JPH044972Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] This invention relates to an optical physical quantity detection device that uses a birefringent optical fiber as a sensor element, and in particular, when a physical change occurs in the birefringent optical fiber, The present invention relates to an optical physical quantity detection device that detects a physical quantity from backscattered light of crosstalk light leaking from one optical axis of a refractive optical fiber to the other optical axis.

[Prior Art] A conventional physical quantity detection device using an optical fiber involves guiding light from a light source through an optical fiber, inputting the light into a conversion element such as an optical crystal, and measuring the intensity, polarization angle, phase, etc. of the transmitted light. A method of measuring a physical quantity from a change, and a method of detecting an optical change by using an optical fiber itself as a sensor element using interferometry or the like are known.

However, the method of using a conversion element as a sensor can only measure the physical quantity of the area where the sensor is installed, and the method of using an optical fiber as a sensor can only measure the average physical quantity of the area where the optical fiber is installed. Therefore, in these methods,
Even if a large number of sensors are installed, it is difficult to determine the location of parts with a fine-grained distribution of physical quantities or significant physical changes.

Therefore, as a way to solve this problem,
OTDR (Optical Time Domain Reflectometry)
is being considered. In an OTDR, backscattered light returning in the direction of incidence is detected among the scattered light of pulsed light incident on an optical fiber, and physical changes and position are determined from the intensity and propagation time of the backscattered light.

[Problems to be solved by the invention] However, in the OTDR, generally a short-time pulsed light is input and the backscattered light is detected, so the level of the backscattered light is extremely low compared to the incident light. For this reason, there have been problems in that the signal-to-noise ratio is poor, it is difficult to accurately detect physical quantities, and the detection accuracy of signals from a distance tends to decrease.

[Purpose of the invention] This invention was devised to solve the problems of the prior art described above. An object of the present invention is to provide an optical physical quantity detection device that can detect with high precision and over a long distance.

[Summary of the invention] In order to achieve the above object, this invention consists of a birefringent optical fiber having two optical axes capable of transmitting linearly polarized light while maintaining its plane of polarization, and a birefringent optical fiber having one of the optical axes. This is caused by a transmission system for inputting linearly polarized pulsed light aligned with the optical axis, and a physical change imparted to the birefringent optical fiber by the linearly polarized pulsed light transmitted aligned with one of the optical axes. and a receiving system for detecting temporal changes in the intensity of backscattered light with respect to crosstalk light to the other optical axis caused by the optical axis. It is possible to detect only the signal from behind the position where crosstalk (light leakage) has occurred between the two optical axes due to a physical change, making it possible to evaluate the position with high precision and also to detect the signal.
High signal-to-noise ratio.

[Example] Hereinafter, an example of this invention will be described in detail with reference to the accompanying drawings.

In FIG. 1, 1 is a light source, and the light source 1 is modulated by a pulse generator 2 to generate pulsed light with a short pulse width. A light source 1 is provided facing one end of a polarization preserving fiber 3 having two orthogonal optical axes, and an isolator 4 and a polarizing prism 5 are provided between the light source 1 and the polarization preserving fiber 3. There is. The isolator 4 receives reflection and scattering of the light emitted from the light source 1 and
This is to block the returning light. The transmitted polarization direction and the reflected polarization direction of the polarizing prism 5 are arranged to match the directions of the two optical axes of the polarization preserving fiber 3, and the polarizing prism 5 converts the pulsed light from the light source 1 into the polarization preserving fiber. The function of the polarizer is to linearly polarize light in the direction of one of the optical axes of the polarization preserving fiber 3, and the backscattered light of the crosstalk light in the direction of the other optical axis of the polarization preserving fiber 3 is transmitted to the receiver 6.
It has the function of an analyzer that separates reflections. In addition, 7
9 is a timing generator that monitors the pulse generation time of the pulse generator 2 and inputs it to the signal processing circuit 8, and 9 is a display that displays the calculation results of the signal processing circuit 8.

The pulsed light emitted from the light source 1 passes through an isolator 4 and a polarizing prism 5, and is converted into linearly polarized light by the polarizing prism 5 that coincides with the optical axis direction of one of the polarization preserving fibers 3.
incident on . The linearly polarized pulsed light incident on one of the optical axes propagates through the polarization preserving fiber 3 without changing its polarization plane.

If there is no physical change in the polarization-preserving fiber 3, linearly polarized pulsed light propagating along one optical axis will not leak to the other optical axis, and the linearly polarized pulsed light will remain aligned along one optical axis. Only pulsed light is propagated. Therefore, in this case, the backscattered light that returns to the incident side is only linearly polarized light that coincides with the one optical axis direction, and all of the backscattered light passes through the polarizing prism 5 and is absorbed by the isolator 4. Since there is no input from the light receiver 6, the signal processing circuit 8 determines that there is no change in the physical quantity in the measurement area along the polarization preserving fiber 3, and displays the result on the display 9.

On the other hand, if a physical change occurs at any point of the polarization preserving fiber 3, for example at point 10, the linearly polarized pulsed light propagating along one optical axis will change to the other optical axis at point 10. After the point 10, the incident light incident along one optical axis and the crosstalk light leaking to the other optical axis propagate. Therefore, from the input end of the polarization-maintaining fiber 3, backscattered light for the incident light that coincides with one optical axis and backscattered light for the crosstalk light that coincides with the other optical axis are emitted. Only the backscattered light of the light is reflected by the polarizing prism 5 and separated. The backscattered light corresponding to the separated crosstalk light is detected by the light receiver 6, and the intensity signal of the backscattered light as shown in FIG. 2 is input to the signal processing circuit 8.

The signal processing circuit 8 performs arithmetic processing based on the pulse generation time of the pulse generator 2 and the detection time and intensity of the backscattered light pulse detected by the light receiver 6, and determines the point where a physical change has occurred based on the time difference between the two. 10
is calculated, and the changed physical quantity is calculated from the backscattered light intensity. This method detects the backscattered light of crosstalk light caused by physical changes to the polarization preserving fiber 3, so it detects the backscattered light of crosstalk light that is caused by physical changes to the polarization preserving fiber 3. can be measured. Furthermore, only signals from behind the point 10 where a physical change has occurred and crosstalk has occurred can be detected. Therefore, the SN ratio of the physical quantity detection signal can be increased, the position where a physical change has occurred and the amount of change can be detected with high precision, and long-distance sensing is possible. Note that the physical quantity to be detected may be any one such as temperature or tension as long as crosstalk occurs due to a change in the physical quantity.

[Effects of the invention] As is clear from the above explanation, this invention provides the following excellent effects.

(1) Since only the backscattered light of crosstalk light due to physical changes is detected, it is not affected by the backscattered light of the incident light.
In addition, it is possible to detect signals only after the point where a physical change has occurred, making it possible to measure according to the physical change.
Therefore, the S/N ratio of the signal is high, and the point where a physical change has occurred and the amount of physical change can be measured with high precision, and long-distance sensing of physical quantity distribution along a birefringent optical fiber is possible.

(2) It does not require conversion elements such as optical crystals in addition to birefringent optical fibers, and can increase the signal-to-noise ratio of signals.
The device can be simplified and costs can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of an optical physical quantity detection device according to the invention, and FIG. 2 is a waveform diagram showing an example of an intensity signal wave of backscattered light for crosstalk light detected by the device. It is. In the figure, 1 is a light source, 2 is a pulse generator, 3 is a polarization preserving fiber, 4 is an isolator, 5 is a polarizing prism, 6 is a light receiver, 7 is a timing generator, 8
9 is a signal processing circuit, 9 is a display, and 10 is a point where a physical change occurs.

Claims (1)

[Claims for Utility Model Registration] (1) A birefringent optical fiber having two optical axes capable of transmitting linearly polarized light while maintaining its plane of polarization;
A transmitting system for inputting linearly polarized pulsed light into a birefringent optical fiber with one optical axis aligned with the birefringent optical fiber; 1. An optical physical quantity detection device comprising: a receiving system for detecting backscattered light intensity of crosstalk light to the other optical axis caused by a physical change applied to the other optical axis. (2) The optical physical quantity detection device according to claim 1, wherein the birefringent optical fiber is a polarization preserving fiber.