JP5584143B2 - Optical amplifier - Google Patents

Description

  The present invention relates to an optical amplifier that amplifies signal light.

  Along with the recent increase in demand for Internet communication and the demand for cost reduction of optical communication systems, a wavelength division multiplexing (WDM) transmission system that multiplexes and transmits signal lights of different wavelengths on a single fiber is introduced. Has been. Furthermore, a system and a network using the method are being studied. With this WDM technology, it is considered to not only increase the transmission capacity but also realize network functions such as routing and restoration. Such a network is called a photonic network. In addition, since the WDM transmission system mainly uses the 1.5 μm band as the communication wavelength band, the optical amplifier used in the photonic network is mainly an Er-doped fiber amplifier (EDFA).

  In a photonic network, not only there is a difference in span loss due to the difference in distance between optical communication nodes, but also the span loss fluctuates due to contact with the fiber, secular change, etc. The number of input wavelengths may change due to obstacles. The span loss fluctuation and the channel number change change the EDFA input optical power used in the network. The EDFA used in WDM transmission needs to amplify a plurality of wavelength signals with the same gain, and is designed so that the wavelength dependence of gain is flat in the signal wavelength band. However, when the wavelength dependency of the gain is flat with a certain signal input optical power, the wavelength dependency of the gain of the EDFA is not flat (gain tilt occurs) when the EDFA input power changes. When the gain tilt occurs, a signal for which a sufficient gain cannot be obtained or a signal whose gain is excessively large is generated, and transmission characteristics may be deteriorated. Therefore, the EDFA used in the photonic network needs to be controlled (gain control) so that the wavelength dependence of the gain is always constant with respect to the fluctuation and change of the input signal light power. is there.

  On the other hand, an optical amplifier having a multistage optical amplifying unit and including an optical variable attenuator and a gain equalization filter between the optical amplifying units has been studied. For example, see Non-Patent Document 1. In such an optical amplifier, the amount of attenuation of the optical variable attenuator is controlled by fluctuation / change in the input signal light power, and there is no need to change the gain equalization filter, and it is always constant with respect to fluctuation / change in the input signal light power In addition, a gain having a flat wavelength dependency can be obtained. In Non-Patent Document 1, it is also considered that the optical amplifier monitors a change in input signal light using a monitoring wave.

  However, in the optical amplifier as described above, it is complicated and difficult to control each optical amplifying unit and the optical variable attenuator.

D.Menashe et al., "Optical Amplifiers for Modern Networks," ICTON2006, pp. 115-118.

  The present invention has been made in view of the above circumstances, and even if there is a change in input signal light power due to a variation in span loss of the optical transmission system or a change in the number of input wavelengths, the gain is maintained while maintaining the wavelength dependency flat. It is an object of the present invention to provide an optical amplifier that suppresses a rapid change in the frequency.

In order to achieve the above object, the present invention provides an erbium-doped fiber that amplifies signal light, a semiconductor laser that generates pumping light to the erbium-doped fiber, and Two or more optical amplifying units including a multiplexer for multiplexing the signal light and the pumping light, and optical power detection means for detecting the input signal light power and the output signal light power of the erbium-doped fiber, Based on the detection result of the power detection means, the amplified spontaneous emission light with respect to the total value of the gain of each optical amplifier, which is the difference between the output signal light power detection value of each optical amplifier and the input signal light power detection value (ASE: Amplified Spontaneous Emission) The drive power to the semiconductor laser of each optical amplifying unit is such that the result of adding and subtracting the correction value becomes a predetermined fixed target value. And a control means configured to control each of the optical amplification sections when the input signal optical power detection value of the first stage optical amplification section is less than or equal to a predetermined input signal optical power value. the ratio of the drive current to the semiconductor laser and the control is a constant, when the input signal light power detection value of the optical amplification portion of the first stage is larger Ri by said predetermined input signal light power value, wherein each of the optical amplifier And switching the control to flatten the wavelength dependence of the gain of the unit, the predetermined input signal light power value is the drive when the input signal light power of each of the optical amplification units is smaller than the maximum value It is the input signal light power value that makes the pumping light power the same in the control in which the ratio of current is constant and the control in which the wavelength dependence of the gain is flattened. Smaller than the value and the predetermined input signal light When the power value is smaller than the warp value, the pumping light power in the control in which the ratio of the drive currents is constant has the characteristic that the pumping light power in the control for flattening the wavelength dependency of the gain is larger, When the input signal light power is smaller than the maximum value and the drive current ratio is constant, the pumping light power of the first stage optical amplification unit becomes excessive, and the pumping light power of the second stage optical amplification unit is It is characterized by a shortage .

The invention according to claim 2 is the optical amplifier according to claim 1, wherein at least one of the two or more optical amplifiers includes a stage before the multiplexer and / or An optical isolator is further provided after the erbium-doped fiber, and an attenuation amount is adjusted between the two or more optical amplification units so as to compensate for a change in power of the signal light input to the optical amplifier. An optical variable attenuator that attenuates light; and a gain equalization filter that performs gain equalization on the signal light attenuated by the optical variable attenuator, wherein the control means includes the optical variable attenuator. It is further configured to adjust the attenuation amount of the attenuation, the light based on the detection result the power detection unit, the optical amplification of the output signal light power detection value of the optical amplification portion of the final stage and the first stage Input signal light power detection value and The result of adding or subtracting the ASE correction value to the gain of the entire optical amplifier including the attenuation, which is the difference, is adjusted so as to be a gain target value that changes according to the input signal light power per wavelength, The gain target value is obtained by calculating the input signal light power per wavelength.

  The invention described in claim 3 is the optical amplifier according to claim 2, wherein the input signal light power per one wavelength is the number of signal light wavelengths and the input signal of the first stage optical amplifier. It is obtained by calculation with the optical power detection value.

  According to a fourth aspect of the present invention, there is provided the optical amplifier according to the second aspect, wherein the branching filter demultiplexes the monitoring wavelength light and the signal light before the first stage optical amplifying unit. Is inserted, and a monitoring wavelength optical power detection means for detecting optical power of the monitoring wavelength is configured to be connected to the duplexer, and the input signal optical power per one wavelength Is obtained from the optical power of the monitoring wavelength detected by the monitoring wavelength optical power detection means.

  The invention described in claim 5 is the optical amplifier according to any one of claims 1 to 4, wherein the ASE correction value is upstream of the first ASE correction value generated by the optical amplifier and the optical amplifier. It is obtained by calculation with the correction value of the second ASE generated by the side optical amplifier.

  The invention according to claim 6 is the optical amplifier according to claim 5, wherein the correction value of the first ASE changes according to the input signal light power of the first stage optical amplifying unit. It is characterized by.

  The invention according to claim 7 is the optical amplifier according to claim 6, wherein the correction value of the first ASE is calculated for each input signal light power 3 dB of the first stage optical amplifier. It is stored in advance in the control means.

The invention according to claim 8 is the optical amplifier according to any one of claims 1 to 7 , wherein the control means determines in advance a drive current to the semiconductor laser of each of the optical amplification units by the control. A first feedforward drive current having a fixed value and a second feedforward drive current that changes in accordance with the input signal light power of the first stage optical amplification unit are added.

The invention according to claim 9 is the optical amplifier according to claim 8 , wherein the control means is configured such that the change rate of the input signal light power of the first stage optical amplifier is a predetermined change rate value. It is characterized in that the addition is performed under the condition that the value exceeds.

  As described above, according to the optical amplifier of the present invention, since appropriate control is performed for each optical amplifying unit and optical variable attenuator, input is performed due to variations in span loss of the optical transmission system and changes in the number of input wavelengths. Even if there is a change in the signal light power, the wavelength dependency of the gain can be kept flat while realizing low noise, and an abrupt change in gain can be suppressed.

It is a block diagram which shows the structural example of the optical amplifier by this invention. It is a figure which shows the dependence with respect to the input signal optical power interval of the error of the correction value of ASE which generate | occur | produces in optical amplifier itself. It is a block diagram which shows the structural example of the optical amplifier using a monitoring wave by this invention. It is a conceptual diagram which shows the mode of the change of the target gain value at the time of attenuation amount control of an optical variable attenuator. It is a conceptual diagram which shows the control method of the drive current of a semiconductor laser in drive current ratio constant control. (A) is a figure which shows typically the wavelength dependence of the 1st step | paragraph optical amplifier gain of drive current ratio constant control and optical amplifier flat gain control, (b) is a drive current ratio constant control and optical amplifier. It is a figure which shows typically the wavelength dependence of the 2nd step | paragraph optical amplification part gain with flat gain control. It is a figure which compares the noise figure of drive current ratio constant control and optical amplifier flat gain control. It is a figure which compares the excitation light power of drive current ratio constant control and optical amplification part flat gain control. It is a figure which shows the excitation light power in the control method which switches drive current ratio constant control and optical amplification part flat gain control. It is a figure which shows the noise figure in the control method which switches drive current ratio constant control and optical amplification part flat gain control. It is a figure which shows typically the drive current proportional to input signal optical power in feedforward drive current.

  FIG. 1 is a block diagram showing a configuration example of an optical amplifier according to the present invention. In the optical amplifier 100, a first-stage optical amplification unit 100a, an optical variable attenuator 110, a gain equalization filter 111, and a second-stage optical amplification unit 100b are sequentially connected. The optical amplifier 100 further includes a controller 112 that controls the drive current of the pumping semiconductor laser and the amount of attenuation of the optical variable attenuator 110 in the first and second stage optical amplification units.

  In the first-stage optical amplifying unit 100a, an optical isolator 102a that propagates the input signal light in one direction, a multiplexer 103a that combines the signal light and the pumping light, and an amplifying medium, erbium is included in the core. The doped erbium-doped fiber 104a and the optical isolator 105a that propagates and outputs the amplified signal light in one direction are sequentially connected. A semiconductor laser 108a serving as an excitation light source that generates excitation light to the erbium-doped fiber 104a is connected to the multiplexer 103a. In addition, in order to provide the optical power detection value to the controller 112, branching devices 101a and 106a for branching a part of the optical power are respectively inserted on the input and output sides of the first stage optical amplifying unit 100a. Furthermore, photodiodes 107a and 109a for detecting optical power are connected to the branching units 101a and 106a. The second stage optical amplifier 100b has the same structure as the first stage optical amplifier 100a.

  In this configuration example, the structure of the optical amplifier having two optical amplifying units is shown. However, the optical amplifier includes three or more optical amplifying units, and an optical variable attenuator and a gain equalization filter are provided between the optical amplifying units. Can also be provided.

  In FIG. 1, input signal light (a WDM signal light in which a plurality of wavelengths are multiplexed) is first amplified by the first-stage optical amplifying unit 100a, then attenuated by the optical variable attenuator 110, and further gain and the like. After the transmission filter 111 passes through the optical filter 111 and the gain variation among the plurality of wavelengths is made equal to or less than a predetermined value, the gain is amplified by the second-stage optical amplifying unit 100b and output. Here, when there is a change in the input signal light power per wavelength, the optical variable attenuator 110 can change the attenuation amount to compensate for the change amount. Thereby, gain equalization can be performed so that the gain equalization filter 111 in the subsequent stage is not affected by the change in the input signal light power. Therefore, the output signal light power per wavelength that is amplified and output by the optical amplifying unit 100b at the subsequent stage is always a constant value regardless of the change in the input signal light power per wavelength (it is constant when a control error is included). Range).

  The controller 112 in the optical amplifier 100 performs a calculation based on the optical power detected by each photodiode, and functions to adjust the drive current of the semiconductor laser that is a pumping light source based on the calculation result. The operation of the controller 112 will be described below.

The input / output power detection value of each optical amplification unit is logarithmically converted by the controller 112. That is, the first amplification input side is converted to P ₁ , the first amplification output side is converted to P ₄ , the second amplification input side is converted to P ₃ , and the second amplification output side is converted to P _2. . Alternatively, a similar logarithmic value can be obtained by amplifying the detected value (current value) in each photodiode with a LOG amplifier after photoelectric conversion.

The controller 112 adds / subtracts the ASE correction value to / from the total value of the gain (P ₄ −P ₁ ) of the first stage optical amplifying unit and the gain (P ₂ −P ₃ ) of the second stage optical amplifying unit. To be constant, ie

So that the drive current I ₁ of the semiconductor laser 108a in the first-stage optical amplifier 100a and the drive current I ₂ of the semiconductor laser 108b in the second-stage optical amplifier 100b are controlled. Here, G _agc is a predetermined fixed target value. As described above, since the attenuation of the optical variable attenuator is controlled so as to compensate for the amount of change in the input signal light power, when the pumping semiconductor laser of each optical amplifier is controlled, the change in the input signal light power There is no need to consider the amount. Also,

Is an ASE correction value. here,

Is a fixed correction value for ASE generated in the optical amplifier upstream of itself when an optical amplifier is used in a multi-relay optical transmission system using a plurality of optical amplifiers. In addition,

Is a correction value for ASE generated in the optical amplifier, and the value changes according to the power P ₁ of the input signal light, and is thus displayed as a function of P ₁ .

Since the value of Expression (4) changes according to the input signal light power P _1, it is measured in advance at a constant input signal light power interval and stored as a table in the storage circuit in the controller 112. Thereby, the value of Formula (4) can be acquired from the table stored by the controller according to the detection result of the input signal light power.

  Since the optical amplifier control error increases as the error in Expression (4) increases, the interval of the input signal light power used for table formation is less than the input signal light power detection accuracy (usually about 0.1 dB), and Expression (4) It is required that there is no error. However, in order to eliminate the error of the equation (4), if the value of the equation (4) is tabulated in the memory circuit in the controller for every 0.1 dB of the input signal light power, there is a problem that the scale of the memory circuit becomes large. . Therefore, it is necessary to widen the interval between the input signal light powers used for table formation.

  FIG. 2 shows the dependency of the error of equation (4) on the interval of the input signal light power. Since the detection accuracy of gain control is also about the input signal light power detection accuracy (usually about 0.1 dB), the error in equation (4) needs to be suppressed to 0.1 dB or less. From FIG. 2, if the input signal light power interval is 3.8 dB or less, the error of the equation (4) can be suppressed to 0.1 dB or less. Usually, an input signal light power interval is an integer, which is often convenient for calculations. Therefore, the value of equation (4) is tabulated in a memory circuit with an input signal light power interval of 3 dB or less, and desired input is performed by interpolation calculation. It is preferable to obtain the value of equation (4) for the signal light power. On the other hand, even when the storage circuit scale in the controller is sufficiently large and can be tabulated every 0.1 dB of the input signal light power, when the value of Equation (4) is measured, the input signal light power interval should be 3 dB or less. Thus, measurement can be simplified.

The attenuation amount of the optical variable attenuator 110 is a gain target in which the result of adding or subtracting the ASE correction value to the gain (P ₂ -P ₁ ) of the entire optical amplifier changes according to the input signal light power per wavelength. To be value, ie

It can be controlled by the controller 112 so as to satisfy. Here, the left side of the equation (5) is the gain of the entire optical amplifier including the attenuation amount of the optical variable attenuator and correcting the ASE generated upstream and itself. Also, on the right side

Is a gain target value that varies according to the input signal light power per wavelength,

Is the input signal light power per wavelength. The gain target value of the entire optical amplifier changes according to the input signal light power per wavelength. In order to keep the output signal light power per wavelength constant, the amount of change in the input signal light power per wavelength is compensated. Because. The compensation is performed by controlling the attenuation amount of the optical variable attenuator.

  Furthermore, Formula (5) will be described together with Formula (1). From formula (5) -formula (1),

It becomes. (P ₃ −P ₄ ) on the left side of Expression (8) is the difference between the value of the signal light power output from the gain equalization filter 111 and the value of the signal light power input to the optical variable attenuator 110. is there. That is, the sum of the attenuation amount of the optical variable attenuator 110 and the increase / decrease value of the signal light power by the gain equalization filter 111. Since the gain equalization filter 111 performs predetermined gain equalization and the increase / decrease value of the signal light power is a fixed value, the change in (P ₃ -P ₄ ) is the change in the attenuation amount of the optical variable attenuator 110. Equivalent to. Since the second term on the right side of Equation (8) is a fixed value, the change on the right side of Equation (8) corresponds to the change in the first term, that is, changes according to the input signal light power per wavelength. Therefore, Expression (8) indicates that when the input signal light power per wavelength is increased (decreased), the attenuation of the optical variable attenuator is increased (decreased) by that amount. That is, the amount of attenuation of the optical variable attenuator is controlled so as to compensate for the amount of change in the input signal light power.

The input signal light power per wavelength shown in Expression (7) is obtained by the controller acquiring the wavelength number information N _ch from the operation system, and

Can be obtained from.

Alternatively, when there is a change in the transmission fiber loss on the input side of the optical amplifier, a change in input signal light power per wavelength is provided to the controller by using monitoring signal light having a wavelength different from that of the signal light. You can also. FIG. 3 shows a configuration example of an optical amplifier using a monitoring wave. In the optical amplifier 300 of FIG. 1, a demultiplexer 313 for demultiplexing the monitoring signal light and the signal light is inserted in front of the first-stage optical amplification unit, and the photodiode 314 is demultiplexed. Connected to the device 313. In FIG. 3, the monitoring signal light is input to the photodiode 314 via the branching filter 313, and the optical power value of the monitoring wavelength detected by the photodiode 314 is provided to the controller. Since the change in the monitoring wavelength optical power is equal to the change in the input signal optical power per wavelength, the gain target value of the entire optical amplifier is G _alc (P ₅ ) using the monitoring wavelength optical power detection value P _5. can do. FIG. 4 is a conceptual diagram showing how the target gain value changes during attenuation control of the optical variable attenuator. As shown in FIG. 4, the gain target value of the entire optical amplifier is a value that is proportional (negative proportionality coefficient) to the input signal light power or the monitoring wavelength light power per wavelength (both in dBm units). It becomes.

  As described above, the drive current of the semiconductor laser of the optical amplification unit is controlled so that the total value of the gain of the first-stage optical amplification unit and the gain of the second-stage optical amplification unit satisfies Equation (1). In addition, another constraint condition is provided between the first-stage optical amplification unit and the second-stage optical amplification unit to further control the driving current of the semiconductor laser of the optical amplification unit, and the control method varies depending on the constraint condition, Become.

FIG. 5 is a conceptual diagram showing a method for controlling the drive current of the semiconductor laser in the constant drive current ratio control. I ₁ is the drive current of the semiconductor laser of the first stage optical amplifier, and I ₂ is the drive current of the semiconductor laser of the second stage optical amplifier. As shown in FIG. 5, control is performed so that I ₁ / I ₂ is constant below the drive current limit of each semiconductor laser. Therefore, such a control method is called constant drive current ratio control. On the other hand, as a method for controlling the driving current I ₁ of the semiconductor laser of the first stage optical amplifying unit and the driving current I ₂ of the semiconductor laser of the second stage optical amplifying unit, the gain is constant in each optical amplifying unit. In other words, there is a method of controlling I ₁ and I ₂ so that each optical amplifying unit is in a gain-flat amplification state (hereinafter referred to as optical amplifying unit flat gain control). As described below, the constant drive current ratio control has an advantage.

FIGS. 6A and 6B schematically show the wavelength dependence of each optical amplification unit gain in the constant drive current ratio control and the optical amplification unit flat gain control. Here, the drive current ratio R = I ₁ / I ₂ at the maximum input signal light power is the same for the constant drive current ratio control and the flat gain control of the optical amplifier. When the maximum input signal power is used, the constant drive current ratio control is the same as the optical amplifier flat gain control, and the gain is flat with respect to the wavelength in both the first optical amplifier and the second optical amplifier. (Flat gain) (gain curve shown by dotted line). When the input signal light power becomes smaller than the maximum value, in the optical amplification unit flat gain control, the gains of both the first-stage optical amplification unit and the second-stage optical amplification unit remain as the dotted gain curve. On the other hand, in the constant drive current ratio control, as shown by the solid line gain curve, the gain of the first-stage optical amplifier is increased (particularly increased on the short wavelength side) and the gain of the second-stage optical amplifier is decreased. (In particular, it is greatly reduced on the short wavelength side). However, as a result, a flat gain characteristic can be obtained by combining the two. This is because the drive current ratio is constant, and when the input signal light power decreases, the pump light power becomes excessive in the first stage optical amplifier, whereas the pump light power is insufficient in the second stage optical amplifier. It is because it will be in a state.

Comparing the noise figure between the constant drive current ratio control and the optical gain flat gain control when the input signal light power is smaller than the maximum value, the constant drive current ratio control is smaller and lower as shown in FIG. It becomes noise. The gain and noise figure of the first stage optical amplifier are G ₁ and NF ₁ respectively, the noise figure of the second stage optical amplifier is NF ₂ , and the first stage optical amplifier including the attenuation of the optical variable attenuator And the transmittance between the second stage optical amplifiers is L, the noise figure NF of the optical amplifier is

It is expressed. In the constant drive current ratio control, when the input signal light power decreases, the first-stage optical amplifier gain G ₁ increases, and as a result, the noise figure NF decreases.

  On the other hand, when the excitation light power is compared between the constant drive current ratio control when the input signal light power is smaller than the maximum value and the optical gain flat gain control, the constant drive current ratio control is shown in FIG. However, the excitation light power is increased. However, since the optical amplifier is equipped with a semiconductor laser that generates the pumping light power required at the maximum input signal light power, any pumping light power lower than that can be generated without any problem. Therefore, in the constant drive current ratio control, it is not a problem that the pumping light power becomes larger than in the optical amplifier flat gain control.

  However, if it is necessary to reduce the pumping light power when the input signal light power is low, the input signal light power that makes the pumping light power the same in the constant drive current ratio control and the optical gain flat gain control is maximized. By making it smaller, the pumping light power can be lowered. In FIG. 9, the input signal light power is

Is shown. Since the input signal light power in Expression (11) is the input signal light power at the intersection of the pumping light power curve with constant drive current ratio control and the pumping light power curve with optical amplifier flat gain control, the intersection input signal will be described below. Called optical power. Further, as shown by the thick line in FIG. 9, when the input signal light power is less than or equal to the intersection input signal light power, the drive current ratio constant control is performed, and when the input signal light power is greater than the intersection input signal light power, switching to the optical amplifier flat gain control is performed. . Thereby, low noise property can be obtained. The noise figure of this switching method is indicated by a thick line in FIG. In FIG. 9 in which the intersection input signal light power is smaller than the maximum value, the drive current ratio at the maximum input signal light power is the same in the constant drive current ratio control and the optical amplifier flat gain control. 8 is the maximum value, it can be seen that the excitation light power of the constant drive current ratio control can be reduced by reducing the intersection input signal light power from the maximum value. As shown in FIG. 10, when the input signal light power is equal to or less than the intersection input signal light power, the constant drive current ratio control has a smaller noise figure, and the input signal light power is larger than the intersection input signal light power. The noise figure is smaller in the partial flat gain control. Therefore, low noise can be obtained by the above switching method. Note that the value of the intersection input signal light power can be determined in advance based on the desired pumping light power characteristic and the desired noise figure characteristic.

As described above, the controller performs feedback control by the calculation of Expression (1) and Expression (2). In addition, when the input signal light power changes suddenly when the number of signal wavelengths changes suddenly, for example, when the input signal light power change speed is ± 10 dB / μs or more, the controller drives the feed forward. it can be added to the current value I _F to the drive current of the feedback control. Feed forward drive current value _IF is

Where,

Is a feedforward drive current value of a fixed value determined in advance,

Is a feedforward drive current value that changes according to the input signal light power detected by the optical power detection means, and is proportional to the input signal light power as shown in FIG.

  By adding this feedforward control to the feedback control, the output signal light power (the output signal light power of one wave whose input did not change) when the number of signal wavelengths rapidly decreases from a maximum of 96 waves to one wave. Can be suppressed to 0.2 dB. As a result, an effect of reducing the fluctuation of the output signal light power was seen as compared with 1.5 dB of only feedback control.

  As described above, the optical amplifier according to the present invention has a wavelength dependency of gain while realizing low noise even when there is a change in input signal light power due to a variation in span loss of the optical transmission system or a change in the number of input wavelengths. Can be maintained flat, and a sudden change in gain can be suppressed.

100, 300 Optical amplifiers 100a, 300a First stage optical amplifiers 100b, 300b Second stage optical amplifiers 101a, 106a, 101b, 106b, 301a, 306a, 301b, 306b Branch units 102a, 105a, 102b, 105b, 302a, 305a, 302b, 305b Optical isolators 103a, 103b, 303a, 303b Multiplexers 104a, 104b, 304a, 304b Erbium-doped fibers 107a, 109a, 107b, 109b, 307a, 309a, 307b, 309b, 314 Photodiodes 108a, 108b, 308a, 308b Semiconductor laser 110, 310 Optical variable attenuator 111, 311 Gain equalization filter 112, 312 Controller 313 Demultiplexer

Claims (9)

An erbium-doped fiber that amplifies signal light, a semiconductor laser that generates pump light to the erbium-doped fiber, a multiplexer that combines the signal light and the pump light, and input signal light of the erbium-doped fiber Two or more optical amplifying units including optical power detection means for detecting power and output signal optical power,
Based on the detection result of the optical power detector, the natural amplification is performed with respect to the total gain of each optical amplifying unit, which is the difference between the output signal optical power detection value of each optical amplifying unit and the input signal optical power detection value. Control means configured to control the drive current to the semiconductor laser of each of the optical amplifying units so that the result of adding and subtracting the emission light (ASE: Amplified Spontaneous Emission) correction value becomes a predetermined fixed target value. Further comprising
The control means controls the ratio of the drive current to the semiconductor laser of each optical amplifying unit to be constant when the input signal optical power detection value of the first stage optical amplifying unit is less than or equal to a predetermined input signal optical power value. If, when the input signal light power detection value of the optical amplification portion of the first stage is larger Ri by said predetermined input signal light power value, and a control to flatten the wavelength dependence of the gain of each optical amplifier Configured to switch ,
The predetermined input signal light power value is a control in which the ratio of the drive current is constant and a control for flattening the wavelength dependence of the gain when the input signal light power of each of the optical amplifiers is smaller than the maximum value. And the input signal light power value at which the pumping light power is the same,
Each optical amplifying unit is configured such that when the input signal light power is smaller than the maximum value and smaller than the predetermined input signal light power value, the pump light power in the control in which the ratio of the drive current is constant depends on the wavelength dependence of the gain. Has a characteristic that becomes larger than the pumping light power in the control of flattening,
When the input signal light power to each optical amplifying unit is smaller than the maximum value and the drive current ratio is constant, the pumping light power of the first stage optical amplifying part becomes an excessive state, and the second stage optical amplification An optical amplifier characterized in that the pumping light power of the part is in an insufficient state . In at least one of the two or more optical amplification units, the optical amplification unit further includes an optical isolator in a front stage of the multiplexer and / or a rear stage of the erbium-doped fiber,
An optical variable attenuator that attenuates signal light by adjusting an attenuation amount so as to compensate for a change in power of the signal light input to the optical amplifier between the two or more optical amplification units, and the light A gain equalization filter that performs gain equalization on the signal light attenuated by the variable attenuator;
The control means is further configured to adjust the attenuation amount of the optical variable attenuator, and the attenuation amount is determined based on the detection result of the optical power detection means, and the output signal optical power of the final stage optical amplifying unit. The result of adding / subtracting the ASE correction value to / from the gain of the entire optical amplifier including the attenuation amount, which is the difference between the detection value and the input signal optical power detection value of the first stage optical amplification unit, is 1 2. The gain target value is adjusted so as to change according to input signal light power per wavelength, and the gain target value is obtained by calculating the input signal light power per wavelength. Optical amplifier.   3. The light according to claim 2, wherein the input signal light power per wavelength is obtained by calculation of the number of signal light wavelengths and an input signal light power detection value of the first stage optical amplification unit. amplifier. A demultiplexer for demultiplexing the monitoring wavelength light and the signal light is inserted before the first stage optical amplifying unit, and a photodiode is connected to the demultiplexer. Further comprising a monitoring wavelength optical power detection means for detecting the optical power of
3. The optical amplifier according to claim 2, wherein the input signal light power per wavelength is obtained from the optical power of the monitoring wavelength detected by the monitoring wavelength optical power detection unit.   The ASE correction value is obtained by calculating a correction value of a first ASE generated by the optical amplifier and a correction value of a second ASE generated by an optical amplifier upstream of the optical amplifier. The optical amplifier according to claim 1.   6. The optical amplifier according to claim 5, wherein the correction value of the first ASE changes in accordance with the input signal light power of the first stage optical amplifying unit.   7. The optical amplifier according to claim 6, wherein the correction value of the first ASE is stored in advance in the control unit for every 3 dB of input signal light power of the first-stage optical amplification unit.   The control means includes a first feedforward driving current having a predetermined fixed value and a driving current to the semiconductor laser of each optical amplifying unit by the control, and an input signal optical power of the first stage optical amplifying unit. 8. The optical amplifier according to claim 1, wherein a second feedforward drive current that changes in accordance with the second feedforward drive current is added.   9. The light according to claim 8, wherein the control means performs the addition under a condition that a change speed of an input signal light power of the first stage optical amplifying unit exceeds a predetermined change speed value. amplifier.

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