JPH02140636A - Backscattering light measuring instrument - Google Patents

Abstract

PURPOSE:To attain measurement with high accuracy a backscattering light generated in an optical waveguide by supplying an output of a photodetector to a signal processing part through a differential amplifier. CONSTITUTION:Photodetectors 7, 10 are provided on each of tails T1, T2, and a differential amplifier 11 is provided as a processing means for negating each other the same phase components included in the outputs of the detectors 7, 10. Also, the detector 7 is arranged in the emission end of the tail T1 of a fiber type photocoupler 2, and the detector 10 is arranged in the multiplexed light emission end of a fiber type photocoupler 9. Moreover, the coupler 9 is provided on the tale T2, the emitted light of a light source 1 is coupled to the coupler 2, and also, a multiplexed light from the coupler 2 is emitted to the other tail. Subsequently, the outputs of the detector 7, 10 are supplied to a signal processing part 8 through the amplifier 11. In this state, by subtracting that which is obtained by doubling an output deltaIC2 of the detector 10 from an output deltaIC1 of the detector 7 by the amplifier 11, an intensity noise is erased. In such a manner, a value for showing the relative electric field amplitude of a back Rayleigh scattering light can be measured by a high signal-to-noise ratio.

Description

[Detailed description of the invention] [Industrial application field] The present invention is used to search for discontinuities in a leading wavepath.

In particular, the present invention relates to a device that highly accurately measures backscattered light generated within an optical waveguide with a spatial resolution of 1 mm or less.

The present invention provides a backscattered light measurement device that combines a reference light with a reference light generated in a leading waveguide and measures the combined light, in which the combined light is measured independently at two positions and included in the output. By canceling out the in-phase components of the light source, deterioration in the signal-to-noise ratio caused by fluctuations in the light intensity of the light emitted from the light source is improved.

[Conventional technology]

FIG. 4 is a block diagram showing a conventional backscattered light measuring device.

The light emitted from the light source 1 is split into two light beams by a fiber type optical coupler 2. As the light source 1, for example, a high brightness light emitting diode (SLDSsuperlumi-nes) is used.
cent diode) is used. One of the branched light beams is emitted from one output end of the fiber-type optical coupler 2, and is turned into a parallel beam by the objective lens 3. This parallel beam is reflected by a movable total reflection mirror 4 provided on a movable table 5, and then enters the fiber optical coupler 2 via the objective lens 3 again. This light beam is used as a reference light.

The other branched beam enters the optical waveguide 6 to be measured. This light flux generates backscattered light within the optical waveguide 6 to be measured. The backscattered light generated at each point of the optical waveguide 6 to be measured enters the fiber optical coupler 2 again and is combined with the above-mentioned reference light. This combined light is detected by a photodetector 7. The signal processing unit 8 determines the intensity of backscattered light generated within the optical waveguide 6 to be measured based on the output of the photodetector 7.

When the movable total reflection mirror 4 is moved, the optical path length of the reference light changes, so the delay time for the backscattered light changes. Using this, each point in the optical waveguide 6 to be measured can be made to correspond to the position of the movable total reflection mirror 4, and the intensity of the backscattered light generated at each point can be determined.

Assuming that the amount of change in delay time due to the movement of movable total reflection mirror 4 is r, and the position in optical waveguide 6 to be measured corresponding to this amount of change in delay time is 2, the output of photodetector 7 is I(r) =c (1+r(2) cos(2πνo
t))-...(1) It is expressed as follows. Here, C is a constant proportional to the light intensity, r'
(z) is a dimensionless value proportional to the optical electric field of the backscattered light generated at point 2 in the optical waveguide 6 to be measured (r(Z)'' represents the relative light intensity), and ν is the movable total reflection mirror 4 This is the beat frequency that occurs as the .

Therefore, by measuring the amplitude value of the AC component included in the output of the photodetector 7, the value of r' (z) can be determined, and the intensity distribution of the backscattered light generated in the optical waveguide 6 to be measured can be determined. Can be done.

[Problem that the invention seeks to solve]

However, the value of r (z) is usually about 10 -6, which is very small compared to 1, and has the disadvantage that it is hidden by noise caused by changes in the light intensity of the light emitted from the light source. To explain this in detail, when the beat signal of equation (1) is measured with a bandwidth δν, the detected signal is δI=c(r'(z)+ηδν) ・ −・−(
2). Here, η is the relative noise amount per unit bandwidth and unit optical power, and is an amount that characterizes optical intensity noise.

For this reason, conventional devices have the disadvantage of not being able to measure backscattered light with a high signal-to-noise ratio.

The present invention solves the above problems, improves the deterioration of the signal-to-noise ratio due to intensity noise due to light intensity fluctuations of the light source, and backscattered light measurement that enables highly accurate measurement of backscattered light generated in a leading waveguide. The purpose is to provide equipment.

[Means for solving problems]

In the backscattered light measurement device of the present invention, the backscattered light and the reference light are multiplexed in a plurality of directions, a photodetector is provided in each of two of the plurality of directions, and the signal processing means is configured to combine the two directions. The present invention is characterized by comprising processing means for mutually canceling in-phase components included in the output of the photodetector.

[For production]

By utilizing the fact that the combined light is emitted in a plurality of directions when the backscattered light and the reference light are combined, the emitted light in at least two directions is detected. By canceling the in-phase component of this detection output, the influence of intensity noise is removed and the signal-to-noise ratio is improved.

〔Example〕

FIG. 1 is a block diagram of a backscattered light measuring device according to a first embodiment of the present invention.

This device includes a light source 1, an input means for inputting light emitted from the light source 1 into an optical waveguide to be measured 6, an optical branching means for branching a reference light from the output light, and an input end of the optical waveguide to be measured 6 for introducing the reference light. A fiber-type optical coupler 2 is provided as an optical multiplexing means for combining the backscattered light appearing in the backscattered light. A photodetector 7 that includes a movable total reflection mirror 4 and detects the output light of the fiber type optical coupler 2.
．． 10, and includes a differential amplifier 11 and a signal processing unit 8 as signal processing means for determining the intensity distribution of backscattered light in the longitudinal direction of the optical waveguide 6 to be measured from the output of the photodetector 7.10.

The fiber type optical coupler 2 has a two-power, two-output structure, and emits the light incident on the first tail T+ from the light source 1 from the third tail T3 and the fourth tail T.
The light incident on the third tail T3 and the light incident on the fourth tail T4 are combined and emitted from the first tail T and second tail T2.

Here, the feature of this embodiment is that the photodetector 7.1
0 is provided for each of the two tails T, , T2, and a differential amplifier 11 is provided as a processing means for mutually canceling the in-phase components contained in the outputs of the two photodetectors 7.10.

The photodetector 7 is connected to the first tail T1 of the fiber optic coupler 2.
The photodetector 10 is arranged at the output end of the combined light of the fiber type optical coupler 9.

A fiber type optical coupler 9 is provided in the second tail T2, which couples the light emitted from the light source 1 to the fiber type optical coupler 2, and outputs the combined light from the fiber type optical coupler 2 to another tail. do.

The output of the photodetector 7.10 is supplied to the signal processing unit 8 via the differential amplifier 11.

The detection outputs of photodetector 7.10 are respectively)c,=(
, (1+r'(Z)cos(2yrνot))Icz=
C2(1-T'(z) cos(2rr va't>
). Therefore, the center frequency ν. , a signal passing through a filter with bandwidth δν is δJc, = C+ (r'(z) cos(2πνat
)+K(t)δν) δIe2=C2(-r'cos(2π°ν.t)+
K(t)δν). Here, K (t) is a coefficient due to intensity noise, and its double mean square root is equal to η.

Here, since the combined light incident on the photodetector 10 is split into three directions by the fiber type optical coupler 9, C2=1/2CI. Therefore, by subtracting twice the output δIe2 of the photodetector 10 from the output δIcl of the photodetector 7 using the differential amplifier 11, δL+ 2δIc2=2 c r r'(z) c
os(2rr vo t ), and the intensity noise term K
(t) δν is eliminated.

In this way, the value of r' (z), which indicates the relative electric field amplitude of the backward Rayleigh scattered light, can be measured with a high signal-to-noise ratio.

In experiments, the signal-to-noise ratio was improved by two orders of magnitude compared to the conventional device.

FIG. 2 shows a block diagram of a backscattered light measuring device according to a second embodiment of the present invention.

This embodiment device includes a light source 1, an input means for inputting light emitted from the light source 1 into an optical waveguide to be measured 6, an optical branching means for branching a reference light from the output light, and an optical branching means for inputting the reference light into an optical waveguide to be measured 60. A fiber-type optical coupler 12 with three power outputs is provided as an optical multiplexing means for multiplexing the backscattered light appearing at the end,
An objective lens 3 and a movable total reflection mirror 4 provided on a movable table 5 are provided as optical path length difference control means for changing the optical path length difference between the backscattered light and the reference light, and the output light of the fiber type optical coupler 12 is detected. A differential amplifier 11 and a signal processing unit 8 are provided as signal processing means for determining the intensity of backscattered light at each point of the optical waveguide 6 to be measured from the output of the photodetector 7.10. Be prepared.

The fiber type optical coupler 12 has a tail T
. The light enters the tails T3, T4 and T,
emitted to. Further, the reference light that has entered the tail T3 and the backscattered light of the optical waveguide 6 to be measured that has entered the tail T5 are combined and output to the tail T and the tail T2.

Here, the feature of this embodiment is that the photodetector 7.1
0 are provided at the output ends of the tails T+ and T2, respectively,
The signal processing means includes a differential amplifier 11 as a processing means for canceling out common-mode components contained in the outputs of the two photodetectors 7.10.

The differential amplifier 11 determines the difference between the outputs of the two photodetectors 7.10, and the signal processing unit 8 processes the value.

According to this embodiment, the signal-to-noise ratio is 2 as in the first embodiment.
Improved by orders of magnitude. Furthermore, the optical system is simpler than that of the first embodiment.

FIG. 3 shows a block diagram of a backscattered light measuring device according to a third embodiment of the present invention.

This embodiment device includes a light source 1, and a fiber-type optical coupler 2 and a fixed optical fiber as an input means for inputting light emitted from the light source 1 into an optical waveguide 6 to be measured and as an optical branching means for branching a reference light from the emitted light. A reflector 4' is provided, and a beam splitter 13 is provided as an optical combining means for combining the reference light with the backscattered light appearing at the input end of the optical waveguide 6 to be measured, and the optical path length difference between the backscattered light and the reference light is changed. A fixed prism 15 and a movable prism 14 provided on a movable stage 16 are provided as optical path length difference control means, and a photodetector 7.10 for detecting the output light of the fiber type optical coupler 12 is provided. 7
．． A differential amplifier 1 is used as a signal processing means for determining the intensity of backscattered light at each point of the optical waveguide 6 to be measured from the output of the differential amplifier 1.
1 and a signal processing section 8.

The fiber type optical coupler 2 branches the emitted light from the light source 1 and makes it incident on the fixed total reflection mirror 4' and the optical waveguide 6 to be measured. The reference light reflected by the fixed total reflection mirror 4' and the backscattered light generated in the optical waveguide 6 to be measured are combined by the fiber type optical coupler 2.

Beam splitter 13, movable prism 14, and fixed prism 15 constitute a bulk Michelson interferometer.

The light emitted from the fiber-type optical coupler 2 is converted into a parallel beam by an objective lens, and enters a beam splitter 13 . This incident light is split into two by a beam splitter 13, folded back by a movable prism 14 and a fixed prism 15, and then combined again. The combined light is emitted in two directions.

Here, the feature of this embodiment is that the photodetector 7.1
0 is provided for each of the two emission directions of the beam splitter 13, and the signal processing means is provided with a processing means for mutually canceling the in-phase components contained in the outputs of the two photodetectors 7.10.

The backscattered light of the optical waveguide 6 to be measured and the reference light are once combined by the fiber type optical coupler 2. When this combined light is split by the beam splitter 13, both components contain backscattered light components. However, since its intensity is sufficiently lower than that of the reference light, the light branched toward the movable prism 14 can be used as the reference light. At this time, the output of the photodetector 7.10 is equivalent to the case where C,=02 in equation (3). Therefore, by taking the difference between the two outputs using the differential amplifier 11, intensity noise can be reduced.

In this embodiment, a fiber type phase modulator 17 is further provided at the output end of the fiber type optical coupler 2, thereby phase modulating the reference light. The fiber phase modulator 17 has a structure in which an optical fiber is wound around a PZT cylindrical electrostrictive vibrator.

By applying an alternating current voltage to this fiber type phase modulator 17, the reference light traveling within the fiber is phase modulated.

At this time, only the driving frequency component of the fiber phase modulator 17 is detected among the interference intensity signals generated as the movable prism 14 moves. Thereby, as in the first and second embodiments, only the interference component between the reference light and the backscattered light generated at each point in the optical waveguide 6 to be measured can be extracted.

In the experiment, the signal-to-noise ratio was improved by two orders of magnitude as in the first and second embodiments. Furthermore, in the first embodiment and the second embodiment, there is a loss in optical intensity due to the use of the Phino rectangular optical coupler, and the DC components of the two outputs may not necessarily have the same value. On the other hand, in the case of this embodiment, since two outputs naturally obtained from the Michelson interferometer are used, there is no extra loss and there is an advantage that the measurement system is simple.

〔Effect of the invention〕

As explained above, the backscattered light measuring device of the present invention includes:
The light intensity noise of light emitted from a light source, which is a main noise source when measuring the light intensity of backscattered light generated within an optical waveguide, can be reduced by more than two orders of magnitude compared to the conventional method.

Therefore, there is an effect that the intensity distribution of directionally scattered light can be measured with a high signal-to-noise ratio.

The backscattered light measuring device of the present invention can be used to search for discontinuous points such as failure points, scattering points, etc. in optical integrated circuits and other leading waveguides, and has the effect of being able to search for their positions very precisely.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a backscattered light measuring device according to a first embodiment of the present invention. FIG. 2 is a block diagram of a backscattered light measuring device according to a second embodiment of the present invention. FIG. 3 is a block diagram of a backscattered light measuring device according to a third embodiment of the present invention. FIG. 4 is a block diagram of a conventional backscattered light measuring device. 1... Light source, 2.9.12... Fiber type optical coupler, 3... Objective lens, 4... Movable total reflection mirror, 4'.
... Fixed total reflection mirror, 5.16... Moving table, 6... Optical waveguide to be measured, 7.10... Photodetector, 8... Signal processing unit, 11... Differential amplifier , 13... Beam splitter, 14... Movable prism, 15... Fixed prism, 17... Fiber type phase modulator. If: J2 Figure No. 5 --- 3 Figure 5] Komanju poop shoulder 1

Claims (1)

[Scope of Claims] 1. A light source, an input means for inputting light emitted from the light source into the optical waveguide to be measured, an optical branching means for branching a reference light from the emitted light, and a light branching means for branching the reference light from the output light to the optical waveguide to be measured. Optical multiplexing means for multiplexing backscattered light appearing at the input end of a wavepath; Optical path length difference control means for changing the optical path length difference between the backscattered light and the reference light; and Detecting the output light of the optical multiplexing means. A backscattered light measuring device comprising: a photodetector for detecting a signal; and a signal processing means for determining the intensity distribution of backscattered light in the longitudinal direction of the optical waveguide to be measured from the output of the photodetector, wherein the plurality of optical multiplexing means The structure is such that the combined light is emitted in a direction, the photodetectors are provided in two of the plurality of directions, and the signal processing means converts the in-phase components included in the outputs of the two photodetectors to each other. A backscattered light measuring device comprising a canceling processing means.