JPH0331736A - Method and instrument for measuring curvature distribution of optical fiber - Google Patents

Abstract

PURPOSE:To measure the influence itself of the bend of an optical fiber without requiring reference data (initial data) by measuring a Brillouin gain spectrum and finding out the ratio of a gain peak related by a reference acoustic waveguide mode to a gain peak related by a high-order waveguide mode. CONSTITUTION:A pulse-like signal beam S1 emitted from a light source 1 is connected to one end of an optical fiber 3 to be measured through a multiplexer/ demultiplexer 4 and a beam S2 emitted from a light source 2 is connected to the other end of the optical fiber 3. A difference between optical frequency values of the two light sources 1, 2 is controlled by an optical frequency control device 7 to excite plural acoustic waveguide modes having different degrees. Since the generation efficiency or attenuation rate of the high-order acoustic waveguide mode strongly depends upon the curvature of the optical fiber 3, the ratio of Brillouin gain of the high-order acoustic waveguide mode on the specific position of the optical fiber to that of the reference acoustic waveguide mode is found out by a signal processor 6 and the curvature distribution of the optical fiber 3 is measured, so that reference data are not required.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for measuring the longitudinal curvature distribution of an optical fiber and an apparatus therefor. (Prior Art) Bending of an optical fiber not only causes loss, but also causes deterioration in strength due to bending strain, which shortens the rupture life. Conventionally, there has been no method to directly measure the curvature distribution in the longitudinal direction of an optical fiber from only the fiber end face, and it has been estimated indirectly by measuring changes over time in the loss distribution in the longitudinal direction of the optical fiber. That is, the 0TDR method (for example, M,
Barnaski et al., 0ptical N.
tae domain reflectometer
,^I)pl, Opt,,Vol,16,9P,23
75-2379.1977), it is possible to measure the loss distribution in the longitudinal direction of an optical fiber, so it is possible to compare the loss distribution before and after laying the optical fiber, or measure the seasonal fluctuations of the loss distribution, so that it is possible to check the installation process and the environment. The degree of bending in the longitudinal direction of the optical fiber caused by temperature changes, etc. was estimated. [Problem to be solved by the invention] However, since the above-mentioned conventional technology is based on comparative measurement, if there is no reference measurement value, even if the loss distribution at a certain time can be measured, the loss distribution at that time cannot be measured. There was a problem that the curvature distribution could not be estimated. In addition, in practice, bending is particularly problematic near the connection point, but both connection loss and bending loss due to processing the extra length of the connection part change over time, making it extremely difficult to branch out these areas. There was a problem. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and apparatus for measuring curvature distribution of an optical fiber, which does not require reference data (initial data) and is capable of measuring the effect of bending by separating it from other effects. It is in. [Means for Solving the Problems] In order to achieve such an object, the present invention measures the Brillouin gain spectrum of one section of the optical fiber to be measured or for each of a plurality of sections of the optical fiber. a first step; and a step of determining the ratio of the maximum gain involved in the fundamental acoustic mode to the maximum gain involved in the higher-order acoustic modes for each of the one or more Brillouin gain spectra measured in the first step. and a third step of calculating the curvature of the optical fiber for one section of interest or for each of a plurality of sections based on the ratio obtained in the second step. shall be. That is, in the first embodiment of the present invention, the method for measuring the Brillouin gain spectrum for each of a plurality of sections performed in the first step includes inputting pulsed light from one end of the optical fiber to be measured, and inputting the pulsed light from the other end of the optical fiber. By inputting continuous light and controlling the frequency difference Δν between pulsed light and continuous light, multiple fixed frequency differences Δ
For each ν, measure the time change in the amount of continuous light emitted from the input end of the pulsed light due to the Brillouin light amplification effect caused by the interaction with the pulsed light, and then It is characterized by obtaining a Brillouin gain spectrum by determining the amount of Brillouin optical amplification light as a function of the frequency difference Δν. A second aspect of the present invention is that the method for measuring the Brillouin gain spectrum for each of a plurality of sections performed in the first step involves injecting mutually synchronized pulsed light from both ends of the optical fiber to be measured, and The incident time of the two pulsed lights is delayed so that the two pulsed lights meet at , and the relative frequency difference Δν between the two pulsed lights is continuously or discretely changed. It is characterized by obtaining a Brillouin gain spectrum from the amount of light emitted from one optical fiber. A third aspect of the present invention provides a Brillouin gain spectrum measuring means for measuring a Brillouin gain spectrum of one section of the optical fiber under test or for each of a plurality of sections of the optical fiber, and a Brillouin gain spectrum measuring means. gain peak ratio calculation means for calculating the ratio of the maximum gain involved in the fundamental acoustic mode to the maximum gain involved in the higher-order acoustic mode for each of the one or more Brillouin gain spectra obtained by the calculation; The present invention is characterized by comprising curvature distribution calculation means for calculating the curvature of the optical fiber for one section or for each of a plurality of sections based on the calculated ratio value. In a fourth aspect of the present invention, the Brillouin gain spectrum measuring means includes a first light source that generates a first signal light in the form of pulse-modulated light, and a first light source that propagates oppositely to the first signal light in an optical fiber to be measured. a second light source that emits a continuous or pulse-modulated second signal light; an optical frequency control means for controlling the difference in optical frequency between the first signal light and the second signal light;
means for controlling the relative delay time of the signal light and the second signal light;
A multiplexing/demultiplexing means for inputting the signal light into the optical fiber to be measured and extracting the second signal light subjected to the Brillouin optical amplification effect generated between the first signal light and the second signal light, and the multiplexing/demultiplexing means A photodetection means for converting the second signal light subjected to the Brillouin optical amplification effect from the photodetection means into an electrical signal, and a signal processing for processing and analyzing the temporal waveform or amplitude and phase of the signal detected by the photodetection means. It is characterized by having a means. [Function] The present invention utilizes the stimulated Brillouin scattering phenomenon, which is a nonlinear interaction between sound waves and light waves within an optical fiber. That is, a plurality of acoustic waveguide modes with different orders are excited by opposingly injecting light with slightly different frequencies from both ends of the optical fiber to be measured. This acoustic waveguide mode interacts with the incident light wave and amplifies the low-frequency light.The present invention focuses on the fact that the generation efficiency or attenuation rate of higher-order acoustic waveguide modes strongly depends on the curvature of the optical fiber. Since the curvature distribution of the optical fiber is measured from the ratio of the Brillouin gain of the higher-order acoustic waveguide mode and the fundamental acoustic waveguide mode, basic data (initial data) is not required, and the effect of bending itself can be compared with other effects. Can be measured separately from In this way, the present invention is significantly different from conventional techniques in which curvature changes are estimated by comparative measurements of loss distributions over time. (Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a measuring device according to one embodiment of the present invention. Here, 1 is a first light source that generates pulsed light that is amplitude-modulated or frequency-modulated in a pulsed manner, and emits light with a narrow spectral linewidth. The first light source 1 pulse-drives an acousto-optic modulator, a field-effect optical modulator, or the like with light emitted from a single longitudinal mode oscillation laser such as a CW full-oscillation YAG laser or a CW full-oscillation DFB laser. This can be realized by adopting a configuration in which the amplitude is modulated in a pulsed manner. Alternatively, the Ss light source 1 can also be realized by applying internal modulation when injecting current to a single longitudinal mode oscillation laser such as a DFB laser. 2
is a CW fully oscillating second light source, which is a YAG laser. It is a light source with a narrow spectral linewidth, such as a DFB laser. 3 is an optical fiber to be measured, 4 is a multiplexer/demultiplexer, and 5 is a Ge
-＾po(＾valanche Photo-diod
6 is a signal processing device that receives the electrical signal from the photodetector 5 and processes and analyzes its temporal waveform or amplitude (No. 8); 7 is a first light source 1 and a second light source; This is an optical frequency control device for controlling the difference in optical frequency of the light source 2 to a desired value.The operation of this measuring device is described, for example, in IEICE Technical Report 0QE88-
80 (1988, 10, 25) or patent application 1988
Although it is detailed in No.-154828, it is as follows. The pulsed light emitted from the first* source 1 , that is, the first signal light S, is coupled to the optical fiber 3 to be measured via the multiplexer/demultiplexer 4 . On the other hand, the emitted light from the second light source 2, that is, the second signal light S, is transmitted from the terminal 3-2 of the optical fiber under test 3 opposite to the terminal 3-1 to which the first signal light S1 is coupled.
It is coupled to the optical fiber 3 to be measured. At this time, the optical frequency f of the first signal light 5 and the optical frequency f of the second signal light S are approximately fl-fl-f color (1)
It is controlled by the optical frequency control device 7 so that it becomes . Here, f6 is the Brillouin shift frequency specific to the optical fiber 3 to be measured. For example, in the case of a quartz-based optical fiber, the frequency is approximately 13 GHz at a wavelength of 1.3 μm.
As long as 1) is satisfied, the second signal light S is Brillouin optically amplified by the first signal light S1. At this time, the waveform of the second signal light S detected by the photodetector 5 is derived as follows. 'fS2 signal optical signal light emission 2 by Brillouin optical amplification,
The gain g at position 2 is G '' when g −1 < l, g = 1 +A−P+(z)
It is expressed as (2). Here, A is a proportional coefficient, and 2 represents the position where the first signal light S+ (pulse signal) propagating in the optical fiber 3 under test exists, and here, the terminal 3-1 of the optical fiber 3 under test is used as a reference. #P
l(Z) is the optical power of the first signal light S at position Z. Similarly, Pz(z) is the optical power of the second signal light S2 at position 2, and the optical loss and length of the optical fiber 3 to be measured are a [neper/m] and L
[ml] Furthermore, since the frequency difference between the signal lights S and 52 is small, assuming that the loss of the optical fiber 3 to be measured for both signal lights is the same, P + (z) - P I (0) exp (-az
) (3-1)P2 (Z) ・h
(L) exp [-a (L-z)]
(3-2). Equation (2), Equation (3) and position 0
Considering the attenuation rate exp (-α2) of the second signal light S2 due to optical fiber loss between ~Z, the power P2 (Z) of the second signal light S2 incident on the photodetector 5 is P2 (Z)- g”P2(z) ・exp (-a z
)−P2(L)・exp(−aL) ◆＾−PL(0) −P2(L) −exp(a
L) ·exp(-a Z) (4) Here, the loss when the second signal light S2 passes through the multiplexer/demultiplexer 4 is ignored. The first on the right side of equation (4)
The term is a DC component, and the second term is a component increased by Brillouin optical amplification. The time t from when the first (g light S) enters the optical fiber 3 to be measured until the second signal light S2 subjected to Brillouin light amplification at the position Z is detected by the photodetector 5
is t-2x/v (5
), the waveform of the received light signal level is as shown in FIG. In FIG. 2, the attenuation rate of the component increased by Brillouin optical amplification represents the optical loss α of the optical fiber 3 to be measured. By the way, as mentioned earlier, in order to observe the Brillouin optical amplification phenomenon as shown in FIG.
The difference between the frequency f of , and the optical frequency f of the second signal light S2 is,
As shown in equation (1), it must be equal to a specific frequency f, but it is known that there is generally more than one such frequency f8. In other words, since the wavelength of the sound wave involved in Brillouin scattering is 1/2 of the wavelength of the light wave, the standard G
Although the eO2-doped core optical fiber is a single mode waveguide for light waves, it is a multimode waveguide for sound waves, and each of these multiple acoustic waveguide modes contributes to Brillouin scattering amplification. As a result, the Brillouin gain spectrum has a plurality of maximum values, as illustrated in FIG. Note that how to obtain the Brillouin gain spectrum will be described later. As a result of detailed analysis, for example, N, 5hibata et al.
al. -Identification of longit
acoustic modes guide
ed in the core region of
a singla-mode optjcal f
Iber by Br1llouin gain
spectral measurements, O
pt, Lett, Vol, 13. No, 7. pp, 59
5-597.1988. As described in detail in Fig. 3, the gain peak (maximum value) a is due to the fundamental acoustic waveguide mode, and the gain peaks b and c are due to the Brillouin wave involving the first and second higher-order waveguide modes, respectively. It turns out to be the gain peak. The ratio of these peak gains depends on the structure of the optical fiber to be measured; for example, in the case of FIG. 3, the ratio of the gains of peaks a and b is about 6:1. By the way, it is well known that loss increases when an optical fiber is bent. Figure 4 shows actual measured data on the increase in loss observed when two types of optical fibers with different structures are bent.It shows that regardless of the difference in the structure of the optical fibers, the loss increases rapidly as the radius of curvature becomes smaller. is confirmed. Therefore, in order to use an optical fiber with low loss, the design must be designed with enough margin to minimize the increase in loss even when the bending of the minimum radius that actually occurs in the extra length processing section at the connection point, etc. This is necessary, and optical fibers are normally used in such a design state. On the other hand, if the wavelength of the light source is made shorter than the design wavelength used for communication, a multimode state will occur at wavelengths from a certain wavelength called the cutoff wavelength to short wavelengths. At this time, the fundamental mode is relatively strong against the bending of the optical fiber (that is, it does not cause an increase in loss), but the higher-order modes are extremely weak against the bending and are accompanied by a large loss. The data shows that this was actually measured, and even for a relatively gentle bend with a radius of about 30 mm, where no increase in loss was observed for the fundamental mode, there was a large loss of about 8 dB/m for the first higher-order mode. There are losses. This phenomenon occurs in almost the same way for the acoustic waveguide mode. That is, for example, Ahmed 5afaajjazi
et al,, “^analysis of Weakl
y Cuiding Fiber＾coustic W
aveguide, IEEE, Trans, UFF
This is because, as reported in C-33, 1°pp, 59-68.198a, the acoustic waveguide mode in a practical optical fiber has a wave group approximately of the same shape as the leading wave mode. . What has been said above can be summarized as follows. In other words, ■ An optical fiber, which is a single mode waveguide for light, is
It serves as a multimode waveguide for sound waves. ■ In an optical fiber bent to a radius of about 30 mm, the loss increase in both the fundamental mode of light waves and sound waves is small (less than about tdB/m), but the higher mode of sound waves exceeds 8 dB/m. It involves a large increase in losses. Here, when the Brillouin gain spectrum is measured for a bent optical fiber, as shown in FIG. 6, the contribution of higher-order modes is significantly reduced. Therefore, the ratio of the gain peak a due to the acoustic fundamental mode to the gain peak b due to the acoustic first higher-order mode increases, and is 50:1 in the case shown in FIG. 6. From the above, it can be seen that the ratio of the fundamental mode peak and higher order mode peak of the Brillouin gain spectrum is a measure of the bending of the optical fiber under test. FIG. 7 shows the dependence of this gain peak ratio on the radius of bending, and depicts how the gain peak ratio increases as the radius of bending decreases. The Brillouin gain spectrum can be measured at any arbitrary location in the longitudinal direction of the optical fiber as described below. That is, first of all, in the measuring device shown in FIG. 1, the optical frequency difference Δν=f, -f2 between the first light source 1 and the second light source 2 is fixed to a certain value ν1, and the measurement as shown in FIG. Measure the immediate distribution of Brillouin gain. In addition,
In Figure 2, the value obtained by subtracting the DC component from the received signal level is the Brillouin gain G shown in Figures 3 and 4.
corresponds to Next, the frequency difference Δν between the first light source 1 and the second light source 2 is fixed to a value ν2 that is slightly different from ν, and the distance distribution of the Brillouin gain similar to that in FIG. 2 is measured. Thereafter, similarly, the distance distribution of the Brillouin gain is repeatedly measured a plurality of times by changing the frequency difference Δν between the two light sources little by little using the optical frequency control device 7. FIG. 8(^) conceptually shows the flow of such measurement processing, and FIG. 8(B) shows a comprehensive display of the measurement data obtained by a series of these measurements and stored in the signal processing device 6. This is what I did. Next, the signal processing device 6 determines a specific location of the optical fiber based on the above measurement data, for example, 2=2. If we look at the point 2. and calculate the gain G at this location as a function of the light source frequency difference Δυ, we will find that the gain G at this location is 2. The Brillouin gain spectrum at is obtained. Furthermore, by sequentially changing the location 2 of interest and repeating the same process for each location, any location Z + * z2 + Z
A Brillouin gain spectrum for 3*Z4... is obtained. FIG. 9 conceptually shows the flow of this arithmetic processing performed by the signal processing device 6. Based on the Brillouin gain spectrum data obtained as described above, the signal processing device 6 calculates the gain peak ratio of the aforementioned fundamental mode and higher-order mode for each location of interest in the optical fiber to be measured. For example, as shown in FIG. 7, the obtained gain peak ratio gives the curvature distribution of the optical fiber. Here, as in an optical fiber in a straight conduit, a region with relatively little bending can be practically known in advance, so the gain peak ratio of the fundamental mode and higher-order mode in this region can be calculated as ( In other words, it can be used as a reference value for a portion (without bending). FIG. 1θ shows an example of measurement of the distribution of the gain peak ratio in the longitudinal direction of the optical fiber, and shows the presence of a steep bend at point A and a relatively gentle bend at point B. Here, the distance resolution ΔZ on the horizontal axis is expressed as Δz−v/2 (6
). This distance resolution Δ2 is exactly the same as the resolution of the conventional 0TDR method. The flowchart in FIG. 11 shows an example of the measurement processing procedure of the embodiment of the present invention described above. It is assumed that this processing procedure is stored in advance in a program ROM (memory) (not shown) in the signal processing device 6. Note that since the contents of each processing step have already been described, their explanation will be omitted. In the above description of the present invention, the second light source 2 is
Although the explanation has been made assuming that the second light source 2 emits W light,
may be a modulated light source such as, for example, the first light source. However, in that case, the first signal light SI (for example, optical pulse) from the first light source 1 and the second light source 2
Since Brillouin optical amplification occurs only at the position (for example, zo) where the second signal light S2 (for example, a light pulse) from mzQ, which is suitable for extracting information from a specific position (zo above), is the first signal light S
By relatively controlling the emission time between L and the second signal light S, it is possible to set it to any desired position. Moreover, if the frequency difference Δν between the first and second light sources is changed here, a Brillouin gain spectrum as shown in FIG. 3 is obtained. It is desirable that the change in Δν be continuous, but since the bandwidth of the Brillouin gain is about 50 to 100Mtlx, if the increments of Δν are sufficiently small (for example, about 10M1lz), it is practical even if the change is discrete. I can't help it. Furthermore, when the frequency @fI of the first light source is higher than the frequency f2 of the second light source, the power of the signal light S emitted from the second light source and passed through the optical fiber 3 under test is measured and the Brillouin gain is calculated. When the spectrum is determined, a gain spectrum as shown in FIG. 9 is obtained. At this time, when the power of the signal light SI emitted from the first light source and passed through the optical fiber 3 under test is measured to obtain the Brillouin gain spectrum, the first
Since the light from the light source is used to amplify the light from the second light source, a spectrum is obtained that is attenuated in a specific frequency range. In this case, if you reverse the polarity of the measured value, the 9th
Exactly the same gain spectrum as shown in the figure is obtained. [Effects of the Invention] As explained above, according to the present invention, the Brillouin gain spectrum is measured for each focused location of the target fiber to be measured, and the gain peak associated with the fundamental acoustic waveguide mode and the higher-order waveguide are detected. By determining the ratio of the gain peaks associated with the modes, it is possible to know the distribution of bends added to the optical fiber. In particular, in the present invention, since light waves always propagate in the optical fiber in the fundamental mode, they are insensitive to normal bending, whereas in the case of sound waves, the fundamental mode is not easily affected by bending. However, since the bending loss of high-order mode sound waves is extremely large, this method has the advantage of being able to measure bending distribution with extremely high sensitivity compared to conventional methods that compare loss distributions. In addition, while conventional methods require time-series comparisons of loss distributions, this is not necessary when using the present invention, and the presence or absence of bending effects can be easily detected by comparing locations. There are advantages. Furthermore, in the present invention, when loss other than bending and loss due to bending exist simultaneously, as in the vicinity of a connection point, a remarkable effect is obtained in that it becomes possible to evaluate each separately.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of a measuring device according to an embodiment of the present invention, Fig. 2 is a waveform diagram showing an example of a received light waveform measured by the present invention, and Fig. 3 is a diagram showing an example of a received light waveform measured by the present invention. Figure 4 is a characteristic diagram showing the Brillouin gain spectrum, Figure 4 is a characteristic diagram showing bending loss of an optical fiber, Figure 5 is a characteristic diagram showing bending loss of the first higher mode in an optical fiber, Figure 6 is a characteristic diagram showing bent light. Characteristic diagram showing the Brillouin gain spectrum in the fiber. Figure 7 is a characteristic diagram showing the bending radius dependence of the Brillouin gain peak ratio of the acoustic fundamental mode and higher-order mode. Figure 8 (^) is the measurement of the distance distribution of Brillouin gain. Figure 8 (B) is a waveform diagram showing the comprehensive display of data. Figure 9 is a diagram showing the calculation processing flow of the Brillouin gain spectrum for each location of interest. Figure 1θ is the longitudinal axis of the gain peak ratio. A characteristic diagram showing the directional distribution. FIG. 11 is a flowchart showing the measurement processing procedure of the embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... First light source, 2... Second light source, 3... Optical fiber to be measured, 4... Multiplexer/demultiplexer, 5... Photodetector, 6... Signal processing device, 7... Optical frequency control device.

Claims (1)

[Claims] 1) A first step of measuring the Brillouin gain spectrum of one section of the optical fiber under test or for each of a plurality of sections of the optical fiber, and measurement in the first step. A second step of calculating the ratio of the maximum gain involved in the fundamental acoustic mode and the maximum gain involved in the higher-order acoustic modes for each of the one or more Brillouin gain spectra obtained.
and a third step of calculating the curvature of the optical fiber for the one section of interest or for each of a plurality of sections based on the ratio determined in the second step. A method for measuring curvature distribution of an optical fiber, characterized in that: 2) The method for measuring the Brillouin gain spectrum for each of the plurality of sections performed in the first step includes inputting pulsed light from one end of the optical fiber to be measured, and inputting continuous light from the other end of the optical fiber. Then, by controlling the frequency difference Δν between the pulsed light and the continuous light, for each of the plurality of fixed frequency differences Δν,
measuring the time change in the amount of the continuous light emitted from the input end of the pulsed light under the effect of Brillouin light amplification caused by the interaction with the pulsed light, and measuring the Brillouin light based on the measurement for each section of interest; 2. The method for measuring curvature distribution of an optical fiber according to claim 1, wherein the Brillouin gain spectrum is obtained by determining the amount of amplified light as a function of the frequency difference Δν. 3) The method for measuring the Brillouin gain spectrum for each of the plurality of sections performed in the first step includes inputting mutually synchronized pulsed light from both ends of the optical fiber to be measured, and measuring the Brillouin gain spectrum in the one section of interest. A delay is given to the incident time of the two pulsed lights so that the two pulsed lights meet, and a relative frequency difference Δν between the two pulsed lights is changed continuously or discretely, and at this time, the two pulsed lights are 2. The method for measuring curvature distribution of an optical fiber according to claim 1, wherein the Brillouin gain spectrum is obtained from the amount of light emitted from at least one of the optical fibers. 4) Brillouin gain spectrum measuring means for measuring a Brillouin gain spectrum of one section of the optical fiber under test or for each of a plurality of sections of the optical fiber; gain peak ratio calculation means for calculating the ratio of the maximum gain involved in the fundamental acoustic mode to the maximum gain involved in the higher-order acoustic mode for each of the one or more Brillouin gain spectra; and curvature distribution calculation means for calculating the curvature of the optical fiber for each section or for each of a plurality of sections based on the value of the ratio. . 5) The Brillouin gain spectrum measuring means includes a first signal light source that generates a first signal light in the form of pulse-modulated light.
a light source; a second light source that emits a continuous or pulse-modulated second signal light that propagates opposite to the first signal light in the optical fiber to be measured; the first signal light and the second signal light; an optical frequency control means for controlling a difference in optical frequency between the first signal light and the second signal light, a means for controlling a relative delay time between the first signal light and the second signal light, and making the first signal light enter the optical fiber to be measured. , and a multiplexing/demultiplexing means for extracting a second signal light that has undergone a Brillouin optical amplification effect generated between the first signal light and the second signal light, and the Brillouin optical amplification effect from the multiplexing/demultiplexing means. and a signal processing means for processing and analyzing the temporal waveform or amplitude and phase of the signal detected by the light detection means. 5. The optical fiber curvature distribution measuring device according to claim 4.