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United States Patent (19) 11 Patent Number: 5,343,322 Pirio Et Al

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United States Patent (19) 11 Patent Number: 5,343,322 Pirio Et Al USOO5343322A United States Patent (19) 11 Patent Number: 5,343,322 Pirio et al. 45 Date of Patent: Aug. 30, 1994 54 SYSTEM OF VERY-LONG-DISTANCE mum Dispersion in Optical Fibres, Optics Communica DIGITAL TRANSMISSION BY OPTICAL tions, vol. 48, No. 3, Dec. 1983, pp. 181-184. FIBER WITH COMPENSATION FOR Gnauck et al.: Dispersion Penalty Reduction using an DISTORTIONS AT RECEPTION Optical Modulator with Adjustable Chirp, IEEE Photon ics Technology Letters, vol. 3, No. 10, Oct. 1991, pp. 75 Inventors: Francis Pirio; Jean Thomine, both of 96-918. Paris, France K. Iwashita et al. Chromatic Dispersison Compensation in 73 Assignee: France Telecom, Paris, France Coherent Optical Communications Journal of Lightwave Technology, vol. 8, No. 3, Mar. 1990, New York US pp. 21 Appl. No.: 995,816 367-375. 22 Filed: Dec. 23, 1992 Blow and Doran Non linear effects in optical fibres and fibre devices IEEE Proceedings, Jun. 1987, pp. 138-144. 30) Foreign Application Priority Data Agrawal; Non linear fiber optics, Academic Press, Chap Dec. 31, 1991 FR France ................................ 91 6488 ter 2, Chapter 6. Gnauck and al; Dispersion penalty reduction using a opti 51 Int. CI. ...................... H04B 10/12; H04B 10/00 cal modulator with adjustable chirp OFC 1991. Post 52 U.S. C. .................................... 359/173; 359/161; deadline in 17. 385/28 Koyama et al.; Compensation of nonlinar pulse distortion 58 Field of Search ........ 359/130, 161, 173, 181-182, in optical fiber by employing prechirp technique ECOC91. 359/188, 195, 111; 385/3, 27-28 WCC7-2, pp. 469-472. 56) References Cited Primary Examiner-Richard E. Chilcot, Jr. U.S. PATENT DOCUMENTS Assistant Examiner-K. Negash 4,067,642 1/1978 King et al. ............................ 385/27 Attorney, Agent, or Firm-Merchant & Gould Smith, 4,261,639 4/1981 Kogelnik et al. ... 385/27 Edell, Welter & Schmidt 4,969,710 1/1990 Ticket al. ......... ... 385/123 X 57 ABSTRACT 4,979,234 12/1990 Agrawal et al. .. ... 359/173 5,119,447 6/1992 Trisno ............... A 385/3 A system for very-long-distance transmission of a digi 5,166,821 11/1992 Huber ... ... 359/173 tal signal between a transmitter station and a receiver 5,224,183 6/1993 Dugan ................................... 385/24 station, wherein the transmitter and receiver stations 5,261,016 l/1993 Poole ..................................... 385/28 are connected by a monomode optical fiber with nega FOREIGN PATENT DOCUMENTS tive chromatic dispersion at the operating wavelength of the system, having a length of at least one thousand 2269309 2/1990 Japan . kilometers. The receiver station comprises device to 2240683 8/1991 United Kingdom ................ 359/16 compensate for the distortions due to the non-linear OTHER PUBLICATIONS effects and to the chromatic dispersion introduced by the transmission line, the compensation device carrying Koch and Alferness: Dispersion Compensation by Active out a positive chromatic dispersion of the received sig Predistorted Signal Synthesis, Journal of Lightwave nal, the amplitude of the positive chromatic dispersion Technology, vol. LT-3, No. 4, Aug. 1985, pp. 800-805. being a function notably of the amplitude of the nega Saito et al.: Prechirp Technique for Dispersion Compensa tive chromatic dispersion induced by the optical fiber as tion for a High-Speed Long-Span Transmission, IEEE well as of the mean on-line optical power of the signal Photonics Technology Letters, vol. 3, No. 1, Jan. 1991, transmitted on the optical fiber. pp. 74-76. Blow et al.: Nonlinear Limits on Bandwidth at the Mini 12 Claims, 2 Drawing Sheets 2 22 23 25 RECEIVER / STATION TRANSMITTER RECEIVER STATION 24 U.S. Patent Aug. 30, 1994 Sheet 1 of 2 5,343,322 SZ ZZ U.S. Patent Aug. 30, 1994 Sheet 2 of 2 5,343,322 N011.d3D3? 30010010Hd No. 5,343,322 1. 2 of 1 W) or for very large propagation distances at rea SYSTEM OF VERY-LONG-DISTANCE DIGITAL sonable levels of power (some thousands of kilometers TRANSMISSION BY OPTICAL FIBER WITH in a periodic amplification system). COMPENSATION FOR DISTORTIONS AT When there is no chromatic dispersion, the Kerr RECEPTION effect induces a self-phase modulation of the optical pulse: the instantaneous frequency diminishes at the BACKGROUND OF THE INVENTION start of the pulse and then increases at its end, propor 1. Field of the Invention tionally to the derivative of the optical power. This The field of the invention is that of very-long-dis induces a widening of the spectrum and a spectral con tance digital transmission (several thousands of kilome 10 position that fosters a substantial widening for negative ters) by optical fiber in systems using on-line optical chromatic dispersions. amplification The distortion provided by the transmission fiber One of the main factors limiting the bit rate in very should be considered as the combination of the chro long-distance systems such as these is the distortion matic dispersion (the first phenomenon) and of the non induced by the transmission fiber. 15 linear effects (the second phenomenon). For, while these distortions may be more or less over The combination of these two effects may be de looked in standard fiber-optic systems (setting up links scribed by a non-linear equation with partial derivatives over distances in the range of some humdreds of kilome of distance and time, known as Schrödinger's non-linear ters), they have, on the contrary, very disturbing effects equation, the resolving of which is discussed notably in on long-distance transmission systems. 20 the work by G. Agrawal, "Non-Linear Fiber Optics', The invention relates to a system of transmission on an optic fiber line enabling compensation, at reception, Academic Press. for this distortion induced by the transmission line, in The numerical resolution of this equation shows that the case of transmission lines having a length of at least there are two forms of behavior which are qualitatively a thousand kilometers, such as those used for trans 25 very different depending on the sign of the chromatic ocean links. dispersion (D): The distortion contributed by the transmission fiber * First case: Did O. In this case, phenomena of insta arises out of the combined existence of two phenomena bility of modulation are observed. The pulses that occur in the monomode fibers: chromatic disper "burst' into very short pulses at the end of 1000 to sion and non-linear effects. 30 2000 km and the optical spectrum widens consider The first phenomenon is that of chromatic dispersion. ably: this may give rise to problems related to the This phenomenon results from the frequency depen optical passband. dency of the refractive index of silica. It entails different * Second case: D-30. There is no instability of modu propagation times depending on the operating wave lation and the pulses keep a certain degree of integ length. In general, chromatic dispersion tends to widen 35 rity while the spectrum widens quasi-monotoni the pulses of the digital trains and, hence, to create cally during the propagation, while keeping rea inter-symbol interferences. sonable widths. However, the pulses widen greatly In the commonly-used fibers, the chromatic disper temporally, thus creating inter-symbol interfer sion is zero around 1.3 um and takes a positive value of ences. These interferences become very trouble about 17 ps/nm/km around 1.55 um. It is also possible 40 some for example, as soon as the chromatic disper to use dispersion-shifted fibers which are designed to sion goes beyond 0.05 ps/nm/km in terms of abso have zero chromatic dispersion in the region of 1.55 lute value for bit rates of 5 Gbits/s on distances of in. 6000 to 8000 km. Very-long-distance transmission systems (covering The most efficient case is naturally the second one, several thousands of kilometers) work at 1.55 um. The 45 that of a negative chromatic dispersion. However, to excessive value of the chromatic dispersion of the com make the very-long-distance systems work in negative monly used fibers at this wavelength rules out their use. dispersion mode, the values of chromatic dispersion of Hence, dispersion-shifted fibers will be used in these the fibers used must obligatorily be very low. C2SeS. 2. Description of the Prior Art It must be noted that the effect of distortion by chro 50 Any method of compensation for the two phenomena matic dispersion depends greatly on the spectral com that are the cause of the distortion in the fiber (chro ponents of the pulses: if a pulse shows variations in matic dispersion and non-linear effects) is therefore of optical phase that are positive at its start and negative at great value since it can be used to overcome the draw its end, it will be greatly widened by a positive chro back of the low values imposed on the chromatic dis matic dispersion. The converse is true for negative dis 55 persion. Indeed, by using a method of compensation for persions. the distortion provided by the transmission fiber, it is The second phenomenon relates to the non-linear possible to envisage two strategies. effects. The most important non-linear effect in a fiber is In a first strategy, for given characteristics of nega the Kerr effect. This effect, which is described for ex tive chromatic dispersion of the transmission fiber, the ample in the document by K. W. Blow and N. J. Doran, 60 compensation can be used to increase the product: line “Non-linear Effects in Optical Fibers and Fiber De bit rate * range of the link. vices' (IEEE Proceedings, June 1987, pp. 138-144) In a second strategy, for a fixed line bit rate and a reflects a linear dependency of the refractive index of fixed range, the compensation makes it possible to use silica with respect to the optical power.
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