Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
2000-11-30
2004-02-10
Chan, Jason (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S152000, C398S154000, C398S155000, C398S158000, C398S161000, C398S162000, C398S202000, C398S209000, C398S184000, C398S195000, C385S011000, C385S015000, C385S040000, C356S073100
Reexamination Certificate
active
06690889
ABSTRACT:
The invention relates to the field of optical transmission of digital signals and more particularly transmission at high bit rates on long-haul 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.
BACKGROUND OF THE INVENTION
An optical fiber transmission system typically includes:
a transmit terminal using at least one optical carrier wave whose power and/or optical frequency it modulates with the data to be transmitted,
an optical transmission line consisting of at least one section of monomode fiber conveying the signal sent by the transmit terminal, and
a receive terminal receiving the optical signal transmitted by the fiber.
The performance of an optical transmission system, in terms of signal quality and bit rate in particular, is limited in particular by the optical properties of the line, which is subject to physical phenomena which degrade the optical signals. Of all the phenomena that have been identified, attenuation of the optical power and chromatic dispersion initially appeared to be the most constraining, and means have already been proposed for remedying at least some of the resulting degradation.
The attenuation in fibers of a given type depends on the signal carrier wavelength. Monomode fibers installed in the last decade, referred to as “standard fibers”, have a minimum attenuation for a wavelength of around 1.5 &mgr;m, which has led to the choice of carriers around this value. What is more, to increase transmission distances further, attenuation has been compensated by means of optical amplifiers disposed at the upstream or downstream end of the line or all along the line.
The problem of chromatic dispersion is significant with standard fibers (approximately 17 ps/(km.nm) at 1.5 &mgr;m). One solution is to insert at least one dispersion compensating fiber (DCF) into the line.
Until now, the forms of compensation referred to above have ignored another unfavorable phenomenon referred to as “polarization mode dispersion”. Under current optical transmission operating conditions, this phenomenon has long been considered to be negligible compared to chromatic dispersion, but this no longer applies when attempts are made to increase further the length of the line, and above all the bit rate.
Even in the absence of chromatic dispersion, and although the carrier wave supplied by a laser diode in the transmit terminal is totally polarized, the fibers are subject to polarization dispersion, one effect of which, for example, is that a pulse sent by the transmit terminal is received in a deformed state after it has propagated in a fiber, and has a duration greater than its original duration.
This deformation is due to the birefringence of the fiber, because of which the optical signal is depolarized during transmission. To a first approximation, the signal received at the end of the line fiber can be considered as made up of two orthogonal components, one of which corresponds to a state of polarization for which the propagation speed is maximum (fastest principal state of polarization) and the other to a state of polarization for which the propagation speed is minimum (slowest principal state of polarization). In other words, a pulsed signal received at the end of the line fiber can be considered to be made up of a first pulsed signal polarized with a preferred state of polarization and arriving first and a second pulsed signal propagating with a retarded propagation state and arriving with a time-delay referred to as the “differential group delay” (DGD), which depends in particular on the length of the line fiber. These principal states of polarization (PSP) therefore characterize the line.
Consequently, if the transmit terminal sends an optical signal consisting of a very short pulse, the optical signal received by the receive terminal consists of two successive and orthogonally polarized pulses having a relative time shift equal to the DGD. Because detection by the terminal consists of supplying in electrical form a measured value of the total optical power received, the detected pulse has a duration that is increased as a function of the DGD.
The DGD can be of the order of 50 picoseconds for 100 kilometers of standard fiber. The deformation of the pulses received by the receive terminal can cause errors in decoding the transmitted data and polarization dispersion therefore constitutes a factor limiting the performance of optical lines, whether analogue or digital.
The skilled person knows how the fabricate monomode fibers with a low polarization dispersion (approximately 0.05 ps/km). However, the problem remains with “standard” fibers already installed, which have very high polarization dispersions, constituting a major technical obstacle to increasing the transmitted bit rates. What is more, this problem will also arise for fibers with low polarization dispersion when it becomes necessary to increase the bit rate further.
Also, the skilled person knows how to make fibers with high polarization dispersion, also referred to as polarization maintaining fibers (PMF). These can be used in short lengths to obtain a fixed differential group delay with invariant principal states of polarization. By judiciously disposing a component of this kind (or any means of generating a differential time-delay between two orthogonal modes of polarization) in series with a transmission line subject to polarization dispersion, it is possible to achieve optical compensation of the polarization dispersion, either by using a polarization maintaining fiber with the same DGD as the line but with the fast and slow principal states of polarization interchanged, or by having a principal state of polarization of the combination of the line and the polarization maintaining fiber coincide with the state of polarization of the source in the transmit terminal. To this end, a polarization controller is placed between the line and the polarization maintaining fiber.
An important aspect of the polarization mode dispersion phenomenon is that the DGD and the principal states of polarization of a line vary in time as a function of many factors, such as vibration and temperature. Thus, unlike chromatic dispersion, polarization dispersion must be considered a random phenomenon. In particular, the polarization dispersion of a line is characterized by a parameter referred to as the polarization mode dispersion (PMD) delay, defined as the average value of the measured DGD.
To be more precise, it can be shown that polarization dispersion can be represented by a random rotation vector &OHgr; in Poincaré space in which the states of polarization are usually represented by a state of polarization vector S, referred to as Stokes' vector, whose tip lies on a sphere.
FIG. 1
shows the principal 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; are in 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 and the symbol {circle around (x)} designates the vector or “cross” product.
The modulus of &OHgr; is the value of the DGD, i.e. the propagation time-delay between two waves polarized with the two principal states of polarization of the line.
A consequence of the random nature of polarization dispersion is that compensation has to be adaptive and the differential group delay of the polarization maintaining fiber chosen has to be at least equal to the maximum differential group delay to be compensated. The compensation must ideally be such that the direction e of the principal states of polarization of the line as a whole between the signal sent and the signal received coincides at all times with the direction of the polarization vector S of the received signal. In other words, the above-defined angle &P
Desthieux Bertrand
Ollivier François-Xavier
Penninckx Denis
Alcatel
Chan Jason
Phan Hanh
Sughrue & Mion, PLLC
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