Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2001-11-27
2004-03-09
Negash, Kinfe-Michael (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S017000, C398S065000, C398S152000, C398S147000
Reexamination Certificate
active
06704510
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical communication networks, and more particularly to a method and apparatus for monitoring the quality of optical channels of an optical link having polarization-mode dispersion (PMD) to intelligently determine and distinguish PMD problems in the optical link.
BACKGROUND OF THE INVENTION
In a typical optical communications system, an optical signal in the form of a series of light pulses is emitted from a modulated optical transmitter comprising a laser diode. Each light pulse is of extremely short duration, such as 40 ps, and is roughly Gaussian shaped as a function of time. In the frequency domain, this signal comprises numerous frequency components spaced very closely about the nominal center frequency of the optical carrier, such as 193,000 GHz. As this type of modulated optical signal passes through an optical fiber, different frequency components of the optical signal travel at slightly different speeds due to an effect known as chromatic dispersion. In the course of the optical signal traveling through a very long fiber, such as fibers that are 200 km in length, chromatic dispersion causes a single pulse of light to broaden in the time domain, and causes adjacent pulses to overlap one another, interfering with accurate reception. Fortunately, many techniques are known for compensating for chromatic dispersion.
Another form of dispersion is becoming a limiting factor in optical communications systems as progressively higher data rates are attempted. Polarization-mode dispersion (PMD) arises due to birefringence in the optical fiber. This means that for two orthogonal directions of polarization, a given fiber can exhibit differing propagation speeds. A light pulse traveling through a fiber will probably, unless some control means are employed, have its energy partitioned into polarization components that travel at different speeds. As with chromatic dispersion, this speed difference causes pulse broadening and restricts the usable bandwidth of each optical carrier.
Schemes to actively compensate for PMD generally involve detecting the presence of polarization-dependent timing differences and either: a) applying delay elements to one or the other polarization to realign the timing of pulses; or b) controlling the polarization state of the optical signal upon entry into the fiber, or at intermediate points along the fiber, such that birefringent effects are minimized or canceled out. Active compensation techniques are required because the PMD of a given fiber varies over time due to temperature and pressure changes along the fiber and due to aging. A fiber that is installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces. A fiber buried underground can be sensitive to loads such as street traffic or construction work.
A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Conventionally, a properly designed optical link can maintain a bit-error-rate (BER) of 10
−13
or better. Noise, attenuation, and dispersion are a few of the impairments that can render an optical signal marginal or unusable at the receiver. Generally, when an optical channel degrades to a bit-error-rate of 10
−8
, a communications system will automatically switch to an alternate optical channel that has a better BER.
One common method of analyzing the quality of a modulated optical signal is a so-called “eye diagram”, shown in FIG.
1
. The eye diagram consists of overlaying successive frames of time-domain traces of the signal, with each frame corresponding to one period of the nominal periodicity of the modulation. As portrayed, the vertical axis represents instantaneous intensity of the received signal, and the horizontal axis corresponds to time. Many successive traces of transmitted “ones” and “zeros” define a region or window within the middle of the display. In the time axis, the window is bound on either side by the transitional leading and trailing edges of the pulses. Using this technique, a large clear area or “window” in the center with no encroachment from any side represents a good signal in that the presence or absence of a pulse during each clock cycle is clearly distinguishable.
Noise added to a signal appears as “fuzziness” of the lines defining the window. Sufficient noise can even obliterate the appearance of the window, representing a bad signal in that “ones” and “zeros” are no longer distinguishable. Impairments in the time axis, such as chromatic dispersion or polarization mode dispersion, cause the transitional areas of the display to close in upon the window from either side. Overlapping of pulses can require more stringent synchronization of the receiver's decision point, or even render the signal unusable.
A given optical receiver will automatically adapt to receive a modulated optical signal. Automatic gain control (AGC), frequency control, and phase lock-in are typically applied in sequence so that a threshold decision circuit can best sample the signal and decode every pulse. Superimposed upon the eye-diagram, an optimal point of operation for a threshold decision circuit would intuitively be at the center of the window, as shown by the “+” in FIG.
1
. This means that the intensity threshold is about halfway between the zero values and one values observed on average.
Timewise, the center of the window corresponds to sampling the pulses in the middle of their duration when they tend to be of maximum intensity also shown by the “+” in FIG.
1
. Intuitively, one can see how this choice for an operating point would be the most robust against either noise or timing impairments, which cause the window to shrink.
A received optical signal can undergo some degree of change in, for example, pulse width without having an immediate impact on the BER, as observed by this optimally positioned main decision circuit. A particular type of receiver has been developed having at least two independent decision circuits of the type just described. Reference is made to an article entitled “Q-factor Measurement for High Speed Optical Transmission Systems”, authored by A. J. Ramos which is from proceedings of the SubOptic '97 conference, San Francisco Calif. 1997,891, as well as an article entitled “Margin Measurement in Optical Amplifier Systems” authored by Bergano, Kerfort and Davidson, Photonics Technology Letters, 5(1993)304, the teachings of both are incorporated herein by reference. A main decision circuit is dedicated to actual communications reception and is maintained at the optimum point, once it is established, within the center of the window. But for analyzing signal quality to a finer degree and for measuring degradation before it impacts the BER of the actual communications, an auxiliary decision circuit is used to probe the extents of the operating window. Robustness to timewise disturbances is gauged by directing the auxiliary decision circuit to sample at various time offsets relative to the optimum point. Findings by the auxiliary circuit may even be used to fine-tune the optimum decision point settings of the main decision circuit.
The auxiliary decision circuit is set to a given timing offset and its output is monitored for BER, especially in comparison to the output of the main decision circuit. The BER measurement at each operating point can typically take several minutes. Gradually, BER data is accumulated for every offset value. As expected, a plot of this data resembles an inverted Gaussian curve with a minimum BER occurring some optimum offset, as shown in
FIG. 2. A
similar plot is derived by varying the amplitude threshold of the auxiliary decision circuit.
All of this BER data may be summarized into a “Q” factor or quality metric for the received signal. In general terms, the broader the range of timings over which a low BER can be sustained, the greater the Q factor of the signal. A
Mao Xiaoping Charles
Robinson Andrew Niall
Negash Kinfe-Michael
WorldCom, Inc.
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