JPS628761B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical transmission line, and particularly to a high-power optical transmission line suitable for transmitting ultra-high-speed, large-capacity information over long distances.

In recent years, with the progress in reducing the loss of single-mode optical fibers, there has been a strong demand for ultra-high-speed, large-capacity information transmission systems that operate in the low-dispersion wavelength range. The need to lengthen it gradually became stronger.

By the way, there is a limit to the reduction of loss in the optical fiber used, so for long-distance optical information transmission, it is necessary to use a photodetector with as high sensitivity as possible and a high-power laser as the light source for transmission. .

However, when high-power laser light is transmitted through an optical fiber, stimulated light scattering such as stimulated Raman scattering and stimulated four-photon mixing tends to occur, and as the stimulated scattered light becomes stronger, the quality of the signal light deteriorates. A problem arises. In conventional optical transmission lines, high-power lasers cannot be used as light sources because no effective measures have been taken to prevent the growth of stimulated scattered light, and this has made it difficult to use optical transmission lines with long relay intervals. The disadvantage was that it was difficult to install.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical transmission line that eliminates the drawbacks of the conventional optical transmission line and is capable of transmitting ultra-high-speed, large-capacity information over long distances.

The present invention is an optical transmission line characterized in that a bandpass filter is provided at at least one of the connection points where a plurality of optical fibers are connected in series to prevent stimulated scattered light from growing to a large extent.

According to the present invention, high-power signal light can be transmitted to an apogee through one transmission line, so that the signal light can be easily branched into a plurality of optical fibers at the far end. Conventionally, the signal light was branched into multiple optical fibers at the transmitting end, and the optical fibers were bundled and laid down to the far end, but compared to this, the present invention reduces the amount of optical fiber required. Since the cost can be significantly reduced, an economical optical transmission system can be realized.

Stimulated light scattering that occurs when high-power laser light is transmitted through an optical fiber includes stimulated Raman scattering, stimulated Brillouin scattering, stimulated four-photon mixing, and the like.

These stimulated light scatterings are particularly likely to occur in single-mode optical fibers with small core diameters, which are capable of ultra-high-speed, large-capacity information transmission.

Furthermore, in the wavelength band of 1.3 to 1.5 μm, which is suitable for long-distance transmission, even if a less intense laser beam of 1 W or less is used, the stimulated scattered light is likely to be generated strongly by transmitting through a long optical fiber.

Therefore, it is clear that in order to transmit ultra-high-speed, large-capacity information over long distances, it is necessary to take some measures to prevent stimulated scattered light from becoming strong.

[For the limits of optical fiber input to avoid signal degradation due to stimulated light scattering when a monochromatic laser light source is used and laser light is transmitted through a single optical fiber, see Applied Optics, No. 11, November 1972 issue
Vol. 2489-2494, in an article by RG Smith. ] Now, among stimulated light scattering, stimulated Brillouin scattering can cause loss of signal light when a monochromatic laser light source is used, but since the gain bandwidth of Brillouin scattering is narrow at about 100 MHz, multi-axis scattering If the spectral band of the signal light is broadened by using a laser beam with different modes, the threshold for the occurrence of stimulated Brillouin scattering can be raised, and the contribution of stimulated Brillouin scattering can be ignored compared to stimulated Raman scattering. I can do it as if it were wet. In addition, stimulated four-photon mixing is caused by the loss of signal light or the crosstalk between signals of different wavelengths due to the generation of new anti-Stokes light when light of other wavelengths is present in addition to signal light. Although the growth of stimulated four-photon mixing can be a cause of talk and the like, the growth of stimulated four-photon mixing can be almost completely suppressed by inhibiting the growth of stimulated Raman scattering.

Therefore, the basic structure of the present invention is that an optical fiber is divided into a plurality of sections, and a bandpass filter is provided at one or more connection points of the sections, and stimulated scattering centered on stimulated Raman scattering is achieved. The feature is that the light is attenuated or removed from the optical transmission path to prevent stimulated scattered light from growing significantly.

Next, regarding the high-power optical transmission line according to the present invention,
This will be explained in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of an embodiment according to the present invention. In this figure, 1 is the incident light,
2a, 2b, 2c are optical fibers, 3a, 3b are optical fiber connection elements, 4a, 4b are band filters,
5a and 5b are stimulated scattered lights taken out from the optical fiber transmission line, and 6 is light emitted from the optical transmission line.

In this embodiment, the optical fiber connecting element 3
In particular, self-occurring lenses are used for a and 3b, and the signal light is reflected by using the reflection characteristics of bandpass filters 4a and 4b, which are made of multilayer interference filters. The optical fiber 2b is efficiently coupled to the subsequent optical fiber 2b.

The characteristics of the bandpass filters 4a and 4b are that they have higher transmittance on the lower frequency side than the signal light, and have higher transmittance than the signal light.
Stimulated Raman scattered light generated on the low frequency side of about 100 to 1000 cm ^-1 is efficiently removed from the optical transmission path before it grows large.

FIG. 2 is a schematic diagram showing the relationship between transmission distance and light intensity within an optical fiber. In the figure, the length
la, lb, and lc correspond to the fiber lengths of the optical fibers 2a, 2b, and 2c in the above-described embodiment, respectively. The solid line indicates the light intensity when a bandpass filter is used, 21 is the signal light intensity, and 22 is the stimulated scattered light intensity.

In addition, the dashed-dotted line indicates the light intensity when the optical fibers 2a, 2b, etc. are fusion-spliced straight without using a bandpass filter,
is the signal light intensity, and 32 is the stimulated scattered light intensity.

Since the stimulated scattered light grows rapidly exponentially, when a certain threshold value is exceeded, the signal light is attenuated very significantly. Therefore, long-distance signal transmission is
In this case (conventional case), it is difficult. [The growth process of stimulated scattered light in optical fibers, especially the growth process of stimulated Raman scattered light, is described in the IEEE Journal of Quantum Electronics (IEEE, Journal of Quantum Electronics).
Quantum Electronics), May 1978 issue,
It is described in the paper by J. Auyeung et al., Volume 14, pages 347 to 352. ] As is clear from FIG. 2, according to the present invention, the rapid growth of stimulated scattered light can be effectively prevented, so high-power signal light can be transmitted over a much longer distance than before. .

For example, a 1.3μm YAG laser beam may be used as the high-output signal light, and a low-dispersion, low-loss single-mode optical fiber may be used as the optical fiber, which enables ultra-high-speed, large-capacity, long-distance transmission of information. Become.

In order to interpose a bandpass filter at the connection point of the optical fiber, attenuation of the signal light by the optical connection circuit for this purpose is unavoidable to some extent. However, the advantage of increasing the transmission distance by being able to transmit laser light with much higher output than before is immeasurably large.

The signal light intensity gradually attenuates due to the transmission loss of the optical fiber, so if the signal light intensity weakens, the single length of the optical fiber is lengthened, and a unit fiber of a bandpass filter is inserted in the optical transmission line on the side far from the transmitting end. The number per length can be reduced.

As described above, according to the present invention, low dispersion and
Since high-power laser light can be transmitted through a low-loss optical fiber, a high-power optical transmission line that can transmit ultra-high-speed, large-capacity information over long distances can be obtained.

Note that the present invention is not limited to only the configurations seen in the above-described embodiments, and several modifications are possible. For example, a diffraction grating reflector can be used as the bandpass filter instead of a multilayer interference filter. Furthermore, instead of using a multilayer interference filter, a wavelength-selective absorber may be used to selectively attenuate only the stimulated scattered light.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the configuration of an embodiment according to the present invention. FIG. 2 is a schematic diagram showing the relationship between transmission distance and light intensity within an optical fiber. In the figure, 1...incident light, 2a, 2b, 2
c... Optical fiber, 3a, 3b... Optical fiber connection element, 4a, 4b... Bandwidth filter, 5a, 5
b... Stimulated scattered light taken out from the optical fiber transmission line, 6... Light emitted from the optical transmission line, la, lb, lc... Fiber length, 21, 22... Signal light intensity when using band filters, respectively. , stimulated scattered light intensity, 3
1, 32...These are the signal light intensity and stimulated scattered light intensity, respectively, when no bandpass filter is used.

Claims (1)

[Claims] 1. In an optical transmission line formed by connecting a plurality of optical fibers in series, a bandpass filter is provided at at least one of the connection points of the optical fibers to prevent the stimulated scattered light from growing to a large extent. Characteristic optical transmission line.