JPH02238339A - Method and apparatus for evaluating characteristic of optical fiber - Google Patents

Abstract

PURPOSE:To improve the accuracy in characteristic evaluation of optical fiber to be measured by analyzing the signal waveform of the second signal light which is affected by Raman light amplification action when the second signal light and the modulated first signal light are propagated in the optical fiber in the opposing manner. CONSTITUTION:First signal light S1 which undergoes amplitude modulation in pulse pattern is emitted from a first light source 1. The signal light S1 is coupled into an optical fiber to be measured 3 at a fiber end 3-1 through a synthesizing and splitting device 4. Meanwhile, second signal light S2 from a second light source 2 is coupled into the optical fiber 3 at a fiber end 3-2. When the Raman frequency shift inherent to the optical fiber 3 is made to be fR, the frequencies f1 and f2 are adjusted so that f1-f2=fR is obtained. In this way, the signal light S2 undergoes Raman light amplification by the signal light S1. The change in waveform is detected with a photodetector 5, and the characteristics of the optical fiber are evaluated. At this time, the change in waveform becomes the value of 100-1,000 times in comparison with conventional OTDR, and the highly accurate evaluation can be performed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to optical fiber loss, components contained in the optical fiber, changes in tension applied to the optical fiber, effects of temperature changes, etc. The present invention relates to a method for evaluating the distribution of various characteristics over a direction, and an apparatus therefor.

[Prior Art] As a method for measuring the loss distribution in the longitudinal direction of an optical fiber, an OTDR (for example, M. K. Barnoski,
et al. ．． “Optical Ti
me domain reflectometer
” Appl. Opt., Vo1.16, p.
p. 2375-2379.1977). O.T.
DR performs analysis in the time domain, whereas OFDR analyzes backward Rayleigh scattered light in the frequency domain (for example, F.P.κapron.et al., 'As
pect of optical frequency
ncy-domain reflectometry
,Tech Digest of I00G'81,
P. 106. 1981) have also been proposed. O.T.
Along with DR and OFDR, this technology is nearing completion.

In addition, as a method for measuring the temperature distribution over the longitudinal direction of an optical fiber, there is a method of detecting a temperature-dependent change in the intensity of backward Rayleigh scattered light or the intensity of backward Raman scattered light generated in the optical fiber using OTDH (for example, A.H. llartog, “＾distribu
ted temperature sensor ba
sed on1quidcore optical
fibers, J. Lightwave
Technol. ．． LT-1, pp. 498-50
9.1983 and J. P. Dakin at al.
．． , 'Distributed optical
ibre Raman temperature se
nsor using asemiconductor
light source and detect
rElectron. Lett. , Vol. 21
, pp. 569-570.1985). [Problem to be solved by the invention] However, since the backward Rayleigh scattered light and the backward Raman scattered light are very weak, the S of the received signal is
Even if the N ratio is improved, it is very difficult to increase the measurement accuracy.

Therefore, the purpose of the present invention is to reduce loss in optical fibers, components contained in optical fibers, changes in tension applied to optical fibers,
An object of the present invention is to provide a method for evaluating characteristics of an optical fiber and an apparatus therefor, which can measure the distribution of various characteristics over the longitudinal force direction of an optical fiber, such as the influence of temperature changes, with high precision.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides modulated first signal light from a first light source and a second light source that propagate oppositely in an optical fiber to be measured. The signal waveform of the second signal light that has been affected by the Raman light amplification effect is analyzed using the nonlinear interaction called Raman light amplification between the second signal light and the second signal light from the Raman light amplification effect.

That is, in the first embodiment of the present invention, the first signal light in the form of modulated light from the first light source is input into the optical fiber to be measured, and the second signal light from the second light source is input into the optical fiber to be measured. The first signal light and the second signal light are propagated through the fiber in opposition to the first signal light.
A Raman light amplification effect is generated between the signal light and the second signal light that has undergone the Raman light amplification effect, and the second signal light generated by the Raman light amplification effect is detected based on the detected second signal light. It is characterized by evaluating the characteristics of the optical fiber under test by analyzing temporal waveform changes or amplitude and phase changes of signal light.

A second aspect of the present invention provides a first method in the optical fiber to be measured.
Let the speeds of the signal light and the second signal light be ■ and v2, respectively, and the average speed V is 2/v- (t/v+
) + (1/v2), and the length of the optical fiber to be measured is L. First, the first signal light in the form of modulated light from the first light source is injected into the optical fiber to be measured. The second signal light is inputted from one first terminal of the fiber, and the second signal light from the second light source is inputted into the optical fiber to be measured from the other second terminal of the optical fiber to be measured. 1st
The first signal light is propagated in the optical fiber under test opposite to the signal light to cause a Raman light amplification effect between the first signal light and the second signal light, and the first signal light is propagated through the optical fiber under test. The time of incidence on the first terminal is jln, and the first
Distance lm z Ill from the terminal along the optical fiber under test
The time at which the first signal light propagates to the position at which the second signal light is subjected to Raman light amplification by the first signal light at that position reaches the first terminal of the optical fiber to be measured is ta.
ut, and the time from time Fn to time tout is j (-jout-j+n-2z/v), and the second signal light that has undergone Raman optical amplification and passed through the first terminal of the optical fiber under test is Based on the detected second signal light, the temporal waveform change of the second signal light caused by the Raman light amplification effect is defined as "PI (vt/z)", and then the first
A first signal light in the form of modulated light from a light source is input into the optical fiber under test from a second terminal of the optical fiber under test, and a second signal light from the second light source is input into the optical fiber under test. By inputting the second signal light from the first terminal of the optical fiber to be measured, the second signal light is propagated through the optical fiber to be measured, facing the first signal light, and the signal light is transmitted between the first signal light and the second signal light. The time when the first signal light enters the second terminal of the optical fiber to be measured is j' In,
The first signal light propagates from the second terminal of the optical fiber to be measured to a position a distance fiZ' apart along the optical fiber to be measured, and at that position, the second signal light is subjected to Raman light amplification by the first signal light. The time when the signal light reaches the second terminal of the optical fiber to be measured is t'. ．． Binding, start time from time j' In.
The time until out is t' (- t'. uL- j
'In-2z' /v), the second signal light that is subjected to the Raman light amplification effect and has passed through the second terminal of the optical fiber to be measured is detected, and the Raman light is generated based on the detected second signal light. The temporal waveform change of the second signal light caused by the amplification effect is WP2 (vt' /2), and the temporal waveform change WP
+(vt/2) and the temporal waveform change W, . 2 (vt'
/2) to variable conversion (vt' /2-L-vt/2
) Temporal waveform change WP2 (L-vt/2)
Product, iyp, (vt/2) WP2 (L-v
t/2), find the distribution of factors that change the Raman gain in the optical fiber under test in the longitudinal direction of the optical fiber under test, and calculate WP+(vt/2), and WP2(L-
vt/2), WP+ (vt/2)/WP2(L-
The method is characterized in that the loss of the optical fiber to be measured is determined from (vt/2).

In a third aspect of the present invention, the optical fiber to be measured has a plurality of Raman frequency shifts fRH (n=1, 2,...),
By adjusting the optical frequency difference between the first signal light and the second signal light to fRI, one of the Raman frequency shifts, a Raman light amplification effect is produced between the first signal light and the second signal light, and the Raman light Detect the second signal light that has been amplified, and let the temporal waveform change of the detected second signal light be W, and then,
By adjusting the optical frequency difference between the first signal light and the second signal light to the other one of the Raman frequency shifts fn'' (≠fRI), the Raman light is shifted between the first signal light and the second signal light. Amplification is caused and the second signal light that has undergone the Raman light amplification is detected, and the temporal waveform change of the detected second signal light is Wj, and the ratio of the temporal waveform changes W and W is By analyzing, the distribution of factors that change the Raman gain in the optical fiber under test over the longitudinal direction of the optical fiber under test is evaluated separately from the loss characteristics of the optical fiber under test.

A fourth aspect of the present invention includes a first light source that generates a first signal light in the form of modulated light, and a second signal light that propagates opposite to the first signal light in an optical fiber to be measured. A second light source and a multiplexer/demultiplexer that inputs the first signal light into the optical fiber to be measured and extracts the second signal light that has undergone the Raman light amplification effect generated between the first signal light and the second signal light. means, a light detection means for converting the second signal light subjected to the Raman light amplification effect from the multiplexing means into an electrical signal, and processing the temporal waveform or amplitude and phase of the signal detected by the light detection means. It is characterized by being equipped with signal processing means for analysis.

A fifth aspect of the present invention includes a first light source that generates a first signal light in the form of modulated light, and a second signal light that propagates opposite to the first signal light in an optical fiber to be measured. A second light source and a multiplexer/demultiplexer that inputs the first signal light into the optical fiber to be measured and extracts the second signal light that has undergone the Raman light amplification effect generated between the first signal light and the second signal light. and receiving a second signal light subjected to Raman light amplification from the multiplexing/demultiplexing means,
an optical frequency filter that passes the optical frequency of the second signal light subjected to the Raman optical amplification effect but blocks the optical frequency of the first signal light; and an optical frequency filter that converts the output light from the optical frequency filter into an electrical signal. It is characterized by comprising a detection means and a signal processing means for processing and analyzing the temporal waveform or amplitude and phase of the signal detected by the photodetection means.

A sixth aspect of the present invention includes a first light source that generates a first signal light in the form of modulated light, and a second signal light that propagates in an optical fiber to be measured in opposition to the first signal light. A second light source and a multiplexer/demultiplexer that inputs the first signal light into the optical fiber to be measured and extracts the second signal light that has undergone the Raman light amplification effect generated between the first signal light and the second signal light. and receiving a second signal light subjected to Raman light amplification from the multiplexing/demultiplexing means,
an optical frequency filter that passes the optical frequency of the second signal light subjected to the Raman optical amplification effect but blocks the optical frequency of the first signal light; and a photodetector that converts the output light from the optical frequency filter into an electrical signal. means, a signal processing means for processing and analyzing the temporal waveform or amplitude and phase of the signal detected by the optical detection means, and a first signal light and a second signal light.
It is characterized by comprising means for changing the polarization state of at least one of the signal lights.

[Work] The principle of Raman light amplification is as follows.

When light is incident on a certain substance, for example, an optical fiber, light is generated whose frequency is shifted by the natural frequency of the material components that make up the optical fiber. Light shifted to the lower frequency side than the incident light is called Stokes light, and light shifted to the higher frequency side is called anti-Stokes light. In addition, the amount of frequency shift (Raman frequency shift) is determined by each individual substance, and is approximately 100
~4000cm-'(cm-' is a unit of wave number and is proportional to frequency, 1c+n-' is 30 G tl z
(equivalent to). If the incident light intensity is further increased, phase-aligned Stokes light can be obtained. This is a phenomenon called guided Raman scattering.

Therefore, if the optical frequency difference between the first signal light (optical frequency is f) and the second signal light (optical frequency is f2) is made to match the Raman frequency shift (fR), the above problem can be solved. Through the guided Raman disturbance, it becomes possible to amplify the second signal light when f, -f2-fp, and to amplify the first signal light when f2-f, .fR.

As described above, the present invention evaluates the characteristics of an optical fiber from the change in the waveform of the signal light caused by the nonlinear effect of Raman light amplification. By measuring the weak backward Rayleigh scattered light or backward Raman scattered light, the loss distribution in the longitudinal force direction of the optical fiber can be determined.
This is significantly different from conventional technology that evaluates temperature distribution.

[Example] Hereinafter, the present invention will be explained in detail with reference to the drawings.

Example 1 FIG. 1 shows Example 1 of the present invention. Here, 1 is a first light source that generates pulsed light (first signal light) whose amplitude is modulated in a pulsed manner. The first light source is, for example, a directly pulse-modulated semiconductor laser or a pulsed Nd:YAG laser.
These include lasers, color center lasers, dye lasers, etc. 2 is a CW oscillation second light source that generates a second signal light, for example, a CW oscillation semiconductor laser or a CW oscillation Nd:
These include YAG laser, color center laser, dye laser, etc. 3 is an optical fiber to be measured, 4 is a multiplexer/demultiplexer, 5 is a photodetector, and 6 is a signal that receives an electrical signal from the photodetector 5 and processes and analyzes the temporal waveform or amplitude and phase of the signal. It is a processing device.

The operation of this evaluation device is as follows.

Pulsed light emitted from the first light source 1, that is, the first signal light S1, 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 s2, is transmitted to the terminal (first terminal) opposite to the terminal (first terminal) 3-1 of the optical fiber under test 3 to which the first signal light S1 is coupled. 2 terminal) 3-2, and is coupled to the optical fiber 3 to be measured. At this time, the optical frequency f of the first signal light S1. and second
The optical frequency f2 of the signal light s2 is f + - f2 - f *
(1) The optical frequencies of the first light source 1 and the second light source 2 are adjusted in advance. Here, fR is a Raman frequency shift specific to the optical fiber 3 to be measured; for example, in the case of a quartz-based optical fiber, it is approximately 430 cm.
-' is. When formula (1) is satisfied, the second
The signal light S2 is Raman optically amplified by the first signal light S1. At this point, the second signal light S detected by the photodetector 5
The waveform of 2 is derived as follows.

The gain g at position Z for the second signal light S2 due to Raman optical amplification is: When g-1<<1, g=t 1 AP I(z)
It is expressed as (2). Here, A represents a comparison coefficient, and Z represents the position where the first signal light St (pulse signal) propagating in the optical fiber under test 3 is present. It is assumed that the position of incidence on the terminal 3-1 is determined as a reference. P(Z) is the optical power of the first signal light s1 at position B. Similarly, let P2 (z) be the optical power of the second signal light s2 at position B. Let α[nep
er/ml and α2 [neper/m]. Further, the length of the optical fiber 3 to be measured is assumed to be L [ml]. At this time, p+(z) =P+(o) ・exp(-a
lz) (3-1)h(Z) =P2(L)
- exp[- a2(L-z)] (3-
2). Equation (2), Equation (3), and attenuation rate exp of the second signal light S2 due to optical fiber loss between the positions Oyz
Considering (-α2Z), the power P2 (Z) of the second signal light S2 that is applied to the photodetector 5 is P2 (Z);
g−h (Z) ”eXp (− Q 2Z)=P2(
L)・exp(-Q2L) 18・P,(o)-P2(L)-exp(-Q2L
)・eXp(− α+Z) (4). Here, the loss caused by the second Shinbune light beam S2 passing through the multiplexer/demultiplexer 4 is ignored. The first term on the right side of equation (4) is a DC component, and the second term is a component increased by Raman light amplification. The time t from when the first signal light S1 enters the optical fiber 3 to be measured until the second signal light S2, which has been Raman amplified at position B, is detected by the photodetector 5 is the time t when the signal light 51, S
VI. Assuming V2 and its average speed as ■, t =
2z/v (5-1)
2/v-1/v, +l/v2
(5-2), the waveform of the received light signal level is as shown in FIG.

In FIG. 2, the optical loss α of the optical fiber 3 to be measured for the first signal light S1 is determined from the attenuation rate of the component increased by Raman light amplification or the difference in light reception at any point.
1 can be found. When comparing the invention and the conventional OTDR, it is immediately clear that the following two points are different.

(1) FIF p lI of signal waveform by conventional OTDH
The attenuation rate for i[t z is exp (-2α+z), and the signal level attenuates rapidly with distance Z, whereas the attenuation rate of the signal waveform in the present invention is exp (-2α+z).
z) and the signal level distance ll! The attenuation for tZ is small.

(2) As shown in equation (4), by increasing the optical power of the second light source 2, the level of the received light signal can also be increased.

Therefore, according to the present invention, the level of the received light signal can be significantly increased compared to the conventional OTDR, and measurement accuracy is improved.

In the above explanation, it is assumed that the optical fiber 3 to be measured has uniform losses α1 and α2, but even if the loss is not uniform in the longitudinal direction of the optical fiber, the measurement system shown in FIG. Accordingly, the longitudinal distribution of loss in the optical fiber 3 to be measured can be measured. That is, for example, in such an optical fiber, the loss can be measured by performing the above consideration for each section where the loss is uniform.

Note that the distance resolution according to the present invention is proportional to the pulse width of the first signal light S1, and for example, when the pulse width is Ions, the distance resolution is 1 m. This is the same as the conventional OTDR. The distance resolution also depends on the Raman gain bandwidth, but since the Raman frequency shift amount of a silica fiber is as much as 200 cm-' ([iMt+z), the influence of the Raman gain bandwidth can be ignored in practice.

Embodiment 2 FIG. 3 shows a second embodiment of the present invention in which an optical frequency filter 8 is disposed before the photodetector 5 in contrast to the first embodiment of the invention shown in FIG. The optical frequency filter 8 has a characteristic of passing the frequency f2 of the second signal light S2 but blocking the frequency f1 of the first signal light S1. Since the Raman frequency shift is very large as described above, a dielectric multilayer interference filter, a diffraction grating, or the like can be used as the optical frequency filter 8. The operation of this second embodiment is almost the same as that of the first embodiment shown in FIG. 1, but by using the optical frequency filter 8, the following effects can be obtained.

(1) Backward Rayleigh scattered light (conventional OTD
R) is not incident on the photodetector 5 to prevent it from becoming noise to the second signal light S2.

(2) Preventing the strong Fresnel reflection of the first signal light Sl from entering the photodetector 5 from the input end and output end of the optical fiber under test 3 or the connector connection point in the optical fiber under test 3 . Normally, when detecting an excessive signal, the weak signal immediately after the excessive signal cannot be measured due to tailing or overshoot of the excessive signal due to saturation of the detection system. In other words, a dead zone problem occurs. However, according to the second embodiment, by employing the optical frequency filter 8, this dead zone problem, which is a problem in the conventional OTDR, can be solved.

Example 3 FIG. 4 shows Example 3 of the present invention. Here, 9 is a polarization control device that controls the polarization state of the second signal light S2, and is arranged between the second light source 2 and the optical fiber 3 to be measured. 1st
In the first and second embodiments of the present invention shown in FIGS. It was implicitly assumed. However, these conditions are only satisfied with special optical fibers such as polarization-maintaining fibers, or with multimode optical fibers in which polarization is randomized.

On the other hand, the gain g due to Raman optical amplification has polarization dependence, in that it takes a maximum value when the polarization directions of the signal lights S1 and 52 match, and takes a minimum value when they are orthogonal. Therefore,
When measuring a general single mode optical fiber, the fourth
As shown in the figure, it is necessary to provide a polarization control device 9. Here, the gain g is determined by simple analysis as g-1+〇. 5Gmax[l+B-cos (2θ
+φ)1 (6) is found. 1+Gma
x represents the maximum gain obtained when the polarization directions of signal light St and 52 match. Further, B and φ are quantities that depend on Z, and −1≦B≦1. θ is the second signal light S
2 is the polarization direction when it enters the optical fiber 3 to be measured.

Since a normal optical fiber has slight mode birefringence, the polarization state of the optical signal propagating through it differs depending on the position Z. Therefore, the signal waveforms measured using the measurement systems of Examples 1 and 2 shown in FIGS. 1 and 3 for ordinary optical fibers are actually shown in FIG. 5 (A
), the waveform becomes fluctuating. Note that in FIGS. 5(8) to 5(C), the DC component is omitted. Further, the horizontal axis represents time as a value converted to the length of the optical fiber using equation (5). This fluctuation reflects the state of polarization of the signal lights St and S2 at each position Z of the optical fiber 3 to be measured.

The polarization control device 9 can be configured with a polarizer and a local wavelength plate for linearly polarizing the second signal light S2 from the light source 2. In this case, by rotating the local wavelength plate, the second signal light S2 in any polarization direction can be made to enter the optical fiber 3 to be measured. FIG. 5(A) is the result of an experiment in which a linearly polarized second signal light S2 with a certain polarization direction θ is incident on the optical fiber 3 under test using the measurement system shown in FIG. Shows the waveform. Further, FIG. 5(B) shows a waveform when a linearly polarized second signal light S2 with a polarization direction θ+π/2 orthogonal thereto is made incident on the optical fiber 3 to be measured.

As can be seen from Equation (6), the polarity of the fluctuations in Figures 5 (^) and (B) are reversed. Therefore, the signal processing device 6 averages FIGS. 5(A) and 5(B) to obtain an output with a waveform shown in FIG. 5(C). In the waveform of FIG. 5(C), fluctuations are removed,
It can be seen that a smooth curved waveform reflecting the optical loss of the optical fiber 3 to be measured, similar to that shown in FIG. 2, is obtained.

In addition, the signal level is about 100 to 1
It has been confirmed through experiments that this amount reaches 1,000 times.

In the above explanation, in order to obtain the waveforms shown in FIGS. 5(A) and 5(B), the polarization state of the second signal light S2 is changed by rotating the wavelength plate constituting the polarization control device 9. let me,
A total of two measurements were performed, but by changing the polarization state of the second signal light S2 at high speed, it was possible to make the measurements as shown in Fig. 5 (
The waveform C) can be obtained.

/ For that purpose, an optical fiber (an optical fiber with a large mode birefringence, such as a polarization-maintaining optical fiber, is more desirable) is wound around an electrostrictive element such as PZT, and the optical fiber is stretched periodically or randomly. It is sufficient to use a polarization control device 9 that changes the polarization state of the second signal light S2 passing therethrough by applying lateral pressure distortion.

Alternatively, an electro-optic effect type element such as a LiNbO3 crystal is also suitable as the high-speed polarization control device 9.

Furthermore, as the polarization control device 9, an element (devilizer) that cancels the polarization of incident light may be used.

Lyot's Deborahizer is well known as this Deborahizer, and devices made by laminating and bonding birefringent crystals, and recently, optical fiber types have also been developed (
K. B6hm. et al. ,”Performance
e'of Lyot depolarizers wi
thbirefringent single-mo
de fibers, J. Lightwave
Techno1. , LT-1, pp. 71-74.19
83. reference). When such a devolaizer is used, it is possible to configure the polarization control device 9 with passive components, so that the measurement accuracy and reliability of the present invention can be improved.

Furthermore, in the above explanation, the polarization control device 9 is placed between the optical bifurnace to be measured 3 and the second light source 2 in order to change the polarization state of the second signal light S2. However, the same effect can be obtained even if the polarization state of the first signal light S1 is changed.

Therefore, the polarization control device 9 may be placed between the first light source 1 and the multiplexer/demultiplexer 4 or between the multiplexer/demultiplexer 4 and the optical fiber 3 to be measured. Alternatively, the polarization control device 9 may be placed midway through the optical fiber 3 to be measured, and the signal light 51 and S
It is also possible to change the polarization states of both of the two.

Example 4 Next, an example of a method for measuring the longitudinal distribution of components contained in an optical fiber according to the present invention will be described.

Raman frequency shift h for the material constituting the optical fiber
fn and Raman gain g have already been investigated in detail; for example, the relationship between the Raman frequency shift and Raman gain of a silica-based optical fiber whose core is doped with P205 is shown in Figure 6. .

In Figure 6, the wave number of Raman frequency shift is 420 cm.
-' (hereinafter referred to as fRs) and 1320cm-'
(hereinafter referred to as fRp)
i and P-0 bond frequencies, and the Raman gain peak of the latter is P20. This was caused by the addition of . Also, the Raman gain at fRp is P20
It is known that the amount increases in proportion to the amount of 5 added.

Therefore, in the measurement system of the third embodiment of the present invention shown in FIG.
If a Raman light amplification effect is caused during this period and the signal light S2 subjected to the Raman light amplification effect is measured, the signal waveform WP(Z) obtained by this measurement (L(Z) is expressed by the formula (4)
) proportional to the second term on the right side), the amount of P205 added at each position Z of the optical fiber 3 to be measured, that is, the amount of P205 over the longitudinal direction of the optical fiber 3 to be measured.
You can know the distribution of the amount added.

However, at this time, the influence of optical loss in the optical fiber 3 to be measured must be removed from the signal waveform w, (Z). There are two methods for this removal:

Removal method 1 (bidirectional measurement) Regarding the above signal waveform WP (Z), first, the signal waveform obtained by inputting the first signal light St from one terminal 3-1 of the optical fiber 3 to be measured is expressed as W, E( Z), and then the signal waveform obtained by inputting the first signal light S1 from the other end 3-2 of the optical fiber 3 to be measured is WP2 (Z). Since optical fiber loss has no directionality, each signal waveform is expressed as follows.

WPr (z)-k-exp (-a lz) -F
(z) (7-1)WP2 (Z)-1(
-exp (-Cl r Z) ・F (L-z)
(7-2) Here, k is the proportionality coefficient, F (z)
is a Raman gain proportional to the amount of P205 added at position Z of the optical fiber 3 to be measured. Also, position Z is at time t
There is a relationship shown in equation (5-1) between At this time, wpm(z)-wp+(z)wp2(t.-z)-k
2・exp(-a IL)'F2(Z)WP! /2 (
Z)-WP+ (2)/WP2(L-z)-exp (
2゜+z)・exp(a+L)? 8-2). In other words, the addition amount distribution (F(z) 2 proportional) from WPlx (2) to P20,
) can be measured separately.

Removal Method 2 In addition to the above signal waveform WP(Z), the waveform obtained when the optical frequency difference between the signal light Sl and 52 is made to match the above fns is assumed to be Ws (z). At this time, WP(z
)-k-exp (-QIZ)・p(Z) (
9-1) Ws(z)-k-exp (-a+z)・G(
z) (9-2). Here, G (z)
is Si・-0-Si depending on the addition amount distribution of P205.
This shows how the Raman gain due to vibration changes over the longitudinal direction of the optical fiber 3 to be measured. F(z)/G(
The value of z) can be determined in advance as a function of the amount of P205 added, so WP(Z)/WS(Z)-F(Z
)/G(Z) (to) by calculating the value of P20 and the amount of addition in the longitudinal direction from the optical loss? Can be measured remotely.

Here, what must be noted is that the signal waveform WP
(z) and Ws (z), by changing the optical frequency of the second signal light S2 without changing the optical frequency of the first signal light 51, the first signal light S1 and the second signal light S2
It is necessary to match the optical frequency difference of 1 to the Raman frequency shift 1 -fRp,f■. This is because when the optical frequency of the first signal light S1 is changed, Equation (9-1),
This is because the values of the loss α in (9-2) would be different, and thus the equation (10) would no longer hold true.

The above has described the case in which P205 is added to a silica-based optical fiber, but other materials, such as Ge02.
The same applies to the case where rare earth elements such as B203, AJ2 203, etc., or Er are added.

Furthermore, all of the above-mentioned additive materials such as P20 are added at the time of manufacturing the optical fiber in order to control the refractive index of the optical fiber. It goes without saying that the distribution of substances diffused therein can also be measured by the present invention. Important such substances include hydrogen (H2) and deuterium (D
There is 2). The Raman frequency shifts due to H 2 , +1 0 , and D2 are approximately 4155 cm-', respectively.
, 3610cm-', 2970cm-'. Therefore, by matching the optical frequency difference between the signal lights S1 and 52 with these Raman frequency shifts, the distribution of each substance can be measured according to the present invention. For example, it is a well-known fact that diffusion of hydrogen into an optical fiber increases optical fiber loss. With conventional OTDRs, although it was possible to measure the increase in loss, the cause was not known. On the other hand, according to the present invention, it is also possible to determine whether the cause of the increase in loss is hydrogen.

In the above, the first signal light S1 has been described as being intensity-modulated in a pulsed manner, but the first signal light S1 may be a single pulse, or alternatively, as shown in FIG. It may be a repeated pulse of tc. In that case, the signal processing device 6 can perform averaging processing to improve the signal-to-noise ratio of the received signal, making it possible to perform more accurate measurements.

Alternatively, the first signal light 51 may be modulated by a pseudo-random code (M-sequence code, Golay code, etc.) as shown in FIG. 7(B). At this time,
Based on the principle of the correlation method, the detected second signal light S2 is subjected to correlation processing by the signal processing device 6, so that
Compared to case A), the S/N ratio is improved in proportion to the code length (for example, κ. Okada et at.,
'Optical cable faultlocat
ion using correlation tec
hniqueElectron. Lett. , vol.
16, p. 629. (see 1980). Traditional OTD
In R, when weak backscattered light and strong Fresnel reflected light coexist, a problem arises in the linearity of the detection system, so the code length cannot be made sufficiently long. On the other hand, in the present invention, as already explained,
By providing the optical frequency filter 8, strong signals such as Fresnel reflected light are not mixed, so it is possible to maximize the features of the correlation method.

Furthermore, as shown in FIG. 7(C), the first signal light S
t is the signal light whose intensity is modulated at the frequency F, and by changing the frequency F and obtaining the frequency characteristics of the amplitude and phase of the second signal light S2, it is possible to perform the same measurement as in the time domain obtained so far. It is also possible to carry out measurements. In this case as well, for the same reason as in the case of the correlation method, there is no problem with the linearity of the detection system, and highly accurate measurement is possible.

The present invention has been described above with reference to the measurement of optical fiber loss and the longitudinal distribution of components contained in the optical fiber. However, it is known that Raman gain depends on changes in tension applied to the optical fiber and changes in temperature. Therefore, in the same way as measuring the distribution of components contained in an optical fiber, changes in tension applied to the optical fiber can be measured. According to the invention, it is possible to measure various quantities of temperature changes along the length of an optical fiber.

In addition, the present invention is not limited to the above description, and can be implemented with various modifications without departing from the gist thereof.

For example, Figure 1. In the embodiment of the present invention shown in FIG. 3 or 4, a branching type multiplexer/demultiplexer 4 is provided, which is similar to the dichroic mirror shown in FIG. 8 as shown below.
Replaced by IO.

In FIG. IO, for example, a dichroic mirror IO
As shown in FIG. 9, the first signal light SL (frequency f1) enters the optical fiber 3 to be measured without suffering the pass loss of the dichroic mirror 10, and the second signal light SL (frequency f1) S2 (frequency f2) is also dichroic mirror 1
The light enters the photodetector 5 without undergoing a zero transmission loss. This results in a significant reduction in insertion loss compared to the case where the branch type multiplexer/demultiplexer 4 shown in FIGS. 1, 3, and 4 is used.

At this time, even without the optical frequency filter 8, the Fresnel reflected light and the backward Rayleigh scattered light by the first signal light St having the frequency f do not enter the photodetector 5, and the signal light S2 having the frequency f2 does not enter the photodetector 5. only detected.

In the above description of the present invention, the first signal light St
The optical frequency f1 at the time of marking has been set to satisfy the relationship of equation (1). However, the first signal light 51 and the second signal light
Even when the position of the signal light S2 is changed and the first signal light S1 is Raman optically amplified by the second signal light S2, that is, when f2L=fR (11), all the measurements according to the present invention described so far are still the same. It is possible. However, at this time, the second signal light S2
loses optical power by Raman amplifying the first signal light S1, so the waveform of the second signal light S2 detected by the photodetector 5 is concave from the DC component, as shown in Fig. It becomes something of a shape.

Further, in the present invention described above, the second light source 2 is CW.
Although the second light source 2 has been described as one that emits light, the second light source 2 may be a modulated light source like the first light source 1, for example. However, in that case, the first light source 1
The signal light Sl (for example, optical pulse) and the second signal from the second light source 2
Since Raman light amplification occurs only at the position (say 20) where the signal light S2 (for example, a light pulse) meets the optical fiber 3 under test, this method It is suitable when it is desired to extract information from 2.) (for example, changes in loss, changes in component amounts, changes in temperature, changes in tension, etc.). Zo is the first signal light S1 and the second signal light S
By relatively controlling the emission time between 2 and 2, it is possible to set it to an arbitrary position.

[Effects of the Invention] As explained above, according to the present invention, the first signal light which is the modulated signal light propagating in the optical fiber to be measured;
By using the Raman light amplification effect between the second signal light and the second signal light propagating oppositely through the optical fiber under test, the characteristics of the optical fiber under test can be determined from the waveform change of the second signal light caused by the Raman light amplification effect. can be evaluated. Here, the signal level of the waveform change is about 100 to 1000 times larger than the signal level obtained by conventional OT [JR], and furthermore, the signal level obtained by the present invention is The attenuation rate corresponding to the loss (ie, loss) is half that of a conventional OTDR. Therefore, according to the present invention, (1) Even when a low-output light source such as a semiconductor laser is used, the SN ratio is much higher than that of the conventional OTDR, and the characteristic evaluation of optical fibers is highly accurate. Equipment is available.

(2) Since the optical frequencies of the first signal light and the second signal light to be measured are different, by using an optical frequency filter, the powerful Fresnel reflected light from the first signal light enters the photodetector. Diagnose with prevention.

(3) The measurement immediately after the connector connection point where a powerful Fresnel reflected light pulse occurs was previously an impossible area (dead zone), but in the present invention, such an impossible area does not exist, and the measurement immediately after the connector connection point Measurement is also possible.

(4) Correlation processing or measurement in the frequency domain is easily possible without the problem of linearity in the detection system.
The S/N ratio of the measurement signal can be significantly improved.

(5) The amount of waveform change of the second signal light measured by the present invention depends on the optical frequency difference between the first signal light and the second signal light, and the optical frequency difference is determined from the constituent components of the optical fiber. It becomes maximum when it matches the specific Raman frequency shift determined,
Furthermore, since the Raman gain is proportional to the amount of the component, it is possible to determine the component constituting or included in the optical fiber and to measure the amount of distribution of the component in the longitudinal direction of the optical fiber.

(6) The amount of waveform change of the second signal light measured by the present invention varies depending on the loss of the optical fiber, the tension applied to the optical fiber, the temperature, etc. Tension applied to optical fiber. It is possible to measure various quantities of temperature.

As can be seen from the above description, the present invention can be applied not only to evaluating the characteristics of optical fibers during optical fiber manufacturing, but also to distributed remote measurement using tension applied to optical fibers or temperature changes. However, it is extremely effective.

(7) Also, the Raman gain bandwidth is very wide (e.g. 2
00 cm-'), it is possible to measure the distribution of various quantities (loss, components, tension, temperature) in the longitudinal direction of the optical fiber with high-level resolution. (8) Furthermore, the Raman gain bandwidth is extremely wide. Therefore, the conditions regarding the frequency width of the oscillated light of the light source are very relaxed, and even a Fabry-Bello type semiconductor laser with a wide oscillated light frequency width can be used as a light source. It has a very narrow gain bandwidth (e.g. 100MH
z), this shows that Raman optical amplification is very advantageous in practice compared to Brillouin optical amplification, which requires a frequency-controlled oscillator mode oscillation laser.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing the configuration of Example 1 of the present invention. FIG. 2 is a waveform diagram showing an example of a waveform measured by the present invention. FIG. 3 is a configuration diagram showing the configuration of Example 2 of the present invention. Figure 4 is a configuration diagram showing the configuration of Embodiment 3 of the present invention, Figures 5 (8) to (C) are explanatory diagrams explaining the influence of polarization on the measurement waveform, and Figure 6 is an illustration of the optical fiber. Waveform diagrams showing the relationship between Raman frequency shift and Raman gain; Figures 7 (A) to (C) are waveform diagrams showing various modulation waveforms of the first signal light S1; Figure 8 is a waveform diagram showing the waveform of the multiplexer/demultiplexer using a dichroic mirror. FIG. 9 is a characteristic diagram showing the transmission characteristics of the dichroic mirror; FIG. 10 is a waveform diagram showing an example of the waveform measured by the present invention. 1... First light source, 2... Second light source, 3... Optical fiber to be measured, 4... Multiplexer/demultiplexer, 5... Photodetector, 6... Signal processing device 8... Optical frequency filter, 9... Polarization control device, 10... Dichroic mirror

Claims (1)

[Claims] 1) A first signal light in the form of modulated light from a first light source is made to enter the optical fiber to be measured, and a second signal light from a second light source is made to enter the optical fiber to be measured. propagate opposite to the first signal to cause a Raman light amplification effect between the first signal light and the second signal light, and the second signal light subjected to the Raman light amplification effect is of the optical fiber to be measured by detecting the detected second signal light and analyzing the temporal waveform change or amplitude and phase change of the second signal light caused by the Raman light amplification effect based on the detected second signal light. An optical fiber characteristic evaluation method characterized by evaluating characteristics. 2) Let the speeds of the first signal light and the second signal light in the optical fiber under test be v_1 and v_2, respectively, and the average speed v is 2/v=(1/v_1
)+(1/v_2), and the length of the optical fiber to be measured is L, first, a first signal light in the form of modulated light from the first light source is introduced into the optical fiber to be measured. The light is input from one first end of the optical fiber to be measured, and the second light from the second light source is
By inputting the signal light into the optical fiber to be measured from the other second terminal of the optical fiber to be measured, the second
Propagating the signal light in the optical fiber to be measured, facing the first signal light, to cause a Raman light amplification effect between the first signal light and the second signal light; The time when the signal light enters the first terminal of the optical fiber to be measured is t_i_n, and the first signal is transmitted from the first terminal of the optical fiber to a position a distance z apart along the optical fiber to be measured. The time at which the light propagates and the second signal light, which has been subjected to Raman light amplification by the first signal light at that position, reaches the first terminal of the optical fiber to be measured is t_o_u_t, and from time t_i_n to time t_o_u_t. The time until t
(=t_o_u_t-t_i_n=2z/v), detect the second signal light that has been subjected to the Raman light amplification effect and passed through the first terminal of the optical fiber to be measured, and inject the detected second signal light into Based on this, the temporal waveform change of the second signal light caused by the Raman light amplification effect is set as W_P_1 (vt/2), and then the first signal light in the form of modulated light from the first light source is determined as W_P_1 (vt/2). enters the optical fiber under test from the second terminal of the optical fiber under test, and a second signal light from the second light source enters the optical fiber under test from the first terminal of the optical fiber under test. By making the second signal light incident, the second signal light is propagated through the optical fiber under test, facing the first signal light, and Raman light amplification is performed between the first signal light and the second signal light. The time when the first signal light enters the second terminal of the optical fiber to be measured is t'_i_n, and the distance from the second terminal of the optical fiber to be measured along the optical fiber to be measured is z′
the time at which the first signal light propagates to a distant position, and the second signal light, which has been subjected to Raman light amplification by the first signal light at that position, reaches a second terminal of the optical fiber under test; is t'_o_u_t, and the time from time t'_i_n to time t'_o_u_t is t'(=t'_o_u_t-t'_i_n=2z'/
v), detecting the second signal light that has undergone the Raman light amplification action and passed through the second terminal of the optical fiber to be measured, and performing the Raman light amplification action based on the detected second signal light; The temporal waveform change of the second signal light caused by W_P_2(vt'/2) is defined as W_P_2(vt'/2), and the temporal waveform change W
_P_1 (vt/2) and the temporal waveform change W_P_
Variable conversion to 2 (vt'/2) (vt'/2 → L-vt/
2) Temporal waveform change W_P_2(L-vt/2
), W_P_1(vt/2)W_P_2(L-v
t/2), the distribution of the factors that change the Raman gain in the optical fiber under test in the longitudinal direction of the optical fiber under test is determined, and the W_P_1(vt/2) and the W_P_2(L-v
t/2), W_P_1(vt/2)/W_P_2(
2. The optical fiber characteristic evaluation method according to claim 1, wherein the loss of the optical fiber to be measured is determined from L-vt/2). 3) The optical fiber to be measured has a plurality of Raman frequency shifts f_R_N (n=1, 2, . . . ), and the first
By adjusting the optical frequency difference between the signal light and the second signal light to one of the Raman frequency shifts f_R_i, a Raman light amplification effect is caused between the first signal light and the second signal light; Detect the second signal light subjected to Raman light amplification, set the temporal waveform change of the detected second signal light as W_i, and then determine the optical frequencies of the first signal light and the second signal light. The difference is expressed as another one of the Raman frequency shifts f_R_j(
≠f_R_i), a Raman light amplification effect is caused between the first signal light and the second signal light, the second signal light subjected to the Raman light amplification effect is detected, and the detected second signal light is detected. The temporal waveform change of the second signal light is assumed to be W_j, and by analyzing the ratio of W_i and W_j of the temporal waveform change, the factors that change the Raman gain in the optical fiber under test can be determined. 2. The optical fiber characteristic evaluation method according to claim 1, wherein the longitudinal distribution is evaluated separately from the loss characteristics of the optical fiber to be measured. 4) a first light source that generates a first signal light in the form of modulated light; a second light source that emits a second signal light that propagates in the optical fiber to be measured in opposition to the first signal light; A multiplexing/demultiplexing means for inputting the first signal light into the optical fiber to be measured and extracting a second signal light subjected to a Raman light amplification effect generated between the first signal light and the second signal light. and a photodetection means for converting the second signal light subjected to the Raman light amplification effect from the multiplexing means into an electrical signal, and processing the temporal waveform or amplitude and phase of the signal detected by the photodetection means. 1. An optical fiber characteristic evaluation device comprising: a signal processing means for analyzing the characteristics of an optical fiber. 5) a first light source that generates a first signal light in the form of modulated light; a second light source that emits a second signal light that propagates in the optical fiber to be measured in opposition to the first signal light; A multiplexing/demultiplexing means for inputting the first signal light into the optical fiber to be measured and extracting a second signal light subjected to a Raman light amplification effect generated between the first signal light and the second signal light. and receiving the second signal light that has undergone the Raman light amplification effect from the multiplexing/demultiplexing means, and allows the optical frequency of the second signal light that has undergone the Raman light amplification effect to pass, but the optical frequency of the second signal light that has undergone the Raman light amplification effect is passed. an optical frequency filter that blocks optical frequencies; a photodetector that converts the output light from the optical frequency filter into an electrical signal; and a processor that processes the temporal waveform or amplitude and phase of the signal detected by the photodetector. 1. An optical fiber characteristic evaluation device comprising: signal processing means for analysis. 6) a first light source that generates a first signal light in the form of modulated light; a second light source that emits a second signal light that propagates in an optical fiber to be measured in opposition to the first signal light; A multiplexing/demultiplexing means for inputting the first signal light into the optical fiber to be measured and extracting a second signal light subjected to a Raman light amplification effect generated between the first signal light and the second signal light. and receiving the second signal light that has undergone the Raman light amplification effect from the multiplexing/demultiplexing means, and allows the optical frequency of the second signal light that has undergone the Raman light amplification effect to pass, but the optical frequency of the second signal light that has undergone the Raman light amplification effect is passed. an optical frequency filter that blocks optical frequencies; a photodetector that converts the output light from the optical frequency filter into an electrical signal; and a processor that processes the temporal waveform or amplitude and phase of the signal detected by the photodetector. An optical fiber characteristic evaluation device comprising: a signal processing means for analysis; and a means for changing the polarization state of at least one of the first signal light and the second signal light.