Abstract:
A wavelength selective optical switch particularly usable as a programmable N×M optical switch in a multi-wavelength communication system. The switch uses a grating that separates multi-channel optical signals into a plurality of optical channels, and combines a plurality of optical channels into multi-channel optical signals. Programmable mirrors switch each optical channel to any of a plurality of fibers coupled to the switch.

Description:
RELATED APPLICATION 
     This application is a divisional of prior U.S. application Ser. No. 11/399,215, filed Apr. 6, 2006 now abandoned, which is a continuation of prior U.S. application Ser. No. 10/460,899, filed Jun. 12, 2003 now U.S. Pat. No. 7,058,251, the entire contents of which are hereby incorporated by reference herein and made a part of this specification, and claims the benefit of U.S. Provisional Application Nos. 60/388,358 filed Jun. 12, 2002, and 60/397,944 filed Jul. 23, 2002, the disclosures of which are also incorporated fully herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of optical communications, and more particularly, to a wavelength selective optical switch for use in optical multiplexing. 
     BACKGROUND OF THE INVENTION 
     For several decades, fiber optics have been used for communication. Specifically, fiber optics are used for data transmission and other telecommunication applications. Despite the enormous information carrying capacity of fiber, as compared to conventional copper cable, the high cost of installing fiber optics presents a barrier to full implementation of fiber optics, particular as the “last mile”, from the central office to residences and businesses. 
     One method of increasing carrying capacity without incurring additional installation costs has been to multiplex multiple signals onto a single fiber using various methods, such as time division multiplexing, where two or more different signals are carried over the same fiber, each sharing a portion of time. Another more preferred multiplexing method is wavelength division multiplexing (WDM), where two or more different wavelengths of light are simultaneously carried over a common fiber. 
     Wavelength division multiplexing can separate a fiber&#39;s bandwidth into multiple channels. Dividing bandwidth into multiple discreet channels, such as 8, 16, 40, or even as many as 160 channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM), is a relatively lower cost method of substantially increasing telecommunication capacity, using existing fiber optic transmission lines. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber-optic line or other optical waveguide and then later separated again into the individual signals or channels at the opposite end or other point along the fiber-optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength sub-ranges) is of growing importance to the fiber-optic telecommunications field and other fields employing optical instruments. 
     Optical multiplexers are known for use in spectroscopic analysis equipment and for the combination or separation of optical signals in wavelength division multiplexed fiber-optic telecommunications systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters. 
     Approaches for selectively removing or tapping a channel, i.e., selective wavelengths, from a main trunk line carrying multiple channels, i.e., carrying optical signals on a plurality of wavelengths or wavelength sub-ranges, is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. Hicks, shows filter taps, as well as the use of gangs of individual filter taps, each employing high performance, multi-cavity dielectric pass-band filters and lenses for sequentially removing a series of wavelength sub-ranges or channels from a main trunk line. The filter tap of Hicks, returns a multi-channel signal to the main trunk line as it passes the desired channel to a branch line. One known demux is disclosed in Pan et al., U.S. Pat. No. 5,652,814, FIG. 25. In Pan et al., the WDM input signal is cascaded through individual filter assemblies, consisting of a fiber collimator, thin film filter, and a fiber-focusing lens. Each filter is set for a given wavelength. However, aligning the fibers for each wavelength is costly and errors in the alignment contribute significantly to the system losses. Further, FIG. 13 of Pan et al. teaches the use of a dual fiber collimator, thin film filter, and a dual fiber focusing lens to selectively DROP and ADD a single wavelength or range of wavelengths. As discussed above, aligning the collimators is expensive. 
     Polarization dependent loss (PDL) is also a problem in WDM system because the polarization of the light drifts as it propagates through the fiber and furthermore this drift changes over time. Thus, if there is PDL in any component, the drifting polarization will change the signal level, which may degrade the system operation. 
     Other multiplexer devices may be employed to add or drop channels in WDM systems. These systems are commonly known as optical add/drop multiplexers, or OADM. Another OADM, disclosed by Mizrahi in U.S. Pat. No. 6,185,023, employs fiber Bragg gratings to demux and mux signals in a WDM system. This method requires optical circulators and multiple components. 
     However, the multi channel OADM designs discussed above are not programmable by the end user. That is, each multiplexer is designed and manufactured to mux (add) specific channels; or when used in reverse each multiplexers is also designed and manufactured to demux (drop) specific channels. This limitation mandates that the optical system&#39;s parameters be fixed before installation. Changes are not possible without replacing the fixed optical multiplexers with different designed multiplexers. This is expensive. 
     One known programmable OADM is discussed in Boisset et al, International Publication No. WO01/13151. In Boisset et al., the desired add/drop channel is programmed by translating a segmented filter. To achieve this translation however, a large mechanical mechanism is employed. A further limitation to Boisset et al. is that only a single channel may be added or dropped per device. Designers may employ multiple devices, deployed in series, and programmed as necessary to add/drop the correct channel; however, this approach requires multiple devices and has multiple points of failure. Furthermore, the size of such a device would be overly large and therefore not practical for many applications where space is limited. 
     An OADM disclosed by Patel et al., U.S. Pat. No. 5,414,540 uses bulk gratings to demultiplex and multiplex WDM input and output signal and compact liquid crystal switches. Because the device uses polarization to switch the light path, the arbitrarily polarized incident beam must be converted into a singular polarization prior to switching by the liquid crystal. Patel teaches the use of a birefringent crystal and a Wollaston prism to separate the incident beam into two polarizations state located between the focusing lens and the liquid crystal. While the OADM disclosed by Patel is relatively compact; it only provides 2×2 switching for each wavelength. There is an Input and Add channel that may be selectively sent to either the Output or Drop channel. If higher dimensionality switching is required, then additional components are required. The additional components require additional space, add attenuation, and add cost to the system. A 2×2 switch has four sub beams incident on the liquid crystal (because of the conversion from an arbitrary polarized beam to a single polarization for the liquid crystal switch) and four sub beams leaving the liquid crystal. Thus, the aperture of the lens focusing the light on the grating must be a minimum of 4× larger than that required for a single sub beam in one polarization. 
     An OADM disclosed by Ranalli et al., U.S. Pat. No 6,285,500, that uses bulk gratings to demultiplex and multiplex WDM input and output signal and compact liquid crystal switches. Because the device uses polarization to switch the light path, the arbitrarily polarized incident beam must be converted into a singular polarization prior to switching by the liquid crystal. Ranalli teaches the use of half-wave plates and a thin film polarization beamsplitter located before the lens that focuses light onto the liquid crystal. Because of the optical arrangement, the aperture of the lens focusing the light on the grating must be a minimum of 2× larger than that required for a single sub beam in one polarization. While the OADM disclosed by Ranalli is relatively compact; it only provides 2×2 switching for each wavelength. There is an Input and Add channel that may be selectively sent to either the Output or Drop channel. If higher dimensionality switching is required, then additional components are required. The additional components require additional space, add attenuation, and add cost to the system. 
     A OADM disclosed by Patel et al., U.S. Pat. No. 6,327,019, uses bulk gratings to demultiplex and multiplex WDM input and output signal and compact liquid crystal switches. The OADM disclosed by Patel provides for dual 2×2 switching for each wavelength. There are two Input and two Add channels that may be selectively sent to either the two Output or two Drop channels. If higher dimensionality switching is required, then additional components are required. The additional components require additional space, add attenuation, and add cost to the system. Because liquid crystals use polarization to switch the light path, the arbitrarily polarized incident beam must be converted into a singular polarization prior to switching, which doubles the required aperture of the lens. Thus, the dual 2×2 switch has eight sub beams incident on the liquid crystal and eight sub beams leaving the liquid crystal. Thus, the aperture of the lens focusing the light on the grating must be a minimum of 8× larger than the aperture required for single incident beam in one polarization. 
     An OADM disclosed by Aksyuk, et al, U.S. Pat. No. 6,204,946 uses a bulk grating to demultiplex and multiplex WDM input and output signal and Micro Electrical Mechanical Systems (MEMS) to provide the switching. This is another relatively compact switch, but it only provides 2×2 switching for each wavelength. There is an Input and Add channel that may be selectively sent to either the Output or Drop channel. If higher dimensionality switching is required, then additional components are required. The additional components require additional space, add attenuation, and add cost to the system. Because Aksyuk uses circulators to separate the Input and Add channels from the Output and Drop channels, the aperture of the lens focusing the light on the grating must be a minimum of 2× larger than the of a single incident beam. 
     Another known programmable OADM is discussed Tomlinson, U.S. Pat. No. 5,960,133, uses a bulk gratings to demultiplex and multiplex WDM input and output signal, and MEMS mirrors to switch. The OADM disclosed by Tomlinson is programmable and provides for dual 2×2 switching. Tomlinson teaches a switch that does not require the use of circulators, potentially increasing the system efficiency. However, the aperture of the lens focusing the light on the grating must be a minimum of (1+Sqrt[2])× larger than the of a single incident beam for a 2×2 switch. Furthermore, for a dual 2×2 without circulators, the aperture of the lens focusing the light on the grating must be a minimum of Sqrt[10]× larger than that of a single incident beam. Thus, the size and expense of the focusing lens required grows quickly when moving from a single to dual switching. 
     A programmable optical multiplexer/demultiplexer, disclosed by Marom et al, in US Pat. App. 02/0196520, independently assigns every input optical channel in a signal to depart from any desired output port, which provides the functionality of 1×N switching for every wavelength. Marom teaches the use of a bulk grating to multiplex/demultiplex WDM input and output signal, and MEMS mirrors to switch. The demultiplexer device can also be operated in the reverse direction, and thus achieve programmable optical multiplexer functionality. However, the size and expense of the lens required by the demultiplexer also grows linearly with port count. A 1×5 port programmable optical multiplexer/demultiplexer requires a lens to focus light on the MEMs mirrors with an aperture at least 5× as large as that of a single incident beam. 
     Optical gratings are a periodic structure, which diffract light according to the wavelength. They can be used in either reflection or transmission. Gratings can be produce by modulating the surface height of a substrate or by modulating the index of refraction of a structure. 
     The spectral resolving power, R=λ/Δλ, of a grating is a measure of its ability to separate adjacent spectral lines, where A is average wavelength of a line and Δλ is the limit of resolution. The theoretical resolving power is
 
 R=Nd  cos Γ(sin α+sin β)/λ
 
where N is the number of groves, d is the groove spacing, Γ is the angle between the incident light path and the plane perpendicular to the groves, α is the angle of incidence on the grating and β is the angle of diffraction. If the grating is planar and the groove spacing is uniform, then the resolving power is proportional to the ruled with of the grating, N d. Spectral resolving power is an important design parameter; the greater the resolving power the greater the optical separation between channels, and ultimately the channels a grating-based system can accommodate. For low-loss transmission of OC-768 channels and a channel spacing of 100 GHz, it is preferred that the resolution be 20 GHz or finer.
 
     Of course, a larger grating can be employed to increase the spectral resolving power, however, that requires a combination of more physical space and faster or longer focal length lenses that are more expensive. Another approach has been to decrease the spacing of the grating grooves, d. However, the maximum theoretical efficiency of the grating decreases for small groove separations. When the separations between the grooves spacing is comparable to the wavelength of light, it is possible to get gratings that operate with high efficiency (&gt;90%) for any incident polarization state. As the groove spacing approaches half the wavelength of light, it is possible to get high efficiency for only light polarized parallel to the grooves. For even smaller grooves separations, it is not possible to get high efficiency in either polarization state. Thus, there is a practical limit to increasing spectral resolving power through decreased grating groove separations. The relationship between grating efficiency, polarization, and groove shape is well known in the art and described in Diffraction Grating Handbook, Ch. 9, 4th Ed, Richardson Grating Laboratory, C. Palmer, (2000), which is hereby incorporated by reference. Each bulk diffraction grating device requires a minimum number of grating grooves to achieve a given spectral resolution. The minimum size is determined by the optical configuration of the device and the grating parameters. 
     One desired application for optical multiplexing and demultiplexing systems is in optical wavelength switch. An optical wavelength switch demultiplexes optical signals, switches the signals, and then and multiplexes to a plurality of optical ports. 
     The ability to switch to a number of optical ports in wavelength switches introduces another limiting design factor. In order to switch to a number of physical ports the size of the device must not only accommodate the space needed for the ports, but the optics must also direct the optical signals to those ports. As the number of ports increases the optical directing means (typically a moveable mirror) must be capable of directing the optical beams across a larger physical area where the optical ports are located. Also, as the optical beams must exit the ports within an acceptance angle so as to be coupled into the optical fiber, the ports must be physically located within a certain placement angle from the directing means. As the placement angle increases, the optical directing means generally becomes more expensive and the insertion loss increases. An additional lens may be used to focus the beam—however, this adds component cost and size to the device. 
     If the optical beams inside the device are made larger so as to increase spectral resolution the device size must increase, and in some cases larger lenses must be used. For example, an optical switch of the type disclosed by Marom et al. US 2002/0196520 A1, with one input port and four output ports (1×4) might be capable of switching 64 wavelengths spaced at 100 GHz. If the same design were used to switch 16 ports the grating and the grating aperture would likely need to be 4× larger to accommodate 100 GHz channels or if the grating was the same size, the system could switch 16 wavelength channels spaced at 400 GHz. The device disclosed by Marom cannot provide adequate spectral resolution for a large number of ports and a large number of wavelengths using small compact lenses that are easy to manufacture. 
     An optical wavelength switch disclosed by Waverka et al. WO 01/37021 uses a bulk diffraction grating and MEMS mirrors to provide 1×N switching. However, this design has a major drawback. Because the image is translated at the spectral focal plane by the MEMS mirrors, the incident angle on the grating changes with switch position, which in turn changes the angular dispersion provided by the grating. Thus, the device is unable to achieve adequate spectral resolution for a large number of ports and a large number of wavelengths with low losses. Waverka also teaches the use of cylindrical optics to produce an elliptical beam that minimizes the size of the grating. However, because the cylindrical optics are used symmetrically to both collimate light for the grating and to focus the light on the switch array, the footprint of the optical beam at the switch is a very high aspect ratio ellipse. Thus, very long thin, hard to fabricate switches are required. 
     It is an object of the present invention to provide improved optical switching that reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, a wavelength selective optical switch, can establish a reconfigurable connection between any two fibers from a plurality of fibers in a fiber array, independently for each optical wavelength that enters the switch. One of a plurality of cylindrical lenses receives a first multi-channel optical signal from an optically coupled fiber in the array, the first multi-channel optical signal is directed through an anamorphic lens to a grating. The grating diffracts the first multi-channel optical signal according to the wavelengths of each individual optical channel, and directs each channel through a rotationally symmetric lens that focuses the individual optical channels near one of a plurality of programmable mirrors. Each mirror is associated with a particular individual optical channel. Depending upon the programmed state of the mirror, the individual optical channel is directed to any one of the fibers in the fiber array by way of the rotationally symmetric lens, the grating, the anamorphic lens, and one of the plurality of cylindrical lenses. By changing the programmed state of the mirror, the individual optical channel may be switched to any of the fibers in the fiber array. 
     The invention may be used as a programmable optical demultiplexer, wherein one of the plurality of cylindrical lenses receives a first multi-channel optical signal from an optically coupled fiber in the array, the first multi-channel optical signal is directed through the anamorphic lens to the grating. The grating diffracts the first multi-channel optical signal according to the wavelengths of each individual optical channel, and directs each channel through a rotationally symmetric lens that focuses the individual optical channels near one of a plurality of programmable mirrors. Each mirror is associated with a particular individual optical channel. Depending upon the programmed state of the mirrors, individual optical channels are directed to any one of the fibers in the fiber array by way of the rotationally symmetric focusing lens, the grating, the anamorphic lens, and one of the plurality of cylindrical lenses. By changing the programmed state of the mirrors, any of the individual optical channels may be switched to any of the fibers in the fiber array. Further, in the case where two or more of the individual optical channels are switched to a single fiber, upon illuminating the grating the two or more individual optical channels are multiplexed into a second multi-channel light signal. 
     The device may also be operated in the “opposite direction” as a programmable multiplexer; that is two or more of the plurality of cylindrical lenses each receives one or more different individual optical channels from optically coupled fibers in the array, the individual optical channels are directed through the anamorphic lens to the grating. The grating diffracts the first multi-channel optical signal according to the wavelengths of each individual optical channel, and directs each channel through a rotationally symmetric lens that focuses the individual optical channels near one of a plurality of programmable mirrors. Each mirror is associated with a particular individual optical channel. Each of the mirrors is programmed to reflect each of the individual optical channels to any one of the fibers in the fiber array by way of the rotationally symmetric focusing lens, the grating, the anamorphic lens, and one of the plurality of cylindrical lenses. By changing the programmed state of the mirrors, all of the individual optical channels may be switched to any of the fibers in the fiber array. 
     In accordance with the first aspect of the invention, the programmed state of the mirrors is such that a mirror connection may be established at any place along the fiber array. In this regard, the device can be programmed to establish optical connectivity, for each optical channel, between any of the fibers in the array. That is the device can operate as an N×1×M switch; directing N unique individual optical channels received from one fiber in a fiber array to any of the M fibers in the fiber array. 
     The device may also direct two or more individual optical channels centered at the same wavelength and received from two or more fibers in the fiber array to other fibers in the array. However, the switching matrix is more restrictive as the same mirror is used for the direction of all the individual optical channels centered at the same wavelength. In this manner, each of the individual optical channels centered at the same wavelength are directed to the fiber in the fiber array that is opposite the mirror&#39;s connection. For example, consider a nine port device coupled to a nine fiber array (the consecutive fibers numbered 1 through 9) which receives a first individual optical channel centered at wavelength x on port  1 , and receives a second individual optical channel centered at wavelength x on port  2 . If the corresponding mirror connection for wavelength x is set such that the light at wavelength x entering the switch from fiber  3  also leaves from fiber  3 , then the first individual optical channel at wavelength x will be directed to fiber  5 , and the second at wavelength x to fiber  4 . In this manner, the device does not operate as an N×1×M switch, but still provides numerous switching options. Such options will be clear to one skilled in the art. 
     In accordance with a second aspect of the invention, a wavelength selective optical switch, can establish a reconfigurable connection between any two fibers from a plurality of fibers in a fiber array, independently for each optical wavelength that enters the switch. One of a plurality of cylindrical lenses receives a first multi-channel optical signal from an optically coupled fiber in the array, the first multi-channel optical signal is directed through an anamorphic lens, to a beam splitter. The beam splitter separates light that is s-polarized from light that is p-polarized, and directs both out of the beam splitter through a first and second quarter waveplate. The s-polarized light illuminates a first grating, and the p-polarized light illuminates a second grating. The gratings diffract the respective s-polarized and p-polarized first multi-channel optical signal according to the wavelengths of each individual optical channel, and direct the respective s-polarized and p-polarized light of each individual optical channel back through the quarter waveplate into the beam splitter, which recombines the s-polarized and p-polarized light of each channel and directs the individual optical channels through a rotationally symmetric lens that focuses the individual optical channels near one of a plurality of programmable mirrors. Each mirror is associated with a particular individual optical channel. Depending upon the programmed state of the mirror, the individual optical channel is directed to any one of the fibers in the fiber array by way of the rotationally symmetric lens, the beam splitter, waveplates and gratings, the beam splitter, anamorphic lens, and one of the plurality of cylindrical lenses. By changing the programmed state of the mirror, the individual optical channel may be switched to any of the fibers in the fiber array. 
     The device may also be operated in the “opposite direction” as a programmable multiplexer; that is two or more of the plurality of cylindrical lenses each receives one or more different individual optical channels from optically coupled fibers in the array, the individual optical channels are directed through the anamorphic lens to the beam splitter. The beam splitter separates the s-polarized and p-polarized states and directs each to the first and second gratings. The gratings diffracts the first multi-channel optical signal according to the wavelengths of each individual optical channel, and directs each channel back through the beam splitter recombining the s-polarized and p-polarized states, and directing the individual optical channel through the rotationally symmetric lens that focuses the individual optical channels near one of a plurality of programmable mirrors. Each mirror is associated with a particular individual optical channel. Each of the mirrors is programmed to reflect each of the individual optical channels to any one of the fibers in the fiber array. By changing the programmed state of the mirrors, all of the individual optical channels may be switched to any of the fibers in the fiber array. 
     In accordance with the second aspect of the invention, the programmed state of the mirrors is such that a mirror connection may be established at any place along the fiber array. In this regard, the device can be programmed to establish optical connectivity, for each optical channel, between any of the fibers in the array. That is the device can operate as an N×1×M switch; directing N unique individual optical channels received from one fiber in a fiber array to any of the M fibers in the fiber array. 
     The device may also direct two or more individual optical channels centered at the same wavelength and received from two or more fibers in the fiber array to other fibers in the array. However, the switching matrix is more restrictive as the same mirror is used for the direction of all the individual optical channels centered at the same wavelength. In this manner, each of the individual optical channels centered at the same wavelength are directed to the fiber in the fiber array that is opposite the mirror&#39;s connection. For example, consider a nine port device coupled to a nine fiber array (the consecutive fibers numbered 1 through 9) which receives a first individual optical channel centered at wavelength x on port  1 , and receives a second individual optical channel centered at wavelength x on port  2 . If the corresponding mirror connection for wavelength x is set such that the light at wavelength x entering the switch from fiber  3  also leaves from fiber  3 , then the first individual optical channel at wavelength x will be directed to fiber  5 , and the second at wavelength x to fiber  4 . In this manner, the device does not operate as an N×1×M switch, but still provides numerous switching options. Such options will be clear to one skilled in the art. 
     In accordance with several aspects of the invention one or more wave plates may be employed to reduce polarization dependent loss (PDL). The one or more wave plates rotates the polarization so that light that is s-polarized on a first pass is p-polarized on a second pass and there is no net polarization dependent loss (PDL) for light traveling through the device. Similarly, a polarization converter such as a rutile crystal may be used in combination with wave plates to reduce PDL. 
     In accordance with several aspects of the invention, the grating or gratings may operate at or near Littrow to increase the diffraction efficiency. In accordance with several aspects of the invention one or more transmission gratings may be employed. In accordance with several aspects of the invention, a beam displacer made of birefringent crystals or multi-layer coated polarization beamsplitters may be employed to separate and combine optical beams. Different aspects of the invention may also be employed together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(A)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 1(B)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 1(C)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 2(A)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 2(B)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 2(C)  is a perspective view of a first embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 3(A)  is a perspective view of a second embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 3(B)  is a perspective view of a second embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 3(C)  is a perspective view of a second embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 4  is a perspective view of a second embodiment of a wavelength selective optical switch detailing the optical polarization states at various locations within the device. 
         FIG. 5(A)  is a perspective view of a third embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 5(B)  is a perspective view of a third embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 5(C)  is a perspective view of a third embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 6(A)  is a perspective view of a fourth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 6(B)  is a perspective view of a fourth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 6(C)  is a perspective view of a fourth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 7(A)  is a perspective view of a fifth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 7(B)  is a perspective view of a fifth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 7(C)  is a perspective view of a fifth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 8(A)  is a perspective view of a sixth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 8(B)  is a perspective view of a sixth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 8(C)  is a perspective view of a sixth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 8(D)  is a perspective view of a sixth embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 9(A)  is a perspective view of a seventh embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 9(B)  is a perspective view of a seventh embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 9(C)  is a perspective view of a seventh embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 9(D)  is a perspective view of a seventh embodiment of a wavelength selective optical switch detailing the optical paths through the device. 
         FIG. 10  is a perspective view of a seventh embodiment of a wavelength selective optical switch detailing the optical polarization states at various locations within the device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The wavelength selective optical switch of the invention has numerous applications, including use in fiber optic telecommunications systems. For purposes of illustration, the embodiments described below detail demultiplexing, switching, and multiplexing in a multi-channel fiber optic telecommunication systems. Exemplary references to an optical channel, or simply to a channel, should be understood to mean an optical signal with a centered wavelength and an upper and lower wavelength. Channel spacing is measured from the center of the first channel to the center of an adjacent channel. 
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 1(A) ,  FIG. 1(B) , and  FIG. 1(C) .  FIG. 1(A) ,  FIG. 1(B) , and  FIG. 1(C)  detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. The wavelength selective optical switch allows for demultiplexing, multiplexing and switching separate optical channels to any one of a plurality of optical ports. The wavelength selective optical switch of  FIG. 1  may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     A first embodiment of the wavelength selective optical switch device of  FIG. 1(A) ,  FIG. 1(B) , and  FIG. 1(C)  comprises a Cylindrical Lens Array  103  optically coupled to an Input Fiber  101 , an Anamorphic Lens  105 , a Grating  109 , a Rotationally Symmetric Lens  111 , a Array of Programmable Mirrors  113 , a first Output Fiber  101 - a , and a second Output Fiber  101 - b . A cylindrical lens has at least one surface that is formed like a portion of a cylinder
 
 z ( x )= cx^ 2/{1+Sqrt[1−(1 +k ) c^ 2 x^ 2 ]}+Ax^ 4 +Bx^ 6 +Cx^ 8+ Dx^ 10
 
where z(x) is the sag, c is the curvature at the pole of the surface, x is the distance from the center of the lens along the x-axis, k is the conic constant, and A, B, C, D are aspheric coefficients. Note that in this case that sag is independent of the y-coordinate. An anamorphic lens, usually having one more cylindrical surfaces, has a different magnification along mutually perpendicular meridians. The device of  FIG. 1  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like.
 
     Cylindrical Lens Array  103 , Anamorphic Lens  105 , and Rotationally Symmetric Lens  111  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical performance. 
     The Array of Programmable Mirrors  113  is responsible for steering optical signals. However, other beam steering devices, such as a liquid crystal or the like, may also be employed. Cassarly et-al teach one such liquid crystal beam steering device in U.S. Pat. No. 5,107,357, which is fully incorporated by reference herein. It will be clear to one skilled in the art that beam steering devices may be used in any of the described embodiments. In addition, whichever means is employed for steering the optical signals may also steer the optical signals in more than one axis. This permits, among other things, the steering of optical signals from one port to another port without directing the optical signal to a third port. This allows one port in the system to be steered to another port without interfering with any other ports that might be in use at the time. 
     A prism may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, a prism may be used to balance the thermal affects on Grating  109 . When the prism and Grating  109  are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain a prism. 
     Quarter-wave plate (QWP)  107  may also be employed between the Rotationally Symmetric Lens  111  and grating  109  to reduce polarization dependent loss (PDL) in the system. The QWP  107  oriented at 45 deg to the grating lines rotates the polarization so that light that is s-polarized at the grating on the first pass is p-polarized on the second pass and there is no net polarization dependent loss (PDL) for light traveling between the Input Fiber  101  and any of the Output Fibers ( 101 - a  through  101 - b ). 
     A multi-channel light signal  115  enters the device through the Input Fiber  101 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  103 . The Cylindrical Lens on the Cylindrical Lens Array  103  collimates the multi-channel light signal  115  in the x-axis and directs it through the Anamorphic Lens  105 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Anamorphic Lens  105  collimates and focuses the multi-channel light signal  115  in the y-axis and directs it through QWP  107 , and onto Grating  109 . The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     The Grating  109  diffracts the individual Channels  117  and  119  of the multi-channel light signal  115  (hereafter channels) towards the Rotationally Symmetric Lens  111 . The Rotationally Symmetric Lens  111  is preferably telecentric, so that the central ray, or chief ray, of each channel is parallel at the MEMS plane. This minimizes the tilt required by the MEMS mirrors. In a telecentric lens, the aperture stop is located at the front focus of the lens, resulting in the exit pupil being at infinity. The Rotationally Symmetric Lens  111  focuses the Channels  117  and  119 , near the Programmable Mirror on the Mirror Array  113 . More specifically, Rotationally Symmetric Lens  111  focuses Channel  117  near the Programmable Mirror associated with Channel  117 , and focuses channel  119  near the Programmable Mirror associated with channel  119 . By focusing the channels in two axes the optical beam size is reduced and the size of the Programmable Mirrors  117  and  119  and Mirror Array  113  may be reduced. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the two of Output Fibers  101 - a  or  101 - b . In this regard, each the channel is reflected back through the Rotationally Symmetric Lens  111  which collimates the channels toward Grating  109 . Grating  109  multiplexes the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 1 , the Programmable Mirrors are programmed so as to switch Channel  117  to Output Fiber  101 - a  and Channel  119  to Output Fiber  101 - b . Therefore, Channel  117  is reflected from its Programmable Mirror through Rotationally Symmetric Lens  111  which collimates the Channel towards Grating  109 . Grating  109  diffracts Channel  117  through QWP  107  and Anamorphic Lens  105 . Anamorphic Lens  105  focuses Channel  117  in the y-axis toward Cylindrical Lens  103 , which focuses Channel  117  in the x-axis and into Output Fiber  101 - a . Similarly, Channel  119  is reflected from its Programmable Mirror through Rotationally Symmetric Lens  111  which collimates the Channel towards Grating  109 . Grating  109  diffracts Channel  119  through QWP  107  and Anamorphic Lens  105 . Anamorphic Lens  105  focuses Channel  119  in the y-axis toward Cylindrical Lens  103 , which focuses Channel  119  in the x-axis and into Output Fiber  101 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  101 - a  and  101   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  117  or Channel  119  may be switched to either Output Fiber  101 - a  or  101 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, receiving an optical Channel  117  via Port  101 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  101 . 
     Turning next to  FIG. 2(A) ,  FIG. 2(B) , and  FIG. 2(C) .  FIG. 2(A) ,  FIG. 2(B) , and  FIG. 2(C)  detail different views of the same device. This embodiment operates similarly to the embodiment detailed  FIG. 1(A) ,  FIG. 1(B) , and  FIG. 1(C)  above; however, it further employs one or more polarization converters. The operation of a polarization converter is well known in the art. Ducellier teaches one such polarization converter in U.S. Pat. No. 6,411,409, which is fully incorporated by reference herein. As explained, a birefringent crystal beam displacer is oriented in such a way as to separate the input light into two sub-beams with s-polarizations and p-polarizations. A half-wave plate (HWP) covers the p-polarized sub-beam to convert it to s-polarization. Thus, the light leaves the polarization converter with a larger beam, but it is entirely in the s-polarization, which has the highest diffraction efficiency at the high frequency gratings. The birefringent crystal beam displacer preferably uses a uniaxial birefringent crystals such as calcite (CaCO3), yrttrium orthovandate (YV04) or rutile (TiO2) to separate the beams. Another common polarization converter uses a polarization beam splitter and a waveplate. The waveplate is usually a single half-wave plate oriented at 45 degrees with respect to the groove axis positioned in the path of one of the two sub-beams. 
     The embodiment of present invention detailed in  FIG. 2(A) ,  FIG. 2(B) , and  FIG. 2(C)  employs one or more polarization converters. Polarization Converter  201  is positioned in the optical path between Input Fiber  101  and the Diffraction Grating  109  and converts multi-channel light signal  115  to entirely s-polarized light. Accordingly, when the larger beam width and entirely s-polarized multi-channel light signal  115 , illuminates Grating  109 , it does so at the highest diffraction efficiency. 
     Optional Polarization Converter  203 , operated in the opposite direction as Polarization Converter  201 , is positioned in the optical path between Diffraction Grating  109  and the Array of Programmable Mirrors  113 . Polarization Converter  203  re-converts the entirely s-polarized light back to both p-polarized and s-polarized light. Additionally, the size of the combined p-polarized and s-polarized beam leaving the polarization converter is smaller than that of the entirely s-polarized sub-beam entering the converter. This reduces the footprint of the beam at the MEMS mirrors and which enables the use of a smaller size of the MEMS mirror without incurring additional insertion losses. It will be clear to one skilled in the art that there are many ways to ensure that the grating efficiency is maximized by illuminating only with s-polarized light. 
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 3(A) ,  FIG. 3(B) , and  FIG. 3(C) .  FIG. 3(A) ,  FIG. 3(B) , and  FIG. 3(C)  detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. This embodiment allows for demultiplexing, multiplexing and switching separate optical channels to any one of a plurality of optical ports. This embodiment may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     A Littrow grating is a grating that operates at or near Littrow. Littrow is a special, but common case, in which the angle of incidence of the light on the grating is equal to the angle of diffraction] for which the grating equation becomes:
 
 ml= 2 d  sin( a )
 
where a is the incident angle (same as the diffracted angle), m is the grating order, I is the wavelength, and d is the grating groove spacing. For a reflection grating, rays diffract off the grating back toward the direction from which they originated. In one embodiment, the grating is used near the Littrow condition. Further, using the gratings near the Littrow condition takes advantage of the high diffraction efficiency near the Littrow condition.
 
     The embodiment of the wavelength selective optical switch, detailed in  FIG. 3(A) ,  FIG. 3(B) , and  FIG. 3(C) , comprises a Cylindrical Lens Array  303  optically coupled to an Input Fiber  301 , an Anamorphic Lens  305 , a Polarization Beam Splitter (PBS)  307 , Littrow Gratings  311  and  313 , QWP  315 , QWP  317 , QWP  319 , a Rotationally Symmetric Lens  321 , a Array of Programmable Mirrors  323 , a first Output Fiber  301 - a , and a second Output Fiber  301 - b . The device of  FIG. 3  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     Anamorphic Lens  305  and Rotationally Symmetric Lens  311  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical performance. 
     A prism may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, a prism may be used to balance the thermal affects on Gratings  311  and  313 . When the prism and Gratings  311  and  313  are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain a prism. 
     QWP  319  may also be employed to reduce polarization dependent loss (PDL) in the system. QWP  319  oriented at 45 degrees to the grating lines rotates the polarization of light, so that light that is s-polarized at the grating on the first pass is p-polarized on the second pass. The net result is no polarization dependent loss (PDL) for light traveling between the Input Fiber  301  and any of the Output Fibers  301 - a  and  301 - b.    
     A multi-channel light signal  315  enters the device through the Input Fiber  301 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  303 . The Cylindrical Lens on the Cylindrical Lens Array  303  collimates the multi-channel light signal  315  in the x-axis and directs it through the Anamorphic Lens  305 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Anamorphic Lens  305  collimates and focuses the multi-channel light signal  315  in the y-axis and directs into the PBS  307 . The PBS separates multi-channel light signal  315  into its s-polarized and p-polarized states. 
     Turning briefly to FIG.  4 ., the polarization states of multi-channel light signal  315  are described in detail. Multi-channel light signal  325  enters the PBS  307  and strikes upon the Beam Splitting Surface  309 . The s-polarized optical component reflects off of Beam Splitting Surface  309  and exits the PBS  307 . This s-polarized optical component  325 -S passes through QWP  315 , which converts the polarization state to right-circular 325-RC, and illuminates Littrow Grating  313 . Littrow Grating  313  diffracts the individual channels of light (now left-circular polarized after diffracting of Littrow Grating  313 ) back through QWP  315  which converts their polarization to a p-polarized state  325 -P, and into the PBS  309 . Because these individual channels are now p-polarized they transmit through Beam Splitting Surface  309  and exit the PBS  307 , passing though QWP  319  that converts the polarization states from p-polarized to left-circular 325-LC. 
     In much the same fashion as described above with the s-polarized optical component, the p-polarized optical component transmits through Beam Splitting Surface  309 , exits PBS  307 , and passes though QWP  317  which converts the polarization state from p-polarized to left-circular, and illuminates Littrow Grating  311 . Littrow Grating  311  diffracts the individual channels of light (now right-circular polarized) back through QWP  317  that converts their polarization to an s-polarized state, and into the PBS  309 . The s-polarized optical component reflects off of Beam Splitting Surface  309  and exits the PBS  307  passing though QWP  319  that converts the polarization states from s-polarized to right-circular 325-RC. 
     Turning again to  FIG. 3(A) ,  FIG. 3(B) , and  FIG. 3(C) , Grating  313  and  311  diffracts the individual Channels  327  and  329  of the multi-channel light signal  325  (hereafter channels) through the PBS  307  and towards the Rotationally Symmetric Lens  321 . The Rotationally Symmetric Lens  321  is preferably telecentric, so that the central ray, or chief ray, of each channel is parallel at the MEMS plane. This minimizes the tilt required by the MEMS mirrors. In a telecentric lens, the aperture stop is located at the front focus of the lens, resulting in the exit pupil being at infinity. The Rotationally Symmetric Lens  321  focuses Channels  317  and  319  in both the x-axis and z-axis, near the Programmable Mirror on the Mirror Array  313 . More specifically, Rotationally Symmetric Lens  321  focuses Channel  327  near the Programmable Mirror associated with Channel  327 , and focuses channel  329  near the Programmable Mirror associated with channel  329 . By focusing the channels in both the x-axis and z-axis, the optical beam size is reduced. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the two of Output Fibers  301 - a  or  301 - b . In this regard, each the channel is reflected back through the Rotationally Symmetric Lens  321  which collimates the channels in both the x-axis and z-axis and directs the channels through PBS  307  and onto Gratings  311  and  313 . Gratings  311  and  313  multiplex the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 3 , the Programmable Mirrors are programmed so as to switch Channel  327  to Output Fiber  301 - a  and Channel  329  to Output Fiber  301 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  301 - a  and  301   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  317  or Channel  319  may be switched to either Output Fiber  301 - a  or  301 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, by receiving an optical Channel  327  via Port  301 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  301 . 
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 5(A) ,  FIG. 5(B) , and  FIG. 5(C) .  FIG. 5(A) ,  FIG. 5(B) , and  FIG. 5(C)  detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. The wavelength selective optical switch allows for demultiplexing, switching separate optical channels, and multiplexing to any one of a plurality of optical ports. The wavelength selective optical switch of  FIG. 5  may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     The embodiment of the wavelength selective optical switch device of  FIG. 5(A) ,  FIG. 5(B) , and  FIG. 5(C)  comprises a Cylindrical Lens Array  503  optically coupled to an Input Fiber  501 , an Anamorphic Lens  505 , a transmissive Grating  513  operating near Littrow, a Rotationally Symmetric Lens  521 , a Array of Programmable Mirrors  523 , a first Output Fiber  501 - a , and a second Output Fiber  501 - b . The device of  FIG. 5  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     Anamorphic Lens  505  and Rotationally Symmetric Lens  521  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical prescription. 
     A prism may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, a prism may be used to balance the thermal affects on Grating  513 . When the prism and Grating  513  are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain a prism. 
     The embodiment of present invention detailed in  FIG. 5(A) ,  FIG. 5(B) , and  FIG. 5(C)  employs one or more polarization converters. Polarization Converter  502  is positioned in the optical path between Input Fiber  501  and the Grating  513  and converts multi-channel light signal  525  to entirely s-polarized light. Accordingly, when the larger beam width and entirely s-polarized multi-channel light signal  525 , illuminates Grating  513 , it does so at the highest diffraction efficiency. 
     Optional Polarization Converter  524 , operated in the opposite direction as Polarization Converter  502 , is positioned in the optical path between Grating  513  and the Array of Programmable Mirrors  523 . Polarization Converter  524  re-converts the entirely s-polarized light back to both p-polarized and s-polarized light. Additionally, the size of the combined p-polarized and s-polarized beam leaving the polarization converter is smaller than that of the entirely s-polarized sub-beam entering the converter. This reduces the footprint of the beam at the MEMS mirrors and which enables the use of a smaller size of the MEMS mirror without incurring additional insertion losses. 
     A multi-channel light signal  525  enters the device through the Input Fiber  501 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  503 . The Cylindrical Lens on the Cylindrical Lens Array  503  collimates the multi-channel light signal  525  in the x-axis and directs it through the Anamorphic Lens  505 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Anamorphic Lens  505  collimates and focuses the multi-channel light signal  525  in the y-axis and directs it through Grating  513 . The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     The Grating  513  diffracts the individual Channels  527  and  529  of the multi-channel light signal  525  (hereafter channels) through QWP  519  and towards the Rotationally Symmetric Lens  521 . The Rotationally Symmetric Lens  521  focuses the Channels  527  and  529 , in both the x-axis and z-axis, near the Programmable Mirror on the Mirror Array  523 . More specifically, Rotationally Symmetric Lens  521  focuses Channel  527  near the Programmable Mirror associated with Channel  527 , and focuses channel  529  near the Programmable Mirror associated with channel  529 . By focusing the channels in both the x-axis and z-axis, the optical beam size is reduced and the size of the Programmable Mirrors and Mirror Array  523  may be reduced. Further, the focal length may be reduced thereby compacting the device. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the two of Output Fibers  501 - a  or  501 - b . In this regard, each the channel is reflected back through the Rotationally Symmetric Lens  521  which collimates the channels in both the x-axis and z-axis and directs the channels through Grating  513 . Grating  513  multiplexes the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 5 , the Programmable Mirrors are programmed so as to switch Channel  527  to Output Fiber  501 - a  and Channel  529  to Output Fiber  501 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  501 - a  and  501   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  527  or Channel  529  may be switched to either Output Fiber  501 - a  or  501 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, by receiving an optical Channel  527  via Port  501 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  501 . 
     A seventeen port grating-based optical switch for sixty four 100 GHz spaced channels, employing one embodiment of the invention, is detailed in  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C) .  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C)  detail different views of the same device. For clarity, in  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C) , only the center and extreme ports, and 2 optical channels, are depicted. The wavelength selective optical switch allows for demultiplexing, switching separate optical channels, and multiplexing to any one of a plurality of optical ports. The wavelength selective optical switch of  FIG. 6  may be dynamically programmed to demultiplex, switch, and multiplex any combination of channels to any of a plurality of optical ports. 
     The embodiment of the wavelength selective optical switch device of  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C)  comprises a Cylindrical Lens Array  603  optically coupled to an Input Fiber  601 , a Cylindrical Lens  605 , a prism  607 , a transmission Grating  609  operating near Littrow, a Rotationally Symmetric Lens  611 , an Array of Programmable Mirrors  613 , a first Output Fiber  601 - a , and a second Output Fiber  601 - b . The device of  FIG. 6  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     Cylindrical Lens  605  and Rotationally Symmetric Lens  611  are both comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to reduce the lens aberrations over a large range of frequencies (6.4 THz), operating temperatures (−20° C. to 70° C.), and field of view. Cylindrical Lens  605  and Rotationally Symmetric Lens  611  have numeric apertures of 0.2 and 0.235, respectively. Table 1 lists the optical prescription for the wavelength selective optical switch in CODE V format. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Optical Prescription for seventeen port grating-based optical switch 
               
             
          
           
               
                 OBJ: 
                   
                 RDY 
                   
                 THI 
                 RMD 
                 GLA 
                   
                   
               
               
                   
               
             
          
           
               
                   
                   
                 INFINITY 
                   
                 3.146570 
                   
                   
                   
                   
               
               
                 1: 
                   
                 INFINITY 
                   
                 0.450000 
                   
                 SILICON_SPECIAL 
               
               
                 2: 
                   
                 INFINITY 
                   
                 0.000000 
               
               
                   
                 RDX: 
                 −8.10984 
               
               
                   
                 Lens spacing: 
                 1.3347E+00 
               
               
                   
                 A: 
                 1.2682E−03 
               
               
                 3: 
                   
                 INFINITY 
                   
                 0.453430 
                   
                 AIR 
               
               
                 4: 
                   
                 −3.30747 
                   
                 2.919365 
                   
                 SF11_SCHOTT 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 5: 
                   
                 −3.74895 
                   
                 11.188256 
                   
                 AIR 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 6: 
                   
                 −39.82847 
                   
                 2.000000 
                   
                 SF15_SCHOTT 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 7: 
                   
                 8.25289 
                   
                 3.069427 
                   
                 NBAK1_SCHOTT 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 8: 
                   
                 −7.03286 
                   
                 0.214935 
                   
                 AIR 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 9: 
                   
                 −6.44129 
                   
                 2.000000 
                   
                 NBK10_SCHOTT 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 10: 
                   
                 9.62630 
                   
                 2.938672 
                   
                 NSK2_SCHOTT 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 11: 
                   
                 −15.90151 
                   
                 0.328897 
                   
                 AIR 
               
               
                   
                 CYL: 
               
               
                   
                 RDX: 
                 INFINITY 
               
               
                 12: 
                   
                 INFINITY 
                   
                 3.000000 
                   
                 SF14_SCHOTT 
               
               
                   
                 SLB: 
                 “prism” 
               
               
                 13: 
                   
                 INFINITY 
                   
                 3.000000 
                   
                 AIR 
               
               
                   
                 XDE: 
                 0.000000 
                 YDE: 
                 0.000000 
                 ZDE: 
                 0.000000 
               
               
                   
                 ADE: 
                 15.219671 
                 BDE: 
                 0.000000 
                 CDE: 
                 0.000000 
               
               
                 14: 
                   
                 INFINITY 
                   
                 0.000000 
                   
                 AIR 
               
               
                   
                 XDE: 
                 0.000000 
                 YDE: 
                 0.000000 
                 ZDE: 
                 0.000000 
               
               
                   
                 ADE: 
                 0.1e21 
                 BDE: 
                 0.000000 
                 CDE: 
                 0.000000 
               
               
                 15: 
                   
                 INFINITY 
                   
                 0.000000 
                   
                 AIR 
               
               
                   
                 XDE: 
                 0.000000 
                 YDE: 
                 −2.190327 
                 ZDE: 
                 17.000000 
                 GLB 
                 G12 
               
               
                   
                 ADE: 
                 −76.238409 
                 BDE: 
                 0.000000 
                 CDE: 
                 0.000000 
               
               
                 16: 
                   
                 INFINITY 
                   
                 2.000000 
                   
                 SILICA_SPECIAL 
               
               
                 STO: 
                   
                 INFINITY 
                   
                 2.000000 
                   
                 SILICA_SPECIAL 
               
               
                   
                 SLB: 
                 “grt” 
               
               
                   
                 GL2: 
                 AIR 
               
               
                   
                 GRT: 
               
               
                   
                 GRO: 
                 −1.000000 
                 GRS: 
                 0.000909 
               
               
                   
                 GRX: 
                 0.000000 
                 GRY: 
                 1.000000 
                 GRZ: 
                 0.000000 
               
               
                 18: 
                   
                 INFINITY 
                   
                 2.000000 
                   
                 AIR 
               
               
                 19: 
                   
                 INFINITY 
                   
                 10.626347 
                   
                 AIR 
               
               
                   
                 XDE: 
                 0.000000 
                 YDE: 
                 −3.824794 
                 ZDE: 
                 0.000000 
               
               
                   
                 ADE: 
                 −56.872509 
                 BDE: 
                 0.000000 
                 CDE: 
                 0.000000 
               
               
                 20: 
                   
                 40.02527 
                   
                 5.798156 
                   
                 NLASF31_SCHOTT 
               
               
                   
                 SLB: 
                 “foc” 
               
               
                 21: 
                   
                 −510.83375 
                   
                 5.947632 
                   
                 NLAK10_SCHOTT 
               
               
                 22: 
                   
                 127.58156 
                   
                 1.702233 
                   
                 AIR 
               
               
                 23: 
                   
                 19.84076 
                   
                 4.276553 
                   
                 NSF1_SCHOTT 
               
               
                 24: 
                   
                 25.60107 
                   
                 4.125666 
                   
                 SF1_SCHOTT 
               
               
                 25: 
                   
                 12.99810 
                   
                 11.900816 
                   
                 AIR 
               
               
                 26: 
                   
                 −21.31335 
                   
                 2.894729 
                   
                 NLAK10_SCHOTT 
               
               
                 27: 
                   
                 68.54462 
                   
                 12.995558 
                   
                 NLASF31_SCHOTT 
               
               
                 28: 
                   
                 −31.91252 
                   
                 6.790847 
                   
                 AIR 
               
               
                 29: 
                   
                 43.81835 
                   
                 12.994567 
                   
                 SF57_SCHOTT 
               
               
                 30: 
                   
                 −47.90572 
                   
                 12.994310 
                   
                 SFL57_SCHOTT 
               
               
                 31: 
                   
                 138.80596 
                   
                 5.065914 
                   
                 AIR 
               
               
                 32: 
                   
                 INFINITY 
                   
                 0.000000 
               
               
                   
                 XDE: 
                 0.000000 
                 YDE: 
                 0.000000 
                 ZDE: 
                 0.000000 
                 DAR 
               
               
                   
                 ADE: 
                 0.371634 
                 BDE: 
                 0.000000 
                 CDE: 
                 0.000000 
               
               
                 IMG: 
                   
                 INFINITY 
                   
                 0.000000 
               
               
                   
               
             
          
         
       
     
     The embodiment of present invention detailed in  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C)  employs a Volume Holographic Grating  609  with 1100 grooves/mm made on a substrate with low coefficient of thermal expansion, such as fused silica. Because this grating has poor efficiency in the p-polarization, the s- and p-polarization are split (not shown) and the s-polarization is switched by the optics shown in  FIG. 6(A) ,  FIG. 6(B) , and  FIG. 6(C) . The p-polarization is rotated by 90°, so that it is s-polarized, and sent through a set of optics that are identical to the s-polarized optics. This technique of splitting the two polarizations and running each through an identical set of optics is known as polarization diversity. 
     Prism  607  is employed to compensate for changes in the grating groove spacing with temperature. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, prism  607  is used to balance the thermal affects on Grating  609 . When Prism  607  and Grating  609  are properly designed and configured the effects of temperature on the system are greatly reduced. Prism  607  is preferable made of a glass with a large change in the optical path length with temperature, such as SF14 by Schott, to minimize the prismatic power required. 
     A multi-channel light signal  615  enters the device through the Input Fiber  601 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  603 . The Cylindrical Lens on the Cylindrical Lens Array  603  collimates the multi-channel light signal  615  in the x-axis and directs it through the Anamorphic Lens  605 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Cylindrical Lens  605  collimates and focuses the multi-channel light signal  615  in the y-axis and directs it through Grating  609 . 
     The Grating  609  diffracts the individual Channels  617  and  619  (hereafter channels) of the multi-channel light signal  615  towards the Rotationally Symmetric Lens  611 . The Rotationally Symmetric Lens  611  focuses the Channels  617  and  619 , near the Programmable Mirror on the Mirror Array  613 . More specifically, Rotationally Symmetric Lens  611  focuses Channel  617  near the Programmable Mirror associated with Channel  617 , and focuses channel  619  near the Programmable Mirror associated with channel  619 . By focusing the channels, the optical beam size is reduced and the size of the Programmable Mirrors and Mirror Array  613  may be reduced. Further, the focal length may be reduced thereby compacting the device. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the Output Fibers  601 - a  or  601 - b . In this regard, each the channel is reflected back through the Rotationally Symmetric Lens  611  which collimates the channels and directs the channels through Grating  609 . Grating  609  multiplexes the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 6 , the Programmable Mirrors are programmed so as to switch Channel  617  to Output Fiber  601 - a  and Channel  619  to Output Fiber  601 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  601 - a  and  601   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  617  or Channel  619  may be switched to either Output Fiber  601 - a  or  601 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system supports both a large number of Output Fibers, and a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, by receiving an optical Channel  617  via Port  601 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  601 . 
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 7(A) ,  FIG. 7(B) , and  FIG. 7(C) .  FIG. 7(A) ,  FIG. 7(B) , and  FIG. 7(C)  detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. This embodiment allows for demultiplexing, multiplexing and switching separate optical channels to any one of a plurality of optical ports. This embodiment may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     The embodiment of the wavelength selective optical switch, detailed in  FIG. 7(A) ,  FIG. 7(B) , and  FIG. 7(C) , comprises a Cylindrical Lens Array  703  optically coupled to an Input Fiber  701 , an Anamorphic Lens  705 , a first Polarization Beam Splitter (PBS)  707 , Half-Waveplate (HWP)  709 , Littrow Gratings  711  and  713 , HWP  715 , a second PBS  717 , QWP  719 , Rotationally Symmetric Lens  721 , a Array of Programmable Mirrors  723 , a first Output Fiber  701 - a , and a second Output Fiber  701 - b . The device of  FIG. 7  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. 
     Anamorphic Lens  705  and Rotationally Symmetric Lens  711  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical performance. A prism may optionally be used in any embodiment of the system. 
     A multi-channel light signal  725  enters the device through the Input Fiber  701 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  703 . The Cylindrical Lens on the Cylindrical Lens Array  703  collimates the multi-channel light signal  725  in the x-axis and directs it through the Anamorphic Lens  705 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Anamorphic Lens  705  collimates and focuses the multi-channel light signal  725  in the y-axis and directs it into the first PBS  707 . The PBS separates multi-channel light signal  725  into its s-polarized and p-polarized states. 
     The s-polarized optical component of Multi-channel light signal  725  reflects off of the Beam Splitting Surface  708  of PBS  707  and exits PBS  707 . The s-polarized optical component then diffracts through Littrow Grating  713  and passes though HWP  715  which converts the s-polarization state to a p-polarized state. The p-polarized optical component of Multi-channel light signal  725  transmits through the Beam Splitting Surface  708  of PBS  707 , exits PBS  707 , and passes though HWP  709  which converts the p-polarization state from p-polarized to s-polarized. This s-polarized light diffracts through Littrow Grating  711 . 
     Grating  711  diffracts the individual Channels  727  and  729  (hereafter channels) of the multi-channel light signal  725  into PBS  717 . Grating  713  diffracts the individual channels through HWP  715  which converts the s-polarization state to a p-polarized state. 
     Both the p-polarized and s-polarized states of the individual channels enter second PBS  717 ; the s-polarized state reflects off of the Beam Splitting Surface  718  of PBS  717  and exits PBS  717 . The p-polarized state transmits through the Beam Splitting Surface  718  of PBS  717 , and exits PBS  717  recombined with the s-polarized state. 
     The individual channels are directed through QWP  719  and through Rotationally Symmetric Lens  721 . The Rotationally Symmetric Lens  721  focuses Channels  727  and  729  in both the x-axis and y-axis, near the Programmable Mirror on the Mirror Array  723 . More specifically, Rotationally Symmetric Lens  721  focuses Channel  727  near the Programmable Mirror associated with Channel  727 , and focuses channel  729  near the Programmable Mirror associated with channel  729 . By focusing the channels in both the x-axis and y-axis, the optical beam size is reduced. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the two of Output Fibers  701 - a  or  701 - b . In this regard, each the channel is reflected back through the device in reverse and is directed toward that appropriate output fiber. In the presently detailed case of  FIG. 7 , the Programmable Mirrors are programmed so as to switch Channel  727  to Output Fiber  701 - a  and Channel  729  to Output Fiber  701 - b . The optical configuration is such that the optical signals directed to and entering Output Fibers  701 - a  and  701   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  727  or Channel  729  may be switched to either Output Fiber  701 - a  or  701 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, by receiving an optical Channel  727  via Port  701 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  701 . 
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 8(A) ,  FIG. 8(B) ,  FIG. 8(C) , and  FIG. 8(D) .  FIG. 8(A) ,  FIG. 8(B) ,  FIG. 8(C) , and  FIG. 8(D) . detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. The wavelength selective optical switch allows for demultiplexing, multiplexing and switching separate optical channels to any one of a plurality of optical ports. The wavelength selective optical switch of  FIG. 8  may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     A first embodiment of the wavelength selective optical switch device of  FIG. 8(A) ,  FIG. 8(B) ,  FIG. 8(C) , and  FIG. 8(D)  comprises First Cylindrical Lens Array  803  optically coupled to an Input Fiber  801 , a First Anamorphic Lens  805 , a First Grating  807 , a First Rotationally Symmetric Lens  809 , an Array of programmable Transmissive Beam Steerers (TBS)  810 , a Second Anamorphic Lens  815 , a Second Littrow Grating  817 , a Second Anamorphic Lens  815 , a Second Cylindrical Lens Array  813 , a first Output Fiber  811 - a , and a second Output Fiber  811 - b.    
     The device of  FIG. 8  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. 
     The First and Second Cylindrical Lens Arrays  803  and  813 , First and Second Anamorphic Lenses  805  and  815 , and First and Second Rotationally Symmetric Lenses  809  and  819  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical performance. 
     The Array of programmable TBS  810  is responsible for steering optical signals. However, other beam steering devices, such as a liquid crystal or the like, may also be employed. It will be clear to one skilled in the art that beam steering devices may be used in any of the described embodiments. 
     A prism may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, a prism may be used to balance the thermal affects on the First and Second Gratings  807  and  817 . When the prism and gratings are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain a prism. 
     A multi-channel light signal  821  enters the device through the Input Fiber  801 , and is directed through one of the Cylindrical Lenses on the First Cylindrical Lens Array  803 . The Cylindrical Lens on the First Cylindrical Lens Array  803  collimates the multi-channel light signal  821  and directs it through the First Anamorphic Lens  805 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The First Anamorphic Lens  805  collimates and focuses the multi-channel light signal  821  and directs it onto First Grating  807 . The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     First Grating  807  diffracts the individual Channels  823  and  825  of the multi-channel light signal  821  (hereafter channels) towards the First Rotationally Symmetric Lens  809 . The First Rotationally Symmetric Lens  809  is preferably telecentric, so that the central ray, or chief ray, of each channel is parallel at the TBS plane. This minimizes the tilt required by the TBS. In a telecentric lens, the aperture stop is located at the front focus of the lens, resulting in the exit pupil being at infinity. The First Rotationally Symmetric Lens  809  focuses the Channels  823  and  825 , in both the x-axis and y-axis, near the TBS Array  810 . More specifically, Rotationally Symmetric Lens  809  focuses Channel  823  near the Programmable Mirror associated with Channel  823 , and focuses channel  825  near the Programmable Mirror associated with channel  825 . By focusing the channels in both the x-axis and y-axis, the optical beam size is reduced and the size of the TBS  810  may be reduced. 
     Depending upon the programmed state of the TBS  810 , each channel may be switched to any one of the two of Output Fibers  811 - a  or  811 - b . In this regard, each the channel is transmitted through the Second Rotationally Symmetric Lens  819  which collimates the channels in both the x-axis and y-axis toward Second Grating  817 . Second Grating  817  multiplexes the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 8 , TBS  810  is programmed so as to switch Channel  823  to Output Fiber  811 - a  and Channel  825  to Output Fiber  811 - b . Therefore, Channel  823  is directed by its corresponding beam steerer on TBS  810  through Second Rotationally Symmetric Lens  819  which collimates the Channel towards Second Grating  817 . Second Grating  817  diffracts Channel  823  through Second Anamorphic Lens  815 . Second Anamorphic Lens  815  focuses Channel  823  toward Second Cylindrical Lens  803 , which focuses Channel  823  into Output Fiber  811 - a . Similarly, Channel  825  is transmitted through Second Rotationally Symmetric Lens  819  which collimates the Channel towards Second Grating  817 . Second Grating  817  diffracts Channel  825  through Second Anamorphic Lens  815 . Second Anamorphic Lens  815  focuses Channel  825  toward Second Cylindrical Lens  813 , which focuses Channel  825  into Output Fiber  811 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  811 - a  and  811   b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  823  or Channel  825  may be switched to either Output Fiber  811 - a  or  811 - b  by simply changing the angle of direction of the associated TBS. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, receiving an optical Channel  813  via Port  811 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  801 - a  or  801 - b.    
     A two channel grating-based optical switch, employing one embodiment of the invention, is detailed in  FIG. 9(A) ,  FIG. 9(B) ,  FIG. 9(C) , and  FIG. 9D ). FIG. (A),  FIG. 9(B) ,  FIG. 9(C) , and  FIG. 9(D)  detail different views of the same device. It is of note that while only two channels are used in this example, a substantially larger number of channels and optical ports may be employed. This embodiment allows for demultiplexing, multiplexing and switching separate optical channels to any one of a plurality of optical ports. This embodiment may be dynamically programmed to demultiplex, multiplex and switch any combination of wavelengths to any of a plurality of optical ports. 
     The embodiment of the wavelength selective optical switch, detailed in FIG. (A),  FIG. 9(B) ,  FIG. 9(C) , and  FIG. 9(D) , comprises a Cylindrical Lens Array  903  optically coupled to an Input Fiber  901 , an Anamorphic Lens  905 , a Polarization Beam Splitter (PBS)  907 , Littrow Gratings  911  and  915 , Faraday Rotators  909  and  913 , QWP  916 , Rotationally Symmetric Lens  917 , a Array of Programmable Mirrors  923 , a first Output Fiber  901 - a , and a second Output Fiber  901 - b . The device of  FIG. 9  may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like. The cylindrical and anamorphic lenses produce a beam with an elliptical footprint on the grating. The major axis of the ellipse is perpendicular to the grooves so that the spectral resolution is maximized, while the overall size of the grating is less than that if a conventional rotationally symmetric collimating lens were used. 
     Anamorphic Lens  905  and Rotationally Symmetric Lens  917  may be comprised of multiple lens elements. It is well known in the art that lenses may be comprised of multiple lens elements to achieve a particular optical performance. 
     A prism may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. As the temperature changes, the refractive index of the prism changes, which in turn, changes the dispersion of the prism. Accordingly, a prism may be used to balance the thermal affects on Gratings  911  and  915 . Littrow Grating  911  and  915  may be optically coupled to one of the prism&#39;s surface. When the prism and Gratings  911  and  915  are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain a prism. 
     QWP  916  may also be employed to reduce polarization dependent loss (PDL) in the system. QWP  916  oriented at 45 deg to the grating lines rotates the polarization of light traveling through the QWP so that light that is s-polarized at the grating on the first pass is p-polarized on the second pass. The net result is no polarization dependent loss (PDL) for light traveling between the Input Fiber  901  and any of the Output Fibers  901 - a  and  901 - b.    
     A multi-channel light signal  925  enters the device through the Input Fiber  901 , and is directed through one of the Cylindrical Lenses on the Cylindrical Lens Array  903 . The Cylindrical Lens on the Cylindrical Lens Array  903  collimates the multi-channel light signal  915  in the z-axis and directs it through the Anamorphic Lens  905 . When beam size is large, the geometrical limit holds and all the rays are parallel in a collimated beam. As the beam size decreases, diffraction becomes important and it is preferable to locate the beam waist at the Grating. The Anamorphic Lens  905  collimates and focuses the multi-channel light signal  915  in the y-axis and directs it into the PBS  907 . The PBS separates multi-channel light signal  925  into its s-polarized and p-polarized states. 
     Turning briefly to  FIG. 10 , the polarization states of multi-channel light signal  925  are described in detail. Multi-channel light signal  925  strikes PBS  907  and the s-polarized optical component reflects, while the p-polarized component transmits through PBS  907 . The s-polarized component of Multi-channel light signal  925  striking PBS  907  is not parallel to the y-axis, because the micro cylindrical collimators array  901 ,  901 - a , and  901 - b  are not in the xy-plane. The s-polarized optical component  925 -SB passes through Faraday Rotator (FR)  909 , which rotates the polarization state by 45 degrees such that the light  925 -SG is s-polarized at the Littrow Grating  911 . A Faraday rotator is a non-reciprocal optical device that rotates the polarization plane of both forward and backward transmitted beam in a certain direction, regardless of the transmission direction of the beam. Littrow Grating  911  diffracts the individual channels  919 -SG and  921 -SG of light back through FR  909  that rotates the light a further 45 degrees so that the light  919 -PB and  921 -PB is p-polarized in the reference frame of PBS  907 . Because individual channels  919 -PB and  921 -PB are now p-polarized they transmit through PBS surface  907  and exit the PBS  907 , passing though QWP  916  that converts the p-polarized light to left circularly polarized light  919 -LC and  921 -LC. 
     Preferably, the input beam  925  at the PBS  907 , and diffraction gratings  911  and  915  are oriented such that the s-p coordinates at the grating are rotated by 45 degrees from the s-p coordinates at the gratings. For example, in one embodiment, the incident beam makes a 51 degree angle with the y-axis and is in the y-z plane and the PBS is rotated by 38 degrees around the y-axis by 38 degrees. One skilled in the art will recognize that many orientations of the incident beams  925 , PBS, and diffraction grating are possible. 
     In much the same fashion as described above with the s-polarized optical component, the p-polarized optical component  925 -PB transmits through PBS  907  and passes though FR  913  which rotates the polarization state from p-polarized in the reference frame of the PBS to s-polarized in the reference frame of the grating, and illuminates Littrow Grating  911 . Littrow Grating  911  diffracts the individual channels of light back through FR  913  that converts their polarization to an s-polarized state in the reference frame of PBS  907 , and into PBS  909 . The s-polarized optical component  919 -SB and  921 -SB reflects off of PBS  907 , passing though QWP  916  that converts the s-polarized light to right circularly polarized light  919 -RC and  921 -RC 
     Turning again to  FIG. 9(A) ,  FIG. 9(B) ,  FIG. 9(C) , and  FIG. 9(D) , Gratings  911  and  915  diffracts the individual Channels  919  and  921  of the multi-channel light signal  925  (hereafter channels) through PBS  907 , QWP  916 , and towards Rotationally Symmetric Lens  917 . The Rotationally Symmetric Lens  917  is preferably telecentric, so that the central ray, or chief ray, of each channel is parallel at the mirrors plane. This minimizes the tilt required by the MEMS mirrors. In a telecentric lens, the aperture stop is located at the front focus of the lens, resulting in the exit pupil being at infinity. The Rotationally Symmetric Lens  917  focuses Channels  919  and  921  in both the x-axis and y′-axis (not shown), near the Programmable Mirror on the Mirror Array  923 . More specifically, Rotationally Symmetric Lens  917  focuses Channel  919  near the Programmable Mirror associated with Channel  919 , and focuses channel  921  near the Programmable Mirror associated with channel  921 . By focusing the channels in both the x-axis and y′-axis (not shown), the optical beam size is reduced. 
     Depending upon the programmed state of the Programmable Mirrors, each channel may be switched to any one of the two of Output Fibers  901 - a  or  901 - b . In this regard, each the channel is reflected back through the Rotationally Symmetric Lens  917  which collimates the channels in both the x-axis and y′-axis (not shown) and directs the channels through PBS  907  and onto Gratings  911  and  913 . Gratings  911  and  913  multiplex the channels switched to the same output fiber and diffracts the resulting beam toward that output fiber. In the presently detailed case of  FIG. 9 , the Programmable Mirrors are programmed so as to switch Channel  919  to Output Fiber  901 - a  and Channel  921  to Output Fiber  901 - b.    
     The optical configuration is such that the optical signals directed to and entering Output Fibers  901 - a  and  901 - b  enter the Output Fibers within the cone of acceptance thereby reducing system loss. It will be clear to one skilled in the art that either Channel  919  or Channel  921  may be switched to either Output Fiber  901 - a  or  901 - b  by simply changing the angle of reflection of the associated Programmable Mirror. In this regard, the system may be scaled to accommodate both a large number of Output Fibers, but also a large number of channels. 
     It will be clear to one skilled in the art that the system may be operated in the opposite direction. For example, by receiving an optical Channel  927  via Port  901 - a , multiplexing it with one or more received optical channels, and directing the multiplexed optical signal via Port  901 .