Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-17
2002-01-15
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06339489
ABSTRACT:
The present invention relates to apparatus for dynamically compensating, at least in part, polarization dispersion in an optical fiber transmission system. A long-haul optical fiber transmission system comprises:
a transmitter terminal essentially constituted by a laser diode transmitting a completely polarized optical signal;
a monomode optical fiber conveying the signal transmitted by the transmitter terminal; and
a receiver terminal receiving the optical signal conveyed by the fiber.
All types of fiber suffer from polarization dispersion: a pulse transmitted by the transmitter terminal is received distorted. Its duration is greater than its original duration. Such distortion is due to the fact that the optical signal becomes depolarized while it is being conveyed: the signal received at the end of the link fiber may be considered to be constituted by two orthogonal components, one of which corresponds to a polarization state for which the propagation speed is at its maximum (fast main polarization state), the other component corresponding to a polarization state for which the propagation speed is at its minimum (slow main polarization state). In other words, a pulse signal received at the end of the link fiber can be considered to be made up of a first pulse signal polarized in a privileged polarization state, and arriving first, and of a second pulse signal propagating in a delayed propagation state, and arriving with a delay referred to as the “instantaneous differential delay” which depends in particular on the length of the link fiber.
If the transmitter transmits an optical signal made up of a very short pulse, the optical signal received by the receiver terminal is made up of two successive pulses that are polarized orthogonally and that have a time offset equal to the instantaneous differential delay. This delay may be about 20 picoseconds for a link that is 100 kilometers long and that is made using monomode fiber of the kind manufactured a few years ago. The distortion of the pulses received by the receiver terminal can cause errors in decoding the transmitted data. Therefore polarization dispersion constitutes a limiting factor on the performance of optical links, both analog links and digital links.
It is now known how to manufacture monomode fibers having low polarization dispersion (about 0.05 picosecond/km). However, a large fraction of monomode fibers that have been installed over the last ten years have very high polarization dispersion, and that constitutes a major technical obstacle to increasing transmitted data rates. In addition, the same problem may well arise again even with low polarization dispersion fibers, assuming the race for ever higher data rates continues.
It is known how to make high polarization dispersion fibers that make it possible, by using short segments, to procure a fixed differential delay. Such fibers are also referred to as “polarization-maintaining fibers”. By appropriately disposing such a component (or any apparatus for generating a differential delay between two orthogonal polarization modes) in series with a transmission link having high polarization dispersion, it is possible to compensate the polarization dispersion optically. That may be achieved either by using a polarization-maintaining fiber having the same differential delay as the link, but with the slow main polarization state and the fast main polarization state being swapped over, or by causing a main polarization state of the assembly constituted by the link and the polarization-maintaining fiber to coincide with the polarization state of the transmission source. To that end, a polarization controller is used that is placed between the link and the polarization-maintaining fiber.
The value of the differential delay and the main polarization states of a link vary over time as a function of many factors, such as vibration and temperature. Compensation apparatus must therefore be adaptive, and the differential delay of the polarization-maintaining fiber must be chosen to be not less than any of the differential delay values that are to be compensated.
U.S. Pat. No. 5,473,457 describes a method and apparatus for compensating polarization dispersion in an optical transmission system. That apparatus comprises:
a polarization controller and a segment of polarization-maintaining fiber interposed between the link fiber and the receiver terminal, in that order;
servo-control apparatus controlling the polarization controller as a function of an error signal;
means for modulating the frequency of the laser constituting the source of the transmitter terminal; and
apparatus for measuring the intensity modulation of the received signal, which apparatus comprises a polarizer splitting the signal delivered by the polarization-maintaining fiber into two orthogonally-polarized signals, the axis of the polarizer being disposed at an angle of 45° relative to the intrinsic axes of the polarization-maintaining fiber.
The polarization controller makes it possible to steer the polarization of each of the components of the optical signal delivered by the link fiber, the polarization being steered by turning it through a given angle defined by the value of a command signal applied to the servo-control apparatus. The servo-control means implemented to compensate the polarization dispersion serve to align the slow main state of the link to be compensated with the fast main state of a polarization-maintaining fiber and to align the fast main state of the link to be compensated with the slow main state of said polarization-maintaining fiber which is selected so that its differential delay coincides with the mean polarization dispersion of the link to be compensated. The frequency modulation of the optical signal is converted into polarization modulation by the polarization dispersion of the assembly constituted by the link fiber and by the polarization-maintaining fiber, and then into intensity modulation by the polarizer situated in the apparatus for measuring the intensity modulation of the received signal. The depth of the intensity modulation is directly proportional to the instantaneous differential delay generated by the polarization dispersion, and as weighted by the impact of the polarization of the transmission source (effective differential delay). The intensity modulation signal is used as an error signal to control the polarization controller which is inserted between the link fiber and the polarization-maintaining fiber, the polarization controller being controlled in such a manner as to minimize the intensity modulation signal, and thus to minimize the effective differential delay produced by the assembly comprising the link and the polarization-maintaining fiber. Ideally, the polarization dispersion of the link fiber is exactly compensated by the polarization dispersion, of opposite sign, created by the polarization-maintaining fiber.
That known apparatus suffers from the following drawbacks:
not only does it require the transmitter terminal to be modified to modulate the frequency of the optical signal, but it also requires the receiver terminal to be modified in order to compensate the polarization dispersion; and
it needs to be optimized specifically for each link to be compensated even when the link does not have different polarization dispersion.
An object of the invention is to provide optical apparatus that is situated entirely at the receiver terminal so that no modification needs to be made to the transmitter terminal, and that does not need to be optimized specifically for each link.
The invention provides apparatus for compensating polarization dispersion in an optical transmission system comprising a transmitter terminal for transmitting a polarized optical signal, a link optical fiber, optional optical amplifiers, and a receiver terminal, the apparatus comprising:
a first polarization controller;
means for generating a differential delay between two orthogonal polarization modes, said controller and said means being interposed between the link fiber and the receiver
Bruyere Franck
Francia Christian
Alcatel
Chan Jason
Leung Christina Y.
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