Patent Application
20110318019
1. Field of the Invention
Embodiments of the present disclosure relate to the field of optical communication systems. More particularly, the present disclosure relates to the use of dispersion compensation modules to optically aid in dispersion compensation of optical DWDM signals performed by digital signal processing circuits.
2. Discussion of Related Art
In long haul optical communication systems, various modulation schemes are used to transmit optical signals using dense wavelength division multiplexing (DWDM) where a plurality of optical channels each at a particular wavelength propagate over fiber optic cables. These long-haul optical systems can be undersea fiber optic communication systems that operate at high data rates per channel. Unfortunately, as these bit rates increase, so do the transmission penalties associated with the transmission fiber's dispersion and nonlinear index of refraction. After propagation over long distances, the impact of these effects create processing and decoding difficulties at the receiver which may compromise the integrity of the transmitted information.
One type of signal distortion caused by propagation of an optical signal in a fiber is chromatic dispersion which is the result of waveguide dispersion (i.e. different wavelengths of light propagate in the fiber at different speeds) and material dispersion (i.e. phase velocity of the light varies with different wavelengths). Another fiber effect is polarization mode dispersion (PMD) which is the result of imperfections in the fiber that lead to different propagation speeds for orthogonal polarization components of an optical signal.
Various techniques have been employed to reduce the deleterious effects of dispersion over long distance DWDM transmission systems. For example, certain modulation techniques such as Quadrature Phase Shift Keying(QPSK) and Polarization Multiplexed QPSK (PM-QPSK) may be implemented which improves transmission tolerances to chromatic dispersion and PMD as well as increasing spectral efficiency by simultaneously transmitting on two orthogonal polarizations. Another compensation technique employs in-line dispersion compensating fiber (DCF) disposed within each of the fiber spans of the transmission line to compensate for dispersion as it accumulates throughout the transmission path. For example,
Generally, coherent receivers detect both amplitude and phase to decode the received optical data signals. Because certain of these long-haul systems extend across the Atlantic and Pacific Oceans, there is significant cost and complexity with installing DCF within each of the fiber spans (referred to as “in-line”). Thus, there has been interest in using fiber optic cables with no in-line dispersion compensation and only using SMF as the transmission medium for undersea communication systems. However, if in-line DCF is not used, accumulated dispersion over the transmission line may be too large to compensate for by using only DSP techniques. For example, optical transmission over trans-Atlantic distances of approximately 6500 km with no in-line compensation results in accumulated dispersion in the range of a 120000 to 140000 ps/nm; and optical transmission over trans-Pacific distances of approximately 10000 km results in accumulated dispersion that may exceed 200000 ps/nm. Accumulated dispersion at these levels result in the overlapping of a large number of data bits, thereby compromising transmission performance. By way of example, at 40 Gb/s transmission rates using PM-QPSK results in approximately 102-103 bits overlapping and for 100 Gb/s transmission rates using PM-QPSK results in approximately 103-104 bits overlapping. In order to compensate for these high dispersion values using only DSP, it would require implementing finite impulse response (FIR) filters with a plurality of taps at the receiver terminals or by performing Fourier transforms on a relatively large number of sampled received bits. Consequently, this would require a prohibitive number of application specific integrated circuits for DSP processing at the receiver resulting in loss of receiver optical to signal noise ratio sensitivity and limit the amount of dispersion that can be compensated for using DSP. Accordingly, an object of the present disclosure is to overcome these problems and provide a transmission line utilizing SMF with no in-line dispersion compensation and to optically aid DSP dispersion compensation at a system terminal.
Exemplary embodiments of the present disclosure are directed to managing dispersion compensation in optical communication systems. In an exemplary embodiment, a communication system terminal includes a dispersion compensation unit (DCU) communicatively coupled to an optical transmission line. The DCU has a first and second optical amplifiers and a dispersion compensation module (DCM) disposed therebetween. The DCM is configured to compensate for at least a first portion of optical dispersion associated with the optical signals supplied from the transmission line. A receiver is communicatively coupled to the second amplifier and is configured to convert the plurality of optical signals into corresponding electrical signals. The receiver includes a digital signal processor configured to compensate for a second portion of dispersion associated with each of the corresponding electrical signals.
In another exemplary embodiment, a communication system includes a transmitter configured to supply a modulated optical signal having a plurality of optical channels. A first dispersion compensation module (DCM) is communicatively coupled to the transmitter and is configured to pre-compensate for at least a first portion of dispersion associated with the modulated optical signal. A first dispersion compensation unit (DCU) is disposed between the first DCM and an optical transmission line. The DCU comprises a first and second optical amplifiers and a second DCM disposed therebetween. The second DCM is configured to pre-compensate for at least a second portion of optical dispersion associated with the modulated optical signal supplied to the transmission line. A second dispersion compensation unit (DCU) is communicatively coupled to the optical transmission line and includes a third and fourth optical amplifiers and a third dispersion compensation module (DCM) disposed therebetween. The DCM is configured to compensate for at least a first portion of accumulated optical dispersion associated with the optical signals supplied from the transmission line. A receiver is communicatively coupled to the fourth amplifier and is configured to convert the plurality of optical signals into corresponding electrical signals. The receiver includes a digital signal processor configured to compensate for a second portion of accumulated dispersion associated with each of the corresponding electrical signals.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
Presently disclosed embodiments provide an optical transmission system utilizing optically aided digital signal processing (DSP) dispersion compensation at a system terminal without the use of in-line dispersion compensation. This allows the system to utilize a transmission line comprising single mode fiber (SMF) with a large effective area to reduce the effects of fiber non-linearities while still adequately compensating for accumulated dispersion. By employing optically aided DSP dispersion compensation, the plurality of Application Specific Integrated Circuits (ASIC) required to perform non-optically assisted DSP is avoided as well as obviating the need for associated power requirements with these ASICs. This also allows lower cost SMF to be installed along the entire transmission line between the terminals.
It is important to note that transmission line 130 does not include in-line dispersion compensation and utilizes single mode fiber. As referenced above, dispersion compensating fiber (DCF) has previously been used along the fiber spans of an optical communication system to compensate for accumulated dispersion. However, the use of DCF contributes an additional signal propagation path having its own attenuation and nonlinear properties. In addition, the amount of accumulated dispersion that can be compensated for using DSP at the receiver 125 while maintaining adequate overall signal quality is limited. Thus, receive terminal 120 in accordance with the present embodiment includes one or more dispersion compensation units (DCU) 1261 . . . 126N communicatively coupled to transmission line 130 and receiver 125. In particular, each DCU is configured to receive the transmitted optical signals and provide an amount of dispersion compensation in the optical domain for the received signals propagating over transmission line 130 prior to transmission to receiver 125 which employs electronic dispersion compensation utilizing digital signal processing (DSP) techniques. In this manner, each DCU provides dispersion compensation to optically aid the DSP dispersion compensation performed at receiver 125. The number of DCU's required to adequately compensate for dispersion prior to supplying the optical signals to receiver 125 will depend on the amount of dispersion compensation required based on a number of factors including, but not limited to, length of signal propagation over transmission line 130, channel bandwidth, fiber type, etc.
As an example of one preferred embodiment, DCU 126N is optically coupled to DCU 1261 and is configured to receive the optical signals from DCU 1261 and supply these signals to the receiver 125, to an additional DCM or to another DCU depending on the amount of dispersion compensation required. DCU 126N comprises a first optical amplifier 128, a second optical amplifier 142 and one or more DCM's 1411 . . . 141N disposed therebetween. Although DCU 126N physically may share second amplifier 128 with DCU 1261, amplifier 128 is logically considered a first amplifier of DCU 126N. Similar to amplifiers 127 and 128, amplifier 142 may be, for example, an erbium doped fiber amplifier having a single or multiple gain stages. Each of the DCMs 1411 . . . 141N is configured to compensate for at least a portion of optical dispersion associated with the optical signals supplied from DCU 1261. Receiver 125 is communicatively coupled to amplifier 142 of DCU 126N. If only one DCU 1261 is employed based on the amount of dispersion compensation required, then receiver 125 is coupled to second amplifier 128. Although transmitter 110 and receiver 125 are illustrated as separate elements, it should be understood that the transmitter 110 may also be included in receive terminal 120 and vice versa to accommodate bidirectional transmission therebetween.
Receiver 125 is configured to receive the optical data signals from at least one of DCU's 1261 . . . 126N and convert these optical signals, each at a particular wavelength, into corresponding electrical signals. Receiver 125 may be a coherent receiver which detects not only an optical signal's amplitude, but phase and polarization as well. In particular, coherent detection mixes the received optical data signals from the DCU with light from a local oscillator which is tuned to a wavelength close to the wavelength or channel of the optical data signal. This combined signal is detected by a photo-detector in receiver 125 which outputs a photocurrent containing a component at a frequency that is the difference between the received optical data signal and the local oscillator frequency. This differential signal contains the amplitude and phase carried by the optical data signal from transmitter 110. In other words, it detects whole characteristics of the received signal across the spectrum and converts these optical signals into electrical signals.
Fiber Bragg grating 215 is an optical fiber that contains a series of perturbations or modulation of its core refractive index over a given length. The grating is configured to reflect light propagating therethrough when the wavelength of that light corresponds to the modulation periodicity. However, the grating period varies along its length and different wavelengths are reflected by different portions of the grating. This induces different time delays of the reflected optical signals based on their respective wavelengths. As noted above, the optical data signals transmitted over line 130 travel long distances and are subject to dispersion (chromatic and PMD) which stretches or broadens the optical pulses that comprise these data signals. Accordingly, the effect of reflecting these optical pulses using grating 215 is to compress, to some degree, the received optical pulses thereby compensating for the accumulated dispersion. The reflected optical signals are then supplied back to circulator 210 and are output via port 2103.
DCM 129N further includes a second optical circulator 220 coupled to second optical amplifier 128. Circulator 220 includes a plurality of input and output ports 2201 . . . 2203. A first of the plurality of input ports 2201 receives the optical signal from first optical circulator 210. The received signals are circulated to a first output port 2202. Another fiber Bragg grating 225 is coupled to the output port 2202 and is configured to reflect a portion of the received optical data signal back toward port 2202 to provide dispersion compensation for the received optical data signal as described above. The reflected optical signals are then supplied back to circulator 220 and are output via port 2203 to optical amplifier 128, to another DCM or to another DCU.
The receive terminal 320 includes a DCU defined by amplifiers 327, 328 and DCM's 3291 . . . 329N disposed therebetween. Each of the DCM's 3291 . . . 329N is configured to compensate for a portion of accumulated optical dispersion associated with the optical data signals which propagated over transmission line 330. Receive terminal 320 includes a receiver communicatively coupled to amplifier 328 and is configured to convert the optical data signals into corresponding electrical signals. As described with reference to receiver 125 of
Embodiments of the present disclosure may be implemented at a transmitter and receiver of an optical communication system. A processor may be used to effectuate operations associated with a communication system, as is known to one of ordinary skill in the art. A processor as used herein is a device for executing stored machine-readable instructions for performing tasks and may comprise any one or combination of hardware, software, and firmware. A processor may also comprise memory storing machine-readable instructions executable for performing tasks. A processor acts upon information by manipulating, analyzing, modifying, converting, or transmitting information for use by an executable procedure or an information device, and/or by routing the information to an output device. A processor may use or comprise the capabilities of, e.g., a controller or microprocessor. A processor may be electrically coupled with any other processor, enabling interaction and/or communication therebetween. A processor comprising executable instructions may be electrically coupled by being within stored executable instructions enabling interaction and/or communication with executable instructions comprising another processor. A user interface processor or generator is a known element comprising electronic circuitry or software, or a combination of both, for generating display images or portions thereof.
An executable application comprises code or machine readable instructions for conditioning the processor to implement predetermined functions, such as those of an operating system, a context data acquisition system, or other information processing system, e.g., in response to user command or input. An executable procedure is a segment of code or machine readable instruction, sub-routine, or other distinct section of code or portion of an executable application for performing one or more particular processes. These processes may include receiving input data and/or parameters, performing operations on received input data and/or performing functions in response to received input parameters, and providing resulting output data and/or parameters.
The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.
