JPH01145545A - Testing method of reflection with high resolution and apparatus therefor - Google Patents

Abstract

PURPOSE:To realize a testing method of reflection with high distance resolution, by a method wherein a difference between the delay times of a reflected light signal from an optical fiber and a reference light from their emission to reception is adjusted to be zero or a value close thereto. CONSTITUTION:An optical wave divider 2 divides an emission light from a light source 1 into a probing light and a reference light. A light frequency shifter 3 shifts the frequency of the probing light by DELTAf from a frequency (f) of the light source 1, and the probing light is made to enter an optical fiber 5 through an optical directional coupler 4. In this constitution, an optical delay circuit 9 is adjusted so that a beat electric signal by a composite wave formed of the probing light reflected or back- scattered at some point in the fiber 5 and of the reference light be the maximum, and then the optical delay circuit 9 is adjusted so that a belt due to reflection or the like at another point in the fiber 5 be the maximum. When a difference between a delay time for one point and a delay time for another point is denoted by DELTAT and a light velocity in the fiber 5 by V, on the occasion, a distance between one point and another point can be determined by V.DELTAT/2, and a high distance resolution is obtained even by using a light source of which a coherence time is short.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a reflection test method and apparatus for detecting reflected light or backscattered light from a test object by a heterodyne method and measuring the characteristics of the test object. It is something.

[Prior Art] The object to be tested in the present invention is not particularly limited as long as it transmits and reflects the light from the exploration light source. However, in the following explanation, the optical fiber that has the widest range of applications will be explained.

By measuring Fresnel reflection and backward Rayleigh scattered light from optical fibers, it can be used as a reflection test device to measure optical loss distribution of optical fibers and search for fault points in optical fibers.
TDR (e.g., M., Barnoski.

et al,, ”0ptical time dom
ain reflectometer”.

Appl, Opt, Vol. 16 (1977),
pp. 2375-2379) is well known.

However, Fresnel reflection and backward Rayleigh scattered light are very weak and therefore extremely difficult to detect. Furthermore, in order to improve the resolution of the 0TDH distance, it is necessary to narrow the width of the optical pulse input to the optical fiber, but in this case, the level of the backward Rayleigh scattered light decreases in proportion to the optical pulse width. Further, since the reception band needs to be widened in proportion to the reciprocal of the optical pulse width, noise increases and the S/N ratio of the backscattered Rayleigh optical signal deteriorates significantly. Heterodyne 0, which uses a heterodyne detection method, is a method to improve the S/N ratio by lowering the minimum detectable optical power level.
TDR has been reported.

■Problems to be Solved by the Invention] However, in order to detect beat electrical signals by heterodyne detection, a single longitudinal mode oscillation light source (such as a DFB laser,
It is very difficult to obtain an optical pulse with a narrow optical pulse width from such a light source. This is because if an optical pulse is obtained by directly modulating a semiconductor laser or the like, the oscillation wavelength will change due to chirping. Also, LiNbO3
This is because when using a high-speed external modulator such as, the insertion loss is extremely large. As explained above, it is very difficult to increase distance resolution in conventional reflection testing methods and devices.

An object of the present invention is to provide a reflection testing method with high distance resolution and an apparatus that can easily implement the method.

[Means for Solving the Problems] In order to achieve such an object, the method of the present invention divides the emitted light from the light source into an exploration light and a reference light having an optical frequency difference of Δf. The probe light is input from the test object, and the reference light is combined with the reference light that is generated as reflected light or backscattered light of the probe light in the test object and is received by a photodetector.The beat of frequency Δf generated by the light reception is In a test method that determines the intensity of light in the opposite direction from the test object or the location of reflection and scattering by analyzing electrical signals, the probe light is emitted from the light source and then reflected inside the test object. The difference between the delay time from when the reference light is emitted from the light source until it is received by the photodetector after being scattered or backwards is zero or close to zero. An optical delay circuit that can be adjusted to a value is provided so that the beat electrical signal generated by combining the probe light, reflected or backscattered light, and the reference light at a certain point in the test object is maximized. Adjust the optical delay circuit, and then adjust the optical delay circuit so that the beat electrical signal generated by combining the reference beam with the light that has been reflected or backscattered at other points in the test object is maximized. When adjusting the distance between one point and another point, the difference between the delay time of the delay circuit for one point and the delay time of the delay circuit for another point is ΔT, and the speed of light in the object under test is V. , ■·ΔT/2.

The apparatus of the present invention includes a light source, a branching means for branching the emitted light from the light source into an exploration light and a reference light, a means for providing a frequency difference of Δf between the optical frequencies of the exploration light and the reference light, and a means for providing a frequency difference of Δf between the optical frequencies of the exploration light and the reference light. An optical directional coupler that combines and separates the reflected light signal, in which the probe light is reflected or scattered in the test object and travels backward, and the light signal that travels forward in the test object, and the reflected light signal and the reference light signal. A multiplexing means for multiplexing light, a light detection means for receiving the light multiplexed by the multiplexing means, a means for processing a beat electric signal of frequency Δf generated by the light reception, and a reflected signal and a reference light. Each of them is characterized by being equipped with an optical delay circuit for adjusting the delay time difference between the light emitted from the light source and the time when the light is received by the light detection means to zero or a value close to zero.

[Function] In order to improve the distance resolution, the conventional technology requires a wide reception band that is equal to or larger than the reciprocal of the optical pulse width incident on the optical fiber, and the reflection position of the reflected optical signal depends on the delay time of the reflected optical signal. It was determined by direct measurement in real time.

However, in detecting the reflected light signal from the optical fiber, the present invention separates the emitted light from the light source into an exploration light and a reference light (corresponding to the local light in heterodyne detection), which have different optical frequencies, and detects the reflected light signal from the optical fiber. By providing an optical delay circuit to adjust the delay time difference between the reflected light signal and the reference light from when they are emitted from the light source to when they are received by the photodetector to τ or a value close to it, the reception S/N ratio can be improved. Even if the reception bandwidth is narrowed in order to increase the frequency, it is possible to always receive the beat electrical signal generated by combining and receiving the reflected optical signal from the optical fiber and the reference light. Further, the reflection position of the reflected optical signal changes the delay time of the optical delay circuit so that the beat electric signal can be received.

It can be found from the value given.

[Embodiment 1] Hereinafter, embodiments of the wooden invention will be described in detail with reference to the drawings.

FIG. 1 shows an embodiment of the wooden invention. 1 is a light source, 2 is a light splitter that divides the emitted light from light source 1 into exploration light and reference light, 3
is a frequency shifter that changes the optical frequency of the exploration light emitted from the optical directional coupler 2 to an optical frequency shifted from the optical frequency f of the light source 1 by Δf. For the frequency shifter 3, an acousto-optical frequency modulator such as TaO2°PbMoO4 or tellurite glass can be used. Reference numeral 4 inputs the probe light into the optical fiber 5 to be measured, and generates a reflected optical signal of the Fresnel reflection or backward Rayleigh scattering light generated in the optical fiber 5 by the probe light and propagates backward through the optical fiber 5 to be measured, and the reflected light signal to be measured. An optical directional coupler separates the probe light traveling forward through the measurement optical fiber 5, a multiplexer 6 combines the reflected optical signal and the reference light, and 7 receives the combined light and converts it into an electrical signal. 8 is a signal processing device for processing a beat electric signal of frequency Δf generated by receiving the combined light by photodetector 7;
The delay time from when the reference light is reflected or scattered by the optical fiber 5 to be measured until it is received by the photodetector 7, and the delay time from when the reference light is emitted from the light source 1 until it is received by the photodetector 7. This is an optical delay circuit that can adjust the difference between τ to τ or a value close to it. The optical delay circuit 9 can be realized by various methods as shown in FIGS. 2 to 4. That is, FIG. 2 shows the prism 10, FIG. 3 shows the mirror 12 facing the half mirror 11, and FIG. 4 shows the optical fiber 13. The collimating lens 15 and optical fiber 16 in the parallel beam system consisting of lens 14 and lens 15° optical fiber 16 are moved from the dashed line position (2 = 0) to the solid line position (z = zo) in each figure, respectively, to change the optical path. This can be achieved by changing the length.

At that time, if the speed of light in vacuum is C, then the amount of change in delay time is 220 in Figures 2 and 3, respectively.
/C, and in FIG. 4 it is 2°/C.

Next, it will be explained that the reflected signal can be measured with high resolution according to the present invention.

FIG. 5 is a diagram showing the amplitude of a normalized light wave interference signal with respect to the delay time difference between two light waves emitted from the same light source, that is, the amplitude of a beat '2ft signal. Generally, even if light is emitted from the same light source, light that is separated by a certain delay time will no longer interfere. This time is called coherence time Tc. In the following description of the present invention, the half-width of the beat electric signal is assumed to be the coherence time, as shown in FIG.

Further, TCNλ2/cΔλ (λ is the center oscillation wavelength of the light source, Δλ is the oscillation wavelength width). Therefore, in Figure 1,
First, the optical delay circuit 9 is configured so that the reflected optical signal from a certain point (point A) in the optical fiber to be measured and the reference optical signal interfere with each other, that is, so that the beat electric signal of frequency Δf is maximized. , and then readjust the optical delay circuit 9 so that the reference optical signal interferes with the reflected optical signal from another point (point B) in the optical fiber under test.
The relationship between the amplitude of the beat electrical signal and the delay time as shown in the figure is obtained. At this time, if the delay times of the optical delay circuit when measuring points A and B are τA and τb, and the difference between them is ΔT (=A at τ8−), then the distance between points A and B is It can be determined from ΔT/2. here,
(2) is the speed of light in the optical fiber to be measured. The reason for dividing by 2 is that ΔT is the time required for light to travel back and forth between point A and point B. Now, when the above ΔT becomes 2Tc or less, as can be seen from FIG. 6, point A and point B become indistinguishable. Therefore, the distance resolution of the test method and test device of the present invention is V-Tc.

Next, we will explain with specific numerical values.

Now, the light source has an oscillation wavelength (λ) of 1.3 μm and an oscillation wavelength width (Δ
λ) Consider a 100 nm light emitting diode (LED).

At this time, since 7(yλ2/cΔλ = 56 fs, if the speed of light in the optical fiber is 2 x lo' + n/s, the distance resolution is about 10 μm. To achieve such high resolution using conventional technology, The exploration light has an optical pulse width of 0.
．． An optical pulse of 1 ps is required, and the reception bandwidth thereof requires a minimum optical pulse width of 107 Hz, which is practically impossible. The distance resolution when using a semiconductor laser (LD), which is expected to have higher output than the above LED, is L
Since the oscillation wavelength width of D is approximately Δλ=0.1 nm, it is determined to be 1 cm by a similar calculation. Even in this case, 10 GH2 is required as a reception bandwidth, and it is quite difficult to obtain the same performance with the conventional technology.

Although distance measurement and distance resolution have been described above, it goes without saying that the level of a reflected light signal can also be measured by the present invention. Since the signal measured in the present invention is a beat electric signal generated by receiving light that is a combination of reflected light No. 3 and reference light, based on the principle of heterodyne reception,
The amplitude of the beat electrical signal is proportional to the parallel plate product of the optical intensity of the reflected optical signal and the optical intensity of the reference beam. Therefore, the level of the reflected optical signal can be measured from the amplitude of the beat electrical signal. In addition, by increasing the light intensity of the reference light, Beat Electric 4
No. 8 is also increased, and an SN ratio close to the shot noise limit can be achieved. Furthermore, in the present invention, the bandwidth when receiving the beat electrical signal can be narrowed to a bandwidth determined by the stability of the optical frequency shifter and the receiving band filter. Even if the frequency Δf of the beat electric signal is 100 MHz, it is easy to receive it with a reception bandwidth of 1k112 or less.

On the other hand, in the conventional technology including the conventional heterodyne 0TDR, the bandwidth must be 10GH2 in order to obtain a distance resolution of 1 cm, which corresponds to the case where the above-mentioned LD is used. Therefore, in the present invention, by narrowing the receiving band, the S/N ratio can be improved by 70 dB or more compared to the conventional technology.

Although the modulation method of the light source used in the present invention has not been mentioned above, the distance resolution of the wood of the present invention is determined by the coherence time of the light source, and unlike the conventional 0TDn, the light source is not necessarily There is no need to pulse drive. In addition, although we have considered reflected light signals from discrete points such as point A and point B above, it is also possible to receive reflected light signals from continuously distributed reflection points, such as back Rayleigh scattered light from an optical fiber. . At this time, by slightly shifting the delay time of the optical delay circuit and measuring the amplitude of the beat electric signal, a backward Rayleigh scattered light signal from the optical fiber as shown by the black circles in FIG. 7 can be obtained. This waveform is exactly the same as that measured with a conventional heterodyne 0TDR.

FIG. 8 shows another embodiment of the invention. In the figure, 17 is an acousto-optic frequency modulator (ultrasonic light modulator). The acousto-optic frequency modulator 17 in this embodiment serves both as the optical splitter 2 and the optical frequency shifter 3 shown in FIG. That is, the light emitted from the light source 1 enters the acousto-optic frequency modulator 17, and its O-th order diffracted light (transmitted light) is guided to the optical delay circuit 9. Further, the first-order diffracted light whose optical frequency has been shifted by Δf is guided to the optical directional coupler 4. The optical power branching ratio of the 0th-order and 1st-order diffracted lights can be changed by adjusting the driving power of the acousto-optic frequency modulator 17. Other operations in this embodiment are similar to those in the embodiment shown in FIG. In this embodiment, the number of parts can be reduced compared to the embodiment shown in FIG. 1, and excessive loss in the optical splitter 2 or the optical frequency shifter 3 can be eliminated.

FIG. 9 shows another embodiment of the invention. In the figure, reference numeral 18 denotes an optical isolator that allows the reference light from the optical delay circuit 9 to pass through to the optical directional coupler 4, but does not allow the light from the optical directional coupler 4 traveling in the opposite direction to pass through. In this embodiment, the optical directional coupler 4 also serves as the optical multiplexer 6 in FIG. That is, the probe light enters the optical fiber 5 to be measured through the optical directional coupler 4, and the reflected optical signal reflected or scattered in the optical fiber 5 to be measured passes through the optical directional coupler 4 again to the photodetector 7. be guided. At the same time, the reflected light signal and the reference light are combined by the optical directional coupler 4. The purpose of using the optical isolator 18 is that when the exploration light enters the optical path through which the reference light propagates through the optical directional coupler 4, reflected light or backscattered light is generated from the optical components that make up the optical path. This is to prevent those reflected light signals from becoming noise to the reflected light signal from the optical fiber 5 to be measured. Since the transmission loss of the optical isolator 18 is almost negligible, this embodiment can reduce the insertion loss for the reflected optical signal by the amount of the optical directional coupler 6 compared to the embodiment shown in FIG.

[Effects of the Invention] As explained above, according to the present invention, the delay time from when the exploration light is emitted from the light source until it is reflected or scattered backwards in the test object and received by the photodetector is By providing an optical delay circuit that can adjust the difference in delay time from when the reference light is emitted from the same light source as the exploration light source until it is received by the photodetector to T or a value close to it,
Even when using a light source with a short coherence time, it is possible to obtain a beat electrical signal corresponding to the interference component between the probe light and the reference light. Since the measurement resolution of the reflection position is proportional to the coherence time of the light source, the present invention, which allows the use of a light source with a short coherence time, makes it possible to realize a reflection testing method and apparatus with high distance resolution. Furthermore, the present invention
Since heterodyne detection is adopted as a signal reception method, and narrow band reception is possible in the present invention, it is possible to receive signals with an excellent S/N ratio.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, FIGS. 2 to 4 are diagrams each showing the configuration of an optical delay circuit, and FIG. 5 is a diagram showing the temporal coherence of the light source. Figure 6 is a diagram showing the relationship between the amplitude and delay time of the beat electrical signal when there are reflected signals from two points (points A and B), and Figure 7 is when back Rayleigh scattered light from an optical fiber is measured. FIG. 8 is a block diagram showing the configuration of another embodiment of the present invention. FIG. 9 is a diagram showing the configuration of still another embodiment of the present invention. It is a block diagram. DESCRIPTION OF SYMBOLS 1... Optical beam, 2... Optical splitter, 3... Optical frequency shifter, 4... Optical directional coupler, 5... Optical fiber to be measured, 6... Optical multiplexer, 7... Photodetector, 8... Signal processing device, 9... Optical delay circuit, 17... Ultrasonic optical modulator, 18... Optical isolator.

Claims (1)

[Claims] 1) Separate the emitted light from the light source into exploration light and reference light having an optical frequency difference of Δf, enter the exploration light from one end of the test object, and The light in the opposite direction generated as reflected light or backscattered light of the probe light and the reference light are combined and received by a photodetector, and the beat electric signal of frequency Δf generated by the received light is analyzed, thereby detecting the In a method for conducting a test to determine the intensity of light in the opposite direction from an object or the position of reflection/scattering, the probe light is emitted from the light source and then reflected or scattered backward in the object under test. The difference between the delay time until the light is received by the photodetector and the delay time from when the reference light is emitted from the light source until it is received by the photodetector is set to zero or a value close to it. An adjustable optical delay circuit is provided so that the beat electric signal generated by combining the reference light and the light that has been reflected or backscattered at a certain point in the test object by the probe light is maximized. Then, the optical delay circuit is adjusted, and the beat electric signal generated by combining the reference light and the light reflected or backscattered at other points in the test object is maximized. When adjusting the optical delay circuit, the distance between the certain point and another point is determined by the difference between the delay time of the delay circuit for the certain point and the delay time of the delay circuit for the other point. ΔT
, a high-resolution reflection test method characterized in that the speed of light in the test object is determined from V·ΔT/2. 2) a light source; branching means for branching the light emitted from the light source into an exploration light and a reference light; means for providing a frequency difference of Δf between the optical frequencies of the exploration light and the reference light; an optical directional coupler that separates a reflected optical signal in which the probe light is reflected or scattered in the object under test and travels backward, and an optical signal that travels forward in the object under test; a combining means for combining the reflected light signal and the reference light; a light detecting means for receiving the light multiplexed by the combining means; and a means for processing a beat electric signal of frequency Δf generated by the received light. , comprising an optical delay circuit for adjusting a delay time difference between the reflected signal and the reference light, from when they are emitted from the light source until they are received by the photodetecting means, to zero or a value close to it. A high-resolution reflection testing device featuring: