JPH0583201A - Optical amplifier with output terminal open state detection circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] To prevent problems such as high optical power of field workers when the output end of the optical amplifier is opened, and to prevent oscillation, etc. by returning reflected light from the open end to the amplifier. To aim. Constitution: When the output of the optical amplifier 1 is not sustained with the optical fiber of the transmission line 2, the reflected light detecting means 4 for detecting the increase of the reflected light from the open end 3, and the reflected light detecting means 4 according to the increase of the reflected light. The output power of the optical amplifier 1 is reduced so that the optical amplifier 1
When it is reconnected to the optical fiber of the transmission line 2, the reflected light detecting means 4 detects the decrease of the reflected light, and the gain control means 5 for increasing the optical power again is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier with an output end release detection circuit, and particularly when the optical amplifier is used as an optical post amplifier (optical power amplifier) in the subsequent stage of an optical transmitter,
When used as an optical intermediate repeater, the present invention relates to an optical amplifier having a function of reducing the optical output level from the amplifier when it is open without being connected to a fiber which is a transmission line.

[0002]

2. Description of the Related Art In recent years, research and development of optical amplifiers have become active. The optical amplifiers include Er-doped fiber amplifiers, semiconductor optical amplifiers, etc., and the applicable fields are (1) optical amplification intermediate repeater,
(2) Optical post amplifier (booster amplifier), (3) Optical printer, etc. are considered.

Especially in the applications of (1) and (2), the optical output level may be close to 100 mW as the performance of the optical amplifier is improved. An important issue is prevention of oscillation and the like due to feedback of reflected light from the end to the amplifier. Conventionally,
Such a problem does not occur in the optical communication device, and is a new problem that occurs with the application of the optical amplifier.

In order to solve this problem, it is necessary to pay attention to the following points. Allowable light output level (Pa). It is necessary to reduce the light output level from the open end below a certain allowable value. Its value is the safety standard established by public institutions,
It needs to be decided according to the work environment of site workers, safety measures, etc. Detection of opening. When the optical amplifier is in the operating state and is released from the state of being connected to the transmission line, the optical amplifier needs to have a function of automatically detecting the release. The place to be opened may be the output end of the optical amplifier, or the place once entered the transmission path and separated from the optical amplifier, and the place connected by a connector or the like may be opened. It is necessary to be able to handle both cases.
However, if the optical output power at the open end is equal to or less than the above allowable value, it may not be detected. Therefore, it is desirable to be able to monitor the optical output power at the open end. Furthermore, it is required that no malfunction occurs when the connection is normal with a normal connector or the like. Method of reducing light output. When it is detected that the output end is open, the output level of the optical amplifier is lowered so that the output level at the open end becomes equal to or less than the allowable value. There are various ways to reduce the optical output level,
It is necessary to match the APC (or AGC) system of the optical amplifier. Although a method of completely shutting down the light output level is also conceivable, it is necessary to consider the light output level in relation to the method of returning matters. Return of output level when connected. When the open end is near the optical amplifier, it is possible to manually restore. However, in the case of handling the opening at a place away from the optical amplifier, it is required to automatically return. A means for measuring the output power from an optical amplifier. If the output terminal is opened and the output level is reduced or shut down, a means for measuring the output power of the optical amplifier during steady operation is required. It is necessary to consider the method. A system that can meet the above various demands is required.

[0005]

FIG. 1 is a block diagram showing the principle of the present invention. In the figure, 1 is an optical amplifier, 2 is a transmission line, 3 is an open end of the transmission line, 4 is reflected light detection means, and 5 is gain control means. The output of the optical amplifier 1
The reflected light detecting means 4 detects an increase in the reflected light from the open end 3 when it is not maintained with the optical fiber of the transmission line 2.
The gain control means (5) reduces the output power of the optical amplifier 1 according to the increase of the reflected light, and according to the decrease of the reflected light when the optical amplifier 1 is reconnected to the optical fiber of the transmission line 2.
Increase the optical power again.

2 to 5 show various aspects of the principle of the present invention, and the same numbers as in FIG. 1 indicate the same parts. In FIG. 2, the reflected light detecting means 4 is a two-input, two-output optical branching device or optical coupler connected to the output end of the optical amplifier 1, and one of the output of the optical branching device or the optical coupler is the transmission line 2
Of the optical branching device or the optical coupler is connected to the output of the optical amplifier 1 and the other is connected to the output end of the optical coupler. It is connected to a reflected light detection circuit 6 for monitoring the reflected light from the open end 3 of the transmission line 2, and the output of the reflected light detection circuit 7 is connected to the gain control circuit 5.

As shown in FIG. 3, two optical branching devices or optical couplers 4 are provided, and one of the output terminals of the preceding optical coupler 4 connected to the output of the optical amplifier 1 has an automatic gain of the optical amplifier 1. Alternatively, it may be connected to a photodetector (APC photodetector 8) for controlling output power. In this case, the output terminal of the optical coupler 4 in the subsequent stage is connected to the reflected light detection circuit 6, the output of the reflected light detection circuit is connected to the signal processing circuit 7, and the outputs of the APC light receiver 8 and the signal processing circuit 7 are gains. It is connected to the input of the control circuit 5.

As shown in FIG. 4, two optical splitters or optical couplers 4 are provided, and one of the output terminals of the optical coupler 4 at the preceding stage connected to the output of the optical amplifier 1 is connected to the reflected light detection circuit 6. The output of the reflected light detection circuit is connected to the signal processing circuit 7
Is connected to the output terminal of the optical coupler 4 in the subsequent stage
Receiver for automatic gain or output power control (APC
It may be connected to the light receiver 8). Also in this case, the outputs of the APC light receiver 8 and the signal processing circuit 7 are connected to the input of the gain control circuit 5.

As shown in FIG. 5, as a means for increasing / decreasing the output power of the optical amplifier 1, an optical variable attenuator 9 is provided between the optical coupler 4 connected to the optical amplifier 1 and another optical coupler 4. May be inserted. The optical amplifier 1 is an Er-doped fiber amplifier, and as a means for increasing / decreasing the output power of the Er-doped fiber amplifier, it is preferable to change the pumping power of the pumping laser by changing the bias current.

The optical amplifier 1 is a semiconductor optical amplifier, and as a means for increasing or decreasing the output power of the semiconductor optical amplifier, the bias current may be varied to increase or decrease. The detection signal of the reflected light from the open end is amplified to a predetermined level using an amplifier, and when the reflected light increases, the output voltage exceeds the first threshold value and becomes a constant output voltage (V1), and the reflected light decreases. Then, it is preferable that a control signal for increasing or decreasing the output of the optical amplifier is obtained by inputting to a hysteresis comparator which becomes a constant voltage (V2) when the voltage falls below the second threshold.

The output of the hysteresis comparator is the optical amplifier 1
It is preferable to feed back to the reference voltage of the APC circuit so that the output power of the optical amplifier is kept constant while the reflected light is increased or decreased. It is preferable to control the optical output power in the optical amplifier and input the control signal to the optical variable attenuator.

The variable optical attenuator may be an MZ intensity modulator. The amplifier may be a logarithmic amplifier.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, as a means for detecting the opening of the output end, the method of detecting the reflected light from the output end in the optical amplifier is generally excellent as in the case of detecting the fault point of the optical fiber. Normally, the open end of the optical fiber (assuming it has been cleaned) is exposed to air. The refractive index of the glass of the optical fiber is about 1.4 to 1.5, and the reflected light to the rear has a value attenuated by 14 dB of the output light. As a situation where the output end is opened, first consider the configuration shown in FIG.

FIG. 6 is a diagram for considering the output end open detection condition in the embodiment of the present invention. In FIG. 6, the optical amplifier 60 includes an optical amplifier 61 and an optical monitor branching device 62, the output of the optical amplifier 60 is PA (dBm), and the branching loss in the optical monitor branching device 62 is Ld (dB ), The loss in the optical attenuation medium 63 is Lf (dB), and the optical output at the open end 64 is Po.
(dBm), and the branch output of the optical monitor branching device 62 is Pmon.

At this time, the following equation holds. Po = PA-Lf Pmon = Po -14-Lf-Ld = PA-14 -2Lf-Ld where 14 dBm is subtracted from Po because the reflection at the glass end of the optical fiber is assumed to be 4%. is there. For example, if PA = 20dBm and Ld = 15dBm, Po = 20 -L
f, Pmon = -9-2Lf.

The factors which give the detection limit of the reflected light power will be examined below. Residual reflection from the connected optical connector in the latter stage of the optical amplifier
The monitor output of the reflected light at the morphism optical amplifier at the output terminal of the connector
Pmonc is usually a PC (Physical contact) type connector, which has a return loss of 30 dB or more, which is more than 14 dBm at the glass end (when the connector end is not contaminated
40dBm) can be expected. In this case, PA is 20 dBm and ld is 18
In dBm, Pmon is as follows. Pmonc = PA-30-Ld (= -28) Rayleigh scattering and stimulated Brillouin scattering When the output power of the optical amplifier is small, Rayleigh scattering is mainly observed. The power is about 3 according to the experimental value.
It is a value attenuated by 5 to 40 dB. Therefore, it is sufficiently smaller than the residual reflection of 30 dBm from the PC connector. Pray = PA-(35 ~ 40)-Ld When the output power of the optical amplifier increases and exceeds a certain threshold, stimulated Brillouin scattering occurs. At this time, all the optical power exceeding the threshold value is scattered backward, and a very large power is returned as compared with the residual reflection from the PC connector. Normally, the output power of the optical amplifier is set below the threshold value of stimulated Brillouin scattering depending on the modulation / demodulation method, so it is considered that there is no problem, but caution is required. The level of thermal noise of the reflected light detecting photodetector and that of the dark current receiver are about 20 pA / √Hz, and the dark current is about 10 to 50 nA when PIN is used. When the photoelectric conversion coefficient of the light receiver is 1 A / W and the band is 1 kHz, the thermal noise is -47 dBm optical power,
Further, when the dark current is -43 dBm, it cannot be detected. Since the reflection from the connector of -1 is considered to be -31 dBm, the thermal noise and dark current are sufficiently smaller than the reflection from the connector. From the above examination, the largest factor that gives the detection limit that can detect that the output end of the optical amplifier is opened is the residual reflection of the optical connector near the optical amplifier.
On the other hand, when the output end of the optical amplifier is opened, the residual reflection value of this optical connector must be sufficiently smaller than that of Pmon in order to be able to detect the opening.
Therefore, Pmon >> Pmonc -14 -2Lf >> -30 → 8 >> Lf (1) This condition gives the limit of the area in which the release can be detected.

Further, in order for Po to be at an allowable level at the detection limit, it is necessary to satisfy the condition Pa> Po >> PA-L-8. For example, if Pa is 0 dBm, the output of the optical amplification unit will be 9 dBm or less. Normally, the current problem is when PA is about 20 dBm, and Pa becomes a considerably high value from the detection limit.

Therefore, in the case of the method of simply detecting the light reflection power, the position where the connector or the like is opened is close to the optical amplifier to the extent that Expression (1) is satisfied, and Po is used at other open positions. Only when the optical power level is below the allowable optical power level Pa. The reflected light detection level Pmon when the output end of the optical amplifier is not opened is the residual reflection level Pmonc from the optical connector. The detection level increases above this value Pmonc by opening the output end of the optical amplifier. It is detected that it is released due to this increase. The detection threshold Pth1 is set between Pmon and Pmonc.

When the opening is detected, the output power of the optical amplifier is reduced. The reduction method depends on the configuration of the optical amplifier. It is possible to shut it down completely, but to detect when the open ends are rejoined,
Consider the case where the optical power is not set to zero. The set value of the optical power is set to the allowable level Pa (for example, Pa = 0 dBm).

When the optical power is reduced, the level of reflected light is also reduced. Reflections from the connector, Rayleigh scattering, etc., are attenuated by the same amount. Since the dark current of the receiver, thermal noise, etc. do not change, the detection limit is the reflection from the connector or
It is determined by the larger dark current of the receiver. Second threshold Pth2
Is set between the reflection level from the open end and the above detection limit. When the open end is connected again, the reflection level falls below the detection limit and exceeds Pth2. At this time, it is determined that the open end is closed, and the output power of the optical amplifier is returned to the original state. The reflected light level at that time becomes the residual reflection level from the connector.

The above is the basic operation principle. Other attendant issues are discussed below. (a) APC Normally, the optical amplifier is used with a constant output power. Therefore, the problem is how to process the APC when changing the above optical power. In order to stabilize the detection level of reflected light, I want APC operation even when the optical output power is reduced.

In order to deal with the above, a method of feeding back the detection signal of the reflected light power to the reference level of the comparison operation circuit for APC is used. According to this method, APC can be operated without stopping. FIG. 7 is a diagram showing the configuration of an optical amplifier with an output end release detection circuit according to an embodiment of the present invention based on the principle. In FIG. 7, the output of the optical amplifier 71 is connected to one input end of a 2 × 2 optical branching device 72 for monitoring an optical signal. One of the outputs of the optical branching device 72 is connected to the transmission line 73, and the other is input to the APC output light monitor light receiver 74. The output power monitor light receiver 74 for APC monitors the optical power, and the monitor output is input to the APC control circuit 76 via the amplifier 75.
The PC control circuit 76 performs the APC operation so that the output of the APC output light monitor light receiver 74 becomes constant.
The other input end of the 2 × 2 optical branching device 4 is connected to the reflected light monitor light receiver 77. The monitor output of the reflected light monitor light receiver 77 is input to the hysteresis comparator 79 via the amplifier 78.

FIG. 8 is a time chart for explaining the operation of the amplifier shown in FIG. 7 and 8, time t1
When the end of the transmission path 73 is opened and the reflected light power that is the output of the amplifier 78 increases and exceeds Pth1, the hysteresis comparator 79 switches and the reference voltage of its output becomes Vlow, which causes The output power of the optical amplifier 71 decreases. APC works in that state. Also, at time t
When the 2d transmission line 73 is reconnected and the reflected light power decreases to Pth2 or less, the hysteresis comparator 79 switches again and its output reference voltage becomes Vhigh, which causes the output light power of the optical amplifier 71 to change. To increase. By performing such an operation, the optical power can be changed in the APC state.

As another careful note, it is required that the 2 × 2 optical branch has sufficiently high directivity and the crosstalk between the input terminals is small. For example, a 2 × 2 optical fiber coupler is effective. For the branching ratio, for example, a 20: 1 branch is used in order to reduce the loss of the main signal system. Further, the reflected light power from the optical monitor for APC needs to be sufficiently small. For example, the value should be 10 dB or more smaller than the residual reflected light of the connector (hence the return loss is 40 dB or more). Since a very large dynamic range is required for the output of the optical power monitor, it is necessary to prevent the output from being buried in the log amp dark current. To improve the reception SN, the gain of the optical amplifier is modulated, There is also a configuration for synchronous detection.

Chattering may occur at the time of connection, and a low-pass filter is inserted in the output stage of the reflected light monitor photodetector to detect the average value.
It is effective to improve SN. Also, in order to avoid uncertain which operating state the optical output is in when the optical amplifier is turned on, the hysteresis comparator is
A circuit is needed to reset the output to Vhigh.

[0026]

As is apparent from the above description, according to the present invention, the optical output level from the optical amplifier is automatically set in the open state where the output of the optical amplifier is not connected to the optical fiber which is the transmission line. As a result, it is possible to prevent troubles due to high optical power of the field worker and also to prevent oscillation of reflected light from the open end due to feedback to the amplifier.

[Brief description of drawings]

FIG. 1 is a principle block diagram of the present invention.

FIG. 2 is a block diagram showing a first aspect of the principle of the present invention.

FIG. 3 is a block diagram showing a second aspect of the principle of the present invention.

FIG. 4 is a block diagram showing a third aspect of the principle of the present invention.

FIG. 5 is a block diagram showing a third aspect of the principle of the present invention.

FIG. 6 is a diagram for considering an output end open detection condition in the embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of an optical amplifier with an output end release detection circuit according to an embodiment of the present invention.

FIG. 8 is a time chart explaining the operation of the amplifier of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical amplifier 2 ... Transmission path 3 ... Open end 4 ... Reflected light detection means 5 ... Gain control means

Claims (9)

[Claims] 1. The output of the optical amplifier (1) is transmitted through a transmission line (2).
Reflected light detecting means (4) for detecting an increase in reflected light from the open end (3) when not being sustained with the optical fiber of
The output power of the optical amplifier (1) is decreased according to the increase of the reflected light, and the reflected light detecting means (4) is provided when the optical amplifier (1) is reconnected to the optical fiber of the transmission line (2). (3) An optical amplifier with an output end release detection circuit, comprising a gain control means (5) for detecting the decrease of reflected light by (1) and increasing the optical power again. 2. The reflected light detecting means (4) is a two-input, two-output optical brancher or optical coupler at the output end of the optical amplifier (1), and the output of the optical brancher or optical coupler is One is connected to the open end (3) of the transmission line (2), the other is non-reflection terminated so that reflected light does not return, and one of the input side of the optical branching device or optical coupler is The light is incident on an optical receiver connected to the output of the amplifier (1) and the other end of which is used to monitor the reflected light from the open end (3) of the transmission line (2). An optical amplifier with the output end release detection circuit described. 3. The output end release according to claim 2, wherein one of the output terminals of the optical branching device or the optical coupler is connected to a photodetector for automatic gain or output power control of the optical amplifier (1). Optical amplifier with detection circuit. 4. The output end release detection according to claim 2 or 3, wherein an optical variable attenuator is inserted after the optical amplifier (1) as means for increasing or decreasing the output power of the optical amplifier (1). Optical amplifier with circuit. 5. The optical amplifier (1) is an Er-doped fiber amplifier, and as a means for increasing or decreasing the output power of the Er-doped fiber amplifier, the pump power of the pump laser is increased or decreased by changing the bias current. An optical amplifier with an output end release detection circuit according to claim 2 or 3. 6. The optical amplifier (1) is a semiconductor optical amplifier, and as a means for increasing or decreasing the output power of the semiconductor optical amplifier, a bias current is varied to increase or decrease. An optical amplifier with the output end release detection circuit described. 7. A detection signal of reflected light from the open end is amplified to a predetermined level by using an amplifier, and when the reflected light increases, exceeds a first threshold value and becomes a constant output voltage (V1). Then, when the reflected light decreases and falls below the second threshold value, a constant voltage (V2) is input to the hysteresis comparator,
A control signal for increasing or decreasing the output of the optical amplifier is obtained.
An optical amplifier with an output end release detection circuit according to claim 1. 8. The output of the hysteresis comparator is fed back to the reference voltage of the APC circuit of the optical amplifier (1) so that the output power of the optical amplifier is kept constant while the reflected light is increased or decreased. An optical amplifier with an output end release detection circuit according to claim 7. 9. The output end release according to claim 4, wherein the optical output power is controlled in the optical amplification section, and the control signal according to claim 7 is input to the optical variable attenuator. Optical amplifier with detection circuit.