1. Field of the Invention
This invention relates to an optical amplifier repeater for use with an optical amplification multi-stage repeating system.
2. Description of the Related Art
In the field of very long haul optical transmission across an ocean over several thousands kilometers, development of an optical amplification multi-stage repeating system which employs a repeating system in which a plurality of optical amplifier repeaters are interposed at a plurality of stages like a chain in an optical transmission line in order to compensate for the loss involved in transmission of signal light is proceeding. An optical amplifier repeater amplifies and outputs signal light inputted thereto as it is, and allows achievement of considerable reduction in cost by reduction in number of parts in each repeater and is expected to achieve improvement in reliability comparing with a conventional a regeneration repeater which performs, after it converts an optical signal inputted thereto into an electric signal, synchronization regeneration (to re-arrange code pulses to correct time positions) and waveform regeneration (to return code pulses into pulses having a correct waveform free from a distortion) and then converts the resulted electric signal back into an optical signal to be outputted. Further, it has an advantage in that it is also possible to increase the transmission rate by improvement in terminal equipments on the opposite sides of it in future.
By the way, in an optical amplification multi-stage repeating system which employs such an optical amplifier repeater as described above, it is necessary to normally supervise the conditions of each optical amplifier repeater and each optical transmission line and, when a trouble occurs, promptly specify and repair a location where the trouble has occurred. Accordingly, it is demanded to provide an optical amplifier repeater which can achieve an increase in reliability in detection of occurrence of a trouble and specification of the trouble occurrence location and allow realization of stabilized communication.
A construction of an optical amplification multi-stage repeating system is shown in FIG. 13. The optical amplification multi-stage repeating system is constructed such that, as shown in FIG. 13, a downstream side terminal equipment 1 and an upstream side terminal equipment 2 are connected to each other by an ascending line 4 and a descending line 5 each formed from an optical fiber by way of a plurality of optical amplifier repeaters 3. Supervision of the condition of the ascending line 4 is performed by introducing part of signal light sent from the downstream side terminal equipment 1 using the ascending line 4 into the descending line 5 at each of the optical amplifier repeaters 3 and measuring the signal light at the downstream side terminal equipment 1. 0n the other hand, supervision of the condition of the descending line 5 is performed by introducing part of signal light sent from the upstream side terminal equipment 2 using the descending line 5 into the ascending line 4 at each of the optical amplifier repeaters 3 and measuring the signal light at the upstream side terminal equipment 2. It is to be noted that, in the following description, an optical transmission route for introducing signal light of the ascending line into the descending line or introducing signal light of the descending line into the ascending line will be referred to as loop back path.
Further, detection of a trouble point on an optical transmission line is performed by sending a predetermined optical pulse or the like from the downstream side terminal equipment 1 using the ascending line 4, introducing part of reflected light of the ascending line 4 into the descending line 5 at each of the optical amplifier repeaters 3 and measuring the reflected light at the downstream side terminal equipment 1. Meanwhile, detection of a trouble point of the other transmission line is performed by sending a predetermined optical pulse or the like from the upstream side terminal equipment 2 using the descending line 5, introducing part of reflected light of the descending line 5 into the ascending line 4 at each of the optical amplifier repeaters 3 and measuring the reflected light at the upstream side terminal equipment 2. It is to be noted that, in the following description, an optical transmission route for introducing reflected light of the ascending line into the descending line or introducing reflected light of the descending line into the ascending line will be referred to as return path.
A construction of the conventional optical amplifier repeater having a loop back path described above will be described with reference to FIG. 14. The optical amplifier repeater shown includes an ascending optical amplifier 6A, a descending optical amplifier 6B, a first optical coupler 7 and a second optical coupler 8. Each of the first optical coupler 7 and the second optical coupler 8 is a two-input two-output optical coupler having two input ports and two output ports.
An input side 4A of the ascending line is connected to the input side of the ascending optical amplifier 6A, and a first input port 7A of the first optical coupler 7 is connected to the output side of the ascending optical amplifier 6A. An output side 4B of the ascending line is connected to a first output port 7D of the first optical coupler 7. An input side 5A of the descending line is connected to the input side of the descending optical amplifier 6B, and a first input port 8A of the second optical coupler 8 is connected to the output side of the descending optical amplifier 6B. An output side 5B of the descending line is connected to a first output port 8D of the second optical coupler 8. A second output port 7E of the first optical coupler 7 and a second input port 8B of the second optical coupler 8 are connected to each other by a first optical transmission line 9A, and a second input port 7B of the first optical coupler 7 and a second output port 8E of the second optical coupler 8 are connected to each other by a second optical transmission line 9B.
Thus, signal light sent from the input side 4A of the ascending line is amplified by the ascending optical amplifier 6A and sent to the output side 4B of the ascending line by way of the first input port 7A and the first output port 7D of the first optical coupler 7. Meanwhile, signal light sent from the input side 5A of the descending line is amplified by the descending optical amplifier 6B and sent to the output side 5B of the descending line by way of the first input port 8A and the first output port 8D of the second optical coupler 8. Further, part of signal light inputted to the first optical coupler 7 is outputted from the second output port 7E of the first optical coupler 7 and sent to the output side 5B of the descending line by way of the first optical transmission line 9A and the second input port 8B and the first output port 8D of the second optical coupler 8. Part of signal light inputted to the second optical coupler 8 is outputted from the second output port 8E of the second optical coupler 8 and sent to the output side 4B of the ascending line by way of the second optical transmission line 9B and the second input port 7B and the first output port 7D of the first optical coupler 7. In this manner, an ascending loop back path is constituted from the first optical coupler 7, the first optical transmission line 9A and the second optical coupler 8 while a descending loop back path is constituted from the second optical coupler 8, the second optical transmission line 9B and the first optical coupler 7.
Subsequently, a construction of another conventional optical amplifier repeater having a loop back path and a return path described above will be described with reference to FIG. 15. The optical amplifier repeater shown includes an ascending optical amplifier 10A, a descending optical amplifier 10B, a first optical coupler 11, a second optical coupler 12, a third optical coupler 13 and a fourth optical coupler 14. Each of the first to fourth optical couplers 11, 12, 13 and 14 is a two-input two-output bidirectional optical coupler having two input ports and two output ports.
The input side of the ascending optical amplifier 10A is connected to an input side 4A of an ascending line, and the output side of the ascending optical amplifier 10A is connected to a first input port 11A of the first optical coupler 11. A first output port 11D of the first optical coupler 11 is connected to an output side 4B of the ascending line. The input side of the descending optical amplifier 10B is connected to an input side 5A of a descending line, and the output side of the descending optical amplifier 10B is connected to a first input port 13A of the third optical coupler 13. A first output port 13D of the third optical coupler 13 is connected to an output side 5B of the descending line.
A first input port 12A of the second optical coupler 12 is connected to a second output port 11E of the first optical coupler 11 by way of a first optical transmission line 15A, and a first output port 12D of the second optical coupler 12 is connected to a second input port 13B of the third optical coupler 13 by way of a second optical transmission line 15B. A first input port 14A of the fourth optical coupler 14 is connected to a second output port 13E of the third optical coupler 13 by way of a third optical transmission line 15C, and a first output port 14D of the fourth optical coupler 14 is connected to a second input port 11B of the first optical coupler 11 by way of a fourth optical transmission line 15D. A second input port 14B of the fourth optical coupler 14 is connected to a second input port 12B of the second optical coupler 12 by way of a fifth optical transmission line 15E.
Thus, signal light sent from the input side 4A of the ascending line is amplified by the ascending optical amplifier 10A and sent to the output side 4B of the ascending line by way of the first input port 11A and the first output port 11D of the first optical coupler 11. Part of the signal light inputted to the first optical coupler 11 is outputted from the second output port 11E of the first optical coupler 11 and sent to the output side 5B of the descending line by way of the first optical transmission line 15A, the second optical coupler 12, the second optical transmission line 15B and the third optical coupler 13. By this, a loop back path for the ascending line is constructed. Further, reflected light from the output side 4B of the ascending line is sent to the output side 5B of the descending line by way of the first optical coupler 11, the fourth optical transmission line 15D, the fourth optical coupler 14, the fifth optical transmission line 15E, the second optical coupler 12, the second optical transmission line 15B and the third optical coupler 13. By this, a return path for the ascending line is constructed.
Meanwhile, signal light sent from the input side 5A of the descending line is amplified by the descending optical amplifier 10B and sent to the output side 5B of the descending line by way of the first input port 13A and the first output port 13D of the third optical coupler 13. Part of the signal light inputted to the third optical coupler 13 is outputted from the second output port 13E of the third optical coupler 13 and sent to the output side 4B of the ascending line by way of the third optical transmission line 15C, the fourth optical coupler 14, the fourth optical transmission line 15D and the first optical coupler 11. By this, a loop back path for the descending line is constructed. Further, reflected light from the output side 5B of the descending line is sent to the output side 4B of the ascending line by way of the third optical coupler 13, the second optical transmission line 15B, the second optical coupler 12, the fifth optical transmission line 15E, the fourth optical coupler 14, the fourth optical transmission line 15D and the first optical coupler 11. By this, a return path for the descending line is formed.
Further detailed description of the loop back paths and the return paths in the second conventional optical amplifier repeater described above will be given below. For the convenience of description, the optical transmission lines in the optical amplifier repeater are divided into 16 paths with the directions taken into consideration, and reference characters from a to p are applied to them as shown in FIG. 15. In particular, a normal loop back path of the ascending line is constituted from the paths a-g-i-e. A normal loop back path of the descending line is constituted from the paths d-k-m-b. Further, a normal return path of the ascending line is constituted from the paths c-n-o-i-e. A normal return path of the descending line is constituted from the paths f-j-p-m-b.
Since the maximum transmission distance of an optical amplification multi-stage repeating system ranges over up to several thousands kilometers, it is necessary to accurately detect occurrence of a trouble as well as to specify and rapidly repair a location at which the trouble has occurred with a high degree of accuracy. However, according to the prior art shown in FIG. 14, while, for example, in the ascent, part of signal light sent from the input side 4A of the ascending line by way of the optical amplifier 6A is sent to the output side 5B of the descending line by way of the first optical coupler 7, the first optical transmission line 9A and the second optical coupler 8, since a loop (annular light transmission path) is formed from the first optical coupler 7, the first optical transmission line 9A, the second optical coupler 8 and the second optical transmission line 9B, circulation light which has circulated the loop once, twice or more is superimposed as noise with part of the signal light for supervision of a trouble. The noise (in the specification of the present application, such noise will be referred to as circulation noise) makes a factor to deteriorate the signal to noise ratio (SNR) of signal light, and detection of a trouble cannot be achieved with a high degree of accuracy. It is to be noted that this quite similarly applies to the descent.
Meanwhile, according to the second conventional optical amplifier repeater shown in FIG. 15, the loop back paths have the same problem as the conventional optical amplifier repeater of FIG. 14. In particular, referring to FIG. 15, as an abnormal loop back path of the ascending line, paths a-g-i-k-m-g-i-e are constructed while, as abnormal loop back paths of the descending line, paths d-k-m-g-i-k-m-b are constructed, and the signal to noise ratio is deteriorated by light which circulates in the abnormal loop back paths.
Further, as abnormal return paths of the ascending line, paths c-n-o-i-k-m-g-i-e (hereinafter referred to as first abnormal return path), paths c-n-l-j-p-m-g-i-e (hereinafter referred to as second abnormal return path) and further paths c-n-l-j-h-n-o-i-e (hereinafter referred to as third abnormal return path) are constructed. As abnormal return paths of the descending line, paths f-j-p-m-g-i-k-m-b (hereinafter referred to as first abnormal return path), paths f-j-h-n-o-i-k-m-b (hereinafter referred to as second abnormal return path) and further paths f-j-h-n-l-j-p-m-b (hereinafter referred to as third abnormal return path), and the signal to noise ratio is deteriorated by circulation light which circulates in those abnormal return paths.
In this manner, according to the prior art, there is a problem in that light which circulates between a plurality of optical couplers constituting loop back paths and return paths is superimposed as circulation noise with signal light or reflected light for detection of a trouble and deteriorates the signal to noise ratio (SNR) of the signal light or reflected light, resulting in failure in detection of a trouble with a high degree of accuracy. Further, the conventional optical amplifier repeaters have another problem in that the intensity of signal light is sometimes rendered unstable by a variation in optical loss caused by deterioration of an optical transmission line with respect to time or the like.