Patent Publication Number: US-7720329-B2

Title: Segmented prism element and associated methods for manifold fiberoptic switches

Description:
PRIORITY CLAIM TO RELATED US APPLICATIONS 
   To the full extent permitted by law, the present United States Non-Provisional patent application claims priority to and the full benefit of United States Provisional patent application entitled “Segmented Prism Element and Associated Methods for Manifold Fiberoptic Switches,” filed on Nov. 7, 2006, having assigned Ser. No. 60/857,441, incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates generally to all-optical fiber optic communications and datacom switches, and more specifically pertains to fiber optic switches used in multi-wavelength networks. 
   BACKGROUND 
   Modern communications networks are increasingly based on silica optical fiber which offers very wide bandwidth within several spectral wavelength bands. At the transmitter end of a typical point-to-point fiber optic communications link an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1525-1565 nanometer transmission band (the so-called C-band), and the resulting modulated optical signal is coupled into one end of the silica optical fiber. On sufficiently long links the optical signal may be directly amplified along the route by one or more amplifiers, for example, optically-pumped erbium-doped fiber amplifiers (EDFAs). At the receiving end of the fiber link a photodetector receives the modulated light and converts it back to its original electrical form. For very long links the optical signal risks becoming excessively distorted due to fiber-related impairments such as chromatic and polarization dispersion and by noise limitations of the amplifiers, and may be reconstituted by detecting and re-launching the signal back into the fiber. This process is typically referred to as optical-electrical-optical (OEO) regeneration. 
   In recent developments, the transmission capacity of fiber optic systems has been greatly increased by wavelength division multiplexing (WDM) in which multiple independent optical signals, differing uniquely by wavelength, are simultaneously transmitted over the fiber optic link. For example, the C-band transmission window has a bandwidth of about 35 nanometers, determined partly by the spectral amplification bandwidth of an EDFA amplifier, in which multiple wavelengths may be simultaneously transmitted. All else being equal, for a WDM network containing a number N wavelengths, the data transmission capacity of the link is increased by a factor of N. Depending on the specifics of a WDM network the wavelength multiplexing into a common fiber is typically accomplished with devices employing a diffraction grating, an arrayed waveguide grating, or a series of thin-film filters. At the receiver of a WDM system, the multiple wavelengths can be spatially separated using the same types of devices that performed the multiplexing and then separately detected and output in their original electrical data streams. 
   Dense WDM (DWDM) systems are being designed in which the transmission spectrum includes 40, 80, or more wavelengths with wavelength spacing of less than 1 nanometer. Current designs have wavelength spacing of between 0.4 and 0.8 nanometer, or equivalently a frequency spacing of 50 to 100 GHz respectively. Spectral packing schemes allow for higher or lower spacing, dictated by economics, bandwidth, and other factors. Other amplifier types, for example Raman, that help to expand the available WDM spectrum are currently being commercialized. However, the same issues about signal degradation and OEO regeneration exist for WDM as with non-WDM fiber links. The expense of OEO regeneration is compounded by the large number of wavelengths present in WDM systems. 
   Modern fiber optic networks are evolving to be much more complicated than the simple point-to-point “long haul” systems described above. Instead, as fiber optic networks move into the regional, metro, and local arenas they increasingly include multiple nodes along the fiber span, and connections between fiber spans (e.g., mesh networks and interconnected ring networks) at which signals received on one incoming link can be selectively switched between a variety of outgoing links, or taken off the network completely for local consumption. For electronic links, or optical signals that have been detected and converted to their original electrical form, conventional electronic switches directly route the signals to their intended destination, which may then include converting the signals to the optical domain for fiber optic transmission. However, the desire to switch fiber optic signals while still in their optical format, thereby avoiding expensive OEO regeneration to the largest extent possible, presents a new challenge to the switching problem. Purely optical switching is generally referred to as all-optical or OOO switching. 
   Switching 
   In the most straightforward and traditional fiber switching approach, each network node that interconnects multiple fiber links includes a multitude of optical receivers which convert the signals from optical to electrical form, a conventional electronic switch which switches the electrical data signals, and an optical transmitter which converts the switched signals from electrical back to optical form. In a WDM system, this optical/electrical/optical (OEO) conversion must be performed by separate receivers and transmitters for each of the W wavelength components on each fiber. This replication of expensive OEO components is currently slowing the implementation of highly interconnected mesh WDM systems employing a large number of wavelengths. 
   Another approach for fiber optic switching implements sophisticated wavelength switching in an all-optical network. In a version of this approach that may be used with the present invention, the wavelength components W from an incoming multi-wavelength fiber are demultiplexed into different spatial paths. Individual and dedicated switching elements then route the wavelength-separated signals toward the desired output fiber port before a multiplexer aggregates the optical signals of differing wavelengths onto a single outgoing fiber. In conventional fiber switching systems, all the fiber optic switching elements and associated multiplexers and demultiplexers are incorporated into a wavelength cross connect (WXC), which is a special case of an enhanced optical cross connect (OXC) having a dispersive element and wavelength-selective capability. Additionally, such systems incorporate lenses and mirrors which focus light to a single focal point and lenslets which collimate such light. 
   Advantageously, all the fiber optic switching elements can be implemented in a single chip of a micro electromechanical system (MEMS). The MEMS chip generally includes a two-dimensional array of tiltable mirrors which may be separately controlled. U.S. Pat. No. 6,097,859 to Solgaard et al., incorporated herein in its entirety, describes the functional configuration of such a MEMS wavelength cross connect which incorporates a wavelength from an incoming fiber and is capable of switching wavelength(s) to any one of multiple outgoing fibers. The entire switching array of several hundred micro electromechanical system (MEMS) mirrors can be fabricated on a chip having dimension of less than one centimeter by techniques well developed in the semiconductor integrated circuit industry. 
   Solgaard et al. further describes a large multi-port (including multiple input M and multiple output N ports) and multi-wavelength WDM cross-connect switch (WXC) accomplishing this by splitting the WDM channels into their wavelength components W and switching those wavelength components W. The Solgaard et al. WXC has the capability of switching any wavelength channel on any input port to the corresponding wavelength channel on any output fiber port. Again, a wavelength channel on any of the input fibers can be switched to the same wavelength channel on any of the output fibers. 
   A complex WDM or white-light network is subject to many problems. For example, the different optical signals which are propagating on a particular link or being optically processed may have originated from different sources across the network. Also, in a WDM system, the WDM wavelength output power may vary from transmitter to transmitter because of environmental changes, aging, or differences in power injected into the WDM stream. Different optical sources for either a WDM or white-light system are additionally subject to different amounts of attenuation over the extended network. Particularly, for a wavelength-routed transparent network, the WDM spectrum on a given fiber contains wavelength components which generally have traversed many diverse paths from different sources and with different losses and different impairment accumulation, such as degradation of the optical signal-to-noise ratio or dispersion broadening. Further, wavelength multiplexing and demultiplexing usually rely on optical effects, such as diffraction or waveguide interference, which are very sensitive to absolute wavelength, and which cannot be precisely controlled. Additionally, the prior art is disadvantageously limited to complex multi input and output fiber port, single dedicated wavelength channel MEMS mirrors, and multi wavelength WDM cross-connect switches. 
   EDFAs or other optical amplifiers may be used to amplify optical signals to compensate loss, but they amplify the entire WDM signal and their gain spectrum is typically not flat. Therefore, measures are needed to maintain the power levels of different signals at common levels or at least in predetermined ratios. 
   Monitoring 
   Monitoring of the WDM channels is especially important in optical telecommunication networks that include erbium doped fiber amplifiers (EDFAs), because a power amplitude change in one channel may degrade the performance of other channels in the network due to gain saturation effects in the EDFA. Network standard documents, such as the Bellcore GR-2918, have been published to specify wavelength locations, spacing and signal quality for WDM channels within the networks. Network performance relative to these standards can be verified by monitoring wavelength, power and signal-to-noise ratio (SNR) of the WDM channels. 
   A multi-wavelength detector array or spectrometer may be integrated into the free space of a WXC and utilized to monitor wavelength channels, power and signal-to-noise ratio (SNR) in telecommunication networks. Typically, a portion of the WDM channels are diverted by a splitter or partially reflective mirror to an optical power monitor or spectrometer to enable monitoring of the WDM channels. Each MEMS mirror in today&#39;s WDM cross-connect switch is dedicated to a single wavelength channel. Whether it tilts about one or more axes, such mirror may be used to control the amount of optical power passing through WXC for such single wavelength channel. In addition, a detector array or spectrometer may be external to the free space of the WXC or OXC, and may be utilized to monitor white light (combined wavelength channels) power, and signal-to-noise ratio of optical signal via input/output fiber port taps or splitters. More specifically, the prior art consists of costly large two-dimensional detector arrays or spectrometer utilized to monitor multiple input or output wavelength channels, power and signal-to-noise ratio. 
   Monitoring and switching are part of a feedback loop required to achieve per-wavelength insertion loss control and such systems comprise three classic elements: sensor for monitoring, actuator for multi wavelength switching and attenuating, and processor for controlling wavelength switching, selection and equalization. The actuator in today&#39;s WXC products is typically a MEMS-based micromirror or a liquid crystal blocker or reflector. The sensor is typically a modular optical power monitor, comprising a mechanical filter for wavelength selection and a photodetector. Depending on the system, the three elements can be co-located in the same device, or can exist as separate standalone cards connected by a backplane. 
   In general, higher levels of integration of the sensor, actuator, and processor are attractive from a size, cost, speed, and simplicity of operation standpoint. The proposed new solution reaps these benefits because of a very high level of integration. 
   Therefore, it is readily apparent that there would be a recognizable benefit from a cost effective fiber optic switch with dual channel selector for all-optical communication networks in which each switching node demultiplexes the aggregate multi-wavelength WDM signal from input fibers into its wavelength components, spatially switches one of many single-wavelength components from different input fibers for each wavelength channel, and wherein such switch multiplexes the switched wavelength components to one output fiber for retransmission; and wherein such wavelength components power may be monitored and varied by controllable attenuation, resulting in a higher level of integration of the sensor, actuator, and processor and enabling multiple switches in a single device capable of utilizing common optical components. 
   BRIEF DESCRIPTION 
   Briefly described in a preferred embodiment, the present invention overcomes the above-mentioned disadvantages and meets the recognized need for such an invention by providing a fiber optic switch utilizing a segmented prism element comprising a fiber optic switch used in multi-channel optical communications networks and having one or more arrays of micro electromechanical system (MEMS) mirrors, wherein at least a first array of MEMS mirrors utilized to select and switch wavelengths from a number of input fiber ports (N) to an output fiber port (M), and wherein λn from multiple fiber ports (N) is focused on λn mirror via the use of such segmented prism element, wherein at least a second array of MEMS mirrors using and sharing the same free space optics as the first MEMS array is utilized to produce yet another fiber optic switch, wherein the second switch is utilized to select individual wavelengths or spectral components from its input fiber ports to send to its output fiber port for optical power or other monitoring purposes, thus, enabling a cost effective, high level of integration N×1, or alternatively a 1×N switch capable of internal feedback monitoring and dynamic insertion loss control of a switching node in telecommunication networks. 
   According to its major aspects and broadly stated, the present invention in its preferred form is a fiber optic switch enabled by the segmented prism element (SPE), comprising input fiber ports, free space optics (FSO) (including but not limited to various lenses, a diffraction grating for spatially separating/combining the wavelength components of the aggregate multi-wavelength WDM signal, and the SPE), a first array of MEMS mirrors whose individual mirrors correspond to unique wavelengths operating within the WDM network (for example, mirror # 1  corresponding to λ # 1  and receiving λ # 1  from all input fiber ports, wherein by tilting MEMS mirror # 1 , the preferred optical path is generated via beam steering between an input fiber port and the output fiber port of the N×1 configuration, this being repeated independently for every wavelength in the WDM network and for every MEMS mirror), wherein such switch multiplexes the MEMS-steered wavelength components from various input fiber ports to one output fiber port for re-transmission in the WDM network, and wherein the above switching functionality, whether in duplicate or variation thereof, is repeated one or more times within the same physical switching device (i.e., common housing) using one or more additional arrays of MEMS mirrors while simultaneously sharing the other free space optic (FSO) components described above. Analogously, the light direction may be arbitrarily reversed from the above description so that wavelengths may be switched from a single input fiber port to any of a number of output fiber ports (1×N) without restriction on which wavelength is routed to which output port. Alternatively, there may be a mixture of multiple input fiber ports and multiple output fiber ports, with the restriction that there cannot be an arbitrary switching assignment of input ports to output ports for any given wavelength. 
   Accordingly, a feature and advantage of the present invention is its ability to focus wavelength components from any or all of the input fiber ports onto a single MEMS mirror, enabling such mirror to select the input port wavelength component to be switched to the output fiber port in an N×1 switch, and to do so for manifold switches operating independently and in parallel while sharing all FSO components within the same physical housing. 
   Another feature and advantage of the present invention is its ability to focus wavelength components from the input fiber ports onto MEMS mirrors, enabling such mirror to select the output fiber port wavelength component to be switched to the output fiber port in a 1×N switch by simple rotation or tilt of the mirror, wherein the MEMS mirrors are only required to tilt around a single common axis of rotation in order to execute switching commands. 
   Still another feature and advantage of the present invention is its ability to provide one or more taps or splitters for coupling power from input and/or output fiber ports. 
   Yet another feature and advantage of the present invention is its ability to utilize switches to provide monitoring input fiber ports utilized to receive tapped or other multi-wavelength WDM signals for the purpose of optical power or other quality-of-signal measurements. 
   Yet another feature and advantage of the present invention is its ability to reuse the same free space optics (various lenses, a diffraction grating for spatially separating/combining the wavelength components of the aggregate multi-wavelength WDM signal, and the SPE) for manifold switches existing in the same physical housing. 
   Yet another feature and advantage of the present invention is its ability to provide an optical path between manifold switches (i.e., an optical bridge) to create a form of M×N switch, wherein in a preferred embodiment the optical bridge may be formed with a simple mirror placed at the SPE between two switches in the manifold switch. 
   Yet another feature and advantage of the present invention is its ability to provide for ganged switching functionality of the manifold switch, wherein the MEMS mirrors corresponding to a certain WDM wavelength are tilted synchronously between all arrays of MEMS mirrors in the manifold switch, wherein the same switch state is created for all switches in the manifold switch on a per wavelength basis. 
   Yet another feature and advantage of the present invention is flexibility wherein an almost limitless range of configurations may be obtained, wherein configuration variations may include number of input and output fiber ports, number of switches in the manifold, ganged switching operations, bridging between switches in the manifold, number and spacing of wavelengths in the WDM system, and the like. 
   Yet another feature and advantage of the present invention is its ability to be calibrated such that systematic effects are canceled and the switching performance improved, wherein systematic effects to be canceled may include, for example, imperfect MEMS mirrors, assembly and component imperfections, environmental effects, and the like, and wherein the obtained calibration data is stored in an electronic memory that can be accessed in real-time in support of switching control and command. 
   Yet another feature and advantage of the present invention is its ability to utilize a second array, or more, of MEMS mirrors for selecting one wavelength component from any of the wavelength components of any of the tapped ports for each wavelength of the multi-wavelength WDM signal, and wherein such switch directs the selected wavelength component to one monitoring output fiber port for optical power monitoring. 
   Yet another feature and advantage of the present invention is its ability to provide more MEMS mirrors in an array than there are wavelengths in the WDM network such that various spectral characteristics of the aggregate multi-wavelength WDM signal may be measured when utilizing the switching functionality for monitoring purposes. For example, by placing MEMS mirrors between the mirrors designated for WDM wavelengths a measure of inter-wavelength noise can be obtained, leading to a form of signal-to-noise measurement. Further, by adding even more mirrors to the MEMS array the spectral location of the various multi-wavelength components of the WDM signal may be verified, leading to a form of absolute wavelength measurement. 
   Yet another feature and advantage of the present invention is its ability to utilize a multi-mode fiber in the fiber array leading to the photodetector when utilizing the switching functionality for monitoring purposes, wherein the larger core of a multimode fiber increases the confidence that the true power of the intended measurement is being captured with sufficient margin for MEMS mirror pointing errors, environmental and aging effects, and the like, wherein the coupling of light from free space into a fiber is vastly less sensitive to positional errors for a multi-mode fiber than for the single-mode fibers typically used for telecom/datacom networks. 
   Yet another feature and advantage of the present invention is its ability, during signal monitoring, to record the power levels during signal measurement as the associated MEMS mirror is swept through a range of angle on either side of the expected peak power coupling angle, wherein the peak signal recorded during this sweep, or the peak of a curve-fit through the data points so taken, represents the truest measure of the intended signal, wherein the detected peak signal is maximally isolated from the potentially detrimental effects of MEMS mirror pointing errors, environmental and aging effects of the system, and the like, wherein this sweep and peak-detect approach is aided by the use of a multi-mode fiber in the fiber array leading to the photodetector. 
   Yet another feature and advantage of the present invention is its ability to provide one or more fiber ports carrying aggregate multi-wavelength WDM signals for the purpose of monitoring said WDM signals, wherein the origin of the WDM signals is arbitrary. 
   Yet another feature and advantage of the present invention is its ability to self-monitor the aggregate multi-wavelength WDM signals at the input and/or output fiber ports of a manifold switch. 
   Yet another feature and advantage of the present invention is its ability to monitor signals within fibers, wherein signals to be monitored may be produced by wideband optical power taps placed on the fibers to be monitored, wherein other approaches make only approximate measurements of signals by sampling them in free-space and therefore neglecting free-space-to-fiber coupling effects. 
   Yet another feature and advantage of the present invention is its ability, with regard to signal monitoring, to be calibrated such that systematic effects are canceled and the measurement accuracy increased, wherein systematic effects to be canceled may include the path-dependent insertion loss of various optical paths through the system, imperfect MEMS mirrors, tap device characteristics, assembly and component imperfections, environmental effects, and the like, wherein so obtained calibration data is stored in an electronic memory that can be accessed in real-time in order to provide corrections to signal measurements in real-time. 
   Yet another feature and advantage of the present invention is its ability to utilize the measurement of power levels of WDM wavelengths obtained via the described self-monitoring functionality as a form of feedback to the 1×N or N×1 switch, wherein the insertion loss of each wavelength through the switch may be actively adjusted to correct for mirror tilt errors, environmental effects, and the like, or similarly to produce desired spectral distributions of the aggregate multi-wavelength WDM signals (for example, making the power levels of all wavelengths equal via the selective attenuation of every wavelength), wherein the insertion loss of each wavelength is controlled by the tilting of the associated MEMS mirror in the 1×N or N×1 mirror array, wherein tilting the MEMS mirror away from its optimal angle of lowest insertion loss steers the free space beam arriving at the output port(s) and therefore misaligns the beam with respect to the output fiber port(s) and introduces progressively larger insertion loss as the MEMS mirror is further tilted. 
   Yet another feature and advantage of the present invention is its compatibility with using MEMS mirrors that can tilt around 2 independent axes of rotation, wherein the primary tilt axis is required for fiber-to-fiber switching and the secondary tilt axis my be used for auxiliary purposes, wherein such auxiliary uses of the secondary tilt axis may include insertion loss control, correction of component and assembly imperfections, environmental and aging effects, and the like. 
   Yet another feature and advantage of the present invention is its ability to provide a means of power equalization, or other arbitrary spectral power distribution, of wavelengths wherein many beams from diverse sources are interchanged among network fibers. 
   Yet another feature and advantage of the present invention is its ability to provide uniformity of power levels across the WDM spectrum, or other arbitrary spectral distribution, so that dynamic range considerations at receivers and amplifier, non-linear effects, and cross talk impairments can be minimized. 
   Yet another feature and advantage of the present invention is its ability to provide dynamic feedback control since the various wavelengths vary in intensity with time and relative to changes in optical channel routing history among the components. 
   Yet another feature and advantage of the present invention is its ability to provide a fiber optic switch with a means of power equalization of wavelengths, and thus provide an aggregate multi-wavelength WDM signal enabling compensation for internal variations of optical characteristics, misalignments, both integral to the device and as a result of both manufacturing and environmental variation, non-uniformity, aging, and of mechanical stress encountered in the switch. 
   Yet another feature and advantage of the present invention is its ability to provide wavelength switching and monitoring in an optical network while reducing the cost and complexity of such optical network. 
   Yet another feature and advantage of the present invention is its applicability for non-WDM, or “white light” switching devices by the simple removal of the diffraction grating and the subsequent simplification of the MEMS array to a single MEMS mirror for each optical fiber in the system. 
   These and other features and advantages of the present invention will become more apparent to one skilled in the art from the following description and claims when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present version of the invention will be better understood by reading the Detailed Description of the Preferred and Alternate Embodiments with reference to the accompanying drawing figures, in which like reference numerals denote similar structure and refer to like elements throughout, and in which: 
       FIG. 1  is a schematic illustration of a six input port by one output fiber port wavelength cross-connect (WXC) switch according to an embodiment of the present invention; 
       FIG. 2  is a schematic illustration of six input port by one output fiber port, dual channel MEMS mirror, output port taps, monitoring input fiber ports, monitoring output fiber port, and monitor wavelength cross-connect switch according to a preferred embodiment of the present invention; 
       FIG. 3A  is a schematic illustration of an optical segmented prism element included in the WXC of  FIG. 1 ; 
       FIG. 3B  is a schematic illustration of an optical segmented prism element and facet angle equations; 
       FIG. 4  is a plan view of a single axis tiltable mirror useable with the present invention; 
       FIG. 5A  is a functional block diagram of a one input port by five output fiber port wavelength cross-connect switch with power monitor and feedback control according to an alternate embodiment of the present invention; 
       FIG. 5B  is a functional block diagram of a five input port by one output fiber port wavelength cross-connect switch with power monitor and feedback control according to a preferred embodiment of the present invention; 
       FIGS. 6A and 6B  are cross sectional views illustrating two kinds of mismatch in optically coupling a wavelength component beam to the waveguide substrate according to an alternate embodiment of the present invention; 
       FIG. 7A  is a top view of a fiber holder according to a preferred embodiment of the present invention; 
       FIG. 7B  is schematically illustrated optical concentrator array using planar waveguide included in the N×1 WXC of  FIGS. 1 ,  2  and  5 B according to an alternate embodiment of the present invention; 
       FIG. 7C  is schematically illustrated optical concentrator array using planar waveguide included in the 1×N WXC of  FIG. 5A  according to an alternate embodiment of the present invention; 
       FIG. 8  is a schematic view of the front end optics included in the WXC of  FIGS. 1 and 2 ; 
       FIG. 9A  is a front face view of a first illustrative channel MEMS mirror and five incident beams from the five input fiber ports according to an illustrative embodiment of the present invention; 
       FIG. 9B  is a front face view of a second channel MEMS mirror and two incident beams from the two monitoring input fiber ports according to an illustrative embodiment of the present invention; 
       FIG. 9C  is a front face view of a third channel MEMS mirror and five incident beams from the five input fiber ports according to a preferred embodiment of the present invention as shown in  FIG. 12 ; 
       FIG. 10  is a schematic illustration of a six input port by one output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 11  is an illustration of a typical single-row MEMS mirror array according to an embodiment of the present invention; 
       FIG. 12  is a three-dimensional schematic of a MEMS mirror according to an embodiment of the present invention of  FIGS. 9A and 9B ; 
       FIG. 13  is a schematic illustration of a six input port by one output fiber port wavelength cross-connect switch according to preferred embodiment of the present invention; 
       FIG. 14  is a three-dimensional schematic of a wavelength cross-connect switch according to an embodiment of the present invention; 
       FIG. 15  is a schematic illustration of a wavelength cross-connect with SPE-based architecture for creating manifold switches within the same package switch according to an embodiment of the present invention; 
       FIG. 16  is a schematic illustration of a wavelength cross-connect with SPE-based architecture FCLA-based optics of  FIG. 10  according to an embodiment of the present invention; 
       FIGS. 17A and 17B  are schematic illustrations of a 4-input-fiber by 4-output-fiber optical switch, made up of four 1×N and four N×1 wavelength selectable switches; 
       FIG. 18A  is a schematic illustration of a five input port by one output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 18B  is a schematic illustration of an optical segmented prism element included in the WXC of  FIG. 18A ; 
       FIG. 19A  is a schematic illustration of a six input port by one output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 19B  is a schematic illustration of an optical segmented prism element included in the WXC of  FIGS. 1 and 2 ,  19 A; 
       FIG. 20  is a schematic illustration of an input port by six output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 21  is a schematic illustration of an input port by six output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 22A  is a schematic illustration of a six input port by six output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; 
       FIG. 22B  is a schematic illustration of a six input port by six output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention; and 
       FIG. 23  is a schematic illustration of a six input port by six output fiber port wavelength cross-connect switch according to an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED AND SELECTED ALTERNATIVE EMBODIMENTS 
   In describing the preferred and selected alternate embodiments of the present version of the invention, as illustrated in  FIGS. 1-23 , specific terminology is employed for the sake of clarity. The invention, however, is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish similar functions. 
   Referring now to  FIG. 1 , there is illustrated a schematic illustration of a six input fiber port by one output fiber port wavelength cross-connect switch  10 . However, it is emphasized that this 6×1 embodiment is illustrated only for simplicity, and that by increasing the number of input fiber ports by N, then an N×1 switch  10  is contemplated herein, wherein N represents the number of input fiber ports. Preferably, wavelength cross-connect switch  10  can be operated in either direction, wherein N of N×1 represents N input fiber ports and one output fiber port, or one input port and N output fiber ports. In the preferred 6×1 wavelength cross-connect switch  10  shown in  FIG. 1 , six input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22 , and one output fiber port  64  are optically coupled to fiber concentrator array (FCA)  52  (fiber port concentrator), preferably in a linear alignment, wherein preferably all-fibers (alternatively planar waveguides)  32 ,  34 ,  36 ,  38 ,  40 ,  42 , and  46  are used to bring the respective beams of fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22  and  64  closer together on output face  44  of fiber concentrator  52  (fiber port concentrator) adjacent the optics. Further, planar waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  46  are also preferably used to output the beams in parallel in a predominantly linearly spaced grid wherein planar waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  have curved shapes within fiber concentrator  52  and are optically coupled to input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22 . A beam, also known as optical signal, is multi-wavelength WDM signals and such signals travel in free space, fiber, waveguides, and other signal carriers. 
   Although, other coupling arrangements are possible, preferred fiber concentrator  52  offers some additional advantages over other coupling arrangements. For example, its planar waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  concentrate and reduce the spacing between input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22  from  125  micrometers, representative of the fiber diameters, to the considerably reduced spacing of, for example, 40 micrometers, which is more appropriate for the magnifying optics of switch  10 . Each of waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  is preferably coupled to the respective  12 ,  14 ,  16 ,  18 ,  20 ,  22  input fiber port. Waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  preferably extend along a predominately common plane directing the multi wavelength signals to output in free space and to propagate in patterns having central axes which are also co-planar. 
   The free-space beams output by waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  of fiber concentrator  52  are preferably divergent and preferably have a curved field. For simplicity, this discussion will describe all the beams as if they are input beams, that is, output from fiber concentrator  52  to free-space optics (FSO)  74 . The beams are in fact, optical fields coupled between optical elements. As a result, the very same principles as those discussed as input beams apply to those of the beams that are output beams which eventually reenter fiber concentrator  52  for transmission onto the network. 
   The beams output from fiber concentrator  52  into the free space of wavelength cross-connect switch  10  preferably pass through front end optics (FE)  56 . Outputs of waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  of face  44  preferably are placed at or near the focal point of front end optics  56 . Front end optics  56  accepts the light beams coming from or going to all fibers via input and output fiber ports. For light beams emerging from a fiber or input port, front end optics  56  preferably captures, projects and collimates the light in preparation for spectral dispersion by diffraction grating  62 . The reverse of this happens for light beams converging toward a fiber; that is, the principles of operation are identical in either case, and independent of the direction of the light. It should be noted that common diffraction gratings do not operate exactly as shown in  FIG. 1 , more specifically the input and diffracted beams do not lie in the same plane as shown in  FIG. 1 . 
   Although a single lens is illustrated in  FIG. 1 , front end optics  56  may generally consist of two or more lenses, and may become progressively sophisticated as the demands of wavelength cross-connect switch  10  increases (e.g., the number of fibers, the range of wavelengths, the number of input and output fiber ports, the spacing of the MEMS mirrors, etc.). For example, in a two lens front end optics  56 , the first lens (closest to the fibers or input fiber ports) may be used to produce customary flat-field and telecentric beams that easily accommodate simple fiber arrays or fiber concentrator  52 , and the second lens may perform the majority of the collimation task. As the demands of wavelength cross-connect switch  10  increase, front end optics  56  may further employ advanced features, such as aspheric optical surfaces, achromatic designs, and the like. Unlike traditional approaches wherein a separate lens must be critically aligned to every fiber, front end optics  56  described herein are preferably common to every fiber, thereby enabling a realization of significant savings in assembly time and cost relative to previously known switch systems. 
   The collimated beams exiting front end optics  56  propagate substantially within a common plane, and are incident upon diffraction grating  62 , a wavelength dispersive element, wherein diffraction grating  62  preferably comprises grating lines extending perpendicular to the principal plane of wavelength cross-connect switch  10 . The beams may overlap when they strike diffraction grating  62 , wherein diffraction grating  62  preferably separates the input port beams WDM (optical signal) into corresponding sets of wavelength-separated beams, λ 1  through λn (wavelengths) for each input port beam, where n is the number of wavelengths in each input port beam. Diffraction grating  62  angularly separates the multi-wavelength input beams into wavelength-specific sub-beams propagating in different directions parallel to the principal optical plane, or alternatively serves to recombine single-wavelength sub-beams into a multi-wavelength beam. Diffraction grating  62  is preferably uniform in the fiber direction, wherein the preferred uniformity allows use of diffraction grating  62  for signals to and from multiple input and output fibers. 
   The line density of diffraction grating  62  should preferably be as high as possible to increase spectral dispersion, but not so high as to severely reduce diffraction efficiency. Two serially arranged gratings would double the spectral dispersion. However, a single grating with a line density of approximately 1000 lines/millimeter has provided satisfactory performance. Diffraction grating  62  is preferably aligned so that the beam from front end optics  56  has an incident angle of preferably 54 degrees on grating  62 , and the diffracted angle is about 63 degrees. The difference in these angles results in optical astigmatism, which may be compensated by placing a prism between front end optics  56  and diffraction grating  62 . In brief, the diffraction efficiency of a grating is generally dependent on the characteristics of the polarization of the light with respect to the groove direction on the grating, reaching upper and lower diffraction efficiency limits for linear polarizations that are parallel p-polarization and perpendicular s-polarization to the grooves. 
   In addition, polarization sensitivity of the grating may be mitigated by introducing a quarter-wave plate (not shown) after diffraction grating  62  or elsewhere in switch  10  whose optical axis is oriented at forty-five degrees to the diffraction grating limiting diffraction efficiency polarization states described previously. It is contemplated herein that such quarter-wave plate may be placed elsewhere in switch  10 . Preferably, every wavelength-separated sub-beam passes twice through the quarter-wave plate so that its polarization state is effectively altered from input to output fiber port. That is, diffraction grating  62  preferably twice diffracts any wavelength-specific sub-beam, which has twice passed through the quarter-wave plate. For example, considering the two limiting polarization cases the sub-beam passes once with a first limiting polarization (for example, p-polarization) and once again with a polarization state that is complementary to the first polarization state (for example, s-polarization) from the perspective of diffraction grating  62 . As a result, any polarization dependence introduced by diffraction grating  62  is canceled. That is, the net efficiency of diffraction grating  62  will be the product of its S-state and P-state polarization efficiencies, and hence independent of the actual polarization state of the input light. 
   In the wavelength division multiplexing (WDM) embodiments of the invention, each input fiber port  12 ,  14 ,  16 ,  18 ,  20 ,  22  is preferably capable of carrying a multi-wavelength WDM optical signal having wavelengths λ 1  through λn. Wavelength cross-connect switch  10  is preferably capable of switching the separate wavelength components from any input port to planar waveguide  46  of fiber concentrator  52 , which is preferably coupled to output fiber port  64 . This architecture applies as well to a WDM reconfigurable add/drop multiplexer (ROADM), such as a 1×6 ROADM in which fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22  are associated respectively with the input (IN) (fiber port  12 ), five (5) DROP ports (fiber ports  14 ,  16 ,  18 ,  20 ,  22 ), and output (OUT) (fiber port  64 ). Or, in the 6×1 ROADM, input (IN) (fiber port  12 ), five (5) ADD ports (fiber ports  14 ,  16 ,  18 ,  20 ,  22 ), and output (OUT) (fiber port  64 ). In operation, fiber ports  14 ,  16 ,  18 ,  20 ,  22 , (local ports) are switched to/from by wavelength cross-connect switch  10 , either are added (ADD) to the aggregate output (OUT) port  64  or dropped (DROP) from the aggregate input (IN) port  12 . 
   Back end optics (BE)  66  projects the wavelength-separated beams onto segmented prism element (SPE)  68  (steering element). Back end optics  66  creates the “light bridge” between diffraction grating  62  and segmented prism element  68  to switching mirror array  72 . Considering the case of light diffracting from diffraction grating  62  and traveling toward back end optics  66 , such back end optics  66  preferably capture the angularly (versus wavelength) separated beams of light, which is made plural by the number of fibers, and wherein back end optics  66  create parallel beams of light. The parallel beams are obtained via a preferred telecentric functionality of back end optics  66 . In addition, because all beams are preferably at focus simultaneously on the flat MEMS plane of switching mirror array  72 , then back end optics  66  must also preferably perform with a field-flattening functionality. After light reflects off of a MEMS mirror and back into back end optics  66 , the reverse of the above occurs; the principles of operation are identical in either case and are independent of the direction of the light. 
   Although a single lens is illustrated in  FIG. 1 , back end optics  66  may generally consist of two or more lenses or mirror, and may become progressively sophisticated as the demands of wavelength cross-connect switch  10  increases (e.g., the number of fibers, the range of wavelengths, the number of input and output fiber ports, the spacing of the MEMS mirrors, etc.). The focal length of back end optics  66  (or the effective focal length in the case of multiple lenses) is preferably determined from the rate of angular dispersion versus wavelength of diffraction grating  62  and the desired mirror spacing of switching mirror array  72 . If the angular separation between two neighboring wavelengths is denoted by A and the spacing between their associated MEMS micro-mirrors is denoted by S, then the focal length of back end optics  66  (F) is approximated by F=S/tan(A). Because the angular dispersion of common gratings is relatively small, and/or as the spectral separation between neighboring wavelengths is decreased, then back end optics  66  focal length may become relatively large. Preferably, however, a physically compact optical system may be retained by providing back end optics  66  with a telephoto functionality, thereby reducing the physical length of back end optics  66  by a factor of two or more. A three-lens system is generally sufficient to provide all of the functionalities described above, and the lenses themselves can become increasingly sophisticated to include aspheric surfaces, achromatic design, etc., as the demands of wavelength cross-connect switch  10  increase (e.g., depending on the number of fibers, the range of wavelengths, the number of input and output fiber port, the spacing of the MEMS mirrors, etc.). The focal length calculations set forth here with respect to the back end optics are applicable to the front end optics as well. 
   Such a preferred multi-lens back end optics  66  system, by virtue of its increased degrees-of-freedom, additionally allows for active optical adjustments to correct for various lens manufacturing tolerances and optical assembly tolerances that otherwise would not be available. Segmented prism element  68 , although physically existing in the beam path of back end optics  66 , is preferably designed utilizing passive monolithic element containing multiple prism or lenses and preferably functions almost independently of back end optics  66 . 
   Referring now to  FIG. 3A , there is illustrated a schematic illustration of a preferred optical segmented prism element included in the WXC of  FIG. 1  (the number of segments or facets varies with the number of signals present in the WXC). Segmented prism element  68  preferably refracts wavelength-separated beams from back end optics  66  and steers such beams onto switching mirror array  72  based on the refractive indices of each segment. Segmented prism element  68  (steering element) preferably refracts λn from each input port  12 ,  14 ,  16 ,  18 ,  20 ,  22  onto λn mirror of switching mirror array  72  assigned to λn. For example, λ 1  mirror of switching mirror array  72  has λ 1 ( 12 )-λ 1 ( 22 ) from all input fiber ports  12 - 22  projected onto λ 1  mirror surface via segmented prism element  68 , and by titling λ 1  mirror of MEMS switching mirror array  72 , wavelength cross-connect switch  10  preferably switches one selected λ 1  ( 12 - 22 ) from input fiber ports  12 - 22  to output fiber port  64  and blocks the remaining unselected λ 1 (s) from input fiber ports  12 - 22 , and so forth for λ 2 -λn. Each λn mirror of switching mirror array  72 , in this example, has five input beams projected simultaneously onto the surface of such mirror, all at wavelength λn, wherein those five beams are preferably demultiplexed and focused by free space optics  74  from input fiber ports  12 ,  14 ,  16 ,  18 ,  20 . It should be recognized that utilizing segmented prism element  68  enables refracting and/or steering of multiple wavelengths onto a single mirror from one or more input fiber ports  12 - 22  or refracting light to any arbitrary point rather than prior art switches, which use lenses or mirrors to focus individual wavelengths to individual dedicated mirrors based on one focal point. Further, it should be recognized that utilizing segmented prism element  68  enables multiple N×1 switches to be packaged as a single unit. Still further, it should be recognized that utilizing segmented prism element  68  enables the potential elimination of lenslets for each optical fiber port, thereby reducing the number of elements and the overall cost of the switch. 
   Segmented prism element  68  preferably is manufactured from fine-anneal glass with class-zero bubble imperfections whose facets are very finely polished and are coated with anti-reflection material. Further, the type of glass may be chosen to have certain optical properties at the desired wavelengths of operation, including but not limited to optical transparency and refractive index. The angular deflection imparted by each facet of segmented prism element  68  is preferably a function of both the angle of the facet and the refractive index of the glass as shown in FIG.  3 B—Segmented Prism Element  68  “Light Deflection Principles and Equations”; hence, in principle segmented prism element  68  can be made from a wide variety of glass types. This allows further optimization of the glass material per the criteria of cost, ease of fabrication, etc. As an example, the type of glass known as BK7 is a common high-quality, low cost glass that is preferably suitable for this application. 
   Another criterion for glass selection may be its change in optical properties relative to temperature. Since the refractive index of all materials changes with temperature, which could in turn produce undesirable changes in the effective facet angles  102  produced by segmented prism element  68 , then for demanding applications, a glass with a very low thermo-optic coefficient may be chosen at the desired operational temperature range. For example, the common glasses known as K5 and BAK1 have very low thermo-optic coefficients at room temperature. In addition to the precision polishing of the segmented prism element  68  from bulk glass, segmented prism element  68  may also be fabricated using castable glass materials, such as sol-gel. Prism elements fabricated in such fashion should exhibit improved performance consistency compared with those fabricated using traditional polishing techniques. The materials for fabrication of segmented prism element  68  are not limited to glass but may also include high quality plastic materials such as ZEONEX (Zeon Chemicals L.P.). As such, the cost of manufacturing segmented prism element  68  may be further lowered by using plastic injection molding techniques. 
   An alternative to fabricating segmented prism element  68  from a single monolithic piece of glass or plastic is to fabricate each facet section, and/or groups of facet sections, individually and then vertically stack them to create a single composite element. 
   In a preferred embodiment, segmented prism element  68  is polished from bulk BK7 glass and has dimensions of length 40 millimeters, height 15 millimeters, width at the base of 4 millimeters and width at the top of 3.18 millimeters. Facet angles  102  for the six input fiber wavelengths and one output fiber wavelength model preferably are 11.82, 8.88, 5.92, 2.96, 0.00, −2.96, −5.92 degrees. For ease of fabrication so that the edges of adjacent facets are coincident, especially with regard to fabrication by polishing, segmented prism element  68  preferably is designed to have varying degrees of thickness for each facet, resulting in the above stated angles of deflection, wherein such angles of deflection preferably position the six input λ 1 ( 12 )-λ 1 ( 22 ) wavelengths on λ 1  mirror and so on for λ 2 -λn mirrors. It should be noted, however, that segmented prism element  68  may be designed and manufactured having facet angles  102  different than set forth herein, depending on the fiber spacing, number of input fiber ports, number of wavelength components per input port, lenses, grating, MEMS mirror configuration, and the like. 
   Referring again to  FIG. 1 , the distance between switching mirror array  72 , segmented prism element  68 , and the vertical location of the beam at segmented prism element  68 , and free space optics  74  as well as other factors including fiber spacing, number of input fiber ports, number of wavelength components per input port, lenses, grating, MEMS mirror configuration and output fiber ports preferably determines the facet angle required to enable all six input port wavelengths to be positioned on each MEMS mirror assigned to the specific wavelength of switching mirror array  72 . Because the vertical location of the various fiber port components are different as they intercept the segmented prism element  68 , the facet angles of segmented prism element  68  preferably vary accordingly in order to combine all of wavelengths λn at a common mirror λn of switching mirror array  72 . The analogous situation exists for the selected input port wavelength λn reflecting from mirror λn of switching mirror array  72  directed to output fiber port  64 . This is illustrated in  FIG. 3B , and wherein the facet angle A can be determined from equation β. In finding the preferred facet angle A from equation β, the known variables are the input beam angle, α, the distance between the input beam vertical location at segmented prism element  68  relative to mirror λn of switching mirror array  72 , y, and the distance between the segmented prism element  68  and the MEMS, z, leaving the only free variable as the refractive index material of segmented prism element  68  (η). Equation β is transcendental in A and may be solved by iteration or various algorithms. 
   Referring now to  FIG. 4 , there is illustrated a top view of a single axis tiltable (moveable) mirror. Switching mirror array  72  (as seen in  FIGS. 1 and 2 ) is preferably formed as a two-dimensional array (preferably two rows of 40 mirrors) of single-axis tiltable mirrors, with one mirror, single cell (mirror)  260  of switching mirror array  72 . Cell  260  is one of many such cells arranged typically in a two-dimensional array in a bonded structure including multiple levels of silicon and oxide layers in what is referred to as multi-level silicon-over-insulator (SOI) structure. Cell  260  preferably includes frame  262  supported in support structure  264  of switching mirror array  72 . Cell  260  further includes mirror plate  268  having reflective surface  270  twistably supported on frame  262  by a pair of torsion beams  266  extending from frame  262  to mirror plate  268  and twisting about axis  274 . In one MEMS fabrication technique, the illustrated structure is integrally formed in an epitaxial (epi) layer of crystalline silicon. The process has been disclosed in U.S. Provisional Application Ser. No. 60/260,749, filed Jan. 10, 2001, (now abandoned) is incorporated herein by reference in its entirety. However, other fabrication processes resulting in somewhat different structures may be used without affecting or departing from the intended scope of the present invention. 
   Mirror plate  268  is controllably tilted about axis  274  in one dimension by a pair of electrodes  272  under mirror plate  268 . Electrodes  272  are symmetrically disposed as pairs across axis  274  respective torsion beams  266 . A pair of voltage signals V(A), V(B) is applied to the two mirror electrodes  272 , while a common node voltage signal V(C) is applied to both mirror plate  268  and frame  262 . 
   Circumferentially lateral extending air gap  278  is preferably defined between frame  262  and mirror plate  268  so that mirror plate  268  can rotate with respect to frame  262  as two parts. Support structure  264 , frame  262 , and mirror plate  268  are driven by the common node voltage V(C), and electrodes  272  and mirror plate  268  form plates of a variable gap capacitor. Although  FIG. 4  illustrates the common node voltage V(C) being connected to mirror plate  268 , in practice, the electrical contact is preferably made in support structure  264  and torsion beams  266  apply the common node voltage signal to both frame  262  and mirror plate  268 , which act as a top electrode. Electrical connectivity between frame  262  and mirror plate  268  can be achieved through torsion beams  266  themselves, through conductive leads formed on torsion beams  266 , or through a combination of the two. Electrodes  272  are formed under mirror plate  268  and vertical air gap  279  shown into the page is further defined between electrodes  272  and mirror plate  268  and forms the gap of the two capacitors. 
   Torsion beams  266  act as twist springs attempting to restore mirror plate  268  to its neutral tilt position. Any voltage applied across either electrode  272  and mirror plate  268  exerts an attractive force acting to overcome torsion beams  266  and to close the variable gap between electrodes  272  and mirror plate  268 . The force is approximately linearly proportional to the magnitude of the applied voltage, but non-linearities exist for large deflections. The applied voltage can be a DC drive or an AC drive per Garverick et al. set forth below. In practice, the precise voltages needed to achieve a particular tilt are experimentally determined. 
   Because each of two electrodes  272  forms a capacitor with mirror plate  268 , the amount of tilt is determined by the difference of the RMS voltages applied to the two capacitors of the pair. The tilt can be controlled in either direction depending upon the sign of the difference between the two RMS voltages applied to V(A) and V(B). 
   Referring again to  FIG. 1 , there are many ways of configuring the MEMS array of micromirrors and their actuation as wavelength switching array (WSA)  75 . The following is an example: The MEMS array may be bonded to and have an array of solder bumps contacting it to control circuitry  78 , preferably including high-voltage circuitry needed to drive the electrostatic actuators associated with each of the mirrors. Control circuitry (controller)  78  controls the driver circuit and hence the mirrors in a multiplexed control system including address lines, data lines, and a clock line, driven in correspondence to an oscillator. The control is preferably performed according to pulse width modulation (PWM), a method for controlling the mirror tilt, as Garverick has described in U.S. Pat. No. 6,543,286, issued Apr. 8, 2003, and U.S. Pat. No. 6,705,165, issued Mar. 16, 2004, incorporated herein by reference in their entirety. In these methods, a high-voltage square-wave common node drive signal is supplied through a power transistor to the common electrical node comprising all the mirrors while the driver array delivers phase delayed versions of the square-wave signal to each individual electrode, the amount of delay determining the RMS voltage applied across the electrostatic actuator electrodes of each mirror. In addition, Garverick has described in U.S. Pat. No. 6,788,981, issued Sep. 7, 2004 and incorporated herein by reference in its entirety, a method wherein an analog control system for an array of moveable mechanical elements, such as tiltable mirrors, formed in a micro electromechanical system (MEMS) is disclosed. 
   Control circuitry  78  preferably receives switch commands from the external system to effect switching of the wavelength separated channels between the input and output fibers. Preferably, the coarse pointing constants, which are primarily representative of the physical characteristics of the MEMS array and its driver circuit, may be stored in an electrically programmable read-only memory. 
   Referring to  FIGS. 1 and 4 , the angle of a mirror in switching mirror array  72  is preferably actively tilted by control circuitry  78  applying a voltage V(A), V(B) to electrodes  272  of switching mirror array  72  so that the selected input port sub-beam λn is preferably reflected to land precisely at the center of concentrator waveguide  46  associated with the particular output fiber port  64  after retracing its path through free space optics  74 . The mirror is preferably actively tilted by control circuitry  78  to the required angle such that the sub-beam, after reflection off the mirror, is properly aligned to planar waveguide  46  associated with output fiber port  64 . Preferably, cell  260  (λ 1  mirror) assigned to λ 1  of switching mirror array  72  tilts its mirror plate  268 , which has projected on its reflective surface  270  λ 1 ( 12 )-λ 1 ( 22 ) from the six input fiber ports  12 - 22 , and by control circuitry  78  applying a predetermined voltage V(A), V(B) to electrodes  272  of switching mirror array  72 , tilts mirror plate  268  thereby selecting λ 1  from any of the six input fiber ports  12 - 22  (the other λ 1 (s) being not selected are reflected into free space) and the selected λ 1  is reflected to land precisely at the center of planar waveguide  46  associated with output fiber port  64  after retracing its path through free space optics  74 . Wavelength cross-connect switch  10  switches one selected λ 1  from input fiber ports  12 - 22  to output fiber port  64  and blocks the remaining unselected λ 1 (s) from input fiber ports  12 - 22 , and so forth for λ 2 -λn. 
   The described embodiment was based on 40 channels (n=40) in the ˜1530-1562 nanometer band. However, the design is easily adapted to conform to various regions of the optical spectrum, including S-band, C-band, and L-band, and to comply with other wavelength grids, such as the 100 GHz, 50 GHz, etc. grids published by International Telecommunication Union (ITU). 
   The described design provides several advantages for facilitating its easy insertion into WDM systems of either a few wavelengths, or for dense WDM (DWDM) systems having many wavelengths. For example, the design of the present invention produces lower polarization mode dispersion (PMD) and low chromatic dispersion relative to previous designs. Low PMD and chromatic dispersion naturally follows from the free-space optics. 
   Other types of MEMS mirror arrays may be used, including dual axis gimbal structure cells, those relying on flexing elements other than axial torsion beams, and those moving in directions other than tilting about a central support axis. In particular, dual axis gimbaled mirrors facilitate hitless switching in regards to 1×N mode of operation. Wavelength dispersive elements other than diffraction gratings also may be used. The concentrator, although important, is not crucial to many of the aspects of the invention. Further, the concentrator may be implemented in an optical chip serving other functions such as amplification, splitter or wavelength conversion. 
   A white-light cross connect, that is, an optical switch that switches all λs on a given fiber together, can be adapted from the system of  FIGS. 1-5  by eliminating the diffraction grating or DeMux/Mux. Although the invention has been described with respect to a wavelength cross connect, many of the inventive optics can be applied to white-light optical cross connects that do not include a wavelength dispersive element. Although tilting micromirrors are particularly advantageous for the invention, there are other types of MEMS mirrors than can be electrically, magnetically, thermally, or otherwise actuated to different positions or orientations to affect the beam switching of the invention. 
   Referring now to  FIG. 2 , a schematic illustration of a six input fiber port by one output fiber port with integrated optical switching and monitoring system  11  is shown. Optical switching and monitoring system  11  preferably includes elements and configuration of switch  10  including six input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22 , additionally auxiliary monitoring fiber port  23 , fiber concentrator array (FCA)  52 , planar waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42 , additionally  41 ,  43  and  45 , FSO  74  including front end optics (FE)  56 , diffraction grating  62 , back end optics (BE)  66 , segmented prism element (SPE)  68 , switching mirror array  72 , control circuitry  78 , WSA  75 , output fiber port  64  and output monitoring fiber port  25 . 
   According to a preferred embodiment of the invention, optical switching and monitoring system  11  is incorporated preferably by fabricating output tap  80  and planar waveguide  41  into fiber concentrator  52 , whereby tap  80  preferably couples about 10% of the optical power from output fiber port  64  of planar waveguide  46  into planar waveguide  41  which directs the multi wavelength output beam to output in free space and propagate in a pattern having a central axis which is co-planar with outputs from waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  42  of  FIG. 1  in free space optics  74 . 
   Alternatively, an optical switching and monitoring system with feedback monitoring of the output fiber may be implemented externally (off-board of the optical switching and monitoring system  11 ) by fusing the output fiber with an monitoring fiber or via use of face plate connector and a splitter or jumper to couple about 10% of the optical power from output fiber port  64  fiber into monitoring fiber port  21 , which is coupled to planar waveguide  41 . Planar waveguide  41  outputs its multi-wavelength beam in free space propagating in a pattern having a central axis which is co-planar with outputs from waveguides  32 ,  34 ,  36 ,  38 ,  40  in free space optics  74 . 
   Optical switching and monitoring system  11  preferably includes auxiliary monitoring fiber port  23  which is preferably coupled to planar waveguide  43 , and preferably outputs its multi-wavelength beam in free space propagating in a pattern having a central axis which is co-planar with outputs from waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  41 ,  42 ,  43  in free space optics  74 , thus enabling an auxiliary multi-wavelength beam to be monitored by optical switching and monitoring system  11 . An external signal not found on input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22  may be input into auxiliary monitoring fiber port  23  and optical switching and monitoring system  11  may be utilized to monitor or read the power of each wavelength of a multi-wavelength beam input on auxiliary monitoring fiber port  23 , and to output such data to a user interface (User i/f) port  77  shown in  FIGS. 5A and 5B . It is contemplated herein that more than one auxiliary monitoring port may be provided in a similar fashion. 
   Free space optics  74  preferably position the two multi-wavelength beams of monitoring fiber ports  21  and  23  propagating from planar waveguides  42  and  43  onto monitoring mirror array  73  second row (row B). Cell  260  assigned to λ 1  mirror of monitoring mirror array  73  tilts its mirror plate  268  (shown in  FIGS. 4 and 9B ), which has projected on its reflective surface  270  λ 1 ( 21 ) and λ 1 ( 23 ) from the two monitoring fiber ports  21  and  23  and by control circuitry  78  applying a voltage V(A), V(B) to electrodes  272  of monitoring mirror array  73  tilting mirror plate  268  selects λ 1  either from monitoring fiber ports  21  or  23  from two monitoring fiber ports  21  and  23  (the other λ 1  being not selected is reflected away from the waveguides) and the selected λ 1  is preferably reflected to land precisely at the center of concentrator waveguide  45  associated with the particular output monitoring fiber port  25  after retracing its path through free space optics  74 . 
   Optical switching and monitoring system  11  is capable of simultaneously switching one selected λ 1  from input fiber ports  12 - 22  to output fiber port  64  and blocking the remaining unselected λ 1 (s) from input fiber ports  12 - 22 , and so forth for λ 2 -λn, and switching one selected λ from monitoring fiber ports  21  and  23  to output monitoring fiber port  25  and blocking the remaining unselected λ from monitoring fiber ports  21  or  23  as well as all other λs from monitoring fiber ports  21  and  23  and so forth for λ 2 -λn individually. Output monitoring fiber port  25  preferably receives the selected single wavelength λ switched by MEMS mirror array  73  (row B) after it has passed through free space optics  74 . Output monitoring fiber port  25  preferably is coupled to optical power monitor  79 . 
   Power monitor (optical measurement device)  79  preferably is a photodiode, preferably measuring the power level of wavelength λn switched by monitoring mirror array  73  (row B), measuring one wavelength at a time. As monitoring mirror array  73  (row B) selects wavelength λn and routes it to waveguide  45  coupled to output monitoring fiber port  25 , power monitor  79  preferably measures the power of such wavelength λn. Alternatively, power monitor  79  may be any device capable of measuring power of one or more wavelengths by scanning the multi-wavelength components, as well as analyzing signal to noise ratios by spectrum analyzing the wavelength bandwidth, polarization-dependent properties and the like. The optical intensities for all wavelength-separated signals are preferably converted to analog or digital form by power monitor  79  and supplied to control circuitry  78 , which preferably adjusts switching mirror array  72  as set forth herein to adjust the power of wavelength λn to conform to one or more predetermined criteria. 
   Other forms of power monitoring are possible as long as the time necessary for resolutions of differences in wavelength channel power levels is sufficient for power adjustments. If the adjustments are intended to only address aging and environmental effects, the resolved measurement time may be relatively long. On the other hand, fast feedback may be necessary for initializing switch states, for compensating for transient changes in power level such as occurs from the combination of polarization-dependent loss and polarization fluctuations which vary at the wavelength level, for stabilizing against vibration, and for alarm signaling to protection circuitry. Moreover, other parameters may be measured such as optical signal to noise ratio (OSNR), center wavelength, transient behavior, or bit error rate with an appropriate detector. 
   Moreover, various configurations of optical switching and monitoring system  11  are contemplated herein, including taps or splitters for all or a selected number of input and output fiber ports, including their associated planar waveguide, free space optics, MEMS mirrors and the like. 
   Referring now to  FIG. 5A , a functional block diagram of a one input port by five output fiber port 1×N (N=5) optical switching and monitoring system  10 . 1  wavelength cross-connect switch with power monitor and feedback control is illustrated according to an alternate embodiment of the present invention. In optical switching and monitoring system  10 . 1 , forty wavelengths enter input port (In)  12  and are demultiplexed (DeMux)  302  into forty separate wavelengths λ 1 -λ 40 , the optical cross-connect (OXC)  304  switches the forty wavelengths, multiplexes (Mux)  306 , and outputs the forty wavelengths to their switch selected output (Out  1 - 5 )  13 ,  15 ,  17 ,  19 ,  21 . Forty wavelengths in and forty wavelengths out; however, the forty wavelengths out are distributed across the output fiber ports (Out  1 - 5 )  13 ,  15 ,  17 ,  19 ,  21  as selected by the optical cross-connect switch  304 . About 10% of the optical power of each output (Out  1 - 5 )  13 ,  15 ,  17 ,  19 ,  21  is tapped or split off (Output taps)  308  to a 5:1 combiner  310 , which is coupled to an 80 channel selector  312 . Channel selector  312  preferably selects one wavelength of the forty internal or forty external (Aux. OPM In)  23  and feeds such wavelength to the photo diode (PD)  314 . The output from the photodiode is passed to the equalization control circuit  316  and/or to user interface  77  (User i/f). The equalization control circuit  316  preferably controls the per wavelength variable optical attenuator (VOA)  318  which adjusts the wavelength transmitted power to conform to one or more predetermined criteria. Switch commands  71  are provided by a network management system or network alarming system external to optical monitoring system  10 . 1  for wavelength selection from input to output switching, for wavelength selection for power monitoring, and/or power monitoring. 
   Referring now to  FIG. 5B , a functional block diagram of five input fiber ports by one output fiber port N×1 (N=5) optical switching and monitoring system  10 . 2  wavelength cross-connect switch with power monitor and feedback control is illustrated according to preferred embodiment of the present invention. In optical switching and monitoring system  10 . 2 , forty wavelengths enter each input port (In  1 - 5 )  12 ,  14 ,  16 ,  18 ,  20  and are demultiplexed (DeMux)  304  into five sets of forty separate wavelengths λ 1 -λ 40 , the optical cross-connect (OXC)  304  selects and switches forty wavelengths, multiplexes (Mux)  306  and outputs forty selected wavelengths to output (Out)  64 . About 10% of the optical power of output (Out)  64  is tapped or split off (Output Tap)  308  to an 80 channel selector  312 . The channel selector  312  selects one wavelength of the forty internal or forty external (Aux. OPM In)  23  and feeds such wavelength to the photo diode (PD)  314 . The output from the photodiode is passed to the equalization control circuit  316  and/or to user interface  77  (User i/f). The equalization control circuit  316  controls the corresponding wavelength variable optical attenuator (VOA)  318  which adjusts the transmitted power to conform to one or more predetermined criteria. Switch commands  71  are provided by a network management system or network alarming system external to optical monitoring system  10 . 2  for wavelength selection from input to output switching, for wavelength selection for power monitoring, and/or power monitoring. 
   User interface  77  preferably is an interface enabling information to pass from the optical switching and monitoring system to outside of the optical switching and monitoring system, and from outside the optical switching and monitoring system into the optical switching and monitoring system, wherein such systems include but are not limited to manual settings, network management systems and/or network alarming systems. Information may include, but is not limited to, wavelength routing information, wavelength selection for power monitoring, wavelength to be switched from input to output, switch status, wavelength power levels, wavelength power level settings, and the like. 
   The optical switching and monitoring systems described above in  FIGS. 1 ,  2  and  5  is preferably internal to the optical switching and monitoring system and has the advantage of monitoring all the free space optics and mirrors of such switch. However, an external optical monitoring system is possible wherein photodiode  79  is external and coupled to the optical switching and monitoring system via monitoring fiber  25  (shown in  FIG. 2 ), with the advantage of monitoring all the optics and mirrors of the switch, as well the insertion losses between the optical switching and monitoring system and the network fibers. 
   Equalization is achieved in the above embodiments with relatively minor additions to the hardware other than the optical power monitor and taps. Mirrors  72  used for switching between channels and for optimizing transmission are used additionally for the variable attenuation of the output power, thereby effecting variable transmission through optical switching and monitoring system  11 . To achieve such variable attenuation external to the switch would otherwise require separate attenuators in each of the multiple wavelengths of each of the optical channels. Moreover, the control functions can be incorporated into the same control circuitry  78 . 
   There are two principal types of misalignment or mismatch between the beam and waveguide to attain variable attenuation of the wavelength output power (transmission coefficient). Referring now to  FIG. 6A , a cross sectional view illustrates a mismatch in optically coupling a wavelength component beam to the waveguide substrate according to a preferred embodiment of the present invention. Positional mismatch occurs when, as illustrated in the cross-sectional view of  FIG. 6A , central axis  112  of wavelength λn beam  110  is offset slightly from central axis  114  of waveguide  116  of fiber concentrator  52 . The figure, being suggestive only, does not illustrate the smooth variation of the optical fields both inside and outside of the illustrated wavelength λn beam  110  and waveguide  116  and across the lateral interface.  FIG. 6A  further assumes that the two modal fields have the same width, which is the typical object of optical design. Slightly tilting mirror λn of switching mirror array  72  (row A) to deliberately misalign or mismatch wavelength λn beam  110  entry into waveguide  116  of fiber concentrator  52 , resulting in a degraded coupling and in loss of wavelength λn beam  110  optical power in waveguide  116 . In a typical embodiment, coupling is attenuated by about 1 dB per micrometer of positional mismatch. 
   On the other hand, angular mismatch occurs when, as illustrated in the cross-sectional view of  FIG. 6B , wavelength λn beam  110  is angularly inclined with respect to waveguide  116  even if their central axes  112  and  114  cross at their interface  118 . Angular mismatch degrades the coupling because a phase mismatch occurs between the two fields at the interface arising from the axial z-dependence of the two complex fields. In a typical embodiment, coupling is degraded by about 1 dB per degree of angular offset but the angular dependence depends strongly upon the optics. It should be appreciated that a beam can be both positionally and angularly mismatched with a waveguide. It should be yet further appreciated that the mismatch can occur at the input fiber (if no concentrator) and its beam field defined by the rest of the optical system. 
   Referring now to  FIG. 7A , there is illustrated a fiber concentrator  120  that relies upon optical fiber included in the switch of  FIGS. 1 ,  2  and  5 . Fiber holder  122  is patterned by precision photolithographic techniques with a series of preferably V-shaped grooves (or other channel configuration) in the general planar pattern shown in fiber holder  122  of  FIG. 7A . Single-mode or multi-mode optical fibers  124  having cores  126  surrounded by claddings  127  and buffer  128 . In this application, optical fibers  124  are stripped of their protective buffer  128  and cladding  127 , or have their cladding  127  reduced or tapered toward output face  44  of fiber holder  122  to enable close linear placement of cores  126 . Typical core and cladding diameters are respectively 8.2 micrometers and 125 micrometers. Among other favorable attributes, the concentrated fiber core spacing reduces the amount of “dead space” between fibers which would otherwise increase the total mirror tilt range. Tapered fibers  124  are preferably placed into the grooves with their tapered ends forming transition to free-space optics  74 . The all-fiber design eliminates the tedious alignment and in-path epoxy joint of combination waveguides, as shown in  FIGS. 7B and 7C . The design also eliminates polarization-related effects arising in planar waveguides. 
   Fiber concentrator  120  interfaces widely separated optical fibers  124  with the closely configured free space optics  74  and wavelength switching array  75  of WXC of  FIGS. 1 and 2 . Multiple fibers  124  are typically bundled in a planar ribbon. V-shaped grooves in fiber holder  122  hold the reduced cladding  128  fibers with a spacing of, for example, 40 micrometers. Although a core of each fiber  124  has a relatively small size of about 8 micrometers, its outer glass cladding results in a fiber diameter of approximately 125 micrometers. The large number of fibers, which can be handled by the single set of free-space optics  74  of the invention, arranged along an optical axis make it difficult to process a large number of fiber beams with such a large spacing between them because the outermost fiber beams are so far from the optical axis capabilities of the mirrors. Also, as discussed in more detail below, a significant amount of optical magnification is required between these fibers and the MEMS mirror array, and the MEMS design and function is greatly simplified as a result of concentrating the fiber spacing. 
   Referring now to  FIG. 7B , a schematically illustrated optical fiber concentrator array  52 , using planar waveguide included in the 5×1 WXC according to a preferred embodiment of the present invention, and included in the switch of  FIGS. 1 ,  2  and  5 B. Single-mode optical fibers  124  having cores  126  surrounded by claddings  127  (shown in  FIG. 7A ) are butt coupled to fiber concentrator  52 . In the 6×1 switch  10  shown in  FIG. 1 , six input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  22  and output fiber  64  are preferably optically coupled to fiber concentrator  52  in a linear alignment and are preferably optically coupled to input fiber ports  12 - 22  and output fiber  64  to bring their beams closer together on output face  44  of fiber concentrator  52  adjacent the optics, and to output the beams in parallel in a linearly spaced grid. Returning to  FIG. 7B , fiber concentrator  52  preferably has curved shaped planar waveguides  32 ,  34 ,  36 ,  38 ,  40  and  46  corresponding to input fiber ports  12 ,  14 ,  16 ,  18 ,  20 , and output fiber  64  within fiber concentrator  52  to preferably concentrate and reduce the spacing between fiber input fiber ports  12 ,  14 ,  16 ,  18 ,  20 ,  64  from 125 micrometers, representative of the fiber diameters, to the considerably reduced spacing of, for example 30 or 40, micrometers and preferably no more than 50 micrometers which is more appropriate for the magnifying optics of switch  10  and an optimum tilt range of the mirrors. Each of waveguides  32 ,  34 ,  36 ,  38 ,  40 , and  46  is preferably coupled to respective  12 ,  14 ,  16 ,  18 ,  20  input port and output fiber  64 . Further, waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  41  and  46  preferably extend along a common plane directing the wavelengths to output in free space and to propagate in patterns having central axes which are also preferably co-planar. 
     FIG. 7B  further discloses the fabricating of output tap  80  and planar waveguide  41  into fiber concentrator  52 , whereby tap  80  preferably couples about 10% of the optical power from output fiber port  64  of planar waveguide  46  into planar waveguide  41 , which directs the multi wavelength output beam to output in free space and to propagate in a pattern having a central axes which is preferably co-planar with outputs from waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  46  of  FIG. 1  in free space and switched by monitoring mirror array  73  (row B) after it has passed through free space optics  74 . 
   Fiber concentrator  52  may include auxiliary monitoring fiber port  23 , coupled to planar waveguide  43 , wherein fiber concentrator  52  preferably outputs its multi-wavelength beam in free space propagating in a pattern having a central axis which is preferably co-planar with outputs from waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  41  in free space optics  74 , thereby enabling an external multi-wavelength beam to be monitored by optical switching and monitoring system  11 . An external signal not found on input port  12 ,  14 ,  16 ,  18 ,  20 ,  22  may be input into auxiliary monitoring fiber port  23  and optical switching and monitoring system  11  may be utilized to monitor or read the power of each wavelength of a multi-wavelength beam on auxiliary monitoring fiber port  23  and to output such data to a user interface (User i/f)  77  port shown in  FIGS. 5A and 5B . It is contemplated herein that additional auxiliary monitoring fiber port  23  may be added in a similar fashion. 
   Potential limitations on the free space optics  74  and wavelength switching array  75  occur when configuring larger numbers of fibers than the present invention, if arranged along an optical axis of input fiber ports  12 ,  14 ,  16 ,  18 , and output fiber  64 . Absent a fiber concentrator  52 , adding additional fibers makes it difficult to switch such increased number of fiber beams with such a large spacing between such fibers because the outermost fiber beams are so far off the center optical axis capabilities of the mirrors in the preferred embodiment between input fiber ports  16  and  18 . Also, as discussed in more detail below, a significant amount of optical magnification is required between these fibers and the MEMS mirror array, and the MEMS design and function are greatly simplified as a result of concentrating the fiber spacing. 
   Referring now to  FIG. 7C , a schematically illustrated optical concentrator array  53  is shown wherein planar waveguides are included in the 1×5 WXC according to an alternate embodiment of the present invention. Single-mode optical fibers  124  having cores  126  surrounded by claddings  128  (shown in  FIG. 7A ) are butt coupled to concentrator  53 . Illustrated in the 1×5 wavelength cross-connect switch  10 . 1  shown in  FIG. 5A , one input port  12  and five output fiber ports  13 ,  15 ,  17 ,  19 ,  21  are preferably optically coupled to fiber concentrator  53  in a linear alignment and are preferably optically coupled to fiber input port  12 , and output fiber ports  13 - 21  to bring their beams closer together on output face  44  of fiber concentrator array  53  adjacent the optics, and to output the beams in parallel in a linearly spaced grid. Fiber concentrator  53  preferably has curved shaped planar waveguides  32 ,  33 ,  35 ,  37 ,  39 ,  47  and  49  within fiber concentrator  53  to preferably concentrate and reduce the spacing between fiber input fiber ports  12 ,  13 ,  15 ,  17 ,  19 ,  21  from 125 micrometers, representative of the fiber diameters, to the considerably reduced spacing of, for example, 30 or 40 micrometers and preferably no more than 50 micrometers which is more appropriate for the magnifying optics of switch  11  and an optimum size and spacing of the mirrors. Each of waveguides  33 ,  35 ,  37 ,  39 ,  47  is preferably coupled to the respective input fiber port  12 , and output fiber ports  13 ,  15 ,  17 ,  19 ,  21 . Further, waveguides  33 ,  35 ,  37 ,  39 ,  47  preferably extend along a common plane directing the multi wavelength beams to output in free space and to propagate in patterns having central axes which are also preferably co-planar. 
     FIG. 7C  further discloses the fabricating of output taps  80  and planar waveguide  49  into fiber concentrator  53  whereby taps  80  preferably couple about 10% of the optical power from output fiber ports  13 ,  15 ,  17 ,  19 ,  21  via waveguides  33 ,  35 ,  37 ,  39 ,  47  into planar waveguide  49 , which directs the multi wavelength output beam to output in free space and to propagate in a pattern having a central axes which is preferably co-planar with outputs from waveguides  32 ,  33 ,  35 ,  37 ,  39 ,  47  in free space and switched by monitoring mirror array  73  (row B) after it has passed through free space optics  74 . 
   Concentrator  53  may include auxiliary monitoring fiber port  23 , coupled to planar waveguide  43  wherein fiber concentrator  53  preferably outputs its multi-wavelength beam in free space propagating in a pattern having a central axis which is preferably co-planar with outputs from waveguides  32 ,  33 ,  35 ,  37 ,  39 ,  47  in free space optics  74 , thereby enabling an external multi-wavelength beam to be monitored by optical switching and monitoring system  10 . 1  or  11 . An external signal not found on input (N) may be input into auxiliary monitoring fiber port  23  and optical switching and monitoring system  10 . 1  or  11  may be utilized to monitor or read the power of each wavelength of a multi-wavelength beam on auxiliary monitoring fiber port  23  and to output such data to a user interface (User i/f) port shown in  FIGS. 5A and 5B . It is contemplated herein that additional auxiliary monitoring fiber port  23  may be added in a similar fashion. 
   Fiber concentrator  52  and  53  can be easily formed by a conventional ion exchange technique, such as is available from WaveSplitter Technologies of Fremont, Calif. For example, waveguides  32 ,  34 ,  36 ,  38 ,  40 ,  41 ,  45 ,  33 ,  35 ,  37 ,  39 ,  47 ,  49  are formed by doping such beam path to obtain a higher refractive index than the surrounding undoped glass, and thus, can serve as optical waveguides. However, a half-elliptical shape is optically disadvantageous. Therefore, after completion of ion exchange, a vertical electric field is applied to the substrate to draw the positive ions into the glass substrate to create nearly circular doped regions. These serve as the planar optical waveguides surrounded on all sides by the lower-index glass. Other methods are available for forming planar waveguides. 
   Fibers  124  of  FIGS. 7B and 7C  are aligned to fiber concentrator  52  and  53  at input face  127  of fiber concentrator  52  and  53 . Preferably, the fiber end faces are inclined by about 8 degrees to the waveguides in order to virtually eliminate back reflections onto fibers  124 . Other types of concentrator chips and fiber holder substrates are available. 
   Fiber concentrator  52  and  53  preferably creates a relatively narrow spread of parallel free-space beams in a linear arrangement for wavelength cross-connect switch  10  and  11 . Even when multiple fibers are connected to wavelength cross-connect switch  10  and  11 , the fiber beams are concentrated to an overall width of only about 1 millimeter. The design allows shorter focal length lenses and significantly reduces the overall size of the package. It is also more reliable and highly tolerant to environmental stress than previously described systems. Without a concentrator, the number of fibers connected to wavelength cross-connect switch  10  and  11  would be limited. 
   An example of front end optics  56  is illustrated in more detail in the cross-sectional view of  FIG. 8 . The free-space beams output by the waveguides, whether planar or fiber, of fiber concentrator  52  or  53  are divergent and form a curved field. This discussion will describe all the beams as if they are input beams, that is, output from the concentrator to the free-space optics. The beams are in fact optical fields coupled between optical elements. As a result, the very same principles apply to those of the beams that are output beams which eventually reenter fiber concentrator  52  or  53  for transmission onto the network. 
   The beam output from fiber concentrator  52  or  53  enters into the cross connect pass through field-flattening lens  220 , in order to flatten what would otherwise be a curved focal plane of the collimator lens. Field-flattening lens  220  accepts a flat focal plane for the multiple parallel beams emitted from the concentrator. In the reverse direction, field-flattening lens  220  produces a flat focal plane and parallel beams compatible with the end of the concentrator  42  to assure good coupling to waveguides in the concentrator. 
   In many optical systems, an image is formed on a curved, non-planar surface, typically by beams non-parallel to each other. In many applications such as photographic imaging systems, such minor deviations from a flat field are mostly unnoticeable and inconsequential. However, for a cross connect based on free-space optics, parallel single-mode fibers, small parallel beams, and planar mirror arrays, a curved image can degrade coupling efficiency. Performance is greatly improved if the optics produce a flat focal plane at output face  44 , and on the return trip it will be imaged onto fiber concentrator  52  or  53  waveguide ends. Hence, the ends of the input waveguides in fiber concentrator  52  or  53  are imaged onto the ends of the output waveguides in fiber concentrator  52  or  53 , and the efficiency of coupling into the single-mode waveguides strongly depends on the quality of the image. Without the field-flattening lens, it would be very difficult to build a WXC with more than a few fiber ports because the error in focus would significantly increase for fibers displaced away from the optical axis. Field-flattening lens  220  preferably is designed as an optical element with negative focal length, and is thicker at its periphery than at its optical axis in the center. The basic function of the thicker glass at the periphery is to delay the focus of the beams passing therein. The delayed focus serves to create a flat plane of focus points for all beams, rather than a curved plane of foci that would occur otherwise. A field-flattening lens may be implemented as a singlet lens, a doublet, aspheric, or other lens configuration. 
   A field-flattening lens may, in the absence of further constraints, produce an optical field in which the off-axis beams approach the flat focal plane at angles that increasingly deviate from normal away from the optical axis. Such non-perpendicular incidence degrades optical coupling to fibers arranged perpendicular to the flat focal plane. Therefore, performance can be further improved if the beams are made to approach the focal plane in parallel and in a direction normal to the flat focal plane. This effect of producing parallel beams is referred to as telecentricity, which is aided by long focal lengths. 
   After field-flattening lens  220 , the beams pass through a collimating doublet lens  222 , preferably consisting of concave lens  224  joined to convex lens  226 . Doublet lens  222  may be a standard lens such as Model LAI-003, available from Melles Griot, which offers superior collimating and off-axis performance. The effective focal length of the assembly may be about 14 mm. Collimating lens  222  is illustrated as following the field-flattening lens  220 , which is preferred, but their positions can be reversed with little change in performance. 
   As an aid to reducing the overall insertion loss of WXC in  FIGS. 1 ,  2 ,  4 ,  5  (although not a strict requirement), prism  228 , which may be a simple wedge, preferably is placed between collimating lens  222  and diffraction grating  62 . Prism  228  pre-corrects for the astigmatism introduced by diffraction grating  62 . The wedge angle of the prism, along with the type of glass from which it is made, allows elliptically shaped (or astigmatic) beams to be created. If prism  228  is composed of common optical glass, the wedge angle is typically on the order of 25 degrees to compensate for the type of diffraction grating  62  considered for the invention. The ellipticity counteracts a similar ellipticity that is an undesirable by-product of diffraction gratings. The net result of the prism and grating is a distortion-free optical beam that can be efficiently processed by the remaining optical components in the system and ultimately coupled with high efficiency back into the small core of a single-mode fiber. Field-flattening lens  220 , collimating doublet lens  222 , and prism  228  are collectively and individually referred to as front-end optics  56 . 
   Referring now to  FIG. 9A , a front face view of first channel MEMS mirror (row A) and five incident beams from the five fiber input fiber ports is illustrated, according to an illustrative embodiment of the present invention. Mirrors  72  and  73  (shown in  FIG. 9B ) of the mirror array are preferably formed within a single substrate  264  (shown in  FIG. 4 ) in a rectangular two-dimensional array, which is arranged in a switching or monitoring dimension and a wavelength dimension. A typical mirror reflective surface  270  (shown in  FIG. 4 ), is illustrated in the plan view of  FIGS. 9A ,  9 B,  9 C includes switching mirror array  72  (row A) preferably having dimensions of about 200 micrometers in the x-axis direction and about 250 micrometers in the y-axis direction. The optics are designed to irradiate each mirror of switching mirror array  72 , preferably with five elliptically shaped spots  320 . As stated earlier, for example, λ 1  mirror of switching mirror array  72  has λ 1 ( 12 )-λ 1 ( 20 ) from all five input fiber ports  12 - 20  projected onto λ 1  mirror surface via segmented prism element  68 , and by tilting λ 1  mirror of switching mirror array  72  of switch  10  or  11 , switches one selected λ 1  ( 12 - 20 ) from fiber input fiber ports  12 - 20  to fiber output port  64  and drops the remaining unselected λ 1 (s) from input fiber ports  12 - 20 , and so forth for λ 2 -λn. In addition, the five elliptically shaped spots  320  are shown in a non-overlapping manner; however, spots  320  may overlap one another on each mirror of switching mirror array  72  (as shown in  FIGS. 9C and 12 ). λ 1 ( 12 )-λ 1 ( 20 ) represented by spots  320  preferably have a diameter on an x-axis of about 100 micrometers and a diameter on a y-axis of 150 micrometers. The MEMS mirrors of switching mirror array  72  preferably spans about 10 millimeters in the x-axis direction (into the page in  FIG. 2 ). It is contemplated by this invention herein that other dimensions are feasible for switching mirror array  72 . 
   Referring now to  FIG. 9B , a front face view of a second channel MEMS mirror (row B) and two incident beams from the two monitoring input fiber ports is illustrated according to an illustrative embodiment of the present invention. A typical mirror reflective surface  270  (shown in  FIG. 4 ), is illustrated in the plan view of  FIG. 9B  includes monitoring mirror array  73  (row B) preferably having dimensions of 200 micrometers in the x-axis direction and 250 micrometers in the y-axis direction. The optics are designed to irradiate each mirror of monitoring mirror array  73 , preferably with two elliptically shaped spots  420 . As stated earlier, for example λ 1  mirror of monitoring mirror array  73  has λ 1 ( 21 ) and λ 1 ( 23 ) from two monitoring fiber ports  21  and  23  projected onto λ 1  mirror surface via segmented prism element  68 , and by tilting λ 1  mirror of monitoring mirror array  73  switch  11  switches one selected λ from monitoring fiber ports  21  or  23  to output monitoring fiber port  25  and blocks the remaining unselected λ from monitoring fiber ports  21  and  23  as well as all other λs from monitoring fiber ports  21  and  23 . In addition, the two elliptically shaped spots  420  are shown in a non-overlapping manner; however, spots  420  may overlap one another on each mirror of monitoring mirror array  73 . λ 1 ( 21 ) and λ 1 ( 23 ), represented by spots  420  preferably have a diameter on an x-axis about 100 micrometers and a diameter on a y-axis of 150 micrometers. The MEMS mirrors of monitoring mirror array  73  span about 10 millimeters in the x-axis direction (into the page in  FIG. 2 ). It is contemplated by this invention herein that other dimensions are feasible for monitoring mirror array  73 . 
   Referring now to  FIG. 9C , a front face view of third channel MEMS mirror  72  (row A) and five incident beams from the five input fiber ports is illustrated, according to a preferred embodiment of the present invention. The five incident beams are preferably shown in an overlapping manner. 
   Referring now to  FIG. 10  there is a schematic illustration of a six input port by one output fiber port wavelength cross-connect switch depicting an alternative prior art apparatus for accomplishing an N×1 wavelength selective switch. The wavelength cross-connect switch of  FIG. 10  does not include a segmented prism element to focus the light beams onto the MEMS mirror array. Rather, the wavelength cross-connect switch of  FIG. 10  uses a simple lens (or lenses) in its back end optics. Such an embodiment limits the wavelength cross-connect switch of  FIG. 10  to a single N×1 or 1×N architecture, and precludes use of multiple N×1 or 1×N switches in a single package, as shown in the other figures. Additionally, wavelength cross-connect switch of  FIG. 10  employs a fiber collimating lens array (FCLA)  502  in place of a FCA and FE optics. The FCLA  502  places a small collimating lens  504  at the output of each fiber. This combination of FCLA  502  and collimating lens  504  generally increases the cost and complexity of the system, especially as more fibers are added, since each fiber requires a dedicated collimating lens  504  with varying demanding alignment specifications. 
   Referring now to  FIG. 11  there is an illustration of a typical single-row MEMS mirror array λ 1 -λn, showing primary axis  506  and optional secondary axis  508  of rotation. Each 1×N or N×1 switch in the preferred embodiment uses one such row. A single MEMS chip may have several such rows. 
   Referring now to  FIG. 12  there is a three-dimensional schematic of a MEMS mirror of  FIGS. 9A and 9C . As illustrated in  FIG. 12  light beams from/to the input/output fibers  1 - 7  preferably are all steered onto the switching mirror array  72  by the SPE  68  (shown in  FIGS. 1 and 2 ) in an overlapping manner. The seven incident beams from the seven input fiber ports are preferably shown in an overlapping manner. It should be recognized that rotation of λn mirror about its primary axis  506  couples a selected λn by reflecting such selected λn to the output fiber  64  (shown in  FIGS. 1 and 2 ), and thus such rotation determines which λn is selected for monitoring or switching. 
   Referring now to  FIG. 13  is a schematic illustration of a six input port by one output fiber port wavelength cross-connect switch representing an N×1 switch and is an alternative depiction of the preferred embodiment of the present invention shown in  FIG. 1 . This depiction emphasizes the present invention&#39;s ability to share all free space optics (FSO)  74 , including front end optics (FE)  56 , diffraction grating  62 , back end optics (BE)  66 , segmented prism element (SPE)  68 , and the elimination of collimating lenses  504  of  FIG. 10 . 
   Referring now to  FIG. 14  there is a three-dimensional schematic of a wavelength cross-connect switch according to an embodiment of the present invention. The wavelength cross-connect switch of  FIG. 14  may represent an N×1 or 1×N embodiment of the present invention. 
   Referring now to  FIG. 15  there is a schematic illustration of a dual wavelength cross-connect switch  12  with SPE-based architecture for creating manifold or multi-packaged switches within the same package.  FIG. 15  uses the same ‘cutaway’ view as  FIGS. 1 and 13  to illustrate an advantage of the present invention&#39;s SPE-based architecture for creating manifold or multi-packaged switches within the same package, while reaping the benefits of re-use of free space optics (FSO)  74  and MEMS control circuitry  78 . By adding an additional row of mirrors  72 . 1  to the existing switching mirror array  72 , adding additional waveguides to FCA  52 , and adding additional facets to SPE  68 , a dual or second N×1 switch  10 . 3  is defined and is shown in the lower-left and upper-right parts of  FIG. 15 . The wavelength cross-connect switches  10  and  10 . 3  of  FIG. 15  operate independently of one another (that is, their light paths do not interact and such switches are capable of independent switching), while sharing the same housing and common components. It should be recognized that SPE  68  is capable of refracting light beams at arbitrary angles; thus, allowing multiple steering points for λn, on multiple mirror rows, to exist.  FIG. 15  illustrates a ‘cutaway’ view of one wavelength λn, and that each MEMS mirror shown represents a row of mirrors coming out of the page, each mirror corresponding to a different wavelength λn separated out by diffraction grating  62  and positioned by SPE  68 . 
   Although this figure depicts two independent switches  10  and  10 . 3 , the concept can easily be extended to three, four, or an arbitrary number of switches by adding more rows of MEMS mirrors  72 , more FCA  52  waveguides, and more SPE  68  facets. If desired, each N×1 or 1×N switch in the package can have a different value of ‘N’, down to N=1. Also, any arbitrary combination of N×1 or 1×N configured switches can be used by altering the external fibering. All of this is possible because of the SPE  68 &#39;s ability to refract an arbitrary number of rays at arbitrary angles, although at some point of increasing the number of switches SPE  68  may become impractically complex. 
   Use of common components by multiple internal N×1 or 1×N switches enables advantages in physical size, thermal output, electrical power consumption, ease of manufacture, and materials and labor costs, when compared to a solution involving multiple switches built and packaged independently. 
     FIG. 16  illustrates the SPE-based architecture of the present invention, in combination with FCLA-based optics with lenslets  504  of  FIG. 10 . The SPE architecture can be used with this type of optical input, as well as the FCA  52  and FE  56  shown in  FIGS. 1 ,  2 , and  15 . An advantage of this approach is that the complexity of SPE  68  is significantly reduced. 
   Referring now to  FIGS. 17A AND 17B  there is a schematic illustration of an advantage of the present invention.  FIG. 17A  is prior art that illustrates schematically a 4-input-fiber by 4-output-fiber optical switch, made up of four 1×N and four N×1 wavelength selectable switches. This is a common switch architecture used in telecom industry. In  FIG. 17B , is a schematic illustration of the same 4×4 switch  12 , but utilizing an embodiment of the present invention of  FIG. 15 , wherein two switches are co-packaged in the same device. It should be recognized that the switch shown in  FIG. 17B  utilizes the advantages listed above in the description of  FIG. 15 , compared to the “one-switch-per-device” architecture of  FIG. 17A . Although both figures show a 4×4 switch, the concept can easily be extended to any value of M×N. Likewise, although  FIG. 17B  illustrates two switches in each device, any number of switches can be combined using the concept of the present invention. Other telecom architectures that employ multiple optical switches, such as East-West dual rings, can also benefit from the embodiments of this invention. 
     FIG. 18A  is a schematic illustration of additional aspects and advantages of the present invention. In this embodiment, SPE  68  is constructed with prisms that direct beams to two rows of mirrors. The lower row switching mirror array  72  is used to switch 5×1 signals as shown in  FIG. 15 . The upper row monitoring mirror array  73  is used to switch 2×1 monitoring signals as a separate switch. The output of the 2×1 monitoring switch is directed to a photodetector  79  serving as an integrated optical power monitor (OPM). By sequentially switching each mirror  73  in the array to send selected signal to photodetector  79 , while dropping all other wavelengths, such switch obtains, in a short period of time, the optical power of all wavelengths of monitoring fiber port  21 . It is contemplated that optical switching and monitoring system is capable of monitoring two fiber ports  21  and  23  sequentially, and this concept is expandable to an arbitrary number of monitoring ports and/or wavelengths. Each wavelength of monitoring fiber port  21  is monitored one at a time, by tilting monitoring mirror array  73  to the correct angle to couple its light into output monitoring fiber port  25  from tap  81 , wherein the tapped signal from output fiber port  64  is coupled to monitoring fiber port  21 . It should be recognized that a key advantage of using one WSX as a fiber switch, and the other as a channel selector for an OPM, is that the ‘sensor’ and ‘actuator’ of the optical power feedback loop are both contained in the same module, and benefit from re-use of internal components as described above. 
   Referring now to  FIG. 18B  there is a schematic illustration of a preferred embodiment of  FIG. 3A , illustrating the design flexibility afforded by SPE  68 , wherein such SPE  68  is fabricating to have varying refraction angles (facet angles). Facet angles of deflection for the five input fiber wavelengths, two input monitor fiber wavelengths, one output monitoring fiber wavelength, and two output fiber wavelength model preferably are 16.1, 13.3, 6.1, 2.8, 0.00, −2.8, −5.7, −6.5, −7.9 degrees. The angles shown in this example correspond to the 5×1 plus 2×1 embodiment shown in  FIG. 18A . 
   Referring now to  FIG. 19A , which illustrates the combination switch plus OPM of  FIG. 18A , this time with the FCLA  502  and lenslet  504  as illustrated in  FIG. 10 . 
   Referring now to  FIG. 19B  there is illustrated a variation of  FIG. 3A , illustrating the design flexibility afforded by SPE  68 , by fabricating the SPE with arbitrary refraction angles. The angles shown in this example correspond to the 5×1 plus 2&#39;1 embodiment shown in  FIG. 19A . It should be recognized from  FIGS. 3A ,  18 B, and  19 B that SPE  68  is a versatile element that can be designed with an arbitrary set of prisms to accomplish refraction for a variety of embodiments and variations of the present invention. Both “FCA” plus “FE” type optical architectures can also be accommodated; and arbitrary combinations of N×1 or 1×N switches can also be accommodated by changing the number of facets and/or their refraction angles of SPE  68 . 
   Referring now to  FIG. 20  there is an illustration of a variation of  FIG. 19A , in which switching mirror array  72  operates a 1-input and 6-output optical switch. In order to monitor power on all fibers for control loop purposes, each output fiber port is tapped using taps  81  and fed to a 6-in and 1-out switch co-packaged with the first switch. The 6-in and 1-out switch sends its output to photodetector  79  for monitoring. Although it is not shown, the same concept could be applied to a WSX using “FCA” plus “FE” type optical architecture, and to arbitrary numbers of fibers. 
   Referring now to  FIG. 21  there is a variation of  FIG. 20 , in which power combiner  508  is used to combine all wavelengths from the six output fiber ports, which are tapped using taps  81  and couples them to monitoring fiber port  21 . In this embodiment, the optical switching and monitoring system has fewer ports than in the example of  FIG. 20 , possibly reducing the number of components simplifying the design. Although it is not shown, the same concept could be applied to a WSX using “FCA” plus “FE” type optical architecture, and to arbitrary numbers of fibers. 
   Referring now to  FIG. 22A  there is illustrated an alternative embodiment of the present invention. In  FIG. 22A , one of the independent switches is configured to accept M inputs and focus them onto one row of a first switching mirror array  72 . Each mirror in switching mirror array  72  (one mirror per wavelength) tilts to select one of the inputs for reflection onto a fixed (stationary) mirror  510 , which can be patterned directly onto SPE  68  or placed elsewhere in the system. Fixed mirror  510  reflects the signal onto one row of a second switching mirror array  72 . Each mirror in second switching mirror array  72 . 1  tilts to the angle necessary to project its beam to the selected one of N outputs. The system thus operates as an M-input by N-output switch, since any input can be coupled to any output. Although it is not shown, the same concept could be applied to a WSX using “FCA” plus “FE” type optical architecture, and to arbitrary numbers of fibers. This alternative embodiment of the present invention is different from the M×N optical switches described in U.S. Pat. No. 6,097,859 (Solgaard et al) because each wavelength in the present invention can only exit the unit on one output fiber at a time. In the referenced Solgaard patent, the M×N establishes multiple in-to-out paths on the same wavelength; however, the present invention teaches a simpler design, using fewer mirror rows, for example. 
   Also, in this and other inventions which incorporate two mirrors in the light path, an additional advantage can be gained when using Pulse Width Modulated (PWM) signals to drive the mirrors, as described in U.S. Pat. No. 6,543,286 (Garverick, et al), U.S. Pat. No. 6,705,165 (Garverick, et al), and U.S. Pat. No. 6,961,257 (Garverick, et al). By operating each of the two mirrors in the path with complementary pulse trains, any insertion loss (IL) ripple caused by mechanical vibration of the mirrors can be reduced by operating each mirror with a complementary pulse train. This causes any mechanical vibration in one mirror to occur 180 degrees out of phase with the other mirror, thus canceling IL ripple in the optical signal. 
   Referring now to  FIG. 22B  there is illustrated an alternative embodiment of the present invention which accomplishes the same M×N switching functionality of  FIG. 22A . In this embodiment, there is no stationary mirror. Instead the input-side switch is configured as an N×1, and the output side as a 1×N. The output of the first switch is coupled to the input of the second, either by fiber splicing, jumpering via fiber connectors  83 , on-chip patterning of waveguides, or the like. Although it is not shown, the same concept could be applied to a WSX using “FCA” plus “FE” type optical architecture, and to arbitrary numbers of fibers. This alternative embodiment of the present invention is different from the M×N optical switches described in U.S. Pat. No. 6,097,859 (Solgaard et al) because each wavelength in the present invention can only exit the unit on one output fiber at a time. In the referenced Solgaard patent, the M×N establishes multiple in-to-out paths on the same wavelength; however, the present invention teaches a simpler design, using fewer mirror rows, for example. 
   Referring now to  FIG. 23  there is illustrated an alternative embodiment of the present invention. In  FIG. 23  the switch is configured as two identical, independent switches. In this embodiment first and second switching mirror arrays  72  and  72 . 1  are operated such that they move synchronously. Although it is not shown, the same concept could be applied to a WSX using “FCA” plus “FE” type optical architecture, and to arbitrary numbers of fibers, numbers of co-packaged switches, and arbitrary port designations (input versus output). 
   With respect to the above description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, position, function and manner of operation, assembly and use, are intended to be encompassed by the present invention. Moreover, where the references are made to a 1×5 or 5×1 optical cross-connect switch, the concepts are also applicable to other fiber counts such as 1×N, N×1 or N×N. 
   The invention disclosed and claimed relates to the various modifications of assemblies herein disclosed and their reasonable equivalents, and not to any particular fiber count or wavelength count optical wavelength cross-connect switch. Although the invention has been described with respect to a wavelength cross connect, many of the inventive optics can be applied to white-light optical cross connects that do not include wavelength dispersive elements. Although tilting micromirrors are particularly advantageous for the invention, there are other types of MEMS mirrors than can be electrically actuated to different positions and/or orientations to affect the beam switching of the invention. 
   The foregoing description and drawings comprise illustrative embodiments of the present invention. Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.