JPS6153710B2 - - Google Patents

Description

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

[Prior Art] When strong light enters an optical fiber, Stokes light is generated due to stimulated Raman scattering, which is a nonlinear optical effect of the optical fiber. In this case, it is well known that when a signal light having the same wavelength as the Stokes light is input into the optical fiber at the same time as the strong light, optical energy is transferred to the signal light and the signal light is amplified. The strong light in this case is called pump light.

Induction 4, another nonlinear optical effect of optical fiber
It is also well known that in optical fibers in which photon mixing occurs, signal light corresponding to the wavelength of Stokes light or anti-Stokes light produced by stimulated four-photon mixing is similarly amplified.

Conventionally, when amplifying light using such nonlinear optical effects of optical fibers, amplification by stimulated Raman scattering and amplification by stimulated four-photon mixing are considered to be separate, and it is difficult to use them simultaneously. That had not been thought of. For this reason, the power of the pump light is dispersed to both, making it impossible to perform high gain amplification.

[Objective] Therefore, in view of the above-mentioned points, the object of the present invention is to
The object of the present invention is to provide an optical amplification method that provides a larger amplification gain than that obtained with each individual Stokes beam by stimulated Raman scattering and stimulated four-photon mixing.

[Structure of the Invention] In order to achieve the above object, the present invention provides an optical amplification method in which pump light and signal light are input into an optical fiber and the signal light is amplified by the nonlinear optical effect of the optical fiber. My induction 4
The phase matching condition for generating photon mixing is satisfied by the inherent birefringence of the optical fiber, or the birefringence generated by mode dispersion, lateral pressure, or bending, and the stimulated four-photon The mixed Stokes light wavelength is made to match the Stokes light wavelength caused by stimulated Raman scattering, which is another nonlinear optical effect of the optical fiber, and the signal light having the same wavelength as the matched Stokes light wavelength is sent into the optical fiber at the same time as the pump light. to amplify the signal light.

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

First, in order for stimulated four-photon mixing to occur efficiently, a state of phase matching must be established. That is, when the wave number vectors of the pump light, Stokes light, and anti-Stokes light are k _P , k _S , and k _A , Δk must be 0 in k _S +k _A −2k _P =Δk.

In general, Δk is the sum of the amount of phase mismatch Δkm between the pump light and Stokes light, which is determined by the material of the optical fiber, and the amount Δkw, which is determined by the properties of the waveguide mode, and these two cancel each other out. A state of phase matching occurs at matching wavelengths.

Figure 1 shows the amount of phase mismatch Δkm with respect to fused silica when the wavelength of the pump light is 1.0 μm. Here, the horizontal axis indicates the amount of frequency shift from the frequency of the pump light.

In order to cancel Δkm as shown in Figure 1, we can use the modal dispersion of a multimode fiber, the waveguide dispersion of a single mode fiber, and the polarization dispersion of a birefringent fiber. things are being done. Since these dispersions can be varied by changing the structure of the fiber or the environmental conditions of the fiber, the amount of frequency shift satisfying Δk=Δkm+Δkw=0 will also vary.

The Stokes optical wavelength of the type of guided four-photon mixing that obtains phase matching by birefringence can be adjusted to some extent by the inherent birefringence of the fiber, as described above, but It can be precisely adjusted by the birefringence generated by applying lateral pressure or bending.

As an example, FIG. 2 shows the relationship between the applied pressure and the amount of frequency shift at which phase matching occurs when pressure is applied to a birefringent fiber to change the amount of birefringence. Here, curve 1 shows the relationship obtained theoretically, and points 2a, 2b, 2c, and 2d show experimental measurement results.

Figure 3 shows the optical fiber used as a model for the calculation and the state of pressure applied to it, where 3 is a core with a diameter of 2 μm and contains GeO ₂ and P ₂ O ₅ , and 4
and 5 is for applying stress to this core.
A stress applying section doped with B ₂ O ₃ , 6 a cladding made of SiO ₂ , and 7a and 7b flat plates arranged in parallel to face each other with the optical fiber in between so as to apply external pressure to the optical fiber.

As can be seen from FIG. 2, by changing the external pressure in the X-axis direction from 0 to about 0.9 Kg/cm, the amount of frequency shift changes from about 1100 cm ^-1 to 0.

On the other hand, stimulated Raman scattering is closely related to the molecular vibrations and lattice vibrations of the material constituting the optical fiber, so the amount of frequency shift of the Stokes light due to stimulated Raman scattering is determined by the material constituting the optical fiber. The measured frequency shift amount in the case of the optical fiber shown in FIG. 3 was 446 cm ^-1 .

Therefore, in the present invention, as shown in FIG. 2, by setting the external pressure to approximately 0.7 kg/cm, the wavelength of the Stokes light due to stimulated four-photon mixing and the stimulated Raman By matching the wavelength of Stokes light caused by scattering, it is possible to obtain light that is stronger than light obtained only by stimulated Raman scattering.

Here, if a signal light having the same wavelength as the matched Stokes light wavelength is input into the optical fiber at the same time as the pump light, the signal light can be amplified, but in the present invention, the Stokes light is By matching the wavelength with the wavelength of Stokes light produced by stimulated four-photon mixing, a larger amplification gain can be obtained.

In addition to the above-mentioned externally applied pressure, changing the frequency shift amount of stimulated four-photon mixing is also possible by bending the fiber, changing the temperature of the fiber, or by changing the structure of the optical fiber. This is also possible, for example, by changing the core diameter or the relative refractive index of the core and cladding.

[Effect] As explained above, the Stokes light caused by stimulated Raman scattering of an optical fiber can be changed by selecting the material used for the optical fiber, and the Stokes light caused by stimulated four-photon mixing can be changed by This can be changed depending on the structure of the fiber and the external environmental conditions of the optical fiber, so in the present invention, the wavelengths of these Stokes lights are made to match.
By determining the material, structure, etc. of the optical fiber, it is possible to obtain optical power stronger than that obtained with each individual Stokes beam. Moreover, by using signal light that matches the wavelength of the Stokes light, there is an advantage that a larger amplification gain can be obtained than that obtained with each individual Stokes light.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between the frequency shift amount of fused silica at a wavelength of 1.0 μm and the amount of phase mismatch between pump light and Stokes light, and Figure 2 is a characteristic diagram showing the relationship between the amount of frequency shift at a wavelength of 1.0 μm and the amount of phase mismatch between the pump light and Stokes light. Figure 3 is a characteristic diagram showing the relationship between the pressure applied to the polarization and the amount of wavelength shift caused by stimulated four-photon mixing. FIG. 3... Optical fiber core containing GeO ₂ and P ₂ O ₅ ,
4, 5... Stress applying part provided for applying pressure to the core 3, 6... Quartz cladding, 7a, 7
b...Parallel plate for applying lateral pressure.

Claims (1)

[Claims] 1. An optical amplification method in which a pump light and a signal light are input into an optical fiber and the signal light is amplified by a nonlinear optical effect of the optical fiber, which includes: generating stimulated four-photon mixing among the nonlinear optical effects. satisfying the phase matching condition for by the inherent birefringence of the optical fiber or the birefringence caused by mode dispersion, lateral pressure or bending,
And for the same pump light source, the Stokes light wavelength of the stimulated four-photon mixing and the Stokes light wavelength due to stimulated Raman scattering, which is another nonlinear optical effect of the optical fiber, are made to match, and the Stokes light wavelength is the same as the matched Stokes light wavelength. An optical amplification method, characterized in that the signal light having a wavelength of 1 is introduced into the optical fiber simultaneously with the pump light to amplify the signal light.