Patent Publication Number: US-2003235361-A1

Title: Polarization independent optical switch networks

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to optical networks and, more particularly, to a method to realize polarization independent optical networks, comprising devices made from birefringent materials such as III-V compounds, LiNbO 3 , and LiTaO 3 .  
       [0002] In a normal single mode (SM) fiber, a signal consists of both polarizations. However, the polarization states are not maintained in a standard SM fiber. During its journey, light couples from one polarization to the other randomly. Thus, it would be highly advantageous that all the elements in an optical telecommunications network be polarization independent.  
       [0003] In optical telecommunications networks, both discrete and integrated optical devices are used, which are built of birefringent materials, such as the materials mentioned above. Such materials exhibit two indices of refraction of different value, an ordinary index n o  and an extraordinary index n e . Optical signals that pass through these devices are subject to a split into two orthogonal polarization components that propagate one with the ordinary index n o , and the other with the extraordinary index n e . In general, the two polarization components have different travel or “passage” times. This difference causes a polarization-dependent time displacement of the signal, denominated “Polarization Mode Dispersion” (PMD).  
       [0004] In addition the two polarization components generally have different values of optical loss for a given optical path. The difference in optical loss of the two polarization components causes a polarization-dependent optical loss of the signal denominated “Polarization Dependent Loss” (PDL).  
       [0005] In particular the passage time t necessary to cover an optical path length L in a material having an index of refraction n is given by the following expression:  
         t=nL/c    (1)  
       [0006] where c=3*10 8  m/sec is the speed of light in vacuum. The difference in the passage times or the differential delay Δt of the two orthogonal polarization components having indices of refraction n o  and n e  in the optical path with length L is given by:  
         Δt =( n   o   −n   e ) L/c=ΔnL/c    (2)  
       [0007] where Δn is the difference between the ordinary and the extraordinary indices of refraction or birefringence.  
       [0008] The temporal displacement between the two polarization components can degrade the optical signal with high penalties in terms of the Bit Error Rate (BER) in the case of a high value of the ΔnL product.  
       [0009] Integrated optical switches are well known. For a recent review of the art using LiNbO 3  substrates, see H. Nakajima, “Development on guided-wave switch arrays.”  IECE Trans. Commun.  Vol. E-82B, pp. 349-356, 1999. Waveguides are created in the substrate material by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light from one waveguide to another, the waveguides are coupled in a switch element by local electro-optical, therno-optical or acousto-optical manipulation of their indices of refraction. In the following, “integrated devices” and “substrates” are used interchangeably. By appropriately combining waveguides and switches, a switch matrix is formed to switch light from a plurality of input waveguides among a plurality of output waveguides. A variety of switch matrix geometries are known. In order to achieve higher port counts, several architectures have been proposed that are based on multiple interconnected substrates. These include the three-stage Clos network [C. Clos, “A study of non-blocking switching networks.”  Bell System Tech. Jour.,  pp. 407-424, March 1953], and the two stage architecture proposed by Spanke referred to herein as the Spanke network [R. A. Spanke, “Architectures for large non-blocking optical space switches.”  IEEE Jour. Quantum Electronics,  Vol. QE-22, pp.964-968, 1986].  
       [0010] For example in a Planar Lightwave Circuit (PLC) made in x-cut lithium niobate consisting of waveguides having propagation along the y-axis (y-propagating) and thus with the optical z-axis in the plane of propagation, the TM (Transverse Magnetic) polarization component perpendicular to the plane of propagation has an ordinary index of refraction (n o ), and the TE (Transverse Electric) polarization component parallel to the plane of propagation has an extraordinary index of refraction (n e ). At wavelengths around 1550 nm, n o  of LiNbO 3  is equal to about 2.226, while n e  is about 2.154. Similar differences of the index of refraction between the TE and the TM components are observed in waveguides made for example, by diffusion of titanium in this substrate. In a device of length of about 60 mm a PMD of about 15 psec (15*10 −12  sec) is calculated.  
       [0011] In high-speed digital optical telecommunication networks working at a rate of 10 Gbps (10 Gbit/sec), the temporal distance between two successive impulses (bits) is of the order of 50 psec. A temporal displacement of 15 psec (or with a higher value in the case of concatenated devices) of the two polarization components of the same bit, induced by a birefringent device, will cause super-positions between successive bits and worsen the quality of the transmission in terms of the BER. In addition, for a device made on such a substrate, the optical loss due to absorption by the metal electrodes is for example much higher for the TM polarized light than for the TE polarized light. Differences in optical loss between the two polarization components occur also due to different losses for the different states of polarization in waveguide bends and in waveguide intersections. These loss factors add up to an overall PDL. Such PDL is highly undesirable in optical telecommunication networks since it causes differences in signal amplitude between different States Of Polarization (SOP), which can be further enhanced by other polarization dependent components included in these systems, until the amplitude of some SOPs may become lower than the system detection limit and increase the system&#39;s BER. When constructing an optical network from multiple devices made of such birefringent materials, such that multiple devices are concatenated, the PMD and PDL add up.  
       [0012] In view of the above-listed disadvantages of PMD and PDL in optical networks, it would be highly advantageous to have low PMD and low PDL optical networks, or in other words polarization-independent optical networks.  
       SUMMARY OF THE INVENTION  
       [0013] The present invention is of architecture of, and a method for, realizing polarization-independent symmetric optical networks, in particular multi-substrate PLC-implemented networks. Herein, “symmetric networks” are optical networks with a symmetric topology between their inputs and outputs in terms of their PMD and PDL, i.e. networks in which the substrate interconnects are in the center of the optical path, such that the PMD and the PDL of the first section of the optical path is equal to the PMD and the PDL of the second section of the optical path. Polarization Independent Optical Switch Networks according to the present invention generally include:  
       [0014] a) multiple substrates of birefringent material, such that each optical path can be divided into two sections, an input section and an output section that have (i) equal PMD (an equal ΔnL product) and (ii) equal PDL.  
       [0015] b) a fiber interconnect section interconnecting each input section to its associated output section, and  
       [0016] c) one or more Polarization Maintaining (PM) fibers used for the fiber interconnection such that a PM fiber itself is used to convert the polarization between the input and the output sections.  
       [0017] PMD and PDL are compensated in the optical paths of the input and output sections. This happens because the polarization component TM endowed with a speed c/n o  and with an optical loss Loss(TM) in the optical path of the input section, is converted to TE polarization between the input and the output sections, and is endowed with a speed c/n e  and with an optical loss Loss (TE) in the optical path of the output section. Similarly, the polarization component TE endowed with a speed c/n e  and with an optical loss Loss(TE) in the optical path of the input section, is converted to TM polarization between the input and the output sections, and is endowed with a speed c/n o  and with an optical loss Loss (TM) in the optical path of the output section. In symmetrical optical networks, the PMD and the PDL for the input and the output sections are equal for each optical path, thus complete PMD and PDL compensation is achieved by the TE-TM conversion between the input and output sections. Degradation of the signals in terms of BER is thus avoided.  
       [0018] According to the present invention, there is provided in a first embodiment a substantially polarization independent optical network in which symmetry is maintained in terms of polarization dependent loss and polarization mode dispersion, comprising, a first bi-refringent optical device having at least one output port with a first ordinary axis and a first extraordinary axis, a second bi-refringent optical device having at least one input port with a second ordinary axis and a second extraordinary axis, the first and second optical devices connected along an optical path between one of the at least one input and output ports, and a polarization conversion stage that includes a polarization maintaining fiber, the fiber having a fast axis and a slow axis and configured to rotate each linear polarization component of a signal traveling along the optical path by 90°.  
       [0019] According to one feature of the first embodiment of the polarization independent optical network of the present invention, the configuration of the fiber to rotate each linear polarization component includes a first connection between the first device and the fiber, in which the slow axis of the fiber coincides with the first extraordinary axis of the first device, and in which the fast axis of the fiber coincides with the first ordinary axis of the first device, and a second connection between the second device and the fiber, in which the fast axis of the fiber coincides with the second extraordinary axis of the second device, and in which the slow axis of the fiber coincides with the second ordinary axis of the second device.  
       [0020] According to another feature of the first embodiment of the polarization independent optical network of the present invention, the configuration of the fiber to rotate each linear polarization component includes a first connection between the first device and the fiber, in which the fast axis of the fiber coincides with the first extraordinary axis of the first device, and in which the slow axis of the fiber coincides with the first ordinary axis of the first device, and a second connection between the second device and the fiber, in which the slow axis of the fiber coincides with the second extraordinary axis of the second device, and in which the fast axis of the fiber coincides with the second ordinary axis of the second device.  
       [0021] According to another feature of the first embodiment of the polarization independent optical network of the present invention, each of the first and second devices are selected from the group consisting of discrete devices and integrated devices.  
       [0022] According to the present invention, there is provided in a second embodiment a polarization independent optical network in which symmetry is maintained in terms of polarization dependent loss and polarization mode dispersion, comprising a first bi-refringent optical device having at least one output port with a first ordinary axis and a first extraordinary axis, a second bi-refringent optical device having at least one input port with a second ordinary axis and a second extraordinary axis, the first and second optical devices connected along an optical path between the at least one input and output ports, and a polarization conversion stage providing the connection along the optical path, the stage including at least two concatenated polarization maintaining fibers having equal ΔnL and configured to rotate each polarization component of a signal traveling along the optical path by 90°.  
       [0023] According to one feature of the second embodiment of the polarization independent optical network of the present invention, the at least two concatenated polarization maintaining fibers having equal ΔnL include two identical fibers.  
       [0024] According to another feature of the second embodiment of the polarization independent optical network of the present invention, the at least two concatenated polarization maintaining fibers having equal ΔnL include a first fiber having a first fast axis and a first slow axis, and a second fiber having a second fast axis and a second slow axis, and the configuration to rotate each polarization component includes a first connection between the first device and the first fiber, in which the first slow axis coincides with the first extraordinary axis and in which the first fast axis coincides with the first ordinary axis, a second connection between the first and the second fibers in which the first slow axis coincides with the second fast axis and the second slow axis coincides with the first fast axis, and a third connection between the second fiber and the second device, in which the second slow axis coincides with the second extraordinary axis, and in which the second fast axis coincides with the second ordinary axis.  
       [0025] According to yet another feature of the second embodiment of the polarization independent optical network of the present invention, each of the first and second devices are selected from the group consisting of discrete devices and integrated devices.  
       [0026] According to the present invention, there is provided in a third embodiment a polarization independent optical network in which symmetry is maintained with regard to polarization mode dispersion and polarization dependent loss, comprising an input optical section including a first plurality of first elements, each with at least one output, and an output optical section including a second plurality of second elements, each with at least one input, each of the at least one outputs of the first elements is connected by an optical path to at least one of the outputs of at least one of the second elements, each of the input and output optical sections have equal PMD and PDL for each optical path in the network, and a PM fiber based polarization conversion stage inserted in each optical path, and configured to rotate by 90° each linear polarization component of a signal traveling along each optical path.  
       [0027] According to one feature of the third embodiment of the polarization independent optical network of the present invention, each of the first and second elements are selected from the group consisting of discrete devices and integrated devices.  
       [0028] According to another feature of the third embodiment of the polarization independent optical network of the present invention, each of the integrated devices is a planar lightwave circuit device.  
       [0029] According to yet another feature of the third embodiment of the polarization independent optical network of the present invention, the network is further characterized in that each element of the first and second pluralities has at least one port with an ordinary and an extraordinary axis, and in that the PM fiber based polarization conversion stage includes a PM fiber that has a fast axis and a slow axis, and in that each PM fiber based polarization conversion stage includes: a first connection between an output port of the element of the first plurality and the PM fiber, in which the slow axis of the fiber coincides with the extraordinary axis of the output port of the first element, and in which the fast axis of the fiber coincides with the ordinary axis of the same output port of the first element, and a second connection between the PM fiber and the at least one input port of at least one second element, in which the fast axis of the fiber coincides with the extraordinary axis of the at least one input port of the at least one second element, and in which the slow axis of the fiber coincides with the ordinary axis of the at least one input port of the at least one second element.  
       [0030] According to yet another feature of the third embodiment of the polarization independent optical network of the present invention, the network is further characterized in that each element of the first and second pluralities has at least one port with an ordinary and an extraordinary axis, and in that the PM fiber based polarization conversion stage includes at least two concatenated PM fibers having equal ΔnL, each of the PM fibers having a fast axis and a slow axis.  
       [0031] According to yet another feature of the third embodiment of the polarization independent optical network of the present invention, the at least two concatenated PM fibers having equal ΔnL include two, first and second identical fibers, and each fiber based polarization conversion stage includes: a first connection between each port of the first element of the first plurality and the first fiber, in which the extraordinary axis of the port of the first element coincides with the slow axis of the first fiber, and in which the first ordinary axis of the port of the first element coincides with the fast axis of the first fiber, a second connection between the first and the second fibers, in which the slow axis of the first fiber coincides with the fast axis of the second fiber, and the fast axis of the first fiber coincides with the slow axis of the second fiber, and a third connection between the second fiber and the at least one port of at least one second element of the second plurality, in which the fast axis of the second fiber coincides with the ordinary axis of the at least one port of the at least one second element, and in which the slow axis of the second fiber coincides with the extraordinary axis of the at least one port of the at least one second element. According to the present invention there is provided a method for obtaining polarization independence in an optical network in which symmetry is maintained with regard to polarization mode dispersion and polarization dependent loss, comprising: providing at least two bi-refringent elements, each two of the at least two elements connected in an optical path carrying TM and TE polarized signal components, and using a PM fiber-based polarization conversion stage inserted in each optical path to rotate each polarization component by 90°, whereby the rotation yields a substantially PMD and PDL compensated network.  
       [0032] According to one feature of the method of the present invention for obtaining polarization independence in an optical network, the step of providing at least two bi-refringent elements includes providing a first bi-refringent optical device having at least one output port with a first ordinary axis and a first extraordinary axis, and a second bi-refringent optical device having at least one input port with a second ordinary axis and a second extraordinary axis, and the step of using a PM fiber-based polarization conversion stage includes providing a polarization maintaining fiber configured to perform the rotation.  
       [0033] According to another feature of the method of the present invention for obtaining polarization independence in an optical network, the step of providing at least two bi-refringent elements includes providing a first bi-refringent optical device having at least one output port with a first ordinary axis and a first extraordinary axis, and a second bi-refringent optical device having at least one input port with a second ordinary axis and a second extraordinary axis, and the step of using a PM fiber-based polarization conversion stage inserted in each optical path includes providing at least two concatenated polarization maintaining fibers having equal ΔnL and configured to perform the rotation.  
       [0034] According to yet another feature of the method of the present invention for obtaining polarization independence in an optical network, the substep of providing at least two concatenated polarization maintaining fibers having equal ΔnL and configured to perform the rotation includes providing two identical fibers.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0035] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:  
     [0036]FIG. 1 shows two birefringent elements interconnected with a PM fiber aligned against opposite polarizations of the optical components at its input and output;  
     [0037]FIG. 2 shows two birefringent elements interconnected with two equal lengths PM fibers interconnected such that the fast axis of one fiber is aligned against the slow axis of the second fiber;  
     [0038]FIG. 3 shows a multiple-substrate Spanke network incorporating the PDL and PMD compensation architecture of the present invention; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0039] The present invention discloses architecture of, and a method to realize a, polarization-independent switch network, and is particularly advantageous for high port count, multi-substrate networks. According to the present invention, polarization conversion is performed in symmetric (in terms of PMD and PDL) multi-substrate networks by using a PM fiber for the inter-substrate interconnect, and by performing a TE⇄TM conversion, between the input and the output sections. In one simple embodiment, the TE⇄TM conversion is performed by aligning one PM fiber&#39;s fast (F) axis against the extraordinary (E) axis of the input substrate and against the ordinary (O) axis of the output substrate, or by aligning the PM fiber&#39;s slow (S) axis against the extraordinary axis of the input substrate and against the ordinary axis of the output substrate. The two connections above can be termed respectively [(F-E/S-O)→(S-E/F-O)] and [(S-E/F-O)→(F-E/S-O)].  
     [0040]FIG. 1 shows a first embodiment of a PMD and PDL-compensated optical network  10  according to the present invention. In this specific case, network  10  comprises two identical PLC waveguide devices  12  and  14  (having typically a plurality of integrated waveguides that are not shown) made for example in an x-cut LiNbO 3  birefringent material. Devices  12  and  14  are reversible such that each of their inputs and outputs may be interchanged. For example, device  12  has an input (or output) port  16  and an output (or input) port  18 , and device  14  has an input (or output) port  20  and an output (or input) port  22 . Network  10  is symmetric such that devices  12  and  14  have equal PDL and equal PMD. In network  10 , devices  12  and  14  are optically connected by a polarization conversion stage  30 , so that port  18  (serving here as an output port) of device  12  is connected to port  22  (serving here as an input port) of device  14 . An optical fiber  32  connects the outside world to port  16 , which serves as an input port of the network, as well as the input of device  12 . Port  20  serves as an output of device  14 , as well as of network  10 , being connected to the outside world by an optical fiber  40 .  
     [0041] An incoming optical signal through fiber  32  is split into its TE and TM components at network input  16 . The TM component (referred to hereafter as original TM”) travels as a TM signal along device  12  with a speed c/n o  and a loss “Loss(TM)” associated with the TM polarization. At port  18 , the original TM polarization component enters polarization conversion stage  30 , and converts to TE polarization (referred to hereafter as “secondary TE”). The secondary TE component is coupled into device  14  at port  22  and travels along device  14  as a TE signal with speed c/n e  and optical loss “Loss(TE)”. Similarly the TE component (“original TE”) entering the network travels along device  12  with TE polarization, converts to TM (“secondary TM”) in polarization conversion stage  30 , and travels with TM polarization through device  14 . Thus each polarization component travels half of the optical path with a TM polarization with speed c/n o  and loss Loss(TM), and half of the optical path with a TE polarization with speed c/n e  and loss Loss(TE). In summary, an optical signal with an arbitrary polarization entering input port  16  is split into its two orthogonal polarization components TM and TE. Each of these polarization components travels half way as TM and half way as TE. They are recombined again at network output  20 , such that the PMD and PDL generated in the first half of the optical path (in device  12 ), are fully compensated for in the second half of the optical path (in device  14 ), and are essentially zero.  
     [0042] In prior art, such polarization conversion in mid-span has been used for PMD and PDL compensation on birefringent substrates by using two equal length chips with a quartz half plate in between, such as in a LiNbO 3  based photonic switch, see for example T. Murphy et. al., “A strictly non-blocking 16×16 electrooptic photonic switch module” ECOC&#39;00 Munich, 2000. This method for polarization conversion however considerably complicates the manufacturing of such devices, and requires special process steps for producing the quartz half plate on the chip facet and the subsequent interconnection of the two chips. This “half-plate” method is also limited to the interconnection of only two optical chips, and is thus limited to optical devices or networks comprised of only two substrates.  
     [0043] In contrast with prior art “half plate” methods, the polarization conversion in the method disclosed herein and performed in stage  30  is being done with the use of at least one portion of a single mode Polarization Maintaining (PM) optical fiber  50 . In the particular case shown in FIG. 1, PM optical fiber  50  is connected to output port  18  of device  12  and to input port  22  of device  14 . Suitable PM optical fibers are for example those presenting elements of internal tension called “PANDA”, or those with an oval inner clad. Other PM fibers are known and can be used as well for the purposes of the present invention. The transverse cross-section of these fibers has an axis called “slow” and an axis called “fast”, the two axes perpendicular one to the other. Linearly polarized light entering either the “slow” or the “fast” optical axis of such a fiber maintains its linear polarization between the fiber&#39;s input and output.  
     [0044] In use, an optical signal entering system  10  for example through fiber  32  is split into its TM and TE components at the network input  16 . Each polarization component then travels with its own speed and loss through one or more integrated waveguides and waveguide devices (such as switches and filters) on device  12 . At port  18 , PM fiber  50  is oriented (with respect to the integrated optical waveguide in which it travels on device  12 ) so that the slow axis of the PM fiber coincides with the extraordinary axis of device  12  and the PM fiber fast axis coincides with the ordinary axis of device  12 . At port  22 , the polarization components of the signal traveling on fiber  50  are aligned to device  14  oppositely: the fast axis of PM fiber  50  coincides with the extraordinary axis of device  14 , and the slow axis of PM fiber  50  coincides with the ordinary axis of device  14 . In other words, fiber  50  is “twisted” (polarization wise) by 90° between ports  18  and  22 , “rotating” each polarization component. A polarization component traveling as a TM (TE) polarization along device  12  will travel as a TE (TM) polarization along device  14 . The “twisted” PM fiber thus acts as a polarization converter.  
     [0045] This method of using a twisted PM fiber for performing polarization conversion in the middle of the optical path generating the PMD and PDL eliminates any special process steps for producing polarization converter  30 . Specifically, the method of the present invention uses fiber interconnect  50  itself for the polarization conversion, instead of prior art half-plate and similar solutions. A major advantage of using a fiber for this purpose is that any number of substrates (chips) can be interconnected and compensated for PMD and PDL, and not only typically two as with sandwiching half-plates between two substrates. For networks in which each optical path is exactly identical and symmetric between the input and output stages, the PDL and PMD resulting from the birefringent devices  12  and  14  is fully compensated for. However there is still a residual PMD as a result of the PMD of the PM fiber itself.  
     [0046] In PM fibers, signals with polarization parallel to the slow axis propagate according to a first index of refraction, with a speed lower than the signals having polarization parallel to the fast axis of the fiber, that propagate according to a different value of the index of refraction. The typical birefringence of these fibers, that is the difference between the indices of refraction related to the two axes, is of the order of Δn˜0.0001−0.001. An example for a “PANDA” fiber suitable for the wavelength of 1550 nm is that of the Fujikura Firm identified by the letters SM(C) 15-P. The birefringence of the PANDA SM(C) 15-P is about Δn=4.5×10 −4 . If the PM fiber interconnect has a length L&gt;1 m than the total PMD associated with the PM fiber alone is Δt=ΔnL/c&gt;1.5 psec. In this case the PMD of the PM fiber itself has also to be compensated for high bit rate transmission.  
     [0047] According to the present invention, the PMD due to the PM fiber interconnect can also be compensated for as shown in FIG. 2 by preferably using two PM fibers of equal ΔnL product (instead of one fiber, e.g. fiber  50  of FIG. 1) for the interconnect. Most preferably, the two-fiber section would be identical and have equal length. Alternatively, each side may be concatenated from any number of fiber sections as long as the total ΔnL product is equal on both sides of the polarization conversion interconnect.  
     [0048]FIG. 2 shows, in a second embodiment of the method and architecture of the present invention, an optical network  100  that is basically identical with network  10  of FIG. 1, except that PM fiber  50  is replaced by two concatenated fibers having the same ΔnL product, a first PM fiber section  102  and a second PM fiber section  104 . Fibers  102  and  104  are interconnected using preferably a PM fiber connector  106 , such that the fast axis of fiber  102  is aligned against the slow axis of fiber  104  and vice versa. In general, it can be stated that the connection of the first section (fiber  102 ) to the first substrate (device  12 ) is identical with the second connection of the second section (fiber  104 ) to the second substrate (device  14 ). That is, the connection between each device and its respectively connected fiber section can be written either as (F-E/S-O) or as (S-E/F-O). Since fibers  102  and  104  are chosen to be of equal ΔnL (equal PMD), their connection is a “mid-span” connection, with the attending conversion obtained by this polarization conversion stage termed “mid-span” polarization conversion. Using such “mid-span” polarization conversion, complete PMD compensation for the PMD of the PM fiber interconnect section ( 102 + 104 ) is achieved in the same manner described before for connecting any two birefringent elements. This effect is additional to the compensation achieved for the PMD and PDL of devices  12  and  14 , which like in network  10  compensate the PMD and PDL of one another, since polarization rotation has been performed in the polarization conversion stage  30 . In this case the polarization conversion in stage  30  is achieved by the polarization rotation in fiber interconnect  106 .  
     [0049] As mentioned in the example above, using the PM fiber for mid-span polarization conversion enables to interconnect a multi-substrate optical network, and achieve full PMD and PDL compensation in the case that the network topology is symmetric and the input section is identical to the output section in terms of PMD and PDL for each optical path. We call such a PMD and PDL compensated network a “polarization independent network”. In particular, according to the present invention, full PMD and PDL compensation may be achieved in the two-stage Spanke optical switching network mentioned above. As shown by example in FIG. 3, a N×N Spanke network  150  is based on partitioning the architecture over several substrates, eliminating the need for integrated optical waveguide crossovers, and allowing for large switch dimension without complex integration on individual substrates. Network  150  includes an input section  152  built of N, 1×N switches  154 , and an output section  156  built from N, N×1 switches  158 . Typically the numbers of input waveguides and output waveguides, both chosen to be N in this example, are equal powers of 2, up to a practical maximum of 512. In FIG. 3, only two switches  154   a, b  out of the N switches of input section  152 , and only two switches  158   a, b  of the N switches of output section  156  are shown. Switches  154  a, b and  158   a, b  are interconnected by four fiber “substrate” interconnects  160 ( a - d ). Only these four interconnects are shown, out of N 2  substrate interconnects in the N×N switch network. The network is symmetric, such that each optical path can be divided into its input section (substrate) and output section (substrate), which are identical, and in particular have identical PMD and PDL. Each substrate interconnect  160 ( a - d ) (always a single PM fiber as in FIG. 1, or a two-section PM fiber as in FIG. 2) is in the center of the optical path between an input substrate and an output substrate, and performs TE⇄TM polarization conversion.  
     [0050] Such a network may be built for example from active x-cut LiNbO 3  1×N switches. In this case the polarization of the light is maintained in each single substrate. By converting the polarization in the middle of the optical path between the two substrates, e.g. in the fiber interconnect region  160 , PMD and PDL can be compensated for as explained in the example above. Specifically, for any optical path between any of the N input ports to any of the N output ports, mid-span polarization conversion is performed in the interconnect region  160 , between two identical substrates in the network, and PMD and PDL are fully compensated for. That is, each optical path contains two equal length and equal loss birefringent substrates, one in the input section  152  and one in the output section  156 , mid-span polarization conversion is performed in  160 , such that PMD and PDL generated in  152  is compensated for in  156 . The polarization conversion may be done either by using a single PM fiber like shown in FIG. 1, (in this case residual PMD due to the PM fiber itself is present) or by two equal ΔnL and preferably identical PM fibers like shown in FIG. 2 (in this case the PMD of the PM fiber itself is also compensated for).  
     [0051] All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.  
     [0052] While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.