Patent Publication Number: US-2009220192-A1

Title: Wavelength selective switch with reduced chromatic dispersion and polarization-dependent loss

Description:
PRIORITY CLAIM TO RELATED US APPLICATIONS 
     To the full extent permitted by law, the present United States Non-Provisional patent application claims priority to and the full benefit of United States Provisional patent application entitled “Wavelength Selective Switch Having Distinct Planes of Operations”, filed on Feb. 28, 2008, having assigned Ser. No. 61/067,635 and United States patent application entitled “Wavelength Selective Switch Having Distinct Planes of Operations”, filed on Jul. 23, 2008, having assigned Ser. No. 12/220,356, incorporated entirely herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to optical communications, and more specifically relates to wavelength division multiplexing. 
     BACKGROUND OF THE INVENTION 
     Modern communications networks are increasingly based on silica optical fiber which offers very wide bandwidth within several spectral wavelength bands. At the transmitter end of a typical point-to-point fiber optic communications link, an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1525-1565 nanometer transmission band (the so-called C-band), and the resulting modulated optical signal is coupled into one end of the silica optical fiber. On sufficiently long links, the optical signal may be directly amplified along the route by one or more amplifiers, for example, optically-pumped erbium-doped fiber amplifiers (EDFAs). At the receiving end of the fiber link, a photodetector receives the modulated light and converts it back to its original electrical form. For very long links, the optical signal risks becoming excessively distorted due to fiber-related impairments, such as, chromatic and polarization dispersion, and by noise limitations of the amplifiers, and may be reconstituted by detecting and re-launching the signal back into the fiber. This process is typically referred to as optical-electrical-optical (OEO) regeneration. 
     In recent developments, the transmission capacity of fiber optic systems has been greatly increased by wavelength division multiplexing (WDM) in which multiple independent optical signals, differing uniquely by wavelength, are simultaneously transmitted over the fiber optic link. For example, the C-band transmission window has a bandwidth of about 35 nanometers, determined partly by the spectral amplification bandwidth of an EDFA amplifier, in which multiple wavelengths may be simultaneously transmitted. All else being equal, for a WDM network containing N number of wavelengths, the data transmission capacity of the link is increased by a factor of N. Depending on the specifics of a WDM network, the wavelength multiplexing into a common fiber is typically accomplished with devices employing a diffraction grating, an arrayed waveguide grating, or a series of thin-film filters. At the receiver of a WDM system, the multiple wavelengths can be spatially separated using the same types of devices that performed the multiplexing, and, then separately detected and output in their original electrical data streams. 
     Dense WDM (DWDM) systems are being designed in which the transmission spectrum includes 40, 80, or more wavelengths with wavelength spacing of less than 1 nanometer. Current designs have wavelength spacing of between 0.4 and 0.8 nanometer, or equivalently a frequency spacing of 50 to 100 GHz respectively. Spectral packing schemes allow for higher or lower spacing, dictated by economics, bandwidth, and other factors. Other amplifier types, for example Raman, that help to expand the available WDM spectrum are currently being commercialized. However, the same issues about signal degradation and OEO regeneration exist for WDM as with non-WDM fiber links. The expense of OEO regeneration is compounded by the large number of wavelengths present in WDM systems. 
     Modern fiber optic networks are evolving to be much more complicated than the simple point-to-point “long haul” systems described above. Instead, as fiber optic networks move into the regional, metro, and local arenas, they increasingly include multiple nodes along the fiber span, and connections between fiber spans (e.g., mesh networks and interconnected ring networks) at which signals received on one incoming link can be selectively switched between a variety of outgoing links, or taken off the network completely for local consumption. For electronic links, or optical signals that have been detected and converted to their original electrical form, conventional electronic switches directly route the signals to their intended destination, which may then include converting the signals to the optical domain for fiber optic transmission. However, the desire to switch fiber optic signals while still in their optical format, thereby avoiding expensive OEO regeneration to the largest extent possible, presents a new challenge to the switching problem. 
     Switching 
     In the most straightforward and traditional fiber switching approach, each network node that interconnects multiple fiber links includes a multitude of optical receivers, which convert the signals from optical to electrical form, a conventional electronic switch which switches the electrical data signals, and an optical transmitter which converts the switched signals from electrical back to optical form. In a WDM system, this optical/electrical/optical (OEO) conversion must be performed by separate receivers and transmitters for each of the W wavelength components on each fiber. This replication of expensive OEO components is currently slowing the implementation of highly interconnected mesh WDM systems employing a large number of wavelengths. 
     Another approach for fiber optic switching, implements sophisticated wavelength switching in an all-optical network. In one version of this approach, the wavelength components W from an incoming multi-wavelength fiber are de-multiplexed into different spatial paths. Individual and dedicated switching elements then route the wavelength-separated signals toward the desired output fiber port before a multiplexer aggregates the optical signals of differing wavelengths onto a single outgoing fiber. In conventional fiber switching systems, all the fiber optic switching elements and associated multiplexers and de-multiplexers are incorporated into a wavelength selective switch (WSS), which is a special case of an enhanced optical cross connect (OXC) having a dispersive element and wavelength-selective capability. Additionally, such systems incorporate lenses and mirrors which focus and reflect light, and lenslets which collimate such light. 
     Advantageously, all the fiber optic switching elements can be implemented in a single chip of a micro electromechanical system (MEMS). The MEMS chip generally includes a two-dimensional array of tiltable mirrors which may be separately controlled. U.S. Pat. No. 6,097,859 to Solgaard et al. describes the functional configuration of such a MEMS wavelength selective switch (WSS), which accepts wavelengths from an incoming fiber and is capable of switching them to any one of multiple outgoing fibers. The entire switching array of up to several hundred micro electromechanical system (MEMS) mirrors can be fabricated on a chip having dimensions of less than one centimeter by techniques well developed in the semiconductor integrated circuit industry. 
     Solgaard et al. further describe a large multi-port (including multiple input M and multiple output N fiber ports) and multi-wavelength WDM wavelength selective switch (WSS), accomplishing this by splitting the WDM channels into their wavelength components W and switching those wavelength components W. The WSS of Solgaard et al. has the capability of switching any wavelength channel on any input fiber port to the corresponding wavelength channel on any output fiber port. Again, a wavelength channel on any of the input fibers can be switched to the same wavelength channel on any of the output fibers. Each MEMS mirror in today&#39;s WDM wavelength selective switch is dedicated to a single wavelength channel whether it tilts about one or more axes. 
     As fiber port counts increase, however, the size of the optics of such WDM wavelength selective switches grows quickly. In turn, the size of the device increases, and the switching element(s) must provide a greater spatial path deflection of the wavelength components. For example, where a MEMS mirror array is employed, the increased size of the device requires a greater tilt angle, increasing the cost of the MEMS mirror array, and increasing the defect rate. Furthermore, many such WDM wavelength selective switches require elements dedicated to a particular special path, i.e., tuned for a particular fiber port. Such dedicated elements increase costs by virtue of their number, but also typically require extremely high performance characteristics and low tolerances, which, likewise, increases costs. 
     Therefore, it is readily apparent that there is a need for an improved WDM wavelength selective switch that allows for increased fiber port counts without substantially increasing the size of the device, and at the same time, reduces the performance requirements for the components thereof, including the switching elements. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly described in a preferred embodiment, the present invention overcomes the above-mentioned disadvantages and meets the recognized need for such a system by providing a wavelength selective switch (WSS) with reduced chromatic dispersion and/or low polarization-dependent loss and utilizing aperture-shared optics to increase the fiber port capacity and optical performance of the WSS, while simultaneously reducing the performance requirements for individual components thereof, wherein optimization of optical performance in functionally distinct orthogonal planes is enabled. 
     According to its major aspects and broadly stated, the present WSS in its preferred form, comprises a reduced chromatic dispersion and/or low polarization-dependent loss optical system utilizing two or more dispersive elements or a double-pass single dispersive element and a turning mirror, wherein the optical signal makes two passes through said single dispersive element when reflected from said turning mirror, and wherein the two or more dispersive elements or double pass single dispersive element have reduced dispersion compared to a functionally equivalent or similar single dispersive element, thus resulting in reduced chromatic dispersion effects and increased wavelength dispersion. In an alternate embodiment, the WSS may comprise a plurality of fiber ports in operable communication with a dispersive element adapted to separate an optical signal into wavelength components, and a switching element adapted to direct a selected wavelength component of an optical input signal from an input fiber port to a selected one of the other fiber ports for output (a 1×N switch). In another alternate embodiment, the switching element is adapted to direct a selected wavelength component of an optical input signal from a selected one of a plurality of input fiber ports to a single output fiber port (an N×1 switch). 
     More specifically, the present WSS preferably comprises a plurality of fiber ports substantially aligned within a switching plane, a plurality of optical elements as disclosed herein operable with each wavelength component of each input or output signal associated with each fiber port, including (1) a plurality of lenses, or their equivalent, (2) a diffraction grating or other dispersive element, or two or more dispersive elements, or a double-pass single dispersive element and a turning mirror, or its equivalent, and (3) a plurality of individually controllable mirrors, each associated with a selected wavelength, or their equivalent. In an alternate embodiment, each of the plurality of mirrors is preferably aligned within a dispersion plane, wherein the dispersion plane is substantially orthogonal with respect to the switching plane. Some elements of the wavelength selective switch, such as the diffraction grating and certain lenses, are designed to be active only in the dispersion plane. While other elements of the wavelength selective switch, such as certain other lenses, are designed to be active only in the switching plane. Still other elements of the wavelength selective switch, such as certain lenses, are designed to be active in both planes. 
     For example, in a preferred embodiment of the present WSS the plurality of optical elements includes, an optical telescope comprising two preferably spherical lenses, i.e., a first telescope lens and a second telescope lens, disposed between the fiber port/free-space interface and the first cylindrical lens. The first telescope lens is preferably disposed at a distance from the fiber port/free-space interface approximately equal to the focal length of the first telescope lens, and the second telescope lens is preferably disposed at a distance from the fiber port/free-space interface approximately equal to the sum of the focal length of the second telescope lens and twice the focal length of the first telescope lens. The second telescope lens is preferably further disposed at a distance from the first telescope lens approximately equal to the sum of the focal length of the second telescope lens and the focal length of the first telescope lens. The first and second telescope lenses are active in both the switching plane and the dispersion plane, and essentially form a “telescope” in front of the fiber array. A first cylindrical lens (L 1 ) is preferably disposed at a distance from the second telescope lens, approximately equal to the sum of the focal length of the second telescope lens, and the focal length of the first cylindrical lens thereof, wherein the first cylindrical lens is active in the switching plane and passive in the dispersion plane. A second cylindrical lens (L 2 ) is preferably disposed at a distance from the first cylindrical lens approximately equal to the sum of the focal length of the second cylindrical lens and the focal length of the first cylindrical lens thereof, wherein the second cylindrical lens is active in the switching plane and passive in the dispersion plane. A third cylindrical lens (L 4 ) is preferably disposed at a distance from the interface between the second telescope lens and first cylindrical lens approximately equal to the sum of focal length thereof, wherein the third cylindrical lens is active in the dispersion plane and passive in the switching plane. Depending upon the selected embodiment, either the diffraction grating, dispersive element, two or more dispersive elements, or a double-pass single dispersive element and a turning mirror is preferably disposed at a distance from the third cylindrical lens approximately equal to the focal length of the third cylindrical lens, wherein the diffraction grating is preferably active in the dispersion plane and passive in the switching plane. The diffraction grating is additionally preferably disposed at a distance from the interface between the second telescope lens and first cylindrical lens approximately equal to the sum of twice the focal length of the first cylindrical lens and twice the focal length of the second cylindrical lens. The diffraction grating, dispersive element, or two or more dispersive elements, or double-pass single dispersive element and turning mirror is additionally preferably disposed at a distance from the interface between the second telescope lens and first cylindrical lens approximately equal to twice the focal length of the third cylindrical lens. A third spherical lens (L 3 ) is preferably disposed at a distance from the diffraction grating approximately equal to the focal length of the third spherical lens, wherein the third spherical lens is active in both the dispersion plane and the switching plane. An array of MEMS mirrors is preferably disposed at a distance from the third spherical lens approximately equal to the focal length of the third spherical lens. 
     The mirrors are preferably formed as a MEMS mirror array, wherein each mirror is preferably tiltable about an axis perpendicular to the switching plane and within the dispersion plane, wherein rotation of a selected mirror about its axis directs a selected wavelength component of an input signal to a selected output fiber port. 
     In the dispersion plane, an input optical signal of a first fiber port preferably enters free-space upon exiting a fiber optic cable, or waveguide, associated therewith, is magnified by the first and second spherical lenses, passes substantially unaltered through the first cylindrical lens, passes substantially unaltered through the second cylindrical lens, is focused by the third cylindrical lens, is angularly dispersed into constituent wavelength components by the diffraction grating, whereafter each constituent wavelength component is focused on an associated one of the plurality of mirrors by the third spherical lens. 
     In the switching plane, an input optical signal of a first fiber port preferably enters free-space upon exiting a fiber optic cable, or waveguide, associated therewith, is magnified by the first and second spherical lenses, is focused by the first cylindrical lens, passes substantially unaltered through the third cylindrical lens, is focused by the second cylindrical lens, is focused by the third cylindrical lens, passes unaltered through the diffraction grating, and is focused on an associated one of the plurality of mirrors by the third spherical lens. Each of the mirrors is selectively adjusted by tilting about the axis to cause the associated wavelength component to travel to a selected output fiber optic cable, or waveguide, thereby connecting the input fiber port and the output fiber port (for the associated wavelength component). 
     After reflection, in the switching plane, each wavelength component passes back through the third spherical lens, and being focused thereby, passing unaltered through the diffraction grating, being focused by the second cylindrical lens, passing substantially unaltered through the third cylindrical lens, and finally being focused on the output fiber port by the first cylindrical lens and by the first and second spherical lenses. In the dispersion plane, the selected wavelength component reflected from each mirror passes back through the third spherical lens, being focused thereby; passes back through the diffraction grating, where it is combined with the other selected wavelengths to form a single WDM beam; passes unaltered through the second cylindrical lens; passes through the third cylindrical lens being focused thereby; passes unaltered through the first cylindrical lens; and finally being focused on the output fiber port by the first and second spherical lenses before reaching the output fiber port. 
     Thus, the wavelength selective switch preferably comprises optical elements selected to optimize performance of the switch in two distinct planes, wherein the fiber port/free-space interfaces, the diffraction element, and the switching element are all disposed at focal points of the optics in both planes. Accordingly, each of the fiber port/free-space interfaces, the dispersive element, and the switching element are disposed at locations where the optical signal exhibits a Gaussian beam waist in both planes simultaneously. 
     The telescope preferably functions to reduce excessively large beam widths at the diffraction grating, thereby allowing a reduction in its size, and therefore the cost, of the diffraction grating. The telescope preferably further functions to alleviate design constraints for the first and second cylindrical lenses imposed by the need for narrow beam widths at the switching mirrors in the dispersion plane for achieving a desired spectral passband shape with smaller mirror dimensions, the need to limit beam widths at the switching mirrors in the switching plane for limiting the switching mirrors&#39; height to width aspect ratios, and the need to reduce mirror tilt angles required for switching between fiber ports spaced a given distance apart. 
     Further, the wavelength selective switch comprises a plurality of fiber ports arranged in a fiber port array, a plurality of optical elements operable with each of the plurality of fiber ports, a dispersion element operable with each of the plurality of fiber ports to separate at least one optical signal into a plurality of wavelength components, and a switching element operable with each of the plurality of wavelength components and controllable to guide a selected one of the plurality of wavelength components to a selected one of the plurality of fiber ports, wherein each of at least one of the plurality of optical elements, the dispersion element, and the switching element affects an optical property of at least one optical signal in a first plane, and wherein each of said at least one of the plurality of optical elements, the dispersion element, and the switching element does not affect said optical property in a second plane, said first plane being generally orthogonal to said second plane. 
     Moreover, the wavelength selective switch still further comprises a means for modifying the size of the optical beam field in at least one of said two generally orthogonal planes, wherein the means provides an additional degree of design freedom by relaxing requirements on at least one of said plurality of optical elements, the dispersion element, the switching element, or the wavelength selective switch. 
     In an alternate embodiment, the wavelength selective switch may include a two-dimensional fiber port array and mirrors that can tilt on two axes, wherein multiplication of the fiber port count may be accomplished by selectively steering one or more wavelength components to one of a plurality of columns of fiber ports in the dispersion plane. 
     In still another alternate embodiment, the wavelength selective switch may include at least one two-dimensional fiber port array, at least one beam steering element, and mirrors that can tilt on two axes, wherein multiplication of the fiber port count may be accomplished by selectively steering one or more wavelength components to one of a plurality of columns of fiber ports in the dispersion plane. 
     In yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include two or more dispersive elements and a turning mirror. 
     In still yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include two or more dispersive elements with lower dispersion and/or line density than or compared to a functionally equivalent or similar single dispersive element. In still another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include a single dispersive element and a turning mirror, wherein the optical signal makes two passes through the single dispersive element when reflected from the turning mirror. 
     In yet still another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include a beam splitter and two or more dispersive elements, wherein the two or more dispersive elements may receive circularly-polarized light from the beam splitter. 
     In yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include two or more dispersive elements and a turning mirror, integrated into a wavelength selective switch. 
     In yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include a single dispersive element and a turning mirror, wherein the optical signal makes two passes through the single dispersive element when reflected from the turning mirror, integrated into a wavelength selective switch. 
     In yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include two or more dispersive elements, integrated into a wavelength selective switch. 
     In yet another alternate embodiment, the chromatic dispersion and/or low reduced polarization-dependent loss optical system may include two or more transmissive dispersive elements. 
     In yet another alternate embodiment, the reduced chromatic dispersion and/or low polarization-dependent loss optical system may include two or more reflective dispersive elements. 
     Accordingly, a feature of the present WSS is its ability to independently select optical elements to optimize performance in one plane in which the optical element is active, without affecting the beam in the other plane. This simplifies design and allows greater flexibility. 
     Another feature of the present WSS is its ability to allow beams to overlap each other in the switching plane optical apertures of the various lenses. This allows for higher fiber port counts for one-dimensional fiber port arrays than previous wavelength selective switches, whose components must dedicate a portion of their optical aperture to each fiber port&#39;s beam, causing the components to grow unacceptably large as large numbers of fiber ports are added. 
     Another feature of the present WSS is its ability to utilize a simple fiber port array for interfacing fibers to free space. 
     Yet another feature of the present WSS is its ability to enable the same wavelengths from one or more optical signals to overlap one another in the WSS while sharing an optical aperture of the optical elements without cross talk occurring between the same wavelengths. 
     Yet another feature of the present WSS is its ability to increase fiber port count multiplicatively by expansion to two-dimensional fiber port arrays, and at lower cost, with better performance than other solutions. 
     Yet another feature of the present WSS is the fiber ports are “colorless”, meaning that there is no limitation to which wavelengths can be switched to/from the fiber ports. 
     Yet another feature of the present WSS is its ability to have “hitless” switching, meaning that a wavelength can be switched to/from one port to another (i.e., an optical route can be established and/or changed) without affecting any other established optical routes, when the beam steering mechanism (e.g., a tiltable micro-mirror) has two axes of steering. 
     Yet another feature of the present WSS is the optical power loss of any established route can be increased in a controlled manner by purposely “detuning” the beam steering mechanism away from its setting that provides minimum optical loss. One use of this feature is to equalize the optical power levels of all routes at the output port (in N×1 operation) or ports (in 1×N operation). 
     Yet another feature of the present WSS is that a relatively high number of optical ports can be accommodated. For example, designs incorporating 42 ports (e.g., a 1×41 or 41×1 WSS) have been developed, although the practical upper limit of port count has not been established. Additionally, there is a variation of the present invention that allows for a multiplicative expansion (e.g., 2×, 3×, etc.) to the number of ports with minimal impact to the basic design. 
     Yet another feature of the present WSS is its ability to maintain a low anticipated insertion loss; for example, less than 5 dB. 
     Yet another feature of the present WSS is its ability to achieve optical performance parameters within established telecom industry-standard specifications (e.g., polarization-dependent loss (PDL), chromatic dispersion (CD), polarization mode dispersion (PMD), etc.). 
     These and other features of the WSS will become more apparent to one ordinarily skilled in the art from the following detailed description of the invention and claims when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present WSS will be better understood by reading the detailed description of the invention with reference to the accompanying drawings, in which like reference numerals denote similar structure and refer to like elements throughout, and in which: 
         FIG. 1  is a plan view illustrating a Gaussian beam path in a switching plane of a wavelength selective switch according to a preferred embodiment, with an orthogonal view of the same components and light beams in the dispersion plane. 
         FIG. 2  is schematically illustrated optical concentrator array using planar waveguide included in the N×1 WSS of  FIG. 1  according to a preferred embodiment; 
         FIG. 3  is a diagram illustrating the relative beam intensity of a Gaussian beam in logarithmic units; 
         FIG. 4  is a diagram illustrating the relative beam intensity of a Gaussian beam of  FIG. 3  in logarithmic units; 
         FIG. 5  is a schematic illustration and formulas representing the transformation of a Gaussian beam passing through a lens; 
         FIG. 6  is a schematic illustration of the beam check points of the wavelength selective switch of  FIG. 1 ; 
         FIG. 7  is a schematic illustration of the wavelength selective switch of  FIG. 1  without first and second telescoping lenses according to an alternate embodiment; 
         FIG. 8  is a schematic illustration of the wavelength selective switch of  FIG. 1  with the addition of a beam steering element according to an alternate embodiment; 
         FIG. 9  is a schematic illustration of two serial transmissive dispersive elements according to an alternate embodiment; 
         FIG. 10  is a schematic illustration of two serial reflective dispersive elements according to an alternate embodiment; 
         FIG. 11  is a schematic illustration of a single transmissive dispersive element with two passes through it according to an alternate embodiment; 
         FIG. 12  is a schematic illustration of a single reflective dispersive element with two passes across it according to an alternate embodiment; 
         FIG. 13  is a schematic illustration of an alternative single reflective dispersive element with two passes across it according to an alternate embodiment; 
         FIG. 14  is a schematic illustration of an alternative dual reflective dispersive element with polarization beam splitter centered there between according to an alternate embodiment; 
         FIG. 15  is a schematic illustration of the wavelength selective switch of  FIG. 1  having quarter wave plate inserted between the diffraction grating and third spherical lens according to an alternate embodiment; and 
         FIG. 16  is a schematic illustration of the end-face of the fiber port array of  FIG. 1  according to an alternate embodiment. 
     
    
    
     It is to be noted that the drawings presented are intended solely for the purpose of illustration and that they are, therefore, neither desired nor intended to limit the invention to any or all of the exact details of construction shown, except insofar as they may be deemed essential to the claimed invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing the preferred embodiments of the present invention, as illustrated in the drawings, specific terminology is employed for the sake of clarity. The invention, however, is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. 
     For example, although the figures and description refer to single-element lenses, it should be understood that each such lens may be replaced by a plurality of elements, including one or more non-planar mirror(s), whereby the same function may be achieved. Such a plurality of elements may additionally offer enhanced performance characteristics. Moreover, such lens may be obtained by various techniques including but not limited to a single glass material, two or more glass materials in a compound fashion, a curved reflective surface, a diffractive surface, a holographic surface, or from combinations thereof. Similarly, while the term optical fiber will henceforth be used exclusively with reference to the means of conducting an optical signal to and from the fiber port array, it should be understood that any waveguide, or combination thereof may be implemented to provide an optical input signal to a free-space interface of the fiber port, and to receive an optical output signal therefrom. Furthermore, while the selective element is described as an array of tiltable switching mirrors, it will be understood that alternate selective elements may be used, including liquid crystal devices, two-dimensional mechanically deformable mirrors, or the like. 
     Referring to  FIG. 1 , wavelength selective switch for switching wavelengths from one or more optical signals, the signals comprising one or more optical wavelengths, each optical wavelength constituting a work piece (WSS) switch  100  employs a bimodal optical system, meaning that it has two distinctly different operational characteristics in orthogonal planes. A key and novel feature of switch  100  is that there is a very large design trade space afforded by the bimodal system that significantly improves the optical performance and fiber port capacity of switch  100  while simultaneously lowering the performance requirements of individual components. Moreover, the bimodal system enables independent selection of optical elements to optimize performance in one plane in which the optical element is active, without affecting the beam in the other plane, thus, simplifying design and allows greater design flexibility. The two optical planes provided by the bimodal optical system are uniquely optimized for the two basic processes that must take place in switch  100 : 1) the separation and recombination (i.e., demultiplexing and multiplexing) of wavelengths in a wavelength-division multiplexing (WDM) signal, 2) the switching of light between fiber ports. The optical plane that performs the fiber port switching in the invention is referred to as the “switching plane”  200 , and the optical plane that performs WDM multiplexing is referred to as the “dispersion plane”  300  since a diffraction grating is preferably employed in this plane to angularly disperse the WDM wavelength components. A feature of the switching plane is its ability to allow beams to overlap each other in the switching plane optical apertures of the various lenses and other optical elements of switch  100 . This allows for higher fiber port counts for one-dimensional fiber port arrays than previous wavelength selective switches, whose components must dedicate a portion of their optical aperture to each fiber port&#39;s beam, causing the components to grow unacceptably large as large numbers of fiber ports are added. 
     In a preferred embodiment of switch  100 , chosen for purposes of illustration, the optical system is shown in  FIG. 1 . It is contemplated herein that although simple single-element lenses are shown in  FIG. 1  it is understood that in practice each lens may in fact be comprised of multiple elements, such as doublet and triplet lenses, in order to provide improved optical performance. In addition, it is further contemplated that the same functionality performed by optical lenses in  FIG. 1  can be performed by non-planar mirrors. The top half of  FIG. 1  illustrates the switching plane  200 , the lower half the dispersion plane  300 . Three lenses in  FIG. 1 , L 1 , L 2 , and L 4 , are cylindrical lenses (denoted “cyl”) and such lenses have optical “power” in one plane but appear as simple flat pieces of glass in the orthogonal plane. The other three lenses in  FIG. 1 , L a , L b , and L 3 , are traditional spherical lenses (denoted “sph”) and therefore appear identical in both planes. 
     Referring again to  FIG. 1 , switch  100  preferably comprises fiber port array  110 , six lenses, three of which are spherical and three of which are cylindrical, comprises optics or optical elements  120 , an array of tiltable switching mirrors comprises switching element  130 , and a diffraction grating comprises dispersive element  140  (only “active” in the dispersion plane), wherein switching plane  200  and dispersion plane  300  are defined. As will be understood by those skilled in the art, switch  100  preferably further includes a baseplate, housing, mounting elements, adhesive, shock absorbing elements, mirror drive electronics, and the like, and as known in the art. 
     Referring to  FIG. 2 , fiber port array  110  preferably includes waveguides  111  adapted to receive and secure optical fibers  112 - 118  in a selected position and/or orientation. In the switch illustrated in  FIG. 1 , each of optical fibers  112 - 117  is substantially aligned, defining a switching plane, and comprising a one-dimensional array. In  FIG. 2 , each of optical fibers  112 - 118  preferably comprises a termination point defining an interface with free-space, wherein optical signals propagating within an optical fiber and a waveguide  111  may exit the fiber and waveguide  111 , and propagate through free-space. Optical fibers  112 - 118  preferably terminate at the edge of the fiber array radiate light containing WDM signals into free-space, which is then captured and manipulated by the various lenses of switch  100 . Similarly, optical signals propagating in free-space that encounter the termination point, at least within a certain range of angular displacements, may enter into, and propagate within the optical fiber. Each such termination point is preferably aligned along line  119 A within switching plane  200 , i.e., disposed at locations where the optical signal exhibits a Gaussian beam waist in both planes simultaneously, for the purpose of but not limited to enabling a condensed core-to-core spacing represented by S and reduced mirror tilt angles required for switching between fiber ports spaced a given distance apart. It is contemplated herein that with regard to the fiber port array  110  that any mention of “fibers” is synonymous with “waveguides” since the fibers that comprise the optical ports of the system may be coupled (i.e., transitioned) to planar waveguides within the fiber array, as shown in  FIG. 2  (note that only seven (7) fibers are illustrated for clarity and that N number of fibers is contemplated). As it may benefit switch  100 , this fiber-to-waveguide transition preferably facilitates the condensation of the core-to-core spacing of waveguides  111  at the edge of the array represented by line  119 A, and further to aid in the implementation of a large number of fibers in fiber port array  110 . Preferably, in  FIG. 2  the core-to-core spacing has been condensed to a value represented by S at the free-space edge of fiber port array  110 . The light emitting from a fiber or waveguide in fiber port array  110  diverges immediately at the free-space edge along line  119 A of the array; hence, there is a beam waist for each fiber at this edge. The width of the beam waist at this location is determined by the fundamental fiber mode. For typical singlemode fiber this beam waist is about 10.4 microns defined at the conventional e −2  Gaussian profile points as shown in  FIG. 4 . 
     In the prior art typically a very small lens (i.e., a lenslet) is placed directly in front of every fiber in the fiber array, but this has the disadvantages of: 1) the optical quality of the tiny lenslets must be very high, 2) the alignment of each lenslet to its associated fiber is extremely critical, 3) the overall vertical height of the optics grows quickly in the switching plane  200  direction as the number of optical fibers  112 - 118  is increased, 4) the highly customized nature of a fiber/lens array results in a very limited number of commercial sources. The present WSS circumvents these problems by using a fully aperture-shared optical (FASO) system; in other words, every beam of light from every optical fiber  112 - 118  passes through every lens, mirror and grating aperture in switch  100 , and occupies a significant portion of the total aperture, such that multiple beams can overlap one another on a given optical element. Preferably the fiber-to-fiber spacing in the fiber array can be condensed to 30 microns or less. This leads to a very compact optical system for switch  100  and relatively small tilt angles for a high port-count switch  100 . The types of fiber port arrays  110  needed for operation of switch  100  are commonly available from a number of commercial sources. Also, the lenses required for operation of switch  100  are also easily obtained from many commercial sources. Therefore, an a key feature of the WSS is that only the switching element  130  and dispersive element  140  are uniquely designed for switch  100 , being the only customized components of switch  100 . 
     Referring now to  FIGS. 3 and 4  switch  100  preferably takes full advantage of the fact that the light beam that emits from optical fibers  112 - 118  of fiber port array  110  has a predominately Gaussian intensity profile and therefore such light beam propagates in free-space according to well-established Gaussian propagation theory. The intensity profile of a Gaussian beam is illustrated in  FIG. 3  (logarithmic units) and  FIG. 4  (linear units). It is clear from  FIG. 3  that there is no convenient “edge” in which to define the diameter of a Gaussian beam, and in fact it theoretically has a diameter that extends to infinity based on the proportion shown in  FIG. 3 . In practice, however, a Gaussian beam will be truncated (i.e., clipped) by some limiting aperture in an optical system. By convention the diameter of a Gaussian beam is often described as the width of the beam where the relative intensity has fallen to a value of 13.5% (−8.7 dB) of its peak, and is denoted herein by the symbol D o  (see equations in  FIGS. 3 and 4 ). This beam width is also commonly known as the e −2  or 1/e 2  beam width (see right-hand axis in  FIG. 3 ). 
     Referring now to  FIG. 5  the transformation of a Gaussian beam B passing through a lens L is described by the relationships illustrated therein, where λ is the wavelength of light. Such relationship is further explained in S. A. Self, “ Focusing of Spherical Gaussian Beams ,” Applied Optics, vol. 22, pp. 658 (1983) and incorporated entirely herein by reference. An important result from Gaussian propagation theory is that points along the optical beam path of minimal beam diameter, called a “beam waist”, can occur simultaneously at the front and back focal planes of a lens. In  FIG. 5 , Equation 2 gives the distance S 2  of the conjugate or output beam waist formed by a lens as a function of the input waist distance S 1  in front of the lens. From this equation, when the input beam waist is located at the front focal plane of the lens (i.e., S 1 =f) then the output beam waist will be located at the back focal plane of the lens (i.e., S 2 =f). This result will be referred to as the F-to-F rule, which enables optimization of WSS performance by control and modification of optical beam parameters and positioning of an optical element. However, the diameter of the two beam waists formed under the F-to-F rule are not generally equal as shown by Equation 3 in  FIG. 5 , except in the special case of Z R1 =f (Note: Z R1  is defined by Equation 1 in  FIG. 5 ). 
     Referring again to  FIG. 1 , optics  120  preferably comprises first cylindrical lens  121 , third cylindrical lens  123 , second cylindrical lens  125  and first spherical lens  122 , second spherical lens  124 , third spherical lens  126 . Preferably, optical telescope lenses  128  comprise first spherical lens  122  and second spherical lens  124 , are disposed between fiber port array  110  and the first cylindrical lens  121  and perform in a telescopic manner. It is contemplated herein that optical telescope lenses  128  may comprise one or more telescopic optical elements and such elements may perform a telescopic function. First spherical lens  122  is preferably disposed at a distance from fiber port array  110  free-space interface line  119 A approximately equal to the focal length f a  of first spherical lens  122 . Second spherical lens  124  is preferably disposed at a distance from fiber port array  110  free-space interface line  119 A approximately equal to the sum of the focal length f b  of second spherical lens  124  and twice the focal length f a  of first spherical lens  122 . Moreover, second spherical lens  124  is preferably further disposed at a distance from first spherical lens  122  approximately equal to the sum of the focal length f b  of second spherical lens  124  and the focal length f a  of first spherical lens  122 . Preferably, optical telescope lenses  128  are active in both the switching plane  200  and the dispersion plane  300 . Optical telescope lenses  128  comprising first spherical lens  122  and second spherical lens  124  shown in  FIG. 1 , and labeled L a  and L b , essentially form a “telescope” in front of fiber port array  110 . Although it is not necessary in an idealized system, optical telescope lenses  128  are a key feature that leads to the realization of relaxed specifications, performance requirements and/or reducing a design constraint of at least one of optics  120 , dispersive element  140 , and switching element  130  and for many of the other components in the optical system of switch  100 . 
     The telescope lenses  128  preferably further function to alleviate design constraints for first spherical lens  122  and second spherical lens  124  imposed by the need for narrow beam widths at switching mirror array  131   a - n  in the dispersion plane  300  for achieving a desired spectral passband shape with smaller mirror dimensions, the need to limit beam widths at switching element  130  in the switching plane  200  for limiting the switching mirrors&#39; height to width aspect ratios, and the need to reduce mirror tilt angles required for switching between fiber ports  110 - 117  spaced a given distance apart. 
     First cylindrical lens  121  is preferably disposed at a distance from second spherical lens  124  approximately equal to the sum of the focal length f b  of second spherical lens  124  and the focal length f 1  of first cylindrical lens  121 . First cylindrical lens  121  is preferably active in switching plane  200  and passive in dispersion plane  300 , i.e. first cylindrical lens focuses optical signals passing therethrough within switching plane  200 , but has substantially no effect on optical signals passing therethrough in dispersion plane  300 , as depicted by ray-tracings  191  and  195  in switching plane  200  verses dispersion plane  300 . Third cylindrical lens  123  is preferably disposed at a distance from line  119 B (positioned at the beam waist between second spherical lens  124  and first cylindrical lens  121 ) approximately equal to focal length f 4  of third cylindrical lens  123 . Third cylindrical lens  123  is preferably active in dispersion plane  300  and passive in switching plane  200 , i.e. third cylindrical lens focuses optical signals passing therethrough within dispersion plane  300 . Second cylindrical lens  125  is preferably disposed at a distance from line  119 B approximately equal to the sum of focal length f 2  thereof and twice focal length f 1  of first cylindrical lens  121 . Moreover, second cylindrical lens  125  is preferably disposed at a distance from first cylindrical lens  121  approximately equal to the sum of the focal length f 2  of second cylindrical lens  125  and the focal length f 1  of first cylindrical lens  121 . Second cylindrical lens  125  is preferably active in switching plane  200  and passive in dispersion plane  300 , i.e. second cylindrical lens focuses optical signals passing therethrough within switching plane  200 . Third spherical lens  126  is preferably disposed at a distance from second cylindrical lens  125  approximately equal to the sum of focal length f 3  of third spherical lens  126  and focal length f 2  of second cylindrical lens  125 . Moreover, third spherical lens  126  is preferably disposed at a distance from third cylindrical lens  123  approximately equal to the sum of the focal length f 4  of third cylindrical lens  123  and the focal length f 3  of third spherical lens  126 . Third spherical lens  126  is preferably active in both switching plane  200  and dispersion plane  300 , i.e. the third spherical lens focuses optical signals passing therethrough within switching plane  200  and dispersion plane  300 . 
     Preferably, optics  120  is a key design feature of switch  100  and based on the particular design and configuration of optics  120 , such optics enables relaxed specifications, performance requirements and/or reduces a design constraint of dispersive element  140 , switching element  130 , and/or other optics  120 . Moreover, it is contemplated herein that optics  120  may include one or more spherical and one or more cylindrical lenses and the like. 
     Although simple single-element lenses are shown in  FIG. 1  for optics  120  it is contemplated herein that in practice each lens may in fact be comprised of multiple elements, such as doublet and triplet lenses, in order to provide improved optical performance of switch  100  and/or optics  120 . Further, the shape of the lenses surfaces is not restricted to be purely spherical or cylindrical in shape, as the case may be, but may have a higher-order “aspheric” shape in order to improve optical performance of switch  100  and/or optics  120  as desired. Further, there is no restriction on the types of glass that the lenses are fabricated from which provides significant flexibility in optimizing the performance of each lens. Further, the optical performance of switch  100  and/or optics  120  preferably will benefit by having every lens surface coated with an anti-reflection coating to eliminate “ghost” reflections which may essentially become optical noise in switch  100 . In addition, it is contemplated herein that the same functionality performed by optical lenses of optics  120  can often be performed by non-planar mirrors. 
     Switching element  130  is preferably formed as tiltable switching mirror array  131   a - 131   n  comprising N number of individually controllable mirrors, each mirror associated with a respective one of N number of wavelengths of an optical signal. Each mirror in switching mirror array  131   a - 131   n  is preferably tiltable about axis  133 , which is preferably oriented perpendicular to switching plane  200  and within dispersion plane  300 . Rotation of a selected mirror in switching mirror array  131   a - 131   n  about axis  133  may direct a corresponding wavelength component of an input signal to a selected output fiber port. Tiltable switching mirror array  131  is preferably disposed at a distance from third spherical lens  126  approximately equal to focal length f 3  thereof, aligned along line  133  within switching plane  200  and dispersion plane  300 , i.e., disposed at locations where the optical signal exhibits a Gaussian beam waist in both planes simultaneously, for the purpose of but not limited to enabling condensed spacing between each mirror of switching mirror array  131   a - 131   n , reduced mirror size, and reduced mirror tilt angles required for switching between fiber ports spaced a given distance apart. 
     Tiltable switching mirror array  131   a - 131   n  preferably is fabricated by the known semiconductor-based micro-electromechanical system (MEMS) technique, but switching element  130  is not restricted to use mirrors fabricated by such technique. Indeed, switching element  130  has the capability to efficiently use mirrors that are substantially larger than typical MEMS mirrors and therefore achievable by other traditional means of mechanical fabrication, perhaps at a significantly lower cost. 
     It is contemplated herein that tiltable switching mirror array  131  of switching element  130 , which serves to steer the beams of light may be replaced by other beam steering mechanisms including, but not limited to, phased-array devices such 2-D pixilated mechanically deformable mirrors and liquid crystals (e.g., liquid-crystal-on-silicon, or LCOS). Herein, for convenience only tiltable MEMS mirrors are used for illustrating the operation of switching element  130  since the functionality of such mirrors within the optical system is known in the art. 
     Dispersive element  140  is preferably formed as diffraction grating  141  and is preferably disposed at a distance from line  119 B approximately equal to the sum of twice focal length f 1  of first cylindrical lens  121  and twice focal length f 2  of second cylindrical lens  125 . Diffraction grating  141  is additionally preferably disposed at a distance from line  119 B approximately equal to twice focal length f 4  of third cylindrical lens  123 . Moreover, diffraction grating  141  is preferably disposed at a distance from third cylindrical lens  123  approximately equal to focal length f 4  of third cylindrical lens  123  and/or diffraction grating  141  is preferably disposed at a distance from third spherical lens  126  of approximately equal to focal length f 3  of third spherical lens  126 . Diffraction grating  141  is preferably active in dispersion plane  300  and passive in switching plane  200 , wherein an optical signal emitted from one of optical fibers  112 - 117  propagating through free-space to diffraction grating  141  is preferably separated into N wavelength components. Moreover, diffraction grating  141  is disposed at a location where the optical signals exhibits a Gaussian beam waist in both planes simultaneously, for the purpose of but not limited to reducing excessively large beam widths at the diffraction grating, thereby allowing a reduction in its size, and therefore the cost, of the diffraction grating. In a preferred embodiment optical signals propagate in a substantially telecentric fashion in the active plane (dispersion plane) of the dispersive element as they ingress, and then egress, from the optical switching element. 
     As a convenience to illustrating the concepts of switch  100  all of the diagrams herein show the use of a transmissive-type of diffraction grating in a functional manner, but not in the true manner in which light is diffracted by such gratings. Equally applicable to switch  100  are reflective-type diffraction gratings. 
     Design Parameters 
     Referring now to  FIG. 6  a further description of the optical functionality of switch  100  as shown in  FIG. 1  is aided by defining several beam check points  150  as shown in  FIG. 6 . The beams at checkpoint  1  (CP 1 )  151  represent a magnified version of the end-face of the fiber array owing to two applications of the F-to-F rule; once through first spherical lens  122  (L a ) and once through second spherical lens  124  (L b ). The divergence angle (or cone angle) of the light beams at CP 1   151  is reduced relative to the divergence angle of the light emitting from fiber port array  110  by the magnification factor of the telescope, denoted M T , which is calculated from the ratio of the focal lengths of first spherical lens  122  to second spherical lens  124 , or M T =f b /f a , which enables optimization of WSS performance by control and/or modification of optical beam parameters and positioning of one or more optical elements. 
     Referring still to  FIG. 6  it is contemplated herein that the F-to-F rule is preferably used consistently and advantageously in both bimodal planes throughout switch  100  in positioning optical components, dispersive element  140 , and switching element  130  of switch  100 . For example, the beams at CP 2   152  represent the conjugate beam waists of CP 1   151 , the beams at CP 3   153  represent the conjugate beam waists of CP 2   152 , and so on. As such, a beam waist is formed between every pair of lenses in the system and where dispersive element  140  and switching element  130  are positioned, as disclosed in  FIG. 1 . Preferably, in optical switch  100  switching element  130  is positioned at beam waists CP 4   154  and CP 7   157  in order to eliminate vignetting (clipping) and diffraction losses which are introduced by tiltable switching mirrors of switching element  130 . Moreover, it is also preferred that after beam  191 ,  195  has propagated a full round trip through optical switch  100  and arrived back at fiber port array  110  that its beam waist should be substantially identical in size to the beam waist that originally emitted from a fiber or waveguide  111  so that the beam can be efficiently coupled back into a similar fiber or waveguide  111 . A preferred objective in designing switch  100  is to design switch  100  with beam waists in both bimodal planes at switching element  130  in the switching plane  200  and at tiltable switching mirror array  131   a - n  in the dispersion plane  300  since this represents the halfway point in a roundtrip of switch  100 . Therefore, adherence to the F-to-F rule, together with the use of lenses of sufficiently optical quality, insures that the end-to-end optical insertion loss of switch  100  will be minimized. 
     The beam paths through the bimodal switch  100  can be determined from traditional geometric optics, also called raytracing. Referring to switching plane  200  in the upper half of  FIG. 6 , raytracing reveals that the F-to-F rule preferably and advantageously produces beams that propagate parallel to the optical axis A (i.e., in a telecentric fashion) at CP 3   153  where the dispersive element  140  is positioned. Such telecentricity is critical for efficient and proper operation of dispersive element  140 ; otherwise, the same wavelength λn from various fibers would not exactly overlay on the designated switching mirror of tiltable switching mirror array  131   a - n  associated with that wavelength λn. Raytracing through third spherical lens  126  (L 3 ) preferably reveals that all beams from every fiber in the fiber array of a particular wavelength λn will converge on the specific switching mirror of tiltable switching mirror array  131   a - n  associated with that wavelength λn. The controlled tilting of this switching mirror for wavelength λn will then create an optical path, or route, between two chosen fibers in fiber port array  110 , which is equivalent to connecting two of the optical ports in switch  100  (on a per wavelength basis). 
     Referring to the dispersion plane  300  in the lower half of  FIG. 6 , raytracing reveals that every beam from every fiber travels in-line with the optical axis A until they reach the dispersive element  140  at CP 6   156 . Preferably, dispersive element  140  will angularly disperse the wavelength components of the WDM signal, and since dispersive element  140  is positioned precisely at the front focal plane of third spherical lens  126  (L 3 ) then the various wavelength λn components, after passing through third spherical lens  126  (L 3 ), will propagate parallel to each other (i.e., in a telecentric fashion) as they approach the switching mirrors of tiltable switching mirror array  131   a - n  positioned at CP 7   157 . This is advantageous to having switching mirrors fabricated on a planar substrate such that their nominal tilt angle in the dispersion plane is zero. In a perfect switch  100  the switching mirrors would therefore only need to tilt in the switching plane in order for switch  100  to function. Moreover, the design of switch  100  utilizing the F-to-F rule has simultaneously provided the desired telecentricity of the optical beams at CP 7   157  and allowed for beam waists to occur at both CP 6   156  and CP 7   157 . It is noted herein that diffraction grating  141  does not operate in the exact manner as shown in  FIG. 1  and  FIG. 6 , but are functionally illustrated as shown for convenience of discussion. Details concerning the diffraction grating are discussed below. 
     It is noted that beam waists are coincident in both planes of switch  100  with the exception that there is not a beam waist in dispersion plane  300  associated with the beam waist located at CP 2   152  in switching plane  200 . It is also noted that all beam crossing locations in switching plane  200 , other than at CP 4   154  (switching mirrors of tiltable switching mirror array  131   a - n ), occur in free-space which significantly reduces the chance of scatter-induced optical crosstalk between fiber ports. 
     Preferably, third spherical lens  126  (L 3 ) simultaneously performs two very different functions: 1) creates convergent beams in switching plane  200 , and 2) creates telecentric beams in dispersion plane  300 . Preferably, it is desirable to start the design of switch  100  by defining dispersive element  140  and switching element  130  parameters early in the design process since these two components are the most unique and, especially in the case of switching element  130 , requires customized components. For these reasons the properties of dispersive element  140  and switching element  130  preferably are allowed to dictate the optical requirements for third spherical lens  126  (L 3 ), which means that third spherical lens  126  (L 3 ) is primarily optimized for dispersion plane  300 . 
     Referring now to the equation below, if Θ nm  represents the difference in dispersive element  140  angles between two adjacent wavelengths λ m  and λ n , and if S mn  represents the switching mirror of tiltable switching mirror array  131   a - n  center-to-center spacing at the same wavelengths, then the required focal length of third spherical lens  126  (L 3 ) can be calculated from: 
     
       
         
           
             
               f 
               3 
             
             = 
             
               
                 S 
                 mn 
               
               
                 2 
                  
                 
                   tan 
                    
                   
                     ( 
                     
                       
                         θ 
                         mn 
                       
                       2 
                     
                     ) 
                   
                 
               
             
           
         
       
     
     At this point in the design of switch  100  all of the other lenses are free variables, meaning that their focal lengths can be selectively chosen to satisfy specific requirements of a WSS design for switch  100 . A great deal of flexibility is afforded by the WSS in selecting telescope lenses  128 , first spherical lens  122  (L a ) and second spherical lens  124  (L b ) in order to reduce the performance burden of individual components while also meeting other system-level performance requirements. Referring again to  FIGS. 1 and 6  it is contemplated, however, that the focal lengths of lenses, first cylindrical lens  121  (L 2 ), third cylindrical lens  123  (L 4 ), and second cylindrical lens  125  (L 2 ) are not completely independent since the focal length of third cylindrical lens  123  (L 4 ) must equal the sum of one focal length f 1  of first cylindrical lens  121  (L 1 ), and one focal length f 2  of second cylindrical lens  125  (L 2 ), i.e., (f 4 =f 1 +f 2 ) [ FIG. 1 ] so that beam waists will exist simultaneously at CP 3   153  and CP 6   156 , which enables optimization of WSS performance by control and/or modification of optical beams and positioning of one or more optical elements. 
     A critical performance specification for a WSS is the spectral passband associated with each WDM wavelength channel. The passband directly relates to the size of the beam waist in dispersion plane  300  at switching mirror of tiltable switching mirror array  131   a - n  located at CP 7   157 . As a rule-of-thumb an adequately broad, flat-topped passband shape is provided for each WDM channel if the e −2  beam width at switching mirror of tiltable switching mirror array  131   a - n , denoted D 7 , preferably is no larger than ¼ the width of the switching mirror, denoted W m , or restated: D 7 &lt;W m /4. For example, preferably with a switching mirror λ n  width of 100 microns D 7  should not be greater than 25 microns. The above rule-of-thumb assumes that the edge-to-edge gap g between adjacent switching mirrors is less than approximately 5% of the width of a mirror. The beam waist D 7  at CP 7   157  preferably is the conjugate of the beam waist D 6  at CP 6   156 . Hence the beam width on a switching mirror of tiltable switching mirror array  131   a - n , D 7 , preferably can be made sufficiently narrow by making the beam waist D 6  at CP 6   156  sufficiently wide. 
     The required beam width D 6  preferably can be calculated from Equation 3 in  FIG. 5  using the previously calculated value for f 4 . Advantageously the design of switch  100  allows for a relatively long focal length f 4  of third cylindrical lens  123  (L 4 ), which in turn helps produce a relatively large beam waist D 6  at CP 6   156 , according to Equation 3 in  FIG. 5 . However, there is a balance to be considered since an excessively wide beam waist D 6  at CP 6   156  results in a larger and more expensive diffraction grating  141  than would otherwise be required by the system passband specifications. Preferably, the actual value of f 4  that is required to obtain the most efficient or desired D 6  at CP 6   156  is dependent on the size of the beam waist D 5  at CP 5   155 , according to Equation 3 in  FIG. 5 , since D 6  is the conjugate beam waist of D 5 . A key feature of the invention is that the beam waist D 5  at CP 5   155  is selectable by the amount of optical magnification M T  provided by optical telescope lenses  128  (L a  and L b ). Therefore, the required value for f 4  is a function of the telescope magnification M T . For practical WSS design, preferably the magnification provided by optical telescope lenses  128  (L a  and L b ) telescope is critical for avoiding excessively large D 6  beam widths. 
     Turning attention now to switching plane  200  in the upper half of  FIG. 6  it is observed that first cylindrical lens  121 , and second cylindrical lens  125  (L 1  and L 2 ) effectively form another telescope. The reason for forming a telescope here preferably is to maintain beam telecentricity from CP 1   151  to CP 3   153  which, as discussed previously, allows third spherical lens  126  (L 3 ) to create convergent beams at switching mirror plane of switching element  130  of CP 4   154 . Preferably, the optical magnification of first cylindrical lens  121  and second cylindrical lens  125  (L 1  and L 2 ) telescope should be minimized for the purpose of reducing the amount of switching mirror tilt required for directing beams between fiber ports. Preferably, the magnification of this second telescope is minimized as f 1  is increased and f 2  is decreased. In addition, this in effect serves to reduce the optical aperture of second cylindrical lens  125  (L 2 ) and third spherical lens  126  (L 3 ) in switching plane  200 . First cylindrical lens  121 , and second cylindrical lens  125  (L 1  and L 2 ) are only operative in switching plane  200 ; hence their focal lengths f 1  and f 2  are variable to the extent that they satisfy the previously mentioned requirement that f 4 =f 1 +f 2  which enables optimization of WSS performance by control and/or modification of optical beam parameters and positioning of one or more optical elements. 
     However, another consideration in optimizing switch  100  is the height of the beam waist on the switching mirror of switching element  130  at CP 4   154 , denoted D 4 . The fabrication and operation of the switching mirrors of switching element  130  preferably is aided by limiting the height of the mirrors to reasonable values; for example, switching element  130  generally benefits from a height to width aspect ratio of 10 or less. The beam diameter D 4  results from repeated applications of Equation 3 in  FIG. 5  from CP 1   151  to CP 4   154 . Therefore, to reduce the size of D 4  then the size of D 3  at CP 3   153  preferably should be increased, which occurs when the size of D 2  at CP 2   152  is decreased, which occurs when f 1  is decreased. However, decreasing f 1  for this purpose is in opposition to increasing f 1  to lower the magnification of first cylindrical lens  121 , and second cylindrical lens  125  (L 1  and L 2 ) telescope as mentioned previously for reducing switching mirror tilt of switching element  130 . Hence, the optimal value of f 1  for switch  100  preferably results from balancing switching mirror tilt angle against the height to width aspect ratio of the switching mirrors of switching element  130 . 
     The general effects of varying the focal lengths of first cylindrical lens  121 , and second cylindrical lens  125  (L 1  and L 2 ) are illustrated in Table 1. In this table ‘SP’ stands for switching plane  200 , ‘DP’ stands for dispersion plane  300 , and ‘F/#’ stands for the ratio of the focal length of a lens divided by its aperture width (It is noted herein that the higher the F/# the easier the lens is to design). There are 4 categories of effects: a check mark means beneficial, an ‘X’ mark means detrimental, an ‘˜X’ means mildly detrimental, and ‘na’ means no significant effect. Preferably, this table may be used as a general guide to improve particular performance parameters of switch  100  by varying the focal lengths of first cylindrical lens  121 , and second cylindrical lens  125  (L 1  and L 2 ). However, it is contemplated herein that in optical design practice there are additional design parameters that may also be varied to achieve particular performance goals in switch  100 , which enables optimization of WSS performance by control and/or modification of optical beam parameters and positioning of one or more optical elements. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 General parametric effects of varying 
               
               
                 the focal length of L1 and L2 
               
            
           
           
               
               
               
            
               
                   
                 effect of 
                 effect of 
               
               
                   
                 increasing focal 
                 increasing focal 
               
               
                 Parameter 
                 length of L1 (f1) 
                 length of L2 (f2) 
               
               
                   
               
               
                 SP F/# of L1 
                 ✓ 
                 na 
               
               
                 SP F/# of L2 
                 ✓ 
                 na 
               
               
                 SP F/# of L3 
                 ✓ 
                 X 
               
               
                 DP F/# of L3 
                 ~X 
                 X 
               
               
                 DP F/# of L4 
                 na 
                 na 
               
               
                 channel passband shape 
                 ✓ 
                 ✓ 
               
               
                 spot aspect ratio at MEMS 
                 X 
                 X 
               
               
                 switching mirror height/width ratio 
                 X 
                 ✓ 
               
               
                 switching mirror tilt angle 
                 ✓ 
                 X 
               
               
                 DP width of grating 
                 na 
                 X 
               
               
                 overall height of optics 
                 ✓ 
                 X 
               
               
                 optical track length 
                 na 
                 X 
               
               
                   
               
            
           
         
       
     
     Above it was preferably noted that a smaller size of D 2  at CP 2   152  is an aid to limiting the switching mirror aspect ratio. Preferably, D 2  can be made smaller if D 1  at CP 1   151  is made larger, which is advantageously provided by telescope lenses  128  (L a  and L b ). Moreover, in support of the earlier use of telescope lenses  128  (L a  and L b ) to preferably provide magnification at CP 5   155  in the dispersion plane  300  (note that since first spherical lens  122  (L a ) and second spherical lens  124  (L b ), (L a  and L b ), are spherical lenses as shown in  FIG. 6  then the beam properties at CP 1   151  and CP 5   155  are identical); hence telescope lenses  128  (L a  and L b ) telescope preferably helps resolve issues in both the switching plane  200  and dispersion plane  300  simultaneously, and this is a key feature of the present WSS. 
     Referring to  FIG. 7  an illustration of an alternate embodiment of the invention with the L a  and L b  telescope elements removed. This alternative embodiment of switch  100 B preferably comprises port array  110 , optics  120 , switching element  130 , and dispersive element  140 , wherein switching plane  200  and dispersion plane  300  are defined. Port array  110  preferably includes fiber channel array  111  adapted to receive and secure optical fibers  112 - 117  in a selected position and/or orientation. In the switch illustrated in  FIGS. 1-2 , each of optical fibers  112 - 117  is substantially aligned within, and defining a switching plane  200 , comprising a one-dimensional array. Each of optical fibers  112 - 117  preferably comprises a termination point defining an interface with free-space, wherein optical signals propagating within an optical fiber may exit the fiber and propagate through free-space. Similarly, optical signals propagating in free-space that encounter the termination point, at least within a certain range of angular displacements, may enter into, and propagate within the optical fiber. Each such termination point is preferably aligned along line  119 A within switching plane  200 . 
     Optics  120  preferably comprises first cylindrical lens  121 , third cylindrical lens  123 , second cylindrical lens  125  and third spherical lens  126 . First cylindrical lens  121  is preferably disposed at a distance from line  119 A approximately equal to the focal length f 1  of first cylindrical lens  121 . First cylindrical lens  121  is preferably active in switching plane  200  and passive in dispersion plane  300 , i.e. first cylindrical lens focuses optical signals passing therethrough within switching plane  200 , but has substantially no effect of optical signals passing therethrough in dispersion plane  300 , as depicted by ray-tracings  191  and  195  in switching plane  200 . Third cylindrical lens  123  is preferably disposed at a distance from line  119 A approximately equal to focal length f 4  of third cylindrical lens  123 . Third cylindrical lens  123  is preferably active in dispersion plane  300  and passive in switching plane  200 , i.e. third cylindrical lens focuses optical signals passing therethrough within dispersion plane  300 . Second cylindrical lens  125  is preferably disposed at a distance from line  119 A approximately equal to the sum of focal length f 2  thereof and twice focal length f 1  of first cylindrical lens  121 . Second cylindrical lens  125  is preferably active in switching plane  200  and passive in dispersion plane  300 , i.e. second cylindrical lens focuses optical signals passing therethrough within switching plane  200 . First spherical lens  126  is preferably disposed at a distance from second cylindrical lens  125  approximately equal to the sum of focal length f 2  of second cylindrical lens  125  and focal length f 3  of first spherical lens  126 . First spherical lens  126  is preferably active in both switching plane  200  and dispersion plane  300 , i.e. first spherical lens focuses optical signals passing therethrough within switching plane  200  and dispersion plane  300 . 
     Similar to  FIG. 1 , switching element  130  is preferably disposed at a distance from first spherical lens  126  approximately equal to focal length f 3 , wherein rotation of a selected switching mirror of tiltable switching mirrors array  131   a - n  about axis  133  may direct a corresponding wavelength component of an input signal to a selected output port. Dispersive element  140 , preferably formed as diffraction grating  141  is preferably disposed at a distance from line  119 A approximately equal to the sum of twice focal length f 1  of first cylindrical lens  121  and twice focal length f 2  of second cylindrical lens  125 , wherein an optical signal emitted from one of optical fibers  112 - 117  propagating through free-space to diffraction grating  141  is preferably separated into N wavelength components. Diffraction grating  141  is preferably active in dispersion plane  300  and passive in switching plane  200 . In use, switch  100 B may be used as a 1×5 switch, wherein a selected one of optical fibers  112 - 117 , such as optical fiber  113 , may be used as an input fiber port, and the remaining ones of optical fibers  112 - 117 , i.e. optical fibers  112  and  114 - 117 , may be used as output fiber ports. Accordingly, an optical signal propagating through optical fiber  113  may enter free-space at line  119 A, preferably generally perpendicularly thereto, along axis A. As illustrated by ray-trace  191  and  195 , the optical signal may propagate as a Gaussian beam, generally parallel to axis A, wherein the beam width expands as the beam propagates further from line  119 A. In switching plane  200 , first cylindrical lens  121  preferably focuses the beam with a focal point generally at position P 1 , but first cylindrical lens  121  preferably has substantially no effect on the beam in dispersion plane  300 , wherein ray-tracing  191  illustrates continued growth of the beam width. The beam is then preferably focused in dispersion plane  300  by third cylindrical lens  123  with a focal point generally at position P 2 , but is substantially unaltered thereby in switching plane  200 . The beam is then preferably focused by second cylindrical lens  125  in switching plane  200 , whereafter the beam propagates generally parallel to axis A, and preferably remains substantially unaltered in dispersion plane  300  (i.e. the focusing of the beam at position P 2  is preferably not disturbed by second cylindrical lens  125  in the dispersion plane  300 ). In dispersion plane  300 , diffraction grating  141 , disposed generally at position P 2  as described above, preferably separates the beam into N wavelength components and, in conjunction with spherical lens  126 , preferably focuses each wavelength component onto a face of a corresponding mirror of mirror array  131 . In switching plane  200 , however, diffraction grating  141  preferably has substantially no effect on the beam, which preferably continues to propagate generally parallel to axis A until each wavelength component is focused onto the face of the corresponding mirror of mirror array  131 , generally at position P 3 . 
     After reflection by mirror array  131 , wavelength components of the optical signal desired to be output on a selected optical fiber at fiber port array  110  are preferably focused by spherical lens  126  having a focal point generally at position P 2  in switching plane  200 , as well as in dispersion plane  300 . Diffraction grating  141  preferably combines, for each optical fiber of fiber port array  110 , the wavelength components selected for output thereon, if any in the dispersion plane  300 . In switching plane  200 , second cylindrical lens  125  and first cylindrical lens  121  preferably focus such combined wavelength components on the selected optical fiber generally at position  119 A. In the illustration of  FIG. 1 , the wavelength component of the input optical signal from optical fiber  115  associated with the mirror has been selected for output on optical fiber  113 , whereas in  FIG. 7 , the wavelength component of the input optical signal from optical fiber  113  associated with the mirror has been selected for output on optical fiber  117 . In dispersion plane  300  of  FIG. 1 , third cylindrical lens  123  preferably focuses each wavelength component of the optical signal generally at position  119 A, while in switching plane  200 , first cylindrical lens  121  and second cylindrical lens  125  likewise focuses each wavelength component of the optical signal generally at position  119 A. 
     Each of mirrors  131   a - 131   n  is preferably controlled by control device C operable to supply mirror control voltages therewith. As illustrated by ray-tracing  191 , a tilt angle of a selected switching mirror of tiltable switching mirror array  131   a - n , such as mirror  131   n , will control onto which fiber port, if any, the associated wavelength component λ n  will be output. Additionally, the tilt about axis  133  of one or more of mirrors  131   a - 131   n  may optionally be controlled such that a spot formed by an associated wavelength component on fiber port array  110  generally at position  119 A may be off-center of the selected fiber port. The degree of tilt about axis  133  of mirror  131   n  may preferably control the signal strength of the output wavelength component, whereby equalization of signal strengths of different wavelength components may be accomplished, or whereby other selective adjustment of the output signal strength of any or all wavelength components may be adjusted. 
     It is important to note that the placement of each of lenses  121 - 127 , diffraction grating  141 , and mirror array  131  causes the beam to exhibit a beam waist (i.e. a local minimum value for beam width), in at least one of switching plane  200  and/or dispersion plane  300 . Specifically, ray-tracing  191  and  195  preferably exhibits a beam waist generally at positions P 1 , P 2 , and P 3 ; while ray-tracing  195  preferably exhibits a beam waist at positions P 2 , P 1 , and  119 A. In switching plane  200 , reduction of the beam width at position P 1  preferably allows reduction of the beam width at position P 2  in switching plane  200 ; in turn, the beam width at position P 3  is reduced, whereby clipping and diffraction losses at mirror array  131  may be avoided. In dispersion plane  300 , avoiding a beam waist at position P 1  preferably allows the beam width at position P 2  to be sufficiently large to achieve a narrow beam waist at position P 3 , whereby a desired spectral passband may be achieved with smaller dimension mirrors (which also facilitates achievement of a desired aspect ratio for the mirrors). Although the beam width at position P 2  is relatively wide in dispersion plane  300 , as discussed above, the beam nevertheless preferably exhibits a beam waist at position P 2  in the switching plane  200 , whereby a beam waist will also be exhibited at position P 3 , further reducing the beam width at position P 3 . Such reduction of the beam width in both switching plane  200  and dispersion plane  300  preferably reduces clipping or signal loss at the mirrors. 
     Referring now to  FIG. 8  is an illustration of an alternate embodiment of the invention shown in  FIG. 1  with the addition of beam steering element (BSE)  162  in the dispersion plane  300 . A WSS with BSE has been disclosed in U.S. Provisional Application, filed Nov. 7, 2006, entitled a Segmented Prism Element and Associated Methods for Manifold Fiberoptic Switches, U.S. patent application filed Jun. 12, 2007, entitled Segmented Prism Element and Associated Methods for Manifold Fiberoptic Switches, U.S. patent application filed Oct. 18, 2007, entitled Beam Steering Element and Associated Methods for Manifold Fiberoptic Switches, U.S. patent application filed Oct. 25, 2007, entitled Beam Steering Element and Associated Methods for Manifold Fiberoptic Switches, U.S. patent application filed Oct. 30, 2007, entitled Beam Steering Element and Associated Methods for Manifold Fiberoptic Switches, are incorporated herein by reference in their entirety. This alternative embodiment of switch  100 C preferably comprises the addition of beam steering element  162 , to switch  100  of  FIG. 1 , preferably positioned between optical telescope lenses  128 , (comprising first spherical lens  122  and second spherical lens  124 ) and first cylindrical lens  121  and operative in the dispersion plane  300  i.e. beam steering element steers optical signals passing therethrough within dispersion plane  300 . Preferably, beam steering element  162  enables the number of optical fiber ports in switch  100 C to be increased in a multiplicative fashion. Moreover, with the use of two dimensional fiber array  110 C containing multiple columns of optical fibers  112 A and with the addition of beam steering elements  162  after second spherical lens  124  of optical telescope lenses  128  (L a  and L b ) the number of optical ports may be doubled or further increased as illustrated in  FIG. 8 , as with a 2-column fiber array  110 D or 3-column fiber array  110 E. Lastly, switch  100 C preferably includes two-axis switching mirrors for tiltable switching mirror array  132   a - n , which includes a first and second tilting axis enabling switching between columns in 2-column fiber array  110 D or 3-column fiber array  110 E. For example, a 1×41 WSS switch  100 C may be expandable to a 1×83 WSS switch when utilizing fiber array  110 D comprising two columns of fibers containing 42 fibers in each column. It is contemplated herein that architectures using three or more columns of fibers, employing three or more beam steering elements, to further increase the port count of switch  100 C. 
     Referring again to  FIG. 8  is an illustration of, yet another alternate embodiment of the invention shown in  FIG. 1 , which includes the addition of beam steering element (BSE)  162  in the switching plane  200 . This alternative embodiment of switch  100 C preferably comprises the addition of beam steering element  162 , to switch  100  of  FIG. 1 , positioned between optical telescope lenses  128 , (comprising first spherical lens  122  and second spherical lens  124 ) and first cylindrical lens  121  and operative in the switching plane  200  i.e. beam steering element steers optical signals passing therethrough within switching plane  200 . Preferably, beam steering element  162  enables light from certain fiber ports to be directed to another set (2 nd  linear array of mirrors) of two-axis switching mirrors of tiltable switching mirror array  132   a - n . This configuration of switch  100 C essentially creates two independently operating WSS systems within the same optical system (switch  100 C). The 2 nd  WSS may be used for a number of purposes, including but not limited to, optical power monitoring of channels within the associated fiber ports. 
     It is contemplated herein that an ideal place to position beam steering element  162  is at CP 3   153  or between separation element  140  and third spherical lens  126  (L 3 ) indicated in  FIG. 6  where the beams from the various fiber ports have gained some physical separation. 
     It is still further contemplated that beam steering element (BSE)  162  may be positioned within switch  100  for the purpose of selecting beams from a portion of fiber array  110  to be directed to another set (one or more rows or columns of linear array of mirrors extending out of the page) of two-axis switching mirrors of tiltable switching mirror array  132   a - n  (an additional switching element  130 ). 
     It is still further contemplated herein that beam steering element (BSE)  162  of switch  100 , which serves to steer the beams of light may be replaced by other beam steering mechanisms including, but not limited to, optical prisms, reflectors, diffractive elements, holographic elements, liquid crystals, liquid crystals on silicon, photonic crystals, and combinations thereof in the art. 
     It is yet further contemplated herein that switching element  130  may comprise dual axis mirror  132   a - n  wherein first axis (shown in  FIG. 7   130 ) is utilized for switching wavelengths or optical signals and second axis (shown in  FIG. 7   133 ) is utilized to attenuate the power level of individual wavelengths or optical signals to obtain equal power levels. Such equalization and attenuation is set forth in United States patent application entitled “Variable Transmission Multi-Channel Optical Switch”, issued on Sep. 28, 2004, having U.S. Pat. No. 6,798,941, which is incorporated herein by reference in its entirety. 
     Enhanced Diffraction Grating 
     It is still further contemplated that several features of the invention may be enhanced as the amount of wavelength dispersion by diffraction grating  141  is increased. It should be noted that there are several ways to accomplish increased wavelength dispersion. The simplest approach is to use a diffraction grating that has very high line density, usually expressed as the number of grooves per millimeter (grooves/mm) in the dispersion direction. However, the polarization dependence of the diffraction efficiency of gratings increases as the number of grooves/mm increases, and this places a practical limit on the selection of gratings used in this manner in order to maintain a sufficiently low polarization-dependent loss (PDL) for the overall optical system. 
     Another approach to achieving increased wavelength dispersion is to utilize two or more diffraction gratings  141  in series. In this manner, diffraction gratings  141  having lower grooves/mm density, and intrinsically lower PDL, can be combined to increase wavelength dispersion while maintaining low total PDL. Referring now to  FIG. 9 , there is illustrated a partial section of switch  100  (the section between third cylindrical lens  123  and first spherical lens  126 ) comprising two transmissive gratings  141 A and  141 B configured in a very compact arrangement and utilizing non-moveable turning mirror  134  between second cylindrical lens  125  and third spherical lens  126 , achieving increased wavelength dispersion, low PDL, and reduced chromatic dispersion. Alternatively, the goal of achieving increased wavelength dispersion, low PDL, and reduced chromatic dispersion, similar to that in  FIG. 9 , may also be implemented with two reflective gratings  141 C and  141 D positioned between second cylindrical lens  125  and third spherical lens  126  as shown in  FIG. 10 . Still further, it is contemplated herein that the desired wavelength dispersion increase, low PDL and reduced chromatic dispersion may be accomplished with implementation of two passes through a single grating, as illustrated in  FIG. 11  with transmissive grating  141 E and turning mirror  134  positioned between second cylindrical lens  125  and third spherical lens  126 . Still further, it is contemplated herein that two passes over a single grating may be implemented as illustrated in  FIG. 12  with reflective grating  141 F and turning mirror  134  positioned between second cylindrical lens  125  and third spherical lens  126 . Still further, it is contemplated herein that two passes over a single grating may be implemented as illustrated in  FIG. 13  with reflective grating  141 G and turning mirror  134  positioned between second cylindrical lens  125  and third spherical lens  126 . It is contemplated herein that other variations to the basic approaches for the diffraction grating improvement in switch  100  illustrated in  FIGS. 9-13 , could be configured utilizing two, three or more diffraction gratings or utilizing two, three or more diffraction gratings and one or more turning mirrors. Additionally, it is contemplated herein that combinations of reflective and transmissive dispersive elements may be used to optimize switch size and light path geometry, as desired. Finally, dispersive elements other than gratings, such as optical prisms, reflectors, diffractive elements, holographic elements, liquid crystals, liquid crystals on silicon, photonic crystals, and combinations thereof and the like, are also contemplated herein. 
     Even when using lower-PDL gratings in the manner suggested by  FIGS. 9-13 , there may be further need to reduce the PDL of the overall switch  100  in order to reach the demanding specifications typical of today&#39;s telecom industry. Since the gratings are likely to be the dominant polarization-dependent component in switch  100 , a technique set forth in  FIG. 14  may be employed, utilizing polarization beam splitter  164  in order to ensure that only circularly-polarized light is incident on gratings  141 H and  141 I. In  FIG. 14 , light (optical signal or beam) enters beam splitter  164  from the left and may have any arbitrary electric-field polarization state. The purpose of the beam splitting layer within beam splitter  164  is to decompose the incoming light beam of generally arbitrary polarization state into two orthogonal linearly-polarized states, one having an electric field oscillation in the plane of the page (labeled S-polarization) and the other having an electric field oscillation perpendicular to the plane of the page (labeled P-polarization). The splitting layer within beam splitter  164  may be designed to consistently reflect or transmit either one of these orthogonal linear polarization states. For the sake of exemplary discussion, and not as manner of limitation, the splitting layer of the beam splitter  164  has been designed to transmit light that is P-polarized and to reflect light that is S-polarized. As such, the decomposed P-polarized component of the incoming light beam is preferably transmitted by the splitting layer within the beam splitter  164  toward the quarter waveplate (QWP)  166 A. The optical “fast” axis of QWP  166 A is oriented at a 45 degree angle relative to the P-polarization of the light, producing a left-hand-circular (LHC) polarization state that strikes the grating  141 H. Upon reflection from the grating  141 H, the polarization state of the light beam obtains a right-hand-circular (RHC) state. As this RHC-polarized beam enters back into QWP  166 A, it is converted into an S-polarized state. Since in this exemplary discussion the splitting layer of the beam splitter  164  has been designed to reflect S-polarized light, the beam will be reflected out of the beam splitter  164  and toward the lens  126 . An analogous situation exists for the decomposed S-polarized component of the incoming light that is reflected by the beam splitting layer within the beam splitter  164  toward the QWP  166 B. In this second exemplary case, the light that strikes grating  141 I will have a RHC-polarization state. The diffraction gratings  141 H and  141 I have equal diffraction efficiencies for RHC-polarized and LHC-polarized light; hence, regardless of the amount of incoming light that is decomposed into either S- or P-polarization states, the net amount of light that leaves the beam splitter  164  toward lens  126  will be the same and will be independent of the polarization state of the incoming light. As such, any natural polarization-dependent diffraction efficiency properties of the gratings are effectively removed. 
     Another relatively simple approach to reduce PDL is illustrated in  FIG. 15 , whereby quarter-waveplate  168 C is preferably inserted in switch  100 D (similar switch as illustrated in  FIG. 1 ) between diffraction grating  141  and third spherical lens  126  just prior to the third spherical lens  126 . The optical axis of quarter-waveplate  168 C is preferably oriented at an angle of 45° with respect to dispersion plane  300  so that any plane-dependent polarization effects prior to third spherical lens  126  are negated. The location of quarter-waveplate  168 C between diffraction grating  141  and third spherical lens  126  is just prior to third spherical lens  126  is advantageous since: 1) the operation of a QWP has some sensitivity to the incident angle of light, and the angles of the beams are not excessive at this location, and 2) PDL can be increasingly mitigated as quarter-waveplate  168 C is placed ever closer to tiltable switching mirror array  131 . Hence, in particular applications it may be feasible to move the QWP closer to the mirror array (between third spherical lens  126  and mirror array  131 ), such as in the form of a “cover glass” just above tiltable switching mirror array  131 . It is contemplated herein to “deposit” or otherwise directly place a QWP directly on the surface of the switching mirrors of tiltable switching mirror array  131 . Moreover, depending on the design of third spherical lens  126 , quarter-waveplate  168 C preferably may be incorporated as an integral component of third spherical lens  126 . For example, quarter-waveplate  168 C may be bonded to a plano surface of third spherical lens  126 , or “sandwiched” between two halves of a doublet lens. 
     Additional design and performance flexibility may be afforded to switch  100  by expanding the optical beam or spot/beam cross section size (i.e., lateral beam width) in fiber port array  110 . As an example, referring back to  FIG. 2  illustrates optical beam expansion in switching plane  200  direction at free-space edge along line  119 A of fiber port array  110 , but the general concept is not solely limited to beam expansion in this plane. Beam expansion in switching plane  200  is preferably achieved during waveguide  111 &#39;s fabrication processes by implementing a gradual widening of waveguide  111  at or near free-space edge along line  119 A so the propagating beam remains in the state of a single Gaussian-like beam. It is noted that this beam expansion process is independent of the direction of light propagation at the free-space edge along line  119 A of fiber port array  110 . By way of illustration, optical beam expansion is accomplished in  FIG. 2  by enlarging waveguides  111  to the width m. 
     Still further design and performance flexibility may be afforded to switch  100  by implementing anamorphic telescope lenses  128  (L a  and L b ) in place of the standard telescope lenses  128  (L a  and L b ) lenses illustrated in  FIG. 1 , which is operable in one of switching plane  200  or dispersion plane  300 . Anamorphic telescope lenses preferably can be accomplished by making the surfaces of telescope lenses  128  (L a  and L b ) non-spherical or by adding additional lenses to L a  and L b  telescope, etc. Anamorphic optical elements change, magnify or distort an optical property in one dimension, axis or plane but not another. A cylindrical lens is one example of an anamorphic optical element. Introducing such characteristics to switch  100  provides relaxation to the previous requirement that the focal length f 4  of third cylindrical lens  123  must equal the sum of the focal length (f 1 ) of first cylindrical lens  121  and the focal length (f 2 ) of second cylindrical lens  125  (L 1  and L 2 ), i.e., (f 4 =f 1 +f 2 ), which enables optimization of WSS performance by control and/or modification of optical beam parameters and positioning of one or more optical elements, and therefore enables even further independent optimization of switching plane  200  and dispersion plane  300 . 
     After a beam of light (representing a particular wavelength λ n  that entered through a particular fiber port; i.e., an optical signal) has made a full round trip though switch  100  and is about to be coupled back into a chosen output fiber (as selected by the angular position of the associated switching mirror λ n  of tiltable switching mirror array  131 ) coupled to waveguides  111  of fiber port array  110 , the efficiency of coupled light energy into the fiber may be selectively reduced by purposely detuning switching mirror λ n  of tiltable switching mirror array  131  away from the angular position that produces maximum coupling efficiency. In effect, purposely detuning switching mirror λ n  provides a means of actively controlling the insertion loss of every optical signal that is chosen to be coupled to an output fiber. Further, if a means external to switch  100  is provided for monitoring of the optical power levels of every optical signal in every output fiber then the aforementioned process of controlled insertion loss can be used to bring every optical signal to a common, or equalized, level of power. Said equalization of optical signals has several significant benefits in an optical network, so the ability to perform signal power equalization is a highly desirable feature of the invention. The degree of optical power coupling into a fiber may be analytically estimated by traditional “overlap integral” methods. Such analytical estimate is further explained in R. E. Wagner, W. J. Tomlinson, “ Coupling efficiency of optics in single - mode fiber components ,” Applied Optics, vol. 21, pp. 2671 (1982) and is incorporated herein by reference. 
     Referring again to  FIG. 2 , in the optical system of switch  100 , angular detuning of a switching mirror λ n  of tiltable switching mirror array  131  for the intentional purpose of de-optimizing the coupling efficiency, or equivalently generating excess insertion loss, results in the applicable beam of light moving laterally on end-face  170  of fiber port array  110  while the angle-of-incidence of the light on end-face  170  of fiber port array  110  remains essentially the same. For this specific scenario an approximate solution to the general coupling overlap integral has been disclosed by St-Amant, et al, in Y. St-Amant, D. Gariepy, D. Rancourt, “ Intrinsic properties of the optical coupling between axisymmetric Gaussian beams ,” Applied Optics, vol. 43, no. 30, pp. 5691 (2004) and incorporated herein by reference in its entirety. 
     For the design case of very closely spaced core-to-core separations s in fiber port array  110  it is preferably advantageous to move the beams orthogonal to the line of fibers B in order to prevent optical crosstalk to neighboring fibers, as illustrated in  FIG. 16 . 
     It should be recognized that the various design parameters set forth herein, enable optimization of WSS performance by control and/or modification of optical beam parameters, positioning of one or more optical elements, and magnifying an optical signal, in an optical switch to enable optimal performance of such optical switch, reducing design constraints, and providing an additional degree of design freedom by relaxing performance requirements, relaxed specifications, and/or reducing a design constraint of at least one of dispersive element  140 , switching element  130 , or other optics  120  in the optical system of switch  100 . 
     It should further be recognized that the various design parameters set forth herein, including positioning of optical elements, dispersive element  140 , switching element  130  and/or other optics  120  proximate the beam waist(s) and/or focal point(s) of optical elements within switch  100  both preserves the Gaussian shape of the optical beams throughout the optical switch  100  and reduces the overall optical path length for each optical signal and/or wavelength within switch  100 . 
     The use of ‘a’ or ‘an’ in the following claims is to be interpreted as—does not require more than one but it permits more than one. In addition the use of “array’ herein includes one and more than one row. 
     Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.