Patent Publication Number: US-2005123241-A1

Title: Polarization independent frequency selective optical coupler

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority from U.S. Provisional Patent Application Ser. No. 60/526,277 filed Dec. 3, 2003 entitled “Integrated Bi-directional Transceiver Planar Lightwave Circuit”; and U.S. Provisional Patent Application Ser. No. 60/543,262 filed Feb. 11, 2004 entitled “Polarization Independent Frequency Selective Optical Coupler”; the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The invention relates generally to the field of planar lightwave circuits and in particular to a polarization independent frequency selective optical coupler.  
      In recent years fiber to the home (FTTH) has become popular as a means for supplying broadband communications services to the home user. To implement this technology at minimum cost, a single optical fiber is utilized bi-directionally by providing for optical transmission at a different wavelength in each direction. Typically, transmission downstream, which is from the central office to the home, is accomplished at a wavelength of approximately 1.5 μm, and transmission upstream, which is from the home to the central office, is accomplished at a wavelength of approximately 1.3 μm. In an exemplary embodiment, downstream transmission is accomplished at multiple wavelengths in the vicinity of 1.5 μm. Selection of the wavelengths used for the downstream and upstream transmission is primarily a function of the economics of the transmission sources, with transmitting lasers at 1.5 μm being more expensive than transmitting lasers at 1.3 μm, and the need to multiplex and subsequently demultiplex the combined signals at a low cost.  
      Implementation of such an FTTH scheme requires the installation of a bi-directional optical transceiver at each customer premises, thus reducing the cost of such a bi-directional optical transceiver is a major factor in the cost of implementation of FTTH solutions.  
      Prior art bi-directional optical transceivers are typically comprised of multiple discrete elements including: a transmitter such as a laser diode; a detector such as a photo-detector; and an optical filter. In the event of multiple downstream wavelengths, multiple detectors are typically required. Each of these components, typically packaged in a transistor outline metal can, is assembled into a bi-directional optical transceiver. Assembly of such a device is costly, in particular in light of the added difficulty and cost of alignment of multiple optical parts. Thus it is desirable to reduce the cost of a bi-directional optical transceiver by utilizing planar lightwave circuits.  
      Planar lightwave circuits are typically formed on a substrate. Various types of waveguides having differing refractive index are known to the prior art. For the purposes of this document, the refractive index of a waveguide will be defined at a wavelength of 1.5 μm. Silicon dioxide (SiO 2 ) is often used as a cladding material and exhibits a refractive index of 1.4445. A waveguide material known to the prior art is SiON, which can be deposited in a range of refractive indexes, typically from 1.48-1.55. SiON is hereinafter termed a low index waveguide material. A high index waveguide material known to the prior art is Si 3 N 4 , which can be deposited in a range of refractive indexes, typically from 2.0-2.2. Other high index waveguide materials are known to those skilled in the art. For the purposes of this document, waveguides whose refractive indexes are in excess of 1.6 are hereinafter called high index waveguides.  
      Planar waveguides exhibit both a height and a width. In order for a waveguide to be polarization independent, that is to exhibit the same effective refractive index for both the TE and TM modes, it is necessary for the height and width of the planar waveguide to be nearly identical. Practically, this is not currently economically achievable, in particular for Si 3 N 4  waveguides that are commercially restricted to a width range of 1-2 μm and a height range of 0.05-0.3 μm. It is difficult to produce a waveguide exhibiting a width of less than 0.5 μm.  
      One design consideration for planar waveguide circuits is whether the planar waveguide will be single mode or multimode over the desired waveband. It is to be noted that as the refractive index of the waveguide core material increases the planar waveguide supports multiple modes, i.e. modes in addition to the fundamental TM and TE modes, for a decreasing height and width. Thus, modifying the waveguide height and width may change the waveguide from single mode to multi-mode. It is to be understood that the term single mode operation includes both the TM and TE modes. The single mode having both a TM and TE mode is sometimes referred to as the fundamental mode. In multiple mode operation, or multi-mode as it is sometimes referred to, both a TM and TE mode exist for both the fundament mode and for each present high order mode.  
      A further design consideration for planar waveguide circuits is the effective refractive index, indicated hereinafter as N eff , of the planar waveguide for a given mode. For a core material with a given refractive index, the larger the dimensions in terms of height or width, the larger the effective refractive index, until the effective refractive index approaches the material refractive index. It is to be noted however, that as indicated above for commercially producible planar waveguides, and in particular high index waveguides, the N eff  of the TM and TE modes differ.  
      The N eff  of a planar waveguide is a function of wavelength, denoted λ, with a shorter wavelength experiencing a larger N eff  and a longer wavelength experiencing a smaller N eff . For a waveguide in which the N eff  is significantly less than the material refractive index due to the dimensions of the waveguide, changing the height or width of the waveguide affects the slope of the relationship between N eff  and λ. In particular, for a waveguide core having a given refractive index and width in which N eff  is a function of λ, increasing the height will increase the slope.  
      Planar waveguide circuits known to the prior art include couplers formed by placing two waveguides in close vicinity of one another so that their respective mode profiles overlap each other to form an evanescent coupler region. The transfer of energy is determined by the coupled mode wave equations and is a function of N eff  of each of the waveguides, as described in detail in “Optical Electronics in Modern Communications”, Oxford University Press (1977), 5 th  edition, at page 522, section 13.8 whose contents are incorporated by reference. Effective coupling is achieved when the effective indexes of the two waveguides match.  
      An article entitled “Integrated Optic Adiabatic Devices on Silicon” by Y. Shani et al. published in the IEEE Journal of Quantum Electronics, Vol. 27, No. 3, Page 556-566, March 1991, whose contents are incorporated herein by reference, describes an asymmetric y-coupler for polarization splitting; an adiabatic full coupler; an adiabatic 3 db coupler and an asymmetric y-coupler for a 1.3-1.55 μm multiplexer. The asymmetric y-coupler is not polarization independent. The article further describes a polarization splitter using a birefringent high index waveguide, and an improved performance splitter utilizing double filtering. A wavelength division multiplexer based on an adiabatic Y-branch is further described. Unfortunately, such a wavelength division multiplexer is not polarization independent.  
      It is understood by those skilled in the art that a wavelength division multiplexer is a specific example of a frequency selective optical coupler.  
      Grating-assisted couplers are known to the art and described for example in an article entitled “Grating-Assisted Codirectional Coupler Filter Using Electrooptic and Passive Polymer Waveguides” by S. Ahn et al. published in the IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 5, September/October 2001, Pages 819-825 whose contents are incorporated herein by reference.  
      Thus there is a need for an improved PLC based polarization independent frequency selective optical coupler.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art PLC based polarization independent frequency selective optical couplers. This is provided in the present invention by a polarization independent, frequency selective optical coupler for coupling an optical signal having at least one wavelength and an arbitrary polarization state, the polarization independent, frequency selective optical coupler comprising: a first waveguide; and at least one additional waveguide in close proximity to the first waveguide thus forming at least one evanescent coupling region; the at least one evanescent coupling region exhibiting a first phase match condition coupling the TM mode of the optical signal and a second phase match condition coupling the TE mode of the optical signal, the first phase match condition being different than the second phase match condition. The phase match conditions are in one embodiment a function of the size of the waveguides, and in another embodiment a function of at least one grating written thereon.  
      The invention provides for a polarization independent, frequency selective coupler for coupling an optical signal having at least one wavelength and an arbitrary polarization state, the polarization independent, frequency selective optical coupler comprising: a first waveguide; and at least one second waveguide, a first portion of the at least one second waveguide being in close proximity to the first waveguide thus forming at least one evanescent coupling region, the at least one evanescent coupling region exhibiting a first phase match condition coupling the TM mode of an optical signal propagating in the first waveguide to the at least one second waveguide and a second phase match condition coupling the TE mode of the optical signal propagating in the first waveguide to the at least one second waveguide, the first phase match condition being different than the second phase match condition.  
      In one embodiment the polarization independent frequency selective coupler further comprises a first grating written on a first portion of the at least one second waveguide within the at least one evanescent coupling region, the first grating being associated with the first phase match condition, and a second grating written on a second portion of the at least one second waveguide within the at least one evanescent coupling region, the second grating being associated with the second phase match condition. In a further embodiment the first grating exhibits a first period and the second grating exhibits a second period, the first period being different than the second period. In one yet further embodiment the first and second gratings at least partially overlap. In another yet further embodiment the first portion is substantially identical with the second portion, the first grating being superimposed over the second grating.  
      In one embodiment the at least one second waveguide forming the at least one evanescent coupling region comprises a first sub-portion having a first characteristic height and width being associated with the first phase match condition and a second sub-portion having a second characteristic height and width being associated with the second phase match condition. In a further embodiment the polarization independent frequency selective coupler further comprises a grating written on at least a portion of one of the first sub-portion and the second sub-portion. In a yet further embodiment the grating exhibits a uniform period over the at least a portion of one of the first sub-portion and the second sub-portion, and optionally the first waveguide comprises core material exhibiting a refractive index between 1.48 and 1.55 at 1.5 μm and the at least one second waveguide comprises core material exhibiting a refractive index in excess of 1.6 at 1.5 μm.  
      In one embodiment the at least one second waveguide forming the at least one evanescent coupling region comprises a first sub-portion having a first characteristic height and width being associated with the first phase match condition and a second sub-portion having a second characteristic height and width being associated with the second phase match condition, and optionally a) wherein the first waveguide comprises core material exhibiting a refractive index between 1.48 and 1.55 at 1.5 μm and the at least one second waveguide comprises core material exhibiting a refractive index in excess of 1.6 at 1.5 μm or b) wherein the first waveguide comprises core material exhibiting a refractive index in excess of 1.6 at 1.5 μm and the at least one second waveguide comprises core material exhibiting a refractive index between 1.48 and 1.55 at 1.5 μm.  
      In one embodiment wherein the optical signal comprises at least one wavelength, the at least one second waveguide supports at least one high order mode at the at least one wavelength. In one further embodiment at least one of the first phase match condition coupling the TM mode and the second phase match condition coupling the TE mode is associated with the at least one high order mode. In another further embodiment the at least one second waveguide forming the at least one evanescent coupling region comprises a first sub-portion having a first characteristic height and width being associated with the first phase match condition and a second sub-portion having a second characteristic height and width being associated with the second phase match condition. Optionally, at least one of the first phase match condition coupling the TM mode and the second phase match condition coupling the TE mode is associated with the first sub-portion and is further associated with the at least one high order mode.  
      In one embodiment the optical signal comprises at least one wavelength; the at least one second waveguide forming the at least one evanescent coupling region supports, at the at least one wavelength, at least two modes, at least one of the at least two modes being a high order mode; the at least one second waveguide comprising: a first sub-portion having a first characteristic height and width being associated with the first phase match condition coupling the TM mode and being further associated with a first one of the at least two modes; and a second sub-portion having a second characteristic height and width being associated with the second phase match condition coupling the TE mode and being further associated with a second one of the at least two modes; the second one of the at least two modes being different than the first one of the at least two modes.  
      In one embodiment the first waveguide comprises core material exhibiting a refractive index between 1.48 and 1.55 at 1.5 μm. In another embodiment the at least one second waveguide comprises core material exhibiting a refractive index in excess of 1.6 at 1.5 μm. In yet another embodiment the at least one second waveguide comprises core material exhibiting a refractive index between 2.0 and 2.2 at 1.5 μm. In yet another embodiment the first waveguide comprises core material exhibiting a refractive index between 1.48 and 1.55 at 1.5 μm and the at least one second waveguide comprises core material exhibiting a refractive index between 2.0 and 2.2 at 1.5 μm.  
      In one embodiment the first waveguide comprises core material exhibiting a refractive index between 2.0 and 2.2 at 1.5 μm and the at least one second waveguide comprises core material exhibiting a refractive index between 2.0 and 2.2 at 1.5 μm. In a further embodiment the height of the first waveguide is between 0.15 and 0.3 microns and the width of the first waveguide is between 0.8 and 1.3 microns. In a yet further embodiment the height of the at least one second waveguide is between 0.15 and 0.3 microns and the width of the at least one second waveguide is between 2 and 7 microns.  
      In one embodiment the at least one second waveguide comprises two waveguides. In a further embodiment a first one of the two waveguides is associated with the TM mode. In a yet further embodiment a second one of the two waveguides is associated with the TE mode. In another further embodiment a first one of the two waveguides is associated with the TM mode, and a second one of the two waveguides is associated with the TE mode. In a yet further embodiment the polarization independent frequency selective coupler further comprises an output waveguide, the output waveguide being in close proximity to a second portion of the first one of the two waveguides forming an evanescent coupling region, the output waveguide further being in close proximity to a second portion of the second one of the two waveguides forming an evanescent coupling region. Optionally, the length of the first one of the second two waveguides and the second one of the second two waveguides is selected so that the propagation time of the TM mode and the TE mode of the optical signal from the input of the first waveguide to the output of the output waveguide are substantially equivalent.  
      The invention also provides for a method of polarization independent frequency selective coupling for an optical signal having at least one wavelength and an arbitrary polarization state, the polarization independent frequency selective optical coupling comprising: coupling by a first phase match condition the TM mode of the optical signal in at least one evanescent coupling region; and coupling by a second phase match condition the TE mode of the optical signal in the at least one evanescent coupling region, the first phase match condition being different from the second phase match condition.  
      The invention also provides for a polarization independent, frequency selective coupler for coupling an optical signal having at least one wavelength and an arbitrary polarization state, the polarization independent, frequency selective optical coupler comprising: a first waveguide; and a second waveguide, a first portion of the second waveguide being in close proximity to the first waveguide thus forming an evanescent coupling region, the evanescent coupling region exhibiting a first phase match condition coupling the TM mode of an optical signal propagating in the first waveguide to the second waveguide and a second phase match condition coupling the TE mode of the optical signal propagating in the first waveguide to the second waveguide, the first phase match condition being different than the second phase match condition.  
      In one embodiment at least one of the first and second phase match conditions are a function of one of a grating, a high order mode and a characteristic height and width.  
      The invention also provides for a polarization independent, frequency selective coupler for coupling an optical signal having at least one wavelength and an arbitrary polarization state, the polarization independent, frequency selective optical coupler comprising: a first waveguide acting as an input waveguide; a second waveguide; a third waveguide; and a fourth waveguide acting as an output waveguide; a first portion of the second waveguide being in close proximity to the first waveguide thus forming an evanescent coupling region exhibiting a first phase match condition coupling the TM mode of an optical signal propagating in the first waveguide to the second waveguide, a second portion of the second waveguide being in close proximity to the fourth waveguide thus forming an evanescent coupling region; a first portion of the third waveguide being in close proximity to the first waveguide thus forming an evanescent coupling region exhibiting a second phase match condition coupling the TE mode of the optical signal propagating in the first waveguide to the third waveguide, a second portion of the second waveguide being in close proximity to the fourth waveguide thus forming an evanescent coupling region, the first phase match condition being different than the second phase match condition.  
      In one embodiment the lengths of the second waveguide and the third waveguides are selected so that the propagation time of the TM mode and the TE mode of the optical signal from the input of the first waveguide to the output of the fourth waveguide are substantially equivalent.  
      Additional features and advantages of the invention will become apparent from the following drawings and description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.  
      With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:  
       FIG. 1   a  illustrates an embodiment of a bi-directional optical transceiver planar lightwave circuit (PLC) structure having a single downstream wavelength in accordance with the principle of the present invention;  
       FIG. 1   b  illustrates another embodiment of a bi-directional optical transceiver planar lightwave circuit (PLC) structure having a single downstream wavelength in accordance with the principle of the present invention;  
       FIG. 2   a  illustrates a first embodiment of a bi-directional optical transceiver PLC structure having multiple downstream wavelengths in accordance with the principles of the current invention;  
       FIG. 2   b  illustrates a second embodiment of a bi-directional optical transceiver PLC structure having multiple downstream wavelengths in accordance with the principles of the current invention;  
       FIG. 2   c  illustrates a third embodiment of a bi-directional optical transceiver PLC structure having multiple downstream wavelengths in accordance with the principles of the current invention;  
       FIG. 2   d  illustrates a fourth embodiment of a bi-directional optical transceiver PLC structure having multiple downstream wavelengths in accordance with the principles of the current invention;  
       FIG. 2   e  illustrates a fifth embodiment of a bi-directional optical transceiver PLC structure having multiple downstream wavelengths in accordance with the principles of the current invention;  
       FIG. 3   a  illustrates a high level schematic diagram of an embodiment of the frequency selective optical coupler of  FIGS. 1   a - 1   b  and  2   a - 2   c  in accordance with the principle of the current invention;  
       FIG. 3   b  illustrates a high level schematic diagram of an embodiment of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  in accordance with the principle of the current invention;  
       FIG. 3   c  illustrates a high level schematic diagram of an embodiment of the polarization independent frequency selective dual optical coupler of  FIGS. 2   c - 2   e  in accordance with the principle of the current invention;  
       FIG. 4   a  illustrates a high level schematic diagram of a first embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention;  
       FIG. 4   b  illustrates a high level schematic diagram of a second embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention;  
       FIG. 4   c  illustrates a high level schematic diagram of a third embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention;  
       FIG. 4   d  illustrates a high level schematic diagram of a fourth embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention;  
       FIG. 4   e  illustrates a high level schematic diagram of a fifth embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention;  
       FIG. 4   f  illustrates a high level schematic diagram of a sixth embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention; and  
       FIG. 4   g  illustrates a high level schematic diagram of a seventh embodiment of an evanescent coupling region of the polarization independent frequency selective optical coupler of  FIGS. 2   a ,  2   b  and  3   b  and of the polarization independent frequency selective dual optical coupler of  FIG. 2   c - 2   e  and  3   c  in accordance with the principle of the current invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present embodiments enable a PLC based polarization independent frequency selective optical coupler. In one embodiment the PLC based polarization independent frequency selective optical coupler comprises a first waveguide and a second waveguide forming an evanescent coupling region, the first and second waveguides exhibiting a plurality of unique coupling conditions. One coupling condition is operable on the TM mode, and a separate different coupling condition is operable on the TE mode. In another embodiment the PLC based polarization independent frequency selective optical coupler comprises a first waveguide and a second set of waveguides, each waveguide of the second set of waveguides forming an evanescent coupling region with the first waveguide, and each of the coupling waveguides being associated with one of the TM and TE modes.  
      Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.  
       FIG. 1   a  illustrates an embodiment of a bi-directional optical transceiver planar lightwave circuit (PLC) structure in accordance with the principle of the invention, generally denoted  10 , comprising: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; frequency selective optical coupler  30 ; planar waveguide  40 ; high index planar waveguide  50 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; and detector  70 , which in a preferred embodiment comprises a photo-detector. Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of frequency selective optical coupler  30 . One end of high index planar waveguide  50  is connected to a second port of frequency selective optical coupler  30  and a second end of high index planar waveguide  50  is connected to transmitter  60 . One end of planar waveguide  40  is connected to a third port of frequency selective optical coupler  30 , and a second end of planar waveguide  40  is connected to detector  70 .  
      In operation, an incoming optical signal propagating through input/output optical fiber  14  is connected through fiber attachment  16  to planar waveguide  20 . Fiber attachment  16  is preferably an embedded pigtail assembly. The incoming optical signal represents downstream transmission at a first wavelength, typically on the order of 1.5 μm, and propagates through planar waveguide  20  to frequency selective optical coupler  30 . The incoming optical signal propagates through frequency selective optical coupler  30  to planar waveguide  40  and ultimately to detector  70 . Transmitter  60  is operable to transmit an upstream optical signal at a second wavelength, typically on the order of 1.3 μm, the second wavelength being distinct and separated from the first wavelength. The upstream optical signal output from transmitter  60  propagates through high index planar waveguide  50  to frequency selective optical coupler  30 , and is coupled through frequency selective optical coupler  30  to planar waveguide  20 . It is to be noted that transmitter  60 , which typically comprises a laser diode, operates with a specific polarity. Thus, in an exemplary embodiment, frequency selective optical coupler  30  is a polarization dependent coupler that is selected to couple optical signals having the polarization output by transmitter  60  to planar waveguide  20 . Generally, frequency selective optical coupler  30  couples the downstream wavelength to planar waveguide  40 , and the upstream wavelength from high index planar waveguide  50  to planar waveguide  20 . The upstream optical signal propagates through first planar waveguide  20  through connector  16 , and propagates through input/output optical fiber  14 . Thus device  10  of  FIG. 1   a  is thus operable to supply bi-directional optical transmission at two distinct wavelengths.  
       FIG. 1   b  illustrates another embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  80 , comprising: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; frequency selective optical coupler  30 ; planar waveguide  40 ; extinction enhancement grating  90 ; high index planar waveguide  50 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; and detector  70 , which in a preferred embodiment comprises a photo-detector. Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of frequency selective optical coupler  30 . One end of high index planar waveguide  50  is connected to a second port of frequency selective optical coupler  30  and a second end of high index planar waveguide  50  is connected to transmitter  60 . One end of planar waveguide  40  is connected to a third port of frequency selective optical coupler  30 , and a second end of planar waveguide  40  is connected to detector  70 . Extinction enhancement grating  90  is written on planar waveguide  40  between frequency selective optical coupler  30  and detector  70 .  
      In operation bi-directional optical transceiver PLC structure  80  operates in a manner similar to that described above in relation to bi-direction optical transceiver PLC structure  10  of  FIG. 1   a , with an improved signal to noise (S/N) ratio as a result of the operation of extinction enhancement grating  90 . Extinction enhancement grating  90  is preferably selected to be a notch filter suppressing all but the designated downstream wavelength and passing only the designated downstream wavelength to detector  70 .  
       FIG. 2   a  illustrates a first embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  100 , supporting two downstream wavelengths. PLC structure  100  comprises: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; frequency selective optical coupler  30 ; planar waveguide  40 ; high index planar waveguide  50 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; first and second detectors  70 , which in a preferred embodiment each comprise a photo-detector; polarization independent frequency selective optical coupler  110 ; and high index planar waveguide  120 . Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of frequency selective optical coupler  30 . One end of high index planar waveguide  50  is connected to a second port of frequency selective optical coupler  30  and a second end of high index planar waveguide  50  is connected to transmitter  60 . A second port of frequency selective optical coupler  30  is connected to a first port of polarization independent frequency selective optical coupler  110 , preferably through a planar waveguide. One end of high index planar waveguide  120  is connected to a second port of polarization independent frequency selective optical coupler  110 , and a second end high index planar waveguide  120  is connected to first detector  70 . One end of planar waveguide  40  is connected to a third port of polarization independent frequency selective optical coupler  110 , and a second end of planar waveguide  40  is connected to second detector  70 .  
      In operation, an incoming optical signal propagating through input/output optical fiber  14  is connected through fiber attachment  16  to planar waveguide  20 . Fiber attachment  16  is preferably an embedded pigtail assembly. The incoming optical signal comprises downstream transmission at a first and second wavelength, with a first wavelength being typically on the order of 1.5 μm, and a second wavelength being typically on the order of 1.49 μm. The incoming optical signal, comprising first and second wavelengths, propagates through planar waveguide  20  to frequency selective optical coupler  30  and propagates through frequency selective optical coupler  30  to polarization independent frequency selective optical coupler  110 . Polarization independent frequency selective optical coupler  110  is operable to couple out a first downstream wavelength to high index planar waveguide  120 , and the first downstream wavelength thus propagates through high index planar waveguide  120  to detector  70 . The second downstream wavelength passes through polarization independent frequency dependent optical coupler  110  to planar waveguide  40  and ultimately to second detector  70 . Transmitter  60  is operable to transmit an upstream optical signal at a third wavelength, typically on the order of 1.3 μm, the third wavelength being distinct and separated from the first and second wavelengths. The upstream optical signal output from transmitter  60  propagates through high index planar waveguide  50  to frequency selective optical coupler  30 , and is coupled through frequency selective optical coupler  30  to planar waveguide  20 . It is to be noted that transmitter  60 , which typically comprises a laser diode, operates with a specific polarity. Thus, in an exemplary embodiment, frequency selective optical coupler  30  is a polarization dependent coupler that is selected to couple optical signals having the polarization output by transmitter  60  to planar waveguide  20 . Generally, frequency selective optical coupler  30  couples first and second downstream wavelengths to polarization independent frequency selective optical coupler  110 , and the upstream wavelength from high index planar waveguide  50  to planar waveguide  20  The upstream optical signal propagates through first planar waveguide  20  through connector  16 , and propagates through input/output optical fiber  14 .  
      It is to be noted that the architecture of bi-directional optical transceiver PLC structure  100  is distinctive in having upstream frequency selective optical coupler  30  closer to input/output optical fiber  14  than polarization independent frequency selective optical coupler  110 , and thus the output of transmitter  60  does not propagate through polarization independent frequency selective optical coupler  110 . Thus device  100  of  FIG. 2   a  is operable to supply bi-directional optical transmission at two distinct downstream wavelengths and a separate distinct upstream wavelength.  
       FIG. 2   b  illustrates a second embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  150 , supporting two downstream wavelengths. PLC structure  150  comprises: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; frequency selective optical coupler  30 ; planar waveguide  40  having extinction enhancement grating  90  written on a portion thereof; high index planar waveguide  50 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; first and second detectors  70 , which in a preferred embodiment each comprise a photo-detector; polarization independent frequency selective optical coupler  110 ; and high index planar waveguide  120 . Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of frequency selective optical coupler  30 . One end of high index planar waveguide  50  is connected to a second port of frequency selective optical coupler  30  and a second end of high index planar waveguide  50  is connected to transmitter  60 . A second port of frequency selective optical coupler  30  is connected to a first port of polarization independent frequency selective optical coupler  110 , preferably through a planar waveguide. One end of high index planar waveguide  120  is connected to a second port of polarization independent frequency selective optical coupler  110 , and a second end high index planar waveguide  120  is connected to first detector  70 . One end of planar waveguide  40  is connected to a third port of polarization independent frequency selective optical coupler  110 , and a second end of planar waveguide  40  is connected to second detector  70 . Extinction enhancement grating  90  is written on a portion of planar waveguide  40  between second detector  70  and polarization independent frequency selective optical coupler  110 .  
      In operation bi-directional optical transceiver PLC structure  150  operates in a manner similar to that described above in relation to bi-direction optical transceiver PLC structure  100  of  FIG. 2   a , with an improved S/N ratio as a result of the operation of extinction enhancement grating  90 . Extinction enhancement grating  90  is preferably selected to be a notch filter suppressing all but the designated downstream wavelength destined for second detector  70  and passing only the designated downstream wavelength to second detector  70 .  
       FIG. 2   c  illustrates a third embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  200 , supporting two downstream wavelengths. PLC structure  200  comprises: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; frequency selective optical coupler  30 ; high index planar waveguide  50 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; first and second detectors  70 , which in a preferred embodiment each comprise a photo-detector; polarization independent frequency selective dual optical coupler  210 ; and first and second high index planar waveguides  120 . Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of frequency selective optical coupler  30 . One end of high index planar waveguide  50  is connected to a second port of frequency selective optical coupler  30  and a second end of high index planar waveguide  50  is connected to transmitter  60 . A second port of frequency selective optical coupler  30  is connected to a first port of polarization independent frequency selective dual optical coupler  210 , preferably through a planar waveguide. One end of first high index planar waveguide  120  is connected to a second port of polarization independent frequency selective dual optical coupler  210 , and a second end of first high index planar waveguide  120  is connected to first detector  70 . One end of second high index planar waveguide  120  is connected to a third port of polarization independent frequency selective dual optical coupler  210 , and a second end of second high index planar waveguide  120  is connected to second detector  70 . A fourth port of frequency selective dual optical coupler  220 , the through port, is unused.  
      In operation, an incoming optical signal propagating through input/output optical fiber  14  is connected through fiber attachment  16  to planar waveguide  20 . Fiber attachment  16  is preferably an embedded pigtail assembly. The incoming optical signal comprises downstream transmission at a first and second wavelength, with a first wavelength being typically on the order of 1.5 μm, and a second wavelength being typically on the order of 1.49 μm. The downstream optical signal, comprising first and second wavelengths, propagates through planar waveguide  20  to frequency selective optical coupler  30  and through frequency selective optical coupler  30  to polarization independent frequency selective dual optical coupler  210 . Polarization independent frequency selective dual optical coupler  210  is operable to couple out a first downstream wavelength to first high index planar waveguide  120 , and a second downstream wavelength to second high index planar waveguide  120 . The first downstream wavelength thus propagates through first high index planar waveguide  120  to first detector  70  and the second downstream wavelength thus propagates through second high index planar waveguide  120  to second detector  70 . Any remaining downstream optical signal not coupled to first and second high index planar waveguide  120  is dissipated in unconnected fourth port  220 .  
      Transmitter  60  is operable to transmit upstream signals at a third wavelength, typically on the order of 1.3 μm, the third wavelength being distinct and separated from the first and second wavelengths. The upstream optical signal output from transmitter  60  propagates through high index planar waveguide  50  to frequency selective optical coupler  30 , and is coupled through frequency selective optical coupler  30  to planar waveguide  20 . It is to be noted that transmitter  60 , which typically comprises a laser diode, outputs an optical signal exhibiting a specific polarity. Thus, in an exemplary embodiment, frequency selective optical coupler  30  is a polarization dependent coupler that is selected to couple optical signals having the polarization of the output optical signal of transmitter  60  to planar waveguide  20 . Generally, frequency selective optical coupler  30  couples first and second downstream wavelengths to polarization independent frequency selective dual optical coupler  210 , and the upstream wavelength from high index planar waveguide  50  to planar waveguide  20 . The upstream optical signal propagates through first planar waveguide  20  through connector  16 , and propagates through input/output optical fiber  14 . It is to be noted that the architecture of bi-directional optical transceiver PLC structure  200  is distinctive in having upstream frequency selective optical coupler  30  closer to input/output optical fiber  14  than polarization independent frequency selective dual optical coupler  210 , and thus the output of transmitter  60  does not propagate through polarization independent frequency selective dual optical coupler  210 . Furthermore a single polarization independent dual frequency selective optical coupler  210  saves space and couples out only the desired first downstream wavelength to first detector  70  and second downstream wavelength to second detector  70 . Thus device  200  of  FIG. 2   c  is operable to supply bi-directional optical transmission at two distinct downstream wavelengths and a separate distinct upstream wavelength.  
       FIG. 2   d  illustrates a fourth embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  250 , supporting two downstream wavelengths. PLC structure  250  comprises: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; polarization independent frequency selective dual optical coupler  210 ; planar waveguide  40 ; first and second high index planar waveguides  120 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; and first and second detectors  70 , which in a preferred embodiment each comprise a photo-detector. Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of polarization independent frequency selective dual optical coupler  210 . One end of planar waveguide  40  is connected to a second port, the through port, of polarization independent frequency selective dual optical coupler  210 , and a second end of planar waveguide  40  is connected to transmitter  60 . One end of first high index planar waveguide  120  is connected to a third port of polarization independent frequency selective dual optical coupler  210 , and a second end of first high index planar waveguide  120  is connected to first detector  70 . One end of second high index planar waveguide  120  is connected to a fourth port of polarization independent frequency selective dual optical coupler  210 , and a second end of second high index planar waveguide  120  is connected to second detector  70 .  
      In operation, an incoming optical signal propagating through input/output optical fiber  14  is connected through fiber attachment  16  to planar waveguide  20 . Fiber attachment  16  is preferably an embedded pigtail assembly. The incoming optical signal comprises downstream transmission at a first and second wavelength, with a first wavelength being typically on the order of 1.5 μm, and a second wavelength being typically on the order of 1.49 μm. The incoming optical signal, comprising first and second wavelengths, propagates through planar waveguide  20  to polarization independent frequency selective dual optical coupler  210 . Polarization independent frequency selective dual optical coupler  210  is operable to couple out a first downstream wavelength to first high index planar waveguide  120 , and a second downstream wavelength to second high index planar waveguide  120 . The first downstream wavelength thus propagates through first high index planar waveguide  120  to first detector  70  and the second downstream wavelength thus propagates through second high index planar waveguide  120  to second detector  70 .  
      Transmitter  60  is operable to transmit upstream signals at a third wavelength, typically on the order of 1.3 μm, the third wavelength being distinct and separated from the first and second downstream wavelengths. The upstream optical signal output from transmitter  60  propagates through planar waveguide  40  to polarization independent frequency selective dual optical coupler  210 , and is passed through to planar waveguide  20 . The upstream optical signal propagates through first planar waveguide  20 , through connector  16  and propagates through input/output optical fiber  14 . It is to be noted that the architecture of bi-directional optical transceiver PLC structure  250  is distinctive in having a single polarization independent dual frequency selective optical coupler  210  which thus saves space and is operable to couple out the desired first downstream wavelength to first detector  70  and the second downstream wavelength to second detector  70 . Thus device  250  of  FIG. 2   d  is operable to supply bi-directional optical transmission at two distinct downstream wavelengths and a separate distinct upstream wavelength.  
       FIG. 2   e  illustrates a fifth embodiment of a bi-directional optical transceiver PLC structure in accordance with the principle of the invention, generally denoted  300 , supporting two downstream wavelengths. PLC structure  300  comprises: substrate  12 ; input/output optical fiber  14 ; fiber attachment  16 ; planar waveguide  20 ; polarization independent frequency selective dual optical coupler  210 ; planar waveguide  40 ; first and second high index planar waveguides  120 ; first and second extinction enhancement gratings  90 ; transmitter  60 , which in a preferred embodiment comprises a laser diode; and first and second detectors  70 , which in a preferred embodiment each comprise a photo-detector. Input/output optical fiber  14  is connected at fiber attachment  16  to a first end of planar waveguide  20 , and a second end of planar waveguide  20  is connected to a first port of polarization independent frequency selective dual optical coupler  210 . One end of planar waveguide  40  is connected to a second port, the through port, of polarization independent frequency selective dual optical coupler  210 , and a second end of planar waveguide  40  is connected to transmitter  60 . One end of first high index planar waveguide  120  is connected to a third port of polarization independent frequency selective dual optical coupler  210 , and a second end of first high index planar waveguide  120  is connected to first detector  70 . First extinction enhancement grating  90  is written on a portion of first high index planar waveguide  120  between the third port of polarization independent frequency selective dual optical coupler  210  and first detector  70 . One end of second high index planar waveguide  120  is connected to a fourth port of polarization independent frequency selective dual optical coupler  210 , and a second end of second high index planar waveguide  120  is connected to second detector  70 . Second extinction enhancement grating  90  is written on a portion of second high index planar waveguide  120  between the fourth port of polarization independent frequency selective dual optical coupler  210  and second detector  70 .  
      In operation bidirectional optical transceiver PLC structure  300  operates in all respects in a manner similar to that described above in relation to bi-direction optical transceiver PLC structure  250  of  FIG. 2   d , with an improved S/N ratio as a result of the operation of first and second extinction enhancement grating  90 . First and second extinction enhancement gratings  90  are preferably respectively selected to be a notch filter suppressing all but the designated first and second downstream wavelengths to be detected respectively by first and second detectors  70 .  
       FIG. 3   a  illustrates a high level schematic diagram of an embodiment of frequency selective optical coupler  30  of  FIGS. 1   a - 1   b  and  2   a - 2   c  in accordance with the principle of the invention. The core area of high index planar waveguide  50  is placed in close proximity to the core area of planar waveguide  20  that continues as planar waveguide  40  defining an evanescent coupling region  350 . It is to be understood that planar waveguide  40  is an extension of planar waveguide  20 , and the term planar waveguide  40  is meant to include the portion of either planar waveguide  20  and/or planar waveguide  40  in evanescent coupling region  350 . High index planar waveguide  50  is shown curved, however this is not meant to be limiting in any way. N eff  for the TM and TE modes of high index planar waveguide  50  are dissimilar, and thus high index planar waveguide  50  is formed with the appropriate refractive index and dimensioned to exhibit an N eff  which matches the N eff  of planar waveguide  20 ,  40  in coupling region  350  for the mode of the output signal of transmitter  60 . Thus light in the mode (TM or TE) for which the N eff  of high index planar waveguide  50  matches that of planar waveguide  20 ,  40  will couple from high index planar waveguide  50  to planar waveguide  20 ,  40  over evanescent coupling region  350  and propagate to input/output optical fiber  14  of  FIGS. 1   a - 1   b  and  2   a - 2   c.    
       FIG. 3   b  illustrates a high level schematic diagram of an embodiment of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  in accordance with the principle of the invention. The core area of high index planar waveguide  120  is placed in close proximity to the core area of planar waveguide  40  defining an evanescent coupling region  360 . It is to be understood that planar waveguide  40  is an extension of planar waveguide  20 , and the term planar waveguide  40  is meant to include the portion of either planar waveguide  20  and/or planar waveguide  40  in evanescent coupling region  360 . High index planar waveguide  120  is shown curved, however this is not meant to be limiting in any way. The operation of coupling in evanescent coupling region  360  will be explained further hereinto below in reference to  FIGS. 4   a - 4   g . Evanescent coupling region  360  is shown as a single evanescent coupling region, however this is not meant to be limiting in any way. A plurality of evanescent coupling regions may be formed as will be explained further hereinto below, without exceeding the scope of the invention.  
       FIG. 3   c  illustrates a high level schematic diagram of an embodiment of polarization independent frequency selective dual optical coupler  210  of  FIGS. 2   c - 2   e  in accordance with the principle of the invention. The core area of first high index planar waveguide  120  is placed in close proximity to the core area of planar waveguide  40  defining a first evanescent coupling region  360 . It is to be understood that planar waveguide  40  is an extension of planar waveguide  20 , and the term planar waveguide  40  is meant to include the portion of either planar waveguide  20  and/or planar waveguide  40  in evanescent coupling region  360 . The core area of second high index planar waveguide  120  is placed in close proximity to the core area of planar waveguide  40  defining a second evanescent coupling region  360 . First and second high index planar waveguide  120  are shown curved, however this is not meant to be limiting in any way. The operation of coupling in first and second evanescent coupling region  360  will be explained further hereinto below in reference to the various embodiments of  FIGS. 4   a - 4   g . Each of first and second evanescent coupling regions  360  is shown as a single evanescent coupling region, however this is not meant to be limiting in any way. A plurality of evanescent coupling regions may be formed as will be explained further hereinto below, without exceeding the scope of the invention. It is to be understood that first evanescent coupling region  360  is not required to be of the same embodiment as second evanescent coupling region  360 , and may in fact be different without exceeding the scope of the invention.  
       FIG. 4   a  illustrates a high level schematic diagram of a first embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of high index planar waveguide  120  having written thereon a first grating  410  and a second grating  420 , being in close proximity to a portion of planar waveguide  40 . Gratings  410  and  420  differ in a manner to be further described hereinto below. High index planar waveguide  120  is preferably formed and dimensioned to be operative in a single mode region of operation for at least the desired downstream wavelength. Preferably, high index planar waveguide is formed so as to exhibit a steep N eff  vs. wavelength slope, thus being operable to form a discriminating filter. First grating  410  is formed to match the phase of a first one of the TM and TE modes at the desired downstream wavelength in planar waveguide  40  and high index planar waveguide  120 . Matching the phase creates a coupling condition for the mode with the matched phase. Second grating  420  is formed to match the phase of a second one of the TM and TE modes at the desired downstream wavelength in planar waveguide  40  and high index planar waveguide  120 . In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns, with a width of between 0.8 and 1.3 microns.  
      In operation, first grating  410  is operative to couple a first one of the TM and TE modes of the desired downstream wavelength propagating in planar waveguide  40  to high index planar waveguide  120 . Second grating  420  is operative to couple a second one of the TM and TE modes of the desired downstream wavelength to high index planar waveguide  120 , thus enabling a polarization independent frequency selective optical coupler. Advantageously, high index planar waveguide  120  is formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
       FIG. 4   b  illustrates a high level schematic diagram of a second embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of high index planar waveguide  120  having written thereon a combined grating  430  being in close proximity to a portion of planar waveguide  40 . High index planar waveguide  120  is formed and dimensioned to be operative in a single mode region of operation for both downstream signals.  
      Preferably, high index planar waveguide is formed so as to exhibit a steep N eff  vs. wavelength slope, thus being operable to form a discriminating filter. Combined grating  430  comprises first grating  410  and second grating  420  as described above in relation to  FIG. 4   a  superimposed on each other. The TM and TE modes are orthogonal to each other, and thus a grating written for the TM mode may be superimposed over a grating written for the TE mode without interference. In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns, with a width of between 0.8 and 1.3 microns.  
      The operation of combined grating  430  is in all respects similar to that described above in relation to gratings  410  and  420  of  FIG. 4   a , and thus the operation of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a - 2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c  as implemented utilizing the evanescent coupling region  360  of  FIG. 4   b  is thus in all respects similar to that described above in relation to the operation of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a - 2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c  as implemented utilizing the evanescent coupling region  360  of  FIG. 4   a.    
       FIG. 4   c  illustrates a high level schematic diagram of a third embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 , the portion of high index planar waveguide  120  exhibiting varying heights and/or widths defining a first region  450  and a second region  460 . An optional uniform grating  465  is written on both first region  450  and second region  460 . High index planar waveguide  120  is preferably formed and dimensioned to be operative in a single mode region of operation for at least the desired downstream wavelength over both first and second regions  450 ,  460 . The slope of the relationship between N eff  and wavelength differs for each of first and second regions  450 ,  460  and is selected in combination with the period of optional uniform grating  465 . N eff  of region  450  in combination with optional uniform grating  465  matches the phase in planar waveguide  40  for a first one of the TM and TE modes at the desired downstream wavelength. N eff  of region  460  in combination with optional uniform grating  465  matches the phase of planar waveguide  40  for a second one of the TM and TE modes at the desired downstream wavelength.  
      In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns, with a width of between 0.8 and 1.3 microns.  
      In operation, first region  450  is operative to couple a first one of the TM and TE modes of the desired downstream wavelength to high index planar waveguide  120 . Second region  460  is operative to couple a second one of the TM and TE modes of the desired downstream wavelength to high index planar waveguide  120 , thus enabling a polarization independent frequency selective optical coupler. Advantageously, high index planar waveguide  120  is formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
       FIG. 4   d  illustrates a high level schematic diagram of a fourth embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 , the portion of planar waveguide  40  exhibiting varying heights and/or widths defining a first region  480  and a second region  490 . An optional uniform grating  470  is optionally written on the portion of high index planar waveguide  120  defining evanescent coupling region  360 . High index planar waveguide  120  is preferably formed and dimensioned to be operative in a single mode region of operation for at least the desired downstream wavelength. The slope of the relationship between N eff  and wavelength differs for each of first and second regions  480 ,  490  and is selected in combination with the period of optional uniform grating  470  of high index planar waveguide  120 . N eff  of region  480  in combination with optional uniform grating  470  and the N eff  of high index planar waveguide  120  matches the phase for a first one of the TM and TE modes at the desired downstream wavelength. N eff  of region  490  in combination with optional uniform grating  470  and the N eff  of high index planar waveguide  120  matches the phase for a second one of the TM and TE modes at the desired downstream wavelength.  
      In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns, with a width of between 0.8 and 1.3 microns.  
      In operation, first region  480  is operative to couple a first one of the TM and TE modes of the desired downstream wavelength to high index planar waveguide  120 . Second region  490  is operative to couple a second one of the TM and TE modes of the desired downstream wavelength to high index planar waveguide  120 , thus enabling a polarization independent frequency selective optical coupler. Advantageously, high index planar waveguide  120  is formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
       FIG. 4   e  illustrates a high level schematic diagram of a fifth embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 , the portion of high index planar waveguide  120  exhibiting varying heights and/or widths defining a first region  500  and a second region  510 . At least a portion of high index planar waveguide  120  is formed and dimensioned to be operative in a multi-mode region of operation for at least the desired downstream wavelength. By multi-mode region of operation is meant the region of operation in which high order modes are present, typically for each high order mode both the TM and TE modes exist. Planar waveguide  40  is formed and dimensioned to be operative in the single mode region of operation. The relationship between N eff  and wavelength differs for each mode in the multi-mode operation, and exhibits a different set of relationships in each of first and second regions  500 ,  510 . The width and/or height of first region  500  is selected so that N eff  for the TM mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TM mode for one of the supported modes in first region  500 . The width and/or height of second region  510  is selected so that N eff  for the TE mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TE mode for another of the supported modes in first region  500 .  
      In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns. In such an embodiment the width of planar waveguide  40  is preferably between 0.8 and 1.3 microns and the width of high index planar waveguide  120  is between 2 and 7 microns.  
      In operation, first region  500  is operative to couple the TM mode of the desired downstream wavelength to high index planar waveguide  120 , where it propagates in the TM mode of either the fundamental or a supported high order mode. Second region  510  is operative to couple the TE mode of the desired downstream wavelength to high index planar waveguide  120 , where it propagates in the TE mode of either the fundamental or a supported high order mode, thus enabling a polarization independent frequency selective optical coupler. The TE and TM modes are each coupled in different modes supported by high index planar waveguide  120 . Advantageously, high index planar waveguide  120  is formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
      While the above has been described in an embodiment in which high index planar waveguide  120  supports multi-mode operation, it is to be understood that the requirement for multi-mode operation need not be satisfied over the entire length of high index planar waveguide  120 . In particular, one of first region  500  and second region  510  may be dimensioned to support single mode operation without exceeding the scope of the invention. In such an embodiment, a first one of the TM and TE modes is coupled into the fundamental mode supported in the single mode region of high index planar waveguide  120 , and a second one of the TM and TE modes is coupled into a supported high order mode of high index planar waveguide  120  in the region for which multi-mode operation is supported.  
       FIG. 4   f  illustrates a high level schematic diagram of a sixth embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . Evanescent coupling region  360  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 . High index planar waveguide  120  is formed and dimensioned to be operative in a multi-mode region of operation for at least the desired downstream wavelength. By multi-mode region of operation is meant the region of operation in which high order modes are present, typically for each high order mode both the TM and TE modes exist. Planar waveguide  40  is formed and dimensioned to be operative in the single mode region of operation. The relationship between N eff  and wavelength differs for each and every mode in the multi-mode operation. The width and/or height and the refractive index of high index planar waveguide  120  in combination with the width and/or height and the refractive index of planar waveguide  40  is selected so that N eff  for the TM mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TM mode for one of the supported modes in high index planar waveguide  120  and that N eff  for the TE mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TE mode for another of the supported modes in high index planar waveguide  120 .  
      In one embodiment, both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of both the portion of planar waveguide  40  and the portion of high index planar waveguide  120  within evanescent coupling region  360  is between 0.15 and 3.0 microns. In such an embodiment the width of planar waveguide  40  is preferably between 0.8 and 1.3 microns and the width of high index planar waveguide  120  is between 2 and 7 microns.  
      In operation, coupling region  360  is operative to couple the TM mode of the desired downstream wavelength to high index planar waveguide  120 , where it propagates in the TM mode of either the fundamental or a supported high order mode. Coupling region  360  is further operative to couple the TE mode of the desired downstream wavelength to high index planar waveguide  120 , where it propagates in the TE mode of either the fundamental or a supported high order mode, the propagation mode of the TM and TE modes being different. In particular, if for example the TM mode is propagating in the fundamental mode, the TE mode is propagating in a high order mode. This enables a polarization independent frequency selective optical coupler. Advantageously, high index planar waveguide  120  is formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
       FIG. 4   g  illustrates a high level schematic diagram of a seventh embodiment of an evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of  FIG. 2   c - 2   e  and  3   c . The embodiment of  FIG. 4   g  comprises planar waveguides  40  and  690 ; high index planar waveguides  120 ′ and  120 ″; evanescent coupling region  360  comprising sub-regions  610  and  620  separated by region  700  of planar waveguide  40 ; evanescent coupling regions  670  and  680  separated by region  710  of planar waveguide  690 ; and detector  70 . Evanescent coupling region  360  of polarization independent frequency selective optical coupler  110  of  FIGS. 2   a ,  2   b  and  3   b  and of polarization independent frequency selective dual optical coupler  210  of FIG.  2   c - 2   e  and  3   c  comprises one or more sub-regions  610  and  620 . High index planar waveguide  120  comprises two high index planar waveguides  120 ′ and  120 ″. In an exemplary embodiment, planar waveguide  690  comprises the input planar waveguide to detector  70 . In an alternative embodiment, planar waveguide  690  comprises a planar waveguide connected to the input of detector  70 . Preferably, planar waveguide  690  is formed and dimensioned to be operative in single mode operation.  
      Evanescent coupling sub-region  610  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 ′. Evanescent coupling sub-region  620  is defined by a portion of planar waveguide  40  being in close proximity to a portion of high index planar waveguide  120 ″. Evanescent sub-region  610  and  620  are shown herein as being sub-regions of a single evanescent coupling region  360  separated by region  700  however this is not meant to be limiting in any way. Evanescent sub-regions  610  and  620  may be formed at a distance from each other thus forming separate and distinct evanescent coupling regions without exceeding the scope of the invention.  
      The width and/or height and the refractive index of high index planar waveguide  120 ′ in combination with the width and/or height and the refractive index of planar waveguide  40  is selected so that N eff  for the TM mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TM mode in high index planar waveguide  120 ′. Furthermore, the width and/or height and the refractive index of high index planar waveguide  120 ′ in combination with the width and/or height and the refractive index of planar waveguide  690  is selected so that N eff  for the TM mode supported in planar waveguide  690  matches N eff  of the TM mode in high index planar waveguide  120 ′. The width and/or height and the refractive index of high index planar waveguide  120 ″ in combination with the width and/or height and the refractive index of planar waveguide  40  is selected so that N eff  for the TE mode of the fundamental mode supported in planar waveguide  40  matches N eff  of the TE mode in high index planar waveguide  120 ″. Furthermore, the width and/or height and the refractive index of high index planar waveguide  120 ″ in combination with the width and/or height and the refractive index of planar waveguide  690  is selected so that N eff  for the TE mode supported in planar waveguide  690  matches N eff  of the TE mode in high index planar waveguide  120 ′. Preferably, high index planar waveguide  120 ′ and high index planar waveguide  120 ″ are each formed and dimensioned to be operative in single mode operation for at least the desired downstream wavelength.  
      Evanescent coupling region  670  is defined by a portion of planar waveguide  690  being in close proximity to a portion of high index planar waveguide  120 ′. Evanescent coupling region  680  is defined by a portion of planar waveguide  690  being in close proximity to a portion of high index planar waveguide  120 ″. Evanescent coupling regions  670  and  680  are herein described as being separate evanescent coupling regions separated by region  710  of planar waveguide  690 , however this is not meant to be limiting in any way. Evanescent coupling regions  670  and  680  may be formed as sub-portions of a larger single evanescent coupling region without exceeding the scope of the invention.  
      In one embodiment, planar waveguides  40  and  690  and high index planar waveguides  120 ′ and  120 ″ are comprised of core material having a refractive index between 2.0 and 2.2, preferably between 2.0 and 2.1. In a further embodiment the height of the portions of planar waveguides  40  and  690  within evanescent coupling regions  610 ,  620 ,  670  and  680  is between 0.15 and 3.0 microns and the width is between 0.8 and 1.3 microns. In one further embodiment the height of high index planar waveguides  120 ′,  120 ″ is between 0.15 and 3.0 microns and the width is between 2 and 7 microns.  
      In operation, evanescent coupling region  610  is operative to couple the TM mode of the desired downstream wavelength to high index planar waveguide  120 ′, and evanescent coupling region  670  is operative to couple the TM mode of the desired downstream wavelength to planar waveguide  690 . The TM mode is thus double filtered, having been filtered by both frequency selective evanescent coupling regions  610  and  670 . Evanescent coupling region  620  is operative to couple the TE mode of the desired downstream wavelength to high index planar waveguide  120 ″ and evanescent coupling region  680  is operative to couple the TE mode of the desired downstream wavelength to planar waveguide  690 . The TE mode is thus double filtered, having been filtered by both frequency selective evanescent coupling regions  620  and  680 . Both the TM and TE modes of the optical signal have thus been double filtered and propagate in single mode waveguide  690  to detector  70 .  
      The length of high index planar waveguides  120 ′ and  120 ″ are selected so as to ensure equal propagation times for both the TM and TE modes from the input of planar waveguide  40  to detector  70 . This minimizes any polarization mode dispersion. In particular, the propagation time of the TM mode through high index planar waveguide  120 ′ and region  710  of planar waveguide  690  may be longer or shorter than the propagation time of the TE mode through the portion  700  of planar waveguide  40  and high index planar waveguide  120 ″. The length of either high index planar waveguide  120 ′ or  120 ″ is thus adjusted to compensate for any difference in overall propagation time between the paths of the TE and TM modes. This enables a polarization independent frequency selective optical coupler having minimal polarization mode dispersion. Advantageously, high index planar waveguides  120 ′ and  120 ″ are formed and dimensioned to improve the discrimination of the frequency selective coupling, and apodization is utilized to reduce the side lobes.  
      The above has been described with the TM mode propagating through high index planar waveguide  120 ′ and the TE mode propagating through high index planar waveguide  120 ″ however this is not meant to be limiting in any way. In particular, in another embodiment the TE mode propagates through high index planar waveguide  120 ′ and the TM mode propagates through high index planar waveguide  120 ″ without exceeding the scope of the invention.  
      It is to be appreciated that either high index planar waveguide  120 ′ or high index planar waveguide  120 ″ may be produced with a grating within one or more of evanescent coupling regions  610 ,  620 ,  670  and  680  without exceeding the scope of the invention. Furthermore, extinction grating  90  of  FIG. 2   e  may be written on planar waveguide  690 , and/or on high index planar waveguides  120 ′ and  120 ″ without exceeding the scope of the invention. It is to be noted that the configuration of  FIG. 4   g  thus advantageously supplies a filtered polarization independent output on single mode waveguide  690 .  
      It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.  
      Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein.  
      All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.  
      It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description.