Abstract:
A versatile, wavelength-slicing methodology, referred to herein as spectrum division multiplexing (SDM), provides new avenues and technologies for optical communication applications. Specifically, SDM separates a composed optical signal into a group of output spectra. Each output spectrum carries a multiple of optical communication signal channels. The bandwidth of each channel and spacing between adjacent channels may differ from one output spectrum to another. A critical building block of SDM technology is a spectrum filter with periodic passbands, referred to herein as an optical spectrum synthesizer (OSS). The cascade of OSS devices, the combinations of OSS with prior art components and modules, and other new devices to be used in conjunction with OSS, lead to new spectrum devices that add new dimensions to existing and new optical network architectures. The invention of OSS leads to new Spectrum Division Multiplexing and management devices based on cascading OSS devices. Examples of these devices include Spectrum Exchanger, Spectrum Multiplexer, Spectrum Demultiplexer and Spectrum Add Drop Module. The combinations of OSS and other prior art devices also lead to several new Spectrum devices and modules. Examples of these include, Spectrum Switch, Spectrum Cross-Connect and Spectrum Long Haul Transport Modules. Other devices designed to be used in conjunction with OSS, e.g., 1/n Multiplexer and 1/n Demultiplexer, can also be used to form new devices and modules. In SDM methodology, spectra (group of channels), instead of single channels, are managed collectively thereby offering both flexibility and efficiency for next generation high channel count optical networks.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of pending applications Ser. No. 09/573,330 filed May 18, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of optical communications and more particularly to a method and apparatus for symmetric and asymmetric wavelength/spectrum slicing for use in dense wavelength division multiplexing (DWDM) applications.  
           [0004]    2. Background Art  
           [0005]    Optical communications is an active area of new technology and is crucial to the development and progress of several important technologies, e.g., Internet and related new technologies. A key technology that enabled higher data transmission rate is the dense wavelength division multiplexing (DWDM) technology. In the DWDM technology, optical signals generated from different sources operating at predetermined, densely spaced center wavelengths, are first combined to form a single optical output. This single optical output is then transmitted, frequently amplified during transmission, through an optical fiber. The single optical output is then de-multiplexed, a process to separate individual data channels and each channel is then directed to its own destinations. In the DWDM technology, each data channel is assigned to a center frequency and the spacing between any two adjacent channels is a constant (e.g., 200 GHz or 100 GHz, per ITU standard). It is also understood that all channels are given frequency windows with identical widths. The width of these windows is kept great enough to pass information associated with these data channels and at the same time as narrow as possible to prevent cross-talk between different data channels. It is generally understood that the narrower the frequency spacing between different data channels, the greater the transmission capacity a DWDM system will have at a given bit rate.  
           [0006]    Several multiplexing and de-multiplexing devices are essential to the operation of a DWDM system. FIG. 1A is a diagram illustrating the operation of a group of devices known as optical filters. An optical filter ( 100 ) has the function of separating signals within a predetermined frequency window ( 104 ) from the input spectrum ( 102 ). The remaining signals are output as OUT 2  ( 106 ). In a DWDM system, to de-multiplex composite data, an optical filter is employed to separate signals associated with a particular data channel as depicted in FIG. 1A. Because each channel requires a specific filter, a DWDM de-multiplexer will require n optical filters in cascade in order to separate all of n channels into separate outputs. Using these filter cascades in the reverse direction will enable the construction of a multiplexer with which individual signal channels with different center wavelengths, can be combined together to form a single composite optical output signal. There are several types of optical filters and brief descriptions are provided for two types of commonly available filters. In FIG. 1B, a filter made with optical fiber, known as fiber Bragg grating (FBG) ( 110 ), is illustrated. In a FBG, the index of refraction of the optical fiber is periodically modified. The period of the modification, d, is related to the center wavelength λ m  of the given filter as λ m =2 n d/m. Where m is the order of the Bragg grating and n is average of the index of refraction of the fiber. Another type of filter frequently used in DWDM systems is a multi-layer interference filter ( 120 ). These filters are constructed with several, sometimes many layers of different optical materials with varying thickness such that a desired transmission (or reflection) curve centered near a predetermined channel center-frequency is obtained as depicted in FIG. 1C.  
           [0007]    In the filter approach to DWDM, each data channel is associated with a specific optical filter. The DWDM system therefore consists of many filters, each of which has to be connected or placed in a particular location and/or orientation. A more systematic way to construct a DWDM system is to use wavelength dispersion devices such that many channels can be multiplexed or de-multiplexed with a single device. In FIG. 2A, a device commonly known as an arrayed waveguide grating (AWG) ( 200 ) is displayed. As depicted, these AWG can be used to separate all data channels simultaneously. The output channels ( 204 - i ) can be connected directly to individual optical fibers. When using an AWG in the reverse direction, many different signal channels can be combined into a single optical fiber. A prism ( 210 ) can also be used to multiplex or de-multiplex optical signals. As illustrated in FIG. 2B, due to dispersion, i.e., the index of refraction is different for different frequencies so that the exit angle is different for channels having different center frequencies. Different output channels ( 214   i ) are separated in space and connected into individual fibers. Another commonly used device is a diffraction grating ( 220 ), an optical surface which is modified periodically (with a period d) such that when light is directed to this surface, the angle of incidence (α) and diffraction (β) are related to the wavelength of the incoming light, λ according to: d (sin α+sin β)=mλ, where m is an integer commonly referred as the order of diffraction. Such a diffraction grating is illustrated in FIG. 2C.  
           [0008]    A third type of wavelength separating and combing devices are known as interleavers. FIG. 3A provides a function diagram of an interleaver ( 300 ). These interleavers separate a composite optical signal ( 302 ) into two complementary signals in which the odd data channels are branched into one output ( 304 ) and the even channels are directed into the other output ( 306 ). In an interleaver application, the frequency space is divided into two parts, 50% for output  1  and 50% for output  2 , as illustrated in FIG. 3B. Two typical interleaver devices are depicted in FIG. 3C and FIG. 3D. In FIG. 3C, an interleaver design based upon a Gires-Toumois (GT) mirror and a Michelson interferometer is displayed ( 320 ). This prior art interleaver was first described by Dingel and Izutsu in a publication (Optics Letters, Jul. 15, 1998, vol 23, pages 1099-1101) and is incorporated herein by reference as relevant background material. In this device, the input signal ( 322 ) is coupled to a 50% beam splitter ( 321 ) through a collimating lens ( 329 ). A GT mirror ( 325 ) and a regular mirror ( 327 ) are used to form the interferometer. The odd channels return to one output fiber ( 324 ) through a lens ( 329 ) whereas the even channels return to the other fiber ( 326 ) through another lens ( 329 ). This type of interleaver and related devices have been disclosed in a recent U.S. Pat. (No. 6,169,626 issued Jan. 2, 2001). This patent is also incorporated herein by reference as relevant background material. In FIG. 3D, another prior art interleaver ( 330 ) based on a 50% beam splitter and a GT mirror is displayed. This prior art device has been disclosed recently in U.S. Pat. No. 6,169,604 issued on Jan. 2, 2001 to Cao. This patent is therefore incorporated herein by reference as relevant background material. In this prior art device, the input signal ( 332 ) is coupled to a 50% beam splitter ( 331 ) through a collimating lens ( 339 ). Two sections of a phase modified GT mirror ( 335 ) are used as two mirrors of the interferometer. The odd channels return to one output fiber ( 334 ) through lens ( 339 ) whereas the even channels return to the other fiber ( 336 ) through another lens ( 339 ).  
           [0009]    These prior art interleavers can provide some flexibility to DWDM system designers and engineers. In FIG. 4, two stages of interleavers ( 400 ,  410 ,  420 ) are cascaded to provide four outputs ( 414 ,  416 ,  424 ,  426 ) each carrying one fourth of the original data channels. The frequency spacing of the adjacent data channels for a particular output is therefore four times the spacing between adjacent data channels in the input signal ( 402 ). Another practical configuration, as demonstrated in FIG. 4B, utilizes both the interleaver ( 430 ) and wavelength dispersion devices ( 440 ,  450 ). In this configuration, the optical alignment and/or temperature stability requirements for the dispersion devices are significantly less stringent when the channel spacing is increased to twice that of the original spacing. In a different configuration shown in FIG. 5, an interleaver ( 500 ), or a two-stage cascade of interleavers, is followed by individual filters. In this configuration, filters with a larger channel spacing and hence lower tolerance (e.g., 200 GHz filters) can be used to construct DWDM systems with a smaller channel-spacing (e.g., 100 GHz or 50 GHz).  
           [0010]    In many optical network applications, one needs to separate a group of signal channels and redirect these channels. This is accomplished via prior art add-drop modules. In FIG. 6, a DWDM long haul system ( 600 ) with multiple add-drop channels is illustrated. The optical signals of different center wavelengths ( 602 ) are combined through a DWDM multiplexer ( 603 ) and amplified via  605 . At a branching point ( 606 ), a group of channels is dropped through add-drop modules, and replaced with signals from alternate sources. This modified composite signal is transferred to a demultiplexer, separated into individual channels and sent to their corresponding receivers ( 608 ).  
           [0011]    There are several prior art add/drop module designs. In FIG. 6B, a particular prior art design ( 610 ) utilizing interference filters is illustrated. The incoming signal ( 612 ) is directed to the first interference filter ( 614 ) where signals associated with the channel to be dropped pass through as the drop output ( 613 ). The remaining signal channels reflect from the first filter ( 614 ) to the second filter ( 615 ), and are combined with the add input ( 616 ) to form the output ( 618 ).  
           [0012]    One of the disadvantages of this prior art add/drop methodology is that when a group of signal channels is added and dropped, many filtering components and modules must be used. Frequently, DWDM Multiplexers and Demultiplexers are also required. There is therefore a need for a single device that can be used to accomplish the multichannel add-drop function in a single step.  
         SUMMARY OF THE INVENTION  
         [0013]    In accordance with the present invention, a new methodology for optical network data transport and routing is disclosed. In this Spectrum Division Multiplexing (SDM) method, the dense signal channels are arranged into groups of channels (spectra) and are transported accordingly. A critical enabling component of SDM is a spectrum filter, Optical Spectrum Synthesizer (OSS). OSS separates a composite, multi-channel optical communication signal into two groups of channels. Each output signal has a different spectrum that allows the selection of a different group of channels or the passage of different frequency regions of the original optical spectrum. Specifically, each spectrum can be characterized as comprising periodic pass bands. The width and period of the pass bands can be designed to accommodate specific network requirements. The two output spectra are complements of each other, but may have different pass bandwidths. An OSS can be used to separate two groups of channels having different OC protocols requiring different bandwidths, e.g., one output is used to pass OC-192 channels whereas the other is used to pass OC-768 channels. With a modification to the OSS, a Spectrum Exchanger (SE) is formed. The SE has the function of exchanging two groups of channels of two input signals and can be used as an Optical Spectrum Add/Drop (OSAD) device. An OSAD module provides the network system designer with a means to add and drop a group of signal channels collectively. A Spectrum De-Multiplexer (SDEMUX), constructed by cascading n OSS devices, separates a composite multi-channel optical signal into n spectra each containing a different subgroup of the incoming channels. The SDEMUX has a similar functional structure in comparison with DEMUX devices used in prior DWDM technology. Instead of having outputs each carrying an individual signal channel, each output of SDEMUX carries a subgroup of channels. The individual channels contained in a particular output of SDEMUX can be further separated using a 1/n DEMUX where the separation between adjacent channels is n times the spacing of a prior art DEMUX. Similarly, a Spectrum Multiplexer (SMUX) is obtained by using the SDEMUX in the reverse direction. A 1/n MUX can be constructed by using a 1/n DEMUX in the reverse direction. In an additional embodiment, a long haul transmission system is disclosed which utilizes SMUX, SDEMUX and EDFA devices. An alternate long haul system is also disclosed consisting of 1/n MUX, 1/n DE-MUX and EDFA devices. An OSAD Module can also be implemented with a cascade of two OSS devices. The combination of a SDEMUX with an optical switch allows the formation of a Spectrum Switch (SS) where different groups of signal channels can be switched simultaneously. The SS can be connected to form a Spectrum Cross-Connect in a way similar to the construction of a conventional optical cross-connect using conventional optical switches. Another device comprises two (or more) OSS devices connected with a branch coupler. Such a device maximizes the usage of frequency space and hence can be used to achieve a higher overall data throughput rate in a network system. In still another embodiment of the invention, a Spectrum Processor is disclosed in which flexible usage of the frequency space is enabled by dividing that frequency space to accommodate different OC protocols and provide a group of channels all within a specific frequency window and with a different channel spacing and width. The term Nano-T™ as used herein is a trademark used on a product of the assignee of the present invention. The Nano-T™ product is described in co-pending application Ser. No. ______ filed on Feb. ______, 2001 and that application is hereby incorporated herein in its entirety by reference.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:  
         [0015]    [0015]FIGS. 1A through 1C (prior art) are simplified diagrams illustrating conventional filters and their use in DWDM technology. FIG. 1A is a block diagram illustrating the operation of a generic filter device. FIG. 1B depicts a fiber Bragg grating filter. FIG. 1C represents a multi-layer interference filter;  
         [0016]    [0016]FIGS. 2A through 2C (prior art) are simplified diagrams illustrating conventional dispersion multi-channel devices and their use in DWDM technology. FIG. 2A is a diagram illustrating the operation of an arrayed waveguide grating (AWG) device. FIG. 2B represents a prism wavelength dispersion device. FIG. 2C shows the operation of a conventional grating device;  
         [0017]    [0017]FIGS. 3A through 3D (prior art) are simplified diagrams illustrating conventional interleaver devices and their use in DWDM technology. FIG. 3A is a block diagram illustrating the operation of an interleaver. FIG. 3B displays the output frequency spectra associated with two output signals. FIG. 3C shows an interleaver based on a GT mirror and a regular mirror and FIG. 3D depicts the operation of an interleaver based upon a GT mirror and a Michelson interferometer;  
         [0018]    [0018]FIGS. 4A through 4B (prior art) are schematic diagrams illustrating DWDM applications utilizing interleavers. FIG. 4A is a block diagram of three interleavers in a cascade. The four outputs each carries ¼ of the signal channels from the original composed input signal. FIG. 4B is a schematic diagram illustrating the combination of an interleaver and two multi-channel dispersion devices (prisms);  
         [0019]    [0019]FIG. 5 (prior art) depicts a device composed of interleaver and filters. Each output of the device carries only one signal channel;  
         [0020]    [0020]FIGS. 6A through 6B (prior art) are diagrams illustrating a multichannel add/drop function in an optical network. FIG. 6A depicts a multichannel add/drop arrangement in a long haul system. FIG. 6B shows a filter based add-drop module;  
         [0021]    [0021]FIGS. 7A through 7C are diagrams illustrating the methodology of SDM and the operation of a versatile interleaver, OSS, according to embodiments of the present invention. FIG. 7A is diagram illustrating a systematic way of grouping signal channels into spectra. FIG. 7B is a block diagram of an OSS and FIG. 7C displays the spectra associated with output signals;  
         [0022]    [0022]FIGS. 8A through 8C are diagrams illustrating the construction of OSS based upon a design with two Nano Tuner (Nano-T) GT mirrors and a 50% beam splitter. FIG. 8A displays a generic OSS. FIG. 8B depicts a modified OSS device having two inputs and two outputs. FIG. 8C shows an OSS constructed with a polygon 50% beam splitter and two Nano-T reflectors;  
         [0023]    [0023]FIGS. 9A through 9C are diagrams illustrating the function of Spectrum Exchanger (SE) in accordance with embodiments of the present invention wherein FIG. 9A displays the operation of a Spectrum Exchanger. FIG. 9B illustrate a symbol for this device. FIG. 9C shows the operation of an Optical Spectrum Add/Drop module based on the SE;  
         [0024]    [0024]FIGS. 10A through 10B are diagrams depicting the separation of a composite optical signal into two outputs of signals carrying different protocol channels;  
         [0025]    [0025]FIG. 11 is a diagram illustrating a Spectrum DeMultiplexer (SDEMUX) constructed with three OSS devices;  
         [0026]    [0026]FIGS. 12A and 12B are 1/3 DEMUX and 1/3 MUX devices according to the present invention;  
         [0027]    [0027]FIGS. 13A and 13B are diagrams illustrating long haul systems according to the present invention. In FIG. 13A a system using 1/n MUX, EDFA and 1/n DEMUX is depicted whereas in FIG. 13B a system based on SDEMUX, EDFAs and SMUX is shown;  
         [0028]    [0028]FIGS. 14A through 14C are diagrams illustrating Spectrum Add-Drop Module and one application based on the present invention. In FIG. 14A an SADM constructed using two OSS is displayed whereas in FIG. 14C a system using SADM is displayed, a symbol for this device is illustrated in FIG. 14B;  
         [0029]    [0029]FIGS. 15A and 15B are diagrams illustrating the construction of a 1×4 Spectrum Switch. In this case, a 4×4 switch follows an SDEMUX to allow flexible redirection of subgroups of signal channels, FIG. 15B illustrates a symbol for this device;  
         [0030]    [0030]FIG. 16 is a diagram which shows the construction of a 4×4×4 Spectrum Cross-Connect. Eight SS are connected to form this SCC;  
         [0031]    [0031]FIGS. 17A and 17B are diagrams illustrating a module for which overlapping spectra were generated as the outputs. Device of this type can be used to maximize the net data throughput rate by allowing certain amount of crosstalk between adjacent channels; and  
         [0032]    [0032]FIGS. 18A and 18B are diagrams which illustrate a Spectrum Processor in accordance with an embodiment of the present invention wherein FIG. 18A illustrates the frequency space usage and FIG. 18B illustrates the structure of a Spectrum Processor module.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    In the following the details of various preferred embodiments of the present invention are disclosed. The preferred embodiments are described with the aid of the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
         [0034]    [0034]FIGS. 7A through 7C are diagrams illustrating the methodology of spectrum division multiplexing (SDM) and the operation of a versatile interleaver, referred to herein as an Optical Spectrum Synthesizer (OSS), according to embodiments of the present invention. Moreover, hereinafter, the terms “spectrum filter”, “asymmetric interleaver”, “1/n interleaver”, “spectrum splitter” are used interchangeably to describe various embodiments of the present invention.  
         [0035]    In FIG. 7A, a systematical way of organizing a collection of signal channels into smaller groups is disclosed. In the particular example illustrated in FIG. 7A, sixteen signal channels are divided into four groups of channels, referred to herein as four Spectra. Moreover, hereinafter, the terms “spectrum”, and “spectra” are used interchangeably with “group of signal channels”, and “groups of signal channels” to describe various embodiments of the present invention.  
         [0036]    In FIG. 7B, an OSS preferably has two outputs. One output has a group of broader periodic pass bands with a predetermined bandwidth and period as depicted in FIG. 7C. The other output has a group of narrower periodic pass bands, which complements that of output  1 . The labels of output  1  and  2  are not critical and the outputs can also be labeled as N and B for narrow and broad output. When the bandwidth of the N output is set to be identical to that of the output B, the device becomes a conventional interleaver as displayed in FIGS. 3A through 3D.  
         [0037]    Referring now to FIG. 8A, a preferred embodiment of an OSS ( 800 ) comprises a 50% broadband non-polarizing beam splitter ( 803 ) and two Nano-T GT mirrors ( 805  and  807 ). The thickness of each optical cavity of the Nano-T GT mirrors is predetermined to obtain desired output spectra with proper channel spacing and/or wavelength separation. The incoming light ( 802 ), preferably a parallel beam with a small angular divergence, is directed to the beam splitter ( 803 ) at a predetermined incident angle with respect to the surface normal of  803 . The branched beam then enters two Nano-T GT mirrors where upon exiting, the phases of the light beams are modified. The two light beams are then recombined and re-branched to form two outputs. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a graded index lens known as a GRIN lens. In one preferred embodiment, the reflective surfaces of the Nano-T GT mirrors have reflectivities of approximately 18%, and 99.5% respectively.  
         [0038]    In order to match the center frequencies of the pass bands of output  1  and  2  to that of a standard communication grid (e.g., ITU grid), the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one or both of the optical cavities may be constructed with piezoelectric materials such that the free-spectra-range of each of the optical cavities may be controlled. Another preferred way to adjust the free-spectra-range of the “air-spaced” GT mirror is to set and control the gas mixture and the pressure of the “air-spaced” cavity. The temperature environment may also be controlled in a way to enhance the performance of the OSS. One or more electrical heaters and coolers are placed close (within a few decimeters) to the two optical cavities to ensure best performance. The temperature sensitivity of the etalon can be reduced by using material with low thermal expansion. Temperature is important because typically a 1 degree C. change in temperature can have an effect on the critical product of width and index of refraction comparable to the required precision to achieve the desired outputs.  
         [0039]    Referring now to FIG. 8B, a preferred embodiment of a modified OSS ( 810 ) comprises a 50% broadband non-polarizing beam splitter ( 813 ), two Nano-T GT mirrors ( 815  and  817 ). The thickness of each optical cavity of the Nano-T GT mirrors is predetermined to obtain desired output spectra with proper channel spacing and/or wavelength separation. Two incoming light beams ( 811 ,  812 ), preferably parallel beams with small angular divergences, are directed to the beam splitter ( 813 ) at predetermined incident angles with respect to the surface normal. The branched beams then enter two Nano-T GT mirrors where upon exiting, the phase of the light beams are modified. The two light beams are then recombined and re-branched to form two outputs. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a graded index lens known as a GRIN lens. In one preferred embodiment, the reflective surfaces of the Nano-T GT mirrors have reflectivities of approximately 18%, and 99.5% respectively.  
         [0040]    In order to match the center frequencies of the pass bands of output  1  and  2  to that of a standard communication grid (e.g., ITU grid), the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one or both of the optical cavities may be constructed with piezoelectric materials such that the free-spectra-range of each of the optical cavities may be controlled. Another preferred way to adjust the free-spectra-range of the “air-spaced” GT mirror is to set and control the gas mixture and the pressure of the “air-spaced” cavity. The temperature environment may also be controlled in a way to enhance the performance of the OSS. One or more electrical heaters and coolers are placed close (within a few decimeters) to the two optical cavities to ensure best performance. The temperature sensitivity of the GT mirrors can be reduced by using material with low thermal expansion. Temperature is important because typically a 1 degree C. change in temperature can have an effect on the critical product of width and index of refraction comparable to the required precision to achieve the desired outputs.  
         [0041]    Referring now to FIG. 8C, a preferred embodiment of a modified OSS ( 820 ) comprises a 50% broadband non-polarizing beam splitter ( 823 ) and two Nano-T GT mirrors ( 825  and  827 ). The thickness of each optical cavity of the Nano-T GT mirrors is predetermined to obtain desired output spectra with proper channel spacing and/or wavelength separation. Two incoming light beams ( 821 ,  822 ), preferably parallel beams with small angular divergences, are directed to the beam splitter ( 823 ) at predetermined incident angles with respect to the surface normal. The branched beams then enter two Nano-T GT mirrors where upon exiting, the phase of the light beams are modified. The two light beams are then recombined and re-branched to form two outputs  824  and  826 . These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a graded index lens known as a GRIN lens. In one preferred embodiment, the reflective surfaces of the Nano-T GT mirrors have reflectivities of approximately 18%, and 99.5% respectively.  
         [0042]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication grid (e.g., ITU grid), the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one or both of the optical cavities may be constructed with piezoelectric materials such that the free-spectra-range of each of the optical cavities may be controlled. Another preferred way to adjust the free-spectra-range of the “air-spaced” GT mirror is to set and control the gas mixture and the pressure of the “air-spaced” cavity. The temperature environment may also be controlled in a way to enhance the performance of the OSS. One or more electrical heaters and coolers are placed close (within a few decimeters) to the two optical cavities to ensure best performance. The temperature sensitivity of the GT mirrors can be reduced by using material with low thermal expansion. Temperature is important because typically a 1 degree C. change in temperature can have an effect on the critical product of width and index of refraction comparable to the required precision to achieve the desired outputs.  
         [0043]    Referring now to FIG. 9A, the modified OSS disclosed above may be used to exchange a portion of the signal channels based upon another preferred embodiment  900  of the present invention. A periodic passband contained in input signal  1  ( 901 ) will be directed to outputI ( 904 ) whereas the complementary periodic passband will be directed to output  2  ( 906 ). For signal channels contained in input signal  2  ( 902 ), the corresponding periodic passbands will be directed to output  1  and  2 , to fill the vacated regions of the full spectra. The net effect is that the two spectra contained in inputs  1  and  2  are exchanged at the outputs  1  and  2 .  
         [0044]    Referring now to FIG. 9B, the modified OSS disclosed above may also be used to perform add/drop function in a single step according to a preferred embodiment  910  of the present invention. A periodic passband contained in input signal ( 912 ) will be dropped to outputl ( 914 ) whereas the complementary periodic passband will be directed to output ( 916 ). The added signal channels are sent through the spectrum add port ( 911 ). These periodic passbands will be directed to the output ( 916 ) fiber.  
         [0045]    In FIG. 10A, an OSS preferably has two outputs. One output has a group of broader periodic pass bands with a predetermined bandwidth and period as depicted in FIG. 10B. The other output has a group of narrower periodic pass bands, which complements that of output  1 . The labels of output  1  and  2  are not critical and the output can be better labeled as N and B outputs. When the bandwidth of the N output is set to be identical to that of B, the device becomes a symmetrical interleaver as displayed in FIGS. 3A through 3D. In a preferred embodiment, a phase correction element and spectrum filter element may also be introduced to each output to enhance the OSS performance. In a preferred embodiment of the present invention, one of the outputs is used to carry channels with one OC protocol, e.g., OC-192, the other output is used to carry channels of a different OC protocol to best utilize the frequency space and maximize the data throughput rate. In a different embodiment of the present invention, different OC protocols may be carried in one or both of the outputs.  
         [0046]    [0046]FIG. 11A is a diagram illustrating a one to four Spectrum De-Multiplexer (SDEMUX) constructed with three OSS devices. In a preferred embodiment of the present invention, three OSS devices are in a cascade with appropriate spectrum filters and/or phase correction elements to form a SDEMUX. In a preferred embodiment of the present invention, the optical spectrum is evenly divided into four complementary spectra with the same pass channel bandwidths. In a different preferred embodiment of the present invention, the optical spectrum is divided into four complementary spectra having different pass channel bandwidths. In another preferred embodiment of the present invention, the number of the optical spectra or output groups, n, is greater than one. When n is equal to two, the SDEMUX is simply an OSS, whereas when n is equal to four, the SDEMUX device is as illustrated in FIG. 11A. In additional embodiments of the present invention, a particular SDEMUX can be used in the reverse direction as a SMUX. In these cases, n different and complementary spectra are combined through a SMUX to form a single composite output signal. FIG. 11B illustrates a proposed symbol  1100  for a SDEMUX.  
         [0047]    Referring now to FIGS. 12A through 12B, a group of three 1/3 DEMUX and a group of three 1/3 MUX are illustrated, respectively. According to a preferred embodiment of the present invention, n 1/n DEMUX devices and n 1/n MUX devices are constructed for a SDEMUX or SMUX device. Each 1/n DEMUX (and 1/n MUX) carries a subgroup consisting of 1/n of the total number of channels. In a different preferred embodiment of the present invention, each 1/n DEMUX (and 1/n MUX) carries a spectrum, which uses a fraction of the whole frequency space, and in certain cases this fraction may be set to 1/n. The 1/n DEMUX inputs  1201 ,  1222  and  1242  produce outputs  1204 ,  1224  and  1244 , respectively. The 1/n MUX inputs  1212 ,  1232  and  1252  produce outputs  1214 ,  1234  and  1254 , respectively.  
         [0048]    [0048]FIG. 13A is a diagram illustrating a long haul system according to a preferred embodiment  1300  of the present invention wherein grouped input signals  1362 ,  1372 ,  1382  and  1392  are transported to grouped output signals  1364 ,  1374 ,  1384  and  1394 . In this case, a long haul system is formed using a SDEMUX  1304 , n optical fibers  1310 ,  1320 ,  1330  and  1340 , EDFAs (Erbium Doped Fiber Amplifiers) and a SMUX  1306 . Due to a much larger channel spacing compared with a conventional long haul system using only one optical fiber or several optical fibers with broadband filters, nonlinear effects are significantly reduced. A much higher optical power can therefore be lunched into each of the n fibers thereby significantly increasing the distances between amplification and/or recondition stations. FIG. 13B is a diagram illustrating a long haul system according to a preferred embodiment of the present invention. In this case, a long haul system is assembled using n 1/n-MUX, n optical fibers  1360 ,  1370 ,  1380  and  1390 , EDFAs and n 1/n-DEMUX devices. Due to a much larger channel spacing compared with a conventional system using fewer optical fibers, nonlinear effects are significantly reduced. A much higher optical power can therefore be launched into each of the n fibers thereby significantly increasing the distances between amplification and/or recondition stations. In a different embodiment of the present invention, a combination of conventional DWDM devices, SMUX, SDEMUX, 1/n MUX, 1/n DEMUX and EDFA devices are arranged in a way to achieve a long haul transport system consisting of more than one fiber to transport the composed signal spectrum with a larger channel spacing in each of the fibers.  
         [0049]    Referring now to FIGS. 14A, 14B and  14 C, an Optical Spectrum Add-Drop module (OSAD)  1400  is assembled using two OSS based upon a preferred embodiment of the present invention. A group of signal channels can be added and removed simultaneously. This device can be used to direct network data traffic in a collective way. In another preferred embodiment of the present invention, the status of many channels can be monitored using a SADM in a parallel way to speed up network data management and routing. FIG. 14B depicts a proposed symbol for this new device and FIG. 14C illustrates a long haul implementation using the SADM. Input spectrum signal  1402  has spectrum signal  1406  dropped and spectrum signal  1408  added to produce output spectrum signal  1404  in FIGS. 14A and 14B.  
         [0050]    In FIG. 14C, input spectrum signals  1453  are combined in SMUX  1454  with input spectrum signals  1432  from 1/4 MUX  1430 . The dropped spectrum signals  1448  via 1/4 DEMUX  1440  and leave output spectrum signals  1458  at SDEMUX  1456 .  
         [0051]    [0051]FIGS. 15A and 15B disclose a preferred construction of a 1×4 Spectrum Switch (SS). In this case, a 4×4 optical switch  1520  follows an SDEMUX  1510  that allows flexible redirection of subgroups of signal channels  1508 . In other preferred embodiments, lxn SS is constructed with the combination of 1 to n SDEMUX and an nxn optical switch. FIG. 15B illustrates a proposed symbol for the spectrum switch  1500 .  
         [0052]    Referring now to FIG. 16, a 4×4×4 Spectrum Cross-Connect (SCC)  1600  is disclosed. The construction of this SCC has a similar structure in comparison with a conventional optical cross-connect where different channels in a conventional cross connect are replaced by subgroups of channels in a SCC. According to a preferred embodiment of the present invention, eight 1×4 SS are connected to form this SCC. A general n×n×m SCC uses 2 n 1×m SS connected in a way similar to a conventional n×n×m optical cross connect. Inputs  1 - 4  ( 1601 ,  1602 ,  1603  and  1604 ) are cross connected to become outputs  1 - 4  ( 1605 ,  1606 ,  1607  and  1608 ).  
         [0053]    [0053]FIGS. 17A and 17B are diagrams illustrating a module  1700  and spectra for which overlapping spectra input  1702  are passed as the outputs  1714 ,  1716 ,  1724  and  1726 , according to a preferred embodiment of the present invention. A wavelength insensitive branch coupler is used to branch the original composed data into two or several parts. An OSS is then used to split the composed signal into two spectra. These spectra are used in a collective way to process and pass data at a higher throughput rate than conventional methods by allowing certain degrees of crosstalk between adjacent channels. The crosstalk between adjacent channels is then removed through electronic and/or optical decoding of the original data.  
         [0054]    In another preferred embodiment of the present invention, a Spectrum Processor  1800  is disclosed where a flexible usage of the frequency space is enabled. As illustrated in FIG. 18A, the frequency space is divided to accommodate different OC protocols as well as to provide a group of channels all within a specific frequency window and with a different channel spacing and width. Such a SP module can be made with a combination of OSS  1810 ,  1820  and  1830  and filters  1814 ,  1824 ,  1834  and  1840  as illustrated in FIG. 18A generating the spectra of FIG. 18B.  
         [0055]    Having thus disclosed various embodiments of the present invention, it being understood that numerous alternative embodiments are contemplated and that the scope of the invention is limited only by the appended claims and their equivalents, what is claimed is: