JP3379104B2 - Optical amplifier for WDM transmission - 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 amplifier for wavelength division multiplex transmission that collectively amplifies signal lights of multiple wavelengths in a wavelength band of 1.55 μm in a wavelength division multiplex transmission system.

[0002]

2. Description of the Related Art Conventionally, wavelength-division multiplex transmission has been performed by using multi-wavelength signal light in the wavelength band of 1.55 μm.
In such a wavelength division multiplex transmission system, an optical amplifier for wavelength division multiplex transmission that collectively amplifies signal lights of multiple wavelengths is used. This wavelength division multiplexing optical amplifier has a large gain, a wide wavelength band capable of optical amplification, a flat wavelength dependence of the gain in the wavelength band, an excellent optical SN ratio, and a good noise figure. And a wide dynamic range of the input signal light level are required, and research and development have been made from such a viewpoint.

For example, the one disclosed in Japanese Unexamined Patent Publication No. 5-48207 includes an optical amplifier whose gain for signal light is controlled to be constant, and an optical attenuator having a variable attenuation amount for signal light in the subsequent stage. It is a combination. Although the output signal light level from the optical amplifier also changes when the input signal light level changes, the output signal light level from the optical attenuator is kept constant by controlling the attenuation amount of the optical attenuator.
By doing so, an attempt is made to widen the dynamic range of the input signal light level with a good noise figure.

Further, the wavelength dependence of the gain for the signal light in the optical amplifier also depends on the gain. Therefore, in general, the gain of the optical amplifier and the gain equalizer having a fixed gain equalization characteristic is used while the gain of the optical amplifier is controlled to be a constant average gain. The wavelength dependence is flattened.

Also, for example, the one described in the document "Miwa Saeki, et al.," Optical Fiber Amplifier with Dynamic Equalizing Function ", NEC Technique, Vol. 51, No. 4, pp. 45-48 (1998)". Is a combination of a pre-stage optical amplifier, a gain equalizer, and a post-stage optical amplifier in this order, in which the gain equalizer is an optical circulator and an arrayed-waveguide diffraction grating (AWG: Arra).
yed Waveguide Grating). With such a configuration, the level of the signal light of each wavelength is detected, and the gain equalization characteristic of the gain equalizer is dynamically adjusted based on the detection result, and the wavelength of the entire gain is adjusted. Dependency is flattened.

[0006]

However, the conventional optical amplifier for wavelength division multiplexing has the following problems. In the one disclosed in the above publication, if the constant gain control in the optical amplifier is to be maintained in the region where the input signal light level is large as in the region where the input signal light level is small, the output of the optical amplifier is necessarily increased. Required, and also
In order to make the output signal light level of each wavelength of the latter-stage optical amplifier constant, it is necessary to increase the attenuation amount of the optical attenuator to the same extent as the increase amount of the input signal light level. Therefore,
If the dynamic range of the input signal light level is to be widened, pump light of large power must be supplied to the optical amplifier, which causes an increase in power consumption and a shortened life of the pump light source.

Further, in the device described in the above-mentioned document, since the insertion loss of the gain equalizer for flattening the wavelength dependence of the overall gain is 15 dB, the gain of the pre-stage optical amplifier is as high as 30 dB. I can't help it. Further, in order to supply the pumping light of large power to the pre-stage optical amplifier, the pre-stage optical amplifier alone has two pumping light sources. Therefore,
The one described in this document also causes an increase in power consumption and a shortened life of the excitation light source.

The present invention has been made in order to solve the above-mentioned problems, and wavelength division multiplexing capable of widening the dynamic range of the input signal light level without requiring the supply of pumping light of large power. An object is to provide an optical amplifier for transmission.

[0009]

An optical amplifier for wavelength division multiplex transmission according to the present invention comprises a pre-stage optical amplifier and a post-stage optical amplifier. The pre-stage optical amplifier includes (1) a first amplification optical fiber that optically amplifies the input signal light and outputs (2)
Vs. minimum input signal level at the maximum number of wavelengths to be used
Excitation light path of the minimum pump power that causes gain saturation Te
A first pumping means for supplying the first pumping light to the first amplification optical fiber as the upper limit of the power, and (3) the maximum number of wavelengths.
When the input signal light is input , the amplification gain of the signal light in the first amplification optical fiber decreases in proportion to the increase amount of the level of the input signal light without causing gain saturation. And a first pumping light control means for controlling the first pumping means. The second-stage optical amplifier includes (1) a second amplification optical fiber that optically amplifies and outputs the signal light input from the first-stage optical amplifier, and (2) a second amplification optical fiber in the second amplification optical fiber. And (3) a second pumping means for supplying a second gain so that the level of the signal light output from the second amplification optical fiber becomes a second gain or an output target value.
And a second excitation light control means for controlling the excitation means.

The wavelength division multiplexing optical amplifier according to the present invention operates as follows. The input signal light is sequentially optically amplified by the pre-stage optical amplifier and the post-stage optical amplifier and output. At the time of optical amplification of the signal light in the former optical amplifier,
A first supply for supplying a first pumping light to a first amplification optical fiber
Excitation means, the first excitation light controlling means, the maximum wave of
Does not cause gain saturation when inputting a long number of signal lights
In addition, it is controlled so that the amplification gain of the signal light in the first amplification optical fiber decreases in proportion to the increase amount of the level of the input signal light. The first excitation light control means is
For the minimum input level at the maximum number of wavelengths to be used
The pump light power is set to the minimum pump light power that causes gain saturation.
As the upper limit of the excitation light, excitation light below this upper limit (preferably below 90%, more preferably below 80%) is supplied.
In the optical amplification of the signal light in the post-stage optical amplifier, the second pumping means for supplying the second pumping light to the second amplifying optical fiber is the signal light output from the second amplifying optical fiber. Is controlled to the second gain or output target value. By performing such control,
Minimize the deterioration of noise figure and gain flatness without continuously supplying pump light with excessive power to keep the gain of the pre-stage optical amplifier substantially constant over the entire range of input signal light level to be used. However, constant output or gain control can be performed as a whole.

In the wavelength-division-multiplexing transmission optical amplifier according to the present invention, the first pumping light control means sets the level of the signal light output from the first amplification optical fiber to the first output target value. It is characterized in that the first excitation means is controlled so.
In this case, the output constant control is also performed in the pre-stage optical amplifier. Each of the first and second output target values is not limited to a value at a certain point but a value having a certain range, and the signal light output from each of the pre-stage optical amplifier and the post-stage optical amplifier. Is controlled to fall within the range. In this way, both the front-stage optical amplifier and the post-stage optical amplifier perform constant output control, so that the gain of the first optical amplification optical fiber decreases with respect to the input signal light level of the front-stage optical amplifier by simple control. The first pumping means is controlled so that the pumping light of excessive power is not continuously supplied to keep the gain of the pre-stage optical amplifier substantially constant over the entire range of the input signal light level to be used, It is possible to perform constant output or gain control as a whole while suppressing deterioration of noise figure and gain flatness to a minimum.

The wavelength-division-multiplexing optical amplifier according to the present invention further comprises an input signal light level detecting means for detecting the level of the signal light input to the pre-stage optical amplifier, and the first (or second) pumping light is provided. The control means sets the first (or second) gain or the output target value according to the level of the signal light detected by the input signal light level detection means. In this case, in the front stage (or rear stage) optical amplifier, the first (or second) in the first (or second) pumping light control means is based on the input signal light level detected by the input signal light level detection means. ) Is set or the target output value is set, and constant gain or output control is performed so that the output signal light level becomes the set first (or second) target gain or output target value. By doing so, the wavelength dependence of the overall gain can be kept small even if the input signal light level changes.

The wavelength division multiplexing optical amplifier according to the present invention further comprises an input signal light level detecting means for detecting the level of the signal light input to the pre-stage optical amplifier, and the first (or second) pumping light is provided. The control means sets the gain of optical amplification in the first (or second) amplification optical fiber according to the level of the signal light detected by the input signal light level detection means, and based on this gain, the first (or Alternatively, the second excitation means is controlled. In this case, in the front stage (or rear stage) optical amplifier, the gain of optical amplification in the first (or second) amplification optical fiber is set based on the input signal light level detected by the input signal light level detecting means. Then, the first (or second) excitation means is controlled based on this gain. By doing so, the output signal light level is set to the first level in the front stage (or rear stage) optical amplifier.
The gain or output constant control is performed so that the (or second) gain or output target value is achieved, and the wavelength dependence of the overall gain can be kept small even if the input signal light level changes.

Further, in this case, the front stage (or rear stage) optical amplifier includes the first optical amplifying section and the second optical amplifying section, and the first (or second) pumping light control means detects the input signal light level. It is preferable that the gain of optical amplification in each of the first optical amplification section and the second optical amplification section is set according to the level of the signal light detected by the means. In this case, even if the first optical amplifying unit and the second optical amplifying unit alone cannot perform complete gain or constant output control, the front stage (including the first optical amplifying unit and the second optical amplifying unit Alternatively, the entire optical amplifier (in the latter stage) can perform complete gain or constant output control.

Further, the wavelength division multiplexing optical amplifier according to the present invention is provided between the input optical signal level detecting means for detecting the level of the optical signal input to the pre-stage optical amplifier and the pre-stage optical amplifier and the post-stage optical amplifier. The optical attenuator having a variable attenuation with respect to the signal light, and the attenuation control means for controlling the attenuation in the optical attenuator according to the level of the signal light detected by the input signal light level detection means. Is characterized by. In this case, the signal light is optically amplified by the pre-stage optical amplifier whose output is controlled to be constant, and then attenuated by the optical attenuator by an attenuation amount corresponding to the input signal light level detected by the input signal light level detecting means. Then, the light is amplified by the post-stage optical amplifier whose output is controlled to be constant. By doing so, the wavelength dependence of the overall gain can be kept small even if the input signal light level changes.

Further, the optical amplifier for wavelength division multiplex transmission according to the present invention further comprises a gain equalizer provided between the front-stage optical amplifier and the rear-stage optical amplifier, which equalizes the wavelength dependence of the gain for the signal light as a whole. It is characterized by being provided. In this case, the signal light is optically amplified by a pre-stage optical amplifier whose output is controlled to be constant, gain-equalized by a gain equalizer, and then optically amplified by a post-stage optical amplifier whose output is controlled to be constant. By doing so, the wavelength dependence of the overall gain can be further reduced.

Further, in the optical amplifier for wavelength division multiplex transmission according to the present invention, the first amplification optical fiber is an Al co-doped Er-doped optical fiber, and the second amplification optical fiber is Al.
The / P co-doped Er-doped optical fiber and the Al co-doped Er-doped optical fiber are connected in this order. In this case, the gain of the pre-stage optical amplifier is small,
Even if the level of the signal light input to the subsequent optical amplifier is low, the wavelength dependence of the overall gain can be kept small. In the second amplification optical fiber, the Al / P co-doped Er-doped optical fiber and the Al co-doped Er-doped optical fiber may be directly connected, or another amplification optical fiber may be provided between them. May intervene. Further, the Al co-doped Er-doped optical fiber is one in which the P element is not substantially added.

In the wavelength division multiplexing optical amplifier according to the present invention, the first pumping means supplies the first pumping light to the first amplifying optical fiber at least from the front side. In this case, even if the pre-stage optical amplifier has a low gain and the population of the first amplification optical fiber is low, the population inversion in the vicinity of the signal light incident side of the first amplification optical fiber is high. Since the state can be raised, the deterioration of the noise figure can be reduced.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

(First Embodiment) First, a first embodiment of the optical amplifier for wavelength division multiplexing according to the present invention will be described. FIG. 1 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to the first embodiment. In the wavelength division multiplexing optical amplifier according to this embodiment, a pre-stage optical amplifier 100 and a post-stage optical amplifier 200 are connected in this order between an input end 11 and an output end 12. Further, a light receiving element 331 and an optical coupler 332 are provided on the signal light input side of the pre-stage optical amplifier 100,
These constitute input signal light level detection means for detecting the level of the signal light input to the pre-stage optical amplifier 100. Further, a control unit 333 that controls the entire optical amplifier is provided.

The pre-stage optical amplifier 100 includes an optical isolator 151, an optical coupler 122, and an amplification optical fiber (first amplification optical fiber) in order from the signal light input side to the signal light output side.
111, the optical isolator 152, and the optical coupler 132 are connected. Each of the optical isolators 151 and 152 allows light to pass in the forward direction but blocks light in the reverse direction. The amplification optical fiber 111 is a silica-based optical fiber in which a rare earth element is added to the core region, and in particular,
An optical fiber in which an Er element is added to the core region as a rare earth element is preferable, and an optical fiber in which an Al element is co-added to the core region is more preferable.

A pumping light source 121 is connected to the optical coupler 122. The pumping light source 121 is the amplification optical fiber 11
Excitation light to be supplied to 1 (wavelength 0.98 μm or 1.4
8 μm), and for example, a semiconductor laser light source is preferably used. The optical coupler 122 is the excitation light source 1
21. The pumping light output from 21 is input, the pumping light is output to the amplification optical fiber 111, and the signal light output from the optical isolator 151 is input, and the signal light is output to the amplification optical fiber 111. . That is, the pumping light source 121 and the optical coupler 122 constitute a first pumping unit that supplies pumping light to the amplification optical fiber 111.

A light receiving element 131 is connected to the optical coupler 132. The optical coupler 132 is used for the amplification optical fiber 11
A part of the signal light output from 1 is branched and the light receiving element 13
Output to 1. The light receiving element 131 receives the signal light that has arrived from the optical coupler 132 and outputs an electric signal corresponding to the power of the signal light, and for example, a photodiode is preferably used. That is, these light receiving elements 13
1 and the optical coupler 132 constitute a first output signal light level detecting means for detecting the level of the signal light output from the amplification optical fiber 111.

The pre-stage optical amplifier 100 is also provided with a control section (first pumping light control means) 141. The control unit 141 controls the pumping light source 121 so that the amplification gain of the signal light in the amplification optical fiber 111 decreases in proportion to the increase amount of the input signal light level. In particular,
The control unit 141 inputs the electric signals output from the light receiving elements 331 and 131, detects the level of the signal light output from the amplification optical fiber 111 based on the electric signals, and detects the level. The output signal light level that is determined by the control unit 141 or the control unit 333 is such that the gain decreases in proportion to the increase amount of the input signal light level. Alternatively, it is preferable to control the power of the pumping light supplied from the pumping light source 121 to the amplification optical fiber 111 so that the power becomes the output target value). The gain or output target value is not limited to a value at a certain point, but has a certain range, and the gain or output level of the signal light output from the pre-stage optical amplifier 100 is within the range. Controlled to enter. Further, the power of the pumping light supplied to the amplification optical fiber 111 is such that it gives a necessary optical SN ratio, and is 90% or less (more preferably 80% or less) of the pumping light power at which the gain is saturated. Is preferred.

The post-stage optical amplifier 200 includes an optical isolator 251, an optical coupler 222, and an amplification optical fiber (second amplification optical fiber) in order from the signal light input side to the signal light output side.
211 and 212, an optical coupler 224, an optical isolator 252 and an optical coupler 232 are connected. Each of the optical isolators 251 and 252 passes light in the forward direction but blocks light in the reverse direction. Each of the amplification optical fibers 211 and 212 is a silica-based optical fiber in which a rare earth element is added to the core region.
An optical fiber in which an Er element is added to the core region as a rare earth element is suitable. Further, it is more preferable that the amplifying optical fiber 211 has Al element and P element co-doped in the core region, and that the amplifying optical fiber 212 has Al element co-doped in the core region. .

A pumping light source 221 is connected to the optical coupler 222. The pumping light source 221 is the amplification optical fiber 21.
Excitation light to be supplied to 1 and 212 (wavelength 1.48μ
m) is output, and for example, a semiconductor laser light source is preferably used. The optical coupler 222 is a pump light source 221.
The pumping light output from the optical isolator 2 is input, and the pumping light is output to the amplification optical fiber 211.
The signal light output from 51 is input, and the signal light is output to the amplification optical fiber 211. Also, the optical coupler 224
An excitation light source 223 is connected to. Excitation light source 223
Outputs the pumping light (wavelength 1.48 μm) to be supplied to the amplification optical fibers 211 and 212, and for example, a semiconductor laser light source is preferably used. Optical coupler 2
24 receives the pumping light output from the pumping light source 223, outputs the pumping light to the amplification optical fiber 212, inputs the signal light output from the amplification optical fiber 212, and outputs the signal light. Output to the isolator 252. That is, the excitation light source 221 and the optical coupler 2
22 and the pump light source 223 and the optical coupler 224,
It constitutes a second pumping means for supplying pumping light to the amplification optical fibers 211 and 212.

A light receiving element 231 is connected to the optical coupler 232. The optical coupler 232 is used for the amplification optical fiber 21.
A part of the signal light output from 2 is branched and the light receiving element 23
Output to 1. The light receiving element 231 receives the signal light that has arrived from the optical coupler 232 and outputs an electric signal corresponding to the power of the signal light, and for example, a photodiode is preferably used. That is, these light receiving elements 23
1 and the optical coupler 232 constitute second output signal light level detection means for detecting the level of the signal light output from the amplification optical fiber 212.

The post-stage optical amplifier 200 is also provided with a control section (second pumping light control means) 241. The control unit 241 inputs the electric signals output from the light receiving elements 131 and 231 and detects the level of the signal light output from the amplification optical fibers 111 and 212 based on the electric signals, and From the pumping light sources 221, 223 to the amplification optical fiber 21 so that the detected output signal light level becomes a predetermined gain or output target value (second gain or output target value) determined by the control unit 333.
The power of the pumping light supplied to 1 and 212 is controlled. The output target value is not limited to a value at one point, but is a value having a certain range, and the post-stage optical amplifier 20
The level of the signal light output from 0 is controlled to fall within the range.

The optical amplifier for wavelength division multiplex transmission according to this embodiment operates as follows. In the pre-stage optical amplifier 100, the pumping light output from the pumping light source 121 is supplied to the amplification optical fiber 111 via the optical coupler 122. In the latter-stage optical amplifier 200, the pumping light output from the pumping light source 221 is supplied to the amplification optical fibers 211 and 212 via the optical coupler 222, and the pumping light source 223 is supplied.
The pumping light output from the optical fiber is supplied to the amplification optical fibers 212 and 211 via the optical coupler 224.

At this time, when multi-wavelength signal light in the wavelength band of 1.55 μm is input to the input end 11, a part of the signal light is branched by the optical coupler 332, received by the light receiving element 331, and received. An electric signal corresponding to the power of the signal light is output from the light receiving element 331 to the control units 141 and 333, and is sequentially input to the amplification optical fiber 111 via the optical isolator 151 and the optical coupler 122 and is optically amplified. The signal light optically amplified by the amplification optical fiber 111 and output is an optical isolator 152, an optical coupler 132, an optical isolator 251 and an optical coupler 22.
2 is sequentially input to the amplification optical fibers 211 and 212, and is optically amplified. The signal light optically amplified by the amplification optical fibers 211 and 212 and output is the optical coupler 22.
4, the optical isolator 252 and the optical coupler 232 are sequentially passed, and the signal is output from the output terminal 12.

At the time of optical amplification in the pre-stage optical amplifier 100, a part of the signal light output from the amplification optical fiber 111 is branched by the optical coupler 132 and received by the light receiving element 131.
The light receiving element 131 outputs an electric signal corresponding to the power of the received signal light to the control unit 141. Then, based on the electric signal, the control unit 141 causes the level of the signal light output from the amplification optical fiber 111 to increase the gain in proportion to the increase amount of the input signal light level by the control unit 141 or the control unit 333. From the pumping light source 121 to the amplification optical fiber 11 so that the predetermined gain or output target value determined to have a decreasing relationship is achieved.
The power of the pumping light supplied to the No. 1 is controlled. Further, in the pre-stage optical amplifier 100, when the decrease amount of the gain of the pre-stage optical amplifier 100 is set equal to the increase amount of the input signal light level, the output constant control is performed based only on the electric signal from the light receiving element 131. Good.

During optical amplification in the post-stage optical amplifier 200, part of the signal light output from the amplification optical fiber 212 is branched by the optical coupler 232 and received by the light receiving element 231, and the received signal light is received. An electric signal corresponding to the power of the light is output from the light receiving element 231 to the control unit 241. Further, the output electric signal of the front-stage optical amplifier 100 that determines the input signal light level of the rear-stage optical amplifier 200 is similarly output from the light receiving element 131 to the control unit 241. Then, the pump light source 221 is controlled by the control unit 241 so that the level of the signal light output from the amplification optical fiber 212 becomes a predetermined gain or an output target value determined by the control unit 333 based on the electric signals. The power of the pumping light supplied to the amplification optical fibers 211 and 212 from is controlled, and the power of the pumping light supplied from the pumping light source 223 to the amplification optical fibers 212 and 211 is controlled. Further, also in the latter-stage optical amplifier 200, the light receiving element 231
The output constant control may be performed only on the basis of the electric signal from.

Optical amplifier for WDM transmission according to the present embodiment
Has the following wavelength dependence of gain. Figure 2 is a wave
Input signal light level and output in long-division optical amplifier
It is a figure which shows the relationship with signal light level for every wavelength. In general
Is the output signal light level P_out(Detected by the light receiving element 231
The output level) is the input signal light level P_in(Light receiving element
331 level detected by 331).
Moreover, the dependence depends on the wavelength λ. Ie output
Signal light level P_outIs the input signal light level P_inAnd waves
It is a function of long λ. Here, the input in the signal light of wavelength λ
Power signal light level P _inOutput signal light level per 1 dB change of
Bell P_outThe change amount of is represented by DGT (λ). This DGT
(λ) is

[0034]

[Equation 1]

It is represented by the following equation.

FIG. 3 is a diagram showing the relationship between the wavelength of the signal light and the DGT value in the wavelength division multiplexing optical amplifier according to the first embodiment. In this figure, the solid line indicates the front-stage optical amplifier 10
For the increment of the input signal light level of 0, the pre-stage optical amplifier 1
00 gain reduction amount is set to be equal, and the pre-stage optical amplifier 10
The case where the output of both 0 and the post-stage optical amplifier 200 is controlled to be constant is shown. The broken line indicates that the pre-stage optical amplifier performs constant gain control that does not change the gain depending on the input signal light level.
In the conventional case where the post-stage optical amplifier performs constant output control by an optical variable attenuator installed at the post-stage input, when the input level becomes high and the pre-stage optical amplifier performs constant pump optical power control due to the limitation of the pump optical power. Indicates. As shown in this figure, the difference between the DGT values at the wavelengths of 1535 nm and 1560 nm is about 1.2 dB / dB in the conventional case (broken line), whereas in the case of the present embodiment (solid line). Is reduced to about 0.6 dB / dB.

As described above, in this embodiment, the output of both the pre-stage optical amplifier 100 and the post-stage optical amplifier 200 is controlled to be constant, so that the DGT (λ ) Value of wavelength λ
The dependence can be reduced. Therefore, the wavelength dependence of the overall gain is small even when the input signal light level changes,
The dynamic range of the input signal light level can be widened. Further, in this embodiment, since the optical component that gives a large loss to the signal light is not inserted, it is not necessary to increase the gain of the pre-stage optical amplifier 100 as compared with the conventional case, and it is necessary to supply the pumping light with a large power. do not do.

Further, in this embodiment, the post-stage optical amplifier 20 is used.
0, the amplification optical fiber 211 is an Er-doped optical fiber in which the Al element and the P element are co-doped in the core region, and the amplification optical fiber 212 in the subsequent stage is an Er-doped optical fiber in which the Al element is co-doped. It is an optical fiber. Therefore, even if the gain of the former optical amplifier 100 is small and the level of the signal light input to the latter optical amplifier 200 is small, the wavelength dependence of the overall gain can be kept small.

Further, in this embodiment, the pre-stage optical amplifier 1
At 00, the pumping light output from the pumping light source 121 is supplied to the amplification optical fiber 111 from the front. Therefore, even if the pre-stage optical amplifier 100 has a low gain and the amplification optical fiber 111 as a whole has a low population inversion state,
Since the population inversion state in the vicinity of the signal light incident side of the amplification optical fiber 111 can be increased, it is possible to reduce the deterioration of the noise figure.

FIG. 4 is a diagram showing each of the noise figure, the optical SN ratio and the gain deviation in the wavelength division multiplexing optical amplifier. In this figure, the solid line shows the case of this embodiment, and the broken line shows the case of the prior art. As shown in this figure, in the conventional case (broken line), when the input signal light level is equal to or lower than the maximum input level I ₁ where the gain can be kept constant, the pre-stage optical amplifier can maintain the constant gain control. Therefore, although the gain flatness is not deteriorated (FIG. 4 (c)), the noise factor cannot be lowered because the gain of the pre-stage optical amplifier is limited to the gain when I _{1 is} input (FIG. 4 (a)). Also, in the case of the conventional case (broken line)
In addition, when the input signal light level is I ₁ or more, the pre-stage optical amplifier cannot maintain the constant gain control, so the pumping light power becomes constant, and the gain flatness deteriorates as the input signal light level increases (Fig. 4 (c)).

On the other hand, in the present embodiment (solid line),
When the number of wavelengths used is the highest, the input signal light level is the highest.
Is controlled so that the highest gain is obtained at a low level.
Therefore, the noise figure is
It can be set smaller and the input signal light level is higher.
The noise figure deteriorates somewhat in the
(FIG. 4 (a)). Furthermore, comparing with the conventional case (broken line)
Then, in this embodiment (solid line), the gain flatness is excellent.
(FIG.4 (c)). However, the input signal light level I ₁than
In the high region, the optical SNR of a single optical amplifier is
The present embodiment is inferior to the case (FIG. 4B).
However, when conventional optical amplifiers are connected in multiple stages and relay amplification is performed,
Input signal light level I₁Gain in higher regions
Since the flatness is significantly degraded, the gain is smaller than the section loss.
Whereas the signal light of short wavelength causes deterioration of the optical SN ratio,
In the case of this embodiment, the gain flatness is excellent.
Thus, the deterioration of the optical S / N ratio is suppressed even if the multi-stage connection is performed. did
Therefore, compared with the conventional case (broken line), this embodiment (actual
Line), the wide dynamic range of the input signal light level
Optical SN ratio required to maintain good gain flatness over
Can be secured.

This can be explained by using mathematical expressions and FIG. 5 as follows. The amplification gain in the pre-stage optical amplifier 100 is G ₁ [dB], and the noise figure is NF ₁ [d
B]. The amplification gain in the latter-stage optical amplifier 200 is set to G
₂ [dB] and the noise figure is NF ₂ [dB]. Between the front-stage optical amplifier 100 and the rear-stage optical amplifier 200, the attenuation amount α
It is assumed that an _m [dB] optical attenuator is provided. In addition, the input signal light level is P _in [dBm].

At this time, the overall gain G _t [dB] of the wavelength division multiplexing optical amplifier according to the present embodiment is

[0044]

[Equation 2]

It is expressed by the following equation. Then, with respect to the fluctuation ΔP _in of the input signal light level, the amplification gain G ₁ in the pre-stage optical amplifier 100 and the amplification gain G ₂ in the post-stage optical amplifier 200 are respectively set.

[0046]

[Equation 3]

The amount is changed by the amount represented by the following equation. By controlling the amplification gains of the front-stage optical amplifier 100 and the rear-stage optical amplifier 200 in this way, the overall gain G _t of the wavelength-multiplexing transmission optical amplifier is

[0048]

[Equation 4]

It changes by an amount represented by the following equation. That is, by controlling the amplification gain G ₂ in the post-stage optical amplifier 200 according to the increase / decrease in the amplification gain G ₁ in the pre-stage optical amplifier 100, the output constant control can be performed as a whole. Here, the expression (3b) is an expression necessary for making the signal light output level of the entire optical amplifier including the pre-stage optical amplifier 100 and the post-stage optical amplifier 200 constant, and is the signal light output level of the whole optical amplifier. This does not apply if it is not necessary to keep constant.

The overall optical SN ratio [dB] and noise figure NF _{t of the} wavelength division multiplexing optical amplifier according to the present embodiment.
[DB] each

[0051]

[Equation 5]

It is expressed by the following equation. Here, h is Planck's constant and ν is the frequency of the signal light. As can be seen from these equations, when the change amount Delta] f _t of f _t is small relative to the change in Delta] p _in the p _in, even if the noise figure NF _t is deteriorated input signal light level P _in is increased, the whole The optical SN ratio of the optical signal continues to increase. The overall noise figure NF _t is basically irrelevant to the amplification gain G ₂ in the post-stage optical amplifier 200. The amplification gain G ₁ in the front-stage optical amplifier 100 decreases, or the attenuation amount α _m in the middle-stage optical attenuator
Increases, the overall noise figure NF _t increases.

FIG. 5 is a diagram showing the relationship between the input signal light level P _in and the overall noise figure NF _{t in} the wavelength division multiplexing optical amplifier. FIG. 7A is a diagram when the attenuation amount α _m of the middle-stage optical attenuator is fixed, and FIG. 11B is a middle-stage optical attenuator for controlling the output constant and flattening the gain wavelength characteristic. It is a figure in case the amount of attenuation (alpha) _{m of} is increased. Further, in each of FIGS. 10A and 10B, the input signal light level P _{in is shown} for the case of the variable gain control of the present embodiment, the conventional constant gain control, and the conventional previous-stage pumped light power maximum value constant control, respectively. And the overall noise figure NF _t .

The above FIGS. 4 and 5 and equation (2)
As can be seen from the expression (5), the amplification gain G ₁ in the pre-stage optical amplifier 100 is proportional to the increase amount of the input signal light level P _in.
By performing the gain variable control so that the relationship is reduced (equation (3a)), while maintaining the required optical SN ratio over the wide dynamic range of the input signal light level ((5
Expression (a), FIG. 4 (b), and deterioration of the noise figure NF _t can be suppressed (expression (5c), FIG. 4 (a), FIG. 5).
(A)). Further, when the attenuation amount α _m of the optical attenuator in the middle stage is increased in accordance with the increase of the input signal light level P _in for constant output control and flattening of the gain wavelength characteristic, the overall noise figure NF _t generally increases. (Equation (5c)), the gain variable control is performed so that the amplification gain G ₁ in the pre-stage optical amplifier 100 increases in proportion to the decrease amount of the input signal light level P _in (Equation (3a)). The noise figure NF _t can be made smaller as the light level P _in is lower ((5c)).
Formula), FIG. 5 (b)).

(Second Embodiment) Next, a second embodiment of the optical amplifier for wavelength division multiplexing according to the present invention will be described. FIG. 6 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to the second embodiment. Compared with the case of the first embodiment, the wavelength division multiplexing optical amplifier according to the second embodiment is
The difference is that the control unit 141 of the front-stage optical amplifier 100 sets an output target value according to the input signal light level.

FIG. 7 is a diagram showing the relationship between the input signal light level and the output target value in the pre-stage optical amplifier 100 in the wavelength division multiplexing optical amplifier according to the second embodiment. As shown in this figure, the higher the input signal light level detected by the light receiving element 331 is, the smaller the output target value is set in the controller 141 of the pre-stage optical amplifier 100. The slope of the output target value with respect to the input signal light level is, for example, −
It is about 0.6 dB / dB. And the front-stage optical amplifier 1
At the time of optical amplification in 00, the pump light supplied from the pump light source 121 to the amplification optical fiber 111 is controlled by the control unit 141 so that the level of the signal light output from the amplification optical fiber 111 reaches the set output target value. Power is controlled.

That is, in the pre-stage optical amplifier 100, the output target value is set based on the input signal light level, and the output constant control is performed so that the output signal light level becomes the output target value. Therefore, the higher the input signal light level is, the more the signal is output from the front-stage optical amplifier 100 and the latter-stage optical amplifier 2 outputs.
The level of the signal light input to 00 becomes small. Then, it is optically amplified by the post-stage optical amplifier 200 whose output is controlled to be constant. By doing so, the optical amplifier for wavelength division multiplex transmission according to the second embodiment exhibits substantially the same effect as that of the first embodiment, and, in addition to this, the input signal light level It is possible to keep the wavelength dependence of the overall gain small even when f is changed.

In this embodiment, the pre-stage optical amplifier 10 is used.
At 0, the output constant control is performed so that the output signal light level becomes equal to the output target value set based on the input signal light level. In the same manner as this, in the subsequent optical amplifier 200, based on the input signal light level. The output constant control may be performed so that the output signal light level becomes equal to the set output target value. Further, both the front-stage optical amplifier 100 and the rear-stage optical amplifier 200 may perform constant output control so that the output signal light level becomes the respective output target values set based on the input signal light level.

(Third Embodiment) Next, a third embodiment of the wavelength division multiplexing optical amplifier according to the present invention will be described. The configuration of the wavelength division multiplexing optical amplifier according to the third embodiment is substantially the same as that of the second embodiment shown in FIG. However, comparing with the case of the second embodiment,
In the WDM optical amplifier according to the third embodiment, the control unit 141 of the pre-stage optical amplifier 100 sets the optical amplification gain in the amplification optical fiber 111 according to the input signal light level, and based on this gain. The difference is that the excitation light source 121 is controlled by the above.

That is, the higher the level of the input signal light detected by the light receiving element 331, the higher the optical amplifier 100 in the preceding stage.
The gain of optical amplification in the optical fiber for amplification 111 is set to be small, and the optical fiber for amplification 11 from the pumping light source 121 is set.
The control unit 141 controls so that the power of the pumping light supplied to the optical fiber 1 becomes smaller. As a result, the front-stage optical amplifier 1
At 00, the output constant control is performed so that the output signal light level becomes the output target value. Therefore, the higher the input signal light level, the lower the level of the signal light output from the pre-stage optical amplifier 100 and input to the post-stage optical amplifier 200. Then, it is optically amplified by the post-stage optical amplifier 200 whose output is controlled to be constant. By doing so, the optical amplifier for wavelength division multiplex transmission according to the third embodiment exhibits substantially the same effect as that of the first embodiment, and, in addition to this, the input signal light level It is possible to keep the wavelength dependence of the overall gain small even when f is changed.

The pre-stage optical amplifier 1 in this embodiment is
It is preferable that 00 has a multi-stage configuration including the first optical amplification section and the second optical amplification section. In this case, in each of the first optical amplification section and the second optical amplification section, the gain of optical amplification in the amplification optical fiber is set according to the input signal light level, and the pumping light source is controlled based on this gain. By doing so, even if the first optical amplifying unit and the second optical amplifying unit alone cannot perform complete output constant control, the pre-stage light including the first optical amplifying unit and the second optical amplifying unit can be performed. Complete constant output control can be performed in the entire amplifier 100.

Further, in this embodiment, the pre-stage optical amplifier 10 is used.
At 0, the gain is set based on the input signal light level, and the pumping light source 121 is controlled based on this gain. Similarly, the gain is set at the post-stage optical amplifier 200 based on the input signal light level. The pumping light source 221 may be controlled based on this gain, and the post-stage optical amplifier 200 may have a multi-stage configuration.

Further, in both the front-stage optical amplifier 100 and the rear-stage optical amplifier 200, respective gains are set based on the input signal light level, and the pump light sources 121 and 221 are controlled based on the gains. Good. In this case, the pre-stage optical amplifier 100 and the post-stage optical amplifier 200
By appropriately distributing the respective gains, it is possible to perform more complete constant output control of the wavelength division multiplexing optical amplifier including the pre-stage optical amplifier 100 and the post-stage optical amplifier 200.

(Fourth Embodiment) Next, a fourth embodiment of the optical amplifier for wavelength division multiplexing according to the present invention will be described. FIG. 8 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to the fourth embodiment. Compared with the case of the first embodiment, the wavelength division multiplexing optical amplifier according to the fourth embodiment is
An optical attenuator 401 that is provided between the front-stage optical amplifier 100 and the rear-stage optical amplifier 200 and has a variable attenuation amount for signal light.
And a control unit 402 for controlling the amount of attenuation in the optical attenuator 401 according to the input signal light level.

FIG. 9 shows the input signal light level and the optical attenuator 40 in the wavelength division multiplexing optical amplifier according to the fourth embodiment.
It is a figure which shows the relationship with the attenuation amount in 1. As shown in this figure, as the input signal light level detected by the light receiving element 331 is higher, the attenuation amount of the optical attenuator 401 controlled by the controller 402 is set to be larger. The slope of the attenuation amount with respect to the input signal light level is +0.5 dB / d, for example.
It is about B.

In this embodiment, the signal light input to the input terminal 11 is optically amplified by the pre-stage optical amplifier 100 whose output is controlled to be constant, and then attenuated by the optical attenuator 401 by an attenuation amount according to the input signal light level. To be done. Therefore, the higher the input signal light level, the lower the level of the signal light output from the optical attenuator 401 and input to the post-stage optical amplifier 200. Then, the latter-stage optical amplifier 2 whose output is controlled to be constant
The light is amplified by 00. By doing this,
The optical amplifier for wavelength division multiplexing transmission according to the fourth embodiment is the first
In addition to the effect substantially the same as that of the embodiment described above, in addition to this, the wavelength dependence of the overall gain can be kept small even if the input signal light level changes.

Further, as described in the description of the first embodiment, even if the optical attenuator 401 is not provided, since the wavelength λ dependence of the value of DGT (λ) is small, the optical attenuator 401 is provided. Also, in the present embodiment, the amount of change in the attenuation amount of the optical attenuator 401 with respect to the change in the input signal light level can be reduced, and as a result, the dynamic range of the input signal light level can be made wider. In addition, the optical attenuator 401
It is possible to use even an input signal light level exceeding the variable range of the attenuation amount of.

(Fifth Embodiment) Next, a fifth embodiment of the optical amplifier for wavelength division multiplexing according to the present invention will be described. FIG. 10 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to the fifth embodiment. Compared with the case of the first embodiment, the wavelength division multiplexing optical amplifier according to the fifth embodiment has a gain that is provided between the pre-stage optical amplifier 100 and the post-stage optical amplifier 200 to equalize the gain for signal light. Equalizer 50
The difference is that it further includes 0.

In this embodiment, the signal light input to the input terminal 11 is optically amplified by the pre-stage optical amplifier 100 whose output is controlled to be constant, and then gain-equalized by the gain equalizer 500, and the output is controlled to be constant. The light is amplified by the post-stage optical amplifier 200 that has been processed. By doing so, the optical amplifier for wavelength division multiplex transmission according to the fifth embodiment exhibits substantially the same effect as that of the first embodiment, and in addition to this, The wavelength dependence can be further reduced.

The present invention is not limited to the above embodiment, but various modifications can be made. For example, an output target value or gain in the front-stage optical amplifier 100 or the rear-stage optical amplifier 200 is set based on the input signal light level, and an optical attenuator having a variable attenuation amount between the front-stage optical amplifier 100 and the rear-stage optical amplifier 200. Providing 401, and
The gain equalizer 500 may be provided between the pre-stage optical amplifier 100 and the post-stage optical amplifier 200, and any two or more of them may be combined.

[0071]

As described above in detail, according to the present invention, the input signal light is sequentially optically amplified and output by the pre-stage optical amplifier and the post-stage optical amplifier. In the optical amplification of the signal light in the pre-stage optical amplifier, the first pumping means for supplying the first pumping light to the first amplification optical fiber is the level of the signal light input by the first pumping light control means. Is controlled so that the amplification gain of the signal light in the first amplification optical fiber decreases in proportion to the increase amount of The first pumping light control means is a minimum pumping light with which the gain of the first amplifying optical fiber to which the minimum input level is input at the maximum number of wavelengths to be used is saturated when the input signal light level further increases. Excitation light of power or less is supplied as the upper limit. When optically amplifying the signal light in the latter-stage optical amplifier, the second pumping means for supplying the second pumping light to the second optical fiber for amplification uses the second pumping light control means to perform the second amplification. The level of the signal light output from the working optical fiber is controlled so as to reach the second gain or the output target value. By performing such control, even if the gain control range is wide in a region where the input signal light level is high, there is almost no gain limitation of the pre-stage optical amplifier, and a wide dynamic range of the input signal light level can be secured. Also, constant output control can be performed as a whole.

Further, since both the front-stage optical amplifier and the rear-stage optical amplifier perform constant output control, the wavelength dependence of the overall gain is small even when the input signal light level changes.
The dynamic range of the input signal light level can be widened.

Further, the first (or second) output target value in the first (or second) pumping light control means is set based on the input signal light level detected by the input signal light level detecting means, When the output constant control is performed so that the output signal light level reaches the set first (or second) output target value, the wavelength dependence of the overall gain is suppressed even if the input signal light level changes. Can be kept small.

Further, the gain of optical amplification in the first (or second) amplification optical fiber is set based on the input signal light level detected by the input signal light level detecting means, and the first gain is set based on this gain. When the (or second) pumping means is controlled, the front-stage (or rear-stage) optical amplifier performs constant output control so that the output signal light level becomes the first (or second) output target value. Even if the input signal light level fluctuates, the wavelength dependence of the overall gain can be kept small.

Further, after being optically amplified by the pre-stage optical amplifier whose output is controlled to be constant, it is attenuated by the optical attenuator by an attenuation amount corresponding to the input signal light level detected by the input signal light level detecting means, and then output. In the case where optical amplification is performed by the post-stage optical amplifier that is constantly controlled, the wavelength dependence of the overall gain can be kept small even if the input signal light level changes.

Also, when a gain equalizer provided between the front-stage optical amplifier and the post-stage optical amplifier for equalizing the wavelength dependency of the gain for the signal light as a whole is further provided, the wavelength dependency of the overall gain is also provided. Can be further reduced.

The first amplification optical fiber is an Al co-doped Er-doped optical fiber, and the second amplification optical fiber is an Al / P co-doped Er-doped optical fiber and an Al co-doped Er-doped Er.
When the doped optical fiber is connected in this order, the gain of the pre-stage optical amplifier is small, and the wavelength dependence of the overall gain is small even if the level of the signal light input to the post-stage optical amplifier is small. Can be kept.

Further, when the first pumping means supplies the first pumping light to the first amplifying optical fiber at least from the front side, the pre-stage optical amplifier has a low gain and the first amplifying optical fiber. Even if the population inversion is low overall,
Since the population inversion state in the vicinity of the signal light incident side of the amplification optical fiber can be increased, the deterioration of the noise index can be reduced.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to a first embodiment.

FIG. 2 is a diagram showing, for each wavelength, a relationship between an input signal light level and an output signal light level in an optical amplifier for wavelength division multiplexing transmission.

FIG. 3 is a diagram showing a relationship between a wavelength of signal light and a DGT value in the wavelength division multiplexing optical amplifier according to the first embodiment.

FIG. 4 is a diagram showing a noise figure, an optical SN ratio, and a gain deviation in an optical amplifier for wavelength division multiplexing transmission.

FIG. 5 is a diagram showing the relationship between the input signal light level and the overall noise figure NF _t in the wavelength division multiplexing optical amplifier.

FIG. 6 is a configuration diagram of an optical amplifier for wavelength division multiplexing according to each of the second and third embodiments.

FIG. 7 is a diagram showing a relationship between an input signal light level and an output target value in a preceding optical amplifier in the wavelength division multiplexing optical amplifier according to the second embodiment.

FIG. 8 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to a fourth embodiment.

FIG. 9 is a diagram showing a relationship between an input signal light level and an attenuation amount in an optical attenuator in the wavelength division multiplexing optical amplifier according to the fourth embodiment.

FIG. 10 is a configuration diagram of an optical amplifier for wavelength division multiplexing transmission according to a fifth embodiment.

[Explanation of symbols]

100 ... Pre-stage optical amplifier, 111 ... Amplifying optical fiber, 1
21 ... Excitation light source, 122 ... Optical coupler, 131 ... Light receiving element, 132 ... Optical coupler, 141 ... Control unit, 151, 15
2 ... Optical isolator, 200 ... Post-stage optical amplifier, 211,
212 ... Amplifying optical fiber, 221 ... Excitation light source, 222
... Optical coupler, 223 ... Excitation light source, 224 ... Optical coupler, 2
31 ... Light receiving element, 232 ... Optical coupler, 241 ... Control unit,
251, 252 ... Optical isolator, 331 ... Light receiving element,
332 ... Optical coupler, 333 ... Control part, 401 ... Optical attenuator, 402 ... Control part, 500 ... Gain equalizer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/06 H04B 9/00 M 10/14 10/17 10/18 H04J 14/00 14/02 (58) Fields investigated (58) Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H01S 3/06 H01S 3/10

Claims (13)

(57) [Claims] 1. A first amplification optical fiber that optically amplifies and outputs an input signal light, and a minimum pumping light power that causes gain saturation with respect to a minimum input signal light level at the maximum number of wavelengths to be used. Is used as an upper limit of the pumping light power and is used when the first pumping means for supplying the first pumping light to the first amplification optical fiber and the signal light of the maximum number of wavelengths are input.
Controlling the first pumping means such that the amplification gain of the signal light in the first amplification optical fiber decreases in proportion to the increase in the level of the input signal light without causing saturation . A first-stage optical amplifier having one pumping light control means, a second amplification optical fiber for optically amplifying and outputting the signal light input from the front-stage optical amplifier, and a second amplification optical fiber for the second amplification optical fiber. Controlling the second pumping means so that the level of the signal light output from the second amplifying optical fiber and the second amplifying optical fiber has a second gain or an output target value. An optical amplifier for wavelength division multiplex transmission, comprising: a post-stage optical amplifier having a second pumping light control means. 2. The first excitation light control means is the first excitation light control means.
2. The wavelength division multiplexing optical amplifier according to claim 1, wherein the first pumping means is controlled so that the level of the signal light output from the amplifying optical fiber becomes the first gain or the output target value. . 3. An input signal light level detection means for detecting the level of signal light input to said pre-stage optical amplifier is further provided, and said first pump light control means is detected by said input signal light level detection means. The optical amplifier for wavelength division multiplexing according to claim 2, wherein the first gain or the output target value is set according to the level of the signal light. 4. An input signal light level detection means for detecting the level of signal light input to said pre-stage optical amplifier is further provided, and said second pumping light control means is detected by said input signal light level detection means. The optical amplifier for wavelength division multiplexing according to claim 1, wherein the second gain or the output target value is set according to the level of the signal light. 5. An input signal light level detection means for detecting the level of signal light input to said pre-stage optical amplifier is further provided, and said first pump light control means is detected by said input signal light level detection means. The gain of optical amplification in the first amplification optical fiber is set according to the level of signal light,
The optical amplifier for WDM transmission according to claim 1, wherein the first pumping means is controlled based on the gain. 6. The pre-stage optical amplifier includes a first optical amplification section and a second optical amplification section, and the first pumping light control means sets the level of signal light detected by the input signal light level detection means. The optical amplifier for wavelength division multiplex transmission according to claim 5, wherein the gain of optical amplification in each of the first optical amplification section and the second optical amplification section is set accordingly. 7. An input signal light level detecting means for detecting the level of the signal light input to the pre-stage optical amplifier is further provided, and the second pumping light control means is detected by the input signal light level detecting means. The gain of optical amplification in the second optical fiber for amplification is set according to the level of signal light,
The optical amplifier for wavelength division multiplexing transmission according to claim 1, wherein the second pumping means is controlled based on the gain. 8. The second-stage optical amplifier includes a first optical amplification section and a second optical amplification section, and the second pumping light control means sets the level of the signal light detected by the input signal light level detection means. The optical amplifier for wavelength division multiplexing transmission according to claim 7, wherein the gains of optical amplification in the first optical amplification section and the second optical amplification section are set accordingly. 9. An input signal light level detecting means for detecting the level of signal light input to said front-stage optical amplifier; and an attenuation amount for the signal light, which is provided between said front-stage optical amplifier and said rear-stage optical amplifier. 2. An optical attenuator, and an attenuation amount control means for controlling an attenuation amount in the optical attenuator according to the level of the signal light detected by the input signal light level detection means. An optical amplifier for wavelength-division multiplexing transmission as described. 10. The apparatus according to claim 1, further comprising a gain equalizer provided between the pre-stage optical amplifier and the post-stage optical amplifier to equalize the wavelength dependence of the gain with respect to the signal light as a whole. Optical amplifier for WDM transmission. 11. The first amplification optical fiber is made of Al.
An optical amplifier for wavelength division multiplexing according to claim 1, which is a co-doped Er-doped optical fiber. 12. The second amplification optical fiber is made of Al.
The optical amplifier for WDM transmission according to claim 1, wherein a / P co-doped Er-doped optical fiber and an Al co-doped Er-doped optical fiber are connected in this order. 13. The optical amplifier for WDM transmission according to claim 1, wherein the first pumping means supplies the first pumping light to the first amplifying optical fiber at least from the front side. .