Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-09-15
2002-07-09
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C385S011000
Reexamination Certificate
active
06417948
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to dispersion compensation in optical fiber transmission systems, and particularly to a differential delay component for use in a polarization mode dispersion (PMD) compensator.
2. Technical Background
The polarization of transmitted light is an important factor affecting the signal quality and available bandwidth (or channel spacing) in single-mode (SM) optical fibers. While single-mode fibers are usually characterized as carrying or supporting only one mode, they actually carry two degenerate modes each having orthogonal polarization relative to one another. By degenerate, it is meant that a single-mode optical fiber having a circularly-symmetric core would not differentiate between the two polarization modes, which are functionally identical or indistinguishable for most purposes. Given a single-mode fiber core having perfect circular symmetry, the presence of two distinct polarization modes would have negligible impact on optical fiber communications. However, in practice these two modes may be subject to polarization mode dispersion (PMD), in which the two polarization modes encounter dissimilar physical conditions or optical properties within the fiber, and therefore travel at slightly different speeds relative to one another. The different transmission speeds cause the polarization modes to spread or separate, creating a delay or phase offset between the two modes which is analogous to chromatic and other types of dispersion. Polarization mode dispersion can significantly denigrate the integrity of certain high-performance optical communications systems, particularly time-division-multiplexed signals operating on the order of 2.5 Gbit/second or faster.
There are several recognized causes of polarization mode dispersion which can be grouped into two main classes: birefringence and mode coupling. The causes may also be intrinsic to the fiber, or extrinsically induced. For example, the core of conventional single-mode fiber is not perfectly symmetric about the longitudinal axis, and non-uniformities in the index of refraction may vary both radially and axially over distances. These non-uniformities may result from deviations in normal dopant concentrations, physical stresses induced when the fiber is drawn or wound on a spool, or external physical pressures (sometimes called “lateral loading”) induced by operations such as coating and cabling the optical fiber. Mode coupling (or “energy transfer”) may result from coupling sites within the glass itself, fiber-to-fiber contact, contact between a coated optical fiber and other objects, or the bends and twists introduced into the length of optical fiber as it is laid, routed, spliced, or connected.
Longer lengths of optical fiber and complex optical transmission networks can then be viewed as a multiplicity of concatenated birefringent segments, with the output optical field reflecting the sum of the individual birefringences of each segment times their respective lengths. The state of polarization at the output will also fluctuate with time due to environmental conditions and physical changes in the configuration of the optical fiber and network.
Over distances, variations in some of these effects may cancel one another out, so that the resultant polarization mode dispersion at a particular node or receiver is relatively small. Conversely, because these effects are non-uniform, the polarization mode dispersion at one point along a transmission pathway may differ markedly from that at another point, and may also shift significantly over time. A given signal may also encounter different polarization mode dispersion effects when traversing alternate routes, so that the resultant dispersion evident in a signal at one location may depend upon the sum of uncorrected dispersion-causing effects to which that signal was subjected over a longer or unique transmission pathway. Polarization mode dispersion may also be introduced or varied somewhat randomly by the addition or deletion of optically-functioning components in a transmission pathway, such as when operations like amplification, wavelength-division multiplexing, regeneration, or switching are performed.
One approach towards compensating for polarization mode dispersion is specialized single-mode fibers which have polarization mode attenuating or maintaining properties created by intentionally-asymmetric cores. In single-polarization fiber, one polarization mode is transmitted normally, whereas the orthogonal polarization mode is subject to three or four orders of magnitude greater attenuation, effectively stripping that mode and leaving the first for signal transmission. In polarization-maintaining fiber, input light is split into two orthogonal modes along a core having an induced stress or asymmetry which defines an maintains different refractive indices (but similar attenuation values) for each polarization. The two polarization modes may travel at different speeds due to the relative refractive indices, but the light energy does not shift between polarization modes. Polarized light may be aligned with and transmitted along one axis of the polarization-maintaining fiber, in which case a single polarization mode is detected at the receiver. Alternately, both polarization modes may be transmitted, but only one filtered at the detector and used for communication signal transmission.
The use of single-polarization and polarization-maintaining fibers introduces certain limitations and drawbacks into the optical transmission system, such as the need to fabricate a more complex optical fiber geometry, or the need for more specialized transmitter and receiver components capable of aligning or detecting light signals at preferred polarization orientations. The polarization orientations must also be preserved or modified uniformly at various junctions along the optical pathway, such as splices or connections, polarization-dependent optical components, and so forth.
Another approach to the issue of polarization mode dispersion is a class of devices referred to as polarization mode dispersion compensators (or “PMD compensators”). These devices may be inserted into an optical pathway to detect and correct polarization mode dispersion at a given location (such as immediately before an amplifier, router, or receiver module), and may be periodically adjusted to increase or decrease the level of correction. The device may be adjusted depending upon the degree to which a relatively-constant polarization mode dispersion drifts over time, or may monitor and correct for near instantaneous shifts in polarization mode dispersion.
Though various designs and configurations for polarization mode dispersion compensators have been proposed, conceptually they may generally be regarded as having several common characteristics or operations. First, the two polarization modes must be split or differentiated from one another so that the relative time or phase differential (or “differential group delay”) between the two modes can be accurately measured. A polarization transformer may also be used in this step to align each polarization mode with a fixed reference or axis. Second, a delay must be introduced into the pathway of the faster or leading polarization mode to counterbalance the measured differential. This variable differential time delay line may have fast and slow axes aligned with the polarization axes induced by the polarization transformer, and the delay line generally imposes a higher degree of delay along at least one axis compared with standard transmission fiber. Third, the polarization modes will be recombined for further transmission or signal processing (if the actual signal was split and measured, as opposed to tapping off a portion of the signal to be measured independently). Finally, some type of forward or backward feedback loop is established to control and periodically adjust the delay.
The process of measuring the differential between the two polarization modes and feeding that informatio
Chowdhury Dipakbin Q.
Lindquist Robert G.
Rahman Ashiqur
Alden Philip G.
Corning Incorporated
Pascal Leslie
Singh Dalzid
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