JP2006121747A - Remote excitation type wavelength multiplexed light transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the deviation of optical levels of a plurality of channels multiplexed wavelength multiplexed light, in an optical transmission system for remotely exciting an amplifying optical fiber for amplifying wavelength multiplexed light. <P>SOLUTION: Wavelength multiplexed light output from a transmitting station 10 is amplified by an EDF 41 and transmitted to a receiving station 20. An excitation light, generated from a light source 45 installed in the receiving station 20 is supplied to the EDF 41. A control circuit 44 analyzes the wavelength characteristics of the wavelength multiplexed light amplified by the EDF 41 and controls the light emission power of the light source 45 so as to reduce the deviation of light levels of a plurality of channels multiplexed to the wavelength multiplexed light. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a system for remotely controlling an amplifier that amplifies the wavelength multiplexed light in a system that transmits wavelength multiplexed light. In particular, it relates to a remote pumping system for optical fiber amplifiers.

  In an information-oriented society in recent years, an optical fiber has been widely used as a transmission path for transmitting information. The optical fiber is excellent not only for high-speed or large-capacity data transmission but also for long-distance transmission.

  However, even in an optical fiber excellent in long-distance transmission, a signal transmitted through the optical fiber is attenuated as the transmission distance becomes longer. For this reason, for example, in a long-distance optical transmission system connecting between cities and continents, a relay node is usually provided at predetermined intervals, and a signal is amplified at each relay node and transferred to the next relay node.

  Various forms have been developed as optical amplifiers for amplifying optical signals, and an optical fiber amplifier is known as one of them. In particular, in the 1.55 μm band, rare earth doped optical fiber amplifiers into which a rare earth material such as erbium is injected are widely used. The rare earth-doped optical fiber amplifier has a configuration in which a rare earth material or the like injected into an optical fiber is excited by excitation light input separately from signal light, and the signal light is amplified by the excitation energy.

  By the way, in the case of data transmission between continents or the like, a submarine cable is often used particularly in the case of high-speed communication. This submarine cable is usually an optical fiber cable, and an optical amplifier is provided at predetermined intervals. That is, in such a long-distance optical transmission system, an optical amplifier such as an optical fiber amplifier is often submerged in the seabed.

  However, when an optical amplifier or the like submerged in the sea floor is broken or deteriorated, the optical amplifier or the like must be lifted to the sea in order to repair or replace it, and maintenance work is difficult. On the other hand, in order to suppress failure, deterioration, etc. as much as possible, extremely high reliability is required as compared with a normal optical amplifier, and it is necessary to use expensive parts, so that the manufacturing cost increases significantly.

As one means for dealing with the above problem, a remote excitation method has been proposed. The remote pumping system is a configuration in which a light source (and a circuit for controlling the light source) that supplies pumping light to the optical fiber amplifier is provided at a position away from the optical fiber amplifier. Usually, as shown in FIG. A light source for excitation light is provided in the vicinity of the transmitter or receiver. That is, in the remote excitation method, a light source or a control circuit that is likely to cause failure or deterioration is provided on the land, and an optical fiber portion that hardly causes failure or deterioration among the components of the optical fiber amplifier (in the figure, Only EDF (erbium-doped fiber) can be configured to sink to the seabed. Therefore, it is possible to construct a system that is easy to maintain without making the quality (reliability) of the optical fiber amplifier unnecessarily high, and it is possible to reduce the cost.
Japanese Patent Laid-Open No. 4-361583 JP-A-8-204647

  By the way, the amount of information transmitted through a network has been dramatically increased. Under such circumstances, research and development have been conducted on technologies for increasing the capacity of transmission lines. Wavelength division multiplexing (WDM) transmission is a technique for increasing the capacity of transmission lines. Wavelength multiplexing transmission is a method of multiplexing and transmitting a plurality of signal lights having different wavelengths via one optical transmission line, and can transmit information for each wavelength (channel). Then, it has been proposed to introduce this wavelength division multiplexing transmission system into the above-described optical transmission system having a remote excitation configuration. FIG. 26B shows an example in which wavelength division multiplexed light is transmitted in an optical transmission system having a remote excitation configuration.

  When a wavelength-multiplexed light is amplified using an optical fiber amplifier (EDF) in a remote pumping system, the optical fiber is usually maintained at a constant value by maintaining the pumping light power output from the pumping light source (Pump). The amplification operation of the amplifier is stabilized. When pumping light is supplied to the optical fiber amplifier, the wavelength multiplexed light is collectively amplified. That is, when wavelength multiplexed light multiplexes a plurality of channels, signals of a plurality of channels having different wavelengths are collectively amplified.

  However, in general, the amplification factor by the optical fiber amplifier has wavelength dependency. For this reason, if the excitation control of the optical fiber amplifier is not appropriate, the amplification factor for each channel multiplexed in the wavelength division multiplexed light is not uniform, and the optical level differs between the channels. In wavelength division multiplex transmission, as the number of multiplexed channels on one optical fiber increases, a larger pumping energy is required. Therefore, it is desirable to adjust the operation of the optical fiber amplifier in accordance with the number of channels.

  However, the existing remote pumping system has not performed control in consideration of the wavelength dependence of the amplification factor of the optical fiber amplifier and the number of multiplexed channels. For this reason, the level deviation of each channel may occur, or noise may increase due to an inappropriate level of signal light.

  An object of the present invention is to provide a method for reducing the deviation of the optical level of each channel multiplexed in wavelength multiplexed light in an optical transmission system that remotely controls an optical fiber amplifier that amplifies wavelength multiplexed light. .

  The transmission system of the present invention has a configuration in which an amplification optical fiber is provided on a transmission path through which wavelength multiplexed light is transmitted between a transmission station and a reception station, and excitation light is supplied to the amplification optical fiber from a remote location. In a transmission system, the light emission power of a light source that supplies pumping light to the amplification optical fiber from a remote place is equalized for each light level of a plurality of signal lights multiplexed in the wavelength multiplexed light Correct as follows.

  The wavelength characteristic of the gain of the optical amplification unit is controlled by the power of pumping light supplied from the light source. Here, a part of the wavelength multiplexed light amplified by the optical amplification unit is guided to the control circuit, and the light emission power of the light source is feedback controlled according to the wavelength characteristics of the wavelength multiplexed light. Therefore, wavelength multiplexed light having a desired wavelength characteristic can be generated.

  In an optical transmission system that remotely controls an optical fiber amplifier that amplifies wavelength multiplexed light, the pump light supplied to the optical amplification unit is adjusted using the analysis result of the amplified wavelength multiplexed light. It is possible to reduce the deviation of the optical levels of a plurality of multiplexed channels.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an overall configuration diagram of an optical transmission system according to the present embodiment, and is mainly illustrated as a diagram for explaining wavelength multiplexing transmission. The transmission system of the present embodiment is configured to transmit wavelength multiplexed light as signal light from the transmitting station 10 to the receiving station 20. In this wavelength multiplexed light, signals of a plurality of channels (ch1 to chn) can be multiplexed. The wavelength multiplexed light transmitted from the transmission station 10 is amplified by one or more optical amplification units provided on the transmission path and transmitted to the reception station 20.

  Transmitters (Tx) 11-1 to 11-n output signals to be transmitted on signal lights having different wavelengths (wavelengths of the respective signal lights are λ1 to λn, respectively). These signal lights are multiplexed by the multiplexer 12 and transmitted to the transmission line 31a. That is, the transmitting station 10 outputs wavelength multiplexed light including n wavelength components (λ1 to λn) as signal light. The wavelength multiplexed light is amplified by the optical amplifying unit 32 and transmitted to the receiving station 20. The wavelength multiplexed light transmitted through the transmission line 31b is demultiplexed by the demultiplexer 21 according to the wavelength (λ1 to λn) and input to the receivers (Rx) 22-1 to 22-n, respectively. As described above, in the transmission system of the present embodiment, the signal lights transmitted from the transmitters 11-1 to 11-n are transmitted as a wavelength multiplexed light through one transmission path, and are respectively received by the receivers 22. -1 to 22-n.

  As wavelength multiplexed light transmitted from the transmitting station 10 to the receiving station 20, for example, light in the 1550 nm band is used. In this case, the wavelengths λ1 to λn on which the signals of the channels ch1 to chn are placed are selected from the range of 1530 to 1560 nm, for example.

  As the optical amplification section 32, a rare earth-doped optical fiber is used when the wavelength of the signal light is in the 1550 nm band. The rare earth doped optical fiber is, for example, an erbium doped optical fiber into which erbium or the like is injected. Hereinafter, an erbium-doped optical fiber will be described as an example of a rare earth-doped optical fiber. As is well known to those skilled in the art, an erbium-doped optical fiber is pumped by pumping light supplied separately from signal light to be amplified (wavelength multiplexed light transmitted from the transmitting station 10 in FIG. 1). Energy is given, and the signal light passing through the erbium-doped optical fiber is amplified by the excitation energy.

  The transmission system of the present embodiment has a remote pumping configuration, and a light source that generates pumping light and a circuit that controls the light source are provided at a position away from the optical amplification unit (erbium-doped optical fiber). These light sources and light source control circuits are not shown in FIG. 1, but are provided, for example, in the transmission station 10 or in the vicinity thereof, or in the reception station 20 or in the vicinity thereof.

  FIG. 2 is a diagram illustrating wavelength characteristics of optical gain (optical gain) of an erbium-doped optical fiber. FIG. 2 shows the power distribution of the light output when the input light is amplified using the erbium-doped optical fiber. However, considering that the input light has a constant power, this graph is substantially It represents the gain of an erbium-doped optical fiber. The wavelength characteristic of erbium-doped optical fiber is that when the pumping rate (Er inversion distribution rate) is high in the signal transmission wavelength band (1530 to 1560 nm), the gain on the long wavelength side is small compared to the gain on the short wavelength side. When the rate is low, the gain on the short wavelength side is smaller than the gain on the long wavelength side. That is, when the pumping rate of the erbium-doped optical fiber is high, the gain slope with respect to the wavelength is “negative”, and when the pumping rate is low, the gain slope with respect to the wavelength is “positive”.

  The excitation rate of the erbium-doped optical fiber can be controlled by the power of the excitation light. That is, when the power of the pumping light supplied to the erbium-doped optical fiber is increased, the pumping rate increases, and accordingly, the gain slope with respect to the wavelength becomes “negative”. On the other hand, when the pumping light power is reduced, the pumping rate is lowered, and the gain slope with respect to the wavelength becomes “positive”. This is shown in FIG.

  Further, if the components to be injected into the optical fiber are appropriately selected, the gain in the erbium-doped optical fiber changes substantially linearly with respect to the wavelength of the wavelength multiplexed light to be amplified, as shown by the broken line in FIG. Can be configured. In other words, if an erbium-doped optical fiber whose gain changes linearly with respect to the wavelength of the wavelength multiplexed light is provided on the transmission line, the optical level of each channel multiplexed in the wavelength multiplexed light to be transmitted Is expected to change linearly with respect to the wavelength, as shown in FIG.

  Considering this characteristic, if the optical levels of any two channels multiplexed in the wavelength multiplexed light can be matched, it is expected that the optical levels of all the channels match. In this case, when the channel with the shortest wavelength and the channel with the longest wavelength are selected from the channels included in the wavelength multiplexed light, and the optical levels of these two channels are made to coincide with each other, the optical level deviation between the channels is Expected to be minimal. That is, as shown in FIG. 4, if the shortest wavelength (λ1) is assigned to the channel ch1 and the longest wavelength (λn) is assigned to the channel chn, the optical levels of the channels ch1 to chn are equalized. For this purpose, the optical levels of the channels ch1 and chn may be detected and controlled so that they match.

  In the transmission system of the present embodiment, the characteristics described with reference to FIGS. That is, the optical level of each channel multiplexed in the wavelength multiplexed light amplified by the erbium-doped optical fiber is detected, and the power of the pumping light supplied to the erbium-doped optical fiber is changed according to the detection result, thereby erbium-doped light. Correct the wavelength characteristics (gain) of the fiber. This equalizes the light level of each channel. The transmission system of the present embodiment has a remote pumping configuration, and the pumping light is controlled at a point away from the position where the erbium-doped optical fiber is provided.

  FIG. 5 is a configuration diagram of the transmission system according to the first aspect. As described with reference to FIG. 1, the transmitting station 10 transmits wavelength multiplexed light as signal light to the transmission line. This wavelength multiplexed light carries signal light ch1 to signal light chn. Signal light ch1 to signal light chn are signal lights to which wavelengths λ1 to λn are assigned, respectively. That is, the wavelength multiplexed light includes the wavelength components of signal light ch1 to signal light chn. Wavelength multiplexed light transmitted from the transmitting station 10 is amplified in an erbium-doped optical fiber (hereinafter referred to as EDF) and transmitted to the receiving station 20.

  The EDF 41 is supplied with excitation light output from a light source (Pump) 45, and amplifies wavelength multiplexed light (signal light) by the excitation energy. The WDM coupler 42 has a function of multiplexing light of different wavelengths. When the wavelength multiplexed light output from the transmission station 10 and the excitation light output from the light source 45 are incident, the WDM coupler 42 combines them. Output. Therefore, wavelength multiplexed light and excitation light are input to the EDF 41.

  The wavelength multiplexed light transmitted through the transmission paths 43a and 43b is demultiplexed for each wavelength at the receiving station 20. The receiving station 20 will be described with reference to FIG. The wavelength multiplexed light is demultiplexed by the demultiplexer 21 for each wavelength component (λ1 to λn), and the demultiplexed light is input to the receivers (Rx) 22-1 to 22-n, respectively. Light input to the receivers 22-1 to 22-n is light having wavelengths λ1 to λn, and is signal light ch1 to signal light chn. A part of the signal light ch1 to signal light chn is branched by branch couplers 46-1 to 46-n, respectively. These branched signal light ch1 to signal light chn are input to the control circuit (cont1) 44. When receiving the branched lights of signal light ch1 to signal light chn, the control circuit 44 controls the output power of the light source 45 so as to equalize the levels. The configuration and operation of the control circuit 44 will be described in detail later.

  The excitation light output from the light source 45 is supplied to the EDF 41 to make it excited. Here, the gain characteristic of the EDF 41 is controlled by the power of the pumping light as described above. However, the power of the pumping light is adjusted by the control circuit 44 to a value that equalizes the optical level of each channel multiplexed in the wavelength multiplexed light. Therefore, the wavelength multiplexed light transmitted through the transmission paths 43a and 43b is amplified by the EDF 41 so that the optical level of each channel is equalized when received by the receiving station 20.

  The transmission system shown in FIG. 5 constitutes a feedback system as described above. In this feedback system, the gains of the EDF 41 are controlled, and the gains are controlled based on the wavelength characteristics of the wavelength multiplexed light amplified by the EDF 41.

  As described above, the transmission system shown in FIG. 5 is a remote transmission system that supplies pumping light to an erbium-doped optical fiber in consideration of the optical level of each channel multiplexed in wavelength multiplexed light in a system that transmits wavelength multiplexed light. Since the power of the excitation light source is adjusted, the deviation of the light level of each channel at the receiving station is minimized.

  If an optical amplifier incorporating the EDF 41, the control circuit 44, and the light source 45 in one unit is used instead of the system of the remote excitation configuration shown in FIG. 5, the optical level of each channel at the time of output from the optical amplifier. Even when the deviation is minimized, at the time of reception by the receiving station 20, the waveform is attenuated by transmission via the transmission path 43b, and the deviation of the optical level of each channel is not necessarily minimized. This point is improved in the remote excitation configuration as shown in FIG.

  A WDM coupler and a branch coupler will be described. As shown in FIG. 7A, the WDM coupler can multiplex lights having different wavelengths. That is, in the system of FIG. 5, when signal light (wavelength multiplexed light) and pump light are input to the WDM coupler, the two lights are combined and output from one output port. Further, as shown in FIG. 7B, the WDM coupler can demultiplex the light, which is a combination of lights having different wavelengths, based on the wavelength component. That is, when the signal light (wavelength multiplexed light) and the light combined with the excitation light are input, the WDM coupler demultiplexes the input light into the signal light and the excitation light and outputs the demultiplexed light.

  As shown in FIG. 7C, the branch coupler branches the input light at a predetermined ratio. That is, when signal light is input to the branch coupler, the signal light is branched and output as it is, and when pump light is input, the pump light is branched and output as it is. In this case, the branch coupler functions as a beam splitter. Further, as shown in FIG. 7D, when light is input from two ports, the branch coupler combines and outputs them.

  FIG. 8 is a block diagram of the control circuit 44 shown in FIG. Photodiodes (PD) 51-1 to 51-n receive signal light ch1 to signal light chn, respectively, and output a voltage corresponding to the light level. That is, each output of the photodiodes 51-1 to 51-n is a signal representing the optical level of the channels ch1 to chn. Each output of the photodiodes 51-1 to 51-n is input to the analog switches 52 and 53 as the optical level of each channel. The outputs of the photodiodes 51-1 to 51-n are input to the comparators 54-1 to 54-n, respectively.

  Each of the comparators 54-1 to 54-n compares the optical level of each channel with the threshold value Vth, and outputs the comparison result at the TTL level. This threshold value Vth is determined as follows. That is, in the wavelength division multiplexing transmission system, the signal is transmitted through a predetermined channel, but the optical level of the channel transmitting the signal is higher than the optical level of the channel not transmitting the signal. In FIG. 9, channels 2 to n transmit signals, whereas channel 1 does not transmit signals. The threshold value Vth is set as a value for determining whether or not each channel is transmitting a signal. The comparators 54-1 to 54-n output “L” level when the received detection light level is larger than the threshold value Vth. That is, the comparators 54-1 to 54-n output the “L” level when the corresponding channel is transmitting a signal.

  The output of the comparator 54-1 is input to the first selection terminal of the analog switch 52. When the “L” level is input to the first selection terminal of the analog switch 52, that is, when the output of the comparator 54-1 is the “L” level, the analog switch 52 is applied with the voltage applied to the first input terminal. Is output. That is, when channel 1 is transmitting a signal, analog switch 52 outputs the detected light level of channel 1. On the other hand, when the output of the comparator 54-1 is at “H” level, that is, when the channel 1 is not transmitting a signal, the analog switch 52 does not output the voltage applied to the first input terminal.

  A logical product of the output of the comparator 54-1 and the output of the comparator 54-2 is input to the second selection terminal of the analog switch 52. Therefore, when the output of the comparator 54-2 is at the “L” level, the “L” level is input to the second selection terminal of the analog switch 52, and the analog switch 52 applies the voltage applied to the second input terminal. Output. That is, when the channel 1 is not transmitting a signal and the channel 2 is transmitting a signal, the analog switch 52 outputs the detected light level of the channel 2.

  Thus, the analog switch 52 outputs the optical level of the channel with the shortest wavelength among the channels transmitting the signal. Similarly, the analog switch 53 outputs the optical level of the channel with the longest wavelength among the channels transmitting signals. Therefore, for example, as shown in FIG. 9, if channel 1 is not transmitting a signal and channels 2 to n are transmitting a signal, analog switch 52 outputs the optical level of channel 2, and analog The switch 53 outputs the optical level of channel n. The outputs of the analog switches 52 and 53 are input to the divider 55.

  Divider 55 is, for example, an operational amplifier. The divider 55 is a part of the feedback system described above, and operates so that the difference between the output of the analog switch 52 and the output of the analog switch 53 becomes “0”. The amplifier 56 amplifies the output of the divider 55. The excitation light source drive circuit 57 includes, for example, a power transistor, and drives the light source 45 by supplying a current according to the output of the amplifier 56. The light source 45 is, for example, a laser diode, and outputs light having power corresponding to the current supplied by the excitation light source driving circuit 57 as excitation light.

  As described above, the control circuit 44 controls the light emission power of the light source 45 so that the light level of the channel with the shortest wavelength among the channels transmitting signals matches the light level of the channel with the longest wavelength. .

  In FIG. 5 or FIG. 6, the configuration is shown in which each signal light demultiplexed by the demultiplexer 21 is further branched and input to the control circuit 44. A configuration in which part of the wavelength multiplexed light is input to the control circuit 44 and the control circuit 44 extracts the wavelength component of each channel may be employed.

  FIG. 10 is a configuration diagram of the transmission system according to the second aspect. In the system of the first aspect shown in FIG. 5, one erbium-doped optical fiber (EDF 41) is provided on the transmission path between the transmitting station 10 and the receiving station 20. In the system, two erbium-doped optical fibers (EDFs 41 and 47) are provided. In the system of the second aspect, the pumping light generated by the light source 45 is branched using the branching coupler 48, and the branched pumping lights are supplied to the EDFs 41 and 47, respectively. When supplying the pumping light to the EDF 47, the WDM coupler 49 is used to multiplex the wavelength multiplexed light and the pumping light.

Thus, in the system of the second aspect, the gains of the plurality of erbium-doped optical fibers are adjusted simultaneously.
FIG. 11 is a configuration diagram of the transmission system of the third aspect. In the system of the third aspect, an optical preamplifier 61 is further provided in addition to the system of the second aspect. The optical preamplifier 61 is an optical amplifier including, for example, an erbium-doped optical fiber and a laser light source, and amplifies wavelength multiplexed light transmitted via the transmission path 43b. The gain of the optical preamplifier 61 is controlled by the control circuit 44. As described above, in the system according to the third aspect, after the optical level deviation of each channel multiplexed in the wavelength multiplexed light is adjusted on the transmission line, the receiving station 20 corrects the level deviation again.

  With the above configuration, the following merits are obtained. That is, in the remote pumping configuration, the pumping light is attenuated in the transmission path before being transmitted to the EDF 41 or 47, so it is necessary to increase the emission power of the pumping light to a predetermined value or more. On the other hand, in order to increase the light emission power of the excitation light, a large current is required for driving the light source, and in reality, there is a limit to increase the light emission power of the excitation light. For this reason, it is not easy in practice to increase the range (dynamic range) that can be taken as the emission power of the excitation light of the light source for remote excitation. Here, since the gain of the erbium-doped optical fiber is controlled by the pumping light power input thereto, if the dynamic range of the pumping light emission power is narrow in this way, the deviation of the light level of each channel is sufficiently large. Adjustment may not be possible. The system of the third aspect improves this point. That is, by providing an optical preamplifier in the receiving station, a large dynamic range can be achieved with a small current consumption, and the optical level deviation of each channel can be adjusted efficiently.

  FIG. 12 is a configuration diagram of the transmission system of the fourth aspect. In the system of the fourth aspect, the erbium-doped optical fiber (EDF47) provided on the transmitting station 10 side is supplied with pumping light from the receiving station 20 to the erbium-doped optical fiber (EDF41) provided on the receiving station 20 side. In the configuration, excitation light is supplied from the transmission station 10. The excitation light supplied to the EDF 47 is generated by the light source 71 provided in the transmission station 10. The light source 71 may be driven with a constant current, or may be driven with ALC (automatic level control). The gain of the EDF 41 is controlled in the same manner as the system of the first aspect shown in FIG.

  According to the above configuration, the power consumption for generating the excitation light can be reduced when compared with the system of the second aspect shown in FIG. That is, in the system of the second aspect, it is necessary to transmit the excitation light generated by the light source 45 to the EDF 47, but in the system of the fourth aspect, power that can excite the EDF 41 is sufficient. Further, the transmission distance from the light source 71 to the EDF 47 is smaller than the transmission distance from the light source 45 to the EDF 47, and it is not necessary to increase the light emission power of the light source 71 so much.

  FIG. 13 is a configuration diagram of the transmission system of the fifth aspect. In the system of the fifth aspect, the number of channels (wavelength multiplexing number) for transmitting signals among the channels multiplexed in the wavelength multiplexed light is detected, and the pumping light is controlled according to the number of channels.

  In general, when a wavelength multiplexed light is amplified using an optical fiber amplifier, the larger the number of channels transmitting signals among the channels multiplexed in the wavelength multiplexed light, the larger the excitation energy is required. Also, in optical amplification in a transmission system, the amplification factor of the optical amplifier must be appropriately controlled. That is, if the amplification factor is too small, the signal light is not transmitted to the receiving device. Conversely, if the amplification factor is too large, noise increases due to nonlinear effects in the transmission path. Therefore, in a system that transmits wavelength multiplexed light while amplifying it for each relay station, it is desirable to adjust the pumping light supplied to the optical fiber amplifier in accordance with the number of channels for transmitting signals.

  In the transmission system of the fifth aspect, the pumping light supplied to the erbium-doped optical fiber is controlled in consideration of this. That is, the control circuit (cont2) 72 detects the number of channels that transmit signals, and adjusts the light emission power of the light source 71 according to the detected number.

  The branch couplers 73-1 to 73-n branch the signal lights (signal light ch1 to signal light chn) output from the transmitters 11-1 to 11-n, respectively, and guide them to the control circuit 72. That is, a part of each signal light (signal light ch1 to signal light chn) before being multiplexed by the multiplexer (multiplexer 12 shown in FIG. 1) is guided to the control circuit 72. Therefore, the control circuit 72 can detect the output levels of the transmitters 11-1 to 11-n.

  FIG. 14 is a configuration diagram of the control circuit 72 shown in FIG. The branched lights of the signal lights output from the transmitters 11-1 to 11-n are received by the photodiodes (PD) 81-1 to 81-n, respectively. The photodiodes 81-1 to 81-n output a voltage corresponding to the light reception level. That is, the photodiodes 81-1 to 81-n detect the output levels of the transmitters 11-1 to 11-n. Outputs of the photodiodes 81-1 to 81-n are input to the comparators 82-1 to 82-n, respectively.

  The comparators 82-1 to 82-n compare the voltage levels received from the photodiodes 81-1 to 81-n with a preset threshold value Vth. This threshold value Vth is a value for determining whether or not a signal is included. That is, as mentioned with reference to FIG. 9, since the optical level of the channel transmitting the signal is higher than the optical level of the channel not transmitting the signal, a threshold value Vth for determining this level difference is set. Thus, it can be determined whether or not each channel is transmitting a signal. The comparators 82-1 to 82-n output “H” level when the corresponding channel transmits a signal, and output “L” level when the signal is not transmitted.

  The analog switch 83 receives the output signals of the comparators 82-1 to 82-n. Then, the number of “H” level signals is detected from the received signals, thereby recognizing the number of channels transmitting the signals. The analog switch 83 includes n voltage setting terminals. Voltages V1 to Vn are applied to the voltage setting terminals, respectively. The analog switch 83 outputs a voltage applied to a voltage setting terminal corresponding to the number of channels. That is, if the number of channels transmitting signals is “m”, the voltage Vm is output. The voltage Vi (i = 1, 2,..., N) is a voltage value indicating the pumping light power.

  The output of the analog switch 83 is amplified by the amplifier 84 and input to the excitation light source driving circuit 85. The excitation light source drive circuit 85 includes, for example, a power transistor, and drives the light source 71 by supplying a current according to the output of the amplifier 84. The light source 71 outputs light having power corresponding to the current supplied by the excitation light source driving circuit 85 as excitation light.

Thus, the control circuit 72 controls the light emission power of the light source 71 in accordance with the number of channels transmitting signals.
FIG. 15 is a configuration diagram of the transmission system of the sixth aspect. In the system of the fifth mode shown in FIG. 13, the erbium-doped optical fiber (EDF47) is provided on the transmission path between the transmitting station 10 and the receiving station 20, but the system of the sixth mode is used. In the system, two erbium-doped optical fibers (EDFs 41 and 47) are provided. In the system of the sixth aspect, the pumping light generated by the light source 71 is branched using the branching coupler 91, and the branched pumping lights are supplied to the EDFs 41 and 47, respectively. Thus, in the system of the sixth aspect, the gains of the plurality of erbium-doped optical fibers are adjusted simultaneously.

  FIG. 16 is a configuration diagram of a transmission system according to a seventh aspect. The system according to the seventh aspect is a system that combines the first aspect shown in FIG. 5 and the fifth aspect shown in FIG. That is, the EDF 47 provided on the transmitting station side is supplied with pumping light adjusted according to the number of channels for transmitting signals, and the EDF 41 provided on the receiving station side has a deviation in the optical level of each channel. Excitation light adjusted to be minimized is supplied.

  FIG. 17 is a configuration diagram of a transmission system according to the eighth aspect. The system of the eighth aspect is a modification of the system of the seventh aspect shown in FIG. That is, in the transmission system of the eighth aspect, the wavelength multiplexed light (signal light) transmission path and the pumping light transmission path are separated, and a bidirectional pumping configuration is adopted.

  The pumping light output from the light source 71 is branched by the branching coupler 91 and supplied to the EDFs 41 and 47 as forward pumping light. At this time, when a part of the excitation light supplied from the light source 71 to the EDF 47 passes through the EDF 47 without being consumed, the excitation light (residual excitation light) that has passed through the EDF 47 as shown in FIG. ) Is demultiplexed from the wavelength multiplexed light by the WDM coupler 92 and guided to the transmission line 93b. The excitation light demultiplexed by the WDM coupler 92 is transmitted through the transmission path 93b and supplied to the EDF 41. Therefore, only the wavelength multiplexed light is guided to the transmission line 43c.

  On the other hand, the excitation light output from the light source 45 is supplied to the EDFs 41 and 47 in the same manner as the excitation light output from the light source 71. However, the excitation light output from the light source 71 is supplied as forward excitation light, whereas the excitation light output from the light source 45 is supplied as backward excitation light.

  According to the above configuration, since the residual pumping light that has passed without being consumed by the first erbium-doped optical fiber can be used as the pumping light to the second erbium-doped optical fiber, the use efficiency of the pumping light is increased.

  In the above configuration, the initial value of the distribution of the excitation light supply by the light source 45 and the light source 71 may be determined in advance. For example, in the case of a system in which the minimum number of channels to be used is “m”, the light source can be amplified so that wavelength-multiplexed light in which m-channel signals are multiplexed with only the excitation light from the light source 45 can be amplified to a predetermined level. A light emission power of 45 is set. When the number of multiplexed channels increases, the light source 71 emits light according to the increased number.

The reason why the wavelength-multiplexed light transmission path (transmission path 43c) and the pumping light transmission path (93b) between the EDF 41 and the EDF 47 are separated is as follows.
If the use efficiency of the pump light is increased as described above without separating the wavelength-multiplexed light transmission path and the pump light transmission path, in the system shown in FIG. The path 93b is removed. With such a configuration, the residual excitation light output from the light source 71 and passing through the EDF 47 is supplied to the EDF 41 via the transmission path 43c. Similarly, the residual excitation light output from the light source 45 and passing through the EDF 41 is supplied to the EDF 47 through the transmission path 43c.

  However, in an optical transmission system, an optical isolator may be provided before and after an optical amplifier or the like for the purpose of preventing reflection or the like. FIG. 18 shows an example in which optical isolators 101a and 101b are provided at the front and rear stages of the EDF 47. When the optical isolator is provided in this way, the residual excitation light output from the light source 45 and transmitted through the transmission path 43 c is blocked by the optical isolator 101 b and is not supplied to the EDF 47. That is, the reason why the wavelength-multiplexed light transmission path and the pumping light transmission path between the EDF 41 and the EDF 47 are separated is to realize bidirectional pumping while preventing reflection. If the wavelength-multiplexed light transmission line and the pumping light transmission line are separated, particularly when three or more stages of optical amplifiers are provided, it becomes easy to control the pumping light for each optical amplifier.

  FIG. 19 is a configuration diagram of the transmission system according to the ninth aspect. The system of the ninth aspect is premised on a configuration in which wavelength division multiplexed light as signal light is bidirectionally transmitted between a local station and a remote station. Here, the transmission path from the local station to the remote station is referred to as an “uplink transmission path”, and the transmission path from the remote station to the local station is referred to as a “downlink transmission path”.

  In the transmission system of the ninth aspect, a part of the wavelength multiplexed light output from the local station and amplified by the erbium-doped optical fiber (EDF 131) is guided onto the downlink transmission path for transmitting the wavelength multiplexed light from the remote station to the local station. Then, a part of the amplified wavelength multiplexed light is received by the local station. Then, the local station uses an erbium-doped optical fiber (EDF131) provided on the upstream transmission line so as to minimize the deviation of the optical levels of the channels multiplexed in the wavelength multiplexed light output from the local station and amplified. ) To adjust the power of the excitation light supplied to. Further, the power of pumping light supplied to the erbium-doped optical fiber (EDF 132) provided on the downstream transmission line is similarly adjusted.

  Details will be described below. Here, the uplink transmission path will be described. The wavelength multiplexed light output from the local station is called “wavelength multiplexed light (FL)”, and the wavelength multiplexed light output from the remote station is called “wavelength multiplexed light (FR)”.

  Wavelength multiplexed light (FL) output from the local station 110 is amplified by the EDF 131 and transmitted to the remote station 120. Excitation light generated by the light source 111 provided in the remote station 110 is supplied to the EDF 131. The wavelength multiplexed light (FL) amplified by the EDF 131 is branched by the branch coupler 133, and the branched wavelength multiplexed light (FL) is guided to the branch coupler 134 provided on the downlink transmission path. The branch coupler 134 combines the wavelength multiplexed light (FR) output from the remote station 120 and the wavelength multiplexed light (FL) branched by the branch coupler 133 and guides it to the downstream transmission path. Therefore, the local station 110 receives the wavelength multiplexed light obtained by combining the wavelength multiplexed light (FR) and the wavelength multiplexed light (FL).

  When the local station 110 receives the wavelength multiplexed light in which the wavelength multiplexed light (FR) and the wavelength multiplexed light (FL) are combined, as described with reference to FIG. Multiplexed light is demultiplexed for each wavelength. Each demultiplexed signal light is branched by a branch coupler and guided to a control circuit (cont3) 112. The operation of the control circuit 112 is basically the same as that of the control circuit 44. That is, the light emission power of the light source 111 is adjusted so that the deviation of each light level of the channel multiplexed with the received wavelength multiplexed light is minimized.

  Note that channels having different wavelengths may be used for the upstream transmission path and the downstream transmission path. That is, among the wavelengths λ1 to λn used as the wavelength multiplexed light, for example, as the wavelength multiplexed light (FL) transmitted through the upstream transmission path, the wavelengths λ1, λ3, λ5,. . . On the other hand, wavelength multiplexed light (FR) transmitted through the downstream transmission path has wavelengths λ2, λ4, λ6,. . . Is used. With this configuration, when wavelength multiplexed light obtained by combining wavelength multiplexed light (FL) and wavelength multiplexed light (FR) is input to the local station 110, the wavelength λ1 is selected from the wavelength multiplexed light. , Λ3, λ5,. . . Only the signal light having the signal can be guided to the control circuit 112. That is, only the signal light corresponding to the channel multiplexed with the wavelength multiplexed light (FL) can be input to the control circuit 112. In this case, the control circuit 112 emits light from the light source 111 so as to equalize the channel multiplexed in the wavelength multiplexed light (FL) without being affected by the wavelength multiplexed light (FR) output from the remote station 120. Power can be adjusted.

  FIG. 20 is a configuration diagram of a transmission system according to the tenth aspect. In the system of the tenth aspect, a part of the wavelength multiplexed light output from the transmitting station and amplified by the optical amplifier is sent back to the transmitting station, where the received wavelength multiplexed light is analyzed to adjust the pumping light power. To do.

  The wavelength multiplexed light output from the transmitting station 10 is amplified by the EDF 47 and transmitted to the receiving station 20. The EDF 47 is supplied with excitation light generated by a light source 71 provided in the transmission station 10. The wavelength multiplexed light amplified by the EDF 47 is branched by the branch coupler 142, and the branched wavelength multiplexed light is guided by the WDM coupler 143 and sent back to the transmitting station 10. Note that the path for returning a part of the wavelength multiplexed light to the transmitting station 10 may use a transmission path for supplying pumping light as shown in FIG. 20, or may transmit through a transmission path provided separately. It may be.

  The wavelength multiplexed light transmitted back to the transmitting station 10 is guided to the control circuit (cont4) 141 by the branch coupler 144. The control circuit 141 has the same function as the demultiplexer 21 shown in FIG. 1 or FIG. 6, and extracts the signal light corresponding to each channel by demultiplexing the received wavelength multiplexed light for each wavelength component. Then, the control circuit 141 adjusts the light emission power of the light source 71 so as to minimize the deviation of the light levels of the plurality of channels multiplexed in the wavelength multiplexed light.

  FIG. 21 is a configuration diagram of the transmission system according to the eleventh aspect. In the system of the eleventh aspect, similar to the system of the tenth aspect, a part of the wavelength multiplexed light output from the transmitting station and amplified by the optical amplifier is sent back to the transmitting station, where the received wavelength multiplexed light is converted. The pumping light power is adjusted by analysis. However, in the system of the tenth aspect, the wavelength multiplexed light is sent back to the transmitting station via the transmission path that supplies the pumping light. In the system of the eleventh aspect, the wavelength multiplexed light is transmitted toward the receiving station. In this configuration, a part of the wavelength multiplexed light is sent back to the transmitting station via the transmission path.

  The wavelength multiplexed light output from the transmitting station 10 passes through the optical circulator 151, is amplified by the EDF 47, and is transmitted to the receiving station 20. The EDF 47 is supplied with excitation light generated by a light source 71 provided in the transmission station 10. The wavelength multiplexed light amplified by the EDF 47 is branched by the branch coupler 142, and the branched wavelength multiplexed light is guided to the transmission line 43a by the optical circulator 151. This wavelength multiplexed light is transmitted via the transmission path 43a and input to the transmitting station 10.

  The wavelength multiplexed light sent back to the transmitting station 10 is demultiplexed into signal light corresponding to each channel by passing through the multiplexer 21 in the reverse direction. The signal light corresponding to each channel is branched by branch couplers 152-1 to 152-n and guided to the control circuit 112. The control circuit 112 adjusts the light emission power of the light source 71 so as to minimize the deviation of the light level of each channel.

  FIG. 22 is a configuration diagram of a transmission system according to the twelfth aspect. The system of the twelfth aspect is configured to adjust the deviation of each light level of the channel multiplexed in the wavelength multiplexed light based on the power of the residual pumping light that has passed through the erbium-doped optical fiber that amplifies the wavelength multiplexed light. It is. If the length of the transmission line and the gain characteristics of the erbium-doped optical fiber that amplifies the wavelength multiplexed light are known, the emission power of the pumping light supplied to the erbium-doped optical fiber and the erbium-doped optical fiber pass without being consumed Based on the power of the residual pumping light, it is possible to estimate deviations in the light levels of a plurality of channels multiplexed in the wavelength multiplexed light. The transmission system according to the twelfth aspect uses this characteristic.

  The wavelength multiplexed light output from the transmitting station 10 is amplified by the EDF 47 and transmitted to the receiving station 20. The EDF 47 is supplied with excitation light generated by a light source 71 provided in the transmission station 10. The residual pump light that has passed through the EDF 47 is demultiplexed from the wavelength multiplexed light by the WDM coupler 161. This residual pump light is guided to the transmitting station 10 by the branch coupler 162.

  The residual pumping light sent back to the transmitting station 10 is guided to the control circuit (cont5) 164 by the branch coupler 163. The control circuit 164 estimates the optical level deviation of the plurality of channels multiplexed in the wavelength multiplexed light based on the residual pumping light and the light emission power of the light source 71, and according to the estimation result, the optical level of each channel. The light emission power of the light source 71 is adjusted so as to minimize the deviation.

  FIG. 23 is a configuration diagram of a modified example of the transmission system of the twelfth aspect. In the system shown in FIG. 23, the positions where the WDM couplers 49 and 143 are provided are different from the transmission system of the twelfth aspect.

  FIG. 24 is a configuration diagram of a transmission system according to the thirteenth aspect. Similar to the system of the twelfth aspect, the system of the thirteenth aspect is a plurality of light multiplexed by the wavelength multiplexed light based on the power of the residual pumping light that has passed through the erbium-doped optical fiber that amplifies the wavelength multiplexed light. It is the structure which adjusts the deviation of the optical level of a channel. However, the pump light generated on the transmitting station side is adjusted in the system of the twelfth aspect, whereas the pump light generated on the receiving station side is adjusted in the system of the thirteenth aspect.

  The wavelength multiplexed light output from the transmitting station 10 is amplified by the EDF 41 and transmitted to the receiving station 20. Excitation light generated by a light source 45 provided in the receiving station 20 is supplied to the EDF 41. The residual pumping light that has passed through the EDF 41 is demultiplexed from the wavelength multiplexed light by the WDM coupler 171 and guided to the control circuit (cont5) 172. The operation of the control circuit 164 is basically the same as that of the control circuit 164 shown in FIG. 22, and a plurality of channels multiplexed in wavelength multiplexed light based on the received residual pumping light and the light emission power of the light source 45. The light level deviation of the light source 45 is estimated, and the light emission power of the light source 45 is adjusted so as to minimize the deviation of the light level of each channel according to the estimation result.

  FIG. 25 is a configuration diagram of a transmission system according to the fourteenth aspect. As in the system of the twelfth aspect, the system of the fourteenth aspect is a plurality of signals multiplexed on the wavelength multiplexed light based on the power of the residual pumping light that has passed through the erbium-doped optical fiber that amplifies the wavelength multiplexed light. It is the structure which adjusts the deviation of the optical level of a channel. However, in the system of the twelfth aspect, the residual pumping light is sent back to the transmitting station using the WDM coupler. In the system of the fourteenth aspect, the residual pumping light is sent back to the transmitting station using the reflection device. is there.

  The wavelength multiplexed light output from the transmitting station 10 is amplified by the EDF 47, passes through the reflection device 181, and is transmitted to the receiving station 20. The EDF 47 is supplied with excitation light generated by a light source 71 provided in the transmission station 10. The reflection device 181 is constituted by, for example, a fiber grating, and reflects only the light of the wavelength component of excitation light (1480 nm in this embodiment) and transmits the light of other wavelength components. Therefore, the wavelength multiplexed light output from the transmitting station 10 passes through the reflection device 181, but the residual excitation light that has passed through the EDF 47 is reflected by the reflection device 181. The reflected residual pump light is guided to the control circuit 164 by the WDM coupler 49 and the branch coupler 163. As described above, the control circuit 164 estimates the optical level deviation of the plurality of channels multiplexed in the wavelength multiplexed light based on the received residual pumping light and the light emission power of the light source 45, and according to the estimation result. The light emission power of the light source 45 is adjusted so as to minimize the deviation of the light level of each channel.

  In addition, as a method of supplying pumping light to an erbium-doped optical fiber, a forward pumping system that supplies pumping light in the same direction as the transmission direction of signal light, and a rear that supplies pumping light in a direction opposite to the transmission direction of signal light. Although there is an excitation system, the present invention is not limited to either one in each embodiment.

  In addition, when supplying pumping light to an erbium-doped optical fiber, in many embodiments, the pumping light is joined to the transmission path for signal light using a WDM coupler provided immediately before or after the erbium-doped optical fiber. However, after the signal light and the pumping light are combined on one optical fiber at the transmitting station or the receiving station, they may be transmitted to the erbium-doped optical fiber. A configuration in which signal light and excitation light are combined and transmitted may increase noise, but is advantageous in terms of cost because a transmission path for transmitting excitation light becomes unnecessary.

  Furthermore, in the examples shown in the above embodiments, the light source that generates the excitation light and the circuit that controls the excitation light are provided in the transmission station or the reception station. However, the light source and the control circuit are provided in the transmission station or the reception station. You may provide outside. However, even when the light source and the control circuit are provided outside the transmitting station or the receiving station, it is desirable to provide them in the vicinity of the transmitting station or the receiving station in view of convenience such as maintenance.

  In each of the above embodiments, an example in which wavelength multiplexed light is amplified using an erbium-doped optical fiber has been shown. However, the present invention is widely applied to various optical fiber amplifiers including rare-earth doped optical fibers. Furthermore, the present invention is not limited to an optical fiber amplifier, and can be widely applied to an optical amplifier whose gain can be remotely controlled. For example, the present invention can also be applied to a semiconductor optical amplifier.

  In each of the above embodiments, the gain of the erbium-doped optical fiber is controlled by controlling the power of the pumping light, but the erbium-doped optical fiber is adjusted by adjusting the optical level of the signal light input to the erbium-doped optical fiber. A system that controls the gain of the optical fiber may also be used. In this case, an optical level adjuster (for example, an optical attenuator) is provided in front of the erbium-doped optical fiber, and the optical level adjuster is remotely controlled.

  Further, in the above-described embodiments, the configuration in which the optical level of each channel is equalized is shown. However, the optical level of each channel may have a predetermined tendency. For example, it is possible to amplify so that the light level increases as the wavelength increases.

1 is an overall configuration diagram of a transmission system according to an embodiment. It is a figure which shows the wavelength characteristic of the optical gain of an erbium dope optical fiber. It is a figure which shows the inclination of the gain with respect to excitation light power and a wavelength. It is a figure which shows the optical level of each channel contained in wavelength multiplexing light. It is a block diagram of the transmission system of a 1st aspect. It is a figure explaining the structure of a receiving station. It is a figure explaining a WDM coupler and a branch coupler. It is a block diagram of the control circuit of FIG. It is a figure explaining the difference in the state which is not transmitting the state which is transmitting the signal. It is a block diagram of the transmission system of a 2nd aspect. It is a block diagram of the transmission system of a 3rd aspect. It is a block diagram of the transmission system of a 4th aspect. It is a block diagram of the transmission system of a 5th aspect. It is a block diagram of the control circuit 72 shown in FIG. It is a block diagram of the transmission system of a 6th aspect. It is a block diagram of the transmission system of a 7th aspect. It is a block diagram of the transmission system of an 8th aspect. It is a figure which shows the structure of the optical amplification part which provided the optical isolator. It is a block diagram of the transmission system of a 9th aspect. It is a block diagram of the transmission system of a 10th aspect. It is a block diagram of the transmission system of the 11th aspect. It is a block diagram of the transmission system of a 12th aspect. It is a block diagram of the modification of the transmission system of a 12th aspect. It is a block diagram of the transmission system of a 13th aspect. It is a block diagram of the transmission system of a 14th aspect. (A) is a figure explaining a general remote excitation system, (b) is a system block diagram at the time of introducing a remote excitation system into the system which transmits wavelength multiplexing light.

Explanation of symbols

10 transmitting station 20 receiving station 41, 47 EDF (erbium doped optical fiber)
45, 71 Light source 44, 72 Control circuit

Claims (1)

An amplification optical fiber is provided on a transmission path through which wavelength multiplexed light is transmitted between a transmission station and a reception station, and a transmission system configured to supply excitation light to the amplification optical fiber from a remote location,
Remote pumping that corrects the light emission power of a light source that supplies pumping light to the amplification optical fiber from a remote location so that the optical levels of the plurality of signal lights multiplexed in the wavelength multiplexed light are equalized. Wavelength multiplex optical transmission system.

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