JP2651086B2 - Optical amplification system - 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 amplifying system for compensating for transmission loss and improving receiving sensitivity by using an optical waveguide which exhibits an optical amplifying function by exciting light.

[0002]

2. Description of the Related Art As an optical amplifying means applicable to an optical CATV system, switching of an optical line, etc., an optical amplifying system using an optical fiber having an optical amplifying function has already been developed. Has reached. For such an optical amplification system, erbium (E
It is reported that when a silica-based single mode optical fiber doped with r) is used as an optical amplifier, light in the 1.55 μm band can be amplified.

As shown in FIG. 9, the above-described optical amplification system includes an excitation light source 31 for optical excitation, a multiplexer 32 for multiplexing an optical signal and excitation light, and a rare earth element in a core. And an optical fiber 33 to which an optical isolator 34 is added.

[0004]

In general, the optical amplification system illustrated in FIG. 9 guides pump light into an optical fiber 33.
The pumping level in the optical fiber 33 is increased.
When this enhanced level returns, the optical fiber
The optical signal guided into the bus 33 is amplified. Such a light increase
Width systems can be adapted to very high speed transmissions due to their high gain and very fast response speed. But
In the state where the pumping light is continuously introduced into the optical fiber 33,
For a long period of time of several milliseconds or more,
3 does not exist, the state of the excitation level is increased.
In this state, the optical signal is stored in the optical fiber 3
When guided inside 3, it amplifies the optical signal with a high amplification rate.
It has the following characteristics. Because of this, the optical signal is
33 continues to be not input, and immediately after that, an optical signal
When input into the optical fiber 33, the optical signal is extremely
Amplified at a high amplification rate and becomes an intense light pulse
And cause destruction and saturation of the subsequent system.

FIGS. 10A and 10B show waveforms when a conventional optical amplification system is applied to a line switching system. The waveform before amplification shown in FIG.
As apparent from the amplified waveform shown in FIG.
Then, the rising portion of the signal is amplified to generate an intense pulse.

The present invention has been made in view of such technical problems, and has as its object to provide an optical amplification system having an optical amplification characteristic independent of the waveform of an input optical signal.

[0007]

According to the first aspect of the present invention, there is provided an optical amplifying unit including an optical amplifying optical waveguide, wherein the optical amplifying unit includes an optical amplifying unit. When an optical signal is not input into an optical waveguide in an optical amplification system for amplifying an optical signal transmitted through the optical waveguide by an optical amplification effect in the optical waveguide generated when the optical signal is supplied. The pump light is not supplied into the optical waveguide, and the pump light is supplied into the optical waveguide when an optical signal is input into the optical waveguide.

In order to enhance the response of the optical amplification, it is desirable that auxiliary pumping light be supplied into the optical waveguide.

[0009] The above-mentioned optical amplifying unit is a third aspect of the present invention.
As described in, an optical waveguide for optical amplification, a multiplexer for supplying excitation light into the optical waveguide, an excitation light source for generating excitation light, and a drive for driving the excitation light source An optical waveguide for optical amplification, an optical switch for turning on / off the supply of pumping light into the optical waveguide, and generating pumping light. And a driving circuit for driving the excitation light source. Optical signals are detected in the drive circuits and optical switches of these optical amplification units,
A photodetector for inputting the detection signal is connected.

As described above, in order to enhance the response of the optical amplification, a bias power supply is connected to the excitation light source of the optical amplification unit, or a bias power supply is connected to the excitation light source of the optical amplification unit. Wherein the optical amplification unit also includes an auxiliary pumping light source and a driving circuit thereof.
As described in the above, an optical excitation auxiliary unit for inputting auxiliary excitation light into the optical waveguide in a direction opposite to the optical signal is connected to the subsequent stage of the optical amplification unit. As described, an auxiliary unit for steady-state optical amplification is connected to a stage preceding the optical amplification unit.

According to a ninth aspect of the present invention, there is provided an optical amplifying unit including an optical amplifying optical waveguide, and the optical waveguide is supplied with excitation light. In an optical amplification system for amplifying an optical signal transmitted in an optical waveguide due to the optical amplification effect in the optical waveguide that occurs when the optical waveguide is constantly supplied with excitation light and is constantly excited. When the optical signal is not input into the optical waveguide, the dummy signal light is supplied into the optical waveguide, and when the optical signal is input into the optical waveguide, the dummy signal light is not supplied into the optical waveguide. It is characterized by the following.

In the case of the optical amplifying system according to the ninth aspect, as described in the tenth aspect, the optical amplifying unit is configured to supply an optical waveguide for amplifying light and pumping light into the optical waveguide. An excitation light source and a drive circuit for driving the excitation light source are included, and a dummy signal light source is connected via an optical switch to a stage preceding the optical amplification unit.

[0013]

In the optical amplifying system according to the first aspect of the present invention, only when an optical signal is input into the optical waveguide, the excitation light is supplied into the optical waveguide, and the optical signal is not input into the optical waveguide. At this time, the excitation light is not supplied into the optical waveguide. The pumping light supplied into the optical waveguide in this manner is supplied into the optical waveguide with a small time delay after the optical signal is input into the optical waveguide, and is thus input into the optical waveguide. Optical amplification does not occur in the optical waveguide when the optical signal rises, and the optical amplification characteristics do not depend on the waveform of the optical signal.

In the above-described optical amplification system, when auxiliary pumping light is supplied into the optical waveguide, when the full-scale pumping light is input into the optical waveguide, the inside of the optical waveguide is brought into a high pumping state. Therefore, operation stability and responsiveness are further improved.

In the optical amplification system according to the ninth aspect, when no optical signal is input into the optical waveguide that is constantly excited, a dummy signal light is supplied into the optical waveguide, and the optical signal is constantly emitted. When an optical signal is input into the excited optical waveguide, the dummy signal light is not supplied into the optical waveguide. That is, since the optical waveguide of the optical amplification system operates in the saturation region by amplifying the dummy signal light when the optical signal is not input, even when the optical signal is input instead of the dummy signal light, Operate in normal state. Therefore, even with this optical amplification system, there is no response to the rising portion of the optical signal.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an optical amplification system illustrated in FIG. 1 will be described. The optical amplification system illustrated in FIG. 1 includes an optical transmission system extending from an optical demultiplexer 1 to an optical isolator 9 via an optical transmission line 2, an optical multiplexer 4, and an optical waveguide 5, and from the optical demultiplexer 1. , Optical transmission line 3, photodetector 6, drive circuit 7, excitation light source 8
And a photoelectric conversion system extending through the optical multiplexer 4 through
The optical multiplexer 4, the optical waveguide 5, the drive circuit 7, the excitation light source 8, and the like constitute an optical amplification unit 10.

In the above, the optical demultiplexer 1 is, for example,
It comprises a beam splitter for splitting the signal light at a ratio of 1:20. The optical transmission lines 2 and 3 are made of, for example, a silica-based coated optical fiber having a core and a clad. The optical multiplexer 4 includes an optical coupler for multiplexing the signal light and the pump light, for example, a WDF (Wave Division Module). The optical waveguide 5 for optical amplification is composed of a silica-based or fluoride-based single mode optical fiber having a core and a clad, and the outer periphery thereof is covered with plastic. The core of the optical waveguide 5 is made of, for example, a silica-based or fluoride-based host glass made of Er (erbium), Nd.
(Neodymium), a rare earth element is added, and as other additives, alkaline earth elements such as Be (beryllium), oxides of YAG (yttrium-aluminum-garnet alloy), YLF (yttrium- lanthanoid) -Fluorine alloy), and may include one or more oxides and transition metal ions. As another example, the core of the optical waveguide 5 is made of ZBLAN (ZrF ₄ —BaF ₂ —LaF ₃₎ doped with Er, Nd, or the like.
-AlF ₃ -NaF) -based fluoride glass, and as another example, BaF ₂ , AlF ₃ , NdF ₃
It is made of a fluoride glass containing, for example. The cladding of the optical waveguide 5 is also made of silica-based or fluoride-based glass containing the above-described doping component, but in the case of this cladding, the refractive index is lower than that of the core, as is obvious. The detector 6 is, for example, a well-known photodiode (PD), and the drive circuit 7 is, for example, an electric circuit including a commercial power supply. The excitation light source 8 has a 0.8 μm band, a 0.98 μm band,
Alternatively, it is composed of a semiconductor laser for oscillating light of a required wavelength such as a 1.48 μm band. Optical isolator 9
For example, a polarization-independent optical element is used to suppress the oscillation phenomenon of the amplifier (optical waveguide 5) due to reflection or the like.

In the case of the optical amplification system illustrated in FIG.
The optical signal is amplified as described below. In FIG. 1, an input optical signal s is split into two optical signals s ₁ and s ₂ via an optical splitter 1 (s ₁ : s ₂ = 20: 1), and these optical signals s ₁ and s _{When 2} is input to two optical transmission lines 2 and 3, an input optical signal s ₁ transmitted through one optical transmission line 2
Enters an optical waveguide (Er ³⁺ -doped optical fiber) 5 via an optical multiplexer 4, and a control optical signal s ₂ transmitted through the other optical transmission line 3 is converted into an electric signal by a photodetector 6. Is input to the drive circuit 7. The drive circuit 7 is turned on when an electric signal from the photodetector 6 is input, and
Is driven (lights up), but the excitation light source 8 is not driven when the electrical signal from the photodetector 6 is off and the signal is off.

In the above, when the drive circuit 7 turned on turns on the excitation light source 8, the excitation light from the excitation light source 8 is input into the optical waveguide 5 via the optical multiplexer 4.
At this time, since the relaxation time of the excited electrons is about 10 ms, the optical amplification system operates with a delay corresponding to the relaxation time. If necessary, the drive circuit 7 delays the optical amplification system. Since the optical amplification system illustrated in FIG. 1 operates in this manner, the optical signal s ₁ is not optically amplified when the optical signal entering the optical waveguide 5 rises, and the optical signal s ₁ is slightly delayed after the rise. ₁ for amplifying a. Therefore, such an optical amplification system performs a predetermined optical amplification without causing the above-described problem.

FIG. 2 shows a waveform before amplification when the optical amplification system illustrated in FIG. 1 is applied to a line switching system. As is apparent in comparison of FIG. 2 and FIG. 10 (B), and when by the optical amplification system of Figure 1 is optically amplified without response to the rise of the input optical signal.

FIG. 3 shows a modification of the optical amplification system shown in FIG. In the illustrated example, in order to enhance the response of optical amplification by the optical waveguide 5, the optical amplification unit 1
The bias power supply 11 is connected to the 0 excitation power supply 7.
In this case, the excitation light source 8 supplies the bias power supply 11 even when the driving circuit 7 to which the optical signal s ₂ is not input is in the off state.
Since the optical amplifier steadily amplifies light at a low level in response to the bias current from the optical amplifier, even if a time delay of the pumping light due to the optical signal s ₂ occurs, the pumping power is preliminarily pumped by the bias power supply 11. Due to the excitation energy, the optical waveguide 5 causes the optical signal s ₁
Is immediately amplified. Therefore, in the case of this example, the optical amplifier does not amplify at the time of the rise of the optical signal s ₁ , and the responsiveness of the optical amplification system is further increased depending on the highly excited state of the optical waveguide 5. The auxiliary excitation light generated when the excitation light source 8 is biased as described above has the same or similar wavelength as the main excitation light, but in order to bring the optical waveguide 5 into a weakly excited state, The output is lower than the main pump light.

Next, the optical amplification system illustrated in FIG. 4 will be described. In the case of the optical amplification system illustrated in FIG.
The basic technical items are the same as those in FIG.
1 in that the optical amplification unit 10 has an optical switch 12 and that the photodetector 6 is connected to the optical switch 12.

In the case of the optical amplification system illustrated in FIG.
The excitation light source 8 is turned on via the drive circuit 7, and the photodetector 6 turns off the optical switch 12 when there is no input optical signal s. In the above, the input optical signal s is split into two optical signals s ₁ and s ₂ via the optical splitter 1, and these optical signals s ₁ and s ₂ are input to the respective optical transmission lines 2 and 3. The input optical signal s ₁ transmitted through one optical transmission line 2 is
The control optical signal s _{2 that} is incident on the optical waveguide 5 via the optical multiplexer 4 and transmitted through the other optical transmission line 3 is converted into an electric signal by the photodetector 6 and input to the optical switch 12. . The optical switch 12 that is turned on in response to the control optical signal s ₂ from the photodetector 6 inputs the pump light from the pump light source 8 to the optical waveguide 5 from the optical multiplexer 4. In this case, it operates the optical detector 6 via a control optical signal s _2, a light detector 6
The excitation light is input to the optical waveguide 5 with a delay from the input optical signal s ₁ by an amount corresponding to the switching of the optical switch 12 by the detection signal from the optical switch 12. Therefore, the optical amplification system illustrated in FIG. 4 does not amplify the optical signal s ₁ input to the optical waveguide 5 when the optical signal s ₁ rises, but amplifies the optical signal s ₁ with a slight delay after the rise. It does not raise the problems already mentioned.

FIG. 5 shows a modification of the optical amplification system shown in FIG. In the case of the optical amplification system illustrated in FIG. 5, the optical amplification unit 10 also includes an auxiliary pumping light source 13 and a driving circuit 14 for enhancing the responsiveness of optical amplification in the optical waveguide 5. It is interposed between the optical multiplexer 4 and the optical switch 12. Also in the case of this example, since the optical waveguide 5 is always kept in a weakly excited state via the excitation light source 13 which is driven at a lower output than in normal use, the optical signal s ₁ is input to the optical waveguide 5. And the optical signal s
_{When a} predetermined pumping light is input from the pumping light source 8 to the optical waveguide 5 by turning on the optical switch 12 based on ₂ , the optical waveguide 5 enters a high pumping state as in the example of FIG. The auxiliary excitation light generated from the excitation light source 13 has a lower output than the main excitation light, as described above.

Next, the optical amplification system illustrated in FIG. 6 will be described. In the case of the optical amplification system illustrated in FIG.
The optical demultiplexer 1, the photodetector 6, the optical amplification unit 10 and the like are the same as those illustrated in FIG. 1 or FIG. 4, but the optical multiplexer 15 and the pump light source 16 Excitation auxiliary unit 1 including a driving circuit 17
8 is different from those illustrated in FIGS. 1 and 4. The optical multiplexer 15, pumping light source 16, driving circuit 17 and the like in FIG. 6 are the same as or similar to those described above, but the pumping light source 16 is for always keeping the optical waveguide 5 in a weakly excited state. The incident direction of the optical multiplexer 15 is opposite to the direction in which the signal light is transmitted.

In the case of the optical amplification system illustrated in FIG.
Optical amplifier unit 10, based on the control signal light s _1, 1
Alternatively, the operation is the same as described with reference to FIG. Also in this optical amplification system, the optical waveguide 5 is always kept in a weakly excited state via the excitation light source 16 driven at a lower output than in normal use.
High excited state.

As a modification of the optical amplification system illustrated in FIG. 6, the optical excitation auxiliary unit 18 is an optical amplification unit 1
It may be connected before 0. In this case, the incident direction of the optical multiplexer 15 is the same as the direction in which the signal light is transmitted.

Next, the optical amplification system illustrated in FIG. 7 will be described. The optical amplifying system illustrated in FIG. 7 also has the same optical demultiplexer 1, photodetector 6, and optical amplifying unit 10 as those illustrated in FIG. 1 or FIG. In the preceding stage, the optical multiplexer 19 and the optical waveguide 2
1 and 4 in that an optical amplification auxiliary unit 23 including an excitation light source 21 and a drive circuit 22 is connected. The optical multiplexer 19, the optical waveguide 20 for optical amplification, the excitation light source 21, the drive circuit 22, etc. in FIG.
The same as or similar to the above, except that the excitation light source 1
Numeral 6 is for maintaining the optical waveguide 5 in a weakly excited state at all times.

In the case of the optical amplification system illustrated in FIG.
The input optical signal s is transmitted via the optical demultiplexer 1 to two optical signals s ₁ ,
s is ₂ demultiplexed, when these optical signals s _1, s ₂ is input to the two optical transmission paths 2, the input optical signal s ₁ for transmitting one of the optical transmission path 2, the optical multiplexer 19. Optical waveguide 2
0, a control optical signal s ₂ that enters the optical waveguide 5 via the optical multiplexer 4 and is transmitted through the other optical transmission line 3 is converted into an electric signal by the photodetector 6 and drives the optical amplification unit 10. The signal is input to the circuit 7 or the optical switch 12.

As a modified example of the optical amplification system illustrated in FIG. 7, an optical amplification auxiliary unit 23 is an optical amplification unit 1
May be connected to the subsequent stage of 0, the optical multiplexer 19 in this case, this light incident direction is opposite to the transmission direction of the signal light s _1.

Next, the optical amplification system illustrated in FIG. 8 will be described. The optical amplification system illustrated in FIG. 8 includes an optical splitter 1, an optical transmission line 2, an optical switch 24, an optical waveguide 5
And a photoelectric conversion system extending from the optical demultiplexer 1 to the optical transmission line 3, the photodetector 6, and the optical switch 24. The optical switch 24 includes a dummy signal light source 25. It is connected. In this case, the optical waveguide 5, the excitation light source 8, the drive circuit 7 and the like constitute an optical amplification unit 10, and the optical switch 24, the dummy signal light source 25, the drive circuit 26 and the like constitute an optical dummy unit 27. I have. When excited through the drive circuit 26, the dummy signal light source 25 generates a dummy signal light having a wavelength corresponding to the signal light s or a dummy signal light having a wavelength similar to the signal light s as pulse light or continuous light. can do.

In the case of the optical amplification system illustrated in FIG.
The inside of the optical waveguide 5 is constantly excited via the excitation light source 8. When the signal light s is not input to the optical amplification system, since the optical switch 24 closes the contacts a and c, the dummy signal light s ₃ from the dummy signal light source 25 is transmitted through the optical switch 24 to the optical waveguide. 5. Therefore, for example, a signal light s of 100 KH bits or more
But the absence during a few milliseconds or longer, the optical waveguide 5, while amplifying the dummy signal light s ₃ to maintain the operation in the saturation region. When the signal light s is input to the optical amplification system,
The optical signal s is transmitted via the optical demultiplexer 1 to two optical signals s ₁ and s ₂
The optical signals s ₁ and s ₂ are transmitted through two optical transmission lines 2 and 3. Hereinafter, when the control light signal s ₂ to be transmitted through the optical transmission line 3 is input to the optical switch 24 is converted into an electric signal by the photodetector 6, the contact b by the switching operation of light switch 24 based on this, the c Since the switch is closed, the input optical signal s ₁ transmitted through the optical transmission line 2 enters the optical waveguide 5 already in an excited state at the same time as the switching, and is amplified there. That is, in the case of the optical amplification system illustrated in FIG.
Amplifies the dummy signal light s ₃ and operates in the saturation region, so that the optical waveguide 5 operates in a normal state even when the optical signal s ₁ is input instead of the dummy signal light s ₃ . Therefore, the optical amplification system also does not respond to the rising portion of the optical signal s _1.

As a modification of the optical amplification system illustrated in FIG. 8, an optical dummy unit 27 may be connected to the subsequent stage of the optical amplification unit 10. In this case, the optical dummy unit 27 is connected via an optical multiplexer (same as the previous example) for injecting light in a direction opposite to the transmission direction of the signal light s ₁ .
It is connected to the subsequent stage of the optical amplification unit 10.

In the case of the optical amplification system according to the present invention, for example, as means for exciting the fluorescent substance in the optical waveguide 5,
Well-known backward excitation, double-sided excitation, and the like can also be employed. As another embodiment of the present invention, for example, in an optical amplification system in which the signal light s and the pumping light are input to the optical amplification unit 10, the signal light s may be input to the optical waveguide 5 later than the pumping light. . In the optical amplification system according to the present invention, the optical signal is an optical signal.
The state that is not input into the waveguide is described in the above description.
As you can see, if the signal light is cut off or
The input interval between one signal light and the next signal light exceeds a certain time.
The case.

[0035]

In the optical amplifying system according to the present invention, optical amplification does not occur at the rise of the input optical signal, and the optical amplification characteristics do not depend on the waveform of the optical signal. It does not cause, and therefore, provides a safe and stable optical amplification system.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical amplification system according to the present invention.

FIG. 2 is a diagram showing optical amplification characteristics of the optical amplification system according to the present invention.

FIG. 3 is a block diagram showing a modification of the first embodiment.

FIG. 4 is a block diagram showing a second embodiment of the optical amplification system according to the present invention.

FIG. 5 is a block diagram showing a modification of the second embodiment.

FIG. 6 is a block diagram showing a third embodiment of the optical amplification system according to the present invention.

FIG. 7 is a block diagram showing a fourth embodiment of the optical amplification system according to the present invention.

FIG. 8 is a block diagram showing a fifth embodiment of the optical amplification system according to the present invention.

FIG. 9 is a block diagram showing a conventional optical amplification system.

10A is a diagram illustrating a waveform before amplifying an optical signal in a conventional optical amplification system, and FIG. 10B is a diagram illustrating an optical amplification characteristic after amplifying an optical signal in a conventional optical amplification system. FIG.

[Explanation of symbols]

1 Demultiplexer 2 Optical transmission path 3 Optical transmission path 4 Optical multiplexer 5 Optical waveguide 6 Photodetector 7 Drive circuit 8 Excitation light source 9 Isolator 10 Optical amplification unit 11 Bias power supply 12 Optical switch 13 Excitation light source 14 Drive circuit 15 Optical multiplexing Device 16 Excitation light source 17 Drive circuit 18 Auxiliary unit for optical excitation 19 Optical multiplexer 20 Optical waveguide 21 Excitation light source 22 Drive circuit 23 Auxiliary unit for optical amplification 24 Optical switch 25 Dummy signal light source 26 Drive circuit 27 Optical dummy unit s Optical signal s ₁ Split optical signal s ₂ Split optical signal s ₃ Dummy signal light

Claims (10)

(57) [Claims] An optical amplification unit including an optical waveguide for optical amplification is provided, and light is transmitted through the optical waveguide by an optical amplification effect in the optical waveguide generated when the optical waveguide receives supply of excitation light. In the optical amplification system for amplifying the optical signal, when the optical signal is not input into the optical waveguide, the excitation light is not supplied into the optical waveguide, and when the optical signal is input into the optical waveguide, An optical amplification system, wherein excitation light is supplied into an optical waveguide. 2. The optical amplification system according to claim 1, wherein the auxiliary pumping light is supplied into the optical waveguide. 3. An optical amplification unit for driving an optical waveguide for optical amplification, a multiplexer for supplying excitation light into the optical waveguide, an excitation light source for generating excitation light, and an excitation light source. of including a drive circuit, the drive circuit of the optical amplifier unit, an optical amplification system according to claim 1, wherein the photodetector for detecting an input optical signal is connected. 4. An optical amplification unit comprising: an optical waveguide for optical amplification;
On the supply of the pumping light into the optical waveguide, the optical <br/> switch for turning off the excitation light source for generating excitation light, and includes a drive circuit for driving the excitation light source, optical amplifier unit The optical amplification system according to claim 1, wherein a photodetector for detecting an input optical signal is connected to the optical switch. 5. The optical amplification system according to claim 3, wherein a bias power supply is connected to an excitation light source of the optical amplification unit. 6. The optical amplifying system according to claim 3, wherein the optical amplifying unit includes an auxiliary pumping light source and a driving circuit therefor. Downstream of 7. The optical amplifying unit, the optical signal and retrograde to claim claim 3 or photoexcitation auxiliary unit because the excitation light to enter into the light guide is connected to an auxiliary 5. The optical amplification system according to 4. 8. The optical amplification system according to claim 3, wherein an auxiliary unit for steady-state optical amplification is connected to a stage preceding the optical amplification unit. 9. An optical amplification unit including an optical amplification optical waveguide, wherein the optical waveguide is transmitted through the optical waveguide by an optical amplification effect in the optical waveguide generated when excitation light is supplied to the optical waveguide. In an optical amplification system for amplifying an optical signal, when the inside of an optical waveguide is constantly supplied with pumping light and is constantly excited, and the optical signal is not input into the optical waveguide, a dummy signal light is generated. An optical amplification system, wherein a dummy signal light is not supplied into an optical waveguide when the optical signal is supplied into the optical waveguide and the optical signal is input into the optical waveguide. 10. An optical amplification unit, comprising: an optical waveguide for optical amplification, an excitation light source for supplying excitation light into the optical waveguide, and a drive circuit for driving the excitation light source. 10. The optical amplifying system according to claim 9, wherein a dummy signal light source is connected via an optical switch in a preceding stage of the optical amplification system.