Abstract:
A multi-repeater optical transmission system comprises a plurality of repeaters which are respectively provided with oscillators for generating oscillation waves of different frequencies. The respective oscillation waves are frequency-modulated according to repeater information signals respectively generated in the repeaters. According to the frequency-modulated oscillation waves, respective pumping sources are operated to emit pumping lightwaves for pumping Er-doped fiber amplifiers. The respective pumping lightwaves carrying the repeater information signals are sent out to a transmission line through a wavelength division multiplexing fiber coupler.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present Invention relates to an optical transmission system which transmits a signal via a plurality of optical repeaters each including an in-line optical amplifier such as an optical fiber amplifier and, more specifically, to a system for transferring repeater information signals which are respectively generated in the optical repeaters. 
     2. Description of the Prior Art 
     In a multiple-repeater optical transmission system that transmits an optical signal via a plurality of repeaters, some failure may occur in a transmission line or a repeater, resulting in a possible serious failure in the transmission system. Therefore, it is necessary to always supervise the repeaters and transmission lines and to immediately inform a receiving terminal and a control terminal of a failure upon its detection. 
     An example of a supervisory method in an optical transmission system Is disclosed in Japanese Unexamined Patent Publication No. 2-266245. In this method, an Er(erblum)-doped fiber is used as an in-line optical amplifier, and pumping light is modulated by a pulse pattern whose period is shorter than the relaxation time of pumping carriers. This pumping light modulation causes the pulse pattern to be sent to an optical fiber transmission line. An optical loss and a failure position of the optical fiber transmission line is determined by calculating a time correlation between the above pulse pattern and a pulse pattern of backscattered light returning from the optical fiber transmission line. Although this type of supervisory method is effecting in supervising the optical fiber transmission line, it cannot provide effective information when a failure occurs in a repeater. 
     By analogical inference from this supervisory method using the pumping light modulation, a system may be conceivable in which each repeater is provided with a supervisory device and failure information in each repeater is superimposed on a main optical signal and is transmitted downstream. More specifically, a carrier of a predetermined frequency is digitally modulated in accordance with the failure information, and pumping light for the Er-doped fiber is generated in accordance with the modulated carrier. Accordingly, this system can achieve both optically amplification of the main signal through the Er-doped fiber pumped by the pumping light and transfer of the failure information carried by the pumping light in a state of being superimposed on the main signal. 
     However, the above failure information transfer system has the following problem. Since the modulation frequency for superimposition of failure information is the same for all the repeaters, when failures occur simultaneously in two or more repeaters, respective pieces of failure information cannot be transferred being superimposed on the main signal. Therefore, it is difficult to identify two or more failure repeaters, respectively. This is a factor of reducing the reliability of the optical transmission system. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an information transfer method and system which can improve the reliability of a multiple-repeater optical transmission system. 
     Another object of the invention is to provide an information transfer method and system which enables a network controller to identify which repeater is suffering from some failure. 
     A multi-repeater optical transmission system according to the present invention is comprised of a plurality of repeaters which are respectively provided with oscillators generating oscillation waves of different frequencies. The respective oscillation waves are used as carriers for frequency-modulation. More specifically, the respective oscillation waves are frequency-modulated in accordance with repeater information signals generated in the respective repeaters. In other words, such frequency-modulation causes the oscillation wave to carry the repeater information signal. According to the frequency-modulated oscillation waves, respective pumping sources are operated to emit pumping lightwaves for pumping optical fiber amplifiers. At the same time, the respective pumping lightwaves carrying the repeater information signals are sent out to a transmission line through an optical fiber coupler. 
     In a receiving terminal in the optical transmission system, received line-signal light is converted into an electrical line signal. The line signal is divided into a plurality of signals having different frequencies respectively corresponding to the oscillation waves generated in the repeaters. The respective signals having different frequencies are demodulated into the repeater information signals which have been generated in the repeaters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating an embodiment of an optical transmission system according to the present invention; 
     FIG. 2 is a block diagram showing a more specific configuration of a repeater of the embodiment; 
     FIG. 3 is a graph showing frequency characteristics of the relative modulation efficiency and the relative gain in a repeater; 
     FIG. 4 is a frequency spectrum diagram showing an example of assigning respective frequencies to the repeaters of the embodiment; 
     FIG. 5 is a block diagram showing an example of a receiving terminal of the embodiment; and 
     FIG. 6 is a block diagram showing another example of a receiving terminal of the embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, in a multiple-repeater optical transmission system comprising a plurality of optical fiber transmission lines TL 1  -TL n  and a plurality of repeaters R 1  -R n  connecting adjacent transmission lines to each other, each repeater R i  (i=1, 2, 3, . . . , n) is comprised of a known failure supervisory system SV i , an oscillator PG i , a modulator MOD i , a pumping source PS i , and an optical fiber amplifier FA i . When detecting a failure, for instance, within the repeater R i , the failure supervisory system SV i  generates an alarm information signal ALM i  indicating &#34;abnormality&#34;. The oscillator PG i  generates an oscillation wave having a frequency different from that of the oscillator PG j  of any other repeater R j . As described later, the frequencies f 1  -f n  for those oscillation waves, which are in a predetermined range, are assigned in advance to the respective repeaters (see FIG. 4). 
     The modulator MOD i  frequency-modulates the oscillation wave of frequency f i  generated by the oscillator PG i  in accordance with the alarm information signal ALM i , and supplies a modulated oscillation wave TS i  to the pumping source PS i . While the repeater R i  operates normally and therefore the alarm Information signal ALM i  indicates &#34;normal&#34;, the modulated oscillation wave TS i  is, for instance, an oscillation wave having a predetermined constant frequency. The pumping source PS i  emits pumping light PL i  for pumping the optical fiber amplifier FA i  in accordance with the frequency-modulated oscillation wave TS i . The optical fiber amplifier FA i  of each repeater R i  amplifies the received main signal, and supplies the amplified main signal and the pumping light PL i  to the downstream repeater R i+1  via the transmission line TL i . 
     FIG. 2 shows a more specific configuration of each repeater R i . In this example, frequency shift keying (FSK) is employed as a modulation scheme, and an Er-doped fiber is employed as an optical amplifier. In FIG. 2, the FSK modulator 101FSK-modulates the oscillation wave of frequency f i  generated by the oscillator PG i , and supplies a resulting FSK-modulated oscillation wave TS i  to a laser driver 102. The laser driver 102 drives a laser diode 103 in accordance with the FSK-modulated oscillation wave TSi, and the laser diode 103 emits pumping light PL l , which is input to a WDM (wavelength division multiplexing) fiber coupler 105. The pumping light PL i  is wavelength-multiplexed with the transmitting signal light by the WDM fiber coupler 105. 
     Inputting to the Er-doped fiber 104 through the WDM fiber coupler 103, the pumping light PL i  pumps the Er-doped fiber 104 to thereby amplify the input signal light. Further, the pumping light PL i  is wavelength-multiplexed with the transmitting signal light through the WI)M fiber coupler 105 and is sent out to the optical fiber transmission line TL i . For example, the wavelengths of the main signal light for transmission and the pumping light PL i  for pumping the Er-doped fiber 104 are set at 1.52-1.55 μm and 1.46-1.48 μm, respectively. 
     As shown In FIG. 3, the usable range of the FSK modulation central frequencies f 1  -f n  that are assigned to the respective repeaters R 1  -R n  is determined in consideration of the gain of the Er-doped fiber 104 and the modulation efficiency of the pumping light PL i . 
     A characteristic curve G in FIG. 3 represents a variation of the gain of a certain Er-doped fiber with respect to the modulation central frequency f i  of the pumping light PL i  when the main signal light and the modulated pumping light PL i  are input to the Er-doped fiber. In the range where the modulation central frequency f i  of the pumping light PL i  is lower than the frequency range R, the gain of the Er-doped fiber 104 becomes lower, so that the signal light may be prevented from reaching a receiving terminal if there exist a large number of repeaters. 
     A characteristic curve M represents a variation of the amplitude modulation efficiency of the pumping light PL i  with respect to the modulation central frequency f i  of the pumping light PL i  when the pumping laser diode 103 is subjected to amplitude-modulation by the FSK-modulated oscillation wave TS i . The range where the modulation central frequency f i  of the pumping light PL i  is higher than the frequency range R is not suitable for practical use, because in that range the amplitude modulation efficiency is too low. 
     It is therefore necessary to set the central frequency f i  of the FSK-modulated oscillation wave TS i  within the frequency range R, where both characteristics G and M are appropriate as shown in FIG. 3. In this embodiment, the usable frequency range R is 0.7-4 kHz. Therefore, as shown in FIG. 4, the oscillation frequencies f 1  -f n  of the oscillators PG 1  -PG n  are set, for instance, at regular intervals in the frequency range R (0.7-4 kHz). In this manner, the n alarm information signals ALM 1  -ALM n  can be transmitted by wavelength multiplexing. 
     As shown in FIG. 5, a receiving terminal has a configuration which can reproduce a plurality of alarm information signals. Upon reception of signal light from the optical fiber transmission line TL n , an optical receiver 201 separates main signal light from the received light and converts it to an electrical signal. The separated main signal is decoded into an original data by a decoder 202. On the other hand, the remaining of the input signal other than the main signal is frequency-divided by a divider 203 into signal components TS 1  -TS n  of frequencies f 1  -f n  , which are demodulated into alarm information signals ALM 1  -ALM n  by FSK demodulators DEM 1  -DEM n , respectively. The alarm information signals ALM 1  -ALM n  are supplied to an alarm information signal processor 204. If there exists an alarm information signal indicating &#34;abnormality&#34;, the alarm information signal processor 204 sends out alarm information identifying the repeater corresponding to the alarm information signal indicating &#34;abnormality&#34; to a network controller (not shown). 
     A description will be made of transfer of alarm information signals in the optical transmission system that is constructed as described above. It is assumed that in the two repeaters R 1  and R n  the failure supervisory systems SV 1  and SV n  have detected some failures and have output alarm information signals ALM 1  and ALM n  indicating &#34;abnormality&#34; to modulators MOD 1  and MOD n , respectively. 
     According to the alarm information signal ALM 1 , the modulator MOD 1  of the repeater R 1  FSK-modulates an oscillation wave of frequency f 1  that is supplied from the oscillator PG 1 . The pumping source PS 1  is subjected to modulation in accordance with the FSK-modulated oscillation wave TS 1 , so that pumping light PL 1  carrying the alarm information signal ALM 1  pumps the optical fiber amplifier FA 1  comprising an Er-doped fiber. As a result, main signal light is optically amplified by the optical fiber amplifier FA 1 , and is wavelength-multiplexed with the pumping light PL 1  that carries the alarm information signal ALM 1 . The resulting signal light is sent out to the optical fiber transmission line TL 1 . 
     The repeater R 2  operates normally. Therefore, upon reception of the signal light carrying the main signal and the alarm information signal ALM 1 , the optical fiber amplifier FA 2  of the repeater R 2  optically amplifies the signal light, multiplexes it with the pumping light PL 2  that carries an alarm information signal ALM 2  indicating &#34;normal&#34;, and finally sends out the resulting signal light to the transmission line TL 2 . Similarly, alarm information signals ALM 3  -ALM n-1  each indicating &#34;normal&#34; are superimposed in the respective repeaters R 3  -R n-1 . The repeater R n  receives and optically amplifies the signal light on which the main signal and the alarm information signals ALM 1  -ALM n  -1 are superimposed, further superimposes alarm information signal ALM n  indicating &#34;abnormality&#34; on the signal light, and sends out the resulting signal light to the receiving terminal. 
     In the receiving terminal, as shown in FIG. 5, the optical receiver 201 separates the main signal, and the divider 203 frequency-divides the remaining input signal into the respective components of frequencies f 1  -f n , which are FSK-demodulated into the respective alarm information signals ALM 1  -ALM n . Upon reception of the alarm information signals ALM 1  -ALM n , the alarm information signal processor 204 searches for alarm information signals indicating &#34;abnormality&#34;. In this example, since the alarm information signals ALM 1  and ALM n  indicate &#34;abnormality&#34;, the alarm information signal processor 204 sends out, to the network controller, alarm information including location information indicating the repeaters R 1  and R n  in which failures have occurred. In response, the network controller quickly performs emergency control processing such as switching of transmission lines. 
     FIG. 6 shows another example of a receiving terminal. Upon reception of signal light from the optical fiber transmission line TL n , a wavelength divider 301 divides the received signal light into main signal light and the remaining. The main signal light is converted to an electrical main signal by an optical/electrical (O/E) device 302 and the main signal is decoded into an original data by a decoder 202. On the other hand, the remaining light is converted to an electrical signal by an O/E device 303 and the electrical signal is frequency-divided by a divider 203 into signal components TS 1  -TS n  of frequencies f 1  -f n , which are demodulated into alarm Information signals ALM 1  -ALM n  by FSK demodulators DEM 1  -DEM n , respectively. The alarm information signals ALM 1  -ALM n  are supplied to an alarm information signal processor 204. If there exists an alarm information signal indicating &#34;abnormality&#34;, the alarm information signal processor 204 sends out alarm information Identifying the repeater corresponding to the alarm information signal indicating &#34;abnormality&#34; to a network controller (not shown). 
     Although in the above embodiment the optical fiber amplifier is an Er-doped fiber, it is not limited thereto but may be other rare-earth-element-doped fibers as long as they are in-line optical amplifiers that can be pumped by pumping light. 
     As described above in detail, in the failure information transmission scheme according to the invention, oscillation waves of different central frequencies are frequency-modulated in accordance with failure information generated in a plurality of repeaters, and are transferred by frequency multiplexing. Therefore, plural pieces of failure information can be transferred together by using the pumping light of a single wavelength.