Abstract:
A filter device and method are presented for filtering a multi-channel randomly polarized light signal to separate therefrom at least one specific channel. The device comprises a polarizer assembly, and a filter structure. The polarizer assembly is operable for processing the multi-channel randomly polarized light signal to split it into two multi-channel light components of a predetermined polarization identical for both of said two multi-channel light components; and for processing two identically polarized light components to produce a randomly polarized light signal. The filter structure is operable to process said two multi-channel light components of said predetermined polarization to select from each of said two light components the specific channel, and thereby produce two first output light components of the specific channel propagating through spatially separated first light paths.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a national stage application of International Patent Application PCT/IL02/00853” filed Oct. 24, 2002, which claims priority from U.S. Provisional Patent Application Ser. No. 60/330,603 filed Oct. 25, 2001, the contents of both of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention is generally in the field of optical devices for use in optical communication systems, and relates to polarization insensitive filtering of a multi-channel optical signal. 
   BACKGROUND OF THE INVENTION 
   Optical transmission systems, which are based on wavelength division multiplexing (WDM), achieve high information capacities by aggregating many optical channels, each carrying a specific wavelength band, onto a signal strand of optical fiber. A tunable filter is a critical optical element with several crucial roles in WDM communication systems. A tunable filter, which can redirect and route wavelengths, is used in conjunction with tunable lasers to create a tunable transmitter, midway in the fiber, in wavelength add and drop applications and at the receiving end in conjunction with a broad band detector for a tunable receiver. 
   It is also known that an optical signal traveling through an optical fiber is subject to variations in its polarization state due to the birefringence of conventional single mode optical fibers. Therefore, an optical signal of a given wavelength λ 0  with single polarization state, after traveling some distance through a fiber, will have two signal components of two orthogonal polarization modes (directions), respectively, for example (but not limited to) TE and TM polarization modes, i.e., λ 0   TE  and λ 0   TM . An optical filter must be able to filter the optical signal irrespective of its polarization state. Hence, it is of utmost importance to provide polarization independent tunable filters, which meet the following criteria: 
   Common central wavelength λ 0 , namely, the central wavelengths of both polarizations must be equal (i.e. λ 0   TE =λ 0   TM ; 
   The filter shape for both modes must be equal, i.e., g(λ) TE =g(λ) TM ; 
   The filter loss for both polarization states must be identical i.e., L TE =L TM . 
   The following two solutions for the above problem are known in the art:
         1. Careful control of the layer structure and refractive index of a filter device, which provides for a polarization insensitive device (K. Worhoff et al, “Birefringence Compensation Applying Double-Core Waveguiding Structures”, IEEE Photon. Tech. Lett., vol. 11, pp. 206–208, (1999));   2. Polarization diversity, i.e., splitting the incoming signal into its constituent polarization modes and independently filtering each polarization separately.       

     FIG. 1  illustrates a prior art device (WO 01/22139) utilizing a polarization diversity scheme. This scheme is aimed at managing the polarization dependence of a microring resonator which employs two resonators serially arranged and constructed to separately filter transverse electric (TE) and transverse magnetic (TM) polarization components of a predetermined wavelength in a randomly polarized DWDM optical signal, and to recombine the separately filtered components prior to output from an optical component employing the inventive polarization diversity scheme. Here, the polarization splitting and subsequent tunable filtering are achieved by using optimized filters for the TE and TM signal. 
   SUMMARY OF THE INVENTION 
   There is a need in the art to facilitate polarization independent wavelength-selective filtering by providing a novel optical filter device and method. 
   The main idea of the present invention consists of applying frequency-selective filtering to two identically polarized light components either by, respectively, two identical filter elements, namely, the filter elements being optimized to the same polarization condition and centered to substantially the same frequency, or by a single bi-directional filter element optimized to the polarization condition of the input light components. The device according to the present invention thus provides for independent filtering of each polarization mode using the same filter design and construction, thus, saving on filter design, specialized construction and tunable filter control for both polarization states. This approach ensures that the above-defined criteria (i.e., common central wavelength, equal filter shapes, and identical filter loss for both polarization states) are met within manufacturable tolerances. 
   There is thus provided according to one aspect of the present invention, a method of filtering a multi-channel randomly polarized light signal to select therefrom at least one specific channel the method comprising:
         (i) processing the multi-channel randomly polarized light signal to split it into two light components of the same predetermined polarization propagating along two spatially separated light paths, respectively;   (ii) passing said two polarized light components through a filter structure centered to the specific channel and operable to process each of said multi-channel polarized light components to select therefrom said specific channel into a first output light component, thereby producing two first output light components of the specific channel propagating through spatially separated first light paths;   (iii) combining said first light paths into a first dropping output path.       

   The method may also comprise rotation of the polarization of one of the first output light components into that of the other one and then recombining the first output light components into a first randomly polarized light component, and/or rotation of the polarization of one of the second output light components into that of the other one and then recombining the second output light components into a second randomly polarized light component. The polarization rotation of the first/second output light component is carried out prior to or concurrently with step (iii). 
   The processing of the input multi-channel randomly polarized light signal comprises splitting this light signal into two orthogonal polarization components, and applying polarization rotation to either one these light components to rotate it into that of the other light component, or to polarizations of both of these light component to rotate it into another polarization identical for both light components. 
   The passage of the two polarized light components through the filter structure may be implemented by using two identical filter elements, namely, centered to the same wavelength and having the same polarization condition. Alternatively, the passage of the two polarized light components through the filter structure may be implemented by using a single bi-directional filter element. 
   The method may also comprise further filtering of the multi-channel output of the said filter structure by passing it through a further filter structure for filtering therefrom another channel, which may and may not be directed to the same dropping channel. 
   According to another aspect of the present invention, there is provided a filter device for filtering a multi-channel randomly polarized light signal to separate therefrom at least one specific channel, the device comprising:
         (a) a polarizer assembly for processing the multi-channel randomly polarized light signal to split it into two multi-channel light components of a predetermined polarization identical for both of said two multi-channel light components; and for processing two identically polarized light components to produce a randomly polarized light signal;   (b) a filter structure operable to process said two multi-channel light components of said predetermined polarization to select the specific channel from each of said two light components, and thereby produce two first output light components of the specific channel propagating through spatially separated first light paths.       

   The polarizer assembly comprises a polarization splitting/combining element that splits the input multi-channel randomly polarized light signal into two orthogonally polarized light components and/or combines two orthogonally polarized light components into a single randomly polarized light signal, and a polarization rotator accommodated either in the optical path of one of the identically polarized light components, or in the optical path of both such light components. The polarization splitting and polarization rotation can be realized using for example one of the following assemblies: Rohmboid prisms and half-wave wave retarders, planar optics waveguides (an input waveguide and two output waveguides, where by careful choice of geometry each polarization is directed to a different output waveguide), and birefringence crystals and half-wave retarders. If a birefringent crystal polarizing splitting element is used, the filter element is tunable by using mechanical, electro optic, thermo optic, free carriers injection or piezoelectric changes in the size or refractive index of the resonator rings or cladding. 
   The filter structure may comprise two identical filter elements, in which case the polarizer assembly includes a polarization splitter and rotator, or a single bi-directional filter element, in which case the polarizer assembly includes a polarization splitter-and-combiner and a polarization rotator. The splitter-and-combiner element may be in the form of an isolator or circulator that splits the incoming light from an input port into two outputs with identical polarization states (while counter-propagating light in these outputs is combined and directed to a different output port), and either combines incoming light from these two ports and redirects them to an additional output port or insures no incoming light from these two ports will return to the first port and optionally directs them to an additional output port. 
   The device may comprise a polarization splitting isolator or circulator element separating the two polarizations of the incoming signal and transforming them to two outputs of the same polarization while counter-propagating light in these outputs is combined and directed to a different output port 
   The isolator/circulator based polarization splitting and rotation can be realized using for example one of the following configurations: standard circulator and a polarization splitter and half-wave retarder on one polarization channel, and polarization splitter and Faraday polarization rotator on both polarization channels. In the latter case, the filter element is tunable by using mechanical, electro optic, thermo optic, free carriers injection or piezoelectric changes in the size or refractive index of the resonator rings or cladding. 
   The bi-directional filter element may have two input/output ports and two additional output ports. In this case, light entering from each input is filtered. The extracted signal from each input is directed and outputted from the other input. The remaining channels (not filtered) exit at the additional outputs. Alternatively, the bi-directional filter element has two input ports and two output ports, in which case light from each input port is filtered, and the filtered signal is directed to an output port while not filtered light exits from the other input port. The bi-directional filter element, as well the two-element filter structure, may utilize a ring resonators (closed loop resonator), or of a combination of two or more closed loop resonator coupled to each other either parallel or serially, or a combination of both. 
   Preferably, the device also comprises an additional polarization combiner assembly that may be accommodated in the first light paths or the second light paths, depending on the filter structure type, or may be composed of two polarization combiner elements accommodated in, respectively, the first light paths and the second light paths. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic illustration of the prior art device utilizing polarization diversity; 
       FIG. 2A  is a schematic illustration of a polarization insensitive filter device according to one embodiment of the invention utilizing a filter structure composed of two identical filter elements; 
       FIG. 2B  schematically illustrates one specific example of the implementation of a polarizer assembly suitable to be used in the device of  FIG. 2A ; 
       FIG. 3A  is a schematic illustration of a polarization insensitive filter device according to another embodiment of the invention utilizing a filter structure composed of a single bi-directional filter element; 
       FIGS. 3B and 3C  show two specific examples, respectively, of the implementation of the filter device of  FIG. 3A ; 
       FIGS. 4A and 4B  schematically illustrate two examples, respectively, of optical systems utilizing cascaded filter devices according to the invention; 
       FIGS. 5A to 5F  exemplify various implementations of the filter device according to the invention utilizing ring-like resonator structures; 
       FIGS. 6A to 6C  exemplify three different implementations, respectively, of for OADM optical systems utilizing multiple filter structures using multiple output waveguides for multiple dropped channels, and a single output waveguide for multiple dropped channels; and 
       FIGS. 7A to 7C  illustrate graphs of the transfer function of ring resonators based filters for, respectively, a single resonator, a cascaded double resonator, and parallel coupled ring resonators. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2A , there is illustrated a polarization independent, wavelength-selective filter device  100  (e.g., channel dropping filter) according to one embodiment of the invention. The filter device  100  comprises a polarizer assembly and a filter structure. In the present example, the filter structure is composed of two identical filter elements  203  and  204 . Accordingly, the polarizer assembly comprises a first assembly including a polarization splitting element  201  and a polarization rotating element  202 ; and a second assembly including a polarization rotating element  207  (and optionally also a polarization rotating element  205 ), and a polarization combining element  208  (and optionally also a polarization combining element  206 ). The filter elements  203  and  204  are identical in their polarization conditions and functions with respect to light passing therethrough, namely, both are drop filters tuned to the same center wavelength, and optimized, for example, to TE polarized light. For example, the filter element may include a ring resonator, or a closed loop compound resonator as described in WO 01/27692 assigned to the assignee of the present application, as will be exemplified further below. The device  100  operates in the following manner. 
   A randomly polarized (linear, circular or elliptic) light signal L in , which is composed of a multitude of WDM optical channels (λ 1 , λ 2 , . . . , λ N ), enters the device at a light path  101 , which may be an optical waveguide or optical fiber. The polarization splitting element  201  splits the incoming light signal L in  into two light components of orthogonal polarizations L TM  and L TE  and direct them to light paths  102  and  103  (preferably confined in optical waveguides or fibers). For example the TM polarization component L TM  is confined to the light path  102  and the TE polarization component L TE  is confined to the light path  103 . The polarization rotating element  202 , which in the present example, is accommodated in the light path  102 , applies a 90 degree polarization rotation to the polarization of the light component L TM . Hence, two identically polarized light components L TE  propagate through the light paths  103  and  104  to enter the filter elements  204  and  203 , respectively. 
   The element  202  (polarization retarder) may be of any known type capable of rotating polarization of light passing therethrough, for example transferring one polarization to its orthogonal polarization. Generally, the polarization rotating element  202  may be accommodated in either one of the optical paths  102  and  103 , depending on the polarization condition of the filter elements  203  and  204 , to rotate the polarization of the respective light component to that of the other light component, or may be accommodated in both optical paths  102  and  103  to rotate the polarizations of both light components to the same third polarization state. In other words, the input light passage through the polarization rotating element results in two light components of identical polarization. The polarization splitting and rotation can be realized using a variety of approaches, for example one of the following: Rohmboid prisms and half-wave retarders, planar optics waveguides, birefringent crystals and half-wave retarders. 
     FIG. 2B  exemplifies one possible implementation of the polarizer assembly. As shown, the polarization splitting element  201  is in the form of a birefringent crystal allowing the propagation of one polarization component, e.g., L TE  of the input light signal L in  in the original direction (of the input light) and deflecting the other polarization component L TM  of the input light a certain angle from the original direction at both input and output facets of the crystal. The polarization components L TE  and L TM  thus emerge from the crystal in a spaced-apart parallel relationship. The polarization rotating element  202  is a half-wave plate accommodated in the light path of the polarization component L TM . Hence, two identically polarized light components, TE-polarized in the present example, are produced to enter two filter elements  203  and  204 , respectively. 
   As indicated above, in the present example, the polarization rotating element  202  is associated with the light path  102 , and thus two TE polarization components propagate through the light paths  103  and  104  without loss of information or disruption of the optical signal. A similar result, namely, the provision of two light components of identical polarizations, would be achieved by rotating the TE polarization in the light path  103  to the TM one, or by rotating both the TE and TM polarizations to a third polarization state equal for both light components. 
   Turning back to  FIG. 2A , the filter element  203  receives the light component L TE  from the light path  104 , selects therefrom a specific wavelength component, e.g., λ 1  component, and directs it to a drop path  109  while allowing propagation of all other wavelength channels from the light path  104  to an output light path  105 . The filter element  204  receives the light component L TE  from the light path  103 , selects therefrom the wavelength component λ 1  and directs it to a drop path  112  while allowing propagation of all other wavelength channels from the light path  103  to an output light path  107 . In other words, the TE polarization component from the light path  103  transverses a similar route as the light component from  102 , but does not undergo polarization rotation prior to being filtered. 
   To provide for a polarization insensitive channel dropping functionality of the device  100 , the polarization of TE-polarization component at the light path  109  (or that of the light path  112 ) is rotated by the element  207  to the TM polarization state, and the two orthogonally polarized light components in the light paths  111  and  112  are recombined by the polarization combining element  208  to produce a randomly polarized output light signal L (λ1)   out  propagating to a dropping channel  113 . Preferably, the TE light component in the light path  105  (or that in the light path  107 ) also undergoes the 90 degree polarization rotation by the element  205 , and two orthogonally polarized light components of the light paths  106  and  107  are recombined by the polarization combining element  206  to thereby produce a multi-channel randomly polarized output light signal L (λ2 . . . λN)   out  propagating to an output path  108 . The output light signal in the path  113  contains substantially the dropped wavelength λ 1 , and the output light signal in the path  108  contains all the other wavelength components of the input light signal. 
     FIG. 3A  illustrates a polarization independent, wavelength-selective filter device  200 A according to another embodiment of the invention. In this embodiment, a filter structure is composed of a single bi-directional filter element  302  centered to a specific channel. Accordingly, a polarizer assembly of the device  200 A may comprise a single isolator- or circulator-type polarization splitting/combining/rotating structure  301 , or also a second polarizer assembly including a polarization rotating element  303  and a polarization combining element  304 . 
   The structure  301  can be realized using a variety of approaches, for example, utilizing a standard circulator and polarization splitter and half-wave retarders, or a polarization splitter (e.g., birefringent crystal) and Faraday polarization rotators. The bi-directional filter element  301  may include a ring resonator or a closed loop compound resonator as described in the above-indicated publication WO 01/27692. These designs will be exemplified further below. 
   In one implementation of the device  200 A, the circulator polarization splitting/combining/rotating structure  301  operates to split incoming light L in  from one port (associated with light path  101 ) into two outputs L 1  and L 2  (associated with light paths  102  and  103 ) with identical polarization states, and operates to combine incoming light from these two paths and redirect them to an additional port (associated with output light path  106 ). The bi-directional filter element  302  has two input/output ports (associated with light paths  102  and  103 ), and may optionally have two additional output ports  104  and  105 . Light entering from each input is filtered. The extracted signal from each input is directed and outputted from the other input The remaining channels (not filtered) may exit at the additional outputs. 
   In another implementation of the device  200 A, the isolator or circulator polarization splitting/combining/rotating structure  301  operates to split incoming light from the input port (light path  101 ) into two output light components (light paths  102  and  103 ) with identical polarization states, in a manner ensuring no incoming light from these two output ports will return to the first port and optionally direct them to an additional output port  106 . Possible implementations of the polarizer assembly and light combining assembly will be described below with reference to  FIGS. 3B and 3C . The bi-directional filter element  302  has two input ports (associated with light paths  102  and  103 ), which may also serve as output ports, and has two output ports (associated with light paths  104  and  105 ). Light from each input port is filtered, and the filtered signal is directed to the output port. Light not filtered may exit from the other input port. 
   Thus, the device  200 A can operate as follows. A randomly polarized (linear, circular or elliptic) light signal L in  composed of a multitude of WDM optical channels (λ 1 , λ 2 , . . . , λ N ) enters the device at light path  101 , which may be an optical waveguide or optical fiber. The polarizer assembly  301  splits the input light signal into two orthogonal polarization components, transforms them to an identical polarization (e.g., TE polarization), and outputs the identically polarized light components at the light paths  102  and  103  that are preferably confined in optical waveguides or fibers. The filter element  302 , which is a polarization sensitive drop filter optimized to the polarization of the circulator output (e.g., TE polarization), operates in the following manner: One or more optical channels (e.g., λ 1  wavelength channel) from the polarized light entering the filter at the light path  102  is output at the light path  103 , while the rest of the light from the path  102  (composed of all the input channels except for the selected/dropped channel(s), i.e., λ 2  . . . λ N ) is output at the light path  104 . The dropped optical channel or channels (λ 1  in the present example) from the light component entering the filter at the light path  103  is output at the light path  102 , and the rest of the light from the light path  103  (composed of all the input channels except for the dropped channel(s), i.e., λ 2  . . . λ N ) is output at the light path  105 . The dropped channels λ 1  from the light paths  102  and  103  then re-enter the circulator-based polarizer assembly  301 , which combines them and outputs randomly polarized light L λ1  at light path  106 . The output light components (λ 2  . . . λ N ) composed of all the input channels except for the dropped channel(s) propagate through the light paths  104  and  105 . The polarization of one of these light components (that of the light path  104  in the present example) can be rotated by the element  303  to the orthogonal polarization component (to TM polarization in the present example) propagating through a light path  107 , and the two polarization components in the light paths  105  and  107  can be recombined by the element  304  to provide a randomly polarized output light signal L λ1-λN  in a light path  108  composed of all the channels of the input light, except for those dropped at the light path  106 . 
   Alternatively, although not specifically shown, the filter element  302  can operate as follows: The dropped optical channel(s) (e.g., λ 1 ) of the polarized light entering the filter at the light path  102  is output at the light path  104 , and the rest of the light from the path  102  composed of all the input channels except for the dropped channel(s), i.e., λ 2  . . . λ N ) is output at the light path  103 . The dropped optical channel(s) of light entering the filter at the light path  103  is output at the light path  105 , and the rest of the light from the path  103  composed of all the input channels except for the dropped channel(s) is output at the light path  102 . The dropped light components in the light paths  104  and  105  are then combined, for example, by rotating the polarization of one of them to the orthogonal polarization by the element  303 , and then combining by the element  304  the two polarization components in the light paths  107  and  105  into a randomly polarized dropped output propagating through the light path  108 . The rest of the channels in the light paths  102  and  103  which were not dropped re-enter the isolator/circulator based polarizer assembly  301 , which either eliminates back reflections (isolator) or combines them and outputs randomly polarized light at the light path  106 . Generally speaking depending on the construction and operation of the bi-directional filter element  302 , the dropped channel(s) is output either at light path  106  or light path  108 , and all other channels of the input light are output at the other light path. 
     FIG. 3B  illustrates one specific example of the implementation of the filter device  200 B utilizing a bi-directional tunable filter element. The device  200 B comprises first and second collimator assemblies  303 A and  303 B; a polarizer assembly  301  including first and second beam splitters  301 A and  301 D (each realized using birefringent crystal such as YVO4), two λ/2 retarders  301 B, and first and second Faraday rotators  301 C and  301 E; and a filter structure including a tunable bi-directional filter element  302  having two input/output ports associated with light paths  106  and  107 . The device  200 B operates in the following manner: 
   A randomly polarized multi-channel light beam L in  enters the first collimator assembly  303 A. The collimator could be realized with a single GRIN (graded index) lens for both input path  101  and output path  112  paths, or by separate lenses for each of these light paths. The collimated beam enters the first birefringent crystal  301 A from the light path  101 . The birefringent crystal  301 A splits the light signal L in  into TE- and TM-polarized light components propagating in light paths  102  and  103 , respectively. The TM component L TM  passes through the λ/2 retarder  301 B accommodated in the light path  103 , and is transformed to TE polarization. Both light components, which are TE polarized, pass through the first Faraday rotator  301 C that rotates their polarization by 45°. The resulting polarized light components at light paths  104  and  105  enter the second birefringent crystal  301 D with an optical axis rotated at 45°, so both polarized light components pass unchanged, and then pass through the second Faraday rotator  301 E. The latter rotates the polarization of both light components by −45°, and as a result, both light components entering the second collimator assembly  303 B at both its input/output ports (light paths  106  and  107 ) are in TE polarization state. Similar to the first collimator, the second collimator could be realized either by a single lens for both light paths or by separate lenses. The TE polarized beams then enter the bi-directional filter element  302 . The light component entering the filter at one input/output port goes through the filter and comes out through the other input/output port as a filtered signal (selected or dropped channel). The dropped output light components (which are both TE-polarized) pass through the second Faraday rotator  301 E, and while being −90° rotated with respect to the input light in the paths  105  or  104 , are diverted by the second crystal  301 D: they are deflected at the input of the crystal  301 D to light paths  108  and  109  and then again deflected to emerge from the crystal along light paths  110  and  111 . The two output light components, which are now TM polarized, then pass through the first Faraday rotator  301 C where they are aligned back to the TE polarization state. The light component in the light path  111  passes through the λ/2 retarder  301 B, is transformed to TM polarization state, and is then combined with the TE polarized component in the light path  110  by the first crystal  301 A into a randomly polarized dropped (filtered) output signal emerging from the device in a dropping output channel  112 . 
     FIG. 3C  illustrates another specific example of the implementation of the filter device  200 B. Here, the λ/2 retarder and +45° Faraday rotator are replaced by −45° Faraday rotators  301 B accommodated in light paths  102  and  110 , and +45° Faraday rotators accommodated in light paths  103  and  111 . This configuration introduces identical effect on the light polarization as described in the previous example. 
   Referring to  FIGS. 4A and 4B , there are illustrated optical systems  300 A and  300 B, each utilizing cascaded channel dropping filter devices  400 . The filter device  400  is of the type described above with reference to FIGS.  2  and  3 A– 3 B. The filter elements of different filter devices in the system  300 A (or  300 B) are preferably tuned to different wavelengths, respectively, and the system thus enables the dropping of multiple channels. In the example of  FIG. 4A , dropped channel(s) of each filter device is output in a separate waveguide or fiber  113 . In the example of  FIG. 4B , the dropped channels  113  of the filter devices  400  are united to a single output waveguide or fiber  114 . The cascaded filters structure may serve as an optical add-drop multiplexer (OADM) which is a key element in modern optical communication systems based on WDM. 
     FIGS. 5A to 5F  illustrate several implementations of the filter structures suitable to be used in the device of the present invention. These filter structures utilize ring or close loop resonator structures, and various configurations provide different filtering and tuning characteristics. Ring resonators are described in the literature, and have been the subject of extensive research. Ring resonators are notoriously hard to optimize for both polarizations concurrently, yet provide highly attractive characteristics for both filtering and tuning, and hence are an important and necessary element for WDM systems. 
   The configuration of  FIG. 5A  is suitable for use in the device  100  of  FIG. 2 , and therefore the same reference numbers are used to identify those components which are common in the examples of  FIGS. 2 and 5A . As shown in  FIG. 5A , a filter structure  203  is composed of a single ring resonator, and has one input associated with a light path (waveguide)  104  for passing input multi-channel polarized light and two outputs associated with light paths (waveguides)  105  and  109  for outputting, respectively, dropped channel(s) (e.g., λ 1  component) and all other channels (λ 2 , . . . , λ N ) of the input light. 
   The configurations of  FIGS. 5B and 5C  are suitable for use in the above-described two implementations of the device  200  of  FIG. 3A , and therefore the same reference numbers are used to identify those components which are common in the examples of FIGS.  3 A and  5 B– 5 C. The device  302  has two input/output ports associated with light paths  102  and  103 , and two additional output ports associated with light paths  104  and  105 . Multi-channel polarized light (λ 1 , λ 2  . . . λ N ) components of identical polarizations exist in the input light paths  102  and  103 . In the example of  FIG. 5B , the ring resonator filter element  302  selects an optical channel (e.g., λ 1  wavelength component) from the light path  102  and drops it to the light path  103 , while allowing all other channels in the light path  102  (λ 2  . . . λ N ) to propagate to the output light path  104 , and selects the same optical channel λ 1  from the light path  103  and drops it to the light path  102 , while allowing all other channels in the light path  103  (λ 2  . . . λ N ) to propagate to the output light path  105 . In the example of  FIG. 5C , the ring resonator filter element  302  selects the optical channel λ 1  from the light path  102  and drops it to the light path  104 , while allowing all other channels in the light path  102  to be output through the light path  103 , and selects the same optical channel λ 1  from the light path  103  and drops it to the light path  105 , while allowing all other channels in the light path  103  (λ 2  . . . λ N ) to be output through the light path  102 . 
     FIGS. 5D and 5E  exemplify the double-ring designs of a filter element  203  suitable to be used in a filter device of the present invention: two ring resonators  203 A and  203 B accommodated in a cascade fashion ( FIG. 5D ) thereby performing the so-called double-stage filtering, and parallel coupled ring resonators  203 A and  203 B ( FIG. 5E ). Such parallel coupled ring resonators presents a closed loop compound resonator structure formed by two ring resonators  203 A and  203 B accommodated in a spaced-apart parallel relationship between two waveguides  104 – 105  and  109  and optically coupled to each other via respective segments of these waveguides. This concept is described in the above-indicated publication WO 01/27692 assigned to the assignee of the present application.  FIG. 5F  shows a filter element  203  in the form of multiple cascaded ring resonators. 
   Reference is now made to  FIGS. 6A to 6C , schematically illustrating various implementations of an OADM system based on cascaded channel drop filter elements. In the example of  FIG. 6A , the multiple filter elements (two such elements  203 A and  203 B being shown in the figure) are associated with multiple output waveguides  109 A and  109 B, respectively. A polarized multi-channel light signal (λ 1  . . . λ N ) propagates in a light path  104 , and upon reaching an interaction region with the ring resonator  203 , undergoes frequency filtering by the element  203 A: a specific channel, e.g., λ 1 , is directed to the output light path  109 A, and all other channels λ 2  . . . λ N  propagate to a light path  105 , which serves as an input light path  104  for the next filter element  203 B tuned to a further channel, e.g., λ 2 , to filter it out of the input light and direct to the further output light path  109 B, and so on. 
   In the example of  FIG. 6B , the multiple filter elements  203 A,  203 B, and so on are associated with a single output waveguide  109 . Each filter element operates similar to that of the above-described example with reference to  FIG. 2 . 
   In the example of  FIG. 6C , each filter element is a bi-directional filter similar to that of  FIG. 3A , and all the filter elements are associated with a common output path for a dropped channel, and a common output path for all the other channels of the input light. The operations of these devices are shown in the figures in a self-explanatory manner. 
     FIGS. 7A to 7C  illustrate the transfer functions of tunable filters based on ring resonators in the serial and parallel configurations. Graphs G 1 , G 2  and G 3  of  FIGS. 7A–7C , respectively, correspond to the transfer function (transmission as a function of frequency) of a single resonator ( FIG. 5A  above), cascaded double resonator or two-stage resonator ( FIG. 5D ) and parallel coupled ring resonators or close loop compound resonator ( FIG. 5E ). 
   The low loss propagation of the optical mode in the ring waveguide is obtained by utilizing a high refractive index contrast between the waveguide and surrounding material. The ring may be composed of optical glass with a refractive index between 1.6 and 2.3, may be made from Silicon (Si, refractive index of 3.5), or may be a combination of layered materials as described in WO 01/81962 assigned to the assignee of the present application. 
   The ring manifests itself on frequencies to which it is resonant. The resonant frequency of the ring, f 0 , is given by, 
                   f     0   ⁢               =     Mc     2   ⁢           ⁢   π   ⁢           ⁢     Rn   ef                 (   1   )               
wherein R is the ring radius measured from the center of the ring to the middle of the ring waveguide, n ef  is the effective refractive index of the ring waveguide, M is an integer value, and c is the speed of light in vacuum.
 
   The coupling between the ring and waveguide is an important quantity in determining the characteristics of the resulting device since it determines the optical bandwidth and photon lifetime, hence the modulation efficiency. 
   The power exchange between the waveguide and ring is denoted by k 2  and can be calculated by computing the overlap integral of the modes of the ring and waveguide multiplied by the length of an interaction region between them. The optical bandwidth, Δf (FWHM), is then given by the formula: 
                   Δ   ⁢           ⁢   f     =       k     π   ⁢       1   -   k           .   FSR             (   2   )               
wherein k is the coupling coefficient, and FSR is the resonator free spectral range.
 
   An individual ring (configuration of  FIG. 5A ) can be described as a two-port device. The throughput function describing the ring optical amplitude characteristics is given by: 
                   T   ⁡     (   ω   )       =       1       1   -   k         ⁢         (     1   -   k     )     ⁢     (     1   -     ⅇ     j   ⁢           ⁢   ω         )         1   -       (     1   -   k     )     ⁢     ⅇ     j   ⁢           ⁢   ω                       (   3   )               
while the drop function is given by:
 
                   D   ⁡     (   ω   )       =     -     k     1   -       (     1   -   k     )     ⁢     ⅇ     j   ⁢           ⁢   ω                       (   4   )               
wherein ω is the normalized radial frequency given by ω=2πf/FSR.
 
   The matrix describing one ring is given by: 
   
     
       
         
           
             M 
             Ring 
           
           = 
           
             
               T 
               
                 - 
                 1 
               
             
             ⁡ 
             
               [ 
               
                 
                   
                     
                       
                         T 
                         2 
                       
                       - 
                       
                         D 
                         2 
                       
                     
                   
                   
                     D 
                   
                 
                 
                   
                     D 
                   
                   
                     1 
                   
                 
               
               ] 
             
           
         
       
     
   
   Complex structures are obtained by multiplying the matrixes of the corresponding sections, as presented in WO 01/27692 assigned to the assignee of the present application. 
   Thus, the present invention provides for a channel dropping filter device, which acts irrespective of the polarization of an optical signal containing a plurality of optical channels. By using a polarization splitter, retarder element, and two identical channel dropping filters or one bi-directional channel dropping filter, efficient polarization free energy transfer of specific channel(s) from the input optical signal can be provided, as well as the polarization free transmission of the plurality of channel (s) which are not dropped. By using a plurality of filters, a 1×N channel dropping filter structure is provided. A plurality of output waveguides provides for N output dropped channels. Yet another embodiment involves tunable filters which provide for multiple channels at the output of the waveguide. 
   Those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore descried without departing from its scope defined in and by the appended claims. 
   In the method claims, which follow, characters that are used to designate claim steps are provided for convenience only, and do not apply to any particular order of performing the steps.