JPH0346052B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an optical physical quantity detection device using an optical fiber as a sensor element, and in particular, the refractive index of the optical fiber changes when the optical fiber undergoes a physical change. The present invention relates to an optical physical quantity detection device that detects a physical quantity by utilizing the fact that crosstalk (light leakage) occurs between two propagation paths having different values.

[Prior Art] In conventional physical quantity detection devices using optical fibers, light emitted from a light source is incident on a conversion element such as an optical crystal through an optical fiber, and physical quantities are detected from changes in the transmitted light and reflected light. There is a method to detect it.

In addition, physical quantity detection devices that use the optical fiber itself as a sensor element use a phase difference (interferometer) to detect optical path length changes based on changes in refractive index, expansion and contraction, etc. caused by the physical environment (temperature, pressure, etc.) of the optical fiber. There is also a method of detection based on changes in transmitted light intensity, etc.

[Problems to be Solved by the Invention] However, in the above-mentioned method using a conversion element such as an optical crystal, the structure of the sensor part becomes complicated and the coupling state between the optical fiber and the conversion element remains for a long period of time. There are problems in that the detection device may change or the detection device becomes complicated. Furthermore, since physical information can only be obtained as point information from the sensor section, many sensors must be installed when examining the average or distribution of physical quantities over a wide area.

In addition, with the above-mentioned method that uses the optical fiber itself as a sensor element, it is possible to detect physical quantities as line information along the optical fiber, but it is difficult to determine the position where a physical change has occurred, and the physical quantity to be measured is difficult to detect. There was a problem that stable measurements could not be made because they were susceptible to external disturbances.

[Object of the Invention] This invention was devised to solve the problems of the above-mentioned prior art, and an object of the invention is to simultaneously and easily detect the location and amount of physical change. An object of the present invention is to provide a physical quantity detection device that can perform stable physical quantity detection at low cost.

[Summary of the Invention] In order to achieve the above object, the present invention includes an optical fiber having two propagation paths having different curvatures, a light source that enters pulsed light into one of the optical fibers as the propagation path, A photoreceiver that receives the pulsed light propagating through one of the propagation paths and the pulsed light which crosstalks due to physical changes imparted to the optical fiber and propagates through the other propagation path; It is equipped with a calculation unit that detects the light intensity ratio of both pulsed lights and the delay time based on the output of the pulsed light, and calculates the presence or absence of a physical change and the amount of change in the physical quantity from the light intensity ratio of both pulsed lights. , the position where a physical change has occurred is determined from the delay time of both pulsed lights.

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

In FIG. 1, reference numeral 1 denotes a polarization-preserving fiber having two mutually orthogonal optical axes with different refractive indices, and the polarization-preserving fiber 1 is disposed within a region where a physical quantity (temperature, etc.) is to be detected. Polarizing prisms 2 and 3 are provided at the input end and output end of the polarization preserving fiber 1 as polarizers and analyzers. The transmitted polarization direction and the reflected polarization direction of the polarizing prisms 2 and 3 are arranged to coincide with the two optical axes of the polarization preserving fiber 1. A light source 4 is provided on the transmission side of the polarizing prism 2 with respect to the input end of the polarization preserving fiber 1. The light source 4 is adapted to emit pulsed light having a short pulse width in response to a pulse signal from a pulse generating circuit 5. In addition, a polarizing prism 3 is connected to the output end of the polarization preserving fiber 1.
Light receivers 6 and 7 are provided on the transmission side and the reflection side, respectively. The light intensity signals detected by the light receivers 6 and 7 are input to a calculator 8. Further, a display 9 is connected to the calculator 8 to display the results of the calculation.

The pulsed light emitted from the light source 4 is transmitted through the polarizing prism 2, converted into linearly polarized waves that coincide with one optical axis of the polarization preserving fiber 1, and is incident on the polarization preserving fiber 1. At this time, if there is no change in the physical state of the polarization preserving fiber 1, the linearly polarized pulsed light incident on one optical axis of the polarization preserving fiber 1 will leak to the other optical axis. Therefore, only pulsed light aligned with one optical axis is propagated. Therefore, the light emitted from the polarization-maintaining fiber 1 is linearly polarized pulsed light that coincides with one optical axis, and all of this pulsed light passes through the polarizing prism 3, is detected by the light receiver 6, and is sent to the arithmetic unit 8. is input. Since there is no input from the light receiver 7, the calculator 8 determines that there is no change in the physical quantity in the measurement area along the polarization preserving fiber 1, and the result is displayed on the display 9.

A certain point or part 1 of the polarization preserving fiber 1
If a physical change occurs at 0, the linearly polarized pulsed light incident on one optical axis of the polarization preserving fiber 1 will move to the part 1 where the physical change has occurred.
0, leakage occurs to the other optical axis and crosstalk occurs. After the part 10, the original pulsed light incident on one optical axis of the polarization preserving fiber 1 and the crosstalk pulsed light leaked from the original pulsed light to the other optical axis are transmitted through two propagation paths. are propagated, respectively, and both pulsed lights are emitted from the output end of the polarization-preserving fiber 1. Of this emitted light, the original pulsed light passes through the polarizing prism 3 and is received by the light receiver 6, and the crosstalk pulsed light is reflected by the polarizing prism 3 and is received by the light receiver 7. Intensity signals of these pulsed lights detected by the light receivers 6 and 7 are input to a calculator 8. Since both propagation paths of the polarization preserving fiber 1 have different refractive indices, the original pulse light a and the crosstalk pulse light b input to the calculator 8 have a delay time t as shown in FIG. arise. In this example, the curvature of the propagation path that transmits the linearly polarized original pulsed light a that coincides with one optical axis is smaller than the curvature of the other propagation path that transmits the crosstalk pulsed light b.

The larger the amount of physical change in the part 10, the greater the amount of crosstalk pulsed light b, that is, the light intensity. The physical quantity of part 10 is calculated from the ratio. Furthermore, in the arithmetic unit 8, since the delay time t between the original pulsed light a and the crosstalk pulsed light b is proportional to the fiber length from the part 10 where a physical change has occurred to the output end of the polarization preserving fiber 1, The position of the region 10 where the physical change has occurred is calculated from the delay time t of both pulsed lights.

Next, the relationship between the amount of physical change and the amount of crosstalk will be described using a temperature change as an example of a physical change.

Polarization-maintaining fibers reduce the amount of crosstalk between the two propagation paths by slightly changing the curvature of the propagation paths corresponding to the two optical axes; the closer the refractive indices of both propagation paths are, the more , the amount of crosstalk increases. Therefore, if the temperature coefficients of refractive index of the two propagation paths are set to different values,
The crosstalk changes depending on the temperature, and by detecting the amount of crosstalk, the temperature can be detected conversely.

Regarding other physical quantities, if distortion is added to the polarization-preserving fiber due to changes in the physical quantities, and this distortion causes a change in the difference in the curvature of both propagation paths of the polarization-preserving fiber, then Measurement can be performed in the same manner as in the example of temperature detection described above.

In the above embodiment, the polarization maintaining fiber 1 was used as the optical fiber, and the physical quantity was detected using the crosstalk between the two linearly polarized waves in the direction of the optical axis. The same measurement as in the above embodiment is also possible by using a twin-core fiber provided with two cores with different ratios and utilizing crosstalk between the two cores (propagation paths).

[Effects of the Invention] In summary, the present invention provides the following excellent effects.

(1) The optical fiber itself is a sensor and does not require an optical conversion element such as an optical crystal. For this reason,
The device configuration can be simplified, the handling is easy, and the device can be provided at low cost.

(2) The position where a physical change has occurred and the amount of change can be detected simultaneously, easily and accurately.

(3) Average physical quantities and physical quantity distributions along the optical fiber can be measured.

(4) There are no loss-causing factors due to intervening elements or coupling conditions, and it is less susceptible to external disturbances, allowing stable, low-loss, long-distance sensing.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical physical quantity detection device according to the present invention, and FIG. 2 is a diagram showing an example of pulsed light emitted from an optical fiber (polarization preserving fiber) of the device. . In the figure, 1 is a polarization preserving fiber, 2 and 3 are polarizing prisms, 4 is a light source, 5 is a pulse generation circuit, 6,
7 is a light receiver, 8 is a computing unit, 9 is a display, 10 is a site where a physical change has occurred, a is an original pulsed light, b is a crosstalk pulsed light, and t is a delay time.

Claims (1)

[Scope of Claims] 1. An optical fiber having two propagation paths with different curvatures, a light source that inputs pulsed light into one of the propagation paths of the optical fiber, and a pulsed light that propagates through the one propagation path. A photoreceiver that receives the pulsed light and the pulsed light that crosstalks due to physical changes imparted to the optical fiber and propagates through the other propagation path, and a light intensity of both pulsed lights based on the output of the photoreceiver. 1. An optical physical quantity detection device comprising: an arithmetic unit that detects a ratio and a delay time. 2. The optical fiber is a polarization preserving fiber having mutually orthogonal optical axes with different curvature indexes,
2. The optical physical quantity detection device according to claim 1, wherein the optical physical quantity detection device is configured such that linearly polarized pulsed light is incident on one optical axis of the polarization preserving fiber from the light source. 3. The optical physical quantity detection device according to claim 1, wherein the optical fiber is a twin-core fiber consisting of two cores having different curvature indexes and one cladding layer surrounding them.