PATENT ABSTRACT
A method of transmitting an optical communications signal, comprising receiving a first signal, encoding the signal with a differential or duobinary encoding scheme, encoding the signal with an oscillating signal component, and sub-carrier modulating the signal onto a sub-carrier of an optical carrier signal. The invention also relates to corresponding systems and apparatuses.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 60/314,600, filed Aug. 24, 2001, and from U.S. patent application Ser. No. 09/441,805, filed Nov. 17, 1999, which claims priority from U.S. provisional patent application No. 60/108,751, filed Nov. 17, 1998, now abandoned, all of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed generally to the transmission of signals in optical communications systems. More particularly, the invention relates to systems, devices, and methods for producing upconverted modulated optical signals. 
     The development of digital technology provided the ability to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was recognized that in order to make effective use of information resources it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems. Optical transmission systems can provide high capacity, low cost, low error rate transmission of information over long distances. 
     The transmission of information over optical systems is typically performed by imparting the information in some manner onto an optical carrier by varying characteristics of the optical carrier. In most optical transmission systems, the information is imparted by using an information data stream to either directly or externally modulate an optical carrier so that the information is imparted at the carrier frequency or on one or more sidebands, with the later technique sometimes called upconversion or sub-carrier modulation (“SCM”). 
     SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200, 5,432,632, and 5,596,436, generally produce a modulated optical signal in the form of two mirror image sidebands at wavelengths symmetrically disposed around the carrier wavelength. Generally, only one of the mirror images is required to carry the signal and the other image is a source of signal noise that also consumes wavelength bandwidth that would normally be available to carry information. Similarly, the carrier wavelength, which does not carry information in an SCM system, can be a source of noise that interferes with the subcarrier signal. Modified SCM techniques have been developed to eliminate one of the mirror images and the carrier wavelength. However, “traditional” SCM techniques do not work well at high bit rates (e.g., greater than 2.5 gigabits per second). For example, mixer linearity, frequency flatness, frequency bandwidth, and group delay tend to be problematic. It is also difficult to keep power levels balanced and well controlled. Such problems and difficulties can result in significant performance degradation and/or increased cost. Modified SCM techniques have also been disclosed to utilize Manchester encoding in place of electrical carriers, such as described in U.S. Pat. Nos. 5,101,450 and 5,301,058. 
     Initially, single wavelength carriers were spatially separated by placing each carrier on a different fiber to provide space division multiplexing (“SDM”) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (“TDM”), on the single wavelength carrier in the SDM system as a means to better use the available bandwidth. The continued growth in demand has spawned the use of multiple wavelength carriers on a single fiber using wavelength division multiplexing (“WDM”). 
     In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information on each wavelength, but also by increasing the number of wavelengths, or channel count, in the system. However, conventional systems already have the capacity to transmit hundreds of channels on a single fiber, and that number will continue to increase. As such, the cost of transmitters, receivers, and other devices can constitute a large portion of a system&#39;s cost. Therefore, the size and cost of systems will increase significantly as the number of WDM channels increase. Accordingly, there is a need to reduce the cost and size of devices in optical systems while at the same time maintaining or increasing system performance. 
     BRIEF SUMMARY OF THE INVENTION 
     The systems, devices, and methods of the present invention address the above-stated need for lower cost, higher capacity, longer distance optical communications systems, devices, and methods. The present invention is directed to improved systems, devices, and methods for producing sub-carrier modulated optical signals. The present invention can be employed, for example, in multi-dimensional optical networks, point to point optical networks, or other devices or systems which can benefit from the improved performance afforded by the present invention. 
     One embodiment of the present invention is a transmitter including an optical carrier source, an electrical to optical converter, a parser, and first and second Manchester encoders. The electrical to optical converter has an optical input connected to the optical carrier source, an optical output, and first and second electrical data inputs. The parser has a data input and first and second data outputs. The first Manchester encoder has a data input connected to the first data output of the parser and an encoded data output connected to the first electrical input of the electrical to optical converter. The second Manchester encoder has a data input connected to the second data output of the parser and an encoded data output connected to the second electrical input of the electrical to optical converter. 
     Another embodiment of the present invention includes two or more optical carrier sources, and two or more corresponding electrical to optical converters. In some embodiments, the optical carrier sources produce optical carriers with the same optical wavelength, and in other embodiments the optical carrier sources produce optical carriers having different wavelengths. 
     Other embodiments of the present invention utilize other variations and combinations of devices, such as forward error correction encoders, differential encoders, filters, interfaces, and multiplexers. In other embodiments, the data signal is separated into two or more lower bit rate signals for at least a portion of the transmitter. In other embodiments, the parser produces more than two parsed signals. 
     Those and other embodiments of the present invention, as well as receivers, systems, and methods according to the present invention, will be described in the following detailed description. The present invention addresses the needs described above in the description of the background of the invention by providing improved systems, devices, and methods. These advantages and others will become apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIGS. 1 and 2  show examples optical communications systems; 
         FIG. 3  shows an embodiment of a transmitter that can be used in the optical communications system; 
         FIG. 4  shows timing diagrams illustrating one example of Manchester encoding; 
         FIG. 5  shows one example of a frequency spectrum for a Manchester encoded signal; 
         FIG. 6  shows one example of a frequency spectrum for an upconverted optical signal generated from the Manchester encoded signal of  FIG. 5 ; 
         FIG. 7  shows another embodiment of the transmitter including a filter; 
         FIG. 8  shows one example of a frequency spectrum for a filtered Manchester encoded signal; 
         FIG. 9  shows one example of a frequency spectrum for an upconverted optical signal generated from the Manchester encoded signal of  FIG. 8 ; 
         FIGS. 10 and 11  show additional embodiments of the transmitter; 
         FIGS. 12 and 13  show other examples of frequency spectrums for upconverted optical signals 
         FIGS. 14 and 15  show other embodiments of the transmitter; 
         FIG. 16  shows a circuit schematic of one embodiment of the parser, Manchester encoders, and differential encoders; 
         FIG. 17  shows another embodiment of the filter portion of the transmitter; 
         FIG. 18  shows one embodiment of the transmitter interface; 
         FIGS. 19-22  shows several embodiments of a receiver; 
         FIG. 23  shows one embodiment of the receiver interface; and 
         FIGS. 24 and 25  show several embodiments of filters which may be used with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an optical communications system  10  which includes optical paths  12  connecting network elements  14 . Advantages of the present invention can be realized with many system  10  configurations and architectures, such as an all optical network, one or more point to point links, one or more rings, a mesh, other architectures, or combinations of architectures. The system  10  illustrated in  FIG. 1  is a multi-dimensional network, which can be implemented, for example, as an all optical mesh network, as a collection of point to point links, or as a combination of architectures. The system  10  can employ various transmission schemes, such as space, time, code, frequency, phase, polarization, and/or wavelength division multiplexing, and other types and combinations of multiplexing schemes. The system  10  can also include more or less features than those illustrated herein, such as by including a network management system (“NMS”)  16  and changing the number, location, content, configuration, and connection of network elements  14 . 
     The optical paths  12  can include guided and unguided paths or waveguides, such as one or more optical fibers, ribbon fibers, and free space devices, and can interconnect the network elements  14  establishing links  18  and providing optical communication paths through the system  10 . The paths  12  can carry one or more uni- or bi-directionally propagating optical signal channels or wavelengths. The optical signal channels can be treated individually or as a single group, or they can be organized into two or more wavebands or spectral groups, each containing one or more optical signal channel. 
     The network elements  14  can include one or more signal processing devices including one or more of various optical and/or electrical components. The network elements  14  can perform network functions or processes, such as switching, routing, amplifying, multiplexing, combining, demultiplexing, distributing, or otherwise processing optical signals. For example, network elements  14  can include one or more transmitters  20 , receivers  22 , switches  24 , add/drop multiplexers  26 , interfacial devices  28 , amplifiers  30 , multiplexers/combiners  34 , and demultiplexers/distributors  36 , as well as filters, dispersion compensating and shifting devices, monitors, couplers, splitters, and other devices. One embodiment of one network element  14  is illustrated in  FIG. 1 , although many other variations and embodiments of network elements  14  are contemplated. Additional examples of network elements  14  are described in U.S. patent application Ser. Nos. 09/817,478, filed Mar. 26, 2001, and 09/253,819, filed Feb. 19, 1999, both of which are incorporated herein by reference. 
     The optical transmitters  20  and receivers  22  are configured respectively to transmit and receive optical signals including one or more information carrying optical signal wavelengths, or channels, via the optical paths  12 . The transmitters  20  include an optical carrier source that provides an optical carrier and can utilize, for example, coherent or incoherent sources, and narrow band or broad band sources, such as sliced spectrum sources, fiber lasers, semiconductor lasers, light emitting diodes, and other optical sources. The transmitters  20  often include a narrow bandwidth laser as the optical carrier source. The optical transmitter  20  can impart information to the optical carrier by directly modulating the optical carrier source or by externally modulating the optical carrier. Alternatively, the information can be upconverted onto an optical wavelength to produce the optical signal, such as by utilizing Manchester encoding as described hereinbelow. Examples of optical transmitters  20  are described in U.S. Pat. No. 6,118,566, issued Sep. 12, 2000, which is incorporated herein by reference. 
     Similarly, the optical receiver  22  can include various detection techniques, such as coherent detection, optical filtering, and direct detection. Tunable transmitters  20  and receivers  22  can be used to provide flexibility in the selection of wavelengths used in the system  10 . 
     The switches  24  can take many forms and can have different levels of “granularity”. “Granularity” refers to the resolution or precision with which the switching is performed. For example, WDM switches  24  can switch groups of wavelengths, individual wavelengths, or portions of wavelengths. Before being switched, the signals can be demultiplexed into the appropriate level of granularity, and after being switched the signals can be multiplexed into the desired format, using the same or different modulation schemes, wavelengths, or other characteristics. 
     Switches  24  can have electrical, optical, or electrical/optical switch “fabrics”. The switch “fabric” describes the domain and/or manner in which the signal switching occurs. Switches  24  having an electrical fabric convert incoming optical signals into electrical signals, the electrical signals are switched with electronic equipment, and the switched electrical signals are converted back into optical signals. Such switching is often referred to as “O-E-O” (“optical-electrical-optical”) switching. In contrast, switches  24  having an optical switch fabric perform the switching with the signals in the optical domain. However, switches  24  having an optical switch fabric can still perform O-E-O conversions, such as when demultiplexing or multiplexing optical signals, or in other related interface devices or operations. 
     There are many optical switch fabrics, some of which use micro-electromechanical systems (“MEMS”), such as small, electrically-controlled mirrors, to selectively reflect an incoming optical signal to a desired output. Other optical switch fabrics use a variable index of refraction device to controllably change the index of refraction of an optical signal path, such as by forming a gas pocket in an optically transparent liquid medium, in order to change the direction of the optical signal. Yet another example of an optical switch fabric is the use of an optical path in which the optical gain and/or loss can be controlled so that an optical signal can be either passed or blocked. Some examples of switches  24  having an optical fabric are described in U.S. patent application Ser. No. 09/119,562, filed Jul. 21, 1998, and No. 60/150,218, filed Aug. 23, 1999, and PCT Patent Application PCT/US00/23051, filed Aug. 23, 2000, all of which are incorporated herein by reference. 
     Switches  24  can be grouped into two categories: interfacial switches and integrated switches. Interfacial switches  24 , sometimes referred to as “dedicated” switches, perform one or more O-E-O conversions of the signals. The O-E-O conversions can be either in the switch  24  itself or in a related component, such as a multiplexer  34  or demultiplexer  36 . Interfacial switches  24  are located within or at the periphery of networks  10  and point to point links  18 , such as between two or more point to point links  18 , between two or more networks  10 , or between a network  10  and a point to point link  18 . Interfacial switches  24  optically separate the links  18  and/or networks  10  because optical signals are converted into electrical form before being passed to the next optical link  18  or network  10 . Interfacial switches  24  are a type of interfacial device  28 , which is discussed in more detail hereinbelow. In contrast, integrated switches  24  are optically integrated into the network  10  and allow optical signals to continue through the network  10 , via the integrated switch  24 , without an O-E-O conversion. Integrated switches  24  are sometimes called “all-optical switches”, “O-O ” switches, or “O-O-O” switches. A switch  24  can have both an integrated switch  24  portion and a interfacial switch  24  portion, such that some signals are switched without an O-E-O conversion, while other signals are subjected to an O-E-O conversion. 
     Add/drop multiplexers  26  and other devices can function in a manner analogous to integrated switches  24  so that, in general, only optical signals which are being “dropped” from the network  10  are converted into electronic form. The remaining signals, which are continuing through the network  10 , remain in the optical domain. As a result, optical signals in an all-optical system  10  (e.g., systems  10  having integrated switches  24  and integrated add/drop multiplexers  26 ) are not converted into electrical form until they reach their destination, or until the signals degrade to the point they need to be regenerated before further transmission. Of course, add/drop multiplexers  26  can also be interfacial devices  28  which subject signals to an O-E-O conversion. 
     Interfacial devices  28  optically separate and act as interfaces to and between optical networks  10  and/or point to point links  18 . Interfacial devices  28  perform at least one optical to electrical (“O-E”) or electrical to optical (“E-O”) conversion before passing signals into or out of the link  18  or network  10 . Interfacial device  28  can be located within or at the periphery of networks  10 , such as between two or more networks  10 , between two or more point to point links  18 , and between networks  10  and point to point links  18 . Interfacial devices  28  include, for example, cross-connect switches, IP routers, ATM switches, etc., and can have electrical, optical, or a combination of switch fabrics. Interfacial devices  28  can provide interface flexibility and can be configured to receive, convert, and provide information in one or more various protocols, encoding schemes, and bit rates to the transmitters  20 , receivers  22 , and other devices. The interfacial devices  28  also can be used to provide other functions, such as protection switching. 
     The optical amplifiers  30  can be used to provide signal gain and can be deployed proximate to other optical components, such as in network elements  14 , as well as along the optical communications paths  12 . The optical amplifiers  30  can include concentrated/lumped amplification and/or distributed amplification, and can include one or more stages. The optical amplifier can include doped (e.g. erbium, neodymium, praseodymium, ytterbium, other rare earth elements, and mixtures thereof) and Raman fiber amplifiers, which can be locally or remotely pumped with optical energy. The optical amplifiers  30  can also include other types of amplifiers  30 , such as semiconductor amplifiers. 
     Optical combiners  34  can be used to combine the multiple signal channels into WDM optical signals for the transmitters  20 . Likewise, optical distributors  36  can be provided to distribute the optical signal to the receivers  22 . The optical combiners  34  and distributors  36  can include various multi-port devices, such as wavelength selective and non-selective (“passive”) devices, fiber and free space devices, and polarization sensitive devices. Other examples of multi-port devices include circulators, passive, WDM, and polarization couplers/splitters, dichroic devices, prisms, diffraction gratings, arrayed waveguides, etc. The multi-port devices can be used alone or in various combinations with various tunable or fixed wavelength transmissive or reflective, narrow or broad band filters, such as Bragg gratings, Fabry-Perot and dichroic filters, etc. in the optical combiners  34  and distributors  36 . Furthermore, the combiners  34  and distributors  36  can include one or more stages incorporating various multi-port device and filter combinations to multiplex, demultiplex, and/or broadcast signal wavelengths λ i  in the optical systems  10 . 
     The NMS  16  can manage, configure, and control network elements  14  and can include multiple management layers that can be directly and indirectly connected to the network elements  14 . The NMS  16  can be directly connected to some network elements  14  via a data communication network (shown in broken lines) and indirectly connected to other network elements  14  via a directly connected network element and the optical system  10 . The data communication network can, for example, be a dedicated network, a shared network, or a combination thereof. A data communications network utilizing a shared network can include, for example, dial-up connections to the network elements  14  through a public telephone system. Examples of an NMS  16  are described in U.S. patent application Ser. No. 60/177,625, filed Jan. 24, 2000, and PCT Patent Application PCT/US01/02320, filed Jan. 24, 2001, both of which are incorporated herein by reference. 
       FIG. 2  shows another embodiment of the system  10  including a link  18  of four network elements  14 . That system  10  can, for example, be all or part of a point to point system  10 , or it may be part of a multi-dimensional, mesh, or other system  10 . One or more of the network elements  14  can be connected directly to the network management system  16  (not shown). If the system  10  is part of a larger system, then as few as none of the network elements  14  can be connected to the network management system  16  and all of the network elements  14  can still be indirectly connected to the NMS  16  via another network element in the larger system  10 . 
       FIG. 3  shows a transmitter  20  including an interface  50 , a Manchester encoder  52 , an optical carrier source  54 , and an E/O converter  56  having a data input  58 . The transmitter  20  can also include components other than those illustrated herein, such as amplifiers, phase shifters, isolators, filters, signal distorters, protocol processors, and other electrical, optical, and electro-optical components. The transmitter  20  can upconvert one or more data signals onto one or more sidebands of the optical carrier λ o , without requiring the data signals to be modulated onto an electrical carrier source. The upconverted optical signal Λo of the present invention does not require a Manchester decoder at the receiver  22 . Rather, the sideband signal can be received in a manner analogous to other upconverted data signals. 
     The interface  50  provides an interface for data signals to be transmitted and can provide a connection to other systems, networks, or links. The interface  50  can be a simple connector or it can be a more sophisticated device, such as one which performs SONET section monitoring and termination functions or other functions, such as transforming the format of the signals entering the system  10  (e.g., an optical to electrical converter or changing a signal from RZ to NRZ format), transforming a single stream of data into plural lower bit rate streams, etc. The interface  50  can be, for example, the receiver end of an optical short reach interface which receives and converts a high bit rate optical signal into two or more lower bit rate electrical signals. The conversion of a single, high bit rate signal into two or more lower bit rate signals is advantageous, for example, when a high bit rate signal can be processed more efficiently in several lower bit rate streams. 
     The Manchester encoder  52  encodes incoming data signals with a Manchester encoding scheme. The encoder  52  can be implemented, for example, as an integrated circuit, such as an application specific integrated circuit, a general purpose integrated circuit, a field programmable gate array, or other integrated circuits. 
     The Manchester encoding scheme typically encodes each bit of data as a two part bit code, with the first part of the bit code being the complement of the data, and the second part being the actual data. Other variations of Manchester encoding, such as where the second part of the bit code is the complement of the data, can also be used with the present invention. Furthermore, although the present invention will be described in terms of Manchester encoding, the present invention is applicable to other encoding schemes, including the modulation of data onto an electrical carrier, which reduce or transform the DC component of data signals and, thereby, provide for signal upconversion in accordance with the present invention. In some embodiments, the transmitter  20  can upconvert data onto one or more sidebands, or it can transmit data at the optical carrier wavelength λ o . For example, the Manchester encoder  52  can be activated for upconversion and deactivated, so that data signals pass through unencoded, for transmission at the optical carrier wavelength λ o . In other embodiments, the transmitter  20  can include a bypass circuit around the Manchester encoder  52  for transmission at the optical carrier wavelength λ o . 
     The optical carrier source  54  provides an optical carrier having a center carrier wavelength λ o , such as a continuous wave optical carrier, to the E/O converter  56 . The optical carrier source  54  can include control circuits (not shown), such as drive and thermal control circuits, to control the operation of the optical carrier source  54 . 
     The E/O converter  56  receives the optical carrier λ o  from the optical carrier source  54  and receives electrical data signals at data input  58 . The E/O converter  56  converts the electrical data signals into optical data signals Λ o . The E/O converter  56  can provide the data on one or more sidebands of the optical carrier λ 0 , which is sometimes referred to as “upconversion” or “subcarrier modulation”. The E/O converter  56  can include, for example, one or more Mach-Zehnder interferometers, other interferometers, or other E/O converters. 
       FIG. 4  shows an example of Manchester encoded data, along with corresponding NRZ data and a clock signal. In that example the Manchester encoded data corresponds with data in NRZ format, although many forms of data can be Manchester encoded, including data in RZ format. In this example, the Manchester encoded data includes a two part bit code, with the first part of the bit code being the complement of the data, the second part being the actual data, and with a transition between the two parts. Other variations of Manchester encoding can also be used with the present invention. One form of Manchester encoding is specified in IEEE Standard 802.3. Other forms and variations of Manchester encoding also exist and are applicable to the present invention. 
       FIG. 5  shows an example of Manchester encoded data in the frequency spectrum. Manchester encoded data typically has an asymmetrical frequency spectrum about data rate frequency f d . Furthermore, the data rate frequency f d  of the data signal affects the frequency spectrum of the Manchester encoded data, so that the greater the data rate f d , the greater the spread of the frequency spectrum of the Manchester encoded signal. Because each bit of a Manchester encoded signal has a transition between states, Manchester encoded data has a frequency component equal to the bit rate. As a result, the electrical data signals are upconverted onto one or more sidebands of the optical carrier λ o  at the electrical to optical converter  56 . Furthermore, the frequency spectrum of the Manchester encoded signal will affect the shape and offset of the sidebands. 
       FIG. 6  shows a signal profile of the optical data signal Λ o  when the Manchester encoded data signal of  FIG. 5  is input to the E/O converter  56 . In that example, the Manchester encoded data signal is upconverted onto a single sideband of the optical carrier λ o  and the optical carrier λ o  is suppressed. The present invention can also be used with other upconversion formats. For example, the carrier does not have to be suppressed, and the Manchester encoded data signals can be upconverted in other formats, such as double sideband signals. 
       FIG. 7  shows another embodiment of the transmitter  20  including a filter  60  for the Manchester encoded signal spectrum. The filtered Manchester encoded signal allows for better performance by, for example, providing a filtered Manchester encoded signal having a frequency spectrum which is more symmetrical about the data rate frequency f d  and more narrow, thereby requiring less bandwidth to transmit the same information. In some embodiments, the filter  60  may be omitted, such as when using a narrow band E/O converter  56  (e.g., a resonantly-enhanced modulator). The filter  60  may also be used to narrow the frequency spectrum in conjunction with other devices, such as differential encoders  69  described hereinbelow, to facilitate other functions, such as to facilitate duobinary encoding. 
       FIG. 8  shows a frequency spectrum for one example of the filtered Manchester encoded signal, with the unfiltered signal shown as a broken line. 
       FIG. 9  shows a signal profile of the optical data signal Λ o  when the Manchester encoded data signal of  FIG. 8  is input to the E/O converter  56 . In that example, the sideband signal is more compact and, therefore, uses less bandwidth than the sideband generated from unfiltered Manchester encoded signals, thereby allowing for increased system performance. 
       FIG. 10  shows another embodiment of the transmitter  20  which includes a forward error correction (“FEC”) encoder  62 . The FEC encoder  62  can utilize, for example, a G.975 compliant (255,239) Reed-Solomon code, or another FEC code or coding scheme. The FEC encoder  62  will add non-information carrying and/or redundant data, sometimes referred to as “overhead”, to the signal, thereby changing the bit rate and frequency spectrum of the Manchester encoded signal. A change in the bit rate and frequency spectrum of the Manchester encoded signals can change the location and frequency spectrum of the sidebands relative to the optical carrier λ o . The amount of overhead added by the FEC encoder  62  will vary depending on the amount of FEC encoding performed on the data signals. 
     In other embodiments, the information can be parsed (or inverse multiplexed) into two or more streams that can be transmitted to the destination. The resulting parsed streams may be lower bit rate streams which can allow for signals to be transmitted over a greater distance without regeneration or with fewer regeneration sites. Alternatively, the signal may be parsed into two or more bit rate streams which are not lower bit rate signals than the received signal, such as when a signal is parsed into identical copies for redundant transport, or when additional information is added to the parsed signals so that the resultant bit rate is not lower. Parsing or inverse multiplexing, when applied to SONET signals constructed from lower bit rate SONET signals can be merely a demultiplexing of the high bit rate SONET signal into its low bit rate SONET components. The information being transmitted can be recovered from the lower bit rate signals without inverse demultiplexing the lower bit rate signals into the higher bit rate signal. Whereas, inverse multiplexing of concatenated SONET signals fragments the information, requiring the IM signals be inverse demultiplexed to recover the information. While inverse multiplexing is known in the art, there are difficulties with the schemes, particularly in concatenated data streams. 
     A primary difficulty with inverse multiplexing is that the inverse multiplexed data streams will travel from the origin through the optical systems at different rates causing a misalignment, or skew, of the data at the destination. In parallel optical systems, transmission path lengths for the inverse multiplexed signals are equalized as much as possible to lessen the skew between the signals. In WDM systems, while a common fiber is used, chromatic dispersion of the different wavelengths carrying the inverse multiplexed signals, as well as the mux/demux structure of the WDM system can greatly increase the skew. 
     Various methods can be applied to compensate for the skewing of inverse multiplexed signals. For example, U.S. Pat. No. 5,461,622 suggests using both framing and pointer bytes in SONET overhead to deskew the information. Unfortunately, the amount of skew introduced by the system  10  can vary with the system conditions, which can degrade the system performance, particularly in WDM systems. For example, variations in the wavelengths one or more of the transmitters used to transmit the inverse multiplexed signals can caused variations in the amount of skew in the system  10 . 
     In one aspect of the present invention, the transmitters  20  are configured to upconvert two or more inverse multiplexed signals onto different subcarriers of a single optical carrier wavelength provide by a transmitter  20 . The frequency spacing between subcarrier can be substantially less than between adjacent carriers, so as to greatly decrease the dispersion and resultant skew between the inverse multiplexed signals during transmission in WDM systems. In addition, transmitting the inverse multiplexed signals on subcarriers of a common optical carrier essentially eliminates path length differences introduced by WDM multiplexing schemes. 
     Various subcarrier modulation techniques can be employed to upconvert the inverse multiplexed data streams onto the subcarriers. Single sideband, suppressed carrier upconversion techniques can be used to minimize unwanted mirror image subcarrier and carrier wavelengths being transmitted along with the signal wavelengths. Although conventional double sideband, non-suppressed carrier, subcarrier modulation techniques also can be employed. An example of single sideband, suppressed carrier transmitters suitable for use in the present invention are described in commonly assigned copending U.S. application Ser. No. 09/185,820 filed Nov. 4, 1998, the disclosure of which is incorporated herein by reference. 
     The number of inverse multiplexed signals may or may not coincide with the number of subcarriers being upconverted on a single transmitter. When the number of inverse multiplexed signals does not correspond to the number of subcarriers, the inverse multiplexed signals can be upconverted onto two or more transmitters transmitting information that provide adjacent signal wavelengths in a wavelength channel plan. For example, placing two subcarriers on each of two adjacent carriers can decrease the dispersion and resultant skew between the inverse multiplexed signals by a factor of 2-3 times compared to the skew using four carriers. 
     Inverse multiplexing can be used to separate and transmit concatenated and unconcatentated higher bit rate information streams, e.g., OC-768c &amp; OC-768, OC-192c &amp; OC-192, etc. The inverse multiplexed signals can be framed with appropriate transmission overhead at lower bit rates to allow the inverse multiplexed signals to be deskewed and recombined into the higher bit rate signal at the end of the link. The deskewing can be performed using the framing A 1  and A 2  bytes in the transmission overhead or additional bytes, as previously discussed. 
     In various embodiments, the receivers are configured to coherently detect two or more of the subcarriers carrying the inverse multiplexed signals. Coherent detection of the subcarriers eliminates much of the path variability introduced by demultiplexing and direct detection of the inverse multiplexed signals. Coherent detection can be performed using a remnant of the carrier wavelength with or without a local oscillator providing a heterodyne signal. In various embodiments, the local oscillator can be locked using the remnant carrier wavelength to ensure proper tracking of any drift in the carriers and subcarriers during operation. In fact, a tunable local oscillator can provide additional flexibility in configuring receivers  22  in the system  10 . In other embodiments, detection techniques other than coherent detection, such as direct detection, may be used. 
       FIG. 11  shows another embodiment of the transmitter  20  including a parser  64  and a coupler  66 . In that embodiment the parser  64  separates the data signal into two signals which are coupled before entering the E/O converter  56  such that the signals are upconverted onto separate sidebands of the optical carrier λ o . The transmitter  20  can be used, for example, to transmit a high bit rate signal as two or more lower bit rate signals. Such a transmitter  20  is advantageous, for example, if a high bit rate signal is provided to a transmitter  20  but desired system performance, such as transmission distance, OSNR, etc., is not practical or cost effective with the higher bit rate signal. In that situation, the higher bit rate signal can be separated into two or more lower bit rate signals which can be recombined or assembled at the receiver  22 . 
     The parser  64  in the illustrated embodiment separates the data signal into two data signals. In other embodiments of the transmitter  20 , the parser  64  can separate the data signal into more than two data signals. The parser  64  can also utilize other parsing schemes, such as separating the data signal into two or more data signals having the same or different bit rates. The parser  64  can also separate the data signal at every bit, at every byte, at every several bits or bytes, or in other intervals, whether uniform or non-uniform. For example, the number of bits or bytes can vary with time or with some other function, such as a parameter of the data signal. Furthermore, the parser  64  can utilize redundancy in the data streams, such that some data is provided on more than one data stream, or no redundancy at all can be used. The parser  64  can include those and other variations and combinations of parsing schemes. In one example, the parser  64  separates a data stream onto two, lower bit rate data streams, and parses the data stream at each bit, sending one bit on one data stream, sending the next bit on the other data stream, and then repeating. 
     The coupler  66  in the illustrated embodiment is a two-by-two, ninety degree electrical coupler, such that the first output produces a signal similar to the signal at the second input plus a ninety degree phase shifted form of the signal at the first input, and the second output produces a signal similar to the signal at the first input plus a ninety degree phase shifted form of the signal at the second input. The coupler  66  couples and phase shifts the parsed data signals so that, for example, when each output of the coupler  66  is used to modulate an arm of a double parallel Mach-Zehnder interferometer or a similar device, each of the parsed signals will be upconverted onto a separate optical sideband, as shown in  FIG. 12 . Other variations of the electrical coupler  66  are also possible. For example, the coupler  66  can have different numbers of inputs and outputs, can induce different phase shifts, and can equally or unequally split and couple the signals to produce different kinds of optical signals. 
     Also in that embodiment, the interface  50  demultiplexes or “deserializes” the incoming data signal into several lower bit rate signals, which are provided by the interface  50  in parallel. Such deserializing of a signal can facilitate processing the signal, such as for FEC encoding and parsing. For example, in some circumstances it is more practical to perform parallel processing on two or more lower bit rate signals than it is to perform the same operation on a single, high bit rate signal. Some, none, or all of the data processing in the transmitter  20  can be performed with several parallel, lower bit rate signals. Multiplexers  68 , sometimes referred to as “serializers”, are also included in that embodiment to combine parallel data signals into a higher bit rate serial data signals. 
       FIG. 12  shows a signal profile of the optical data signal Λ o  when the parsed and coupled data signals of  FIG. 11  are input to the E/O converter  56 . In that embodiment, one of the data signals is upconverted to a data signal at a longer wavelength than the optical carrier λ o , the other sideband is upconverted to a sideband at a shorter wavelength than the optical carrier λ o , and the optical carrier λ o  is suppressed. 
       FIG. 13  shows another signal profile of the optical data signal Λ o . That signal profile can be produced by an embodiment of the transmitter  20  in which the parser  64  separates the data signal into signals having different bit rates and, therefore, different frequencies. As a result, the different data signals will be offset differently from the optical carrier λ o . Typically, the lower bit rate signal will also have more narrow frequency and wavelength spectrums. In other embodiments, the optical data signals can be on opposite sides of the optical carrier λ o , and in other embodiments there can be more than two parsed data signals having more than two different bit rates. 
       FIG. 14  shows another embodiment of the transmitter  20  including differential encoders  69 . The parser  64 , differential encoders  69 , and Manchester encoders  52  can be implemented, for example, as one or more field programmable gate arrays, application specific integrated circuits, general purpose integrated circuits, or other integrated circuits. Furthermore, the differential encoders  69 , as well as other devices, may be implemented in other embodiments of the invention, such as embodiments without the parser  64 . Furthermore, the differential encoder may be replaced with other encoders, such as duobinary encoders. 
       FIG. 15  shows another embodiment of the transmitter  20  in which the parser  64  is used and the coupler  66  is eliminated. In that embodiment, an optical carrier source  54  and an E/O converter  56  are provided for each parsed signal. For example, both parsed data signals can be provided at the same bit rate, but optical carriers λ o  having different wavelengths can be used so that the data signals are upconverted onto different frequencies. In other embodiments, the optical carrier sources  54  can produce optical carriers λ o  having the same wavelength and, for example, one parsed data signal can be upconverted onto a sideband having a longer wavelength than the optical carrier λ o , and the other parsed data signal can be upconverted onto a sideband having a shorter wavelength than the optical carrier λ o . In other embodiments, the parser  64  can separate the data signal into more than two signals, and more than two optical carrier sources  54  and an E/O converters  56  can also be used. 
       FIG. 16  shows a circuit schematic of one embodiment of the parser  64 , differential encoders  69 , and Manchester encoders  52 . That embodiment can be, for example, in the form of an integrated circuit, such as an application specific integrated circuit, a field programmable gate array, a general purpose integrated circuit, other integrated circuits, or discrete components. 
       FIG. 17  shows another embodiment of a portion of the transmitter  20  around the filter  60 . That embodiment includes a first amplifier  70  in front of the filter  60 , a second amplifier  70  after the filter  60 , and a feedback loop including a-processor  72 . The first amplifier  70  and the feedback loop provide controlled signal gain to compensate for variations in the data signal. For example, one or more parameters (e.g., gain and gain profile) of the first amplifier  70  can be controlled through the feedback loop, which can include the processor  72  and/or other circuitry, such as an application specific integrated circuit, a general purpose integrated circuit, a field programmable gate array, and discrete components, to process the feedback signal and control the first amplifier  70 . The second amplifier  70  provides additional gain, and it can be eliminated if sufficient gain is provided by the first amplifier  70 . This embodiment can be modified, such as to utilize a feed-forward loop, to utilize more or less amplifiers  70 , to vary the location of the amplifiers  70 , etc. 
       FIG. 24  illustrates one embodiment of the filter  60 . In that embodiment, the filter  60  includes a low pass stage and a high pass stage which collectively act as a band pass filter. The low pass stage is illustrated as an amplifier, such as a gain limiting amplifier, and the high pass stage is illustrated as a passive filter, such as a passive Bessel filter, although other types of amplifiers, filters, or other devices may be used, and the filter may include active or passive stages. In some embodiments, the order in which the stages are arranged and the number of stages may be changed. In other embodiments, one or more of the amplifiers  70  illustrated in  FIG. 17  may operate as one or more of the filter stages, such as the gain limiting amplifier. In other embodiments, the filter  60  may be a filter other than a band pass filter. The filter  60  may be used, for example, to facilitate duobinary encoding by selecting filter characteristics which compliment the differential encoder  69  or other devices. 
       FIG. 18  shows an embodiment of the transmitter interface  50  including a short reach interface (“SRI”) receiver  74  and a SONET performance monitor  76 . In the illustrated embodiment, the SRI  74  converts the incoming data signal into two or more parallel, lower bit rate signals. For example, the SRI can convert an optical OC-192 signal into sixteen parallel, 622 Mbps electrical signals. The SONET performance monitor  76 , for example, can perform section monitoring and termination functions. 
       FIG. 19  shows a receiver  22  including a filter  80 , an optical to electrical (“O/E”) converter  82 , and an interface  84 . That receiver  22  can receive the optical data signals generated by the transmitters  20  of the present invention without the need for Manchester, differential, or duobinary decoders. The receiver  22  can also include other features, such as FEC decoding, assembling two or more data signals, automatic gain control (“AGC”), clock and data recovery (“CDR”), deserializing, etc. 
     The filter  80  filters one or more signals from the incoming optical data signal Λ o . For example, in a WDM system  10  the filter can be used to select among the several signals and to reduce the noise in the optical data signal Λ o , while in a single channel system  10  the filter  80  can be used to filter noise. In some embodiments, such as single channel systems where noise is not of concern, the filter  80  can be eliminated. The filter  80  can be a single stage or multiple stage filter, can be a single pass or a multiple pass filter, and can utilize one or more types of filters. For example, the filter  80  can have one stage including one or more fiber Bragg gratings and another stage including one or more Mach-Zehnder interferometric filters. The filter  80  can also include other types of filters, such as a fiber Bragg Fabry-Perot filter, a notched filter, a phase shifted filter, a bulk grating, etc., and can, for example, provide one or more filtered signals to one or more receivers  22 . Many other types and combinations of filters  80  are also possible. 
     The O/E converter  82  converts the optical data signal Λ o  into one or more corresponding electrical signals. The interface  84  provides a connection for data being received and is analogous to the interface  50  in the transmitter  20 . 
       FIG. 20  shows another embodiment of the receiver  22  including a FEC decoder  86 . That receiver  22  can be used to receive data signals which are FEC encoded, such as can be transmitted by the transmitter  20  illustrated in  FIG. 10 . 
       FIG. 21  shows another embodiment of the receiver  22  including an assembler  88  that can be used to receive separated data signals, such as those transmitted by the transmitter  20  illustrated in  FIG. 11 . In that embodiment, the received optical signal is split between two filters  80 , each of which filters one of the signals to be received. In other embodiments, the separate filters  80  can be replaced by a single filter (e.g. a bulk grating or an arrayed waveguide) which can separate from the incoming signal Λ o  the two or more data signals of interest. The filtered signals are converted to electrical form by the O/E converters  82 , and the electrical signals are combined by the assembler  88 . In other embodiments, more than two signals can be assembled. The illustrated embodiment also includes a FEC decoder  86  which decodes the forward error correction encoded signals. 
       FIG. 22  shows another embodiment of the receiver  22  that includes automatic gain controllers (“AGC”)  90 , clock and data recovery (“CDR”) circuits  92 , and demultiplexers  94 , which are sometimes referred to as “deserializers”. The demultiplexers  94  separate a serial data signal into plural lower bit rate data signals, which are assembled by the assembler  88 . The assembler  88  produces the assembled data as several separate data signals which are FEC decoded and combined into a single signal by the interface  84 . The demultiplexing or deserializing of the data signal into several lower bit rate signals facilitates further processing of the signal, such as assembling and FEC decoding. For example, in some circumstances it is more practical to perform parallel processing on several lower bit rate signals than it is to perform the same operation on a single, high bit rate signal. Some or all of the data processing in the receiver  22  can be done with several parallel low bit rate signals. 
       FIG. 23  shows an embodiment of the receiver interface  74  including a SONET performance monitor  96  and a short reach interface (“SRI”) transmitter  98 . The SONET performance monitor  96 , for example, can perform section monitoring and termination functions. The SRI  98  combines the parallel data signal into a higher bit rate, serial signal. The receiver interface  74  is analogous to the transmitter interface  50 . 
       FIG. 25  illustrates one embodiment of a filter  80  which may be used, for example, in the receiver  22 . In that embodiment, the filter  80  includes one periodic filter stage, such as a Mach-Zehnder filter, and one band filter, such as a Bragg grating filter. Other types of periodic and band filters may be used in the filter  80 . In other embodiments, the order of the stages may be different, the filter  80  may include more or less stages, different types. of stages, and different types of filters. 
     Many variations and modifications can be made to the present invention without departing from its scope. For example, advantages of the present invention can be realized with different numbers, configurations, and combinations of components in the transmitters  20  and receivers  22 . Similarly, different numbers and forms of electrical and optical data signals can also be utilized with the present invention. Many other variations, modifications, and combinations are taught and suggested by the present invention, and it is intended that the foregoing specification and the following claims cover such variations, modifications, and combinations.