JPS61149836A - Pressure measuring instrument - Google Patents

Abstract

PURPOSE:To reduce the size and cost of a pressure measuring instrument which uses a polarization plane maintaining optical fiber and to improve its precision by connecting the optical fiber to an photoelastic element through a 1/8-wavelength phase device so that its optical axis is slanted by a specific angle. CONSTITUTION:A light source 1, photodetectors 2a and 2b, and the polarization plane maintaining optical fiber 8 are coupled mutually by a directional coupler 17 and a probe part 12 which receives pressure P to be measured is connected to the tip part of the polarization plane maintaining optical fiber 8 to constitute a pressure measuring instrument. The photoelastic element 21 of the probe part 12 is formed as a single-mode transmission line and a reflector 16 is fitted to its tip. Further, the tip surface of the polarization plane maintaining optical fiber 8 is joined with the base end of the photoelastic element 21 through the 1/8-wavelength phase device 15. In this case, the inherent axis of polarization of the tip surface of the optical fiber 8 is set at 45 deg. to the pressure application direction of the photoelastic element 21 and the inherent optical axis of the 1/8-wavelength phase device.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a pressure measuring device using a polarization-maintaining optical fiber, and in particular, it is possible to reduce the size and size of the device by integrating individual optical components.
This article relates to a pressure measuring device that achieves high accuracy and low cost.

[Prior Art] A pressure sensor using a polarization maintaining optical fiber 8 and a photoelastic element 10 as shown in FIG. 6 has already been proposed (Japanese Patent Application No. 58-28710).

As shown in the figure, light from a light source 1 passes through an isolator 3 and is split into two directions by a beam splitter 4. One of them enters the light receiver 2b as a reference light, and the other passes through a half-wave plate 6, a microlens 7a, and enters a polarization-maintaining optical fiber 8 for pressure detection. The half-wave plate 6 can rotate the polarization plane of linearly polarized light to any angle of 0 by changing the direction of its optical axis, so the half-wave plate 6 can be adjusted to input the polarization-maintaining optical fiber 8. The plane of polarization of the linearly polarized light is aligned with one of two mutually orthogonal characteristic polarization axes (U and V axes in FIG. 7) of the polarization-maintaining optical fiber 8. The emitted light propagates in the +2 direction through the polarization-maintaining optical fiber 8 and is emitted from the tip thereof, passes through the microlens 7b and the 1/8 wavelength plate 9, enters the photoelastic element 10, and is reflected by the reflection plate 11. . The light reflected by the reflection plate 11 travels backward along the above-mentioned path and returns to the beam splitter 4, and a part of it is reflected by the beam splitter 4, passes through the analyzer 5, and enters the light receiver 2a.

As shown in FIG.
and make an angle of 45°. Further, the optical axis of the ⅛ wavelength plate 9 coincides with the X axis or the V axis, and forms an angle of 45° with the Ll and V axes.

Assuming that the light that enters the polarization-preserving optical fiber 8 from the light rA1 and travels in the +7 direction is only the U component (polarized wave component in the U direction), it is reflected by the reflective surface 11 and returns to the -Z direction. Immediately after passing through the 178 wavelength plate 9, the U axis and the x and V axes make an angle of 45 degrees, so the X component and the y component of the reflected light are equal in amplitude, and the X component passes through the 178 wavelength plate 9 twice. A phase difference of 90" occurs between the V component and the V component. Therefore,
When no pressure P is applied to the photoelastic element 10, 178
At the time the reflected light passes through the wave plate 9, it becomes circularly polarized light. On the other hand, when pressure P is applied to the photoelastic element 10, a phase difference α occurs between the x and y components of the light propagating within the photoelastic element 10, so that the light passes through the 178 wavelength plate 9 in 12 directions. Immediately after, the x and y components ex%Y are expressed as follows. Here, a is the amplitude for linearly polarized light, ω
is the angular frequency of light, and ψ is the phase, both of which are constants.

The pressure P is a function of the phase difference α, and if the variable α is known, the pressure P can be found. Up&! (X of type 11.

The y component is converted into the U and V components e in the polarization maintaining optical fiber 8.
When converted to u, e v, it becomes, so the receiver 2
If only the V component is received at point a, the output Vs of the light receiver 2a will be as follows. The output VR (CK-
), a stable phase difference α can be achieved by processing the signal with
can be detected. This pressure sensor can detect pressure using a single optical fiber, making it possible to reduce the diameter of the pressure sensor.

[Problems to be Solved by the Invention] However, since the pressure sensor described above is configured by combining individual optical elements (lenses, wave plates, photoelastic elements, etc.), it has the following problems.

■ Cannot be miniaturized below the size of individual optical elements.

■ Since light exits into the external space between the optical elements, a slight deviation in size or position between the optical elements can easily cause large measurement errors.

(2) The price of each individual optical element and the price of assembling these elements constitute the overall price, but there are limits to how low the price of Ii[ can be reduced because many optical elements are used and assembly is difficult.

[Object of the Invention] The present invention was devised to solve the problems of the prior art described above, and an object of the present invention is to provide a pressure measuring device that can promote downsizing, high precision, and low cost. It is about providing.

[Summary of the Invention] In order to achieve the above object, the present invention is provided at the tip side of a single-mode polarization-maintaining optical fiber, and by applying pressure, polarization components are a probe section having a photoelastic element having different phase velocities, a light source provided on the proximal end side of the polarization-maintaining optical fiber, and receiving reference light from the light source and phase-modulated modulated light from the probe section; In the pressure measuring device, the photoelastic element forms a single mode transmission path, and a reflector is bonded to the distal end surface of the photoelastic element, and a single mode 1/8 wavelength phase shifter is connected to the proximal end surface of the photoelastic element. The tip surfaces of the polarization-maintaining optical fibers are joined, and the characteristic polarization axis of the tip surface of the polarization-maintaining optical fibers is aligned with the pressing direction of the photoelastic element and the characteristic optical axis of the 1/8 wave phaser at 45 degrees. It is shaped like a square melon, and each optical element is joined to form an integrated single-mode waveguide.

[Example] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1, the pressure measuring device includes a probe 1 section 12 provided in a pressure measuring section, a single mode polarization maintaining optical fiber 8, a light source 1, a reference light from the light source 1, and a probe section 12. It is mainly composed of a light receiving section 13 that receives phase-modulated light from the light receiving section 13.

The probe section 12 includes a pressure receiving section 14 that receives pressure P, a single mode 1/8 wavelength phase shifter 15, and a reflector 16, and the light receiving section 13 includes a light receiving section 2a.

2b, single mode directional coupler 17. It consists of a single mode optical isolator 18 and a single mode analyzer 19.

As shown in FIGS. 1 and 2, the pressure receiving section 14 of the probe section 12 consists of a pressure receiving block 20 and a photoelastic element 21 embedded therein. When pressure P is applied to the pressure receiving block 20, the photoelastic element 21 Internal stress occurs within the material, resulting in birefringence. Since the pressure P is applied in the y-axis direction,
The phase velocity of the polarized wave component in the axial direction becomes different. For the photoelastic element 21, a single mode optical fiber having no birefringence in the absence of pressure is used. For this single mode optical fiber, it is sufficient to form a single mode transmission path using a material with a large photoelastic effect such as epoxy resin, GaAS, or InP.Since the photoelastic effect is large, a highly sensitive sensor can be used even if the transmission path is short. is obtained. Furthermore, the single mode optical fiber may be a quartz fiber for communications. However, since quartz has a small photoelastic effect, it is necessary to increase the transmission path length to increase sensitivity.

A birefringent polarization-maintaining optical fiber is used for the 1/8 wavelength phase shifter 15. The characteristic polarization axis of the polarization-maintaining optical fiber serving as the 178-wavelength phase shifter 15 is X.

It coincides with the V axis and forms an angle of 45° with the unique polarization axes U and V of the polarization maintaining optical fiber 8. If the wavelengths of the U and V polarized lights of the polarization-maintaining optical fiber 8 are λ and V, respectively, then the wavelength generated by propagating through the polarization-maintaining optical fiber serving as the 1/8 wavelength phase shifter 15 by a length ρ is The phase difference △Φ is as follows. Therefore, the length ρ of the polarization-maintaining optical fiber as the 1/8 wavelength phase shifter 15 is such that ΔΦ is (1/4 + n )
For the length of π, that is, for the reflector 16, a reflecting plate is bonded to the distal end surface of the photoelastic element 21, or a reflecting film is formed.

The distal end surface of the polarization maintaining optical fiber 8 is connected to the proximal end surface of the photoelastic element 21 via the 1/8 wavelength phase shifter 15, but these optical elements 8, 15, and 21 are all single mode guiding. The parameters that determine the waveguide characteristics, such as the core diameter and the refractive index of the core and cladding, are set to be almost the same, and they are joined together by a method such as fusion.

On the other hand, the directional coupler 17 of the light receiving section 13 is one in which two transmission lines are coupled by a coupling section 17e, and the port 17 on the light source side
It has four ports: a, 17b, and probe section side ports 17c, 17d.

Of these, the optical isolator 1 is connected to the port 17a on the light source side.
A light source 1 is connected to the port 17b via an analyzer 19, and a light receiver 2a is connected to the port 17b via an analyzer 19. Also,
The light receiver 2b is connected to the port 17c on the probe section side, and the proximal end surface of the polarization-maintaining optical fiber 8 is connected to the port 17d. Similar to the probe section 12, these optical elements are also integrally joined by a method such as fusion bonding. port 1
7d and the polarization-maintaining optical fiber 8 are adjusted and spliced so that their characteristic polarization axes coincide.

The operation and pressure measurement principle of the pressure measuring device having the above g[J configuration are the same as the pressure sensor shown in FIG. 6 described above.

That is, the light from the light source 1 is separated into two directions by the directional coupler 17, and one of them is inputted into the light receiver 2b from the port 17C as a reference light. The other is port 1 for pressure detection.
7d, enters the polarization maintaining optical fiber 8, propagates in the +Z direction, passes through the 1/8 wavelength phase shifter 15 from its tip, enters the photoelastic element 21, and is reflected by the reflector 16. Further, the reflected light follows a path opposite to the above to the directional coupler 17, where it is split into two ports 17a and 17b. The light emitted from port 17b is detected by analyzer 19.
On the other hand, the light emitted from the port 17a is blocked by the optical isolator 18 and is not returned to the light source 1. By signal processing the output of the light receiver 2a and the output of the light receiver 2b, the phase difference modulated by the photoelastic element 21 can be determined, and the pressure P can be determined from this phase difference.

In the present invention, a photoelastic element 21, a 1/8 wavelength phase shifter 15, a polarization maintaining optical fiber 8. Directional coupler 17, optical isolator 18. The analyzer 19 is integrally joined by fusion bonding or the like to form a single mode waveguide. Therefore, unstable factors such as reflection of light due to misalignment between the connecting surfaces of these optical elements or leakage of light to the outside from between the optical elements can be eliminated. Therefore, stable measurement and highly accurate pressure measurement are possible. Further, in the conventional method, a mechanism is required to hold these optical elements in a fixed positional relationship with each other, and the housing used for the holding mechanism is complicated and expensive, and it is difficult to miniaturize the housing. However, in the method of the present invention, each optical element is joined and integrated by fusion or the like, so a complicated and expensive housing is not necessary, and the size can be reduced. Further, since the transmission lines having the same diameter are connected, the microlenses 7a and 7b as couplers are not required. Furthermore, since the directional coupler 17 and the polarization-maintaining optical fiber 8 are coupled with their intrinsic polarization axes aligned, a 1/2 wavelength plate can also be omitted.

In the above embodiment, the photoelastic element 2 of the pressure receiving part 14
1 was embedded in the pressure receiving block 20, but as shown in FIG. 3, the pressure receiving block 20a.

It may also be of a type in which the photoelastic element 21 is sandwiched between two Obs.

The cross section of the pressure receiving block 20 of the embedded type receiving part 14 may not be rectangular but may be shaped like a drum with depressions on both sides as shown in FIG. In this way, the photoelastic element 21
External forces will be concentrated and applied to the area, improving Kanmaro.

Further, as shown in FIG. 5, a pressure receiving block 20 made of a photoelastic material is formed, and a photoelastic element 2 of a high refractive index transmission path is placed inside the pressure receiving block 20.
1 may be provided. In this case, various methods such as a resin molding method, an ion implantation method, a vapor deposition method, and a diffusion method can be used as a manufacturing method. The cross-sectional shape of the pressure receiving block 20 is selected depending on the application, such as a rectangular shape or a deformed shape such as an hourglass shape.

Furthermore, since it is desirable that the probe section side has a structure that can be detached from the measuring instrument body, the polarization maintaining optical fiber 8
An optical connector may be used between the directional coupler 17 and the directional coupler 17 or in the middle of the polarization maintaining optical fiber 8 so that it can be attached and detached.

In this case, a highly accurate optical connector is used that can simultaneously align the polarization plane of the connection part and the optical axis.

[Effects of the Invention] As is clear from the above description, the present invention exhibits the following excellent effects.

■ Conventionally, the configuration was made by assembling individual optical elements, but each optical element is joined and integrated to create a closed single-mode transmission path, which reduces reflections at connections and external interference. Since unstable optical coupling is removed, highly stable and accurate pressure measurement is possible. This is especially effective in the pub section, which is exposed to severe environmental conditions.

(1) Each optical element can be made into a single mode fiber, the optical system can be easily miniaturized, an ultra-small probe 1 section can be realized, and measurements in narrow spaces, which were previously impossible, become possible.

- Since it is an integrated optical system, the housing of the probe part etc. can be made into a simple structure, making it easy to assemble and manufacturing at low cost.

Furthermore, since there is no need to use expensive optical components such as a wave plate, the cost of the device can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an embodiment of the pressure measuring device according to the present invention, FIG. 2 is a diagram showing the positional relationship between optical elements in the probe section, and FIG. 3'. 4 and 5 are cross-sectional views showing other embodiments of the pressure receiving section of the present invention, FIG. 6 is a schematic configuration diagram showing a conventional pressure measuring device, and FIG. 7 is a cross-sectional view showing the relationship between optical elements in the probe section. FIG. In the figure, 1 is a light source, 2a and 2b are light receivers, 8 is a polarization maintaining optical fiber, 12 is a probe section, 13 is a light receiving section, 14 is a pressure receiving section, 15 is a 1/8 wavelength phase shifter, and 16 is a reflector. , 17
are directional couplers, 17a, 17b, 17c
, 17d is a port, 18 is an optical isolator, 19 is an analyzer, 20 is a pressure receiving block, 21 is a photoelastic element, P is pressure, (1°■ is the characteristic polarization axis of the polarization maintaining optical fiber.

Claims (2)

[Claims] (1) A probe section having a photoelastic element which is provided on the tip side of a single mode polarization maintaining optical fiber and has a photoelastic element whose polarization component has a different phase speed in the direction of pressure and in the direction orthogonal to the direction of pressure when pressure is applied; A pressure measuring device comprising a light source provided on the proximal end side of a polarization-maintaining optical fiber and a light receiving section that receives reference light from the light source and phase-modulated modulated light from the probe section. The element forms a single-mode transmission path, and a reflector is bonded to its distal end surface, and the distal end surface of the polarization-maintaining optical fiber is bonded to its proximal end surface via a single-mode 1/8 wavelength phase shifter, The characteristic polarization axis of the tip surface of the polarization-maintaining optical fiber is 45° with respect to the pressing direction of the photoelastic element and the characteristic optical axis of the 1/8 wavelength phase shifter.
A pressure measuring device characterized by forming an angle of . (2) The light receiving section has a 4-port single mode directional coupler, the light source is connected to one of the light source side ports via a single mode optical isolator, and the other port is connected to the light source through a single mode optical isolator. The reference light receiver is connected via a single-mode analyzer, and the base end of the polarization-maintaining optical fiber is connected to one of the ports on the probe side of the directional coupler, 2. The pressure measuring device according to claim 1, wherein a light receiver for the modulated light is connected to the other port.