JPS63127700A - Acoustic sensor - Google Patents

Abstract

PURPOSE:To form an acoustic sensor having a reflection film with high reliability by applying metallic coating to the surrounding of an optical fiber in the lengthwise direction, forming a reflection film to either end of the optical fiber and opposing the face of reflection film to the face of reflection film formed by a high polymer film. CONSTITUTION:An incident light A is interferred repetitively between the reflection film applied to the end face of the optical fiber 4 an the reflecting film applied to a diaphragm film 5 to be a reflected light C. The reflected light C propagates again through the optical fiber 4, is branched by a light branching device 3, converted into an electric signal by a photodiode 6 and led to an output terminal 9 via an amplifier 7 and a filter 8. The oscillated wavelength of a laser 1 is controlled by a temperature control circuit 10 and the laser 1 is driven by a constant current drive circuit 11. Moreover, the diaphragm film 5 is a polyimide film, whose heat resistance is 400 deg.C thereby enabling the hard coating requiring the heat resistance of 300-350 deg.C. In using the optical fiber coated with a metal in this way, the heat resistance is improved and the acoustic sensor having a reflecting film with high reliability is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an acoustic sensor that converts physical conditions such as pressure and vibration into optical signals.

BACKGROUND OF THE INVENTION In recent years, optical sensors have been applied to many devices and systems for measurement, monitoring, and inspection purposes.

As a conventional technique, for example, "Optical Fiber Probe Microphone" by Ohba et al., Journal of the Institute of Electronics and Communication Engineers Vol. 1. J
There is an acoustic sensor as shown in 64-CI'h2 PP137 to 1431981/2. An example of the conventional acoustic sensor described above will be described below with reference to the drawings. Figure 3 shows the conventional acoustic sensor υ
This shows the configuration. In Fig. 3, 1 is a laser;
2 is an isolator that prevents light from returning to the laser 1; 27 is a vibrating membrane made of mica, and the optical fiber 1
9 are in opposing positions with the housing 13 interposed therebetween. 2
o is a beam splitter that splits the light. 21
is a photoelectric detector that converts light into electricity. 22 is a filter, and 9 is an output terminal. Person represents incident light, C represents reflected light, and B represents the physical state from the outside, which in this case is pressure.

The operation of the acoustic sensor configured as described above will be described below.

First, light from the laser 1 is guided to the optical fiber 19 via the isolator 2. Furthermore, it becomes an incident beam via a beam splitter 2o. The incident light beam is an optical fiber 19
The optical fiber 19 is guided by a reflective film surface formed at one end.
The reflected light C is repeatedly reflected and interfered with the reflective film surface formed on the vibrating membrane 27 which is located in an opposing position. Reflected light C
is split by a beam splitter 20 and converted into an electrical signal by a photoelectric converter 21. Furthermore, it is detected at the output terminal 9 as an output signal via the filter 22. Next, the conversion principle will be explained. The reflected light C changes depending on the pressure B.

FIG. 4 shows the configuration of the converter section. The nine incident light beams are guided from the reflective film with a reflectance of 40% formed on the end face of the optical fiber section 14 to the reflective film surface of the vibrating membrane 27 with a reflectance of 40% at a refraction angle of X, and a portion of the light passes through. A part of it is reflected and returns to the end face of the optical fiber section 14 again. A portion of the returned light becomes reflected light C and propagates through the optical fiber 19 to become an optical signal, and a portion is reflected and guided to the reflective surface of the vibrating membrane 27 again. Thereafter, reflection interference occurs repeatedly between the vibrating membrane 27 and the optical fiber section 14 in the same manner. The above original signal is determined by the intensity reflection coefficient, which is the ratio of the intensity of the nine incident lights and the intensity of the reflected light C. The intensity of the incident light C is I, and the intensity of the reflected light C is I.
Assuming r, the intensity reflection coefficient can be expressed by the following equation.

Here, δ=4π-nO-d-cos(x)/λd=do+Δd r=r,=r2 r: Amplitude reflection coefficient of optical fiber end, diaphragm no= Refractive index of air do: Original fiber end and diaphragm Distance Δd: Displacement of the diaphragm X: Refraction angle from the optical fiber end to air λ: Incident wavelength. Note that the square of the amplitude reflection coefficient is the reflectance.

Now, assume that the incident wavelength λ is 1.3 μm, and the amplitude reflection coefficient of the vibrating membrane 27 made of mica coated with a dielectric multilayer film is the same as the amplitude reflection coefficient of the optical fiber section 14 similarly coated with a dielectric multilayer film, and d .

When set optimally, the intensity reflection coefficient changes depending on the value of d from the first equation. In other words, it can be seen that it changes depending on the displacement Δd of the vibrating membrane 27 due to the external pressure B. Further, FIG. 5 shows a state in which the housing 13 and the original fiber 19 shown in FIG. 3 are fixed.

Generally, an optical fiber has a silicone resin cushioning material 23 around the optical fiber part 14 consisting of a quartz glass core and a cladding, and a polyamide coating 24 around the optical fiber part 14.
It is fixed to the housing 13 with an adhesive 25. The outer diameter of the polyamide coating 24 is 0.9 mm. optical fiber 19
The optical fiber portion 14 from which the silicone resin cushioning material 23 and the polyamide coating 24 have been removed is directly fixed to the nozzle 13 with an adhesive 25. At this time, the adhesion area is φ
The length is 125 μm and 0.5 mm to 1.0 ml. The reflective film 26 and the vibrating film 27 are in opposing positions. Further, the coating for forming the reflective surface is performed after the optical fiber 19 is fixed to the housing 13 with an adhesive 26 and the reflective film surface 26 is polished. The vibrating film 27 is made of mica and has high heat resistance, and a reflective film can be formed by coating the dielectric multilayer film. The reflectance was set to 4o%. Further, the reflectance of the reflective film surface 26 was set to 40 inches.

Problems to be Solved by the Invention However, in the above configuration, the heat resistance of the coating of the optical fiber and the adhesive for bonding the coating of the optical fiber and the housing is low, and the temperature of 300°C to 300°C required for a hard coat is low.
It has drawbacks such as not being able to withstand heat of 50"C and adversely affecting the quality of the reflective film surface during coating. Therefore, the reflective film can only be formed with a soft coating, and it is unreliable as it can be damaged by humidity or external force. There is a problem of performance: C, and it is desired to develop an acoustic sensor that can form a hard reflective film.

In view of the above problems, the present invention provides an acoustic sensor having a highly reliable reflective film surface.

Means for Solving the Problems In order to solve the above problems, the acoustic sensor of the present invention includes a metal coating applied around the length of the optical fiber,
Further, a reflective film is formed on either end of the optical fiber, and the surface of the formed reflective film faces the surface of the reflective film formed on the polymer film.

Effect of the Invention With the above-described configuration, the present invention can apply a hard coating to the fiber end face, and can obtain acoustic information with high quality and reliability.

EXAMPLE Hereinafter, an acoustic sensor according to an example of the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of an acoustic sensor according to an embodiment of the present invention.

In FIG. 1, 1 is a laser, and 2 is an isolator that converts light into parallel light using a lens 1. 3 is an optical splitter that splits the light, and is also called an optical fiber baffler. 4 is an original fiber, and a reflective film is formed on one end surface. Reference numeral 5 denotes a vibrating membrane, and a reflective film is formed on the surface facing the reflective film surface of the optical fiber 4.

6 is a photodiode that converts light into electricity. 7 is an amplifier, 8 is a filter, and 9 is an output terminal. 1
0 is a temperature control circuit that controls the temperature of the laser 1, and 11 is a drive circuit for the laser 1. Note that laser 1. Isolator 2. Filter 8. The output terminal 9 is the same as in the conventional example.

Regarding the acoustic sensor configured as above, the first
The operation will be explained using FIG.

In FIG. 1, light from a laser 1 is converted into parallel light by a lens 12, passed through an isolator 2, and coupled to an optical splitter 3 by the lens 12. The isolator 2 prevents light from returning to the laser 1. Two stages are used here. The light from the optical splitter 3 propagates through the optical fiber 4 and enters nine beams. The nine incident lights repeatedly interfere between the reflective film applied to the end face of the optical fiber 4 and the reflective film applied to the vibration membrane 6, and become reflected light C. The reflected light C propagates through the optical fiber 4 again, is split by the optical splitter 3, converted into electricity by the photodiode 6, and guided to the output terminal 9 via the amplifier 7 and filter 8.The laser 1 is a temperature control circuit. 1°, the oscillation wavelength of the laser 1 is controlled by the temperature, and the laser 1 is driven by a constant current drive circuit 11.

The vibrating membrane 5 is made of polyimide film with a thickness of 7.5 μm.
It is m. This polyimide film has a heat resistance of 400'
C, which allows hard coating that requires heat resistance of 300°C to 350'C. In this embodiment, a multilayer dielectric film is coated to have a reflectance of 40 factors. Similarly, the reflectance of the optical fiber 4 was set to 40%, but the reflectance does not necessarily need to be 40% depending on the application. Naturally, the back side of the polyimide film coated with a reflective film is coated with a non-reflective hard coating. The operating principle is the same as the conventional example.

Next, the structure of the optical fiber 4 and the vibrating membrane 6 will be explained in detail using FIG. 2. In FIG. 2, 14 is a core;
This is an optical fiber section consisting of a cladding, which is the same as in the conventional example. Reference numeral 15 denotes an aluminum coating layer coated with aluminum on the outer periphery of the optical fiber portion 14, and a nickel coating layer 16Fi coated with nickel on the outer periphery thereof. Here, this optical fiber section 14
, an aluminum coating layer 15, and a nickel coating layer 16 constitute the optical fiber 4. The optical fiber 4 is fixedly integrated with the housing 13 via a solder layer 17,
The end face of the original fiber 4 is polished.

The acoustic sensor of this embodiment configured as described above has improved heat resistance because the optical fiber 4 is fixed with solder instead of adhesive, unlike the conventional example. Therefore, heat resistance 300℃ ~ 3
Hard coatings requiring 60'C are possible. Additionally, there is no gas generated during preheating during coating.

The reflective film 18 is formed as described above. The vibrating membrane 5 may be provided to face the reflective membrane 18. As described above, the vibrating membrane 5 has a reflective membrane 18 coated with a note. The surface opposite to the reflective film 18 is coated with a non-reflective coating.

As described above, according to this embodiment, by using a metal-coated optical fiber, an acoustic sensor with improved heat resistance and a highly reliable reflective film can be obtained.

Effects of the Invention As described above, in the present invention, since the metal-coated optical fiber can be fixed and integrated into the housing with solder using the metal-coated original fiber, not only the heat resistance is improved but also no gas is generated at high temperatures. therefore,
Note-coating of the reflective film becomes possible, and an acoustic sensor having a highly reliable reflective film that is free from damage to the coating surface due to the influence of humidity, external force, etc. can be obtained.

Furthermore, unlike the conventional example, the optical fiber does not move in the direction of coming out of the housing due to temperature, and stability is improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an acoustic sensor according to an embodiment of the present invention, FIG. 2 is a cross-sectional view of the acoustic/light converter in this embodiment, and FIG. 3 is a block diagram of a conventional acoustic sensor. FIG. 4 is a diagram of the principle of acoustic/light conversion, and FIG. 6 is a sectional view of a conventional acoustic/light converter. 1... Laser, 4... Optical fiber, 6
... Vibration membrane, 13 ... Housing, 1
4... Optical fiber part, 16... Aluminum coating layer, 16... Nickel coating layer, 17... Solder layer, 18... reflective film. Name of agent: Patent attorney Toshi Nakao and one other person town

Claims (1)

[Claims] An optical fiber having a metal coating applied to the circumference in the length direction, a surface of the reflective film formed with a reflective film on either end of the optical fiber, and a surface of the reflective film formed of a polymer film. An acoustic sensor characterized in that the acoustic sensors are configured such that they face each other.