JPH0363033B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an earthquake sensor capable of efficiently detecting earthquakes using optical fibers.

Conventional technology and problems Conventional seismometers use, for example, a long hole drilled in bedrock.
The hole formed a reciprocating path for laser light from a He--Ne laser device, etc., and was configured to detect earthquakes by detecting interference fringes of the laser light. However, it is not easy to form holes in bedrock with high precision, and local vibrations caused by the movement of automobiles and the like are also detected, making it difficult to distinguish them from earthquakes. Therefore, conventional seismometers have to be installed deep underground to avoid being affected by vibrations caused by automobiles, etc., which limits the locations where they can be installed and also has the disadvantage that the cost of installing a seismometer is extremely high. It was hot.

OBJECT OF THE INVENTION The object of the present invention is to eliminate the influence of local vibrations and to detect only earthquakes with a simple configuration. Examples will be described in detail below.

Embodiment of the Invention FIG. 1 is an explanatory diagram of the principle of the present invention, in which each end of a single mode optical fiber loop 1 is
Laser equipment 2 such as He-Ne laser or semiconductor laser
A splitter 3 splits the laser beam into two parts, and each laser beam propagates through the optical fiber loop 1 and is emitted from each end. A signal corresponding to the phase difference of the light is output from the photodetector 4. The radius of the optical fiber loop 1 is a, the total length of the optical fiber loop 1 is L, the speed of light in vacuum is c, the wavelength of the light is λ, and it rotates around the central axis of the optical fiber loop 1 at an angular velocity of Ω. When I did,
The phase difference θ between the laser beams emitted from both ends of the optical fiber loop 1 is expressed by the following equation.

θ=(2πLa/cλ)Ω (1) This effect is called the Sagnac effect, and the phase difference caused by rotation is the product of the total length L and radius a of the optical fiber loop. Since the total length L is proportional to the number of turns of the optical fiber, the angular velocity of rotation Ω can be detected with high sensitivity even with a relatively small radius by increasing the number of turns.

The present invention utilizes the principle as described above, and FIG. 2 shows a perspective view of an embodiment thereof. In the figure, 10 is a hard substrate, 11 is an optical fiber loop formed by winding an optical fiber once or multiple times, and 12 is a measuring section including a laser device, a photodetector, etc. It is desirable that the optical fiber loop 11 be as long as possible, and the parallel portions are fixed on the rigid substrate 10 so that they do not come into contact with each other.

FIGS. 3 and 4 are explanatory diagrams of detection operations using seismic waves and local vibration waves, and schematically show deformations of the optical fiber loop 11. FIG. As described above, this optical fiber loop 11 is fixed to a hard substrate such as rock, and its length l is as long as possible, for example, 10 km, as described above. It is. Also optical fiber loop 11
The width d of is selected so that the optical fibers arranged in parallel do not touch each other and the desired loop area is obtained. This optical fiber loop 11
When a rotational component of Δφ is added to the center O of , the phase difference ΔZ of the laser beams emitted from both ends of the optical fiber loop 11 becomes the following equation.

△Z=8ΩSn/λc……(2) However, S is the area of the optical fiber loop 11, which is approximately l
Corresponds to ×d. Also, n is the number of turns of the optical fiber loop 11, Ω is the angular velocity of the rotational component, λ is the wavelength of light, and c
is the speed of light in vacuum.

If seismic waves are applied perpendicularly to the longitudinal direction or width direction of the optical fiber loop 11, each side of the optical fiber loop 11 will only move in parallel and no rotational component will occur, and the angular velocity Ω in that case is Since it is zero, the phase difference ΔZ of the laser beam is also zero.

Also, when a seismic wave is applied to the optical fiber loop 11 at a certain angle as shown by the arrow in FIG.
Seismic waves are propagated from one end of the optical fiber loop 11 to the other end, and a rotational component Δφ is generated in the optical fiber loop 11 due to partial movement indicated by a solid line from a dotted line. Therefore, a phase difference ΔZ of the laser beam according to the angular velocity Ω of this rotational component Δφ occurs as shown in equation (2).

This phase difference ΔZ is measured by the measuring unit 12 shown in FIG. 2, and since the phase difference ΔZ substantially follows the magnitude of the earthquake, the earthquake can be detected. In addition to making the phases of the laser beams incident on both ends of the optical fiber loop 11 the same, for example,
It is also possible to provide a phase difference of 90°. Furthermore, since the laser light propagated through the optical fiber loop 11 becomes elliptically polarized, it is also possible to provide a polarizing plate to extract only one direction component.

In addition, in the case of local vibration waves caused by passing automobiles, etc., instead of seismic waves, the vibration waves applied to one end of the optical fiber loop 11 may be attenuated and not propagated to the other end, or there may be multiple vibration waves. By randomly applying local vibration waves to different positions along the length of the optical fiber loop 11, the fourth
Only the local movement shown by the solid line from the dotted line in the figure occurs, and no rotational component occurs. Therefore, the phase difference ΔZ of the laser beam becomes zero.

As mentioned above, by fixing the optical fiber loop 11 as long as possible, such as several tens of kilometers, on a hard substrate such as rock, a configuration is created in which the phase difference △Z of the laser beam is generated only by seismic waves, and local vibration waves are generated. Earthquakes can be detected without being affected by

FIG. 5 shows optical fiber loops 11a to 11d arranged in different directions on a common substrate 20.
This shows an example in which hard substrates 10a to 10d having fixed thereon are arranged. Assuming that the substrate 20 is fixed horizontally to a bedrock etc., the optical fiber loops 11a,
11b can detect earthquake waves in the horizontal direction, and optical fiber loops 11c and 11d can detect earthquake waves in the vertical direction. Further, the wave propagation direction can be determined based on the phase difference detected by each optical fiber loop. Note that it is effective to arrange a rigid substrate on which three optical fiber loops are fixed at 120° intervals on a horizontal plane, since the propagation of waves from any direction within the horizontal plane can be easily determined.

Effects of the Invention As explained above, in the present invention, since an optical fiber loop is fixed on a hard substrate, a phase difference △Z of the laser beam is caused by rotational components of the optical fiber loop due to earthquake waves. By detecting this, earthquakes can be detected without being affected by local vibrations.

[Brief explanation of drawings]

Fig. 1 is an illustration of the principle of the present invention, Fig. 2 is a perspective view of an embodiment of the invention, Figs. 3 and 4 are illustrations of detection operation, and Fig. 5 is a top view of the embodiment of the invention. It is. 10, 10a to 10d are hard substrates, 11, 11
A to 11d are optical fiber loops, and 20 is a common substrate.

Claims (1)

[Claims] 1. An optical fiber loop of such a length that no phase difference occurs due to local vibrations was fixed on a hard substrate, and laser light was incident on both ends of the optical fiber loop, and emitted from both ends. An earthquake sensor characterized in that the seismic sensor is configured to detect the rotational angular velocity of the optical fiber loop based on the phase difference of the laser beam.