JPH1117258A - Light amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light amplifier stably having a large gain, even when the intensity of an input signal light is small. SOLUTION: A closed loop constituted of a pre-light amplifying part 30, an optical demultiplexer 40, a light feedback part 60, and an optical multiplexer 20 constitutes a traveling wave type laser oscillator, and laser oscillation is operated with the passage wavelength of an optical band pass filter 62 of the light feedback part 60. Inversion distribution in an EDF 34 of the pre-light amplifying part 30 is fixed, and the gain of a signal light in the pre-light amplifying part 30 is maintained to be constant according to the laser oscillation. A signal light outputted from the pre-light amplifying part 30 is allowed to pass through an optical filter 70, inputted to a post-light amplifying part 80, further amplified with a constant gain, and outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical amplifier for collectively amplifying multi-wavelength signal light in a WDM optical transmission system.

[0002]

2. Description of the Related Art With the increase in capacity and speed of optical communication, research and development on a wavelength division multiplex (WDM) transmission system have been conducted. In the WDM transmission system, one of the most important optical elements is an optical amplifier that collectively amplifies multi-wavelength signal light. As one of such optical amplifiers, an optical fiber amplifier (EDFA: Er-Doped Fiber A) using an amplifying optical fiber (EDF: Er-Doped Fiber) doped with an Er (erbium) element has conventionally been used.
mplifier).

Since this optical amplifier is provided in each repeater in the optical transmission system, if the gain characteristic is not stable, the network-type optical transmission system in which the number of signal light wavelengths increases or decreases is particularly required. In some cases, even though the signal light reaches the receiving station with sufficient intensity at some times, the intensity of the signal light may decrease at other times and an error may occur in the reception of the signal light. Therefore, several techniques for controlling the gain of an optical amplifier to a constant value have been conventionally known.

For example, a technique is known in which the intensity of each of input light and output light of an optical amplifier is monitored to determine the ratio between the two, and the gain is controlled to be constant based on the value of the ratio.
However, according to this technology, the output light from the optical amplifier includes not only the signal light input to the optical amplifier and optically amplified, but also an ASE (Amplified Spontaneous Emissi) generated inside the optical amplifier.
on) Light is also included. Therefore, when the intensity of the signal light input to the optical amplifier is low, the ASE included in the output light
There is a problem that the ratio of light increases and the intensity of output signal light decreases.

As a technique for solving such a problem, an optical closed loop including an optical amplifier is provided, and a laser is oscillated in the closed loop to keep the state of the optical waveguide having an optical amplifying action in the optical amplifier constant. A technique for maintaining and thereby controlling the gain of an optical amplifier at a constant level has been disclosed (J. Chung et al., "All-optical gain-clampedEDF").
As with different feedback wavelengths for use in
multiwavelength optical networks ", Electron.Lett.,
1996, vol.32, no.23, pp.2159-2161).

FIG. 4 is a configuration diagram of this conventional optical amplifier. As shown in this figure, a part of the output light of the optical amplifying unit 1 is taken out via an optical demultiplexer 2 provided on the output side of the optical amplifying unit 1, and the variable attenuator 3 and the optical bandpass filter 4 are removed. Then, the light is input to the optical amplifier 1 again via the optical multiplexer 5 provided on the input side of the optical amplifier 1. The optical band-pass filter 4 in the closed loop thus configured
Laser oscillation at the center wavelength of
The population inversion in the optical waveguide having the optical amplification function is maintained constant, and the gain of the optical amplifier 1 is controlled to be constant.

[0007]

However, in the above prior art, the energy of the pumping light supplied to the optical waveguide in the optical amplifier 1 is not only consumed for optically amplifying the signal light but also for the laser light. It is also consumed for oscillation. Therefore, the input signal light cannot be sufficiently optically amplified, and an optical output of only about half of the optical output when the optical amplifier 1 alone amplifies the optical signal can be achieved.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an optical amplifier having a large gain stably even when the intensity of an input signal light is small. .

[0009]

According to the present invention, there is provided an optical amplifier comprising: (1) a pre-amplifier for optically amplifying and outputting an input light in a predetermined wavelength band; and (2) an output from the pre-amplifier. An optical demultiplexer that receives the input light and splits the light into a first light and a second light; and (3) an optical feedback unit that passes light other than the signal light wavelength band of the first light; (4) an optical multiplexer for multiplexing the signal light to be amplified with the light output from the optical feedback unit and inputting the multiplexed light to the optical pre-amplifier; and (5) an optical multiplexer of the signal light wavelength band of the second light. An optical filter that allows light to pass through, and (6) a post-stage optical amplifier that controls the gain to be constant based on the ratio between the intensity of the input light and the intensity of the output light, inputs the light output from the optical filter, amplifies the light, and outputs it. And a unit.

According to this optical amplifier, the closed loop including the pre-stage optical amplifier, the optical demultiplexer, the optical feedback unit, and the optical multiplexer includes:
A traveling-wave type laser oscillator is formed, and oscillates at a passing wavelength (wavelength outside the signal light wavelength band) in the optical feedback section. With this laser oscillation, the population inversion in the upstream optical amplifier is fixed, and the gain of the signal light in the upstream optical amplifier is kept constant. The signal light (light in the signal light wavelength band) output from the pre-stage optical amplifier passes through the optical filter,
The light is input to the post-stage optical amplifier, further amplified at a constant gain, and output.

Further, in the optical amplifier according to the present invention, the pre-stage optical amplifier includes: (1) an optical waveguide for optically amplifying and outputting light of a predetermined wavelength band inputted when the pumping light is supplied. (2) pumping means for outputting pumping light and supplying it to the optical waveguide. In this case, when the pumping light output by the pumping unit is supplied to the optical waveguide, the light of a predetermined wavelength band input to the optical waveguide is optically amplified by the optical waveguide.

Further, in the optical amplifier according to the present invention, the rear-stage optical amplifier is provided. (1) an optical waveguide for optically amplifying and outputting light of a predetermined wavelength band inputted when the pumping light is supplied, (2) an excitation means for outputting the pumping light and supplying the light to the optical waveguide, (3) Input light detection means for detecting the intensity of light input to the optical waveguide, (4) output light detection means for detecting the intensity of light input from the optical waveguide, and (5) input light detection means and output light detection means Control means for controlling the excitation light output from the excitation means based on the ratio of the light intensities detected by the respective means. In this case, when the pumping light output by the pumping unit is supplied to the optical waveguide, the light of a predetermined wavelength band input to the optical waveguide is optically amplified by the optical waveguide. Further, the intensity of each of the input light and the output light of the optical waveguide is detected by each of the input light detection means and the output light detection means, and based on the ratio between the two, the control means controls the excitation light output from the excitation means. The intensity is controlled, and the gain of the post-stage optical amplifier is kept constant.

Further, the optical amplifier according to the present invention is characterized in that the optical waveguides of the first and second optical amplifiers are optical fibers. In this case, connection loss with the optical fiber line is small. Further, the optical amplifier according to the present invention is characterized in that the optical waveguides of the first-stage optical amplifier and the second-stage optical amplifier are added with an Er element as a substance having an optical amplification effect. In this case, signal light in the wavelength band of 1.55 μm, which is most frequently used in optical communication, is amplified.

Further, in the optical amplifier according to the present invention, the optical filter is an optical fiber grating in which a Bragg grating having a periodic refractive index change along the optical axis is formed. . In this case, the connection loss with the optical fiber is small.

Further, the optical amplifier according to the present invention further comprises:
It is characterized by further comprising monitoring means for monitoring the intensity of light other than the signal light wavelength band. In this case, the intensity of the laser oscillation light generated in the closed loop including the pre-stage optical amplifier is monitored by the monitoring unit, and the state of the optical amplification operation in the pre-stage optical amplifier can be determined.

[0016]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Hereinafter, an optical fiber amplifier which is one of the optical amplifiers will be described.

FIG. 1 is a configuration diagram of an optical fiber amplifier according to the present embodiment. This optical fiber amplifier optically amplifies the signal light input to the input end optical connector 10 and outputs the amplified signal light from the output end optical connector 90. Wave device 20,
The first-stage optical amplifier 30, the optical splitter 40, the optical filter 70, and the second-stage optical amplifier 80 are cascaded in this order.
Further, an optical feedback unit 60 is provided between the optical demultiplexer 40 and the optical multiplexer 20, and the optical demultiplexer 40 has a light receiving element 5
0 is also connected. These components are optically connected to each other by an optical waveguide such as an optical fiber.

The signal light input to the input end optical connector 10 first reaches the optical multiplexer 20. The optical multiplexer 20 receives the signal light from the input end optical connector 10 and the light output from the optical feedback unit 60, multiplexes the two, and outputs the multiplexed light.

The preamplifier 30 for inputting the light output from the optical multiplexer 20 includes optical isolators 31A and 31B, a pump light source 32, a WDM coupler 33, and an ED.
F34. The optical isolator 31A is an optical multiplexer 2
The light input from 0 is passed to the WDM coupler 33,
It does not allow light to pass in the opposite direction. Excitation light source (for example,
The semiconductor laser light source 32 outputs excitation light having a constant intensity to be supplied to the EDF 34. The WDM coupler 33 receives the light arriving from the optical isolator 31A and the pumping light output from the pumping light source 32, and outputs both to the EDF. When the EDF 34 is supplied with the excitation light,
Light of a predetermined wavelength input from the optical isolator 31A is optically amplified. The optical isolator 31B allows the light output from the EDF 34 to pass, but does not allow the light to pass in the opposite direction. In other words, the pre-stage optical amplifier 30 includes the pump light source 3
2 when the pumping light is supplied to the EDF 34 via the WDM coupler 33, the optical isolator 31A and the WD
The light having a predetermined wavelength input through the M coupler 33 is
, And outputs the optically amplified light via the optical isolator 31B.

The optical demultiplexer 40 receives the light output from the optical amplifier 40, outputs a part of the light to the optical feedback unit 60, and outputs the rest to the optical filter 70. Further, the optical demultiplexer 40 receives the light input from the optical filter 70 side,
A part or the whole is output to the light receiving element 50. The light receiving element (for example, a photodiode) 50 receives the light arriving from the optical demultiplexer 40 and outputs an electric signal according to the intensity.

The optical feedback section 60 is configured by optically cascading a variable attenuator 61 and an optical bandpass filter 62. The optical feedback unit 60 receives the light output from the pre-stage optical amplifying unit 30 and arrives via the optical demultiplexer 40, gives a predetermined loss to the light by the variable attenuator 61, The filter 62 allows light of the light other than the signal light wavelength band to pass through, and outputs only light of the predetermined wavelength band. The light output from the optical feedback unit 60 is input again to the pre-stage optical amplifier 30 via the optical multiplexer 20.

The optical filter 70 receives, from among the lights output from the pre-stage optical amplifying section 30, the light that has reached via the optical demultiplexer 40, and inputs the light in the signal light wavelength band (ie, the signal light). Let it pass. As the optical filter 70, an optical fiber grating in which a Bragg grating having a periodic refractive index change along the optical axis is preferably used. In this case, the optical filter 70 and another optical fiber are Connection loss is small. At this time, the Bragg wavelength in the Bragg grating is matched with the wavelength of the light to be cut off.

The post-amplifier 80 for inputting the light passing through the optical filter 70 includes optical isolators 81A and 81A.
1B, excitation light source 82, WDM coupler 83, EDF 84A
84B, optical demultiplexers 85A and 85B, light receiving elements 86A and 86B, and a control circuit 87. The optical isolator 81A allows the light input from the optical filter 70 to pass through the optical splitter 85A, but does not allow the light to pass in the opposite direction. The optical splitter 85A is an optical isolator 81
The light arriving from A is input and a part of the light is
A, and outputs the remainder to EDFs 84A and 84B. The EDFs 84A and 84B are cascaded with each other, and optically amplify light of a predetermined wavelength input from the optical demultiplexer 85A when the pumping light output from the pumping light source 82 is supplied.

Excitation light source (for example, semiconductor laser light source) 8
2 outputs the excitation light to be supplied to the EDFs 84B and 84A. The WDM coupler 83 receives the pump light output from the pump light source 82 and outputs the pump light to the EDFs 84B and 84A, and outputs the light output from the EDFs 84A and 84B to the optical isolator 81B. The optical isolator 81B allows the light reaching from the WDM coupler 83 to pass, but does not allow the light to pass in the opposite direction. Optical splitter 85
B receives the light arriving from the optical isolator 81B, outputs part of the light to the light receiving element 86B, and outputs the remainder to the output end optical connector 90.

Light receiving element (for example, photodiode) 8
6A and 86B are optical demultiplexers 85A and 8B, respectively.
5B, and outputs an electric signal corresponding to the intensity of each of the input light and the output light of the post-stage optical amplifier 80. The control circuit 87 includes a light receiving element 86A
The electric signal output from each of the optical amplifiers 86B and 86B is input, the intensity of the excitation light output from the excitation light source 82 is controlled based on the ratio between the two, and the gain of the post-amplifier 80 is controlled to be constant.

That is, when the pumping light is supplied from the pumping light source 82 to the EDFs 84A and 84B via the WDM coupler 83, the post-stage optical amplifying section 80 receives the predetermined light inputted through the optical isolator 81A and the optical splitter 85A. The light of the wavelength is optically amplified by the EDFs 84A and 84B,
The optically amplified light is output through the WDM coupler 83, the optical isolator 81B, and the optical demultiplexer 85B. At this time, the gain of the latter-stage optical amplifier 80 is controlled to be constant by the light receiving elements 86A and 86B and the control circuit 87.

The optical fiber amplifier configured as described above operates as follows. In the pre-stage optical amplifier 30, the excitation light is output from the excitation light source 32 and supplied to the EDF 34 via the WDM coupler 33. Further, in the post-stage optical amplifier 80, the pump light is output from the pump light source 82, passes through the WDM coupler 83, and the EDFs 84B and 8B.
4A.

As described above, when the excitation light is supplied to the EDF 34 in the pre-stage optical amplifier 30, the Er element contained in the EDF 34 is excited, and a population inversion occurs. Then, light is generated when the excited Er element transitions to a lower energy state. Of the light, the light feedback unit 60
The light (light other than the signal light wavelength band) that can pass through the optical bandpass filter 62 in the inside passes through the optical demultiplexer 40, the optical feedback unit 60, and the optical multiplexer 20, and is again input to the pre-stage optical amplifier 30. And cause stimulated release in EDF34.

That is, the pre-amplifier 30 and the optical demultiplexer 4
The closed loop including the optical feedback unit 60, the optical feedback unit 60, and the optical multiplexer 20 constitutes a traveling-wave laser oscillator. The wavelength of the laser oscillation in this closed loop is the wavelength in the pass wavelength band of the optical bandpass filter 62, and the oscillation intensity is
It is determined by the transmission loss in the variable attenuator 61. Then, when laser oscillation occurs, the pre-stage optical amplifier 30
Is inverted in the EDF 34 of FIG.

At this time, when the signal light is input to the input end optical connector 10 of the optical fiber amplifier, the signal light is input to the pre-stage optical amplifier 30 via the optical multiplexer 20 and is optically amplified and output. Since the population inversion in the EDF 34 of the upstream optical amplifier 30 is fixed, the gain of the signal light in the upstream optical amplifier 30 is kept constant.

The light output from the pre-amplifier 30 includes the signal light and the laser oscillation light that have been optically amplified, and is input to the optical filter 70 via the optical demultiplexer 40. Of the light input to the optical filter 70, the signal light passes through the optical filter 70, but the laser oscillation light is cut off and reflected by the optical filter 70. Optical filter 70
The laser oscillation light reflected by is received by the light receiving element 50 via the optical demultiplexer 40, and an electric signal corresponding to the intensity is output. Then, by monitoring this electric signal, the optical amplification operation of the pre-stage optical amplifier 30 can be monitored.

On the other hand, the signal light that has passed through the optical filter 70 is input to a post-stage optical amplifier 80, where the signal light is amplified and output.
Further, it is output from the output terminal optical connector 90. In the latter-stage optical amplifying section 80, the intensities of the input light and the output light are detected by the light receiving elements 86A and 8B, respectively, and the excitation supplied from the excitation light source 82 to the EDFs 84B and 84A by the control circuit 87 based on the ratio between the two. The light intensity is controlled, and the gain of the optical amplification of the signal light is kept constant.

As described above, since the population inversion in the EDF 34 of the pre-stage optical amplifier 30 is fixed by the laser oscillation, the gain in the pre-stage optical amplifier 30 is stably maintained constant. In addition, since the signal light having the intensity equal to or higher than a certain level by the first-stage optical amplifier 30 is input to the second-stage optical amplifier 80, the gain in the second-stage optical amplifier 80 is also stably maintained at a constant level. Therefore, the constant gain control of the entire optical fiber amplifier is also stabilized.

Further, even if the intensity of the output signal light is not sufficient only by the gain in the pre-stage optical amplifier 30, the optical amplifier is further amplified by the post-stage optical amplifier 80, so that the gain of the entire optical fiber amplifier is sufficiently increased. Thus, the optical fiber amplifier outputs a signal light having a sufficiently large intensity.

Next, more specific examples of the optical fiber amplifier according to the present embodiment and experimental results will be described.

The EDFs 34, 84A and 8 used here
The concentration, absorption peak and length of the 1.53 μm band of each additive element of each of 4B are values shown in the following table.

[0037]

[Table 1]

The signal light input to the input end optical connector 10 is a 16-wave WDM signal with a wavelength of 1543 nm to 1558 nm in increments of 1 nm, and the intensity of each signal light is -2.
2 dBm, and the total intensity of the 16-wave signal light was -10 dBm. Then, in order to collectively amplify this multi-wavelength signal light,
A semiconductor laser light source that outputs laser light having a wavelength of 0.98 μm is used as the excitation light source 32, a semiconductor laser light source that outputs laser light having a wavelength of 1.48 μm is used as the excitation light source 82, and a transmission wavelength of the optical bandpass filter 62. The center wavelength of the band was 1565 nm, and the cutoff wavelength of the optical filter 70 was also 1565 nm.

FIG. 2 shows a spectrum of light at each of the three points in the optical fiber amplifier configured as described above. FIG. 2A is a spectrum diagram of signal light at an input point (point A in FIG. 1) of the input end optical connector 10, and FIG. 2B is an input point of the optical filter 70 (FIG. 1). B point)
2C is a spectrum diagram of light at an output point of the output end optical connector 90 (point C in FIG. 1).

In the laser oscillator composed of a closed loop including the pre-stage optical amplifier 30, the optical demultiplexer 40, the optical feedback unit 60, and the optical multiplexer 20, the center wavelength of the pass band of the optical band-pass filter 62 is 1565 nm. Causes laser oscillation. With this laser oscillation, the EDF 3 of the pre-stage optical amplifier 30
4 was fixed, and as a result, a stable gain of about 20 dB was obtained in the pre-stage optical amplifier 30. Accordingly, at the input point (point B in FIG. 1) of the optical filter 70, as shown in the spectrum diagram of FIG. 2B, the optically amplified 16-wave signal light and the laser oscillation light having the wavelength of 1565 nm appear. At this time, the intensity of each signal light is about -2.
dBm, and the total intensity of the 16-wave signal light was about 10 dBm.

Of the light input to the optical filter 70,
The laser oscillation light having a wavelength of 1565 nm is reflected by the optical filter 70, and the 16-wave signal light passes through the optical filter 70, is further input to the post-amplifier 80, and is collectively amplified. The gain in the latter-stage optical amplifying section 80 depends on the light receiving element 8
Based on the ratio of the intensity of the input light and the intensity of the output light detected by each of 6A and 86B, the control circuit 87
Is controlled to be constant at 7 dB. Therefore, at the output point (point C in FIG. 1) of the output end optical connector 90, as shown in the spectrum diagram of FIG. 2C, the further amplified 16-wave signal light is output. At this time, since no laser oscillation light is input to the rear-stage optical amplifier 80, the total intensity of the output 16-wave signal light was about 17 dBm, which is the maximum output of the rear-stage optical amplifier 80. As described above, the gain of the entire optical fiber amplifier was 27 dB.

When the pre-stage optical amplifier 30 is used alone without forming a closed loop, 1 at point B is used.
The total intensity of the six-wave signal light is about 12.5 dBm.
However, when laser oscillation is performed in a closed loop as in this embodiment, if the intensity of the input 16-wave signal light increases, the laser oscillation stops, so that the population inversion cannot be fixed and the gain is kept constant. Lost. Therefore, in this embodiment, the maximum value of the total intensity of the 16-wave signal light at the point B is about 10.0 dBm in order to stably maintain the gain in the pre-stage optical amplifying unit 30 and is used alone. This is about 2.5 dBm lower than the case.

FIG. 3 is a diagram showing the dependence of the gain on the intensity of all input signal lights. In this figure, the solid line shows the case of the present embodiment, and the broken line shows the case of the prior art. As in the conventional optical fiber amplifier, the light intensity at each of the points A and C (that is, the intensity of the input light and the output light with respect to the entire optical fiber amplifier) is monitored, and the gain is controlled to be constant based on the ratio between the two. In this case, since the output light includes the ASE light, if the total intensity of the input signal light is reduced as shown by the broken line in FIG. 3, the overall gain is reduced.

On the other hand, in the optical fiber amplifier according to the present embodiment, the former-stage optical amplifier 30 and the latter-stage optical amplifier 8
0, the ASE light generated in the rear-stage optical amplifier 80 is compared with the ASE light component based on the ASE generated in the front-stage optical amplifier 30.
E light is negligibly small. This is because the intensity of the signal light input to the pre-stage optical amplifier 30 is higher than that of the post-stage optical amplifier 80.
Since the intensity of the signal light input to the second stage is higher, most of the population inversion in the EDFs 84A and 84B of the post-stage optical amplifier 80 is used for amplifying the signal light, and is slightly used for generating the ASE light. Because. Therefore, in the optical fiber amplifier according to the present embodiment, as shown by the solid line in FIG. 3, even when the total intensity of the signal light input to the input end optical connector 10 fluctuates, the overall gain is substantially constant at about 27 dB. Is kept.

The laser oscillation light is reflected by the optical filter 70, passes through the optical demultiplexer 40, enters the light receiving element 50 and is received. By monitoring the intensity of the laser oscillation light received by the light receiving element 50, it can be determined whether or not the gain in the pre-stage optical amplifier 30 is controlled to be constant. That is, as described above, when the total intensity of the input signal light exceeds a predetermined value, such as when the wave number of the signal light input to the input end optical connector 10 increases, laser oscillation stops, and the population inversion occurs. Cannot be fixed, and the gain cannot be kept constant. Therefore, when the intensity of the laser oscillation light received by the light receiving element 50 becomes lower than a predetermined value, it is considered that the constant gain control has been disturbed, and an alarm or the like can be issued to that effect. Based on this, maintenance of an optical fiber amplifier and an optical transmission system including the same can be performed.

The present invention is not limited to the above embodiment, and various modifications are possible. In the description of the above embodiment, the optical fiber (EDF) doped with the Er element has been described as having the optical amplification function in each of the first-stage optical amplification unit and the second-stage optical amplification unit. However, the present invention is not limited to this. For example, an optical fiber to which a rare earth element other than the Er element (for example, an Nd (neodymium) element, a Pr (praseodymium) element, or the like) is added as a substance having an optical amplification effect may be used. Instead of a fiber, a planar optical waveguide may be used.

As means for monitoring the laser oscillation light, there is provided a light demultiplexing means for extracting the laser oscillation light from the light traveling from the pre-stage optical amplification unit 30 to the post-stage optical amplification unit 80. Light may be received and detected. Further, in this case, the optical demultiplexing means and the optical filter 70 can also be used. For example, an optical fiber grating having a grating formed obliquely to the optical axis of the optical fiber, the laser oscillation light is Bragg reflected and emitted to the side of the optical fiber,
On the other hand, one that allows the signal light to pass through as it is may be used.

[0048]

As described above in detail, according to the present invention, the closed loop including the pre-stage optical amplifier, the optical demultiplexer, the optical feedback unit, and the optical multiplexer constitutes a traveling-wave laser oscillator. The laser oscillates at the passing wavelength (wavelength outside the signal light wavelength band) in the optical feedback section. With this laser oscillation, the population inversion in the upstream optical amplifier is fixed, and the gain of the signal light in the upstream optical amplifier is kept constant. Signal light (light in the signal light wavelength band) output from the pre-stage optical amplifier
Passes through an optical filter, is input to a subsequent optical amplifier, is further optically amplified at a constant gain, and is output.

With such a configuration, even when the intensity of the input signal light is low, the signal light is optically amplified with a constant gain in the pre-stage optical amplifier, and the signal amplified by the pre-stage optical amplifier is also amplified. Since the light is input to the latter-stage optical amplifier, the latter-stage optical amplifier is also optically amplified with a constant gain.
Therefore, even when the intensity of the input signal light is low, the gain of the optical amplifier is maintained at a sufficiently large value and stably constant.

The pre-stage optical amplifier includes (1) an optical waveguide for optically amplifying and outputting light of a predetermined wavelength band inputted when the pumping light is supplied, and (2) an optical waveguide for outputting the pumping light and outputting the same. And an excitation means for supplying. Also, the post-stage optical amplifier. (1) an optical waveguide for optically amplifying and outputting light of a predetermined wavelength band input when the pumping light is supplied,
(2) pumping means for outputting pumping light and supplying it to the optical waveguide,
(3) input light detection means for detecting the intensity of light input to the optical waveguide; (4) output light detection means for detecting the intensity of light input from the optical waveguide; and (5) input light detection means and output. It is preferable that the control device includes a control unit that controls the excitation light output from the excitation unit based on a ratio of light intensities detected by the light detection units. When the optical waveguides of the first-stage optical amplifier and the second-stage optical amplifier are optical fibers, the connection loss with the optical fiber line is small. In addition, when the Er element is added as a substance having an optical amplifying action, the optical waveguides of the first and second optical amplification sections have a wavelength of 1.55 μm, which is most frequently used in optical communication.
The m-band signal light is amplified.

In the case where the optical filter is an optical fiber grating in which a Bragg grating having a periodic change in the refractive index along the optical axis is formed, the connection loss with the optical fiber is small.

When a monitoring means for monitoring the intensity of light other than the signal light wavelength band is further provided, the intensity of the laser oscillation light generated in the closed loop including the pre-stage optical amplifier is monitored by the monitoring means. The state of the optical amplification operation in the amplification unit can be determined. Further, an alarm or the like can be issued based on the monitoring result by the monitoring means.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier according to an embodiment.

FIG. 2 is a spectrum diagram of light at each of three points in the optical fiber amplifier according to the present embodiment.

FIG. 3 is a diagram illustrating the dependence of gain on the intensity of all input signal light.

FIG. 4 is a configuration diagram of a conventional optical amplifier.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Input end optical connector, 20 ... Optical multiplexer, 30 ... Pre-stage optical amplifier, 31A, 31B ... Optical isolator, 32 ... Excitation light source, 33 ... WDM coupler, 34 ... EDF, 40 ... Optical demultiplexer, 50 ... Light receiving element, 60 ... optical feedback section, 61 ... variable attenuator, 62 ... optical band pass filter, 70 ... optical filter, 80 ... post-stage optical amplifying section, 81A, 81B ... optical isolator, 82 ... excitation light source, 83 ... WDM coupler, 8
4A, 84B: EDF, 85A, 85B: Optical demultiplexer, 8
6A, 86B: light receiving element, 87: control circuit, 90: output end optical connector.

Claims (7)

[Claims] 1. A pre-amplifier for optically amplifying and outputting an input light of a predetermined wavelength band, and a light output from the pre-amplifier and input to a first light and a second light. An optical demultiplexer for demultiplexing, an optical feedback unit for transmitting light other than the signal light wavelength band of the first light, and a signal light to be amplified and a light output from the optical feedback unit. An optical multiplexer for inputting to the optical pre-amplifier; an optical filter for transmitting light in the signal light wavelength band of the second light; and a gain based on a ratio of the respective intensities of the input light and the output light. And a post-stage optical amplifying unit that inputs light output from the optical filter, amplifies the light, and outputs the light. 2. The optical amplifier, comprising: an optical waveguide configured to optically amplify and output the light of the predetermined wavelength band input when the pumping light is supplied; and an optical waveguide that outputs the pumping light to the optical waveguide. The optical amplifier according to claim 1, further comprising: a pumping means for supplying. 3. The post-stage optical amplifier. An optical waveguide for optically amplifying and outputting the light in the predetermined wavelength band input when the excitation light is supplied; an excitation unit for outputting the excitation light and supplying the light to the optical waveguide; and inputting the optical waveguide. Input light detection means for detecting the intensity of light, output light detection means for detecting the intensity of light input from the optical waveguide, and intensity of light detected by the input light detection means and the output light detection means, respectively. 2. The optical amplifier according to claim 1, further comprising: a control unit configured to control the pumping light output from the pumping unit based on the ratio of: 4. The optical amplifier according to claim 2, wherein said optical waveguide is an optical fiber. 5. The optical amplifier according to claim 4, wherein said optical waveguide is doped with Er element as a substance having an optical amplification effect. 6. The optical amplifier according to claim 1, wherein said optical filter is an optical fiber grating in which a Bragg grating having a periodic refractive index change along an optical axis is formed. 7. The optical amplifier according to claim 1, further comprising monitoring means for monitoring the intensity of light other than the signal light wavelength band.