JP3421610B2 - Fiber Raman amplifier and optical fiber communication system using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical fiber communication system and an optical signal processing system.

[0002]

2. Description of the Related Art FIG. 13 shows a basic configuration of a fiber Raman amplifier (hereinafter referred to as "Raman amplifier" for simplicity) according to a conventional technology. As shown in FIG. 13, the Raman amplifier includes an optical fiber 1 as a gain medium, a laser diode (LD) pumping light source 2, and a wavelength multiplexer 3 that multiplexes signal light and pumping light. The pumping light from the laser diode pumping light source 2 is incident on the optical fiber 1 from the rear direction with respect to the signal light propagation direction by using the wavelength multiplexer 3. Similarly, it may be incident from the front direction.

The laser diode excitation light source 2 comprises a laser diode (LD) 2a and a DC power source 2b,
It is driven with a constant current. Therefore, the optical fiber 1 is
It is excited with a constant pump light power. The laser diode 2a is a Fabry-Perot type high-power laser diode, and oscillates in multiple modes. FIG. 14 shows the spectrum of the excitation light emitted from the Raman amplifier shown in FIG. A feature of the Fabry-Perot type laser diode is that it oscillates in multiple modes as shown in FIGS. 14 (a), (b), and (c), and the wavelength spacing between modes is usually 1 to
It is about 2 nm.

The spectrum shape of this multimode oscillation sensitively changes due to factors such as temperature, drive current, internal light reflection, etc. In general, even if the temperature and drive current are controlled to be constant, they exhibit time fluctuations. As shown in FIG. 14C, the excitation light spectrum may be narrowed. 14 (a), (b), and (c) show how the spectral time changes. The Raman gains for these pumping light spectra are shown in FIG. 15 for FIGS. 14A and 14B, and in FIG. 16 for FIG. 14C.
The Raman gain is the signal light gain in the optical fiber. If the pumping light power is constant, the Raman gain is generally constant with respect to the signal light wavelength with a constant wavelength difference.

Therefore, the Raman gain spectra for FIGS. 14 (a) and 14 (b) show time fluctuations as shown in FIGS. 15 (a) and 15 (b). Further, in the Raman gain spectrum for the pumping light spectrum of FIG. 14C, ripples generally appear in the long wavelength region of gain due to the narrowing of the pumping light spectrum. As described above, the conventional technique has a drawback that the Raman gain spectrum varies with time, and the Raman gain spectrum cannot be finely controlled by the spectrum variation amount.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a Raman amplifier which solves the above-mentioned drawbacks of the prior art.

[0007]

FIG. 1 shows a first basic structure of a Raman amplifier according to the present invention. The Raman amplifier includes an optical fiber 1 as a gain medium, a laser diode (LD) pumping light source 2, and a wavelength multiplexer 3 for the signal light and the pumping light. The pumping light from the laser diode pumping light source 2 is sent to the optical fiber 1 using the wavelength multiplexer 3.
In addition, the light is incident from the rear side with respect to the signal light propagation direction.
Similarly, it may be incident from the front direction.

The laser diode pumping light source 2 comprises a laser diode 2a, a direct current power source 2b, an alternating current power source 2c, and a current coupler (for example, bias tee) 2d for direct current and alternating current. The waveform of the alternating current is a sine wave or a pulse. The laser diode 2a is a Fabry-Perot type high-power laser diode, is driven by a current in which alternating current is superimposed on direct current, and its time average value is a direct current value. The pumping light power fluctuates with time corresponding to the time fluctuation of the superimposed current. The AC frequency of the AC current is the response cutoff frequency of the Raman gain (F. Forghieri e
t al., Tech, Digest of Optical Fiber Communicatio
ns Conference, FC6, pp. 294-295, 1994).

Therefore, even if the pumping light power fluctuates with time, the Raman gain is constant without fluctuating with time. FIG. 2 shows the frequency response characteristic of Raman gain. The relationship between the modulation frequency of pumping light power (relative value with the response cutoff frequency as 1) and the modulation component of Raman gain (relative value) is shown. In FIG. 2, when the fluctuation frequency of the pumping light power is sufficiently higher than the response cutoff frequency (for example, the relative value is 100),
The modulation component of Raman gain is sufficiently small (relative value is 0.0
1).

FIG. 3 shows the time variation characteristics of the pumping light power and Raman gain. In the figure, the fluctuation period (f ₁ ) of the excitation light power indicated by the thin line is one fifth of the fluctuation period (f ₀ ) of the excitation light power indicated by the thick line, and f ₀ is a value near the response cutoff frequency. Have. The fluctuation range of the Raman gain indicated by the thin line is about one fifth of the fluctuation range of the Raman gain indicated by the thick line. The length of the optical fiber 1 is about 100 m to about 1 km in the case of a concentrated amplification type Raman amplifier, and about 10 km to about 100 km in the case of a distributed amplification type Raman amplifier.
Is. The response cutoff frequency of Raman gain is about 1 kHz to about 100 kHz.

A second basic configuration of the Raman amplifier of the present invention is shown in FIG. The Raman amplifier is an optical fiber 1 serving as a gain medium, two laser diode (LD) pumping light sources 4,5, and polarization multiplexing that combines pumping lights from the two pumping light sources 4,5 in a polarization region. And a wavelength multiplexer 3 for the signal light and the pumping light. The pumping light from the pumping light sources 4 and 5 is incident on the optical fiber 1 from the direction opposite to the signal light propagation direction using the wavelength multiplexer 3. Similarly, it may be incident from the front direction. Each of the laser diode pumping light sources 4 and 5 includes a laser diode (LD) 4a, 5a, a DC power supply 4b, 5b, an AC power supply 4c, 5c, and a current combiner 4d for a DC current and an AC current.
5d (for example, bias tee). The waveform of the alternating current is a sine wave or a pulse.

Each of the laser diodes 4a and 5a is a Fabry-Perot type high power laser diode, and is driven by a current in which an alternating current is superimposed on a direct current, and its time average value is a direct current value. Laser diode excitation light source 4
The phase of the alternating current of the laser diode excitation light source 5 is shifted by 180 degrees by using the phase adjuster 7 (so-called reverse phase). The pumping light powers from the laser diode pumping light source 4 and the laser diode pumping light source 5 are time-varying corresponding to the time fluctuations of the respective superimposed currents, and the phase is 180
Deviated.

Therefore, the total power of the two pumping lights is constant regardless of time. FIG. 7 shows the time dependence of the drive currents of the laser diode pumping light source 4 and the laser diode pumping light source 5, and the time dependence of the pumping light power and the total power of the laser diode pumping light source 4 and the laser diode pumping light source 5. Comparing the first basic configuration and the second basic configuration according to the Raman amplifier of the present invention described above, there are the following features.

The second basic construction can be constructed only when there are two or more pumping light sources, but there is no limitation on the frequency and amplitude of the alternating current for stabilizing the pumping light spectrum in multiple modes. On the other hand, the first basic configuration can be realized with one pump light source, but has a restriction that the frequency of the alternating current is set sufficiently higher than the response cutoff frequency of the Raman gain. FIG. 4 shows the spectrum of the excitation light emitted from the Raman amplifiers of FIGS. 1 and 6.

Due to the superimposed alternating current, a stable spectrum is shown in multiple modes, and the wavelength interval between the modes is usually about 1 to 2 nm. The spectrum shape of this multi-mode oscillation is generally stable and does not show time fluctuation if the temperature and drive current are controlled to be constant. Moreover, the excitation light spectrum is not narrowed as in the case of the conventional technique shown in FIG.

A Raman gain spectrum for the pumping light spectrum of FIG. 4 is shown in FIG. A Raman gain spectrum without time fluctuation and gain spectrum ripple is obtained. Further, when the temperature or drive current of the laser diode is changed, the spectral shape of the excitation light remains unchanged, and the wavelength centroid shifts. Therefore, the wavelength range of Raman gain can be finely changed by changing the temperature or the drive current. As described above, according to the present invention, it is possible to obtain a Raman amplifier which has no Raman gain variation with time and has a good gain spectrum controllability.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 8 shows a first embodiment of the present invention applied to an optical fiber communication system. This is the case of distributed amplification using the transmission fiber 101 as a gain medium for Raman amplification. The transmission fiber 101 is an 80 km dispersion-shifted fiber. The central wavelength of the excitation light is 1.4
The center wavelength of the signal light is 8 μm and the wavelength is 1.57 μm.

Two pump light sources 104 and 105 are used, and each pump light source 104 and 105 has a laser diode. The two pump light sources 104 and 105 are
Since they have the same configuration, the internal configuration of one pumping light source 105 is omitted in FIG. 8 for simplicity. The single-polarized pump light emitted from the pump light sources 104 and 105 is multiplexed by using the polarization multiplexer 106, and the polarization-multiplexed pump light is generated by using the wavelength multiplexer 103. It is multiplexed with the signal light.

Signal light loss of the transmission fiber 101 (about 20
erbium-doped fiber amplifier (E
DFA) 108 is installed after the Raman amplifier. The gain of the erbium-doped fiber amplifier 108 is about 10 dB. The direct current in the excitation light sources 104 and 105 is 500 mA, and the alternating current is 100 mA. The modulation factor of the laser diode drive current is about 20%.

The Raman gain frequency response cutoff frequency is about 1.
The frequency of 0 kHz and the alternating current is about 1 MHz. The pumping light power emitted from the pumping light sources 104 and 105 is about 120 mW, and the input pumping light power to the transmission fiber 101 is about 200 mW. The Raman gain spectrum in this example is shown in FIG. Central wavelength of light intensity is 1.48
μm, full width at half maximum is about 10 nm.

The Raman gain spectrum in this embodiment is shown in FIG. Center frequency of flat gain is about 1.57μ
m, the flat gain value is about 10 dB. The time variation of Raman gain is 0.1 dB or less. As described above, the Raman gain spectrum is stable because the pumping light spectrum is stable. Further, for example, the center wavelength of the excitation light spectrum can be finely changed by changing the temperature of the laser diode. For example, for a temperature change of 10 degrees, the change in center wavelength is about 5 nm.

[Second Embodiment] FIG. 11 shows the configuration of a second embodiment of the present invention. This is a case of concentrated amplification using the Raman fiber 201 as a gain medium for Raman amplification.
The Raman fiber 201 is an 8 kHz high NA fiber. The center wavelength of the pumping light is 1.43 μm, and the center wavelength of the signal light is 1.57 μm. Two excitation light sources 20
4, 205 are used, and each pumping light source 204, 205 has a laser diode.

Since the pumping light sources 204 and 205 have the same structure, the internal structure of one pumping light source 205 is omitted in FIG. 11 for simplification. Excitation light source 2
The single polarization pump lights emitted from 04 and 205 are multiplexed by using the polarization multiplexer 206, and the polarization-multiplexed pump lights are signal light by using the wavelength multiplexer 203. Has been combined with. The direct current in the excitation light sources 204 and 205 is 50
The current is 0 mA and the alternating current is 100 mA.

The modulation factor of the laser diode drive current is about 20%. The Raman gain frequency response cutoff frequency is about 100 kHz and the alternating current frequency is about 10 MHz. The pumping light power emitted from each of the pumping light sources 204 and 205 is about 120 mW, and the input pumping light power to the Raman fiber 201 is about 200 mW. The excitation light spectrum in this example is similar to that in FIG. The center wavelength of the light intensity is 1.48 μm, and the half width is about 10 nm. The Raman gain spectrum in this example is similar to that in FIG. The center frequency of the flat gain is about 1.57 μm, and the flat gain value is about 10 dB. Raman gain time variation is 0.1
It is below dB.

[Third Embodiment] FIG. 12 shows the configuration of a third embodiment of the present invention. This is a case of concentrated amplification using the Raman fiber 301 as a gain medium for Raman amplification.
The Raman fiber 301 is an 8 km high NA fiber. The center wavelength of the pumping light is 1.48 μm, and the center wavelength of the signal light is 1.57 μm. Two pump light sources 304,
305 is used, and each pumping light source 304, 305 has a laser diode.

The alternating currents of the pumping light sources 304 and 305 drive the laser diode in reverse phase by the phase adjuster 307. The single-polarized pump light emitted from the pump light sources 304 and 305 is multiplexed by using the polarization multiplexer 306, and the polarization-multiplexed pump light is generated by using the wavelength multiplexer 303. It is multiplexed with the signal light. Excitation light source 304, 305
DC current is 500mA, AC current is 100m
It is A. The degree of modulation of the laser diode drive current is about 4
It is 0%.

The frequency of the alternating current can be set arbitrarily, but is, for example, 50 Hz. Excitation light source 304, 305
The output pumping light power from each is about 120 mW, and the input pumping light power to the Raman fiber 301 is about 200 mW.
Is. The Raman gain spectrum in this embodiment is shown in FIG.
Is the same as. The center wavelength of the light intensity is 1.48 μm, and the half width is about 10 nm. The Raman gain spectrum in this example is similar to that in FIG. The center frequency of the flat gain is about 1.57 μm, and the flat gain value is about 10 dB.
In principle, there is no temporal change in Raman gain.

[0028]

As described above in detail based on the embodiments, according to the present invention, there is an effect that a Raman amplifier having no Raman gain variation with time and a good gain spectrum controllability can be obtained. .

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first basic configuration of the present invention.

FIG. 2 is a graph showing a frequency response characteristic of Raman gain.

FIG. 3 is a graph showing time-varying characteristics of pumping light power and Raman gain.

FIG. 4 is a graph showing an excitation light spectrum in the present invention.

FIG. 5 is an explanatory diagram showing a Raman gain spectrum in the present invention.

FIG. 6 is an explanatory diagram showing a second basic configuration of the present invention.

FIG. 7 is a graph showing the time dependence of drive current and pumping light power.

FIG. 8 is an explanatory diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 9 is a graph showing an excitation light spectrum in the first example of the present invention.

FIG. 10 is a graph showing a Raman gain spectrum in the first example of the present invention.

FIG. 11 is an explanatory diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 12 is an explanatory diagram showing a configuration of a third exemplary embodiment of the present invention.

FIG. 13 is an explanatory diagram showing a basic configuration of a conventional technique.

FIG. 14 is a graph showing a conventional excitation light spectrum.

FIG. 15 is a graph showing a Raman gain spectrum in the related art.

FIG. 16 is a graph showing a Raman gain spectrum in the related art.

[Explanation of symbols]

1 optical fiber 2,4,5 Laser pump source 2a Laser diode 2b DC power supply 2c AC power supply 2d current coupler 3 wavelength multiplexer 6 polarization multiplexer 7 Phase adjuster

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H04B 10/17 (56) References JP-A-6-61554 (JP, A) JP-A-8-204267 (JP, A) Kaihei 9-179152 (JP, A) JP 8-32162 (JP, A) JP 2-272432 (JP, A) JP 5-347446 (JP, A) JP 7-176813 ( JP, A) JP 11-87853 (JP, A) Special table 6-500896 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/35 501 H01S 3 / 00-3/30 H04B 10/16-10/17 JISST file (JOIS) WPI (DIALOG)

Claims (3)

(57) [Claims] 1. A laser diode pumped fiber Raman amplifier comprising: an optical fiber as a gain medium of the fiber Raman amplifier; two laser diode pumping light sources; and two pumping lights emitted from the laser diode pumping light source. A multiplexer for multiplexing the signal light, pumping light emitted from the multiplexer is incident on the optical fiber to optically pump the optical fiber, and Raman amplification of the signal light in the optical fiber is performed. Each of the excitation light sources includes a Fabry-Perot laser diode, a direct current power supply, an alternating current power supply, and a current coupler, and the direct current from the direct current power supply and the alternating current from the alternating current power supply are coupled by the current coupling. To stabilize the oscillation wavelength spectrum of the laser diode in multimode oscillation by driving the laser diode with the superimposed current. And each pump source has two pump lights
The total power of is constant without depending on time, and has a phase adjuster that sets the phase of the AC power supply of each pumping light power supply to the opposite phase, and stabilizes the wavelength spectrum characteristic of Raman gain. And a fiber Raman amplifier. 2. The fiber Raman amplifier according to claim 1 , wherein the optical fiber is a transmission fiber, and an optical fiber communication system using a distributed amplification type fiber Raman amplifier. 3. The fiber Raman amplifier according to claim 1 , wherein the optical fiber is a Raman fiber.