JP2000332334A - Optical fiber amplifier - Google Patents

Abstract

(57) Abstract: An object of the present invention is to provide an optical fiber amplifier in which a pump light source has a redundant configuration, by suppressing a jump in the oscillation wavelength of the pump light source and a change in power, thereby reducing the fluctuation of the amplification characteristic. It is an object to provide a fiber amplifier. SOLUTION: In order to oscillate at different wavelengths in a plurality of excitation light sources, the present invention provides a plurality of first excitation light sources 2 having a plurality of first wavelengths whose center wavelengths of reflection wavelength bands are different from each other.
The optical filter 3 is connected. Then, in order to prevent light from another excitation light source 2 from being incident on the excitation light source 2, light having the central wavelength of the first optical filter 3 connected to each of the first optical filters 3 is transmitted and the central wavelength Are connected, and a plurality of second optical filters 4 that block light other than the above are connected. The light obtained by multiplexing the light from each of the second optical filters 4 with the optical multiplexer 5 becomes light with small power fluctuation and enters the rare-earth element-doped optical fiber 1, so that the optical fiber with little fluctuation in amplification characteristics An amplifier can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier in which a pump light source has a redundant configuration, wherein the power of light emitted from the pump light source is stabilized to reduce fluctuations in amplification characteristics. About. With the aim of constructing a multimedia network in the future, an ultra-long distance and large-capacity optical communication device is required. Wavelength division multiplexing (Wavelength-d
ivision Multiplexing, hereinafter abbreviated as “WDM”. Research and development are proceeding from the point of advantage that the method can effectively utilize the broadband and large capacity of the optical fiber. In particular, in an ultra-long-distance optical transmission system such as a submarine cable that crosses the ocean, it is necessary to amplify and relay the WDM optical signal because the WDM optical signal is attenuated during ultra-long-distance optical transmission. As such an optical fiber amplifier, a conventional 3R (Reshaping, Retiming, Regeneratin)
g) Research and development of optical fiber amplifiers as compared to repeaters with functions, because of their advantages such as transmission speed independence, simplification of repeaters, and the ability to amplify WDM optical signals as they are Is thriving.

[0002]

2. Description of the Related Art Conventionally, an optical fiber amplifier shown in FIG. 11 has been known as an optical fiber amplifier having a redundant excitation light source. FIG. 11 is a diagram showing a configuration of an optical fiber amplifier having a conventional pump light source having a redundant configuration. First,
FBGs 112-1 and 112-2 shown by broken lines in FIG.
An optical fiber amplifier having no configuration will be described. Note that FBG is a fiber grating filter (Fi
Ber Grating filter), and will be similarly abbreviated below.

In FIG. 11, a laser diode (Laser Diode, which oscillates a laser beam having a wavelength of 1480 nm) is used.
Hereinafter, it is abbreviated as “LD”. The laser light emitted from 110-1 enters the coupler 115 together with the laser light emitted from the LD 110-2 which oscillates the laser light in the same wavelength band of 1480 nm. These LDs 110-1 and 110-2
Is an erbium doped optical fiber described later.
Fiber, hereinafter abbreviated as “EDF”. ) Is a pumping light source having a redundant configuration for pumping. Note that the LD is also called an injection type semiconductor laser.

[0004] The coupler 115 multiplexes these laser beams, further branches the multiplexed laser beam into two, and emits them as excitation light. One of the emitted pump lights is multiplexed with the upstream optical signal by the coupler 116-1 and enters the EDF 117-1. EDF117-1 is composed of LD110-1 and LD1.
By absorbing the excitation light from 10-2, the EDF 117
The electrons in -1 are excited to form population inversion. When an upward optical signal is incident in a state where the population inversion is formed, the optical signal is guided by the optical signal to generate stimulated emission, and the optical signal is amplified.

[0005] An amplified upstream optical signal is an isolator (hereinafter referred to as "ISO") that transmits light in only one direction.
Abbreviated. ) And is emitted from ISO 118-1. On the other hand, the other pump light emitted from the coupler 115 is combined with the downstream optical signal by the coupler 116-2 and then enters the EDF 117-2 in the same manner as described above. Injected through -2.

In an optical fiber amplifier having such a configuration,
The excitation light source for exciting EDF 117-1, 117-2 is LD
Since there are two of the pump light source 110-1 and the LD 110-2, even if one of the pump light sources, for example, the LD 110-1, stops oscillating due to a failure, the optical signal is amplified by the pump light from the other pump light source LD 110-2. There are advantages that can be.

In the semiconductor lasers used as the LDs 110-1 and 110-2, mode hopping (mode
Due to phenomena such as hopping, power distributed between modes fluctuates with time. Mode hopping is a phenomenon in which the oscillation wavelength of a semiconductor laser jumps from one resonance mode to another resonance mode. FIG. 12 is a diagram illustrating the mode hopping of the semiconductor laser. The horizontal axis in FIG.
The wavelength is expressed in units of nm, and the vertical axis is power.
Each curve in FIG. 12 is a result of measuring a spectrum of a certain semiconductor laser, and is obtained by changing a measurement time.

As shown in FIG. 12, the semiconductor laser oscillates in multiple modes, and the number of oscillation modes and the oscillation power at each oscillation wavelength differ for each measurement time. As described above, in the semiconductor laser, since a phenomenon such as mode hopping occurs, the power of the pump light entering the absorption wavelength band of the 1480 nm band in the EDF varies with time. For this reason, in the above-described conventional optical fiber amplifier, amplification characteristics such as gain characteristics and noise figure change.

Therefore, in the invention described in Japanese Patent Application Laid-Open No. 09-097940, the FBG indicated by a broken line in FIG.
An attempt was made to solve this problem by inserting 112-1 between the LD 110-1 and the coupler 115 and inserting the FBG 112-2 between the LD 110-2 and the coupler 115. These FBGs
Reference numerals 112-1 and 112-2 denote FBGs having a center wavelength of a reflection wavelength band at a predetermined wavelength at which the oscillation wavelength of the LDs 110-1 and 110-2 is to be fixed.

Note that FBGs 112-1 and 11 shown by broken lines
Configurations other than 2-2 are the same as those described above, and a description thereof will be omitted. With this configuration, the FBG11
The LD 110 is reflected by the laser beam having the wavelength
The oscillation wavelength of -1 is fixed to a predetermined wavelength, and FBG1
The laser beam having the wavelength reflected by the laser beam 12-2
Since the oscillation wavelength 0-2 is fixed to the predetermined wavelength, it is possible to suppress the power of the pump light from fluctuating with time.
Here, in the conventional example shown in FIG. 11, since the center wavelengths of the reflection wavelength bands of the FBG 112-1 and the FBG 112-2 are the same, the LD 110-1 and the LD 110-2 are fixed to the same wavelength.

[0011]

However, the above-mentioned FB
LD110-1 by G112-1 and FBG1
Also in the method of fixing the oscillation wavelength of the LD 110-2 by 12-2, the laser light of the LD 110-2 affects the LD 110-1 due to reflection at the joint of each optical component in FIG. Laser light is LD110-2
Therefore, there is a problem that the power of the laser light in each of the LDs 110-1 and 110-2 fluctuates.

In particular, in an ultra-long-distance optical transmission system such as a submarine cable that crosses the ocean, repeaters of several tens or hundreds of stages are required, so that the margin per repeater decreases. Fluctuations in the power of the pumping light source cause significant fluctuations in the amplification characteristics of the optical fiber amplifier used in the repeater, which is a significant problem. In the 980 nm band, which is another excitation wavelength band in the EDF, the absorption wavelength band is one.
Since the wavelength is narrower than the absorption wavelength band of the 480 nm band, the power fluctuation within the absorption wavelength band due to the power fluctuation of the pump light is 14
There is a problem that it becomes larger than in the case of the 80 nm band.

Further, when the drive current and temperature of the pump light source are controlled, the center-of-gravity wavelength of the spectrum of the semiconductor laser fluctuates. As a result, the power of the pump light entering the absorption wavelength band of the EDF fluctuates over time. There's a problem. Here, the center-of-gravity wavelength λc is

(Equation 1) Refers to the wavelength indicated by Here, λj is the j-th wavelength, Pj is the optical power at the j-th wavelength, and N is the number of samples.

In view of the above, it is an object of the present invention to provide an optical fiber amplifier having a small variation in amplification characteristics by suppressing a jump of an oscillation wavelength of a pump light source and a variation in power. And

[0015]

Hereinafter, the principle of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the principle configuration of the first aspect of the present invention. FIG. 2 is a diagram showing the principle configuration of the invention described in claims 2 to 7. The control means 18 indicated by a broken line indicates the characteristic configuration of the invention described in claim 5, and the modulation means 19 indicated by the broken line indicates the characteristic configuration of the invention described in claim 6 and claim 7. I have.

(Claim 1 and Claim 2) In FIG. 1, according to the invention described in claim 1, an optical fiber 1 doped with a rare earth element and a plurality of excitation light sources 2- 1-2-n and the respective excitation light sources 2-1 through 2-n
And a plurality of first optical filters 3-1 to 3-n having different center wavelengths in the reflection wavelength band, and connected to and connected to the respective first optical filters 3-1 to 3-n. A plurality of second optical filters 4-1 to 4-n for transmitting the light of the central wavelength of the first optical filters 3-1 to 3-n and blocking light other than the light of the central wavelength; Two-light filter 4-1 to 4-n
And an optical multiplexer 5 for multiplexing each light emitted from the optical fiber into the optical fiber 1.

In FIG. 2, according to the second aspect of the invention, an optical fiber 10 doped with a rare earth element, a first excitation light source 11 and a second excitation light source 12 for exciting the optical fiber 10, The light emitted from the excitation light source 11 is incident, the first optical filter 13 in which the center wavelength of the reflection wavelength band is set to a predetermined first wavelength, and the light emitted from the second excitation light source 12 are incident, and the first A second optical filter 14 having a predetermined second wavelength different from the wavelength and having the center wavelength of the reflection wavelength band set to the second wavelength;
The third light filter 15 which receives the light emitted from the third light 3, transmits the first wavelength light, and blocks the second wavelength light.
And the light output from the second optical filter 14 is incident,
A fourth optical filter 16 that transmits light of the second wavelength and blocks light of the first wavelength, and multiplexes light emitted from the third optical filter 15 and light emitted from the fourth optical filter 16. An optical multiplexer 17 for causing light to enter the optical fiber 10.

The operation of the optical fiber amplifier having such a configuration will be described based on experimental results. FIG. 3 is a diagram showing the configuration of an experimental apparatus for examining the relationship between the wavelength difference between two pumping light sources and the frequency of occurrence by power fluctuation.

In FIG. 3, a laser beam emitted from a first LD 30, which is an excitation light source that oscillates a laser beam in a predetermined wavelength band, enters a first FBG 32 whose central wavelength of a reflection wavelength band is set to a wavelength λ1. The first FBG 32 is
The laser beam reflected by the first FBG 32 causes the first
It functions to fix the oscillation wavelength of the LD 30 to the wavelength λ1.

On the other hand, the laser light emitted from the second LD 31, which is an excitation light source for oscillating the laser light in a predetermined wavelength band, enters the second FBG 33 in which the center wavelength of the reflection wavelength band is set to the wavelength λ2. The second FBG 33 uses the second
The laser beam reflected by the FBG 33 causes the second LD 31
And has the function of fixing the oscillation wavelength of the light to the wavelength λ2. Then, the laser light emitted from the first FBG 32 and the laser light emitted from the second FBG 33 are multiplexed by the 2 × 2 coupler 34 and further branched. One of the laser beams branched by the 2 × 2 coupler 34 is a photodiode 3
5 and its power is measured. The other laser beam split by the 2 × 2 coupler 34 is terminated at the reflection end face R whose reflectivity can be adjusted arbitrarily.

In such an experimental apparatus, when the temperature of the experimental apparatus, the reflectance at the reflection end face R, and the vibration conditions are changed to provide a wavelength difference between the oscillation wavelengths of the two excitation light sources, An experiment was conducted to measure the amount of power fluctuation of the laser light emitted from the 2 × 2 coupler 34 when no wavelength difference was provided. Here, when the wavelength difference is provided, the wavelength λ1 is set to 978 nm and the wavelength λ2 is set to 98 nm.
It was set to 0 nm. When there is no wavelength difference, the wavelength λ
Both 1 and wavelength λ2 were set to 980 nm. Also,
As shown in FIG. 3, vibration was individually applied to four points of the optical fiber before and after the first FBG 32 and the optical fiber before and after the second FBG 33. In addition,
It is known that when vibration is applied to an optical fiber, the polarization state of laser light transmitted through the optical fiber changes.

Here, the reason why the temperature of the experimental apparatus, the reflectance at the reflection end face, and the vibration conditions were changed is to reproduce the use environment of the optical fiber amplifier. FIG. 4 is a diagram showing the relationship between the wavelength difference between the two excitation light sources and the frequency of occurrence according to the amount of power fluctuation, and is the result of the above-described experiment. The horizontal axis in FIG. 4 is the power fluctuation amount displayed in decibels (dB), and the vertical axis is the frequency displayed in percentage (%). The hatched bar graph shows the result when a wavelength difference is provided between two excitation light sources, and the oblique line bar graph shows the result when no wavelength difference is provided between the two excitation light sources. Each bar graph is a histogram in which the power fluctuation amount of the laser light emitted from the 2 × 2 coupler 34 is totaled for each power fluctuation amount when the above-described experimental conditions are changed.

FIG. 4 shows that when the wavelength difference is provided between the oscillation wavelengths of the two pumping light sources, the frequency of the range where the power fluctuation is small is higher, and conversely, the frequency of the range where the power fluctuation is large is lower. You can see that Therefore, from this experimental result, in order to reduce the power fluctuation amount of the laser light emitted from the 2 × 2 coupler 34, a wavelength difference may be provided between the oscillation wavelengths of the two pump light sources. Therefore, the center wavelength λ1 of the reflection wavelength band in the first FBG 32 and the center wavelength λ2 of the reflection wavelength band in the second FBG 33 may be set to different wavelengths.

On the other hand, the second LD 31 given to the first LD 30
In order to prevent the influence of the above, an optical filter that blocks the laser light of the wavelength λ2 of the second LD 31 and transmits the laser light of the wavelength λ1 of the first LD 30 may be provided between the first LD and the coupler 34. Also, the first LD3 given to the second LD31
0, the wavelength λ1 of the first LD 30
An optical filter may be provided between the second LD 31 and the coupler 34 for blocking the laser light of the second LD 31 and transmitting the laser light of the wavelength λ 1 of the second LD 31.

Therefore, according to the first aspect of the present invention, the first
The center wavelength of the reflection wavelength band in the optical filter 13 and the second wavelength
Since the center wavelength of the reflection wavelength band in the optical filter 14 is different from the center wavelength, power fluctuation of the laser light emitted from the optical multiplexer 17 can be reduced. Also,
The oscillation wavelength of the first excitation light source 11 can be fixed at a predetermined wavelength λ1 by the first optical filter 13. Therefore, if the wavelength .lambda.1 is set to a wavelength within the absorption wavelength band of the optical fiber, the absorption wavelength band of the optical fiber is not deviated. Similarly, the oscillation wavelength of the second excitation light source 12 can be fixed at the predetermined wavelength λ2 by the second optical filter 14. Therefore, if the wavelength .lambda.2 is set to a wavelength within the absorption wavelength band of the optical fiber, the absorption wavelength band of the optical fiber is not deviated.

Further, the oscillation wavelength λ 1 of the first pumping light source 11
And the oscillation wavelength λ2 of the second excitation light source 12
Since the third optical filter 15 that blocks the laser light is provided between the first excitation light source 11 and the optical multiplexer 17, it is possible to suppress the influence of the second excitation light source 12 on the first excitation light source 11. it can. Therefore, the first pumping light source 11 does not leave the absorption wavelength band of the optical fiber 10. A fourth optical filter 16 that transmits the laser light of the oscillation wavelength λ2 of the second excitation light source 12 and blocks the laser light of the oscillation wavelength λ1 of the first excitation light source 11 is provided between the second excitation light source 12 and the optical multiplexer 17. Therefore, the influence of the first excitation light source 11 on the second excitation light source 12 can be suppressed. Therefore, the second excitation light source 12 does not leave the absorption wavelength band of the optical fiber.

In order to suppress the influence of one pumping light source 11, 12 on the other pumping light source 12, 11, the wider the wavelength difference between the two pumping light sources 11, 12, the better. On the other hand, the absorption wavelength band of the optical fiber 10 is determined from the amplification characteristics of the optical fiber amplifier, and is a specific absorption wavelength band in order to realize desired amplification characteristics. For this reason, since the oscillation wavelength of each of the pump light sources 11 and 12 must exist within this specific absorption wavelength band, a wavelength difference cannot be provided beyond this specific absorption wavelength band.

FIG. 5 is a diagram showing the relationship between the oscillation wavelength of the first pump light source and the second pump light source and the absorption wavelength band of the optical fiber when single mode oscillation is performed. FIG. For these reasons, according to the first aspect of the present invention, since the laser light emitted from the optical multiplexer 17 can be stabilized, the amplification characteristics such as the gain characteristic and the noise figure can be stabilized.

(Claim 3) In FIG. 2, according to the invention of claim 3, in the optical fiber amplifier of claim 2, the center wavelength of the first pumping light source 11 is a reflection wavelength band in the first optical filter. And the center wavelength of the second pumping light source 12 substantially coincides with the second wavelength which is the center wavelength of the reflection wavelength band in the second optical filter.

The operation of the optical fiber amplifier having such a configuration will be described based on experimental results. Note that the experimental device is the experimental device shown in FIG. FIG. 6 is a diagram illustrating the relationship between the side mode suppression ratio and the difference between the center wavelength of the reflection wavelength band in the FBG and the center-of-gravity wavelength of the pump light source. The horizontal axis in FIG. 6 is the difference between the center wavelength of the FBG and the center of gravity wavelength of the pump light source expressed in nm, and the vertical axis is the side mode suppression ratio expressed in decibels (dB).

As shown in FIG. 6, the relationship of the side mode suppression ratio with respect to the difference between the reflection center wavelength of the FBG and the center of gravity wavelength of the pump light source is a straight line descending to the right. The smaller the difference from the wavelength, the larger the side mode suppression ratio. A large side mode suppression ratio means that the pump light source mainly oscillates at a predetermined oscillation wavelength.

FIG. 7 is a diagram showing the relationship between the amount of power fluctuation and the side mode suppression ratio. The horizontal axis in FIG. 7 is the side mode suppression ratio, and the vertical axis is the power fluctuation amount. Both axes are displayed in decibels (dB). As shown in FIG. 7, since the relationship of the power fluctuation amount with respect to the side mode suppression ratio is a straight line descending to the right, the larger the side mode suppression ratio, the smaller the power fluctuation amount.

Therefore, from FIG. 6 and FIG.
The smaller the difference between the reflection center wavelength and the center wavelength of the excitation light source, the smaller the power fluctuation. Therefore, according to the third aspect of the present invention, the center wavelength of the first pump light source 11 is made substantially coincident with the center wavelength of the reflection wavelength band of the first optical filter 13, and the center wavelength of the second pump light source 12 is set to the second wavelength. Optical filter 1
Since the center wavelength of the reflection wavelength band 4 is substantially equal to that of the reflection wavelength band 4, the power fluctuation amount of the laser light emitted from the optical multiplexer 17 can be reduced.

Therefore, according to the third aspect of the invention, amplification characteristics such as gain characteristics and noise figure can be stabilized. (Claim 4) In FIG. 2, in the invention according to claim 4, in the optical fiber amplifier according to claim 2, the first optical filter 13
The fourth optical filter 16 is a fiber grating filter.

Since FBGs are used as the first to fourth optical filters 13 to 16, it is possible to realize a narrow-band reflection wavelength characteristic. Therefore, the first optical filter 13 to the fourth optical filter 16 can satisfy desired requirements required for each optical filter with high accuracy. (Claim 5) In FIG. 2, according to the invention of claim 5, in the optical fiber amplifier of claim 2, the first
Control means 1 for controlling the center of gravity wavelength of the excitation light source 11 within the reflection band of the first optical filter 13 and controlling the center of gravity wavelength of the second excitation light source 12 within the reflection band of the third optical filter 15;
8 is further provided.

With this configuration, even if the center-of-gravity wavelengths of the first pumping light source 11 and the second pumping light source 12 are shifted due to temperature fluctuations or the like, the shift can be corrected by the control means 18. The power fluctuation amount of the laser light emitted from the wave device 17 can be reduced.

Therefore, according to the fifth aspect of the invention, it is possible to stabilize amplification characteristics such as a gain characteristic and a noise figure. (Claim 6 and Claim 7) In FIG. 2, according to the invention described in Claim 6, any one of Claims 2 to 5
In the optical fiber amplifier described in the paragraph, the first pump light source 1
A modulation means for modulating the first driving current and the driving current for the second excitation light source 12 is further provided.

In FIG. 2, according to the invention described in claim 7, in the optical fiber amplifier described in claim 6, the modulation means 19 controls the intensity fluctuation of the light output from the optical multiplexer 17 to be minimum. The phase of the driving current of the first excitation light source 11 to be modulated and the phase of the driving current of the second excitation light source 12 to be modulated are controlled. In general, it is known that when a modulation signal is superimposed on a driving current of a semiconductor laser, a center-of-gravity wavelength is stabilized.

Therefore, with this configuration,
Since the center-of-gravity wavelength can be stabilized, the optical multiplexer 17 can be used.
Power fluctuation amount of the laser light emitted from the light source can be reduced. Therefore, according to the inventions described in claims 6 and 7, amplification characteristics such as a gain characteristic and a noise figure can be stabilized.

[0041]

Embodiments of the present invention will be described below with reference to the drawings. (Structure of the First Embodiment)
FIG. 9 is an embodiment of an optical fiber amplifier corresponding to the invention described in claim 5.

FIG. 8 is a diagram showing the configuration of the optical fiber amplifier according to the first embodiment. In FIG. 8, a laser beam emitted from the first LD 40 that oscillates in a multi-mode in a predetermined wavelength band enters a first FBG 42 in which the center wavelength of the reflection wavelength band is set to the wavelength λ1. This first FBG4
Reference numeral 2 denotes a low-reflectance optical filter that transmits most of the laser light from the first LD 40 and reflects part of the laser light. The reflected laser light returns to the first LD 40, and the first LD 4
It functions to fix the oscillation wavelength of 0 to the wavelength λ1. That is, when there is no first FBG 42, the first LD oscillates in multiple modes, but the first FBG 42 causes single mode oscillation.

The center-of-gravity wavelength of the first LD 40 is the first F
The BG 42 is designed to have a center wavelength .lambda.1 of the reflection wavelength band. The laser beam transmitted through the first FBG 42 enters the third FBG 44. This third FBG44
Is an optical filter having a high reflectance, and the center wavelength of the reflection wavelength band is set to a wavelength λ2 which is the center wavelength of the reflection wavelength band in the second FBG 43 described later. And
The wavelength λ1 is set outside the reflection wavelength band of the third FBG 44. Therefore, the third FBG 44 transmits the laser beam having the wavelength λ1 and blocks the laser beam having the wavelength λ2.

On the other hand, the laser light emitted from the second LD 41 that oscillates in a multi-mode in a predetermined wavelength band enters the second FBG 43 in which the center wavelength of the reflection wavelength band is set to the wavelength λ2. The second FBG 43 is a low-reflectance optical filter that transmits most of the laser light from the second LD 41 and reflects part of the laser light. The reflected laser light is the second L
Returning to D41, the oscillation wavelength of the second LD 41 is fixed to the wavelength λ2 by the same operation as the operation of the first FBG 42 described above.

The wavelength of the center of gravity of the second LD 41 is the second F
The BG 43 is designed to have a center wavelength .lambda.2 of the reflection wavelength band. The wavelength λ2 is set to a wavelength different from the wavelength λ1, and these wavelengths λ1 and λ2 are set within an absorption wavelength band of an EDF described later. Also, the wavelength λ1
The wavelength difference between the wavelength .lambda.2 and the wavelength .lambda.2 is determined by amplification characteristics such as gain characteristics and noise figure required for the optical fiber amplifier in the first embodiment.

The first LD 40 and the second LD 41
The temperature and the drive current are controlled by the control CPU 59. The control CPU 59 refers to the memory 58 that stores the relationship between the temperature and the center-of-gravity wavelength, the relationship between the drive current and the center-of-gravity wavelength, and the relationship between the aging degradation and the center-of-gravity wavelength for each of the first LD 40 and the second LD 41.
The processing described below is performed.

The laser beam transmitted through the second FBG 43 enters the fourth FBG 45. The fourth FBG 45 is an optical filter having a high reflectance, and the center wavelength of the reflection wavelength band is set to the wavelength λ1 which is the center wavelength of the reflection wavelength band in the first FBG 42. The wavelength λ2 is the fourth
It is set outside the reflection wavelength band of the FBG 45. Therefore, the fourth FBG 45 transmits the laser beam having the wavelength λ2 and blocks the laser beam having the wavelength λ1.

The laser light emitted from the third FBG 44 and the laser light emitted from the fourth FBG 45 are 2 × 2
The light enters the coupler 46. The 2 × 2 coupler 46 combines these laser lights, and then branches the combined laser light into two and emits them. One of the branched laser lights is 2 × 1
The light enters one port of the coupler 50. And this 2
In the × 1 coupler 50, the light is multiplexed with the input light incident from the other port and emitted.

The laser light multiplexed by the 2 × 1 coupler 50 enters the EDF 51 for amplifying the light. This EDF5
1, the first LD 40 of the combined laser light
The laser light from the second LD 41 excites electrons in the EDF 51 to form a population inversion. Since the input light also enters in a state where the population inversion is formed, the input light of the multiplexed laser light is guided by the input light to generate stimulated radiation, and the input light is amplified.

The laser light emitted from the EDF 51 is emitted as output light via the ISO 52 that transmits light only in one direction. On the other hand, the other laser light branched by the 2 × 2 coupler 46 is multiplexed with the input light by the 2 × 1 coupler 55 as described above, and excites the EFD 56 to amplify the input light. The laser light emitted from the FDF 56 is
And is emitted as output light through

(Correspondence between the present invention and the first embodiment)
Hereinafter, the correspondence between the present invention and the first embodiment will be described. Regarding the correspondence between the invention described in claim 1 and the first embodiment, the optical fiber corresponds to the EDF 51,
The plurality of excitation light sources correspond to the first LD 40 and the second LD 41. The plurality of first optical filters are the first FBG 42,
In correspondence with the second FBG 43, the plurality of second optical filters are the third optical filters.
The optical multiplexer corresponds to the coupler 46, and corresponds to the FBG 44 and the fourth FBG 45.

Regarding the correspondence between the invention described in claims 2 to 4 and the first embodiment, the optical fiber is
The first excitation light source corresponds to the first LD 40, and the second excitation light source corresponds to the second LD 41. And
The first optical filter corresponds to the first FBG 42, and the second optical filter corresponds to the second FBG 43. Further, the third optical filter corresponds to the third FBG 44, and the fourth optical filter corresponds to the fourth FBG44.
The optical multiplexer corresponds to the FBG 45 and the optical multiplexer corresponds to the coupler 46.

The correspondence between the invention described in claim 5 and the first embodiment is that the control means includes a memory 58 and a control CP.
Corresponds to U59. FIG. 9 is a diagram showing the relationship between the center-of-gravity wavelength of the LD and the center wavelength of the reflection wavelength band of the FBG. (A) is when the center wavelength matches the center wavelength, (b) is when the center wavelength is shorter than the center wavelength,
(C) shows the case where the center-of-gravity wavelength is longer than the center wavelength. In each of (a), (b) and (c),
The upper curve is the LD spectrum when multi-mode oscillation occurs, and the lower curve is the reflection wavelength characteristic of the FBG. (D) shows a case where the LD changes from multi-mode oscillation to single-mode oscillation by the FBG. Note that in FIG. 9, the relationship between the center-of-gravity wavelength of the LD and the center wavelength of the reflection wavelength band of the FBG is the first LD 40 and the second LD 4.
1 is similar to that of FIG.

The operation and effect of the first embodiment and the processing in the control CPU 59 will be described below with reference to FIG. When the optical communication system using the optical fiber amplifier starts operation, the control CPU 59 starts the program stored in the memory 58. The control CPU 59 transmits a signal for setting the drive current and the temperature to the first LD 40 so that the center-of-gravity wavelength of the first LD 40 becomes the center wavelength λ1 of the reflection wavelength band of the first FBG 42 with reference to the memory 58. Then, the control CPU 59 refers to the memory 58 and transmits a signal for setting the drive current and the temperature to the first LD 41 so that the center of gravity wavelength of the second LD 41 becomes the center wavelength λ2 of the reflection wavelength band in the second FBG 43.

The first LD 40 and the second LD 41 which have received these signals start multi-mode transmission as shown in FIG. 9A, and the first LD 40 transmits the signal to the first FBG 42 and the second L
D41 emits a laser beam to the second FBG43. The first FBG 42 reflects a laser beam having a wavelength λ1 of a few percent with respect to the incident laser beam, and
Return to By the reflected laser beam having the wavelength λ1, the first LD 40 causes the first FBG 42 as shown in FIG.
A single-mode oscillation occurs at the wavelength λ1, which is the center wavelength of the reflection wavelength band at.

The second FBG 43 reflects a laser beam having a wavelength λ 2 which is several percent of the incident laser beam, and returns the laser beam to the second LD 41. Due to the reflected laser beam having the wavelength λ2, the second LD 41 oscillates in a single mode at the wavelength λ2 which is the center wavelength of the reflection wavelength band in the second FBG 43 as shown in FIG.

The first L which starts single mode oscillation in this way
The laser light of D40 includes the first FBG 42 and the third FBG
The light enters the 2 × 2 coupler 46 via 44. The laser light of the second LD 41 that has started single mode oscillation is
The light enters the 2 × 2 coupler 46 via the FBG 43 and the fourth FBG 45. The laser light multiplexed by the 2 × 2 coupler 46 enters the EDF 51 via the 2 × 1 coupler 50,
The input light is amplified by exciting the EDF 51.

The control CPU 59 adjusts the temperature or the drive current of the first LD 40 and the second LD 41 or the temperature and the drive current of the first LD 40 and the second LD 41 with reference to the memory 58 at regular intervals. This is to correct the shift because the center-of-gravity wavelengths of the first LD 40 and the second LD 41 shift as shown in FIGS. 9B and 9C due to deterioration with time, temperature fluctuation, noise, and the like.

The temperature control is performed by a first L not shown in FIG.
This is performed using a Peltier device that is in contact with D40 and the second LD 41. In such an optical fiber amplifier, the first FBG4
2, the center wavelength .lambda.1 of the reflection wavelength band and the second FBG4
Since the center wavelength .lambda.2 of the reflection wavelength band in 3 is different from the wavelength, the power fluctuation of the laser light emitted from the 2.times.2 coupler 46 can be reduced.

Further, the oscillation wavelength of the first LD can be fixed to the wavelength λ 1 within the absorption wavelength band of the EDFs 51 and 56 by the first FBG 42. Similarly, the second FBG4
3, the oscillation wavelength of the second LD 41 is increased by the EDFs 51 and 56.
Can be fixed to the wavelength λ2 which is within the absorption wavelength band. Therefore, the laser light emitted from the 2 × 2 coupler 46 can be reliably maintained within the absorption wavelength band of the EDFs 51 and 56, and the power fluctuation supplied to the EDFs 51 and 56 can be reduced.

The third FBG 44, which transmits the laser light of the oscillation wavelength λ1 of the first LD 40 and cuts off the laser light of the oscillation wavelength λ2 of the second LD 41, is connected to the first LD 40 and the coupler 4.
6 is provided. For this reason, the laser light of wavelength λ2 emitted from the second LD 41 is
It is possible to prevent the light from being reflected at each joint of the optical components such as the one coupler 50 and 55 and the EDFs 51 and 56 and entering the first LD 40. Therefore, the influence of the second LD 41 on the first LD 40 can be suppressed. Therefore, the oscillation wavelength of the first LD 40 does not deviate from the wavelength λ1. Further, a fourth FBG 45 that transmits the laser light of the oscillation wavelength λ2 of the second LD 41 and blocks the laser light of the oscillation wavelength λ1 of the first LD 40 is connected to the second LD 41 and the coupler 46.
And is provided between them. Therefore, the laser light of wavelength λ1 emitted from the first LD 40 is
It is possible to prevent the light from being reflected at each joint of the optical components such as the couplers 50 and 55 and the EDFs 51 and 56 and entering the second LD 41. Therefore, the influence of the first LD 40 on the second LD 41 can be suppressed. Therefore, the oscillation wavelength of the second LD 41 does not deviate from the wavelength λ2.

Further, the center-of-gravity wavelength of the first LD 40 is
The FBG 42 is designed to have a center wavelength .lambda.1 of the reflection wavelength band. Therefore, the power fluctuation of the first LD 40 can be reduced. The center wavelength of the second LD 41 is the center wavelength λ of the reflection wavelength band in the second FBG 43.
Designed to be 2. Therefore, the power fluctuation of the first LD 40 can be reduced.

Even if the center-of-gravity wavelength fluctuates due to temperature fluctuations and aging during operation of the optical fiber amplifier, the center-of-gravity wavelength of the first LD 40 is set to the center wavelength λ 1 of the reflection wavelength band in the first FBG 42 and to the second LD 4.
The center-of-gravity wavelength of 1 is controlled to be the center wavelength λ2 of the reflection wavelength band in the second FBG 43. Therefore, the first
Power fluctuations of the LD 40 and the second LD 41 can be reduced.

For these reasons, in the optical fiber amplifier according to the first embodiment, the laser light emitted from the coupler 46 can be stabilized, so that the amplification characteristics such as the gain characteristic and the noise figure can be stabilized. it can. Next, another embodiment will be described. (Structure of the Second Embodiment)
An optical fiber amplifier according to an embodiment of the present invention.

FIG. 10 is a diagram showing a configuration of an optical fiber amplifier according to the second embodiment. A feature of the optical fiber amplifier according to the second embodiment is that a small-amplitude modulation signal is superimposed on the drive current of each of the first LD 40 and the second LD 41 with respect to the optical amplifier according to the first embodiment. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, are shown in FIG. 10, and the description thereof is omitted.

In FIG. 10, the laser light emitted from the first LD 40 enters the first FBG 42,
The laser beam transmitted through the BG 42 enters the third FBG 44. On the other hand, the laser light emitted from the second LD 41
The laser beam that has entered the second FBG 43 and transmitted through the second FBG 43 enters the fourth FBG 45. Here, the temperature and the drive current of the first LD 40 and the second LD 41 are controlled by the control CPU 64. The control CPU 64 refers to the memory 58 that stores the relationship between the temperature and the center-of-gravity wavelength and the relationship between the drive current and the center-of-gravity wavelength for each of the first LD 40 and the second LD 41.
The temperature and the drive current are controlled, and the phase of the drive current is controlled based on the output of an analog / digital converter (hereinafter abbreviated as “A / D”) 63 described later.

The laser light emitted from the third FBG 44 and the laser light emitted from the fourth FBG 45 are 2 × 2
The light enters the coupler 46 and is branched after being multiplexed. One part of the branched laser light is branched by the beam splitter 60, and most of the remaining part enters the 2 × 1 coupler 50,
After being multiplexed with the input light, the light enters the EDF 51. EDF
The laser light emitted from 51 is emitted as output light via ISO52.

On the other hand, the other laser light branched by the 2 × 2 coupler 46 is multiplexed with the input light by the 2 × 1 coupler 55 and
The DF 56 is excited to amplify the input light. The laser light emitted from the FDF 56 is emitted as output light via the ISO 57. The laser light split by the beam splitter 60 is incident on a photodiode (PhotoDiode, hereinafter abbreviated as “PD”) 61 that outputs a current according to the power of the input light, and is photoelectrically converted by the PD 61. The output of the PD 61 is input to the A / D 63 after being amplified by the amplifier 62 that amplifies the output at a predetermined magnification.

The control CPU 64 sets A at regular time intervals.
/ D63, take in the current 2 × 2 coupler 4
A measurement value of the power in the laser light emitted from 6 is obtained. The control CPU 64 calculates a difference between the measured value and a predetermined target value, and determines whether the calculation result is within an allowable value. The control CPU 64 outputs a signal to maintain the current phase to the modulator 65 when the value is within the allowable value. On the other hand, when the value is not within the allowable value, the control CPU 64 changes the phase at a predetermined interval from the current phase within a certain range, thereby searching for an optimal phase. Then, the control CPU 64 outputs a signal having a new phase to the modulator 65.

(Correspondence between the present invention and the second embodiment)
Hereinafter, the correspondence between the present invention and the second embodiment will be described. The correspondence between the inventions described in claims 1 to 4 and the second embodiment is the same as in the first embodiment, and a description thereof will be omitted. In the correspondence between the invention described in claim 5 and the second embodiment, the control means corresponds to the memory 58 and the control CPU 64.

The correspondence between the invention described in claim 6 and the second embodiment is that the modulation means corresponds to the modulator 65. The correspondence between the invention according to claim 7 and the second embodiment is that the modulating means includes a beam splitter 60, a PD 61, and an amplifier 6
2, an A / D 63, a control CPU 64, and a modulator 65.

(Operation and Effect of Second Embodiment) In such an optical fiber amplifier, in addition to the operation and effect of the first embodiment, the driving current of the first LD 40 and the second LD 4
Since the first drive current is modulated, the center-of-gravity wavelength of the first LD 40 and the center-of-gravity wavelength of the second LD 41 can be stabilized. Therefore, power fluctuation of the laser light emitted from the 2 × 2 coupler 46 can be reduced.

Further, since the phase of the drive current of the first LD 40 and the phase of the drive current of the second LD 41 are controlled so as to reduce the power fluctuation, the power fluctuation of the laser light emitted from the 2 × 2 coupler 46 is controlled. Can be smaller. For these reasons, in the optical fiber amplifier according to the second embodiment, the laser light emitted from the 2 × 2 coupler 46 can be stabilized, so that the amplification characteristics such as the gain characteristic and the noise figure can be stabilized. it can.

Although the beam splitter 60 is inserted between the 2 × 2 coupler 46 and the 2 × 1 coupler 50 in the second embodiment, the wavelength band of the input light to be amplified and the first L
When the wavelength band of the pumping light from the D40 and the second LD 41 is sufficiently separated, the beam splitter 60 is
It may be inserted between F51 and ISO52. In this case, the PD 61 receives the leakage excitation light that has not excited electrons in the EDF 51 among the excitation light from the first LD 40 and the second LD 41. In this case, since the beam splitter 60 is connected after the EDF 51, the excitation light rate of the EDF 51 does not decrease.

In the first and second embodiments, the EDF is used, but the present invention is not limited to this. Optical fibers doped with other rare earths may be used. For example, an optical fiber doped with neodymium (element symbol: Nd), praseodymium (element symbol: Pr), or the like that amplifies the 1.3 μm band may be used.

[0076]

According to the first to seventh aspects of the present invention, in the optical fiber amplifier in which the pumping light source has a redundant configuration, the pumping light source has different oscillation wavelengths from each other, and another pumping light source is provided by an optical filter. Since the light from the light source is blocked, power fluctuation of the light incident on the rare earth element-doped optical filter can be reduced. For this reason, fluctuations in the amplification characteristics of the optical fiber amplifier can be reduced.

In addition, in the third and fifth aspects of the present invention, the center wavelength of the pump light source is made to substantially coincide with the center wavelength of the reflection wavelength band in the optical filter connected to the pump light source. Power fluctuation of light incident on the rare earth element-doped optical filter can be further reduced. For this reason, fluctuations in the amplification characteristics of the optical fiber amplifier can be further reduced.

In addition, in the inventions according to the sixth and seventh aspects, since the drive current of the excitation light source is modulated, the power fluctuation of the light incident on the rare earth element-doped optical filter can be further reduced. Can be. For this reason, fluctuations in the amplification characteristics of the optical fiber amplifier can be further reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a principle configuration of the invention described in claim 1;

FIG. 2 is a diagram showing a principle configuration of the invention described in claims 2 to 7;

FIG. 3 is a configuration diagram of an experimental apparatus for examining a relationship between a wavelength difference between two pump light sources and an occurrence frequency for each power fluctuation amount.

FIG. 4 is a diagram illustrating a relationship between a wavelength difference between two excitation light sources and an occurrence frequency for each power fluctuation amount.

FIG. 5 is a diagram illustrating the relationship between the oscillation wavelengths of the first and second pump light sources and the absorption wavelength band of the optical fiber when single mode oscillation is performed.

FIG. 6 is a diagram illustrating a relationship between a side mode suppression ratio and a difference between a center wavelength of a reflection wavelength band in the FBG and a center-of-gravity wavelength of an excitation light source.

FIG. 7 is a diagram illustrating a relationship between a power fluctuation amount and a side mode suppression ratio.

FIG. 8 is a diagram illustrating a configuration of an optical fiber amplifier according to the first embodiment.

FIG. 9 is a diagram showing the relationship between the center-of-gravity wavelength of the LD and the center wavelength of the reflection wavelength band of the FBG.

FIG. 10 is a diagram illustrating a configuration of an optical fiber amplifier according to a second embodiment.

FIG. 11 is a diagram showing a configuration of an optical fiber amplifier in which a conventional pump light source has a redundant configuration.

FIG. 12 is a diagram showing a mode hopping state of a semiconductor laser.

[Explanation of symbols]

 Reference Signs List 1 optical fiber 2 excitation light source 3 first optical filter 4 second optical filter 5 optical multiplexer 10 optical fiber 11 first excitation light source 12 second excitation light source 13 first optical filter 14 second optical filter 15 third optical filter 16th 4 optical filter 17 optical multiplexer 18 control means 19 modulation means 40 first LD 41 second LD 42 first FBG 43 second FBG 44 third FBG 45 fourth FBG 46 coupler 51, 56 EDF 58 memory 59 control CPU 64 control CPU 65 modulator

Claims (7)

[Claims] An optical fiber doped with a rare earth element; a plurality of excitation light sources for exciting the optical fiber; and a plurality of excitation light sources connected to the respective excitation light sources and having different center wavelengths of reflection wavelength bands. A first optical filter, a plurality of second optical filters connected to each of the first optical filters and transmitting light of the central wavelength of the connected first optical filter and blocking light other than the central wavelength; An optical fiber amplifier, comprising: an optical multiplexer that combines light emitted from each of the second optical filters into the optical fiber. 2. An optical fiber doped with a rare earth element, a first excitation light source and a second excitation light source for exciting the optical fiber, and light emitted from the first excitation light source is incident on the optical fiber. A first optical filter in which a center wavelength of a reflection wavelength band is set to one wavelength, light emitted from the second excitation light source is incident, and a predetermined second wavelength different from the first wavelength and A second optical filter having a wavelength set to the center wavelength of a reflection wavelength band; and light emitted from the first optical filter is incident, transmits the first wavelength light, and blocks the second wavelength light. A third optical filter, a fourth optical filter that receives light output from the second optical filter, transmits the second wavelength light, and blocks the first wavelength light, and the third optical filter The light emitted from the fourth An optical fiber amplifier comprising: an optical multiplexer that causes light, which is obtained by multiplexing light emitted from an optical filter, to enter the optical fiber. 3. The optical fiber amplifier according to claim 2, wherein a center-of-gravity wavelength of the first pumping light source substantially coincides with the first wavelength which is a center wavelength of a reflection wavelength band in the first optical filter. An optical fiber amplifier, wherein a center-of-gravity wavelength of the second pump light source substantially coincides with the second wavelength which is a center wavelength of a reflection wavelength band in the second optical filter. 4. The optical fiber amplifier according to claim 2, wherein the first to fourth optical filters are fiber grating filters. 5. The optical fiber amplifier according to claim 2, wherein a center-of-gravity wavelength of the first pumping light source is controlled within a reflection wavelength band of the first optical filter, and a center-of-gravity wavelength of the second pumping light source is set to the second center. An optical fiber amplifier, further comprising control means for controlling the wavelength within a reflection wavelength band of the three-optical filter. 6. The optical fiber amplifier according to claim 2, wherein a modulator modulates a drive current of the first pump light source and modulates a drive current of the second pump light source. An optical fiber amplifier, further comprising: 7. The optical fiber amplifier according to claim 6, wherein the modulating means drives the first pumping light source that is modulated so that the intensity fluctuation of the light output from the optical multiplexer is minimized. An optical fiber amplifier for controlling a phase of a current and a phase of a drive current of the second pump light source to be modulated.