JPH0511480B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a sensor that detects underwater acoustic signals using optical fibers.

(Prior Art) Conventionally, as a method for detecting underwater acoustic signals using optical fibers, light passing through an optical fiber placed in the sound field and an optical fiber outside the sound field is made to interfere with each other.
The so-called optical interference method detects the phase of light changed by sound waves (for example, Journal of the Acoustical Society of Japan, Vol. 40, No. 2 (1984), P101-P106), which uses a structure that causes optical transmission loss in part of the optical transmission path. The so-called light intensity modulation method (for example, the so-called optical intensity modulation method, in which a sound wave is applied to the light to cause a change in the transmission loss, and the sound wave is detected as a change in the intensity of the passing light) (for example, the Acoustical Society of Japan, Vol. 40, No. 3 (1984), P175-
180) etc. were announced.

(Problem to be solved by the invention) However, all of the above methods are based on principles and do not take into consideration practically important matters such as the effects of output fluctuations due to temperature fluctuations, optical fiber vibrations, light source fluctuations, etc. There was a drawback.

(Means for Solving the Problems) The present invention comprises a cylindrical container consisting of a water pressure resistant container, a sound insulating structure container, and a rubber container, and has a first tube of the same quality and length that has anti-permeable membranes on both end surfaces. A part of the first optical fiber for a sensor among the optical fibers for a second sensor is installed in the rubber container, a part of the optical fiber for a second sensor is installed in the sound-insulating structure container, and a part of the optical fiber for a second sensor is installed in the sound-insulating structure container, A plurality of small passages communicating with the sound-insulating structure container are provided to fill both containers with liquid, and a piezoelectric body for adjusting the length of the optical fiber for the first and second sensors is provided in the water pressure resistant container. The present invention is an optical fiber hydrophone characterized in that the first and second sensor optical fibers and transmission optical fibers are connected to each other.

(Function) When the optical fiber hydrophone is placed in an underwater sound field, the first sensor optical fiber changes its length and refractive index under the influence of the acoustic signal, and its interference output changes. The optical fiber for the sensor is located in a sound-insulating structure and is not affected by the acoustic signal, so its interference output does not change. These interference outputs are transmitted to a photodetector via an optical fiber for optical transmission, and the difference between them is determined to detect an acoustic signal. Here, the first and second sensor optical fibers provided in the rubber container and the sound-insulating structure container, respectively, are filled and can be kept at the same temperature by the liquid flowing through the small holes, so temperature fluctuations can be prevented. The output fluctuations caused by this are the same, and by finding the difference between both interference outputs, it is possible to eliminate the influence of temperature fluctuations.

(Embodiment) FIG. 2 is a block diagram showing an embodiment of the present invention, in which a light beam 2 emitted from a laser oscillator 1 is expanded by a beam expander 3. The expanded beam 4 is split into two path lights 6 and 7 by a half mirror 5, condensed by lenses 8 and 9, and made to enter the optical fibers from one end face of the transmission optical fibers 10 and 20. It propagates within the transmission optical fiber and enters the first and second sensor optical fibers 11, 21 through the semi-transparent membranes 12, 22 on one end surface of the first and second sensor optical fibers 11, 21. . The incident light undergoes multiple reflections between the semi-transparent films 12, 13 and 22, 23, and a portion of the light is transmitted through the semi-transparent films 13, 23. The transmitted light enters the transmission optical fibers 16 and 26 and is transmitted, the intensity of the light is detected by the photodetectors 17 and 27, and the amplifier 1
Amplify at 8,28. The outputs of the photodetectors 17 and 27 follow the principle of Fabry-Perot interferometer,
First and second sensor optical fibers 11, 21
The length l, the reflection coefficient k of the semi-transparent film, the wavelength λ of light, and the number of multiple reflections n are proportional to the interference output I of the following equation.

I=1/1+Ksim ² β (1) K=4k/1−k ² (2) β=2πnl/λ (3) The interference output I in equation (1) has the characteristics as shown in curve 32 in Figure 3. Since β in equation (3) has a maximum value at every integer multiple of π, either the wavelength λ can be changed or the length l of the first and second sensor optical fibers 11 and 21 can be changed.
is changed to adjust the interference output I. As an example of the method of adjusting the length of the optical fiber, the first and second
A part of the sensor optical fibers 11 and 12 are wound around the piezoelectric bodies 15 and 25, and when there is no acoustic signal, the interference output I of each becomes I ₀ as shown by the dotted line 33 in FIG. Control power sources 19 and 29 are connected to the piezoelectric bodies 15 and 25 so that the output of the system shows the same value.
DC voltage is applied to the piezoelectric bodies 15 and 25, respectively.
The length of the optical fiber is adjusted according to the expansion and contraction of the fiber. In this state, a part of the first sensor optical fiber is exposed to the sound field, the second sensor optical fiber is sound-insulated, and the optical fiber within the range surrounded by the dotted line 14 shown in FIG. 2 is used as the acoustic sensor. Since the length and refractive index of the first sensor optical fiber 11 change depending on the acoustic signal, the interference output I changes depending on the acoustic signal. Since the interference output I on the side not exposed to the acoustic signal is constant, the difference between the outputs of the amplifiers 18 and 28 is determined by the processing circuit 30 and displayed on the display 31, so that the output corresponding to the strength of the acoustic signal is determined. Also, 2
Transmission optical fibers 10 and 2 in one optical system
By making optical fibers 0, 16 and 26, and the first and second sensor optical fibers 11 and 21 of the same quality and the same length, light generated by temperature fluctuations and vibrations of the transmission optical fibers can be reduced. Since the change in intensity is the same in the two optical systems, the amplifiers 18 and 2
By finding the difference between the outputs of 8, the above-mentioned change in light intensity can be eliminated. Fluctuations in the output of the laser oscillator 1 due to variations in the light source can also be eliminated by finding the difference between the two optical systems, since the output of the laser light source 1 is divided into two by the half mirror 5.

FIG. 1 is a cross-sectional view showing an embodiment of the present invention, and the structure is within the dashed line 34 shown in FIG. 2, and this portion is placed in an underwater sound field. The inside of the hollow container 36 is lined with a sound insulating material 38 to form a sound insulating structure. A chamber 37 is formed by one side of the hollow container 36 and a cylindrical rubber container 35 having an inner diameter equal to the outer diameter of the hollow container 36. Cylinders 39 and 40 are fixed in the hollow container 36 and in the chamber 37. Hollow container 36 and sound insulation material 37 have small holes 4
1 in several places. A first sensor optical fiber 11 is wound and fixed around the outer periphery of the cylinder 39 to detect an acoustic signal. Both ends of the first sensor optical fiber 11 are led into a water pressure container 42, one end is wrapped around the piezoelectric body 25, and the other end is directly connected to the transmission optical fibers 10, 16 through the connector 43. Connect with 44. The second sensor optical fiber 21 is wound around the outer periphery of the cylinder 40 in the same structure as the first sensor optical fiber 11 around the cylinder 39, and the same length is wound, and both ends are placed inside the water pressure container 42. After wrapping one end around the piezoelectric body 15, the other end is directly connected to the transmission optical fibers 20 and 26 using the connector 4.
Connect with 3,44. Here, the first and second sensor optical fibers 11 and 21 between the connectors 43 and 44 have the same length. Transmission optical fiber 10, 16,
20 and 26 are introduced into the water pressure container 42 as a cable 46 in which the piezoelectric bodies 15 and 25 are integrated together with the drive cable 45, and are fixed with a waterproof portion 47 to form a waterproof structure. The chamber 37 in the rubber container 35 and the inside of the hollow container 36 are filled with liquid and are connected through a small hole 41. The acoustic signal is transmitted through the rubber container 35 to the liquid in the chamber 37 and acts on the first sensor optical fiber 11, changing the refractive index and length of the optical fiber 11. Since the second sensor optical fiber 21 inside the hollow container 36 is sound-insulated by the sound-insulating material 37, it does not receive any acoustic signals. Since the liquid flows through the plurality of small holes 41 inside the chamber 37 and the hollow container 36, the temperature can be kept at the same level. Therefore, the piezoelectric By adjusting the voltages applied to the bodies 15 and 25, underwater acoustic signals can be detected by using the optical fiber hydrophone having the structure shown in FIG. 1 based on the difference in the interference outputs of the two sets of Fabry-Perot. be able to.

In this explanation, optical fibers are wound around the outer periphery of the cylinder to form the sensor part, but the same shape can be used as the rubber container 3 by arranging the optical fibers in an arbitrary shape such as a flat coil shape or an annular shape.
The same effect can be obtained even if they are placed in the chamber 37 in the chamber 5 and the sound-insulated hollow container 36, respectively.

(Effects of the Invention) As explained above, the present invention comprises a rubber container and a sound-insulating structure container in which the liquid filled in each of them flows through a plurality of small holes, and a first sensor for a first sensor is provided in the rubber container. By providing a portion of the optical fiber and a portion of the second optical fiber for the sensor in the sound-insulating structure container, and connecting the piezoelectric body for length adjustment of the first and second optical fibers for the sensor and the optical fiber for transmission. Since both optical systems are kept at the same temperature by being housed in the same water pressure container, there is an advantage that output fluctuations due to temperature fluctuations can be eliminated by determining the output difference between the two optical systems.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an embodiment of the present invention, FIG. 2 is a block diagram showing an embodiment using the present invention, and FIG. 3 is an output waveform diagram of a Fabry-Perot interferometer. 10, 16, 20, 26... Optical fiber for transmission, 11... Optical fiber for first sensor, 15,
25... Piezoelectric body, 21... Second sensor optical fiber, 35... Rubber container, 36... Hollow container, 3
7... Chamber, 38... Sound insulation material, 39, 40... Cylinder, 41... Small hole, 42... Water pressure container, 43,
44... Connector, 45... Drive cable, 46
... Cable, 47 ... Waterproof part.

Claims (1)

[Claims] 1 Of the first and second sensor optical fibers of the same quality and the same length that have anti-transparent membranes on both end faces, a part of the first sensor optical fiber is placed in a sound field, and a part of the second sensor optical fiber is placed in a sound field. In an optical fiber hydrophone that is placed outside the sound field and detects underwater acoustic signals from the difference in the intensity of light that passes through each optical fiber for each sensor, it consists of a cylindrical container consisting of a water pressure resistant container, a sound insulating structure container, and a rubber container. A cylinder with a part of the first sensor optical fiber wound around the outer periphery is fixed in the rubber container, and a cylinder with a part of the second sensor optical fiber wound around the outer periphery is fixed in the sound-insulating structure container. ,
A plurality of small holes are provided to communicate the rubber container and the sound-insulating structure container, and a liquid is filled in both containers, and a piezoelectric actuator is provided in the water pressure container to adjust the lengths of the first and second sensor optical fibers, respectively. An optical fiber hydrophone characterized in that the first and second sensor optical fibers and the transmission optical fiber are respectively connected within the container.