JPH05118954A - Device for measuring reflection in optical frequency area - Google Patents

Abstract

PURPOSE:To reduce the fluctuation of a signal value to be caused by fading, and measure the signal strength accurately by overlapping the temporal modulation waveform with the single sawtooth frequency sweep waveform, and shifting the frequency obtained by the measurement of multiple times, and sampling the data to perform the addition meaning processing. CONSTITUTION:The laser beam 1 from a light source system 1 is divided-into the measuring light MS and the reference light RF by a first directional coupler 2. The light MS enters an object 7 to be measured, and the light RF enters a heterodyne receiver 4. The reflected light RL at any position of the object 7 is divided by a second directional coupler 3 to enter the receiver 4. The receiver 4 performs the coherent wave detection of the reflected light with the light RF as the local light. The output light of the receiver 4 is sampled by a spectrum analyzer 5, and the output thereof is measured multiple times by a signal processing system 6, and the addition meaning processing is performed. Fluctuation of a measured signal value to be caused by fading is thereby reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency domain reflectometer used for measuring loss distribution of optical fibers and optical fiber parts and searching for fault points.

[0002]

2. Description of the Related Art Optical frequency domain reflectometry is one of distributed sensing techniques having a high distance resolution in which position information is reflected in a carrier frequency of light and a physical quantity is reflected in its reflection intensity.

Specifically, in an optical frequency domain reflectometer, as shown in FIG. 2, a narrow linewidth laser whose frequency is swept linearly with respect to time at a constant time T1 and a constant frequency range F1. Light is incident on the object to be measured, and the light reflected or backscattered in the object to be measured is measured by heterodyne detection using reference light branched in advance from the same light source as local light.

As described above, since the frequency of the laser light linearly changes with time, the beat frequency of the measurement light and the reference light has a constant value proportional to the optical path length difference between the two lights. . Therefore, the reflection position within the object to be measured can be known from the frequency difference. Further, since the reference light intensity is constant, the magnitude of the measured physical quantity can be known from the beat signal intensity that is proportional to the product of the local light and the measured light intensity.

In such a conventional optical frequency domain reflectometry, a high distance resolution can be realized, but since coherent light having extremely excellent coherence is used, a plurality of wavelength distributions distributed within the range of the distance resolution are obtained. As a result of the reflected lights from the individual scatterers interfering with each other according to their phase relationship, a so-called fading phenomenon occurs in which a speckle pattern appears in the measurement signal waveform.

That is, the measurement signal waveform reflects the temporally fixed randomness of the phase relationship between the reflected lights by the plurality of scatterers randomly distributed in each resolution unit, and spatially. It will shake violently. The fluctuation pattern of the measurement signal waveform due to such fading changes by changing the wavelength (frequency) of the incident light. This is because the fluctuation pattern changes in proportion to the frequency of light. Therefore, while varying the frequency of the light incident on the object to be measured over a wide range, the measurement is performed a number of times, and the values are added and averaged to effectively reduce fluctuations in the measurement signal due to fading. (This will be referred to as frequency shift arithmetic processing).

The effect of fading reduction due to the frequency shift addition and averaging processing is large when the frequency variable range is large and the measured data is sampled uniformly and finely over a wide frequency variable range by a spectrum analyzer. It will be noticeable.

[0008]

However, in the conventional optical frequency domain reflectometry device, no active device for reducing the fluctuation of the measurement signal waveform due to such fading is made, and therefore the measurement is performed. It is difficult to obtain high measurement accuracy with respect to the physical quantity, and there are the following problems.

1) In the conventional optical frequency domain reflectometer, it is important to maintain the linearity with respect to the time of the frequency sweep of the laser in order to make the beat frequency correspond to the distance as described above. However, it is difficult due to the characteristics of the light source to sweep the frequency while maintaining this linearity over an extremely wide frequency range. Therefore, even though the light source has a wide frequency variable range, the wide variable range cannot be used for fading reduction.

2) When performing a large number of measurements for the averaging process, the frequency value at the sampling start point for each measurement is not controlled, and the averaging process effectively reduces fluctuations. Not necessarily done.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical frequency domain reflectometer capable of reducing fluctuation of a signal value due to fading and measuring the signal strength with high accuracy. Especially.

[0012]

In order to achieve the above object, according to the present invention, a laser light source whose frequency can be arbitrarily controlled with respect to time and a variable frequency whose lower limit is a constant reference frequency F0 in a cycle T1 are used. The frequency of the laser light source is controlled to have a sawtooth wave shape in the range F1, and the reference frequency F
A light source system having a frequency control unit that changes 0 for each repetition or arbitrarily changes the phase of the sawtooth waveform; and a first light that splits the light emitted from the laser light source into measurement light and reference light. Second branching means for branching the measurement light incident on the measured object and the reflected light reflected in the measured object
Optical branching means, a heterodyne receiver that coherently detects the reflected light by using the reference light as local light, a spectrum analyzer that receives the output of the heterodyne receiver, and an output of the spectrum analyzer over a plurality of measurements. And a signal processing system for performing averaging processing.

[0013]

According to the present invention, the oscillation frequency of the laser light source in the light source system is controlled by the frequency control section within a constant frequency variable range F1 with a constant basic sweep period T1 and a constant reference frequency F0 as the lower limits. In addition to repeating the sawtooth sweep with respect to time, the reference frequency F0 is changed at each repetition, or the phase of the sawtooth waveform is arbitrarily changed, so that the frequency is changed over a plurality of measurements. The shifted laser light is emitted.

The laser light emitted from the light source system is split into the measurement light and the reference light by the first light splitting means, the measurement light is incident on the object to be measured, and the reference light is incident on the heterodyne receiver. To be done. The reflected light at an arbitrary position of the measurement object is branched by the second light branching means into an optical path different from that of the incident measurement light and is incident on the heterodyne receiver.

In the heterodyne receiver, the reflected light is coherently detected by using the reference light as local light. The output light of the heterodyne receiver is sampled by the spectrum analyzer, and its output is added and averaged by the signal processing system over a plurality of measurements.

[0016]

1 is a block diagram showing an embodiment of an optical frequency domain reflectometer according to the present invention. In FIG. 1, 1
Is a light source system and includes a narrow linewidth laser light source whose frequency can be arbitrarily controlled with respect to time, and a frequency control unit which controls the frequency of the narrow linewidth laser light source.

FIG. 3 shows a specific configuration example of the light source system 1. FIG. 3A shows a configuration example in which the injection current control frequency variable semiconductor laser 11a is used as the narrow line width laser light source and the injection current control unit 12a is used as the frequency control unit. Further, (b) of the figure uses a more general frequency variable laser 11b such as a solid-state laser, a gas laser, or a dye laser as a narrow line width laser light source, and a high frequency sweep for applying a high frequency sweep as a frequency control unit. LiNbO
An example of the configuration using an external frequency modulator 12b including a _three- phase modulator is shown.

Reference numeral 2 is a first directional coupler, which splits the light emitted from the laser light source into the measurement light MS and the reference light RF. Reference numeral 3 denotes a second directional coupler, which is the measurement light M incident on the measured object 7.
S and the reflected light RL reflected in the measurement object 7 are branched. A heterodyne receiver 4 coherently detects the reflected light RL using the reference light RF as local light. Reference numeral 5 denotes a spell tram analyzer which receives the output of the heterodyne receiver 4 as an input and samples the input signal. Reference numeral 6 is a signal processing system, which averages the output of the spectrum analyzer 5 over a plurality of measurements.

Next, a specific example of frequency control waveform setting of the frequency control section of the light source system 1 will be described with reference to FIGS.

First, the case of double frequency sweep will be described with reference to FIG. FIG. 4 is a diagram in which the frequency sweep waveforms given in the frequency sweep period T2 and the frequency sweep range F2 are further superimposed on the single sawtooth sweep waveform having the fundamental frequency sweep period T1 and the fundamental frequency sweep range F1. ..
Here, T2> T1 and F2> F1. At this time, the sweep rate in each sweep is given by F1 / T1 + F2 / T2 (1) (generally, F1 / T1> F2 / T2), and the frequency change range is F1, but the overall frequency The change range is F1 + F2. Here, in the conventional single sawtooth frequency sweep, it is generally difficult to sweep the frequency over F1 + F2 at a high sweep rate close to the value given by the equation (1) because of the characteristics of the light source. Be aware of the situation. Here, the frequency sweep at the sweep rate of F2 / T2 is performed for a time longer than T1 over a wide frequency variable range, that is, slowly.

In FIG. 4, S is data of a specific point,
It is a sampling time interval for sampling by the spectrum analyzer 5 and the signal processing system 6. In the spectrum analyzer 5, in order to sample 1-point data having a range of range resolution, a decomposition time given by the reciprocal of the reception bandwidth is required. At this time, N
Assuming that the measurement data of a point is taken and one measurement is completed, the sampling time interval S in which the measurement data of a specific point is taken in is uniquely determined by the reception bandwidth and the number N of measurement points. On the other hand, the fundamental frequency sweep time T1 can be arbitrarily set, but it is desirable to set it so that the frequency shift addition / averaging process is most effectively performed. As an example, when T1 = S ... (2) is set, the frequency of the light at the sampling start time of the reflected light RL from the measurement point is Fstep = (F2 / T2) S. ) Each step changes. In the figure, the intersection of the frequency sweep waveform indicated by a square “□” and the broken line is the frequency at the start of each sampling. The number of times of measurement M = T2 / S (4) In addition averaging of the measurement signal values, the high-frequency sweep rate F1 / T1 + F2 / T2 to F1 is achieved without the measurement values at the same frequency being sampled multiple times. With / T1 maintained, it is most efficiently performed over a wide frequency range F2, and fluctuations due to fading of the measured signal value are effectively reduced.

When the injection current control frequency variable semiconductor laser 11a is used as shown in FIG. 3A, the injection current may be a superposition of two sawtooth sweep currents having different periods and different amounts of change. Alternatively, a step function having a constant step and a time interval T1 may be superposed on a single sawtooth sweep waveform for each basic sweep. In particular, in a frequency variable semiconductor laser represented by a DFB laser, it is difficult to continuously sweep while maintaining linearity at a high sweep rate over a wide frequency variable range, and it is discontinuous in several stages. The frequency is controlled under different conditions. However, using the present invention, the above-mentioned drawbacks of the semiconductor laser do not hinder the purpose of taking a wide frequency variable range.

Next, as shown in FIG. 3B, a case where a general frequency variable laser 11b and an external frequency modulator 12b are used will be described. In this case, a solid-state laser, a gas laser, a dye laser, and the like have a wide frequency variable range, but on the other hand, it is difficult to obtain a high sweep rate because frequency control is performed by temperature change. On the other hand, a large value is not required for the F2 / T2 value in the double frequency sweep. Therefore, the sweeping of F2 / T2 may be performed by direct frequency control of the light source, and the sweeping of the fundamental frequency of F1 / T1 may be performed using the external frequency modulator 12b. Specifically, the phase modulator is controlled so that the phase change amount becomes a quadratic function of time.

FIG. 5 shows "the signal strength variance value (<σI>)-the signal strength average value (<I>)" when the frequency shift addition and averaging process is performed according to the double frequency sweep method of FIG.
The calculated value of the measurement frequency dependence of the ratio (fluctuation amplitude / measurement signal value) ”is shown with the frequency variable range F2 and the resolution z as parameters. In addition, FIG. 6 shows the distance resolution of FIG.
The correspondence table of the frequency variable range and each numerical graph is shown. (A) to (e) in FIG. 5 correspond to the case where the parameter values are selected according to the table of FIG.

Next, an example of setting a frequency control waveform different from the above double frequency sweep will be described with reference to FIG.

FIG. 7 shows a random step waveform that is flat during the period T1 with respect to a single sawtooth frequency sweep waveform (shown by a solid line) having a fundamental frequency sweep period T1 and a fundamental frequency sweep range F1. An example of waveform setting for overlapping (indicated by a broken line) is shown.

In this case, the step difference can be arbitrarily set within the range allowed by the characteristics of the light source, and the frequency can be changed within the range of F2. In this setting example, since the frequency at each measurement time point is random, the efficiency of the frequency shift addition / averaging process does not depend on the fundamental frequency sweep period T1.

Next, a setting example of the frequency control waveform different from those in FIGS. 4 and 7 described above will be described with reference to FIG.

FIG. 8 shows that the interval t (generally t <T1) between the individual unit sweep waveforms having the fundamental frequency sweep period T1 and the fundamental frequency sweep range F1 is changed irregularly or with a constant variation amount. An example of waveform setting in which the waveform is changed stepwise is shown. In this case, the frequency at each sampling time randomly or regularly changes uniformly in the fundamental frequency sweep range F1, and as a result, the averaging process is efficiently performed.

[0030]

As described above, according to the present invention,
A single sawtooth frequency sweep waveform used in conventional optical frequency domain reflectometry is overlaid with an intentionally set temporal modulation waveform to shift the frequency over multiple measurements while By performing sampling and averaging processing, fluctuations in the measured signal value due to fading can be reduced. In this way, it is possible to achieve high measurement accuracy not only for the distance resolution but also for the measured physical quantity.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical frequency domain reflectometry device according to the present invention.

FIG. 2 is a diagram showing a conventional frequency sweep waveform.

FIG. 3 is a diagram showing a configuration example of a light source system according to the present invention.

FIG. 4 is a diagram showing a setting example of a frequency control waveform according to the present invention (double frequency sweep).

FIG. 5 shows “fluctuation amplitude / measured signal value (signal strength variance value /
The figure which shows the theoretical value of the measurement frequency dependence of "signal strength average value)"

FIG. 6 is a diagram showing a correspondence table between FIG. 5 and each setting parameter.

FIG. 7 is a diagram showing another setting example of the frequency control waveform according to the present invention (superposition of saw-tooth sweep and random step waveform).

FIG. 8 is a diagram showing another example of setting the frequency control waveform according to the present invention (control of the sawtooth sweep interval).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source system, 11a ... Injection current control frequency variable semiconductor laser, 11b ... Frequency variable laser, 12a ... Injection current control part, 12b ... External frequency modulator, 2 ... 1st directional coupler, 3 ... 2nd Directional coupler, 4 ... Heterodyne receiver, 5 ... Spectrum analyzer, 6 ... Addition / averaging signal processing system.

Claims (1)

[Claims] 1. A laser light source whose frequency can be arbitrarily controlled with respect to time, and a frequency of the laser light source is controlled in a sawtooth wave shape of a frequency variable range F1 with a fixed reference frequency F0 having a lower limit in a period T1. Together with the reference frequency F0
A light source system having a frequency control unit that changes the frequency of the laser light source every repetition or arbitrarily changes the phase of the sawtooth waveform, and a first optical branch that splits the light emitted from the laser light source into measurement light and reference light. Means, second light splitting means for splitting the measurement light incident on the measured object and the reflected light reflected in the measured object, and a heterodyne receiver for coherently detecting the reflected light with the reference light as local light. An optical frequency domain reflection measurement, comprising: a spectrum analyzer that receives the output of the heterodyne receiver; and a signal processing system that performs an averaging process on the output of the spectrum analyzer over a plurality of measurements. apparatus.