical matching of components is obviated. One significant advantage relates to the fact that the angle of incidence on the grating, in the first and second pass, is inherently matched with the optical arrangement.  
         [0095]    The instant invention further provides an optical device for rerouting and modifying an optical signal device that is substantially more compact and that uses substantially fewer components than similar prior art devices.  
         [0096]    [0096]FIG. 7 shows an alternate arrangement of FIG. 6 a  and FIG. 6 b  that is particularly compact. In this embodiment, the more bulky dispersive element  620  and modifying means  650  are disposed outwardly from the narrower front-end unit  605 .  
         [0097]    [0097]FIG. 8 illustrates an optical platform for use as a dispersion compensator or DGE including a conventional three port optical circulator  880  and having a particularly symmetrical design. A beam of light is launched into a first port  882  of the circulator  880  where it circulates to and exits through port  884 . The beam of light exiting port  884  is passed through the front-end unit  805 , which produces two collimated sub-beams having a same polarization that are transmitted to an upper region of the spherical reflector  810  in a direction parallel to an optical axis OA thereof. The collimated sub-beams of light incident on the spherical reflector  810  are reflected and redirected to the diffraction grating  820  with an angle of incidence β. The sub-beams of light are spatially dispersed according to wavelength and are transmitted to a lower region of the spherical reflector  810 . The spatially dispersed sub-beams of light incident on the lower region of the spherical reflector  810  are reflected and transmitted to the modifying means  850  in a direction parallel to the optical axis of the spherical reflector  810 . Once compensated or attenuated, the sub-beams of light are reflected back to the spherical reflector  810 , the diffraction grating  820 , and the front-end unit  805  along the same optical path. The diffraction grating recombines the spatially dispersed sub-beams of light. The front-end unit  805  recombines the two sub-beams of light into a single beam of light, which is transmitted to the circulator  880  where it is circulated to output port  860 . The front-end unit  805 , diffraction grating  820 , and modifying means  850 , which are similar to components  105 ,  120 , and  150  described above, are each disposed about a focal plane  825  of the spherical reflector  810 . In particular, the diffraction grating  820  is disposed about the focal point of the spherical reflector  810  and the modifying means  850  and front-end unit are symmetrically disposed about the diffraction grating. Preferably, the modifying means is a tunable etalon  850  including a front partially reflective surface  830  and a rear fully reflective surface  840 .  
         [0098]    Notably, an important aspect of the optical design described heretofore relates to the symmetry and placement of the optical components. In particular, the fact that each of the front-end unit, the element having optical power, the dispersive element, and the modifying means are disposed about one focal length (of the element having optical power) away from each other is particularly advantageous with respect to the approximately Gaussian nature of the incident beam of light.  
         [0099]    Referring again to FIG. 8, the input beam of light emerges from the front-end unit  805  essentially collimated and is transmitted via the element having optical power  810  to the diffraction grating  820 . Since the diffraction grating  820  is located at the focus of the element having optical power  810  and the input beams are collimated, the light is essentially focused on the diffraction grating  820 , as discussed above. The 1/e 2  spot size at the grating, 2ω 1 , and the 1/e 2  diameter 2ω 2  the front-end unit  805 , are related by: 
         ω 1 *ω 2   =λ*f/π   
         [0100]    where λ is wavelength and f is the focal length of the element having optical power. Accordingly, one skilled in the art can tune the spot size on the diffraction grating  820  and the resulting spectral resolution by changing the beam size at the front-end unit  805 .  
         [0101]    Moreover, the instant invention allows light beams launched from the front-end unit  805  to propagate to the liquid crystal array  830  with little or no spot expansion, since by symmetry, the spot size at the liquid crystal array is the same as the spot size at the front-end unit. Accordingly, the size of a beam of light launched from the front-end unit  805  can be changed to conform to the cell size of the liquid crystal array and/or vice versa. Alternatively, the size of the beam of light can be adjusted to change the spot size on the grating element  820 , as discussed above. Obviously, the same tuning is achievable with the optical arrangements shown in both FIG. 1 and FIGS. 6 a,    6   b.    
         [0102]    [0102]FIG. 9 illustrates an embodiment in accordance with the instant invention, wherein a single collimating/focusing lens  990  replaces the optical circulator  884  in the dispersion compensator or DGE shown in FIG. 8. Preferably, the lens  990  is a collimating/focusing lens such as a Graded Index or GRIN lens. The GRIN lens  990  is disposed such that an end face  994  thereof is coincident with the focal plane  925  of the spherical reflector  910 . The GRIN lens  990  is orientated such that its optical axis (OA 2 ) is parallel to but not coaxial with the optical axis OA of the spherical reflector  990 . An input  985  and an output  987  port are disposed about an opposite end face  993  of the lens  990 , off the optical axis OA 2 , and are optically coupled to input  999  and output  998  optical waveguides, respectively. Preferably, input  999  and output  998  waveguides are optical fibres supported by a double fibre tube, such as a double bore tube or a double v-groove tube. A single input/output port  992  is disposed about end face  994  coincident with the optical axis OA 2 . The illustrated modifying means includes a tunable etalon  950  including a front partially reflective mirror  930  and a rear fully reflective mirror  940 . Alternatively, the modifying means  950  could include a liquid crystal array  930  and a flat mirror  940  perpendicular to the OA of the spherical reflector  910 . The modifying means may also comprise a pair of liquid crystal arrays, one in the incident path and one in the reflected path. Furthermore, a MEMS array (not shown) can replace the flat mirror  940  to enable individual channel control. All other optical components are similar to those described with reference to FIG. 8.  
         [0103]    In operation, a beam of light is launched from input waveguide  999  into port  985  in a direction substantially parallel to the optical axis (OA 2 ) of the lens  990 . The beam of light passes through the GRIN lens  990 , and emerges from port  992  at an angle α to the optical axis. The angle α is dependent upon the displacement d of port  985  from the optical axis (OA 2 ). The beam of light is transmitted to an upper end of the spherical reflector  910 , where it is directed to the diffraction grating  920  with an angle of incidence β. The resulting spatially dispersed beam of light is transmitted to the spherical reflector  910 , is reflected, and is transmitted to the modifying means  950 . If the diffraction grating  920  is parallel to the focal plane  925 , as shown in FIG. 9, the beam of light incident on the modifying means has an angle of incidence substantially close to α. Each sub-beam of the spatially dispersed beam of light is selectively reflected back to the spherical reflector  910  at a predetermined angle, generally along a different optical path from which it came. Dispersion compensation or variable attenuation is provided by the modifying means  950 . The spherical reflector  910  redirects the modified spatially dispersed beam of light back to the diffraction grating  920  such that it is recombined to form a single modified output beam of light, which is incident on the single port  992  with an angle of incidence close to −α. The output beam of light is passed through the lens  990 , and is directed towards output port  987  where it is transmitted to output optical fibre  998 .  
         [0104]    Advantageously, this simple device, which allows light to enter and exit through two different ports disposed at one end of the device, is simple, compact, and easy to manufacture relative to prior art modifying and rerouting devices.  
         [0105]    Moreover, the instant design obviates the need for a bulky and costly optical circulator, while simultaneously providing an additional degree of freedom to adjust the mode size, which in part defines the resolution of the device, i.e. can adjust the focal length of GRIN lens  990 .  
         [0106]    Preferably, light transmitted to and from the output  998  and input  999  optical waveguides is focused/collimated, e.g. through the use of micro-collimators, thermally expanded core fibers, or lens fibers. Optionally, a front-end unit, e.g. as shown in FIGS. 2 a  or  2   b , which is in the form of an array, couples input/output waveguides  999 / 998  to end face  993 . FIGS. 9 a - 9   d  illustrate various optical input arrangements, which for exemplary purposes are illustrated with the arrangement shown in FIG. 2 a.    
         [0107]    In FIG. 9 a  the input  999  and the output  998  optical fibers are coupled to the GRIN lens  990  via a lenslet array  912 . A spacer  913  is provided in accordance with the preferred tele-centric configuration. This optical arrangement, which does not provide polarization diversity, is suitable for applications that do not involve polarization sensitive components.  
         [0108]    [0108]FIGS. 9 b  and  9   c  depict top and side views of the embodiment where a front-end unit, i.e. as shown in FIG. 2 a,  couples the input/output waveguides  999 / 998  to the GRIN lens  990 . More specifically, the front-end unit includes sleeve  996 , lenslet array  912 , birefringent element  914 , half waveplates  916 , glass plates or second waveplates  918 , and GRIN lens  990 .  
         [0109]    In FIGS. 9 d  and  9   e  there is shown top and side views of an arrangement wherein the birefringent element  914 , half waveplates  916 , and glass plates  918 , which provide the polarization diversity, are disposed about end face  994  of GRIN lens  990  and a spacer  913  the lenslet array  912  are disposed about end face  993 .  
         [0110]    [0110]FIG. 9 f  illustrates an embodiment wherein the input  999  and output  998  optical waveguides are not symmetrically disposed about the optical axis OA 2  of the GRIN lens  990 . In these instances, it is more convenient to compare the fixed distance between the input  999  and output  998  waveguides (D=2d) to the total angle between the input and output optical paths (2α). More specifically, the relationship is given approximately as:  
         D   F     =     2      α                           
 
         [0111]    where F is the focal length of the GRIN lens  990 .  
         [0112]    Of course other variations in the optical arrangement are possible. For example, in some instances, it is preferred that the diffraction grating  920  is disposed at an angle to the focal plane  925 . In addition, the placement of the front end unit/lens  990 , diffraction grating  920 , and modifying means  950  can be selected to minimize aberrations associated with the periphery of the element having optical power  910 . In FIG. 10, an alternative design of FIG. 9, wherein the element having optical power is a lens  910  having two focal planes,  925   a  and  925   b  is illustrated. The diffraction grating  920  is coincident with focal plane  925   b  and the reflector  940  is coincident with focal plane  925   a.  The operation is similar to that discussed for FIG. 9.  
         [0113]    An advantage of the embodiments including a GRIN lens  990 , e.g. as shown in FIG. 9- 9   d,  is that they are compatible with modifying means based on MEMS technology, for both COADM and DGE applications. This is in contrast to the prior art optical arrangements described in FIGS. 1 and 6- 8 , wherein the MEMS based modifying means  150  are preferred for DGE applications over COADM applications.  
         [0114]    In particular, when the single collimating/focusing lens  990  provides the input beam of light and receives the modified output beam of light, the angular displacement provided by each MEMS reflector complements the angular displacement resulting from the use of the off-axis input/output port(s) on the GRIN lens  990 . More specifically, the angular displacement provided by the lens  990 , e.g. α, is chosen in dependence upon the angular displacement of the MEMS device, e.g. 1°.  
         [0115]    A preferred embodiment is illustrated in FIG. 11, wherein an arrangement similar to that shown in FIG. 9 designed to include COADM functionality. Optical circulators  80   a  and  80   b  are coupled to each of the optical waveguides  99   a  and  99   b , respectively, for separating in/out and add/drop optical signals. Optical waveguides  99   a  and  99   b  are optically coupled to micro-lenses  12   a  and  12   b  disposed on one side of the lens  90 .  
         [0116]    The lens  90  is disposed such that an end thereof lies in the focal plane  25  of the spherical reflector  10 . Also in the focal plane are the dispersive element  20  and the modifying means  50 , as described above. However, in this embodiment, the modifying means is preferably a MEMS array  50 . Notably, the MEMS array provides a 2×2 bypass configuration wherein an express signal launched into port  1  of the circulator  80   a  propagates to port  3  of the same circulator  80   a  in a first mode of operation and a dropped signal propagates to port  3  of the second circulator  80   b  in a second mode of operation. Similarly, a signal added at port  1  of the second circulator device  80   b  propagates to port  3  of the first circulator  80   a  in the second mode of operation, but is not collected in the first mode of operation. For exemplary purposes, the beam of light is assumed to include wavelengths λ 1  and λ 2 , however, in practice more wavelengths are typically used.  
         [0117]    In operation, a beam of light carrying wavelengths λ 1  and λ 2 , is launched into port  1  of the first optical circulator  80   a  and is circulated to optical waveguide  99   a  supported by sleeve  96 . The beam of light is transmitted through the micro-lens  12   a  to the lens  90 , in a direction substantially parallel to the optical axis (OA 2 ) of the lens  90 . The beam of light enters the lens  90  through port  85  disposed off the optical axis (OA 2 ) and emerges from port  92  coincident with the optical axis (OA 2 ) at an angle to the optical axis (OA 2 ). The emerging beam of light λ 1 λ 2 , is transmitted to an upper portion of the spherical reflector  10 , is reflected, and is incident on the diffraction grating  20 , where it is spatially dispersed into two sub-beams of light carrying wavelengths λ 1  and λ 2 , respectively. Each sub-beam of light is transmitted to a lower portion of the spherical reflector  10 , is reflected, and is transmitted to separate reflectors  51  and  52  of the MEMS array  50 . Referring to FIG. 11, reflector  51  is orientated such that the sub-beam of light corresponding to λ 1  incident thereon, is reflected back along the same optical path to the lens  90 , passes through port  85  again, and propagates to port  2  of circulator  80   a  where it is circulated to port  3 . Reflector  52 , however, is orientated such that the sub-beam of light corresponding to λ 2  is reflected back along a different optical path. Accordingly, the dropped signal corresponding to wavelength λ 2  is returned to the lens  90 , passes through port  87 , propagates to port  2  of the second circulator  80   b , and is circulated to port  3 .  
         [0118]    Simultaneously, a second beam of light having central wavelength λ 2  is added into port  1  of the second optical circulator  80   b  and is circulated to optical waveguide  99   b . The second beam of light λ 2  is transmitted through the micro-lens  12   b  to the lens  90 , in a direction substantially parallel to the optical axis (OA 2 ) of the lens  90 . It enters the lens  90  through port  87  disposed off the optical axis (OA 2 ) and emerges from port  92  coincident with the optical axis (OA 2 ) at an angle to the optical axis. The emerging beam of light is transmitted to an upper portion of the spherical reflector  10 , is reflected, and is incident on the diffraction grating  20 , where it is reflected to reflector  52  of the MEMS array  50 . Reflector  52  is orientated such that the second beam of light corresponding to λ 2  is reflected back along a different optical path to the spherical reflector  10 , where it is directed to the diffraction grating  20 . At the diffraction grating  20 , the added optical signal corresponding to λ 2  is combined with the express signal corresponding to λ 1 . The multiplexed signal is returned to the lens  90 , passes through port  85 , and returns to port  2  of the first circulator  80   a  where it is circulated out of the device from port  3 .  
         [0119]    Of course, numerous other embodiments may be envisaged, without departing from the spirit and scope of the invention. For example, in practice it is preferred that each reflector of the MEMS array is deflected between positions non-parallel to focal plane  25 , i.e. the deflection is not equivalent to the 45° and 0° deflections illustrated heretofore. In these instances, it is preferred that the optical waveguides coupled to the lens  90  be asymmetrically disposed about the optical axis OA 2 , as illustrated in FIG. 9 d.  For example, FIG. 12 illustrates how strategic placement of the optical waveguides  99  and  98  can complement the angular displacement provided by the MEMS reflector  51 . Moreover, it is also within the scope of the instant invention for the MEMS array to flip in either a horizontal or vertical direction, relative to the dispersion plane. Furthermore, any combination of the above embodiments and/or components are possible, such as a dispersion compensator/COADM combination in which an array of MEMS reflectors redirect individual sub-beams during or after passage through a tunable etalon. Similarly, a dispersion compensator/DGE combination is also possible.