Patent Publication Number: US-7221452-B2

Title: Tunable optical filter, optical apparatus for use therewith and method utilizing same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to U.S. provisional patent application Ser. No. 60/402,127 filed Aug. 7, 2002 and U.S. provisional patent application Ser. No. 60/435,816, filed Dec. 19, 2002, the entire contents of each of which are incorporated herein by this reference. 
   INVENTIVE FIELD 
   The various embodiments of the present invention generally relate to the field of widely tunable optical bandpass filters and particularly to those utilized in dense wavelength division multiplexing or dense wavelength division multiplexing optical networks. 

   BACKGROUND 
   Tunable optical bandpass filters offer significant advantages in dense wavelength division multiplexing or DWDM optical networks. Single channel tunable bandpass filters are used for amplified spontaneous emission suppression after optical amplification, and in receivers as adaptive pre-filters for noise reduction. When used for noise reduction, a fiber-in, fiber-out filter followed by a separate fiber-coupled receiver may be used, or the filter and detector may be integrated into a single tunable receiver unit. Universal line cards based on tunable receivers may reduce the costs of maintaining inventory and spares. Tunable filters may also reduce costs for optical performance monitoring by allowing one monitor to select between multiple channels. More generally, tunable filters can be used in reconfigurable optical add-drop multiplexers (ROADM) which are often the central switching element of a transparent optical node. 
   Commercially available tunable filter technologies include, but are not limited to, fiber Bragg gratings, arrayed waveguide gratings, linearly variable thin film dielectric filters, Mach-Zehender interferometers, fiber Fabry-Perot etalons, Fabry-Perot etalons with deformable semiconductor multi-layer mirrors, and certain devices combining two or more or these elements. See, for example, “Tunable Optical Filters for Dense WDM Networks,” by Dan Sadot and Effraim Bolmovich,  IEEE Communications Magazine , December 1998, pp. 50–55. These and other filter technologies have certain drawbacks that limit or reduce their desirability. For example, some of these devices suffer from slow tuning speed, large form factor, large power consumption, narrow tuning range, large insertion loss, repeating passbands, and/or poor adjacent channel isolation. Additionally, in some such devices the filter bandpass shape cannot be easily modified to range from a broad flat-top to a narrow Gaussian. The optimum filter for a given application may require tailoring the bandpass shape or it may depend on the ease with which a receiver can be integrated into the device. See, for example, “A quantum-limited, optically-matched communication link, D. D. Caplan and W. A. Alter, paper MM2-1, Proceedings of the Optical Fiber Communication Conference, OSA Technical Digest Series, Optical Society of America, Washington, D.C., 2001. Further, in certain of such devices the center wavelength may be adjusted by a voltage-controlled position with no internal wavelength reference, and may require complex temperature mapping. In addition, for some of such devices it may be difficult, if not impossible, to construct as a combined filter and receiver. 
   Fixed diffraction gratings typically have not been used in telecommunications-grade tunable filter applications. Tunable filters incorporating fixed diffraction gratings are commonly multi-element devices that utilize electronically-driven deflection elements in combination with fixed gratings to generate a narrow and tunable transmission function. See, for example, U.S. Pat. Nos. 5,946,128 and 6,141,361. Rotating diffraction gratings, mounted in one of several well-known scanning monochrometer configurations, may be used for spectrum analysis but, generally, are not suitable for telecom-grade tunable filter applications due to their mechanical complexity and size. 
   As such, there is a need for a diffractive tunable filter that does not include many of the foregoing disadvantages and which desirably provides superior optical transmission and tuning characteristics. 
   SUMMARY 
   One embodiment of the present invention provides a tunable optical device for use with an input beam of light comprising a polarization recovery element adapted for receiving the input beam of light and outputting first and second spatially offset beams of polarized light. The invention also comprises a dispersive optical element and a movable mirror for directing the first and second beams of polarized light onto the dispersive optical element and receiving a portion of the first and second beams of polarized light returned from the dispersive optical element. 
   In another embodiment of the present invention a tunable optical device for use with a beam of light comprises a dispersive optical element and a movable mirror adapted for directing the beam of light onto the dispersive optical element, receiving a portion of the beam of light returned by the dispersive optical element. 
   Another embodiment of the present invention provides an optical apparatus for use with a collimated input beam of light. The optical apparatus comprises a polarization recovery element adapted for receiving the arbitrarily polarized input beam of light and outputting first and second collimated beams of polarized light with the same polarization state. A mirror directs the first and second beams of light to the diffractive optical element. The diffractive optical element receives the first and second beams of light and produces first and second returned beams of light of a same polarization state, and directs the first and second returned beams of light to the polarization recovery element. An actuator is coupled to and provides for rotating the mirror. The polarization recovery element receives the first and second returned beams of light and outputs a single output beam of light. 
   Another embodiment of the present invention provides an optical apparatus for use with an arbitrarily polarized input beam of light comprising a polarizing beam splitter adapter for receiving and splitting the arbitrarily polarized input beam into first and second beams of polarized light, and a Faraday rotator. The apparatus further comprises a reflector for directing the first beam of polarized light to the Faraday rotator. A path length compensator is disposed between the beam splitter and the Faraday rotator and delays the travel of the second beam of light to the Faraday rotator. The Faraday rotator aligns the polarization of the first and second beams of polarized light. The apparatus also includes first and second half-wave plates for respectively receiving the first and second aligned beams of polarized light. 
   Other embodiments, devices, elements, components and the like may also be utilized in conjunction with and/or separate of the before mentioned and following embodiments of the present invention as described in greater detail below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings, which are somewhat schematic and are incorporated in and form a part of this specification, illustrate various embodiments of the present invention and, together with the detailed description, serve to explain the principles of the present invention. 
       FIG. 1  is a block diagram illustrating various devices and components which may be utilized in various embodiments of a diffractive tunable filter of the present invention. 
       FIG. 2  is a graph illustrating the first order diffraction efficiency of the tunable optical filter of  FIG. 1 . 
       FIG. 3  is a graph showing the shapes of the filter bandpasses that can be achieved by varying the width of a slit used as a spatial filter in conjunction with a lens in the tunable optical filter of  FIG. 1 . 
       FIG. 4  is a block diagram of a combination of optical elements that produces two adjacent focused spots on the spatial filter. 
       FIG. 5  is a graph showing the relationship between the insertion loss and the deviation from the center frequency for a spectral profile generated using the filter of  FIG. 4 . 
       FIG. 6  is a block diagram of a polarization recovery element which may be utilized in the tunable optical filter of  FIG. 1 . 
       FIG. 7  is a block diagram of another embodiment of a polarization recovery element which may be utilized in the tunable optical filter of  FIG. 1   
       FIG. 8  is a block diagram of a further embodiment of a tunable filter of the present invention. 
   

   DETAILED DESCRIPTION 
   In general, a diffractive tunable filter operable over a wide frequency range for use in a dense wavelength division multiplexing optical network is provided. As is described in greater detail below, for at least one embodiment, single mode optical fibers may be used as the input and output of the diffractive tunable filter such that the wavelength dependence of the transmission spectrum may be Gaussian and well-suited for applications including pre-receiver channel selection, amplified spontaneous emission (ASE) suppression, and optical performance monitoring. In other embodiments of the present invention, a non-Gaussian spectrum, such as a flat top spectrum, may be obtained by using a slit or spatial filter of fixed or variable width as the output aperture. Such an exit slit is well suited to a tunable receiver, where low-loss integration of the slit with the receiver photodiode is possible. 
   The various embodiments of the present invention may utilize a polarization recovery element that is compatible with physically identical or separate input and output ports. The polarization recovery element is generally positioned after a collimating optical element to minimize the insertion and polarization dependent losses. 
   Tunable optical filters can be utilized in a variety of applications, especially in applications where it is desirable to selectively tune a received signal to a plurality of selected frequencies. Such applications include, but are not limited to, light wave and other communication systems, optical sensing systems, and other systems which utilize optical signaling for the transmission of information, control signals, and/or other data. The present invention provides various embodiments of optically tunable filters which are herein described without regards as to the specific application(s), system(s) or process(es) in which such embodiments may be utilized. 
   An overview of the various components and/or devices utilized in a diffractive tunable filter  40  which can be configured to filter a beam of light over a range of frequencies is illustrated in  FIG. 1 . As shown therein, a diffractive tunable filter or filter  40  generally includes, an input path  42 , a polarization recovery element  44 , which is suitably connected to the input path  42 , a reflective optical element or mirror  45 , and a diffractive, dispersive, or reflective optical element  46  (hereinafter, a “diffractive optical element”). The mirror  45  is preferably movable and is desirably positioned relative to a diffractive optical element  46  such that upon receiving a first beam of light, for example via the input path  48 , the mirror  45  directs the first beam along a second path  50  and towards the diffractive optical element  46 . The first beam is diffracted, dispersed, filtered and/or otherwise returned by the diffractive optical element  46  and a portion of such first beam, hereafter the “returned beam” or “diffracted beam,” is propogated back along the second path  50  towards the mirror  45 . The returned beam angle depends upon the light frequency. The portion of the returned beam with frequencies in the filter passband is directed back along the second beam path  50  and the first beam path  48 . More specifically, the mirror  45  is positioned relative to the diffractive optical element  46  such that the first beam is returned, by the diffractive optical element  46  at a predetermined angle based upon the frequency of the light, the angle of incidence of the first beam, upon reflection from a reflective surface on the mirror  45 , to the normal of the diffractive surface on the diffractive optical element  46 . In  FIG. 1 , this angle of incidence is identified by the symbol “θ”. 
   The position of the diffractive optical element  46  can be fixed or movable relative to the axis of rotation for the mirror  45 . The diffractive optical element or grating  46  can be any suitable diffractive element such as a diffractive grating mounted in a Littrow configuration in which the returned output beam, counter propagates along the same path, that is the second path  50 , of the input beam. At the diffractive surface of the diffractive optical element  46 , the angles between the surface normal, and the input beam (θ i ) and diffracted/filtered beam (θ d ) ideally satisfy the grating equation:
 
 mλ=d (sin θ i +sin θ d ).
 
where λ is the wavelength and d is the pitch of the diffraction grating. When operated at the first order Littrow angle, m=1 and θ i =θ d =θ, and the return beam angle θ as a function of wavelength can then be expressed as the Littrow condition, λ=2d sin θ. The Littrow mount maximizes the grating efficiency for a given grating groove density and wavelength. The nominal wavelength of the returned beam is determined by the wavelength that satisfies the Littrow condition. Note that in embodiments using separate input and output apertures, the input spatial filter (the fiber) and the output spatial filter (fiber or aperture) are not identical, and cases where θ i −θ d  is small, but not zero, are also possible. In these cases the actual filter wavelength may be determined from the grating equation and may differ slightly from the nominal value. The filter  40  also includes an actuator  52  to which the mirror  45  is coupled and preferably mounted to facilitate the directing of the first beam along the second path  50  towards the grating  46  such that light of a predetermined wavelength is returned by the grating  46 . The actuator  52  rotates the mirror  45  in order to diffract the first beam onto the second path  50 , so that the propogated light beams are incident upon the grating  46  at such an angle that the incident light is predominately diffracted by the grating  46 , back into the second path  50  and towards the mirror  45  at the predetermined and desired wavelength. In general, the actuator  52  changes the angle at which the light propogated over the second path  50  is incident upon the grating  46  and advantageously changes the angle θ between the incident light and the normal to the surface of the diffractive optical element  46 . Light diffracted back along the input beam paths  50  and  48  is coupled to an output aperture that selects a narrow band of frequencies.
 
   Although filter  40  can be operated over any suitable wavelength range, filter  40  preferably operates across the C-band from 1527 nm to 1567 nm, using a 1200 groove per millimeter (gr/mm) grating which is mounted relative to the mirror to achieve a Littrow-diffraction angle of 68° at a center wavelength of 1547 nm. It is to be appreciated, however, that other diffraction angles, tuning ranges and/or center wavelengths may be utilized. Such other tuning ranges may be obtained by changing the diffraction angle θ and/or the spacing “d” of the grooves in the grating  46 . For example, operation across the L-band from 1566 nm to 1607 nm may be accomplished at a Littrow-diffraction angle of 68° if a grating with 1170 gr/mm is used. For the embodiment shown in  FIG. 1 , the center-pivoted mirror surface  45  is mounted at 45° and the output wavelength may be tuned from 1527 nm to 1567 nm by rotating the mirror  45  through +/−1 degrees. 
   Although any suitable actuator  52  can be utilized, tunable filter  40  preferably utilizes a microactuator, and more preferably a micro-electromechanical or MEMS actuator. A MEMS actuator is generally preferred because of its small size, millisecond response time and low control power requirements. MEMS actuators are typically produced using semiconductor fabrication techniques that offer the additional advantage of low-cost volume production. Ideally, the combined mass of the mirror  45  and actuator  52  are approximately balanced in three dimensions so as to improve the stability of the assembly to vibration. Particularly, suitable actuator  52  designs are electrostatic actuators, such as of the type described in U.S. Pat. Nos. 6,329,737 and 6,469,415 the entire contents of which are hereby incorporated by this reference. Other microactuators may also be utilized, as is practical depending upon filter constraints and specific features and functions of any given embodiment, to rotate and/or translate the mirror  45  and/or the grating  46 . 
   The filter  40  preferably includes a control unit  54  which provides control signals to the actuator  52 . In short, the rotation of the mirror  45  and the operation of the actuator  52  occur under the control of the control unit  54 . Although any suitable control unit can be utilized, the unit  54  can be similar to the type described in U.S. Publication Nos. US-2002-0164125-A1 and US-2003-0026302-A1, the entire contents of which are incorporated herein by this reference. 
   Fixed, rotatable and/or translatable mirrors  45  in combination with fixed, rotatable and/or translatable gratings  46  and/or other components may be used to tune the diffractive filter  40  to a given wavelength. Although such rotation/translation may occur with respect to any combination of axes that maintains the in-plane alignment of the device, the rotation is typically about an axis parallel to the direction of the grating grooves. In general, any combination of fixed or movable mirrors  45 , gratings  46  and other components may be utilized to tune the filter  40  to a given wavelength. Further, while less desirable generally for purposes of efficiency, second order, third order and other order radiation reflected by the grating  46  may be utilized to tune the filter  40  to a predetermined wavelength. 
   The filter may also include a collimating optical element  56  which collimates converging and/or diverging input light beams into parallel or collimated beams on the input path  42 . The filter  40  also includes an output beam path  58  onto which the returned output beam is propogated or output from the polarization recovery element  44 . Further, the output beam path  58 , for this preferred embodiment, is connected to a spatial filter assembly  60 , which includes a focusing lens  62  and a slit  64 . The output of the spatial filter assembly  60  is provided on a spatially filtered beam path  66  to a photodetector  68 , which suitably characterizes the received light provided via the spatially filtered path  68  and generates electrical signals indicative thereof. Each of the components and/or connectors are discussed in further detail below. In an opposite direction of propagation along return beam path  42 , the collimating optical element focuses a plurality of parallel beams of light onto a focal point, for example, for coupling into the optical fiber  72 . 
   Referring again to  FIG. 1 , the filter  40  also includes a connector to an input fiber  72 , such connector currently being provided to the collimating optical element  56 . The collimating optical element  56  may be utilized with any suitable collimating lens. However, the tunable filter  40  preferably utilizes a 1.96 mm focal length lens as the collimating optical element. The range of frequencies over which an input fiber  72  may communicate light is determined by a combination of the fiber mode size, the dispersion of the grating  46  and the focal length of the collimating optical element  56 . In a typical embodiment, an SMF28 single mode fiber is used as the input fiber  72 , however, other suitable optical fibers may also be utilized. 
   As described above, the filter  40  includes a polarization recovery module or element  44 . In general, the polarization recovery element  44  receives an input beam of light, via the input path  42 , conditions the input beam of light, and outputs a first beam of light onto the first path  48 . Additionally, in the opposite transmission direction, the polarization recovery element  44  receives a returned beam of light, via the first path  48 , conditions such returned beam and provides an output beam of light on the output beam paths  42  or  58 . The output beam of light is utilized in accordance with the specific implementation of the filter in an optical network or system or otherwise. Various embodiments of polarization recovery elements are discussed herein. 
   In general, the polarization recovery element  44  is utilized to convert a received input beam of light into a first beam, and then provide the input beam to the tuning assembly comprising diffractive optical element  46  and the mirror  45 . More specifically, the polarization recovery element  44  splits the input beam of light into two parallel p-polarized first beams which propagate via the first path to the mirror  45 . In general, the polarization recovery element  44  is utilized to minimize insertion losses and polarization losses. As is commonly appreciated, for many applications, stringent requirements for insertion loss and polarization dependent loss may exist. Typical telecom dense wavelength division multiplexing applications often require tunable filters to have an insertion loss of less than 2.0 dB and a polarization dependent loss of less than 0.2 dB. Commonly, the insertion loss depends upon the grating efficiency and round-trip fiber coupling efficiency, while the polarization dependent loss depends upon the polarization dependence of the grating efficiency.  FIG. 2  shows the calculated first order diffraction efficiency at 1547 nm and p-polarization versus incidence angle for a commercially available 1200 gr/mm diffraction grating. As shown, a maximum efficiency of 96% is achieved for p-polarization at the Littrow angle of 68°. The diffraction efficiency decreases with increasing angle to 90% at 80° and 75% at 85°. 
   As described above, the polarization recovery element  44  desirably p-polarizes the arbitrarily polarized input light beams such that the incident light, that is the first beam, upon the grating  46 , is diffracted with the 96% grating efficiency shown at the Littrow angle in  FIG. 2 . It is to be appreciated that for the direction parallel to the grating grooves, that is for s-polarization, the efficiency is much lower. In another embodiment of filter  40  which does not include a polarization recovery element  44 , a grating  46  with low polarization dependent loss can be utilized. Gratings of conventional design can offer diffraction efficiencies for s and p polarizations that are similar at or near the Littrow angle, but generally have absolute diffraction efficiencies that are smaller than gratings designed for p-polarization. Alternatively, non-conventional grating designs that combine high diffraction efficiency with low polarization dependent loss, for example, echelle gratings, may advantageously be used. U.S. Pat. No. 6,400,509 B1, the entire contents of which are incorporated herein by reference, describes a non-conventional low polarization dependent loss grating design that may be suitable for certain embodiments of the present invention. However, in general, while grating design changes may be utilized to minimize both insertion loss and polarization dependent loss, various embodiments of the present invention reduce insertion loss and polarization dependent loss by illuminating the grating  46  with p-polarization only. Various polarization recovery element embodiments suitable for illuminating the grating  46  with p-polarization only are discussed in greater detail below with reference to  FIGS. 6 and 7 . 
   In addition to polarizing the light incident upon the grating  46 , it is often desirable to increase the width of the beam of light incident upon the grating. It is commonly appreciated that for grating-based devices that the number of illuminated lines on the grating  46  determines the wavelength resolution of the device. A larger beam width decreases the achievable spectral bandwidth. The number of illuminated lines on a grating  46  is proportional to the secant of the angle θ between the incident beam and the grating surface normal. In order to illuminate more lines on the grating  46 , the tunable filter  40  includes at least one beam width adjuster and preferably first and second beam width adjusters or beam expanders  74  and  76 . A first beam expander  74  can be positioned between the polarization recovery element  44  and the mirror  45 . The first beam expander  74  receives the first beam, via the first path  48 , and expands the diameter of the first beam so that as the beam is reflected by the mirror  45  into the second path  50  and onto the grating  46 , so that more of the grooves in the grating  46  are illuminated. Alternatively and/or additionally, a second beam expander  76  may be positioned in the filter  40 , for example along the second path  50  and between the mirror  45  and the grating  46 , in order to further expand the first beam and thereby illuminate more grooves on the grating  46 . Either or both beam expanders  74  and  76  may be used to expand the first beam such that more of the grooves in the grating  46  are illuminated and the filter spectral bandwidth is decreased. 
   Desirably, the beam expanders  74  and  76  expand the beam in a single direction perpendicular to the diffraction grating rulings, and perpendicular to the mirror rotation axis, by using suitable optical devices such as anamorphic prisms or cylindrical telescopes. The beam expanders  74  and  76  are preferably compatible with and may be used with other embodiments of the filter  40 , and desirably are independent of the polarization recovery element  44  embodiment, if any, employed. 
   When using beam expanders  74  and  76 , the grating incidence angle θ may be configured such that it is near the optimum angle of 68° suggested by the chart of  FIG. 2 . The beam size may then be independently adjusted to provide the desired resolution. Adjustments to the beam size may be accomplished, for example, by changing the relative angles of the beam expanders  74  and  76  on the first and second paths  48  and  50 , or using other well known optical techniques 
   When the input fiber  72  is a single input/output fiber, as is discussed in greater detail below, the spectral bandwidth of the tunable filter  40  is directly related to the resolution of the diffractive optical element  46 . As such, the bandwidth of the tunable filter  40  may be changed by varying the number of illuminated lines on the diffraction grating  46 . In other embodiments, such as a tunable receiver which generates at least one electrical signal based upon information signals contained within an optical signal, the width of the slit filter  64  or other spatial filtering device commonly determines the spectral bandwidth and the bandpass shape will be determined by the combined resolution of the tunable filter  40  and the adjustable spatial filter  60 . The resolution of the tunable filter  40  can be adjusted by changing, that is expanding, the beam diameter in the direction perpendicular to the grating grooves. As such, it is to be appreciated that beam expanders  74  and  76  may also be utilized to further condition the input beam for tuning by the tunable filter  40 . 
   Additionally, it is to be appreciated that when a pair of beam expanders are utilized, the magnitude of the change in resolution with filter wavelength can be minimized. This variation is caused by mirror-angle dependent changes in the input beam diameter at the grating surface. In particular, if Δ g  is equal to the change in grating incidence angle, defined as the angle between the grating normal and the input beam, required to tune the filter between wavelengths λ o  and λ, the beam radius at the grating  46  is equal to:
 
 w   G   =w   p /cos(θ o +Δ g )
 
where w G  is the radius of the input beam as it intersects the grating  46 , w p  is the radius of the beam as it leaves the first beam expander  74 , and θ o  is the incident angle corresponding to the wavelength λ o . The radius at the grating  46 , therefore, increases with increasing angle at the grating  46 . The beam expansion provided by the second beam expander  76  is also a function of the angle between the incident beam and the normal to the input facet of the second beam expander  76  according to the following equation:
 
 w   p   =w   o /cos ( B   o+Δ   m )
 
where w o  is the radius of the beam at the input surface of the second beam expander  76 , B o  is the angle between the input beam and the first surface normal when the filter  40  is tuned to λ o  and Δ m  is the change in mirror rotation angle required to tune the filter  40  between λ o  and λ. If the grating  46  and the second beam expander  76  are oriented such that an increase in incidence angle at the grating  46  corresponds to a decrease in incidence angle at the prism, the change in magnification may be adjusted to correct for the change in beam size at the grating  46 . Therefore, it is to be appreciated that by utilizing beam expanders  74  and  76  the filter  40  may be configured to provide for greater beam size uniformity over the tuning range at the grating  46  and thereby provide for greater filter bandwidth uniformity over the tuning range.
 
   The diffractive tunable filter  40  may also be utilized to further condition and filter light beams, as desired, by combining the tuning assembly, that is the grating  46  and mirror  45 , with other optical, electrical, mechanical and/or other components, including but not limited to those discussed herein. For example, the tuning assembly may be configured to include after the polarization recovery element  44 , such as along output beam path  58 , a spatial filter assembly  60 , such as one having an adjustable width slit. 
   The filter assembly  60  can include a slit  64 , a spatial filter or other type of filter. The slit  64  may include a pinhole or slit in a suitable membrane such as a metal plate. The membrane is desirably located at the focal distance for a given wavelength of a focusing lens  62  or other suitable collimating optical element, which suitably collimates the light provided on the output beam path  58 , that is the returned output beam. Using this configuration, unwanted radiation is blocked by the slit  64  and is not output by the filter  40 . Further, additional tuning and/or conditioning of the returned output beam may be accomplished by providing an adjustable or moveable spatial filter  60 . The spatial filter  60  may be configured closer to or farther away from the focusing lens  62  and thereby the filter  40  adjusted to any given bandwidth. 
   When the mirror  45  is positioned so that a desired wavelength is diffracted by the grating  46 , the angle between the input beam and the grating surface normal is desirably θ and the filter  40  provides a returned beam whose position is centered on the focusing lens  62  and the slit  64 . Ideally, the waist of the focused beam is coincident with the plane of the slit  64 . 
   Additionally, as the wavelength of the diffracted beam is changed Δλ, the center frequency of the light provided on the output beam path  58  to the focusing lens  62  and the slit  64  will vary by a distance 
               Δ   ⁢           ⁢   x     =       Mf   ⁢           ⁢   Δλ       d   ⁢           ⁢   cos   ⁢           ⁢   θ         ,         
wherein M is the magnification of the beam expanders  74  and  76 , f is the focal length of the focusing lens  62 , d is the period of the grating  46 , and θ is the angle which satisfies the Littrow condition. λ=2d sin θ The radius of the output beam at the slit  64  is approximately w=fw 1 /f 1 , where w 1  is the (1/e 2 ) beam radius of the Gaussian input light provided on the input filter  72 , f 1  is the focal length of the collimating optical element  56 .
 
   For purposes of comparison, if the slit  64  is not utilized and the focal length of the collimating optical element  56  and the focusing lens  62  are the same, it is to be appreciated that w=w 1 . In this case, the insertion loss of the tunable optical filter  40  may be calculated as:
 
 IL ( dB )=−10log[exp(−(Δ x/w ) 2 )]
 
which provides the following Gaussian bandpasses:
 
3 dB (full width) bandwidth=1.66 wd  cos θ/( Mf ); and
 
20 dB  bandwidth=4.29 wd  cos θ/( Mf ).
 
   As noted above, these results are proportional to the width of the first beam at the surface of the grating  46  in the direction perpendicular to the grooves. This result applies to the tunable filter  40  when the collimating optical element  56  and the focusing lens element  62  have equivalent focal lengths. Further, for this embodiment, the magnification required for a particular bandwidth can be calculated using typical component values of 1.94 mm for the focal length of a 0.23 pitch, SLW 1.8 SELFOC lens and 5.2 μm for the mode field radius for SMF28 fiber, and assuming the 68° Littrow angle for a 1200 line-per-mm grating. To obtain a 250 pm (32 GHz) 3 dB bandwidth and a 650 pm (82 GHz) 20 dB bandwidth, for example, a magnification of 5.55 is utilized. 
   In certain applications it may be desirable to have a non-Gaussian spectrum or to change the bandwidth of the filter  40  in response to changing spectra of the light on the input fiber  72 . This may be accomplished by using an adjustable slit. For example, by using any suitable microactuator or other motor, the slit jaws may be linearly translated. Additionally, and/or alternatively, spatial filters with spatially varying amplitude transmittance functions may also be placed at the focal point of the focusing lens  62 , or at other locations in the optical path, in order to shape the pass band of the light provided on the output beam path  58 . Additionally, and/or alternatively, spatial filters with spatially varying amplitude transmittance functions may also be placed at the focus of the focusing lens  62 , or at other locations in the optical path, in order to shape the pass band of the light output by slit  64  and for example, provided to the photodetector  68 . 
   Further, the spectrum at the photodetector  68  is generally determined by the transmission function of the slit  64 , which is determined by the relative size of the slit and the input beam. For a Gaussian beam with radius w, the transmission through a slit of full width s is given by 
   
     
       
         
           
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               . 
             
           
         
       
     
   
   The results of the preceding insertion loss minimization approach are shown in  FIG. 3 . More specifically,  FIG. 3  provides the relationship between the Insertion Loss (in decibels) versus the effect of a change in the shifting of a beam of light of w width upon a slit filter when a Gaussian beam is transmitted through a rectangular slit. Further, the above function evolves from a lossy Gaussian for s&lt;&lt;w to a flat top with Gaussian edges for s&gt;&gt; w. In particular, when s=2w, that is when the slit width equals the mode field diameter, the response is very similar to that of a filter coupled to a fiber with a mode field radius equal to w. It is to be appreciated, that flat top spectra obtained this way are useful when the resolution of the system is higher than required for the channel spacing. For example, if a filter with a 20 dB spectral width of 82 GHz is used in a system with 100 GHz channel spacing that requires a 170 GHz 20 dB bandwidth, the slit width can be chosen to give an 88 GHz flat section at the center of the bandpass. 
   The insertion loss minimization approach previously described above with respect to  FIG. 3  is more difficult with a tunable filter having a slit that is coupled to a fiber instead of to a photodetector, where the spectrum additionally depends on the spatial overlap of the signal transmitted by the slit with the mode of the output fiber. If the slit is significantly wider than the fiber mode field diameter, for example, then most of the power transmitted by the slit will not couple to the fiber. This can be avoided by using a slit  64  that is narrower than the mode field diameter of an output fiber (not shown, however, it is to be appreciated that in the embodiment shown in  FIG. 1 , such output fiber would replace, and or be possible on conjunction with, the photodetector  68 ). Further, a mode field diameter at the slit  64  should be narrower than the slit. However, it is to be appreciated that such a configuration generally introduces loss because of the increased beam divergence. For example, when the mode field diameter at the slit  64  is one third that of the mode of an output fiber, the loss will be approximately 5 dB. This loss may be mitigated, for example, by using a cylindrical focus in the focusing lens  62  so that the mode field diameter at the slit remains large along the long axis of the slit. 
   It is to be appreciated, that the distinction between a tunable filter and a tunable receiver is somewhat artificial. When a multimode fiber is used in lieu of a photodetector  68 , the difficulty of fiber coupling is greatly relaxed and the results ascribed to a tunable receiver can be obtained in a fiber-coupled form. Similarly, if a waveguide photodiode is used in a tunable receiver embodiment, the coupling problem is very similar to that of a single mode fiber such that the device will have similar properties to those ascribed to the tunable filter. Suitable waveguide detectors may include multi- or single mode semiconductor waveguides with photo-detection capabilities. Examples of suitable waveguide detectors are described in U.S. Pat. No. 5,054,871 and in U.S. patent application Publication No. 2001/0021299 A1, the entire contents of each are incorporated herein by this reference. 
   In order to provide tighter center frequency control, the tunable filter  40  may include an optical wavelength stabilization system. Several such systems include position detection apparatus that does not require information from an input signal. For example, the tunable filter  40  may stabilize filter operation by sensing the capacitance seen by the mirror voltage input, for example by means of a closed loop servo control system of the type described in U.S. Publication No. US-2001-0036206, the entire contents of which are incorporated herein by this reference. With a typical electrostatic microactuator, capacitance sensing has a typical resolution limit of 1 part in 400. This corresponds to 12.5 GHz in a filter that is designed to tune across 100 channels that are 50 GHz in spacing. 
   A position detector apparatus of the present invention may also be provided which utilizes optical reference beams reflected from mirror  45  to monitor the position of the mirror  45 . Such an optical apparatus, which can be used in conjunction with or as an alternative to the capacitance sensing system referred to above, preferably includes a second reference source  78  which directs a second reference beam  80  at the mirror  45 . The output from the reference source  78 , such as a laser source and preferably a laser diode or light emitting diode, is preferably collimated by a collimating optical element  82  and directed towards the rotatable mirror  45 . The reflected reference beam  84  is incident on a position sensitive detector  86 . As the mirror  45  rotates, the reflected reference beam  84  is translated across the surface of the position sensitive detector  86 . The position sensitive detector  86  outputs a determination of the mirror position  88  to a wavelength control unit  90 . The wavelength control unit  90  compares the mirror position to a set point  92  and generates an output signal  94 , for example an error signal, to the control unit  54 . The position of the mirror  44  is adjusted by means of actuator  52  based upon control signals generated by the control unit  54  according to the error signal  94 . The position sensitive detector  86 , wavelength control unit  90 , control unit  54  and actuator  52  are typically operated as a closed loop servo system that stabilizes the mirror position  88  to the set point value  92  and stabilizes the center frequency of the filter. 
   It may also be desirable to coarsely position the mirror  45  using techniques such as of the type described and/or referred to above and, when a signal is present, lock the mirror  45  position to the mirror position  88  of the target input channel by maximizing the output power. The tunable filter  40  may be configured to accomplish such tuning by including an output power control apparatus. One embodiment of such a control apparatus may utilize the photodetector  68 . As described above, the photodetector  68  receives a filtered beam, via the filtered path  66 , from the spatial filter assembly  60 . The photodetector  68  desirably provides a second electrical signal  96  to a power monitor  98 . In the embodiment shown in  FIG. 1 , the power monitor  98  is configured to monitor the DC bias current as supplied by the photodetector  68 . It is to be appreciated, however, that other devices and/or methods may be utilized to monitor the output power of the filter  40 . Further, the power detector  98  or the photodetector  68 , depending upon embodiment utilized, outputs a power signal  100  to the wavelength control unit  90 . Using the power signal  100  and/or any of the above mentioned mirror position signals, the wavelength control unit  90  generates a tuning error signal when a signal is not present or the filter  40  is being tuned to a new target channel. Advantageously, a locking error signal is generated when the filter  40  is tuned to a channel with an active signal. Such error signals are suitably provided to the control unit  54  to desirably control the position of mirror  45 . 
   In various other embodiments, an improved locking signal may also be generated by modulating the mirror angle at an audio frequency. The resulting amplitude variation on the output beam path  66  may be detected using a phase sensitive detector (not shown), thereby providing a more accurate lock point and reducing the sensitivity of the locking circuit to power variations in the input beam. 
   Thus, it is to be appreciated that the positioning of the mirror  45  may be controlled using various techniques and processes, some of which have been described herein. Other known techniques and processes for controlling a mirror or other optical components may also be utilized in conjunction with various embodiments of the diffractive tunable filter of the present invention. 
   In many dense wavelength division multiplexing applications it is desirable for the filter  40  to function adequately in the presence of mechanical shock and vibration. Shock and vibration applied to the actuator  52  may lead to deviation, both in the plane of the actuator and out of the plane of the actuator, of the returned beam, with concomitant degradation of optical performance. In the tunable filter  40 , measurement and correction systems and servo control electronics and algorithms may be utilized to correct for shock and vibration over certain ranges of frequency and applied force. See, for example, U.S. Pat. No. 6,469,415 and U.S. Publication No. US-2003-0094881, the entire content of which is incorporated herein by this reference. In plane deviation of the output beam of filter  40  often leads to a deviation in the center frequency  88  of the filter  40 , which as discussed above, may be measured using the position sensitive detector  86  and corrected for using the wavelength control unit  90 . However, out-of-plane deviations of the optical beam may require additional measurement and servo systems. 
   The tunable filter  40  may include a two dimensional position sensitive detector which is capable of measuring out-of-plane deviations. Such a two dimensional position sensitive detector generates signals for both out-of-plane deviations of the filter beam as well as providing signals utilized for center frequency control. A correction system for out-of-plane deviation of the filter beam may use a low numerical aperture or NA lens mounted on a linear actuator such as a linear electrostatic or other MEMS microactuator. Examples of suitable lens actuator assemblies are disclosed in U.S. Publication No. US-2001-0036206 and U.S. patent application Ser. No. 10/099,414, the entire contents of each are incorporated herein by reference. Further, it is to be appreciated that the correction subsystem may be placed at any point along the first path  48 , the second path  50 , and/or the output beam path  67 . Additionally, an electronic control unit may be used in certain embodiments to servo and control those lenses, if any, utilized to stabilize the filter. Devices and processes for controlling any such lenses are known in the art. 
   Referring now to  FIG. 4 , an alternative embodiment of a filter assembly  101  which may be utilized to generate a flat top spectral response is shown. In this embodiment, a beam splitter  102 , pathlength compensator  104  and a steering element  100  are utilized to split the light on output beam path  58  into first and second beam components  108  and  110 , prior to passing such beams through the focusing lens  112  and onto the second filtered path  66 . In particular, the light on output beam path  58  is passed through a beam splitter  102  which splits the lights into a first output beam component  108  and a second output beam component  110 . Desirably, the light on the output beam path  58  is split in the plane which is perpendicular to axis of rotation for the mirror  45  (as shown in  FIG. 1 ). Further, the first and second output beam components  108  and  110  are equalized in path length prior to focusing such components onto the focusing lens  112 . In one embodiment, such path length equalization is accomplished using a pathlength compensator  104  which “slows down” the transmission of the first output beam component  108  to the steering element  106  and thereby compensates for the shorter path length through the beam splitter  110  for the first output beam component  108  with respect to the second output beam component  110 . Each of these output components  108  and  110  are then directed by the steering element  106  onto the focusing lens  112  so that the two beams  108  and  110  are focused by the lens  112  onto the second filtered path  66  as two Gaussian signals of 3 dB width w, which are separated by a distance d, wherein the distance d corresponds to a frequency shift of Δf. 
   Advantageously, by splitting the light on the output beam path  58 , using the beam splitter embodiment shown in  FIG. 5 , instead of a single Gaussian signal with a peak centered at f0 with a 3 dB width of δf two superimposed beams are generated, which when summed together result in a nearly flat top peak with a 3 dB width of approximately δf+Δf. As shown in  FIG. 5 , the shape of the resulting peak is determined by the ratio d/w, with small values giving a near-Gaussian peak and large values resulting in a bimodal spectrum. 
   Further, for the embodiment shown in  FIG. 5 , optimum flatness is obtained for d/w near 0.8. However, interference effects between the first and second beam components  108  and  110  may result in significant disturbances of the resulting output beam such as ripples in the pass band. As such, appropriate path length compensation may be important in minimizing ripple and/or other effects. 
   It is to be appreciated that analogous techniques may be employed with more than two beams to produce a broader spectral profile. For example, many other embodiments may be utilized to provide the beam splitter  102 , the path length compensator  104 , and/or the steering element  106 , for example interferomatic devices may be particularly suitable. Further, such devices may be suitably combined in certain embodiments and/or may not be necessary, depending upon specific design details. As such, it is to be appreciated that generation of a flat top spectral profile (or a substantially flat top spectral profile) based upon an output beam generated by a diffractive tunable filter may be accomplished using various devices and configurations thereof. 
   As discussed above, the tunable filter  40  also may include a polarization recovery element  44  (as shown in  FIG. 1 ). In general, the polarization recovery element  44  conditions those light beams received on the input path  42  so that low insertion loss and low polarization dependent loss may be accomplished. The polarization recovery element  44  may be utilized in conjunction with collinearly propagating input and output beams. Such a configuration may be referred to as a “common path” polarization recovery element. 
   With reference to  FIG. 6 , one embodiment of a common path polarization recovery element  113  may include a first polarization beam splitting element  114  (hereafter, the “first polarization element”) which receives an beam of light  116  via the input path  42  (as shown in  FIG. 1 ). Desirably, the input path  42  provides collimated light by passing such input light beams first through a collimating optical element such as element  56 . Commonly, the beam  116  is arbitrarily polarized, which, upon being received by the first polarization element  114  is split into a first p-polarized input beam  118  and an s-polarized input beam  120 , wherein p-polarization refers to light that has been polarized into a plane that is parallel to the plane in which a given light wave oscillates and s-polarization is in a plane that is perpendicular to the plane of oscillation. In particular, the first polarization element  114  may be formed by a multi-layer dielectric coating via which the s-polarized beam  120  is transmitted through the element  114  and the p-polarized beam  118  is reflected at a 90° angle out of the element  114  towards a highly reflecting cube reflector  122  which reflects the p-polarized beam  118  at a 90° angle towards the mirror  44  (not shown in  FIG. 6 ). 
   As further shown in  FIG. 6 , the s-polarized beam  120  is suitably propogated to a polarization element  124 , which rotates the polarization of the s-polarized input beam  120  into a second p-polarized input beam  126 . In particular, the polarizing element  124  may include a half-wave plate (λ/2) that is oriented at 45° relative to the s-polarization direction is so that upon receiving the s-polarized beam  120 , the polarization element  124  rotates the polarization of the beam by 90°, thereby converting the s-polarized beam  120  into a second p-polarized beam  126 . 
   Additionally, since the path length taken by the first p-polarized beam  118  is longer than the path length taken by the second p-polarized beam  126  (wherein both paths are measured through the first polarization element  114 ), a path length compensator  128  (e.g., a length of material with a highly refractive index) can be added to the s-polarized/second p-polarized path (i.e.,  120 – 126 ) so that the first and second p-polarized beams  118  and  126  arrive at the mirror  45  at substantially the same time, thereby minimizing polarization-mode dispersion in the device (as discussed in greater detail below). 
   Additionally, it is to be appreciated that in a common path polarization recovery element  44 , the input beams (i.e., the first and second polarized beams  118  and  126 ) are propogated upon the same pathway as are any returned diffracted beams. As shown in  FIG. 6  by the dashed arrows (which are shown above/below the corresponding beams for purposes of illustration only), once the first and second p-polarized beams are propogated to the mirror  45  they are then returned to the polarization element  44 ). Once such diffracted beams reach the polarization element  44 , the beams are suitably delayed, rotated, and combined in order to generate a returned output beam  130  whose polarization state is the same as that of the beam  116 . As is discussed above, this returned output beam  130  is then provided on the output beam path  58 . 
   The polarization recovery element can also be made using other types of polarizers, especially those based on double refraction in birefringent crystals. An example of such a known device is disclosed in U.S. Pat. No. 5,886,785, the entire contents of which are incorporated herein by this reference. 
   Another embodiment of a combined polarization recovery element  131  in which separate input and output beams may be generated is shown in  FIG. 7 . In this embodiment, a randomly polarized input beam  132  is steered, by a steering element  134  (e.g., a beam steering prism), to a polarization beam splitter  136 . Please note, that for purposes of illustration, in  FIG. 7 , the input beam is represented by a solid line, while the output beam is represented by a dashed line. Such input and output beams are propogated over a common path, such as the first path  48 , to/from the mirror  45  and from/to the polarization beam splitter  136 . As discussed above with reference to other embodiments, the polarization beam splitter  136  splits the input beam  132  into a transmitted s-polarized input beam  138  and a downward reflected first p-polarized input beam  140 . The p-polarized input beam  148  propagates to a reflector  142  (for example, a right angle prism or alternatively a high reflectivity beam directing mirror) which reflects the p-polarized input beam  140  along a path that is parallel to the s-polarized input beam  138 . 
   A path length compensator  144  is included along the path of the s-polarized input beam  138 , so that the s and p polarized input beams ( 138  and  140 ) travel equivalent optical path lengths. The two input beams  138  and  140  propagate through a Faraday rotator  146  which rotates their respective polarizations by 45° in a common direction. The half-wave plates  148  are advantageously oriented such that both input beams  140  and  138  are p-polarized as they travel towards the mirror  44 , for example, on the first path  48 . 
   On the return path from the tuning assembly, the first and second p-polarized output beams  150  and  152 , respectively, travel along the input beam paths and pass through the half-wave plates  148  and the Faraday rotator  146 , as shown by the dashed lines in  FIG. 7 . The light beams propagating from the Faraday rotator  146  to the polarization beam splitter  136  and reflector  142  have polarization states that are orthogonal to the input light beams  138  and  140  counter propagating along the same paths. The s-polarized output beam  150  is reflected by first reflector  142  and passes through the polarization beam splitter  176  without deflection. The p-polarized output beam  152  is combined with the s-polarized output beam  150  by the polarization beam splitter  136  to form the combined output beam  156 . A second reflector  158  (for example, a right angle prism or alternatively a high reflectivity mirror) reflects the combined output beam  136  through the beam steering element  134  and then to an output beam path  58 . 
   Similarly, in other embodiments, it may be desirable to use separate coupling lenses for the input beam  132  and output beam  160 . Two lens embodiments may be used, for example, to provide an increased working distance between the input lens and the tuning assembly or to permit a spatial filter and/or slit to be inserted in the output beam path. This approach is also useful for a tunable receiver, where a spatial filter and a photodiode are used in place of an output fiber. For example, by rotating reflector  158  (in  FIG. 7 ) by 180°, a linear two-lens embodiment may be provided wherein the input and output lenses are located on opposite ends of the device. Similarly, a beam directing mirror or other reflective element may be utilized to direct an output beam in any direction. As such, it is to be appreciated that the various embodiments of the diffractive tunable filter of the present invention may be configured to direct output beams in practically any direction and/or to any suitable device or component. 
   Since the propagation speed of light through typical fiber and optical components is polarization dependent, for any optical component, the time averaged differential time delay between two orthogonal states of polarization is termed the differential group delay. The larger the differential group delay the larger the polarization mode dispersion. The polarization mode dispersion impacts a telecommunication system by delaying the transmission of different polarization components of an optical pulse. Discriminating between optical pulses at the detector becomes increasingly difficult as the polarization mode dispersion increases. Thus, minimization of polarization mode dispersion across the wavelength passband is an objective of all fiber and optical component design. Typical polarization mode dispersion for optical components in the same category as a tunable filter or tunable receiver is commonly &lt;1 ps, and preferably &lt;0.2 ps over the wavelength passband. 
   The polarization recovery element or modules depicted and described above with reference to  FIGS. 6 and 7  prevent the occurrence of an unacceptable level of polarization mode dispersion by utilizing the compensator  128  and  144 , which minimizes any path length differences which would otherwise arise between the two p-polarized beams incident on the mirror  44 , for example, in  FIG. 7  input beams  138  and  140 . The amount of polarization mode dispersion depends on the thickness and refractive index of the optical components utilized. For example, in the embodiments shown in  FIGS. 6 and 7 , the s-polarized beams ( 120  and  138 , respectively) that are transmitted by the beam splitters ( 114  and  136 ) travel approximately the same distance as the corresponding p-polarized beams ( 118  and  140 ) because of the addition of the compensators  128  and  144 . Thus, the embodiments shown in  FIGS. 6 and 7  minimize polarization mode dispersion by substantially compensating for any differences in delay between the different optical paths. Such delays occur because light travels increasingly slower through media with increasing refractive index. 
   A simple embodiment of a polarization mode dispersion compensator is an optically transparent plate. It is highly preferable that the refractive index of the polarization mode dispersion compensator plate material does not substantially change over the wavelength range of interest because changes in refractive index with wavelength reintroduces polarization mode dispersion. There are many choices for the plate material. For radiation in the telecommunication bands, silicon is a good choice because it has minimal dispersion and a high refractive index. Optimal thickness of the polarization mode dispersion compensator plate is determined from knowledge of the total tunable filter optical path difference and refractive index of the plate. In the polarization recovery element shown in  FIG. 7 , for example, the polarization mode dispersion may be reduced from 10 ps to &lt;0.2 ps using a single simple silicon compensator plate with a mechanical thickness of about 0.57 mm. 
   Similar application of polarization mode dispersion compensation may be easily implemented in any polarization recovery element embodiments disclosed herein. Other polarization mode dispersion compensator embodiments may also be suitable for various embodiments of the present invention. These other compensator embodiments include, but are not limited to, prisms, plates or blocks. The use of several transparent optical plates in the polarization mode dispersion compensator could enable a minimal change in the value of polarization mode dispersion compensation across the wavelength range of interest. For example, two plates could be used where the dispersion of the two plates have different signs. The plates need not have the same mechanical thickness. 
   As discussed throughout the above description, the tunable filter  40  provides certain advantageous features and functions. The first of these advantageous features is the capability of a diffractive tunable filter to tune across large wavelength ranges such as the full C- and/or L-bands. While tuning across such wide wavelength ranges, the tunable filter  40  also supports simply adjustable transmission spectral bandwidths. Such adjustments being possible via the use of spatial slit filters, beam expanders and/or other components. Similarly, the tunable filter  40  supports simple adjustments to a transmissions spectral shape. For example, spectral shapes ranging from Gaussian to flat-top may be supported. Other advantages associated with various embodiments of the present invention include: small variations in the transmission spectral bandwidth over the wavelength tuning range; high adjacent channel isolation and high out-of-band isolation; low insertion loss; low polarization dependent loss; low chromatic dispersion and low polarization mode dispersion; fast wavelength tuning with micro-electromechanical rotary actuators; accurate center wavelength control with built-in wavelength reference and servos; center wavelength tracking to the incoming signal; compatible with tunable receiver separately or co-packaged, low-loss exit slit and receiver photodiode; environmentally robust, small form factor module; and the potential for volume manufacturing at low cost. Thus, it is to be appreciated that the tunable filter  40  offers significant features and functional advantages. 
   While the preferred embodiment of the filter  40  has been described with reference to certain devices and components and specifically to an embodiment in which a photodetector  68  may be utilized in conjunction with a slit  64  or other adjustable spatial filter assembly  60 , it is to be appreciated that the filter may also be configured to not utilize such devices and/or components. For example, an output fiber may be utilized in lieu of the photodetector  68 . Thus, it is to be appreciated that when a returned beam reaches the polarization recovery element  44 , the filter  40  may variously distribute such returned beam for use at an output destination. Examples of such an output destination may include, but are not limited to, a receiver such as a photodiode receiver, an output fiber or any other optical device which may be directly or indirectly connected to the filter  40 . In particular and as shown in  FIG. 1  by the phantom lines, the input fiber  72  may be utilized as a common input/output beam path on which the input beam and the output or diffracted beam are transported to/from the filter  40 , in general, and to/from the collimating optical element  56 , in particular. Further, such an input/output fiber may be suitably connected to an optical circulator  168 , the use and configuration of which are well known in the art. The optical circulator  168  desirably receives the input light beams from a second input fiber  170 . Such input light beams then being propagated via the optical circulator  168  and the input fiber  72  to the collimating optical element  56  for further tuning by the filter  40  in accordance with the above descriptions. Output or diffracted beams are propogated by the polarization recovery element  44  to the collimating optical element  56  onto the input/output fiber  72  and then directed onto a separate output fiber  172  by the optical circulator  168 . As such, it is to be appreciated that the filter of the present invention may output returned beams to additional filter assemblies, photodetectors (for use as a tunable receiver) and/or into separate output beam paths. 
   With respect to  FIG. 8 , the tunable filter  161  may be provided which includes a polarization recovery element which also provides the functions of an optical circulator. In particular, for this embodiment, the collimating optical element  56  includes an optical element that steers input beam  162  and output beams  164  onto separate paths, while the beams are propogated from/to a polarization recovery element  44  and to/from the mirror  45  are propogated along a common first path  48 . As such, this embodiment provides for separation between input beams  162  and output beams  164  between the collimating optical element  56  and the polarization recovery element  44 . Further, in this embodiment both the input fiber  72  and an output fiber  166  are connected to the collimating optical element. In general,  FIG. 8  provides another embodiment of the filter  40  wherein the output beam path  58  filter assembly  60  and photodetector  68 , as well as other related components, are not utilized. 
   Further, it is to be appreciated that various other embodiments of the filter  40  of the present invention may include all or less than all of the elements described above. For example, a second embodiment of the present invention may utilize only a collimating optical element  56 , a polarization recovery element  44 , a mirror  45  and a grating  46  to filter and tune a beam of light to a desired wavelength. In yet another embodiment, the filter may or may not include the beam expanders  74  and  76 , focusing lens  62  or filter assembly  60 , slit  64 , photodetector  68 , power monitor detector  98 , wavelength control unit  92  and associated control devices. As such, the various elements set forth hereinabove, as well as other elements commonly known in the art, may be combined in various and numerous embodiments to provide the diffractive tunable filter of the present invention. Any and all such embodiments are considered to be within the scope of the present invention. 
   Additionally, various other embodiments of the present invention may utilize polarization recovery elements that include, for example, two port, single collimating lens designs. Such two port designs, in addition to those set forth above with respect to  FIGS. 7 and 8 , may utilize first and second bi-refringent crystals, first and second faraday rotators, wedges, and half-wave plates to separate input and output beams without using an optical circulator. Such an embodiment is further described in U.S. provisional patent application Ser. No. 60/402,127 filed Aug. 7, 2002, again, the entire contents of which are incorporated herein by reference. 
   Further, the tunable filter of the present invention may be configured to include multiple filter assemblies. Such embodiments may include using two or more slit or spatial filters (with associated focusing lenses) to further tune an output beam to a desired wavelength. Again, this and other alternative embodiments of the present invention are described in the above referenced U.S. provisional patent applications. As such, it is to be appreciated that the present invention may utilize numerous and various combinations of components to provide a diffractive tunable filter. 
   Although many of the embodiments of the present invention shown and described herein utilize only a single polarization recovery element, it should also be appreciated that two or more polarization recovery element can be used in alternative embodiments of the present invention. For example, a first polarization recovery element can be utilized to receive an input beam of light and condition such input beam of light into two or more p-polarized beams of light, for tuning by the tuning assembly. Additionally, a second polarization recovery element may be utilized to combine filtered p-polarized beams of light into a combined output beam of light. Other embodiments of the present invention may also be implemented as various embodiments of a tunable optical receiver.