Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-07-16
2001-09-18
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06292282
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to methods and apparatus for optical communication.
BACKGROUND OF THE INVENTION
The communication bandwidths available with optical fibers are very large, generally many terahertz (THz). Unfortunately, this bandwidth is too large to be effectively used by a single amplitude-modulated data channel in a practical communication system. Many factors limit the practical use of this bandwidth. These factors include limitations on the modulation rates achievable with conventional electronic devices as well as the high cost of high-bandwidth optical components such as modulators and detectors. In some communication systems, fiber dispersion limits the useful fiber bandwidth.
Methods for using more of the available fiber bandwidth include several multiplexing methods that permit data transmission at rates exceeding the rates possible for a single amplitude modulated channel. These methods include time-division multiplexing (TDM), wavelength-division multiplexing (WDM), and optical code-division multiple access (OCDMA). Each of these approaches has significant limitations. In TDM, data from one or more data streams is assigned to selected time slots so that each data stream uses a portion of the fiber bandwidth. TDM allows aggregation of data streams from many users, but does not increase the data rate available to a single data stream. Some examples of TDM systems include commercial communication systems that conform to the synchronous optical network (SONET) standard or the synchronous digital hierarchy (SDH) standard.
Wavelength-division multiplexed (WDM) systems and dense WDM (DWDM) systems transmit data using multiple spectrally displaced narrow-band optical carriers (wavelengths), and each wavelength is temporally encoded. The time-coding is typically a relatively simple amplitude modulation scheme and thus the bandwidth of each wavelength channel is set by limits similar to those of non-WDM systems. WDM systems normally require multiple laser sources, one for each wavelength channel. The number of wavelength channels that can be multiplexed in a single fiber is limited by the spectral channel separation required to compensate for variations in laser wavelength as well as other factors such as the gain bandwidth of optical amplifiers. The available maximum bandwidth is limited by the number of wavelength channels and the per-channel data rate. The spectral channel separations and per-channel data rates of many WDM systems leave significant portions of the fiber bandwidth unused, and higher data rates are theoretically possible.
OCDMA uses coded data bits whose spectrum exceeds the data transmission rate and spans a spectral window. Multiple approaches to the realization of OCDMA have been proposed. Unfortunately, implementation of OCDMA can be difficult and expensive, and improved methods are needed to take advantage of the full bandwidth of optical fibers.
SUMMARY OF THE INVENTION
The present invention provides, inter alia, optical communication methods and apparatus that use time-wavelength (TW) codes.
According to a first aspect of the invention, time-wavelength coders (encoders and decoders) are provided that receive input optical signals having a first predetermined time-wavelength spectrum and generate an output signal having a second predetermined time-wavelength spectrum. The first and second predetermined time-wavelength spectra include a plurality of spectral components. The coders provide optical paths for corresponding spectral components of the input optical signals and the path lengths of the optical paths are determined by time-wavelength codes.
In an illustrative embodiment, a coder comprises an optical fiber that includes a plurality of fiber Bragg gratings defined by spatial variations of an optical property of the fiber. The spatial variations provide predetermined time delays and phase differences for the spectral components. Coherent coders are provided in which the phase differences for the spectral components are predetermined to within about one-half of a wavelength of the associated spectral component. The spatial variations in the optical property of the fiber include variations in at least one of amplitude, spatial period, and spatial phase of the optical property. The Bragg grating can be defined by refractive-index variations, and the refractive-index variations can be in the fiber core, the fiber cladding, or in a coating on the fiber cladding.
In another representative embodiment, a coder comprises a plurality of optical paths corresponding to spectral components of an input, the optical paths having path lengths selected to produce predetermined delays (and phase shifts) for the spectral components. A beam divider directs the spectral components along the respective optical path and a beam combiner recombines the spectral components after propagation along respective optical paths. In some embodiments, the path lengths are predetermined by a time-wavelength code. For a coherent coder, the path lengths and phase shifts are predetermined to significantly less than &lgr;/2, wherein &lgr; is a wavelength of the spectral component received by a respective optical path. In other embodiments, the path lengths are selected to compensate for dispersion in a transmission medium such as an optical fiber or to convert an optical input coded with a first code into an output corresponding to a second code.
In another representative embodiment, an encoder is provided that receives a Fourier-transform-limited optical input and produces at least two Fourier-transform-limited spectral components. These spectral components have bandwidths less than or equal to the bandwidth of the optical input and durations longer than the duration of the optical input.
According to another aspect of the invention, transmitters for an optical communication system are provided. A representative embodiment of such a transmitter comprises a light source that is modulated in response to a data stream, the light source comprising a plurality of spectral components. A time-wavelength encoder receives the modulated optical signal and produces an output signal having a predetermined time-wavelength spectrum.
According to another aspect of the invention, optical receivers are provided for detecting an optical signal having a time-wavelength spectrum determined by a time-wavelength code. A representative embodiment includes a time-wavelength encoder that receives the optical signal and modifies the time-wavelength spectrum of the optical signal. A photodetector receives the optical signal from the encoder and converts the optical signal into an electrical signal.
According to yet another aspect of the invention, optical communication systems arm provided. A representative embodiment includes a first and a second series of optical pulses corresponding to a first and a second data stream. A first encoder receives the first series of optical pulses and produces a first coded output having a time-wavelength spectrum determined by a first time-wavelength code. A second encoder receives the second series of optical pulses and produces a second coded output having a time-wavelength spectrum determined by a second time-wavelength code. A combiner receives and combines the first and second coded outputs. A divider receives the combined first and second coded outputs and produces a first output and a second output that are delivered to first and second decoders, respectively.
According to yet another aspect of the invention, methods are provided for multiplexing a first optical data stream and a second optical data stream, the first and second optical data streams having respective time-wavelength spectra. Time-wavelength codes are selected for encoding and decoding the first and second data streams and the time-dependent spectra of the first and second optical data streams are transformed according to the selected time-wavelength codes. The data streams are combined and transmitted in a transmission medium. The combined optical data streams are received an
Grunnet-Jepsen Anders
Mossberg Thomas
Munroe Michael
Sweetser John N.
Chan Jason
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Sedighian M. R.
Templex Technology, Inc.
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