JP4771530B2 - Optical amplifier - Google Patents

Description

  The present invention relates to an optical amplifier and an optical communication system, and more particularly, to an optical amplifier and a wavelength multiplexed optical communication system that amplify wavelength multiplexed light.

  With the spread of the Internet and image transmission services, the amount of information transmitted over the network is rapidly increasing, and in order to cope with this, the introduction of a wavelength division multiplex (WDM) optical communication system has been introduced. It has been advanced. Further, wavelength division multiplexing optical communication systems have been introduced not only in long-distance trunk lines but also in urban ring (metro ring) networks.

  In a long-haul trunk line system, optical amplifiers are usually provided at predetermined intervals, and each optical amplifier emits wavelength multiplexed light in an ALC (Automatic Level Control) mode or an AGC (Automatic Gain Control) mode depending on the situation. Amplify. Here, the ALC mode is an operation mode in which the output of the optical amplifier is held at a constant level, and the AGC mode is an operation mode in which the gain of the optical amplifier is held at a constant value.

  Each optical amplifier normally operates in the ALC mode in order to stabilize the level diagram. Then, when the number of wavelengths of wavelength multiplexed light increases or decreases, the operation mode of each optical amplifier is switched from the ALC mode to the AGC mode in accordance with a control signal from the terminal station in order to suppress level fluctuation. Here, the time constant (response time) of ALC is set to several tens to several hundreds of milliseconds in order to easily realize the ALC circuit and suppress the influence of PDL (Polarization Dependent Loss). On the other hand, the time constant (response time) of AGC is set to several tens of milliseconds. The “ALC time constant (response time)” is not unambiguous, but for example, when the input / output level of the optical amplifier fluctuates, it is excited so that the output level returns to a certain level that should be maintained. It means the time until the optical power or loss in the variable attenuator is adjusted appropriately. On the other hand, the “time constant (response time) of AGC” is not unambiguous, but for example, when the input / output level of the optical amplifier changes, the gain of the optical amplifier returns to a constant value that should be maintained. Means the time until the pumping light power is properly adjusted. Incidentally, an optical amplifier having an ALC mode and an AGC mode is described in detail in, for example, Japanese Patent Application Laid-Open No. 2000-151515.

  In an urban ring network, a plurality of optical nodes are normally connected in a ring shape, and the above-described optical amplifier is provided in each optical node. Here, in the urban ring network, the number of wavelengths of wavelength division multiplexed light is changed more frequently than the long-distance trunk system for the following reasons.

(1) A path is set / deleted between any optical nodes according to demand.
(2) In IP over WDM suitable for an IP network, it is desired that a protection function be provided by the WDM system. Here, for this protection function, in consideration of a request for bit rate independence, a method of switching wavelengths in the optical layer layer is effective. However, this method causes a transient increase / decrease in the number of wavelengths.

(3) In the future, a time lending service for wavelengths is expected to be provided. In this case, paths are frequently set / deleted between arbitrary optical nodes.
Thus, in an urban ring network, the number of wavelengths of wavelength multiplexed light frequently changes. The optical amplifier at each optical node amplifies the wavelength multiplexed light while appropriately switching the operation mode every time the number of wavelengths changes.
JP-A-11-112434 JP 2000-151515 A JP 2000-232433 A

  By the way, in order to reduce the cost of an optical communication system, it is basically necessary to reduce the cost of components constituting an optical node or an optical amplifier. In one embodiment, a configuration using a relatively inexpensive photodiode as the light receiving element is implemented.

  However, such a light receiving element generally has a narrow dynamic range, and the output level of the optical amplifier needs to be stable in consideration of tilt of wavelength multiplexed light, variation in loss characteristics of optical components, changes in the external environment, and the like. There is. For example, when a 10 Gbps signal is amplified by an optical preamplifier, the dynamic range of an inexpensive light receiving element is about 10 dB. Here, the variation of the loss of the duplexer provided in the receiving circuit of the optical node is about ± 2 dB, and the tilt of the wavelength multiplexed light (the wavelength dependency of the optical level) is about ± 2 dB. Also, assuming that the change in ambient temperature is 60 degrees, the light level fluctuates up to about 1.8 dB when the length of the transmission path is 100 km. Therefore, when the output level of the optical amplifier becomes unstable, a reception error occurs.

However, with existing optical amplifiers, it has been difficult to suppress fluctuations in the output level for the following reasons.
(1) As described above, the optical amplifier normally operates in the ALC mode, and operates in the AGC mode when the number of wavelengths of the wavelength multiplexed light changes. At this time, the wavelength number information is notified by a control signal transmitted via each optical node, for example. However, this control signal is usually interpreted after being converted into an electric signal at each node, and then converted into an optical signal again, and the next optical node is subjected to 3R (signal reproduction, waveform reproduction, timing reproduction) operation. To be transferred to. For this reason, it may take several hundred milliseconds to several seconds for the wavelength number information to arrive at each optical node after the wavelength number of the wavelength multiplexed light has changed. On the other hand, in the ALC mode, normally, the output level of the optical amplifier is controlled to be a constant value corresponding to the number of wavelengths of the wavelength multiplexed light. At this time, the number of wavelengths is notified by the above-described wavelength number information. Therefore, the optical amplifier maintains the output level corresponding to the wavelength number before the change from the change of the wavelength number to the notification of the wavelength number information (in the above example, several hundred milliseconds to several seconds). As a result, the level for each wavelength fluctuates. For example, when the number of wavelengths of wavelength multiplexed light increases from 3 wavelengths to 5 wavelengths, the optical amplifier operating in the ALC mode is multiplexed with 3 wavelengths until the wavelength number information is received. As a result, the output level for each wavelength is greatly reduced.

  (2) In the AGC mode, when the input level fluctuates, the output power changes so that a constant gain is maintained by adjusting the pumping light power according to the fluctuation. However, in general, a value longer than the response time of the optical amplifier is set as the time constant in the AGC mode. Here, the “response time of the optical amplifier” is not unambiguous, but for example, it corresponds to the pumping light power in the amplification medium since the power of the pumping light supplied to the amplification medium of the optical amplifier changes. It means the time until an excited state is obtained. For this reason, when the optical amplifier is operating in the AGC mode, when the input level fluctuates, the pumping light power cannot follow it, and a state in which an appropriate gain cannot be obtained transiently occurs. For example, when the total input power is reduced due to a decrease in the number of wavelengths of the wavelength multiplexed light, the optical amplifier operating in the AGC mode has a period until the pump light power is properly adjusted. Since the wavelength multiplexed light is amplified on the assumption that the state before the decrease continues, the output level for each wavelength temporarily increases.

  (3) In the AGC mode, the pumping light power is controlled so that the ratio between the input level and the output level is constant, but the signal gain is increased by the ASE light generated in the optical amplifying medium (for example, erbium-doped fiber). Deviates from the target value.

  (4) In the AGC mode, the pumping light power is controlled so that the ratio between the input level and the output level is constant, so that the gain control system becomes unstable when the input light stops in the protection operation of the communication system. Become. Therefore, when a transition is made from a state in which the input light is stopped to a state in which the signal light is input, a surge (here, a phenomenon in which the output level of the optical amplifier temporarily becomes excessive) may occur. is there.

As described above, when the input level changes (including the case where the number of wavelengths of wavelength multiplexed light changes), the output level of the existing optical amplifier may fluctuate.
It is an object of the present invention to obtain a stable output level in an optical amplifier that amplifies wavelength multiplexed light.

  The optical amplifier of the present invention is an optical communication system having a function of transmitting wavelength multiplexed light between a plurality of nodes each including an optical amplifier and notifying a control signal indicating wavelength multiplexed light between the plurality of nodes. An optical amplifier provided at an arbitrary node among the plurality of nodes to be used, the optical amplifying medium for amplifying wavelength-multiplexed light, and the wavelength-multiplexed light provided before or after the optical amplifying medium. Attenuating optical attenuator; first monitoring means for monitoring the gain of the optical amplifying medium; second monitoring means for monitoring the output level of the optical amplifier; and gain of the optical amplifying medium at a constant value. In the optical attenuator, the gain control circuit to be held and the output level of the optical amplifier are held at a value corresponding to the wavelength multiplexing number indicated by the control signal notified from another node. Level control circuit for controlling the loss having. Then, when the output level of the optical amplifier changes, the level control circuit controls the loss in the optical attenuator, so that the changed output level returns to a level corresponding to the wavelength multiplexing number indicated by the control signal. The time constant of the level control circuit, which is the time required until the time until the node that created the control signal transmits the control signal until it is notified to the optical amplifiers of all other nodes in the optical communication system It is set longer than the time required for. The gain control circuit calculates a gain of the optical amplifying medium based on detection means for detecting input power and output power of the optical amplifying medium, and input power and output power detected by the detecting means. A calculation unit; and a pumping light control unit that controls the power of the pumping light to be supplied to the optical amplification medium according to the gain calculated by the calculation unit. Then, the calculating means calculates the gain of the optical amplifying medium based on a ratio between the input power obtained by adding a predetermined value corresponding to the spontaneous emission light power generated in the optical amplifying medium and the output power. To do.

  When the number of wavelengths of the input wavelength multiplexed light changes, the output level of the wavelength multiplexed light changes accordingly. However, at this time, the output level for each wavelength does not fluctuate. The level control circuit adjusts the loss in the optical attenuator so that the output level returns to the original state at a speed corresponding to the set time constant when the output level of the wavelength multiplexed light varies. Thereafter, when the new number of wavelengths of the wavelength multiplexed light is notified by the control signal, the operation of the optical amplifier is controlled according to the control signal. Here, the time constant of the level control circuit is longer than the time until the control signal is notified to the optical amplifier. For this reason, the output level of the wavelength multiplexed light does not fluctuate greatly by the level control circuit during the period from when the number of wavelengths of wavelength multiplexed light changes until the optical amplifier receives the control signal. Therefore, the fluctuation of the output power for each wavelength of the wavelength multiplexed light is small.

  In the optical amplifier, the level control circuit may fix the loss in the optical attenuator when notified by the control signal that the number of wavelengths of wavelength multiplexed light has changed. According to this configuration, when the number of wavelengths of wavelength division multiplexed light changes, the operation mode of the optical amplifier changes from a state operating in ALC (automatic level control) to a state operating in AGC (automatic gain control). be able to.

  When calculating the gain of the optical amplifying medium, the influence of spontaneous emission light is eliminated. For this reason, the output level is accurately controlled based on an appropriate gain. Further, the calculation means calculates a gain based on a ratio between input power and output power to which a predetermined value corresponding to spontaneous emission light power is added. Therefore, even when the signal input is stopped, “zero” is not input to the calculating means, and it is avoided that the operation of the level control circuit becomes unstable.

  According to the present invention, in an optical amplifier that amplifies wavelength multiplexed light, the output power of each signal light included in the wavelength multiplexed light is stabilized. In particular, even when the number of wavelengths of wavelength multiplexed light changes, the output power of each signal light is held at a constant value. In addition, since the influence of spontaneous emission light can be eliminated, the operation of the level control circuit is stabilized.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of an example of an optical communication system in which an optical amplifier according to an embodiment of the present invention is used. Here, an optical communication system having a ring-shaped transmission path is taken, but the present invention is not limited to this, and can be applied to an optical communication system in which two points or multiple points are connected by an arbitrary route. .

  This optical communication system has a configuration in which a plurality of optical nodes 1 are connected in a ring shape by optical fibers. Here, each optical node is connected by two optical fibers (clockwise line and counterclockwise line), and a double ring is formed. Then, the wavelength multiplexed light is transmitted through the clockwise line and the counterclockwise line. Specifically, in the clockwise line, the wavelengths λ2, λ4, λ6,. . . Are used to transmit the working signal, and the wavelengths λ1, λ3, λ5,. . . Is used to transmit a standby signal (or a signal with a low priority). On the other hand, in the counterclockwise line, the wavelengths λ1, λ3, λ5,. . . Is used to transmit the working signal, and the wavelengths λ2, λ4, λ6,. . . Is used to transmit a standby signal (or a signal with a low priority).

  Each optical node 1 includes an optical amplifier, amplifies the input wavelength multiplexed light, and transmits it to the next optical node. Each optical node 1 can accommodate one or more client lines. Each optical node 1 receives from the client line a function (drop function) for guiding an arbitrary signal light in the wavelength multiplexed light received from the trunk line (clockwise line or counterclockwise line) to the client line. A function (add function) is provided for multiplexing the signal light to the main line.

  FIG. 2 is a diagram illustrating a control system of the optical communication system according to the embodiment. Each optical node 1 includes a supervisory control (OSC) unit 10. Here, the OSC unit 10 generates / transfers a monitoring control signal for monitoring and controlling the operating state of the optical communication system, and controls operations of the optical amplifier and the like according to the received monitoring control signal. Specifically, the receiver 11 converts the supervisory control signal received from the corresponding optical node into an electrical signal and passes it to the control circuit. The control circuit 12 controls the optical amplifier and the like in the own node according to the received monitoring control signal, and updates the monitoring control signal as necessary. The transmitter 13 converts the monitoring control signal into an optical signal and transmits it to the next optical node. Here, the OSC unit 10 performs 3R (signal reproduction, waveform reproduction, timing reproduction) processing on the monitoring control signal. The monitoring control signal is transmitted using a predetermined wavelength (for example, 1510 nm, 1625 nm). In addition, this supervisory control signal transmits at least wavelength number information indicating the number of wavelengths of wavelength multiplexed light transmitted via the trunk line.

Thus, the state of the optical communication system according to the embodiment is monitored by the monitoring control signal, and the operation is controlled.
FIG. 3 is a configuration diagram of an optical node. Here, as described above, the optical node 1 has a function of amplifying wavelength-multiplexed light transmitted via the trunk line, a drop function, and an add function. Note that the wavelength multiplexed light includes OSC light that transmits a plurality of signal lights and a supervisory control signal.

  The input wavelength multiplexed light is amplified by the optical preamplifier 22 and then demultiplexed for each wavelength by the demultiplexer 23. Note that the WDM coupler 21 provided in the preceding stage of the optical preamplifier 22 guides the OSC light included in the wavelength multiplexed light to the receiver 11. The signal light demultiplexed for each wavelength by the demultiplexer 23 is guided to the corresponding optical switch 24. The multiplexer 25 multiplexes a plurality of signal lights output from the optical switch 24. Then, the wavelength multiplexed light output from the multiplexer 25 is amplified by the post amplifier 26 and then output to the trunk line. Note that the WDM coupler 27 provided at the subsequent stage of the optical preamplifier 26 multiplexes the OSC light with the wavelength multiplexed light.

  The optical switch 31 selects signal light guided from two trunk lines (clockwise line and counterclockwise line). Further, the transponder (add / drop circuit) 32 guides the signal light selected by the optical switch 31 to the client line, and transmits the signal light received from the client line via the optical splitter (branch coupler) 33 to the optical switch 34a, To 34b. The optical switches 34a and 34b guide the received signal light to the corresponding optical switch 24. Each optical switch 24 selects signal light from the trunk line or signal light from the client line in accordance with an instruction from the control circuit 12, and guides the selected signal light to the multiplexer 25 or the transponder 32. .

  4 and 5 are diagrams for explaining a protection operation in the optical communication system according to the embodiment. Here, “protection operation” means an operation of resetting a path or a route for transmitting a signal when a line failure or the like occurs.

  FIG. 4 shows a state in which no failure has occurred (non-protection state). Here, the solid line indicates a path for transmitting the working system signal, and the broken line indicates a path for transmitting the backup system signal (or a signal having a low priority).

  In FIG. 4, three paths (λ # 5, λ # 7, and λ # 9) for transmitting the working system signal from the optical node 1a to the optical node 1d are connected to the counterclockwise line, and the optical node 1b transmits the optical signal. Two paths (λ # 1, λ # 3) for transmitting the working system signal to the node 1c, and two paths (λ # 2) for transmitting the protection system signal from the optical node 1a to the optical node 1b , Λ # 4) is set. Also, on the clockwise line, three paths (λ # 6, λ # 8, λ # 10) for transmitting the working signal from the optical node 1d to the optical node 1a, and from the optical node 1c to the optical node 1b. Two paths (λ # 2, λ # 4) for transmitting the working system signal and two paths (λ # 1, λ #) for transmitting the protection system signal from the optical node 1b to the optical node 1a 3) is set.

  The number of wavelengths of wavelength multiplexed light transmitted via the trunk line differs from location to location. For example, wavelength multiplexed light in which five wavelengths (λ1, λ3, λ5, λ7, λ9) are multiplexed is transmitted on the counterclockwise line between the optical node 1b and the optical node 1c. Therefore, in this case, the optical preamplifier provided for the counterclockwise line in the optical node 1c amplifies the wavelength multiplexed light in which the five wavelengths are multiplexed. On the other hand, no path is set on the counterclockwise line between the optical node 1d and the optical node 1a. Therefore, in this case, wavelength multiplexed light is not input to the optical preamplifier provided for the counterclockwise line in the optical node 1a.

  FIG. 5 shows a state in which a failure has occurred between the optical node 1a and the optical node 1b (protection state). In this case, among the paths set using the transmission path in which the failure has occurred, the path for transmitting the working system signal is reset through another path. Specifically, the three paths (λ # 5, λ # 7, λ # 9) set on the counterclockwise line in FIG. 4 are the three paths set on the clockwise line ( λ # 5, λ # 7, λ # 9). Similarly, the three paths (λ # 6, λ # 8, λ # 10) set on the clockwise line in FIG. 4 are the three paths (λ ##) set on the counterclockwise line. 6, λ # 8, λ # 10). On the other hand, standby signals (or signals with low priority) that have been transmitted using the transmission path where the failure has occurred are stopped.

  As a result, wavelength multiplexed light in which two wavelengths are multiplexed is input to the optical node 1c via the counterclockwise line. That is, when the failure shown in FIG. 5 occurs, the number of wavelengths of the wavelength multiplexed light input to the optical preamplifier provided for the counterclockwise line in the optical node 1c is changed from “5” to “2”. Will be reduced. On the other hand, wavelength multiplexed light in which three wavelengths are multiplexed is input to the optical node 1a via a counterclockwise line. That is, when the failure shown in FIG. 5 occurs, the number of wavelengths of the wavelength multiplexed light input to the optical preamplifier provided for the counterclockwise line in the optical node 1a is changed from “0” to “3”. Will increase.

  Thus, in the optical communication system of the embodiment, when protection is started due to the occurrence of a failure, the number of wavelengths of wavelength multiplexed light input to the optical amplifier varies in one or a plurality of nodes. In the optical amplifier according to the embodiment, even when the number of wavelengths of the wavelength multiplexed light varies in this way, the variation in the output level is suppressed to a small level.

  FIG. 6 is a diagram showing a basic configuration of the optical amplifier according to the embodiment of the present invention. This optical amplifier is provided in each optical node and corresponds to, for example, the optical preamplifier 22 or the optical postamplifier 26 shown in FIG. Then, the wavelength multiplexed light input in the AGC mode or ALC mode is amplified.

  This optical amplifier has a two-stage amplification configuration and includes erbium-doped fibers (EDF) 41a and 41b as optical amplification media. The optical amplifying medium is not limited to the erbium-doped fiber, but may be another form of rare-earth doped fiber. Further, a variable attenuator (VATT) 42 for adjusting the light level is provided between the EDF 41a and the EDF 41b.

  The AGC circuit 43a includes an excitation light source for supplying excitation light to the EDF 41a. Then, the input level / output level of the EDF 41a is monitored, and the pumping light power is adjusted so that the gain of the EDF 41a is maintained at a predetermined constant value. Similarly, the AGC circuit 43b includes an excitation light source for supplying excitation light to the EDF 41b. Then, the input level / output level of the EDF 41b is monitored, and the pumping light power is adjusted so that the gain of the EDF 41b is held at a predetermined constant value. The ALC circuit 44 controls the loss in the variable attenuator 42 so that the output level of the optical amplifier (that is, the output level of the EDF 41b) is held at a predetermined value.

  When the optical amplifier having the above configuration operates in the AGC mode, the loss in the optical attenuator 42 is fixed to an appropriate value. The AGC circuits 43a and 43b appropriately control the gains of the EDFs 41a and 41b. On the other hand, when the optical amplifier operates in the ALC mode, the ALC circuit 44 controls the loss in the variable attenuator 42 so that the output level is maintained at a value corresponding to the number of wavelengths of the wavelength multiplexed light. At this time, the gains of the EDFs 41a and 41b are held at appropriate values by the AGC circuits 43a and 43b. Note that the number of wavelengths of the wavelength multiplexed light is notified using the supervisory control signal described with reference to FIG.

In the optical amplifier configured as described above, the time constant of AGC and the time constant of ALC are set as follows.
(1) The AGC time constant is made sufficiently shorter than the response time of the EDFs 41a and 41b.

(2) The time constant of ALC is set longer than the time required for the supervisory control signal to be transmitted to each optical node.
FIG. 7 is a diagram for explaining the response time of the optical amplification medium. The gain of the optical amplifying medium is controlled by the excitation light. That is, if the pumping light power decreases, the gain of the optical amplifying medium decreases accordingly, and if the pumping light power increases, the gain of the optical amplifying medium increases accordingly. However, it takes a certain time from when the pumping light power changes until the gain of the optical amplifying medium is adjusted according to the change. This response time is generally longer when the pumping light power is lower than when the pumping light power is increased. The response time of the optical amplifying medium is determined depending on the material, and is, for example, about several milliseconds in the case of EDF.

  FIG. 8 is a diagram for explaining the time constant of AGC. Here, as shown in FIG. 8A, the AGC circuit (43a or 43b) calculates the gain based on the ratio between the input power and the output power of the EDF (41a or 41b), and the calculated gain. And a pump light source that generates pump light corresponding to the comparison result.

  As shown in FIG. 8B, during the period when the input power is stable, the output power is also stable, and the gain is also constant. However, the gain of the EDF depends on the input power. Specifically, for example, when the input power is reduced when a certain amount of pumping light is applied, the gain of the EDF tends to increase accordingly. For this reason, when the input power changes, the AGC circuit needs to appropriately adjust the pumping light power accordingly. Here, even when the input power changes, the gain is kept constant if the AGC circuit follows it in a sufficiently short time.

  The pumping light power is basically controlled based on the input power and the output power. That is, in order for the pumping light power to be adjusted appropriately according to the change in input power, a process for calculating the gain based on the ratio between the input power and the output power, and the calculated gain and reference value A process for obtaining the difference is required. For this reason, it is difficult to make the response time of the AGC circuit zero.

  However, it is possible to make this response time sufficiently small. That is, the response time of the AGC circuit greatly depends on the response time of the amplifier that operates as a divider for calculating the gain and the response time of the amplifier for deriving the difference. As a result, the response time of the AGC circuit is shortened. A method for controlling the EDF using a high-speed AGC circuit is described in, for example, OFC2001 PD38-1.

  In the optical amplifier of the embodiment, the AGC time constant (response time) is set to be shorter than the response time of the optical amplification medium. As an example, the time constant of AGC is set to 1/100 or less of the response time of the optical amplifying medium. Therefore, even when the input power changes, the gain is kept constant. That is, even if the number of wavelengths of wavelength multiplexed light changes, the output power per wave does not change unless the input power per wave changes.

  FIG. 9 is a diagram for explaining the time constant of ALC. Here, as shown in FIG. 9A, the ALC circuit (44) has a function of controlling the loss in the variable attenuator (42) based on the output power.

  As shown in FIG. 9B, when the input power changes, the output power changes accordingly. Here, the ALC circuit adjusts the loss in the variable attenuator so that the output power is kept constant. At this time, the change in the loss is expressed using, for example, an exponential function. Specifically, the change in loss is defined, for example, such that the output power is expressed by the following equation. However, “ΔP” represents a change amount of the total output power when the input power is changed, and “T” represents a predetermined time constant. “T” represents a time measured from the time when the input power changes, and “α” represents the output power at “t = 0”.

Pout = ΔP (1−e ^{−t / T} ) + α
According to this equation, the error in the output power at the time when the time T has elapsed since the change in the input power is as small as “ΔP / e”. Hereinafter, the time T may be referred to as “ALC time constant” or “ALC response time”.

  The ALC time constant (response time) is set to be longer than the time required for the supervisory control signal to be transmitted to each optical node. Specifically, for example, the ALC time constant is set to be 10 times or more the time required for the supervisory control signal to be transmitted to each optical node. Here, the time during which the supervisory control signal is transmitted to each optical node is given by the sum of the delay time, signal propagation time, protection time, and wavelength number processing time at each optical node. Note that the delay time in each optical node includes the time required for the 3R operation. The signal propagation time depends on the length of the transmission path. The protection time is defined as a standard such as SONET, and is 50 to 100 milliseconds. The wavelength number processing time is a time required from when the number of wavelengths of wavelength multiplexed light is notified by the monitoring control signal until the setting of the ALC circuit is changed according to the number of wavelengths.

  For example, in order to suppress the fluctuation of the output level of the optical amplifier when the number of wavelengths of wavelength multiplexed light is 40 and the number of wavelengths changes to 0.1 dB or less, according to the above calculation formula, “T = 100 seconds "is obtained.

  If the response time of the optical amplifying medium and the time constant of AGC, or the response time of the optical amplifying medium and the time constant of ALC, or the time constant of AGC and the time constant of ALC are the same or close to each other, Oscillation or the like may occur in the control system. However, in the optical amplifier of the embodiment, the time constant of AGC is set sufficiently short with respect to the response time of EDF. On the other hand, the ALC time constant is set to be longer than the time required for the supervisory control signal to be transmitted to each optical node, and is sufficiently longer than the response time of the EDF. Therefore, the control system does not become unstable due to oscillation or the like.

  FIG. 10 is a configuration diagram of the optical amplifier according to the embodiment. In this optical amplifier, the input wavelength multiplexed light is amplified by the EDFs 41a and 41b, and the output level thereof is adjusted by the variable attenuator 42. The gains of the EDFs 41a and 41b are controlled by the AGC circuits 43a and 43b, and the loss in the variable attenuator 42 is controlled by the ALC circuit 44.

  The gain of the EDF 41a is controlled based on the input power and output power of the EDF 41a. Here, the input power of the EDF 41a is detected by the light receiving element (PD) 51a, and the output power of the EDF 41a is detected by the light receiving element (PD) 52a.

  The amplifier 53a calculates the ratio between the input power and the output power. However, as is well known, amplification using EDF causes spontaneous emission (ASE: Amplified Spontaneous Emission). That is, if the input power to the EDF is “Pin”, the output power is “GPin + Pase”. Here, “G” represents EDF gain, and “Pase” represents ASE optical power. Therefore, if the EDF gain is calculated based on the detected input power and output power, the following values are obtained.

Calculated gain = Output power / Input power
= (GPin + Pase) / Pin
= G + Pase / Pin
= G + Pase / mPinch
In this equation, “m” represents the number of wavelengths (number of channels) of wavelength multiplexed light, and “Pinch” represents the input power per channel. That is, if the gain is calculated using only the input / output power detected by the light receiving element, an error due to ASE occurs. The error depends on the number of wavelengths of wavelength multiplexed light.

  Therefore, in this embodiment, the ASE correction value 1 is added to the input power value detected by the light receiving element 51a in the adder 54a. Here, the ASE correction value 1 corresponds to a value obtained by dividing the ASE optical power output from the EDF 41a by the gain of the EDF 41a, and is given by, for example, “NFhνΔf”. “NF (= 2nsp)” represents a noise figure, “hν” represents energy, and “Δf” represents a band. At this time, since “NF” depends on the input power per channel, it may be set based on the input power. If the AS correction value 1 is determined in this way, the influence caused by the error caused by the ASE and depending on the number of wavelengths is eliminated.

The amplifier 55a drives the excitation light source (LD) 56a based on the difference between the gain calculation value output from the amplifier 53a and a predetermined gain G1 set in advance.
Thus, in the AGC circuit 43a, the ASE correction value 1 is added to the input power value detected by the light receiving element 51a. Therefore, the gain of the EDF 41a is appropriately detected, and the gain of the EDF 41a is correctly controlled using the detection result. In addition, when the input light to the EDF 41a stops, the AGC circuit 43a operates assuming that light corresponding to the ASE correction value 1 is input to the EDF 41a. Therefore, a state where “zero” is input to the AGC circuit 43a is avoided, and the control system is stabilized.

The configuration and operation of the AGC circuit 43b that controls the gain of the EDF 41b are basically the same as those of the AGC circuit 43a. Therefore, the description is omitted.
The response time (AGC time constant) of the AGC circuits 43a and 43b is sufficiently shorter than the response time of the EDFs 41a and 41b as described above. This is realized, for example, by increasing the speed of the amplifiers 53a, 53b, 55a, and 55b.

  The ALC circuit 44 controls the loss in the variable attenuator 42 so that the output power of the optical amplifier matches the set value corresponding to the number of wavelengths of the wavelength multiplexed light. Here, the output power of the optical amplifier is detected by the light receiving element (PD) 52b. Further, the number of wavelengths of the wavelength multiplexed light is notified by the monitoring control signal described above. However, the set value corresponding to the number of wavelengths of the wavelength multiplexed light is corrected by the ASE correction value 3. Note that the ASE correction value 3 may be determined according to a method described in Japanese Patent Application Laid-Open No. 2000-232433, for example. The variable attenuator 42 is controlled by the output of the ALC circuit 44. Here, the switching circuit 57 selects the output of the ALC circuit 44 when the optical amplifier is operating in the ALC mode.

Note that the response time (ALC time constant) of the ALC circuit 44 is longer than the time required for the supervisory control signal to be transmitted to each optical node, as described above.
The optical amplifier according to the embodiment further includes an ALC auxiliary circuit 60. Here, the ALC auxiliary circuit 60 includes amplifiers 61 and 63 and a sample hold circuit 62. Then, the amplifier 61 obtains the loss in the variable attenuator 42 based on the input / output power of the variable attenuator 42. The input power of the variable attenuator 42 is detected by the light receiving element 52a, and the output power is detected by the light receiving element 51b. The sample and hold circuit 62 takes in the output of the amplifier 61 every predetermined time. The amplifier 63 outputs difference data for making the loss in the variable attenuator 42 obtained by the amplifier 61 coincide with the loss value held in the sample hold circuit 62.

  The ALC auxiliary circuit 60 performs the above sampling when the optical amplifier is operating in the ALC mode. However, in the ALC mode, the switching circuit 57 selects the output of the ALC circuit 44. On the other hand, when the operation of the optical amplifier is switched from the ALC mode to the AGC mode, the ALC auxiliary circuit 60 stops the above-described sampling operation, and the sample hold circuit 62 holds the data obtained by the last sampling. In the AGC mode, the control system operates so that the loss in the variable attenuator 42 matches the loss value held in the sample hold circuit 62. In the AGC mode, the switching circuit 57 selects the output of the ALC auxiliary circuit 60. Thereafter, when the operation of the optical amplifier returns from the AGC mode to the ALC mode, the loss in the variable attenuator 42 is controlled by the ALC circuit 44, and the ALC auxiliary circuit 60 performs the above sampling again. . The switching to the operation mode of the optical amplifier will be described in detail later.

  FIG. 11 shows a modification of the optical amplifier shown in FIG. The basic configuration of the optical amplifier shown in FIG. 11 is the same as that of the optical amplifier shown in FIG. 10, but the method for correcting the ASE light in the AGC circuit is different. That is, in the optical amplifier shown in FIG. 10, in order to eliminate the influence of ASE light, the ASE correction value 1 and the ASE correction value 2 are added to the input power values detected by the light receiving elements 51a and 51b, respectively. On the other hand, the optical amplifier shown in FIG. 11 is configured such that the ASE correction value 4 and the ASE correction value 5 are subtracted from the output power values detected by the light receiving elements 52a and 52b, respectively. The ASE correction values 4 and 5 correspond to the ASE optical power generated in the EDFs 41a and 41b, respectively. A configuration for subtracting the ASE correction values 4 and 5 from the output power is disclosed in, for example, Japanese Patent Application No. 11-112434.

  Thus, in the optical amplifier shown in FIG. 10, the pumping light power is controlled so that the calculated gain G = Pout / (Pin + correction value) matches the reference value, whereas in the optical amplifier shown in FIG. The pumping light power is controlled so that the calculated gain G = (Pout−correction value) / Pin matches the reference value. Therefore, it is basically the same in eliminating the influence of ASE in AGC. However, in the configuration shown in FIG. 11, when the input light to the EDF stops, “zero” is input to the AGC circuit 43a. Therefore, the configuration shown in FIG. 10 is more advantageous for the stability of the control system.

FIG. 12 is a diagram illustrating a specific configuration of the optical amplifier according to the embodiment. This optical amplifier is based on the configuration shown in FIG.
In this optical amplifier, various controls are executed by the DSP 71. The DSP 71 is given digital data representing the input power of the optical amplifier, digital data representing the output power of the optical amplifier, digital data representing the loss in the variable attenuator 42, and control information. Here, digital data representing the input power of the optical amplifier is obtained by converting an analog value detected by the light receiving element 51a into digital data. Digital data representing the output power of the optical amplifier is obtained by converting an analog value detected by the light receiving element 52b into digital data. Digital data representing the loss in the variable attenuator 42 is obtained by converting an analog value representing the ratio of the optical power detected by the light receiving elements 52a and 51b into digital data. The control information includes wavelength number information, operation mode switching information, and ASE correction amount information, and is given from the control circuit 12. These pieces of information are transmitted to each optical node by the monitoring control signal described above. Further, the DSP 71 can access the ROM 72 and the EP2ROM 73. The DSP 71 outputs information representing the loss in the variable attenuator 42 based on these data and information. Note that the output of the DSP 71 is converted into analog data by the D / A conversion circuit and is given to the variable attenuator 42.

The DSP 71 mainly executes the following processing.
(1) An instruction is given to the variable attenuator 42 in the ALC mode.
(2) In the ALC mode, the loss in the variable attenuator 42 is detected every predetermined time and written in the EP2ROM 73. At this time, the EP2ROM 73 is updated as needed with newly detected loss values.

  (3) When the operation mode of the optical amplifier is switched from the ALC mode to the AGC mode, updating of the EP2ROM 73 is stopped. Thereby, the data representing the loss in the variable attenuator 42 at the time when the operation mode is switched is stored in the EP2ROM 73. Note that the switching of the operation mode described above is, for example, when the fact that the number of wavelengths has been changed by the monitoring control signal, when the mode switching instruction is received by the monitoring control signal, the monitoring control signal could not be received for a predetermined time or more. Occurs at times.

(4) When the signal input to the optical amplifier is started, the value held in the EP2ROM 73 is instructed to the variable attenuator 42 in the AGC mode.
The above (1) will be described in detail. The output power in the ALC mode is basically determined according to the number of wavelengths of wavelength multiplexed light. Here, as described above, the number of wavelengths of the wavelength multiplexed light is notified by the monitoring control signal. The target output power to be held in the ALC mode is basically given by the product of “output power per wave” and “number of wavelengths”. The “output power per wave” is stored in the ROM 72 in advance.

  As shown in FIG. 13, the DSP 71 calculates a difference ΔP between the actual output power and the target output power detected by the light receiving element 52b for each time ΔT, and further calculates a correction amount “ΔP / n”. Here, “n” is a positive value larger than 1. In the example shown in FIG. 13, “ΔPo” is obtained as the difference between the actual output power and the target output power at time To. Therefore, in this case, the DSP 71 adjusts the loss in the variable attenuator 42 so that the actual output power increases by “ΔPo / n”. Subsequently, at time T1, “ΔP1” is obtained as a difference between the actual output power and the target output power. Therefore, in this case, the DSP 71 adjusts the loss in the variable attenuator 42 so that the actual output power increases by “ΔP 1 / n”. Thereafter, similarly, the loss in the variable attenuator 42 is adjusted every time ΔT, and the actual output power is gradually brought closer to the target output power. The output power adjustment speed (that is, the ALC time constant) can be set to a desired value by appropriately selecting “n” or “ΔT”. For example, if “ΔT” is made longer, the ALC time constant becomes longer.

  The target output power is basically determined based on the number of wavelengths of the wavelength multiplexed light, but may be determined in consideration of noise. In this case, for example, as shown in FIG. 14, noise information transmitted by the monitoring control signal is used. That is, each optical amplifier estimates noise generated in its own device according to the following equation.

Noise generated at each optical node = 1 / OSNRi
= NFi hνΔF / Pin
Here, “OSNR i” represents an optical signal / noise ratio in the i-th optical amplifier. “NFi” represents a noise figure in the i-th optical amplifier, and is stored in the ROM 72 in advance, for example. “Hν” represents energy. “ΔF” is a standardized constant, for example, 0.1 nm. “Pin” represents input power.

Each optical amplifier adds the noise information generated in its own device to the noise information notified using the supervisory control signal, and transfers the addition result to the next optical node. For example, in the example shown in FIG. 14, the following results are obtained as noise information.
Optical node 1a: 1 / OSNRa
Optical node 1b: 1 / OSNRa + 1 / OSNRb
Optical node 1c: 1 / OSNRa + 1 / OSNRb + 1 / OSNRc
Then, the DSP 71 determines the target output power according to the following equation.

Target output power = mPoutch (1 + 1 / OSNR)
Here, “m” represents the number of wavelengths of wavelength multiplexed light, “Poutch” represents output power per wave, and “1 / OSNR” is cumulative noise calculated as described above.

Next, switching of the operation mode of the optical amplifier will be described.
<First embodiment>
The optical amplifier according to the first embodiment operates using the ALC mode as a basic mode, and operates in the AGC mode when the number of wavelengths varies or when a failure occurs.

  FIG. 15 is a diagram illustrating transition of operation modes of the optical amplifier. The optical amplifier operates in the first ALC mode when it is activated. In the first ALC mode, a relatively short time constant is set. When the input power to the optical amplifier is stabilized in the first ALC mode, the AGC mode is entered. The input power is detected by the light receiving element 51a.

  In the AGC mode, when the number of wavelengths of the wavelength multiplexed light is stable, the process proceeds to the second ALC mode. The “number of wavelengths” is periodically notified by a monitoring control signal. Accordingly, it is determined that the number of wavelengths is stable when the notified “number of wavelengths” remains the same for a certain period of time.

  In the second ALC mode, a long time constant is set. In the second ALC mode, (1) when a notification of a change in the number of wavelengths is received, (2) when a failure of the monitoring control signal occurs, (3) when signal input stops, or (4) When a failure (such as a temporary stop of the power supply) occurs, the AGC mode is entered.

  As described above, the optical amplifier according to the embodiment operates in the first ALC mode having a relatively short time constant at the time of activation. Therefore, the output power of the optical amplifier is stabilized within a relatively short time when it is started.

  FIG. 16 is a diagram showing a control flow of the first embodiment. In the ALC mode, the loss in the variable attenuator 42 is adjusted so that the output power of the optical amplifier (that is, the output power of the EDF 41b) matches the target output power. At this time, the actual loss in the variable attenuator 42 is periodically detected, and the latest detection value is held in the EP2ROM 73.

  In the AGC mode, updating of the EP2ROM 73 is stopped. The loss in the variable attenuator 42 is fixed so as to match the loss value data held in the EP2ROM 73.

  FIG. 17 is a diagram illustrating an operation when the number of wavelengths of wavelength multiplexed light varies in the ALC mode (corresponding to the “second ALC mode” in FIG. 15). Here, it is assumed that the number of wavelengths of wavelength multiplexed light has decreased at time T1. In this case, the total input power and total output power of the optical amplifier are reduced. However, the output power per wave does not change. Thereafter, in the ALC mode, the loss in the variable attenuator 42 is adjusted so that the total output power of the optical amplifier is maintained at a constant value. However, since the ALC time constant of the optical amplifier of the embodiment is sufficiently long, the rate of change of the total output power of the optical amplifier is very slow. Therefore, the output power per wave remains almost constant.

  Subsequently, when “change in the number of wavelengths” is notified by the monitoring control signal at time T2, the operation mode is switched from the ALC mode to the AGC mode. Here, the time from the change of the number of wavelengths of the wavelength multiplexed light to the notification of the “number of wavelengths” by the monitoring control signal (period from time T1 to time T2) is, for example, about several tens of milliseconds. On the other hand, the ALC time constant is about several seconds. Therefore, the loss in the variable attenuator 42 hardly changes and the output power per wave hardly changes during the period from when the number of wavelengths changes until the operation mode is switched.

  On the other hand, if it is assumed that the time constant of ALC is not sufficiently long, as shown in FIG. 18, the output power per wave during the period from when the number of wavelengths changes until the operation mode is switched. Will fluctuate. That is, in the example shown in FIG. 18, when the number of wavelengths of wavelength multiplexed light changes at time T1, thereafter, the total output power of the optical amplifier attempts to return to the level before time T1 in a relatively short time. Therefore, this ALC operation also changes the output power per wave.

  The ALC time constant is determined as follows, for example. That is, first, an allowable value of output power fluctuation per wave is determined. Subsequently, based on the configuration of the communication system, the signal processing time (including 3R operation) in each optical node, and the like, the maximum value of the time during which the supervisory control signal is transmitted to each optical node is estimated. The time constant of the ALC is such that the fluctuation of the output power per wave does not exceed the above allowable value during the period from when the number of wavelengths fluctuates until the maximum time estimated as described above elapses. Set to

  In addition, the characteristics of the optical amplifier change due to changes in ambient temperature, aging of components constituting the optical amplifier, and the like. However, the speed of the characteristic change caused by these factors is extremely slow. For example, a fluctuation of about 1 dB occurs between several hours to several years. Therefore, even if the time constant of ALC is set to about several seconds to several tens of seconds, output power fluctuation due to these factors can be suppressed.

  Returning to FIG. After the operation mode is switched at time T2, the optical amplifier operates in the AGC mode. Here, the AGC time constant of the optical amplifier of the embodiment is sufficiently shorter than the response time of the EDF. Therefore, the gain of the optical amplifier is maintained at a constant value as described with reference to FIG. That is, even when the number of wavelengths of wavelength multiplexed light changes, the output power per wave does not vary.

  On the other hand, if it is assumed that the AGC time constant is not sufficiently short, the output power per wave varies as shown in FIG. 19 when the number of wavelengths of wavelength multiplexed light changes. That is, if the time from when the input power is reduced with the change in the number of wavelengths until the pumping light power is appropriately adjusted becomes longer than the response time of the EDF, the pumping light power adjustment is delayed. Therefore, the gain of the EDF becomes excessive. As a result, the output power per wave temporarily increases. That is, a surge occurs.

  FIG. 20 is a diagram showing the transition of the operation mode and the output power per wave when the number of wavelengths of the wavelength multiplexed light is changed. Here, it is assumed that the number of wavelengths of wavelength multiplexed light has decreased from “5” to “3” at time T1. In the ALC mode, the target output power corresponding to “number of wavelengths = 5” is “50”, and the target output power corresponding to “number of wavelengths = 3” is “30”. In FIG. 20, the optical power is represented by a numerical value without a unit.

Prior to time T1, the optical amplifier operates in the ALC mode, and the total output power is maintained at "50". At this time, the output power per wave is “10”.
At time T1, when the number of wavelengths of wavelength multiplexed light decreases from “5” to “3”, the total output power of the optical amplifier decreases from “50” to “30”. At this time, the optical amplifier is operating in the ALC mode. Accordingly, the DSP 71 adjusts the variable attenuator 42 so that the total output power of the optical amplifier returns to “50” until the monitoring control signal including the wavelength number information is received at time T2. However, as described above, since the ALC time constant of the optical amplifier of the embodiment is sufficiently long, the total output power of the optical amplifier at time T2 remains substantially “30”. That is, the output power per wave remains substantially “10” during the period from time T1 to time T2.

  At time T2, the operation mode of the optical amplifier is switched from the ALC mode to the AGC mode. Therefore, thereafter, the loss value of the variable attenuator 42 at the time T2 is held in the EP2ROM 73.

  After time T2, the optical amplifier operates in the AGC mode. At this time, the loss of the variable attenuator 42 follows the value held in the EP2ROM 73. Also, in the AGC mode, the output power per wave does not change because the gain of the optical amplifier is kept constant. In other words, during the period of operation in the AGC mode, the output power per wave remains approximately “10”, and the total output power remains approximately “30”.

  When the same wavelength number information (wavelength number = 3) is detected for a certain period of time while the optical amplifier is operating in the AGC mode, the operation mode returns from the AGC mode to the ALC mode at time T3. At this time, the target output power is set to “30” according to the received wavelength number information. On the other hand, during the period in which the optical amplifier is operating in the AGC mode, the total output power is maintained at substantially “30”. That is, at the time T3, the actual total output power of the optical amplifier substantially matches the target output power. Therefore, the loss value of the variable attenuator 42 is not greatly adjusted after time T3. Therefore, the output power per wave remains substantially “10” after time T3.

As described above, the output power of each signal light included in the wavelength multiplexed light output from the optical amplifier is maintained at a substantially constant value even if the number of wavelengths of the wavelength multiplexed light changes. That is, in the optical amplifier of the embodiment, since the AGC time constant is sufficiently shortened and the ALC time constant is sufficiently long, the output power of each signal light included in the wavelength multiplexed light output from the optical amplifier is always stable.
<Second embodiment>
The optical amplifier of the second embodiment operates using the AGC mode as a basic mode and periodically operates in the ALC mode.

  FIG. 21 is a diagram showing transition of the operation mode of the optical amplifier. Note that the procedure from when the optical amplifier is activated until it transits to the AGC mode after passing through the first ALC mode is the same as in the first embodiment.

  In the AGC mode, the loss in the variable attenuator 42 is fixed to the value held in the EP2ROM 73. The ALC mode is activated periodically or by an external instruction. In the ALC mode, the loss in the variable attenuator 42 is adjusted so that the output power of the optical amplifier matches the target power, and the EP2ROM 73 is updated accordingly. Therefore, when the operation mode returns to the AGC mode, the loss in the variable attenuator 42 is appropriately corrected.

Thus, since the loss in the variable attenuator 42 is appropriately corrected, the output power is stable even when operating in the AGC mode.
<Third embodiment>
As in the first embodiment, the optical amplifier according to the third embodiment operates using the ALC mode as a basic mode. However, the optical amplifier of the third embodiment includes a shutdown mode in addition to the first ALC mode, the second ALC mode, and the AGC mode of the first embodiment.

  FIG. 22 is a diagram showing the transition of the operation mode of the optical amplifier. Here, since the first ALC mode, the second ALC mode, and the AGC mode are basically the same as those in the first embodiment, the description thereof is omitted.

  In the third embodiment, in the AGC mode or the second ALC mode, (1) when the signal input is stopped, or (2) when a failure (such as a temporary stop of the power supply) occurs, the mode is switched to the shutdown mode. . When the signal input is resumed in the shutdown mode, the AGC mode is entered.

  FIG. 23 is a diagram showing a control flow of the third embodiment. In the ALC mode, the loss in the variable attenuator 42 is adjusted so that the output power of the optical amplifier matches the target output power. At this time, the actual loss in the variable attenuator 42 is periodically detected, and the latest detection value is held in the EP2ROM 73. Then, when the input signal is stopped, the mode is changed to the shutdown mode.

  In the shutdown mode, updating of the EP2ROM 73 is stopped. Further, the amplification operation by the EDF is stopped. That is, the drive of the excitation light source is stopped. When the signal input resumes, the AGC mode is entered.

  In the AGC mode, the excitation light source is driven. Further, the loss in the variable attenuator 42 is fixed so as to coincide with the loss value data held in the EP2ROM 73. Then, after a predetermined time has elapsed or when the number of wavelengths of the wavelength multiplexed light is stable, the ALC mode is entered.

Thus, in the third embodiment, when the signal input stops, the operation proceeds to the shutdown mode and the amplification operation stops. Therefore, even in the configuration shown in FIG. 11, “zero” is not input to the AGC circuit. Therefore, the operation of the AGC circuit is prevented from becoming unstable.
<Fourth embodiment>
The optical amplifier of the fourth embodiment has a standby (hot standby) mode as shown in FIG.

  There are a large number of optical amplifiers in the communication system of the embodiment, but some of these may not receive wavelength multiplexed light during normal operation, but may receive wavelength multiplexed light only during protection. is there. In this case, these optical amplifiers are set to a standby mode when the communication system is in a normal operation state. Here, in the standby mode, excitation light having a smaller power than that when the normal amplification operation is performed is supplied to the EDF. That is, in the standby mode, the EDF is excited to a low level. When wavelength multiplexed light is input to the optical amplifier operating in the standby mode, the operation mode immediately changes to the AGC mode. At this time, since the EDF is already excited to some extent, the time until the amplification operation in the AGC mode is started is short.

  In the optical amplifier of the above-described embodiment, the gains of the EDF 41a and the EDF 41b are individually controlled, but the present invention is not limited to this configuration. That is, as shown in FIG. 25, a configuration in which the sum of gains of the EDF 41a and the EDF 41b is controlled to be constant may be employed. In this case, the sum of the gains of the EDF 41a and the EDF 41b is obtained by subtracting the loss by the variable attenuator 42 from the gain of the entire optical amplifier. The excitation light sources 56a and 56b are controlled so that the sum of the gains of the EDF 41a and the EDF 41b becomes a constant target value. With such a configuration, a gain tilt does not occur, and a high output is not required in the pre-amplifier (ie, EDF 41a).

  In the above-described embodiment, a set of erbium-doped fiber optical amplifiers is provided, but linear optical amplifiers may be used instead. The linear optical amplifier is a semiconductor amplifier that performs laser oscillation in the vertical direction of the resonator, and its gain is clamped, so that the gain for a signal is maintained at a constant value regardless of the input level. In this case, the AGC circuit can be omitted or the configuration of the AGC face path can be simplified.

  Further, the optical amplifier of the above-described embodiment has a configuration including a front-stage amplifying unit, a rear-stage amplifying unit, and a variable attenuator provided therebetween, but the present invention is not limited to this. That is, the present invention is also applied to an optical amplifier composed of one amplification unit, as shown in FIG. 26 (a) or FIG. 26 (b).

(Supplementary note 1) An arbitrary optical amplifier among a plurality of optical amplifiers used in an optical communication system for transmitting wavelength division multiplexed light, including an optical amplification medium and an optical attenuator,
A gain control circuit for holding the gain of the optical amplification medium at a constant value;
A level control circuit for controlling the loss in the optical attenuator so that the output level of the optical amplifier is held at a value corresponding to a control signal notified to each optical amplifier in the optical communication system;
An optical amplifier characterized in that a time constant of the level control circuit is longer than a time until the control signal is notified to each optical amplifier in the optical communication system.

(Supplementary note 2) An optical amplifier that includes an optical amplification medium and an optical attenuator and amplifies wavelength multiplexed light,
A gain control circuit for holding the gain of the optical amplification medium at a constant value;
In the first operation mode in which the loss in the optical attenuator is dynamically controlled so that the output level of the optical amplifier is maintained at a constant value, or in the first operation mode as the loss in the optical attenuator. A level control circuit that selectively executes a second operation mode in which a loss value at the end is fixedly set;
An optical amplifier.

(Appendix 3) The optical amplifier according to appendix 1 or 2,
The time constant of the gain control circuit is shorter than the response time of the optical amplification medium.

(Appendix 4) The optical amplifier according to appendix 1 or 2,
The gain control circuit is
Detecting means for detecting input power and output power of the optical amplification medium;
Calculation means for calculating the gain of the optical amplification medium based on the input power and the output power detected by the detection means;
Pumping light control means for controlling the power of pumping light to be supplied to the optical amplification medium according to the gain calculated by the calculation means,
The calculation means calculates the gain of the optical amplifying medium based on a ratio between the input power and a value obtained by subtracting the spontaneous emission light power generated in the optical amplifying medium from the output power.

(Supplementary Note 5) An optical amplifier that includes an optical amplification medium and an optical attenuator and amplifies wavelength multiplexed light,
A gain control circuit for holding the gain of the optical amplification medium at a constant value;
A level control circuit for controlling the loss in the optical attenuator so that the output level of the optical amplifier is maintained at a constant value,
The gain control circuit is
Detecting means for detecting input power and output power of the optical amplification medium;
Calculation means for calculating the gain of the optical amplification medium based on the input power and the output power detected by the detection means;
Pumping light control means for controlling the power of pumping light to be supplied to the optical amplification medium according to the gain calculated by the calculation means,
The calculating means calculates the gain of the optical amplifying medium based on a ratio between the input power to which a predetermined value corresponding to the spontaneous emission light power generated in the optical amplifying medium is added and the output power. An optical amplifier characterized by.

(Appendix 6) The optical amplifier according to appendix 1,
The level control circuit fixes the loss in the optical attenuator when notified by the control signal that the number of wavelengths of wavelength multiplexed light has changed.

(Appendix 7) The optical amplifier according to appendix 6,
In the optical attenuator, the level control circuit is configured to maintain the output level of the optical amplifier at a predetermined value when the number of wavelengths of the wavelength multiplexed light is stabilized when the loss in the optical attenuator is fixed. Return to operating mode to control loss.

(Appendix 8) The optical amplifier according to appendix 1,
The level control circuit fixes a loss in the optical attenuator when the control signal cannot be received continuously for a predetermined time.

(Supplementary note 9) The optical amplifier according to supplementary note 1, wherein
The gain control circuit stops the amplification operation of the optical amplification medium when wavelength multiplexed light is not input to the optical amplifier.

(Supplementary note 10) The optical amplifier according to supplementary note 1, wherein
The level control circuit operates with a first time constant when the optical amplifier is started, and thereafter, the level control circuit is longer than the first time constant and the control signal is notified to each optical amplifier in the optical communication system. It operates with the second time constant longer than the time until.

(Supplementary note 11) The optical amplifier according to supplementary note 1, wherein
The level control circuit reduces the loss in the optical attenuator so that the output level of the optical amplifier is maintained at a value determined in consideration of noise accumulated while wavelength multiplexed light is transmitted to the optical amplifier. Control.

(Supplementary note 12) The optical amplifier according to supplementary note 1, wherein
The optical amplification medium is composed of first and second optical amplification media,
The gain control circuit holds a sum of gains of the first and second optical amplification media at a constant value.

(Supplementary note 13) An arbitrary optical amplifier among a plurality of optical amplifiers used in an optical communication system for transmitting wavelength division multiplexed light,
An optical amplifying medium for amplifying the input wavelength multiplexed light with a constant gain without depending on the input level;
An optical attenuator provided before or after the optical amplification medium;
A level control circuit for controlling a loss in the optical attenuator so that the output level of the optical amplifier is held at a value corresponding to a control signal notified to each optical amplifier in the optical communication system;
An optical amplifier characterized in that a time constant of the level control circuit is longer than a time until the control signal is notified to each optical amplifier in the optical communication system.

(Supplementary note 14) A wavelength division multiplexing optical communication system including a plurality of optical nodes connected in a ring shape,
An optical communication system, wherein each of the plurality of optical nodes includes the optical amplifier described in Appendix 1.

(Supplementary note 15) An optical communication system in which a plurality of optical amplifiers are provided on a transmission line for transmitting wavelength division multiplexed light,
An optical communication system, wherein each of the plurality of optical amplifiers is the optical amplifier according to appendix 1.

It is a block diagram of an example of the optical communication system in which the optical amplifier of embodiment of this invention is used. It is a figure explaining the control system of the optical communication system of embodiment. It is a block diagram of an optical node. It is a figure which shows the state (non-protection state) where the failure has not occurred. It is a figure which shows the state (protection state) in which the failure occurred. It is a figure which shows the basic composition of the optical amplifier of embodiment of this invention. It is a figure explaining the response time of an optical amplification medium. It is a figure explaining the time constant of AGC. It is a figure explaining the time constant of ALC. It is a block diagram of the optical amplifier of embodiment. It is a modification of the optical amplifier shown in FIG. It is a figure which shows the specific structure of the optical amplifier of embodiment. It is a figure explaining the operation | movement of ALC mode. It is a figure explaining the method of determining the target output power of ALC mode. It is a figure which shows the transition of the operation mode of the optical amplifier of a 1st Example. It is a figure which shows the control flow of a 1st Example. It is a figure explaining the operation | movement of the ALC mode of embodiment. It is a figure explaining the operation | movement of the conventional ALC mode. It is a figure explaining the operation | movement of the conventional AGC mode. It is a figure which shows the transition of an operation mode, and the output power per wave. It is a figure which shows the transition of the operation mode of the optical amplifier of a 2nd Example. It is a figure which shows the transition of the operation mode of the optical amplifier of a 3rd Example. It is a figure which shows the control flow of a 3rd Example. It is a figure which shows the transition of the operation mode of the optical amplifier of a 4th Example. It is a block diagram of the optical amplifier of other embodiment. It is a figure which shows the basic composition of the optical amplifier of other embodiment.

Explanation of symbols

1 optical node 12 control circuit 41a, 41b EDF (erbium doped fiber)
42 Variable attenuators 43a and 43b AGC circuit 44 ALC circuits 56a and 56b Excitation light source (LD)
62 Sample hold circuit 71 DSP
72 ROM
73 EP2ROM

Claims (1)

An amplification method using an optical amplifier provided in the node used in an optical communication system for transmitting wavelength multiplexed light between a plurality of nodes,
The optical amplifier is composed of an AGC amplifier and an optical attenuator for ALC operation. The optical amplifier performs AGC operation in the AGC mode and ALC operation in the ALC mode. Is transmitted to the next optical node while performing the operation of converting the control signal into an electric signal at each node and converting it to an optical signal again, in any of the plurality of nodes of the optical communication system. Provided in the node,
The optical amplifier is
An optical amplifying medium that amplifies wavelength multiplexed light and performs AGC operation;
An optical attenuator that is provided at the front stage or the rear stage of the optical amplification medium and attenuates wavelength multiplexed light to perform an ALC operation;
First monitoring means for monitoring the gain of the optical amplification medium;
Second monitoring means for monitoring the output level of the optical amplifier;
A gain control circuit for holding the gain of the optical amplification medium at a constant value;
A level control circuit for controlling the loss in the optical attenuator so that the output level of the optical amplifier is held at a value corresponding to the number of wavelength multiplexing indicated by the control signal notified from another node; And
The optical amplification medium performs an AGC operation based on a monitoring result of the first monitoring means,
In the ALC mode, the optical attenuator performs an ALC operation targeting a level corresponding to the number of wavelengths notified by the control signal based on a monitoring result of the second monitoring unit. In the AGC mode, the AGC mode is in the AGC mode. ALC operation is performed on the target value acquired in
When the node that created the control signal transmits the control signal, in each node, when a change in the number of wavelength multiplexing is notified by the control signal, the ALC mode is changed to the AGC mode, and In the ALC mode, the level which is a time from when the input / output level of the optical amplifier is changed until the loss in the optical attenuator is appropriately adjusted so that the output level returns to a certain level to be held The time constant of the control circuit is longer than the time required from when the input / output level of the optical amplifier changes until the change in the number of wavelengths is notified by the control signal.
An amplification method characterized by the above.

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