JPS5855478B2 - Distance displacement measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for accurately measuring minute displacements over relatively long distances, such as deformations in the earth's crust.

Devices that have recently come into widespread use on the ground to measure distances from several kilometers to several tens of kilometers with high precision are light wave modulation distance measuring devices called geosimeters and pulsed laser reciprocating devices. There is a distance measuring device that measures time.

The former has good accuracy, but is not suitable for long-distance measurement.

The latter is suitable for long-distance ranging, and its accuracy has improved significantly in recent years, but its accuracy is lower than the former, and its value ranges from several α to several 1.
It is on the order of 0crrL.

In recent years, the need for a system to measure changes in the earth's crust over time has been discussed in connection with earthquake prediction.
Since the amount of displacement to be detected is very small over a long distance range, the above-mentioned conventional distance measuring device cannot necessarily perform sufficiently satisfactory distance measurement.

An object of the present invention is to provide an apparatus that can accurately measure minute displacements over time over relatively long distances, such as when measuring crustal deformation.

The present invention projects a laser pulse with a very short duration obtained by a laser toward a reflector at a point to be measured, and also provides a round trip between the point where the laser pulse is projected and the point to be measured. The present invention uses an optical fiber that generates a delay time corresponding to time, and a streak tube to determine the time difference between a laser pulse emitted from the emitting end of the optical fiber and a laser pulse that is reflected and returned.

That is, currently, according to lasers, ns (light reach distance 30
Pulsed light with a width from α (corresponding to α) to ps (corresponding to <0.3 mm) can be obtained, and the time resolution of the streak tube has reached the ps order.

On the other hand, optical fiber has the characteristics of low loss and wide band, and it is known that single mode fiber can provide a band of GH2 or more for a transmission distance of several tens of kilometers.

Therefore, the present invention combines these three new technologies to enable highly accurate measurement of displacement over long distances.

FIG. 1 is a diagram explaining the present invention in detail. In the figure, 1 is a pulse laser oscillator, 2 is its output laser beam, 3 is a beam splitter, 14 is an optical fiber, and 5 is a distance over which displacement is to be measured. A reflector is placed at point 6, and 6 is a mirror.

Now, as shown in the figure, the position of the beam splitter 3 is B1, the output end of the optical fiber 4 is C1, the position of the reflector 5 is D, the position of the mirror 6 is E, and the final destination position of the reflected light is F. .

In addition, the effective length of the optical fiber 4, that is, the speed at which the laser pulse propagates within the optical fiber (referred to as group velocity) is Vg, the fiber length is l, and e is the speed of light in vacuum, and the length is given by c1/vg. If the length is BC, it is possible to set conditions such as BC = BD + DE + EF = 2L.

However, here, if the BD section is the atmosphere, we must consider its refractive index, but for the sake of simplicity, we
, DE, and EF are temporarily set as a vacuum.

By setting these conditions, the laser pulse passes through the optical fiber 4 via the beam splitter 3,
The time when the light reaches the final point C can be made to coincide with the time when the light reflected by the reflector 5 passes through the mirror 6 and reaches the final point F.

Furthermore, by changing the length of the optical fiber 4, the arrival times of the two pulses can be changed appropriately.

Conversely, if the effective length of the optical fiber 4 is fixed, the displacement of the length L will appear as a shift in the arrival time of the two pulses.

By using a streak tube to detect this deviation, highly accurate measurement becomes possible.

That is, although the streak tube can time-resolve photon signals that arrive in an extremely short period of time with high resolution, it has the disadvantage that it cannot take a long sweep length along the time axis.

However, as mentioned above, if the time difference between the two pulses incident on points C and F, that is, the streak tube, is made smaller, the characteristics of the streak tube described above can be utilized, and minute displacements over long distances can be detected. It becomes possible to do so.

Next, the present invention will be explained in detail with reference to FIG.

In FIG. 2, the same reference numerals as in FIG. 1 indicate the same parts.

Among the beams split by the beam splitter 3, 7 is a transmission beam that is projected toward the reflector 5 at the target ranging point, 8 is a fixed delay beam guided to the optical fiber 4, and 9 is the reflector 5 10 is an imaging lens for the reflected light reflected from the mirror 6 toward the transmitting side.

Reference numeral 11 denotes a box that houses the optical fiber 4, and the box includes a coupling lens 12 that focuses the fixed delay beam 8 on the input end 4a of the optical fiber 4, and a coupling lens 12 that focuses the fixed delay beam 8 on the input end 4a of the optical fiber 4, and a coupling lens 12 that focuses the fixed delay beam 8 on the input end 4a of the optical fiber 4. A coupling lens 13 for forming an image of light and a coupling lens 17 for forming an image on a photodetector 16 of the light branched by a light branching device 14 attached to a part of the optical fiber 4 and emitted from the emission end 15 are attached. are doing.

20 to 27 are streak tubes and their control parts; 20 is a slit through which the light from the coupling lenses 10, 13;'l, etc. passes; 21 is a lens; 22 is a photocathode; 24 is a (2) deflection plate for the beam, 25 is a sweep voltage generation circuit that is activated by the output of the photodetector 16 to sweep the voltage of the deflection plate 24, 26 is a channel plate, and 27 is a firefly. It is a light surface.

29 is a television camera that images the fluorescent surface 26 through an imaging lens 28, and 30 is a display device that displays the image.

In this configuration, a pulsed laser beam 2 emitted from a pulsed laser oscillator 1 is split into two by a beam splitter 3, and one is directed toward a reflector 5 installed at a distance in which displacement is to be measured. the other is coupled to the optical fiber 4 via the coupling lens 12.

Here, the length of the optical fiber 4 is such that when the reflected light 9 reaches the slit 20 of the streak tube, the light that has passed through the optical fiber 4 also passes through the coupling lens 13 from the output end 4b. 20 slits at the same time
The length should be such that it reaches .

The light incident on the slit 20 of the streak tube passes through the lens 2
1, the light is imaged on the photocathode 22, converted into electrons, and accelerated by an accelerating section (not shown).

This accelerated electron beam 23 is transmitted to the sweep voltage generation circuit 2
5, it is directed as it passes between the deflection plates 240 whose voltage is controlled by 5, and enters the channel plate 26.

In this channel plate, electrons are amplified and the fluorescent surface 27
It is converted into an image of light.

This image is a so-called streak image that is swept along the time axis by the voltage applied to the deflection plate 24.

The timing at which the deflection plate 24 starts sweeping is determined by detecting a light pulse branched by the light branching device 14 attached to the part 2 of the optical fiber 4 with the photodetector 16 and using it as a trigger signal.

Since the direction of flow of the electron beam 23 by the deflection plate 24 corresponds to the time axis, the streak image appearing on the fluorescent surface 27 is caused by a sweep if there is a time difference between the arrival times of the two light pulses incident on the slit 20. It appears as an image whose position is shifted in the direction.

By measuring this interval, the amount of deviation of the distance measurement point can be determined.In order to facilitate this work, in the embodiment, an image of the fluorescent surface 27 is captured by a television camera 29, and a display device such as a cathode ray tube is used. It is enlarged to 30.

In addition, in this device, by attaching a device to the storage box 11 for the optical fiber 4 to keep the temperature constant, it is possible to prevent the optical fiber 4 from expanding or contracting due to temperature changes and changing its length, and to prevent the refractive index from changing. It is possible to prevent the propagation speed of the optical pulse from changing due to the change.

As described above, in the present invention, the optical fiber delays the laser pulse from the pulsed laser oscillator by the time the laser pulse travels back and forth to the distance measurement point, and the light emitted from the optical fiber and the distance measurement point are Since the time difference with the reflected light is measured using a streak tube that can measure minute time differences, it is possible to measure displacement over long distances.

The measurement accuracy of the present invention will be explained below using specific numerical values.

As mentioned earlier, this system is used to observe minute displacements of the earth's crust over an interval of several kilometers to several tens of kilometers, which is necessary as data for earthquake prediction.

As a result of crustal deformation, it has been reported that recent displacement in the southern Kanto region is from several tens of degrees per 10 km in terms of horizontal displacement.

Now, assuming that the section distance is 10 km, and the refractive index of the optical fiber waveguide is approximately 1.5, the length of the optical fiber will be approximately 13 km.

If a single mode fiber made of quartz glass is used and its diameter is 100 μm, the weight of the fiber of this length is less than 500P.

Therefore, the fiber used as the delay line can be made sufficiently compact by winding it around a cylindrical drum or the like.

As a laser, Nd-YA with a wavelength of 1.067z77L
If a G laser is used, optical fiber loss of about 1 dB/km can be obtained even now, and combined with the high output power of the laser, there is no need to worry about problems caused by loss.

In addition, the dispersion of pulses due to optical fiber transmission is ~8 ps/A at a wavelength of 1.06 μm.
km, and after 13 km of transmission ~10
It is estimated to be about 0 ps, which is 3(1;7
It corresponds to 71.

At first glance, this seems to be the most problematic problem in the system according to the present invention, but if the input waveform of the laser and the input/output conditions to the optical fiber are constant,
Since the output waveform from the fiber is always in the form of constant distortion, it can be treated as a fixed amount of delay by waveform analysis, and does not necessarily cause deterioration in accuracy.

The change in the refractive index of the quartz optical fiber with respect to temperature change is on the order of 3 x 10-6 near room temperature, and the linear expansion coefficient is on the order of 5 x 10'.

If the temperature of the optical fiber storage box is controlled and the change is kept within 0.1°C, the change due to refractive index and expansion will be 3.6 mm and 0.65 mm in terms of length.

On the other hand, as mentioned above, if a mode-locked pulsed Nd-YAG laser is selected as the laser, a high output laser with a pulse width of 10 ps and 8 degrees can be easily obtained, and the streak tube has a resolution of about 10 ps. is a commercially available product.

These are 3rn7IL in terms of length. In actual measurements, it is necessary to correct for atmospheric refractive index fluctuations, but it is possible that this can be kept sufficiently small by measuring meteorological parameters and performing multiple measurements.

Considering all the above, according to the present invention, 10
Assuming a system that measures displacement over a km section, I
Accuracy below CIrL can be expected.

This is accurate enough to detect the horizontal displacement of the earth's crust in the southern Kanto region mentioned above.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the present invention in detail, and FIG. 2 is a configuration diagram showing an embodiment of the present invention. 1...Pulse laser oscillator, 3...Beam splitter-14...Optical fiber, 5...Reflector, 6...Mirror, lo,
12, 13, 17° 21.28...Lens, 1
4... Optical branching device, 16... Photodetector, 20... Slit, 22... Photocathode, 23... Electron beam , 24... Deflection plate, 25... Sweep voltage generation circuit, 26...
... Channel plate, 27 ... Fluorescent surface, 2
9...Television camera, 30...Display device.

Claims (1)

[Claims] 1. A pulsed laser oscillator that projects a laser pulse toward a reflector installed at a local point at one of the two points where the distance displacement is to be measured; An optical fiber that delays the laser pulse by the time it takes for the laser pulse to travel back and forth between points, and a streak tube that measures the time difference between the optical pulse from the exit end of the optical fiber and the reflected light from the reflector. A distance displacement measuring device characterized by: