JP2890654B2 - Distributed optical fiber temperature sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a distributed optical fiber temperature sensor in which a laser pulse from a light source is amplified.

[Prior Art] FIG. 2 shows a block diagram of a conventional distributed optical fiber temperature sensor.

The laser pulse oscillated from the pulse light source (LD or the like) 11 of the light source unit enters the optical fiber 13 to be measured, and the backward Raman scattered light generated in the optical fiber 13 returns to the incident end. The backward Raman scattered light is guided to the measuring device by the optical directional coupler 12, and first, Stokes light (S) and anti-Stokes light (AS) in the backward Raman scattered light are separated and detected. It is converted into an electric signal proportional to the intensity. Each of the electric signals is averaged a predetermined number of times by an averager 15. The signal subjected to the averaging process is transmitted to a signal processing unit such as the computer 16, where the ratio of the signal of the Stokes light to the signal of the anti-Stokes light is calculated, and a process such as conversion into a temperature distribution is performed. Here, a trigger signal is also oscillated from the pulse light source 11 in order to synchronize measurement with the averager 15.

Conventionally, a pulsed light source 11 is a high-output pulsed semiconductor laser (wavelength 850 nm) or a semiconductor laser-pumped YAG laser (hereinafter abbreviated as QSW / DPY) obtained by Q switching.
Wavelengths of 1.06 μm and 1.32 μm) are used.

[Problems to be Solved by the Invention] Problems with the conventional pulse light source will be described.

1) High-power pulsed semiconductor laser In general, a relatively stable pulsed light can be obtained by controlling the current and temperature in a semiconductor laser. However, the radiation angle from the light emitting surface is wide, and the high-powered pulsed semiconductor laser is generally used. Has a problem in connection with an optical fiber because the light emitting surface is larger than that of an ordinary CW (continuous wave) semiconductor laser.

In addition, the wavelength is limited to around 850 nm, and it is not suitable for long distance measurement due to the loss in the fiber at present.

2) QSW ・ DPY In DPY, a beam with good straightness can be obtained, so there is not much problem in coupling with the optical fiber. However, when pulse operation is performed by Q switching, the pulse position as shown in FIG. Jitter and fluctuations in peak values 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 is a signal light source for inputting a laser pulse to an optical fiber to be measured and an excitation light source for inputting excitation light to an optical amplifier. And a light directional coupler that combines the laser pulse and the excitation light and guides the light to the measured optical fiber, and a light direction that guides backward Raman scattered light from the measured optical fiber to the measurement device. Sexual coupler, a detector that detects Stokes light and anti-Stokes light in the backward Raman scattered light, and photoelectrically converts the Stokes light and the anti-Stokes light into electric signals proportional to the respective intensities, and an average that averages a predetermined number of electric signals from the detector. Processing section, a signal processing section for processing the data averaged a predetermined number of times to measure the temperature distribution of the optical fiber to be measured, and amplifying the laser pulse between the light source section and the optical fiber to be measured. The present invention provides a distributed optical fiber temperature sensor characterized in that an optical amplifier is provided.

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

Semiconductor laser for signal (hereinafter, LD) 1, LD2 for excitation
With the optical directional coupler 4 and the rare earth-doped optical fiber 5
And optically coupled. The wavelengths of the LDs 1 and 2 differ depending on the rare earth to be doped. For example, when erbium (Er) is used as a doping source, 1.55 μm is used for a signal and 1.48 μm is used for excitation. The rare earth to be doped is S
m, Ho, etc. are used.

Both wavelength lights are incident on the rare earth doped optical fiber 5,
The rare earth in the optical fiber 5 is excited by the excitation light, and its orbital electrons transition between higher energy levels,
Light having the same wavelength as the signal light is emitted and the signal light is amplified.

The backward Raman scattered light generated by the signal light in the measured optical fiber is guided to the measuring device by the optical directional coupler 4 such as an optical fiber coupler or an acousto-optic device. The backward Raman scattered light and the excitation light are separated by a filter such as a half mirror 6, and are monitored by a signal power monitor (PD) 7 and an excitation power monitor (PD) 8, respectively, to monitor the occurrence of insufficient optical power and the like. Adjust with feedback. Although not shown, the backward Raman scattered light is guided to the measuring device by an optical directional coupler provided between the rare earth-doped optical fiber 5 and the optical fiber to be measured, and the same processing as shown in FIG. The distribution is measured.
Therefore, the optical directional coupler may be provided for each of the monitor and the temperature distribution measurement, or may be used alone.

[Operation] If the core diameter and NA (numerical aperture) of the rare-earth-doped optical fiber 5 are designed and manufactured close to those of the optical fiber to be measured, optical coupling can be easily performed, and if an LD is used as a light source, pulse stability will be improved. Since it can be obtained relatively easily, there is little optical loss, stable pulse position,
An incident laser pulse having a pulse amplitude and a large light amount can be obtained.

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

The signal light wavelength is 1.55 μm, and the pump light wavelength is 1.48 μm. In this case, since the loss and the chromatic dispersion in the optical fiber are small, long distance measurement is possible.

The amplification factor in the optical fiber depends on the pump light power and the optical fiber length, but the pump light power is 70 to 80 mW,
20 to 30 (dB) at 90m optical fiber length (Amplification factor 100 to 1000
Times). When several watts are obtained as output light, the signal light input power has a low output of about several mW to several tens of mW, which makes it possible to use an inexpensive LD which is reliable for communication and as a light source. Also, since the optical power is gained in the optical fiber, the signal light power does not need to be so large, so that the distance resolution can be improved by using a communication high-speed modulation laser (DFB laser).

The distance measurable by this light source is the dynamic range
If 10 (dB) and the loss in the optical fiber is 0.2 dB / km,
The distance will be 25km one way and 50km round trip.

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

In addition, the amount of laser pulse incident on the optical fiber to be measured also increases, and the measurable distance increases.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention and is a block diagram of a main part. FIGS. 2 and 3 show a conventional example. FIG. 1 is a block diagram of a conventional example, and pulse position jitter and peak value fluctuation of a laser pulse. FIG. 6 is a waveform diagram illustrating the above. 1. LD for signal 2. LD for excitation 5. Rare-earth doped optical fiber

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁶ , DB name) G01K 11/12 G01K 11/32 G02B 6/00 G01M 11/02

Claims (2)

(57) [Claims] 1. A light source section comprising a signal light source for injecting a laser pulse into an optical fiber to be measured and an excitation light source to inject excitation light into an optical amplifier, and multiplexing the laser pulse and the excitation light into an optical fiber to be measured. An optical directional coupler that guides light, an optical directional coupler that guides backward Raman scattered light from the optical fiber to be measured to the measuring device, and detects Stokes light and anti-Stokes light in the backward Raman scattered light and detects respective intensity. A detector that photoelectrically converts the electric signal into a proportional electric signal, and an averaging processing unit that averages the electric signal from the detector a predetermined number of times,
A signal processing unit for processing the data averaged a predetermined number of times to measure the temperature distribution of the optical fiber to be measured, and an optical amplifier for amplifying the laser pulse between the light source unit and the optical fiber to be measured. Characteristic distributed optical fiber temperature sensor. 2. The distributed optical fiber temperature sensor according to claim 1, wherein said optical amplifier is a rare earth doped optical fiber.