JPH06347225A - Apparatus for measuring distortion distribution of optical fiber - Google Patents

Abstract

PURPOSE:To use a single light source and to make it possible to achieve the low cost by using the light source used for generating pulse light and continuous wave light as the single wavelength light source, providing an optical fiber in a loop shape, and switching and using both ends of the optical fiber and the variable wavelength light source with an optical switch. CONSTITUTION:A light source 2 is oscillated at a frequency nucw, and the light is emitted into (a) of an optical switch 9. The light is transmitted through an optical fiber 7. At the time when the light reaches the entire length of the fiber 7, the switch is changed to (b), and the light of the light source 2 is applied. At this time the oscillating frequency is made to be nup at the same time. For the frequency nup at this time, the difference DELTAnu is made to agree with a Brillouin frequency nub of the fiber 7. Thereafter, the light becomes the pulse light with an optical pulse generator 3. The light is transmitted through the fiber 7. At this time, the light of nucw undergoes Brillouin optical amplification in the optical fiber 7 by the light/electric-power conversion step from the light of the frequency nup to the light of the frequency nucw until the time when the pulse light reaches the position that is the half point of the fiber 7. The distortion distribution can be measured by analyzing the light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber strain distribution measuring apparatus.

[0002]

2. Description of the Related Art FIG. 6 shows the structure of a conventional optical fiber strain distribution measuring apparatus.

The optical fiber strain distribution measuring apparatus 1 comprises a pump light source 2a, a pulse generator 3 for generating an optical pulse from the pump light source 2a, a probe light source 2b for oscillating continuous wave light (hereinafter referred to as CW), and a probe. Optical demultiplexer 4 that reflects only the optical frequency of the optical source 2b, photodetector 5 that receives the light reflected by the optical demultiplexer 4 and photoelectrically converts it, and averaging that averages the electric signals from the photodetector 5 It is composed of a processing circuit 6 and an arithmetic circuit 8 which measures the strain distribution of the optical fiber 7 to be measured based on the output signal of the averaging processing circuit 6. Next, the operation of the optical fiber strain distribution measuring device configured as described above will be briefly described.

Lights emitted from the pump light source 2a and the probe light source 2b arranged at both ends of the optical fiber 7 are incident on the optical fiber while facing each other. Further, the difference Δν between the optical frequency νP of the pump light source 2a and the optical frequency νcw of the probe light source 2b is Δν =
νp−νcw is matched with the Brillouin frequency shift νp of the optical fiber 7. At this time, the CW light is amplified by the Brillouin light in the optical fiber by the optical power conversion process from the pulsed light (pump light source) to the CW light (probe light source) due to the stimulated Brillouin scattering. The optically amplified CW light passes through the optical demultiplexer 4 and is converted into an electric signal by the light receiver 5. The center wavelength of the optical demultiplexer 4 is the frequency νcw of the CW light.
And a Fresnel reflected light and Rayleigh scattered light due to pulsed light of frequency νp are attenuated. At this time, CW observed on the pump light source 2a side
FIG. 2 schematically shows the time waveform of light. The observed CW optical power Pd is given as follows.

Pd (z, ν) = Pdc + Pd (z, Δν) (1) “Pdc = Pcw · exp (−αL) (2) Pb (z, Δν) = [g (z, Δν) / A] [nW / Z] Pcw ・ exp (−αL) P ・ exp (−αz) ＝ D (z, △ ν) (3) g (z, △ ν) ＝ go Q (νb, △ ν) ＝ go / {1 + [(Δν−νb) / (Δνb / 2)] ² } (4) z = υt / 2 (5) where Pdc is the DC component of the CW light. Also Pb
(Z, Δν) is the incremental power of the CW light amplified by the Brillouin light, and is the signal analyzed by this device. Pp and Pcw are the incident powers of the pulsed light and the CW light to the optical fiber, respectively. W is the pulse width of the pulsed light, α is the loss coefficient of the optical fiber, L is the length of the optical fiber, and Z is the position of the optical fiber.

D (z, Δν) is a coefficient indicating that the pulsed light is attenuated by losing its own energy in the process of amplifying the CW light. G (z, Δν) in the equation (4) is a Brillouin gain spectrum. g
o is the Brillouin gain coefficient, and A is the effective area of the optical fiber. Δνb is the effective Brillouin gain bandwidth and is the spectral line width Δν of the pump light source and probe light source.
It is given by the sum of p, Δνcw and the Brillouin gain bandwidth Δνbi peculiar to the optical fiber.

Z, which represents the strain sensing position coordinate in the optical fiber, has the relationship shown in equation (5) with the time t based on the time when the pulse is incident on the optical fiber.

Brillouin frequency shift νb
Is greatly dependent on the elongation strain applied to the optical fiber and is given by the following equation.

Νb = νb (0) (1 + C · ε) (6) νb (0) is the Brillouin frequency shift when there is no distortion, and C is the distortion coefficient of the Brillouin frequency shift.

As shown in FIG. 3, when the strain ε is applied to the optical fiber only in the section [z ₁ , z ₂ ] as shown in FIG. 3, the signal waveform is changed while discretely changing the optical frequency difference Δν of the light source. When Pb is measured, the waveform group shown in FIG. 3 is obtained.
In the intervals [0, z ₁ ] and [z ₂ , L], the maximum signal power is obtained when ν = νb (0), while the intervals [z ₁ ,
z ₂ ], it can be seen that the maximum signal power is obtained when Δν = νb (ε). In this way, for each position z, the optical frequency difference Δν of the light source that gives the maximum signal power,
That is, by measuring the Brillouin frequency shift νb (ε (z)), the strain distribution ε (z) of the optical fiber can be obtained by using the equation (6). (Journal of the Institute of Electronics, Information and Communication Engineers B-1, Vol. J73-B-1, No2p
(See p. 144-152, February 1990 issue)

[0011]

However, the conventional strain distribution measuring device has a problem that it is expensive because it requires two light sources.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and provide an inexpensive optical fiber strain distribution measuring apparatus.

[0013]

The gist of the present invention resides in that the strain distribution measuring device has only one light source, which reduces the cost of the device.

That is, the above objects of the present invention are as follows. A light receiving circuit that matches only the Brillouin frequency shift of the fiber and detects only continuous wave light,
In an optical fiber strain distribution measuring device provided with an averaging processing circuit for improving the S / N ratio of the received light signal and an arithmetic circuit for measuring the strain distribution of the optical fiber from this signal, the pulsed light and the continuous light are maintained. Optical fiber strain distribution measurement, characterized in that the light source used for generating wave light is one variable wavelength light source, the optical fiber is installed in a loop shape, and both ends of the optical fiber and the variable wavelength light source are switched and used by an optical switch. apparatus.

(2) A modulator using an acousto-optic effect is used in place of the optical switch.
The optical fiber strain distribution measuring device described.

Is achieved by

In the present invention, when the amount of backscattered light generated by pulsed light and returning to the incident end of the pulsed light is sufficiently smaller than the amount of transmitted CW light, measurement is performed without separating frequency νcw and frequency νp. In this case, an optical branching device may be used instead of the optical demultiplexer 4 because there is no problem.

If a modulator utilizing the acousto-optic effect is used instead of the optical switch of the present invention, an optical pulse can be generated by turning on / off a high frequency signal to the modulator,
Further, the role of the optical switch 9 can also be served, and the number of parts can be further reduced.

[0019]

FIG. 1 shows an embodiment of the optical fiber strain distribution measuring apparatus of the present invention, and the optical fiber type strain distribution measuring apparatus of the present invention will be described in detail. The strain distribution measuring apparatus 1 includes a light source 2c whose oscillating light frequency changes according to an applied signal,
An optical switch 9 for switching the light from the light source 2c to both ends of the optical fiber 7 to be measured, and a pulse generation for turning one light emitted from the light switch 2c from the optical switch 9 into an optical pulse. And the optical demultiplexer 4 for the other CW light emitted from the optical switch 9 at the optical frequency νcw.
And a light receiver 5 that photoelectrically exchanges the light guided by the optical demultiplexer 4.
And an averaging processing circuit 6 for averaging the electric signal from the light receiver 5, and an arithmetic circuit 8 for measuring the strain distribution of the optical fiber 7 to be measured based on the output signal of the averaging processing circuit 6. There is.

Next, the operation of the optical fiber strain distribution measuring device configured as described above will be briefly described.

The light source 2c is oscillated at a certain optical frequency νcw and is incident on (A) of the optical switch 9. The incident light is transmitted through the measured optical fiber 7. When the light reaches the entire length of the optical fiber 7 with reference to the time of incidence on the optical fiber 7, the optical switch 9 is switched (B), and the light from the light source 2 is made incident. At this time, the oscillation frequency is set to νp at the same time. The frequency νp at this time is Δν when Δν = νp−νcw
Corresponds to the Brillouin frequency νb of the optical fiber 7. Thereafter, this light becomes pulsed light by the optical pulse generator 3 and is transmitted through the optical fiber 7. At this time,
Until the time when the pulsed light reaches the half position of the optical fiber 7, the frequency ν due to the interaction with the light of the frequency νcw
The light of νcw is amplified by Brillouin light in the optical fiber by the process of converting the electric power from p to the light of νcw.

The strain distribution can be measured by analyzing this light in the same manner as the conventional device.

By the way, with the above method alone, the strain distribution is measured only for half the total length of the optical fiber 7. Therefore, in order to measure the total length, it is necessary to invert the connector connection of the fiber to be measured. Good.

The manner of interaction with the light having the optical frequency νcw based on the time when the optical pulse having the optical frequency νp enters the optical fiber 7 will be described with reference to FIG. When the time t = 0, the light of νcw exists in the entire length (1) of the optical fiber 7, and therefore the light of νcw and the light of νp interact, but the light is the optical fiber 7.
When the time t = 1 / 2υ (υ: speed of light in the optical fiber) to reach the position of half the total length is exceeded, the lights of νcw and νp do not interact with each other.

Another embodiment of the optical fiber strain distribution measuring apparatus of the present invention will be described. In this case, if a modulator utilizing the acousto-optic effect is used for the optical pulse generator 3a, a high frequency signal to the modulator is used. Can be used to generate an optical pulse, and the optical switch 9 can also serve as the optical pulse.

[0026]

The optical fiber type strain distribution measuring apparatus of the present invention can measure the strain distribution even with a single light source, so that the manufacturing cost of the apparatus can be reduced.

[Brief description of drawings]

FIG. 1 shows an optical fiber strain distribution measuring device of the present invention.

FIG. 2 shows a signal measured by an optical fiber strain distribution measuring device.

FIG. 3 shows a final result measured by an optical fiber strain distribution measuring device.

FIG. 4 shows the time course of light interaction.

FIG. 5 shows another embodiment of the optical fiber strain distribution measuring device of the present invention.

FIG. 6 shows a conventional optical fiber strain distribution measuring device.

[Explanation of symbols]

 1 Optical fiber strain distribution measuring device 2 Light source 2a Pump light source 2b Probe light source 2c Variable wavelength light source 3 Optical pulse generator 3a Modulator using acousto-optic effect 4 Optical splitter 5 Optical receiver 6 Averaging processing circuit 7 Optical fiber 8 Calculation Circuit 9 Optical switch

Claims (2)

[Claims] 1. A pulsed light and a continuous wave light are incident from both ends of the optical fiber so as to face each other in the optical fiber, and the difference between the optical frequency of the pulsed light and the optical frequency of the continuous wave light is matched with the Brillouin frequency shift of the optical fiber. Optical fiber strain distribution including a light receiving circuit that detects only continuous wave light, an averaging circuit that improves the S / N ratio of the light receiving signal, and an arithmetic circuit that measures the strain distribution of the optical fiber from this signal In the measuring device, the light source used to generate the pulsed light and continuous wave light is one variable wavelength light source and the optical fiber is installed in a loop,
An optical fiber strain distribution measuring apparatus characterized in that both ends of an optical fiber and the variable wavelength light source are switched and used by an optical switch. 2. The optical fiber strain distribution measuring apparatus according to claim 1, wherein a modulator utilizing an acousto-optic effect is used instead of the optical switch.