Optical fiber amplifier repeating system

Abstract: The invention provides an optical-fiber amplifier comprising: error-signal-generating means for inputting a light signal, on which a first subcarrier is superposed, and generating an error signal according to the difference between the level of the light signal and a predetermined reference level; constant level means for maintaining the level of the light signal at a constant level, based on the error signal outputted from the error-signal-generating means; subcarrier-processing means for detecting and processing the first subcarrier based on the error signal; and subcarrier-superposing means for generating a second subcarrier based on the process of the subcarrier processing means and for superposing the second subcarrier on the light signal maintained at the constant level by the constant level means.

Description: 1. Field of the Invention This invention relates to an optical-fiber amplifier which is to be used in a light communication system and uses an optical fiber doped of a rare earth element. 2. Description of the Related Art: FIG. 1 of the accompanying drawings shows a conventional optical-fiber amplifier of the type described above, the amplifier being to be used for a two-system signal line. In FIG. 1, reference numerals 1a, 1b designate first and second rare-earth-doped optical fibers; 2a, 2b, first and second pumping light sources; 3a, 3b, first and second multiwavelength combining and dividing devices; 4a, 4b, first and second pumping light source drive circuits; 5a, 5b, 5c, 5d, input and output terminals for signal light; and 11a, 11b, first and second isolators. In operation, each of the first and second rare-earth-doped optical fibers 1a, 1b is a single-mode optical fiber doped of a rare earth element such as erbium (Er) and having a length of several meters to several tens meters. The first wavelength division multi/demultiplexing (WDM) device 3a is connected to the first rare-earth-doped optical fiber 1a. The wavelength division multi/demultiplexing (WDM) devices may be optical couplers. The first and second pumping light source 2a, 2b are semiconductor lasers having a wavelength of, for example, 1.48 µm and are driven the first and second pumping light source drive circuits 4a, 4b, respectively. When several mW to several tens mW of first pumping light outputted from the first pumping light source 2a is inputted to the first rare-earth-doped optical fiber 1a via the first wavelength division multi/demultiplexing (WDM) device 3a, the first rare-earth-doped optical fiber 1a assumes an inverted distribution state so that the signal light having a wavelength of 1.53 or 1.55 µm and inputted from the input and output terminal 5a for the signal light is amplified by the action of induced emission for output to the input and output terminal 5b. Likewise, when the second pumping light outputted from the second pumping light source 2b is inputted to the second rare-earth-doped optical fiber 1b via the second wavelength division multi/demultiplexing (WDM) device 3b, the second rare-earth-doped optical fiber 1b assumes a population inversion state so that the signal light inputted from the input and output terminal 5c is amplified for output to the input and output terminal 5d. With this conventional arrangement, it is impossible to improve the reliability of this type optical-fiber light amplifier. The simplest popular optical-fiber amplifier for a single-system signal has a construction such as shown in FIG. 2. This known art is exemplified by Japanese Patent Laid-Open Publication No. Hei 2-241073. In this light amplifier, two sources for pumping light are combined by a combining device. In FIG. 2, reference numeral 1 designates a rare-earth-doped optical fiber; 2a, 2b, first and second pumping light sources; 3, a multiwavelength combining device for combining pumping light and signal light; and 8, a combining device for combining the first pumping light and the second pumping light. However, when the first pumping light and the second pumping light are combined by the combining device 8, the combined light power will be 1/2 of the total light power of the first and second pumping light emitted from the first and second sources 2a, 2b. Further, since the first pumping light and the second pumping light interfere with one another, the output power of the combining device tends to fluctuate and hence to be non-stable. FIG. 3 shows a conventional optical amplifier for a double-system signal, which is a natural expansion of the construction of FIG. 2. First pumping light and second pumping light outputted from the first and second sources 2a, 2b are combined and then divided by a combining and dividing device 8, and the resulting separate parts of pumping light are inputted to first and second rare-earth-doped optical fibers 1a, 1b via first and second multiwavelength combining devices 3a, 3b. However, the output of the combining device 8 is non-stable so that the amplifying characteristic of the rare-earth-doped optical fibers 1a, 1b will not be stable. With this conventional arrangement, the level of pumping light to be inputted to the amplifying media will not be stable. FIG. 4 shows another conventional light amplifier, which is disclosed in "The Impact That An Er-doped Optical-Fiber Amplifier Contributes To Light Communication" by T. Shimada, Oplus E, No. 113, pp. 75-82, 1989. In FIG. 4, reference numeral 1 designates a rare-earth doped optical fiber; 2, a pumping light source; 3, a combining and dividing device; 4, a pumping light source drive circuit; and 5, an optical fiber serving as a signal transmission path. In operation, the rare-earth-doped optical fiber 1 is a single-mode optical fiber doped of a rare earth element such as erbium (Er) and having a length of several meters to several tens meters. The combining and dividing device 3a is connected to the rare-earth-doped optical fiber 1. The combining and dividing device may be an optical coupler. The pumping light source 2 is a semiconductor laser having a wavelength of, for example, 1.48 µm and is driven the pumping light source drive circuit 4. When several mW to several tens mW of pumping light outputted from the pumping light source 2 is inputted to the rare-earth-doped optical fiber 1 via the combining and dividing device 3, the rare-earth-doped optical fiber 1 assumes an inverted distribution state so that the signal light having a wavelength of 1.53 or 1.55 µm and inputted from the optical fiber 5 is amplified by the action of induced emission. The rare-earth-doped optical fiber 1, the pumping light source 2, the combining and dividing device 3 and the pumping light source drive circuit 4 jointly constitute a light amplifying means. In the light amplifier utilizing the action of stimulated emission, spontaneous emission light is emitted along with the action of light amplification. This spontaneous emission light produces noise, which deteriorates the signal-to-noise rate of signal light. The noise resulting from spontaneous emission light is exemplified by shot noise due to spontaneous emission light, beat noise between signal light and spontaneous emission light, and beat noise between one spontaneous emission light and another natural emission light. Spontaneous emission light is free of polarization dependency and has the component parallel to the polarization plane of signal light and that perpendicular thereto. Further, when two or more light amplifiers are to be used as connected in tandem, accumulation of spontaneous emission light brings down the saturation level of signal gain of the light amplifier. With the conventional light amplifier, the noise resulting from spontaneous emission light would deteriorate the signal-to-noise ratio of signal light. If two or more of the conventional light amplifiers are used as connected in tandem, accumulation of spontaneous emission light would bring down the saturation level of signal gain of the individual light amplifier. FIG. 5 shows a conventional light amplification repeating system, which is disclosed in Japanese Patent Laid-Open Publication No. Hei 3-214936. In this conventional light amplification repeating system, when the main signal light c, on which a subcarrier a is superposed, is inputted from a main signal light input terminal 11, the main signal light c is branched by a light branching coupler 14. Part of the branched main signal light c is inputted to a subcarrier processing circuit 20 where a process takes place based on the subcarrier a. A subcarrier generator 15 generates a subcarrier b based on the subcarrier a, and a pumping light source drive circuit 18 drives a pumping light source 19, and a rare-earth-doped optical fiber 12 modulates, in intensity, main signal light c, whereupon main signal light d, on which the subcarrier b is superposed, is transmitted to the repeating system at the subsequent stage. At that time the subcarrier b to be superposed has a frequency different from the subcarrier a. With this arrangement, the subcarrier to be processed by the subcarrier processing circuit must have a different frequency for every light repeating system. FIG. 6 shows a typical light amplification repeating system equipped with light amplifiers and utilizing light pumping. The technology of this kind is disclosed in detail in, for example, Japanese Patent Laid-Open Publication No. Hei 3-214936. In FIG. 6, reference numerals 100a, 100b designate light end offices; 101c, 101d, 101e, light amplification repeaters; 102b, 104a light-to-electric converters; 103a, 103b, subcarrier terminals; 102 a, 104b, electric-to-light converters; 105c, 105d, 105e, 107c, 107d, 107e, light amplifiers; and 106c, 106d, 106e, subcarrier transmitters. In the conventional supervisory method, assuming that an abnormality, such as a fiber breakage between the light amplifiers 105c, 105d or a fault of the light amplifier occurs, the subcarrier transmitter 106c generates a subcarrier to modulate the light amplifier 105d, which is a pumping LD, and transmits the subcarrier to the downstream side. The subcarrier terminal 103b discriminates the context of the transmitted subcarrier and issues an appropriate command. FIG. 7 shows the conventional light amplifier and subcarrier transmitter which are shown in Japanese Patent Laid-Open-Publication No. Hei 3-214936. In FIG. 7, reference numeral 105 designates a light amplifier; 106, a subcarrier transmitter; 111, an erbium-doped fiber; 112a, 112b, light isolators; 113, a light branching device; 114, a light combining device; 115, an excitation LD; 116, a subcarrier processing circuit; 117, a subcarrier generator; and 118, a pumping LD drive circuit. FIGS. 8A, 8B and 8C show the signal and spectrums of the system of FIG. 7. In operation, the signal light is inputted to the light branching device 113 via the isolator 112a. The light branching device 113 branches a small quantity of the power to the subcarrier processing circuit 116 and most of the remaining power to the erbium-doped fiber 111. In the subcarrier processing circuit 116, the state of light input to the light amplifier is detected. In the subcarrier generator 117, a subcarrier is generated based on information from the subcarrier processing circuit 116 and the state of operation of the light amplifier. The information from the subcarrier processing circuit is a breakage of input to the light amplifier when, for example, the upstream fiber happened to be cut off. What to be supervised while the light amplifier is operative may be a fault of the pumping LD. The pumping LD drive circuit drives the pumping LD by a current obtained by superposing the subcarrier on a bias current. The pumping light is modulated in intensity by the subcarrier. FIG. 8A schematically shows the output light from the pumping LD 115 to the light combining device 114 in FIG. 7; a signal slightly modulated by 1 to several % is superposed on a direct current. Since the gain of the light amplifier is very dependent on the pumping light power, it will be modulated as the pumping light is modulated in intensity. Consequently the envelope of signal light is modulated in intensity by the subcarrier. The transmission characteristic of the light amplification repeating and transmission system is a band pass characteristic; it has therefore been customary to premodulate the subcarrier by a sine wave having predetermined binary information. This concept is discussed in detail in, for example, Japanese Patent Laid-Open Publication No. Hei 3-252231 and "Control of Supervision in Light Amplification Repeating and Transmission Method" by Imai et al., Electronic Information Communication Society Spring Meeting B-944, 1992. Transmission of a subcarrier requires a high transmission quality. On many occasions the quality of signal is evaluated on the carrier/noise ratio (C/N ratio). The present inventors discovered that the main factor to deteriorate the C/N ratio of a subcarrier is the intensity noise produced from the light amplifier. The intensity noise is expressed quantitatively by relative intensity noise (RIN). FIG. 8B shows a frequency spectrum of electric power density of the output light from the pumping LD 115 in FIG. 7. As shown in FIG. 8B, in the conventional excitation in which a direct current is modulated by a subcarrier, noise would occur chiefly at the low-frequency side so that the output of the light amplifier 112b of FIG. 7 will be as indicated by the frequency spectrum of FIG. 8C. Consequently the frequency of the subcarrier deteriorates the C/N ratio. FIG. 9 shows the result of calculation of the C/N ratio when the modulation indices of the subcarrier is varied. When RIN is α, the C/N ratio is determined by the spontaneous emission light of the light amplifier; thus 90 dB is obtained with a modulation index of 1%. However, the C/N ratio varies markably, compared to RIN. In fact, the measurement of RIN of the output light from the light amplifier shows that RIN at 5 kHz was -82 dB/Hz. Calculating the C/N ratio using this value, the C/N ratio at a modulation index of, for example, 1 %, was only 20 dB. Consequently an adequate code error cannot be achieved. The mode of supervising operation for any breakage of communication channel in the foregoing circuit will now be discussed. Assuming that the fiber between the light amplifiers 105c, 105d is cut off, "input is cut off" will be judged, based on the fact that the light power branched from the branching device 113 in the subcarrier transmitter 106d of the light amplification repeater 101d is lowered from a threshold preset in the subcarrier processing circuit 116. Instantly the subcarrier generator 117 generates a subcarrier, which will modulate the pumping LD of the light amplifier 105d and will then be transmitted to the downstream side. Also in the light amplification repeater 101e at the next stage, the light power will be lowered to start generating a subcarrier, but the light power outputted from the upstream light amplifier 105d is increased progressively (the time constant at this time depends on the relaxation time of spontaneous emission from the light amplifier) so that the subcarrier processing circuit in the light amplification repeater 101e at the next stage will stop transmission of the carrier signal soon. With this light amplification repeater, the C/N ratio of a supervisory signal would be lowered to deteriorate the quality of the supervisory signal. When detecting any breakage of signal channel, the subcarrier terminal 103b will receive a subcarrier, indicating the fact "input is cut off", not only from the light amplification repeater 101c but also the light amplification repeater 101d. Consequently it was impossible to discriminate as to which information is correct. SUMMARY OF THE INVENTION An object of the invention is to make it possible to detect one of light amplification repeaters at which a signal-line breakage has occured. According to the invention, there is provided a light repeating system composed of a plurality of light repeaters, each with a monitoring function, and a light end office, the system comprising: demodulating means located in the light end office for demodulating subcarriers transmitted from the individual light repeaters; and judging means located in the light end office for judging it abnormal when abnormal information contained in the subcarrier demodulated by the demodulating means continues for a predetermined time period or appears a predetermined number of occasions. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an optical-fiber amplifier of the related art; FIG. 2 is a diagram showing a first system of optical-fiber amplifier of the related art; FIG. 3 is a diagram showing a second system of optical-fiber amplifier of the related art; FIG. 4 is a diagram showing another optical-fiber amplifier of the related art; FIG. 5 is a light repeating system of the related art; FIG. 6 is a block diagram showing a light amplification repeating and transmitting system of the related art; FIG. 7 is a block diagram showing a light amplification repeater of the related art; FIG. 8A is a diagram showing the output light of an excitation laser diode (LD) of the related art; FIGS. 8B and 8C are graphs showing output light spectrums of pumping LD of the related art; FIG. 9 is a graph showing the C/N ratio of a subcarrier according to the related art; FIG. 10 is a block diagram showing a light end office according to an embodiment of the invention; FIG. 11 is a diagram showing a frame format of binary information according to the light end office of the embodiment according to FIG. 10; FIG. 12 is a system diagram of the light end office of the embodiment according to FIG. 10; and FIGS. 13A through 13E are timing diagrams of the light end office of the embodiment according to FIG. 10. DETAILED DESCRIPTION FIG. 10 shows a light end office 103b according to an embodiment of this invention. The subcarrier converted into an electrical signal by a photoelectric converter 102b is synchronous-detected by a detector 81 and is then converted into binary information by a demodulator 82. A detector 83a of a decoder 83 detects carrier information bits to report to a discriminator 83b that an abnormal signal has been received. The discriminator 83b discriminates whether or not the number of subcarrier information bits continue for a predetermined period, or counts whether or not they continue for a predetermined number of successive frames or for a predetermined time period. FIG. 11 shows one example of frame format of binary information. 1 frame consists of 16 bits, every fourth bits of which are frame synchronizing bits. The subcarrier information bits are indicated by A and B in FIG. 11, and each bit will be "0" to indicate an abnormality for the fact "input is cut off". The decoder 83 judges the fact as "input is cut off" from this binary information to issue an alarm. FIGS. 13A through 13E are timing diagrams of individual signals of various parts of the system when the fact "input is cut off" occured. Assuming that the breakage of the fiber occured at a particular time, the input light power of the light amplifier #1 (FIG. 4) varies as shown in FIG. 13A. The output power of the light amplifier #1 falls instantly and then rises slowly as spontaneous emission light increases gradually, as shown in FIG. 13B. The rising time is determined by a relaxation time constant of spontaneous emission of the light amplifier. Having a threshold level preset between the output light power at the time of normal and the output light power at the time of "input is cut off", the light amplifier #1 detects the fact "input is cut off". The output light power of the next-stage light amplifier #2 of FIG. 13C varies very similary to that of FIG. 13B. In this case, since the spontaneous emission light generated from the light amplifier #1 is inputted, the output of the light amplifier #2 will become much larger than the output of the light amplifier #1, exceeding the threshold. The subcarrier transferred from the light amplifier #1 and decoded at its terminal portion will become "HI" when it is below the threshold level, as shown in FIG. 13D. The subcarrier transferred from the light amplifier #2 and decoded will produce "HI" and will then return to "LO" when it restores over the threshold, as shown in FIG. 13E. According to the measurement, this time period was about 3 ms. In the conventional art, since the fact "input is cut off" will be regarded as having occured from both the light amplifiers #1 and #2, it is impossible to judge as to which is correct. Whereas in this invention, the discriminator 83b of the decoder 83 will not issue an alarm until the subcarrier information bit A will become, for example, "0" three consecutive times; this is, the result of judgement will not be issued in the absence of three consecutive coincidences. If the transmission rate of binary information is 1200 bits/s, it will take the time of 16x3 bits, i.e. 40 ms, until an alarm is issued. Even when an error subcarrier has arrived from light amplifier #2 for a period of about 3 ms, the decoder does not judge it as error. It is therefore possible to judge the light amplifier #1 correctly as having issued "input is cut off". In this embodiment, the subcarriers are digital. Alternatively analog subcarrier may be used. In the case of analog subcarrier, a circuit-breakage-detecting-signal separator for analog signal separates a circuit-breakage signal in response to the sampling of subcarrier information bits at the decoder of FIG. 10. Unlike the case of digital subcarriers, only when the separated analog circuit-breakage signal has continued for a predetermined time period, it will be judged that there has occured a fault. In the light end office, since the decoder for decoding a subcarrier from each light amplification repeater includes the discriminating means for detecting a plurality of successive codes from information bits of the subcarriers from the individual amplification repeaters to judge it as back error, it is possible to detect an abnormal with improved accuracy.

Claims: A light repeating system composed of a plurality of light repeaters, each with a monitoring function, and a light end office, said system comprising: (a) demodulating means (82) located in the light end office for demodulating subcarriers transmitted from the individual light repeaters; and (b) judging means (83) located in the light end office for judging it abnormal when abnormal information contained in said subcarrier demodulated by said demodulating means continues for a predetermined time period or appears a predetermined number of occasions.

Inventors: KITAYAMA TADAYOSHI, MATSUSHITA KIWAMI, MIZUOCHI TAKASHI, NAKAGAWA EIICHI, SHIMIZU KATSUHIRO, TAKEMURA NOBUYUKI, KITAYAMA, TADAYOSHI, MATSUSHITA, KIWAMI, MIZUOCHI, TAKASHI, NAKAGAWA, EIICHI, SHIMIZU, KATSUHIRO, TAKEMURA, NOBUYUKI, Kitayama, Tadayoshi, Mitsubishi Denki K. K., Matsushita, Kiwami, Mitsubishi Denki K. K., Mizuochi, Takashi, Mitsubishi Denki K. K., Nakagawa, Eiichi, c/o Mitsubishi Denki K. K., Shimizu, Katsuhiro, Mitsubishi Denki K. K., Takemura, Nobuyuki, Mitsubishi Denki K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H04B10
IPC codes: H01S3, H04B10