JPS62484B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical fiber used in the field of optical communications and a method for optical amplification using such an optical fiber as an amplification medium.

[Prior Art] FIG. 2 shows the loss-wavelength characteristics of a silica-based optical fiber widely used for optical communications. In Figure 2, relatively steep loss peaks can be seen near wavelengths of 1.25 μm and 1.39 μm, but these loss peaks are caused by impurities mixed into the optical fiber.
This was due to the OH group. Absorption due to such impurities has a relatively steep loss peak, but the wavelength is determined by the material itself. Therefore, it is not possible to cause a large loss at any wavelength. Furthermore, in the 1.55 μm wavelength band where optical fiber loss is minimum, in order to eliminate the effects of absorption loss at 1.39 μm, 1.90 μm, and 2.22 μm due to OH groups, it is important to avoid mixing OH groups as much as possible. is important. Therefore,
It is not a practical idea to utilize the absorption loss of OH groups. In fact, at present, as shown in Figure 3, OH
Optical fibers have been manufactured that have extremely low absorption losses due to the radicals.

On the other hand, in optical amplification using stimulated four-photon mixing in an optical fiber, signal light matching the wavelength of Stokes light or anti-Stokes light can be amplified. FIG. 4 shows the relationship between the gain coefficient and pump light power. As can be seen from FIG. 4, there is a tendency for the gain coefficient to become saturated as the pump light power increases. This is because a part of the power of the pump light contributes to the first Stokes oscillation of stimulated Raman scattering, and the pump light power at which the gain coefficient begins to saturate and the critical power of the first Stokes light of stimulated Raman scattering are Almost equal. Therefore, in order to obtain a linear change in the gain coefficient and secure a larger gain coefficient even with a high pump light input, it is necessary to increase the oscillation critical pump light power of the first-order Stokes light. However, conventionally there has been no method for realizing such a request.

[Objective] The present invention was made in view of the above circumstances, and an object of the present invention is to provide an optical fiber having a configuration suitable for optical amplification.

Another object of the present invention is to provide a method for performing optical amplification using the above-mentioned optical fiber.

[Configuration of the Invention] The optical fiber of the present invention includes a clad, a main core buried in the clad and forming a main waveguide, a stress applying section arranged to apply stress to the main core in one direction, and a main core. The auxiliary core is disposed in parallel with the auxiliary core and couples only light of a specific wavelength, and the loss imparting section is disposed along the longitudinal direction near the auxiliary core.

The optical amplification method of the present invention includes a cladding, a main core buried in the cladding and forming a main waveguide, a stress applying part arranged to apply stress to the main core in one direction, and a stress applying part arranged in parallel with the main core. The pump light and the signal light are simultaneously guided to an optical fiber that is equipped with a sub-core that couples only light of a specific wavelength, and a loss imparting section that is disposed along the longitudinal direction near the sub-core. By matching the wavelength of the Stokes light or anti-Stokes light of stimulated four-photon mixing with the wavelength of the signal light, the waveguide mode of the main core is
Only the wavelength of the first-order Stokes light of stimulated Raman scattering is coupled with the waveguide mode of the sub-core, the optical energy is transferred from the main core to the sub-core by mode coupling, and the transferred optical energy is transferred by the loss imparting part. By attenuating, stimulated Raman scattering is suppressed by increasing only the loss of light with a wavelength corresponding to the wavelength of the first Stokes light of stimulated Raman scattering among the waveguide modes of the main core, and the signal light in the main core is suppressed. is amplified by stimulated four-photon mixing.

[Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of the configuration of the optical fiber of the present invention, where 1 is a cladding, 2 is a core (main core) buried in the cladding 1 and forming a main waveguide;
Numeral 3 is a core (sub-core) placed in parallel with the main core 2 in the cladding 1 and couples only light of a specific wavelength to form a sub-waveguide. The loss imparting portions 5 are stress imparting portions disposed opposite to each other with the main core 2 in between so as to impart stress to the main core 2 in one direction.

Optical fibers with such a structure are placed inside a quartz tube using the rod-in-tube method using the VAD method or
By inserting the core base material, stress-applying part base material, clad base material, and loss-applying part base material produced by the MCVD method, and drawing the base materials while heating and integrating them, can be manufactured.

The stress-applying base material is for changing the coefficient of linear expansion from that of the clad base material, and glass doped with impurities such as B ₂ O ₃ can be used. For the purpose of inflicting a loss on the loss-imparting base material,
Glass doped with impurities such as B ₂ O ₃ can be used.

Although two cores are provided in the embodiment shown in the first drawing, the optical fiber of the present invention is not limited to this, and it goes without saying that it may be configured to include three or more cores and a plurality of loss imparting parts. It is.

By applying stress in one direction to the main core 2 by the stress applying section 5, birefringence occurs in the main core 2. Here, the Stokes wavelength of stimulated four-photon mixing is determined by the relationship between the pump wavelength and the magnitude of such birefringence. The magnitude of this birefringence is determined depending on the distance between the main core 2 and the stress applying section 5 and the material of the stress applying section 5.

Therefore, the Stokes wavelength of the stimulated four-photon mixing can be changed to match the signal light wavelength, so the stimulated four-photon mixing of the birefringent core is suitable for optical amplification.

_Now , in the optical fiber shown in _FIG .
If λ _S1 = 1.401 μm), sufficient coupling is generated between the waveguide modes of both cores 2 and 3, and the pump light wavelength λ _P and the signal light wavelength λ matched to the Stokes light wavelength of stimulated four-photon mixing. Coupling at _S can be sufficiently suppressed to achieve wavelength-selective coupling. That is, such coupling can be realized by setting the core diameters of the two cores and the refractive index difference between the core and the cladding to different values.

Figure 5 shows the relationship between the normalized propagation constant β/k of the lowest-order waveguide mode LP ₀₁ mode propagating through cores 2 and 3 in Figure 1 and the wavelength k (=2π/λ, λ is the wavelength). For example, the first Stokes wavelength λ=λ _S1
The propagation constants of both modes are equal at =1.401 μm, and different at λ=λ _P =1.318 μm and λ _S . In this case, the relative refractive index difference Δ between the core and the cladding is 0.4% for the main core 2 and 0.26% for the sub-core 3, and the core radius is 3.4% for the main core 2.
μm, and for the sub-core 3 it was 5.9 μm.

At this time, if a light wave of λ = λ _S1 is incident on the main core 2, after propagating a certain distance, it will be caused by mode coupling.
100% power is transferred to sub-core 3, but λ=λ _P ,
The light wave of λ _S does not migrate to the sub-core with the above-mentioned relative refractive index difference and core diameter settings. λ in this numerical example
The binding at =λ _P and λ _S was about 5% or less.
Since the light wave of λ=λ _S1 transferred from the main core 2 to the sub-core 3 is absorbed by the loss imparting part 4, from the perspective of the main core 2, the coupling apparently contributes as a loss at λ=λ _S1 . Become.

That is, as shown in FIG.
The loss wavelength characteristics of the LP ₀₁ mode are as follows: λ = λ _S1 (=1.401μ
The losses in m) are superimposed.

As is generally known, the Stokes critical pump power in stimulated Raman scattering increases in proportion to the loss, so λ = λ _S1 described in this example.
By using an optical fiber with a large loss for optical amplification, the first-order Stokes critical pump power increases. As a result, as shown in FIG. 7, the relationship between the gain coefficient of amplification by stimulated four-photon mixing and the pump light does not exhibit the saturation characteristic as shown in FIG. 4. Therefore, pump light power can be efficiently used for optical amplification.

In addition, as the material of the loss imparting part 4 shown in the first diagram,
For example, when using a base material of quartz doped with B ₂ O ₃ , the B ₂ O ₃ doped glass has a
(dB/Km) or more, by arranging the loss adding section 4 adjacent to one side of the sub-core 3, the light waves with a wavelength of 1.4 μm or more propagating through the sub-core 3 can be transferred to the loss adding section 4. can be almost attenuated by

When using a normal B ₂ O ₃ doped glass base material as the material for the stress applying part 5, in order to prevent the main core 2 from being affected by the loss due to the loss applying part 4, the main core 2 and the stress The distance from the applying part 5 needs to be larger than the spread of the electromagnetic field distribution of the LP ₀₁ mode of the main core 2, for example, about 3 of the core diameter.
More than twice as much.

Figure 8 shows that at λ _S1 = 1.401 μm, both LP ₀₁ modes are degenerate and the coupling is 100%.
The optimal relative refractive index difference and core radius to minimize the coupling at =λ _P are determined by the ratio V _P of the normalized frequency V values of cores 2 and 3 at λ = λ _P
2 /V _P1 . here,
Δ ₁ = 0.4%, λ _P = 1.318 μm, λ _S1 =
δβ=0 at 1.401 μm. In this case, in order to set the V value ratio V _P2 / _{V P1} to ₂ from FIG. It can be seen that the difference Δ ₂ should be set to 0.2.

9 and 10 show two examples in which the optical amplification method using the optical fiber of the present invention is applied to an optical transmission line. Here, 6 is a signal light source, 15 is a pump light source, 7 is a mirror, 8 is a half mirror, 9 is a lens, 10 is an optical fiber according to the present invention, 10' is another optical fiber, 11 is a spectroscope, 12 , 16 is an optical transmission line, 13 is a transmitting station building, 14 is a receiving station building, 2
5 is a pump light, 26 is a signal light, 26' is an amplified signal light, 35 and 35' are ordinary optical fibers, 1
7 indicates a relay amplifier.

In the example shown in FIG. 9, the optical fiber 10 according to the present invention is installed in the optical transmission line 12, also serving as a transmission line.
The signal light 26 is multiplexed by the pump light 25 and the half mirror 8, and is sent to the main core of the optical fiber 10 through the lens 9.
are combined by

Here, the wavelength of the Stokes light or anti-Stokes light of stimulated four-photon mixing with respect to the wavelength of the pump light is made to match the wavelength of the signal light, and the waveguide mode of the main core is set to the wavelength of the first-order Stokes light of stimulated Raman scattering. The optical energy is transferred from the main core to the sub core by mode coupling, and the transferred optical energy is attenuated by the loss imparting part, thereby combining the waveguide mode of the main core with the waveguide mode of the main core. Of these, only the loss of light with a wavelength corresponding to the wavelength of the first-order Stokes light of stimulated Raman scattering is increased to suppress stimulated Raman scattering, and the signal light is amplified by stimulated four-photon mixing in the main core.

That is, the signal light 26 is amplified in the optical transmission line 12 and then sent to the receiving station 14. Since there is a propagation loss in the optical fiber 10 in actual operation, long-distance transmission is possible while compensating for signal attenuation due to this loss.

In the example shown in FIG. 10, a repeater amplifier 17 is constructed using the optical fiber of the present invention. That is, the signal light 26 that has propagated through the general optical fiber 35 is converted into pump light 2 by the half mirror 8 in the relay amplifier 17.
5, is coupled to the main core of the optical fiber 10 of the present invention, and is optically amplified within the optical fiber 10 in the same manner as in the above example. From the amplified output light,
The pump light 25 is separated by the spectrometer 11 to extract the signal light 26', and this signal light 26' is coupled to the optical fiber 35' of the next stage transmission line. Such a configuration has the advantage that a conventional optical cable can be used as is as the optical transmission line 16.

[Effect] As explained above, according to the optical fiber of the present invention, the oscillation of stimulated Raman scattering can be suppressed, and according to the optical amplification method of the present invention by stimulated four-photon mixing using such an optical fiber, the oscillation of stimulated Raman scattering can be suppressed. Since the gain saturation phenomenon that could not occur does not occur, a gain coefficient proportional to the pump light power can be obtained. Therefore, in contrast to the gain of about 30 dB obtained with conventional optical amplification, according to the present invention, the upper limit of the amplification gain due to stimulated four-photon mixing is due to the fact that the optical fiber end face is destroyed by the pump light power. It is estimated that the gain is 100 dB or more. This makes it possible to eliminate repeaters in ultra-long-distance optical transmission systems such as submarine systems, and for example, F-
When an amplification gain of 100 dB is obtained using the 400M system, it is possible to achieve a distance of about 500 km without repeaters, which has a great practical effect.

In addition, since the present invention performs optical amplification by stimulated four-photon mixing, the wavelength of the signal light that can be amplified can be arbitrarily selected as desired, and the Stokes of stimulated Raman scattering in which the wavelength of the signal light is determined by the material is utilized. Compared to conventional amplification, this method has a greater degree of freedom regarding the wavelength of the signal light, which is also a great practical advantage.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing one embodiment of the optical fiber of the present invention, Fig. 2 is a loss wavelength characteristic diagram of a normal optical fiber, and Fig. 3 is a loss wavelength characteristic diagram of a recent optical fiber without loss of OH groups. , FIG. 4 is a characteristic diagram showing the relationship between gain coefficient and pump light power in the conventional optical amplification method, FIG. 5 is a characteristic diagram showing the dispersion curve of the LP ₀₁ mode of the optical fiber of the present invention, and FIG. A characteristic diagram showing an example of the loss wavelength characteristic of the invented optical fiber, FIG. 7 is a characteristic diagram showing the relationship between the gain coefficient and pump light power in this example, and FIG. 8 is a characteristic diagram showing the relationship between the optimum optical fiber parameter and V _P2 /V _P1 FIGS. 9 and 10 are characteristic diagrams showing the relationship between
FIG. 3 is a diagram showing two examples. DESCRIPTION OF SYMBOLS 1... Clad, 2... Main core, 3... Sub-core, 4... Loss imparting part, 5... Stress imparting part, 6...
... Signal light source, 7 ... Mirror, 8 ... Half mirror, 9 ... Lens, 10 ... Optical fiber of the present invention, 11 ... Spectrometer, 12, 16 ... Optical transmission line,
15... Pump light source, 17... Relay amplifier.

Claims (1)

[Scope of Claims] 1. A cladding, a main core buried in the cladding and forming a main waveguide, a stress applying section arranged to apply stress in one direction to the main core, and the main core. When the pump light and the Stokes light of stimulated four-photon mixing with respect to the wavelength of the pump light or the signal light of the wavelength corresponding to the anti-Stokes wavelength are incident on the main core arranged in parallel, stimulated Raman scattering of the pump light occurs. An optical fiber comprising: a sub-core that couples only light having the wavelength of first-order Stokes light; and a loss imparting section disposed along the longitudinal direction in the vicinity of the sub-core. 2. a cladding, a main core buried in the cladding and forming a main waveguide, a stress applying section arranged to apply stress in one direction to the main core, and arranged in parallel with the main core, Pump light and signal light are simultaneously guided into an optical fiber that includes a sub-core that couples only light of a specific wavelength, and a loss adding section disposed along the longitudinal direction in the vicinity of the sub-core, and the wavelength of the pump light is The wavelength of the Stokes light or anti-Stokes light of stimulated four-photon mixing is matched with the wavelength of the signal light, and the waveguide mode of the main core is controlled only at the wavelength of the first-order Stokes light of stimulated Raman scattering. The waveguide mode of the main core is coupled with the waveguide mode of the main core to transfer optical energy from the main core to the sub-core by mode coupling, and the transferred optical energy is attenuated by the loss imparting section. Among the modes, only the loss of light with a wavelength corresponding to the wavelength of the first Stokes light of the stimulated Raman scattering is increased to suppress stimulated Raman scattering, and the signal light is amplified by stimulated four-photon mixing in the main core. An optical amplification method characterized by: