JPS58160827A - Temperature sensor - Google Patents

Abstract

PURPOSE:To measure temp. change as interference output change, by using the rectangularly crossing 2 transmission paths of an optical fiber retaining a polarized face as the 2 light paths of an interferometer, and utilizing change of the difference between the lengths of 2 paths dependent on temp. CONSTITUTION:Light waves emitted from a light source 51 are converted into a linearly polarized light with a polarizing plate 52. These linearly polarized waves are inclined with a half wavelength plate 53 at the middle of 2 axes of an optical fiber 54 capable of retaining a polarized face, crossing each other at right angles placed in a temp. measurement object 58. At the side of the output, a light receiver 56 receives only the components of the rectangularly crossing 2 light waves in the axial directions of a light detector 55 through this detector 55. This output can be monitored continuously, for example, by a recorder 57 or the like. Temp. change can be thus measured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature sensor using an optical fiber.

Conventionally, a temperature sensor using an optical fiber as shown in FIG. 1 has been proposed. In the figure, light source 1
One light beam is split into two by a beam splitter 12 and input into an optical fiber 13 and an optical fiber 14, respectively. Due to the difference in length of these two optical fibers 13°14,
Within the coherence length of light, two optical fibers 13
, 14 create interference fringes 15. This interference fringe 15 moves as the optical path length difference between the two light waves changes.

Therefore, the amount of relative change in the lengths of the two optical fibers can be measured from the amount of movement of this interference fringe. On the other hand, since the length of optical fibers expands and contracts due to temperature changes, if one of the two optical fibers 13 and 14 is used for temperature measurement, the interference fringes will change with temperature changes due to the above-mentioned principle. , the temperature fluctuation 1 can be measured from the amount of change. This method requires two optical fibers, and one optical fiber must be placed in a constant temperature oven to prevent temperature changes, making the measurement system complex at some sites. Ta.

The present invention provides a temperature sensor that can easily measure temperature changes using only one optical fiber.

The present invention uses two orthogonal transmission lines of a polarization-maintaining optical fiber as two optical paths of an interferometer, and utilizes the fact that the difference in the length of these two optical paths changes depending on temperature. It is characterized by the fact that it is measured as

Figures 2 and 3 show the principle diagrams for detailed explanation of the present invention.
Shown in the figure. In FIG. 2, a light wave 21 is split into two by a beam splinter 22 and separated into a light wave 26 and a light wave 27. These two light waves are folded back by mirrors 23 and 24 and combined again by beam splino/25. The optical output ■ of this composite wave 28 is the optical power of the two optical waves 26°27
, , I2, it is generally expressed by the following formula.

Here, γ represents the degree of interference of the light source, and is generally 0〈γ
It is. ψ is the difference Δd between the optical path length of the optical wave 26 and the optical path length of the optical wave 27, and the optical wave 26 and the optical wave 2 are generated by the difference Δd.
7 and is expressed by the following equation.

Δd ψ=2π・T(2) λ is the wavelength of light.

Therefore, by moving the optical delay path 29, the composite signal 28
The optical output ■ changes sinusoidally according to equation (1). This interferometer is generally called the Mano-Zenda interferometer. Using this polarization-maintaining optical fiber, it can be constructed as shown in FIG. In FIG. 3, a linearly polarized light wave 31 is transferred to a polarization maintaining optical fiber 35.
The light is incident at an angle so that it falls between the two orthogonal axes 33 and 34. To rotate a linearly polarized wave to an arbitrary angle,
This is possible by changing the angle of the half-wave plate 32 with respect to the linearly polarized light wave. In this way, the light waves 38 and 39, which are the components of the linearly polarized light wave 310 to the two orthogonal axes 33 and 34 of the polarization-maintaining optical fiber 35, propagate independently through the polarization-maintaining optical fiber 35. Since no interference occurs between orthogonal light waves, the axis of the analyzer 311 is set so as to be located between the two orthogonal axes 33 and 34 of the polarization maintaining optical fiber 35 on the output side. As a result, a composite wave 36 is obtained by the components of the light wave 38 and the light wave 39 toward the axis of the analyzer 311, and the optical output I is given by equation (1).

This is equivalent to the interferometer shown in FIG. 2, and the beam splitter 22 and beam splitter 25 in FIG. Equivalent to.

On the other hand, in FIG.
Since the optical waveguides along the axes 33 and 34 have different refractive indexes, two light waves 38° 39 that enter simultaneously on the input side also produce a time difference Δτ expressed by the following equation on the output side.

Δτ=CL-Cp-a・(To T) (3) Here, C is the speed of light in air, L is the length of the polarization-maintaining optical fiber 35, and Cp is the photoelastic constant of the polarization-maintaining optical fiber. , a is a proportionality coefficient related to the Young's modulus, Poisson's ratio, and thermal expansion coefficient of the polarization-maintaining optical fiber 35, To is the softening temperature of silica glass, which is approximately 1000°C, and T is the position of the polarization-maintaining optical fiber 35. This is the ambient temperature. Δτ has a relationship with the phase difference ψ between the two light waves 38 and 39 given by the following equation.

2π ψ=Δτ・C・T (4) When equation (3) is substituted into equation (4), the phase difference ψ is given by the following equation.

2π ψ= =L*Cp-a・(To T) (5)
λ Equation (5) shows that when the ambient temperature of the polarization maintaining optical fiber 35 changes, the light wave 38. This shows that the phase difference of the light waves 39 changes. This means that the change in the ambient temperature of the polarization maintaining optical fiber 35 in FIG. 3 corresponds to the movement of the optical delay diagram in FIG. 2. Therefore, in FIG. 3, the output of the analyzer 311 changes according to equation (1) with respect to changes in the ambient temperature of the polarization-maintaining optical fiber 35. FIG. 4 shows an example of experimental results of changes in the output of the analyzer 311 with respect to changes in the ambient temperature of the polarization-maintaining optical fiber 35. It can be seen that the optical output changes sinusoidally in response to temperature changes.

By differentiating equation (5) with respect to temperature T, the ratio of the amount of phase change to a unit temperature change is obtained and is expressed by the following equation.

From equation (6), it can be seen that there is a linear relationship between temperature change and phase change. Therefore, the amount of temperature change can be measured from the change in the optical output level due to the change in phase.

A basic configuration diagram of the present invention is shown in FIG. The configuration of FIG. 5 is based on the principles described above. In the figure, a light source 51
The light wave from is turned into linearly polarized light by the polarizing plate 52. This linearly polarized light wave is transmitted to the temperature measurement target 58 by a half-wave plate 53.
Two orthogonal polarization-maintaining optical fibers 54 placed in
Tilted in the middle of the axis.

On the output side, only the axial components of the two light waves orthogonally crossed by the analyzer 55 are received by the light receiver 56 . This output can be continuously monitored, for example by a recorder 57 or the like.

With such a configuration, temperature changes can be measured based on the above-mentioned mm.

As described above, the present invention uses two orthogonal transmission lines of a polarization-maintaining optical fiber as two optical paths of an interferometer, and takes advantage of the fact that the difference in the length of these two optical paths changes with temperature.
The feature is that temperature changes are measured as interference output changes, and the configuration is simpler than conventional ones.As shown in equation (6), the rate of change in phase per unit temperature change is proportional to the fiber length. Therefore, there is an advantage that the sensitivity and measurement range can be changed by selecting the fiber length. That is, to increase the sensitivity, the fiber length is lengthened, and to widen the temperature range to be measured, the fiber length is shortened. For example, in the example of FIG. 4, the fiber length is 20 m, but when the fiber length is increased to 1 m, the period of temperature change becomes 20 times as shown by the dotted line.

[Brief explanation of drawings]

Figure 1 is a system diagram showing an example of a conventional temperature sensor.
1' Fig. 2 is a system diagram for explaining the present invention in detail, Fig. 3 is a perspective view for explaining the principle of the optical interferometer used in the present invention, and Fig. 4 is a detailed explanation of the present invention. FIG. 5 is a configuration diagram showing an embodiment of the present invention. 11... Light source, 12... Beam splitter, 13
, 14... Optical fiber, 15... Interference fringe, 21
, 26 , 27... Light wave, 22, 25... Beam splitter, 23, 24... Mira, public... Combined wave, 29... Optical delay path, 31... Linearly polarized light wave, 32
... Half-wave plate, 33, 34... Optical axis, 35...
Polarization maintaining optical fiber, 36...Synthetic wave, 37...
Incident plane, 38, 39...Light wave, 31O...Linearly polarized light wave, 311...Analyzer, 51...Light source, 52...
- Polarizing plate, 53... Half-wave plate, 54... Polarization preserving optical fiber, 55... Analyzer, 56... Light receiver, 5
7...Recorder, 58...Temperature measurement object. Patent applicant International Telegraph and Telephone Co., Ltd. Agent Otsuka 1 external person

Claims (1)

[Claims] a light source that generates light; a polarization-maintaining optical fiber that enters the light and transmits the light while preserving the polarization planes of the two optical axis components perpendicular to each other; A temperature sensor comprising means for outputting an output corresponding to an output level due to interference when synthesized, and configured to output an output corresponding to an ambient temperature of the polarization-maintaining optical fiber.