JP2001094181A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier, which performs a control of a gain on the basis of the input power of signal light and can perform control of the gain, which realizes keeping of a constant gain with good accuracy, also in the case where the signal light wavelength distribution of multiple wavelength signal light is varied. SOLUTION: An excitation light power calculating part 21 provided in a gain control circuit 20 normally sets an excitation light power from an excitation light source 11, on the basis of an input power of signal light detected by a photodetector 22 to perform a constant control of a feed forward gain at high speed. At the same time, when the signal light is changed over and a variation is generated in the signal light wavelength distribution of multiple wavelength signal light, variation in the signal light wavelength distribution is detected by a wavelength distribution detecting means 30, and an excitation light power setting method in the calculating part 21 is modified on the basis of the result of the detection. Hereby, an optical amplifier, which always holds constant the gain with good accuracy, is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier capable of collectively amplifying multi-wavelength signal light.

[0002]

2. Description of the Related Art Research and development on large-capacity high-speed communication and long-distance communication using an optical fiber transmission line network have been actively conducted due to social needs due to the advent of an advanced information society.
Here, wavelength division multiplexing (WDM: Wavelength Division Mult)
The iplexing) transmission system performs high-speed, large-capacity optical communication by transmitting multi-wavelength signal light (a plurality of signal lights having different wavelengths) through an optical fiber line. Development and introduction are being promoted in response to the above.

In such a wavelength division multiplexing transmission system, an optical amplifier is used in a relay station or the like in order to compensate for transmission loss in transmitting multi-wavelength signal light over a long distance. The optical amplifier is configured to include a light amplifying section to which a fluorescent substance excitable by the pump light is added, amplify and output the input signal light, and pumping means for supplying the light amplifying section with the pump light. For example, Er (erbium)
Doped fiber amplifier (EDFA: Erbium-Doped Fiber A)
mplifier).

[0004]

SUMMARY OF THE INVENTION In an optical amplifier,
It is important to amplify the multi-wavelength signal light with a constant gain (amplification rate). In response to such a requirement, automatic gain control (AGC: Automati
As one method of performing c Gain Control), it has been proposed to set or change the power of the pumping light from the pumping means based on the input power of the signal light to control the gain in the optical amplifier ( For example, the document "SY Parket al., Elec
tron. Lett., Vol. 32, No. 23, pp. 2161-2162, 1996 "and" SYPark et al., IEEE Photon. Technol. Lett.,
Vol. 10, No. 6, pp. 787-789, 1998 ”). In this method, the gain control is performed by monitoring only the input power of the signal light input to the optical amplifier. Therefore, the control circuit is simplified, and the control can be performed at high speed.

However, the above-described gain control based on the input power has the following problems when the optical amplifier is applied to a transmission line for multi-wavelength signal light. In other words, when the input power of the multi-wavelength signal light changes uniformly over the entire signal wavelength band due to span loss fluctuations, etc., it is possible to accurately stabilize the gain by controlling the pump light power based on the input power. It is.

On the other hand, in a wavelength division multiplexing transmission system, an OXC (Optical Cross Connect) or an OADM (Opt.
icalAdd / Drop Multiplexer) may be used.
At this time, the signal light included in the multi-wavelength signal light is switched, and the signal light wavelength distribution, that is, the number of signal lights and the signal light wavelength arrangement (the wavelength arrangement of each signal light within the wavelength band) fluctuates. The total input power changes, causing a gain variation. With respect to such a gain variation, the pump light power control based on only the input power cannot realize a constant gain with sufficient accuracy due to the wavelength dependence of the gain control.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has been made in consideration of a case where a signal light wavelength distribution of a multi-wavelength signal light is changed in an optical amplifier that performs gain control based on the input power of the signal light. It is another object of the present invention to provide an optical amplifier capable of performing gain control in which constant gain can be accurately realized.

[0008]

In order to achieve the above object, an optical amplifier according to the present invention collectively amplifies a multi-wavelength signal light comprising a plurality of signal lights having different wavelengths within a predetermined wavelength band. (1) an optical amplifier for optically amplifying input multi-wavelength signal light with excitation light and outputting the amplified light; (2) excitation means for supplying excitation light to the optical amplifier.
(3) input power detection means for detecting the input power of the multi-wavelength signal light input to the optical amplifier, and (4) signal light within a predetermined wavelength band for a plurality of signal lights included in the multi-wavelength signal light. Wavelength distribution detecting means for detecting a wavelength distribution; (5)
A pump light power calculating means for calculating a pump light power to be set for the pump means based on the input power and the signal light wavelength distribution; and an optical amplifier by controlling the pump means according to the calculated pump light power. Gain control means for controlling the gain of the optical amplification in the section.

In the above-described optical amplifier, the gain control is performed by setting the pump light power based on two conditions of the input power and the signal light wavelength distribution in the pump light power calculating means provided in the gain control means. At this time, if signal light switching by OXC or OADM is not performed and the signal light wavelength distribution of the multi-wavelength signal light to be optically amplified does not fluctuate, the gain control is based only on the input power, and the feedforward high-speed gain The gain is stabilized by the control.

[0010] On the other hand, when the signal light switching is performed and the signal light wavelength distribution fluctuates, gain control is performed so as to respond to the fluctuation. Therefore, an optical amplifier capable of performing gain control that can always or accurately obtain the effect of stabilizing the gain by preventing or suppressing gain fluctuation that has occurred during signal light switching in normal gain control based on input power. Can be.

Further, the pump light power calculating means includes:
Based on the input power, the pumping light power is calculated using a relational expression for the input power and the pumping light power that makes the gain of the optical amplification constant, and (b) when the signal light wavelength distribution is changed, Based on the signal light wavelength distribution,
It may be characterized by changing the relational expression.

As a specific method of the constant gain control based on the input power, a relationship between the input power and the pumping light power at which a constant gain is obtained is obtained in advance, and the pumping light power is calculated from the input power using the relational expression. There is a method of controlling the excitation means by setting. In this case, when the signal light wavelength distribution changes, the gain control method can be easily changed by changing the relational expression to one corresponding to the changed wavelength distribution. As a method of changing the relational expression, for example, a method of changing the relational expression by applying a relational expression with the same functional form to an arbitrary signal light wavelength distribution and correcting the coefficient of the relational expression There is.

Further, the wavelength distribution detecting means is a signal light number detecting means for detecting the number of signal lights contained in the multi-wavelength signal light and the signal light wavelength arrangement.

Alternatively, the wavelength distribution detecting means is a distribution deviation detecting means for detecting deviation of the signal light wavelength distribution.

As the wavelength distribution detecting means for detecting the signal light wavelength distribution, various configurations can be used. For example, all the signal lights included in the multi-wavelength signal light are demultiplexed to separate each signal light. There is a method in which the presence or absence of the signal light is detected and the number of signal lights and the signal light arrangement are obtained. Further, the wavelength distribution bias and its change may be obtained by dividing the wavelength band of the multi-wavelength signal light into several wavelength regions, detecting the respective signal light powers, and comparing them. The configuration of these wavelength distribution detecting means can be appropriately selected depending on conditions such as the accuracy of gain stabilization required for gain control. These signal light detections may be performed by branching a part of the input signal light, or may be performed by branching a part of the output signal light after the optical amplification.

Alternatively, the wavelength distribution detecting means is a monitoring light detecting means for obtaining information on the signal light wavelength distribution from the monitoring light added to the multi-wavelength signal light. It is possible to detect the signal light wavelength distribution similar to the above by using the monitoring light instead of detecting the signal light.

Further, the optical amplifier may further include a variable gain equalizer for equalizing the wavelength dependence of the gain of the optical amplification with respect to the multi-wavelength signal light output from the optical amplifier. Alternatively, while the loss is substantially constant at a predetermined wavelength within a predetermined wavelength band,
The apparatus may further include a loss slope variable device in which a slope of a loss with respect to a wavelength is variable in a predetermined wavelength band and a predetermined slope is provided for a wavelength dependency of a gain of optical amplification for a multi-wavelength signal light output from the optical amplifier. good. By providing these additional gain adjustment means, the accuracy of gain stabilization can be improved.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an optical amplifier according to the present invention will be described below in detail with reference to the drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

First, a gain control method based on the input power of the signal light input to the optical amplifier will be described. FIG.
An EDFA (Erbium-Doped Fiber Amplifier), which is an example of an optical amplifier capable of collectively amplifying multi-wavelength signal light, is described below.
lifier, Er-doped fiber amplifier).
The EDFA 9 is an optical amplification unit, EDF (Erbium-DopedFi
ber, Er-doped optical fiber) 90, a pump light source 91 as a pumping means, a WDM coupler 92, and two optical isolators 93 and 94. In the above configuration, the excitation light source 9
The signal light input via the optical isolator 93 to the EDF 90 to which the pump light is supplied from 1 is amplified by the EDF 90 and output to the outside via the optical isolator 94.

The gain coefficient γ _s of the optical amplification gain in the EDF 90 in the EDFA 9 is approximately given by the following (1).
Expression: γ _s = g ^* / (1 + P _p ^th P _s / P _p P _sat ) (1) Here, g ^* is the unsaturated gain, P _p ^th is the pump light threshold power, P _s is the signal light power, P _p is the pump light power, and P _sat is the saturation intensity. From this equation (1), in order to keep the gain constant by setting the gain coefficient γ _s to a constant value, the gain control should be performed so that P _p ^th P _s / P _p P _sat = constant (2) I understand. That is, by setting or changing the pumping light power P _p in response to the signal light power P _s, it is possible to realize the automatic gain control. In particular, by utilizing this relationship, it is possible to perform high-speed constant feed-forward gain control based on the input power of the signal light input to the optical amplifier.

The gain control based on the input power utilizing the above relationship will be described with reference to a specific example. In the EDFA 9 shown in FIG.
0.98 μm forward pumping E using a 98 μm semiconductor laser
DFA, and the absorption length product at a wavelength of 1.53 μm of the EDF 90 is 56 dB. For signal light to be subjected to optical amplification, 1ch with a wavelength of 1550 nm (hereinafter, ch is a channel or the number of channels of multi-wavelength signal light,
That represents a signal light or the signal light number included in the WDM signal light) only, -18～0D the input power P _in
It is changed in the range of Bm. In this case, the pumping light power P _p of the pumping light source 91, by controlling so as to satisfy the relationship shown in the graph of FIG. 2 based on the input power P _in of the signal light, the optical amplification of the signal light in the EDF90 1
A constant gain can be obtained at 8.18 dB.

The pump light power P _p [mW] shown in FIG.
And the input power P _in [mW] of the signal light is a linear relationship, and can be expressed as P _p [mW] = 133.54 P _in [mW] +5.2035 (3). Using this relational expression, the excitation light source 91
The pumping light power P _p from by controlling feed forward, it is possible to realize the automatic gain control for any input power. In general, the input power P _in of the excitation light power P _p and the input signal light is approximately follows (4)
Expression: P _p [mW] = K · P _in [mW] + A (4) Here, K is a constant representing the slope of the relational expression, A is a constant representing the intercept, and in the above example, K = 133.54 and A = 5.2035. These constants are determined by the target gain set in the constant gain control, the specific configuration of the optical amplifier, and the like.

Next, let us consider a case where gain control based on input power is similarly performed on a multi-wavelength signal light including a plurality of signal lights having different wavelengths. As the multi-wavelength signal light, it is assumed that signal light of a total of 16 channels is multiplexed in a wavelength band of 1535.0 to 1559.0 nm and a wavelength interval of 1.6 nm. At this time, the input power per signal light is-
30 to -12 dBm / ch (-18 to 0 at all input power)
dBm), the above-described 1ch
The feed-forward control of the pump light power is performed by using the following linear relational expression, P _p [mW] = 133.49P _in [mW] +5.2516 (5), as in the case of only the signal light. The gain can be kept constant.

A uniform change in input power over the entire wavelength band in the multi-wavelength signal light as in the above-described example is caused by a change in loss in the optical transmission line and a change in the intensity of the entire signal light (span loss fluctuation). And so on. On the other hand, OXC (Optical Cross Co.)
nnect) and OADM (Optical Add / Drop Multiplexer)
The signal light wavelength distribution may fluctuate due to switching of the signal light to be transmitted using such a method. In this case as well, the input power (total input power) of the signal light input to the optical amplifier changes. I do. In the following, the signal light wavelength distribution of the multi-wavelength signal light refers to the number of signal lights to be subjected to optical amplification on a transmission line among a plurality of signal lights constituting the multi-wavelength signal light, and within the signal wavelength band. The signal light wavelength arrangement (wavelength of each signal light) of those signal lights will be referred to.

With respect to the change of the total input power due to the change of the signal light wavelength distribution, the wavelength band 153 described above is used.
With 16ch multi-wavelength signal light of 5.0 to 1559.0 nm,
Assuming that the input power per signal light is -12 dBm / ch and the number of signal lights is reduced by 1 ch from 16 ch at a time, the change in gain that occurs with respect to the residual signal light was examined. However, with regard to gain control, control is performed using equation (5) that makes the gain constant with respect to a uniform input power change in 16 channels.

FIGS. 3 and 4 are graphs showing the change in gain of the residual signal light at this time. The horizontal axis indicates the total input power of the signal light, and the vertical axis indicates the change in gain of the residual signal light.
The total input power is reduced from 0 dBm (16 ch) to -12 dBm (1 ch) by reducing the number of signal lights.
To change. Dotted lines shown in the respective graphs are set gains set in gain control assuming a uniform change in input power, and are set before the number of signal lights decreases (total input power = 0 dBm). The gain matches this set gain.

The graph shown in FIG. 3 shows a change in gain with respect to the residual signal light having a wavelength of 1559.0 nm, which is the longest wavelength side, when the signal light of 16 ch is sequentially removed from the short wavelength side. At this time, the gain is 0.29 dB at the maximum.
Fluctuation (gain decrease). In addition, the graph shown in FIG. 4 shows that the wavelength 1535.0n which is the shortest wavelength side when the signal light of 16 ch is sequentially excluded from the long wavelength side.
5 shows a change in gain with respect to m residual signal light. At this time, the gain fluctuates up to 0.51 dB (gain increase).

As described above, in the examples shown in FIGS. 3 and 4, although the gain control is performed based on the change in the input power, a gain variation from the set gain occurs in any case. . Such a gain variation is due to the wavelength dependence of gain control. That is, in the EDF 90 which is an optical amplification unit, the stimulated emission cross section and the absorption cross section have wavelength dependence, and therefore, there is wavelength dependence in characteristics such as saturation intensity.

At this time, the gain coefficient γ _s (1)
As is apparent from the expression and the conditional expression of the gain control shown in the expression (2) based on the expression, the wavelength dependence also occurs _in the relationship between the pump light power _Pp and the input power Pin of the signal light under the condition of the constant gain. . Therefore, in order to perform gain control based on the input power so as to keep the gain constant even when the input power changes due to the fluctuation of the signal light wavelength distribution, it is necessary to set the input power at the time of the fluctuation of the signal light wavelength distribution. It is necessary to change the excitation light power setting rule for calculating the excitation light power to be changed. Specifically, when the gain control based on the input power is performed by setting the pumping light power using the linear relational expression as shown in Expression (4), the slope K and the intercept A, which are the coefficients for determining the relational expression, are used. Must be adjusted according to the signal light wavelength distribution.

The optical amplifier according to the present invention performs high-speed gain control based on a constant pumping light power setting rule based on input power except when the signal light wavelength distribution varies, and at the same time, changes the signal light wavelength distribution by switching the signal light. During fluctuations,
By adjusting the pump light power setting rule such as the relational expression between the input power and the pump light power, it is possible to prevent or suppress the gain fluctuation caused by the fluctuation of the signal light wavelength distribution.

FIG. 5 shows an optical amplifier ED according to the present invention.
1 is a configuration diagram illustrating a first embodiment of an FA (Erbium-Doped Fiber Amplifier, Er-doped fiber amplifier). The EDFA 1 includes an EDF (Erbium-Doped Fiber, Er-doped optical fiber) 10, an excitation light source 11, a WDM coupler 12,
And two optical isolators 13 and 14. The EDF 10, which is an optical amplifier of this optical amplifier,
This is a silica-based optical fiber to which Er element is added, and optically amplifies and outputs an input signal light included in a predetermined optical amplification wavelength band when excitation light of a predetermined wavelength is supplied.

Each of the optical isolators 13 and 14
It allows light to pass in the forward direction but not in the reverse direction. That is, the optical isolator 13 allows the light arriving from the signal light input side to pass through the EDF 10, but does not allow the light to pass in the opposite direction. Also, the optical isolator 14
Allows the light arriving from the EDF 10 to pass through to the signal light output side, but does not allow the light to pass in the opposite direction.

The excitation light to the EDF 10 is supplied by an excitation light source 11 which is an excitation means. The excitation light source 11 is WD
It is connected to an optical fiber line via an M coupler 12. Here, the WDM coupler 12 allows the pumping light output from the pumping light source 11 to reach the EDF 10 and passes the signal light from the optical isolator 13 toward the EDF 10. With this, EDF
A1 is an optical amplifier having a forward pumping (forward pumping) configuration. The excitation light source 11 that excites the EDF 10
For example, a semiconductor laser having a wavelength of 0.98 μm is used.

In the above configuration, when signal light is input via the optical isolator 13 to the EDF 10 to which pump light of a predetermined wavelength is supplied from the pump light source 11, the input signal light is amplified by the EDF 10. Then, the amplified signal light is output to the outside from the EDF 10 via the optical isolator 14 as output signal light.

In the following, the EDFA 9 of FIG.
As in the specific example described above, the pumping light source 11 is 0.98 μm semiconductor laser and the EDFA 1 is 0.98 μm.
The EDFA is a μm forward excitation EDFA, and the absorption length product at a wavelength of 1.53 μm of the EDF 10 is 56 dB. Also,
The signal light to be subjected to optical amplification has a wavelength band of 1535.0 to 1535.0.
The multi-wavelength signal light is obtained by multiplexing 16ch signal lights at 1559.0 nm and a wavelength interval of 1.6 nm.

In the EDFA 1 of this embodiment, the EDFA 1
Automatic gain control (AGC) based on the input power of the signal light input to F10.
l) have done. To realize this gain control, FIG.
The EDFA 1 shown in FIG. 1 includes an optical coupler 15, a photodetector 22,
And a gain control circuit 20.

A part of the signal light input to the EDF 10 is split by an optical coupler 15 provided on a transmission path in front of an optical isolator 13, and the split input signal light component is a light detection signal which is input power detection means. Is detected by the detector 22. The detection result of the input signal light by the photodetector 22 includes the gain control circuit 2 including information on the input power of the signal light.
0 is given.

The gain control circuit 20 has a pump light power calculator 21 for calculating the pump light power to be set for the pump light source 11. The pump light power calculator 21 calculates the pump light source 1 based on the input power of the signal light.
The excitation light power supplied to the EDF 10 from 1 is calculated. The calculation of the pumping light power is performed using a fixed pumping light power setting rule, for example, a linear relational expression expressed by the equation (4), whereby the constant gain control of the optical amplification in the EDF 10 is realized.

In the gain control described above, when there is no signal light switching of the multi-wavelength signal light, the gain K is made constant by setting the pumping light power while keeping the slope K and intercept A in equation (4) constant. Is realized. At this time, since the pumping light power is set only from the input power, high-speed feed-forward gain control is possible. On the other hand, in the case where the signal light wavelength distribution fluctuates due to switching of the signal light and the input power changes, the gain control using such a constant relational expression causes a change in the gain. I will.

Wavelength band: 1535.0 to 1559.0 nm
In the 16ch multi-wavelength signal light, K = 133.49 and A = 5.
The constant gain control is possible by setting the pumping light power using equation (5), which is 2516. Here, in the multi-wavelength signal light, the number of signal lights fluctuates from 16 ch to 4 ch, and 4 ch on the shorter wavelength side (wavelength 1535.0).
nm, 1536.6 nm, 1538.2 nm, and 15
(39.8 nm) is assumed to be residual signal light.
Assuming that the input power per signal light is −12 dBm / ch, the total input power at this time is from 0 dBm to −
It changes to 5.98 dBm.

FIG. 6 shows the gain variation when gain control is performed by the equation (5) with respect to the input power variation due to the signal light number variation (input power indicated as signal light number variation = −12 dBm). / Ch data points). The four points with no correction are the gain fluctuations generated for the signal light of each of the four wavelengths, and the magnitude of the gain fluctuation slightly varies depending on the signal light wavelength, but the maximum is about 0.46 dB (wavelength 1535.0n).
m signal light).

Further, in this state, the input power of the residual signal light of 4 ch is changed from -12 dBm / ch to -30 dBm / c.
FIG. 6 also shows the gain variation when gain control is performed by equation (5) while varying the input power to h (input power indicated as input power variation = −12 to −30d).
Bm / ch data point, corresponding to span loss variation). From these gain fluctuation data, in the gain control using the same equation (5) as before the occurrence of the signal light number fluctuation, the gain fluctuation occurs when the signal light wavelength distribution fluctuates, and the signal light number after the wavelength distribution fluctuation fluctuates. It can be seen that in the gain control in a constant state, the gain is not fixed.

On the other hand, in the linear relational expression used for gain control, the slope K = 119.66 and the intercept A = 5.22019
The result of performing gain control according to a new relational expression obtained by correcting the coefficient as follows: P _p [mW] = 119.66P _in [mW] +5.22019 (6) is shown in FIG. (Common to signal light of each wavelength). According to this relational expression, for any of the four wavelengths of signal light, the occurrence of gain fluctuation due to signal light wavelength distribution fluctuation is prevented, and
Also in gain control based on a change in input power thereafter, gain control in which the gain is accurately fixed is realized.
That is, when the signal light wavelength distribution, such as the signal light speed is switched signal light is varied, in equation (4) for the input power P _in of the excitation light power P _p and the signal light used for gain control, The slope K and the intercept A are the coefficients.
By correcting and changing the value of, the gain is stabilized by high-speed gain control based on the input power even for multi-wavelength signal light having a new signal light wavelength distribution after the number of signal lights has changed. Becomes possible.

In order to perform such gain control, the EDFA 1 according to the embodiment shown in FIG. 5 obtains information on the signal light wavelength distribution on the transmission line of the multi-wavelength signal light, which is to be amplified. The wavelength distribution detecting means 30 is provided. Further, the pumping light power calculation unit 21 of the gain control circuit 20 determines the input power information obtained by the photodetector 22 as the input power detection means and the signal light wavelength distribution information obtained by the wavelength distribution detection means 30 based on the information. To calculate the excitation light power.

That is, the pumping light power calculator 21 calculates the pumping light power so as to realize the feed-forward high-speed gain control based on the input power. A pump light power setting rule such as a relational expression for calculating the pump light power from the power is set or changed. As the information about the signal light wavelength distribution, specifically, information such as the number of signal lights, the arrangement of signal light wavelengths, or the bias of the wavelength distribution is used. In this way, usually, high-speed feed-forward gain control is performed based on the input power, and when the signal light wavelength distribution fluctuates, the relational expression is changed by correcting the coefficient of the relational expression for gain control, thereby switching the signal light. Is performed, the gain of the optical amplifier can be controlled so that the gain can be kept constant with sufficient accuracy for the multi-wavelength signal light.

It is preferable that the rules for setting the pumping light power, such as the relational expression corresponding to each signal light wavelength distribution, be obtained by measurement, simulation calculation, etc., and given to the gain control circuit 20 in advance.

EDFA having such a configuration and function
FIG. 7 shows an example of the time change of the gain in the gain control using the control signal No. 1. Here, FIG. 7A shows a change in the total input power P due to the switching of the signal light, and FIG. 7B shows a gain variation occurring in the residual signal light due to the switching of the signal light. In this example, at time T ₀ , the number of signal lights is reduced by the signal light switching, and the total input power becomes P
₀ is reduced to P ₁ (FIG. 7 (a)).

At this time, as shown in FIG. 7B, immediately after the decrease in the number of signal lights at the time T ₀ , the gain G of the residual signal light is set to the set gain G ₀ set to keep the gain constant. From Δ
To generate the overshoot of the G _1. On the other hand, the high-speed gain control based on the change in the input power detected by the photodetector 22 suppresses the amount of overshoot and returns to the steady gain after the time ΔT ₁ . However, at this stage, the gain control method corresponding to the fluctuation of the signal light wavelength distribution has not been changed, and the gain control by setting the pump light power using the relational expression corresponding to the signal light wavelength distribution before switching the signal light. Has been done. Therefore, the steady-state gain returned after the time ΔT ₁ is shifted from the set gain G ₀ by ΔG ₂ .

On the other hand, the wavelength distribution detecting means 30 detects a change in the number of signal lights due to the switching of the signal lights. The pumping light power calculator 21 corrects the coefficient of the relational expression between the input power and the pumping light power used for the gain control based on the new signal light wavelength distribution after the fluctuation detected by the wavelength distribution detecting means 30. Change the expression. As a result, the deviation Δ from the set gain remaining after the time ΔT ₂
G ₂ is further solved, the gain G of the residual signal beam is restored to set the gain G _0. Thereafter, until the next signal light switching, the gain stabilization in the high-speed gain control based on the input power is performed by the corrected relational expression.

As described above, in the optical amplifier having the configuration of the present embodiment, the gain is made constant by setting the pumping light power based on the input power and the correction of the pumping light power setting method based on the signal light wavelength distribution. Perform gain control. As a result, the response to the gain change is fast, and even when the signal light wavelength distribution changes due to the switching of the signal light, the occurrence of the gain change is prevented or suppressed, and the constant gain with high accuracy is realized. An optical amplifier is obtained.

FIG. 8 is a configuration diagram showing a second embodiment of the EDFA. In the present embodiment, a signal light number detection system 31 which is a signal light number detection means is provided as a wavelength distribution detection means. This signal light number detection system 3
1 includes an arrayed waveguide grating type optical demultiplexer 311, a plurality of photodetectors 312, and a signal light number detection circuit 313.

In this embodiment, the optical isolator 1
A part of the output signal light after optical amplification that has passed through 4 is split by the optical coupler 16, and the split multi-wavelength signal light is split by the optical splitter 311 into signal light of each wavelength for each channel. , Are detected by the corresponding photodetectors 312.

The detection result of each signal light component by the plurality of light detectors 312 is based on a signal light number detection circuit 3 including a counting circuit and the like.
The signal light number detection circuit 313 detects the number of signal lights of the multi-wavelength signal light being transmitted and the signal light wavelength arrangement within the signal wavelength band. The information is output to the gain control circuit 20 as information on the signal light wavelength distribution, and based on the information, the input power and the pump light used for the gain control in the pump light power calculation unit 21 of the gain control circuit 20 are calculated. A power relational expression is set or changed.

In the embodiment shown in FIG. 8, the number of photodetectors 312 corresponding to the number of channels of the signal light included in the multi-wavelength signal light is used. And the signal light wavelength distribution is detected. On the other hand, even when the signal light wavelength distribution fluctuates, depending on the fluctuation condition and the condition of gain stabilization, the generated gain fluctuation may be small and need not be corrected. In such a case, instead of always changing the gain control method for signal light switching, correction of a relational expression or the like is performed only when it is determined that the signal light wavelength distribution is necessary. Is also good.

Here, as an example of the switching condition that does not require the above-mentioned correction, a wavelength band of 1535.0 to 1559.0 is used.
In the case of 16ch multi-wavelength signal light of nm, the input power per signal light is set to −12 dBm / ch, and
Consider a change in the number of signal lights to the channel. However, in the gain control, the equation (5) for making the gain constant with respect to a uniform change in input power in 16 ch is used. The signal light switching is performed symmetrically from the center wavelength side of the 16ch signal light so as not to cause a bias in the signal light wavelength distribution.
channels are sequentially excluded, and finally 2ch on the short wavelength side
(Wavelengths 1535.0 nm and 1536.6 nm) and 2ch on the long wavelength side (wavelengths 1557.4 nm and 155).
(9.0 nm) becomes a residual signal light.

FIG. 9 is a graph showing gain fluctuations in the four-wavelength residual signal light generated at this time. The range of 0.04 to -5.98 dBm in the total input power change shown on the horizontal axis represents the above-described gain change due to the change in the number of signal lights from 16 ch to 4 ch. Further, in this graph, the gain fluctuation after switching the signal light also shows a gain fluctuation caused by a uniform input power fluctuation due to a span loss fluctuation or the like in a range of −5.98 to −23.98 dBm of the entire input power. ing. In any case, no correction was made to the relational expression for gain control.

The gain variation occurring under the above conditions slightly varies at each signal light wavelength as shown in FIG. 9, but the total input power variation (0.04 to 0.04) due to the signal light wavelength distribution variation.
−5.98 dBm) are all sufficiently small. Further, the total input power change due to the span loss fluctuation after the signal light wavelength distribution fluctuation (−5.98 to −23.98)
(dBm), the gain fluctuation is about 0.14 dB at maximum with respect to the input power change of 24 dB, and the gain can be relatively accurately fixed without correcting the relational expression. Understand.

That is, if the signal light is switched and the number of signal lights fluctuates, the wavelength arrangement of the signal light is deviated, or if the state of the deviation changes, it is necessary to change the gain control method. . On the other hand, if the signal light wavelength distribution fluctuates symmetrically with respect to the wavelength distribution and the distribution bias does not change significantly, the gain control change is not necessarily performed even though the number of signal lights fluctuates. Not required.

FIG. 10 is a block diagram showing a third embodiment of the EDFA. The present embodiment is configured to be slightly more simplified than the EDFA shown in FIG. 8 in consideration of the above-mentioned fact regarding the occurrence of gain fluctuation. A detection system 32 is provided. This distribution bias detection system 32
Is configured to include an optical coupler 321, an optical filter 322, two photodetectors 323 and 324, and a distribution bias detection circuit 325.

In this embodiment, the optical isolator 1
A part of the output signal light after optical amplification that has passed through 4 is split by the optical coupler 16, and the split multi-wavelength signal light is further split into two signal light components by the optical coupler 321. Of these two signal light components, one is detected by the photodetector 323 as it is, and the other component that has passed through the optical filter 322 is detected by the photodetector 324.

The detection of the signal light and the detection of the distribution bias by the above configuration will be described with reference to FIG. In FIG. 11, the multi-wavelength signal light is assumed to be 8ch.
The detection of the signal light wavelength distribution is schematically illustrated.

In the photodetector 323, FIG.
As shown in (1), all components of the output signal light are input as they are, and all the signal lights included in the multi-wavelength signal light are detected. On the other hand, a filter having a large loss with respect to a signal light component having a wavelength shorter than the center wavelength of the multi-wavelength signal light is used as the optical filter 322. For example, in FIG. 11B, an optical filter 322 having a large loss on the shorter wavelength side with respect to the wavelength between the fourth and fifth channels for the multi-wavelength signal light of 8 channels. . At this time, as shown in FIG. 11C, only the component of the output signal light having a wavelength longer than the center wavelength of the multi-wavelength signal light is input to the photodetector 324, and the length of the multi-wavelength signal light is reduced. Half of the signal light on the wavelength side is detected.

The detection results of the respective signal light components by the photodetectors 323 and 324 are given to the distribution deviation detecting circuit 325, and the distribution deviation detecting circuit 325
The distribution bias and its change are detected by comparing the signal light power detected at 23 and 324 with its change. The information is output to the gain control circuit 20 as information on the signal light wavelength distribution.

Pump light power calculator 2 of gain control circuit 20
1 changes the relational expression used for gain control, if necessary, based on the information on the distribution bias. That is, even if the number of signal lights fluctuates, if the distribution deviation does not occur as in the example shown in FIG. 9 or if the distribution deviation that has already occurred is not changed to a certain degree or more, the pumping light power calculation unit Numeral 21 does not correct the relational expression used for calculating the excitation light power. On the other hand, when the distribution bias occurs or the already-distributed distribution bias changes to a certain degree or more, the pumping light power calculator 21 corrects the relational expression to change the gain control method.

At this time, there is a case where a certain amount of gain fluctuation occurs with the fluctuation of the signal light wavelength distribution, but the fluctuation amount is sufficiently small as described above with reference to FIG. Therefore, even with such a configuration, it is possible to obtain an optical amplifier in which the occurrence of gain fluctuation is suppressed to be within a predetermined range, and the gain is stabilized with sufficient accuracy.

When the distribution band is detected from the intensity balance of the signal light power in each wavelength region by dividing the wavelength band like the EDFA of FIG. 10, the division number of the wavelength band is shown in FIG. Number 2 in the embodiment
Not limited to For example, according to conditions such as the total number of signal lights included in the multi-wavelength signal light and the amount of gain variation allowed for the optical amplifier, the signal light is divided into three or more wavelength regions and the signal light power in each region is compared. The distribution bias may be obtained. In this case, the distribution bias detection system 32 can be configured using a plurality of optical filters having different loss wavelength characteristics and photodetectors corresponding to the number of divided wavelength regions.

Even when the signal light number detection system 31 having the configuration shown in FIG. 8 is provided, the pump light power calculator 21 determines whether or not to correct the relational expression every time the signal light wavelength distribution changes. By making a determination and performing correction only when needed, it is possible to realize an operation equivalent to the EDFA shown in FIG. The signal light number detection system and the distribution bias detection system shown in FIGS. 8 and 10 are all installed for the output signal light in the above-described embodiment. May be performed on the input signal light.

FIG. 12 is a configuration diagram showing a fourth embodiment of the EDFA. In the present embodiment, a monitoring light detection system 33 is provided as a wavelength distribution detection unit.

In this embodiment, information about the signal light wavelength distribution is obtained from the monitoring light (SV light) added to the multi-wavelength signal light. The monitoring light added to the multi-wavelength signal light input to the EDFA 1 is branched from the signal light by the WDM coupler 17. And the monitoring light detection system 3
In 3, information on the signal light wavelength distribution of the input multi-wavelength signal light is obtained from the monitoring light, and output to the gain control circuit 20. Thereafter, the monitoring light is again combined with the multi-wavelength signal light after the optical amplification via the WDM coupler 18 and output from the EDFA 1. Excitation light power calculator 21
Is the same as in the other embodiments.

The optical amplifier according to the present invention is not limited to the above embodiments, and various modifications and changes in the configuration are possible. For example, as the optical amplifier, an optical amplifier using an optical fiber to which a rare earth element such as Pr or Nd is added in addition to Er can be used. Further, other gain control methods such as a method of obtaining a signal light input / output ratio based on the input power and the output power to control the gain and a method of using the pump light input / output ratio may be used. It is also possible to use a gain adjustment method other than the gain control based on the pump light power.

FIG. 13 is a configuration diagram showing a fifth embodiment of the EDFA. In the present embodiment, in addition to gain control based on input power and signal light wavelength distribution, gain adjustment is performed by a variable gain equalizer 40 installed on a transmission path behind the optical isolator 14.

In this embodiment, the optical isolator 1
4 and a part of the output signal light after optical amplification that has passed through the variable gain equalizer 40 is branched by an optical coupler 41 and analyzed by a spectrum analyzer 42. Based on the analysis result, the gain monitoring method is determined by the gain monitoring system 43 based on the deviation from the gain setting value, and the gain is adjusted by the variable gain equalizer 40 via the control circuit 44.

FIG. 14 is a configuration diagram showing a sixth embodiment of the EDFA. In the present embodiment, in addition to the gain control based on the input power and the signal light wavelength distribution, the gain is adjusted by the loss slope variable device 50 installed on the transmission path behind the optical isolator 14. The loss slope variable device 50 can change the slope of the loss change with respect to the wavelength as shown in FIG. 15, and decreases the gain of the EDFA 1 by increasing the loss, and conversely, reduces the loss. Gain can be increased.
Therefore, it is possible to adjust the gain balance between the short wavelength side and the long wavelength side in the multi-wavelength signal light.

In this embodiment, the optical isolator 1
4 and a part of the output signal light after optical amplification that has passed through the loss slope variable device 50 is branched by the optical coupler 51, and further branched into two signal light components by the optical coupler 52. The two split signal light components are input to optical filters 53 and 54, respectively. The optical filter 53 transmits the short wavelength side of the wavelength band of the multi-wavelength signal light, and the signal light on the short wavelength side transmitted through the optical filter 53 is detected by the photodetector 55. The optical filter 54
Is to transmit the longer wavelength side of the wavelength band,
The signal light on the long wavelength side transmitted through the optical filter 54 is detected by the photodetector 56. Then, the photodetectors 55 and 5
The signal light power detected at 6 is compared in the comparison control circuit 57 to determine the loss slope to be set, and the loss slope variable unit 50 performs gain adjustment.

The EDFA shown in FIGS. 13 and 14 is a modification of the optical amplifier according to the present invention, but various other configuration changes and applications are possible. Further, the optical amplifiers shown in the respective embodiments all have a constant gain. However, a configuration in which the gain is further variable is also possible. That is, by preparing a pump light power setting rule such as a relational expression for each signal light wavelength distribution for a plurality of gains, the set gain can be changed.

As for the relationship between the input power and the pumping light power (setting gain 18.18 dB) for stabilizing the gain shown in FIG. 2, the setting gain is further increased from 18.18 to 14.18 dB.
FIG. 16 shows the relationship between the input power and the pumping light power obtained for each set gain while changing within the range shown in FIG.
However, the signal light is assumed to be 1ch with a wavelength of 1550 nm,
The input power is changed in the range of -18 to 0 dBm. By preparing the relational expressions corresponding to a plurality of set gains, specifically, the values of the slope K and the intercept A, the gain can be stabilized and the set gain can be changed. Variable control becomes possible.

[0077]

As described above in detail, the optical amplifier according to the present invention has the following effects. That is, in the optical amplifier performing the gain constant control based on the input power, the pumping light power calculator of the gain control circuit calculates the signal light wavelength distribution in addition to the input power of the multi-wavelength signal light to be amplified. The pump light power is calculated in consideration of the above. At this time, if the signal light wavelength distribution is constant without switching the signal light, the feedforward high-speed gain constant control based on only the input power is performed. On the other hand, when the signal light is switched and the signal light wavelength distribution fluctuates, the gain constant control corresponding to the fluctuation of the signal light wavelength distribution is performed such as changing the gain control method.

As a result, the gain is normally stabilized at a high speed based on the input power, and the gain is also kept constant even when the signal light is switched and the signal light wavelength distribution of the multi-wavelength signal light is changed. Is realized with high precision, and an optical amplifier that achieves both high-speed and high-precision gain control is realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of an EDFA.

FIG. 2 is a graph showing a relationship between input power and pump light power for making the gain constant.

FIG. 3 is a graph showing a change in gain that occurs in residual signal light when signal light of 16 ch is sequentially excluded from a shorter wavelength side.

FIG. 4 is a graph showing a change in gain that occurs in residual signal light when signal light of 16 ch is sequentially excluded from a longer wavelength side.

FIG. 5 is a configuration diagram showing a first embodiment of an EDFA that is an optical amplifier according to the present invention.

FIG. 6 is a graph illustrating a gain change caused by the EDFA shown in FIG. 5;

FIG. 7 is a graph showing a time change of a gain in the EDFA shown in FIG. 5;

FIG. 8 is a configuration diagram illustrating a second embodiment of the EDFA.

FIG. 9 is a graph showing a gain variation with respect to a symmetrical signal light wavelength distribution variation.

FIG. 10 is a configuration diagram illustrating a third embodiment of the EDFA.

FIG. 11 is a schematic diagram illustrating detection of deviation of signal light wavelength distribution in the EDFA illustrated in FIG. 10;

FIG. 12 is a configuration diagram showing a fourth embodiment of the EDFA.

FIG. 13 is a configuration diagram showing a fifth embodiment of the EDFA.

FIG. 14 is a configuration diagram showing a sixth embodiment of the EDFA.

FIG. 15 is a graph showing characteristics of a loss slope variable device used in the EDFA shown in FIG.

FIG. 16 is a graph showing variable gain control in the EDFA.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... EDFA, 10 ... EDF, 11 ... Excitation light source, 12 ...
WDM couplers, 13, 14 ... optical isolators, 15, 1
DESCRIPTION OF SYMBOLS 6 ... Optical coupler, 17, 18 ... WDM coupler, 20 ... Gain control circuit, 21 ... Excitation light power calculation part, 22 ... Photodetector, 30 ... Wavelength distribution detection means, 31 ... Signal light number detection system, 311 ... Array Waveguide grating type optical demultiplexer, 31
2, photodetector, 313, signal light number detection circuit, 32, distribution deviation detection system, 321, optical coupler, 322, optical filter, 323, 324, photodetector, 325, distribution deviation detection circuit, 33, monitoring light Detection system, 40: variable gain equalizer, 41: optical coupler, 42: spectrum analyzer,
43: gain monitoring system, 44: control circuit, 50: loss inclination variable device, 51, 52: optical coupler, 53, 54: optical filter, 55, 56: photodetector, 57: comparison control circuit.

Claims (7)

[Claims] 1. An optical amplifier for collectively amplifying multi-wavelength signal light composed of a plurality of signal lights having different wavelengths within a predetermined wavelength band, wherein the input multi-wavelength signal light is optically amplified by a pump light. An optical amplifying unit that outputs the pumping light to the optical amplifying unit; an excitation unit that supplies the excitation light to the optical amplifying unit; an input power detection unit that detects an input power of the multi-wavelength signal light input to the optical amplifying unit; Wavelength distribution detecting means for detecting a signal light wavelength distribution within the predetermined wavelength band for the plurality of signal lights included in the multi-wavelength signal light; and the pumping means based on the input power and the signal light wavelength distribution. A pump light power calculating means for calculating a pump light power to be set for the optical amplifier, and controlling the gain of the optical amplification in the optical amplifier by controlling the pump means in accordance with the calculated pump light power. An optical amplifier, characterized in that it comprises a gain control means. 2. The pumping light power calculating means calculates the pumping light power based on the input power using a relational expression between the input power and the pumping light power that makes the gain of the optical amplification constant. 2. The optical amplifier according to claim 1, wherein when the signal light wavelength distribution is changed, the relational expression is changed based on the changed signal light wavelength distribution. 3. An optical amplifier according to claim 1, wherein said wavelength distribution detecting means is a signal light number detecting means for detecting the number of signal lights included in said multi-wavelength signal light and the signal light wavelength arrangement. . 4. The optical amplifier according to claim 1, wherein said wavelength distribution detecting means is a distribution deviation detecting means for detecting deviation of said signal light wavelength distribution. 5. The monitoring light detecting means according to claim 1, wherein said wavelength distribution detecting means is monitoring light detecting means for obtaining information on said signal light wavelength distribution from monitoring light added to said multi-wavelength signal light. Optical amplifier. 6. The variable gain equalizer according to claim 1, further comprising a variable gain equalizer for equalizing a wavelength dependency of a gain of the optical amplification with respect to the multi-wavelength signal light output from the optical amplifier. Optical amplifier. 7. The multi-wavelength signal output from the optical amplifying unit, wherein the loss is substantially constant at a predetermined wavelength within the predetermined wavelength band, and the slope of the loss with respect to the wavelength is variable in the predetermined wavelength band. 2. The optical amplifier according to claim 1, further comprising a loss slope variable device for giving a predetermined slope to the wavelength dependence of the gain of the optical amplification with respect to light.