Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1998-06-23
2001-02-20
Hellner, Mark (Department: 3642)
Optics: measuring and testing
By particle light scattering
With photocell detection
C359S199200
Reexamination Certificate
active
06191854
ABSTRACT:
The present invention relates to an optical telecommunications system and a method for the transmission of optical signals.
The use of optical fibres for sending optical signals, carrying the information to be communicated over a distance, is known in the most recent telecommunications technology.
The optical signals sent along an optical fibre are subject to attenuation, so that it may be necessary to amplify the signal power level from the end of the line close to the transmitting station, by means of a power amplifier (booster).
A preamplifier is usually present at the end of the line close to the receiving station, to bring the power level of the signal into a range of values which is suitable for the receiving equipment.
There are known optical amplifiers based on the properties of a fluorescent dopant, for example erbium, which, if suitably excited by the application of pumping light energy, generates a high emission -in the wave band corresponding to the minimum attenuation of the silica optical fibres.
In-fibre optical amplifiers, for example erbium doped in-fibre amplifiers, are known, for example, from patent application EP 677.902.
To increase the length of the communication line, the use of optical line amplifiers, connected in predetermined positions along the line to periodically increase the power of the transmitted signals, is known.
The length of the optical fibre spans between the power amplifier and the preamplifier, in the case of communication lines without intermediate amplifiers, or the length of the fibre spans between successive line amplifiers, in the case of lines with line amplifiers, is limited by the available optical power at the input of the span.
The applicant has approached the problem of the transmission of signals in optical communication systems with medium or long spans, for example those with a length of between 100 and 1000 km, and preferably with a length of between 200 and 400 km. In particular, the applicant has approached the problem of the transmission of optical signals in communication systems comprising a single optical fibre span with a length of between 200 and 400 km.
It is known that an optical signal propagating along an optical fibre may give rise, in the case of high optical power density to stimulated Brillouin scattering (SBS). This phenomenon may constitute a source of noise which can obstruct or impede the correct reception of the signal at the end of the optical fibre.
It is also known, for example from U.S. Pat. No. 4,560,246(Cotter) and EP 565.035 (Hitachi), that the threshold optical power of the said phenomenon of stimulated Brillouin scattering in an optical fibre is increased in the case of widening, by phase modulation, of the frequency bands of the propagating optical signal.
The bandwidth values required to prevent SBS may be calculated, by a person skilled in the art, from the corresponding power values, for example on the basis of the information in the cited patents. If the bandwidth is not sufficient, this bandwidth may be increased by known methods.
The widening of the said frequency band is not necessary if the signal has a sufficiently wide band to prevent the occurrence of SBS, for example between 0.3 and 1.5 nm, and a power of less than 30 dBm (1 W).
In particular, the applicant has observed that signals of this type can be obtained by direct semiconductor laser modulation, for example with digital signals at 2.5 Gbit/s, and amplification of these, before they are sent to the fibre, to suitable powers. In this case the widening of the line is attributable to the phenomenon of “chirping” of the signal wavelength, related to the direct semiconductor laser modulation, which entails a variation of the signal wavelength during the pulses' emission, particularly at rising or falling edges in the case of a digital modulating signal.
It is known that the optical fibres used in communication lines have chromatic dispersion, due to the combination of the characteristics of the refractive index profile and of its constituent material, which varies with the wavelength of the transmitted signal and is cancelled at a certain value &lgr;
0
of the wavelength.
This phenomenon of chromatic dispersion consists essentially in the fact that the different chromatic components of each pulse, each characterized by its own wavelength, travel in the fibre at different velocities.
Step index (SI) single-mode optical fibres are known. These fibres have a zero dispersion point in the spectral region around 1300 nm and have a positive dispersion per unit wavelength, of approximately 17-18 ps/(nm·km), for signals with wavelengths in the band between approximately 1530 nm and approximately 1560 nm.
Optical fibres with a shifted zero chromatic dispersion point, or DS (dispersion shifted) fibres, are known. The optical characteristics of DS fibres are designed so that they shift the zero chromatic dispersion point to a wavelength in the region between 1500 and 1600 nm, which is commonly used for telecommunications.
Fibres of this kind are defined in ITU-T Recommendation G.653 of March 1993, in which it is specified that the chromatic dispersion of the fibre is cancelled nominally at a wavelength &lgr;
0
of 1550 nm, with a tolerance of 50 nm with respect to this value.
DS fibres are described, for example, in U.S. Pat. Nos. 4,715,679, 4,822,399, and 4,755,022 and are marketed by Corning (USA) under the trade name SMF/DS™ and by Fibre Ottiche Sud (Italy) under the trade name SM DS.
In the framework of the present application the terms “dispersion shifted fibre” will indicate, in general, a single-mode optical fibre having a relatively low dispersion in the third telecommunication window in comparison with standard SI fibres, i.e., a fibre having a zero dispersion wavelength &lgr;
0
in the region between 1450 and 1650 nm.
It is known that high values of chromatic dispersion, for example in the case of intercontinental optical communication lines, with lengths of the order of many thousands of kilometers, must be suitably compensated to permit correct reception of the signals.
For example, in the presence of lines with positive dispersion, negative dispersion optical fibres of the type described in the patents U.S. Pat. No. 5,361,319 or U.S. Pat. No. 5,448,674 or in the patent application JP 1-295,207 may be used.
Other examples of dispersion compensators are optical fibre Bragg filters in with variable pitch (chirped), for example of the type described in the article by F. Ouellette published in Optics Letters, Vol. 12, No. 10, pp. 847-849, in October 1987, or in U.S. Pat. No. 4,953,939.
However, it is considered that compensation is not necessary in the presence of a chromatic dispersion of limited extent, for example of less than 1500 ps
m, such as that which accumulates in SI fibre spans with a length of less than approximately 80 km, or in DS fibre spans with a length of up to 1000 km.
U.S. Pat. No. 5,355,240 (Prigent et al.) describes implementing an optical communication link when transmission of information over the link is subject to non-linear effects to the degree that correction of such effect is necessary. A link of this kind is typically a long-haul link such as an intercontinental link which can be from around 3000 to 10000 km long or even longer. The patent indicates that the described invention may be relevant for shorter links if they have a high mean optical power rating. Correction means for limiting the disadvantageous consequences of chromatic dispersion and/or non-linear effects comprise, at the output of the transmission line, a dispersion compensator adapted to apply dispersion in the opposite direction to and of lower absolute value than the line dispersion.
U.S. Pat. No. 5,343,322 (Pirio et al.) describes a system for very long distance transmission of digital signal by optical fibres, with dispersion compensation at reception. The transmitter and receiver stations are connected by a monomode optical fibre with negative chromatic dispersion at the operating wavelength of the system, having a length of
Donati Fabio
Grasso Giorgio
Ottolenghi Paolo
Righetti Aldo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hellner Mark
Pirelli Cavi E Sistemi S.p.A.
LandOfFree
Optical telecommunications system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical telecommunications system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical telecommunications system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2586205