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  Optical communications using multiplexed single sideband transmission and heterodyne detectionUSPTO Application #: 20060291868Title: Optical communications using multiplexed single sideband transmission and heterodyne detection Abstract: A transmitter subsystem generates an optical signal which contains multiple subbands of information. The subbands have different polarizations. For example, in one approach, two or more optical transmitters generate optical signals which have different polarizations. An optical combiner optically combines the optical signals into a composite optical signal for transmission across an optical fiber. In another approach, a single optical transmitter generates an optical signal with multiple subbands. The polarization of the subbands is varied, for example by using a birefringent crystal. In another aspect of the invention, each optical transmitter generates an optical signal containing both a lower optical sideband and an upper optical sideband (i.e., a double sideband optical signal). An optical filter selects the upper optical sideband of one optical signal and the lower optical sideband of another optical signal to produce a composite optical signal. (end of abstract) Agent: Meyertons, Hood, Kivlin, Kowert & Goetzel, P.C. - Austin, TX, US Inventors: Ting K. Yee, Peter H. Chang, Shin-Sheng M. Tarng, Gregory M. Cutler, Slava Yazhgur, Ji Li, Laurence J. Newell, James F. Coward, Michael W. Rowan, Norman L. Swenson, Matthew C. Bashaw USPTO Applicaton #: 20060291868 - Class: 398152000 (USPTO) Related Patent Categories: Optical Communications, Transmitter And Receiver System, Including Polarization The Patent Description & Claims data below is from USPTO Patent Application 20060291868. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of pending U.S. patent application Ser. No. 09/728,373, "Optical Communications System Using Heterodyne Detection", by Ting K. Yee and Peter H. Chang, filed Nov. 28, 2000, which is a continuation-in-part of pending U.S. patent application Ser. No. 09/474,659, "Optical Communications System Using Heterodyne Detection", by Ting K. Yee and Peter H. Chang, filed Dec. 29, 1999. [0002] This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/265,251, "Fiber Optic Communications Using Optical Single Sideband Transmission Including using Interleaver Filters and Heterodyne Detection and Apparatus for Impairment Compensation using Nonlinear Phase Conjugation," by Ting K. Yee, et al., filed Jan. 30, 2001. [0003] This application relates to pending U.S. patent application Ser. No. 09/746,261, "Wavelength-Locking of Optical Sources," by Shin-Sheng Tarng, et al., filed Dec. 20, 2000. [0004] This application also relates to pending U.S. patent application Ser. No. 09/747,261, "Fiber Optic Communications using Optical Single Sideband Transmission and Direct Detection," by Ting K. Yee and Peter H. Chang, filed Dec. 20, 2000. [0005] This application also relates to pending U.S. patent application Ser. No. 09/854,153, "Channel Gain Control For An Optical Communications System Utilizing Frequency Division Multiplexing," by Laurence J. Newell and James F. Coward, filed May 11, 2001; and pending U.S. patent application Ser. No. 09/569,761, "Channel Gain Control For An Optical Communications System Utilizing Frequency Division Multiplexing," by Laurence J. Newell and James F. Coward, filed May 12, 2000. [0006] This application also relates to pending U.S. patent application Ser. No. 09/405,367, "Optical Communications Networks Utilizing Frequency Division Multiplexing," by Michael W. Rowan, et al., filed Sep. 24, 1999; which is a continuation-in-part of pending U.S. patent application Ser. No. 09/372,143, "Optical Communications Utilizing Frequency Division Multiplexing and Wavelength-Division Multiplexing," by Peter H. Chang, et al., filed Aug. 20, 1999; which is a continuation-in-part of U.S. patent application Ser. No. 09/229,594, "Electrical Add-Drop Multiplexing for Optical Communications Networks Utilizing Frequency Division Multiplexing," by David B. Upham, et al., filed Jan. 13, 1999; which is a continuation-in-part of U.S. patent application Ser. No. 09/035,630, "System and Method for Spectrally Efficient Transmission of Digital Data over Optical Fiber", by Michael W. Rowan, et al., filed Mar. 5, 1998. [0007] The subject matter of all of the foregoing applications is incorporated herein by reference. BACKGROUND OF THE INVENTION [0008] 1. Field of the Invention [0009] This invention relates generally to optical fiber communications, and more particularly, to the use of single sideband transmission and heterodyne detection for optical fiber communications systems. [0010] 2. Description of the Related Art [0011] As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic. [0012] Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as SONET have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and transmits the resulting optical signal across the optical fiber to the receiver. The receiver recovers the original data from the received optical signal. Recent advances in transmitter and receiver technology have also resulted in improvements, such as increased bandwidth utilization, lower cost systems, and more reliable service. [0013] However, current optical fiber systems also suffer from drawbacks which limit their performance and/or utility. For example, optical fibers typically exhibit dispersion, meaning that signals at different frequencies travel at different speeds along the fiber. More importantly, if a signal is made up of components at different frequencies, the components travel at different speeds along the fiber and will arrive at the receiver at different times and/or with different phase shifts. As a result, the components may not recombine correctly at the receiver, thus distorting or degrading the original signal. In fact, at certain frequencies, the dispersive effect may result in destructive interference at the receiver, thus effectively preventing the transmission of signals at these frequencies. Dispersion effects may be compensated by installing special devices along the fiber specifically for this purpose. However, the additional equipment results in additional power loss (e.g., insertion loss) as well as in additional cost, and different compensators will be required for different types and lengths of fiber. Other fiber effects, such as fiber nonlinearities, can similarly degrade performance. [0014] As another example, the transmitter in an optical fiber system typically includes an optical source, such as a laser, and an external modulator, such as a Mach-Zender modulator (MZM). The source generates an optical carrier and the modulator is used to modulate the optical carrier with the data to be communicated. In many applications, linear modulators are preferred in order to increase the performance of the overall system. MZMs, however, are inherently nonlinear devices. Linear operation is approximated by biasing the MZM at its quadrature point and then limiting operation of the MZM to a small range around the quadrature point, thus reducing the effect of the MZM's nonlinearities. However, this results in an optical signal with a large carrier (which contains no information) and a small modulated signal (which contains the data to be communicated). A larger optical signal to noise ratio is required to compensate for the large carrier. [0015] As a final example, optical fibers have an inherently large bandwidth available for the transmission of data, but constructing transmitters and receivers which can take advantage of this large bandwidth can be problematic. First, current approaches, such as the on-off keying and time-division multiplexing of signals used in the SONET protocols, cannot be extended to higher speeds in a straightforward manner. This is because current electronics technology limits the speeds at which these approaches can be implemented and electronics fundamentally will not have sufficient bandwidth to fill the capacity of a fiber. Even if this were not a limitation, current modulation schemes such as on-off keying are not spectrally efficient; more data can be transmitted in less bandwidth by using more efficient modulation schemes. [0016] Current optics technology also prevents the full utilization of a fiber's capacity. For example, in wavelength division multiplexing, signals are placed onto optical carriers of different wavelengths and all of these signals are transmitted across a common fiber. However, the components which combine and separate the different wavelength signals currently place a lower limit on the spacing between wavelengths, thus placing an upper limit on the number of wavelengths which may be used. This also leads to inefficient utilization of a fiber's bandwidth. [0017] The ever-increasing demand for communications bandwidth further aggravates many of the problems mentioned above. In order to meet the increasing demand, it is desirable to increase the data rate of transmission across each fiber. However, this typically can only be achieved by either increasing the bandwidth being utilized and/or by increasing the spectral efficiency of the encoding scheme. Increasing the bandwidth, however, aggravates frequency-dependent effects, such as dispersion. Increasing the spectral efficiency increases the signal to noise requirements. [0018] Thus, there is a need for optical communications systems which more fully utilize the available bandwidth of optical fibers. There is further a need to reduce or eliminate the deleterious effects caused by fiber dispersion, to reduce the power contained in the optical carrier, and to combat the many drawbacks mentioned above. SUMMARY OF THE INVENTION [0019] In accordance with the present invention, an optical communications system is for communicating information across an optical fiber and includes a transmitter subsystem. The transmitter subsystem includes at least two optical transmitters coupled to an optical combiner. Each optical transmitter generates an optical signal containing a subband of information. The optical signals have different polarizations, which preferably are orthogonal polarizations. The optical combiner optically combines the optical signals into a composite optical signal. [0020] In another aspect of the invention, the transmitter subsystem includes an optical transmitter coupled to a polarization controlling device. The optical transmitter generates an optical signal containing at least two subbands of information. The polarization controlling device, for example a birefringent crystal, varies a polarization of the subbands so that the subbands have different polarizations. [0021] The use of different polarizations yields many benefits. For example, subbands with different polarizations will interact less since they have different polarizations. Thus, unwanted effects due to phenomena such as four-wave mixing and cross-phase modulation will be reduced between the differently polarized subbands. Continue reading... Full patent description for Optical communications using multiplexed single sideband transmission and heterodyne detection Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Optical communications using multiplexed single sideband transmission and heterodyne detection patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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