GB2322026A - Optical amplifier which is enabled only in the presence of an input signal - Google Patents

Optical amplifier which is enabled only in the presence of an input signal Download PDF

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Publication number
GB2322026A
GB2322026A GB9809228A GB9809228A GB2322026A GB 2322026 A GB2322026 A GB 2322026A GB 9809228 A GB9809228 A GB 9809228A GB 9809228 A GB9809228 A GB 9809228A GB 2322026 A GB2322026 A GB 2322026A
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United Kingdom
Prior art keywords
electric current
idling
pumping light
optical
light source
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Application number
GB9809228A
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GB9809228D0 (en
GB2322026B (en
Inventor
Tsukasa Takahashi
Hiroyuki Itou
Yuji Tamura
Michikazu Shima
Kazuo Yamane
Yumiko Kawasaki
Keisuke Imai
Yoshinori Ohkuma
Kazuhiro Suzuki
Yasunori Nagakubo
Takashi Tsuda
Yuji Miyaki
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Fujitsu Ltd
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Fujitsu Ltd
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Priority claimed from JP7023640A external-priority patent/JPH0837497A/en
Application filed by Fujitsu Ltd filed Critical Fujitsu Ltd
Publication of GB9809228D0 publication Critical patent/GB9809228D0/en
Publication of GB2322026A publication Critical patent/GB2322026A/en
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Publication of GB2322026B publication Critical patent/GB2322026B/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/2912—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form characterised by the medium used for amplification or processing
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1301—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers
    • H01S3/13013—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers by controlling the optical pumping
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/077—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using a supervisory or additional signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
    • H04B10/0795—Performance monitoring; Measurement of transmission parameters
    • H04B10/07955—Monitoring or measuring power
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/293—Signal power control
    • H04B10/2931—Signal power control using AGC
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
    • H04B10/50572—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the modulating signal amplitude including amplitude distortion
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
    • H04B10/50575—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the modulator DC bias
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/564—Power control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0078—Frequency filtering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1305—Feedback control systems

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optics & Photonics (AREA)
  • Plasma & Fusion (AREA)
  • Optical Communication System (AREA)
  • Lasers (AREA)

Abstract

An optical amplifier which is enabled, after some time delay, when the presence an input signal is detected. The amplifier comprises a doped fibre 140, a pumping light source 144, a photo detector 152 for detecting the intensity of light outputted from the doped fibre, automatic level control means 154 for supplying a controlling electric current to the pumping light source 144 to fix the output level, idling means 158 for supplying an idling electric current lower than a controlling electric current to the pumping light source 144, and means 134 for detecting whether or not the signal light is inputted to the doped fibre 140. Masking means 160 controls the automatic level control means 154 and the idling means 158 such that, when it is detected that the signal light is inputted to the doped fibre 140, first the idling electric current and then the controlling electric current is supplied to the pumping light source 144. This prevents a surge in output.

Description

R(_52254 /001 2322026 -I- OPTICAL AMPLIFIERS This invention relates to
optical amplifiers for 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 transmission 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 emission occurs, and countermeasures against the problem are demandedtfor optical amplifiers and optical transmit- ters.
When pumping light having a predetermined wavelength is introduced from a first end toward a second end or from the second end toward the first end of a doped fiber which is doped with a rare earth element suitable for a wavelength of signal light to be amplified while signal light is introduced from 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. Therefore, in an optical amplifier which employs a doped fiber, automatic level control (ALC) is usually applied 4 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- is ing, can desirably detect that the optical signal has been intercepted.
Accordingly, it is desirable to eliminate an influence of an optical surge arising from a characteristic of a doped fiber.
According to the present 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; means for detecting whether or not the signal light is inputted to said doped fiber; 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; idling means for supplying an idling electric current lower than the controlling electric current to said pumping light source; and masking means for controlling said automatic level control means and said idling means so that, when it is detected that the signal light is inputted to said doped fiber, the idling electric 5- current is first supplied to said pumping light source and then the controlling electric current is supplied to said pumping light source.
By means of the present invention, since the intensity of the pumping light is lowered when it is detected that amplified signal light is not included in light outputted from the doped fiber, when inputting of signal light is restored, production of an optical surge is prevented.
Reference will now be made, by way of example, to the accompanying drawings, in which:
FIG. 1 is a block diagram showing a previously considered erbium-doped fiber amplifier (EDFA), not in accordance with the invention; FIG. 2 is a diagram illustrating a transition response of an output level upon starting of the EDFA; FIG. 3 is a block diagram showing a first embodiment of an EDFA to which the present invention is applied; FIG. 4 is a diagram illustrating a transition of an output level upon starting by an idling current; is a circuit diagram (part 1) showing a example around a masking circuit of FIG. 3; 6 is a circuit diagram (part 2) showing the example around the masking circuit of FIG. 3; 7 is a diagram showing an example of a response electric FIG concrete FIG concrete FIG relationship between an LD electric current and an LD driver controller voltage; FIG. 8 is a time chart upon changing over from idling to ALC based on setting of a time constant; FIG. 9 is a circuit diagram of an output stabilization circuit; FIG. 10 is a diagram illustrating operation of the output stabilisation circuit of FIG. 9; FIG. 11 is a timing chart upon changing over from idling to ALC based on detection of an optical output level; FIG. 12 is a block diagram showing a second embodiment of an EDFA to which the present invention is applied; FIG. 13 is concrete example FIG. 14 is circuit diagram (part 1) showing a around a masking circuit of FIG. 12; circuit diagram (part 2) showing the concrete example around the masking circuit of FIG. 12; FIG. 15 is a block diagram showing a third embodiment of an EDFA; FIG. 16 is a circuit diagram (part 1) showing a concrete example around a masking circuit of FIG. 15; and FIG. 17 is a circuit diagram (part 2) showing the concrete example around the masking circuit of FIG. 15.
Embodiments of the present invention described subsequently relate to prevention of occurrence of an optical surge in an optical amplifier. When an EDFA is applied to an optical communication system, it is demanded to keep the output level fixed from the necessity of system designing. Therefore, ALC is adopted in an EDFA- FIG. 1 is a block diagram showing a previouslyconsidered EDFA (not in accordance with the invention) to which ALC is applied. Signal light supplied by way of an input side optical connector 130 is split into two beams of light by an optical coupler 132, and one of the two split beams of light is converted by photo-electric conversion by a photo-detector 134. The other split beam of light passes through an optical isolator 138 and is supplied to a doped fiber 140. Also pumping light from a laser diode 144 is supplied to the doped fiber 140 by way of an optical coupler 142. Signal light amplified in the doped fiber 140 passes the optical coupler 142, an optical isolator 146 and an optical coupler 148 in this order and is sent out from an output side optical connector 150 into an optical transmission line not shown.
Part of the amplified signal is split by the optical coupler 148 and converted by photo-electric conversion by a photo-detector 152. An output signal of the photo-detector 152 is supplied to an ALC circuit 154. The ALC circuit 154 controls the intensity of pumping light so that the intensity of the amplified signal light may be fixed. The control of the intensity of pumping light is performed, for example, by way of a driving electric current to be supplied to the laser diode 144.
- 9 An ATC (automatic temperature control) circuit 156 is provided additionally to the laser diode 144 so that the temperature of the laser diode 144 may be kept fixed. Data regarding the temperature of the laser diode 144, presence or absence of input signal light and so forth are supplied to a monitor circuit 136. adopts ALC in this manner, there is a problem in that when the power is thrown in (cold start) or when inputting of signal light is restored from an interception condition, an optical surge is likely to be produced.
FIG. 2 is a diagram illustrating a transition response of the optical output level upon starting of the EDFA. Reference numeral 158 denotes a variation of the intensity of pumping light with respect to time, and reference numeral 160 denotes a variation of the output level of the EDFA with respect to time. The output level is stabilized to such a set level as denoted by reference numeral 162 by ALC. In the EDFA, such a delay of, for example, several ms as denoted by reference numeral 164 is provided until amplification of signal light is started after pumping light is introduced into a doped fiber, and accordingly, such an optical surge as denoted by reference numeral 166 is produced by a transient response. In previous ly-cons idered EDFAs, in order to prevent production of In the EDFA which such an optical surge as described above, a countermeasure to retard a response of ALC has been taken. However, there is a subject to be solved in that, when the power is thrown in or when inputting of signal light is restored from.an interception condition, the required time to reach an ordinary output level should be shortened.
Accordingly, the present invention is intended to provide an optical amplifier which can be started at a high speed without producing an optical surge.
- 1 j_ FIG. 3 is a block diagram showing a first embodiment of an EDFA to which the present invention is applied. The present EDFA is characterized in that, in contrast with the previously-considered construction of FIG. 1, it additionally includes an idling electric current generation circuit 158 and a masking circuit 160. The idling electric current has a value lower than a control electric current supplied from the ALC circuit 154 to the laser diode 144, and when the power source of the EDFA is thrown in or when inputting of signal light is restored from an interception condition, the laser diode 144 is driven at a comparatively low level by the idling electric current from the idling electric current generation circuit 158.
A transition response of the output level then is shown in FIG. 4. In this instance, since the idling current is comparatively low, the delay of the doped fiber is dominant for a rising edge of an optical output. Accordingly, since there is no necessity of retarding the response of ALC as with the previously-considered construction of FIG. 1, high speed starting is possible. After starting with the idling electric current is completed, control of the intensity of pumping light is changed over to ALC. As a switching method, a method is listed wherein a control voltage for the idling electric current and another control voltage for ALC are inputted in the form of analog ORing to the control voltage of the driving circuit for the laser diode and one of the voltages is masked in individual operations. To this end, in the present embodiment, the masking circuit 160 is connected to the ALC circuit 154 and the idling electric current generation circuit 158 as shown in FIG. 3.
FIGS. 5 and 6 are circuit diagrams showing a concrete example around the masking circuit 160 of FIG.
3. The cathode of the photo-detector (photo-diode) 152 for detecting the output power of the EDFA is connected to a power source line, and the anode of the photo-detector 152 is grounded by way of a variable resistor RV1. The anode of the photo- detector 152 is connected to the positive side input port of an operational amplifier OP1. The negative side input port of the operational amplifier OP1 is connected to the output port, and the output port is connected to the negative side input port of another operational amplifier OP2 by way of a resistor R1. The positive side input port of the operational amplifier OP2 is connected to a reference voltage source SW by way of another resistor R2. The reference voltage source SV1 is provided to determine a level for ALC.
A capacitor C2 and a resistor R3 are connected in parallel between the negative side input port and the output port of the operational amplifier OP2. The output port of the operational amplifier OP2 is connected to the positive side input port of an operational amplifier OP3 by way of a resistor R4. The output port of the operational amplifier OP3 is connected to the anode of a diode D1, and the cathode of the diode D1 is connected to the input port of an LD driver 162. The cathode of the diode D1 is connected also to the negative side input port of the operational amplifier OP3 by way of a resistor R5. The output port of the LD driver 162 is connected to the anode of the laser diode 144 which serves as a pumping light source, and the cathode of the laser diode 144 is grounded. The positive side input port of an operational amplifier OP4 is connected to a reference voltage source SV2 by way of the resistor R5. The reference voltage source SV2 is provided to determine a value of an idling electric current. The output port of the operational amplifier OP4 is connected to the input port of the LD driver 162 by way of a diode D2. The cathode of the diode D2 is connected to the negative side input port of the operational amplifier OP4 by way of a resistor R6. The input port of the LD driver 162 is grounded by way of a resistor R7.
The cathode of the photo-detector (photo-diode) 134 for detecting an optical input power of the EDFA is connected to the power source line, and the anode is grounded by way of a variable resistor RV2. The anode of the photo-detector 134 is connected to the positive side input port of an operational amplifier OP5, and the negative side input port is connected to the output port. The output port of the operational amplifier OP5 is connected to the positive side input port of an operational amplifier OP6 by way of a resistor R8. The negative side output port of the operational amplifier OP5 is -Is- connected to a reference voltage source SV3 by way of a resistor R9. The reference voltage source SV3 determines a threshold level to be used to detect that inputting of signal light has been intercepted.
The positive side input port of the operational amplifier OP6 is connected to the output port by way of a resistor R10. The output port of the operational amplifier OP6 is connected to one of pairs of input ports of three AND elements Q1, Q2 and Q3. The other input port of the AND element 01 is connected to the power source line. The output port of the AND element Q1 is grounded by way of a resistor Rll and a capacitor Cl connected in parallel. The resistor Rll and the capacitor Cl constitute a time constant circuit. The output port of the AND element Q1 is connected to the positive side input port of an operational amplifier OP7 by way of a resistor R12. The negative side input port of the operational amplifier OP7 is connected to a reference voltage source SV4 by way of a resistor R13. The positive side input port of the operational amplifier OP7 is connected to the output port, and the output port is connected to the other input port of the AND element Q2.
The output port of the AND element Q1 is connected to the positive side input port of an operational amplifier OP8 by way of a resistor R15, and the positive side input port is connected to the output port by way of a resistor R17. The negative side input port of the operatiorial amplifier OP8 is connected to a reference voltage source SV5 by way of a resistor R16. Where the voltage value of the reference voltage source SV4 is represented by Vl and the voltage value of the reference voltage source SV5 is represented by V2, they are set to Vl < V2. The output port of the operational amplifier OP8 is connected to the input port of an invertor INV, and the output port of the invertor INV is connected to the other input port of the AND element Q3. The output port of the AND element Q2 is connected to a junction between the reference voltage source SV1 and the resistor R2, and the AND element Q3 is connected to a junction between the reference voltage source SV2 and the resistor R5.
FIG. 7 shows an example of the relationship between the LD electric current and the LD driver control voltage. The LD electric current is an electric current which flows through the laser diode 144 serving as a pumping light source, and the LD driver control voltage is a voltage signal to be supplied to the input port of the LD driver 162. The relationship between the variation of the LD electric current and the variation of the LD driver control voltage is, for example, linear as seen in FIG. 7.
FIG. 8 is a time chart illustrating changing over from idling to ALC in the embodiment of FIGS. 5 and 6. In the present embodiment, supply of an idling electric current is automatically stopped after lapse of a predetermined time after it is detected that signal light is inputted to the doped fiber based on a time constant provided by the resistor R11 and the capacitor Cl. Details are such as follows.
Now, it is assumed that the power source for the EDFA is thrown in at time To while an optical input is present, and also assumed that an optical input which has been intercepted once while the power source is available is restored at time Ti. Whether or not there is an optical input at time To is detected by comparison of the input level to the'positive side input port of the operational amplifier OP6 (refer to FIG. 6) with the reference voltage source SV3, and when there is an optical input, the input level of one of the inputs to the AND element Q2 exhibits a "high" level. When the output level of the operational amplifier OP6 changes to a "high" level, also the output level of the AND element Ql changes to a "high" level, and simultaneously, the time constant circuit constituted from the resistor Rll and the capacitor Cl is started as a timer. Until after a preset time of the timer elapses, the output level (VT1) of the operational amplifier OP7 is maintained at a "low" level, and also the output level of the AND element Q2 remains at a "low" level. Accordingly, in this instance, the ALC reference voltage to be supplied to the positive side input port of the operational amplifier OP2 becomes equal to zero, and consequently, the ALC control voltage to be supplied from the operational amplifier OP3 to the LD driver 162 is masked.
On the other hand, since the output level (VT2) of the operational amplifier OP8 remains at a "low" level until after the preset time of the timer elapses, the output level of the invertor INV is at a "high" level and the input levels to the AND element Q3 are both at a "high" level. Accordingly, an idling electric current control voltage supplied from the reference voltage source SV2 is supplied to the LD driver 162 then, and consequently, the optical output of the EDFA rises without being accompanied by an optical surge. Since the value Vl of the reference voltage source SV4 (refer to FIG. 6) is lower than the value V2 Of the reference voltage source SV5, the preset time of the timer (first timer) based on the reference voltage source SV4 is shorter than the preset time of the timer (second timer) based on the reference voltage source SV5. Accordingly, the output level (VT1) of the operational amplifier OP7 rises first, and thereupon, masking of the ALC control voltage is Canceled. Thereafter, the output level (VT2) of the operational amplifier OP8 rises, and thereupon, the idling control voltage is masked. Accordingly, after the output level VT2 rises, ALC is performed so that the optical output of the EDFA may be fixed.
In this manner, according to the present embodiment, high speed starting of the EDFA is possible without producing an optical surge. It is to be noted that, since the masking operation upon restoration of an optical input after time Tl is similar to that described above, description thereof is omitted. While, in the foregoing description, the idling electric current control voltage is fixed within the idling period until the output level (VT2) of the operational amplifier OP8 rises, even if the optical input level fluctuates within a certain range, feed forward control may be performed in order to stabilize the optical output level upon idling.
FIG. 9 is a circuit diagram of an output stabi- lization circuit for this object. The cathode of the photo-detector 134 for detecting the optical input level is connected to the power source line, and the anode is grounded by way of the resistor R17. The anode of the photo-detector 134 is connected to the positive side input port of an operational amplifier OP9. The negative side input port of the operational amplifier OP9 is connected to the output port. The output port of the operational amplifier OP9 is connected to the negative side input port of an operational amplifier OP10 by way of a resistor R18. The negative side input port of the operational amplifier OPIO is connected to the output port by way of a variable resistor RV3. The positive side input port of the operational amplifier OP10 is connected to a reference voltage source SV6 by way of a resistor R19. The output of the output stabilization circuit, that is, the output voltage of the operational amplifier OPIO, is set in place of the reference voltage source SV2 of FIG. 5.
FIG. 10 is a view illustrating operation of the output stabilization circuit of FIG. 9. The axis of ordinate indicates the idling electric current control voltage supplied from the output stabilization circuit, and the axis of abscissa indicates the monitor voltage of the optical input level. Denoted by reference numeral 164 is a region in which the optical input is intercepted, and if the level of the optical input exceeds the region, the idling electric current control voltage decreases-as the optical input level increases as represented by reference numeral 166. Consequently, the optical output of the EDFA upon idling can be stabilized. Further, while, in the foregoing description, the timing of changing over from idling to ALC is set based on the preset time of the timer by the time constant circuit, changing over from idling to ALC may be performed by detecting that the optical output level of the EDFA has exceeded a predetermined value.
A timing chart of changing over from idling to ALC based on detection of the optical output level is shown in FIG. 11. In this instance, the voltage signal VTI for canceling masking of the ALC control voltage is unnecessary. In the present embodiment, until the optical output level of the EDFA exceeds a predetermined value denoted by reference numeral 168, the ALC control voltage remains masked, and when the predetermined value is exceeded, masking of the ALC control voltage is canceled. Whether or not the output level of the EDFA has exceeded the predetermined value can be detected using an output signal of the photo-diode 152.
FIG. 12 is a block diagram showing a second embodiment of an EDFA to which the present invention is applied. The present EDFA includes an ACC (automatic electric current control) loop for the laser diode 144 for outputting pumping light. The driving current (bias current) of the laser diode 144 is detected by an LD electric current monitor circuit 170, and such detection signal is supplied to an ACC circuit 172. The ACC circuit 172 controls so that the driving current of the laser diode 144 may be fixed upon idling. Also the present embodiment is provided with the ALC circuit 154 for keeping fixed the optical output level of the EDFA, and a control signal from that one of the ACC circuit 172 and the ALC circuit 154 which is not masked by a masking circuit 174 is supplied to a driver for the laser diode 144 by way of an analog OR circuit 176.
FIGS. 13 and 14 are circuit diagrams showing a concrete example around the masking circuit of FIG. 12. A LD driver 162' has a terminal 178 for outputting an LD electric current monitor voltage. The LD electric current monitor voltage is supplied to the positive side input port of an operational amplifier OP11, and the negative side input port is connected to the output port.
The output port of the operational amplifier OP11 is connected to the negative side input port of an operational amplifier OP12 by way of a resistor R20. A capacitor C3 and a resistor R21 are connected in parallel between the negative side input port and the output port of the operational amplifier OP12. The output port of the operational amplifier OP12 is connected to the positive side input port of the operational amplifier OP4 by way of the resistor R5. The positive side input port of the operational amplifier OP12 is connected to a reference voltage source SV7 by way of a resistor R22.
The reference voltage source SW is provided to generate a reference voltage for ACC, and a junction between the reference voltage source SW and the resistor R22 is connected to the output port of the AND element Q3 of FIG. 14. Elements other than those described here are substantially same as those of the construction of FIGS.
and 6. The present embodiment is different from the embodiment of FIGS. 5 and 6 only in that an idling electric current is obtained by ACC, and since other operations can be recognized readily referring to a time chart of FIG. 8, description of them is omitted herein.
FIG. 15 is a block diagram showing a third embodiment of an EDFA to which the present invention is applied. In the present embodiment, APC (automatic power control) is performed for a pumping light source, and setting of an idling electric current is performed by a loop of the APC. TO this end, a photo-detector (photo diode) 178 for converting backward light of the laser diode 144 by photo-electric conversion is provided in place of the LD electric current monitor circuit 170 of the second embodiment of FIG. 12. An output signal of the photo-diode 177 is supplied to an APC circuit 180.
Control signals from the APC circuit 180 and the ALC circuit 154 are sent to the laser diode 144 by way of the analog OR circuit 176. The masking circuit 174 selec tively masks the APC circuit 180 and the ALC circuit 154 so that, upon cold starting or upon restoration of input ting of an optical signal, an idling electric current is first supplied to the laser diode 144, and then ALC is performed.
FIGS. 16 and 17 are circuit diagrams showing a concrete example around the masking circuit of FIG. 15.
In the present embodiment, in place of using an LD elec tric current monitor voltage from the LD driver 162' in the second embodiment of FIG. 13, an output signal of a photo-diode for receiving backward light of a pump ing light source is supplied to the positive side input port of the operational amplifier OP11. In particular, the cathode of the photo-diode 178 is connected to the power source line, and the anode is grounded by way of a variable resistor RV4. And, the potential variation at a junctionbetween the photo-diode 178 and the variable resistor RV4 is supplied to the positive side input port of the operational amplifier OP11.
Even where an idling electric current is obtained by APC in this manner, an optical amplifier which can be started without producing an optical surge similarly to the preceding embodiment can be provided. It is to be noted that, also in the second embodiment and the third embodiment, such an output stabilization circuit as shown in FIG. 9 can be adopted in a similar manner to the first embodiment. Further, in place of changing over from idling to ALC based on setting of a time constant, changing over from idling to ALC based on detection of an optical output level may be performed.
is

Claims (7)

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; means for detecting whether or not the signal light is inputted to said doped fiber; 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; idling means for supplying an idling electric current lower than the controlling electric current to said pumping light source; and masking means for controlling said automatic level control means and said idling means so that, when it is detected that the signal light is inputted to said doped fiber, the idling electric current is first -27supplied to said pumping light source and then the controlling electric current is supplied to said pumping light source.
is
2. An optical amplifier according to claim 1, further comprising means for adjusting the idling electric current in response to the input level of the signal light.
3. An optical amplifier according to claim 1 or 2, wherein said masking means stops supply of the idling electric current after a predetermined interval of time elapses after it is detected that the signal light is inputted to said doped fiber.
4. An optical amplifier according to claim 1 or 2, wherein said masking means stops supply of the idling electric current when the output level of said photodetector becomes equal to a predetermined value.
5. An optical amplifier according to claim 1, further comprising means for detecting an electric current supplied to said pumping light source, converting the value of the detected electric current into a monitor voltage and outputting the monitor voltage, and means for generating an automatic electric current control (ACC) reference voltage, the magnitude of the idling voltage being set so that the monitor voltage may coincide with the ACC reference voltage.
6. An optical amplifier according to claim 1, -28further comprising means for detecting the intensity of the pumping light, converting the value of the detected intensity into a monitor voltage and outputting the monitor voltage, and means for generating an automatic power control (APC) reference voltage, the magnitude of the idling voltage being set so that the monitor voltage coincides with the APC voltage.
7. An optical amplifier substantially as hereinbefore described with reference to Figures 3 to 11, or to Figures 12 to 14, or to Figures 15 to 17 of the accompanying drawings.
GB9809228A 1994-05-20 1995-03-27 Optical amplifiers Expired - Fee Related GB2322026B (en)

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US6583899B1 (en) 1998-12-31 2003-06-24 Cisco Photonics Italy S.R.L. Automatic protection system for an optical transmission system
EP1017192B1 (en) * 1998-12-31 2006-02-22 Cisco Systems International B.V. Automatic protection system for an optical transmission system
ATE367021T1 (en) * 2000-05-24 2007-08-15 Corvis Corp OPTICAL TRANSMISSION SYSTEMS AND METHODS WITH OPTICAL PROTECTION
DE10146001B4 (en) 2001-09-18 2008-04-03 Nokia Siemens Networks Gmbh & Co.Kg Circuit arrangement and method for safety shutdown of an optical amplifier
DE60133207T2 (en) * 2001-12-13 2009-04-30 Alcatel Lucent Laser transmitter and method for transmitting controlled optical signals

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GB2230912B (en) * 1989-04-22 1993-10-20 Stc Plc Optical amplifier gain control
US5379145A (en) * 1989-12-01 1995-01-03 Scientific-Atlanta, Inc. Laser transmitter for light wave (fiber optic) communication espectially of AM modulated CATV signals having means . . . against damage
IT1247845B (en) * 1991-03-29 1995-01-02 Pirelli Cavi Spa OPTICAL FIBER TELECOMMUNICATION LINE WITH PROTECTION DEVICE FOR OPTICAL AMPLIFIERS
JPH0575547A (en) * 1991-09-10 1993-03-26 Hitachi Ltd Optical transmitter

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EP0637860A1 (en) * 1993-07-01 1995-02-08 Alcatel SEL Aktiengesellschaft Fiber optic amplifier with device for monitoring the pump and input power
US5442479A (en) * 1993-07-01 1995-08-15 Alcatel Sel Aktiengesellschaft Fiber-optic amplifier with a facility for monitoring the pump power and input power

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GB2322027A (en) 1998-08-12
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GB2322025A (en) 1998-08-12
GB2322025B (en) 1998-11-04

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