JP2002303895A - Raman amplification method and its system - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To realize Raman amplification with high efficiency and suppressed wavelength dependence of gain. SOLUTION: In a Raman amplification system for amplifying a plurality of wavelength-multiplexed signal lights by a plurality of pump lights having different wavelengths, modulation means 405 and 406 for modulating each pump light so that a modulation phase is different are provided. The amplification gain of each signal light was made equal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Raman amplification method and a system thereof, and more particularly, to a Raman amplification method and a system thereof having a wide band, high efficiency, and suppressed wavelength dependence of gain.

[0002]

2. Description of the Related Art A Raman amplification system that amplifies signal light by applying the stimulated Raman scattering phenomenon is known. In the Raman amplification system, the amplification band is the wavelength of the pump light,
It is determined by the material of the optical fiber transmission line that is the amplification medium. For example, in a general quartz glass optical fiber transmission line for communication, based on the optical frequency of excitation light, -12 THz
An amplification band is formed at 0 to -30 THz with the vicinity as a peak. Assuming that the wavelength of the excitation light is 1430 nm, the peak is around 1520 nm and the amplification band is around 1430 to 1670 nm.

FIG. 1 is a diagram showing a configuration of a conventional Raman amplification system. In order to expand the amplification band or to obtain a flat gain characteristic in the amplification band, amplification using multi-wavelength pump light is performed. FIG. 1A shows a system configuration in which amplification is performed using two wavelengths of pump light. The Raman amplification system 100 includes an optical fiber transmission line 121,
The pumping light signal optical multiplexer 101 inserted in the pumping light 122 and the pumping light multiplexer 102 connected to the pumping light signal optical multiplexer 101
And the excitation light source 10 connected to the excitation light multiplexer 102
3, 104.

[0004] With such a configuration, the transmitters 131a to 131a to
The signal light output from 131c is transmitted through the optical fiber transmission line 1
21 and is amplified by the pumping light input from the pumping light signal multiplexer 101 and propagates through the optical fiber transmission line 122. FIG. 1B shows a temporal change in the power of the pump light output from the pump light source. The pumping light source 103 always outputs the pumping light of the wavelength λ1 and the pumping light source 104 outputs the pumping light of the wavelength λ2 at a constant optical power.

[0005]

FIG. 2 is a diagram showing a state of amplification in a conventional Raman amplification system. FIG. 2A shows the powers of the signal light and the pump light at the transmission line position L of the optical fiber transmission lines 121 and 122. The transmission line position of the pumping light signal optical multiplexer 101 is L = 0, and the transmission line positions of the transmitters 131a to 131c are L = Ln.

FIG. 2B shows a gain frequency characteristic of the pump light. The gain frequency characteristic at the transmission line position L = 0 is shown. When two different pumping lights having optical frequencies separated by about 10 THz are used in the optical fiber transmission lines 121 and 122 using quartz glass for general communication, the amplification band of the pumping light (λ1) on the short wavelength side is used. Overlap with the excitation light (λ2) on the long wavelength side. Therefore, the pump light (λ
2) pumps the pumping light (λ1) on the short wavelength side, so that there is a problem that the wavelength dependence of the accumulated gain generated until the pumping light is attenuated cannot be suppressed.

FIG. 2C shows the accumulated gain up to the transmission line position L = Le where the pump light is attenuated. Since the accumulated gain is not flat, the level of the output signal light is not constant. Therefore,
As shown in FIG. 2A, the excitation light (λ1) on the short wavelength side
And the pump light (λ2) on the long wavelength side
The optical power on the optical fiber transmission line 121 is different.
Is amplified at the transmission line position L = 0 to an optical power that differs depending on the wavelength.

FIG. 3 is a diagram showing gain frequency characteristics in a conventional Raman amplification system. In consideration of the amplification of the pumping light (λ1) on the short wavelength side, the power of the pumping light output (λ2) on the long wavelength side may be previously lowered and combined with the pumping light output (λ1) on the short wavelength side. Is being done. However, a reduction in efficiency is inevitable, and there is a problem that the design becomes difficult when the number of wavelengths of the excitation light increases.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a broadband, high-efficiency,
It is an object of the present invention to provide a Raman amplification method and its system in which the wavelength dependence of the gain is suppressed.

[0010]

According to the present invention, in order to achieve the above object, the present invention is directed to a method for converting a plurality of signal lights wavelength-multiplexed by a plurality of pump lights having different wavelengths. In the Raman amplification method for amplifying, each of the pump lights is modulated so as to have a different modulation phase.

According to a second aspect of the present invention, in the first aspect, an optical frequency interval between the two pumping lights having different wavelengths is:
It is characterized in that the difference between each optical frequency of the pump light and each optical frequency at which the amplification gain is peaked by each pump light is smaller.

According to a third aspect of the present invention, in the first or second aspect, the effective length of the optical fiber amplifying medium contributing to the amplification of the signal light is at least 10 times the modulation wavelength of the pump light. Features.

According to a fourth aspect of the present invention, in a Raman amplification system for amplifying a plurality of wavelength-multiplexed signal lights by a plurality of pump lights having different wavelengths, each of the pump lights is modulated so as to have a different modulation phase. And a modulating means.

According to a fifth aspect of the present invention, in the fourth aspect, each of the excitation lights is output from a semiconductor laser,
The modulator modulates an applied current of the semiconductor laser.

According to a sixth aspect of the present invention, there is provided a Raman amplification system for amplifying a plurality of wavelength-division multiplexed signal lights with a plurality of pump lights having different wavelengths, a partial modulation means for partially modulating an applied current of a semiconductor laser. External resonance means for oscillating the output of the semiconductor laser partially modulated by the partial modulation means at the wavelength of each of the excitation lights, so that the modulation phases of the respective excitation lights are different. It is characterized by.

According to a seventh aspect of the present invention, the external resonance means according to the sixth aspect includes a fiber grating for reflecting a selected wavelength and an optical isolator for removing return light from an optical fiber. It is characterized by.

According to an eighth aspect of the present invention, in any one of the fourth to seventh aspects, an optical frequency interval between the two pumping lights having different wavelengths is different from each optical frequency of the pumping light and each of the pumping lights. It is characterized in that the gain is smaller than the difference between each optical frequency at which the amplification gain peaks due to light.

According to a ninth aspect of the present invention, in any one of the fourth to eighth aspects, the effective length of the optical fiber amplification medium contributing to the amplification of the signal light is at least 10 times the modulation wavelength of the pump light. There is a feature.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 4 is a diagram showing the configuration of the Raman amplification system according to the first embodiment of the present invention. FIG.
(A) shows a system configuration when amplification is performed using two wavelengths of pump light. The Raman amplification system 400 includes a pumping light signal multiplexer 401 inserted into the optical fiber transmission lines 421 and 422, a pumping light multiplexer 402 connected to the pumping light signal multiplexer 401, and a pumping light multiplexer 402. The excitation light sources 403 and 404 are connected. The excitation light source 403 is connected to the output of the control signal source 406 via the phase adjuster 405, and the excitation light source 404 is directly connected to the output of the control signal source 406.

With this configuration, the transmitters 431a to 431a to
The signal light output from the optical fiber transmission line 431c is
21 and is amplified by the pumping light input from the pumping light signal multiplexer 401 and propagates through the optical fiber transmission line 422. FIG. 4B shows a temporal change in the power of the pump light output from the pump light source. The output of the control signal source 406 has a fixed modulation period, the pump light source 403 outputs pump light of wavelength λ1 alternately, and the pump light source 404 alternately outputs pump light of wavelength λ2 at a constant optical power. I have.

Various means have been proposed for modulating the excitation light. A semiconductor laser is used as the excitation light source.
By directly modulating the applied current of the semiconductor laser, the excitation light can be modulated easily and without any loss in the excitation light.

FIG. 5 is a diagram showing a first example of amplification in the Raman amplification system according to the first embodiment of the present invention. FIG. 5A shows optical fiber transmission lines 421 and 422.
2 shows the power of the signal light and the pump light at the transmission line position L of FIG. The transmission path position of the pumping light signal optical multiplexer 401 is L = 0, and the transmission path positions of the transmitters 431a to 431c are L = Ln.
And

FIG. 5B shows a gain frequency characteristic by the pump light. The gain frequency characteristic at the transmission line position L = 0 is shown. At the transmission line position L = 0, the gain frequency characteristics are the same as those shown in FIG.

FIG. 5C shows the accumulated gain up to the transmission line position L = Le where the pump light is attenuated. By modulating the short-wavelength-side pumping light (λ1) and the long-wavelength-side pumping light (λ2) so that the modulation phases are different from each other, the short-wavelength-side pumping light (λ1) forms a Raman amplification band. Amplification of the included pump light (λ2) on the long wavelength side can be suppressed. Therefore, the accumulated gain has a flat characteristic, and the levels of the plurality of output signal lights are also constant.

As shown in FIG. 5A, the pumping light (λ1) on the short wavelength side and the pumping light (λ2) on the long wavelength side have the same optical power on the optical fiber transmission line 421. Thus, the signal light propagating through the optical fiber transmission line 421 is transmitted at the transmission line position L
At = 0, the light is amplified to the same optical power depending on the wavelength.

In this way, there is no need to lower the power of the long-wavelength-side pumping light output (λ2) from that of the short-wavelength-side pumping light output (λ1) and combine them. A wideband Raman amplification system with reduced dependence can be realized.

As described above, in a general quartz glass optical fiber transmission line for communication, an amplification band is formed with a peak around -12 THz based on the optical frequency of the excitation light. In the present embodiment, by setting the optical frequency interval between the two wavelengths to 12 THz or less, even when the number of wavelengths of the pump light is increased, it is possible to effectively suppress the amplification between the pump lights while avoiding design complexity. be able to.

FIG. 6 is a diagram showing a second example of amplification in the Raman amplification system according to the first embodiment of the present invention. FIG. 6A shows optical fiber transmission lines 421 and 422.
2 shows the power of the signal light and the pump light at the transmission line position L of FIG. The transmission path position of the pumping light signal optical multiplexer 401 is L = 0, and the transmission path positions of the transmitters 431a to 431c are L = Ln.
And FIG. 6B shows the transmission line position L by the pump light.
FIG. 6C shows the gain frequency characteristic at = 0, and FIG. 6C shows the accumulated gain up to the transmission line position L = Le at which the pump light is attenuated.

As shown in FIG. 6A, the transmitter 431
When the powers of the signal lights at the optical fiber input ends a to 431c are different, as shown in FIG. 6B, the short-wavelength-side pump light (λ1) and the long-wavelength-side pump light (λ2) are used. Is adjusted, the signal light transmitted through the optical fiber transmission line 421 is amplified to the same optical power depending on the wavelength at the transmission line position L = 0.

Next, the optical fiber transmission line 421 will be described. In the Raman amplification system, the optical fiber transmission line 421 connected to the pumping light signal optical multiplexer 401 includes:
An optical fiber amplification medium that effectively contributes to Raman amplification is included. The effective length of the optical fiber amplification medium, that is, the transmission line position L = 0 to Le, is set as described below in relation to the pulse modulation of the pump light.

For example, an excitation light pulse-modulated at 50 kHz (a 10 μs pulse is generated at a period of 20 μs) in an optical fiber amplification medium having an effective length of 1 km that contributes to Raman amplification.
Is assumed. The time required for light to pass through an optical fiber of 1 km is about 5 μs, so that an optical fiber amplifying medium of 1 km can occupy only 0.25 pumping light of the modulation period. That is, when the rear end of the pumping light pulse passes through the effective length of the optical fiber amplifying medium, the next pumping light pulse enters from the entrance of the optical fiber amplifying medium after 5 μs. Since it is not present in the optical fiber amplification medium, the amplification of the signal light is interrupted.

If the modulation period of the pulse modulation is set to 200 kHz or more, which is four times, the effective length of the optical fiber amplifying medium is
It can be occupied by pump light having a modulation period of one cycle or more, so that at least signal light is not interrupted. Further, when the modulation period is set to 2 MHz or more, which is 40 times, in the effective length of the optical fiber amplifying medium, it can be occupied by pump light having a modulation period of 10 periods or more. It can be suppressed to a practically negligible level.

FIG. 7 is a diagram showing a configuration of a Raman amplification system according to the second embodiment of the present invention. FIG.
(A) shows a system configuration when amplification is performed using two wavelengths of pump light. The Raman amplification system 600 includes a pumping optical signal optical multiplexer 601 inserted into the optical fiber transmission lines 421 and 422, an optical isolator 602 connected to the pumping optical signal optical multiplexer 601, and a fiber connected to the optical isolator 602. Excitation light source 6 which is composed of grating sections 607 and 608 and is controlled by control signal source 606
03 is connected to the fiber grating units 607 and 608.

With such a configuration, the transmitters 431a to 431a to
The signal light output from the optical fiber transmission line 431c is
21 and is amplified by the pumping light input from the pumping light signal multiplexer 601 and propagates through the optical fiber transmission line 622. The control signal source 606 partially modulates the drive current of the excitation light source 603. Fiber grating section 60
Reference numerals 7 and 608 denote external resonance reflectors whose reflectances are adjusted so that the oscillation wavelengths of the excitation light source 603 alternately occur at different wavelengths. Further, the optical isolator 602 has a sufficiently small light reflectance for wavelengths other than the oscillation wavelength,
That is, the connection is made to sufficiently increase the return loss.

FIG. 7 (b) shows a time change of the drive current of the pump light source, the oscillation wavelength, and the pump light power. The output of the control signal source 606 has a constant modulation period,
03 is modulated. The excitation light source 603 alternately outputs short-wavelength excitation light (λ1) and long-wavelength excitation light (λ2) at a constant optical power.

FIG. 8 is a diagram showing the principle of pumping light oscillation of the Raman amplification system according to the second embodiment of the present invention. FIG. 8A shows that the driving current of the excitation light source 603 is I +
The case of ΔI is shown. The gain of the pump light (λ1) on the short wavelength side is G1, and the fiber grating units 607 and 608
Is the return loss of α1, and the pump light (λ2) on the long wavelength side
Is G2, and the fiber grating 60
The return loss of 7,608 is α2. At this time, since α1> G1α2 <G2, the pumping light (λ2) on the long wavelength side is output.

FIG. 8B shows a case where the driving current of the excitation light source 603 is I-ΔI. Excitation light on short wavelength side (λ1)
Is G1 ', and the fiber grating 60
The return loss of 7,608 is α1, the gain of the pump light (λ2) on the long wavelength side is G2 ′, and the return loss of the fiber grating units 607,608 is α2. At this time, since α1 <G1′α2> G2 ′, the excitation light (λ1) on the short wavelength side is output.

In this manner, different wavelengths can be alternately pumped by one pumping light source, so that there is no need for a pumping light multiplexer for multiplexing two pumping wavelengths, which is simplified. A Raman amplification system can be provided.

[0040]

As described above, according to the present invention,
Since each pump light is modulated so that the modulation phase is different, and the amplification gain of each signal light is made equal, amplification with wide band, high efficiency and suppressed wavelength dependence of gain becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a conventional Raman amplification system.

FIG. 2 is a diagram showing a state of amplification in a conventional Raman amplification system.

FIG. 3 is a diagram showing gain frequency characteristics in a conventional Raman amplification system.

FIG. 4 is a diagram illustrating a configuration of a Raman amplification system according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a first example of amplification in the Raman amplification system according to the first embodiment of the present invention.

FIG. 6 is a diagram illustrating a second example of amplification in the Raman amplification system according to the first embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration of a Raman amplification system according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a principle of pump light oscillation of a Raman amplification system according to a second embodiment of the present invention.

[Explanation of symbols]

100, 400, 600 Raman amplification system 101, 401, 601 Pumping light signal optical multiplexer 102, 402 Pumping light multiplexer 103, 104, 403, 404, 603 Pumping light source 121, 122, 421, 422 Optical fiber transmission line 131a To 131c, 431a to 431c Transmitter 405 Phase adjuster 406, 606 Control signal source 602 Optical isolator 607, 608 Fiber grating section

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) H04B 10/17 F term (Reference) 2K002 AA02 AB30 CA15 DA10 GA10 HA23 5F072 AK06 JJ20 PP07 QQ07 YY17 5K002 AA06 CA13 DA02 FA01

Claims (9)

[Claims] 1. A Raman amplification method for amplifying a plurality of wavelength-multiplexed signal lights with a plurality of pump lights having different wavelengths, wherein each of the pump lights is modulated so as to have a different modulation phase. Amplification method. 2. An optical frequency interval between two pumping lights having different wavelengths is smaller than a difference between each optical frequency of the pumping light and each optical frequency at which an amplification gain is peaked by each of the pumping lights. The Raman amplification method according to claim 1, wherein the Raman amplification method is small. 3. The Raman amplification method according to claim 1, wherein an effective length of the optical fiber amplification medium contributing to the amplification of the signal light is at least 10 times the modulation wavelength of the pump light. . 4. A Raman amplification system for amplifying a plurality of wavelength-multiplexed signal lights with a plurality of pump lights having different wavelengths, comprising a modulating means for modulating each of the pump lights so as to have different modulation phases. A Raman amplification system characterized by the following. 5. The Raman amplification system according to claim 4, wherein each of the pumping lights is output from a semiconductor laser, and the modulation unit modulates an applied current of the semiconductor laser. 6. A Raman amplification system for amplifying a plurality of wavelength-multiplexed signal lights with a plurality of pump lights having different wavelengths, wherein: a partial modulation means for partially modulating a current applied to a semiconductor laser; External resonance means for oscillating the modulated output of the semiconductor laser at the wavelength of each of the pump lights, wherein the modulation phases of the respective pump lights are different. . 7. The Raman amplification system according to claim 6, wherein said external resonance means includes a fiber grating that reflects a selected wavelength, and an optical isolator that removes return light from an optical fiber. . 8. An optical frequency interval between two pumping lights having different wavelengths is smaller than a difference between each optical frequency of the pumping light and each optical frequency at which an amplification gain is peaked by each of the pumping lights. The Raman amplification system according to any one of claims 4 to 7, wherein the Raman amplification system is small. 9. The optical fiber amplification medium according to claim 4, wherein an effective length of the optical fiber amplification medium contributing to the amplification of the signal light is at least 10 times a modulation wavelength of the pump light. Raman amplification system.