JP2859385B2 - Bidirectional optical fiber amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical fiber amplifier that suppresses reflected light and can use an optical pulse tester.

<Prior Art> Conventionally, it is known that an optical fiber having a core portion doped with a rare earth element has optical amplification characteristics. FIG. 4 shows a basic configuration of an optical fiber amplifier using such a rare earth-doped optical fiber. As shown in the figure, an optical isolator 02 is coupled before and after the rare-earth-doped optical fiber 01, and an optical coupler 03 is coupled to the incident side thereof. An excitation light source 04 is coupled to one incident end of the optical coupler 03, and the input signal and the excitation light are wavelength-multiplexed by the optical coupler 03 and input to the rare-earth-doped optical fiber 01. I have. On the other hand, an optical filter 05 is coupled behind the optical isolator 02 on the output side of the rare-earth-doped optical fiber 01.

In such an optical amplifier, if an Er-doped optical fiber is used as the rare-earth-doped optical fiber 01, an optical amplification characteristic in the 1.5 μm band can be obtained, and the input signal and the pump light incident on the optical coupler 03 are wavelength-multiplexed. When the signal is input to the rare-earth-doped optical fiber 01, the rare-earth-doped optical fiber 01 is in a population inversion state by the excitation light, and as a result, the signal light is amplified. Then, the amplified signal light is emitted through the optical filter 05. Here, laser light in the 0.98 μm band or the 1.45 to 1.5 μm band is usually used as the excitation light. Since the amplified signal light includes spontaneous emission light as a noise component, an optical filter 05 is provided for the purpose of removing the spontaneous emission light. The optical isolators 02 provided before and after the rare earth-doped optical fiber 01 are for suppressing reflected light. In other words, the optical coupler 03, the optical filter 05, the optical connector and the like become reflection points of signal light, and if reflected light enters, the optical fiber amplifier may oscillate. Therefore, it is necessary to suppress the reflected light. is there.

<Problems to be Solved by the Invention> Although the amplification characteristics of the rare-earth-doped optical fiber 01 are bidirectional in principle, as described above, the configuration of a normal optical fiber amplifier includes the optical isolator 02, In the figure,
It can only be used as an optical amplifier in the (A direction). Therefore, a transmission line into which such an optical fiber amplifier is inserted is not suitable for a system used as a bidirectional transmission line.

By the way, in recent years, long-distance transmission experiments using an optical repeater using an optical fiber amplifier have been performed. For example,
An experiment with an optical fiber length exceeding 2000 km has been reported by NTT (Optical Fiber Communication Conferrence OF
C'90 PD-2 1990). In order to put such a system into practical use, it is necessary to be able to accurately ascertain the fault point of the optical fiber and the fault point of the optical fiber amplifier, and to perform quick and reliable repair. In a system in which an optical fiber amplifier is inserted in a transmission line, when grasping the state of all the transmission lines using an optical pulse tester, the backscattered light of the pulse light needs to pass through the fiber amplifier in the reverse direction. However, since the conventional optical fiber amplifier has the optical isolator as described above, there is a problem that the optical pulse tester cannot be used over the entire transmission line.

In view of such circumstances, an object of the present invention is to provide a bidirectional optical fiber amplifier that suppresses reflected light and can use an optical pulse tester.

<Means for Solving the Problems> A bidirectional optical fiber amplifier that achieves the above object is an optical fiber amplifier including a rare earth-doped optical fiber and an optical circulator having four ports serving as light input / output terminals. The other end of the first rare earth-doped optical fiber in which the excitation light and the signal light are coupled to one end is connected to the input side of one of the two sets of input / output ports that can be the input / output ports of the optical circulator. And the optical fiber is connected to the output side, while the other end of the optical fiber having a reflective surface at one end is connected to one of the input side and the output side of the other set of the optical circulator, and the input side and The other end of the output side includes a portion that couples the pump light and the signal light, and the other end of the second rare earth-doped optical fiber whose reflection surface is connected to one end is connected, Further, an optical fiber amplifier comprising a rare earth-doped optical fiber and an optical circulator having four ports serving as input / output terminals of light, wherein two sets of input / output ports that can be input / output ports of the first optical circulator are provided. A first optical fiber including a portion that couples the pump light and the signal light is connected to the input side of one set, and one end of a first rare-earth-doped optical fiber is connected to the output side. The other end of the second optical fiber having a reflection surface at one end is connected to one of the input side and the output side of the other set of one optical circulator, and the other of the input side and the output side is provided with excitation light. Two sets that include a portion for coupling with signal light and are connected to the other end of a second rare-earth-doped optical fiber having a reflection surface connected to one end, and can be an input / output port of a second optical circulator. Entering The other end of the first rare earth-doped optical fiber is connected to the input side of one set of output ports, and the third optical fiber is connected to the output side, while the other end of the second optical circulator is connected. A fourth optical fiber having a reflection surface at one end is connected to one of the input side and the output side of the set, and the other of the input side and the output side includes a portion for coupling the pump light and the signal light. A third rare-earth-doped optical fiber having a reflection surface connected to one end side is connected.

<Operation> In the bidirectional optical fiber amplifier of the first configuration, when the signal light and the pump light are combined and input to the first rare-earth-doped optical fiber, the signal light is amplified and is applied to one input side of the optical circulator. Input to a port and output from its output port.

Here, if the pump light is not input from the other input port of the optical circulator, the reflected light of the signal light amplified and output as described above is output from the other input side to the output side and propagated. In doing so, the second rare earth-doped optical fiber acts as an optical attenuator, and the reflected light is suppressed.

When the pumping light is input, the second rare earth-doped optical fiber acts as an optical amplifier, that is, the light propagates in the direction in which the reflected light travels.

On the other hand, in the bidirectional optical fiber amplifier of the second configuration, the operation when the signal light and the pump light are combined and input from one input side of the first optical circulator is the same as that described above. If the pumping light is not input to the second and third rare-earth-doped optical fibers, the reflected light in this case acts as an optical attenuator, and the reflected light is suppressed.
At this time, the second rare earth-doped optical fiber coupled to the first optical circulator suppresses the reflected light before the second optical circulator. Conversely, when pumping light is input to the second and third rare earth-doped optical fibers, they act as optical amplifiers, that is, light propagates in the direction in which the reflected light travels.

<Example> Hereinafter, the present invention will be described based on examples.

FIG. 1 shows a bidirectional optical fiber amplifier according to one embodiment. In the figure, reference numeral 1a denotes a first rare earth-doped optical fiber, and 2 denotes an optical circulator. The optical recirculator 2 is 2-A,
It has four ports 2-B, 2-C, 2-D,
Light can pass from 2-A to 2-B, 2-B to 2-C, 2-C to 2-D, and 2-D to 2-A. The configuration of the optical circulator 2 is described in, for example, "A Trial of Deletion of Dependence Dependence of Optical Circulator", IEICE, Optical and Quantum Electronics Research Group, OQE78-149, 1978, etc.

One end of a first rare-earth-doped optical fiber 1a is connected to the first port 2-A of the optical circulator 2 so that signal light and pump light are input from the other end in a coupled state. Has become. That is, the first optical coupler 3a is connected to the other end of the rare-earth-doped optical fiber 1a, and the first pumping light source 4a is connected to one input end thereof. Also, the second port 2- of the optical circulator 2
B is coupled to a first optical fiber 6a in which an optical filter 5 is interposed.

To the third port 2-C of the optical circulator 2, the other end of the second optical fiber 6b whose one end is a reflection surface 7a is connected. On the other hand, one end of a second rare-earth-doped optical fiber 1b is connected to the fourth port 2-D of the optical circulator 2, and a second optical coupler 3b and a reflection surface 7b are provided at the other end. ing. Further, a second excitation light source 4b is coupled to another input end of the optical coupler 3b. In the drawings, 8a and 8b indicate connection points.

In such an optical fiber amplifier, a case is considered in which a long-distance transmission line optical fiber is connected to the connection point 8a, and there is no reflection point including the connection point 8a on the left side of the rare-earth-doped optical fiber 1a in the drawing.

Here, a case is considered where the signal light propagates only in the direction A (left to right) in the figure, that is, only from the connection point 8a to the connection point 8b. In addition,
In this case, only the first excitation light source 4a needs to be operated.

The signal light input from the connection point 8a is wavelength-multiplexed with the pump light by the optical coupler 3a and input to the first rare earth-doped optical fiber 1a. Of the input wavelength-division multiplexed light, the pumping light pumps the first rare-earth-doped optical fiber 1a, and the signal light is amplified by the rare-earth-doped optical fiber 1a. The amplified signal light is input to the port 2-A of the optical circulator 2 together with the spontaneous emission light. The signal light and the spontaneous emission light input to the port 2-A are output to the port 2-B and input to the optical filter 5. Only the signal light is output from the connection point 8b by the optical filter 5. Optical circulator port 2-
Since the direction between A and the port 2-B is unidirectional, even if the signal light output from the connection point 8b is reflected, it does not return on the path from the port 2-B to the port 2-A. However, the reflected light propagates along a path from port 2-B to port 2-C, port 2-D, and port 2-A. On the other hand, when no pumping light is incident from the second pumping light source 4b, the second rare-earth-doped optical fiber 1b operates as an optical attenuator for reflected light propagating in the direction B (right to left) in the figure. Moreover, experiments show that attenuation of 20 dB or more can be easily realized. Therefore, in such a state, the reflected light generated at the output destination of the connection point 8b is not output to the connection point 8a.

Next, a case where signal light propagates in both directions (A direction and B direction in the figure) between the connection points 8a and 8b will be considered. The process of amplifying the signal light propagating in the direction A in the figure is as described above. Therefore, the signal light propagating in the direction B in the drawing will be described below. In this case, the excitation light source 4b also operates.

After passing through the optical filter 5, the signal light is input to the port 2-B of the optical circulator 2 and output to the port 2-C. The signal light output to the port 2-C is reflected by the reflection surface 7a and output to the port 2-D. The excitation light from the excitation light source 4b incident by the optical coupler 3b excites the second rare earth-doped optical fiber 1b, and the signal light is reflected by the reflection surface 7b. It is amplified and output to port 2-A. The signal light output to the port 2-A is a pumped first rare earth-doped optical fiber.
Amplified by 1a and output to connection point 8a.

The problem of amplifying the signal light in both directions is that the operating state of the optical fiber amplifier is affected by the reflected light generated inside the optical fiber amplifier or at the input / output end, or at a reflection point such as an optical connector on the transmission line. Is to be unstable. For example, in FIG. 1, the signal light input from the connection point 8a is output to the connection point 8b, but if there is a reflection point beyond that, the reflection light is transmitted from the port 2-B to the port 2-C and the port 2-D. , Port 2-A, and the operation as a bidirectional optical amplifier may be unstable. However, this problem limits the simultaneous use of the two pumping light sources 4a and 4b to only the search for a fault point in the optical fiber transmission line, and furthermore, the return loss at the reflection point and the amplification of the two rare earth-doped optical fibers 1a and 1b. The problem can be solved by appropriately setting the relationship with the gain. For example, if the gain of the first rare earth-doped optical fiber 1a is set to 20 dB, the return loss at the reflection point beyond the connection point 8b is set to 30 dB, and the amplification gain of the second rare earth-doped optical fiber 1b is set to 0 dB,
Port 2-B to Port 2-C, Port 2-D, Port 2-
The loop gain to A is -10 dB. In this loop gain, the noise of the optical fiber amplifier may increase and the signal waveform of the optical pulse to be transmitted may deteriorate, but at least there is no unstable element such as oscillation. When an optical pulse tester is used in an optical fiber transmission line including this bidirectional optical fiber amplifier, the influence of waveform deterioration on measurement accuracy can be reduced by averaging the received backscattered light. As a result, it is considered that the influence of the waveform deterioration on the measurement accuracy when searching for a fault point is smaller than the pulse waveform deterioration in a normal optical signal transmission system, which deteriorates the transmission quality.

FIG. 3 shows an example of a bidirectional optical fiber amplifier when a reflection point exists before and after the optical fiber amplifier. As shown in the figure, in the present embodiment, the first rare earth-doped optical fiber 11
By providing the optical circulators 12a and 12b on the front and rear sides of a, the reflected light of the signal light is suppressed. The optical circulators 12a and 12b are the same as the optical circulator 2 described above, and each port 12a-B, 1
Use 2b-B as an input / output port.

A first optical fiber 16a provided with a first optical coupler 13a is coupled to the port 12a-B of the first optical circulator 12a, and the other input end of the first optical coupler 13a is The first excitation light source 14a is coupled. And port 12a-
One end of the first rare earth-doped optical fiber 11a is connected to the output port 12a-C corresponding to B. One end of a second rare-earth-doped optical fiber 11b is coupled to ports 12a-D of the optical circulator 12a, and a second optical coupler 13b and a reflection surface 17b are provided at the other end. Further, a second excitation light source 14b is coupled to the other input end of the second optical coupler 13b. Optical circulator 12a
The other end of the second optical fiber 16b whose one end is a reflection surface 17a is connected to the port 12a-A.

On the other hand, the other end of the first rare earth-doped optical fiber 11a is connected to a port 12b-A of a second optical circulator 12b, and the second optical circulator 12b is the same as that of the above-described embodiment (FIG. 1). Is the same as That is, the port
A third optical fiber 16c having an optical filter 15 interposed is coupled to 12b-B, and a reflection surface 17c is connected to one end of the port 12b-C.
The other end of the fourth optical fiber 16d provided is coupled,
A third rare earth-doped optical fiber 1 is connected to port 12b-D.
One end of 1c is coupled and the other end is connected to a third optical coupler 13
c and a reflection surface 17d, and a third excitation light source 14c is coupled to the other incident end of the optical coupler 13c. In the figure, reference numerals 18a and 18b indicate connection points.

Here, a case is considered where the signal light propagates only in the direction A (left to right) in the figure, that is, from the connection point 18a to the connection point 18b, as in the above-described embodiment. In this case, only the first excitation light source 14a needs to be operated.

The signal light is wavelength-multiplexed with the pumping light from the pumping light source 14a by the optical coupler 13a and input to the ports 12a-B of the first optical circulator 12a. The input wavelength multiplexed light is output to the ports 12a-C, the pump light pumps the first rare earth doped optical fiber 11a, and the signal light is amplified by the rare earth doped optical fiber 11a. The amplified signal light and the spontaneous emission light are transmitted to the port 12b of the second optical circulator 12b.
The signal is input to A, output from the ports 12b-B, and only the signal light is output from the connection point 18b by the optical filter 15.
Port 1 which is a unidirectional section of the optical circulators 12a and 12b
2a-B to port 12a-C and port 12b-A to port
Since the rare-earth element-doped optical fiber 11a is interposed between the ports 12b-B, even if the signal light output to the port 12b-B is reflected, the ports 12b-B to 12b-A and 12a-
There is no return on the route of C, ports 12a-B. If the pumping light sources 14b and 14c are not operated, the second and third rare-earth-doped optical fibers 11b and 11c operate as optical attenuators for the reflected light, so that the output destination of the port 12b-B or the second Before the optical circulator 12b, for example, port 1
The reflected light generated at 2a-A is not output to ports 12a-B. Therefore, the amplification of the signal light propagating in the direction A in the figure is equivalent to the configuration in FIG.

Next, consider a case where signal light propagates in both directions A and B in the figure. The process of amplifying the signal light propagating in the A direction is as described above. Therefore, the signal light propagating in the B direction is considered below. In this case, the excitation light sources 14b and 14c also operate.

After the signal light passes through the optical filter 5, the port 12b-B
And output to ports 12b-C. Port 12b−
The signal light output to C is reflected by the reflection surface 17c, and
Output to 2b-D. Since the excitation light of the excitation light source 14c incident on the optical coupler 13c is reflected by the reflection surface 17d, the excitation light excites the third rare-earth-doped optical fiber 11c, and the signal light is reflected by the reflection surface 1d.
Since the light is reflected by 7d, it is amplified both round trip and third round by the third rare-earth-doped optical fiber 11c and output to the port 12b-A. The signal light output to the port 12b-A is amplified by the first rare-earth-doped optical fiber 11a excited by the pump light incident from the port 12a-B of the first optical circulator 12a, and input to the ports 12a-C. Is done. The signal input to ports 12a-C is output to ports 12a-D. The excitation light of the excitation light source 14b input by the optical coupler 14b excites the second rare-earth doped optical fiber 11b, and the signal light is reflected by the reflection surface 17b.
Therefore, the light is amplified in both directions by the rare-earth-doped optical fiber 11b and output to the ports 12a-A. Port 12a−
The signal light output to A is reflected by the reflection surface 17a and
Output to 2a-B.

When an optical signal is amplified in both directions, there is a possibility that the operation becomes unstable due to the reflected light. However, as in the embodiment shown in FIG. 1, the case where three pump lights are used simultaneously is limited to only the search for a fault point in the optical fiber transmission line, and the return loss at the reflection point and the three rare earth-doped optical fibers 11a, 11a, 11b, 11C
Can be solved by appropriately setting the relationship with the amplification gain.

In the above embodiment, the ports 2-C and 2-D of the optical circulator 2, the ports 12a-D and ports 12a-A of the optical circulator 12a, and the optical circulator 12b
There is no operational problem even if the ports 12b-C and 12b-D are interchanged.

FIG. 3 shows a specific application example of the present invention. Reference numeral 100 denotes a bidirectional optical fiber amplifier of the present invention (shown in FIGS. 1 and 2). At the time of ordinary signal light transmission, as shown in FIG. 3 (a), a unidirectional optical fiber using only one pump light is used so that the amplification directions of all the optical fiber amplifiers are the same. Used as an amplifier. On the other hand, FIG. 3 (b) shows a case where the status of the transmission line is grasped using an optical pulse tester. The optical pulse tester uses a wavelength within the amplification band of the optical fiber amplifier and passing through the optical filter. Each optical fiber amplifier is used as a bidirectional optical fiber amplifier using the remaining pump light. In the configuration of the transmission path, instead of the transmission device for transmission 110, an optical pulse tester 130 is connected, and an optical pulse for measurement is incident. The optical pulse incident from the transmitting side is transmitted through the optical fiber transmission line while being lost by the optical fiber and amplified by the optical fiber amplifier. The backscattered light generated in the transmission path is again lost by the optical fiber, amplified, and input to the optical pulse tester. By analyzing the backscattered light, the entire section including the optical fiber amplifier can be tested at one time. Transmission equipment for reception
Even if an optical pulse tester 130 is connected instead of 120 and a measuring light pulse is incident in the opposite direction to the propagation direction of the signal light, the entire section including the optical fiber amplifier can be tested in a single operation.

As described above, by using the present invention, an optical pulse tester can be used even in a transmission line including an optical fiber amplifier, and the entire transmission line section can be tested using this. it can.

<Effects of the Invention> As described above, according to the present invention, by using an optical circulator at least one of before and after the rare-earth-doped optical fiber, reflected light can be suppressed, and furthermore, light can be suppressed. By measuring the long-distance transmission line including the fiber amplifier once with the optical pulse tester, the fault point of the optical fiber and the fault point of the optical fiber amplifier can be accurately grasped, and the repair can be performed quickly and reliably.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a bidirectional optical fiber amplifier according to one embodiment, FIG. 2 is a block diagram showing a bidirectional optical fiber amplifier according to another embodiment, and FIG. 3 is a bidirectional optical fiber according to the embodiment. FIG. 4 is an explanatory diagram showing an example of use of the amplifier, and FIG. 4 is a configuration diagram showing an optical fiber amplifier according to the prior art. In the drawings, 1a, 1b, 11a, 11b, 11c are rare earth-doped optical fibers, 2, 12a, 12b
Is an optical circulator, 3a, 3b, 13a, 13b, 13c are optical couplers, 4
a, 4b, 14a, 14b, 14c are excitation light sources, 5, 15 are optical filters, 6a, 6
b, 16a, 16b, 16c, 16d are optical fibers, 7a, 7b, 17a, 17b, 17c, 1
7d is a reflection surface, and 8a, 8b, 18a, and 18b are connection points.

Continuation of the front page (56) References JP-A-1-231030 (JP, A) JP-A-2-144527 (JP, A) JP-A-4-29123 (JP, A) (58) Fields investigated (Int) .Cl. ⁶ , DB name) H01S 3/10 H01S 3/07 H01S 3/17 G02F 1/35 501 H04B 10/00 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. An optical fiber amplifier comprising a rare earth-doped optical fiber and an optical circulator having four ports serving as light input / output terminals, wherein two sets of input / output ports which can be input / output ports of the optical circulator are provided. The other end of the first rare-earth-doped optical fiber in which the excitation light and the signal light are coupled to one end is connected to the input side of one set, and the other end is connected to the optical fiber at the output side. The other end of the optical fiber having a reflection surface at one end is connected to one of the input side and the output side of the other set, and the other side of the input side and the output side is a portion that couples pump light and signal light. And a second rare-earth-doped optical fiber having a reflection surface connected to one end thereof and the other end of the second rare-earth-doped optical fiber connected thereto. 2. An optical fiber amplifier comprising a rare earth-doped optical fiber and an optical circulator having four ports serving as light input / output terminals, wherein two sets of input / output ports which can be input / output ports of a first optical circulator are provided. A first optical fiber including a portion for coupling the pump light and the signal light is connected to the input side of one set of the output ports, and one end side of the first rare earth doped optical fiber is connected to the output side. The other end of the second optical fiber having a reflection surface at one end is connected to one of the input side and the output side of the other set of the first optical circulator, and the other of the input side and the output side is The other end of the second rare-earth-doped optical fiber, which includes a portion that couples the excitation light and the signal light and has a reflection surface connected to one end, is connected to the other end, and serves as an input / output port of the second optical circulator. obtain The other end of the first rare earth-doped optical fiber is connected to the input side of one set of the input / output ports of the pair, and the third optical fiber is connected to the output side, while the second optical circulator is A fourth optical fiber having a reflection surface at one end is connected to one of the input side and the output side of the other set, and a portion for coupling pump light and signal light to the other of the input side and the output side And a third rare-earth-doped optical fiber having a reflection surface connected to one end thereof is connected.