JPH03146851A - Method and apparatus for measuring double refractive index - Google Patents

Abstract

PURPOSE:To measure the double refractive indices over a wide range with high accuracy and to measure the distribution of the double refractive indices as well by measuring the magnitude of double refractions by utilizing the mixing of 4 light waves between the light waves propagating in opposition. CONSTITUTION:The light wave 1 which is the continuous wave from a pumping light source L1 is made incident to an optical fiber 2a via a lens l1. The light wave 2 which is the continuous wave from a pumping light source L2 is made incident to the optical fiber 2a via a lens l2. The light wave 1 and the light wave 2 propagate in opposition in the optical fiber 2a. The light wave 3' propagating in the same direction sa the light wave 1 and the light wave 4' propagating in the same direction as the light wave 2 are generated as a result of the mixing of the four light waves of the light wave 1 and the light wave 2 as pumping waves. The light wave 2 and the light wave 4' are converted to the electric signal oscillating at the frequency difference between the light wave 2 and the light wave 4' by a photodetector 4. This frequency is measured by a frequency selecting receiver 5 and is computed by a signal processor, by which the double refractive index is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method and apparatus for measuring the birefringence of a medium.

(Prior art) Conventionally, the birefringence of a medium has been measured using two methods:
ν, Ramaswamy et at,, Opt.
, vo118, no, 24. pp, 4080-4
084.1979), ■Method using POTDR (B
, Y, Ktm and S, Choi,
Electron.

Lett,, vol, 17. no, 5. pp, 1
93-194.1981), ■ Induction 4 between pump light, Stokes light, and anti-Stokes light propagating in the same direction
Method using light wave mixing (R, H, 5tolen et
al,, 0ptics Lett,, vol, 6+
- no, 5. pp. 213-215.1981) have been reported.

However, in the method (2), measurement becomes extremely difficult when the refractive index 71In is small. For example, assuming a birefringence index 1n = 10-6, which is typical for a normal single mode optical fiber, the interval between bright and dark Rayleigh scattered light becomes 1 m to 10 m. Method (2) is suitable when the test medium is long, such as an optical fiber. Using POTDR, it is also possible to measure the distribution of birefringence in the longitudinal direction.

POTDR is a method in which the received back Rayleigh scattered light temporally changes sinusoidally, and the birefringence index is determined from the time period.

Birefringence and time period are inversely proportional to each other, and An
=10-', 10-', the time periods are approximately 10 ns and ins, respectively. Therefore, in order to accurately measure the time period, it is necessary to measure the back Rayleigh scattered light with a time resolution of about 1/10 of the time period, that is, lns and 0.1 ns. It is technically very difficult to measure weak backward Rayleigh scattered light with such high temporal resolution.

■How to do it, Tess! - A method of measuring birefringence by injecting pump light into a medium and measuring the optical frequency difference between the pump light and Stokes light or anti-Stokes light generated by di0 guided four-wave mixing. The optical frequency difference is approximately 1
It is known that it is proportional to 77a(f!='if fit)-. Therefore, using the method (2), the chromatic dispersion is close to zero (for example, in the case of a silica optical fiber, 1
．． 3μm), the measurement error of chromatic dispersion is
This results in a large measurement error for the birefringence an. Furthermore, in that case, the optical frequency difference becomes a very large value, and the wavelength of the Stokes light or anti-Stokes light deviates from the wavelength region where the loss of the test medium is small. Therefore, it is actually impossible to measure Stokes light or anti-Stokes light. Furthermore, method (2) also has the disadvantage that it is not possible to measure the distribution of birefringence.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a method and device for measuring birefringence that can measure birefringence in a wide range with high precision and also measure the distribution of birefringence. I'm in the middle of the day.

(Means for Solving the Problems) The present invention measures the magnitude of birefringence by utilizing four-wave mixing between light waves propagating in opposite directions. In addition, by correlating the propagation time of a light wave generated, amplified, or attenuated by four-wave mixing at a certain point in the test medium from that point to the end of the test medium with the position in the test medium, the continuity of birefringence can be determined. measure the distribution of

Four-wave mixing is a nonlinear optical phenomenon via the real part of the third-order nonlinear constant of the medium. Now, assume that light waves 1 and 3 are propagating in the same direction in the medium, and light waves 2 and 4 are propagating in the opposite direction.

At this time, light wave 1 and light wave 2 were used as pump waves.

In order to efficiently generate four-wave mixing involving light waves 1, 2, 3, and 4, it is necessary to establish the following relationship between these light waves.

fl + rz = r3 + r4 (1) /I k = to 2- to 40 to 3-kl = O (2) where fl + f2. f3. f, is light wave 1, 2, 3
It represents an optical frequency of 4 degrees. Similarly, kl, k2+ k
3+ ka indicates the propagation constant of light waves 1, 2, and 3.4. Equations (1) and (2) are the law of conservation of energy,
Corresponds to the law of conservation of momentum. The momentum Zk in equation (2) can be separated into Akc, which is due to the chromatic dispersion of the medium, and Akb, which is due to the birefringence of the medium.

1=lkc+lkb (3)A
kc is approximately expressed by the following formula.

akC=4π(fz-f4)/v
(4) Here, ■ is the speed of light in the medium. On the other hand, akb
takes various values shown in Table 1 depending on the polarization state of each light wave.

Table 4 Table for explaining the combination of polarization of each light wave in which light wave mixing occurs. Here, s and f represent the slow axis and the fast axis, respectively, which are the principal axes of birefringence of the medium. Table 1 shows the formula (2
) to (4) and the optical frequency difference 1 f = f 2 fa determined using the lhb values shown in Table 1 are also shown. As will be seen, the birefringence An of the medium is 1, dn=2λl f2 f41 /v; combinations 1 to 1.
8 (5), Jn=4λ(f2 f41/v;
It can be obtained from combination 9.10 (6).

Here, λ is the average wavelength of light waves 1.2 and 3.4. As seen from equation (1), fz fa−fz
Since f+, the birefringence, dn, may be determined by measuring fx f+ instead of f2fa.

(Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

Fig. 1 is a diagram showing Embodiment 1 of the present invention, in which Ll is a first pump light source, pct is a polarization control device, ■ is a beam splitter, 11 is a lens, and 2 is a test medium. An optical fiber, L2 is a second pump light source, PO2 is a polarization control device, 12 is a lens, 3 is an optical frequency filter, 4 is a photodetector, 5 is a frequency selection receiver, and 6 is a signal processing device. To operate the apparatus shown in FIG. 1, the following steps may be taken.

A light wave l, which is a continuous wave, from a pump light source L1 is incident on an optical fiber 2 via a lens I11, which is an optical means 1 for guiding it into the test medium. Also, a lens is an optical means 2 for guiding the light wave 2, which is a continuous wave from the pump light source L2, to the test medium! 22 and enters the optical fiber 2. At this time, the light waves 1 and 2 are made to propagate in the optical fiber 2 in opposite directions, as shown in FIG.

At this time, as a result of four-wave mixing using light wave 1 and light wave 2 as pump waves, light wave 3' propagating in the same direction as light wave 1 and light wave 4' propagating in the same direction as light wave 2 are generated. light wave 1,
2. The optical frequencies of 3' and 4' are fl + fz, respectively.
f3+r4, and the propagation constants are + and kz, respectively.
, and 3+k4, between each light wave, equation (1) and equation (
The relationship 2) is established. Light waves 2 and 4' are guided to an optical frequency filter 3 via a beam splitter 1.

It is assumed that the optical frequency filter 3 passes only the optical frequencies of the optical waves 2 and 4' and blocks other optical frequencies. Thereby, reflected light or scattered light of the light wave 1 can be prevented from entering the photodetector 4 and becoming noise. In this way, the light waves 2 and 4' are converted by the photodetector 4 into electrical signals that oscillate at hfa, which is the frequency difference between the light waves 2 and 4'. The frequency of this electrical signal is measured by the frequency selection receiver 5, and the signal processing device 6 uses the formula (
5) Calculate equation (6) to find the birefringence of the optical fiber, !
! lln can be obtained. Note that equation (5), equation (
Which of 6) to use cannot be determined without measuring the polarization state of each light wave. However, by performing the above measurements on various combinations of polarization states between light wave 1 and light wave 2 using the polarization control devices PCI and PO2, signals at frequencies F and 2F can be detected. Here, In can be determined by substituting F into fz L in equation (5>).

Note that light wave 2 and light wave 4 are
When ' is propagated through an optical fiber and received by an optical receiver, in the case of combinations 1, 4, 5°, 8, 9, and 10 in Table 1, the beat electrical signals shown in Table 1 are not detected. Because combinations 1, 4, 5, 8゜9.
10, the polarizations of the light waves 2 and 4 are orthogonal and do not interfere with each other.

In the example explained above, by measuring the frequency of the beat electrical signal between light wave 2 and light wave 4', the frequency difference f2-f
However, it is also possible to measure this frequency difference optically. For example, when a high finesse swept Fabry-Perot interferometer is used as the optical frequency filter 3, the optical frequency difference fz f is determined from the difference in resonator length when light waves 2 and 4' pass through the Fabry-Perot interferometer.
a can be found.

An = 10-6 (corresponds to normal optical fiber), 1
For the case of 0-', 10-' (corresponding to polarization-maintaining optical fiber), from equation (5), the frequency difference fz fa
When the wavelength is λ-1μm, each is 100M
Hz.

ICl3, 10 GHz. These are easily measurable values.

Now, in the first embodiment described above, the birefringence distribution of the optical fiber cannot be measured. Below, we will explain how to make this possible.

FIG. 2 is a diagram showing a second embodiment of the present invention, in which 5 is a frequency selection receiver, 6 is a signal processing device, and 7 is an optical modulator. Here, the continuous wave from the light source L1 is intensity-modulated into pulses by the optical modulator 7. As in the case of Embodiment 1, the pulsed light wave 1 causes four-wave mixing with the light wave 2 propagating in the optical fiber oppositely, and the light wave 1
A light wave 3' propagating in the same direction as light wave 2 and a light wave 4' propagating in the same direction as light wave 2 are generated. The light waves 2 and 4' pass through an optical frequency filter 3 and are converted by a photodetector 4 into beat electric signals of both waves. Light wave 1 and light wave 3'
is blocked by the optical frequency filter 3. The beat electric signal formed by the light wave 2 and the light wave 4' is detected by a frequency selective receiver 5 capable of frequency selective reception, and the frequency of the electric signal as well as the time change in the intensity thereof are measured as a time waveform. These pieces of information are recorded in the signal processing device 6, and are subjected to averaging processing or the like to improve the signal-to-noise ratio, if necessary.

FIG. 3 shows a group of measurement signal waveforms when the above measurements were performed while changing the selected frequency in the frequency selection receiver 5. Now, let Afo be the value when the optical frequency difference f2-f satisfies equation (5). At this time, as shown in FIG. 3, a finite signal strength is obtained; otherwise, the signal strength is zero.

The time when the pulsed light wave l enters the optical fiber 2 is tl
shall be. The time when the light wave 1 reaches the other end of the optical fiber and the light wave 4' generated at the other end reaches the position where the light wave 1 entered the optical fiber 2 is defined as t2. The time difference 1. -12, the time is 2L/V. However, here L is the length of the optical fiber 2. Therefore, a signal waveform with finite signal strength is observed for a time of 2 L/v. The reason why the signal strength is attenuated over time is because the intensity of the light wave 4' generated at a certain position in the optical fiber depends on the intensity of the light wave 1, which is a pump wave, at that position.

FIG. 4 shows a group of measurement signal waveforms when the birefringence of the optical fiber 2 has a distribution, unlike the case of FIG. Here, the birefringence in the interval [21+22] is an
+, the birefringence in other sections is An2 (≠Anl)
It is assumed that Also, in equation (5), 1Jn=I
n, and the optical frequency difference fz when 1Jn=ln2
Let fa be Art and Ar1, respectively.

At this time, the selected frequency IJ in the frequency selection receiver 5
When r+, a finite signal strength can be obtained only at the time interval of [2z, /v, 2z2/v]. Also, when the selected frequency in the frequency selection receiver 5 is Arz, [0, 2z, /v] and [2zz/v, '2L/v]
A finite signal strength is obtained only during the time interval of . In this way, the magnitude of birefringence can be measured from the frequency at which the signal is observed. Furthermore, the position on the optical fiber where the birefringence occurs can be determined from the time at which the signal is observed.

In Example 1.2 described above, the light wave 1 which is the pump light
From the light wave 2, a light wave 3' and a light wave 4' are generated. In this case, the light wave 3' and the light 1 wave 4' are generated by growing from the noise light, so the generation efficiency is not high. It is possible to improve the efficiency by making a light wave that matches the frequency of at least one of the light waves 3' and 4' enter the optical fiber together with the pump light.

Hereinafter, such embodiments of the present invention will be described.

Implementation plate 1 FIG. 5 is a diagram showing Embodiment 3 of the present invention, in which L3 is a frequency-variable third light source, and PO2 is a polarization wave for controlling the polarization state of light wave 3 from light source L3. The control device 8 is a beam splitter for multiplexing the light waves 1 and 3.

As already explained in Examples 1 and 2, when light waves 1 and 2, which are pump lights, propagate oppositely in the test optical fiber 2, light waves 3' and 4' are generated by four-wave mixing. do. The light intensity of the generated light wave 4' is proportional to the product of the light intensities of light wave 1, light wave 2, and light wave 3'.

Therefore, by matching the frequency of light wave 2 of light wave 3 from light source L3 with the frequency of light wave 3', it is possible to obtain light wave 4'' with significantly greater light intensity than in the first and second embodiments. can.

The method of receiving light wave 4'' and light wave 2 is exactly the same as in the second embodiment.

In the above explanation, in order to obtain light wave 4'' with a large light intensity, the optical frequency of light wave 3 was changed to match the optimal value, but it is also possible to change the optical frequency of light wave 1 instead. is easily understood.

Figure 6 is a diagram showing Embodiment 4 of the present invention, with 9, 1
0.11 is a beam splitter, 12 is a photodetector, and 13 is a frequency counter. To operate the present invention, first, the optical frequency fI of the light wave 1 from the light source L1 and the optical frequency f2 of the light wave 2 from the light source L2 are fixed.

Next, while changing the optical frequency f3 of the light wave 3 from the light source L3, the photodetector 4 detects the temporal intensity change of the light wave 4'' generated by four-wave mixing and propagating in the same direction as the light wave 2. In this embodiment 4, instead of measuring the optical frequency difference between light wave 2 and light wave 4'', the optical frequency difference between light wave 1 and light wave 3, which is equivalent to that difference, is measured. Specifically, the light wave 1 from the light source LL that is partially split by the beam splitter 9 and the light wave 3 from the light source L3 that is partially split by the beam splitter 10 are
After the light waves are combined by a beam splinter 11 and converted into a beat electric signal by a photodetector, the frequency of the beat electric signal is measured by a frequency counter 13. The measured frequency is sent to the signal processing bag W6.

With this method, a large beat electrical signal is obtained, so
Compared to Examples 1 to 3, the frequency f+ can be determined with very high accuracy.
L=h f4 can be measured. Furthermore, in this method, there is no need to measure the beat electrical signals of the light waves 2 and 4'', so the optical filter 3 is configured to allow only the light waves 4'' to pass, and the light waves 4'' detected by the photodetector 4 are
It is sufficient to send a time-varying signal of intensity to the signal processing device 6.

In the above explanation, in order to generate light wave 4'' by four-wave mixing, the optical frequency of light wave 3 from light source L3 was changed, but instead, the optical frequency of light wave 1 from light source L1 was changed. As in the case of the third embodiment, it is possible to change .

In addition, in the above explanation of Embodiment 4, the occurrence of four-wave mixing is detected by measuring the light intensity of light wave 4''.However, when four-wave mixing occurs, light wave 4'' is generated simultaneously. , a decrease in the light intensity of light wave 1 and an increase in the light intensity of light wave 3 occur. Therefore, in order to detect that four-wave mixing has occurred, instead of measuring the light intensity of light wave 4', the change in the light intensity of light wave 1 or light wave 3 transmitted through optical fiber 2 is measured. Good too. However, at this time, it is not possible to measure the position in the optical fiber 2 where four-wave mixing occurs, so the birefringence distribution cannot be determined.

Husband I FIG. 7 is a diagram showing a fifth embodiment of the present invention.

L4 is a frequency variable fourth light source, PO2 is a polarization control device, and 14 is light wave 2 from light source L2 and light wave 4 from light source L4.
This is a beam splitter for multiplexing.

To operate the present invention, first, the optical frequency f of the light wave 1 from the light source L1 and the optical frequency f2 of the light wave 2 from the light source L2 are fixed. Next, when the light wave 4 is amplified or attenuated by four-wave mixing between the pulsed light wave 1 from the light source L1, the light wave 2, and the light wave 4 while changing the optical frequency f4 of the light wave 4 from the light source L4. optical frequency difference fz
Search for fa. At this time, the light waves 2 and 4 pass through an optical frequency filter 3 and are converted by a photodetector 4 into beat electric signals of both waves. This electrical signal is detected by a frequency selective receiver 5 capable of frequency selective reception, and the frequency of the electrical signal as well as the temporal change in its intensity are measured as a temporal waveform. This information is sent to the signal processing device 6
The signals are stored in the data and are subjected to averaging processing to improve the signal-to-noise ratio, if necessary.

In the above embodiment, the optical frequency of the light wave 4 is changed, but the optical frequency ft of the light wave 2 from the light source L2 may be changed.

FIG. 8 shows a group of measurement waveforms when the birefringence of the optical fiber 2 has a distribution, as in the case of FIG. 4. Here, the refractive index is an+ in the interval [zl, zz], and the birefringence in other intervals is Jnz(
≠an, ). Also, in equation (5), /
When Jn=, 6n, and /Jn=lJn2, 4
The optical frequency difference f2-14 caused by light wave mixing is Ar
1 and Arz. It is assumed that measurement is performed while changing the optical frequency difference 1'z fa by changing the optical frequency of at least one of the light waves 2 and 4.

In this case, the selected frequency in the frequency selection receiver 5 is set to z
When set to lr+, signal strength ■ when no four-wave mixing occurs. A signal strength greater than [2z1/v
, 222/V.

In addition, the selected frequency in the frequency selection receiver 5 is set to 1Urz.
Then, [0,2z+/v ] and [2z2
/v, 2L/v], a signal strength greater than Io can be obtained. Thus, the birefringence of an optical fiber is 1. can be measured from the frequency at which a signal larger than . Furthermore, the position on the optical fiber where the birefringence occurs can be determined from the time at which the signal is observed.

Although FIG. 8 shows only the case where light wave 4 is amplified by four-wave mixing, if it is attenuated, the signal strength when four-wave mixing occurs is I. The only difference from the above case is that it is smaller than , and everything else is the same as the above case.

The advantage of this embodiment over Embodiment 1.2 is that since the light intensities of light waves 2 and 4 are large, their beat frequencies can be measured with high precision.

Figure 9 shows a sixth embodiment of the present invention. In this example, light sources LL, L2 . L3. In order to efficiently cause four-light wave mixing involving light wave 1, light wave 2, light wave 3, and light wave 4 from L4 in the test optical fiber 2, the optical frequency of each light wave is controlled as follows.

That is, by changing the optical frequency of at least one of the light waves 2 and 4, the optical frequency difference fz
L is changed, and at the same time, light wave 1 and light wave 3
By changing the optical frequency 8 of at least one of the light waves (FIG. 9 shows the case of light wave 3), f 3 f l-f z
The optical frequency difference f3-f is changed while maintaining the relationship f4=lf. At this time, the relationship 2 (5) is satisfied, so if 1. df is the frequency shown in Table 1 (
±2K), the relationship of equation (6) is also satisfied, and four-wave mixing occurs efficiently. At this time, the light wave 4 increases or attenuates. The method of receiving light wave 4 affected by such four-wave mixing is the same as in the fifth embodiment. Note that the above f'a L-fz f
A specific method for changing 1Jf-42-r while maintaining the relationship a is as follows.

First, the light source L is partially split by the beam splitter 9.
1 and the light wave 3 from the light source L3 that has been partially split by the beam splitter 10 are combined by the beam splitter 11, and the combined light wave is converted into a beat electric signal by the photodetector 12. The frequency f3-f of the beat electric signal is measured by the frequency counter 13. The measured frequency is sent to the signal processing device 6. At the same time, the optical frequency difference fz f4 between the light waves 2 and 4 is also measured by the frequency selection receiver 5 and sent to the signal processing device 6. The signal processing device 6 sends an error signal representing the frequency error (f3-fl)-(f2-f4) to the light source L3. The light source 1.3 operates to satisfy equation (5), which is one condition for four-wave mixing, by controlling its own oscillation frequency f3 based on the error signal. While doing this, light source L2
By changing the optical frequency of at least one of the light waves from light a1. is obtained. Therefore, the birefringence distribution of the test optical fiber 2 can be obtained in exactly the same manner as in Example 5.

In the above explanation, the light wave 1 incident on the test optical fiber 2 is
are all pulses. However, light source L
The light wave 3 from 3 may be a pulse. Also, light wave 1
However, it is clear that the light wave 3 may also be a pulse synchronized with the pulse of the light wave 1. Furthermore, the light wave 2 or the light wave 4 may be a pulse. However, in the last case, four-wave mixing occurs only at the position where the pulse of light wave 1 and the pulse of light wave 2 or light wave 4 meet, so the birefringence distribution along the longitudinal direction of test optical fiber 2 is measured. In order to do this, it is necessary to perform measurements without changing the difference between the time when the pulse of light wave 1 enters the test optical fiber 2 and the time when the pulse of light wave 2 or 4 enters the last optical fiber 2. be.

Furthermore, in the embodiment described above, in order to measure the distribution of birefringence, the intensity of the light source L1 or the light source L3 is modulated in a pulsed manner, and the light wave 1 from the light source L1 is faced in the optical fiber. Propagating light wave 4', light wave 4'' or light wave 4
We measured the temporal intensity change due to the four-wave mixing effect. However, as a result of four-wave mixing, the intensity of light wave 4', light wave 4'', or light wave 4 changes, and at the same time the intensity of the light wave from light source 2 also changes. Therefore, light wave 4', light wave 4'', or light wave 4 Instead of measuring the intensity of the light wave 2, changes in the intensity of the light wave 2 may be measured. Since the light wave 2, which is the pump light, loses its own energy due to four-wave mixing, the light intensity of the light wave 2 when four-wave mixing occurs is smaller than the light intensity when four-wave mixing does not occur.

Furthermore, in the above description, the light wave 1 from the light source L1 was intensity-modulated by the optical modulator 7 to obtain pulses.

However, the modulation method may be frequency modulation instead of intensity modulation. However, in this case, the frequency of light wave 1 when the mark is off is selected so that the relationships of equations (5) and (6) do not hold between light wave 1, light wave 2, light wave 3, and light wave 4. It is necessary to keep it.

Further, in the embodiment described above, in order to efficiently cause four-wave mixing, polarization control devices 1.2.3 corresponding to the respective light waves of light wave I, light wave 2, light wave 3, and light wave 4 are used. ．． and 4 were used. However, as can be seen from Table 1, it is not necessary to measure all combinations of polarization states in order to efficiently cause four-wave mixing.

- As an example, consider the case of Example 3 shown in FIG. Focusing on combinations 3 and 7 in Table 1, the polarization of light wave 1 coincides with the fast axis of test optical fiber 2, and the polarization of light wave 3 coincides with the slow (s Ioiv) axis. When the polarization direction of the light wave 2 is the same, the light wave 4'' can be generated by four-wave mixing regardless of the polarization direction of the light wave 2. Therefore,
For example, light wave 1 and light wave 3 can be separated by a polarization beam splitter, etc.
The measurement can be performed by changing the direction of polarization of light waves 1 and 3 using only one polarization control device, while keeping the polarizations of light waves 1 and 3 orthogonal. . In general,
A polarization control device configured with a 1/2 wavelength plate, a 1/4 wavelength plate, etc. has wavelength dependence, but in the present invention, lt+
Since f, l<<l, as described above, one polarization control device can control polarization of two or more waves.

Although only the cases of polarization combination 3 and combination 7 in Example 3 have been described above, the same applies to other examples and other polarization combinations.

Furthermore, even if the axis of birefringence of the test optical fiber 2 changes in the longitudinal direction of the test optical fiber 2, the test light can be At all positions in the longitudinal direction of the fiber 2, measurements can be made with the influence of polarization on the efficiency of four-wave mixing reduced.

Further, although the above explanation has been made using an optical fiber as the test medium, it goes without saying that the present invention is applicable to other media as well.

(Effects of the Invention) As explained above, according to the present invention, the birefringence is determined from the optical frequency difference between the pump light and the light wave whose generated or signal intensity is amplified or attenuated due to the effect of four-wave mixing. be able to. Furthermore, the birefringence index and the position of the test medium can be correlated based on the time difference between when the light waves subjected to the four-wave mixing effect arrive at the end of the test medium. Generally, when strain (lateral pressure, bending, twisting, etc.) is applied to the test medium,
It is known that the birefringence of the test medium varies.

Therefore, the present invention can also be applied as a strain sensor. In particular, when optical fiber is used as the test medium, its low loss properties make it possible to perform strain sensing over very long distances.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing Embodiment 1 of the present invention, FIG. 2 is a diagram showing Embodiment 2 of the present invention, FIG. 3 is a diagram illustrating a group of measurement signal waveforms according to the present invention, and FIG. 4 is a diagram showing the present invention. 5 is a diagram illustrating a third embodiment of the present invention. FIG. 6 is a diagram illustrating a fourth embodiment of the present invention. FIG. 7 is a diagram illustrating a fifth embodiment of the present invention. 8 is a diagram illustrating a group of measurement signal waveforms according to the present invention, and FIG. 9 is a diagram showing a sixth embodiment of the present invention. Ll, L2. L3. L4...Light source PCI, PC2, PC3, PC4...Polarization control device L
8.9.10. IL 14...Beam splitter 2...Test optical fiber 3...Optical frequency filter 4.12...Photodetector 6...Signal processing device 13...Frequency counter 5...Frequency selection reception Machine 7... Optical modulator

Claims (1)

[Claims] 1. Two light waves generated as a result of four-wave mixing between light wave 1 and light wave 2, which propagate in the test medium in opposite directions to each other, are defined as light wave 3' and light wave 4', and light wave 2 and From the difference frequency between the optical frequency of the light wave 4' propagating in the same direction and the optical frequency of the light wave 2, or the optical frequency of the light wave 3' propagating in the same direction as the light wave 1 and the optical frequency of the light wave 1. A method for measuring birefringence, comprising measuring the birefringence of the test medium from the difference frequency. 2. As a result of four-wave mixing between light waves 1 and 3 that propagate in the test medium in the same direction and light wave 2 that propagates in the opposite direction to light wave 1, a light wave propagates in the same direction as light wave 2. 4'' occurs, and the optical frequency of the light wave 2 and the light wave 4
measuring the birefringence of the test medium from the difference frequency between the optical frequency of the light wave 1 and the optical frequency of the light wave 3 when the light wave 4 is generated; A method for measuring birefringence characterized by: 3. As a result of four-wave mixing between light wave 2 and light wave 4 that propagate in the test medium in the same direction and light wave 1 that propagates in the opposite direction to light wave 2, the optical signal intensity of light wave 4 is amplified or A method for measuring birefringence, comprising measuring the birefringence of the test medium from the difference frequency between the optical frequency of the light wave 2 and the optical frequency of the light wave 4 when attenuated. 4. As a result of four-wave mixing between light waves 1 and 3 that propagate in the test medium in the same direction, and light waves 2 and 4 that propagate in the opposite direction to light wave 1, the optical signal intensity of light wave 4 From the difference frequency between the optical frequency of the light wave 1 and the optical frequency of the light wave 3, or from the difference frequency between the optical frequency of the light wave 2 and the optical frequency of the light wave 4 when amplified or attenuated,
A method for measuring birefringence, comprising measuring the birefringence of the test medium. 5. In the method for measuring birefringence according to claim 1, 2, 3, or 4, at least one of the light waves 1 and 3 is intensity-modulated or frequency-modulated. , the light wave 4' or the light wave 4'' generated by the four-wave mixing, or the light wave 4 increased or attenuated by the four-wave mixing, or the temporal intensity of the light wave 2 contributing to the result of the four-wave mixing. A birefringence measuring method characterized by measuring the distribution of birefringence by detecting a change. 6. A light source L1, a light source L2, and a device for controlling the polarization of the light wave 1 from the light source L1. a polarization control device PC1; an optical means 1 for guiding the light wave 1 to a test medium; and a light source L2.
a polarization control device PC2 for controlling the polarization of a light wave 2 from , and optical means 2 for directing the light wave 2 in the opposite direction to the light wave 1 so that the light wave 2 propagates in the test medium; The light wave 1 and the light wave 2 cause a nonlinear interaction called four-wave mixing in the test medium, and a light wave 3' propagates in the same direction as the light wave 1 and a light wave 4' propagates in the same direction as the light wave 2. means for measuring the difference frequency between the optical frequency of the light wave 1 and the optical frequency of the light wave 3';
Alternatively, a birefringence measuring device comprising means for measuring a difference frequency between the optical frequency of the light wave 2 and the optical frequency of the light wave 4'. 7. Light source L1 and light source L3, at least one of which is frequency variable
, a light source L2, a polarization control device PC1 for controlling the polarization of the light wave 1 from the light source L1, an optical means 1 for guiding the light wave 1 to a test medium, and the light source L2
a polarization control device PC2 for controlling the polarization of the light wave 2 from the light wave 2; and optical means 2 for guiding the light wave 2 in the opposite direction to the light wave 1 so that the light wave 2 propagates in the test medium.
and a polarization control device PC3 for controlling the polarization of the light wave 3 from the light source L3, guiding the light wave 3 in the same direction as the light wave 1 so that the light wave 3 propagates in the test medium. an optical means 3; the light wave 1, the light wave 2 and the light wave 3 cause a nonlinear interaction called four-wave mixing in the test medium, and the optical frequency of the light wave 4'' generated as a result of the four-wave mixing; A means for measuring a difference frequency between the optical frequency of the light wave 2 and a means for measuring a difference frequency between the optical frequency of the light wave 1 and the optical frequency of the light wave 3, which is equivalent to the difference frequency. A birefringence measurement device characterized by: 8. a light source L1; a light source L2 and a light source L4, at least one of which is variable in frequency; and a polarization control device PC1 for controlling the polarization of the light wave 1 from the light source L1. , optical means 1 for guiding the light wave 1 into the test medium, and the light source L2.
a polarization control device PC2 for controlling the polarization of the light wave 2 from the light wave 2; and optical means 2 for guiding the light wave 2 in the opposite direction to the light wave 1 so that the light wave 2 propagates in the test medium.
a polarization control device PC4 for controlling the polarization of the light wave 4 from the light source L4; Optical means for guiding 4
and the light wave 1, the light wave 2, and the light wave 4 cause a nonlinear interaction called four-wave mixing in the test medium, and as a result of the four-wave mixing, the light wave 4 has an increased or attenuated signal intensity. 1. A birefringence measuring device comprising means for measuring a frequency and a difference frequency between the optical frequency of the light wave 2. 9. A light source L1 and a light source L3, at least one of which has a variable frequency; a light source L2 and a light source L4, at least one of which has a variable frequency; a polarization control device PC1 for controlling the polarization of the light wave 1 from the light source L1; optical means 1 for guiding the light wave 1 into the test medium; and the light source L2.
a polarization control device PC2 for controlling the polarization of the light wave 2 from the light wave 2; and optical means 2 for guiding the light wave 2 in the opposite direction to the light wave 1 so that the light wave 2 propagates in the test medium.
and a polarization control device PC3 for controlling the polarization of the light wave 3 from the light source L3, guiding the light wave 3 in the same direction as the light wave 1 so that the light wave 3 propagates in the test medium. an optical means 3; a polarization control device PC4 for controlling the polarization of the light wave 4 from the light source L4; and a polarization control device PC4 for controlling the polarization of the light wave 4 from the light source L4; Optical means 4 for guiding
and the light wave 1, the light wave 2, the light wave 3, and the light wave 4 cause a nonlinear interaction called four-wave mixing in the test medium, and as a result of the four-wave mixing, the light wave 4 is amplified or attenuated. means for measuring the difference frequency between the optical frequency and the optical frequency of the light wave 2, or means for measuring the difference frequency between the optical frequency of the light wave 1 and the optical frequency of the light wave 3, which is equivalent to the difference frequency; A birefringence measuring device characterized by comprising: 10. In the birefringence measuring device according to claim 6, 7, 8, or 9, the emitted light from at least one of the light sources L1 and L3 is intensity-modulated or a modulation means for frequency modulating; and the light wave 4' or the light wave 4'' generated by the four-wave mixing, or the light wave 4 amplified or attenuated by the four-wave mixing, or the light wave 2 involved in the result of the four-wave mixing. 1. A birefringence measurement device, comprising: means for detecting temporal intensity changes.