JPH0680412B2 - Acoustic sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acoustic sensor that utilizes an optical phenomenon to replace an acoustic signal with a change in the intensity of light for detection.

2. Description of the Related Art In recent years, semiconductor lasers and optical fibers have come into practical use, and various acoustic sensors utilizing optical phenomena are being developed.

Hereinafter, an example of the conventional acoustic sensor described above will be described with reference to the drawings.

FIG. 2 is a block diagram showing the configuration of a conventional acoustic sensor.

In FIG. 2, 1 is a semiconductor laser, 2 is a first rod lens, 3 is an optical isolator, 4 is a non-polarizing beam splitter, 5 is a second rod lens, and 6 is a single mode optical fiber. 7 is a sensor head, 8 is a third rod lens with a partial reflection coat that constitutes the sensor head 7,
Reference numeral 8r is an end surface of the third rod lens 8 which is partially reflective.

Reference numeral 9 denotes a sound receiving diaphragm which constitutes the sensor head 7, and 9r denotes an end surface of the sound receiving diaphragm 9 which is partially reflective. The partially reflective coat end surface 8r of the third rod lens 8 and the partially reflective coat end surface 9r of the sound receiving diaphragm 9 form a so-called Fabry-Perot interference system. Reference numeral 10 is a light receiving element.

a, b, c, e and f are the semiconductor laser 1 to the sensor head 7
Is a light beam traveling toward the light beam, d is a light beam d of the light beam c diffracted by the non-polarizing beam splitter 4, and g is a third light beam.
Of the rod lens 8 and the partially reflecting coat end surface 9r of the sound receiving diaphragm 9 are repeatedly reflected and interfered with each other, and h is a light beam transmitted through the sound receiving diaphragm 9, i
And j are light beams returned from the sensor head 7, k is a light beam of the light beam j transmitted through the non-polarizing beam splitter 4, and 1 is a light beam of the light beam j diffracted by the non-polarizing beam splitter 4.

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

First, the light beam a emitted from the semiconductor laser 1 becomes a parallel light beam b by the first rod lens 2. Next, the light beam b passes through the optical isolator 3 to become the light beam c. Further, the light beam c is divided into a diffracted light beam d and a transmitted light beam e by the non-polarization beam splitter 4,
Among them, the transmitted light beam e is polarized by the second rod lens 5 and becomes the light beam f propagating inside the single mode optical fiber 6. Next, the light beam f is collimated by the third rod lens 8 and is formed by the partial reflection coat end face 8r of the third rod lens 8 and the partial reflection coat end face 9r of the sound receiving diaphragm 9. The light beam g is repeatedly reflected and interfered by the Perot interference system.

Each time the reflection is repeated, light that is proportional to the distance between the third rod lens 8 and the end surface 9r of the partial reflection coat 8r of the sound receiving diaphragm 9 is transmitted. Among them, the light transmitted through the partial reflection coating end surface 9r of the sound receiving diaphragm 9 becomes the light beam h, while the light passing through the partial reflection coating end surface 8r of the third rod lens 8 is the third.
Is collected by the rod lens 8 and is returned to the single mode optical fiber 6 again to become the light beam i.

Next, the light beam i is divided into a diffracted light beam 1 and a transmitted light beam k by the non-polarization beam splitter 4, and the diffracted light beam 1 is incident on the light receiving element 10 and detected.
Here, the light beam k is ineffective light that does not contribute to the measurement, like the above-described light beam d.

Now, when a sound wave is applied to the sensor head 7, the sound receiving diaphragm 9
Is displaced, and the end face of the partially reflective coat of the third rod lens 8
The distance between 8r and the end surface 9r of the partially reflecting coat of the sound receiving diaphragm 9 changes, and the return light beam i from the sensor head 7 becomes an intensity-modulated light beam proportional to the displacement of the sound receiving diaphragm 9.
A part of the light beam 1 enters the light receiving element 10 as signal light and is detected.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the above configuration, the third rod lens optically couples with the single-mode optical fiber, that is, shapes the outgoing light from the single-mode optical fiber into a parallel light beam. In addition, it has the function of condensing the light extracted as signal light again and making it enter the single-mode optical fiber, and the function of forming an optical Fabry-Perot interference system by partially reflecting the end surface of the fiber. , Optical axis alignment between the single mode optical fiber and the third rod lens is adjusted in three axes of X, Y and Z, and θ,
There has been a problem that it is extremely difficult to adjust the tilt angle of two axes of δ and fine adjustment of a total of five axes at the same time.

In addition, the non-polarizing beam splitter halves the emitted light of the semiconductor laser, and further halves the signal light from the sensor head, so there is a problem in terms of both efficiency and signal-to-noise ratio (hereinafter referred to as SN ratio). Had.

In view of the above-mentioned problems, the present invention provides an acoustic sensor that can easily perform optical axis alignment with an optical fiber, and realizes a high SN ratio with low loss.

Means for Solving the Problems In order to solve the above problems, an acoustic sensor of the present invention includes a semiconductor laser and a first rod lens which is arranged at a light emitting end of the semiconductor laser and shapes the emitted light into parallel light. A polarization beam splitter arranged at the light emitting end of the first rod lens so that its outgoing light has a polarization plane coincident with each other, and a second light beam emitting end of the polarization beam splitter emitting from the semiconductor laser. And an optically coupled polarization-maintaining optical fiber arranged at the light emitting end of the second rod lens from the semiconductor laser with the plane of polarization of the emitted light aligned, and the plane of polarization. A third rod lens which is arranged at a light emitting end of the storage optical fiber from the semiconductor laser and shapes the emitted light into parallel light, and a light output from the semiconductor laser of the third rod lens. The quarter-wave plate which is arranged at the emitting end and converts the emitted light into circularly polarized light and which has a partially reflective coated light emitting end, and a partially reflective coated surface having the partially reflective coated surface And a sound receiving diaphragm arranged to face the quarter-wave plate,
The return light from the Fabry-Perot interference system formed by the sound receiving diaphragm and the quarter-wave plate is converted again into linearly polarized light by the quarter-wave plate, and further the third rod is used. The intensity change is detected through the lens, the polarization-maintaining optical fiber, and the polarization beam splitter in this order.

The present invention has the above-described configuration, wherein the 5-axis simultaneous optical axis alignment between the two parts and the 3-axis simultaneous optical axis alignment between the two parts and the two parts in the optical coupling between the sensor head and the optical fiber are performed. It can be separated into two simultaneous optical axis alignments, and independent optical axis alignments are possible, which not only makes optical axis alignment extremely easy, but also achieves low loss and a high SN ratio. Your heart will be able to do it.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an acoustic sensor in one embodiment of the present invention. In FIG. 1, 1
Is a semiconductor laser, 2 is a first rod lens which is arranged at the light emitting end of the semiconductor laser 1 and shapes the emitted light into parallel light, and 20 is a light emitting end of the first rod lens 2 and its polarization plane. The polarization beam splitters 5 and 5 are arranged so as to coincide with each other, 5 is a second rod lens arranged at the light emitting end of the polarization beam splitter 20 from the semiconductor laser 1, and 21 is a second rod lens.
Optically coupled polarization-maintaining optical fiber disposed at the light-exiting end of the rod laser 5 from the semiconductor laser 1 so that the light-emission light and the polarization plane match, 22 is a sensor head, and 23 is a polarization-preserving light. A third rod lens 24 arranged at the light emitting end of the fiber 21 from the semiconductor laser 1 and shaping the emitted light into parallel light, and 24 is the semiconductor laser 1 of the third rod lens 23.
A quarter having a light emitting end which is arranged at the light emitting end to convert the emitted light into circularly polarized light and which is partially reflection coated.
A wave plate, 24r is an end surface of the partial reflection coating, 9 is a surface having a partial reflection coating, and the sound receiving diaphragm is arranged so that the partial reflection coating surface faces the quarter wavelength plate. It is the end face of the partially reflective coat. End face 24r of partially reflecting coat of quarter-wave plate 24 and end face 9 of partially reflecting coat of sound receiving diaphragm 9
The r and form the Fabry-Perot interference system. Reference numeral 10 is a light receiving element. a, b, e and f are light beams traveling from the semiconductor laser 1 toward the sensor head 22, g
Is a light beam that repeatedly reflects and interferes between the partially reflecting coat end surface 24r of the quarter-wave plate 24 and the partially reflecting coat end surface 9r of the sound receiving diaphragm 9, and h is a light beam that has passed through the sound receiving diaphragm 9. i, j, l are light beams returned from the sensor head 7. The same parts as those in the conventional example shown in FIG. 2 are designated by the same reference numerals.

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

First, the light beam a emitted from the semiconductor laser 1 becomes a parallel light beam b by the first rod lens 2. Next, the light beam b, while maintaining the linearly polarized light, is almost entirely transmitted through the polarization beam splitter 20, condensed by the second rod lens 5, and propagated inside the polarization-maintaining optical fiber 21. become. Next, the light beam f is collimated by the third rod lens 23 and is converted into a quarter wave plate.
The linearly polarized light is converted into circularly polarized light there, and the partially reflected coating end face 24r of the quarter wavelength plate 24 and the sound receiving diaphragm 9
The light beam g is repeatedly reflected and interfered by the Fabry-Perot interference system formed by the end surface 9r of the partial reflection coat of FIG.

Every time the reflection is repeated, light which is proportional to the distance between the quarter-wave plate 24 and the end face 9r of the partial reflection coat of the sound receiving diaphragm 9 and the end face 9r of the partial reflection coat of the sound receiving diaphragm 9 is transmitted. Among them, the light transmitted through the partially reflecting coat end surface 9r of the sound receiving diaphragm 9 becomes a light beam h. On the other hand, the light transmitted through the partially reflective coating end face 24r of the quarter-wave plate 24 is converted from circularly polarized light into linearly polarized light orthogonal to the incident light by the quarter-wave plate 24, and then the third light is transmitted. It is condensed by the rod lens 23 and is returned to the polarization-maintaining optical fiber 21 again to become a light beam i. Next, the light beam i is diffracted by the polarization beam splitter 20, and the diffracted light beam 1 is incident on the light receiving element 10 and detected.

Now, when a sound wave is applied to the sensor head 22, the sound receiving diaphragm 9
Is displaced, and the end face 24r of the partially reflective coating of the quarter-wave plate 24
And the distance from the end surface 9r of the partial reflection coating of the sound receiving diaphragm 9 changes, and the return light beam i from the sensor head 22 becomes an intensity-modulated light beam proportional to the displacement of the sound receiving diaphragm 9. The diffracted light beam 1 enters the light receiving element 10 as signal light and is detected.

As described above, according to the present embodiment, a quarter-wave plate with a partial reflection coating is newly added to the sensor head, and the function of the third rod lens is made to be a single function of coupling with the optical fiber, and the optical Fabry-Perot interference system with partial reflection coat 4
The optical axis alignment between the sensor head and the optical fiber is achieved by using a half-wave plate and a sound receiving vibration plate, and by branching and transmitting the light using a polarization beam splitter and a polarization-maintaining optical fiber. It provides an acoustic sensor that can be easily performed, has a low loss, and has a high SN ratio, and its practical effect is great.

EFFECTS OF THE INVENTION As described above, according to the present invention, the semiconductor laser, the first rod lens arranged at the light emitting end of the semiconductor laser to shape the emitted light into parallel light, and the light emission of the first rod lens. A polarization beam splitter disposed at an end of the polarization beam splitter so as to match the polarization plane thereof, a second rod lens disposed at a light emission end of the semiconductor laser of the polarization beam splitter, and a second rod lens. Of the semiconductor laser, the polarization-maintaining optical fiber optically arranged and arranged so that the polarization plane thereof coincides with that of the emitted light, and the light-emitting end of the polarization-maintaining optical fiber from the semiconductor laser. And a third rod lens for shaping the emitted light into parallel light and a light emitting end from the semiconductor laser of the third rod lens for converting the emitted light into circularly polarized light. A quarter wave plate having a light emitting end that is partially reflection coated, and a sound receiving device that has a partially reflection coated surface and the partially reflection coated surface is arranged to face the quarter wavelength plate. A vibration plate is provided, and the sound receiving vibration plate and the quarter
The return light from the Fabry-Perot interference system formed with the wave plate is reconverted into linearly polarized light by the quarter wave plate, and the third rod lens, the polarization-maintaining optical fiber, and the polarized light are further converted. By detecting the intensity change through the beam splitter in sequence, the 5-axis simultaneous optical axis alignment between the two parts in the optical coupling between the sensor head and the optical fiber is the same. It is possible to separate the optical axis alignment and the simultaneous optical axis alignment of the two axes between the two parts, and it is possible to perform the optical axis alignment independently of each other, which not only makes the optical axis alignment extremely easy, but also has low loss and high It will be possible to make SN ratio,
The practical effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an acoustic sensor in one embodiment of the present invention, and FIG. 2 is a block diagram showing the configuration of a conventional acoustic sensor. 1 ... semiconductor laser, 2 ... first rod lens, 5 ...
… Second rod lens, 9 …… Sound receiving diaphragm, 9r …… Partially reflective coating end face, 10 …… Light receiving element, 20 …… Polarizing beam splitter, 21 …… Polarization-maintaining optical fiber, 22 …… Sensor head , 23 ... third rod lens, 24 ... quarter wave plate, 24r ... partial reflection coating end face.

Claims (1)

[Claims] 1. A semiconductor laser, a first rod lens disposed at a light emitting end of the semiconductor laser and shaping the emitted light into parallel light, and a light emitting end of the first rod lens. A polarization beam splitter arranged so that the plane of polarization of the emitted light coincides with the plane of polarization, a second rod lens arranged at the light emission end of the polarization beam splitter from the semiconductor laser, and the semiconductor laser of the second rod lens. A polarization plane-maintaining optical fiber optically arranged and arranged so that its outgoing light and a polarization plane coincide with each other at the light outgoing end, and is placed at a light outgoing end from the semiconductor laser of the above polarization-maintaining optical fiber, A third rod lens for shaping the emitted light into parallel light and a light emitting end of the third rod lens from the semiconductor laser for converting the emitted light into circularly polarized light and partial reflection. A sound receiving vibration having a quarter-wave plate having a light emitting end that is exposed to light and a partially reflection-coated surface, and the partially reflection-coated surface facing the quarter-wave plate. A plate, the feedback light from the Fabry-Perot interference system formed by the sound receiving diaphragm and the quarter-wave plate is converted into linearly polarized light by the quarter-wave plate. Furthermore, an acoustic sensor configured to detect the intensity change through the third rod lens, the polarization-maintaining optical fiber, and the polarization beam splitter in this order.