Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-07-01
2002-07-16
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C356S073100
Reexamination Certificate
active
06421153
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to optical communication systems, and more particularly to a device and method for accurately determining PMD of optical signals transmitted over a fiberoptic network.
BACKGROUND OF THE INVENTION
In a typical optical communication system, an optical signal in the form of a series of light pulses is emitted from an optical transmitter comprising a modulated laser diode. 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 an optical signal traveling through a very long fiber, such as 200 km, 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 communication 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.
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 communication system will automatically switch to an alternate optical channel in an attempt to improve the 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 optical signal, with each frame corresponding to one period of the nominal periodicity of the modulation. The vertical axis of an eye diagram represents instantaneous intensity (I) of the received signal, and the horizontal axis corresponds to time (T). Many successive traces of transmitted “ones” and “zeros” define a region or window within the middle of the display, defining the “eye diagram.” On the time axis, the window is bound on either side by the transitional leading and trailing edges of the pulses. The eye diagram in
FIG. 1
shows an optical signal
50
,
52
traveling through the optical fiber. A large clear area or “window” in the center, such as the one shown in
FIG. 1
, with a single peak and 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.
Active PMD compensation techniques are required in an optical communication system because the PMD of a given fiber varies over time due to the fiber aging, and due to temperature and pressure changes along the fiber. A fiber installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces (e.g. wind blowing the fiber). A fiber buried underground can be sensitive to loads such as street traffic or construction work. Also, the fiber may not have a perfect circular cross-section, causing varying delays of the polarization components.
In current optical communication systems, PMD changes are typically compensated for by a Polarization Mode Dispersion Compensator (PMDC) which detects the degree of polarization-dependent differential delay suffered by an optical carrier and then adaptively corrects the delay. As polarization characteristics of the fiber change, the PMDC constantly monitors and adjusts the signal in an attempt to minimize the PMD contribution to overall dispersion. A typical PMD compensator splits an incoming optical signal, intensity modulated by a data pulse stream, into two polarizations. The relative timing of the two signal halves is then corrected by introducing a delay into one signal half and then recombining the halves to form a corrected output signal.
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 state-of-polarization (SOP) of the signal upon entry into the fiber, or at intermediate points along the fiber, such that birefringent effects are minimized or canceled out. Existing PMD compensators either include a state-of-polarization (SOP) detector and/or controller in order to feed a consistent polarization orientation into the beam splitter at the front end of the compensator, or rely on the use of one elsewhere in the optical communication system.
SUMMARY OF THE INVENTION
A problem of prior art PMD compensators is that they either assume SOP is not changing in the optical path, or utilize a SOP detector and/or controller to stabilize SOP by feeding a consistent polarization orientation into a beam splitter at the front end of the compensator, with the presumption that any residual SOP changes are caused by PMD. These assumptions are incorrect and cause flawed readings and unnecessary corrections of the optical signal to reduce PMD. These assumptions are incorrect because while changes in PMD cause changes in SOP, the converse is not necessarily true; SOP changes are not always caused by PMD. PMD is a separate phenomenon, distinguishable from SOP, where SOP can change dramatically and abruptly without a corresponding change in PMD. Thus, PMD is not being correctly compensated for in current optical transmission systems. More particularly, existing PMD compensator designs assume that SOP through a given optical path does not change appreciably over time, or that SOP can be controlled by a slow response SOP controller such that any residual SOP deviations are presumed attributable to PMD. This assumption is invalid, resulting in poor performance of existing PMD compensators. The present invention, an SOP-independent PMD detector, solves the problems of the prior art by providing a device and method of determining and compensating real-time PMD variations independently of SOP shifts.
The method and device of the present invention analyzes the pulse shape, for example, the eye diagram, of an optical signal on an optical transmission line to determine PMD. The amount of PMD calculated may then be compensated for. The PMD is observable on an eye diagram of an optical pulse stream suffering from PMD as two overlapping pulses, or peaks, with a saddle point in-between. The relative temporal offset, observable as the distance between the two peaks of the eye diagram, is indicative of and a measure of PMD. The SOP is seen on the eye diagram as a function of relative amplitude of the peaks, and advantageously, is not a factor taken into consideration w
Way David
Xia T. J.
Pascal Leslie
Phan Hanh
WorldCom, Inc.
LandOfFree
Device and method for determining PMD independent of SOP does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Device and method for determining PMD independent of SOP, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Device and method for determining PMD independent of SOP will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2889846