Abstract:
An optical wavelength add/drop multiplexer provides communications between two optical links supporting wavelength division multiplexing (WDM). A wavelength slicer spatially separates the input signal into two sets of channels. An optical filter, such as an interference filter, spatially separates the a subset of the input channels into an array of separated channels. A programmable optical add/drop switch array selectively routes channels from and array of input ports to an array of drop ports, substitutes channels from an array of add ports in place of the dropped channels, and routes the remaining input channels and added channels to an array of output ports. The channels from the output ports of the said add/drop switch array are then combined and transmitted into the second optical link. In an alternative embodiment, a circulated drop filter consisting of an optical circulator and a series of filter Bragg gratings is used to select a predetermined series of input channels to be processed by the add/drop switch array, with the remaining channels being passed by the circulated drop filter as express lanes.

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of the Applicant&#39;s U.S. Pat. No. 5,694,233, entitled “SWITCHABLE WAVELENGTH ROUTER,” filed on Jan. 8, 1997, which is hereby incorporated herein by reference; and U.S. patent application Ser. No. 09/273,920, entitled “OPTICAL WAVELENGTH ADD/DROP MULTIPLEXER,” filed on Mar. 22, 1999, which is hereby incorporated herein by reference, and is related to Applicant&#39;s U.S. Pat. No. 5,115,155, entitled “SYSTEM FOR DEALING WITH FAULTS IN AN OPTICAL LINK,” filed on Nov. 12, 1998, which is hereby incorporated herein by reference; Applicant&#39;s U.S. Pat. No. 6,160,665, entitled “HIGH EXTINCTION RATIO POLARIZATION BEAMSPLITTER,” which is hereby incorporated by reference; Applicant&#39;s U.S. patent application Ser. No. 09/326,250, entitled “N×M DIGITALLY PROGRAMMABLE OPTICAL ROUTING SWITCH USING HIGH EXTINCTION RATIO POLARIZATION BEAMSPLITTER,” which is hereby incorporated herein by reference; Applicant&#39;s U.S. patent application Ser. No. 09/326,251, entitled “FIBER OPTIC SMART SWITCH,” which is hereby incorporated herein by reference; Applicant&#39;s U.S. patent application Ser. No. 09/326,256, entitled “OPTICAL ADD/DROP WAVELENGTH SWITCH USING A HIGH EXTINCTION RATIO POLARIZATION BEAMSPLITTER,” which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates in general to optical communications, and more particularly to using a wavelength slicer for wavelength division multiplex communications. 
     BACKGROUND 
     Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optical communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information over optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology. These optical signals are repeatedly amplified by erbium-doped fiber amplifiers (EDFA) along the network to compensate for transmission losses. The amplified signals reach the receiving end and are detected using WDM filters followed by photo receivers. 
     Fiber optic communications networks are typically arranged with a plurality of terminals in any of a number of topological configurations. The simplest configuration is two terminals communicating data over an optical link. This can be extended to a daisy-chain configuration in which three or more terminals are connected in series by a plurality of optical links. Ring configurations are also used, as well as other two-dimensional mesh networks. In each case, the optical link between two terminals typically includes a plurality of optical fibers for bidirectional communications, to provide redundancy in the event of a fault in one or more of the optical fibers, and for future capacity. 
     Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, demultiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into individual channels or sets of channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels to output ports and permutes these channels according to control signals to create a desired coupling between an input channel and an output port. 
     Note that each carrier has the potential to carry gigabits of information per second. Current technology allows for about forty channels or optical carriers, each of a slightly different wavelength, to travel on a single-mode fiber using a single WDM signal. The operating bands are limited by the EDFA amplifier (C) band, thus the increase in the number of channels has been accomplished by shrinking the spacing between the channels, and by adding new bands. The current standard is 50 and 100 GHz between optical channels, whereas older standards were 200 and 400 GHz spacings. Another characteristic of the WDM signal is the modulation rate. As the modulation rate is increased, more data can be carried. Current technology allows for a modulation rate of 10 Gigabits per second (Gbs). This has been recently increased from 2.5 Gbs. The 10 Gbs standard is SONET OC-192, wherein SONET is synchronized optical network and OC is optical carrier. The increase in the modulation rate translates into a wider signal in the spatial domain. Consequently, the wider signal and smaller spacing means that the signals are very close together (in the spatial domain), and thus are very hard to separate. As a result, crosstalk may occur from adjacent signals. 
     One prior art separation method is to divide the spatial band into four sub-bands, each about 200 GHz wide. The filters used to perform the separation have significant side slopes (i.e., they produce trapezoidal shapes), and thus overlap occurs between the bands. To prevent crosstalk, guard bands are placed at the boundaries of the sub-bands, where no signals are placed. These guard bands consume significant bandwidth, i.e., about 30%. Additional stages could be added to achieve 100 GHz bands, but this increases the bandwidth consumed by the guard bands. 
     Also dropping and adding channels is a problem. For example, in a group of 16 carrier channels, 4 might need to be dropped for distribution to a local metropolitan area and the other 12 carrier channels might need to be passed on to other remote destinations. This is typically accomplished by demodulating all 16 optical carriers to obtain 16 electronic signals, then remodulating the 12 carriers and processing the 4 electrical signals. Optical-to-electrical (O-E) converters are used at switching centers to demodulate all the optical signals, including those not intended for local distribution. The “long-haul” signals are processed to modulate a laser (E-O) converter for launch into optical fiber to their ultimate destinations. The channels vacated by taking off signals for local distribution can now be filled by new carriers to move signals from local switches to remote destination. These electrical-to-optical-to-electrical (OEO) “add/drop” operations are critical to network performance but require that all carriers on a fiber be demodulated, processed, and remodulated in order to pick off even a small fraction of the data flowing on the fiber. In the current art, there is no effective non-OEO method of simultaneously dropping a DWDM carrier with mixed traffic for local distribution while simultaneously passing the carrier through to a remote location. 
     SUMMARY OF THE INVENTION 
     This invention provides an optical wavelength add/drop multiplexer for communications between two optical links supporting wavelength division multiplexing (WDM). A wavelength slicer spatially separates the input signal into two sets of channels. An optical filter such as an interference filter, spatially separates the set of the input channels into an array of separated channels. A programmable optical add/drop switch array selectively routes channels from an array of input ports to an array of drop ports, substitutes channels from an array of add ports in place of the dropped channels, and routes the remaining input channels and added channels to an array of output ports. The channels from the output ports of the said add/drop switch array are then combined and transmitted into the second optical link. A network of wavelength slicers can be used to spatially separate the input signal into a larger number of sets of channels that can either be accessed by a number of add/drop switch arrays, or pass unchanged as “express lanes” to the second optical link. In an alternative embodiment, a circulated drop filter consisting of an optical circulator and a series of fiber Bragg gratings is used to select a predetermined series of input channels to be processed by the add/drop switch array, with the remaining channels being passed by the circulated drop filter as express lanes. 
     A primary object of the present invention is to provide an optical wavelength add/drop multiplexer that can separate multiple channels from an input WDM signal and selectively substitute channels from series of add ports in place of the input channels. 
     Another object of the present invention is to provide an optical wavelength add/drop multiplexer that can be used to augment the channel capacity of an existing central office equipment for optical communications. 
     These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawing. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIGS. 1A and 1B are simplified block diagrams illustrating the functionality of the wavelength slicers; 
     FIG. 2 is a schematic block diagram of a wavelength slicer; 
     FIG. 3 is a spectral diagram of the transmission function of a wavelength slicer for separating adjacent 50 GHz input channels into two sets of output channels; 
     FIG. 4 is a spectral diagram of the transmission function of a wavelength slicer for separating adjacent 50 GHz input channels into two sets of output channels; 
     FIG. 5 depicts a smart switch utilizing a high extinction ratio polarization beamsplitter; 
     FIG. 6 depicts a 4×4 add/drop switch array; 
     FIG. 7 depicts two 4×4 add/drop switch arrays forming an arbitrary add/drop switch; 
     FIG. 8 depicts a 4×4 add/drop switch with wavelength conversion; 
     FIG. 9 depicts a drop/add switch for wavelength management and restoration; 
     FIG. 10 depicts a multi-ring network system; 
     FIG. 11 depicts the a OS-200 signal being sub-divided into four OS-800 signals; 
     FIG. 12 depicts wavelength slot interchange; and 
     FIG. 13 depicts a mesh network system. 
    
    
     DETAILED DESCRIPTION 
     A WDM signal consists of multiple channels with each channel having its own range of wavelengths or frequencies. As used herein, the terms “channel” or “spectral band” refer to a particular range of frequencies or wavelengths that define a unique information signal. Each channel is usually evenly spaced from adjacent channels, although this is not necessary. For example, the wavelength slicers shown in FIG. 1 can separate channels based on a 50 GHz spacing between adjacent channels, as depicted in FIG.  3 . Uneven spacing may result in some complexity in design, but, as will be seen, the present invention can be adapted to such a channel system. This flexibility is important in that the channel placement is driven largely by the technical capabilities of transmitters (i.e., laser diodes) and detectors and so flexibility is of significant importance. 
     The sets of input WDM channels are mutually exclusive, in that there is no overlap in the frequency bands assigned to channels in the different sets. In the preferred embodiment of the present invention, the first and second sets of channels are interdigitally spaced as shown in FIG.  1 . For example, the spacing between adjacent channels in the first set can be 100 GHz, and the spacing between adjacent channels in the second set can be 100 GHz. The resulting interdigital spacing between channels after the first and second sets are combined would be 50 GHz. This type of arrangement allows for network planning. Consider the one 50 GHz signal to comprise two 100 GHz signals or four 200 GHz signals. Similarly, one 25 GHz signal comprises two 50 GHz signal or comprise four 100 GHz signals or eight 200 GHz signals or sixteen 400 GHz signals. These signals can be divided as needed to handle different situations. Note that other channel configurations could be employed, as long as there are at least two mutually exclusive sets of channels. For example, alternating blocks of channels could be used to define the first and second sets of channels. 
     FIG. 1A is a block diagram illustrating the general functionality of an individual wavelength slicer  100  as a component in a larger network system. The input WDM signal is coupled using conventional optical signal coupling techniques to the input port of the wavelength slicer  100 . The wavelength slicer  100  separates the input signal into two sets of channels, which are routed to the output ports as depicted in FIG.  1 A. FIG. 1B depicts a 1×4 slicer  3000 . The input signal is a 25 GHz spaced signal, OS-25, which is divided into four 100 GHz signals by slicer  3000 . Note that each of these signals are orthogonal to each other, i.e. they do not overlap. Also note that they are synchronized in the wavelength domain. This 1×4 slicer can be constructed by cascading two 1×2 slicers of FIG.  1 A. 
     In the preferred embodiment, the wavelength slicer  100  separates alternating adjacent input channels into the first and second sets of output channels. FIG. 3 illustrates the transmission characteristics of a wavelength slicer with a channel spacing of 50 GHz. Each wavelength slicer  100  is inherently bidirectional, and therefore can be used interchangeably either to separate (or demultiplex) an input signal into two sets of output channels, or to combine (or multiplex) two sets of input channels into a combined output WDM signal. Note that no guard bands are needed. Moreover, the system is scalable, e.g. when a 25 GHz spacing is available, a 25 GHz slicer is attached in front of the 50 GHz slicer and a second 50 GHz slicer network may be added to the 25 GHz network. Further note, that by separating the adjacent channels, good isolation between adjacent channels is achieved. 
     FIG. 2 depicts the details of the structure and operation of one possible implementation of a wavelength slicer, for other implementations see “SWITCHABLE WAVELENGTH ROUTER,” U.S. application Ser. No. 08/780,291, filed Jan. 8, 1997, which is hereby incorporated herein by reference. 
     FIG. 2 is a detailed schematic diagram of a wavelength slicer  100 . Each of the optical paths is labeled with either a horizontal double-headed line indicating horizontal polarization (p polarization), or a vertical double-headed line indicating vertical polarization (s polarization, which may be depicted as “·”), or both horizontal and vertical double-headed lines indicating mixed horizontal and vertical polarizations in the optical signal at that point. 
     The input signal  101  enters the first birefringent element  102  that spatially separates horizontal and vertically polarized components of the input signal. The first birefringent element  102  comprises a material that allows the vertically polarized portion of the optical signal to pass through without changing course because they are ordinary waves in the birefringent element  102 . In contrast, horizontally-polarized waves are redirected at an angle because of the birefringent walk-off effect. The angle of redirection is a well-known function of the particular materials chosen. Examples of materials suitable for construction of the birefringent element include calcite, rutile, lithium niobate, YVO 4 -based crystals, and the like. The horizontally-polarized component travels along a path  201  as an extraordinary signal in the first birefringent element  102  while the vertically polarized component  202  travels as an ordinary signal and passes through without spatial reorientation. The resulting signals  201  and  202  both carry the full frequency spectrum of the input signal  101 . 
     At least one of the beam components  201  and  202  are coupled to a polarization rotator  103  which selectively rotates the polarization state of either beam component  201  or  202  by a predefined amount. In the preferred embodiment, the rotator  103  rotates the signals by either 0° (i.e., no rotation) or 90°. In FIG. 2, the vertically-polarized component  202  is rotated by 90° so that both signals  203 ,  204  exiting the polarization rotator  103  have a horizontal polarization. Again, at this stage, both the horizontal and vertical components  202 ,  203  contain the entire frequency spectrum of channels in the input WDM signal  101 . 
     The stacked waveplates element  104  is a stacked plurality of birefringent waveplates at selected orientations that generate two eigen states. The first eigen state carries a first set of channels with the same polarization as the input, and the second eigen state carries a complementary set of channels at the orthogonal polarization. The polarization of the incoming beam and the two output polarizations form a pair of spectral responses, where (H, H) and (V, V) carry the first set of channels from the input spectrum and (H, V) and (V, H) carry the complementary (second) set of channels of the input spectrum, where V and H are vertical and horizontal polarization, respectively. With horizontal polarizations  203 ,  204  input to the first stacked waveplates element  104  as illustrated in FIG. 2, orthogonal vertical and horizontal polarizations are generated with the first set of channels residing in horizontal polarization and the second set of channels residing in vertical polarization. 
     The pairs of optical responses  205 ,  206  output by the first stacked waveplates element  104  are coupled to a second birefringent element  105 . This birefringent element  105  has a similar construction to the first birefringent element  102  and spatially separates the horizontally and vertically polarized components of the input optical signals  205  and  206 . The optical signals  205 ,  206  are broken into vertically-polarized components  207 ,  208  containing the second set of channels and horizontally-polarized components  209 ,  210  containing the first set of channels. Due to the birefringent walk-off effect, the two orthogonal polarizations that carry first set of channels  209 ,  210  in horizontal polarization and second set of channels  207 ,  208  in vertical polarization are separated by the second birefringent element  105 . 
     Following the second birefringent element  105 , the optical elements on the input side of the second birefringent element  105  can be repeated in opposite order, as illustrated in FIG.  2 . The second stacked waveplates element  106  has substantially the same composition as the first stacked waveplates element  104 . The horizontally-polarized beams  209 ,  210  input to the second stacked waveplates element  106 , are further purified and maintain their polarization when they exit the second stacked waveplates element  106 . On the other hand, the vertically-polarized beams  207 ,  208  experience a 90° polarization rotation and are also purified when they exit the second stacked waveplates element  106 . The 90° polarization rotation is due to the fact that the vertically-polarized beams  207 ,  208  carry the second set of channels and are in the complementary state of stacked waveplates element  106 . At the output of the stacked waveplates element  106 , all four beams  211 ,  212  and  213 ,  214  have horizontal polarization. However, the spectral bands defined by the filter characteristics of the stacked waveplates elements  104 ,  106  are separated with the second set of channels on top and the first set of channels below. 
     To recombine the spectra of the two sets of beams  211 ,  212  and  213 ,  214 , a second polarization rotator  107  and a third birefringent element  108  are used. The second rotator  107  intercepts at least two of the four parallel beams  211 - 214  and rotates the polarization of the beams to produce an orthogonally-polarized pair of beams  215 ,  216  and  217 ,  218  for each spectral band at the output of the second polarization rotator  107 . In the case of FIG. 2, the polarization of beams  211  and  213  is rotated by 90°, and beams  212  and  214  are passed without change of polarization. Finally, a third birefringent element  108  recombines the two orthogonally-polarized beam pairs  215 ,  216  and  217 ,  218  using the walk-off effect to produce two sets of channels that exit at the output ports  109  and  110 , respectively. 
     The slicer shown in FIG. 2 can be used to slice a WDM signal into bands, each of which comprises a plurality of wavelength channels, additional slicers would be used to separate the bands down to individual channels. This is depicted in FIG.  4 . 
     FIG. 4 provides an overall schematic diagram  400  of an embodiment of the present invention using two networks  301  and  401  of wavelength slicers. Input WDM signals  10  from an optical link are coupled to the input port of a first wavelength slicer  302 . The input WDM signal comprises multiple channels with each channel having its own range of wavelengths or frequencies. 
     Each wavelength slicer  302 - 306  in the wavelength slicer network  301  spatially separates a set of input WDM channels into two complementary sets of output channels. In the preferred embodiment, each wavelength slicer separates alternating adjacent input channels into first and second sets of output channels as shown in FIG.  1 . The first wavelength slicer  302  separates the network input signal  10  into a first set of channels  11  that are routed to wavelength slicer  303 , and a second set of channels  12  that are routed to wavelength slicer  304 . For example, the initial wavelength slicer  302  can separate channels based on a 50 GHz spacing between adjacent channels. 
     The first set of channels output by the initial wavelength slicer  302  is routed along a first optical path  11  to second and third stage wavelength slicers  303 ,  305 , and  306 . The second set of channels output by the initial wavelength slicer  303  is routed along a second optical path  12  to wavelength slicer  304 . The second state of wavelength slicers  303 ,  304  further separate the input channels into four sets of channels. For example, the second state of wavelength slicers  303 ,  304  separates channels based on a 100 GHz spacing between adjacent channels. This process can be continued by cascading additional stages of wavelength slicers to achieve up to 2N sets of channels, where N is the number of stages. For example, the wavelength slicer network  301  has a partial third state consisting of wavelength slicers  305  and  306 . 
     The output channels from wavelength slicer  304  exit the first wavelength slicer network  301  along optical path  13  without further processing. These output channels are referred to as “express lanes” and pass directly to the second wavelength slicer network  401  used to recombine the optical signals that are to be returned to the optical network, as will be discussed below. Optionally, wavelength slicers  304  and  404  could be eliminated so that the second set of optical signals from the initial wavelength slicer  302  would serve as the express lanes and pass directly to the final wavelength slicer  402  in the second wavelength slicer network  401 . 
     The first set of channels are further subdivided into four sets of channels that are received as inputs by an array of optical filters  307 ,  308 ,  309 , and  310 . These optical filters  307 - 310  separate the input sets of channels into an array of separated channels. The implementation shown in FIG. 4 is based on commercially-available interference filter arrays that can separate up to eight channels. However, other types of optical filters can be employed for spatially separating the channels. The type of optical filter used and the number of filters in the array are purely matters of convenience of design. As an alternative, additional slicers could be used. 
     The array of separated channels are connected to the input ports of a series of programmable optical add/drop switch arrays  500 . Here again, any number of add/drop switch arrays can be employed to handle any desired number of channels based on design requirement. 
     Each add/drop switch array  500  also has a corresponding arrays of output ports, add ports, and drop ports. The add/drop switch array  500  selectively routes channels from the input ports to its drop ports; substitutes channels from the add ports in place of the dropped channels; and routes the remaining input channels and the added channels to the output ports of the add/drop switch array  500 . 
     The array of output channels from the add/drop switch array  500  passes through a regulator  501 , which adjustably regulates the optical power level of each channel. The output channels are then combined so that they can be transmitted through a second optical link  15  in the optical network. The means for combining the output channels consists of a second array of interference filters  407 - 410  and a second wavelength slicer network  401 . These devices are inherently bi-directional, and therefore can be used to multiplex as well as demultiplex the WDM signal for the optical network. Each of the interference filters  407 - 410  in the second array combines eight channels as an inverse operation of that performed by the first array of interference filters  307 - 310 . The second array of interference filters  407 - 410  also serves to purify the spectral characteristics of the output channels and reduce cross-talk. Wavelength slicers  405 ,  406 , and  403  in the second wavelength slicer network  401  multiplex the sets of channels from the second array of interference filters  407 - 410  as an inverse operation to the demultiplexing provided by wavelength slicers  303 ,  305 , and  306  in the first wavelength slicer network  301 . Similarly, wavelength slicers  404  and  402  combine the express lanes  13  with the multiplexed channels from the wavelength slicer  403  to reconstitute the entire WDM signal for the optical network. 
     It should be understood that other means could be readily substituted to combine output channels from the add/drop switch arrays  500  and the express lanes  13  since each channel has a unique wavelength. 
     FIG. 5 depicts an add/drop smart switch  590  for use in the switch array  500 . The array would be comprised of 32 switches  590 , each of which can be individually controlled to selectively replace one of the input channels with one of the add channels. The smart switch  590  is described more fully in Applicant&#39;s U.S. patent application Ser. No. 09/326,251, entitled “FIBER OPTIC SMART SWITCH,” which is hereby incorporated herein by reference. 
     The smart switch  590  includes a polarization beam splitter  502  which is shown in Applicant&#39;s U.S. Pat. No. 6,160,665, entitled “HIGH EXTINCTION RATIO POLARIZATION BEAMSPLITTER,” which is hereby incorporated herein by reference. The beamsplitter  502  operates to pass p light (horizontal or “|”) through the beamsplitting surfaces, and deflects s light (vertical or “·”). Thus, when the input  503  and the add  504  signals are in p light, the input  503  signal is routed to the drop port  506  and the add  504  signal is routed to the output port  505 . This mode is known as the add/drop mode. When the input  503  is s light, the input  503  signal is routed to the output port  505 . This mode is known as the bypass or pass through mode. Note that in this mode there is no add signal. To properly process the signals, the switch  590  includes a collimator  507  to collimate the input/add light, and birefringent separator elements  508  to separate the light into its p and s components and to laterally displace the p and s components. 
     The birefringent element  508  is made of a material that allows the vertically polarized portion of the optical signal to pass through without changing course because they are ordinary waves in the birefringent element  508 . In contrast, horizontally polarized waves are redirected at an angle because of the birefringent walk-off effect. The angle of redirection is a well-known function of the particular materials chosen. Examples of materials suitable for construction of the birefringent elements used in the preferred embodiments include calcite, rutile, lithium niobate, YVO4 based crystals, and the like. 
     The switch also uses a halfwave plate  509  to change one of the polarization components into the other component, (as shown, the plate changes the s component into p light, however it could be placed in front of the p component) and thus all of the light incident onto the rotator is of the same polarization type. Each output/drop port includes a reverse of the input elements. Note that the halfwave plate on the output port is on the opposite branch from plate  509 . This provides balance to the system by having the light from each branch pass through the same number of optical elements. Also, note that since the smart switch  590  performs regulation, the additional regulator  501  is not needed. 
     The PBS  502  is surrounded by four polarization controllers or rotators  510 . The rotators  510  are used to control the polarization of the light received by the PBS. The voltage applied to the rotators causes the light to either pass through unrotated (15 volts) or rotated such that p becomes s and s becomes p (0 volts). For example, if the light incident onto the PBS is made to be p light, the light will pass through the PBS, and if the light is made to be s light, the light will be deflected by the PBS. The switchable polarization rotators  510 ,  514 ,  515  can be made of one or more types of known elements including parallel aligned liquid crystal rotators, twisted pneumatic liquid crystal rotators, ferro electric liquid crystal rotators, pi-cell liquid crystal rotators, magneto-optic based Faraday rotators, acousto-optic and electro-optic polarization rotators. Commercially available rotators using liquid crystal based technology are preferred. 
     The switch  590  includes a partially reflecting mirror  511  which passes about 95% of the incident light, and deflects about 5% of the light. The deflected 5% is indecent onto a detector  512  which measures the amount of light. The controller electronics  513  uses this measurement to control the rotators, as shown input rotator  514  and output rotator  515 . Note that the voltage being applied to the rotators can be varied between 0-15 volts, the voltage level determines how much of the light is rotated. 
     By controlling the voltage level to the input rotator  514 , the add/drop switch can have a drop and continue operation. For example, if the rotator  514  changes both branches of the input signal to half p and half s, then half of the input signal will be routed to the drop port  506 , and half of the input signal will be routed to the output port  505 . This operation allows for the input signal to be split, and thus shared between two network components. The operation also allows for the regulation/attenuation of the output signal  505  by shunting a portion of the input signal to the drop port. This prevents the output signal  505  from this switch to be greater than output signals from other switches in the array  500 . The add signal can be similarly controlled. Note that by controlling the output rotator  515 , the output signal can also be controlled in such a manner. 
     As shown in FIG. 5, the input signal is split into two components during processing, and recombined by a birefringent element  516  prior to outputting. The output rotator  515  can introduce both p and s polarizations into each branch of the signal. Normally, the vertical branch  517  passes through the element, and thus the output collection point  519  is located along this path. The horizontal branch  518  is deflected into the path of the vertical branch  517  for collection. However, if the vertical branch  517  has been changed to include a horizontal component, then this component is deflected out of the element and away from the collection point  519 . Similarly, if the horizontal branch  518  has been changed to include a vertical component, then this component will pass through the element and away from the collection point  519 . Thus, this rotator  505  will provide for regulation/attenuation of the output signal. The drop signal can be similarly controlled. 
     The array of add/drop switches can be formed in matrix arrangement to allow for interconnection or cross-connection of the add and drop ports. FIG. 6 depicts a 4×4 array  1000  of PBSs  700 . In front of each input path to each respective PBS is a polarization controller (not shown) which controls the polarization of the light entering the respective PBS, i.e., the controller could change the light such that the light incident onto the PBS is p light or s light. In the add/drop mode, where the input goes to drop, and the add goes to output, each of the add signals, ABCD, would go to a respective output path, 5678. Similarly, each input signal, EFGH, would go to a respective drop path, 1234. The add/drop mode is accomplished by setting the various rotators to emit only p light. 
     In the bypass mode, each input signal, EFGH, could be directed to any one of the output paths 5678. Thus, inputs EFGH could be outputted to 5678, 5867, etc. The bypass mode is accomplished by setting particular ones of the rotators to s light. For example, to have the E input switched to output 6, the input rotator for unit  1001  would be set to provide s light to the unit. The other units in the E input column would be set to provide p light such that the light would pass through the PBSs, until encountering unit  1001 . The output rotator from  1001  would be set to provide p light such that the light passes through the units of the output 6 row, and consequently be delivered to output 6. Thus, the various rotators are used to control the polarization of the light that is incident onto the various PBSs of the switch module, and thus control their connection. 
     Note that the various combinations of the switch and pass through states can be achieved. For example, input E could be routed to drop, inputs FGH could be routed to outputs 567, respectively, and add D could be routed to output 8. As general rules, input E can be routed to any of 15678, input F can be routed to any of 25678, input G can be routed to any of 35678, input H can be routed to any of 45678, while input A can be routed to 5, input B can be routed to 6, input C can be routed to 7, and input D can be routed to 8. Note that the 4×4 arrangement is by way of example only, as the PBS can be arranged in a N×M array, see “N×M DIGITALLY PROGRAMMABLE OPTICAL ROUTING SWITCH USING HIGH EXTINCTION RATIO POLARIZATION BEAM SPLITTER,” Applicant&#39;s U.S. patent application Ser. No. 09/326,250, which is incorporated herein by reference. The switch described above is actually more than 4×4, as defined by the conventional definition. It has a total of 16 ports, 4 input and 4 output ports are cross connected. The other set of 4/4 input/output ports are used for add/drop operation. This feature is not available in the prior art. Note that each of the switches in the array may be a smart switch, and thus have its respective rotators controlled by varying the applied voltage based on the output (or drop) light. 
     FIG. 7 depicts an arbitrary add/drop switch  1100 . This switch is comprised of two 4×4 modules of FIG. 6, note that only one set of input/output elements are needed. This arrangement permits any input signal ABCD to be dropped to any drop path 1234. Similarly any add signal EFGH can be delivered to any output path 5678. Also the input signals ABCD can be passed through to their respective output paths 5678. Again, each of the switches in the array may be a smart switch, and thus have its respective rotators controlled by varying the applied voltage based on the output (or drop) light. This arrangement allows for a mesh type connection, as shown in FIG. 13, and thus is useable with internet protocol (IP). Consequently, IP protocol data can be routed over WDM. Also, as demonstrated by FIG. 4, different amounts of bandwidth can be allocated to different areas. For example, if the local area needs more bandwidth, then the express lanes could be routed into the local area. If less bandwidth is needed, then more express lanes could be created. This is shown in FIG. 13 by the different thickness of lines, including higher bandwidths (or smaller signal spacing, e.g. 50 GHz) and numbers of lines (numbers of fibers). 
     FIG. 8 depicts the arbitrary add/drop switch of FIG. 7 with wavelength conversion. In addition to operations described in FIG. 7, each input signal λ1, λ2, λ3, λ4, can be routed to any of the detectors  1101 , where each signal is converted into an electrical signal and processed by electronics  1102 . The output electrical signal are then sent to respective lasers  1103  for retransmission as light. Each light output from the lasers can be routed to any of the output paths. Again, each of the switches in the array may be a smart switch, and thus have its respective rotators controlled by varying the applied voltage based on the output (or drop) light. 
     Each of the types of add/drop switches would provide the network of FIG. 4 with different operating capabilities. Note that other add/drop switches as described in Ser. No. 09/326,256 entitled “OPTICAL ADD/DROP WAVELENGTH SWITCH USING A HIGH EXTINCTION RATIO POLARIZATION BEAMSPLITTER,” which is incorporated herein by reference, could be used herein as array  500 . 
     Also note that the network of FIG. 4 could be protected according to the fault handling mechanisms described in Applicant&#39;s U.S. Pat. No. 5,115,155, entitled “SYSTEM FOR DEALING WITH FAULTS IN AN OPTICAL LINK,” filed Nov. 12, 1998, which is incorporated herein by reference. This type of OS protection is useful for handling IP protocol data. Layer  3  protection is provided by the internet protocol. 
     FIG. 9 depicts a drop/add switch for wavelength management and restoration. Note that FIG. 9 includes four fibers  1201 , and four 4×4 modules  1203 . Note that the number of fibers is by way of example only, as more fibers would merely require a scaled drop/add switch. Also note that the modules are shown as being only partially connected to the DEMUXes and MUXes to simplify the figure. Further note that λ1, λ2, λ3, and λ4, can be viewed as λb (blue), λg (green), λy (yellow), and λr (red). Each fiber carries different signals which are encoded by wavelength, e.g., λ1, λ2, λ3, λ4. These signals are demultiplexed by DEMUX  1202 . Each wavelength from each fiber is provided to a particular 4×4 module, e.g. λ1 from each of the four fibers is provided to module  1203 . Within each module  1203 , the operations are as described with respect to FIG.  6 . Thus, particular signals can be dropped or added or re-routed to the output. Note that the diagonal slash through each module indicates the orientation of the PBSs within the module. For a further discussion on multiple wavelength management, see related application “MULTI-WAVELENGTH CROSS CONNECT OPTICAL NETWORK,” Applicant&#39;s U.S. Pat. No. 6,005,697, which is incorporated herein by reference. 
     FIG. 10 depicts a multi-ring network system  2000  connecting seven nodes A, B, C, D, E, F, and G. Note that the ring connecting DEFG is connected by an OS-200-4 line, while another OS-200-4 line ring connects ADC, without connecting B. The ABC ring is connected by an OS-200-2 line. The OS-200-1 ring connects ABCD, and OS-200-3 ring connects nodes BCD. The various switches described above allow this network to have express connections, to perform local add/drops of signals. For example, node E could comprise the switch shown in FIG. 6, where the OS-200 is sliced down to four OS-800 signals and can perform add/drops to those signals. Node D could comprise the wavelength cross-connect switch shown in FIG. 11, each OS-200-4 signals can be divided into four OS-800 signals. Note that FIG. 9 would be simplified in this case, as it would have four 4×4 switches with two input DEMUXs. The two OS-200-4 fibers would be inputted into the input DEMUXs, and processed into the switches from there. An OS-800 signal from one of the fibers can be connected into the other input line, or other signals can be added into the lines, etc. 
     As shown in FIG. 12 a cross-connected node, such as node D in FIG. 10, can facilitate empty slots being filled via wavelength slot interchange. For example, train  4000  has empty slots  4001 ,  4002 . These may be filled via an add/drop switch with wavelength conversion as shown in FIG.  8 . In other words, a signal can have its wavelength changed, and be added into an empty slot. Also the wavelength cross-connect switch shown in FIG. 9 can be used to move a signal with the same wavelength from a different train in the empty slot. 
     The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims. 
     Other types of frequency multiplexers and demultiplexers could be readily substituted in place of the wavelength slicers  101 - 104  shown in FIGS. 1-3. For example, a set of polarization rotators and a polarized beamsplitter can be used to combine the first and second sets of channels, in place of wavelength slicers  101  and  102  in terminal  1 . Demultiplexing can be accomplished by filters or diffraction gratings, although such approaches would tend to be less efficient and more expensive. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.