Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-01-16
2003-03-25
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06538788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a telecommunication system, and more particularly, to noise reduction in a long-distance optical telecommunications system.
2. Discussion of the Background
In a long-distance optical telecommunications system, the transmitted signal generally suffers from effects of nonlinearity and dispersion, which must be taken into consideration when it comes to optimizing the parameters of the system itself. On account of these effects, the signal received at the end of the communication line may have distortion (or variations of form), constituting a limitation on the system's transmission capacity. In order to minimize the distortion, it is possible to use special transmission techniques, which depend on characteristics of the system in question, such as the bit transmission speed (or bit rate), the length of the connection, the spacing between the amplifiers and the number of WDM channels. To quote as examples of these techniques, there is chromatic dispersion compensation by means of dispersion compensation fibers or variable-pitch Bragg gratings, solitonic transmission without chromatic dispersion compensation and solitonic transmission with arrangements for chromatic dispersion compensation, as described, for example, in patent application WO99/08406 filed by the Applicant. The latter-named technique, in some cases, may represent a suitable solution for reducing the distortion in the system.
A further phenomenon, constantly present in optically amplified transmission systems, is represented by the progressive increase in amplifier spontaneous emission noise (ASE) generated along the line by the line optical amplifiers. Each time the signal passes through an optical amplifier, spontaneous emission noise is added to it. At the line end, the influence of ASE noise on the system's performance will be correspondingly greater, the higher the level of this noise (in terms of optical power) in relation to the signal level, that is to say the lower the signal-to-noise ratio (SNR), defined as the ratio of the optical power associated with the signal to the optical power associated with the noise in a pre-established reference band of wavelength. In general, the minimum value needed for the signal-to-noise ratio in order to guarantee correct reception of the signal depends on the characteristics of the system under examination (bit rate, transmitted signal format, receiver characteristics).
When distortion and ASE noise are simultaneously present at the end of the link, the performances of the system change depending on the size of the two contributions. Generally speaking, the impairment of the system's performances due to distortion and noise must not be in excess of established limits, beyond which correct signal reception is no longer guaranteed. In order to maintain the signal impairment within the established limits, constraints are generally imposed when defining the system parameters, and particularly when defining the bit rate, the number of WDM channels to be transmitted, the overall length of the link, the number of amplifiers to be inserted in the link and the output power of the amplifiers.
If the nonlinear effects present in the system are negligible, it may be assumed that, during propagation of the signal, there is no interaction between signal and noise and, therefore, that the ASE noise may be considered as an additional contribution to the signal. In this case, the impairment of the signal received corresponds to the combination of the impairment due to the distortion (calculated as if the ASE noise did not exist) and the impairment due to the ASE noise (calculated as if the distortion did not exist).
If, on the other hand, the nonlinear effects present in the system are not negligible, for example in the case of long-distance transmissions and/or transmissions at a high bit rate, the optical signal and the optical noise propagated along the line interact with one another. This interaction occurs due to the effect of a phenomenon known as “modulation instability”, described for example in G. P. Agrawal, Nonlinear Fiber Optics, Academic Press, pages 134-141 and 267-273. In particular, there is modulation instability in a transmission medium if, together with the chromatic dispersion, there is a particular type of nonlinearity, known as “Kerr effect”, which is found with the refractive index of the medium depending on the intensity of the optical signal passing through the medium itself. In the remainder of this description, when we speak of nonlinearity, we will be referring to the nonlinearities known as “Kerr effect”.
In the case in hand, the phenomenon of modulation instability manifests itself as follows. Consider a transmission line in which there is propagation both of an optical signal S and an optical noise n. The optical noise n is a complex quantity and may be divided into a component n
F
in phase with the signal S and a component n
Q
in quadrature with the signal S. The modulation instability originating at the end of the transmission line may have different effects depending on whether the chromatic dispersion along the line is of normal or anomalous type. In the case of a line operating with anomalous dispersion, the modulation instability causes an amplification of both the in-phase noise component n
E
the quadrature noise component n
Q
to the detriment of the signal S. On the other hand, where the dispersion is of normal type, only the quadrature component n
Q
is amplified to the detriment of the signal S, whereas the in-phase component n
F
is attenuated. These phenomena are described in detail in M. Midrio, “Statistical Properties of Noise Propagation in Normal Dispersion Nonlinear Fibers”, J. Opt. Soc. Am. B. vol. 14, n. 11 November 1997, pages 2910-2914.
In a telecommunications system, at the end of the transmission line the signal and the noise are generally received by a quadratic type photodetector (a photodiode), in which beating occurs between the signal and the noise. In actual fact, however, the beating is only between the signal and the in-phase component n
F
, whereas the quadrature component n
Q
does not cause beating with the signal, but only with itself. This is because the electronic signal received by the photodetector is proportional to the power of the optical radiation received, that is to say to the quantity:
|S+n
F
+i n
Q
|
2
=S
2
+n
F
2
+2·
S n
F
+n
Q
2
In this expression, S
2
represents the effective signal detected by the photodiode. The other terms represent noise contributions. Usually the terms n
F
2
and n
Q
2
are negligible and, therefore, the main contribution to the noise at the receiver is given by 2·S·n
F
, i.e. the term that represents the beating of the signal with the in-phase noise.
Therefore the main contribution to signal impairment due to noise comes from the beating [signal]-[in-phase noise], whereas the beatings [quadrature noise]-[quadrature noise] and [in-phase noise]-[in-phase noise] are non-influential, apart from effects of a secondary order. The presence of this type of signal impairment defines a technical problem that the Applicant has perceived as being very important in the development of optical telecommunications systems, particularly over long-distances (indicatively, distances of more than 500 km) and with high performance, for example with a bit rate greater than or equal to 2.5 Gbit/s.
With regard to continuous transmission of optical signals (i.e., on a single wavelength and with no added information), the effect of modulation instability on the noise is studied, for example, in the above article by M. Midrio. The study presented in this article confirms that, in continuous transmission of signals in a normal dispersion optical fiber, the modulation instability acts by causing a decrease of the noise in-phase component. This behavior is the opposite of that observed in an anomalous dispersion f
Franco Pierluigi
Midrio Michele
Romagnoli Marco
Cisco Technology Inc.
Negash Kinfe-Michael
LandOfFree
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