JPH0685037B2 - Stimulated Raman scattering suppression method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is used for transmission of an optical signal or optical energy using an optical fiber. In particular, it relates to a method for suppressing stimulated Raman scattering that occurs in an optical fiber when the intensity of transmitted light is high.

Here, stimulated Raman scattering occurs due to a third-order linear optical phenomenon in a substance, and when intense light (which is called “excitation light”) propagates in the substance, Is a phenomenon in which strong light with different frequencies is scattered.
The frequency difference between the excitation light and the scattered light is equal to the natural frequencies of the electrons and lattices that make up the substance.

[Conventional technology]

It is known that in a standard optical fiber used for information transmission, stimulated Raman scattering occurs when the intensity of pumping light is about 0.2 w or higher. Regarding this, T. Takashima et al., “Dependence of Raman Gain on Relative Index Difference for GeO ₂ Doped Single Mode Fibers (Dependence of
Raman gain on relative index defference for GeO ₂ −
doped single-made fibers ", Optics Letters Volume 10 (19
(August 1985) See page 420 for details.

When stimulated Raman scattering occurs, it becomes noise in wavelength division multiplexing communication, and the energy of pumping light is reduced, so it is necessary to suppress the occurrence of stimulated Raman scattering.

FIG. 4 shows an example of measuring the gain of stimulated Raman scattered light. In this example, the gain of stimulated Raman scattered light is experimentally observed with respect to the intensity of pumping light propagating in a standard optical fiber.

In this experiment, a wavelength of 1.34μ was measured in an optical fiber with a length of 4.22km.
It was measured by propagating m excitation light. The wavelength of the stimulated Raman scattered light with respect to this excitation light wavelength is 1.42 μm. When a small signal light having the same wavelength as the stimulated Raman scattered light is incident on the incident end of the optical fiber at the same time as the excitation light, the small signal light is amplified by the stimulated Raman scattering process and propagates to the outgoing end of the optical fiber. The Raman gain shown on the vertical axis in FIG. 4 is calculated from the ratio of the value at the incident end of the small signal light to the value at the exit end.
It is a value obtained by subtracting the optical loss of the optical fiber 2. In FIG. 4, the solid line is the trend curve.

Such coupling between the pumping light and the small signal light poses a problem when performing wavelength division multiplexing transmission. For example, it is assumed that an optical signal having a wavelength of 1.34 μm and an optical signal having a wavelength of 1.42 μm are incident on the optical fiber and the light intensity at the incident end is several w. At this time, at the emission end of the optical fiber, the optical signal having a wavelength of 1.42 μm is amplified by obtaining energy with the optical signal having a wavelength of 1.34 μm, and the optical signal having a wavelength of 1.34 μm is attenuated by that amount.

The stimulated Raman scattered light is observed even when the small signal light is not incident from the incident end of the optical fiber. Excitation light intensity is 0.5w
At a certain level, noisy light existing near the entrance end of the optical fiber is amplified, and stimulated Raman scattered light that can be observed even with an avalanche photodiode at room temperature is generated.

FIG. 5 shows a cross-sectional view of a conventional stimulated Raman scattering suppressing optical waveguide.

This conventional optical waveguide is disclosed in the specification of Japanese Patent Application No. 227955/1984 and the drawings and utilizes optical coupling. That is, in the cladding 11, the main waveguide 12 in which the pumping light propagates
And a sub-waveguide 13 parallel to the main waveguide 12, and further provided with a loss imparting section 14 for attenuating the propagation light of the sub-waveguide 13, the main waveguide 12 and the sub-waveguide at the wavelength of the stimulated Raman scattered light.
Strong coupling occurs with 13 and the coupling is sufficiently small at the wavelength of the excitation light.

With this structure, when strong excitation light propagates through the main waveguide 12, even if stimulated Raman scattering occurs in a part of the main waveguide 12, the stimulated Raman scattered light moves to the sub-waveguide 13 and inside this sub-waveguide 13. Decays at. In general, stimulated Raman scattering is a light amplification process, and stimulated Raman scattering is not observed when there is no gain exceeding the loss of the system. Therefore, in the above structure,
Due to the loss of the sub-waveguide 13, stimulated Raman scattering is less likely to occur.

[Problems to be solved by the invention]

However, the stimulated Raman scattering suppressing optical waveguide of the conventional example has a drawback that the waveguide structure is more complicated than that of the standard optical fiber and the optical loss of the main waveguide is large. In addition, since the structure is complicated, the manufacturing cost is high.

It is an object of the present invention to solve the above problems and provide a method of suppressing stimulated Raman scattering with an optical fiber having a simple structure.

[Means for solving problems]

The stimulated Raman scattering suppressing method of the present invention is a method for suppressing stimulated Raman scattering generated when a light flux having a peak power value of 0.1 w or more is propagated in a single mode optical fiber,
It is characterized in that the single mode optical fiber is twisted 0.1 to 1 times per 1 m.

[Work]

The stimulated Raman suppression method of the present invention is most characterized by using a twisted optical fiber.

When propagating a light flux with two or more wavelengths in an optical fiber, the interaction caused by stimulated Raman scattering between these light fluxes is particularly strong when the polarization planes of the two light fluxes match. Does not occur when are orthogonal. For this, see RHStolen, “Polarization Effects in Fiber.
Raman and Brillouin Laser (Polarization E
ffects in Fiber Raman and Brillouin Lasers ”, IEEE J. Quantum Electronics, QE-15, Volume 10, No. 10 (1979), p. 1157.

Further, the polarization plane of the stimulated Raman scattered light when only the excitation light is incident on the optical fiber automatically coincides with the polarization plane of the excitation light.

Therefore, the amplification of the stimulated Raman scattered light can be suppressed by gradually shifting the polarization planes of the excitation light and the stimulated Raman scattered light. In the present invention, the optical fiber is twisted to cause mechanical distortion in the optical fiber, and the polarization plane of the light flux is rotated by the distortion. Since the rotation angle of the plane of polarization differs depending on the wavelength, the planes of polarization of the excitation light and the stimulated Raman scattered light are gradually displaced, and the amplification of the stimulated Raman scattered light can be suppressed.

Conventionally, a light beam is propagated through a twisted optical fiber. However, the purpose thereof is to stabilize the polarization state of light by utilizing circular birefringence generated by twisting, and it is effective when the twisting rate is 1 time / m or more. This is described in Sakai, Machida, and Kimura, “Polarization-maintaining characteristics of twisted single-mode optical fiber”, Research and Practical Report, The Institute of Electrical Communication, Nippon Telegraph and Telephone Public Corporation, Vol. 31, No. 12 (1982), page 2229. It is explained in detail.

On the other hand, the present invention is the same in that it utilizes the fundamental change in physical properties caused by the twist of the optical fiber, but since the purpose is different, the range of the twist ratio used is 0.1 to
1 time / m is different.

〔Example〕

FIG. 1 is a block diagram for measuring an optical fiber according to an embodiment of the present invention.

The light source 1 causes a light flux having a peak power of 0.1 w or more to enter the optical fiber 2. The optical fiber 2 is twisted 0.5 times / m or more and 1 time / m or less. The light receiver 3 measures the luminous flux propagating through the optical fiber 2.

Since the optical fiber 2 is twisted, a mechanical distortion is generated in the optical fiber 2, and the distortion causes the plane of polarization of the light beam to rotate. The ratio of the rotation angle of the plane of polarization to the twist angle is called the optical rotatory power.

FIG. 2 shows the measured values of optical activity. The solid line is the trend line.

As shown in this figure, the rotation angle of the plane of polarization differs depending on the wavelength for a fixed fiber length. Therefore, in the twisted optical fiber 2, the polarization planes of the excitation light and the stimulated Raman scattered light are gradually deviated. Therefore, effective amplification is not performed for the stimulated Raman scattered light, and the stimulated Raman scattered light is reduced for an optical fiber without twist.

FIG. 3 shows the calculated value of the gain of stimulated Raman scattered light. This calculated value is based on the wavelength of 1.34 μm of pumping light and wavelength in the optical fiber.
Raman gain with respect to the twist rate is shown when propagating a small signal light of 1.42 μm. The optical fiber length is 2,
The calculated values for 4, 6, 8 and 10 km are shown. The value obtained by the straight line in FIG. 2 was used as the optical activity.

Thus, the Raman gain decreases as the twist ratio of the optical fiber increases, and the effect is remarkable when the optical fiber length is 4 km or less.

〔The invention's effect〕

As described above, the stimulated Raman scattering suppressing method of the present invention suppresses stimulated Raman scattering by twisting the optical fiber. Therefore, there is an effect that a standard optical fiber can be manufactured with almost no change in the manufacturing process, and an inexpensive optical fiber can configure an optical transmission line in which stimulated Raman scattering is suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration for measuring an optical fiber according to an embodiment of the present invention. FIG. 2 is a diagram showing measured values of optical rotation. FIG. 3 is a diagram showing calculated values of the gain of stimulated Raman scattered light. FIG. 4 is a diagram showing an example of measuring the gain of stimulated Raman scattered light. FIG. 5 is a cross-sectional view of a conventional stimulated Raman scattering suppressing optical waveguide. 1 ... Light source, 2 ... Optical fiber, 3 ... Photoreceiver, 11 ...
Cladding, 12 ... Main waveguide, 13 ... Sub waveguide, 14 ... Loss imparting part.

Claims (1)

[Claims] 1. A method for suppressing stimulated Raman scattering that occurs when a light flux having a peak power value of 0.1 w or more is propagated in a single mode optical fiber, wherein the single mode optical fiber has 0.1 to 1 times per 1 m. A method for suppressing stimulated Raman scattering, which is characterized by applying a twist.