Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-06-23
2004-03-23
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C398S081000, C398S152000, C398S194000
Reexamination Certificate
active
06710904
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the transmission of signals by optical means and more particularly to high bit rate transmission on long-haul links using optical fibers.
The invention relates to a system for dynamically compensating at least some polarization dispersion observed in fiber optic transmission systems.
2. Description of the Prior Art
A fiber optic transmission system typically includes:
a transmitter terminal using at least one optical carrier wave whose optical frequency and/or power it modulates according to the data to be transmitted,
an optical transmission line consisting of at least monomode fiber section conveying the signal emitted by the transmitter terminal, and
a receiver terminal receiving 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 in particular by the optical properties of the link, which is the site of physical phenomena which degrade the optical signals. Of all the various phenomena that have been identified, attenuation of the optical power and chromatic dispersion initially appeared to be the most severely constraining and means have been proposed for at least partially remedying the degradation that they cause.
The attenuation in fibers of a given type depends on the signal carrier wavelength. Monomode fibers installed during the past decade and referred to as “standard fibers” have a minimum attenuation at a wavelength around 1.5 &mgr;m, which shows the benefit of choosing this value for the carriers.
Also, to increase transmission distances further, attenuation has been compensated by means of optical amplifiers at the upstream or downstream end of the link or all along the link.
Chromatic dispersion also depends on wavelength. For standard fibers there is zero chromatic dispersion at 1.3 &mgr;m and the chromatic dispersion at 1.5 &mgr;m is approximately 1.7 ps/(km.nm). The low attenuation at 1.5 &mgr;m has led to the development of new fibers referred to as “dispersion shifted fibers” for which there is zero chromatic dispersion at this wavelength.
Nevertheless, to improve the performance of standard fibers already installed, attempts have been made to correct the effects of chromatic dispersion in these fibers at 1.5 &mgr;m.
One solution is to insert into the link at least one dispersion compensating fiber (DCF). To compensate the chromatic dispersion exactly, it is sufficient for the dispersion compensating fiber to have a length and dispersion characteristics such that the cumulative dispersion along the compensating fiber is equal and opposite to that created along the fiber of the transmission link.
A residual cumulative dispersion value DR can therefore be defined for the whole of the link, including the compensating fiber(s), as the algebraic sum of the cumulative dispersions DL and DC of the dispersion compensating fiber(s) and the fiber of the transmission link. In mathematical terms this can be expressed by the equation:
DR=DC+DL=∫D
1
(z
1
).
dz
1
+∫D
2
(
z
2
).
dz
2
(1)
in which z
1
and Z
2
are the abscissas of points respectively along the dispersion compensating fiber and along the associated link and D
1
and D
2
are the respective chromatic dispersion parameters at the abscissas z
1
and z
2
of the dispersion compensating fiber and of the fiber of the transmission link. The integrals which express the cumulative dispersions DC and DL are respectively calculated along the dispersion compensating fiber and along the associated transmission link fiber, taking the direction of propagation of the waves as the positive direction.
It will be remembered that the dispersion parameter D is related to the propagation constant &bgr; by the equation:
d
2&bgr;
/d&ohgr;
2
=−(2
&pgr;c/&ohgr;
2
)
D,
in which &ohgr; is the angular frequency of the wave and c is the speed of light in a vacuum.
The condition for exact compensation of chromatic dispersion is therefore DR=DC+DL=0.
In reality exact compensation of chromatic dispersion is not always the optimum, because the quality of the compensated signal received also depends on other transmission parameters and in particular on how the transmitted signal is modulated. This applies in particular if the transmitted signal includes a “chirp”, i.e. optical frequency modulation accompanying any amplitude modulation.
In fact, this kind of compensation is applied only when required, i.e. when transmission conditions (fiber type, modulation type, transmission distance and bit rates) would, if not compensated, lead to bit error rates exceeding a commercially acceptable limiting value, typically 10
−15
. Moreover, to minimize the cost of the dispersion compensating fiber, a minimum compensation value is normally chosen that is compatible with the required bit error rate. Thus for sufficiently short links no attempt is made to compensate chromatic dispersion.
Until now, the forms of compensation referred to above have been treated independently and without regard to another unfavorable phenomenon referred to as “polarization modal dispersion”. Under current operating conditions for optical transmission systems, this phenomenon has long been considered as negligible compared to chromatic dispersion. This is no longer the case when the length, and most importantly the bit rate, of the links are to be increased further.
Even in the absence of chromatic dispersion in the usual sense, and although the carrier wave supplied by a laser diode in the transmitter is totally polarized, the fibers are the site of polarization dispersion which has the effect, for example, that a pulse emitted by the transmitter terminal is received in a distorted form after propagating in a fiber and has a duration greater than its original duration.
This distortion is due to the birefringence of the fibers, whose effect is that the optical signal is depolarized during transmission. To a first approximation, the signal received at the end of the link fiber can be considered as made up 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 propagation speed is a minimum (slowest principal state of polarization). In other words, the pulsed signal received at the end of the link fiber can be considered to be made up of a first pulsed signal which is polarized with a privileged state of polarization and arrives first and a second pulsed signal propagating with a retarded state of propagation and arriving with a delay referred to as the differential group delay (DGD) which depends in particular on the length of the link fiber. The above two principal states of polarization (PSP) therefore characterize the link.
Consequently, if the transmitter terminal emits an optical signal consisting of a very short pulse, the optical signal received by the receiver terminal is made up of two successive pulses polarized orthogonally and having a relative time difference equal to the differential group delay. As detection by the terminal entails providing an electrical form of the measured total optical power received, the detected pulse has a temporal width increased as a function of the value of the differential group delay.
The delay can be in the order of 50 picoseconds for a standard fiber 100 kilometers long. The distortion of the pulses received by the receiver terminal can cause errors in decoding the transmitted data and polarization dispersion therefore constitutes a factor limiting the performance of optical links, whether analog or digital.
It is currently possible to manufacture monomode fibers with low polarization dispersion (approximately 0.05 ps/km). However, the problem remains in the case of “standard fibers” already installed and which have very high polarization dispersion, constituting a major technical obstacle to
Hamaide Jean-Pierre
Lanne Stéphanie
Penninckx Denis
Alcatel
Pascal Leslie
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