JP2001144353A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier having a structure, where deterioration of transmission quality is prevented and high speed and high accuracy control for obtaining a constant gain is enabled. SOLUTION: In a representative example of this optical amplifier, an amplified light beam from an EDF 10 is once introduced into an optical filter 22 via an optical circulator 21. The optical filter 22 makes a noise beam in the introduced amplified beam pass and divides signal components from the noise beam, by reflecting the signal components of the respective wavelengths. The noise beam, which has passed through the optical filter 22, is detected with an optical detector 23, and gain control is performed with a gain control circuit 20 so that power fluctuation of the detected noise beam is restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier for collectively amplifying a signal level within a predetermined wavelength band.

[0002]

2. Description of the Related Art Research and development on large-capacity, high-speed optical communication and long-distance optical communication using an optical fiber transmission line network have been actively conducted in response to social needs due to the advent of the advanced information society. Here, wavelength division multiplexing (WDM: Wavelength Divis)
An ion multiplexing transmission system is a transmission system that performs high-speed and large-capacity optical communication by transmitting a WDM signal (a plurality of optical signals having different wavelengths) through an optical fiber line. It is being developed and introduced to meet the needs.

In such a WDM transmission system, in order to compensate for a transmission loss of a WDM signal caused by long-distance transmission,
In general, optical amplifiers are installed at predetermined intervals as relay stations. The optical amplifier includes an optical amplifier that amplifies an input optical signal, such as an optical fiber doped with a fluorescent substance that can be excited by the excitation light, and an excitation light source that supplies the optical amplifier with the excitation light. For example, Er (erbium) -doped fiber amplifier (EDFA: Erbium-Doped F)
iber Amplifier) is known.

[0004] Further, in a WDM transmission system, an OXC (Optical Cross Connect), an OXC (Optical Cross Connect) capable of switching a transmission path and the number of signals, etc., for efficient operation and improvement of reliability.
ADM (Optical Add / Drop Multiplexer) is often applied. In particular, in recent years, with regard to such OXC, OADM, and the like, research on switching large-capacity signals and increasing the switching speed with the increase in transmission capacity has been advanced.

When an optical amplifier is applied to a transmission system provided with an optical device having a signal switching function such as the OXC and OADM, the following problems may occur. That is, in the optical amplifier, it is important to amplify each of the optical signals of a plurality of wavelengths as a WDM signal with a constant gain (amplification factor). On the other hand, when a part of the optical signal is switched at high speed and the number of optical signals (the number of channels) inputted to the optical amplifier changes sharply, each light propagating in the optical fiber line after the change is changed. Transient gain fluctuation occurs in the optical amplifier due to interference between signals (inter-channel interference). For such a problem,
Attempts have been made to suppress gain fluctuations in optical amplifiers by using automatic gain control (AGC) technology to speed up control for maintaining amplification gain in optical amplifiers constant.

For example, regarding gain control in an optical amplifier, an ASE (Amplified Spontaneous Emis
sion: spontaneous emission light), a gain control technique using a part of noise light output due to spontaneous emission light is disclosed in Japanese Patent No. 2778438 (Reference 1) and H. Yoon et al., IEEE Photon. Techno.
l. Lett., vol.11, no.3, pp.316-318, 1999 (Reference 2)
It is described in.

[0007]

As a result of studying the above-mentioned prior art, the inventors have found the following problems. That is, in the optical amplifiers shown in the above-mentioned documents 1 and 2, automatic output control (APC: Automatic Power Control) of noise light included in the amplified light output from the optical amplifier is performed.
There is a problem that the power ratio of the components used in l) cannot be sufficiently secured due to the structure.

For example, the optical amplifier shown in Document 1 uses a part of noise light separated by an optical coupler as monitoring light, and the optical amplifier shown in Document 2 uses an arrayed waveguide grating type light. Demultiplexer (AWG: Arrayed Waveguide Gr)
A part of the noise light separated by ating) is used as monitoring light, and none of the optical amplifiers can sufficiently increase the detected noise light power.

Conversely, this means that the amplified light output to the outside of the optical amplifier (fiber transmission line) contains a considerable amount of noise light not used as monitoring light. As described above, since the conventional optical amplifier does not have a structure for sufficiently removing noise light, there is a problem that the noise characteristics of signal light are deteriorated, and as a result, sufficient transmission quality cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and is an optical amplifier that performs automatic gain control by automatic output control using noise light. By effectively increasing the noise light power, it is possible to effectively prevent the transmission quality from deteriorating,
It is an object of the present invention to provide an optical amplifier having a structure for realizing a control for maintaining the amplification gain for each optical signal constant (gain constant control) with higher speed and higher accuracy.

[0011]

In order to achieve the above object,
An optical amplifier according to the present invention includes at least an optical amplification unit such as an optical fiber doped with a rare earth element or the like, an excitation light source, an optical circulator, an optical filter, a light detection device, and a gain control system. After the noise light separated from the amplified light output from the optical waveguide is guided to a branch line different from the main transmission line through which the amplified optical signal propagates, the noise light is used for gain constant control of the optical amplifier. Features.

Specifically, the optical amplifying unit is an optical component that collectively amplifies an optical signal input together with the pump light,
For example, it includes an amplification optical fiber to which a rare earth element such as Er is added. The excitation light source is an LD (Laser Diode)
And the like, and supplies excitation light to the optical amplification unit. The optical circulator is an optical component for guiding noise light of the amplified light output from the optical amplifier to the branch line, and is a first component for inputting amplified light including an amplified optical signal and noise light. A port, a second port for outputting amplified light from the first port to the branch line, and an optical signal of the amplified light input again via the second port outside the optical amplifier (main transmission line). Third for output to
And a port. The optical filter is an optical component provided on a branch line connected to a second port of the optical circulator, the optical filter allowing noise light of the amplified light output from the second port to pass therethrough, and The light signal is reflected toward the second port. The light detection device is a PD that detects noise light that has passed through an optical filter.
(Photo Diode) and other optical components. The gain control system controls the amplification gain in the optical amplifier by controlling the pump light source so as to suppress the power fluctuation of the noise light detected by the light detection device.

In the optical amplifier according to the present invention, automatic output control is performed by using most of the ASE noise light separated from the amplified light, so that more efficient automatic gain control of optical amplification becomes possible. Since most of the noise light is used by the above-described configuration, the automatic gain control method, specifically, the circuit configuration such as the gain control circuit included in the gain control system can be simplified, and the gain can be reduced. Faster automatic gain control to stabilize (If the power of detected noise light or the like is small, the load resistance of the PD must be increased. Therefore, it is important to increase the detection level of noise light used as monitoring light in order to realize high-speed automatic gain control, since the tracking becomes impossible.

Further, in the optical amplifier according to the present invention, part of the noise light is not separated from a main transmission line such as an optical fiber transmission line network through which the amplified optical signal is to be propagated, but is temporarily amplified by an optical circulator. After the light is guided to a branch line different from the main transmission line, a signal component and a noise component are separated using an optical filter. The separated signal components are reflected by the optical filter and propagate through the main transmission line again, but all the noise components pass through the optical filter and reach the light detection device. With such a configuration,
Most of the noise light generated in the optical amplifier can be used for automatic output control. In this case, the detected noise light power increases, so that the gain change of the optical amplification unit can be measured at higher speed and with higher accuracy. Therefore, compared to the conventional optical amplifier that uses a part of the noise light by branching, the optical amplifier according to the present invention is:
Enables faster and more accurate automatic gain control.

In the above optical filter, the reflectance is set for each wavelength of the amplified optical signal so as to equalize the wavelength dependence of the amplification gain in the optical amplifier (to flatten the signal level at each signal wavelength). It is characterized by having been done. By setting the reflectance in the optical filter in this way, both automatic gain control and gain equalization are realized.

The optical amplifier according to the present invention further comprises an output-side variable optical attenuator between the third port of the optical circulator and the output terminal of the optical amplifier for attenuating the optical signal output from the third port. May be provided. Alternatively, the optical amplifier may further include an input-side variable optical attenuator for attenuating an optical signal to be input to the optical amplifier, between the input terminal and the optical amplifier. By installing a variable optical attenuator on the output side or input side of the optical amplifier in this way,
Automatic control of the output level of the optical signal and the like are further enabled.

Further, the optical amplifier according to the present invention may include a structure for changing the reflectance in the optical filter for each wavelength of the amplified optical signal. With this configuration, it is possible to perform gain equalization control or the like that follows a change in the operation state of the optical amplifier, and the accuracy of automatic gain control is further improved. In this case, the optical amplifier further includes an output control system that controls the output level of the output optical signal by controlling the reflectance of the optical filter. The provision of such an output control system enables various gain control methods in which output control by reflectance control for an optical filter and output control by a variable optical attenuator are combined.

The optical filter preferably includes a fiber grating that transmits noise light of the amplified light and reflects the amplified optical signal. However, other optical filters than the fiber grating can be used as long as they have a similar reflection function. Further, the output control system has a structure for changing the reflectance of the optical filter by applying distortion due to temperature change or deformation to a predetermined portion of the optical filter.

The optical amplifier according to the present invention may include an optical element for dividing a band component in a predetermined wavelength band into two at predetermined frequency intervals, instead of the optical circulator and the optical filter provided on the branch line. . The optical element includes a first port for inputting the amplified light from the optical amplifying unit including the amplified optical signal and the noise light, and the amplification port separated from the amplified light input through the first port. A second port for outputting the converted optical signal, and a third port for outputting noise light separated from the amplified light input through the first port. Even with such a configuration, high-speed and high-precision gain control is possible. In particular, it is preferable that such an optical element divides the band component in the predetermined wavelength band into two at a frequency interval substantially equal to a half of the frequency interval of the optical signal input to the optical amplifier.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the optical amplifier according to the present invention will be described below with reference to FIGS. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 shows an EDFA (Erbium-Doped Fiber Amplifie) which is a first embodiment of the optical amplifier according to the present invention.
FIG. 2 is a diagram illustrating a configuration of (r: Er-doped fiber amplifier).
In FIG. 1, the EDFA 1 has an input terminal 1a for receiving an optical signal propagating through a main transmission line such as an optical fiber transmission network and an output terminal 1b for transmitting amplified light to the main transmission line. An EDF (Erbium-Doped Optical Fiber: EDF) is provided between the input terminal 1a and the output terminal 1b.
It has a structure in which an Er-doped optical fiber 10, an excitation light source 11, a WDM coupler 12, and two optical isolators 13 and 14 are arranged. E, which is an optical amplification unit of the EDFA1,
The DF 10 is a silica-based optical fiber to which an Er element is added, and functions to amplify a signal component included in a predetermined amplification wavelength band while pumping light having a predetermined wavelength is supplied.

The optical isolators 13 and 14 are respectively
It is an optical component that allows input light to pass in the forward direction but not in the reverse direction. That is, the optical isolator 1
3 allows the light arriving from the input terminal 1a to pass through to the EDF 10, but does not transmit the light once arriving to the input terminal 1a. Further, the optical isolator 14 allows the light arriving from the EDF 10 to pass to the output end 1b side, but does not transmit the light once arriving to the EDF 10 side.

The excitation light source 11 is connected to an optical fiber line including the EDF 10 via a WDM coupler 12,
The excitation light is supplied from the excitation light source 11 to the EDF 10. Here, the WDM coupler 12 allows the pumping light output from the pumping light source 11 to pass toward the EDF 10 and also converts the optical signal arriving from the optical isolator 13 into an EDF.
It functions to pass it toward 10. As described above, the EDFA 1 according to the first embodiment is an optical amplifier having a configuration that enables forward pumping (forward pumping).
The excitation light source 11 for exciting the EDF 10 includes:
For example, a semiconductor laser having a wavelength of 1.48 μm can be used.

In the above configuration, when optical signals are inputted via the optical isolator 13 to the EDF 10 to which the excitation light of the predetermined wavelength is supplied from the excitation light source 11, these optical signals are amplified in the EDF 10. Then, the amplified light including the amplified optical signal is transmitted from the EDF 10 to the optical isolator 1.
4 is output. At this time, the amplified light output from the EDF 10 is included in the ASE together with the amplified optical signal.
(Amplified Spontaneous Emission: spontaneous emission light).

In the EDFA 1 of the first embodiment, automatic gain control (AGC) of optical amplification using ASE noise light included in the amplified light is performed.
To realize automatic gain control, the EDFA 1 includes an optical circulator 21, an optical filter 22, a photodetector 23, and a gain control circuit 20 included in a gain control system.

The optical circulator 21 has a first port 211 for inputting the amplified light output from the EDF 10.
(Hereinafter, referred to as an input port), a second port 212 (hereinafter, referred to as a control port) for taking in noise light as monitoring light used in the above-described automatic gain control, and
The output terminal 1 outputs the amplified optical signal of the amplified light as the output light.
This is a three-port optical circulator having three input / output ports of a third port 213 (hereinafter, referred to as an output port) for outputting to the outside via the port b.

Here, a general optical circulator is
It is an optical component having three or more ports and transmitting light traveling in the forward direction with low loss. The three-port optical circulator 21 applied to the EDFA 1 sets the forward direction in the direction indicated by the arrow in FIG. 1, outputs the light input through the input port 211 from the control port 212, and controls the light. The light input through the output port 212 is output to the output port 213.
It is configured to output from.

Input port 21 of optical circulator 21
The amplified light from the EDF 10 input through the control port 21 is output through the control port 212 and is controlled by the control port 21.
2 is input to the optical filter 22 on the branch line optically connected to the optical fiber 2.

The optical filter 22 is an optical component that allows noise light of the amplified light to pass therethrough and reflects the amplified optical signal of the amplified light, and is built in an optical fiber that is a branch line. n fiber gratings 22
_{1 to} 22 _n . Fiber gratings 22 _{1 to} 22 _n
The number n corresponds to the total number of channels of the WDM signal input to the EDFA 1, that is, the total number of WDM signals.

Here, assuming that the wavelengths of the n-channel optical signals as WDM signals are λ ₁ , λ ₂ ,..., Λ _n , _n fiber gratings 22 ₁ , 22 ₂ ,. , 22 _n reflect light of wavelengths λ ₁ , λ ₂ ,..., Λ _n with high reflectivity, respectively. That is, the optical filter 22 including the _n fiber gratings 22 _{1 to} 22 _n reflects all signal components of the WDM signal.
The optical filter 22 of the amplified light output from the EDF 10
Are reflected by the optical circulator 21 again via the control port 212 and output as the output light via the output port 213 and the output terminal 1b.
The signal is output to the outside of A1 (an optical fiber transmission line installed between relay stations).

A part of the amplified light other than the signal component that has passed through each of the fiber gratings 22 _{1 to} 22 _n included in the optical filter 22 without being reflected contains only the ASE noise component generated by the EDF 10. This noise light is detected by a photodetector 23 (photodetection device) such as a PD. The photodetector 23 is electrically connected to the gain control circuit 20, and the result of detection of the noise light level by the photodetector 23 is given to the gain control circuit 20.

If the amplification gain in the EDF 10 is constant, the power of the ASE noise light from the EDF 10 hardly changes even if the signal level (input level) of the optical signal changes. With respect to the noise light having such a property, the gain control circuit 20 (included in the gain control system) controls the excitation light source so that the power of the noise light detected by the photodetector 23 becomes substantially constant. The power of the pumping light supplied from 11 to the EDF 10 is controlled. Thereby, this EDFA1
Is realized.

The control actually performed in the gain control circuit 20 is automatic power control (APC) for only the power of the noise light, but the above-described control of the power of the noise light and the amplification gain is performed. From such a relationship, the effect of automatic gain control can be obtained. Therefore, the automatic gain control method, specifically, the gain control circuit 20
Can be simplified, and the speed of automatic gain control for maintaining the amplification gain constant can be increased.
Also, since the photodetector 23 only needs to measure the power of the entire noise light, it can easily cope with an optical amplifier having a wide dynamic range. Further, in the EDFA 1 of the first embodiment, the speed of the automatic gain control is increased by increasing the power ratio of the noise light used for the automatic output control.

In the conventional gain control using the ASE noise light, only a part of the noise light branched from the main transmission line is detected as monitoring light using an optical coupler or an arrayed waveguide grating type optical demultiplexer. (See References 1 and 2). In this case, of the noise light included in the amplified light output from the optical amplifier, the power ratio of the noise light used for gain control cannot be sufficiently ensured.

On the other hand, the EDFA 1 of the first embodiment
Then, the branching of the noise light used for the automatic output control is not performed on the main transmission line through which the optical signal propagates, and the amplified light from the EDF 10 is transmitted to the control port 2 using the optical circulator 21.
The signal is once output to a branch line optically connected to the signal line 12. The optical filter 22 provided on the branch line reflects only the light (WDM signal) in the signal wavelength band toward the optical circulator 21, while the photodetector 23 removes all the noise light passing through the optical filter 22. By detecting, EDF10 by automatic output control using noise light
Is performed automatically.

In this case, since most of the forward ASE noise light output from the EDF 10 can be used for gain control, the gain can be stabilized at high speed and with high accuracy by automatic gain control.

FIG. 2 is a graph showing the input level dependency of the gain obtained by the EDFA 1 shown in FIG. The graph in FIG. 2 shows that the signal wavelength λ is 1535.
78 to 1558.31 nm, and a wavelength interval Δλ of about 3.2 n
The gain (vertical axis) of each optical signal of 8 channels (corresponding to the number of optical signals) where m is the input level P _in (horizontal axis) of the optical signal is from −35 dBm / ch to −15 per channel.
It is a result measured while changing to dBm / ch.
The graph G210 shows the gain variation of the optical signal having a wavelength of 1535.78 nm, and the graph G220 shows the gain of 1539.12n.
The gain variation of the light of m, graph G230 shows a wavelength of 1542.2.
Gain variation of 6 nm light, graph G240 shows wavelength 154
5. Gain fluctuation of light at 19 nm, graph G250 shows wavelength 1
Gain variation of light at 548.46 nm, graph G260 is gain variation of light at a wavelength of 1551.85 nm, graph G270.
Is a graph showing a gain variation of light having a wavelength of 1555.07 nm, and a graph G280 is a graph showing gain variation of light having a wavelength of 1558.31 nm.

The maximum gain variation caused by the input level change of 20 dB is about 0.6 dB, which indicates that the constant gain control with sufficient accuracy is realized.

FIG. 3 is a graph showing a transient gain variation when the number of signals in the EDFA 1 shown in FIG. 1 changes. In FIG. 3, the signal level is set to −20 dBm / ch per channel, and the transient gain fluctuation Δ of the residual optical signal when the number of signals decreases from 16 ch to 1 ch.
6 is a graph showing G. Wavelength λ
Are used, two optical signals having 1548.46 nm and 1558.31 nm. The graph in FIG.
Is set to the power corresponding to 15 ch, and the wavelength λ is set to 155.46 nm.
A graph simulating a gain variation caused by reducing the number of signals from 16 ch to 1 ch by setting the power of the light of 8.31 nm to the power for one channel and turning on / off the light of the wavelength of 1548.46 nm. The switching of the optical signal that causes the gain fluctuation is performed by the arrow A in the figure.
Was performed at the time indicated by.

In FIG. 3, a dashed line in a graph G310 indicates a transient gain fluctuation caused when the gain control is not performed at the signal switching time A, and a solid line in the graph G320 indicates a gain control at the signal switching time A (shown in FIG. 1). EDFA1 performed by the EDFA 1). At this time, the transient gain fluctuation when the gain control is not performed (graph G310) greatly increases to 6.3 dB, whereas the transient gain fluctuation when the gain control by the automatic output control of the noise light is performed (graph G310). G
320) fluctuates only by about 0.4 dB, and the transient gain fluctuation is greatly suppressed.

As described above, the ED of the first embodiment
The FA1 is capable of achieving a constant gain with high accuracy by automatic output control of noise light, and is capable of significantly suppressing transient gain fluctuations caused by high-speed switching of the number of signals by increasing the speed of automatic gain control. It turns out that it is.

In the EDFA 1 according to the first embodiment shown in FIG. 1, gain control is performed by using an optical circulator 21 and an optical filter 22. The optical circulator 21 and the optical filter 22 serve as gain equalizers. Can also work. Here, regarding the function of the gain equalizer,
This will be described with reference to FIGS. In the following description, the optical signal is assumed to be ₈ channels of wavelengths λ _{1 to} λ 8.

The amplification gain in the EDF 10 has wavelength dependence. Therefore, as shown in FIG.
Even if there is no variation in the input level between the wavelengths of the input optical signal, the amplified light output from the EDF 10 has the difference between the signal wavelengths as shown in FIG. Level deviation occurs.

On the other hand, by adjusting the reflectivity of each signal wavelength in each of the fiber gratings 22 _{1 to} 22 _n constituting the optical filter 22, the output is made as shown in FIG. Gain equalization between signals is realized. That is, the reflectance R of each optical signal in the optical filter 22 is canceled so that the wavelength dependence of the optical amplification shown in FIG.
By setting the wavelength dependence of the above, gain equalization between output signals is realized. Here, the reflectance R (dB) shown in Figure 5, the reflection of the entire optical filter 22 the combined reflectance of the fiber grating 22 _{1-22 8} corresponding to the optical signal of the wavelength lambda ₁ to [lambda] ₈ This is the wavelength distribution of the ratio, and the reflection peak at each wavelength λ _i (i = 1 to 8) corresponds to the reflectance at the fiber grating 22 _i .

The provision of the optical filter 22 enables the EDFA 1 to use the optical circulator 21 and the optical filter 22 for high-speed automatic gain control, and also functions as a gain equalizer. Can be done.

FIG. 6 is a diagram showing a configuration of an EDFA which is a second embodiment of the optical amplifier according to the present invention. This ED
Configuration from the input end 2a to the output end 2b of the FA2 is provided with a structure similar to EDFA1 shown in FIG. 1, mainly, on behalf of the optical filter 22 including the fiber grating 22 ₁ through 22 _n, a variable fiber The difference is that a variable optical filter 24 including gratings 24 _{1 to} 24 _n is applied.

This variable fiber grating 24₁~
24_nIs the optical signal to be amplified (wavelength λ ₁~ Λ_nSignal of
1) at the point of reflection
Ring 22₁~ 22_nIs the same as However, this
Variable fiber grating 24₁~ 24_nEach
So that the reflectivity of the corresponding signal wavelength can be changed.
Has been established.

For example, in the EDFA 1 shown in FIG.
Although gain equalization is realized by setting the reflectance in the optical filter 22, each of the fiber gratings 22 ₁ to 22 _1-
Since the reflectance of 22 _n is fixed, the setting of the gain equalization is performed when the EDFA 1 is manufactured.

On the other hand, the EDF according to the second embodiment
A2 is a fiber grating 24 ₁ whose reflectivity is variable.
Since the optical filter 24 including 〜24 _n is applied, variable gain equalization control according to the operation state of the EDFA 2 can be performed. Thereby, the level of the output signal can be adjusted. For gain equalization using reflection by a fiber grating, see, for example, the document `` S.-K.
Liaw et al., IEEE Photon. Technol. Lett., Vol.11,
no. 7, pp. 797-799, 1999 ".

As such fiber gratings 24 _{1 to} 24 _n with variable reflectivity, for example, a strain is applied to each of the fiber gratings 24 _{1 to} 24 _n by deformation using a piezo element (the fiber grating is expanded and contracted). ), it can be shifted peak wavelength of the reflected wavelength distribution for each fiber grating 24 ₁ to 24 _n, changing the reflectance of each optical signal.

In this case, it is preferable to use a chirped fiber grating as the fiber grating. The chirped fiber grating has a gradual change in the reflectance near the reflection peak wavelength as compared with a normal fiber grating, and the reflectance change control by the shift of the reflection peak wavelength is easier. When the reflectance is changed by the piezo element, the piezo element can respond in about several hundred μs, so that the variable gain equalization control in the order of several hundred μs is possible. Further, other than the piezo element, the reflectance may be changed by another method such as temperature control.

The EDFA 1 in FIG. 1 and the EDFA in FIG.
In No. 2, automatic gain control is performed by the automatic output control using noise light as described above. However, as shown in FIG. 2, there is a case where slight gain fluctuation occurs due to the fluctuation of the input level of the optical signal. There is. This is considered to be due to the fact that the value of the noise figure (NF: Noise Figure) of ASE noise light generation slightly changes depending on the input signal level.

That is, the power P of the ASE noise light
_ASE (ν) is given by P _ASE (ν) =
NF (ν) G (ν) hνΔν. Here, ν is a signal frequency, G (ν) is a gain for the frequency ν, h is a Planck constant, and Δν is a wavelength band of ASE noise light. The influence of the minute change of the noise light due to the change of the noise figure is also described in the variable optical filter 24 shown in FIG.
Is applied, the accuracy of the automatic gain control can be further improved.

FIG. 6 shows a configuration example of a control system (output control system 300) of the variable optical filter 24. That is, in the EDFA 2, a part of the optical signal output via the output port 213 of the optical circulator 21 is separated by the optical coupler 31 as monitoring light. The separated light is split into signal components having wavelengths λ _{1 to} λ _{n in} the arrayed waveguide diffraction grating type optical splitter 32, and n photodetectors corresponding to the level fluctuations of these split signal components are provided. 3
3 is detected.

The detection result of each signal component by the photodetector 33 is given to the output control circuit 30, and the output control circuit 30
Adjusts the reflectivity of the corresponding variable fiber gratings 24 _{1 to} 24 _n based on the detection result so that the output level of each signal component becomes a predetermined value. For example, when a piezo element is applied as a distortion adding structure for applying distortion to the variable fiber gratings 24 _{1 to} 24 _n , the reflectance is set or changed by adjusting the voltage applied to each piezo element. Thus, the second
According to the EDFA 2 according to the embodiment, the variable gain equalization can be realized, and the influence of a minute gain variation due to a change in the noise figure can be reduced.

The optical amplifier according to the present invention is not limited to the embodiments described above, and various modifications and changes in the configuration are possible. For example, as an optical amplifier, Er
In addition, an optical fiber to which a rare earth element such as Pr or Nd is added is applicable. In each of the above-described embodiments, a fiber grating is applied as an optical filter, and the fiber grating can narrow a reflection wavelength band, so that only an optical signal can be efficiently reflected. Other than such a fiber grating, as long as it is an optical component having a similar reflection function, the application of other forms of optical filters is not limited.

FIG. 7 is a diagram showing the structure of an EDFA 3 which is a third embodiment of the optical amplifier according to the present invention. This E
The configuration of the DFA 3 from the input terminal 3a to the output terminal 3b is similar to the EDFA 2 (FIG. 6) according to the second embodiment, but the output of the optical circulator 21 is provided between the optical circulator 21 and the output terminal 3b. Output-side variable optical attenuator 25 for attenuating an optical signal output via port 213
(A first variable optical attenuator) is provided. FIG. 8 is a view showing a structure of an EDFA 4 which is a fourth embodiment of the optical amplifier according to the present invention.
In the structure from the input terminal 4a to the output terminal 4b in the above, the input-side variable optical attenuator 26 (second
This is different from the EDFA 2 according to the second embodiment in that a variable optical attenuator is provided.

As described above, by installing the variable optical attenuator on the output side or the input side of the EDF 10, the output level of the optical signal is adjusted to make the gain constant, or to change the output level if necessary. Automatic level control (ALC) can be realized. Further, various adjustments and settings can be performed by combining output control by reflectance control in the variable optical filter 24 and output control by the output-side variable optical attenuator 25 and the input-side variable optical attenuator 26. The optical filter 22 having a fixed reflectance like the EDFA 1 of FIG.
In the case where is used, automatic level control or the like can be realized by installing a variable optical attenuator on the output side or the input side.

FIG. 9 is a diagram showing a configuration of an EDFA 5 which is a fifth embodiment of the optical amplifier according to the present invention. The EDFA 5 according to the fifth embodiment also takes in an optical signal (WDM signal) from the main transmission line via the input terminal 5a and sends out the amplified optical signal to the main transmission line via the output terminal 5b. However, the structure for separating noise light from amplified light output from the EDF 10 between the input terminal 5a and the output terminal 5b is the EDFA according to the first embodiment.
1 is different from the structure of FIG.

In FIG. 9, an EDFA 5 has an input terminal 5a for receiving an optical signal propagating through the main transmission line.
And an output end 5b for transmitting amplified light to the main transmission line, and between the input end 5a and the output end 5b, an EDF (Erbium-Doped Optical Fiber: Er-doped optical fiber) 10, an excitation light source 11 , A WDM coupler 12, and two optical isolators 13 and 14 are provided. EDF10 which is an optical amplification unit of the EDFA5
Is a silica-based optical fiber doped with an Er element,
In a state where pumping light of a predetermined wavelength is supplied, it functions to amplify a signal component included in a predetermined optical amplification wavelength band.

The optical isolators 13 and 14 are respectively
It is an optical component that allows input light to pass in the forward direction but not in the reverse direction. That is, the optical isolator 1
3 allows the light arriving from the input terminal 5a to pass to the EDF 10, but does not transmit the light once arriving to the input terminal 5a. The optical isolator 14 allows the light reaching from the EDF 10 to pass to the output end 5b side, but does not send the light once reaching the EDF 10 side.

The excitation light supplied to the EDF 10 is emitted from the excitation light source 11. The pump light source 11 is connected to an optical fiber line including the EDF 10 via a WDM coupler 12. Here, the WDM coupler 12 functions to allow the pumping light output from the pumping light source 11 to pass toward the EDF 10 and to allow the optical signal arriving from the optical isolator 13 to pass toward the EDF 10. As described above, the EDFA 5 according to the fifth embodiment in FIG. 9 is an optical amplifier having a configuration that enables forward pumping (forward pumping). The excitation light source 1 that excites the EDF 10
As 1, for example, a semiconductor laser with a wavelength of 1.48 μm can be applied.

In the above configuration, when an optical signal is input via the optical isolator 13 to the EDF 10 to which the excitation light of the predetermined wavelength is supplied from the excitation light source 11, the optical signal is amplified by the EDF 10. Then, the amplified light including the amplified optical signal is output from the EDF 10 to the optical isolator 14. At this time, the amplified light output from the EDF 10 is added together with the amplified optical signal and A
SE (Amplified Spontaneous Emission)
Noise light (forward ASE light) caused by the above.

In the EDFA 5 according to the fifth embodiment, automatic gain control (AGC) of optical amplification using ASE noise light included in the amplified light is performed. In order to realize this gain control, the EDFA 5 includes an optical element 400 for separating noise light and an optical signal from amplified light, a photodetector 23, and a gain control circuit 20 included in a gain control system.

The optical element 400 is an optical component generally used for multiplexing or separating WDM signals.
As shown in (a), an input port 401 for capturing an optical signal in a predetermined wavelength band, and output ports 402 and 403 for extracting light obtained by dividing an input optical signal into two at a constant frequency interval are provided. ing. Optical element 400
Is, for example, a WDM signal at 50 GHz intervals is input port 4
01, the output ports 402, 40
3 and outputs WDM signals at 100 GHz intervals as shown in FIGS. 10 (b) and 10 (c). vice versa,
The optical element 400 is an input WDM at 50 GHz intervals.
In addition to separating the signals into a set of WDM signals at 100 GHz intervals, a set of WDM signals at
It is also possible to multiplex with WDM signals at intervals of Hz (a set of WDM signals at intervals of 100 GHz is input via ports 402 and 403, respectively, and
A WDM signal at a GHz interval is output). Such an optical element 400 is generally called an interleaver, for example, as described in Oguma, et al., “Flatttened pass-band filterof
double cascaded Mach-Zehnder interferometers “,
It was introduced in the 1999 IEICE Electronics Society Conference C-3-96.

Note that the optical element 400 has the 5
WDM signals at 0 GHz intervals are converted to WDM signals at 100 GHz intervals.
Not only split into two signals but also WD at 100GHz intervals
M signals can be divided into WDM signals at 200 GHz intervals, and conversely, a set of WDM signals at 200 GHz intervals can be divided by 100.
It can be designed to combine with WDM signals at GHz intervals.

In the EDFA 5 (fifth embodiment) shown in FIG.
When the amplified light having the spectrum shown in FIG. 11A (including the ASE noise light together with the optical signal) is output from the EDF 10, the amplified light is taken into the optical element 400 via the input port 401. The optical element 400 is designed to divide the captured amplified light into light in a wavelength band including a signal component and light in a wavelength band including only a noise component. That is, the frequency interval when the band component is divided into two is approximately の of the frequency interval of the optical signal input to the EDF 10. As a result, light including signal components having the spectrum as shown in FIG. 11B is output through the output port 401, while the light of the spectrum shown in FIG. Light containing only noise components is output.

Further, the noise light separated by the optical element 400 is converted to a light detector 23 (light detection device) such as a PD.
Is detected by The photodetector 23 is a gain control circuit 20
The detection result of the noise light level by the photodetector 23 is given to the gain control circuit 20.

The power of the ASE noise light output from the EDF 10 is such that if the amplification gain in the EDF 10 is constant,
Even if the signal level (input level) of the optical signal changes, it hardly changes. For noise light with such properties,
The gain control circuit 20 (included in the gain control system)
The power of the excitation light supplied from the excitation light source 11 to the EDF 10 is controlled so that the power of the noise light detected by the photodetector 23 becomes substantially constant. As a result, this ED
Automatic gain control in FA5 is realized.

The control actually performed in the gain control circuit 20 is automatic power control (APC) for only the noise light power. However, the above-described control of the noise light power and the amplification gain is performed. From such a relationship, the effect of automatic gain control can be obtained. Therefore, the automatic gain control method, specifically, the gain control circuit 20
Can be simplified, and the speed of automatic gain control for maintaining the amplification gain constant can be increased.
Also, since the photodetector 23 only needs to measure the power of the entire noise light, it can easily cope with an optical amplifier having a wide dynamic range. Further, in the EDFA 5 according to the fifth embodiment, the speed of the automatic gain control is increased by increasing the power ratio of the noise light used for the automatic output control.

As described above, the EDFA according to the fifth embodiment
In 5, the band components including the optical signal are separated by 2 at appropriate frequency intervals.
A splittable optical element 400 is applied. Therefore, by appropriately setting the frequency interval to be divided, the noise light and the optical signal having sufficient power are separated from the amplified light with high accuracy, so that the pumping efficiency of the EDF 10 can be improved, and Output terminal 5b of EDFA5
The noise characteristics of the output light finally sent out on the main transmission line via the main transmission line are greatly improved.

[0072]

As described above, according to the present invention, the ASE noise light included in the amplified light is not separated on the main transmission line through which the optical signal should propagate, but on the branch line using the optical circulator. Or use a high-precision light separation element such as an interleaver to remove most of the noise light from the ED.
Since it is configured to be used as the monitoring light for the automatic gain control of F, it is possible to improve the measurement efficiency of the noise light power and effectively prevent the deterioration of the transmission quality. Further, with this configuration, it is possible to detect a change in the gain of the EDF at higher speed and with higher accuracy. Therefore, it is possible to realize high-speed and high-accuracy gain stabilization by automatic gain control.

When such an optical amplifier is applied to a WDM transmission system, the signal level of each signal wavelength is sufficiently high even when high-speed and large-capacity signal switching is performed by OXC, OADM, or the like. A transmission system that is flattened with high accuracy can be realized.

[Brief description of the drawings]

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

FIG. 2 is a graph showing an input level dependency of a gain obtained by the EDFA shown in FIG. 1;

FIG. 3 is a graph showing a transient gain variation when the number of signals changes in the EDFA shown in FIG. 1;

FIG. 4 is a diagram for explaining gain equalization for a WDM signal.

FIG. 5 is a graph showing signal reflection characteristics of an optical filter for realizing the gain equalization shown in FIG.

FIG. 6 is a diagram showing a configuration of an EDFA which is a second embodiment of the optical amplifier according to the present invention.

FIG. 7 is a diagram showing a configuration of an EDFA which is a third embodiment of the optical amplifier according to the present invention.

FIG. 8 is a diagram showing a configuration of an EDFA which is a fourth embodiment of the optical amplifier according to the present invention.

FIG. 9 is a diagram showing a configuration of an EDFA which is a fifth embodiment of the optical amplifier according to the present invention.

10A and 10B are diagrams for explaining the structure and function of an optical element in the EDFA shown in FIG. 9, wherein FIG. 10A is a schematic structure of the optical element, and FIGS. 4 shows a spectrum of a WDM signal divided into two by an element.

FIG. 11 is a graph showing the relationship between W by the optical element of the EDFA shown in FIG.
FIG. 9 is a diagram for explaining a division operation of a DM signal, and FIG.
Shows the spectrum of the input WDM signal, and (b) and (c) show the spectrum of the WDM signal divided into two.

[Explanation of symbols]

1, 2, 3, 4, 5,... EDFA (optical amplifier), 1a, 2
a, 3a, 4a, 5a ... input terminals, 1b, 2b, 3b, 4
b, 5b: output terminal, 10: EDF (optical amplifier), 11 ...
Pumping light source, 12: WDM coupler, 13, 14: optical isolator, 20: gain control circuit (included in gain control system), 21: optical circulator, 211, 401 ...
Input ports (first ports), 212, 402 ... control ports (second ports), 213, 403 ... output ports (third ports), 22 ... optical filters, 22 _{1 to} 22 _n ... fiber gratings, 23 , 33 ... light detection device,
24 variable optical filters, 24 _{1 to} 24 _n variable fiber gratings, 25 variable output optical attenuators (first variable optical attenuators), 26 variable input optical attenuators (second variable optical attenuators), Reference numeral 30 denotes an output control circuit, 31 denotes an optical coupler, 32 denotes an arrayed waveguide grating optical demultiplexer, 300 denotes an output control system, and 400 denotes an interleaver (optical element).

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Claims (9)

[Claims] An optical amplifier for amplifying an optical signal input together with the pump light; an excitation light source for supplying the pump light to the optical amplifier; and an optical amplifier including noise light together with the amplified optical signal. A first port for inputting amplified light from the unit, a second port for outputting amplified light from the first port, and the second port.
An optical circulator having a third port for outputting a part of the amplified light input again via a port, and the noise light included in the amplified light output via a second port of the optical circulator. And an optical filter that reflects the amplified optical signal so that the amplified optical signal of the amplified light is again input to the optical circulator through the second port; and A light detection device that detects the passed noise light; anda control of the amplification light in the optical amplification unit by controlling the excitation light source so as to suppress power fluctuation of the noise light detected by the light detection device. And a gain control system. 2. The optical filter according to claim 1, wherein a reflectance is set for each wavelength of the amplified optical signal so as to equalize wavelength dependence of an amplification gain in the optical amplifier. The optical amplifier according to claim 1. 3. A first attenuator for attenuating the amplified optical signal output from the third port of the optical circulator.
3. A variable optical attenuator, comprising: a variable optical attenuator;
An optical amplifier according to any of the preceding claims. 4. The optical amplifier according to claim 1, further comprising a second variable optical attenuator for attenuating the optical signal to be input to the optical amplifier. 5. The optical filter according to claim 1, wherein the optical filter includes a fiber grating that transmits the noise light of the amplified light and reflects the amplified optical signal of the amplified light. 5. The optical amplifier according to claim 4. 6. The method according to claim 6, wherein each reflectance of the optical filter is adjusted for each wavelength of the amplified optical signal,
The optical amplifier according to claim 1, further comprising an output control system that controls an output level of an optical signal output from a third port of the optical circulator. 7. The output control system has a structure for applying a distortion due to a temperature change or deformation to a predetermined portion of the fiber grating in order to change a reflectance for each wavelength of the amplified optical signal. The optical amplifier according to claim 6, wherein: 8. An optical amplifier for amplifying an optical signal input together with the pumping light, an excitation light source for supplying the pumping light to the optical amplifier, and a band component including the optical signal for every predetermined frequency interval. An optical element for splitting into two, a first port for inputting an amplified light from the optical amplifying unit including noise light together with an amplified optical signal, and an amplified light input through the first port. An optical system having a second port for outputting the separated amplified optical signal, and a third port for outputting noise light separated from the amplified light input through the first port. An element, a light detection device that detects the noise light output through the third port of the optical element, and the excitation light source that suppresses a power fluctuation of the noise light detected by the light detection device. Control The Rukoto, an optical amplifier and a gain control system for controlling the amplifying gain in said optical amplifying section. 9. The optical device according to claim 1, wherein the optical element divides a band component including the optical signal into two at a frequency interval substantially equal to a frequency interval of an optical signal input to the optical amplifier. Item 10. The optical amplifier according to item 8.