Abstract:
A versatile, wavelength-slicing device, referred to herein as an optical spectrum synthesizer (OSS), provides new avenues and technologies for optical communication applications. Specifically, OSS separates a composed optical signal into two 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 to the other. The cascades 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 Wavelength Division Multiplexing and management devices based on cascades of OSS devices. Examples of these devices include 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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of pending application 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 an etalon-based method and apparatus for asymmetric wavelength 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, dense-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). It is also understood that all channels are given frequency windows with identical width, 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-talking between different data channels. It is generally understood that the more narrow the frequency spacing between different data channels, the greater the transmission capacity a DWDM system will have.  
           [0006]    Several multiplexing and de-multiplexing devices are essential to the operation of a DWDM system. FIG. 1(A) is a diagram illustrating the operation of a group of devices known as optical filters. An optical filter has the function of separating signals within a predetermined frequency window from the input spectrum. 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. 1(A). 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 where individual signal channel with different center wavelengths can be combined together to form a single composed optical output signal. There are several types of optical filters and brief descriptions are provided for two types of commonly available filters. In FIG. 1(B), a filter made with optical fiber, known as fiber Bragg grating (FBG), 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. 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. 1(c).  
           [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. 2(A), a device commonly known as an arrayed waveguide grating (AWG) is displayed. As depicted, these AWG can be used to separate all data channels simultaneously. The output channels 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 can also be used to multiplex or de-multiplex optical signals. As displayed in FIG. 2(B), due to dispersion, i.e., the index of refraction is different for different frequency is so that the exit angle is different for channels having different center frequencies. Different output channels are separated in space and connected into individual fibers. Another commonly used device is a diffraction grating, 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)=m λ, where m is an integer commonly referred as the order of diffraction. Such a diffraction grating is illustrated in FIG. 2(c).  
           [0008]    A third type of wavelength separating and combing devices is known as interleavers. FIG. 3(A) provides a function diagram of an interleaver. These interleavers separate a composite optical signal into two complementary signals in which the odd data channels are branched into one output and the even channels are directed into the other output. 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. 3(B). Two typical interleaver devices are depicted in FIG. 3(C) and FIG. 3(D). In FIG. 3(C), an interleaver designed based upon a Fabry-Perot etalon is displayed. In this device, two parallel, partially reflecting surfaces are separated by a distance d. The center wavelengths λ m  associated with transmitted channels are given by λ m =2 n d/m, where m is an integer and n is the index of refraction. In FIG. 3(D), an interleaver based on the Michelson interferometer is illustrated. The center wavelengths λ m  associated with channels branched into OUTPUT  2  are given by λ m =2 n (d 1 −d 2 )/m, where m is an integer and n is the index of refraction along the optical path. The performance and optical characteristics of these interleavers can be enhanced with certain modifications. For example, when a partial reflector is inserted into a Michelson interferometer, based interleaver, parallel to one of the two mirrors, both the reflection and transmission spectra are significantly improved.  
           [0009]    These prior art interleavers can provide more flexibility to DWDM system designers and engineers. In FIG. 4, two stages of interleavers are cascaded to provide four outputs 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. Another practical configuration, as demonstrated in FIG. 5, utilizes both the interleaver and wavelength dispersion devices. 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 of the original spacing. In a different configuration as displayed in FIG. 6, an interleaver, 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).  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with the present invention, an Optical Spectrum Synthesizer (OSS) separates a composed, 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 passing bands. The width and period of the passing bands can be designed to accommodate specific network requirements. The two output spectra are complements of each other, but may have different passing 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. A Spectrum De-Multiplexer (SDEMUX) constructed by cascading n OSS devices, will separate a composed multi-channel optical signal into n spectra each containing a different sub group 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 sub group 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. A Spectrum Add-Drop Module (SADM) can be implemented with a cascade of two OSS devices. The SADM provides the network system designer a means to add and drop a group of signal channels collectively. 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 is also disclosed here where two (or more) OSS devices are 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.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    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:  
         [0012]    [0012]FIGS. 1A through 1C (prior art) are simplified diagrams illustrating conventional filters and their use in DWDM technology.  
         [0013]    [0013]FIG. 1A is a block diagram illustrating the operation of a generic filter device.  
         [0014]    [0014]FIG. 1B depicts a fiber Bragg grating filter.  
         [0015]    [0015]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.  
         [0017]    [0017]FIG. 2A is a diagram illustrating the operation of an arrayed waveguide grating (AWG) device.  
         [0018]    [0018]FIG. 2B represents a prism wavelength dispersion device.  
         [0019]    [0019]FIG. 2C shows the operation of a conventional grating device;  
         [0020]    [0020]FIGS. 3A through 3D (prior art) are simplified diagrams illustrating conventional interleaver devices and their use in DWDM technology.  
         [0021]    [0021]FIG. 3A is a block diagram illustrating the operation of an interleaver.  
         [0022]    [0022]FIG. 3B displays the output frequency spectra associated with two output signals.  
         [0023]    [0023]FIG. 3C shows an Febray-Perot etalon interleaver and  
         [0024]    [0024]FIG. 3D depicts the operation of an interleaver based upon Michelson interferometer;  
         [0025]    [0025]FIG. 4 (prior art) 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;  
         [0026]    [0026]FIG. 5 (prior art) is a schematic diagram illustrating the combination of an interleaver and two multi-channel dispersion devices (prisms);  
         [0027]    [0027]FIG. 6 (prior art) depicts a device composed of interleaver and filters. Each output of the device carries only one signal channel;  
         [0028]    [0028]FIGS. 7A through 7B are diagrams illustrating the operation of a versatile interleaver, OSS, according to an embodiment of the present invention.  
         [0029]    [0029]FIG. 7A is block diagram of an OSS and  
         [0030]    [0030]FIG. 7B displays the spectra associated with output signals;  
         [0031]    [0031]FIGS. 8A through 8C are diagrams illustrating the construction of OSS based upon a design with two Fabry-Perot Etalons.  
         [0032]    [0032]FIG. 8A depicts a device having two solid Etalons and three partially reflective boundaries.  
         [0033]    [0033]FIG. 8B shows an OSS constructed with a solid etalon and an air-spaced etalon.  
         [0034]    [0034]FIG. 8C displays an OSS made with two solid etalons in contact through optical contacting or other process;  
         [0035]    [0035]FIGS. 9A through 9C are diagrams illustrating the construction of OSS based upon a design with three Fabry-Perot Etalons.  
         [0036]    [0036]FIG. 9A depicts a device having three solid Etalons and four partially reflective boundaries.  
         [0037]    [0037]FIG. 9B shows an OSS constructed with two solid etalons and an air-spaced etalon.  
         [0038]    [0038]FIG. 9C displays an OSS made with three solid etalons in contact through optical contacting or other process;  
         [0039]    [0039]FIGS. 10A through 10B are diagrams depicting the separation of a composite optical signal into two outputs of signals carrying different protocol channels;  
         [0040]    [0040]FIG. 11 is a diagram illustrating a Spectrum DeMultiplexer (SDEMUX) constructed with three OSS devices;  
         [0041]    [0041]FIGS. 12A and 12B are ⅓ DEMUX and ⅓ MUX devices according to the present invention;  
         [0042]    [0042]FIGS. 13A and 13B are diagrams illustrating long haul systems according to the present invention.  
         [0043]    In FIG. 13A a system using 1/n MUX, EDFA and 1/n DEMUX is depicted whereas in  
         [0044]    [0044]FIG. 13B a system based on SDEMUX, EDFAs and SMUX is shown;  
         [0045]    [0045]FIGS. 14A through 14C are diagrams illustrating Spectrum Add-Drop Module and one application based on the present invention.  
         [0046]    In FIG. 14A an SADM constructed using two OSS is displayed whereas in  
         [0047]    [0047]FIG. 14C a system using SADM is displayed, a symbol for this device is illustrated in FIG. 14B;  
         [0048]    [0048]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 sub groups of signal channels, FIG. 15B illustrates a symbol for this device;  
         [0049]    [0049]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;  
         [0050]    [0050]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 cross talking between adjacent channels;  
         [0051]    [0051]FIGS. 18A and 18B are diagrams which illustrate a Spectrum Processor in accordance with an embodiment of the present invention wherein  
         [0052]    [0052]FIG. 18A illustrates the frequency space usage and  
         [0053]    [0053]FIG. 18B illustrates the structure of a Spectrum Processor module; and  
         [0054]    [0054]FIG. 19A through 19B are drawings of different designs of OSS, which are interfering etalons achieved by non-uniform, partially reflective coatings. Non-uniform phase correction plates are also indicated.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    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.  
         [0056]    [0056]FIGS. 7A through 7B are diagrams illustrating the operation of a versatile interleaver, referred to herein as an Optical Spectrum Synthesizer (OSS), according to an embodiment 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.  
         [0057]    In FIG. 7A, 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. 7B. 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.  
         [0058]    Referring now to FIG. 8A, a preferred embodiment of an OSS comprises optical cavities with boundaries set by three parallel, partially reflective surfaces. The thickness of each optical cavity is predetermined to obtain output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle of less than 10 degrees with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms another output. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a gradient index lens known as a GRIN lens. In one preferred embodiment, the partially reflective surfaces have reflectivities of approximately 77%, 92% and 40%, respectively.  
         [0059]    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. 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 effect of temperature can also be compensated for through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angle of the OSS thereby achieving a stable operation condition.  
         [0060]    Referring now to FIG. 8B, in another preferred embodiment of OSS, one solid Fabry-Perot etalon is followed by an “air-spaced” Fabry-Perot etalon. The solid etalon is made with an optical disc with two parallel surfaces. Each surface is coated with a predetermined, partially reflective coating. The “air-spaced” etalon is constructed with the addition of a precision spacer and another partially reflective window. One side of this window is coated for partial reflection whereas the other, a wedged surface, is coated for anti-reflection. This construction also creates two optical cavities with boundaries set with three parallel, partially reflective coatings. The thickness of each optical cavity is predetermined to obtain the output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle (i.e., no more than  10  degrees from normal) with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms another output. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is gradient index lens known as a GRIN lens. The thickness or width of the spacer of the air-spaced etalon is about 1.5 times greater than the thickness of the solid etalon to compensate for the lower refractive index of air as compared to the refractive index of the material used in the solid etalon (i.e., quartz, glass, et cetera).  
         [0061]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication channel 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 piezoelectrical materials such that the free-spectra ranges of each or both of the optical cavities may be controlled. Another preferred way to adjust the free-spectra-range of the “air-spaced” etalon is to set and control the gas mixture and the pressure of the “air-spaced” cavity. The temperature environment of both etalons 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 effect of temperature can also be compensated through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angle and the thickness of the OSS thereby achieving a stable operation condition. 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.  
         [0062]    Referring now to FIG. 8C, in another preferred embodiment of OSS, one solid Fabry-Perot etalon is combined with another solid Fabry-Perot etalon. The two solid etalons are preferably in optical contact where the distance between two surfaces is kept minimum. Both etalons are made with optical discs with two parallel surfaces. Each surface is coated with a predetermined partially reflective coating. This construction also creates two optical cavities with boundaries set by three parallel, partially reflective surfaces. The thickness of each of the optical cavities is predetermined to obtain the output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle (i.e., less than 10 degrees) with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms another output. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a gradient index lens known as a GRIN lens.  
         [0063]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication channel 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. The temperature environment of both etalons 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 effect of temperature change can also be compensated through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angles and the thickness of the OSS thereby achieving a stable operation condition.  
         [0064]    Referring now to FIG. 9A, another preferred embodiment of an OSS is made using three optical cavities with boundaries set by four parallel, partially reflective surfaces. The thickness of each optical cavity is predetermined to obtain the output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms another output. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a gradient index lens known as a GRIN lens. In one preferred embodiment, the partially reflective surfaces have reflectivities of approximately 36%, 75%, 75% and 36%, respectively.  
         [0065]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication grid, the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one, two or all three of the optical cavities may be constructed with piezoelectrical materials such that the free-spectra ranges of each of the optical cavities may be controlled. 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 three optical cavities to ensure best performance. The effect of temperature can also be compensated for through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angle of the OSS thereby achieving a stable operation condition. The temperature sensitivity of the air spaced etalons can be reduced by using spacer materials with a low thermal expansion coefficient.  
         [0066]    Referring now to FIG. 9B, in another preferred embodiment of OSS, two solid Fabry-Perot etalons and an “air-spaced” Fabry-Perot etalon form a “sandwich” device. The solid etalons are made with optical discs with two parallel surfaces. Each surface is coated with a predetermined partially reflective coating. The “air-spaced” etalon is constructed with the addition of a precision spacer placed in between the two solid etalons. This construction also creates three optical cavities with boundaries set by four parallel, partially reflective surfaces. The thickness of each optical cavity is predetermined to obtain the output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms another output These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a gradient index lens known as a GRIN lens.  
         [0067]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication channel grid, the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one, two or all three 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” etalon is to set and control the gas mixture and the pressure of the “air-spaced” cavity. The temperature environment of these etalons 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 three optical cavities to ensure best performance. The effect of temperature can also be compensated for through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angles and the thickness of the OSS thereby achieving a stable operation condition. The temperature sensitivity of the air-spaced etalon can be reduced by using spacer materials with a low thermal expansion coefficient.  
         [0068]    Referring now to FIG. 9C, in another preferred embodiment of OSS, three solid Fabry-Perot etalons are combined. The three solid etalons are preferably in optical contacts where the distances between adjacent etalons are kept to a minimum. All three etalons are made with optical discs with two parallel surfaces. Each surface is coated with a predetermined partially reflective coating. This construction also creates three optical cavities with boundaries set by four parallel, partially reflective coatings. The thickness of each optical cavity is predetermined to obtain the output spectra as displayed in FIG. 7B. The incoming light, preferably a parallel beam with a small angular divergence, is directed to the surface of the OSS at a small incident angle with respect to the surface normal of the OSS. The reflected light beam from OSS forms one of the outputs whereas the transmitted light beam forms the other output. These input and output light beams are interfaced/coupled to optical fibers through lenses. A preferred type of lens is a gradient index lens known as a GRIN lens.  
         [0069]    In order to match the center frequencies of the passing bands of output  1  and  2  to that of a standard communication channel grid, the incident angles and/or environment temperature(s) of the OSS are adjusted. In addition, one, two, or three optical cavities may be constructed with piezoelectric materials such that the free-spectra-range of each of the optical cavities may be controlled. The temperature environment of the three etalons 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 three optical cavities to ensure best performance. The effect of temperature change can also be compensated through an a-thermal design where materials with different thermal expansion coefficients are used to change the incident angle and the thickness of the OSS thereby achieving a stable operation condition. It will be understood that the various embodiments of the invention can be implemented using more than just two or three optical cavities.  
         [0070]    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.  
         [0071]    [0071]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 passing channel bandwidths. In a different preferred embodiment of the present invention, the optical spectrum is divided into four complementary spectra having different passing 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 for a SDEMUX.  
         [0072]    Referring now to FIGS. 12A through 12B, a group of three ⅓ DEMUX and a group of three ⅓ MUX are illustrated. 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 sub group 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.  
         [0073]    [0073]FIG. 13A 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 formed using a SDEMUX, n optical fibers, EDFAs (Erbium Doped Fiber Amplifiers) and a SMUX. 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, 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.  
         [0074]    Referring now to FIGS. 14A, 14B and  14 C, a Spectrum Add-Drop Module (SADM) 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.  
         [0075]    [0075]FIGS. 15A and 15B disclose a preferred construction of a 1×4 Spectrum Switch (SS). In this case, a 4×4 optical switch follows an SDEMUX that allows flexible redirection of sub groups of signal channels. In other preferred embodiments, 1×n SS is constructed with the combination of 1 to n SDEMUX and an n×n optical switch. FIG. 15B illustrates a proposed symbol for the spectrum switch.  
         [0076]    Referring now to FIG. 16, a 4×4×4 Spectrum Cross-Connect (SCC) 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 sub groups 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 2n 1×m SS connected in a way similar to a conventional n×n×m optical cross connect.  
         [0077]    [0077]FIGS. 17A and 17B are diagrams illustrating a module and spectra for which overlapping spectra were passed as the outputs, 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 degree of cross talking between adjacent channels. The cross talking between adjacent channels are then removed through electronic and/or optical decoding of the original data.  
         [0078]    In another preferred embodiment of the present invention, a Spectrum Processor 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 and filters as illustrated in FIG. 18B.  
         [0079]    [0079]FIG. 19A and 19B disclose additional embodiments of the present invention. An OSS is implemented with interfering etalons formed by non-uniform coatings. Non-uniform phase correction plates and or additional filters may also be used in the optical path to enhance the properties of the output spectra.  
         [0080]    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.