JP3511241B2 - WDM optical amplifier and optical communication system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical amplifier and an optical communication system for amplifying a wavelength division multiplexing optical signal obtained by multiplexing optical signals having different wavelengths.

The optical wavelength division multiplex transmission system multiplexes optical signals of different wavelengths and transmits the optical signals through an optical transmission line formed of an optical fiber, and enables large-capacity transmission according to the number of multiplexed optical signals. To do.

Further, in the optical wavelength division multiplexing transmission system, an optical amplifying device for amplifying the wavelength division multiplexing optical signal is required, and a predetermined output level is obtained for each multiplexed optical signal. It is required to have amplification characteristics.

[0004]

2. Description of the Related Art An optical amplifier that directly amplifies an optical signal generally uses an erbium (Er) -doped fiber. Since this can amplify an optical signal in the 1.55 μm band widely used in optical communication systems, it is applied to a post-amplifier of a transmitter, a pre-amplifier of a receiver, or an in-line amplifier as a repeater. hand,
It has been attracting attention as a powerful means for increasing the transmission distance at low cost.

In the optical wavelength division multiplexing transmission system, the wavelength division multiplexing in the above-mentioned 1.55 μm band is particularly promising as a technique capable of easily increasing the transmission capacity, but optical signals of different wavelengths are expected. However, the loss in the optical device that is inserted for multiplexing and demultiplexing has a problem that the transmission distance is reduced. So, even in the sense of compensating for this loss,
It is effective to apply an optical amplifier, and the combination of the wavelength multiplexing technique and the optical amplification technique is expected as a powerful means that enables long-distance and large-capacity optical signal transmission.

As a conventional optical output control method for an optical amplifier, a constant optical output control method is generally used. FIG. 12 shows an example of a conventional optical amplifier control system, where 1 is an erbium-doped fiber (EDF) and 2 is
Is a pumping laser diode (LD) that supplies pumping light to the EDF 1, 3 is a multiplexer / demultiplexer for multiplexing the signal light and the pumping light from the pumping LD 2 and demultiplexing the signal light output, 4 is the pumping LD 2 Is a drive circuit for driving.

The signal light input is input to the EDF 1 via an isolator 5 for separating the input side and the EDF 1. The EDF 1 amplifies and outputs the signal light in the presence state of the pumping light that has been applied via the multiplexer / demultiplexer 3.
A signal light output is generated through the isolator 6 by the amplified signal light.

A part of the optical output is branched through a branching device 7 inserted on the output side, converted into an electric signal by a light receiving element 8, and the level is detected by a level detector 9. Then, the level detected by the comparator 10 is compared with the reference voltage V _REF , the drive circuit 4 is controlled according to the error, and the excitation L
By driving D2, the intensity of the pumping light from the pumping LD2 is controlled to keep the signal light output constant.

At this time, a part of the optical input is branched through the branching device 11 inserted on the input side, converted into an electric signal by the light receiving element 12, and the input break detection circuit 13 checks its level. When the disconnection of the optical input is detected, a shutdown signal is generated to stop the operation of the drive circuit 4,
The excitation light from the excitation LD 2 is turned off. When there is no optical input, the gain of the control system for keeping the optical output constant increases, and only noise is amplified. However, if the optical input is restored in this state, an excessive optical output will result. This is because there is a risk that the reception side may be adversely affected.

FIG. 13 shows an example of a conventional wavelength division multiplexing optical amplifier, in which 14 is an optical fiber, 15 is a semiconductor laser amplifier, and 16 is a branch for branching a part of the optical output of the semiconductor laser amplifier 15. It is a vessel. Reference numeral 17 is a drive circuit for driving the semiconductor laser amplifier 15, 18 is a demultiplexer for separating a part of the signal light from the light branched by the branching device 16, 19 is a light receiving element for converting an optical input into an electric signal, and 20 is The gain control circuit controls the gain of the semiconductor laser amplifier 15.

The wavelength division multiplexed optical signals λ _1, λ ₂ are input to the signal light.
In addition, the monitor light λ _m is included. The demultiplexer 18 separates the monitor light λ _m from the signal light output branched by the brancher 16, and the light receiving element 19 converts this into an electric signal. The gain control circuit 20 compares the level of this electric signal with a reference voltage (not shown) and controls the drive circuit 17 according to the error signal so that the gain of the semiconductor laser amplifier 15 becomes constant.

As described above, in the conventional wavelength division multiplexing optical amplifier, the control system for keeping the optical output constant is used as in the case of the single wavelength optical amplifier, so that the optical output level as a whole is always constant. is there.

Further, conventionally, there is known a wavelength division multiplexing optical amplifier for amplifying signal light of two wavelengths, which uses signal light of wavelength band of 1.535 μm and signal light of wavelength band of 1.55 μm. This is because in the case of an optical amplifier using an erbium-doped fiber, a region of 1.52 to 1.54 μm and a region of 1.54 μm from 1.54 μm as a boundary.
This is because there is a high gain portion in the 1.57 μm region.

FIG. 14 shows an example of the configuration of a conventional two-wavelength multiplex optical amplifier, in which the same components as in FIG. 12 are designated by the same reference numerals, and 21 is the signal light in the wavelength 1.535 μm band and the wavelength 1 A multiplexer that combines signal light in the .55 μm band,
Reference numeral 22 is a wavelength division multiplexing (WDM) coupler for multiplexing / demultiplexing the wavelength division multiplexing optical signal and the pumping light.

In the multiplexer 21, the wavelength of 1.535 μm
The signal light 1 in the band and the signal light 2 in the wavelength band of 1.55 μm are multiplexed and input to the EDF 1 via the isolator 5. The pumping light from the pumping LD2 is passed through the WDM coupler 22 to the EDF1.
The signal light 1 and the signal light 2 are amplified in the state where they are input to the optical fiber, and an optical output is generated via the isolator 6.

FIG. 15 shows the gain characteristics of the two-wavelength multiplex optical amplifier shown in FIG.
Is excited by excitation light in the wavelength range of 1.48 μm (hereinafter 1.4
Shows the gain characteristics when performing 8 μm excitation)
(B) shows a gain characteristic when pumping with pumping light in the wavelength band of 0.98 μm (hereinafter abbreviated as 0.98 μm). When 1.48 μm excitation is performed,
As shown in (a), the gain on the 1.55 μm band side increases, and as a result, optical transmission on the 1.535 μm band side may become difficult. When 0.98 μm excitation is performed,
As a result of increasing the gain on the 1.535 μm band side,
Optical transmission on the μm band side may be difficult.

FIG. 16 shows an example of the configuration of a gain measuring system in a wavelength division multiplexing optical amplifier. The same components as those in FIG. 14 are designated by the same reference numerals, and 23 is an optical spectrum for measuring the spectral distribution of optical output. It is an analyzer.

In the multiplexer 21, the signal light 1 (λ = 1.
56 μm) and the signal light 2 (λ = 1.53 μm to 1.57
μm) and is input to the EDF 1 via the isolator 5. Excitation light (λ = 1.48 μm or 0.98 μm) from the excitation LD2 is passed through the WDM coupler 22 to the EDF1.
The signal light 2 is amplified while being input to the optical spectrum analyzer 23, and the amplified signal light 2 is input to the optical spectrum analyzer 23 via the isolator 6, the optical spectrum is separated, and the respective optical output levels are measured. .

[0019]

When the conventional optical amplifier shown in FIG. 12 is used as it is for amplifying wavelength-division multiplexed light, the total optical output level is constant.
The optical level per signal light is (1 / number of multiplexed signals). Therefore, when the number of optical signals changes, the required optical level may not be secured in the receiving section. In addition, if the optical signal of a certain wavelength is cut off for some reason during system operation, the optical amplifier distributes the power of that amount to the signal light of other wavelengths. Increase), which may cause an error in the receiving unit. On the contrary, when the number of optical signals is increased, the level fluctuation (decrease) of other wavelength signal light is caused. This is the first problem in the conventional WDM optical amplifier.

As the operation of the optical amplifier for the wavelength-division multiplexed optical signal, the wavelength dependence of the gain cannot be ignored at present. Furthermore, the level difference of each wavelength light on the receiving side can be calculated by adding the variation of the transmission light source output of the optical terminal device, the variation of the insertion loss of the multiplexer / demultiplexer, the variation of the loss of the transmission line fiber, etc. There is a problem in that the reception unit may not be able to obtain the required reception level. In general, reception error is more likely to occur in the wavelength signal light having a lower level. This is the second problem in the conventional wavelength division multiplexing optical amplifier.

In the conventional wavelength division multiplexing optical amplifier shown in FIG. 13, the level of the monitor light is monitored so that the total optical output is controlled to be constant. Therefore, the optical output may not be constant because the gains of the main signal and the monitor light do not necessarily match. Further, not only the signal light but also the monitor light is required, so that not only the cost is increased, but also the main signal system is normal, but if the monitor system fails, there is a problem that control becomes impossible.

FIG. 17 is a diagram for explaining the gain characteristic in the wavelength division multiplexing optical amplifier, in which (a) is N ₂ / (N ₂ −
When N ₁ ) is small, (b) shows the case where N ₂ / (N ₂ −N ₁ ) is large. Here, N ₁ is the number of photons in the ground state, and N ₂ is the number of photons in the excited state. The vertical axis represents the radiant energy N ₂ σ _e (where σ _e is the radiant cross section) and the absorbed energy N ₁ σ _a (σ _a is the radiant cross section),
The horizontal axis represents the signal light wavelength (μm).

In FIG. 17, the radiant energy N₂ σ_e
And absorbed energy N₁ σ_aIs the difference between the gain of the optical amplifier
Shows. N as shown₂ / (N₂ -N₁ ) Is small
In other words, if the ground state distribution is small enough,
When the state distribution is large enough, the wavelength is 1.535 μm
The near gain g (λ = 1.535 μm) has a wavelength of 1.55 μ
It is larger than the gain g (λ = 1.55 μm) near m.
Conversely, N₂ / (N₂ -N ₁ ) Is large, ie the excitation
If the state distribution is not large enough, g (λ = 1.53)
5 μm) is smaller than g (λ = 1.55 μm).

When the light of the wavelength band of 1.48 μm is used as the excitation light, it is difficult to bring the distribution of the ground state close to 0 because the emission cross section is tailed to the vicinity of 1.45 μm. In particular, when applied to a post-amplifier by two-wavelength multiplexing, N ₂ / (N ₂ −N ₁ ) becomes large and g (λ =
1.535 μm) is smaller than g (λ = 1.55 μm), and a gain tilt occurs.

FIG. 18 shows an example of the gain tilt, and shows the case where the signal light 1 and the signal light 2 are collectively amplified by 1.48 μm excitation, and the vertical axis shows the optical output of the signal light 2.
The horizontal axis represents the wavelength of the signal light 2. In the figure, A shows the case where the wavelength of only the signal light 2 is changed without entering the signal light 1, and B shows the case where the wavelength of the signal light 1 is fixed at 1.56 μm and the wavelength of the signal light 2 is changed. Wavelength from 1.53μm to 1.57μ
It shows the case where the value is changed to m.

As described above, in the case of only a single signal light, a substantially flat amplification characteristic is shown, whereas in the case of two-wavelength collective amplification, the optical output of the signal light 2 is reduced in a short wavelength region, Gain tilt occurs.

On the other hand, when the light of wavelength 0.98 μm band is used as the pumping light, the distribution of the ground state can be made sufficiently small, and the gain wavelength characteristic has the shape of the radiation cross section shown in FIG. Approach. This is because, unlike the excitation light in the wavelength band of 1.48 μm, the excitation light in the wavelength band of 0.98 μm does not emit radiation in the vicinity of the excitation wavelength.

FIG. 19 shows changes in the gain characteristics of the wavelength division multiplexing optical amplifier, and shows the case of 0.98 μm excitation. (A) shows the state of small signal light or large excitation light, and (b) shows the state of large signal light or small excitation light.

As shown in FIG. 19A, when the signal light is small or the pump light is large, the gain near 1.535 μm is larger than the gain near 1.557 μm.
As shown in (b), when the signal light is large or the pump light is small, the gain near 1.535 μm is smaller than the gain near 1.55 μm.

FIG. 20 shows dG / d due to changes in excitation wavelength.
This shows the tendency of the change of λ. In the case of 1.48 μm excitation, when the signal light power is increased by the transition probability of returning to the ground state, the transition from the region (a) to the region (b) in FIG. 19 is 0. It is faster than the case of .98 μm excitation.

In FIG. 20, in the area indicated by the dotted line,
In the case of 0.98 μm excitation, the shape shown in FIG. 19 (a) is maintained, but in the case of 1.48 μm excitation, gain tilt occurs and the shape shown in FIG. 19 (b) is obtained.

The present invention solves the problems of the prior art as described above, and provides a wavelength division multiplexing optical amplifier capable of controlling the output level according to the number of optical signals included in the wavelength division multiplexing optical signal. The purpose is to

[0033]

SUMMARY OF THE INVENTION A wavelength-multiplexed optical amplifier device of the present invention comprises an optical amplifier means for modulating a plurality of optical signals of different wavelengths at different frequencies and amplifying the multiplexed wavelength-multiplexed optical signal, Multiplexed optical signal number information detection means for obtaining information on the number of wavelength-multiplexed optical signals depending on the presence or absence of a modulation frequency component that has modulated the signal, detection means for detecting the output level of the optical amplification means, and detection by this detection means Optical output control means for controlling the output level of the optical amplification means to a level corresponding to the number of wavelength-multiplexed optical signals based on the signal and the information of the number of optical signals by the multiplexed optical signal number information detecting means. It has and.

Further, the multiplexed optical information detecting means includes a converting means for converting the wavelength multiplexed optical signal into an electric signal, and a modulation frequency component detecting means for separating the electric signal converted by the converting means into a modulation frequency component. It is a waste.

FIG. 1 shows the basic configuration (1) of the present invention.
The same reference numerals as those in FIGS. 14 and 16 denote the same names. In FIG. 1, reference numeral 25 denotes a light output control circuit as a light output control means. The signal light input is input to the EDF 1 as the optical amplification means via the branching device 11 and the isolator 5, and the pump light from the pump LD 2 is input to the EDF 1 via the WDM coupler 22 to amplify the signal light. The amplified signal light is output as a signal light via the isolator 6 and the splitter 7. The amplified output level is detected by the light receiving element 8 as the detecting means, and the detection signal is detected.
Enter in 5. Information on the number of optical signals included in the wavelength division multiplexed optical signal is input to the optical output control circuit 25. The light output control circuit 25 inputs a control signal to the drive circuit 4 and
Controls the pumping light power from the pumping LD2. Thereby, the amplification output level is controlled according to the information on the number of optical signals included in the wavelength division multiplexed optical signal.

At this time, a part of the signal light input is branched through the branching device 11 inserted on the input side, converted into an electric signal by the light receiving element 12, and its level is checked by an input break detection circuit (not shown). Thus, when the disconnection of the signal light input is detected, a shutdown signal is generated to stop the operation of the drive circuit 4, and the pumping light from the pumping LD 2 is turned off.

In the configuration of FIG. 1, the variable reference voltage for determining the optical output level is changed according to the information on the number of optical signals included in the wavelength division multiplexed optical signal, and the amplified output level is controlled.

In this case, the output control method is to keep the optical output level per signal constant. That is, when the output of the optical level of Pout is required for one signal, the optical output level of the optical amplifier is set to (Pout × N) when N optical signals are multiplexed.

FIG. 2 shows the basic configuration (2) of the present invention, in which the same elements as in FIG. 1 are indicated by the same numbers, and 27 is a reference voltage corresponding to the number of multiplexed signals at the signal light input. It is a reference voltage generating circuit for generating.

The reference voltage generating circuit 27 generates a reference voltage according to information on the number of multiplexed signals input from the outside, and supplies this to the optical output control circuit 25. The optical output control circuit 25 compares the electric signal corresponding to the signal light output level input from the light receiving element 8 with the variable reference voltage, controls the drive circuit 4 in accordance with the error, and drives the excitation LD 2. Is output. The excitation LD2 is
Excitation light whose intensity is controlled according to the drive current is generated, whereby the optical output is kept constant at a level according to the number of multiplexed signals.

FIG. 3 shows the basic configuration (3) of the present invention.
2, the same reference numerals as those in FIG. 2 indicate the same name parts,
28 is a branching device for branching the signal light input, 29 is a wavelength division multiplexed light
Each wavelength λ₁ ~ Λ_NWavelength separation (WD
M) filter, 30₁ ~ 30 _NIs the wavelength λ₁ ~ Λ
_N, A light receiving element for converting the component light of₁ ~ 3
1_NIs the wavelength λ₁ ~ Λ_NDetection of input loss of component light of
Input disconnection detection circuit, 32 is each input disconnection detection circuit 31₁ ~
31_NGenerates information on the number of multiplexed signals from the detection output of
It is a signal number information generating circuit.

In the configuration shown in FIG. 3, a WDM filter 29 is provided in the monitor section of the signal light input, the wavelength division multiplexed optical signal is separated into each component light, and the light reception element converts it into an electric signal, which is input. The disconnection detection circuit monitors whether or not the signal light of each wavelength is input. Then, the multiplexed signal number information generation circuit 32 generates multiplexed signal number information based on the monitoring result, and the reference voltage generation circuit 27
Then, a reference voltage for controlling the optical output is created based on the information on the number of multiplexed signals, and the intensity of the excitation light of the excitation LD 2 is controlled via the optical output control circuit 25. by this,
The optical output is kept constant at a level according to the number of multiplexed signals.

FIG. 4 shows the principle configuration (4) of the present invention,
The same reference numerals as those in FIG. 1 denote the same names, and 35 denotes a wavelength 1.
A pump LD for generating pump light in the 48 μm band, 36 is a pump LD for generating pump light in the wavelength band of 0.98 μm, and 37 is a multiplexer. Further, FIG. 5 shows a gain characteristic in the case of the principle configuration (4).

In the configuration shown in FIG.
The pumping light in the 48 μm band and the pumping light in the 0.98 μm wavelength band are combined through the multiplexer 37 and are incident on the EDF 1 to obtain the gain characteristic shown in FIG. ), A gain characteristic intermediate between the gain characteristics shown in FIG.
The gain difference between the signal light 1 in the μm band and the signal light 2 in the wavelength 1.55 μm band can be reduced.

[0045]

[Operation] (1). In the wavelength division multiplexing optical amplifier of the present invention,
When amplifying the wavelength-multiplexed optical signal, the optical output level of the optical amplifier is controlled so as to be changed according to the number of multiplexed signals of the wavelength-multiplexed optical signal.

Therefore, it is possible to prevent the signal light input from reaching the required level on the receiving side. Further, when the signal light of a certain frequency is cut off, it is possible to prevent an increase in level and a reception error.

(2). The optical output level of the optical amplifier in (1) can be controlled by controlling the power of the pumping light for the optical amplifier.

(3). As a method of controlling the optical output level of the optical amplifier in (1), the optical output level of the optical amplifier may be controlled so that the optical output per signal in the wavelength division multiplexed optical signal becomes constant.

(4). As a method of controlling the optical output level of the optical amplifier in (1), the optical output level of the optical amplifier may be changed by externally applying a reference input according to the number of multiplexed signals.

(5). The reference input of (4) can be given by switching from outside with a switch.

(6). As a method of controlling the optical output level of the optical amplifier in (1), the number of multiplexed signals at the signal light input may be detected and the optical output level of the optical amplifier may be changed according to the detected number of multiplexed signals. Good.

(7). The determination of the number of multiplexed signals in (6) is
This can be performed by branching the signal light input, separating the multiplexed signal light of each wavelength by the wavelength separation filter, and detecting the presence or absence of the signal light of each wavelength.

(8). The determination of the number of multiplexed signals in (6) is
Modulation of different frequencies is performed in advance on the signal light of each wavelength, and the components of each modulation frequency are separated from the signal obtained by converting the input optical signal into an electrical signal in the optical amplifier. This can be done by detecting the presence or absence of the component.

(9). The modulation frequency for modulating the signal light in (8) is set to a frequency that is suitable for suppressing the stimulated Brillouin scattering that occurs in the optical fiber that transmits the wavelength-multiplexed optical signal, so that stimulated Brillouin scattering is prevented at the same time. can do.

(10). In the wavelength division multiplexing optical amplifier of the present invention, by detecting the level of the signal light of each wavelength that is multiplexed, by controlling the gain of the optical amplifier so that the signal light of the wavelength of the lowest level becomes a required level, The output level of the signal light of all wavelengths is set to a predetermined value or higher.

Therefore, even if the wavelength-multiplexed optical amplifier has a wavelength dependency of gain, the signal level of any wavelength does not become unreceivable because the reception level is below the required level.

(11). To detect the level of the signal light of each wavelength in (10), separate the signal light of each wavelength by a wavelength separation filter, convert the separated signal light of each wavelength into an electrical signal, and detect the level. Can be done by.

(12). The detection of the level of the signal light of each wavelength in (10) was obtained by previously modulating the signal light of each wavelength with a different frequency and converting the input optical signal into an electric signal in the optical amplifier. This can be done by separating each modulation frequency component from the signal and measuring the level of each modulation frequency component.

(13). In the case of (10), a means for detecting the presence or absence of signal light of each detected wavelength is provided, and the signal light of the lowest level is the required level except for the signal light of the wavelength detected to have no input. By controlling the gain of the optical amplifier so that, the output level of the signal light of all wavelengths having the input may be set to a predetermined value or higher.

(14). In the optical amplifier, the wavelength is 1.5
When the 35 μm band signal light and the 1.55 μm band signal light are transmitted to a signal light line made of an erbium-doped fiber and a 0.98 μm band light is incident on the signal light line and excited. Has a gain for the signal light in the wavelength band of 1.535 μm larger than that for the signal light in the wavelength band of 1.55 μm.
When the light of 48 μm band is incident and excited, the wavelength is 1.5
There is a phenomenon that the gain for the signal light in the 35 μm band becomes smaller than the gain for the signal light in the wavelength 1.55 μm band.

In such a case, the wavelength division multiplexing optical amplifier of the present invention has a wavelength of 0.98 μm and a wavelength of 1.48.
Since the light in the μm band is made incident on the signal light line to be excited at the same time, the gain for the signal light in the wavelength of 1.535 μm and the gain for the signal light in the wavelength of 1.55 μm can be equalized. it can.

(15). In the case of (14), the wavelength is 0.98 μm
After combining the band light and the wavelength 1.48 μm band light,
You may make it inject and excite in a signal light line.

(16). In the case of (14), the wavelength is 0.98 μm
The band light and the wavelength 1.48 μm band light may be incident and excited at different positions of the signal light line, respectively.

[0064]

FIG. 6 shows an embodiment (1) of the present invention, in which the same parts as those in FIG.
Reference numeral 1 is an adder circuit for adding a voltage signal corresponding to the multiplexed signal number information, 42 is a level conversion circuit for converting a voltage signal corresponding to the multiplexed signal number information into a reference voltage, and these form a reference voltage generation circuit 27. is doing. Four
Reference numeral 3 is a transistor that drives the excitation LD 2.

The optical input branched by the branching device 28 is sent to the WDM
The filter 29 separates the component light of each wavelength λ _{1 to} λ _N , and the light receiving elements 30 ₁ to 30 _N convert it into an electric signal indicating the level of the component light of each wavelength λ _{1 to} λ _N. Input disconnection detection circuit 31
_{1 to} 31 _N normalize the levels of the component lights of wavelengths λ _{1 to} λ _{N to 1} V when normal and 0 V when disconnected, and output the normalized levels. The adder circuit 41 adds the outputs from the input disconnection detection circuits 31 _{1 to} 31 _N to generate a voltage corresponding to the multiplexed signal number information in the wavelength multiplexed optical signal.

The level conversion circuit 42 level-converts the voltage corresponding to the multiplexed signal number information to generate a reference voltage. The light output control circuit 25 compares the output voltage detected from the light output level from the light receiving element 8 with the reference voltage from the level conversion circuit 42, controls the transistor 43 according to the error voltage, and drives the pump LD 2 Drive current is generated. The pumping LD 2 generates pumping light whose intensity is controlled by this, and the signal light output is
It is always kept constant at a level according to the number of multiplexed signals.

FIG. 7 shows an embodiment (2) of the present invention, showing an example of a method for detecting the presence or absence of each wavelength signal in a wavelength multiplexed optical signal. On the optical terminal device side, 46 ₁ modulates light of wavelength λ ₁ at frequency f ₁ to generate a modulated optical signal, and 46 ₂ modulates light of wavelength λ ₂ at frequency f _2. A modulated optical signal generator for generating a modulated optical signal, and 47 is a wavelength multiplexer (WDM MUX) for synthesizing optical signals of respective wavelengths to generate a wavelength multiplexed optical signal.
Is. Reference numeral 48 is an optical fiber for transmitting an optical signal. On the optical amplifier side, 49 is a light receiving element for converting the optical signal branched by the branching device 28 into an electric signal, 50 ₁ is a band pass filter (BPF) for extracting a signal of frequency f ₁ from the converted electric signal, 50 ₂ is a band pass filter (BPF) for extracting a signal of frequency f ₂ from the converted electric signal,
51 ₁ determines the level of the signal of frequency f ₁ and determines the wavelength λ
₁ , a level determination unit that generates information indicating the presence or absence of an optical signal,
51 ₂ determines the level of the signal of frequency f ₂ and determines the wavelength λ
₂ is a level determination unit that generates information indicating the presence or absence of an optical signal.

On the optical terminal device side, frequency (phase) modulation for wavelength identification is applied to each transmission signal. That is, the transmission optical signal of wavelength λ ₁ is modulated at frequency f ₁ and
_By applying modulation of frequency f ₂ to the transmission optical signal of ₂ ,
It is transmitted as a wavelength-multiplexed optical signal wavelength-multiplexed by the MUX 47.

On the side of the optical amplifier, a part of the signal light input is converted into an electric signal, and the band pass filters 50 ₁ ,
The components of the frequencies f ₁ and f ₂ are extracted by 50 ₂ and the levels are determined by the level determiners 51 ₁ and 51 ₂ , and the presence or absence of signal light of each wavelength, that is, the number of optical signals included in the wavelength multiplexed optical signal. To obtain the information of
Band pass filters 50 ₁ and 50 ₂ and level determiner 5
1 ₁ and 51 _{2 form a} multiplexed optical signal information detecting means. Generally, rather than separating the signal light of each wavelength by the WDM filter as in the embodiment (1) and obtaining information on the number of optical signals, it is converted into an electrical signal once as in the embodiment (2). Then, the method of detecting with a filter is more advantageous in terms of cost.

In an optical fiber, when the transmitted light has a high spectral purity and its level is higher than a certain threshold value, stimulated Brillouin scattering occurs and the light transmission is hindered. By increasing the spectrum band by applying phase or frequency modulation to the signal light, it is possible to increase the level of the transmitted light while suppressing the stimulated Brillouin scattering, so by doing as in Example (2) It is also possible to combine the function of preventing the occurrence of stimulated Brillouin scattering.

In this case, if the modulation frequency is not high to some extent, the stimulated Brillouin scattering cannot be effectively suppressed, and if the frequency is too high, the LD drive current is amplitude-modulated in the modulated optical signal generator. When the frequency of the light generated by the LD is modulated,
The conversion efficiency of amplitude modulation / frequency modulation is reduced, the width of frequency modulation is narrowed, and the effect of suppressing stimulated Brillouin scattering is reduced. Therefore, it is desirable that the modulation frequency for realizing the embodiment (2) is also a frequency suitable for suppressing such stimulated Brillouin scattering.

FIG. 8 shows an embodiment (3) of the present invention, and shows a wavelength division multiplexing optical amplifier used as a post amplifier. The same components as those in FIGS. 3 and 7 are designated by the same reference numerals, 55 is a WDM filter for separating a wavelength-division multiplexed optical signal into optical signals of respective wavelengths, and 56 _{1 and} 56 ₂ are optical signals of wavelengths λ ₁ and λ ₂ , respectively. Light receiving elements 57 _{1 and} 57 ₂ for converting into electric signals are light receiving elements 56 _{1 and} 56 _, respectively.
A level detector for detecting the level of the output signal of ₂ , 58,
Reference numeral 59 is a comparator, and 60 to 63 are diodes.

FIG. 8 shows the case of two-wave multiplexing for simplicity. The WDM filter 55 separates the optical signal of wavelength λ _{1 and} the optical signal of wavelength λ ₂ from the optical output branched by the branching device 7, and the light receiving elements 56 _{1 and} 56 ₂ convert the respective optical signals into electric signals. Then, the level detectors 57 _{1 and} 57 ₂ are
The level of each electric signal is detected.

On the other hand, the WDM filter 29 includes a branching device 28.
The optical signal having the wavelength λ _{1 and} the optical signal having the wavelength λ ₂ are separated from the optical input branched by the optical input, and the light receiving elements 30 _{1 and} 30 ₂ convert the respective optical signals into electric signals, and the input disconnection detection circuit 31 _{1 ,} 31
₂ is for each wavelength λ depending on the level of each electric signal.
Detects disconnection of optical signals of ₁ and λ ₂ .

The comparators 58 and 59 compare the levels of the optical signals of the respective wavelengths detected by the level detectors 57 _{1 and} 57 ₂ with the reference levels V _{REF1 and} V _REF2 , respectively, and compare the results with the diodes 62 and 57. It outputs through the logic circuit which consists of 63, and controls the drive circuit 4. As a result, the intensity of the pumping light output from the pumping LD 2 is fed back by the optical signal of the wavelength that requires the most gain, and control is performed so that the level difference between the optical signals of the respective wavelengths is reduced.

When any _{one of the} input disconnection detection circuits 31 _{1 and} 31 ₂ detects the disconnection of the optical input of the corresponding wavelength and the output becomes 0 V, the disconnection is detected by the logic circuit including the diodes 60 and 61. The output level of the comparator corresponding to the optical input of the changed wavelength decreases. As a result, the drive circuit 4 operates according to the feedback control corresponding to the optical signal of the wavelength where the optical output exists, and drives the pump LD,
This prevents the EDF 1 from entering an excessively excited state.

Therefore, according to the embodiment shown in FIG.
It is possible to compensate for the signal level drop due to the wavelength dependency of gain, which is the second problem described above.

FIG. 9 shows an embodiment (4) of the present invention, in which the same components as those in FIG. 4 are designated by the same reference numerals.

In the embodiment shown in FIG. 9, a wavelength division multiplexed optical signal composed of a signal light 1 having a wavelength of 1.535 μm and a signal light 2 having a wavelength of 1.557 μm is input to the EDF 1 via the isolator 5. On the other hand, the pumping light having a wavelength of 1.48 μm from the pumping LD 35 and the wavelength of 0.
The excitation light of 98 μm is multiplexed via the multiplexer 37, and W
It is input to the EDF 1 via the DM coupler 22 to be in an excited state.

As a result, the EDF 1 performs an amplifying operation on the wavelength-multiplexed optical signal input, and the amplified wavelength-multiplexed optical signal produces a signal light output through the isolator 6.

At this time, since the EDF 1 is excited by the excitation light having the wavelength of 1.48 μm and the excitation light having the wavelength of 0.98 μm, the EDF 1 has the wavelength of 1.535 μm due to the flat gain characteristic as shown in FIG. Signal light and wavelength of 1.557 μm
It is possible to amplify a wavelength division multiplexed optical signal including the signal light of

FIG. 10 shows an embodiment (5) of the present invention, in which the same components as those in FIG. 9 are designated by the same reference numerals, and 65 is a WDM coupler.

In FIG. 10, the pumping light of wavelength 1.48 μm from the pumping LD 35 enters the EDF 1 from the rear through the WDM coupler 22, and the pumping light of wavelength 0.98 μm from the pumping LD 36 passes through the WDM coupler 65. It is incident on the EDF 1 from the front via the front.

As a result, the EDF 1 has a wavelength of 1.48μ.
Since it is excited by the pump light of m and the pump light of wavelength 0.98 μm, the signal light of wavelength 1.535 μm and the wavelength of 1.557 μm are obtained by the flat gain characteristic as shown in FIG.
It is possible to amplify a wavelength-division-multiplexed optical signal composed of m signal lights.

FIG. 11 shows an embodiment (6) of the present invention, in which the same parts as in FIG. 9 are indicated by the same numbers, 67 is a branching device for branching the signal light output, 68 is a wavelength 1 A WDM filter for separating the signal light of .535 μm and the signal light of wavelength 1.557 μm, 69 and 70 are light receiving elements for converting the signal light of wavelength 1.535 μm and the signal light of wavelength 1.557 μ into electric signals, and 71 is Light receiving element 69, 7
A comparator 72 for comparing the output signal level of 0, a pump LD 35 for generating pump light of wavelength 1.48 μm and a pump LD 36 for generating pump light of wavelength 0.98 μm according to the comparison result in the comparator 71. It is a controller that controls so as to increase the excitation light intensity of either of the above.

The signal light output is branched by the branching device 67 and WD
The signal light having a wavelength of 1.535 μm and the signal light having a wavelength of 1.557 μm are separated via the M filter 68, converted into an electric signal, and the respective levels are compared in a comparator 71. As a result of the comparison, for example, when the optical level of the signal light having the wavelength of 1.535 μm is higher, the pump LD 35 is controlled to increase the intensity of the pump light having the wavelength of 1.48 μm. When the optical level of the signal light is higher, the pump LD 36 is controlled to increase the intensity of the pump light having a wavelength of 0.98 μm.

By doing so, the difference between the level of the signal light having the wavelength of 1.535 μm and the level of the signal light having the wavelength of 1.557 μm in the signal light output can be kept to a minimum.

[0088]

As described above, the wavelength-multiplexed optical amplifier device of the present invention has the optical output control means so that the amplified optical output level is in accordance with the information on the number of optical signals included in the wavelength-multiplexed optical signal. Since the pumping light power of the pumping LD is controlled by the above, it is possible to prevent the receiving side from being unable to secure the required received light level, or to prevent an adverse effect such as an error due to an excessive received light level.

Further, when the wavelength-multiplexed optical signal obtained by wavelength-multiplexing the optical signals modulated at different frequencies is amplified by the optical amplification means such as the EDF 1, the modulation frequency component is detected corresponding to the frequency and included in the wavelength-multiplexed optical signal. Since the information on the number of optical signals is obtained, the information on the number of optical signals is automatically obtained and the optical amplification means such as the EDF 1 is controlled to control the optical output level according to the number of optical signals. can do.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle configuration (1) of the present invention.

FIG. 2 is a diagram showing a principle configuration (2) of the present invention.

FIG. 3 is a diagram showing a principle configuration (3) of the present invention.

FIG. 4 is a diagram showing a principle configuration (4) of the present invention.

FIG. 5 is a diagram showing a gain characteristic in the case of the principle configuration (4).

FIG. 6 is a diagram showing an embodiment (1) of the present invention.

FIG. 7 is a diagram showing an embodiment (2) of the present invention.

FIG. 8 is a diagram showing an embodiment (3) of the present invention.

FIG. 9 is a diagram showing an embodiment (4) of the present invention.

FIG. 10 is a diagram showing an embodiment (5) of the present invention.

FIG. 11 is a diagram showing an embodiment (6) of the present invention.

FIG. 12 is a diagram showing an example of a conventional optical amplifier control system.

FIG. 13 is a diagram showing an example of a conventional wavelength division multiplexing optical amplifier.

FIG. 14 is a diagram showing a configuration example of a conventional two-wavelength multiplex optical amplifier.

15 is a diagram showing gain characteristics in the two-wavelength multiplex optical amplifier shown in FIG.

FIG. 16 is a diagram showing a configuration example of a gain measurement system in a wavelength division multiplexing optical amplifier.

FIG. 17 is a diagram illustrating gain characteristics in a wavelength division multiplexing optical amplifier.

FIG. 18 is a diagram showing an example of gain tilt.

FIG. 19 is a diagram showing changes in gain characteristics in a wavelength division multiplexing optical amplifier.

FIG. 20 is a diagram showing a tendency of changes in dG / dλ due to changes in excitation wavelength.

[Explanation of symbols]

1 Erbium-doped fiber (EDF) 2 pump laser diode (LD) 4 drive circuit 5 Isolator 6 Isolator 7 turnout 8 Light receiving element 11 turnout 12 Light receiving element 22 Wavelength Division Multiplexer (WDM) Coupler 25 Optical output control circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04J 14/00 14/02 (72) Inventor Kenji Tagawa 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited (72) Invention Shinya Inagaki 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor, Nobufumi Shunan Minami 2-chome, Kitaichijo Nishi, Chuo-ku, Sapporo, Hokkaido (56) Fujitsu Digital Digital Technology Limited (56) References JP-A-7-202306 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/35 501 H01S 3/06 H01S 3/10

Claims (8)

(57) [Claims] 1. Optical amplification means for amplifying and outputting a wavelength-multiplexed optical signal obtained by modulating a plurality of optical signals of different wavelengths with different modulation frequencies and multiplexing, and converting means for converting the wavelength-multiplexed optical signal into an electrical signal. A modulation frequency component detecting means for separating the electric signal converted by the converting means for each of the modulation frequencies; a detecting means for detecting an output level of the optical amplifying means; a detection signal by the detecting means; based on the number of information of the optical signal by the frequency component detection means, the output level of said optical amplifying means, and a light output control means for controlling the level corresponding to the number of the wavelength-multiplexed optical signal A wavelength division multiplexing optical amplifier characterized by the above. 2. Optical amplification means for amplifying and outputting a wavelength-multiplexed optical signal obtained by modulating a plurality of optical signals of different wavelengths by different phase modulation and multiplexing, and conversion means for converting the wavelength-multiplexed optical signal into an electrical signal. A modulation phase component detection means for separating the electric signal converted by the conversion means for each phase modulation, a detection means for detecting the output level of the optical amplification means, a detection signal by the detection means, and the phase Optical output control means for controlling the output level of the optical amplification means to a level corresponding to the number of wavelength-multiplexed optical signals based on the information on the number of optical signals by the modulation component detection means. A wavelength division multiplexing optical amplifier characterized by the above. 3. Optical amplification means for amplifying and outputting a wavelength-multiplexed optical signal obtained by modulating and multiplexing a plurality of optical signals of different wavelengths with different modulation frequencies, and conversion means for converting the wavelength-multiplexed optical signal into an electrical signal. A modulation frequency component detection means for separating the electric signal converted by the conversion means for each modulation frequency; and a level of an optical signal of each wavelength corresponding to the level of each frequency component by the modulation frequency component detection means. And a light output control means for controlling the output level of the light amplification means to a level corresponding to the number of the wavelength-multiplexed optical signals. 4. An optical amplifying means for amplifying and outputting a multiplexed signal obtained by modulating a plurality of optical signals of different wavelengths by different phase modulation, a converting means for converting the wavelength multiplexed optical signal into an electric signal, and the converting means. Phase modulation component detecting means for separating the electrical signal converted by each phase modulation, and obtained from the level of each phase modulation component by the phase modulation component detecting means, based on the level of the optical signal of each wavelength, An optical output control means for controlling the output level of the optical amplification means to a level corresponding to the number of the wavelength-multiplexed optical signals, a wavelength division multiplexing optical amplifier. 5. A wavelength-multiplexed optical signal by combining a modulated optical signal generator for applying frequency or phase modulation for wavelength discrimination to an optical signal of a plurality of wavelengths and an optical signal of each wavelength from the modulated optical signal generator. And an optical fiber for transmitting a wavelength-multiplexed optical signal from the optical terminal station, and a signal obtained by converting the wavelength-multiplexed optical signal into an electrical signal. An optical communication system comprising: a wavelength-multiplexing optical amplification device that determines the number of multiplexed signals by separating the modulation frequency or each phase modulation component and detecting the presence or absence of each modulation component. 6. The optical communication system according to claim 5, wherein the wavelength identifying frequency is set to a frequency suitable for suppressing stimulated Brillouin scattering in an optical fiber transmitting a wavelength division multiplexed optical signal. An optical communication system characterized by being selected. 7. A plurality of optical signals having different wavelengths are different from each other.
Wavelength-multiplexed optical signal that is modulated and multiplexed at
Optical amplification means for amplifying and outputting, conversion means for converting the wavelength-multiplexed optical signal into an electric signal, and an electric signal converted by the conversion means for the modulation frequency.
Modulation frequency component detection means for separating each number, and the presence or absence of an optical signal of each wavelength from the modulation frequency component detection means
Signal light interruption detecting means for detecting the optical signal and the wavelength division multiplexed optical signal output from the optical amplifying means.
Then, the wavelength division multiplexing optical signal is
Output to detect the intensity of the optical signal of each wavelength.
Force signal intensity detection means, information regarding signal light interruption by the signal light interruption detection means,
The intensity of the optical signal of each wavelength by the output signal light intensity detection means
And the wavelength-multiplexed optical signal based on
The optical amplifier so that the level difference between the wavelengths of
And a light output control means for controlling the output level of each stage . 8. Different optical signals of different wavelengths are provided.
The wavelength-division multiplexed optical signal
Width optical output means, conversion means for converting the wavelength-multiplexed optical signal into an electric signal, and phase modulation of the electric signal converted by the conversion means
The phase modulation component detection means that separates for each, and the presence or absence of the optical signal of each wavelength from the phase modulation component detection means
Signal light break detection means for detecting and wavelength division multiplexed optical signal output from the optical amplification means
Then, the wavelength division multiplexing optical signal is
Output to detect the intensity of the optical signal of each wavelength.
Force signal intensity detection means, information regarding signal light interruption by the signal light interruption detection means,
The intensity of the optical signal of each wavelength by the output signal light intensity detection means
And the wavelength-multiplexed optical signal based on
The optical amplifier so that the level difference between the wavelengths of
And a light output control means for controlling the output level of each stage .