Abstract:
A single Hybrid Optical Integrated Circuit that contains an optical switch having six groups of ports using micro-electron mechanical or six-port MEMS for short, amplifiers, laser pumps (or inputs from laser pumps) in a single package. This is done either by providing alignment features between the MEMS devices and the amplifier silicon substrate, or by building both on a common silicon substrate. The optical switching device provides the expansion capability and add/drop functionality desired via the optical expansion input and output ports and the optical inter-matrix input and output ports, respectively. Additionally, since these separate matrices of divertors are used to selectively couple the ports of the device, less divertors are required than if one large matrix of divertors were used. Further, the optical path length, a major issue in the design of such devices, is less in this structure than it would be in a single MEMS crosspoint providing a similar function. The six-port arrangement facilitates mechanical fabrication compared to eight-port arrangements. The resultant module can be used to implement the complete optical path for one plane of a wavelength-plane optical switch, with built in access to an external wavelength converter, an ability to expand the wavelength plane size by use of expansion ports and an ability to compensate for amplitude errors in the switched signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related in subject matter to co-pending U.S. application Ser. No. 09/511,065, entitled “Switch For Optical Signals”, filed on Feb. 23, 2000, assigned to the Assignee of the present invention and hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems for switching optical signals and more particularly to optical switching devices used as components in such systems. 
     BACKGROUND OF THE INVENTION 
     It is known to use mirrors in micro-machine devices to divert an optical signal from an input of the device to any one of a plurality of outputs of the device. For example, Lih Y. Lin describes such a device in the form of a Micro-Electro-Mechanical System (MEMs) in an article entitled “Free-Space Micromachined Optical-Switching Technologies and Architectures” in OFC99 Session W14-1 Proceedings published Feb. 24, 1999. Optical signals switched by Lin&#39;s device experience a power loss of about 5 dB when switched from an input to an output port of the device. While this amount of loss may be satisfactory for systems that switch optical signals through only one such MEMs switching device, the loss may be excessive for systems having multiple MEMs switching devices in a signal path. For example, a three-stage CLOS switching architecture of these MEMs switching devices in series would have a 15 dB power loss (i.e., 5 dB per stage) across the architecture from input to output. 
     In addition to power loss, another consideration in selecting an optical switching device is the ability to expand the input/output port capacity of an optical switching system built from the devices. This ability can be realized by providing each switching device with a plurality of input throughports, each input throughport aligned with a respective output port, and a plurality of output throughports, each output throughport aligned with a respective input port, as described in the related co-pending U.S. application Ser. No. 09/511,065. Expansion of the switching system can then be achieved by adding more MEMs switching devices to each switching matrix of the system and coupling output ports to input throughports and output throughports to input ports of adjacent MEM switching devices. This expansion can be realized without excessive losses since the maximum loss from port to throughport is significantly less than the maximum loss from port to port. 
     It may also be desirable to provide, in a switching matrix, the ability to add and drop optical signals from the matrix. For example, add/drop functionality is useful for performing wavelength conversion on signals switched by the system. Wavelength conversion is performed in order to alleviate blocking that occurs when two optical signals of the same wavelength need to egress a Wavelength Division Multiplexed (WDM) switching system from the same output port. 
     In view of the above, there is a need for an optical switching device that addresses the requirements of input/output port expansion and add/drop functionality described above. It would further be desirable that such a device be adequate for use in switching systems having multiple such devices in a switched signal path. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved optical switching device. 
     It is our intent to provide a switching element for use in a practical low cost wavelength plane switch. One aspect of this is a MEMS-based optical crosspoint array with six port groups (6P MEMS). Other aspects are providing optical amplification on a per optical wavelength controllable basis in the wavelength plane switch for both reducing/eliminating switch loss and for performing gain-flattening between paths through the switch in a commonly packaged module to control costs. 
     An embodiment of the present invention provides a single Hybrid Optical Integrated Circuit that contains the 6P MEMS, amplifiers, laser pumps (or inputs from laser pumps) in a single package. This is done either by providing alignment features between the MEMS devices and the amplifier silicon substrate, or by building both on a common silicon substrate. 
     According to an aspect of the present invention there is provided an optical switching device comprising: 
     a first optical switch matrix having first and second port groups associated therewith and a plurality of optical inputs and a plurality of optical outputs; 
     a second optical switch matrix having third and fourth port groups associated therewith and a plurality of outputs coupled to the plurality of optical input of the first optical matrix switch; 
     a third optical switch matrix having fifth and sixth port groups associated therewith and a plurality of optical inputs coupled to the plurality of optical outputs; 
     whereby the first optical switch matrix provides primary switching between the first and second port groups, the second optical switch matrix provides additional input port groups using the third and fourth port groups, and the third optical switch matrix provides additional output port groups using the fifth and sixth port groups. 
     According to another aspect of the present invention there is provided an optical switching device comprising: 
     a plurality of optical input ports; 
     a plurality of optical output ports; 
     a first matrix of optical divertors, each divertor being operable to divert an optical signal from one of the optical input ports to any one of a plurality of the optical output ports; 
     a plurality of optical expansion input ports, each one of the optical expansion input ports coupled to a respective optical output port; 
     a plurality of optical expansion output ports, each one of the optical expansion output ports coupled to a respective optical input port; 
     a plurality of optical inter-matrix input ports; 
     a second matrix of optical divertors, each divertor being operable to direct an optical signal from one of the optical inter-matrix input ports to any one of a plurality of the optical output ports; 
     a plurality of optical inter-matrix output ports; and 
     a third matrix of optical divertors, each divertor being operable to divert an optical signal from one of the optical input ports to any one of a plurality of the optical inter-matrix output ports. 
     The optical switching device provides the expansion capability and add/drop functionality desired via the optical expansion input and output ports and the optical inter-matrix input and output ports, respectively. Additionally, since these separate matrices of divertors are used to selectively couple the ports of the device, less divertors are required than if one large matrix of divertors were used. Further, the optical path length, a major issue in the design of such devices, is less in this structure than it would be in a single MEMS crosspoint providing a similar function. 
     Embodiments of the invention may include a plurality of optical amplifiers, each one of the optical amplifiers coupled in series with a respective optical output port or a respective optical inter-matrix output port. 
     The optical amplifiers compensate for the losses introduced into the optical switching path by the optical divertors on a path-by-path basis. This can be used to compensate for the loss of an individual switch stage or, for an externally introduced amplitude error such as that from a non-flat line system. Furthermore, the combination of amplification and switching can be used in multiple stage switches. This ability to compensate for signal power loss means the optical switching device can be used to build large switching matrices, for example 3-stage CLOS switching architectures. 
     Embodiments of the invention may be implemented as monolithic structures on a silicon wafer substrate. In this case optical amplifiers incorporated into an embodiment may take the form of an array of erbium doped Silica (Silicon Dioxide) or Phosphate-glass waveguides fabricated on a silicon or similar substrate, in combination with a an array of pump lasers, thereby enabling the gain of individual amplifiers to be set separately. First V-grooves in the silicon substrate provide alignment for rod lenses at inputs and outputs of the matrices of divertors and are sized to accept rod-lenses (which are large relative to the 6-10 μm SiO 2  waveguides) and to align the centers of these rod-lenses with the centers of the waveguides. The switching matrices are either fabricated directly on the silicon substrate or are fabricated separately and attached to the substrate after alignment with alignment feature, such as wells, etched in the substrate. Second V-grooves in the silicon substrate provide alignment for optical fibers that couple optical signals from the rod lenses or directly to the matrices of divertors. For example, these second V-grooves are dimensioned to align the core of a 125 μ fiber to the to the 6-10 μm SiO 2  waveguides. The facet angles in any V-groove are set by the crystal structure. Implementing the optical switching devices as monolithic structures on silicon wafers allows alignment features to be provided such that components of the devices can be positioned and interconnected within required tolerances. 
     Other aspects of the invention include combinations and subcombinations of the features described above other than the combinations described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further understood from the following detailed description of embodiments of the invention and accompanying drawings, in which: 
     FIG. 1 is a functional block diagram of an optical switching system having a wavelength-plane optical switching sub-system; 
     FIG. 2 a  is a functional block diagram showing the wavelength plane switches of the optical switching sub-system of FIG. 1 in greater detail, each wavelength plane switch includes a six-port optical switching device which is in accordance of an embodiment of the present invention; 
     FIG. 2 b  is a functional block diagram of another implementation of one of the wavelength plane switches of FIG. 2 a , the wavelength plane switch including an eight-port optical switching device in accordance with another embodiment of the present invention; 
     FIG. 3 a  is a functional block diagram of yet another implementation of one of the wavelength plane switches of FIG. 2 a , the wavelength plane switch including four six-port optical switching devices of FIG. 2 a  arranged in an expanded configuration; 
     FIG. 3 b  is a functional block diagram of still another implementation of one of the wavelength plane switches of FIG. 2 a , the wavelength plane switch including four eight-port optical switching devices of FIG. 2 b  arranged in an expanded configuration; 
     FIG. 4 is a plan view of an implementation of the six-port optical switching matrix of FIG. 2 a;    
     FIG. 5 shows the optical amplifier waveguide of FIG. 4 in greater detail; 
     FIG. 6 a  is a cross-sectional view of the wavelength-plane switch of FIG. 4 taken along the line AA; 
     FIG. 6 b  shows part of the cross-sectional view of FIG. 6 a  in greater detail; 
     FIG. 7 a  is a cross-sectional view of the wavelength-plane switch of FIG. 4 taken along the line BB; and 
     FIG. 7 b  shows part of the cross-sectional view of FIG. 7 a  in greater detail. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a functional block diagram illustrating a WDM optical switching system  10 . The system  10  switches Dense WDM (DWDM) optical signals, each signal consisting of M optical signal channels. Each of the M channels carries an optical signal modulated on an optical carrier of a wavelength unique to that channel. Incoming DWDM optical signals are split, or demultiplexed, into their component optical signal channels, which are then switched by the system  10 , and then combined, or multiplexed, into outgoing DWDM optical signals. The system  10  has N input ports and N output ports to receive and transmit the incoming and outgoing DWDM optical signals, respectively. 
     The system  10  includes a wavelength-plane optical switching sub-system  12 , a plurality N of 1 to M demultiplexers  16 , a plurality N of M to 1 multiplexers  18 , a wavelength converting switch  14  and a controller  20 . After input preamplifier  21 , a plurality N of fibers  22  are coupled to the plurality N of demultiplexers  16  at the ingress of the system  10 , each fiber  22  coupled to a respective demultiplexer  16 . Each of the demultiplexers  16  has one input and M outputs. For the purpose of example M=40 and N=24. A plurality N of array of optical interconnections  24 , each of width M couple the N×M outputs of the demultiplexers  16  to M×N port inputs (Pi) of the optical switching sub-system  12 . Similarly, a plurality N of array of optical interconnections  26 , each of width M, couple N×M port outputs (Po) of the optical switching sub-system  12  to N×M inputs of the multiplexers  18 . Each of the N multiplexers  18  has M inputs and one output. A plurality N of fibers  28  are coupled to the plurality of multiplexers  18  at the egress of the system  10 , each fiber  28  coupled to a respective multiplexer  18  and an output preamplifier  29 . The switching sub-system  12  is a wavelength plane structure in that it includes a distinct switching matrix, or matrices, for switching each one of the M unique wavelengths. 
     The optical switching sub-system  12  includes a plurality K of matrix output ports (Mo) and a plurality K of matrix input ports (Mi) for coupling optical signal channels to the wavelength converting switch  14 . A plurality M of optical interconnection arrays  30 , each of width K, couple K×M matrix output ports (Mo) to the wavelength converting switch  14  at its ingress. Similarly, the egress of the wavelength converting switch  14  is coupled to K×M matrix input ports (Mi) via a plurality M of optical buses  32 , each of width K. The wavelength converting switch  14  has a plurality R of inputs for adding optical signal channels  34  and a plurality R of outputs for dropping optical signal channels  36 . 
     The controller  20  has a bi-directional port  42  for receiving external input, for example from other switching systems or from a network controller, and changing the operation of the system  10  in response to the input as required. The controller  20  also communicates node status to the higher level network controller (not shown in FIG.  1 . The controller  20  is coupled to the optical switching sub-system  12  via an optical or electrical communication link  40  and to the wavelength converting switch  14  via a similar link  38  for effecting this change in operation. 
     In operation, the system  10  receives DWDM optical signals from the fibers  22  after pre-amplification. The amplification provided in a conventional optical amplifier is on overall around 24 dB. This is achieved in two stages, a pre-amplifier and a post-amplifier, which might each have a gain of up to 19 dB. This allows combinations of filters, add-drop components or bulk (WDM) fixed impairment compensators (e.g. chromatic dispersion compensator) to be added up to an optical loss value of about 14 dB, between the output of the pre-amplifier and the input of the post-amplifier while maintaining an overall gain of 24 dB. 
     Hence, if standard pre-amplifiers are used on the switch input and standard post-amplifiers are used on the switch output, then the switch node would be limited to a maximum loss of 14 dB. However, the likely range of losses across a switch node such as that shown in FIG. 1 is: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Minimum Loss 
                 Maximum Loss 
               
               
                   
                   
               
             
             
               
                   
                 WDD 
                   6 dB 
                 8 dB 
               
               
                   
                 Optical interconnect 
                 0.2 dB 
                 1 dB 
               
               
                   
                 Switch 
                   4 dB 
                 7 dB 
               
               
                   
                 Optical interconnect 
                 0.2 dB 
                 1 dB 
               
               
                   
                 WDM 
                   6 dB 
                 8 dB 
               
               
                   
                 Total 
                 16.4 dB  
                 25 dB  
               
               
                   
                   
               
             
          
         
       
     
     The variability of the switched path losses and reducing the overall loss. This is provided by the individually settable EDWA&#39;s, which, to provide an overall node loss of 14 dB, would have to exhibit a gain in the range 2.4-11 dB. Given that up to +/−2 dB of amplitude error/ripple may be present on the inputs, then this range would have to be increased to 0.4-13 dB. This is within the range of a 3-5 cm, active optical path EDWA. (Note that while we are showing the EDWA&#39;s as straight tracks, the high difference between the refractive index of the glass/SiO2 and the surrounding medium (air) permits tightly coiled or folded structures, too. 
     Each DWDM optical signal is demultiplexed into its M component optical channel signals by a respective demultiplexer  16 . The optical channel signals are coupled to the optical switching sub-system  12  via the N arrays of optical interconnections  24 . The optical switching sub-system  12  receives the optical channel signals at its port inputs (Pi) and switches them individually to its port outputs (Po) or matrix outputs (Mo) according to the state, or configuration, of the optical switching sub-system  12  as determined by the controller  20 . The wavelength converting switch  14  has the capability of converting the wavelength of an optical channel signal to another one of the unique wavelengths of the system  10  as determined by the controller  20 . Optical channel signals can also be added  34  or dropped  36  by the wavelength converting switch  14  under the direction of the controller  20 . Added or converted optical channel signals on the wavelength converting switch  14  are received at the matrix inputs (Mi) of the optical switching sub-system  12  via the M K-wide buses  32 . Optical channel signals received at the matrix inputs (Mi) are individually switched to the port outputs (Po) where they are transmitted to the N multiplexers  18  via the N buses  26 . Each of the N multiplexers receives optical channel signals at its M inputs and multiplexes these signals into a DWDM optical signal which is output on one of the fibers  28 . 
     The optical switching sub-system  12  also includes a plurality (N×M) expansion inputs (Ei) and a plurality (N×M) expansion outputs (Eo) for the purpose of building out, or expanding, the optical switching sub-system  12 . This expansion will be described later in more detail. 
     FIG. 2 a  shows the optical switching sub-system  12  being comprised of a plurality M of identical wavelength-plane switch  12   a  to  12   m . Each wavelength-plane switch  12   a  to  12   m  switches optical channel signals of a unique wavelength. That is, each of the wavelength-plane switches  12   a  to  12   m  provides switching for one of the M unique wavelengths used by the system  10 . Each wavelength-plane switch  12   a  to  12   m  has a plurality N of port inputs Pi′, which, taken over the M wavelength-plane switches  12   a - 12   m , sum to provide the N×M port inputs Pi of the optical switching sub-system  12  in a wavelength-plane architectured switch. Similarly, each wavelength-plane switch  12   a  to  12   m  has N port outputs (Po′), N expansion inputs (Ei′), N expansion outputs (Eo′), K matrix inputs (Mi′), and K matrix outputs (Mo′), which, taken over the M wavelength-plane switches  12   a  to  12   m , result in their corresponding inputs/outputs of the optical switching sub-system  12 . 
     The wavelength-plane switch  12   a  will now be described in further detail. The wavelength-plane switch  12   a  includes a six-port optical switching device  49  in the form of a MEMs device or devices. The six-port optical switching device  49  includes an N×N four-port matrix  50  of optical divertors, which could be a MEMs optical switching device for example. The matrix  50  has N inputs (I) on one side and N outputs (O) on an adjacent side. The matrix  50  is capable of switching an optical signal arriving at any one of the inputs (I) to any one of a plurality of the outputs (O) by activation of one or more of the optical divertors of the matrix  50 . In the case of a MEMs optical switching device, as shown in expanded detail of matrix  50 , each optical divertor is a planar mirror, which is part of a grid arrangement of other like divertors, such that, in an activated state, the mirror intercepts an optical signal from its corresponding row input and diverts the signal to its corresponding column output. This is usually done by arranging the mirrors so that the plane of each mirror in an activated state is normal to the substrate as well as at a 45° angle with respect to its row and column in the grid. In the inactive state the mirror lies parallel to the substrate. The matrix  50  has N through inputs (Ti) on the side opposite the outputs (O). Each through input (Ti) corresponds to a respective output (O) such that an optical signal input at the through input (Ti) can pass to its respective output (O) when all of the optical divertors in alignment between the through input (Ti) and the output are in a non-activated state. Similarly, the matrix  50  has N through outputs (To), each aligned with a corresponding input (I), such that an optical signal entering at the input passes to the through outputs (To) if none of the optical divertors in its path are in an activated state. For illustrative purposes only, matrix  50  is shown with two signals Sc 1  and Sc 2  entering its inputs (I). Signal Sc 1  is deflected through 90 degrees by an active MEMS mirror to exit from one of its outputs (O), while signal Sc 2  passes straight through to exit one of its through outputs (To). The N inputs (I) of the matrix  50  are coupled to the N port inputs (Pi′) of the wavelength-plane switch  12   a , and the N outputs (O) of the matrix  50  are coupled to the N port outputs (Po′) of the wavelength-plane switch  12   a . Optionally, an optical amplifier  55  is connected in series between each output (O) of the matrix  50  and each port output (Po′). These optical amplifiers  55  compensate, partially or fully, for power lost in switching an optical signal from an input (I) to an output (O). This power loss is typically 5 dB with current MEMs optical switching devices. However, the gain of the optical amplifiers  55  is not limited to that value (5 dB), but could be another value to compensate for other losses or provide additional gain as required. 
     This overall power loss is typically around 20 dB with switches based on the structure of FIG.  1  and using current MEMS optical switching devices, which exhibit about 5 db of switching loss and consists of collimating beam errors, diffraction losses, mirror reflectivity losses, mirror non-flatness losses, mirror pointing accuracy losses. Of these only two, the collimating beam error and diffraction losses affect the through paths to/from the expansion ports, which as a result show much lower losses (typically of 5 db). 
     The six-port optical switching device  49  also includes a K×N matrix  54  of optical divertors, which typically would be of the same technology as the N×N matrix  50 . The K×N matrix  54  has K inputs (I) coupled to the K matrix inputs Mi′, N through inputs (Ti) coupled to the N expansion inputs Ei′, and N outputs (O) coupled to the N through inputs (Ti) of the N×N matrix  50 . 
     The six-port optical switching device  49  further includes an N×K matrix  52  of optical divertors, which typically would be of the same technology as the N×N matrix  50  and K×N matrix  54 . The N×K matrix  52  has N inputs (I) coupled to the N through outputs To of the N×N matrix  50 , N through outputs (To) coupled to the N expansion outputs Eo′, and K outputs (O) coupled to the K matrix outputs Mo′. Optionally, an optical amplifier  57  may be series connected between each of the K outputs of the N×K matrix  52  and the K matrix outputs Mo′. The presence of an optical amplifier with optional variable gain is an integral part of building a photonic switch that is adaptable to gain requirements of the optical network in which the photonic switch is employed. 
     In operation, matrix  50  routes inputs which contain wavelengths being routed directly to outputs, or takes no action, the default “no action” resulting in the projected input light beam being passed on to the N×K matrix  52 . This either takes action to switch/divert the light into one of its outputs, in which case the input is coupled to a drop port/lambda converter input, or matrix  52  takes no action, in which case the light passes straight through to the “expansion out” port. More specifically, the wavelength-plane switch  12   a  receives optical channel signals at its port inputs Pi′ and the N×N matrix  50  routes them individually to either the N×K matrix  52  or the port outputs Po′. Signals routed to the N×K matrix  52  are then individually routed to either a respective one of the N expansion outputs Eo′ or to any one of a plurality of the K matrix outputs Mo′ by the N×K matrix  52 . The N×K matrix  52  provides the capability to select any K of N signals passed through the N×N matrix  50  and direct these signals to any of K inputs of the wavelength-converting switch  14 . This capability eases design restrictions placed on the wavelength-converting switch  14 . For example, tunable transponders used to convert a signal channel from one wavelength to another can be arranged in K groups, with each transponder being tunable over a range of M/K channel wavelengths. In this way, tunable transponders with ranges smaller than M channel wavelengths can be used to provide a tunable range of M channel wavelengths, thereby reducing the cost of the system  10 . 
     The N×K matrix  52  permits optical traffic concentration of traffic exiting to the wavelength converting switch  14  converter function or being dropped, while the K×N matrix  54  provides the inverse function for added traffic. This permits the amount of add/drop and/or wavelength conversion equipment to be a provisioned entity, dependent upon the needs of the switch  10 . The matrices  52 ,  54  allow from 1 to K add/drop or wavelength converters to be provided per wavelength plane. 
     This provides concentration of N:1 step 1 through K which is a relatively small concentration group. This can be expanded by adding an optical switch or array of optical switches across P planes of the wavelength converting switch  14  with each of these new optical switches accessing one or more ports of each of the matrices  52  or  54 . 
     Signals received at the matrix inputs Mi′ from the wavelength converting switch  14 , or from the expansion inputs Ei′, are coupled to the K×N matrix  54 . Signals received at the expansion inputs Ei′ are individually passed to a respective one of the outputs (O) of the K×N matrix  54  and on to the N×N matrix  50  where they individually pass from a through input (Ti) to a corresponding output (O) and are then coupled to the port outputs Po′. Signals received at the matrix inputs Mi′ can be switched to any K of N outputs (O) of the K×N matrix  54 . As before with the N×K switch  52  this switching capability of the K×N matrix  54  provides for flexibility in the design and provisioning levels of the wavelength converting switch  14 . Signals so switched by the K×N matrix  54  are then coupled to the through inputs (Ti) of the N×N matrix  50  where they are individually passed to respective outputs (O) of the N×N matrix  50  and are then coupled to the port outputs Po′. 
     FIG. 2 b  shows a second embodiment of the wavelength-plane switches  12   a  to  12   m . Each of the wavelength-plane switches includes an eight-port optical switching device  59 . This device  59  makes use of the through output port To of the K×N matrix  54  and the through input port of the N×K matrix  52  to add new expansion output port Eo 2  and expansion input port Ei 2 , respectively. The use of these devices in an expanded wavelength-plane switch will be described later. 
     The seventh Ei 2  and eighth Eo 2  port opposite to Mi′ and Mo′, respectively, allow for cascading of the add-drop I/O without use of the splitter, combiner. This has to be differentiated from the use of a larger 4-port MEMS device to do this job. This can be done since the worst case optical path length (measured in number of mirrors passed) would be (2N+K+1) in our proposal vs. (2N+2K−1) in a generic  4  port large MEMS device of (N+K) ports (though we would still only use the same paths in either case with the same optical loss, but the generic  4  port device has to be designed to work if the 2N+2K−1 path is activated). In addition, it is believed that large monolithic 2D-MEMS structures become very complex to design, fabricate and yield at sizes above 32×32, corresponding to 1024 mirrors and mirrors drivers. This leads to the concept of tiling parts. 
     FIG. 3 a  shows an expanded wavelength-plane switch  12   a ′. The expanded wavelength-plane switch  12   a ′ is comprised of four identical six-port optical switching devices  49   a-d , each being identical to the six-port optical switching device  49  described previously with reference to FIG. 2 a . The expanded wavelength-plane switch  12   a ′ has 2N port inputs Pi″ and 2N port outputs Po″. An optical signal received at any one of the 2N port inputs Pi″ can be switched to any one of the 2N port outputs Po″. The expanded wavelength-plane switch  12   a ′ also has 2N expansion outputs (Eo″), each corresponding to a respective port input Pi″, and has 2N expansion inputs (Ei″), each corresponding to a respective port output Po″. An optical signal received at any one of the port inputs Pi″ can be passed to an expansion output Eo″ corresponding to the port input via intervening optical switching devices  49   a-d  when the intervening devices  49   a-d  have no activated diverters in the path of the signal. Similarly, an optical signal received at any one of the expansion in puts Ei″ can be passed to a port output Po″ corresponding to the expansion input via intervening optical switching devices  49   a-d  when the intervening devices  49   a-d  have no activated divertors in the path of the signal. The expanded wavelength-plane switch  12   a ′ further includes 2K matrix inputs Mi″ and 2K matrix outputs Mo″. An optical signal received at any one of the 2K matrix inputs Mi″ can be switched to any one of the port outputs Po″ via the optical switching devices  49   a-d  and other components used to interconnect the devices  49   a-d  as will be described later. Similarly, an optical signal received at any one of the 2N port inputs Pi″ can be switched to any one of the 2K matrix outputs Mo″. 
     The interconnection of the devices  49   a-d  will now be described. The 2N port inputs Pi″ are coupled to the devices  49   a  and  49   b  by an optical bus  60  of width 2N of which one half  60   a  of width N is coupled to the port inputs Pi′ of the device  49   a  and the other half  60   b  of width N is coupled to the port inputs Pi′ of the device  49   b . The 2N port outputs Po″ are coupled to the devices  49   b  and  49   d  by an optical bus  62  of width 2N of which one half  62   a  of width N is coupled to the port outputs Po′ of the device  49   b  and the other half  62   b  also of width N is coupled to the port outputs Po′ of the device  49   d . The N port outputs Po′ of the device  49   a  are coupled to the N expansion inputs Ei′ of the device  49   b  by an optical bus  64  of width N. Similarly the N port outputs Po′ of the device  49   c  are coupled to the N expansion inputs Ei′ of the device  49   d  by an optical bus  68  of width N. The N expansion outputs Eo′ of the device  49   a  are coupled to the N port inputs Pi′ of the matrix  49   c  by an optical bus  66  of width N. Similarly, the N expansion outputs Eo′ of the device  49   b  are coupled to the N port inputs Pi′ of the device  49   d  by an optical bus  70  of width N. 
     The 2N expansion inputs Ei″ are coupled to the devices  49   a  and  49   c  via an optical bus  72  of width 2N of which one half  72   a  is coupled to the N expansion inputs Ei′ of the device  49   a  and the other half  72   b  also of width N is coupled to the N expansion inputs Ei′ of the device  49   c . The 2N expansion outputs Eo″ are coupled to the devices  49   c  and  49   d  by an optical bus  74  of width 2N of which one half  74   a , of width N, is coupled to the N expansion outputs Eo′ of the device  49   c  and the other half  74   b , also of width N, is coupled to the N expansion outputs Eo′ of the device  49   d.    
     The expanded wavelength-plane switch  12   a ′ further includes two groups of K 1:2 optical splitters  78 ,  84  and one group of K 2:1 optical combiners  92  for the purpose of coupling the 2K matrix inputs Mi″ and 2K matrix outputs Mo″ to the devices  49   a  to  49   d.    
     The 2K matrix inputs Mi″ are coupled to the groups of splitters  78 ,  84  by an optical bus  76  of width 2K, of which one half  76   a , of width K, is coupled to the inputs of the first group of splitters  78 , and the other half  76   b , also of width K, is coupled to the inputs of the second group of splitters  84 . Two optical buses  80  and  82 , each of width K, couple the outputs of the first group of splitters  78  to the matrix inputs Mi′ of the devices  49   c  and  49   a , respectively. Similarly, two optical buses  86  and  88 , each of width K, couple the outputs of the second group of splitters  84  to the matrix inputs Mi′ of the devices  49   d  and  49   b  respectively. 
     The 2K matrix outputs Mo″ are coupled to the group of combiners  92  by an optical bus  90  of width 2K. Optical interconnections  90   a  and  90   b , both of width 2K, are input into the combiners  92 . The interconnection  90   a  is split into two optical interconnections  94 , 96  and coupled to the matrix outputs Mo′ of the devices  49   b  and  49   d , respectively. The other half  90   b  of the interconnect of the first group of combiners  92 , and the other half  90   b , also of width K, is coupled to the outputs of the second group of combiners  98 . Two optical buses  94  and  96 , each of width K, couple the inputs of the first group of combiners  92  to the matrix outputs Mo′ of the matrices  12   a   2  and  49   d , respectively. Similarly, two optical buses  100  and  102 , each of width K, couple the inputs of the second group of combiners  98  to the matrix outputs Mo′ of the matrices  49   a  and  49   c , respectively. 
     FIG. 3 b  shows another embodiment of the expanded wavelength-plane switch  12   a ″. This embodiment uses the 8-port optical switching device  59  of FIG. 2 b  to eliminate the need for splitters  78 , 84  and combiners  92 , 98 . The interconnection of the devices  59   a  to  59   d  is similar to that of the expanded wavelength-plane switch  12 ′ of FIG. 3 a  except with respect to the expansion outputs Eo 2 ′ of device  59   a  which are connected to the matrix inputs Mi′ of device  59   c  and the expansion inputs Ei 2 ′ of the device  59   b  and  59   d  which are connected to the matrix outputs Mo′ of the devices  59   a  and  59   c , respectively. Operation of this expanded wavelength-plane switch  12   a ″ is the same as that of expanded matrix  12   a ′ of FIG. 3 a  from the perspective of its inputs Pi″, Mi″, Ei″ and outputs Po″, Mo″, and Eo″. 
     FIG. 4 shows a Hybrid Optical Integrated Circuit (HOIC) for implementing the entirety of an individual wavelength plane in an expandable wavelength plane switch with integral add/drop; access to wavelength conversion capabilities and including amplification, both for offsetting switch loss and/or switch node loss, but also for providing compensation of other per-wavelength amplitude errors, e.g. from external concatenated line system elements. The HOIC consists of several components, those being an N×N 2D-MEMS device  50  for port-to-port switching from port V to port R, an N×K 2D-MEMS device  52  for extracting drop traffic and/or traffic to be wavelength converted from the input at part V into the drop port at port S. and a K×N 2D-MEMS device  54  for inserting added traffic and/or wavelength converted traffic at port W into any of the N outputs from the module at port R. 
     The HOIC also includes an expansion output port at Port T to allow further per-wavelength-plane expansion, and an expansion input port at Port U to allow further per-wavelength plane expansion. 
     FIG. 4 shows the optical wavelength-plane switch  12   a  of FIG. 2 in a plan view. The wavelength-plane switch  12   a  is implemented as a monolithic optical switching device on a silicon substrate  110 , however a multi-chip hybrid structure is also possible. The N×N matrix  50  of divertors, the N×K matrix  52  of divertors, and the K×N matrix  54  of divertors are fabricated on the substrate  110 , or alternatively, are fabricated separately and then affixed to the substrate  110 . The K×N matrix  54  is positioned to be adjacent the N×N matrix  50  with the outputs (O) of K×N matrix  54  aligned with the through inputs (Ti) of the N×N matrix  50  in a manner that provides optical coupling between the outputs (O) and the through inputs (Ti). Since the rod lenses at the Ei′ input of  54  (port  118 ) project a nominally parallel beam across the top of the MEMS mirror array, as long as that beam is parallel, it can be projected across the top of 1, 2 or even 3 properly aligned MEMS switch arrays with a level of increased loss due to beam divergence and diffraction, dependent upon the level of error in the forming of that parallel beam. Hence, the optical coupling between the (O) ports of  54  and the (Ti) ports of  50  is a matter of aligning the beams exiting  54  to pass over  50  at the right point. Likewise for the other path direction. The N×K matrix  52  is positioned to be adjacent the N×N matrix  50  such that the inputs (I) of the N×K matrix  52  are aligned and optically coupled to the through outputs (To) of the N×N matrix  50 . 
     Optical signals are directed to/from the matrices  50 ,  52 ,  54  via optical fibers positioned on the substrate  110  and aligned with corresponding inputs or outputs of the matrices  50 ,  52 ,  54  by alignment features provided on the substrate  110 . These alignment features will be described in more detail later. N optical fibers  112 ,  114 ,  116 ,  118  for each of the N port inputs Pi′, N port outputs Po′, N expansion inputs Ei′, and N expansion outputs Eo′, respectively, are used to direct optical signals on/off the wavelength-plane switch  12   a  as shown in FIG. 2 a . Likewise, K optical fibers  120 ,  122  for each of the K matrix inputs Mi′ and K matrix outputs Mo′, respectively, perform the same function. 
     The wavelength-plane switch  12   a  further includes a plurality N of rod lenses  124 , each of which is aligned with an output (O) of the N×N matrix  50 , a plurality K of rod lenses  128 , each of which is aligned with an output (O) of the N×K matrix  52 , and a plurality N of rod lenses  132 , each of which is aligned with a through output (To) of the N×K matrix  52 . Similarly, the matrix  12   a  includes a plurality N of rod lenses  113 , each of which is aligned with an input (I) of the N×N matrix  50 , and a plurality K of rod lenses  121 , each of which is aligned with an input (I) of the K×N matrix  54 . Each lens of the plurality of N of rod lenses  124  is aligned with a respective waveguide of a plurality N of Erbium doped silica (SiO 2 ) or phosphate-glass waveguides  126 , each of which is aligned with a respective fiber of the plurality of N fibers  114 . Similarly, each lens of the plurality K of rod lenses  128  is aligned with a respective waveguide of a plurality K of Erbium doped silica (SiO 2 ) or phosphate-glass waveguides  130 ; each of which is aligned with a respective fiber of the plurality of K fibers  122 . The rod lenses  124 ,  128  focus optical signals received from the matrices  50 ,  52  into the waveguides  126 ,  130 . The waveguides  126 ,  130  are part of the optical amplifiers  55 ,  57  previously mentioned. Each lens of the rod lenses  132  is aligned with a respective fiber of the plurality N of optical fibers  116  for the purpose of focusing optical signals from the matrix  52  into the optical fibers  116 . 
     Erbium doped waveguide amplifiers, fabricated by constructing Erbium doped SiO 2  waveguides of about  10  micron by  10  microns on a silicon substrate are also included. These are energized from an array of lasers coupled into the waveguides by a coarse WDM coupler, which is designed to couple between itself and the amplifier waveguide at wavelengths around 980 nm; but not at the wavelengths of interest to the traffic (typically 1500-1550 nm), since such coupling would extract traffic signal optical power from the amplifier, reducing its gain. The amplifier tracks are therefore 10 micron×10 micron SiO 2  on Silicon tracks placed with their centers on a pitch that matches that of the MEME mirrors (typically 80 microns) although other spacings are possible by introducing curved sections into the tracks. Metal pillars 10 microns thick can be built between the amplifier tracks to provide connectivity to, and to support, the pump lasers which could then be in the form of a high power VCSEL array. In this case the laser facets would be located such that they emitted light down towards the end of the coarse WDM coupler and the end of that coupler would incorporate a 45 degree mirror to couple the light into the coupler. Alternatively conventional lasers could be located in etched pits and butt-coupled to the coupler. 
     The amplifiers, lasers, couplers and switches could be fabricated on a single substrate  110  or they could be fabricated as individual piece parts that are then precision-aligned onto the substrate  110 . This alignment can be facilitated by making each of the piece parts on a Silicon substrate since several known methods exist to fabricate extremely precise alignment structures between silicon substrates. These methods include the use of MEMS latches (as detailed by Lin), multiple V-grooves with locking rods. Several partitions are possible, along lines Z, Y and even X. Line Z would separate the amplifiers from the MEMS structures and, assuming that the matrices  52 ,  50  are accurately located with respect to their portion of the substrate  110 , then the rod lenses  124  . . .  128  (in conjunction with return equivalent features) could provide the necessary alignment. If not sufficient, additional V-groove features can be used to provide further alignment. Line Y would allow the K×N matrix  54  to be fabricated and if necessary pigtailed separately to the unit comprising matrices  50 ,  52 . This pigtailing is advantageous since it would allow for a product yielding stage at a level of two attached pigtails per module instead of four. However, since the pigtails would not yet be assembled into the package walls, care would be required in handling these units. To illustrate this advantage let us assume that we have a pigtailing yield of 50% per pigtail. Then a four pigtail unit would have a yield of 12.5% whereas a two pigtail unit would have a yield of 25%. As long as the two units can be butt coupled with a yield well above 50%, this approach is favourable. This can be taken one stage further by exploiting line X. 
     The matrices  50 ,  52  and  54  in the form of MEMS devices would be mounted onto substrate  110  or its component parts. In a conventional four port MEMS making electrical connections to the MEMS is very difficult since all four faces are occluded by optical connections. In the case of this wavelength-plane switch  12   a  however each MEMS device has two faces that are not directly coupled into entry or exit optical paths, although there are still free space light beams fairly close to the MEMS surface. Hence, by not fully butt-coupling the MEMS, but rather mounting them precisely, close to each other which can be done via alignment features on the MEMS die or by alignment features on the substrate  110 , then relatively narrow trenches can be formed at P. This would permit an array of wirebonds to link the MEMS chip to the substrate  110  on two sides of the MEMS. This linking is obviously further facilitated by exploiting the partitioning of  110  along X, Y, since this would provide better access for the wire bonding process, or other equivalent process to link the MEMS to the substrate  110 . The tracks to the wire bond pads could be run as features on the substrate underneath the MEMS matrices  50 ,  52 ,  54  as long as appropriate features are designed into substrate  110  to ensure precise vertical alignment of the MEMS at a pre-determined height above the substrate. This is not a normal way of mounting a silicon device, due to its relatively poor power dissipation capabilities, but in this case it is acceptable since there is very little power to dissipate. 
     Having access to two sides of the MEMS device facilitates the use of a scanned row and column approach for energizing the mirrors, resulting in a need for 2N connections (or N+K connections) instead of N 2  (or N×K) connections. However, to do this a simple electronic latch would have to be integrated into the MEMS substrate at each row/column intersection. This can be done with a pair of complementary dual gate FETS and a small integrated charge storage device, for example, a capacitor, though other known prior art scan/latch schemes can also be used. The MEMS may be energized from a matrix mirror driver, if the aforementioned scan latch functions are implemented in the MEMS or alternatively the matrix mirror drive could be replaced with a dedicated mirror driver array, which would probably be implemented in one or more integrated circuits per MEMS because of the high pin-out. This requires 256 pins for the 16×16 version of MEMS, rising to 1024 pins for the 32×32 version. In contrast, the scanning technique would permit a 16×16 using 32 (or 33) drive lines, and 32×32 using 64 (or 65) drive lines for an improvement of 8:1 and 16: 1, respectively. The mirror driving ASICs can themselves be driven from a module control ASIC that also controls the optical amplifier gains in dependence upon external commands. This is done by changing the value of the d.c. current through the pump laser which varies the pump power, and in turn varies the amplifier gain. 
     Several alternative implementations exist, for instance locating the control ASIC and pump lasers external to the package. In the event of a matrixed mirror driver the entire MEMS array could be controlled by 2 (N+K) pins which for a practical N=32, K=16 would give a relatively achievable pinout of 96 pins. 
     The HOIC further includes ribbon cable terminations with physical pitch expanders to couple to the MEMS pitch, amplifier pitch and with hermetic package wall seals at R, S, T, V, V, W. The ribbon pitch is 250 microns whereas the MEMS, amplifiers pitch may be between 700-1000 microns, for example purposes 800 microns will be assumed. Hence, a 16 fiber ribbon, at 15×250 microns outer center to outer center pitch, or about 3.75 mm, would be expanded up to 15×800 micron=12 mm pitch. The fiber ribbon expanders may be joined via a sub-mount to the rod lenses prior to final assembly, especially if a rod-lens-array is used. 
     FIG. 5 shows an optical amplifier  55   a  of the plurality of optical amplifiers  55  in greater detail. The optical amplifier  55   a  is an Erbium-doped waveguide amplifier (EDWA) including a pump laser  200   a , an erbium doped amplifying waveguide track  126   a  and a WDM coupler element  202   a . The amplifying waveguide track  126   a  is coupled to couple a free space optical input, via a rod lens  124   a  and is coupled to a single mode fiber  114   a.    
     In operation, the pump laser  200   a  provides the optical energy to operate the amplifier and is coupled into the amplifying waveguide using the WDM coupler element  202   a . The amplifying waveguide and the coupling WDM can be implemented as an array of multiple instantiations of amplifiers on a single silicon substrate, with the waveguides  202   a ,  126   a  being implemented in a suitable optical material such as SiO 2  or phosphate-glass. Each optical amplifier of the plurality of optical amplifiers  55  and  57  is identical to the optical amplifier  55   a , however, this is not necessary. For example, different gains in the plurality of optical amplifiers  55  and  57  may be desired, in which case the amount of Erbium doping, laser pump power, and coupling distance between the waveguide and pump laser signal could be varied. 
     As the operation of Erbium-doped waveguide amplifiers is well understood in the art it will not be explained in detail here. Briefly, an optical signal is received by the rod lens  124   a  coupled into the Erbium-doped waveguide  126   a , in which the erbium ions have been excited to a high energy state by the pump optical source. The signal photons at the wavelength to be amplified stimulate or trigger the release of energy from these higher energy states causing the emission of further photons, at the signal photon wavelength, creating an effective amplification. In doing so the power of the optical signal is increased and it emerges from the waveguide  126   a  where it is received by the fiber  114   a  and egresses the wavelength-plane switch  12   a.    
     In FIG. 5,  124   a ,  126   a , and  114   a  are contiguous and by the use of a bonding agent or index-matching agent provide a continuous optical path there through. The core, cladding, fiber creating on  114   a  is not shown. In practice  202   a  would be a short part of the length of  126   a . The rod lenses  124  are aligned with the center of an erect mirror from matrix  50 . The MEMS devices are mounted well below the mounts for the rod lenses. This is shown in FIG. 6 b.    
     FIG. 6 a  is a cross-sectional view of the wavelength-plane switch  12   a  taken along the line AA in FIG.  4 . Alignment features previously referred to will now be described. The matrices  50  and  52  are shown in outline affixed to the substrate  110  in an alignment well  154  which provides a spacing  156  between the matrices  50 ,  52 . The rod lenses are aligned with the erect MEMS mirrors. The alignment well  154  provides alignment of the matrices  50 ,  52  (and  54  not shown) when the matrices are not fabricated on the substrate  110  and need to be affixed to the substrate during manufacturing of the wavelength-plane switch  12   a . The alignment well lowers the MEMS chips to the point where the mirror centers align with the rod lenses. 
     A groove  138   a  having a V-shaped cross-section is etched into the planar substrate  110  to position the rod lens  124   a  in alignment with the waveguide  126   a  and the output (O) of the N×N matrix  50 . The groove  138   a  positions the rod lens  124  axially parallel to the plane of the substrate  110  and at a desired length relative to the surface substrate  110  into which the groove  138   a  has been etched. There is a similar groove  138  etched in the substrate  110  for each rod lens of the plurality of lenses  124 . Likewise, a groove  140   a  of V-shaped cross-section is etched into the substrate  110  for aligning the lens  128   a  in height and direction with a respective one of the waveguides  130  and an output (O) of the N×K matrix  52 . A plurality of such grooves is provided for the remaining lenses of the plurality of rod lenses  128 . A plurality of such grooves, each orthogonal to the groove  140   a  in the plane of the substrate  110 , is provided for aligning the plurality of rod lenses  132 . One such groove  150   n  for aligning the rod lens  132   n  is shown in FIG. 6. A spacing  158  is provided between the rod lens  132   n  and the matrix  52 . The size of the spacing  158  is determined by the space between the alignment well  154  and the groove  150   n.    
     Each fiber of the pluralities of fibers  112 ,  114 ,  116 , 118 , 120 , and  122  is provided with a corresponding alignment groove of V-shaped cross-section etched into the substrate  110 . One such groove  152   n  is shown in FIG. 6 for one fiber  116   n . The groove  152   n  is shown as being shallower in depth than any of the grooves  138   a ,  140   a , and  150   n , which are shown in FIG. 6 as having the same depth. However, the depth and width of any particular groove depends on the size of its corresponding rod lens or fiber and the height of the lens/fiber relative to the surface of the substrate  110  to adequately align the lens/fiber with an output of a matrix  50 ,  52  or waveguide  126 ,  130  for the purpose of optical signal transmission therethrough. 
     FIG. 6 b  shows further details of the cross-sectional view of the wavelength-plane switch  12   a  shown in FIG. 6 a . Shown in greater detail are the rod lenses  124   a ,  124   b ,  124   c , grooves  138   a ,  138   b ,  138   c , and MEMS mirrors  160   a ,  160   b ,  160   c . MEMS mirrors  160   a  and  160   c  are illustrated in an active state in which they deflect incident beams through 90 degrees in a plane parallel to the substrate  110 . MEMS mirror  160   b  is shown in an inactive state in which a beam will pass over it without deflection. FIG. 6 a  also generally illustrates the alignment of MEMS mirrors  160  and rod lenses  124  as provided by V-grooves  138  which center the rod lenses  124  on the center of the MEMS mirrors  160 . In FIG. 6 a  and  6   b  the MEM substrate is shown in a well  154  in substrate  110  and V-grooves  138  are etched into the substrate  110 . An alternative embodiment, not illustrated, is to laminate both the MEMS matrix  50  and a substrate bearing V-grooves  138  to a common substrate. In this case, the upper surface of the common substrate would correspond to the bottom of well  154 . 
     FIG. 7 a  is a cross-sectional view of a portion of the wavelength-plane switch  12   a  taken along the line BB in FIG. 4. A plurality of alignment grooves of V-shaped cross-section etched into the substrate  110  are provided for the plurality of fibers  114  and  122 , as stated previously. FIG. 7 shows one of these grooves  170   a  for one of the fibers  1   14   a , and their alignment with the waveguide  126   a , the groove  138   a , and the rod lens  124   a . The alignment well  154  for the MEMS matrix  50  is also shown in relation to these grooves  138   a  and  160   a.    
     FIG. 7 b  shows further details of the cross-sectional view of FIG. 7 a . The rod lenses are typically 500-800 μm in diameter, whereas the amplifying waveguide is typically 6-10 μm on a side. Hence the FIGS. 6 a ,  6   b ,  7   a  and  7   b  to not attempt to show the relative sizes, that is the figures are not to scale. The fiber  114   a  is further shown to include a fiber optical channel  114   a ′, enclosed in fiber cladding  114   a ″, and surrounded by a plastic coating  114   a ″′. The axial alignment of the rod lens  124   a , the silicon waveguide  126   a , and the fiber optical channel  114   a ′ is shown. This is accomplished by the V-grooves  170   a  and  138   a  for positioning the fiber  114   a  and rod lens  124   a , respectively, in relative axial alignment. 
     There are co-planarity requirements on several design features illustrated in FIGS. 4,  6  and  7 . Chief amongst these is that the physical size of the V-grooves to align the fibers to the rod lenses (e.g.  124   a  in FIG. 7 b ) places the fiber core in a position where its center is one-half of the height of the amplifying waveguide height (height of  126   a ) above a reference plane  172  plane parallel to the substrate  110 . This causes the rod lenses to accurately couple to fiber on to amplifying waveguides while all being at the same height above the MEMS for centering on all of the centers of the MEMS mirrors. 
     The V-grooves are precisely etched exploiting the crystal structure of the silicon. The size of the grooves is determined by the need to accurately align components in two axes; that of across the groove and that of perpendicular to the substrate. Alignment in longitudinal degree of freedom is provided by other means (e.g. end to the groove). The vertical alignment perpendicular to the substrate is very important for rod lens—fiber coupling (achieved by the use of two different sizes of V-groove) and rod lens—amplifying waveguide alignment. This is achieved by using a specific dimension of V-groove, that is dependent upon the rod lens diameter to place the center of the rod lens in alignment with the center of the amplifying waveguide. 
     Modifications, variations and adaptations to the embodiments of the invention described above are possible within the scope of the invention, which is defined by the claims.