JP3077705B2 - Optical fiber amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier which suppresses reflected light and can be used in an optical pulse tester.

[0002]

2. Description of the Related Art Hitherto, it has been 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 provided before and after a rare earth-doped optical fiber 01.
Are coupled to each other, and an optical coupler 03 is coupled to the incident side. An excitation light source 04 is coupled to one input end of the optical coupler 03, and the input signal and the excitation light are wavelength-division multiplexed by the optical coupler 03 so that the rare-earth-doped optical fiber 0
1 is input. 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, 1.5
The optical amplification characteristics in the μm band can be obtained, and when the input signal and the pumping light incident on the optical coupler 03 are wavelength-multiplexed and input to the rare-earth-doped optical fiber 01, the rare-earth-doped optical fiber 01 receives the pumping light. As a result, the signal light is amplified. Then, the amplified signal light is emitted through the optical filter 05. Here, a laser beam in a 0.98 μm band or a 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 serve as reflection points for signal light, and when the reflected light enters, the optical fiber amplifier may oscillate. Therefore, it is necessary to suppress the reflected light. is there.

[0004]

The amplification characteristics of the rare-earth-doped optical fiber 01 are bidirectional in principle. However, as described above, the ordinary optical fiber amplifier configuration includes the optical isolator 02, so It can be used only as an optical amplifier in the direction A in the figure). 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.

In recent years, long-distance transmission experiments have been conducted using an optical repeater using an optical fiber amplifier. For example, NTT has reported an experiment in which the optical fiber length exceeds 2000 km (Optical Fiber Communicat).
ion Conferrence OFC'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 optical fiber amplifier in the opposite 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.

[0006] 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.

[0007]

An optical fiber amplifier according to the present invention, which achieves the above object, comprises a rare earth-doped optical fiber, an optical circulator having four ports serving as light input / output terminals, and a pair of upstream and downstream optical fibers. An optical fiber amplifier interposed in a downstream optical fiber transmission line, wherein a first pump light is provided to one input side of one of two sets of input / output ports which can be input / output ports of the first optical circulator; An input optical fiber of the upstream line is connected via a first rare earth-doped optical fiber including a portion that couples the signal light with the output light, and an output optical fiber of the upstream line is connected to the output side, and The input side of the other set of input / output ports of the first optical circulator is connected to one end of a second rare earth-doped optical fiber including a portion for coupling the second pumping light and the signal light, and to the output side. The other end of the optical fiber whose end is a non-reflection end is connected, while the third pump light is connected to the input side of one of two sets of input / output ports that can be input / output ports of the second optical circulator. output optical fiber of the downstream line is connected to the third through the rare-earth doped optical fiber is connected to the downlink of the input side optical fiber and the output side comprising a moiety that binds the signal light, also, the first One end of a fourth rare earth-doped optical fiber including a portion for coupling the fourth pump light and the signal light is connected to the input side of the other set of input / output ports of the two optical circulators, and one end is connected to the output side. The other end of the optical fiber serving as a non-reflection end is connected, and the other end of the fourth rare earth-doped optical fiber connected to the second optical circulator is connected to the first optical circulator. Rare earth doped optical fiber The other end of the second rare-earth-doped optical fiber connected to the upstream connection end of the second rare earth-doped optical fiber connected to the first optical circulator is connected to the second optical circulator. Characterized in that the third rare earth-doped optical fiber is coupled to the downstream connection end side of the third rare earth doped optical fiber via a second optical coupler.

[0008]

In the optical fiber amplifier having the above construction, when the signal light from the input optical fiber on the upstream line is combined with the first pumping light and input to the first rare earth doped optical fiber, the signal light is amplified and the light is amplified. The light is input to one input port of the circulator and output from the output port. On the other hand, when there is reflected light of the output light, the output light is output from the output port to the other input port. Here, unless the second pumping light is coupled to the second rare-earth-doped optical fiber connected to the other input port, the second rare-earth-doped optical fiber acts as an optical attenuator for the reflected light. As it works, the reflected light is suppressed. In addition, the same operation is performed when signal light is input from the input optical fiber of the downlink. on the other hand,
When searching for a fault point on the uplink using an optical pulse tester, the first to fourth pump lights are respectively coupled to the first to fourth rare earth-doped optical fibers. When an optical pulse is input from the uplink, the optical pulse is amplified as described above and output to the output optical fiber of the uplink. The backscattered light is amplified by the second rare-earth-doped optical fiber, and further amplified by the third rare-earth-doped optical fiber to which the downstream input fiber of the second optical circulator is connected. Is output to the output side optical fiber. Further, when searching for a point of failure in the downlink, when an optical pulse is output from the input optical fiber of the downlink, the backscattered light at the output optical fiber of the downlink is output by the fourth rare earth doped optical fiber and the fourth optical fiber. The signal is amplified by one rare-earth-doped optical fiber and output to an output optical fiber on the upstream line.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

FIG. 1 shows a bidirectional optical fiber amplifier according to one embodiment. In the figure, reference numeral 11 denotes a first rare earth-doped optical fiber, and reference numeral 21 denotes a first optical circulator. The optical circulator 21 has four ports 21A, 21B, 21C, and 21D.
1B to 21C, 21C to 21D, 21D to 2
Light can pass through to 1A. The configuration of the optical circulator 21 is described in, for example, “An attempt to remove the polarization dependency of an optical circulator”, IEICE,
Optics and Quantum Electronics Workshop Material, OQE78-1
49, 1978, and the like. One end of the first rare earth-doped optical fiber 11 is connected to the first port 21A of the optical circulator 21, and the signal light and the pump light are input from the other end in a coupled state. I have. That is, a first pumping light coupler 31 is connected to the other end of the rare-earth-doped optical fiber 11, and a first pumping light source 41 is connected to one input end thereof. Further, a first optical fiber 61 having a first optical filter 51 interposed is coupled to the second port 21B of the first optical circulator 21. A second rare-earth-doped optical fiber 1 to which a second pumping light coupler 32 is coupled is connected to a third port 21C of the first optical circulator 21.
2 are coupled at one end, and the second excitation light coupler 3
A second pumping light source 42 for inputting pumping light to the second rare-earth-doped optical fiber 12 is coupled to the other input terminals of the two. On the other hand, the other end of the second optical fiber 62 whose one end is the non-reflection end 71 is coupled to the fourth port 21D of the first optical circulator 21. With the above configuration, the first optical fiber amplifier 101 having the input terminal 81 and the output terminal 91 and amplifying the signal light propagating in the direction A in the drawing is configured.

The second optical fiber amplifier 102 has the same configuration as the first optical fiber amplifier 101. That is, one end of the third rare-earth-doped optical fiber 13 is connected to the first port 22A of the second optical circulator 22 having the same structure as the above-described first optical circulator 21, and the third end is connected to the other end. A third excitation light coupler 33 to which the excitation light source 43 is coupled is connected, and a third optical fiber 63 having a second optical filter 52 interposed is coupled to the second port 22B. Further, one end of a fourth rare-earth-doped optical fiber 14 to which a fourth excitation light source 44 is coupled via a fourth excitation light coupler 34 is coupled to the third port 22C, Port 2 of
In 2D, the fourth optical fiber 6 whose one end is a non-reflection end 72
4 are connected at the other end. With the above configuration, the second optical fiber amplifier 102 having the input terminal 82 and the output terminal 92 and amplifying the signal light propagating in the direction B in the drawing is configured.

Further, each of the optical fiber amplifiers 101, 10
The optical fibers 65 and 66 provided with optical couplers 111 and 112 are coupled to the input ends 81 and 82 of the optical fiber 2, respectively. And the other end of the second rare-earth-doped optical fiber 12 are coupled to each other. Thereby, the third port 21C of the first optical circulator 21
Light incident on the second rare-earth-doped optical fiber 12 from
The signal is input to the input terminal 82 of the second optical fiber amplifier 102 via the second optical coupler 112. Also, the third port 22C of the second optical circulator 22
Is incident on the fourth rare earth-doped optical fiber 14 from
The signal is input to the input terminal 81 of the first optical fiber amplifier 101 via the first optical coupler 111. Thus, the optical fiber amplifier 100 of the present invention is configured.

The optical fiber amplifier 100 described above has
For example, an input optical fiber and an output optical fiber of an uplink are connected to the optical fiber amplifier 101, and an input optical fiber and an output optical fiber of a downlink are connected to the optical fiber amplifier 102, respectively.

The operation of the optical fiber amplifier 100 during signal transmission will be described. In this case, in the optical fiber amplifiers 101, 102, the pump light sources 41, 4
3, only the excitation light sources 42, 44 are not used.

Here, it is assumed that the signal light propagates in the direction A in FIG. The signal light propagating in the direction A through the optical fiber 65 passes through the optical coupler 111, is input from the input end 81, is wavelength-multiplexed with the pump light by the pump light coupler 31, and is transmitted to the first rare-earth-doped optical fiber 11. Is entered. Of the input wavelength-division multiplexed light, the pump light pumps the first rare-earth-doped optical fiber 11 and the signal light is amplified by the rare-earth-doped optical fiber 11. The amplified signal light is transmitted to the optical circulator 21 together with the spontaneous emission light.
Is input to the port 21A. The signal light and spontaneous emission light input to the port 21A are output to the port 21B and input to the optical filter 51. Only the signal light is output from the output terminal 91 by the optical filter 51. Since the path between the port 21A and the port 21B of the optical circulator 21 is unidirectional, even if the signal light output from the output terminal 91 is reflected, it does not return on the path from the port 21B to the port 21A. However, this reflected light is output from the port 21B to the port 21C, and the second rare earth-doped optical fiber 12
Propagate to At this time, since no excitation light is incident from the second excitation light source 42, the second rare-earth-doped optical fiber 12 operates as an optical attenuator for reflected light, and
According to experiments, an 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 output end 91 does not return to the first rare earth-doped optical fiber 11.

Next, a case is considered where the signal light propagates in the direction B in the figure and enters from the input terminal 82. Optical fiber 66 to B
The signal light propagating in the direction passes through the optical coupler 112, is input from the input end 82, is multiplexed with the pump light by the pump light coupler 33, and is input to the third rare earth-doped optical fiber 13. Since the rare-earth-doped optical fiber 13 is excited, the signal light is amplified, passes through the ports 22A to 22B of the optical circulator 22, and is output from the output terminal 92. Here, even if the signal light is reflected after the output end 92, the reflected light propagates from the port 22B to the port 22C to the fourth rare earth-doped optical fiber 14. At this time, since the excitation light source 44 is not used, the rare-earth-doped optical fiber 14 operates as an optical attenuator for reflected light. Therefore, the reflected light generated after the output end 92 does not return to the third rare-earth-doped optical fiber 13.

As described above, in the optical fiber amplifier 100, the signal light propagating in the direction A and the direction B is stably amplified by the optical fiber amplifiers 101 and 102, respectively.

Next, the operation when the optical pulse tester is used will be described. In this case, the optical fiber amplifier 10
Excitation light sources 42 and 44 are used in addition to the excitation light sources 41 and 43 in 1, 102. That is, a line (hereinafter, referred to as a line) connected to the optical fiber amplifier 101 using an optical pulse tester.
The pumping light source 42 is used when searching for a fault point on the uplink (referred to as an up link), and the pumping light source 44 is used for searching for a fault point on a link (hereinafter referred to as a downlink) connected to the optical fiber amplifier 102. I do.

First, a description will be given of a case where an uplink search is performed using an optical pulse tester. When the test pulse light from the optical pulse tester propagates in the direction A and is input from the input terminal 81, the optical fiber amplifier 101
, And is output to the output terminal 91. Backscattered light generated in the optical fiber after the output terminal 91 by the test pulse light enters the optical fiber amplifier 101 from the output terminal 91 and propagates from the port 21B to the port 21C. The backscattered light is multiplexed with the excitation light by the excitation light coupler 32 and input to the second rare-earth-doped optical fiber 12. Since the rare-earth-doped optical fiber 12 is excited by the excitation light, the backscattered light is amplified and then coupled to the downstream optical fiber 66 via the optical coupler 112 and input to the input end 82 of the optical fiber amplifier 102. Is done. This backscattered light propagates through the optical fiber amplifier 102 in the B direction, is amplified, and is output from the output terminal 92. As a result, the backscattered light generated in the optical fiber after the output end 91 of the upstream line can be propagated to the optical pulse tester through the downstream line, and a stable fault point search can be performed.

The same applies to the case of searching for a fault point on the downlink using an optical pulse tester. When the test pulse light of the optical pulse tester is input in the B direction from the input terminal 82, the test pulse light is amplified by the optical fiber amplifier 102 and output from the output terminal 9
2 output. Further, the backscattered light generated by the test pulse light in the optical fiber after the output terminal 92 propagates down the line in the direction A, and from the output terminal 92 to the optical fiber amplifier 10.
2 and output from port 22B to port 22C. The backscattered light is multiplexed with the pumping light by the pumping light coupler 34, input to the fourth rare earth-doped optical fiber 14, amplified, and then coupled to the upstream optical fiber 65 via the optical coupler 111. Is done. The backscattered light input from the input end 81 to the optical fiber amplifier 101 propagates through the optical fiber amplifier 101 in the direction A, is amplified, and is output from the output end 91. As a result, the downstream end 9
The backscattered light generated in the second and subsequent optical fibers can be propagated to the optical pulse tester through the uplink, and a stable fault point search can be performed.

When the pumping light sources 42 and 44 are used while the pumping light sources 41 and 43 are used, the reflection points after the output terminals 91 and 92, the rare-earth-doped optical fibers 12 and 14, and the optical fiber amplifiers 101 and 102 are configured. If the loop that generates the gain causes the operation of the optical fiber amplifiers 101 and 102 to become unstable, the operation may be unstable. The optical fiber amplifier 101
The case where the excitation light source 42 is used will be considered.
The reflected light generated after the output end 91 propagates in the up line in the direction B, and propagates through the port 21B, the port 21C, the rare-earth-doped optical fiber 12, the optical fiber 66, the input end 82, and the rare-earth-doped optical fiber 13. Then, if there is a reflection point again after the output end 92, this reflected light propagates to the port 22B -port 22C, but the pumping light source 44 is not used, so that the rare-earth-doped optical fiber 14 operates as an optical attenuator. Then, the reflected light is attenuated by the rare-earth-doped optical fiber 14. Therefore, since the reflected light generated at the reflection point after the output end 92 does not return to the rare-earth-doped optical fiber 11,
The optical fiber amplifier 101 operates stably. Further, the optical fiber amplifier 102 is similarly stable when the pump light source 14 is used.

As described above, in the present invention,
When searching for a fault point on a line, the optical pulse tester must have a structure in which a test pulse light is input to the line and receives backscattered light from the opposite line. However, a general optical pulse tester has a structure in which a light source for measurement pulse light and a receiver for backscattered light are connected to the fiber to be measured by an optical coupler. An optical pulse tester that connects the receiver to different optical fibers can be easily realized.

Next, a specific application example of the optical fiber amplifier of the present invention will be described with reference to FIGS.

FIG. 2 shows a normal signal transmission, in which 100 _1-
100 _n is the optical fiber amplifier 1 for upstream and downstream described above.
01, 102 integrated optical fiber amplifier, 201
A and 201B are transmission devices for transmission, 202A and 202B are transmission devices for reception, 301 and 302 are stations, and 401 ₁ to 40.
1 _{n + 1} and 402 _{1 to} 402 _{n + 1} indicate optical fibers in each relay section. That is, the uplink line 401 is constituted by optical fibers 401 _{1 to} 401 _{n + 1} connected between the transmission device for transmission 201A and the transmission device for reception 202A, and between the optical fibers 401 _{1 to} 401 _{n + 1} . Are each provided with an optical fiber amplifier 101. Downlink 402 is connected to transmission device 201B for transmission and transmission device 20 for reception.
2B and optical fibers 402 _{1 to} 402
_{n + 1} and each of the optical fibers 402 _{1 to} 402 _1
An optical fiber amplifier 102 is interposed between _{n + 1} . In such a configuration, if only the pumping light sources 41 and 43 (see FIG. 1) of all the optical fiber amplifiers 101 and 102 for the uplink and the downlink are used, the A in the uplink 401 and the D in the downlink 402 are used. Signal transmission can be performed stably in the B direction.

FIG. 3 shows an uplink 4 using an optical pulse tester.
01 shows a configuration for searching for a fault point, and 500 in FIG.
Is a pulse tester. The pulse tester 500 includes a transmission device 201A for transmission and a transmission device 202 for reception shown in FIG.
Instead of B, the station 301 is arranged, and its light source is connected to the optical fiber 401 ₁ of the upstream line 401 and the light receiver is connected to the optical fiber 402 _{n + 1} of the downstream line 402.

First, a description will be given of a case where a fault point search for a single repeater section optical fiber is performed. Specifically described fault point search of the optical fiber 401 _2. All of the uplink for-downlink optical fiber amplifier 101, 102 using the excitation light source 41, 43 (see FIG. 1), the optical fiber amplifier 100 optical fiber amplifier 101 for ₁ uplink in addition to the excitation light source 41, An excitation light source 42 is also used. Here, when the measurement pulse light enters the upstream line 401 from the optical pulse tester 500, the measurement pulse light is transmitted on the line 401, and the gain of each upstream optical fiber amplifier 101 and each of the relay section optical fibers 401 _{1 to} 401 _{1 n + 1} loss,
Reach the end. On the other hand, the backscattered light generated in the optical fiber 401 ₂ propagates in the direction B. The backscattered light is diverted to the down-link 402 by the up-link optical fiber amplifier 101 of the optical fiber amplifier 100 ₁ , and propagates in the down-link 402 in the B direction. As a result, it is possible to receive the backscattered light generated in the optical pulse tester 500 in the optical fiber 401 _2, it is possible to perform fault point search of the optical fiber 401 _2.

Next, a description will be given of a case where a failure point search for all the lines 401 is performed. All upstream optical fiber amplifiers 1
01 uses pump light sources 41 and 42 (see FIG. 1), and all downlink optical fiber amplifiers 102 use only pump light source 43. Here, the optical pulse tester 500 is connected to the uplink 4
At 01, a measurement pulse light is incident. The measurement pulse light is transmitted on the upstream line 401 to each of the optical fiber amplifiers 100 ₁ to 100 _1.
It receives the gain of the _n uplink optical fiber amplifier 101 and the loss of each of the optical fibers 401 _{1 to} 401 _{n + 1} and reaches the end. Optical fibers 401 _{1 to} 401 _{n + 1 in} each relay section
The backscattered light generated therein propagates in the B direction. Then, the backscattered light is diverted to the downlink 402 by each uplink optical fiber amplifier 101 and propagates in the downlink 402 in the B direction. As a result, in the optical pulse tester 500, each of the relay section optical fibers 401 _{1 to} 401 of the upstream line 401 is output.
_{Since the} backscattered light generated in _{n + 1} can be received, a fault point search can be performed.

As described above, by using the optical fiber amplifier of the present invention, it is possible to search for a fault using an optical pulse tester even in a transmission line including an optical fiber amplifier.

[0029]

As described above, according to the present invention, the same amplification characteristics as those of the prior art can be obtained, and a fault point search of the optical fiber transmission line including the optical fiber amplifier can be performed by using the optical pulse tester. It can be performed stably.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier according to one embodiment.

FIG. 2 is an explanatory diagram showing a normal use example of the optical fiber amplifier of one embodiment.

FIG. 3 is an explanatory diagram showing an example of use of the optical fiber amplifier of one embodiment when searching for a damage point.

FIG. 4 is a configuration diagram showing an optical fiber amplifier according to the related art.

[Explanation of symbols]

 11-14 Rare earth doped optical fiber 21,22 Optical circulator 31-34 Excitation light coupler 41-44 Excitation light source 51,52 Optical filter 61-66 Optical fiber 71,72 Non-reflection end 81,82 Input end 91,92 Output end REFERENCE SIGNS LIST 100 Optical fiber amplifier 101, 102 Optical fiber amplifier 111, 112 Optical coupler

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 3/06 G02F 1/35 501 H04B 10/00 JICST file (JOIS)

Claims (1)

(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 and interposed in a pair of upstream and downstream optical fiber transmission lines. A first rare earth-doped optical fiber including a portion for coupling the first pumping light and the signal light is provided on the input side of one of the two sets of input / output ports that can be the input / output ports of the first optical circulator. The input side optical fiber of the upstream line is connected via the output side, and the output side optical fiber of the upstream line is connected to the output side, and the input side of the other pair of input / output ports of the first optical circulator is connected to the input side. One end of a second rare-earth-doped optical fiber including a portion coupling the second pumping light and the signal light is connected to one end, and the other end of the optical fiber whose one end is a non-reflection end is connected to the output side. Second light The input side of one of the two sets of input / output ports, which can be the input / output ports of the calculator, is connected to the downstream side of the downstream line via a third rare earth-doped optical fiber including a portion for coupling the third pump light and the signal light. the is connected to the input side optical fiber and the output side is connected to the output side optical fiber of the down times line, also on the input side of the other set of input and output ports of said second optical circulator fourth excitation One end of a fourth rare-earth-doped optical fiber including a portion that couples light and signal light is connected to one end, and the other end of the optical fiber whose one end is a non-reflection end is connected to the output side. The other end of the fourth rare earth-doped optical fiber connected to the optical circulator is connected to the upstream connection end of the first rare earth-doped optical fiber connected to the first optical circulator via a first optical coupler. Combined and also on First second optical coupler downlink connection end of the third rare-earth doped optical fiber in which the other end side of the second rare earth doped optical fiber is connected to the second optical circulator connected to the optical circulator An optical fiber amplifier, wherein the optical fiber amplifier is coupled through the optical fiber amplifier.