An optical regenerator and an optical transmission system

Abstract: A a first clock extraction device (4') included in the receiving section of the optical regenerator recovers a receiving clock. The receiving clock follows the jitter contained in a receiving optical signal caused by fiber wavelength dispersion. The receiving optical signal is detected in synchrony with the receiving clock. A transmission section includes a transmitter (8, 9) for generating a transmission optical signal changed in phase in such a manner as to suppress the stimulated Brillouin scattering and a second clock extraction device (4'') for regenerating the transmission clock not following the jitter. The data of the receiving optical signal is processed at a processing unit (6), and intensity-modulated on the basis of the data thus processed. The timing of this modulation is synchronized with the transmission clock. The receiving clock is synchronized with the jitter, and therefore no logic error occurs. Since the transmission clock does not follow the jitter, the jitter is prevented from accumulating.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical regenerator and an optical transmission system for multi-repeater optical transmission, or more in particular to a high-quality optical transmission technique employing a measure against what is called the stimulated Brillouin scattering in order to produce a high launch power. In an optical transmission system, implementation of practical optical amplifiers has made it possible to produce a high-output optical transmission signal from an optical transmitter. The power of the optical transmission signal that can be applied to an optical fiber coupled to the repeater, however, is limited by the stimulated Brillouin scattering (hereinafter referred to simply as "SBS") of the optical fiber. A method for improving the power of the optical transmission signal that can be applied to the optical fiber is described in ECOC Journal, pp.657-660, (1991) by P.M. Gabla, et al. A similar technique is disclosed in EP-A-581 525. According to this technique, the laser beam carrying an information signal and to be coupled into the optical fibre is phase modulated with at least twice the frequency of the highest signal frequency. The phase modulation changes the optical spectrum and generates additional optical carriers. The new spectrum enables the power of the laser beam to be increased before reaching the threshold of stimulated Brillouin scattering. To refresh an optical signal travelling through an optical transmission system, optical regenerators are used. An example of such a regenerator is given in EP-A-571 134.This document does not relate to the problems associated with stimulated Brillouin scattering. SUMMARY OF THE INVENTION The inventors have studied the following problem: According to P.M. Gabla, et al., SBS can be suppressed by modulating the phase or frequency of the signal input to the optical fiber so that the phase is uncontinuously. In the case where the input signal to an end of the optical fiber, i.e., the frequency of the output signal of a repeater is different, however, the time of arrival of the signal at the other end of the optical fiber is diverged due to the wavelength dispersion of the optical fiber, with the result that a jitter appears in the signal waveform at the other end. Detailed description will be made with reference to Fig. 1. In Fig. 1, reference numeral 1 designates an anti-SBS transmitter including a semiconductor laser 9 as a CW light source which emits at 1.3 µm or 1.5 µm drived by a predetermined magnitude of current. Numeral 8 designates an amplitude modulator for frequency-modulating with a sinusoidal wave (several kHz to several MHz) the predetermined current applied to the semiconductor laser. As a result, the output phase or frequency of the semiconductor laser 9 is modulated, thereby generating a carrier signal which has an uncontinuous phase capable of suppressing SBS. In the process, the output amplitude of the semiconductor laser is also modulated. This amplitude modulation, however, is negligibly small. The amplitude of this carrier signal is modulated in accordance with the data signal by an external modulator 10 and makes up an optical transmission signal. When a signal with a discrete phase is applied to an optical fiber 11, the output waveform of the optical fiber contains a jitter for the reason described above. The observed frequency of the jitter coincides with the frequency of the frequency modulator 8. As shown in Fig. 2, the frequency modulator 8 may be eliminated and the phase of the output of the laser 9 may be modulated directly using a phase modulator 100 to suppress SBS. Repeaters 20 and 30 which have been studied by the inventors are shown in Figs. 3A and 3B. In these repeaters, a received signal containing the jitter is logically decided at edge of the clock pulse extracted from the received signal and then transmitted as an optical signal (by regeneration repeating). The modulation frequency of the anti-SBS frequency modulation is assumed to be fSBS, and the cut-off frequency of the jitter transfer function for a clock extraction device to be fC1. The configuration with the relation fSBS < fC1 held is shown in Fig.3A, the jitter transfer function for the clock extraction device 4' in Fig. 4A. Since the jitter frequency fSBS of the receive signal is lower than the cut-off frequency of the jitter transfer function, the clock phase extracted by the clock extraction device 4' follows the jitter of the data. As a consequence, the data phase coincides with the clock phase at a decision block 5, and therefore no logical error of the data is caused due to the jitter. In spite of this, the jitter on input side passes through the optical regenerator 20 and appears in a similar form in the optical intensity waveform on the output side. The frequency modulation for SBS suppression is required to be performed for each optical regenerator. After transmission through the optical fiber 11, therefore, the frequency modulation is converted into a jitter by wavelength dispersion, resulting in an increased jitter. In this way, when fSBS is smaller than fC1, the problem is that the jitter is accumulated during the multi-repeater transmission. Now, the configuration for the relation fSBS > fC1 held is shown in Fig.3B, and the jitter transfer function for the clock extraction device 4" in Fig. 4B. In view of the fact that the input jitter frequency fSBS is higher than the cut-off frequency of the jitter transfer function, the clock extraction device 4" suppresses the jitter of frequency fSBS. No jitter by anti-SBS frequency modulation, therefore, appears on the output. Since the clock extracted the clock extraction device 4" fails to follow the data jitter, therefore, an error occurs between the clock phase and the data phase at the decision block 5, which is likely to cause a logic error of the data. In this regenerators, the transmission quality is deteriorated due to the jitter caused when the SBS is suppressed. It is an object of the present invention to provide an optical regenerator and an optical regeneration method of improved quality which overcomes the above problem arising from SBS suppression. This object is solved by the optical regeneration repeater of claim 1 and the optical regeneration method of claim 11. Claim 14 is directed to an optical transmission system having an optical generator of the invention. The subclaims are directed to preferred embodiments of the invention. A practical implementation of the invention is an optical regenerator in which, in the presence of jitters in the received optical signal due to phase or frequency modulation performed for SBS suppression, logical errors and jitter accumulation are reduced. Such a regenerator is suitable for a transmission system where an optical signal is repeated by a plurality of regenerators. In order to achieve the above-mentioned object, there is provided according to one embodiment of the invention an optical regenerator in which an optical transmission signal frequency-modulated so that the phase of the signal changes to suppress the SBS and further intensity-modulated by data is transmitted as an optical output, the optical regenerator having two functions of clock extraction for decision of the received signal and clock regeneration for a transmission clock. In other words, as shown in Fig.5. there is provided an optical regenerator comprising a first clock extraction device 4' for deciding the received signal and a second clock extraction device 4" for regenerating the transmission clock, wherein the relation holds that fC2 < fSBS < fC1, where fSBS is the modulation frequency for frequency modulation of the optical transmission signal, fC1 is the cut-off frequency of the jitter transfer function of the first clock extraction device 4', and fC2 is the cut-off frequency of the jitter transfer function for the second clock extraction device 4". In this optical regenerator, suppose the first clock extraction device 4' and the second clock extraction device 4" are cascaded as shown in Fig. 5. The first clock extraction device 4' follows the input and the second clock attraction device 4" can easily suppress the jitter through the extraction devices 4'. This configuration is desirable as the jitter tolerance and the jitter transfer characteristic can be improved simultaneously. Also, in order to achieve the above-mentioned object of the invention, there is provided according to another embodiment of the present invention a multi-repeater optical transmission system for transmitting an optical transmission signal frequency-modulated and further intensity-modulated by data as an optical output, in which the optical transmission signal of the transmitter is frequency-modulated with frequency fSBS, a corresponding repeating receiver includes a first clock extraction device 4' for deciding the received signal and a second clock extraction device 4" for regenerating the transmission clock as a clock for transmission data, as shown in Fig. 5, and the relation is held that fC2 < fSBS < fC1 where fC1 is the cut-off frequency of the jitter transfer function for the first clock extraction device and fC2 the cut-off frequency of the jitter transfer function for the second clock extraction device. Even when a jitter occurs in the received signal due to the frequency modulation for SBS suppression, the relation fSBS < fC1 causes the phase of the input clock to the decision block to follow the phase of the input data and therefore no error occurs. In view of the fact that the transmission clock holds the relation fC2 < fSBS, on the other hand, the frequency component of jitter for anti-SBS frequency modulation is suppressed, and therefore jitters are not added independently or accumulated at the time of multiple repeating operations. In this way, the present invention is provided to solve the two problems of jitter and the logical error due to the input jitter at the same time. In the foregoing description, assume that the anti-SBS transmitter shown in Fig. 2 is used. In the case where the phase uncontinuity of the transmission signal is converted to the instantaneous frequency or in the case where the modulation by the frequency modulator 8 is not by sinusoidal wave, as shown in Figs. 4C and 4D, the instanteneous frequency corresponding to the modulation frequency fSBS has a certain bandwidth. In this case, the relations hold that fC2 < fSBSL and fSBSH < fC1, where fC1 is the cut-off frequency of the jitter transfer function of the first clock extraction device 4' and fC2 is the cut-off frequency of the jitter transfer function of the second clock extraction device 4". BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and technical advantages of the present invention will be readily apparent from the following description of the preferred exemplary embodiments of the invention in conjunction with the accompanying drawings, in which: Figs. 1 and 2 are schematic diagrams showing an anti-SBS transmitter and jitter generation; Figs.3A and 3B are diagrams showing the configuration of a repeater studied by the inventors; Figs. 4A to 4D are diagrams showing the jitter transfer characteristic of the clock extraction devices; Fig. 5 is a diagram showing the configuration of an optical regenerator according to a first embodiment of the invention; and Fig. 6 is a diagram showing the configuration of an optical regenerator according to a second embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail below. An example configuration of an optical regenerator according to a first embodiment is shown in Fig.5. Numeral 40 designates an optical regenerator unit, numeral 2 a photodiode, numeral 3 an equalizing amplifier, and numeral 4' a first clock extraction device. The clock extraction device, which includes such functions as nonlinear extraction, is represented by a tank circuit related to the jitter characteristic. Numeral 5 designates a decision block numeral 6 a digital signal processing section for data processing required for regenerator, numeral 7 a D-FF (D-type flip-flop), and numeral 4" a second clock extraction device. The clock supply for driving the digital processing section 6 is not shown as it is not important in terms of configuration. The D-FF 7, which indicates that the phase (timing) of the output data is determined by the clock supplied from the clock extraction device 4", may be included in the digital processing section 6 depending on the particular configuration. Numeral 8 designates a amplitude modulation oscillator for frequency modulation suppressing SBS, numeral 9 a semiconductor laser, and numeral 10 an external modulator. First, the operation at the transmitting end will be explained. When the current supplied to the semiconductor laser 9 is modulated by an oscillator 8 of frequency fSBS in order to reduce SBS, the light spectrum of the output of the semiconductor laser is diverged. This light is modulated by data through the external modulator 10 thereby to produce an optical signal intensity-modulated according to the data. Next, the operation at the receiving end will be explained. The received optical signal through dispersive optical fiber contains a jitter according to the principle shown in Fig. 1. This received optical signal is converted into a current by the photodiode 2, amplified by the equalizing amplifier 3, and then is distributed between the decision block 5 and the first clock extraction device 4'. The first clock extraction device 4', which generally includes a nonlinear extractor, for example, full-wave rectifier, a tank circuit and a limitting amplifier, may alternatively use a PLL. Provided, however, that fSBS < fC1 where fC1 is the cut-off frequency of the jitter transfer function. In the case where a tank circuit is used for the clock extraction device 4', Q1 is determined as Q1 = fB/(2fC1), where fB is the transmission bit rate. In actual transmission systems, it is necessary that 500 < Q1 < 1500, and fSBS is required to be determined after fC1 has been determined for various reasons. The clock recoverd at the clock extraction device 4' is supplied as a timing signal for logic decision on the equalizing amplification signal to the decision block 5 on the one hand and is applied to the second clock extraction device 4" at the same time. Given the relation that fSBS < fC1, the jitter of frequency fSBS passes through the clock extraction device 4'. As a result, the clock supplied to the clock extraction device 4' follows the jitter of the equalizing amplification signal applied to the decision block 5. There occurs, therefore, no logic error of data which otherwise might be caused by the jitter generated for SBS suppression. The clock extraction device 4" is for removing the jitter of the clock recoverd at the clock extraction device 4', and includes a tank circuit alone or a PLL. The clock extraction device 4" is input the clock including the jitter of frequency fSBS, and in order to suppress this jitter, the relation is held fC2 < fSBS. The transmission data is output syncronized with the clock regenerated at the clock extraction device 4" using the D-FF 7. As a consequence, the jitter of frequency fSBS input to the optical repeater 40 is removed from the output. Also, no logic error of data occurs which otherwise might be caused by the jitter of frequency fSBS. As will be seen from the foregoing embodiment, the object of the invention is achieved by establishing the relation fC2 < fSBS < fC1, where fSBS is the frequency-modulating frequency fSBS for SBS suppression, fC1 is the cut-off frequency of the jitter transfer function for the first clock extraction device, and fC2 is the cut-off frequency of the jitter transfer function for the second clock extraction device. The difference between fC1 and fSBS, which may be small, is preferably about one order of magnitude considering a margin. Also, the difference between fSBS and fC2 should preferably be one order of magnitude. For example, according to the embodiment under consideration, fSBS is set to 10 kHz to 1 MHz, fC1 to 3 MHz to 10 MHz, and fC2 to 1 kHz to 0.1 MHz for 10 Gb/s transmission. Unlike in the aforementioned embodiment in which the first clock extraction device and the second clock extraction device are connected in cascade, parallel connection of the first and second clock extraction devices has a similar same effect. Also, according to the above-mentioned embodiment, the semiconductor laser 9 for SBS suppression is frequency-modulated with a sinusoidal wave. A similar effect is obtained, however, by the frequency modulation with other than a sinusoidal wave or by the phase modulation instead of frequency modulation as far as fC2 is set to less than the lower limit (fSBSL) of the spectrum of the modulation signal and fC1 to more than the upper limit (fSBSH) of the spectrum of the modulation signal. When a multiplicity of repeaters as described with reference to the foregoing embodiment are connected, jitters will neither accumulate nor cause a logic error. As a result, a transmission system comprising such repeaters can realize high-quality optical transmission between remote sites. A repeater 50 according to another embodiment for 10 Gb/s optical regenerate is shown in Fig. 6. In Fig. 6, the component parts which perform the same operation as the corresponding parts in Fig. 5 are designated by the same reference numerals respectively and will not be fully described. A first clock extraction device 54', like the first clock extraction device 4' of Fig. 5, is for recovering the clock in order not to cause any logic error due to jitters. The first clock extraction device 54' includes a full-wave rectifier 61, a tank circuit 62 and a limitting amplifier 63. The transmission optical signal is converted into a first electrical signal at a photodiode 2, which first electrical signal is amplified at an amplifier 3 and is distributed between the first clock extraction device 54' and the decision block 5. The first clock extraction device 54' recovers the clock signal 80 (10 GHz) on the basis of the first electrical signal distributed. The clock signal 80 is divided to 1/16 in frequency by a demultiplexer 51, and after being further reduced to 155 MHz by a frequency divider circuit not shown and built in a receive data processing section 52, is sent to a first PLL circuit 71 as a second clock signal 81. The first PLL circuit 71 is a filter for removing the jitter contained in the second clock signal. A third clock 82 free of the jitter is sent to a second PLL circuit 72 and further to a transmission data processing section 55. The first electrical signal identified at the decision block 5 is parallely converted in frequency by the demultiplexer 51 and an internal frequency dividing circuit into a second electrical signal. This second electrical signal is processed at an arithmetic circuit of the processing section 52. The clock signal 82 free of jitters is increased in frequency from 155 MHz up to 10 GHz by the second PLL circuit 72 (to form a clock signal 83). The transmission data processing section 55 operates at a timing of the 155-MHz clock signal 84. The frequency of the transmission data rate is increased up to 10 GHz by a multiplexer 56 and a frequency multipher circuit not shown and built in the processing section 55. The transmission data is applied to a modulator driver 57 at a timing of the clock signal 83 by means of the D-FF 7. An elastic buffer 53 arbitrates between the phase of writing data at CLOCK 81 including the jitter and the phose of readiry data at CLOCK 84 including nogitter. As a result, data are positively delivered from the receiver 90 to the transmitter 91. As far as the second PLL circuit 72 is provided with the function of removing jitters, the first PLL circuit 71 is done without. According to the embodiment under consideration, the cut-off frequency fC1 of the tank circuit 62 is set to 3 MHz to 10 MHz, and the cut-off frequency fC2 of loop filter for the first PLL circuit 71 to 1 kHz. The cut-off frequency of loop filter the second PLL circuit 72 could not be reduced below 3 MHz for the reason of reducing random jitter. Consequently, the first PLL circuit 71 is required. The repeater 50 according to the present embodiment, like the repeater 40 in Fig. 5, effectively prevents the logic error and the jitter accumulation even when a jitter is contained in the received signal.

Claims: An optical regeneration repeater comprising: means (2, 3, 5, 6) for receiving a receiving optical signal changed in phase in such a manner as to suppress stimulated Brillouin scattering and consequently containing a jitter, said means regenerating data; means (8, 9) for generating a transmission optical signal changed in phase in such a manner as to suppress stimulated Brillouin scattering; means (10) for modulating and transmitting said transmission optical signal on the basis of the data regenerated; first clock extraction means (4') for regenerating from said receiving optical signal the receiving clock for identifying the receiving optical signal, said receiving clock following said jitter; and second clock extraction means (4") for regenerating the transmission clock for giving a timing for modulating the transmission optical signal with the data regenerated, said transmission clock being independent from the jitter. An optical regeneration repeater according to Claim 1, wherein the relation holds: fC2 < fSBS < fC1, where fSBS is the frequency corresponding to the phase change of the receiving optical signal, fC1 is the cut-off frequency of the jitter transfer function of the first clock extraction means, and fC2 is the cut-off frequency of the jitter transfer function of the second clock extraction means. An optical regeneration repeater according to Claim 1, wherein said second clock extraction means (4") recovers said transmission clock on the basis of said receiving clock. An optical regeneration repeater according to Claim 2, wherein said fSBS has a bandwidth, and the relation holds:fC2 < fSBSL and fSBSH < fC1, where fSBSH and fSBSL are the maximum value and the minimum value of the bandwidth respectively. An optical regeneration repeater according to Claim 2, wherein said means for generating a transmission optical signal includes a semiconductor laser (9) providing a source of CW oscillation and an amplitude modulator (8) for modulating the current applied to said semiconductor laser with said frequency fSBS. An optical regeneration repeater according to Claim 1, wherein said means for generating the transmission optical signal includes a semiconductor laser (9) providing a CW oscillation source oscillating at a predetermined frequency and a phase modulator (100) for phase-modulating the output light of said semiconductor laser. An optical regeneration repeater according to Claim 2, wherein said first or said second clock extraction means includes a tank circuit or a PLL circuit. An optical regeneration repeater according to Claim 7, wherein said first clock extraction means further includes a full-wave rectifier (61) and a limiting amplifier (63). An optical regeneration repeater according to Claim 2, wherein said second clock extraction means includes at least two PLL circuits (71, 72) coupled in series. An optical regeneration repeater according to Claim 2, further comprising means for reducing the frequency of said receiving clock and forming a third clock (81), wherein said second clock extraction means (72) regenerates said transmission clock on the basis of the third clock. A method of optical regeneration comprising the steps of: receiving a receiving optical signal changed in phase in such a manner as to suppress stimulated Brillouin scattering and consequently containing a jitter, and regenerating data from the receiving optical signal by deciding said data with the receiving clock following said jitter; generating from said receiving clock a transmission clock independent from said jitter, by suppressing said jitter in said receiving clock; and intensity-modulating a transmission optical signal changed in phase in such a manner as to suppress the stimulate Brillouin scattering, in synchrony with the transmission clock and on the basis of the data regenerated. An optical regeneration repeater according to claim 1, comprising: processing means (6) for processing the receiving optical signal and forming processed data for transmission by using said transmission clock. An optical regeneration repeater according to claim 1, comprising: a first clock extraction circuit (54') for regenerating a receiving clock signal for identifying a first electrical signal output from said receiving means (2, 3), said receiving clock signal following said jitter; means (51) for reducing the frequency of said receiving clock signal and the frequency of said first electrical signal thereby to form a second clock signal and a second electrical signal; first processing means (52) for processing the second electrical signal in synchrony with said second clock signal and forming receiving data; a first PLL circuit (71) for removing the jitter from said second clock signal and regenerating a third clock signal; a second PLL circuit (72) for regenerating a transmission clock signal from said third clock signal; second processing means (55) for forming transmission data from said receiving data in synchrony with said third clock signal; an elastic buffer (53) for synchronizing the transfer of the receiving data from said first processing means to said second processing means; and means (56) for increasing the frequency of said transmission data. An optical transmission system comprising a plurality of optical regeneration repeaters according to any one of claims 1 to 10, 12 and 13. An optical transmission system according to Claim 14, further comprising a repeater amplifier between said regenerators.

Inventors: FUNADA TAKASHI, HAGIMOTO KAZUO, TAKEYARI RYOJI, WAKAI YUKIHIKO, FUNADA, TAKASHI, HAGIMOTO, KAZUO, TAKEYARI, RYOJI, WAKAI, YUKIHIKO
Applicants: HITACHI LTD, NIPPON TELEGRAPH & TELEPHONE, HITACHI, LTD., NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1995
Main classification: H04B10
IPC codes: H04L7, H04B10, H04B3, H04L25