Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-28
2001-01-23
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C385S011000, C385S037000
Reexamination Certificate
active
06178021
ABSTRACT:
The invention concerns a device for compensating polarization dispersion of channels in a wavelength-division multiplex signal.
The invention applies to wavelength-division multiplex (WDM) transmission systems.
BACKGROUND OF THE INVENTION
All types of fiber are subject to polarization dispersion: a pulse sent by a sending terminal is deformed when it is received. Its duration is greater than its original duration. The deformation is due to the fact that the optical signal is depolarized during transmission. The signal received at the end of the connecting fiber may be considered to comprise two orthogonal components, one corresponding to a maximum propagation speed polarization state (fastest main polarization state) and the other corresponding to a minimum propagation speed polarization state (slowest main polarization state).
In other words, a pulse signal received at the end of the connecting fiber may be considered to comprise a first pulse signal polarized in accordance with an advanced polarization state and arriving first and a second pulse signal propagating in accordance with a retarded propagation state and arriving with an instantaneous differential delay that depends in particular on the length of the connecting fiber.
If the sending terminal sends an optical signal consisting of a very short pulse, the optical signal received by the receiving terminal comprises two successive pulses polarized orthogonally and having a relative time shift equal to the instantaneous differential delay. This delay can be 20 ps for a 100 km link comprising a monomode fiber of the kind manufactured a few years ago.
Deformation of the pulses received by the receiving terminal can cause errors in decoding the transmitted data and polarization dispersion is therefore a factor limiting the performance of optical links, whether analog or digital.
We now know how to fabricate monomode fibers with low polarization dispersion (approximately 0.05 ps/km
½
). However, a high proportion of monomode fibers installed in the last decade have very high polarization dispersion, which constitutes a major technical obstacle to propagation of the transmitted bit rates. Furthermore, if the bit rate race continues, there is nothing to prevent this problem appearing in the low polarization dispersion fibers that we now know how to produce.
We know how to make fibers with high polarization dispersion, also known as maintained polarization fibers, which enable a fixed differential delay to be achieved through the use of short segments. By judiciously placing a component of this kind (or any arrangement generating a differential delay between two orthogonal polarization modes) in series with a transmission link subject to high polarization dispersion, it is possible to compensate the polarization dispersion. This can be achieved either using a maintained polarization fiber having the same differential delay as the link, but interchanging the slow and fast main polarization states, or by having a main polarization state of the combination of the link and the maintained polarization fiber coincide with the polarization state of the sending source. To achieve this a polarization controller is placed between the link and the maintained polarization fiber.
The value of the differential delay and the main polarization states of a link vary in time in accordance with many factors, such as vibration and temperature. A compensator system must therefore necessarily be adaptive and the differential delay of the maintained polarization fiber must be made at least equal to all differential delay values to be compensated.
Polarization dispersion is a difficult problem to solve in the context of upgrading existing optical fiber networks with channels having bit rates of 10 Gbit/s and above.
It is estimated that the maximum polarization dispersion that can be tolerated is 10% of the duration of a bit, for example 10 ps at 10 Gbit/s and only 2.5 ps at 40 Gbit/s.
Polarization dispersion compensator systems have been designed. However, there is no means of solving the problem of polarization dispersion in a wavelength-division multiplex network other than juxtaposing identical single-channel compensator systems after demultiplexing. One example of this is the system disclosed in French Patent Application No. 96 16194 filed Dec. 30, 1996 by the Applicant and concerning a single-channel transmission system of this kind.
OBJECTS AND SUMMARY OF THE INVENTION
The aim of the present invention is to solve the above problem and to propose solutions which have an optimum architecture, in particular in terms of cost.
The present invention consists in a system for compensating polarization dispersion for wavelength-division multiplex systems. The solutions proposed in accordance with the invention are based on the use of cascaded multiplexers and demultiplexers to compensate the polarization dispersion of all the channels forming the multiplex.
System architectures are disclosed for implementation in-line or at the end of the transmission line.
More particularly, the invention consists in a system for compensating polarization dispersion of channels in a wavelength-division multiplex signal, the system comprising a plurality of cascaded polarization control modules respectively associated with said channels and a differential delay generator delivering a compensated multiplex signal, wherein each module comprises:
a drop and insert multiplexer having a first input for receiving a multiplex input signal, a first output for an associated channel signal extracted from said multiplex input signal, a second input for inserting a modified channel signal and a second output for delivering a modified multiplex output signal, and
a polarization controller adapted to receive said extracted channel signal and to deliver said modified channel signal,
and wherein the system further comprises a control loop for controlling the polarization controllers in response to respective optical properties of the channel signals of said compensated multiplex signal.
Generally speaking, the above optical properties are obviously chosen to be representative of the quality of the compensated multiplex signal so as to measure the degrees of polarization of the channel signals directly or indirectly. The control loop then optimizes the quality of the compensated multiplex signal by operating on the polarization controllers.
In one particular instance, the control loop comprises demultiplexer means receiving the compensated multiplex signal, extraction outputs for said channel signals extracted therefrom and control means adapted to receive at least a portion of the signals from said extraction outputs, to measure the degrees of polarization thereof and to control the polarization controllers to maximize said measured degrees of polarization.
In accordance with another feature, the demultiplexing means include a plurality of demultiplexers in cascade, this embodiment being suited to the use of multiplexers having a structure using fiber segments including in-fiber Bragg gratings (IFBG).
In accordance with another feature, the control means for each channel include:
a feedback loop polarization controller,
means for measuring the total intensity of the portion of the signal from one of said extraction outputs,
means for measuring the intensity of a component of that portion in a fixed polarization direction and for controlling the loop polarization controller to maximize that intensity,
means for calculating the degree of polarization from said measured total intensity and the measured intensity in the fixed polarization direction, and
feedback means for controlling the polarization controller of the corresponding channel in accordance with the degree of polarization calculated in this way in order to maximize the degree of polarization.
In accordance with another feature of the invention, the multiplexers and demultiplexers used have a structure using fiber segments including in-fiber Bragg gratings.
REFERENCES:
patent: 5508839 (1996-04-01), Ono
patent: 5960133
Bruyere Franck
Penninckx Denis
Alcatel
Lieu Vu
Pascal Leslie
Sughrue Mion Zinn Macpeak & Seas, PLLC
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