Pulse or digital communications – Multilevel
Patent
1995-12-13
1999-02-02
Vo, Don N.
Pulse or digital communications
Multilevel
375289, 359185, 359246, H04L 2534
Patent
active
058675345
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a fiber optic transmission method having reduced sensitivity to chromatic dispersion caused by the fiber; a transmission device and system for implementing this method.
The maximal transmission distance over a repeaterless optical fiber is limited by attenuation and chromatic dispersion. Erbium-doped optical fiber amplifiers can correct the attenuation but the chromatic dispersion remains a major limitation in the case of signals having a high bit rate, for example 10 Gbit/s. Various prior art methods can compensate chromatic dispersion on a given fiber section, at least in part.
Other prior art methods are directed to making the transmission less sensitive to chromatic dispersion, without changing the latter. For example, the article "10 Gbit/s unrepeated three-level optical transmission over 100 km of standard fibre" by X. Gu and L. C. Blanck published in Electronics Letters, 9th Dec. 1993, vol. 29, n.degree. 25, describes a method consisting of: signal, i.e. a signal which can assume values approximately equal to +1, 0 and -1, by reducing the bandwidth of a binary signal; obtain an amplitude modulated carrier at three different levels; particular method of compensating chromatic dispersion; fiber using a photoreceiver supplying an electrical signal representative of the amplitude of the received optical signal, and comparing this signal with two reference values to reconstitute two binary signals in parallel; and
The use of duobinary coding and amplitude modulation reduces the bandwidth occupied by the signal transmitted. This reduces the effect of chromatic dispersion and therefore significantly increases the maximal transmission distance. Another advantage is that the optical transmitter and optical receiver can have a narrower bandwidth than in the case of conventional transmission in which the carrier is amplitude modulated by a binary signal.
However, this prior art method has a drawback: it is necessary to discriminate three different amplitude levels in the electrical signal supplied by the photodetector. Discriminating three levels is much more difficult than discriminating two levels as required for amplitude modulation by a binary signal. Also, the signal
oise ratio is lower than that which would be obtained with a binary signal.
SUMMARY OF THE INVENTION
An object of the invention is to propose an optical transmission method which is less sensitive to chromatic dispersion but in which demodulation is easier.
The invention consists in an optical transmission method having reduced sensitivity to dispersion, wherein: can assume three meaningful values: a maximal value, a minimal value and an intermediate value between the maximal value and the minimal value; and modulation signal; with: the filtered modulation signal, the phase of the modulated carrier being shifted approximately 180.degree. in one case relative to the other; and signal; discriminated in the envelope of the power of the modulated carrier and the data pattern transmitted is deduced therefrom.
The above method has reduced sensitivity to chromatic dispersion because the bandwidth of the signal transmitted is reduced by coding it in the form of a ternary signal as compared with conventional amplitude modulation by a binary signal. For example, it enables transmission over distances exceeding 150 km at 10 Gbit/s on a fiber having a chromatic dispersion of 17.5 ps
m.km. It also has the advantage of facilitating implementation of the optical carrier demodulator means since demodulation concerns only the amplitude and consists in discriminating only two distinct levels. Finally, the signal
oise ratio is better than in the situation where demodulation consists in discriminating three levels.
In a preferred embodiment of the invention, to code a data pattern to be transmitted in the form of a ternary signal: already in this form; differential coding; and obtain a ternary signal.
In another embodiment of the invention, to code a data pattern to be transmitted in the form of a ternary
REFERENCES:
patent: 4922249 (1990-05-01), Cointot et al.
patent: 5144669 (1992-09-01), Faulkner et al.
patent: 5623518 (1997-04-01), Pliffner
Gu et al, "10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fibre", Electronics Letters, vol. 29, No. 5, Dec. 09, 1993, Stevenage, GB, pp. 2209-2211.
Vodhanel et al. "10 Gbit/s modulation performance of distributed feedback lasers for optical fibre communication systems", IEEE Global Telecommunications Conference, vol. 3, Dec. 1990, San Diego, pp. 1534-1540.
Patent Abstracts of Japan vol. 13, No. 113 (E-729) 17 Mar. 1989 corresponding to JP,A,63 284 957 (Mitsubishi) dated Nov. 22, 1988.
Price Alistair
Uhel Roland
Alcatel Cit
Vo Don N.
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