METHOD AND APPARATUS FOR TRANSMITTING INFORMATION

Abstract: An optical communications system comprises a laser (10) controlled to output a stream of soliton pulses (12) coupled to a beamsplitter arrangement (14, 20, 22) which converts the pulse stream (12) to a stream of pulses (24) having twice the pulse rate. This stream of pulses (24) is modulated to form a binary coded signal by modulator (24) as a series of binary digits each having one of two values in which a series of n optical soliton pulses, where n is an integer > 1 is coupled into an optical fibre (4) at each occurrence of one of the two values. The optical fibre (4) and repeaters (6) provide a transmission link to a receiver (8). The present invention allows one to increase the average power within a bit without having to operate with shorter pulses that would otherwise be necessary with one soliton pulse per bit of prior art systems in order to avoid the limits inherent in single soliton pulse per bit prior art systems. The result is that the ASE noise limit is moved to longer pulses so allowing higher bit rates notwithstanding that there is a greater number of solitons in each bit of data than with single soliton per bit schemes of modulation. The method also reduces GH the jitter and thus opens the window of operation by pushing the G-H limit to longer pulses.

Description: METHOD AND APPARATUS FOR TRANSMITTING INFORMATION The present invention relates to a method of transmitting information where information is represented a series of binary digits (bits) each having one of two values. The present invention also relates to a transmitter for generating such information for transmission, a transmission system for conveying such information, and a communications system for communicating such information between parties. The present invention in its different aspects finds particular, but not exclusive, application to situations in which information is to be conveyed by means of an optical fibre waveguide in which spaced-apart, discrete, optical fibre amplifiers are used to compensate for losses caused during propagation along the optical fibre. For a receiver of a particular bandwidth at the end of such a communications system to be able to detect transmitted signals within a given error rate it must receive signals having a signal-to-noise ratio (S/N) greater than some minimum value. In an optical fibre transmission line with in-line optical fibre amplifiers, noise is generated by amplified spontaneous emission (ASE) in the amplifiers. The total noise generated by the optical transmission line therefore depends on the number of amplifiers in the line and the ASE noise generated by each amplifier. The ASE noise is a function of the gain of amplifier which is given -by g = 0.23L s γ/n dB ... (1) where L s is the system length in km; n is the total number of amplifiers, all assumed the same; and γ is the system loss in dB/km. The soliton pulses propagating down the optical fibre transmission line will lose energy and be subject to intermittent amplification. In order to have propagation in which the distance average power of the pulse is equal to a single soliton power it can be shown that an Nth order soliton has to be launched into the optical transmission line where N is given by N 2 = log(g)/(l - 1/g) ... (2) The minimum average power Pmin necessary to achieve an -14 S/N ratio sufficient to give a 10 bit error rate is P mιn = 10 "4 Bn[exp(45L s /n) -1] mW ... (3) where B is the bandwidth in G/bits. It can be shown that the pulse width required to generate the required Nth-order soliton pulse of the desired minimum average power P and bandwidth B is given by -ASE = °' 658 N2 BD/P • • • (4) where D is the dispersion. Equation (4) puts a constraint on the maximum soliton pulse width, t, to achieve the desired S/N ratio. Another source of noise which becomes increasingly important at higher bit rates is the Gordon-Haus effect: see J.P.Gordon and H. A. Haus, Random Walk of Coherently Amplified Solitons in Optical Fibre Transmission, Optics Lett 11 665-7 (1986). This effect induces an error due to fluctuations in arrival times, t a , which occurs from the combined action of an ASE induced frequency fluctuation and dispersion. The mean square jitter can be expressed as where n' s are photon numbers. The error rate due to this effect can be calculated. The operating nonlinear dynamics imposes a second requirement that the soliton period be rather longer than the amplifier spacing L A , namely ts p acn g > ( 0. 3DL A α ) 1 /2 . . . ( 6 ) where a is a safety factor of about 10. Once these two conditions of equations (4) and (7) are satisfied one observes essentially distortionless propagation of single pulses over arbitrarily large distances. In Figure 1 is shown a plot of amplifier spacing against pulse duration showing the three limiting processes (equations 4,5 and 6) for the example of a 6000km system length. The G-H effect is the only one which depends on bit rate and this is plotted for three bit rates (10, 8 and 5 Gbit/s). To operate a prior art soliton transmission system, t, the soliton pulse width, must be less than the G-H limit, less than t ASE and greater than t c . According to a first aspect of the present invention there is provided a method of transmitting information as a series of binary digits, each having one of two values, in which a series of n optical soliton pulses, where n is an integer > 1, is coupled into an optical fibre at each occurrence of one of the two values. According to a second aspect of the present invention there is provided an optical transmitter for transmitting information as a series of binary digits each having one of two values comprising a source of optical soliton pulses arranged to provide a series of n optical soliton pulses, where n is an integer >1, at each occurrence of one of the binary digit values, and optical coupling means for coupling the series of pulses into an optical fibre. The present invention allows one to increase the average power within a bit without having to operate with shorter pulses that would otherwise be necessary with one soliton pulse per bit of prior art systems in order to avoid the limits imposed by equations 5 and 6 on single soliton pulse per bit' prior art systems. The result is that the ASE noise limit is moved to longer pulses so allowing higher bit rates notwithstanding that there is a greater number of solitons in each bit of data than with single soliton per bit schemes of modulation. The method also reduces GH the jitter and thus opens the window of operation by pushing the G-H limit to longer pulses. The reason for this is that the pulse will be independently subject to a jitter so that an error will occur only if both pulses are sufficiently shifted. A simple delta ffuunnccttiicon model for the pulses given a n reduction in the jitter. A potential problem with the multiple pulse per bit modulation scheme of the present invention is pulse-pulse (i. e. soliton-soliton) interaction. This is a well known phenomenon which leads to a collapse of the pulses if they are initially in phase. The total energy per bit is unaltered by the collapse and so this will not, in principle, affect the system performance. It is desirable in such cases that there is no interaction with the adjacent bit pulses. It is then preferable that each series of n optical soliton pulses representative of a binary digit is separated from the next series by a time interval greater than the pulses within each series. When pulse-pulse interaction is significant it is also desirable that each pulse in each series of n optical soliton pulses is O or π/2 radians out of phase relative to the adjacent pulses in the series. A convenient way to achieve variations in pulse spacing and phase control for two solitons per bit is a transmitter comprising means for generating a stream of optical soliton pulses; a beam splitter located to split the stream of optical pulses into two subsidiary streams, a pair of reflective means to reflect the two subsidiary streams back to the beam splitter, the reflective means being positioned such that positions of the two subsidiary streams are combined by the beam splitter to form an output stream of optical soliton pulses alternate ones of the subsidiary streams. Series of pulses greater then two can be readily produced by multiplexing the appropriate number of such transmitters. The distance of one of the reflective means, for example a mirror, can be made adjustable to provide equidistantly spaced or paired optical pulses of the desired phase relationship. Once this phase is fixed the collapse effect on propagation will not reduce the energy in the bit interval. The proposed scheme can also operate in the regime of complete overlap of the pulses in the centre of the bit. This is equivalent to operating with solitons of the initial form Nsech(t) with N an integer equivalent to the number of pulses per bit. This latter multiple soliton modulation scheme will have the same benefit as for the increase in average power but will not have the benefit of the reduced jitter. According to a third and a fourth aspect of the present invention an optical transmission system comprises an optical transmitter optically coupled to an optical fibre communications network having two or more spaced apart optical amplifiers and an optical communications system comprising the above transmission system and an optical receiver coupled to the optical fibre communications network for receiving the transmitted optical soliton pulses. An embodiment of the present invention illustrating its several aspects and its principle of operation will now be described with reference to the accompanying drawings of which: Figure 1 is a graph of the factors determining the operational limits of a prior art, 6000km, single soliton pulse per bit, soliton communications system; Figure 2 is a graph of the factors determining the operational limit of a 6000km system of Figure 1 but using a double soliton pulse per bit according to the method of the present invention; Figures 3(a), (b) and (c) are diagrammatic representations of a binary information signal transmitted according to the present invention having uniform and non-uniform pulse spacings respectively; and Figure 4 is an embodiment of an optical communications system according to the present invention. Figure 1 shows the calculated limits of t spacιng , t ASE and the Gordon-Haus limit for 5, 8 and 10 Gbit/s data rates for single soliton/bit transmission. Transmission is possible for amplifier spacings for which there is a pulse width that is simultaneously less than t ήSE , greater than t cj and greater than the G-H bit rate limit. This window of operation is shown shaded in Figure 1. It can be seen that for large inter-amplifier spacing of say 50km one is limited to data rates of up to about 5Gbits/s Referring to Figure 2, there is shown the calculated limits for the two soliton per bit transmission of the present invention in which the reduced jitter benefit has not been included as this may not pertain with soliton-soliton collapse. The effect of increasing the average power without having to operate with a shorter soliton pulse can be seen to open the operating window to that shown by the shaded area of Figure 2. Thus an 80km amplifier spacing can be achieved for data rates under 5Gbit/s or 50km amplifier spacing at 8 Gbit/s data rate. For close spacing of amplifiers, 10 Gbit/s operation is possible which could not be achieved previously. Referring now to Figure 3, there are shown two modulation schemes of the present invention in which an exemplary data stream of binary digits of Figure 3(a) have been encoded with two soliton pulses per bit in which the presence of a pulse represents a value of 1. In Figure 3(b) the soliton pulses are equidistantly spaced in the bit period. In Figure 3(c) the soliton pulse pairs are bunched towards the centre of the bit period so the time interval between each series of two pulses in a bit is greater than the time interval between the pulses within a bit period. Referring to Figure 4, an optical communication system comprises an optical transmitter 2, a 6, 000km optical fibre transmission line with spaced apart erbium fibre amplifiers 6, and an optical receiver 8. The transmitter 2 comprises a mode-locked, semiconductor laser 10 which generates in a known manner a stream of optical soliton pulses which is directed at a bulk optic beamsplitter 114. The stream 12 is divided by the beamsplitter 14 into two subsidiary streams 16 and 18 (with pulses labelled PI and P2, respectively) which are reflected back to the beamsplitter 14 by mirrors 20 and 22. The distance 1 of the mirror 22 from the beamsplitter 14 is adjustable. The distance is set, in Figure 4, to obtain equidistant interleaving of the subsidiary streams 16 and 18 to form a output stream 24 of pulses of twice the rate provided by the laser 10. The mirror 22 can be adjusted to tune the phases of pulses P2 relative to pulses PI. The output stream 24 is modulated by modulator 26 in response to electrical signals from a controller 28 modulated with a binary information signal. The data rate is such that two soliton pulses lie in a single bit period. The modulated soliton pulse stream is then coupled into the optical fibre transmission line 4, 6 for onward propagation to the receiver 8 by standard optical means ( not explicitly shown).

Claims: CLAI MS 1. A method of transmitting information as a series of binary digits each having one of two values in which a series of n optical soliton pulses, where n is an integer > 1, is coupled into an optical fibre (4) at each occurrence of one of the two values. 2. A method as claimed in claim 1 in which each series of n optical soliton pulses are produced by generating a continuous stream of optical soliton pulses (24) and modulating the stream at l/n h the pulse repetition rate. 3. A method as claimed in claim 1 in which each series of n optical soliton pulses representative of a binary digit is separated from the next series by a time interval greater than between consecutive pulses within each series. 4. A method as claimed in claim 3 in which each pulse in each series of n optical soliton pulses is 0 or π radians out of phase relative to the adjacent pulses in the series. 5. A method as claimed in claim 2 in which the optical soliton pulses are generated by means of a mode-locked semiconductor laser (10). 6. An optical transmitter for transmitting information as a series of binary digits each having one of two values comprising a source of optical soliton pulses (10) arranged to provide a series of n optical soliton pulses, where n is an integer > 1, at each occurrence of one of the binary digit values, and optical coupling means for coupling the series of pulses into an optical fibre (4). 7. A transmitter as claimed in claim 6 including a generating means (10, 14,20,22) for generating a continuous stream of optical soliton pulses (24) having a predetermined pulse repetition rate and a modulating means (26) arranged to modulate the stream at l/n th the pulse repetition rate. 8. A transmitter as claimed in claim 6 in which each series of n optical soliton pulses representative of a binary digit is separated from the next series by a time interval greater than the time interval between pulses within each series. 9. A transmitter as claimed in claim 8 in which each pulse in each series of n optical soliton pulses is O or π radians out of phase relative to the adjacent pulses in the series. 10. A transmitter as claimed in claim in any one of claims 6 to 9 in which the generating means (10, 14,20,22) comprises: a source of optical soliton pulses (10); a beam splitter ' (14) located to split the stream of optical pulses (12) into two subsidiary streams (16, 18); a pair of reflective means (20,22) to reflect the two subsidiary streams (16, 18) back to the beam splitter (14); the reflective means (20,22) being positioned such that the two subsidiary streams (16, 18) are combined by the beam splitter (14) to form an output stream (24) of optical soliton pulses alternate pulses of which derive from different ones of the subsidiary streams (16, 18). 11. A transmitter as claimed in claim 10 in which the distance of at least one of the reflective means (16, 18) from the beamsplitter (14) is adjustable. 12. A transmitter as claimed in either of claims 10 and 11 in which the reflective means (16, 18) are positioned to provide equidistant spacing of the pulses in the output stream (24). 13. A transmitter as claimed in either of claims 10 and 11 in which the reflective means (20,22) are positioned to provide that the output stream (24) of optical soliton pulses comprises pairs of pulses each pair separated from the next pair by a time interval greater than the time interval between pulses of each pair. 14. An optical transmission system comprising an optical transmitter as claimed in any one of claim 6 to 13 optically coupled to an optical fibre communications network (4,6) having two or more spaced apart optical amplifiers (6). 15. A system as claimed in claim 14 in which the optical amplifiers (6) are optical fibre amplifiers. 16. An optical communications system comprising an optical transmission system (2) as claimed in either one of claims 14 and 15 and a receiver (8) coupled to the optical fibre communications network (4,6) for receiving the transmitted optical soliton pulses.

Inventors: BLOW KEITH JAMES, DORAN NICHOLAS JOHN, BLOW, KEITH, JAMES, DORAN, NICHOLAS, JOHN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: H04B10
IPC codes: G02B6, G02F1, H04B10