JPH04248434A - Remote tester of optical line - Google Patents

Abstract

PURPOSE:To provide a wide measuring distance range and high distance resolving power in a remote tester of an optical line using a light frequency region reflecting method. CONSTITUTION:Reference light is preliminarily divided to pass through a newly inserted delay line optical fiber 4 for reference light having proper length to make the light path difference between the reference light and measuring light as short as possible and even the reflected light (measuring light) from a remote place is set so that the light path length difference with the reference light becomes sufficiently shorter than the coherence length of a light source and, by inserting a frequency shifter 3 in the light path of measuring light or reference light, the reflected lights from two different places same in the absolute value of the light path length difference can be separated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a remote testing device for optical lines that applies an optical frequency domain reflection method.

0002

[Prior Art] Optical frequency domain reflection method is a method in which position information is reflected in the carrier frequency of light, and physical quantities are reflected in its intensity.
It is a distributed sensing technology with high distance resolution.

Specifically, a narrow linewidth laser beam whose frequency is swept linearly with respect to time is incident on an object to be measured, and the light reflected within the object is demultiplexed in advance. Measurement is performed by heterodyne detection using the reference light as local light. At this time, the difference in optical path length between the measurement light and the reference light will uniquely correspond to the difference in frequency, so by measuring the beat frequency of the measurement light and the reference light, it is possible to You can know the location.

Conventionally, although the optical frequency domain reflection method has a high distance resolution, it generally cannot cover a wide measurement distance range, so it has been developed mainly as an evaluation technique for microscopic optical components.

On the other hand, when the optical frequency domain reflection method is used as a method for measuring the loss distribution of an optical fiber, a measurement system has been constructed as shown in FIG.

Here, 11 is a narrow linewidth laser light source, 12 is a beam splitter for separating incident light and reflected light, and 13 is a beam splitter for separating incident light and reflected light.
1 is an optical fiber to be measured, and 14 is a heterodyne receiver. The frequency of the narrow linewidth laser light from the narrow linewidth laser light source 11 is operated so as to vary linearly with time as shown in FIG. The light that returns from point B (optical fiber end) of the optical fiber to be measured 13 by Fresnel reflection and the light that returns from point A (
After combining the light returned by Rayleigh scattering from the middle of the optical fiber, heterodyne detection is performed.

At this time, there is a constant optical path length difference between the reflected light from point B and the reflected light from point A, and a constant time difference is always maintained. The frequencies of the two lights differ by an amount corresponding to this time difference, but the frequency difference is constant. In this way, the optical path length difference and the frequency difference can be made to correspond, and position information can be added to the frequency of the beat signal. This method uses, as the reference light, not light source light that has been demultiplexed in advance, but light that returns from one end face of an optical fiber by Fresnel reflection.

[0008]

[Problem to be Solved by the Invention] However, in principle, this conventional method cannot be used when the coherence distance of the laser light source is shorter than the distance of about twice the length of the optical fiber, so it cannot be used for measurement. There were limits to the possible fiber lengths. In addition, if the optical path length difference between the measurement beam and the reference beam is long and the time difference between these two beams becomes large, the influence of frequency fluctuation from the linear frequency change of the laser beam and the thermal frequency of the light source will increase. Frequency measurement errors increased due to fluctuations, and it was not possible to achieve the high accuracy expected from an ideal optical frequency domain reflection method.

In other words, the conventional method has the disadvantage that it is not possible to simultaneously satisfy a wide measurement distance range and high distance resolution.

SUMMARY OF THE INVENTION An object of the present invention is to provide a remote testing device for optical lines that has a wider measurement distance range and higher distance resolution than conventional methods.

[0011]

[Means for Solving the Problems] In order to achieve the above object, claim 1 provides a narrow linewidth laser light source capable of temporally linear frequency sweep and a measurement of the emitted light of the laser light source. a first directional coupler for splitting into light and reference light; a frequency shifter inserted to give a frequency shift to the measurement light or reference light; and measurement light incident on the optical line under test. and a second directional coupler for demultiplexing the reflected light reflected within the optical path; A delay line optical fiber having a length comparable to the optical path length until it is input to the heterodyne receiver and receiving the reference light as input, and a reference light and the reflected light that are output from the delay line optical fiber. Heterodyne receiver for multiplexing and heterodyne detection
A spectrum analyzer that receives the output of the heterodyne receiver as input, and a remote testing device for an optical line were constructed.

0012

[Operation] According to claim 1, the emitted light of the laser light source is
The directional coupler splits the measurement light into a reference light, and the frequency shifter applies a frequency shift to the measurement light. The reference light is input into the delay line optical fiber, and the reference light and the reflected light output from the delay line optical fiber are combined by a heterodyne receiver and subjected to heterodyne detection. input into the spectrum analyzer.

[0013]

Embodiment FIG. 1 is a block diagram showing an embodiment of the present invention.

1 is a narrow linewidth range capable of linear frequency sweep.
2 is a first directional coupler for splitting the emitted light from the laser light source into measurement light and reference light; 3 is a frequency shifter for imparting a frequency shift to the measurement light; . 4 is a delay line optical fiber for reference light, and its length is selected so that the difference in optical path length between the reference light and measurement light is shorter than the coherence length of the laser light source. (The delay line optical fiber itself is preferably variable in length by some method.) 5 is for separating the measurement light incident on the optical path to be measured and the reflected light reflected within the optical path; This is a second directional coupler. 6 is an optical fiber line to be measured. 7 is a heterodyne receiver for combining the reflected light and the reference light and performing heterodyne detection. 8 is a spectrum analyzer which receives the output of the heterodyne receiver as an input.

In this configuration, by selecting the length of the delay line optical fiber 4 for the reference light so that the difference in optical path length between the reference light and the measurement light is smaller than the coherence length of the light source, reflection from a distance can be prevented. Even light can be measured with good accuracy.

The reason for this will be described below. In the optical frequency domain reflection method, the phase correlation between the reflected light (measurement light) and the reference light, which have a specific time difference, is maintained, and the output of the heterodyne receiver 7, which receives both lights as input, always has a specific It is assumed that a beat signal corresponding to the frequency difference is generated, but as the time difference increases (that is, as the optical path length difference increases), this phase correlation will no longer be maintained and the beat signal will no longer be observed. . The optical path length difference that can maintain this phase correlation is called the coherence length. By inserting the delay line optical fiber 4 into the optical path of the reference light and making the optical path length difference shorter than the coherence length, the phase correlation can be maintained. It is possible to maintain the beat signal in a good condition at all times and improve measurement accuracy.

The optical frequency domain reflection method is based on the assumption that the frequency of narrow linewidth laser light is swept linearly with respect to time; Due to thermal frequency fluctuations of the laser light source, etc., a certain amount of variation occurs in the non-linearly corresponding frequency difference. Figure 4 shows this situation. The curve in the figure shows how the frequency f of the light source changes with respect to time t. T1 and T2 are time differences between the reference light and the measurement light, and T1=T2. However, the frequency change df1,
Since df2 is not linear with respect to time, the corresponding frequency difference is not uniquely determined and has variations (d
f1≠df2).

However, by shortening the optical path length difference, the time difference can be reduced and the degree of this variation can be reduced. This improves measurement accuracy.

The necessity of the frequency shifter 3 can be explained as follows. In this configuration, if twice the length of the optical fiber line to be measured 6 is longer than the length of the delay line optical fiber 4, the optical path length difference between the measurement light and the reference light can be either positive or negative. It can be. In such a case, reflected light from two different points with the same absolute value of optical path length difference will be measured with the same frequency difference (Figure 5 (
a)). Therefore, in order to avoid such a situation, it is necessary to apply a frequency shift fs to the measurement light to prevent frequency overlap. Specifically, the following two methods may be used.

When the optical path length difference Ls at a certain position is longer than the coherence length Ls of the light source, the frequency shifter 3 shifts the frequency so that the frequency difference corresponding to this position becomes exactly 0 (FIG. 5(b)). ). In this case, reflected light from outside the range of the coherence length Ls is not observed as a beat signal for the reason described above, so no frequency overlap occurs.

The frequency of the beat signal corresponding to the reflected light from the output end of the optical fiber line 6 to be measured becomes 0.
The frequency fs at the optical path length difference of 0 is shifted (Fig. 5
(c)). In this case, there is no reflected light corresponding to the broken line in the figure. Incidentally, there is also a method of similarly adjusting the frequency of the beat signal corresponding to the reflected light from the input end to zero. Of course, it is clear that the amount of frequency shift may be greater than fs.

As the frequency shifter 3 described above, an ultrasonic light modulation element or the like may be used. Further, in the above description, the frequency shift is applied to the measurement light, but depending on the case, the frequency shift may be applied to the measurement light.

[0023]

As described above, according to claim 1, by inserting a delay line optical fiber into the optical path of the reference light, the difference in optical path length between the reference light and the measurement light is shortened to maintain phase correlation. This improves the measurement accuracy, and by inserting a frequency shifter into the optical path of the measurement light or reference light, the correspondence between the reflection position and the frequency is guaranteed. In this way, the measurable range can be significantly expanded while maintaining measurement accuracy, and the optical frequency domain reflection method has excellent features not found in conventional methods. remote testing equipment can be realized.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a measuring device showing an embodiment of the present invention.

[Figure 2
] Principle diagram of optical frequency domain reflection method

[Figure 3] Diagram showing the frequency sweep of narrow linewidth laser light

[
Figure 4: Diagram showing the influence of linear frequency change

[Figure 5] Diagram showing how to use a frequency shifter

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... Narrow linewidth laser light source, 2... First directional coupler, 3...
Frequency shifter, 4... Delay line optical fiber for reference light, 5... Second directional coupler, 6... Optical fiber line to be measured, 7... Heterodyne receiver, 8... Spectrum analyzer.

Claims (1)

[Claims] 1. A narrow linewidth laser light source capable of temporally linear frequency sweep, and a first laser light source for splitting the emitted light of the laser light source into measurement light and reference light. A directional coupler, a frequency shifter inserted to give a frequency shift to the measurement light or reference light, and a frequency shifter that separates the measurement light incident on the optical path under test and the reflected light reflected within the optical path. a second directional coupler for the purpose of achieving a second directional coupler, and an optical path length that is approximately the same as the optical path length from when the emitted light of the laser light source exits the first directional coupler until it is input to the heterodyne receiver. a delay line optical fiber having a length of , and a spectrum analyzer whose input is the output of the heterodyne receiver.