Optical-fiber amplifier

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: BACKGROUND OF THE INVENTION 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 Erdoped 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 convention 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 (GB-A-2 242 091) 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 ∝, 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 It is an object of the invention to provide an optical fiber amplifier and a light repeating system which can demodulate and sample a first subcarrier from level fluctuation of main signal light, on which the first subcarrier was superposed from the front stage, when received the main signal light and which can superpose a second subcarrier of an arbitrary frequency (including the same frequency as the first subcarrier) on main signal light or spontaneous emission light for transmission. The object is solved by the features of the main claim. According the invention, there is provided an optical-fiber amplifier which includes 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. The constant level means eliminates the intensity modulation due to the first subcarrier and samples the first subcarrier from the error-signal-generating means. Since the subcarrier-superposing means superposes the second subcarrier on the main signal light, which has become to have a constant level, it is possible to superpose the second subcarrier, whose arbitrary frequency (of course, may be equal to the first subcarrier) is regardless of the frequency of the first subcarrier of the main signal light, on the main signal for transmission. According to the invention, there is also provided a light amplification repeating system with a monitoring function using a light amplifier. Since the spontaneous emission light from the light amplifier is utilized as the subcarrier for a supervisory signal, it is possible to transmit the subcarrier to the subsequent stage even when the main signal light is not inputted such as due to a circuit breakage. 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 repeating system using an optical fiber amplifier according to an embodiment of the invention; FIGS. llA and llB are diagrams illustrating the operation of the light repeating system when a subcarrier is modulated by amplitude shift keying; FIGS. 12A and 12B are diagrams illustrating the operation of the light repeating system when a subcarrier is a base-band PCM signal; FIG. 13 is a graph showing a spontaneous emission light spectrum of the light amplifier when a main signal is not inputted; FIG. 14 is a graph showing a spontaneous emission light spectrum of the light amplifier when a main signal is inputted; FIG. 15 is a block diagram showing a modified light repeating system of the embodiment of the invention; FIG. 16 is a block diagram showing another modified light repeating system of the embodiment of the invention. DETAILED DESCRIPTION Fig. 10 shows a light repeating system according to an embodiment using the optical fiber amplifier of the invention. In FIG. 10, reference numeral 201 designates an input terminal for main signal light; 202, 217, rare-earth-doped optical fibers; 203, 216, WDM optical coupler; 204, a light branching coupler; 205, a light receiver; 206, a differential amplifier; 207, a reference potential generator 208; 214, a pumping light source drive circuit; 215, a pumping light source; 210, a demodulator; 211, a separator; 212, a multiplexer; 213, a subcarrier generator; 218, an output terminal for main signal light; a, a first subcarrier; b, a second subcarrier; c, main signal light on which the first subcarrier is superposed; d, main signal light on which the second subcarrier is superposed; e, pumping light; g, a level error signal; and h, amplified light. Further, 225 designates an error signal generating means for inputting the light signal, on which the first subcarrier is superposed, to compare it with a predetermined reference value and for generating an error signal based on the result of comparison; 230, a constant level circuit for making an inputted light signal to be a constant level, based on the error signal; 240, a subcarrier processing means for detecting and processing the first carrier based on the error signal generated from the error signal generator 225; and 250, a subcarrier superposing means for generating a second subcarrier based on the process of the subcarrier processing means 240 and for superposing the second subcarrier on the light signal. The operation of the light repeating system of this embodiment will now be described with reference to FIG. 10. The main signal light c, on which the first subcarrier a as a low-frequency signal is superposed, is inputted to the input terminal 201 from the front-stage light amplification repeating system. The light-branching coupler 204 branches part of the main signal light c, on which the first carrier signal a is superposed, from the input terminal 201; one of the branched output sides is connected to the rare-earth-doped optical fiber 217, while the other output side is connected to the light receiver 205 which receives part of the main signal light c. When the light level of the main signal light c, on which the first subcarrier a is superposed, is constant, the light output of the pumping light source 209 is regulated by the pumping light source drive circuit 208 so as to be a predetermined constant value. But, for example, when the main signal light c is modified intensely with the first subcarrier a, i.e. a low-frequency signal, the light level of the main signal light c will not be constant; in this instance, the constant level amplifying means 230 operates so as to make the light level constant. As the light level varies, the light level at the light receiver 205 also varies. The light receiver 205 converts the light level photo- electrically. Meanwhile the differential amplifier 206 compares the light level with the level of the reference potential generator 207 and outputs the difference in level as a level error signal g resulting from the first subcarrier a superposed on the main signal light. The pumping light source drive circuit 208 regulates the output level of the pumping light e of the pumping light source 209 based on the level error signal g, and the regulated light enters the rare-earth-doped optical fiber 202 from the WDM optical coupler 203. Partly since the rare-earth-doped optical fiber 202 is a single-mode fiber having a length of several meters to several tens of meters and doped of a rare earth element such as erbium (Er), and partly since the pumping light e of the pumping light source 209 is excited behind the WDM optical coupler 203, the rare-earth-doped optical fiber 202 causes inverted distribution so that the main signal light from the input terminal 201 is amplified by stimulated emission. As a result, the pumping light source drive circuit 208 of the constant level amplifying means 230 controls the pumping light source 209 in such a manner that the first subcarrier a and the level error signal g are inverted in phase, and the pumping light e is excited at the rare-earth-doped optical fiber 202, thus obtaining the amplified light h whose light level is stable. The subcarrier processing means 240 controls the light repeating system using the first subcarrier a, as the level error signal g of the differential amplifier 206 is demodulated by the demodulator 210, is separated by the separator 211 and is processed by the subcarrier processing circuit 220. The multiplexer 212 inputs the signal from the separator 211, the first subcarrier a and the signal from the subcarrier processing circuit 220, and then multiplexes these signals over the signal from the light repeating system. The subcarrier superposing means 250 outputs the main signal light, which the second subcarrier b is superposed on the second amplified light h stabilised in a constant level in the fiber, by the same action as mentioned above, as the second subcarrier b is generated from the subcarrier generator 213 by the repeating system, as the pumping light source 215 is controlled by the pumping light source drive circuit 214 and as the pumping light f of the pumping light source 215 is excited at the rare-earth-doped optical fiber 217 behind the WDM optical coupler 216. As mentioned above, according to this embodiment, the light amplification repeating system equipped with light amplifier comprises: a first optical fiber, which is doped of a light amplifying medium, for inputting main signal light, on which a first subcarrier is superposed, and for outputting a first amplified light; a first pumping light source for generating first pumping light to excite the first optical fiber; a first WDM optical coupler connected to the first optical fiber to multiplex wavelength; a first pumping light source drive circuit for driving the first pumping light source; a light-branching coupler for branching the first amplified light; a light receiver for converting the branched output light of the light-branching coupler photoelectrically; a reference potential generator for selecting a reference potential to compare an electrical signal from the light receiver with the predetermined reference value; a differential amplifier for comparing the output signals from the light receiver and the reference potential generator and for outputting a level error signal based on the result of comparison; a demodulator for inputting the level error signal, i.e. the first subcarrier sampled, from the differential amplifier and for demodulating the inputted signal; a second optical fiber doped of a light amplifying medium for inputting the first amplified light and for outputting second amplified light; a second pumping light source for generating second pumping light to excite the second optical fiber; a second WDM optical coupler connected to the second optical fiber for multiplexing the second excitation light over the first amplified light; a second pumping light source drive circuit for driving the second pumping light source; and a subcarrier generator connected to the second pumping light source drive circuit for superposing the second subcarrier on the second pumping light from the second pumping light source. In this case, the first subcarrier a and the second subcarrier b may have a common carrier frequency. Each of the first and second subcarriers a and b may be a carrier wave modified with supervisory information (in this case, 1, 0, 1, 0, 1...) by amplitude shift keying, as shown in FIG. 11A. The demodulator for demodulating the subcarrier samples a modulated signal of FIG. 11B by inputting the main signal light, on which the subcarrier is superposed, by the light receiver, then converting the inputted main signal light photoelectrically and finally transmitting the converted signal through a band-pass filter which permits only the modulated-frequency component of the subcarrier to pass. Then when the modulated signal is square-law-detected or synchronous-detected, the light amplification repeating system can obtain supervisory information. Of course, an alternative modulation (e.g., FSK or PSK) may be used. FIGS. 12A and 12B show supervisory information directly modulated by PCM. The light amplification repeating system can obtain supervisory information in the same method as the ASK modulation. According to this embodiment, since the light repeating system has a second light amplifier for modulation, the subcarrier b outputted from the subcarrier generator 213 can control the pumping light source 215, totally regardless of the index of modulation of the front-stage subcarrier a. In this embodiment, the main signal light c is inputted in the light amplification repeating system all the time. However, if breakage of the fiber happens to occur upstream of the light amplification repeating system or the upstream light amplification repeating system happens to be faulty, the main signal light c cannot be inputted to the light amplification repeating system. In this instance, with the system having the same construction as the previous embodiment, assuming that the pumping light from the pumping light source 215 is modulated and that the second subcarrier b is transferred with spontaneous emission light as a carrier wave, it is possible to transfer the subcarrier even in the absence of the main signal light c. FIG. 13 shows a spontaneous emission light spectrum of the light amplifier, when the main signal light c is not inputted, and FIG. 14 shows a spontaneous emission light spectrum of the light amplifier when the main signal light c is inputted. In the presence of the main signal light c inputted, since the energy of the light amplifier is used in amplification of the main signal light c, the level of the spontaneous emission light is low. But in the absence of the main signal light c inputted, no stimulated emission occurs to increase the level of the spontaneous emission light. When the main signal light c is cut off, the second subcarrier b should be transferred with this high-label spontaneous emission light as a carrier wave. In this embodiment, the rare-earth-doped optical fiber 202, 217 is excited by backward pumping. Alternatively, the same result as this embodiment can be expected also in the case of forward pumping as shown in FIG. 15. Designated by 219 is a band-pass filter. Further, in this embodiment, two WDM couplers are used. Alternatively the same result can be expected even with a single WDM coupler (Fig. 16).

Claims: An optical-fiber amplifier comprising: a first rare earth element doped optical fiber (202) receiving a main light signal (c) having a first subcarrier superposed thereon, a first pumping light source (209) for generating first pumping light (e) introduced in the first optical fiber (202) through a first wavelength division multiplexing optical coupler (203) connected to the first optical fiber (202), a second rare earth element doped optical fiber (217) receiving an amplified light signal (h) from the first optical fiber (202), a second pumping light source (215) for generating second pumping light (f) introduced in the second optical fiber (217) through a second wavelength division multiplexing optical coupler (216) connected to the second optical fiber (217), the optical-fiber amplifier further comprising: a) error-signal-generating means (225) for receiving a light signal, on which a first subcarrier (a) is superposed, and generating an error signal (g) according to the difference between the level of said light signal and a predetermined reference level; wherein said error-signal-generating means (225) includes: a light-branching coupler (204) for branching off a light signal from the main light signal (h) amplified in said first doped optical fiber (202) ; light-receiving means (205) for receiving the light signal from said light-branching coupler (204) and for converting the light signal into an electrical signal; and differential amplifying means (206) for receiving the output signal from said light-receiving means (205) and a reference signal from reference-potential-generating means (207) and for outputting the difference therebetween as said error signal (g); b) constant level means (230) for maintaining the level of said amplified light signal (h) at a constant level, based on said error signal (g) outputted from said error-signal-generating means (225) ; wherein said constant level means (230) includes: said first source (209) for emitting said first pumping light (e); first driving means (208) for driving said first source, based on said error signal (g) outputted from said differential amplifying means (206) ; and said first rare-earth-element-doped optical fiber (202) for amplifying to a constant level the main light signal (c), on which said first subcarrier is superposed, by said first pumping light emitted from said first source; c) subcarrier-processing means (240) for detecting and processing said first subcarrier (a) based on said error signal (g); wherein said subcarrier-processing means (240) includes: means for demodulating (210) said error signal outputted from said differential amplifying means (206); means for separating (211) a demodulated error signal outputted from said demodulating means (210); means for processing (220), as a first subcarrier, one of the separate demodulated error signals outputted from said separating means (211); and superposing means (212) for receiving the other demodulated error signal outputted from said separating means (211) and for superposing said other demodulated error signal on said first subcarrier from said processing means (220) to output the resulting signal to a subcarrier-superposing means (250) and d) said subcarrier-superposing means (250) for generating a second subcarrier (b) based on the process of said subcarrier processing means (240) and for superposing said second subcarrier (b) on said amplified light signal (h) maintained at the constant level by said constant level means (230) ; wherein said subcarrier-superposing means (250) includes: means for generating (213) said second subcarrier signal (b) based on the process of said subcarrier-processing means (240) ; said second source (215) for emitting said second pumping light (f) ; second driving means (214) for driving said second source based on said second subcarrier (b) outputted from said subcarrier-generating means (213); and said second rare-earth-doped optical fiber (217) for superposing said second subcarrier (b) on said amplified light signal (h), which is maintained at the constant level by said constant level means (230), by using said second pumping light emitted from said second source (215). 2. A light amplification repeating system with a monitoring function comprising a light amplifier according to claim 1.

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, C/O MITSUBISHI DENKI KK, MATSUSHITA, KIWAMI, C/O MITSUBISHI DENKI KK, Mizuochi, Takashi, c/o Mitsubishi Denki KK, NAKAGAWA, EIICHI, C/O MITSUBISHI DENKI KK, SHIMIZU, KATSUHIRO, C/O MITSUBISHI DENKI KK, TAKEMURA, NOBUYUKI, C/O MITSUBISHI DENKI KK
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01S3
IPC codes: H04B10, H01S3