JPS62232525A - Acoustic sensor - Google Patents

Abstract

PURPOSE:To facilitate the alignment of an optical axis, by a method wherein the fed-back beam of a Fabry-Perot interferometer formed of a sound receiving vibration plate and a 1/4 wavelength plate is re-converted to linear polarized beam which is, in turn, passed through a rod lens, a polarizing front preserving optical fiber and a polarizing beam splitter successively. CONSTITUTION:When a sonic wave is applied to a sensor head 2, a sound receiving vibration coated end surface 24r of a 1/4 wavelength plate 24 and the partial reflecting coated end surface 9r of the vibration plate 9 changes and the laser beam (i) fed back from the sensor head 22 comes to laser beam of which the intensity is modulated so as to be proportional to the displacement of the vibration plate 9 and the diffracted laser beam 1 thereof is incident on a beam receiving element 10 to be detected. By this method, the simultaneous alignment of five optical axes between two parts in the optical coupling of the sensor head and an optical fiber can be separated into the simultaneous alignment of three axes between two parts and that of two optical axes between two parts and the alignment of the optical axis becomes possible respectively independently and not only the alignment of the optical axis can be performed in an extremely easy manner but also a high SN ratio can be achieved with low loss.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an acoustic sensor that uses optical phenomena to replace an acoustic signal with a change in the intensity of light.

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

Hereinafter, an example of the above-mentioned conventional audio system will be described with reference to the drawings.

FIG. 2 shows 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 partially reflective coating that constitutes the sensor head 7;
8r is the partially reflective coated end surface of the third rod lens 8.

9 is a sound receiving diaphragm forming the sensor head 7, and 9τ is an end surface of the sound receiving diaphragm 9 with a partial reflection coating. The partially reflective coated end face 8r of the third rod lens 80 and the partially reflective coated end face 9τ of the sound receiving diaphragm 9 form a so-called Fabry-Bello interference system. 10 is a light receiving element.

a, b, c, e, and f are light beams traveling from the semiconductor laser 1 toward the sensor head 7, d is a light beam diffracted by the non-polarizing beam splitter 4 of the light beam C, and q
is a light beam that repeatedly reflects and interferes between the partially reflective coated end surface 8r of the third rod lens 80 and the partially reflective coated end surface 9r of the sound receiving diaphragm 9, h is the light beam transmitted through the sound receiving diaphragm 9, i and j is the light beam returned from the sensor head 7, k is the light beam of the light beam j transmitted through the non-polarizing beam splitter 4, and l is the light beam of the light beam j that is diffracted at the non-polarizing beam splitter 4.

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

First, a light beam a emitted from a semiconductor laser 1 is turned into a parallel light beam b by a first rod lens 2. Next, the light beam b passes through the optical isolator 3 and becomes the light beam C. Further, the light beam C is split into a diffracted light beam d and a transmitted light beam e by a non-polarizing beam splitter 4,
Among them, the transmitted light beam e is polarized by the second rod lens 5 and becomes a light beam f that propagates inside the single mode optical fiber 6. Next, the light beam f is made into parallel light by the third rod lens 8, and a Fabry bellows is formed by the partially reflective coated end surface 8r of the third rod lens 8 and the partially reflective coated end surface 9r of the sound receiving diaphragm 90. A light beam q is repeatedly reflected and interfered with in the interference system.

Each time the reflection is repeated, the partially reflective coated end face 8r of the third rod lens 8 and the partially reflective coated end face 9 of the sound receiving diaphragm 9
At r, light proportional to the distance between the two at that time is transmitted. Among them, the light transmitted through the partially reflective coated end surface 9r of the sound receiving diaphragm 9 becomes a light beam h, and on the other hand, the third
The light transmitted through the partially reflective coated end surface 8r of the rod lens 8 is condensed by the third rod lens 8, and is returned to the single mode optical fiber 6 again to become a light beam i.

Next, the light beam i is split into a diffracted light beam l and a transmitted light beam by a non-polarizing beam splitter 4, of which the diffracted light beam l enters a light receiving element 10 and is detected.

Here, the light beam is invalid light that does not contribute to measurement, similar to the above-mentioned light beam d.

Now, when a sound wave is applied to the sensor head 7, the sound receiving diaphragm 9
is displaced, the distance between the partially reflective coated end surface 8r of the third rod lens 8 and the partially reflective two 1 end surface 9r of the sound receiving diaphragm 9 changes, and the feedback light beam i from the sensor head 7 is directed to the sound receiving diaphragm 9. The result is a light beam whose intensity is modulated in proportion to the displacement of . A part of the light beam l is sent to the light receiving element 1 as a signal light.
o and is detected.

Problems to be Solved by the Invention However, in the above configuration, the third rod lens does not perform optical coupling with the single-mode optical fiber, that is, it shapes the light emitted from the single-mode optical fiber into a parallel light beam. Furthermore, it has the function of condensing the light to be extracted as signal light again and inputting it into a single mode optical fiber, and the function of forming an optical Fabry-Bello interference system by applying a partial reflection coating to the end face. , the optical axis alignment of the single mode optical fiber and the third rod lens is achieved by adjusting the position of the three axes of X, Y, and Z, and by θ
, δ, and fine adjustment of a total of six axes must be performed at the same time, which is extremely difficult.

In addition, since the non-polarizing beam splitter reduces the output light of the semiconductor laser by half and also reduces the signal light from the sensor head by half, there are problems in terms of efficiency and signal-to-noise ratio (hereinafter referred to as SN ratio). It had

In view of the above-mentioned problems, the present invention provides an acoustic sensor that can easily align the optical axis with an optical fiber and achieves a high signal-to-noise ratio with low loss.

Means for Solving the Problems In order to solve the above problems, the acoustic sensor of the present invention includes a semiconductor laser and a first rod lens arranged at the light emitting end of the semiconductor laser and shaping the emitted light into parallel light. a polarizing beam splitter disposed at the light emitting end of the first rod lens so that the plane of polarization matches the emitted light; and a polarizing beam splitter disposed at the light emitting end of the semiconductor laser of the polarizing beam splitter. a second rod lens; a polarization-maintaining optical fiber that is disposed and optically coupled to a light output end of the second rod lens from the semiconductor laser with a plane of polarization matching that of the output light; a third rod lens disposed at the light emitting end of the semiconductor laser of the wavefront preserving optical fiber and shaping the emitted light into parallel light; and a third rod lens disposed at the light emitting end of the semiconductor laser of the third rod lens. a quarter wavelength plate that converts the emitted light into circularly polarized light and has a light output end that is partially reflective coated;
a sound-receiving diaphragm disposed opposite to the wavelength plate, the feedback light from the Fabry-Bello interference system formed by the sound-receiving diaphragm and the quarter-wave plate is divided into four parts. It is configured to reconvert the light into linearly polarized light using the one-wavelength plate, and detect changes in its intensity through the third rod lens, the polarization-maintaining optical fiber, and the polarization beam splitter in this order.

Effect of the Invention With the above-described configuration, the present invention can achieve simultaneous optical axis alignment of five axes between two parts in optical coupling between a sensor head and an optical fiber, and simultaneous optical axis alignment of three axes between two parts. It can be separated into two simultaneous optical axis alignments between parts, allowing each optical axis to be aligned independently, which not only makes optical axis alignment extremely easy, but also enables optical axis alignment with low loss and high S/N ratio. You will be able to adjust your mind.

EXAMPLE An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an audio system in one embodiment of the present invention. In Figure 1, 1
2 is a semiconductor laser, 2 is a first rod lens arranged at the light emitting end of the semiconductor laser 1 and shapes the emitted light into parallel light, and 20 is a lens arranged at the light emitting end of the first rod lens 2 with the emitted light and the polarization plane. 5 is a second rod lens placed at the light emitting end of the semiconductor laser 1 of the polarization beam splitter 20; a polarization-maintaining optical fiber arranged at the light output end so that the plane of polarization matches the output light and optically coupled; 22, a sensor head; 2;
3 is arranged at the light output end of the semiconductor laser 1 of the polarization maintaining optical fiber 21 and shapes the output light into parallel light.
The rod lens 24 is arranged at the light output end of the semiconductor laser 1 of the third rod lens 23, converts the output light into circularly polarized light, and has a quarter-wavelength light output end coated with a partial reflection coating. plate, 24r is its partially reflective coated end surface;
Reference numeral 9 denotes a sound receiving diaphragm having a surface coated with a partial reflection coating, and the sound receiving diaphragm is disposed with the partially reflection coated surface facing the quarter wavelength plate, and 9r denotes an end surface of the partial reflection coating. The partially reflective coated end face 24r of the quarter-wave plate 24 and the partially reflective coated end face 9r of the sound receiving diaphragm 9 form a Fabry-Perot interference system. 1o is a light receiving element.

a, b, e, and f are light beams traveling from the semiconductor laser 1 toward the sensor head 22; q is the quarter-wave plate 2;
A light beam that repeatedly reflects and interferes between the partially reflective coated end surface 24r of No. 4 and the partially reflective coated end surface 9r of the sound receiving diaphragm 9, h is a light beam transmitted through the sound receiving diaphragm 9, i, j, 1
is the light beam returned from the sensor head 7. In addition,
The same parts as in the conventional example shown in FIG. 2 are given the same numbers.

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

First, a light beam a emitted from a semiconductor laser 1 is turned into a parallel light beam b by a first rod lens 2. Next, while maintaining linear polarization, the light beam b is almost entirely transmitted through the polarizing beam splitter 20, is focused by the second rod lens 5, and is propagated inside the polarization-maintaining optical fiber 21. become. Next, the light beam f is made into parallel light by the third rod lens 23, and is divided into four parts.
The linearly polarized light is incident on the wavelength plate 24, where it is converted into circularly polarized light, and the Fabry light is formed by the partially reflective coated end face 24r of the quarter wavelength plate 24 and the partially reflective coated end face 9r of the sound receiving diaphragm 9. A light beam q is repeatedly reflected and interfered with in the Perot interference system.

Every time the reflection is repeated, the partially reflective coated end face 24r of the quarter wavelength plate 24 and the partially reflective coated end face 9 of the sound receiving diaphragm 9
At r, light proportional to the distance between the two at that time is transmitted. Among them, the light transmitted through the partially reflective coated end surface 9r of the sound receiving diaphragm 9 becomes a light beam h. Also, on the other hand, 4
The light transmitted through the partially reflective coated end surface 24r of the quarter-wave plate 24 is again converted from circularly polarized light to linearly polarized light perpendicular to the incident light by the quarter-wave plate 24, and then focused by the third rod lens 23. The light is then transferred back to the polarization-maintaining optical fiber 2
1 and becomes a light beam l.

Next, the light beam i is diffracted by the polarizing beam splitter 2o, and the diffracted light beam l enters the light receiving element 1° and is detected.

Now, when a sound wave is applied to the sensor head 22, the sound receiving diaphragm 9 is displaced, and the distance between the partially reflective coated end surface 24r of the quarter wavelength plate 24 and the partially reflective coated end surface 9r of the sound receiving diaphragm 9 changes. , the feedback light beam i from the sensor head 22 becomes a light beam whose intensity is modulated in proportion to the displacement of the sound receiving diaphragm 9. The diffracted light beam l is sent to the light receiving element 1 as a signal light.
0 and is detected.

As described above, according to this embodiment, a quarter-wave plate with a partial reflection coating is newly added to the sensor head, and the function of the third 1-butre lens is made into a single function of coupling with an optical fiber. A Fabry-Perrault interference system with a partially reflective coating 4
By using a half-wave plate and a sound receiving diaphragm, and splitting and transmitting light using a polarizing beam splitter and a polarization-maintaining optical fiber, the optical axis alignment between the sensor head and the optical fiber is achieved. The present invention provides an acoustic sensor that is easy to manufacture, has low loss, and has a high signal-to-noise ratio, and its practical effects are outstanding.

Effects of the Invention As described above, the present invention provides a semiconductor laser, a first rod lens disposed at the light emitting end of the semiconductor laser and shaping the emitted light into parallel light, and a light emitting device of the first rod lens. a polarizing beam splitter disposed at an end with a plane of polarization matching that of the emitted light; a second rod lens disposed at an end of the polarizing beam splitter from which the light emitted from the semiconductor laser is emitted; and the second rod lens. a polarization-maintaining optical fiber arranged and optically coupled to the light output end of the semiconductor laser so that the plane of polarization matches that of the output light; and a light output end of the polarization-maintaining optical fiber from the semiconductor laser. and a third rod lens arranged at the light emitting end of the semiconductor laser to convert the emitted light into circularly polarized light and partially reflect the emitted light. a quarter wavelength plate having a coated light emitting end; and a sound receiving diaphragm having a partially reflective coated surface and disposed with the partially reflective coated surface facing the quarter wavelength plate. , the above-mentioned sound-receiving diaphragm and the above-mentioned quarter
The feedback light from the Fabry-Perot interference system formed by the wavelength plate is reconverted into linearly polarized light by the quarter wavelength plate, and then the third rod lens, the polarization maintaining optical fiber, and the polarized light are By sequentially detecting the intensity changes through the beam splitter, it is possible to simultaneously align the five axes between the two parts in the optical coupling between the sensor head and the optical fiber, and to align the three axes between the two parts. Simultaneous optical axis alignment and alignment between two parts
It is possible to separate the simultaneous optical axis alignment of the axis and independently align the optical axis, which not only makes it extremely easy to align the optical axis, but also enables low loss and high S/N ratio.
Its practical effects are significant.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an acoustic sensor according to an embodiment of the present invention, and FIG. 2 is a block diagram showing the configuration of a conventional acoustic sensor. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... First rod lens, 5... Second rod lens, 9...
...Sound receiving diaphragm, 9r...partial reflection coat end face, 1o...light receiving element, 20...
Polarizing beam splitter, 21...Polarization maintaining optical fiber, 22...Sensor head, 23...
...Third rod lens, 24...1/4
Wave plate, 24τ... Partially reflective coated end face. Name of agent: Patent attorney Toshio Nakao and 1 other person1-
11 horizontal pile - T''' - 27 nod spring 5 - - - 24 N 9 - - 1 receiving party T exhibition machine 9 to - part of 6 tots anti-approximate 'co-L creation jshi / Q- + Absolute () 2'°-- Sea circle σ- Melon 7° Wa, Ri Fig. 2

Claims (1)

[Claims] a semiconductor laser; a first rod lens disposed at a light output end of the semiconductor laser to shape the output light into parallel light; a polarizing beam splitter arranged to coincide with each other, a second rod lens arranged at a light emitting end of the semiconductor laser of the polarizing beam splitter, and a light emitting end of the second rod lens from the semiconductor laser. a polarization-maintaining optical fiber that is arranged and optically coupled with the polarization plane of the emitted light, and a polarization-maintaining optical fiber that is arranged at the light emitting end of the semiconductor laser of the polarization-maintaining optical fiber and parallelizes the emitted light. a third rod lens that shapes the light; and a light output end of the third rod lens that is disposed at the light output end of the semiconductor laser, converts the output light into circularly polarized light, and has a partially reflective coated light output end. a quarter-wave plate having;
a sound-receiving diaphragm having a partially reflective-coated surface, the partially-reflecting-coated surface facing the quarter-wave plate; The feedback light from the Fabry-Perot interference system formed by the wavelength plate is reconverted into linearly polarized light by the quarter wavelength plate, and then the third rod lens, the polarization maintaining optical fiber, and the polarized light are An acoustic sensor configured to detect changes in intensity sequentially through beam splitters.