JPH042933A - Distribution-type optical fiber temperature sensor - Google Patents

Abstract

PURPOSE:To obtain a stable incident laser pulse by providing a photoamplifier for amplifying the laser pulse between a light source part and a optical fiber to be measured. CONSTITUTION:A wavelength light from a signal LD (semiconductor laser) 1 and an exciting LD 2 enters, through a directional photocoupler 4, an optical fiber 5 into which a rare earth substance is doped. The rare earth substance in the optical fiber 5 is excited by the light, and its orbital electrons make transition to and from a higher energy potential. Accordingly, a light of the same wavelength as the signal light is emitted, and the signal light is amplified. Subsequently, the rear Raman scattering light generated by the signal light in the optical fiber to be measured is guided by the coupler 4 to a measuring apparatus. Then, the rear Raman scattering light and exciting light are separated by a half mirror 6 and monitored by a signal power and an exciting power monitors 7, 8. The lack of power of the light or the like is monitored, fed back and adjusted. The rear Raman scattering light is guided to the measuring apparatus by a directional photocoupler between the optical fiber 5 and optical fiber to be measured to measure the temperature distribution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a distributed optical fiber temperature sensor that amplifies laser pulses from a light source.

[Prior Art] A block diagram of a conventional distributed optical fiber temperature sensor is shown in FIG.

A laser pulse emitted from a pulsed light source (such as an LD) 11 in the light source section is incident on the optical fiber to be measured 13, and backward Raman scattered light generated in the optical fiber 13 returns to the input end. The backward Raman scattered light is transmitted to the optical directional coupler 1.
2, the light is guided to the measuring device, and first, Stokes light (S) and anti-Stokes light (AS) in the backward Raman scattered light are separated and detected, and each is converted into an electric signal proportional to the intensity by the detector 14. The electrical signals are each average or -15
Averaging processing is performed a predetermined number of times.

The averaged signal is transmitted to a signal processing unit such as the computer 16, where the ratio of the signals of Stokes light and anti-Stokes light is calculated, and processing such as conversion to temperature distribution is performed. Here, a trigger signal is also generated from the pulse light source 11 in order to synchronize the measurement with the averager 15.

Conventionally, the pulsed light source 11 uses a high-power pulsed semiconductor laser (wavelength: 850 nm) or a semiconductor laser-excited YAG laser (hereinafter referred to as QSW-D) obtained by Q-switching.
Abbreviated as PY. Wavelengths of 1.06 μm and 1.32 μm) are used.

[Problem to be solved by the invention] This section describes the problems with conventional pulsed light sources.

1) In the case of high-power pulsed semiconductor lasers In general, semiconductor lasers can provide relatively stable pulsed light if current and temperature are controlled, but the radiation angle from the light emitting surface is wide, and high-power pulsed semiconductor lasers Since the light-emitting surface of the laser is larger than that of a normal CW (continuous wave) semiconductor laser, there were problems in coupling it with an optical fiber.

In addition, the wavelength is limited to around 850 nm, and currently it is not suitable for long-distance measurements due to loss in the fiber.

2) In the case of QSW-DPY With DPY, a beam with good straightness is obtained, so there is no problem in coupling with an optical fiber, but when pulse operation is performed with QSW, pulse jitter and the like shown in Figure 3 occur. , peak value fluctuations sometimes became a problem.

The present invention has the advantages of both light sources and solves the disadvantages.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a signal light source that injects a laser pulse into an optical fiber to be measured, and an excitation light source that injects excitation light into an optical amplifier. a light source section consisting of a light source section, a light directional coupler that combines the laser pulse and the excitation light and guides the light to the optical fiber to be measured, and a light direction that guides the backward Raman scattered light from the optical fiber to be measured to the measurement device. a detector that detects Stokes light and anti-Stokes light in the backward Raman scattered light and photoelectrically converts them into electrical signals proportional to their respective intensities; and an averager that averages the electrical signals from the detector a predetermined number of times. a signal processing unit that processes data averaged a predetermined number of times to measure the temperature distribution of the optical fiber to be measured, and an optical amplifier that amplifies the laser pulse between the light source unit and the optical fiber to be measured. A distributed optical fiber temperature sensor is provided.

FIG. 1 shows a block diagram of the main parts of the distributed optical fiber temperature sensor of the present invention.

Semiconductor laser for signal (hereinafter referred to as LD) 1, LD for excitation 2
, an optical directional coupler 4 and a rare earth doped optical fiber 5
optically combine with

The respective wavelengths of LDI, 2 differ depending on the rare earth doped, but for example, when using erbium (Er) as the doping source, the wavelength for the signal is 1.55 μm, and the wavelength for the excitation is 1.48 μm.
Use μm. Other rare earth elements used for doping include Sm and Ho.

The light of the same wavelength enters the rare earth doped optical fiber 5, and the rare earth in the optical fiber 5 is excited by the excitation light, and its orbital electrons transit between higher energy levels and emit light of the same wavelength as the signal light. The signal light is then amplified.

Backward Raman scattered light generated by the signal light in the optical fiber to be measured is guided to the measuring device by the optical directional coupler 4 such as an optical fiber 7 coupler or an acousto-optic element. The backward Raman scattering light and the excitation light are separated by a filter such as a half mirror 6, and each is connected to a signal power monitor (PD) 7.
This is monitored by an excitation power monitor (PD) 8, and the occurrence of an optical power shortage or the like is monitored and feedback is provided for adjustment. Although not shown, the backward Raman scattered light is guided to the measurement device by an optical directional coupler provided between the rare earth-doped optical fiber 5 and the optical fiber to be measured, and is guided to the second measurement device.
Processing similar to that shown in the figure is performed, and the temperature distribution is measured. Therefore, an optical directional coupler may be provided for monitoring and temperature distribution measurement, or one optical directional coupler may be used for both purposes.

[Function] If the core diameter and NA of the rare earth-doped optical fiber 5 are designed and manufactured close to those of the fiber to be temperature measured, optical coupling can be easily performed, and if an LD is used as the light source, the pulse stability can be relatively improved. Since it is easy to obtain, an incident laser pulse with a stable pulse position, pulse amplitude, and large light intensity can be obtained with little optical loss.

[Example] As an example, a case where Er is used as the rare earth element will be described.

The signal light wavelength is 155 μm, and the excitation light wavelength is 1.48 μm. In this case, since the wavelength causes less loss and chromatic dispersion in the optical fiber, long-distance measurement is possible.

The amplification factor in the fiber depends on the pump light power and optical fiber length, but the pump light power is 70 to 80 mW.
, 20 to 30 (dB) (amplification factor of 100 to 1000 times) with an optical fiber length of 90 m. When obtaining several watts of output light, the signal light input power is a low output of several mW to several tens of mW, which is reliable for communication purposes, and allows the use of an inexpensive LD as a light source. Also, since the optical power is stored in the optical fiber, the signal optical power is not very thick (
Since it is not necessary to use high-speed modulated laser (DF) for communication,
It is also possible to improve the distance resolution using the B laser.

The measurable distance with this light source is a dynamic range of 1
0 (dB), loss in optical fiber 0.2 dB/km
If so, the measurement distance will be 25km one way and 50km round trip.

[Effects of the Invention] The present invention has the above-mentioned excellent effects, and by particularly selecting a rare earth element to be doped, light of various wavelengths can be amplified.

Furthermore, the amount of laser pulses incident on the optical fiber to be measured increases, and the measurable distance also increases.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the present invention and is a block diagram of the main part, and FIGS. 2 and 3 show a conventional example. FIG. 3 is a waveform diagram for explanation. 1... LD for signal 2... LD for excitation 5... Rare earth doped optical fiber

Claims (2)

[Claims] (1) A light source section consisting of a signal light source that inputs a laser pulse to the optical fiber under test and a pump light source that inputs pump light to the optical amplifier, and combines the laser pulse and the pump light and guides the light to the optical fiber under test. an optical directional coupler that guides the backward Raman scattered light from the optical fiber under test to the measuring device, and a light directional coupler that detects the Stokes light and anti-Stokes light in the backward Raman scattered light, each of which is proportional to the intensity. a detector that photoelectrically converts the electrical signal into an electrical signal; an averaging processing unit that averages the electrical signal from the detector a predetermined number of times; and measures the temperature distribution of the optical fiber to be measured by processing the data that has been averaged a predetermined number of times. What is claimed is: 1. A distributed optical fiber temperature sensor, comprising: a signal processing section for amplifying the laser pulse; and an optical amplifier for amplifying the laser pulse between the light source section and the optical fiber to be measured. (2) The distributed optical fiber temperature sensor according to claim 1, wherein the optical amplifier is a rare earth doped optical fiber.