JPH0865249A - Optical amplifier suppressing generation of optical surge - Google Patents

Abstract

PURPOSE: To provide the optical amplifier especially suppressing the generation of optical surge to be used for an optical communication system. CONSTITUTION: This optical amplifier is composed of an optical amplifying part 11 for amplifying an input signal, gain monitor means 12 for monitoring the gain of the optical amplifying part, averaging circuit 13 for calculating the average value of gain values provided by the gain monitor means, output monitor means 14 for monitoring the output level of the optical amplifying part, gain control means 15 for controlling the gain of the optical amplifying part, and control circuit 16 for controlling the exciting level of the gain control means 15 so that the instantaneous gain value can be less than the added value of the average gain value and a prescribed set allowable value by comparing the instantaneous gain value obtained by the gain monitor means with the average gain value obtained by the average circuit 13 and controlling the exciting level of the gain control means 15 so as to fix the output when the instantaneous gain value does not exceed the added value of the average gain value and the prescribed set allowable value but lowering the gain in the other case.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier, and more particularly to an optical amplifier used in an optical communication system, which suppresses the occurrence of an optical surge. In recent years, introduction of an optical communication system using an optical amplifier such as an erbium-doped optical fiber amplifier (EDFA) in place of some of the regenerative optical repeaters in a trunk optical communication system has been considered. When an optical amplifier such as an EDFA is used as a repeater, optical-electrical conversion and synchronization / regeneration are unnecessary, so that the structure of the repeater is simple and can be used regardless of the transmission speed. There are various advantages. The 1992 IEICE National Convention Spring SB-9-3 describes the configuration of an optical transmission system using an EDFA and the above-mentioned features of the EDFA.

[0002]

2. Description of the Related Art In general, an optical amplifier is provided with (1) a constant gain control system, (2) a constant optical output control system (ALC control), and (3) a pumping level fixing system of a pumping means (in order to stabilize its amplification characteristic). It is controlled by a pumping light power constant method (APC control) or the like. In each of the methods, as their names indicate, (1) makes the gain of the optical amplifier constant, (2) makes the optical output power of the optical amplifier constant, and (3) makes the pumping light power input to the optical amplifier constant. To control.

For example, in the case of an optical repeater requiring a high output, a two-stage connection structure of optical amplifiers is adopted, and the optical amplifier at the first stage has the constant gain control system of (1) or the constant gain control system of (3). The pumping level fixing system of the pumping means is used, and the optical output constant control system of the above (2) is used for the optical amplifier in the subsequent stage. 1992 IEICE National Convention Spring SB-9
2 shows a configuration method of an optical amplification type linear repeater using an EDFA, and in the case of the above two-stage configuration, when the input signal light power is -20 dBm, NF 7.0 dB, net gain 31 dB, An example of an optical amplifier with signal light output power of +11 dBm is shown.

[0004]

As described above, the optical amplifier is stabilized by various control methods. However, when the optical amplifier is used as a repeater, in addition to the steady state in which the optical input signal is considered to be at a constant level, the optical amplifier has no optical input state caused by disconnection of the transmission line or mis-insertion of the device connector, and transmission. It is required to be able to appropriately deal with momentary fluctuations in the level of the optical input signal that may occur due to device failures, switching, etc. (Journal of the Institute of Electronics, Information and Communication Engineers B-II VOL. J74-BI No2 pp. 1
44-150, 1991.2). Conventionally,
There has been a problem that an optical surge is generated from the following optical amplifier when the above-mentioned optical non-input state or the level fluctuation of the optical input signal occurs.

FIG. 14 is an explanatory diagram showing an example of an optical surge generated from an optical amplifier of a constant optical output control method (ALC control) due to the occurrence of a light non-input state. FIG. 14 (a)
Shows a case where the input level of the optical signal to the optical amplifier instantaneously becomes zero due to, for example, incorrect insertion / removal of a connector. In this case, as shown in FIG. 14B, the optical amplifier increases the pumping level to increase the gain of the amplifier in order to keep the optical output constant. However, FIG.
4 (a), after the optical non-input state, when the optical input returns to the original input level more rapidly than the response time of the optical amplifier (about several ms in the case of EDFA), In this connection, the gain control means of the optical amplifier cannot follow it, and as a result, as shown in FIG. 14 (c), a so-called optical surge in which an optical level more than the specified level is output momentarily occurs.

FIG. 15 is an explanatory diagram showing an example of an optical surge generated from an optical amplifier of a constant optical output control system due to a decrease in the level of an optical input signal. FIG. 15A shows the case where the input level of the optical signal to the optical amplifier is suddenly returned to the original input level after being lowered for a certain period of time. Also in this case, as shown in FIG. 15B, the optical amplifier increases the pumping level and the gain of the amplifier to keep the optical output constant, but the input level becomes steeper than the response time of the optical amplifier. When returning to the original level, an optical surge is instantaneously generated as shown in FIG. 15C for the same reason as described in FIG.

There are two processes for increasing the gain of an optical amplifier which causes an optical surge. The first is by constant output control. That is, when the input level decreases, it is necessary to increase the gain for constant output control, and this is because the excitation level is increased to increase the gain. Second, I am using an optical amplifier in a gain saturation state,
This is because the gain of the optical amplifier increases even when the pumping level does not change when the input level or the input signal level decreases. FIG. 16 shows the relationship between the gain of the optical amplifier and the input level when the pumping level is fixed. The gain decreases as the input level increases (the output power saturates). The region where the gain decreases is called the gain saturation state. When there is no input or when the input signal level drops, the gain of the optical amplifier is saturated according to the time constant of an optical amplification medium such as EDF (about several ms in the case of EDF) even if the pumping level is constant. The gain level increases to the gain level at the time of non-saturation.

By changing the response speed of the light output constant control (ALC control) to be slower than the time of the no-input state and the time when the input level is lowered as shown in FIGS. Gain fluctuation due to the first gain increase factor (gain fluctuation due to ALC control) by averaging output fluctuations
Can be suppressed to some extent. But slow ALC
In the control, when there is no input or when the input level decreases, the excitation level is almost constant, and therefore the increase in gain cannot be suppressed due to the second gain increasing factor. Further, even when the level fixing method (APC control) of the pumping means is used, the pumping light level cannot be changed (decreased), and thus the increase in the gain of the optical amplifier cannot be prevented. The occurrence of an optical surge becomes remarkable especially in a configuration in which optical amplifiers are connected in two stages in order to obtain high output characteristics and when the output of the optical amplifiers in the subsequent stages is controlled to be constant. In order to generate the signal light power +11 dBm in the latter stage as in the example of the above-mentioned reference, the latter stage optical amplifier further amplifies the signal of a large level which is amplified in the former stage and reaches the gain saturation region in the latter stage. Become. In the gain saturation region, increasing the input level reduces the gain of the amplifier. Therefore, on the contrary, a decrease in the signal level in the saturation region increases the gain of the amplifier itself, and results similar to those in FIG.

When the above-mentioned optical surge occurs, an input exceeding an allowable level may be input to the optical amplifier, and eventually the optical amplifier may be destroyed or the connector may be burned. On the other hand, in order to avoid the occurrence of the optical surge, it is possible to use a constant gain control method that is not affected by input fluctuations. In that case, however, another difficult problem arises because the gain of the optical amplifier is fixed. Occurs. For example, there is a problem of level diagram setting in a multi-stage repeater optical communication system. The upper limit value of the output level of the optical amplifier is predetermined in order to reduce the occurrence of non-linear phenomenon, and it is desirable to set the upper limit value of the output of the optical amplifier from the viewpoint of improving S / N. For that purpose, it is necessary to calculate the loss and its variation for each transmission line between the repeaters to determine the gain of each amplifier. There is also a problem that the error of the level diagram of each optical amplifier accumulates in the downstream transmission section. Even if the optical amplifier is controlled by the APC control method, the output level follows the input level, so that the same problem as described above occurs. For example,
In a land-based repeater, it is difficult to design a level diagram because the input level of each optical amplifier is different.

[0010]

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to
In view of the above problems, it is an object of the present invention to provide an optical amplifier capable of suppressing the occurrence of an optical surge and stabilizing the level diagram over a long period of time. According to the first basic configuration of the optical amplifier according to the present invention shown in FIG. 1, the optical amplifier 11 for amplifying an input signal, the gain monitor 12 for monitoring the gain of the optical amplifier 11, and the gain monitor 12 are provided. An averaging circuit 13 for averaging the obtained gain values, an output monitor 14 for monitoring the output level of the optical amplifier 11, and a gain controller 15 for controlling the gain of the optical amplifier 11. Then, the instantaneous gain value obtained by the gain monitor means 12 and the average gain value obtained by the averaging circuit 13 are compared, and the instantaneous gain value is set to the average gain value by a predetermined set allowable value. If the sum of the gain and the sum does not exceed, the excitation level of the gain control means 15 is controlled so that the output becomes constant, and in other cases, the gain is lowered so that the instantaneous gain value becomes the average gain value. And predetermined setting Optical amplifier for a control circuit 16 for controlling the excitation level of said gain control means (15) to be equal to or less than the sum of the allowable value is provided.

The optical amplifier further includes optical input monitor means, and when an abnormal state such as a sudden decrease in input level or no input state continues for a set time or more, the amplifying operation of the optical amplifying section 11 is performed. And the control circuit 16
Controls the gain control means 15 so that the output becomes constant only when the optical amplification section 11 is started up, instead of the above control. Further, the optical amplification section 11 is composed of an erbium-doped optical fiber amplifier, and the gain monitor means 12 replaces the optical signal from the optical amplification section 11 with its spontaneous emission light (A).
It is realized by monitoring the level of (SE).

Further, according to the second basic configuration of the optical amplifier according to the present invention shown in FIG. 3, an optical amplifier 21 for amplifying an input signal and an input monitor means 22 for monitoring the optical input level of the optical amplifier 21. , An averaging circuit 23 for averaging the instantaneous values of the input levels obtained by the input monitoring means 22, an output level monitoring means 24 for monitoring the output level of the optical amplification section 21, the optical amplification section 21. Gain control means 25 for controlling the gain of the optical amplifier 21, gain monitor means 26 for monitoring the gain of the optical amplification section 21, and hold circuit 27 for holding the output value of the gain monitor means 26 in response to a predetermined trigger signal. Then, the instantaneous value of the optical input level from the input monitor means 22 is compared with the average value of the optical input level from the averaging circuit 23, and [average value-instantaneous value] is a predetermined set value. In the case of the above, the trigger signal is sent to the hold circuit 27, the hold value of the hold circuit 27 and the gain value from the gain monitor means 26 are compared, and the gain value from the gain monitor means 26 is compared. Is controlled to be equal to or less than the value held by the hold circuit 27, and when the [average value-instantaneous value] is recovered to be equal to or less than a predetermined set value B, the light output is kept constant. An optical amplifier including a control circuit 28 for controlling the gain control means 25 is provided.

The optical amplifier further stops the amplifying operation of the optical amplifying section 21 when an abnormal state such as a sudden decrease in input level or no input state continues for a set time or longer, and the control circuit 28, instead of the above control, controls the gain control means 25 so that the output becomes constant only when the optical amplification section 21 is started up. In addition, the optical amplifier 21
Is an erbium-doped optical fiber amplifier, and the gain monitor means 26 is realized by monitoring the level of spontaneous emission light (ASE) from the optical amplification section 21.

Further, according to the third basic configuration of the optical amplifier according to the present invention shown in FIG. 5, the optical amplifier 30 in the preceding stage and the optical amplifier 40 in the succeeding stage connected in cascade to the optical amplifier 30 are connected in two stages. The optical amplifier 30 is controlled so that its output is constant, and the optical amplifier 40 at the latter stage is an optical amplifier whose gain is controlled to be constant. The optical amplifier 30 is further controlled so that the output becomes constant only when the optical amplifier is started up. Further, the optical amplification section 30 and the optical amplification section 40 are composed of an erbium-doped optical fiber amplifier.

[0015]

In the first basic configuration of the present invention, the optical amplifier 1
1 amplifies the input optical signal, gain monitor 12 monitors the gain of the optical amplifier 11, and the averaging circuit 13
Monitors the instantaneous gain value obtained by the gain monitor means 12 for a predetermined time and outputs its average value. The output level monitor means 14 monitors the optical output value of the optical amplification section 11, and the gain control means 15 controls the gain of the optical amplification section 11.

The control circuit 16 compares the instantaneous gain value obtained by the gain monitor means 12 with the average gain value obtained by the averaging circuit 13 as shown in the control flowchart of FIG. 2 (S101, 102), when the instantaneous gain value is less than the sum of the average gain value and the given allowable value, the excitation level of the gain control means 15 is controlled so that the output monitored by the output level monitoring means 14 becomes constant. (S10
2, 103, 105, 106), in other cases, the gain control is performed such that the gain instantaneous value is equal to or less than the sum of the gain average value and the given allowable value by lowering the gain of the optical amplification unit 11. The excitation level of the means 15 is controlled (S1
02, 104).

According to this configuration, if the time during which the input fluctuates is shorter than the averaging target time of the average value circuit 13, the average gain obtained from the average value circuit 13 is the gain corresponding to the input in the steady state. . The gain of the optical amplifier 11 is controlled by the control circuit 1.
As a result of 6, the average gain does not rise above the set value (allowable increase). Further, the input itself does not exceed the steady value, so that the output of the optical amplifier does not increase from the steady value to the increase allowable value or more. As a result, the amount of light surge can be suppressed or controlled. Furthermore, the optical amplifier 1
The output of 1 is kept constant by the control circuit 16 within the variable gain range. Further, the average value circuit 13 slowly follows the aging deterioration and the level fluctuation due to temperature, in which the amount of change per averaging target time is originally small and changes slowly by updating the average value, so that the light output value is sufficient. Can be kept constant.

In the second basic configuration of the present invention, the optical amplification section 21 amplifies an input optical signal, and the input monitor means 22 monitors the optical input level of the optical amplification section 21. Further, the averaging circuit 23 monitors the instantaneous value of the optical input level obtained by the input monitor means 22 for a predetermined time and outputs the average value, and the output level monitor means 24 of the optical amplifier 21. Monitor the output value. Further, the gain control means 25
Controls the gain of the optical amplification section 21, and the gain monitor means 2
Reference numeral 6 monitors the output value of the optical amplification section 21, and the hold circuit 27 receives the trigger signal from the control circuit 28 and holds the output value of the gain monitor means 26.

The control circuit 28 compares the instantaneous value of the optical input level from the input monitor means 22 with the average value of the optical input level from the averaging circuit 23 as shown in the control flowchart of FIG. Depending on the condition, the gain control means 25 is instructed to switch the control to either the method of controlling the light output constant or the method of controlling the gain constant (S201, 202). Control circuit 28
When the [average value-instantaneous value of input level] exceeds the set value A, the optical output constant control mode is shifted to the following gain limiter control mode. A trigger signal is sent to the hold circuit 27 to hold the gain value in the steady state, and the held value from the hold circuit 27 is compared with the gain value from the gain monitor means 26 to determine the gain value from the monitor 26 as the hold circuit. Control is performed so that the value held from 27 is not exceeded (S2
02, 203, 205, 206). When the [average value-instantaneous value of input level] is restored to the set value B or less, the gain limiter control mode is shifted to the constant optical output control mode (S206, 204).

According to this structure, when an abnormal decrease in the input level is detected, the current gain value is held before the gain is changed so that the gain of the optical amplifying section 1 becomes equal to or less than that value. Take control. Therefore, the gain is fixed below the steady state value, and no optical surge occurs even when the input level returns to the original level. In the steady state, the light output constant control is performed, so that the output level is stable. For switching between the gain control and the constant optical output control, a hysteresis characteristic is provided with reference to two set values A and B, thereby preventing erroneous switching between the two and setting the switching value freely. The degree is being expanded.

In the third basic configuration of the present invention, of the optical amplifiers 30 and 40 cascade-connected in two stages, the optical amplifier 30 in the preceding stage is controlled so that its optical output becomes constant, and the optical amplifier in the subsequent stage. 40 is controlled so that its gain is constant. The optical amplifier 31, 31 inside each optical amplifier 30, 40,
41 amplifies the input optical signal, and gain control means 35,
Reference numeral 45 controls the gain of each of the optical amplifiers 31 and 41. Also, the output level monitor means 34 of the optical amplifier 30.
Monitors the optical output level of the optical amplifier 31, and the gain monitor means 44 of the optical amplifier 40 monitors the gain of the optical amplifier 41.

The control circuit 36 of the optical amplifier 30 in the preceding stage performs a constant optical output control, and the control circuit 46 of the optical amplifier 40 in the subsequent stage performs a constant gain control. An example of each of those control flows is shown in FIG. FIG. 6A shows an example of a control flow of the control circuit 36 that performs the constant light output control in the preceding stage. As shown in the flow chart, it is first determined whether or not the output obtained by the output level monitor means 34 is within the target value range (S301, 30).
2). Therefore, when it is determined to be within the range, the control level of the gain control means 35 at that time is maintained (S304). On the other hand, if it is determined that the value is out of the range, then [light output <target value] is determined, and if the light output is smaller than the target value, the gain control means increases the gain of the optical amplifier 31. 35 (S303,
305), and if the optical output is larger than the target value, the gain control means 35 is controlled so that the gain of the optical amplification section 31 becomes small (S303, 306). This keeps the light output constant.

FIG. 6B shows an example of the control flow of the control circuit 46 for performing the constant gain control in the subsequent stage. As shown in the flow chart, the gain monitor means 4
First, it is judged whether or not the gain obtained in 4 is within the range of the target value (S401, 402). Therefore, when it is determined to be within the range, the control level of the gain control means 35 at that time is maintained (S404). On the contrary, when it is determined that the value is out of the range, next, [gain <target value]
If the gain is smaller than the target value, the gain control means 45 is controlled so that the gain of the optical amplifying section 41 is increased (S403, 405). If the gain is larger than the target value, the optical amplification is performed. The gain control means 45 is controlled so that the gain of the section 41 becomes small (S403, 40).
6). This keeps the gain constant.

According to this structure, the optical amplifier 3 having a two-stage structure is used.
At 0 and 40, the amplified output of the optical amplifier 30 of the previous stage is given as an input to the optical amplifier 40 of the subsequent stage, which operates in a very strong gain saturation state as compared with the optical amplifier 30 of the previous stage because of the level of the input. are doing. Therefore, if the optical amplifier 40 in the subsequent stage is controlled to have a constant optical output, a large optical surge is likely to occur as pointed out in the conventional example. Therefore, in this configuration, the optical amplifier 40 in the subsequent stage is controlled to have a constant gain to suppress the amount of optical surge in the subsequent stage. At the same time, the optical amplifier 30 in the previous stage has a constant optical output control (low-speed ALC).
By doing so, fluctuations and variations in the optical input level are absorbed, and a stable optical output level is transmitted to the subsequent stage. Since the input level of the optical amplifier in the previous stage is small, the amount of gain reduction from the non-saturation gain due to gain saturation is also small. This allows
It is possible to configure an optical amplifier with a small fluctuation in optical output and a small amount of optical surge.

[0025]

FIG. 7 is a circuit diagram showing an embodiment of an optical amplifier based on the first basic configuration of the present invention shown in FIG. In terms of the relationship between FIG. 7 and FIG. 1, the erbium (Er ³⁺ ) doped fiber (EDF) 53 of FIG. 7 corresponds to the optical amplification unit 11 of FIG. Further, the photodetector (PD) 54, the optical demultiplexer 55 and the photodetector (PD) 56 of FIG. 7, the optical multiplexer 51 and the pump laser diode (LD) 52 are the gain monitor means 12 and the output level of FIG. They correspond to the monitor means 14 and the gain control means 15, respectively. Further, a control circuit 59 including an averaging circuit 57 and an adding circuit 58 of FIG.
Correspond to the control circuit 16 of FIG. 1, respectively.

In FIG. 7, an erbium-doped fiber (EDF) 53 is used as an optical amplification element, and a 1.48 μm semiconductor laser (LD) 52 is used as its excitation light source. The excitation light from the semiconductor laser (LD) 52 is
It is given to the EDF 53 together with the optical input signal via the optical multiplexer 51. Further, since the level of spontaneous emission light (ASE) emitted from the side surface of the fiber of the EDF 53 is proportional to the gain of the EDF amplifier, the spontaneous emission light is monitored by the photodetector (PD) 54 composed of a photodiode so that the gain monitor means is provided. Has been realized (Reference: IEICE Journal OC
S91-32, Kazuo Aida, "Method for controlling gain of EDFA by detecting spontaneous emission light from side surface of fiber", patent application 03-131326, publication 04-356984). The photo detector (PD) 56 monitors the optical signal output level after amplification via the optical demultiplexer 55.

The control circuit 59 constitutes a constant gain control circuit and a constant output control circuit together with the averaging circuit 57 and the adding circuit 58. The constant gain control circuit uses the operational amplifier 1 to measure the instantaneous gain value monitored by the photo detector (PD) 54,
The value of [average gain value + set allowable value] obtained via the averaging circuit 57 and the adding circuit 58 is compared. [Gain instantaneous value <
In the case of [gain average value + set allowable value], that is, [gain instantaneous value−gain average value] is less than or equal to the allowable value, the transistor 1 (Tr1) is turned on, and the drive current to the excitation diode 52 for constant gain control. The pull-in path opens. On the contrary, when the [instantaneous gain value-gain average value] is equal to or more than the allowable value, the transistor 1 (Tr1) is turned off and the path is closed to stop the excitation light emission of the excitation diode 52. Increases in gain beyond the norm allowance are prevented. The constant gain control circuit actually forms a feedback loop including the EDF 53 as a whole circuit, and makes the instantaneous gain within the range of the gain average value to the allowable value by the negative feedback amplification operation.

Next, the operation of the constant output control circuit will be described. The output level signal monitored by the photo detector (PD) 56 via the optical demultiplexer 55 is the operational amplifier 2 and the resistor R5.
3, R54, and is input to a minus amplifier whose reference potential ref is ground. When the monitor signal is smaller than the reference potential ref, the transistor 2
(Tr2) is turned on, and the drive current drawing path to the excitation diode 52 is opened for constant output control. On the contrary, when the monitor signal is larger than the reference potential ref, the transistor 2 (Tr2) is turned off, and the drive current drawing path to the excitation diode 52 is closed for constant output control. The constant output control circuit actually forms a feedback loop including the EDF 53 as a whole circuit and equalizes the output level to the level of the reference potential ref by the negative feedback amplification operation.

Finally, by connecting the transistor 1 (Tr1) of the constant gain control circuit and the transistor 2 (Tr2) of the constant output control circuit in series, the control flow as described above with reference to FIG. 2 is performed. It becomes possible to execute. In the case of the present embodiment, the present invention is realized by using an electronic circuit, particularly an analog circuit, but as another realizing means, it is digitally realized by using a processor circuit and various functions realized by its programming. Functions similar to those of this embodiment, such as an averaging circuit, a hold circuit, and a control circuit, may be realized.

FIG. 8 is a circuit diagram showing an example in which optical input monitor means is newly added to the embodiment of FIG. In this example, only the newly added optical input monitor means is different from FIG. 7 described above. The optical input monitor means includes an optical demultiplexer 60, a photo detector (PD) 61, and a control circuit 62.
Consists of The control circuit 62 stops the operation of the optical amplifier when the input level abnormality continues for a set predetermined time or longer. If the state in which the input level including the input disconnection is lower than usual continues for a long time, the average value of the gain gradually increases, and the average gain becomes larger than the gain in the steady state, which may cause an optical surge.

The control circuit 62 is for preventing the occurrence of such a case. For example, the control circuit 62
When the abnormal state described above is detected by the circuit connection shown by the alternate long and short dash line in FIG.
The ef potential may be lowered to the negative power supply potential V _EE , thereby stopping the operation of the optical amplifier. Further, as a case different from the above, the gain cannot be sharply increased when the optical amplifier is started up depending on the circuit configurations shown in FIGS. 7 and 8, so that the steady operation is started from the time when the circuit is started up. It is also possible to add a circuit (not shown) that always turns on Tr1 only for the time until the transfer, thereby shortening the startup time of the optical amplifier.

FIG. 9 is a circuit diagram showing a modification of the embodiment of FIG. 7 in which the gain is monitored on the output side. FIG. 10 is a diagram showing the relationship between the wavelength bands of the signal light and the spontaneous emission light (ASE). 9 and 7, the spontaneous emission light (ASE) emitted from the side surface of the fiber is monitored by the photodetector (PD) in order to monitor the gain in FIG. 9, whereas the gain is output in FIG. 9. It is different in that it is monitored on the side. Therefore, a new optical splitter 63, optical filter A,
B64 and 65 are used.

In FIG. 9, a part of the output of the optical amplifier is
The optical filter B65 is branched by the optical branching device 63.
Pass through. At that time, only the spontaneous emission light portion is selected. Therefore, the photo detector (PD) 54 can monitor only the level of spontaneous emission light. FIG. 10A shows
The relationship between the signal light wavelength band and the spontaneous emission light wavelength band around the signal light wavelength band, and FIGS. 10B and 10C show the bandpass characteristics of the optical filter A64 that passes light in the signal light wavelength band, and The band pass characteristics of the optical filter B65 that blocks light in the signal light wavelength band and passes only the ASE portion are schematically shown.

FIG. 11 is a circuit diagram showing an embodiment of an optical amplifier based on the second basic configuration of the present invention shown in FIG. FIG. 12 schematically shows the relationship between the input signal level in FIG. 11 and the control state of the optical amplifier. In FIG. 11, the same parts as those in FIGS. 7 and 8 are designated by the same reference numerals, and will not be further described here. In addition, regarding the relationship between FIG. 11 and FIG.
The holding circuit 66 of 1 corresponds to the holding circuit 27 of FIG.
Further, the control circuit 68, the switching device 67, the comparator 1,
2, the subtraction circuits 69 and 70, and the constant gain control circuit and constant output control circuit correspond to the control circuit 28 in FIG.

Here, the control operation of the optical amplifier according to this configuration will be described with reference to FIGS. 11 and 12. FIG.
1, the comparators 1 and 2 further determine each predetermined set value from the input level instantaneous value monitored by the photo detector (PD) 61 via the optical demultiplexer 60 and the smoothed output by the averaging circuit 57. The average values of A and B subtracted by the subtraction circuits 69 and 70 are compared. As shown in FIG. 12, the set values A and B give so-called Schmidt type threshold values, and the set value A is used as the threshold value when the abnormality is detected when the input level decreases from the steady state, and conversely The set value B is used to detect the recovery from the state to the steady state. As described above, by providing the detection threshold with the hysteresis characteristic, it is possible to suppress the flutter during the abnormality detection and the recovery detection, stabilize each operation, and further improve the degree of freedom in setting the threshold. In the example of FIG. 11, the control circuit 68 performs the sequence control during the series of abnormality detection and recovery detection.

In the steady state, the control circuit 68 monitors only the output of the comparator 1 (set value A) for detecting abnormality as valid. Also, during this period, the output constant control circuit outputs the output of the transistor 1 (Tr
The switch 67 is controlled to connect to the base of 1). Next, when the comparator 1 detects an abnormal state due to a decrease in the input level, the control circuit 68 triggers the holding circuit 66 to hold the gain value at the time of detecting the abnormality. Then, the switch 67 is switched to connect the output of the constant gain control circuit to the base of the transistor 1 (Tr1). Next, only the output of the comparator 2 (setting value B) for detection of recovery is monitored as being effective, and preparation is made for recovery from the abnormal state.

Further, if the state where the input level is lower than normal (including the input interruption) continues for a long time, the average value of the input level also gradually decreases, and there is a possibility that the control returns to the constant output control even in the abnormal input state. Therefore, when the abnormal input state continues for more than the set time, the operation of the optical amplifier may be stopped as shown in the above example. When the comparator 2 detects the recovery from the abnormal state, the control returns to the steady state described above.

FIG. 13 is a circuit diagram showing an embodiment of an optical amplifier based on the third basic configuration of the present invention shown in FIG. Note that, in FIG. 13, the same reference numerals are attached to the same functional portions as those in FIG. 7, and they will not be described further here (however, when using in two places, A and B are added after the reference numerals). Are distinguished by attaching.). In FIG.
The former optical amplifier 71 has the configuration of the constant optical output control circuit described in FIG. 7, and the latter optical amplifier 73.
Similarly has a basic circuit configuration of the constant gain control circuit of FIG. It is considered that these operations can be easily inferred from the description of FIG. 7, and therefore the description thereof is omitted here.

In this configuration, the optical amplifier 73 in the subsequent stage performs constant gain control, and the optical amplifier 71 in the previous stage uses an output constant control circuit having a time constant that is sufficiently slower than the expected input interruption time or input drop time. Light output constant control (low speed ALC)
I do. As a result, fluctuations and variations in the optical input level can be absorbed, stable optical levels can be transmitted to the subsequent stage, and the amount of light surge generated due to the gain saturation state when the input level decreases can be reduced.

[0040]

As described above, according to the present invention,
It is possible to provide an optical amplifier capable of suppressing the occurrence of an optical surge, keeping the output of the optical amplifier constant over a long period of time, and stabilizing the level diagram. According to the first configuration of the present invention, the output of the optical amplifier does not increase from the steady value to the increase allowable value or more, and therefore, the amount of optical surge can be suppressed or controlled. Furthermore, the output of the optical amplifier is normally (when there is no sudden input change) kept constant by the control circuit, and the amount of change per averaging target time is essentially minute and changes slowly. On the other hand, since the average value is updated to follow it slowly, the light output value can be kept constant.

According to the second configuration of the present invention, the gain is limited to the steady state value or less, and the optical surge does not occur even when the input level returns to the original level. In the steady state, the light output constant control is performed, so that the output level is stable. For switching between the constant gain control and the constant optical output control, a hysteresis characteristic is provided with two set values A and B as a reference, thereby preventing erroneous switching between the two and setting the switching value freely. The degree is being expanded.

Further, according to the third configuration of the present invention, the optical amplifier in the subsequent stage is controlled to have a constant gain to suppress the amount of optical surge in the subsequent stage, and at the same time, the optical amplifier in the previous stage is controlled to have a constant optical output (low speed ALC). By doing so, it becomes possible to absorb fluctuations and variations in the optical input level and transmit a stable optical output level to the subsequent stage, and as a result, an optical amplifier with a small optical output fluctuation and a small amount of optical surge is configured. it can.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first basic configuration of an optical amplifier according to the present invention.

2 is a flowchart showing an example of a control flow of the control circuit of FIG.

FIG. 3 is a block diagram showing a second basic configuration of the optical amplifier according to the present invention.

FIG. 4 is a flowchart showing an example of a control flow of the control circuit of FIG.

FIG. 5 is a block diagram showing a third basic configuration of the optical amplifier according to the present invention.

6 is a flowchart showing an example of a control flow of the control circuit of FIG.

7 is a circuit diagram showing an embodiment of an optical amplifier based on the first basic configuration of the present invention in FIG.

8 is a circuit diagram showing an embodiment in which a light input monitor means is newly added to the embodiment of FIG.

9 is a circuit diagram showing a modification of the embodiment of FIG. 7 configured to monitor the gain on the output side.

FIG. 10 is a diagram showing a relationship between wavelength bands of signal light and spontaneous emission light (ASE).

11 is a circuit diagram showing an embodiment of an optical amplifier based on the second basic configuration of the present invention in FIG.

FIG. 12 is an explanatory diagram schematically showing the relationship between the input signal level and the control state of the optical amplifier in FIG.

13 is a circuit diagram showing an embodiment of an optical amplifier based on the third basic configuration of the present invention in FIG.

FIG. 14 is an explanatory diagram showing an example of an optical surge generated from an optical amplifier of a constant optical output control system when an optical non-input state occurs.

FIG. 15 is an explanatory diagram showing an example of an optical surge generated from the optical output constant control type optical amplifier due to the level reduction of the optical input signal.

FIG. 16 is a diagram showing the relationship between the gain and the input level of the optical amplifier when the pumping level is fixed.

[Explanation of symbols]

 11 ... Optical amplification section 12 ... Gain monitor means 13 ... Averaging circuit 14 ... Output level monitor circuit 15 ... Gain control means 16 ... Control circuit 22 ... Input monitor means 27 ... Hold circuit

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Claims (13)

[Claims] 1. An optical amplification section (11) for amplifying an input signal,
A gain monitor means (12) for monitoring the gain of the optical amplification section (11), an averaging circuit (13) for taking an average value of the gain values obtained by the gain monitor means (12), the optical amplification Output monitor means (14) for monitoring the output level of the section (11), gain control means (15) for controlling the gain of the optical amplification section (11), and gain monitor means (12). If the instantaneous gain value is compared with the average gain value obtained by the averaging circuit (13) and the instantaneous gain value does not exceed the sum of the average gain value and the predetermined set allowable value, By controlling the excitation level of the gain control means (15) so that the output becomes constant, and otherwise reducing the gain, the gain instantaneous value and the gain average value are set to a predetermined set allowance. The above profits should be An optical amplifier, characterized by a control circuit for controlling the excitation level of the control means (15) (16). 2. The optical amplifier section (11) further comprising an optical input monitor means, and when an abnormal state such as a sudden decrease in input level or no input state continues for a set time or longer.
The optical amplifier according to claim 1, wherein the amplifying operation of (1) is stopped. 3. The control circuit (16) controls the gain control means (15) so that the output is constant instead of the control only when the optical amplification section (11) is started up. The optical amplifier described. 4. The optical amplifier according to claim 1, wherein the optical amplification section (11) is an erbium-doped optical fiber amplifier. 5. The optical amplifier according to claim 4, wherein the gain monitor means (12) monitors the level of spontaneous emission light (ASE) from the optical amplification section (11). 6. An optical amplification section (21) for amplifying an input signal,
An input monitor means (22) for monitoring the optical input level of the optical amplification section (21), and an averaging circuit (23) for taking an average value of the instantaneous values of the input level obtained by the input monitor means (22). ), Output level monitor means (24) for monitoring the output level of the optical amplification section (21), and gain control means (2) for controlling the gain of the optical amplification section (21).
5), a gain monitor means (26) for monitoring the gain of the optical amplification section (21), a hold circuit (27) for holding the output value of the gain monitor means (26) in response to a predetermined trigger signal, Then, the instantaneous value of the optical input level from the input monitor means (22) is compared with the average value of the optical input level from the averaging circuit (23), and [average value-instantaneous value] is a predetermined set value. When it becomes equal to or higher than A, the trigger signal is sent to the hold circuit (27) to hold the hold value of the hold circuit (27) and the gain monitor means (26).
The gain monitor means (26) by comparing the gain value from
The gain control means (25) is controlled so that the gain value from the holding circuit (27) is equal to or less than
A control circuit (28) for controlling the gain control means (25) so that the optical output becomes constant when the [average value-instantaneous value] recovers to a predetermined set value B or less. Characteristic optical amplifier. 7. The amplifying operation of the optical amplifying section (21) is stopped when an abnormal state such as a sudden decrease in input level or no input state continues for a set time or longer. Optical amplifier. 8. The control circuit (28) controls the gain control means (25) so that the output is constant instead of the control only when the optical amplification section (21) is started up. The optical amplifier described. 9. The optical amplifier according to claim 6, wherein the optical amplification section (21) is an erbium-doped optical fiber amplifier. 10. The optical amplifier according to claim 9, wherein the gain monitor means (26) monitors the level of spontaneous emission light (ASE) from the optical amplification section (21). 11. The optical amplifier (30) of the preceding stage comprises a two-stage connection structure of an optical amplifier (40) of the succeeding stage connected in cascade to the optical amplifier (30) of the preceding stage, and the output of the optical amplifier (30) of the preceding stage is constant. Controlled, and the optical amplifier (4
0) is an optical amplifier characterized in that its gain is controlled to be constant. 12. The optical amplifier according to claim 11, wherein the optical amplifier (30) is controlled so that the output becomes constant only when the optical amplifier is started up. 13. The optical amplifier according to claim 11, wherein the optical amplification section (30) and the optical amplification section (40) are erbium-doped optical fiber amplifiers.