Device for compensating polarization dispersion in an...

Optical waveguides – Polarization without modulation

Reexamination Certificate

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C398S150000, C398S159000

Reexamination Certificate

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06792168

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on French Patent Application No. 01 11 133 filed Aug. 27, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119.
BACKGROUND OF THE INVENTION
1. Field of the invention
The invention relates to transmission of signals by optical means and relates more particularly to transmission at high bit rates on long distance lines using optical fibers.
The invention relates to a device for dynamically compensating at least some of the polarization dispersion that is observed in optical fiber transmission systems.
2. Description of the prior art
An optical fiber transmission system typically includes:
a transmitter terminal which modulates the power and/or optical frequency of a polarized optical carrier wave as a function of information to be transmitted,
an optical transmission line including a section of monomode fiber for routing the signal transmitted by the transmitter terminal, and
a receiver terminal which receives the optical signal transmitted by the fiber.
The performance of an optical transmission system, in particular in terms of signal quality and bit rate, is limited among other things by the optical properties of the line, which is subject to physical phenomena that degrade the optical signals. Means have been proposed for at least partly remedying the degradation caused by the phenomena that appeared at first to be the most severe, which include attenuation of the optical power and chromatic dispersion.
Another undesirable phenomenon is polarization mode dispersion. If the lengths of optical transmission lines, and more importantly their bit rates, are to be increased further, this phenomenon is no longer negligible compared to the chromatic dispersion.
Optical fibers are subject to polarization dispersion, one effect of which is that a polarized light pulse transmitted by the transmitter terminal and received after propagating in a fiber is distorted and has a duration greater than its original duration. The distortion is due to the fact that the optical signal is depolarized during transmission because of the birefringence of the fibers. To a first approximation, the signal received at the end of the connecting fiber can be considered to consist of two orthogonal components, one corresponding to a state of polarization for which the propagation speed is a maximum (fastest principal state of polarization) and the other corresponding to a state of polarization for which the speed of propagation is a minimum (slowest principal state of polarization). In other words, a pulse signal received at the end of the connecting fiber can be considered to comprise a first pulse signal polarized with a privileged state of polarization and arriving first and a second pulse signal with a slower speed of propagation and arriving with a time-delay, known as the differential group delay (DGD), which depends among other things on the length of the line. The differential group delay and the two principal states of polarization (PSP) therefore characterize the line.
Consequently, if the transmitter terminal transmits an optical signal consisting of a very short pulse, the optical signal received by the receiver terminal consists of two successive orthogonally polarized pulses having a relative time shift equal to the DGD. As detection by the terminal consists in supplying a measurement in electrical form of the total optical power received, the detected pulse has its duration increased as a function of the DGD. This time-delay can be the order of 50 picoseconds for 100 kilometers of standard fiber. Accordingly, for a binary signal whose bit rate is 10 gigabits per second, the time-delay can be as much as half a bit period, which is not acceptable. The problem is obviously even more critical at higher bit rates.
One important aspect of polarization mode dispersion is that the differential group delay and the principal states of polarization of a line vary in time as a function of many factors, including vibration and temperature. Accordingly, unlike chromatic dispersion, polarization dispersion must be considered a random phenomenon. In particular, the polarization dispersion of a line is characterized by a polarization mode dispersion delay (PMD) defined as the average of the measured DGD values.
To be more precise, it can be shown that the polarization dispersion can be represented by a random rotation vector &OHgr; in the Poincaré space in which the states of polarization are usually represented by a polarization state vector S, known as the Stokes vector, the end of which is situated on a sphere.
FIG. 1
shows the main vectors involved: the state of polarization vector S, the polarization dispersion vector &OHgr;, and the principal states of polarization vector e. &PHgr; is the angle between S and &OHgr;.
The vectors e and &OHgr; have the same direction and the following equation applies: ∂S/∂&ohgr;=&OHgr;{circle around (x)}S, where &ohgr; is the angular frequency of the optical wave, the symbol {circle around (x)} designating a vector product.
The modulus of &OHgr; is the value of the group delay difference, i.e. of the propagation time-delay between two waves polarized in accordance with the two principal states of polarization of the line.
One principle of polarization dispersion compensation consists in inserting between the line and the receiver a compensator device which has a differential group delay and principal states of polarization which can be represented in the Poincaré space by a vector &OHgr;c such that the resultant vector &OHgr;t obtained from the sum &OHgr;+&OHgr;c is at all times parallel to S or zero. These two cases are shown by
FIGS. 2 and 3
, respectively.
One consequence of the random character of polarization dispersion is that a compensator must be adaptive and include a differential group delay generator DDG (for example a polarization-maintaining fiber) which provides a differential group delay at least equal to the maximum differential delay values to be compensated. In practice the aim of compensation must be for the direction e of the principal states of polarization of the line as a whole (including the compensator) to coincide at all times with the direction of the polarization vector S of the received signal. In other words, the angle &PHgr; previously defined must be kept as small as possible.
One embodiment of a device for compensating polarization mode dispersion is described in U.S. Pat. No. 6,339,489.
FIG. 4
shows an example of an optical transmission system including the above kind of compensator device.
This system is a wavelength division multiplex system designed to convey a plurality of spectral channels in the form of signals Se&lgr;, Se&lgr;′, Se&lgr;″ with respective carrier wavelengths &lgr;, &lgr;′, &lgr;″. Each channel, for example the channel Se&lgr;, comes from a transmitter terminal TX transmitting an optical signal taking the form of amplitude modulation of a polarized carrier wave. The channels are combined in a multiplexer MUX whose output is coupled to an optical transmission line LF. This line is typically an optical fiber, but more generally can include diverse optical components (not shown), such as optical amplifiers on the upstream and/or downstream side of the fiber and/or chromatic dispersion compensators. The line can also be composed of a plurality of sections of fiber with optical amplifiers between them.
The end of the line is connected to a receiver terminal, for example the terminal RX, via a demultiplexer DEMUX whose function is to extract the spectral channel Sr addressed to the receiver RX.
The system includes a device CM between the demultiplexer DEMUX and the receiver RX for compensating polarization dispersion so that the receiver RX receives a compensated optical signal Sc. The device CM includes a polarization controller PC, a generator DDG for generating a compensating differential group delay DGDc

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