JPH063639B2 - Optical / acoustic sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoacoustic sensor used in a measurement information transmission system such as a place where electromagnetic induction is strong or a place where explosion proof is required, such as steel or a chemical plant.

Conventional technology Optical fiber sound pressure sensor (1) The optical fiber itself becomes a sensor having a measuring function, and (2) The optical fiber simply functions as an optical transmission medium, and the sensor is attached to the end face for measurement. There are cases.

The configuration of such a conventional photoacoustic fiber sensor and the signal transmission method thereof will be described.

The configuration of a conventional optical / acoustic fiber sensor is shown in FIG.
Here, 1 is a sound wave, 2 is a movable optical fiber (hereinafter referred to as a movable fiber), 3 is a ferrule (fiber fixing jig), 4 is an optical fiber, 5 is a fixed end of the optical fiber 4 (hereinafter, fixed fiber). , 6 is incident light, and 7 is outgoing light. Now, when the sound wave 1 is incident on this sensor, the movable fiber 2 vibrates and an axis shift occurs between the movable fiber 2 and the fixed fiber 6. Therefore, when laser light is incident on the optical fiber 4, the amount of emitted light 7 changes due to the axis shift between the movable fiber 4 and the fixed fiber 5. This change in the amount of light is proportional to sound wave 1.

FIG. 7 is a schematic diagram of wavelength division multiplexing transmission, in which 8a, 8b ... 8n are laser light sources of wavelengths λi ... λn, 9a, 9b ... 9n are optical fibers,
10 is an optical combiner for combining the laser beams of the optical fibers 9a, 9b, 9n, 11 is an optical fiber, 12 is an optical demultiplexer, 13a, 13b,
.. 13n are optical fibers for transmitting the laser light demultiplexed by the optical demultiplexer 12 and are connected to the acoustic fiber sensors of FIG.

Now, laser beams having different wavelengths λ from the laser light source 8 are incident on the optical fibers 9a, 9b ... 9n. Optical multiplexer 1 for the above laser light
It is multiplexed at 0, transmitted through one optical fiber 11, and is incident on the optical demultiplexer 12. Light having different wavelengths emitted from the optical demultiplexer 12 becomes incident light 6 of the movable fiber sensor of FIG. 1 through the optical fiber.

Fig. 8 is a schematic diagram of WDM transmission (outgoing side from sensor)
Is. In FIG. 8, 9a, 9b, ... 9n are optical fibers, 1
Reference numeral 0 is an optical multiplexer, 11 is an optical fiber, and 12 is a demultiplexer, which have the same configurations as in FIG. 14a, 14b3, 14n
Is a photodetector for detecting the laser light of wavelengths λ ₁ , λ _2, ... λ _n demultiplexed by the demultiplexer 12. Here will be described the operation of the Figure 8 Figure 6 photoacoustic sensor of the emitted light 7 (wavelength lambda ₁ ~
lambda _n) are respectively optical fibers 9a, 9b, is incident from ... 9n to the optical multiplexer 10, is incident to the optical fiber 11, it is transmitted. The transmitted light beam is incident on the optical demultiplexer 12, demultiplexed into respective wavelengths λ _{1 to} λn, and incident on the light receiver 14.
To be detected.

Problems to be Solved by the Invention As described above, in the case of transmitting an input / output signal in the conventional optical / acoustic sensor, the transmission path has one input for each sensor and one output for each sensor.
It was customary to use two books in total. A method has also been proposed in which the transmission system is wavelength-multiplexed using a single transmission path as described above, but each sensor does not have wavelength selectivity, so a demultiplexer and a multiplexer are required respectively. . The optical demultiplexer and the multiplexer are interference film type, diffraction grating type, etc., but in reality, they are expensive and do not have sufficient performance.

Means for Solving the Problems In order to solve the above problems, the optical / acoustic sensor of the present invention makes a laser beam incident on a first optical branching device and transmits the emitted light through a transmission optical fiber, An optical fiber and a microlens are respectively provided in a plurality of outgoing branch paths of the optical splitter which are incident on the optical splitter of No. 2, and the lens end face and the vibrating film are spaced from each other at different intervals. A parallel plane interferometer provided with a vibrating film facing each other in parallel is provided, laser light is incident on the parallel plane interferometer after passing through the optical fiber and the microlens, and reflected light from the parallel plane interferometer is changed to the first The light is incident on the first light branching device through the optical branching device 2 and the optical fiber for transmission, and the branched reflected light is received by the light receiving device. It is configured to retrieve the output.

The laser light is made incident on the second optical branching device, and a plurality of outgoing branching paths are provided with parallel plane interferometers having different intervals between the lens end face and the vibrating membrane to form a sensor. , A sensor for detecting a minute displacement of the vibrating film by utilizing the multiple reflection (multi-beam interference) of the laser light incident on the parallel plane interferometer. in previously prepared sensor to different intervals in accordance with the wavelength lambda ₁ to [lambda] _n, the sensor itself to lambda ₁ ~
It is a sensor with wavelength selectivity of λ _n . By arbitrarily changing the wavelength of the incident light, a sensor corresponding to the wavelength λ is operated from a remote location (incident laser light source), and the reflected light flux passes through the first optical branching device near the light source and then changes to the wavelength λ. By extracting the corresponding sensor and output, it is possible to selectively operate any sensor from a remote location.

In addition, some of the λ _{1 to} λ _n laser light sources having different wavelengths are combined and transmitted by a transmission optical fiber, the plurality of sensors are operated, and the reflected light is returned to the first optical branching device and then branched. It is possible to distinguish which sensor output is obtained by performing spectral analysis with a detector and detecting the wavelength λ and its output.

Embodiments The structure and operation of the present invention will be described in the following embodiments. FIG. 1 shows the basic configuration of an embodiment of the optical / acoustic sensor relating to the present invention. Here, 21 is a laser light source, 22
Is an optical branching device for branching the laser light from the laser light source 21 and the reflected light, 23 is an optical fiber for transmitting the laser light, 24
Is a microlens, 25 is a vibrating film that vibrates by acoustic vibration, 26 is an air layer between the microlens 24 and the diaphragm 25, 27 is a photodetector for detecting the laser beam from the demultiplexer 22, and 28 is a photoreceiver. It is an amplifier that amplifies the signal of 27.
Further, FIG. 2 is a diagram for explaining the operation principle of the optical / acoustic sensor, in which the vertical axis represents the reflection coefficient of light. The horizontal axis represents the distance between the vibrating membrane 25 and the microlens 24. 30 is the waveform of the input sound, 32 is the operating point, 31
Is the intensity-modulated light output.

In FIG. 1, the laser light emitted from the laser light source 21 enters the optical branching device 22, passes through the optical fiber 23, and then the microlens 2
It is incident on 4. Here, the air layer 26 of the microlens 24
The end surface on the side and the surface of the vibrating film 25 on the side of the air layer 26 are coated with metal or the like so as to cause light reflection, and the microlens 24
And the vibrating film 25 are arranged so as to be parallel to each other with the air layer 26 interposed therebetween to form a parallel plane interferometer 29. Here, d is the distance between the microlens 24 and the vibrating membrane 25 of the parallel plane interferometer. A part of the laser beam emitted from the microlens 24 is reflected by the vibrating film 25 and enters the microlens 24. A part of the light is transmitted through the end surface of the microlens 24, a part of the light is reflected again, returns to the vibrating film 25, is reflected again by the vibrating film 25, and enters the microlens 24. In this way, the laser light is repeatedly reflected by the air layer 26. The entire reflected light flux enters the light receiver 27 through the optical fiber and the branching device 22, and is converted into an air signal. It is known that the total luminous flux reflected by the parallel plane interferometer as described above has a characteristic 33 shown in FIG.

The distance d between the microlens 24 and the vibrating membrane 25 of the parallel plane interferometer 29 is determined, and the operating point is determined as 32. Now, when the vibrating membrane 25 vibrates like 30 due to the sound wave, its output light is modulated as 31 and the vibration of the vibrating membrane 25 can be detected by the output light. FIG. 3 is an operation explanatory diagram of the optical / acoustic sensor. Where the horizontal axis is the phase difference Is.

Note that n ₀ is the refractive index of the air layer 26, λ is the wavelength of the laser light, and d
Is the microlens 24 of the parallel plane interferometer 29 and the vibrating membrane 2.
5 is the interval.

As is well known, the phase difference δ is a periodic function that repeats every 2π (where m is an integer.) The vertical axis is the intensity reflection coefficient of reflected light with respect to incident light. Further, γ is the amplitude reflectance when the vibrating film 25 and the microlens 24 are viewed from the air layer 26 in the parallel plane interferometer 29, and the amplitude reflectance γ.
= 0.9, γ = 0.6, γ = 0.3.

Here, the distance d between the microlens 24 and the vibrating membrane 25 is
When it increases, δ = 4πn ₀ d / λ, so δ increases along the horizontal axis in FIG. 3. Now, with the reflection coefficient γ = 0.9, the distance d between the microlens 24 and the vibrating film 25 is set to determine the operating point. When the distance d is set so that the operating point is the point 34 and sound pressure is applied to the vibrating membrane, the intensity-modulated light output is obtained as described above. However, if the operating point is set to the point 35 in FIG. 3, the light output does not change even if the vibrating membrane vibrates. That is, sharp wavelength selectivity can be provided by changing the above d.

FIG. 4 shows a first embodiment using the wavelength tunable laser of the optical / acoustic sensor of the present invention. Here, 36 is a wavelength tunable laser, 22 is a first optical branching device, 23 is an optical fiber, 37 is a second optical branching device, 38a, 38b, ... 38n are parallel plane interferometers described in FIG. Is a light receiver. The same reference numerals are given to those described in the basic configuration diagram of FIG. Also, the parallel plane interferometers 38a, 38b, ... 38n change the distance d between the microlens and the vibrating membrane to d ₁ to d _N , respectively, to change the wavelengths λ _{1 to} λ _N.
It is configured to have wavelength selectivity with respect to. When a certain wavelength λ ₁ is emitted from the wavelength tunable laser light source 36, the laser light passes through the optical branching device 22, the optical fiber 23, and the second optical branching device 37 and is incident on the parallel plane interferometer 38a, which is reflected and reflected. It returns to the branching device 22 of No. 1 and the branched light is detected by the photodetector 27.

Therefore, the wavelengths λ ₁ , λ ₂ , ... λ _{N of the} wavelength tunable laser light source 36
By arbitrarily selecting the spacing d ₁ corresponding to the wavelength of the laser beam, d _2, ... plane parallel interferometer 38a which lives d _n, 38b,
... 38n can be operated and its output light is received by the receiver 2
By detecting in 7, it is possible to operate an arbitrary sensor from the laser light source side. By making the optical fiber 23 long, it is possible to perform remote measurement.

FIG. 5 is a configuration diagram showing a second embodiment of the present invention using a plurality of laser light sources.

In FIG. 5, N laser light sources have different λ ₁ to λ _N of the wavelength 17 of the light source. Reference numeral 18 is a first optical branching device, 14 is a second optical branching device, 19 is an optical spectroscope, and 20
Are _N photodetectors corresponding to wavelengths λ ₁ ... λ _N.

The laser beams λ ₁ , λ ₂ , ... λ _N emitted from the light source 1 are combined by the first optical branching device 18 and are incident on the optical fiber 3 to reach the second optical branching device 14. The laser light emitted from the second optical branching device 14 is modulated by a voice signal in the sensor head unit 15 having a different selected wavelength and returned. The returned signal is separated by the optical spectroscope 19 and detected by the photodetector 20.

With this configuration, the light receivers λ ₁ , λ ₂ ... λ _N
, One sensor for any sensor λ ₀ ... λ
You can choose from ₁₀ actions.

EFFECTS OF THE INVENTION The optical / acoustic sensor of the present invention is provided with an air layer in which the distance d between the vibrating film of the optical parallel plane interferometer, which is a constituent element, and the microlens is different. The desired sensor corresponding to the wavelength can be selected from a remote place by selecting the wavelengths λ ₁ , λ ₂ , ... λ _n of the incident laser light.

Also, in a system that supplies a sensor with a laser light source that multiplexes laser light sources with different wavelengths and performs wavelength-multiplexed transmission with a single optical fiber, the sensor head has wavelength selectivity, so the light receiving side disperses the light into wavelengths. By selecting, the sensor head unit corresponding to the wavelength can be selected.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of an optical / acoustic sensor in one embodiment of the present invention, FIG. 2 is a characteristic diagram showing an operating principle of the optical / acoustic sensor, and FIG. FIG. 4 is a block diagram showing the configuration of the same optical / acoustic sensor using a wavelength tunable laser, FIG. 5 is a block diagram showing the configuration of the same wavelength multiplex transmission optical / acoustic sensor, and FIG. FIG. 7 is a sectional view of a movable optical fiber sensor, FIG. 7 is a block diagram of a sensor on the incident light side of conventional wavelength division multiplexing transmission, and FIG.
It is a block diagram of the outgoing radiation side sensor of the conventional wavelength division multiplexing transmission. 21 ... laser light wave, 22 optical branching device, 23 optical fiber,
24 ... Microlens, 25 ... Vibration film, 26 ... Air layer, 27 ... Photoreceiver, 28 ... Amplifier, 29 ... Parallel plane interferometer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuji Hattori 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-57-197690 (JP, A) JP-A-58-169007 (JP, A)

Claims (2)

[Claims] 1. A laser beam is incident on a first optical branching device, and its outgoing light is transmitted through a transmission optical fiber to enter a second optical branching device. An optical fiber and a microlens are respectively provided in the path, and a parallel plane interferometer in which a vibrating film is provided in parallel facing the lens end face is provided at different intervals between the lens end face and the vibrating film. Laser light is incident on the parallel plane interferometer via the optical fiber and the microlens, and reflected light from the parallel plane interferometer is passed through the second optical branching device and the transmission optical fiber to the first optical branch. An optical / acoustic sensor that injects the output light that has been split into light into a light source using a light receiver. 2. The optical / acoustic sensor according to claim 1, wherein the reflected light is extracted from the first branching device, separated by a light spectroscope, and output is extracted by a light receiver.