GB2322027A

GB2322027A - An optical amplifier or transmitter which is shut down in the abscence of an input signal

Info

Publication number: GB2322027A
Application number: GB9809322A
Authority: GB
Inventors: Tsukasa Takahashi; Hiroyuki Itou; Yuji Tamura; Michikazu Shima; Kazuo Yamane; Yumiko Kawasaki; Keisuke Imai; Yoshinori Ohkuma; Kazuhiro Suzuki; Yasunori Nagakubo; Takashi Tsuda; Yuji Miyaki
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1994-05-20
Filing date: 1995-03-27
Publication date: 1998-08-12
Anticipated expiration: 2015-03-27
Also published as: GB9809322D0; GB9809228D0; GB2322026A; GB9809227D0; GB2322027B; GB2322026B; GB2322025A; GB2322025B

Abstract

An optical amplifier or transmitter which is shut down for a preset time when the absence of an input signal is detected. The optical amplifier comprises a doped fibre 274, a pumping light source 278, interception detection means 268,270 for detecting that inputting of the signal light to said doped fibre is intercepted, a photo detector 284 for detecting the intensity of light outputted, automatic level control means 288 for supplying a controlling electric current to said pumping light source 278 so that the output level of said photo detector 284 may be fixed, and shutting down means 266 for intercepting the input to the doped fibre 274 for a predetermined period of time in response to an output signal from said interception detection means 268,270. This prevents an optical surge. The transmitter is constructed similarly. Embodiments that intercept the output are also disclosed.

Description

Hú-5225JIOO-3 2322027 OPTICAL AMPLIFIERSAND OPTICAL TRANSMISSION APPARATUS

This invention relates to an optical amplifiers and optical transmission apparatus. This application is a divisional of British patent application No.9506179.2 filed on 27th March 1995.

In recent years, optical amplifiers which employ a waveguide structure (referred to as "doped fiber" in the present application) doped with a rare earth element including an EDF (erbium-doped fiber) have been developed, and construction of systems which include a large number of optical repeaters including such optical amplifiers interposed in an optical tran8mission line has been proposed. In order to put optical amplifiers of the type just mentioned into practical use, it is essentially required to adopt a controlling technique which conforms to characteristics of a doped fiber, and various control systems have been proposed. For example, optical amplifiers of the type mentioned have a problem in that an optical surge is likely produced, for example, due to the presence of a delay time after signal light and pumping light are introduced into a doped fiber until stimulated emis7-slon occurs, and countermeasures against the problem are demanded for optical amplifiers and optical transmit- ters.

When pumping light having length is introduced from a first or from the second end toward the fiber which is doped with a rare for a wavelength of signal light signal light is introduced from a predetermined waveend toward a second end first end of a doped earth element suitable to be amplified while the first end toward the second end of the doped fiber, stimulated emission occurs in the doped fiber so- that the signal light is amplified. The wavelength of-the pumping light depends upon the wavelength of the signal light and the doped element. A laser diode is normally used as a pumping light source for outputting pumping light. Since the amplification factor relies upon the intensity of pumping light, it is advantageous for construction of various control systems to employ as a pumping light source a laser diode which exhibits a variable output level.

For example, when an optical amplifier which is based on the principle of stimulated emission described above is applied to an optical communication system, it is demanded to keep fixed the output level of the optical amplifier from the necessity of system designing. ThereforiE-, in an optical amplifier which employs a doped fiber, automatic level control (ALC) is usually applied so that the output level of the optical amplifier may be fixed. In order to perform ALC, light outputted from the doped fiber is split into two beams of light. The first split light is sent out into an optical transmission line while the second split light is converted into a photoelectric current by a photo-detector. The photoelectric current is converted by current/voltage conversion and then compared with a reference voltage, and the bias electric current to a laser diode serving as a pumping light source is controlled so that the difference obtained by the comparison may be zero or fixed.

If the input of signal light in the optical amplifier in which ALC is proceeding is intercepted, then the pumping light is increased as a result of the ALC. If inputting of signal light is restored in this condition, then a surge is outputted from the optical amplifier from the reason that there is a delay of several ms before stimulated emission in the doped fiber. If the optical surge reaches a receiver, then there is the possibility that this may make an excessive input to a photo-electric converter (photo-detector) of the receiver and destroy a component of the receiver. Therefore, an optical amplifier used as an optical repeater, particularly an optical amplifier applied to multi-stage repeat- ing, can desirably detect that the optical signal has been intercepted.

Where an optical amplifier is used as a preamplifier on the-receive side, the power of signal light inputted is so low that, if it is tried to split the inputted signal light and detect interception of the signal light based on the thus split light, the power of the inputted signal light may be lower than a detection limit of the photo-detector. Meanwhile, if it is tried to split signal light inputted to an optical amplifier and detect interception of the signal light, then the NF (noise figure) of the optical amplifier is deteriorated by an amount by which the power of the signal light supplied to the doped fiber is decreased.

Accordingly, it is desirable to make it possible.to detect interception of signal 1 ight inputted to an optical amplifier readily.

It is also desirable to prevent deterioration of the NF of an optical amplifier which arises from detection of interception of signal light.

It is also desirable to eliminate an influence of an optical., sarge arising from a characteristic of a doped fiber.

According to a first aspect of the invention, there is provided an optical amplifier, comprising: a doped fiber doped with a rare earth element and having a first end and a second end for guiding signal light from said first end toward said second end thereof; a pumping light source for outputting pumping light; optical coupling means optically connected to said doped fiber and said pumping light source for introducing the pumping light into said doped fiber; interception detection means for detecting that inputting of the signal light to said doped fiber is intercepted; a photo-detector for detecting the intensity of light outputted from said second end of said doped fiber; automatic level control means for supplying a controlling electric current to said pumping light source so that the output level of said photo-detector may be fixed; and shutting down means for intercepting the output of said doped fiber for a predetermined period of time in response to an output signal of said interception detection means.

According to a second aspect of the invention, there is provided an optical transmission apparatus, comprising:

signal light generating means for generating signal light modulated in accordance with an input signal; input interception detection means for detecting that the input signal is intercepted; and shutting down means for intercepting the output of said signal light generating means for a predetermined period of time in response to an output signal of said input interception detection means.

Reference will now be made, by way of example, to the accompanying drawings, in which:

FIG.1 is a block diagram showing a first erbiumdoped fiber amplifier (EDFA) useful for understanding the present invention, but not in accordance with the same; FIG.2 is a diagram illustrating a characteristic of calculation means of FIG.1; FIG.3 is a block diagram of a delay circuit of FIG. 1; FIG.4 is a time chart illustrating operation of the delay circuit of FIG. 3; FIG.5 is a block diagram showing a second EDFA useful for understanding the present invention, but not in accordance with the same; FIG.6 is a time chart of a behaviour of an optical fiber amplifier (when the optical input interception continues for a long period of time); FIG.7 is a time chart of a behaviour of r-he optical fiber amplifier (when the optical input interception continues for a short period of time); _ FIG.8 is a block diagram of a first optical transmission apparatus, useful for understanding the invention but not in accordance with the same, employing external modulation; FIG.9 is a block diagram of another optical transmission apparatus, useful for understanding the invention but not in accordance with the same, employing direct modulation; FIG.10 is a block diagram showing a further optical transmission apparatus, useful for understanding the invention but not in accordance with the same; FIG.11 is a block diagram showing a still further optical transmission apparatus, useful for understanding the invention but not in accordance with the same; FIG.12 is a block diagram showing an example of a mark rate detection circuit; FIG.13 is a block diagram showing an example of a shutting down circuit; FIG.14 is a time chart of the examples of FIGS.10 and 11; FIG.15 is a block diagram showing a first embodiment of an EDFA to which the present invention is applied; FIG.16 is a time chart illustrating operation of the EDFA of FIG.15; FIG.17 is a block diagram showing a second embodiment of an EDPA; FIG.18 is a time chart illustrating operation of the EDFA of FIG.17; FIG.19 is a block diagram showing a third embodiment of an EDFA to which the present invention is applied; FIG.20 is a time chart illustrating operation of the EDFA of FIG.19; FIG.21 is a block diagram showing a first embodiment of an optical transmission apparatus to which the present invention is applied; FIG.22 is a time chart illustrating operation of the optical transmission apparatus of FIG.21; FIG.23 is a circuit diagram showing a concrete example of an input interception detection circuit; FIG.24 is a circuit diagram showing another concrete example of an input interception detection circuit; FIG.25 is a circuit diagram showing a concrete example of an LD driver; FIG.26 is a block diagram showing a second embodiment of an optical transmission apparatus to which the present invention is applied; FIG.27 is a block diagram showing a third embodiment of an optical transmission apparatus to which the present invention is applied; and FIG.28 is a block diagram showing a fourth embodiment of an optical transmission apparatus to which the present invention is applied.

Before describing embodiments of the present invention, the background of the invention will be explained.

FIG.1 is a block diagram showing a first erbiumdoped fiber amplifier (EDFA) which is useful for understanding the invention, but not in accordance with the same. This EDFA is of the forward excitation type. In particular, a wave combiner 66 serving as optical coupling means is provided on the upstream side in the propagation direction of signal light with respect to the doped fiber 2, and pumping light from the laser diode 6 is inputted to 9_ the doped fiber 2 in the same direction as signal light by way of the wave combiner 66. Output light of the doped fiber 2 is split into two beams of light by an optical splitter 68. One of the two split beams of light is converted by photo-electric conversion by a photodetector 70. The other split beam of light of the optical splitter 68 is supplied to an optical filter 72, and output light of the optical filter 72 is split into two beams of light by an optical splitter 74. One of the two split beams of light of the optical splitter 74 is converted by photo-electrIc conversion by a photo-detector 76 while the other split beam of light is sent out to the optical transmission line not shown.

For the optical filter 72, for example, an optical band-pass filter for removing an ASE (Amplified Spontaneous Emission) component from output light of the doped fiber 2 can be employed. Output signals of the photodetectors 70 and 76 are supplied to calculation means 78, and an output signal of the calculation means 78 is supplied to one of a pair of input ports of a comparator 80. A reference voltage from a reference voltage source 82 is supplied to the other input port of the comparator 80. For the calculation means 78, a subtracter or a divider can be employed, and properties of it will be hereinafter described. The comparator 80 is provided to detect that inputting of signal light has been intercepted or restored, and an output signal of the comparator 80 is supplied to the LD controller 22 by way of a delay circuit 84. An output signal of the photodetector 76 is supplied to one of a pair of input ports of a comparator 86 for ALC, and a reference voltage from a reference voltage source 88 is supplied to the other input port. An output signal of the comparator 86 is supplied to the LD controller 22. The delay circuit 84 is a circuit for providing a protection time in order to prevent a malfunction which occurs, for example, when the input power to the EDFA fluctuates instantaneously.

Now, the signal level supplied from thp photodetector 70 to the calculation means 78 is represented by A; the signal level supplied from the photo-detector 76 to the calculation means 78 is represented by B; and the output level of the calculation means 78 is represented by C. In the present amplifier, since ALC is proceeding, if the optical input signal power is within a saturation gain region of the doped fiber 2, then the pumping power is low compared with a non-saturation gain region, and the value of A is low. Further, since ALC is proceeding, the value of B is fixed- As the optical input signal power decreases, also the output power of the EDFA decreases, but control is performed so as to raise the intensity of pumping light so that the output is kept fixed. If the input power further decreases, the pumping power increases, but either it is limited by a limiter or APC is entered. The value of A then is a high value compared with that when the input power is high. The value of B decreases as the input signal power decreases.

The input/output characteristic of the calculation means 78 is illustrated in FIG. 2 together with the gain characteristic of the EDFA. Reference numeral 90 denotes the relationship between the potential of the EDFA and the optical input power, and reference numeral 92 denotes the relationship between the output level of the calculation means 78 and the optical input power. If the optical input power varies from a high value to a low value or interception of optical inputting occurs, the value of C increases as the optical input power decreases. Accordingly, interception information and restoration information can be obtained by comparison of the output level of the calculation means 78 with the reference voltage from the reference voltage source 82.

In order to prevent a malfunction caused by a fluctuation of the input power, the delay circuit 84 is provided between the comparator 80 and the LD controller 22.

FIG. 3 is a block diagram showing an example of a construction of the delay circuit 84. An input signal is split into two signals, and one of the two signals is supplied to.a timer 96 by way of an invertor 94. The other is supplied directly to another timer 98. Output signals of the timers 96 and 98 pass invertors 100 and 102, respectively, and are supplied to a flip-flop 104. And, an output of the flip-flop 104 makes an output of the present delay circuit. The potential at the input port of the delay circuit is represented by (1); the potential at an input port of the timer 96 is represented by (2); the potentials at input ports of the invertors 100 and 102 are represented by (3) and (4). respectively; the potentials at the two input ports of the flip-flop 104 are represented by (5) and (6), respectively; and the potential at the output port of the delay circuit is represented by (7).

FIG. 4 is a time chart illustrating operation of the delay circuit shown in FIG. 3. The potentials (1) to (7) are shown in the form of binary information. When the optical input is intercepted, the potential (1) changes to "low", but when there is an optical input, the potential (1) exhibits a "high" level. The potential (7) corresponding to a signal supplied to the LD controller 22 in order to control pumping light exhibits a "high" level when the input is intercepted, but exhibits a "low" level when there is an input. It is to be noted that, in -FIG. 3, the timer 96 is a protection timer when the input is intercepted, and the output thereof changes to a "high" level after it is detected for a fixed period of time that the input is intercepted. Meanwhile, the timer 98 is a protection timer when the input is restored, and outputs a "high" level after lapse of a fixed period of time after restoration is detected. By delaying it for the fixed period of time to determine that the input has been intercepted or the like using such a delay circuit as described above, when instantaneous interception of the input or instantaneous restoration of the input occurs, a malfunction based on such instantaneous interception or restoration can be prevented. It is to be noted that, as an IC for realizing such a delay circuit as discribed above, MB3771 by Fujitsu can be employed.

With this IC, the delay time can be set arbitrarily by the capacitance of a capacitor to be connected external- ly.

FIG.5 is a block diagram showing a second EDFA which is useful for understanding the invention, but not in accordance with the same. The present EDFA is characterised in that in contrast with the construction of FIG. 1, an optical output power controller 106 is provided on the output side of the optical splitter 74. For the optical output power controller 106, an optical shutter or an optical attenuator can be employed. And, an output signal of the comparator 80 which is outputted by way of the delay circuit 84 is supplied to the optical output power con troller so that the optical output of the EDFA is turned on or off in response to interception information or restoration information. The optical output power con troller 106 operates in response to input interception information so that the optical output of the EDFA is attenuated or intercepted, and operates in response to restoration information so that the optical output of the EDFA is restored. It is to be noted that, also in this EDFA which employs the optical output power controller 106, the intensity of pumping light may be lowered in response to input interception information.

The problem addressed by the present invention will now be explained in more detail.

As repeaters for optical communication systems, repeaters of the type wherein an optical signal is con verted once into an electric signal and then the electric signal is converted back into an optical signal after it is shaped by waveform shaping and repeaters of the type wherein a received optical signal is directly amplified and repeated by an optical amplifier have been put into practical use. Repeaters of the former type have been used since an initial stage of development of optical communication systems whereas repeaters of the latter type have been developed in the last several years and are at such a stage where they have begun to be partially put into practical use. The most significant characteristic of optical amplifiers including EDFAs would be the flexibility that they can be continuously used as they are even if the transmission rate of the optical communication system changes. Particularly, in an,optical submarine communication system, once a repeater is laid on the bottom of the sea, a vast sum of money is required to draw up the same, and accordingly, such an optical amplifier having flexibility as described above provides - is a significant an optimum repeater. Naturally, it advantage that, also on land, the optical amplifier can cope flexibly with a change in use of an optical communication system. In an EDFA, a circuit which detects input light and stops, when no input is detected, a driving electric current of a pumping light source is used.

FIGS. 6 and 7 are views illustrating a behavior of an EDFA upon interception of an optical signal. As indicated by reference numeral 182 in FIG. 6, if an optical signal is intercepted, then an interception detection circuit outputs an interception detection signal having a delay peculiar to the circuit as indicated by reference numeral 184. In response to the interception detection signal, the electric current of the pumping light source increases once and then is intercepted as indicated by reference numeral 186, and upon restoration, the electric current returns to a predetermined value after a delay time peculiar to the pumping light source. Then, the optical output increases once as the electric current of the pumping light source increases as indicated by reference numeral 188.

The example of FIG. 6 relates to a case wherein the optical input interception is comparatively long, and while no problem is caused particularly since the optical signal is restored after the electric current of the pumping light source increases once. However, when the optical input interception is comparatively short as shown in FIG. 7, an optical surge is produced in the optical output since the optical signal is restored while the electric current of the pumping light source remains increased. It is to be noted that charts denoted by reference numerals 190, 192, 194 and 196 in FIG. 7 correspond to the charts denoted by reference numerals 182, 184, 186 and 188 in FIG. 6, respectively. If an optical surge is produced in a certain optical amplifier, then this phenomenon propagates by way of all optical amplifiers incorporated in the optical repeater system, and in the worst case, such an accident that a light reception circuit of an optical reception apparatus is destroyed occurs. Accordingly, in an optical communication system which employs an optical amplifier, it is essential to take a countermeasure to prevent occurrence of the phenomenon described above.

FIG.8 is a block diagram of a first optical transmission apparatus (not in accordance with the invention) to which external modulation is applied. Reference numeral 201 denotes a laser module including a laser diode 201a and a photo-diode 201b, reference numeral 205 denotes a Mach-Zehnder inter- ferometer, reference numeral 206 denotes an optical splitter, reference numeral 207 denotes a photo-electric conversion circuit, reference numeral 208 denotes an automatic bias control circuit, reference numeral 209 denotes a drive circuit, and reference numeral 210 denotes a capacitor. In the present apparatus, when the mark rate of the input signal is close to 1 or 0, a low frequency signal is supplied to the MachZehnder interferometer 205, and consequently, a low frequency band is blocked by the capacitor 210. Accordingly, no optical signal is outputted from the Mach-Zehnder interferometer 205. However, if an ordinary condition is restored and an input signal whose mark rate is approximately 1/2 is inputted, then an optical signal is sent out immediately.

FIG.9 is a block diagram showing a second optical transmission apparatus (not in accordance with the invention, but useful for understanding the same) to which direction modulation is applied. Reference numeral 211 denotes a laser module including a laser diode 211a and a photo-diode 211b, reference numeral 212 denotes a pulse electric current supply circuit, reference numeral 213 denotes a bias electric current supply circuit, reference numeral 202 denotes an APC circuit, and reference numeral 204 denotes a mark rate detection circuit.

In the present apparatus, when the mark rate of an input signal is close to 0, only a bias electric current is supplied to the laser diode 211a while no pulse electric current is supplied, and accordingly, the optical output is effectively intercepted. However, if an ordinary condition is restored and an input signal whose mark rate is approximately 1/2 is supplied, then an optical output is supplied immediately. In particular, in the above type of cptical transmission apparatus, even where external modulation is applied or where direct modulation is applied, an optical output is sent out immediately when the mark rate returns from I or 0 to about 1/2, and consequently, there is the possibility that an optical surge may be produced in optical amplifi ers disposed in an optical transmission line, which is a problem when it is tried to assure the reliability of the system.

Accordingly, it is desirable to provide an optical transmission apparatus wherein no optical surge is produced in optical amplifiers when the mark rate of an input signal changes to 1 or 0.

FIG. 10 is a block diagram showing a third optical transmission apparatus (not in accordance with the invention, but useful for understanding the same), and an external modulation is applied to the optical transmission apparatus. Reference numeral 201 denotes a laser module including a laser diode 201a and a photo-diode 201b, reference numeral 202 denotes an APC circuit, reference numeral 203 denotes a shutting down circuit, reference numeral 204 denotes a mark rate detection circuit, reference numeral 205 denotes a Mach-Zehnder interferometer, reference numeral 206 denotes an optical splitter, reference numeral 207 denotes a photoelectric conversion circuit, reference numeral 208 denotes an automatic bias control circuit, reference numeral 209 denotes a drive circuit, and reference numeral 210 denotes a capacitor.

FIG.11 is a block diagram showing a fourth optical transmission apparatus (not in accordance with the invention, but useful for understanding the sarre).

Direct modulation is applied to the optical transmission apparatus. Reference numeral 211 denotes a laser module including a laser diode 211a and a photo-diode 211b, reference numeral 212 denotes a pulse electric current supply circuit, reference numeral 213 denotes a bias electric current supply circuit, reference numeral 202 denotes an APC circuit, reference numeral 203 denotes a shutting down circuit, and reference numeral 204 denotes a mark rate detection circuit.

In particular, the above apparatus (Figs. 10 and 11) are charac- terized in that they comprise a shutting down circuit for stopping a laser diode by way of an APC circuit in response to an output of a mark rate detection circuit. Since the apparatus of Figs. 10 and 11 share a common operating principle,, operation is described with reference to FIG. 10. Further, for simplification, description will be given of a case wherein an input is intercepted. If an input signal is intercepted, then the mark rate detection circuit 204 which is an average value detection circuit varies its output level in response to the interception time. In particular, if an interception is detected, then the output level changes from an average value to "0", but if an input signal is restored, then the output level changes from "0" to an average value. This signal is received by the shutting down circuit 203, which detects a falling edge and generates a pulse of a predetermined time in response to the detection signal. If the electric current of the laser diode is stopped by way of the APC circuit 202 in response to the pulse, then the optical output is stopped for the period.

If the duration of the pulse is set to a time TS longer than a time T shown in FIG. 7, then no optical surge will be produced in the optical amplifier. Since interception of an input signal is described as an example in the foregoing description, the case wherein the mark rate of an input signal is 0 is described. If a similar level variation is provided also when the mark rate detection circuit 204 detects the mark.rate 1, the shutting down circuit 203 operates similarly, and accordingly, the optical output can be stopped also for the period of TS

FIG. 12is a block diagram showing an example of a mark rate detection circuit. Reference numeral 240 denotes a buffer gate, reference numeral 241 denotes a one-bit delay circuit, reference numeral 242 denotes an exclusive OR circuit, reference numeral 243 denotes a counter, reference numeral 244 denotes a latch circuit, reference numeral 245 denotes a digital to analog (D/A) conversion circuit, reference numeral 246 denotes a first OR circuit, and reference numeral 247 denotes a comparator. It is to be noted that FIG. 12 illustrates a mark rate detection circuit of the type which transmits a level variation to the shutting down circuit 203 even if it detects that the mark rate is close to 0 or close to If the mark rate of the input signal is approximately 1/2, then the output of the buffer gate 240 and the output of the one-bit delay circuit 241 exhibit different logic levels in the probability of about 1/2, and accordingly, also the output of the exclusive OR circuit 242 exhibits "1" in the probability of about 1/2. This is counted by the counter 243, and when a predetermined output bit exhibits "l", the counter output is latched by the latch circuit 244 and supplied to the D/A conversion circuit 245 and besides the count value is reset. An analog output of the D/A conversioncircuit 245 is compared with a predetermined reference voltage, and "1" is outputted to the shutting down circuit 203.

On the other hand, when the mark rate of the input signal is 1 or 0, since the output of the buffer gate 240 and the output of the one-bit delay circuit 241 exhibit a same logic level, no counting operation of the counter 243 proceeds. Accordingly, even if the predetermined time elapses, the output of the D/A conversion circuit 245 remains equal to 0 volt. This voltage is compared with a predetermined reference voltage Vref, and "0" is outputted to the shutting down circuit 203. Although the mark rate is described to be 1 or 0 for simplified description, since counting proceeds slowly where the mark rate is close to 1 or close to 0, also the output of the D/A conversion circuit 245 remains at a low voltage, and operation of the entire circuit is similar to that described above.

In this instance, however, if it is waited that "1" appears at a predetermined output bit of the counter 243, then after a long interval of time, "V' appears at the predetermined bit so that the circuit operates similarly as that when the mark rate is 1/2, and accordingly, it is desired to stop counting after lapse of a predetermined time. This signal is a counting stopping signal of FIG. 12. The period of the counting stopping signal should be set, using, for example, an external clock signal, equal to a time required for I'V' to appear at the predetermined bit when the mark rate is 1/2. Further, while the foregoing description proceeds on the assumption that a fixed reference voltage Vref is applied as the reference voltage to the comparator 247, if the reference voltage is varied in accordance with the output voltage of the comparator 247, then a hysteresis is provided to the comparison characteristic so that chattering which often arises upon interception of an input signal can be suppressed.

The mark rate detection circuit shown in FIG. 12 is suitable for an optical transmission apparatus, to which external modulation is applied, which does not require a mark rate detection voltage for APC because the output voltage of the D/A conversion circuit 245 with respect to a variation of the mark rate exhibits a maximum value when the mark rate is 1/2 and exhibits 0 volt when the mark rate is 1 or 0. On the contrary, in an optical transmission apparatus, to which direct modula tion is applied, wherein an output voltage of a mark rate detection circuit is used as a reference voltage for APC, since the characteristic described above is not suitable for APC, a known mark rate detection circuit which detects an average value of an input signal should be used In place of the mark rate detection circuit of FIG. 12.

FIG. 13 is a block diagram showing an example of a shutting down circuit. Reference numeral 231 denotes a differentiation circuit, and reference numeral 232 denotes a monostable multivibrator. A falling edge of the output of a comparator of a mark rate detection circuit from "V' to "0" is differentiated by the differentiation circuit 231 so that a trigger pulse to be supplied to the monostable multivibrator 232 is produced. In response to the trigger pulse, the monostable multivibrator 232 outputs a pulse of a duration TS This pulse is supplied to the APC circuit 202 so that operation for APC is intercepted for the period of time of TS to stop the driving electric current of the laser diode.

FIG. 14 is a time chart of the embodiment of FIGS. 10and 11. In the present time chart, a case of interception of an input is assumed, but the time chart similarly applies when the mark rate is 1. If an input signal is intercepted, then the mark rate detection circuit 204 outputs a mark rate detection signal from the comparator 247. The mark rate detection signal is differentiated at a falling edge thereof by the differentiation circuit 231 of the shutting down circuit 203 so that a falling edge differentiation signal is produced. In response to the falling edge differentiation signal, the monostable multivibrator 232 outputs "V' for a predetermined period of time. Consequently, even if the input signal is restored, the optical output is stopped, and accordingly, the optical output is stopped in accordance with interception of the input signal, but is restored when the monostable multivibrator 232 returns to "0". Accordingly, if the time TS is set longer than the time T in which the electric current of the pumping light source increases once and then decreases after the input signal is intercepted, then no optical surge is produced in any optical amplifier.

As described above, the effect that an optical transmission apparatus wherein an optical amplifier does not produce an optical surge when the mark rate of an input signal returns to a normal mark rate 1/2 after the mark rate-of the input signal changes to 1 or 0 can be provided is provided. Accordingly, a significant effect is provided in that the reliability of an optical communication system is enhanced. It is to be noted that, in the time chart of FIG. 14 reference numeral 252 denotes an input signal, reference numeral 254 denotes a mark rate detection signal, reference numeral 256 denotes a falling edge differentiation signal, reference numeral 258 denotes an output signal of the monostable multivibrator, and reference numeral 260 denotes an optical output. Further, an electric current of the pumping light source is denoted at reference 262 for reference.

The technical subject presented in FIGS. rO and 7 can be solved not only in an optical transmission apparatus but also in an optical amplifier of an optical repeater. In particular, the present invention can provide an optical amplifier, comprising a doped fiber doped with a rare earth element and having a first end and a second end for guiding signal light from the first end toward the second end thereof, a pumping light source for outputting pumping light, optical coupling means optically connected to the doped fiber and the pumping light source for introducing the pumping light into the doped fiber, interception detection means for detecting that inputting of the signal light to the doped fiber is intercepted, a photodetector for detecting the intensity of light outputted from the second end of the doped fiber, automatic level control means for supplying a controlling electric current to the pumping light source so that the output level of the photo- detector may be fixed, and shutting down means for intercepting the output of the doped fiber for a predetermined period of time in response to an output signal of the interception detection means.

Preferably, the shutting down means includes an optical shutter provided on the upstream side in the signal light propagation direction of the doped fiber. Or, the shutting down means includes an optical shutter provided on the downstream side in the signal light propagation direction of the doped fiber.

FIG-1.5 is a block diagram showing a first embodiment of an EDFA to which the present invention is applied. Signal light supplied to an input side optical connector 264 is inputted to an optical splitter 268 by way of an optical shutter 266. The optical shutter 266 corresponds to the optical output power controller 106 in the embodiment of FIG. 5, and may be an optical attenuator. 0 ne of two split beams of light of the optical splitter 268 is converted by photo-electric conversion by a photodetector 270, and the other split beam of light is supplied to a doped fiber 274 by way of an optical isolator 272. A wave combiner 276 is provided on the downstream side of the doped fiber 274 in the signal light propagation direction, and pumping light from a laser diode 278 serving as a pumping light source is supplied to the doped fiber 274 by way of the wave combiner 276.

The signal light amplified in the doped fiber 274 passes the wave combiner 276 and an optical isolator 280 in this order and is supplied to an optical splitter 282. One of a pair of split beams of light of the optical splitter 282 is converted by photo-electric conversion by a photodetector 284, and the.other split beam of light is sent out to an optical transmission line not shown by way of an output side optical connector 286. An output signal of the photo-detector 284 is supplied to an ALC circuit 288. The ALC circuit 288 supplies a control electric current to the laser diode 278 so that the output level of the photo-detector 284 may be fixed. An output signal of the photo-detector 270 is supplied to the ALC circuit 288 and a monostable multivibrator 290. The monostable multivibrator 290 is provided to set a present time based on input interception information obtained by the photo-detector 270, and sends a control signal to a drive circuit 292 of the optical shutter 266 so that the optical shutter 266 may be closed for the fixed period of time.

FIG. 16is a time chart illustrating operation of the EDFA of FIG. 15 and shows waveforms at various points of the EDFA. Reference character P1 denotes an input waveform to the optical shutter 266; P2 an output waveform of the optical shutter 266; P3 an output waveform of the photodetector 270; P4 an output waveform of the monostable multivibrator 290; P5 an output waveform of the ALC circuit 288; and P6 an output waveform of the EDFA.

If inputting of signal light is intercepted at the timing indicated by reference numeral 294, then the output level of the photo-detector 270 drops after a little delay as indicated by reference numeral 296. Using the level drop as a trigger, the monostable multivibrator 290 supplies a signal to the drive circuit 292 to change the optical shutter 266 from an open state to a closed state, and the closed state is continued for a fixed period of time T. As the optical input drops, the driving current of the laser diode 278 increases as indicated by reference numeral 298 by an operation of the ALC circuit 288. In this instance, since the optical shutter 266 is in a closed state, no optical surge is produced on the optical output of the EDFA. In the example shown, inputting of signal light is restored at a comparatively early stage at a timing indicated by reference numeral 300. However, when the optical shutter 266 is in a closed state, the optical output is lost for a very short period of time. However, when the disadvantage that an optical surge may be produced to destroy a light reception element of a light reception apparatus to cause system down is-taken into consideration, the effect of the present embodiment is high in that the system down is prevented.

FIG. 17 is a block diagram showing a second embodiment of an EDFA to which the present invention is applied. In the present embodiment, an optical shutter is provided on the output side in place of the optical shutter provided on the input side in the first embodiment of FIG. 15. In particular, an optical shutter 302 is provided between the optical splitter 282 and the output side optical connector 286. The optical shutter 302 may be an optical attenuator. It is similar as in the first embodiment of FIG. 15that the optical shutter 302 is driven by the drive circuit 292 and an output signal of the photo-detector 270 is supplied by way of the monostable multivibrator 290.

FIG. 18is a time chart illustrating operation of the EDFA of FIG. 17. Reference character P11 denotes an input waveform of the optical splitter 268; P12 an output waveform of the photo-detector 270; P13 a driving waveform of the laser diode 278; P14 an output signal level of the monostable multivibrator 290; P15 an input waveform of the optical shutter 302; and P16 an output waveform of the optical shutter 302. In the present embodiment, since the optical shutter 302 is provided on the output side, an optical surge is produced as indicated by reference numeral.304 on the input waveform to the optical shutter 302. However, this is intercepted by the optical shutter 302, and consequently, no optical surge is produced on the output waveform P16 of the optical shutter 302..

FIG. 19 is a block diagram showing a third embodiment of an EDFA to which the present invention is applied. The present EDFA includes a latch circuit 306 in place of the monostable multivibrator 290 of the first embodiment of FIG. 47. The latch circuit 306 sends a control signal to the drive circuit 292, when the output level of the photo-detector 270 drops, so that the optical shutter 266 may be changed over from an open state to a closed state. On the other hand, when an unlatching signal is inputted to the latch circuit 306, the latch circuit 306 sends a control signal to the drive circuit 292 to change over the optical shutter 266 from a closed state to an open state.

FIG. 20 is a time chart illustrating operation of the EDFA of FIG. 19. Reference character P21 denotes an input waveform of the optical shutter 266; P22 an output waveform of the optical shutter 266; P23 an output wave form of the photo-detector 270; P24 an output signal level of the latch circuit 306; P25 an input waveform of the laser diode 278; P26 an output waveform of the EDFA; and P27 a waveform of an unlatching signal supplied to the latch circuit 306. In the present embodiment, in place of the optical shutter closed for a fixed period of time by a monostable multivibrator in the first embodiment of FIG. 15 or the second embodiment of FIG.

17. the period of time for which an optical shutter is closed is determined using a latch clear pulse 308 of an unlatching signal.

In the present embodiment, the latch circuit 306 may have a function of notifying to an operator or the like that an interception condition of a signal input takes place and the EDFA is in a stopping condition. The operator can restore an ordinary operation if, by input ting a latch clear pulse by operation from the outside, inputting of signal light is restored from an intercep tion condition at the point of time. It is to be noted that, while the optical shutter is provided on the input side of the EDFA in the third embodiment of FIG.

19 in which a latch circuit is adopted, the optical shutter 266 may be provided on the output side of the EDFA.

An example of optical transmission apparatus embodying the present invention which can address the technical subject described hereinabove with reference to FIGS. 6 and 7 comprises signal light generating means for generating signal light modulated in accordance with an input signal, input interception detection means for detecting that the input signal is intercepted, and shutting down means for intercepting the output of the signal light generating means for a prede- termined period of time in response to an output signal of the input interception detection means. Several embodiments of the same will be described below.

Fig- 21 is a block diagram showing a first embodiment of an optical transmission apparatus to which the present invention is applied. Reference numeral 310 denotes a laser diode serving as a light source. The laser diode 310 is driven by an LD driver 312. The LD driver 312 has an input port 312A for an input signal (data), an input port 312B for a shutting down signal, an input port 312C for a control signal (voltage signal) VIB for a bias electric current to be supplied to the laser diode 310, an input port 312D for a.control signal (voltage signal) VIP for a pulse electric current to be supplied to the laser diode 310, and an output port 312E for mark rate data. The input ports 312C and 312D and the output port 312E are connected to an APC circuit 318. Forward light of the laser diode 310 is sent out into an optical transmission line not shown, and backward light of the laser diode 310 is converted by photo- electric conversion by a photo-diode (photo-detector) 314. An output signal of the photo-diode 314 is supplied to an optical output detection circuit 316.

The intensity of output light of the laser diode 310 is reflected on an output signal of the optical output detection circuit 316. Part of the output signal is supplied to the APC circuit 318, and in response to this, the APC circuit 318 sends a control signal to the LD driver 312 so that the output level of the laser diode 310 may be fixed. The other part of the output signal of the optical output detection circuit 316 is supplied to a monostable multivibrator 324. An output signal of the monostable multivibrator 324 is supplied to one of a pair of input ports of an OR circuit 326. A shutting down signal from the outside is supplied to the other input port of the OR circuit 326. Part of an input signal to be supplied to the input port 312A of the LD driver 312 is split and supplied to an input data detection circuit 320. An output signal of the input data detection circuit 320 and an output signal of the OR circuit 326 are supplied to input ports of another OR circuit 322, and an output signal of the OR circuit 322 is supplied to the input port 312B of the LD driver 312.

FIG.. 22 is a flow chart illustrating operation of the optical transmission apparatu's of FIG. 21. Reference character P31 denotes a waveform of an input signal to be supplied to the input port 312A of the LD driver 312; P32 a waveform of an input data detection signal outputted from the input data detection circuit 320; P33 a waveform of a shutting down signal supplied to the input port 312B of the LD driver 312; P34 an output waveform of the laser diode 310; P35 a waveform of an optical output alarm signal from the optical output detection circuit 316; and P36 an output waveform of the monostable multivibrator 324.

If the input signal is intercepted as indicated by P31, then the input data detection signal exhibits a high level for the period of time as indicated by P32. The timing at which the input signal is intercepted is represented by TA Since the light reception level of the photo-diode 314 drops when the input signal is intercepted, an optical output alarm signal is developed as indicated by P35 in response to such drop. The monostable multivibrator 324 receives the optical output alarm signal and changes the output signal thereof to a high level for the predetermined period of time T. The timing at which one pulse of the monostable multivibrator 324 ends is represented by TB. While the output signal of the monostable multivibrator 324 remains at a high level, a shutting down signal is inputted to the LD driver 312 as indicated by P33, and consequently, the output of the laser diode 310 is intercepted as indicated by P34. In this manner, also in the present embodiment, production of an optical surge is prevented. It is to be noted that the reason why inputting of a shutting down signal from the outside to the OR circuit 326 is enabled is that it is intended to'make it possible to intercept the output of the laser diode 310 by an operator in case of emergency.

Several means are available as means for detect ing that an input signal has been intercepted. In the embodiment of FIG. 21, means for directly detecting an interception from an input signal to the LD driver 312 and means for detecting an interception based on the intensity of backward light of the laser diode 310 are both employed. In addition to them, also means which makes use of mark rate data supplied from the LD driver 312 to the APC circuit 318 can be adopted. This will be hereinafter described.

A concrete example.of an input interception detection circuit will be described with reference to FIGS. 2 3 and 24--- The input interception detection circuit is included, for example, in the input data detection circuit 320 or the optical output detection circuit 316 of FIG. 21.. A pair of resistors R101 and R102 are connected in series between an input port 328 and the positive side input port of an operational amplifier 340. A junction between the resistors R101 and R102 is grounded by way of a capacitor CIOI. The positive side input port of the operational amplifier 340 is connected to the negative side input port of an operational amplifier 342. Resistors R103, R104 and R105 are connected in series in this order between a positive power source line +V and a negative power source line - V. The resistors R103 and R104 are connected to the negative side input port of the operational amplifier, and a junction point between the resistors R104 and R105 is connected to the positive side input port of the operational amplifier 342. The output port of the operational amplifier 340 and the output port of the operational amplifier 342 are connected to the two input ports of an OR circuit 344. And, the output port of the OR circuit 344 is connected to an output port 346 of the input intercepion detection circuit.

The present circuit is provided to perform detection of an ave3age value of an input signal, and when an average level of an input signal goes out of a range between the potential at a junction P41 between the resistors R103 and R104 and the potential at a junction P42 between the resistors R104 and R105, a signal indicating that the input signal is intercepted is outputted __40- from the output port 346.

In the example of the input interception detection circuit shown in FIG. 24 a resistor R106 and a diode D101 are connected in this order between the input port 328 and the positive side input port of the operational amplifier 340. The direction of the diode D101 is set so that the cathode thereof may be connected to the operational amplifier 340. The cathode of the diode D101 is grounded by way of a capacitor C102. Since the other portions are similar to those of the circuit of FIG. 23, description thereof is omitted. The present input interception detection circuit Is provided to detect a peak of an input signal. In particular, when a peak value of an input signal goes out of the range between the potential at the point P41 and the potential at the point P42, a signal indicating that the input signal is intercepted is outputted from the output port 346.

FIG. 25 is a circuit diagram showing a concrete example of the LD driver shown in FIG. 21 The LD driver is constituted from a combination of electric-current switches formed from transistors, and since the construction and operation of it can be recognized very readily by those skilled in the art, the input and output ports of the LD driver 312 shown in FIG. 21 are indicated in the circuit diagram in place of describing it. It is to be noted that reference numeral 348 in FIG. 25 denotes an electric current output port to the laser diode 310.

FIG. 26 is a block diagram showing a second embodiment of an optical transmission apparatus to which the present invention is applied. The present embodiment is characterized in that, in contrast with the embodiment of FIG. 21, mark rate data from the output port 312E of the LD driver 312 are supplied to the monostable multivi brator 324. If an input signal is intercepted, then the mark rate becomes equal to 0, and consequently, by monitoring the variation of the mark rate data, the timing TA (refer to FIG. 22.) at which the input signal is intercepted can be obtained, and in response to this, the monostable multivibrator 324 can be started.

FIG. 27 is a block diagram showing a thirdembod iment of an optical transmission apparatus to which the present invention is applied. In the present embodiment, in place of a shutting down signal inputted from the OR circuit 322 to the LD driver 312 in the firstembodiment of FIG. 21, a gate circuit 350 is employed. In particular, in the present embodiment, the shutting down input port of the LD driver 312 is not used. The gate circuit 350 functions to intercept control signals VIB and VIp for an LD electric current to be supplied from the APC circuit 318 to the LD driver 312 in response to a shutting down signal. Consequently, similar effects to those of the firstembodiment of FIG. 21 are obtained.

FIG. 28 is a block diagram showing a fourthembodiment of an optical transmission apparatus to which the present invention is applied. In particular, a peculiar optical modulator 352 and an OM driver 362 for control ling driving of the optical modulator 352 are used for external operation. For the optical modulator 352, for example, a Mach-Zehnder optical modulator can be used. A laser diode 351 serving as a light source is controlled so that it may output light of a fixed intensity. To this end, a photo-diode (photo-detector) 356 for converting backward light of the laser diode 351 by photoelectric conversion is provided, and an output signal of the photo-diode 356 is supplied to an LD output detection circuit 358. And, an output signal of the LD output detection circuit 358 is supplied to an APC circuit 360. The APC circuit 360 supplies a bias electric current to the laser diode 351 so that the intensity of output light of the laser diode 351 may be fixed.

Forward light of the laser diode 351 is modulated, for example, by intensity modulation by an optical modulator 352, and modulated light from the optical modulator 352 is split into two beams of light by an optical splitter 354. One of the split beams of light is sent out into the optical transmission line not shown, and the other split beam of light is supplied to the photo- detector 314. The OM driver 362 has an input port 362A for an input signal (data), an input port 362B for a shutting down signal, an output port 362C for mark rate data, and an output port 362D for a driving signal of the optical modulator 352.

In this manner, also in the optical transmission apparatus to which external modulation is applied, the optical transmission apparatus which can prevent production of an optical surge can be provided by adopting the construction similar to that of the f irst embodiment of FIG. 21- It is to be noted that, where external modulation is applied, other concrete examples than the concrete example of the detection means for an input interception described in connection with direct modulation can be adopted. In particular, since generally the driving signal from a driving circuit (OM driver) for an optical amplifier is a voltage signal, input interception can be detected directly using the voltage signal. Further, where external modulation is applied, if an OM driver does not have a shutting down input port, then such a gate circuit as shown in FIG. 27 may be provided between the APC circuit 360 and the laser diode 351 so that shutting down may be performed using the gate circuit.

While several concrete examples of input interception detection means are described above, they may be adopted in combination or adopted solely.

As described above, an embodiment of the present invention can facilitate detection of interception and restoration of an optical signal of an optical amplifier. Embodiments of the invention can also prevent production of an optical surge, and can produce other useful effects as described hereinbefore in the description of the embodiments.

While specific embodiments of the present invention have been described in the foregoing description, the present invention is not limited to details of the embodiments., except as -defined in the appended claims, and all alterations and modifications which belong to the scope of equivalency of the claims shall be included in the scope of the present invention.

Claims

CLAIMS:

1. An optical amplifier, comprising: a doped fiber doped with a rare earth element and having a first end and a second end for guiding signal light from said first end toward said second end thereof; a pumping light source for outputting pumping light; optical coupling means optically connected to said doped fiber and said pumping light source for introducing the pumping light into said doped fiber; interception detection means for detecting that inputting of the signal light to said doped fiber is intercepted; a photo-detector for detecting the intensity of light outputted from said second end of said doped fiber; automatic level control means for supplying a controlling electric current to said pumping light source so that the output level of said photo-detector may be fixed; and shutting down means for intercepting the output of said doped fiber for a predetermined period of time in response to an output signal of said interception detection means.

2. An optical amplifier according to claim.1, wherein said shutting down means includes an optical shutter provided on the upstream side in the signal light propagation direction of said doped fiber-

3. An optical amplifier according to claim 1, wherein said shutting down means includes an optical shutter provided on the downstream side in the signal light propagation direction of said doped fiber.

4. An optical transmission apparatus, comprising: signal light generating means for generating signal light modulated in accordance with an input signal; input interception detection means for detecting that the input signal is intercepted; and shutting down means for Intercepting the output of said signal light generating means for a predetermined period of time in response to an output signal of said input interception detection means.

5. An optical amplifier substantially as hereinbefore described with reference to Figures 15 and 16, or to Figures 17 and 18, or to Figures 19 and 20 of accompanying drawings.

6. optical transmission apparatus substantially as hereinbefore described with reference to Figures 21 to 25, or to one of Figures 26 to 28 of the accompanying drawings.