JP3527627B2 - Optical fiber amplifier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier used as a linear repeater in an optical fiber communication system.

[0002]

2. Description of the Related Art In an optical fiber communication system, an optical fiber amplifier is inserted as a linear repeater for each appropriate transmission distance, and a long distance transmission is achieved by amplifying an attenuated optical signal transmitted through the optical fiber. It is possible. An optical fiber amplifier using stimulated emission can simultaneously amplify a plurality of signal lights having different wavelengths within an amplification band at the same time. Therefore, a wavelength division multiplexing optical transmission system (WDM transmission system) using this has been actively developed and studied in recent years. ing. In addition, a lightwave network (WDM) that uses different frequencies of a plurality of signals for path identification
The transmission method) is also being actively studied.

Generally, in an optical fiber amplifier using a three-level laser transition such as erbium ion (Er ³⁺ ), the gain spectrum changes depending on the input signal light intensity. WD
In the case of amplifying a plurality of signals having different frequencies as in the M transmission / transmission system, even if the gain is optimized to be flat with respect to a certain input signal light intensity, the gain deviation will occur if the input signal light intensity changes. Occurs, and as a result, the output intensity of each signal light varies.

This channel-to-channel variation results in a reduction of the transmittable span between the optical fiber amplifiers. In general, the input signal light intensity to the optical fiber transmission line in optical fiber communication should be such that signal deterioration due to nonlinear phenomena such as four-wave mixing, cross-phase modulation, self-phase modulation, stimulated Brilliant scattering, and Raman crosstalk between channels does not occur. Is set to the upper limit. In addition, the transmission distance is such that the signal light to each optical fiber amplifier becomes small and the signal / noise ratio (SN ratio)
Is set so that it does not get worse.

Here, if the inter-channel intensity varies, the input signal light intensity to the optical fiber transmission line is 1
The signal light with the highest intensity is determined so as not to exceed the upper input limit, and the optical fiber amplifier input is set so that the channel with the lowest intensity does not fall below the lower limit of the optical fiber amplifier input. The transmittable span is the optical fiber transmission line input strength /
Although it is determined by the input intensity of the optical fiber amplifier, when the signal light intensity is set as described above, the transmittable span is reduced by the output intensity deviation between the maximum channel and the minimum channel. For example, if the output deviation between the maximum and minimum channels is 10 dB, the span that can be transmitted is
It will be shortened by 10 dB (about 40 km). Therefore, it is necessary to suppress the gain deviation due to the change of the input signal light intensity as much as possible.

As a method of suppressing this, there is a method of controlling the gain of the optical fiber amplifier to be constant. By controlling the gain to be constant, it is possible to make the output deviation between the channels constant, even if the input intensity varies. As a constant gain control method, (1) any one channel is cut out by an optical filter, and the intensity ratio between the input and output is monitored and fed back to the excitation source (excitation light intensity, semiconductor laser bias current, etc.), (2) A method of monitoring the input-output intensity ratio of all channels and feeding it back to the excitation source, (3) Laser oscillation in the amplification medium, or a control signal light source to keep the total input intensity constant. Generally, a method of making the gain constant optically by keeping it at, or the like. Among them, the methods (1) and (2) are advantageous from the viewpoints of simplicity of control and cost. However, with respect to (1), it is difficult to monitor a specific wave in a network such as a WDM transmission network in which the signal wavelength input to an optical fiber amplifier may fluctuate arbitrarily. Therefore, in the point that it can be applied to a WDM transmission network, the applicable range of (2) is the widest among the above methods, and the constant gain control can be performed accurately. The present invention is applied to the method (2).

FIG. 8 shows the configuration of a conventional optical fiber amplifier corresponding to the method (2). In the figure, a part of the input signal light is branched by the optical branching device 2-1 and the light is detected by the photodetector 3-1.
And the input signal light intensity Pin is detected. on the other hand,
The input signal light is amplified via the optical fiber amplifier 1.
A part of the output signal light is branched by the optical branching device 2-2 and input to the photodetector 3-2, and the output signal light intensity Pout is detected. The optical fiber amplifier 1 includes a rare earth-doped fiber 11, a pumping light source 12-1 that performs forward pumping, a pumping light source 12-2 that performs backward pumping, and a WDM coupler 13-1 that inputs pumping light to the rare earth-doped fiber 11. 13-2
And the optical isolators 14-1, 1 arranged at the input and output ends
And 4-2.

The gain detector 4 monitors the ratio Pout / Pin of the input signal light intensity Pin and the output signal light intensity Pout as the gain A. The error signal extraction unit 5 has a gain A for a predetermined value.
Error signal is extracted and the gain control unit 6 determines that the error signal is 0
The intensity of the excitation light output from the excitation light sources 12-1 and 12-2 is controlled so that

[0009]

By the way, in the constant gain control in the conventional optical fiber amplifier shown in FIG. 8, the ratio Pout / of the input signal light intensity Pin and the output signal light intensity Pout is set.
Pin (gain A) is monitored and controlled so that it is constant. However, the output light of the optical fiber amplifier that uses stimulated emission is the amplified signal light and the amplified spontaneous emission (ASE).
Is added. At this time, if the control shown in FIG.
Since the output signal light intensity and ASE are added to t, Pout / Pin is not an accurate signal gain, and Pout / Pin
Even if / Pin is controlled to be constant, accurate constant gain control cannot be performed. The control error increases as the intensity of the input signal light becomes smaller and the ratio of ASE to the output light intensity increases.

Therefore, it is necessary to remove the influence of ASE,
In general, ASE power does not correspond 1: 1 with gain. Therefore, it is necessary to monitor the ASE intensity and subtract it from Pout to remove the influence, but the control becomes complicated.

It is an object of the present invention to provide an optical fiber amplifier capable of removing the influence of ASE by simple control and performing constant gain control with high accuracy.

[0012]

The optical fiber amplifier according to the present invention is forward-pumped to the rare earth-doped fiber. Then, the input signal light power Pin and the output signal light power P
When detecting the gain from out, P _ASE is input as a constant value (≠ 0) corresponding to the spontaneous emission light power generated in the rare earth-doped fiber, and (Pout-P _ASE ) / Pin is set as the gain. The forward pumping light power is controlled so as to be constant (claim 1).

Further, the optical fiber amplifier of the present invention is forward-pumped and backward-pumped to the rare earth-doped fiber. Then, when the gain is detected from the input signal light power Pin and the output signal light power Pout, (Pout−
P _ASE ) / Pin is used as a gain, and the forward pumping light power Pf and the backward pumping light power Pb are controlled so that this gain becomes constant. However, it is controlled so that Pf> Pb always holds (Claim 2).

Further, when controlling so that Pf> Pb always holds, the forward pumping light power fluctuation ΔPf and the backward pumping light power fluctuation ΔPb with respect to the fluctuation of the input signal light power are always | ΔPf | <| ΔPb | The pumping light power is changed as described above (claim 3). Further, ΔPf = 0 (claim 4) or | ΔPf | = α | ΔPb | (α <1) (claim 5).

Further, when controlling so that Pf> Pb always holds, the forward pumping light power Pf is not the predetermined minimum value including 0 with respect to the fluctuation of the input signal light power when the backward pumping light power Pb is not a predetermined minimum value. The backward pumping light power Pb is controlled after being fixed to a predetermined value, and when the backward pumping light power Pb reaches a predetermined minimum value including 0, the backward pumping light power Pb is fixed and the forward pumping light power Pf is set. Control (claim 6).

Further, the optical fiber amplifier of the present invention is an optical fiber amplifier in which a plurality of optical amplifiers and at least one optical component whose loss does not change are connected in cascade, and the entire input signal optical power Pin and output signal Pin Optical power Pout
Similarly, when detecting the gain from (Pout- _PASE ) / P
The gain is set to in, and the pumping light power of each optical amplifier is controlled so that this gain becomes constant (claim 7).

Note that the gain detecting section is provided with the input signal light power P.
In and the output signal light power Pout are inputted as an electric signal, and the electric circuit is constituted by an electric circuit in which a fixed circuit constant corresponding to a predetermined spontaneous emission light power P _ASE is set (claim 8).

As described above, the optical fiber amplifier of the present invention uses the ASE power P _ASE given as a constant. However, in reality, the effect of ASE cannot be regarded as a constant. That is, the value of ASE and the gain are not always 1
It does not correspond to one-to-one. In the case of an optical fiber amplifier, the equation for propagating the signal light power I _s and the ASE power I _{ASE in} the amplification medium is dI _s / dz = (σ _s N ₂ −σ _a N ₁ ) I _s dI _ASE / dz = (Σ _s N ₂ −σ _a N ₁ ) I _ASE + hνσ _s N ₂
Δν. Here, z is the position in the light traveling direction of the amplification medium,
N ₂ and N ₁ are the densities of the upper and lower levels of the amplification level, σ _s and σ _a are the stimulated emission cross section and the stimulated absorption cross section of the amplification medium at the wavelengths of the signal light and the ASE light, respectively.
ν and Δν respectively indicate the center frequency spread of ASE (reference: “Optical amplifier and its application” Ishio, Nakagawa, et al.
Ohmsha, Chapter 5).

As shown in this equation, I _ASE is an equation similar to I _s that represents amplification, and h ν σ _s N ₂ Δν indicating spontaneous emission light
Is added. This equation is the intensity (gain) of I _s
Is determined by the integrated value of the upper level density and the lower level density of the entire amplification medium regardless of the distribution of the upper level density and the lower level density in the amplifying medium, whereas I _ASE is the upper level density. It also shows that it depends on the distributions of the position density and the lower level density in the amplification medium. That is, the gain of the amplifier and the output ASE power are not independently determined, and the output ASE power cannot be treated as a constant. Therefore, it is usually necessary to perform complicated control such as monitoring the ASE power and eliminating its influence.

However, by keeping the pumping light power on the signal input side of the amplification medium strong, if the gain is constant, ASE
Power can be treated as a constant. This is because when the gain in the amplification medium near the signal input end is high, most of the output ASE power is occupied by the component that the ASE generated near the signal light input end of the amplification medium passes through the amplification medium and is amplified. Is.

Based on this finding, the present invention performs forward pumping (claim 1) and sets the forward pumping light power Pf as high as possible compared to the backward pumping light power Pb (claim 2).
Under Pf> Pb, the forward pumping light power fluctuation ΔPf is set smaller than the backward pumping light power fluctuation ΔPb (claims 3 to 5). Under Pf> Pb, the input is small and the backward pumping power is 0. However, when the control cannot be completed, there is a procedure of controlling by the forward pump light power (claim 6), and the method of varying the pump light is limited. This gives A
The SE power can be treated as a constant.

[0022] The ASE of the influence becomes a problem when the monitoring conventional Pin and Pout by automatic gain control, removed by subtracting electric circuits the preset P _ASE from the monitor value of Pout as constants. That is,
By controlling the optical amplification section (rare earth-doped fiber) so that A = (Pout− _PASE ) / Pin becomes constant, AS
Good gain constant control that eliminates the influence of E becomes possible.
Further, the present invention can be applied as it is even to a situation in which the ASE accumulated in the input signal is included by connecting the optical fiber amplifiers in multiple stages.

[0023]

DETAILED DESCRIPTION OF THE INVENTION

(First Embodiment: Claim 1) FIG. 1 shows a first embodiment of the present invention. In the present embodiment, the forward pumping type optical amplification section 1A is used.

In the figure, the input signal light is an optical splitter 2-1.
Then, a part of it is branched and input to the photodetector 3-1, and the input signal light power Pin is detected. On the other hand, the input signal light is amplified via the optical amplifier 1A. A part of the output signal light is branched by the optical branching device 2-2 and input to the photodetector 3-2, and the output signal light power Pout is detected. The optical amplification unit 1A includes a rare earth-doped fiber 11, a pumping light source 12 that performs forward pumping, a WDM coupler 13 that inputs pumping light to the rare earth-doped fiber 11, and optical isolators 14-1 and 14 arranged at input and output ends. -2 and.

The gain detecting section 4A has an input signal light power Pin.
Then, the output signal light power Pout and the ASE power _PASE are input, and (Pout- _PASE ) / Pin is monitored as the gain A. Note that P _ASE is a constant that does not depend on the input, and this value is set by an electric circuit. The error signal extraction unit 5
The error signal of the gain A with respect to the predetermined value is extracted, and the gain control unit 6A changes the pumping light power output from the pumping light source 12 so that the error signal becomes 0, and controls the gain.

As described above, by adopting the forward pumping configuration, a high pumping state can be obtained in the vicinity of the signal light input end of the rare earth-doped fiber 11, so that the ASE power P _ASE corresponds to the gain of about 1: 1. Can be treated as a constant. Therefore, by setting (Pout- _PASE ) / Pin as the gain detection value, it is possible to remove the influence of ASE that causes an error when monitoring, and it is possible to realize good gain constant control. Since the calculation of (Pout- _PASE ) / Pin can be realized using an electric circuit, high-speed control within a time constant of 1 ms becomes possible.

Although the ASE power P _ASE needs to be initialized, it can be obtained as follows. P _ASE
May measure the power of the spontaneously emitted light that is actually amplified, but it can be obtained as a fitting parameter by the following procedure.

First, the control loop is closed to perform control. Then, the pumping light condition for achieving the desired amplification spectrum at the assumed maximum input signal power is determined, and the signal light gain spectrum (signal gain at two or more wavelengths apart from each other within the amplification band) at that time is measured. To do. Next, the input value of P _ASE is adjusted so that the signal light gain spectrum at the assumed minimum input signal light power matches the signal light gain spectrum measured at the maximum input signal light power. The final value of P _ASE is the predetermined P _ASE. Predetermined P
_ASE can be used as a constant, and good constant gain control is possible even with input fluctuations. The initial setting of the ASE power P _ASE is the same in the following embodiments.

(Second Embodiment: Claims 2 and 3) FIG.
Shows a second embodiment of the present invention. In the present embodiment, a forward pumping type and a backward pumping type optical amplifying section 1B are used.

In the figure, the input signal light is an optical branching device 2-1.
Then, a part of it is branched and input to the photodetector 3-1, and the input signal light power Pin is detected. On the other hand, the input signal light is amplified via the optical amplifier 1B. A part of the output signal light is branched by the optical branching device 2-2 and input to the photodetector 3-2, and the output signal light power Pout is detected. The optical amplification unit 1B includes a rare earth-doped fiber 11, a pumping light source 12-1 that performs forward pumping, and a pumping light source 12- that performs backward pumping.
2 and WD for inputting pumping light into the rare earth-doped fiber 11
It is composed of M couplers 13-1 and 13-2 and optical isolators 14-1 and 14-2 arranged at the input and output ends.

The gain detecting section 4A has an input signal light power Pin.
Then, the output signal light power Pout and the ASE power _PASE are input, and (Pout- _PASE ) / Pin is monitored as the gain A. Note that P _ASE is a constant that does not depend on the input, and this value is set by an electric circuit. The error signal extraction unit 5
The error signal of the gain A for a predetermined value is extracted, and the gain control unit 6B sets the pump light source 12- so that the error signal becomes zero.
By changing the pumping light power output from 1, 12-2,
Control the gain.

The feature of this embodiment is that, when pumping light is input from both ends of the rare earth-doped fiber 11, both the powers of the forward pumping light and the backward pumping light are changed simultaneously. As a changing method, the forward pumping light power Pf and the backward pumping light power Pb are set to Pf> Pb, and the forward pumping light power fluctuation ΔPf and the backward pumping light power fluctuation ΔPb are | ΔPf | The control is performed so that <│ΔPb│. FIG. 2B shows how the power of the forward pumping light and the backward pumping light varies with respect to the extraction error signal.

In this way, the forward pumping light power and the backward pumping light power are varied non-uniformly, the population inversion near the signal light input end of the rare earth-doped fiber 11 is kept high, and the ASE power can be treated as a constant. by determining the gain (Pout-P _ASE) / Pin with P _ASE, it is possible to precisely gain control. Further, this pumping light control method is also effective in not increasing the noise figure.

(Third Embodiment: Claim 4) FIG. 3 shows a third embodiment of the present invention. This embodiment is the same as the second embodiment, in which the forward-pumped and backward-pumped optical amplifier 1 is used.
B is used.

The feature of this embodiment is that when the pumping light is input from both ends of the rare earth-doped fiber 11, the gain control unit 6 is provided.
In C, the forward pumping light power Pf is kept high and constant, and the backward pumping light power Pb is changed to control the gain. FIG. 3B shows power fluctuations of the forward pump light and the backward pump light with respect to the extraction error signal.

As a result, the population inversion near the signal light input end of the rare earth-doped fiber 11 is kept high, and the ASE from the output end is mainly the amplified ASE generated near the input end. Become. Therefore, the ASE power strongly reflects the gain, and P
_ASE can be regarded as a constant. As a result, (Pout
By controlling −P _ASE ) / Pin, it is possible to accurately control the constant gain even if P _ASE is a constant. Further, this pumping light control method is also effective in not increasing the noise figure.

(Fourth Embodiment: Claim 5) FIG. 4 shows a fourth embodiment of the present invention. This embodiment is the same as the second embodiment, in which the forward-pumped and backward-pumped optical amplifier 1 is used.
B is used.

The feature of this embodiment is that, when pumping light is input from both ends of the rare earth-doped fiber 11, both the powers of the forward pumping light and the backward pumping light are changed simultaneously. As a changing method, the forward pumping light power Pf and the backward pumping light power Pb are set to Pf> Pb, and the forward pumping light power fluctuation ΔPf and the backward pumping light power fluctuation ΔPb are set to ΔPf <ΔPb while the forward pumping light power is relatively high. Control so that. The process up to this point is the same as in the second embodiment, but in the present embodiment, regarding the relationship between ΔPf and ΔPb, in the gain control unit 6D, | ΔPf | = α | ΔPb | (where α <1) Control.

For example, when the backward pumping light power Pb is changed from 0 to Pb _max , the forward pumping light power Pf is Pf = (Pf _max −Pf _min ) × Pb / Pb _max + Pf _min (Pf _min > 0) = Α × Pb + β (α <1) according to the relational expression. FIG. 4B shows the power fluctuations of the forward pump light and the backward pump light with respect to the extraction error signal. Here, Pf _max and Pb _max are the maximum pumping light powers of the forward pumping light and the backward pumping light, respectively. In this case, when the input signal light is at the minimum allowable input level, the forward pumping light power becomes Pf _min and the backward pumping light power becomes 0.
The pumping light power near the input end is high. Further, even when the input light changes, the forward pumping light power always becomes larger than the backward pumping light power.

As described above, even when the powers of both the forward pumping light and the backward pumping light are moved, the ratios of the respective moving methods are set as described above, so that the signal light input end of the rare earth-doped fiber 11 near the input end. The population inversion is kept high,
By the control of (Pout- _PASE ) / Pin, the constant gain control can be accurately performed. Since this control method fluctuates both pumping light powers, it is possible to cope with a large input fluctuation and the control is simple. Moreover, the control method of this excitation light is
It is also effective in not increasing the noise figure.

(Fifth Embodiment: Claim 6) FIG. 5 shows a fifth embodiment of the present invention. This embodiment is the same as the second embodiment, in which the forward-pumped and backward-pumped optical amplifier 1 is used.
B is used.

The feature of this embodiment is that in the gain control section 6E, when the forward pumping light power Pf and the backward pumping light power Pb are Pf> Pb and the pumping light is inputted from both ends of the rare earth-doped fiber 11, the backward pumping light is supplied. If the optical power Pb is not a predetermined minimum value including 0, the forward pumping light power Pf is fixed and the backward pumping light power Pb is controlled. When the backward pumping light power Pb reaches a predetermined minimum value including 0, the forward pumping light power Pf is controlled. That is, the method of changing the pumping light power is prioritized. Figure 5
(b) and (c) show the power fluctuations of the forward pump light and the backward pump light with respect to the extraction error signal.

As a result, the population inversion in the vicinity of the signal light input end of the rare earth-doped fiber 11 is kept high and (Pout-P
By the control of _ASE ) / Pin, the gain constant control can be accurately performed. Further, this pumping light control method is also effective in not increasing the noise figure.

(Sixth Embodiment: Claim 7) FIG. 6 shows a sixth embodiment of the present invention. The present embodiment is applied to a configuration in which a plurality of optical amplification units are connected in cascade.

The feature of this embodiment is that a plurality of optical amplifiers 1B are provided.
(Or 1 A) and optical components 8 such as an optical isolator whose loss does not change are connected in cascade, and a pair of Pin monitor and Pout monitor are installed at the input and output ends of the entire amplifier,
The constant gain control is performed according to the detected value.
When the optical component 8 whose loss does not change is inserted as in the present embodiment, (Pout- _PASE ) / Pin is kept constant by the methods shown in the first to fifth embodiments. By performing such control, it becomes possible to perform constant gain control. Also in this case, the control accuracy can be improved by increasing the pumping light power in the vicinity of the signal light input end of the rare earth-doped fiber 11 in each optical amplifier. Also,
This is also effective in not increasing the noise figure.

(Seventh Embodiment) FIG. 7 shows a seventh embodiment of the present invention.
2 shows an embodiment of the present invention. The present embodiment is applied to a configuration in which a plurality of optical amplification units are connected in cascade.

The feature of this embodiment is that a plurality of optical amplifiers 1B are provided.
(Or 1 A) and the optical components 9 with variable loss are connected in cascade, the constant gain control is performed for each optical amplification unit by the method described in the first to fifth embodiments. is there.

That is, when the optical component 9 with variable loss is inserted between the optical amplifiers, the control such that (Pout- _PASE ) / Pin is constant is performed in each optical amplifier. This enables constant gain control of the entire amplifier. Also in this case, the control accuracy can be improved by increasing the pumping light power in the vicinity of the signal light input end of the rare earth-doped fiber 11 by the method shown in the first to fifth embodiments. . This is also effective in not increasing the noise figure.

In the seventh embodiment, an optical attenuator is often inserted between the optical fiber amplifiers as the optical component 9 whose loss varies. By inserting the optical attenuator, even when the input light intensity to each optical fiber amplifier is high, it is possible to prevent the input of the optical fiber amplifier in the subsequent stage from becoming too high and losing the constant gain control. That is, the input dynamic range of the optical fiber amplifier can be increased. In such a case, it is not possible to monitor Pin and Pout of the optical fiber amplifier and perform constant gain control. However, the method shown in the first to fifth embodiments is applied to each optical fiber amplifier. If constant gain control is performed, it is possible to perform control with the gain deviation suppressed satisfactorily.

[0050]

As described above, the optical fiber amplifier according to the present invention can be used for rare-earth-doped fibers by performing forward pumping, or by increasing forward pumping light power even when performing forward pumping and backward pumping. A high pumping state is obtained near the signal light input end, and the ASE power P _ASE can be treated as a constant corresponding to the gain. Therefore, the gain can be detected by (Pout- _PASE ) / Pin, and good constant gain control can be realized by removing the influence of ASE.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical fiber amplifier according to the present invention.

FIG. 2 is a diagram showing a second embodiment of the optical fiber amplifier according to the present invention.

FIG. 3 is a diagram showing a third embodiment of the optical fiber amplifier according to the present invention.

FIG. 4 is a diagram showing a fourth embodiment of the optical fiber amplifier according to the present invention.

FIG. 5 is a diagram showing a fifth embodiment of the optical fiber amplifier according to the present invention.

FIG. 6 is a diagram showing a sixth embodiment of the optical fiber amplifier according to the present invention.

FIG. 7 is a diagram showing a seventh embodiment of the optical fiber amplifier according to the present invention.

FIG. 8 is a diagram showing the configuration of a conventional constant gain control circuit for an optical fiber amplifier.

[Explanation of symbols]

1A Forward pump type optical amplifier 1B Forward pumping and backward pumping type optical amplifier 2 Optical splitter 3 Photodetector 4,4A gain detector 5 Error signal extraction unit 6,6A, 6B, 6C, 6D, 6E Gain control unit 8 Optical components whose loss does not fluctuate 9 Optical components with variable loss 11 Rare earth doped fiber 12 Excitation light source 13 WDM coupler 14 Optical isolator

Continued front page       (56) Reference JP-A-9-162476 (JP, A)                 JP-A-6-308548 (JP, A)                 JP-A-7-38187 (JP, A)                 JP-A-5-292036 (JP, A)                 JP-A-8-111664 (JP, A)

Claims (8)

(57) [Claims] 1. A rare earth-doped fiber used as an amplification medium, an optical amplifier including a pumping means for inputting forward pumping light from an input end of the rare earth-doped fiber, an input signal optical power Pin and an output signal of the optical amplifier. A light detecting unit that detects the optical power Pout, a gain detecting unit that detects a gain in the optical amplifying unit from a detection value of the light detecting unit, a gain detected by the gain detecting unit, and a preset predetermined gain. An optical fiber including an error signal extraction unit that extracts the error signal of (1), and a gain control unit that controls the forward pumping light power according to the error signal to control the gain in the optical amplification unit to be constant. In the amplifier, the gain control unit controls the forward pump power.
Is generated by the optical amplifier with a predetermined gain during operation.
Naturally, the emitted light power P _ASE is a constant value (≠ 0).
And the gain detection unit controls the spontaneous emission light power P of the constant value.
An optical fiber amplifier having a configuration in which _ASE is input and (Pout- _PASE ) / Pin is output to the error signal extraction unit as a detection gain of the optical amplification unit. 2. A rare earth-doped fiber used as an amplification medium, and an optical amplifier including a pumping means for inputting forward pumping light and backward pumping light from both an input end and an output end of the rare earth-doped fiber, and the optical amplifier. Of the input signal light power Pin and the output signal light power Pout, a gain detector for detecting the gain in the optical amplifier from the detection value of the light detector, and a gain detector for detecting the gain. An error signal extraction unit that extracts an error signal between a gain and a preset predetermined gain, and a gain in the optical amplification unit that varies the forward pumping light power Pf and the backward pumping light power Pb according to the error signal. In the optical fiber amplifier, the gain control section controls the forward pumping light power P f and the forward pumping light power P f to be constant.
By controlling the serial backward pumping light power P b to always be Pf> Pb, the optical amplification unit is operating in the predetermined gain
The spontaneous emission light power P _ASE sometimes generated is a constant value (≠ 0)
And the gain detection unit is configured such that the spontaneous emission light power P having the constant value is
An optical fiber amplifier having a configuration in which _ASE is input and (Pout- _PASE ) / Pin is output to the error signal extraction unit as a detection gain of the optical amplification unit. 3. The optical fiber amplifier according to claim 2, wherein the gain control section has a forward pumping light power fluctuation ΔPf and a backward pumping light power fluctuation ΔPb with respect to a fluctuation of the input signal light power.
The optical fiber amplifier is characterized in that the pumping means is controlled so that | ΔPf | <| ΔPb | always holds. 4. The optical fiber amplifier according to claim 3, wherein the gain controller sets ΔPf = 0. 5. The optical fiber amplifier according to claim 3, wherein the gain controller includes | ΔPf | = α | ΔPb |
An optical fiber amplifier characterized in that the pumping means is controlled so as to satisfy <1). 6. The optical fiber amplifier according to claim 2, wherein the gain control unit does not respond to fluctuations in the input signal light power unless the backward pump light power Pb is a predetermined minimum value including 0. The backward pumping light power Pb is controlled after fixing the power Pf to a predetermined value, and when the backward pumping light power Pb reaches a predetermined minimum value including 0, the backward pumping light power Pb is fixed and then the forward pumping light Pb. An optical fiber amplifier having a structure for controlling power Pf. 7. An optical fiber amplifier in which a plurality of optical amplification units according to claim 1 or 2 and at least one optical component whose loss does not change are connected in cascade. The light detection means for detecting the input signal light power Pin and the output signal light power Pout, the detection value of the light detection means, and the spontaneous emission light power generated at the time of operation of each of the light amplification parts with a predetermined gain set in advance. Enter P _ASE as the corresponding constant value (≠ 0),
A gain detection unit that outputs (Pout- _PASE ) / Pin as a detection gain in the plurality of optical amplification units as a whole, and an error signal between the gain detected by the gain detection unit and a preset predetermined gain is extracted. An error signal extraction unit, and the forward pumping light power Pf and the backward pumping light power Pb of each of the optical amplifying units based on the control method according to any one of claims 2 to 6 according to the error signal. And a gain control unit that controls the gains of the plurality of optical amplification units to be constant. 8. The optical fiber amplifier according to claim 1, wherein the gain detecting section uses the input signal light power Pin and the output signal light power Pout detected by the light detecting means as electric signals. An optical fiber amplifier characterized by comprising an electric circuit in which a fixed circuit constant corresponding to a predetermined spontaneous emission light power P _ASE is set.