JPS63132131A - Evaluating device for mode coupling - Google Patents

Abstract

PURPOSE:To detect an interference signal with both propagation mode components and to measure the lengthwise distribution of the degree of mode coupling quantitatively with high resolution by splitting coherent light into two, transmitting those split light beams to optical paths set to a necessary optical path difference, and multiplexing them and making the composite light incident on a transmission line to be measured which has two propagation modes. CONSTITUTION:The coherent light from a light source 1 is split into two light waves having a constant frequency difference by a frequency shifting device 5. One of the split light waves is made incident on a half-mirror 11 through a phase shifter 7 and a polarizer 8. Further, the other light wave is made incident on the half-mirror 1 in different mode through a reflecting mirror 6 and a polarizer 9, and the half-mirror 11 multiplexes both light waves. This composite light wave is made incident on a birefringent fiber 12 to be measured and propagated in different modes HE11X and HE11Y. Then, one propagation mode component is extracted by an analyzer 13 after being propagated in a fiber 12, and the function of the optical path length difference of the amplitude of the interference signal is recorded in a waveform recording device 16.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a mode in which the position and degree of coupling between two propagation modes in an optical transmission path can be measured with high resolution. This invention relates to a combination evaluation device.

(Prior Art) Conventionally, as a device for evaluating power coupling between propagation modes in a transmission path, there is a device that evaluates the strength of a propagation signal at the end of the transmission path. However, in this conventional device, the amount of power coupling is integrated over the length of the transmission path, so the dependence in the longitudinal direction of the transmission channel cannot be obtained and power coupling cannot be appropriately evaluated.

Another conventional example involves injecting a light pulse into a transmission line in order to determine the dependence in the longitudinal direction, and evaluating the mode coupling from the backscattered light that propagates in the opposite direction that occurs in the transmission line. Yes (M, Nakazawa
etal, “M 6aSurelentS 0f
polariZation mode coupt
ings along p.

Iarization-11atainingSi
ngl-mode optical fib
ers”, J, opt, SOc, Am, A.

vat-1, pp285-292.1984).

However, in a device that uses this backscattered light,
First, the intensity of the backscattered light is 30 to 4 compared to the intensity of the incident light.
It will drop by 0dB. Therefore, it is necessary to increase the intensity of the incident light. Further, since a light pulse is used as the incident light, the resolution in the longitudinal direction is about the same as the pulse width, and as a typical numerical example, when the pulse width is 1 μs, it is about 100 m.

In addition, the pulse height (intensity) is required to be several kW due to the light receiving sensitivity limit of the photodetector, and the extinction ratio of the optical fiber to be measured is limited to 20 ciB/kmli degrees or less.
The dynamic range with respect to the degree of mode coupling becomes narrow.

Furthermore, when the intensity of the optical pulse exceeds several kW, stimulated Raman scattering and Prillouin scattering are induced in the transmission path, and these scattered lights are detected as noise. Therefore, correction is required.

(Problems to be solved by the invention) In the conventional example in which the strength of the propagation signal is evaluated at the end of the transmission line, the dependence in the longitudinal direction of the transmission line cannot be obtained, and mode coupling cannot be properly performed with high resolution. There was a problem that it could not be evaluated.

In addition, in the conventional example in which a light pulse is used as the incident light and mode coupling is evaluated from the backscattered light, the intensity of the backscattered light decreases, so the intensity of the incident light is reduced to several k.
There were problems in that it was necessary to increase the distance above W, the distance resolution was low, and the dynamic range with respect to the degree of mode coupling was narrow.

This invention was made based on the above circumstances, and it is possible to quantitatively measure the distribution of the degree of mode coupling in the longitudinal direction with high resolution, and it is possible to achieve a mode coupling that allows a large dynamic range for the degree of mode coupling. The purpose is to provide an evaluation device.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention includes a light source that emits coherent light, and a light source that emits coherent light, and a light source that emits light that is coherent.
A demultiplexing means for splitting, a means for variably setting a required optical path length difference between the two demultiplexed lights, and a means for combining the two lights transmitted through each optical path set to the required optical path length difference. means for making the light incident on a transmission line under test having two propagation modes; means for extracting light transmitted in one of the two propagation modes in the transmission line under test; and means for photoelectrically converting the extracted light. and means for detecting the amplitude of the interference signal interfering in the transmission path under test as a function of the optical path length difference.

(Operation) Coherent light emitted from a light source is split into two, and after the split lights are transmitted through each optical path set to the required optical path length difference, they are combined into two light beams. The signal is input to a transmission line under test having a propagation mode.

The multiplexed light is transmitted through the transmission line under test in each propagation mode. Then, of the transmitted light, only the light transmitted in one propagation mode is extracted and photoelectrically converted, and the amplitude of the interference signal that interfered in the transmission path to be measured is detected as a function of the optical path length.

Using the light that has passed through the transmission path under test in this way, interference signals due to both propagation mode components are detected, and the degree of mode coupling is detected temporally separated by the group delay time difference between the two propagation modes. Ru.

In this way, the longitudinal distribution of the degree of mode coupling is measured constantly with high resolution, and the dynamic range of the degree of mode coupling is expanded.

(Embodiment) First, an embodiment of the present invention will be conceptually explained using FIG.

In FIG. 1, 1 is a light source that emits coherent light waves;
2 is a variable optical path length device, 3 is a transmission line (waveguide), and 4 is a photodetector.

The coherent light waves emitted from the light source 1 enter the transmission line 3 and are propagated as modes 1 and 2. At this time, due to the variable optical path length bag ff2, the difference between the optical path lengths of mode 1 and mode 2 becomes ΔL as described later.
It has been adjusted to suit.

Now, if the distance of the transmission line 3 is divided into N and the coupling coefficient between both modes at the position of the distance zi is hi, then the photodetector 4
The time average light intensity Is detected in can be expressed as follows.

IS-ku1φ1212>+<lφ22+2>+2Re(
<φ12・φ” 22 >) =11)φ+2(Z,
t)-Σ (hi/(1-ht)) days (1-hk
) ・fA〈ω〉expj((βweight−β2)z+β2L−ω
t) (lω...(2)φ22 (Z,
t) -mouth <1-hk)・fA(ω)e
xpj −(β 2L+kOΔ l−ω t)d ω...ku3) Here, φ12 and φ22 are 1! which has been mode-converted from mode 1 to mode 2, respectively. represents the electric field propagated without transformation in field and mode 2, and Re represents its real part. β1 and R2 are propagation constants in mode 1 and mode 2, kO is the wave number of light in free space, and ΔL is the optical path difference given by the variable optical path length device 2.

Further, A(ω) is the amplitude spectrum of the light source 1, and the oscillation power spectrum S(ω)(-A(ω)・A(
As a general approximation, ω)1) can be expressed as a Gaussian distribution with half width Δω, S(ω)−(1/rTΔω)exp ・[−(ω−ωo)2/2Δω2] ・・・(4) I'm assuming. (4) In the formula, ■ is the light intensity of light source 1, ω
0 is the oscillation center frequency.

At this time, the interference term, which is the third term in equation (1), is calculated as shown in the following equation.

2Re(<φ12・φ22 ° 〉] -F7rl Σ (h i/(1-h i))N
N ・ 口 (1-hk) 口 (1-h
fL) expk=1 1-1 ・(-Δω2 R2/2)CosR...(5) R-(β1-R2)Zi+k(1ΔL-(6)
From equations (5) and (6) above, the amplitude of the interference signal is R-0
The maximum value is taken at . That is, for the optical path where power coupling occurs at position Zi, ΔL −−(β 2− β + )Z i
The interference intensity is maximum when /ko - (7) is satisfied.

In this case, place 1! The interference intensity for Zt is 1/ of the maximum value.
Half width Δ2 of position Zi at e (coherence distance 111 of light source 1
From equations (5) and (6), 1[) is ΔZ-2/(β1-
R2) Δω2 ... is given by (3). Therefore, the resolution for the position Z is approximately Δ2.

Equation (8) above is expressed as the coherence time τC of light source 1 plus 2π/Δ
It can be expressed as the product of ω and the group velocity difference δVg-(d(β1-R2)/dω)t between both modes.

Next, in order to normalize the value of the coupling coefficient hi, if settings are made so that the light beam emitted from the light source 1 and split into two is propagated in the same mode, the above equation (2) becomes (3 )
Similar to the formula, it becomes:

φ +2 (Z, t) -mouth (1-hk)
-fA (ω)expj (R2L-ωt) dω・・
・(9) As a result, the interference signal (optical beat signal) is 2Re×φ12
φφ22">- J-τrfu τ (1-hk)2 becomes -1 ・ Icos (ko ΔL )-(10). If the ratio of equation (5) and the above equation (10) is 1,
11q is as shown in the following equation.

IR-Σ (h i / (1-h i ) ) exp
・(-Δω2R'/2)...(11) Considering the mode coupling per Δ2 given by the above equation (8), the above (1
Equation 1) can be approximated by the following equation.

IRt-hi/(1-hi)...0■ From the formula, the coupling coefficient hi is given by hi-11/(1+ll2i)...■.

Furthermore, the extinction ratio η between propagation modes is defined as the integration of the power-converted coupling coefficient with respect to the transmission path length, and is defined as η=f h2dz (to). η is η−Σ h
i2... (15) Here, N-L/Δ2 can be calculated.

Next, one embodiment of the present invention will be described based on FIGS. 2 and 3.

Note that in FIG. 2 and FIG. 4, which will be described later, the same or equivalent components as those in FIG.

First, to explain the configuration, in FIG. 2, 5 is a frequency shift device, and this frequency shift device 5 is used to split the coherent light from the light source 1 into two light waves that differ by a certain frequency difference. A demultiplexing means is configured.

As the optical path length variable device 2, for example, one constructed of a movable reflecting mirror 20 and two fixed reflecting mirrors 21 as shown in FIG. 3 is used.

6 is a reflecting mirror, 7 is a retarder, 8.9 is a polarizer, and 11 is a semi-transparent mirror. The semi-transparent mirror 11 constitutes means for recombining the two separated and polarized light waves.

12 is an optical fiber to be measured (transmission line to be measured) having two propagation modes (birefringence); 13 is an analyzer; the analyzer 13 detects the optical component of only one of the two propagation modes; A means for taking out is constructed. 14 is an intermediate frequency filter (band pass filter), 15 is an amplifier, 16 is a waveform writing device, and the intermediate frequency filter 14
The waveform recording device 116 constitutes means for detecting the amplitude of the interference signal as a function of the optical path length difference.

Next, the action will be explained.

Coherent light emitted from the light source 1 passes through the frequency shift device 5 and is split into two light waves that differ by a constant frequency difference Δν.

One of the split light waves passes through the retarder 7 and polarizer 8 and enters the semi-transparent mirror 11, and the other light wave passes through the variable optical path length device 2, the reflecting mirror 6 and the polarizer 9, and enters the semi-transparent mirror 11. !11
is incident on the Then, it is divided into two polarized light beams.
The two light waves are multiplexed again by the semi-transparent mirror 11 and are input to the optical fiber to be measured 12 having birefringence.

At this time, the polarization directions of the respective divided light waves are set to be orthogonal to each other and coincide with the birefringence axis of the optical fiber 12 to be measured.

The two light waves propagate in the optical fiber 12 to be measured as HEfi and HEζ modes, respectively, and after exiting the optical fiber 12 to be measured, only the component in the direction of one birefringence axis is extracted by the analyzer 13. It is photoelectrically detected by the photodetector 4.

At this time, H generated in the optical fiber 12 to be measured
Depending on the degree of power coupling between the E ++ mode and the II E n mode, it is possible to obtain the interference intensity corresponding to the position 2 on the optical fiber 12 to be measured, as explained using FIG. 1 above.

However, in this embodiment, since a constant angular frequency difference Δωb (Δωb(Δω)) is given between the two interfering light waves, the above equation (5) becomes as follows.

2Re(<φ12 'φ22@>) N N;□Σ(
hi/(1-hi)) i = 1 = 1 ・ (1-hk) e −”cos Δωbt+
ko Δ L ) (16) That is, the interference signal is heterodyne detected at the frequency Δν, and its amplitude value is related to the degree of mode coupling at the position aZ.

Next, FIG. 4 shows an embodiment of the pond of this invention.

This embodiment allows evaluation of changes in the degree of mode coupling relative to distance dependence.

In this embodiment, the retarder and polarizer in the embodiment (FIG. 2) described above are omitted, and a mode filter 17 is provided in place of the analyzer.

The coherent light wave emitted from the light WA1 passes through the frequency shift device 5 and is split into two different light waves with a constant frequency difference Δν.

One of the split light waves is directly incident on the semi-transparent 11111, and the other light wave is incident on the optical path length variable device 2 and the semi-transparent 13j.
The light passes through It 6 and enters the semi-transparent mirror 11 . The two divided light waves are then combined again by the semi-transparent mirror 11 and input into the optical fiber 12 to be measured.

The two light waves each travel through the optical fiber 12 under test.
propagates as Po+ mode and LP11 mode,
After exiting the optical fiber 12 to be measured, only one mode of light passes through the mode filter 17 and is photoelectrically detected by the photodetector 4.

The interference signal detected by the photodetector 4 is expressed by the following equation, similar to equation (16) above.

2Re (<φ12・φ22”>) N i=1 ;E77”CΣhi/(1-hi) -1 −(1-hk)e −'cos ・(Δωb t + k(1ΔL)・...(17) Here, C is a deterioration coefficient of interference efficiency based on the difference in spatial and temporal radio wave distribution between LPo+ mode and LPu mode, and since its value is constant, the degree of mode coupling depends on distance. Relative changes can be observed with respect to gender.

Next, FIGS. 5A to 5D show specific examples of measurement results based on the measurement system of FIG. 4 described above.

The optical fiber 12 to be measured has a total length of 100 m, and the spectral width of the light source 1 used is approximately 500 GHz.

(A) to (C) in FIG. 5 are the amplitudes of the interference signal (optical beat signal) expressed by the above formula (η), the horizontal axis is the optical path length difference ΔL, and the illustrated example is 8゜19mm/din
It is. Further, the vertical axis is the amplitude of the optical beat signal and is a linear scale.

First, (A) in FIG. 5 shows the ΔL dependence of the beat amplitude when the two light waves in FIG. 1 (Reference: N.

Shibata eta+ ti Measur
ellents of Polarisat
ion Mode oispers+on
bY 0pttcal' Heterodyne
Detection”, E 1ectron
1ett, 20. ppio55~1057.19
84>.

FIG. 5(B) shows the dependence of the beat imaging width on ΔL, and the vertical axis is smaller by 25 dB compared to FIG. 5(A). The two peaks visible at both left and right ends are due to a mismatch between the birefringence axis of the optical fiber 12 to be measured and the polarization direction of the incident light when the optical fiber 12 to be measured is excited. Corresponds to the position. In the example shown, the peak-to-peak distance is 39.0 mm and the group delay difference δτQ is 13 CI.
Corresponds to S.

The amplitude level between the peaks in (8) of FIG. 5 is -35 dB with respect to the peak level in (A) of FIG.
From this, it can be seen that the degree of coupling in the longitudinal direction of the optical fiber 12 to be measured is approximately constant.

The distance resolution in this measurement is given by the above equation (8), and the actual EL value τc (=0.3 pS) and the group velocity difference δV
From g (=L/δτg), the distance resolution ΔZ is approximately 0.2 m
becomes.

(C) and (D) in FIG. 5 are 36 mm from the input end of the optical fiber 12 under test under the same conditions as in FIG. 5 (A).
At the positions of m and 37 m, the optical fiber to be measured 12
The beat amplitude is shown when external pressure is applied to cause mode coupling. Two peaks indicating the external pressure application area can be seen in the center of FIG. 5(C). FIG. 5(D) is an enlarged horizontal axis view of these two peak portions. These 2
From the height of two peaks, the mode coupling coefficient is 1-. Approximately 1×10″5 per 20 cm was obtained as 2 (in terms of electric power).

[Effects of the Invention] As explained above, according to the configuration of the present invention, only the light transmitted in one propagation mode among the light transmitted in the two propagation modes through the transmission line under test is extracted. The amplitude of the interference signal that has been photoelectrically converted and interfered in the transmission line to be measured is detected as a function of the optical path length difference set by the optical path length difference setting means. In this way, the interference signal due to both propagation mode components is detected using the light transmitted through the transmission line under test, and the degree of mode coupling is detected temporally separated by the group delay time difference between the two propagation modes. There is an advantage that the longitudinal distribution of the degree of mode coupling can be quantitatively measured with high resolution, and the dynamic range of the degree of mode coupling can be expanded.

[Brief explanation of the drawing]

Fig. 1 is a block diagram for conceptually explaining a mode coupling evaluation device according to the present invention, Fig. 2 is a block diagram showing an embodiment of the invention, and Fig. 3 is an optical path applied to the above embodiment. FIG. 4 is a block diagram showing another embodiment of the present invention, and FIG. 5 is a diagram showing an example of the measured value of the beat amplitude with respect to the optical path length difference obtained in the above embodiment. be. 1: Light source, 2: Optical path length variable device, 4: Photodetector, 5: Frequency shift device (demultiplexing means), 8.9: Polarizer, 11: Semi-transparent mirror (combining means), 12: Target Measurement optical fiber (transmission line to be measured), 13: Analyzer (means for extracting one propagation mode component), 14: Intermediate frequency filter, 16: Together with the intermediate frequency filter, detect the amplitude of the interference signal as a function of the optical path length difference. Waveform recording g constituting the means
17: Mode filter (means for extracting one propagation mode component). Agent Patent Attorney Yasuo Miyoshi II H
Optical path length; l! "/k. 5th C (A) Light 1 length 1! R/k. Figure 5 ([3) Optical path length difference 〃. Figure 5 (C) Optical path length M k. Figure 5 (D) Procedure Authorized (spontaneous) 1. Indication of the case Japanese Patent Application No. 61-276850 2. Name of the invention Mode combination evaluation device 3. Person making the amendment Relationship with the case Patent applicant address (residence) Within Chiyoda-ku, Tokyo Sachi * IT No. 1-6 Name (422) Nippon Telegraph and Telephone Corporation Representative Tsune Shinfuji 4, Agent address 1-2-3 Toranomon, Minato-ku, Tokyo 105
No. 7・ζτ, 1, ++'1. ? , 5, Subject of amendment (1) Specification (2) Drawing 6, Contents of amendment (+> The entire text of the specification is amended as shown in the attached sheet. (2) (A> to Amend (D) as in the attached sheet. Above description 1, title of the invention: mode coupling evaluation device 2, claims: a light source that emits coherent light, and a light source that emits light that is coherent; A demultiplexing means for dividing into two, a means for variably setting the required optical B length difference between the two demultiplexed lights, and a means for combining the two lights transmitted through each optical path set to the required optical path length difference. means for inputting the light into a transmission line under test having two propagation modes; means for extracting the light transmitted in one of the two propagation modes in the transmission line under test; 3. Detailed description of the invention [Purpose of the invention] (Industrial Field of Application) The present invention relates to a mode coupling evaluation device that can measure the position and degree of coupling between second propagation modes in an optical transmission path with high resolution. (Prior Art) Conventionally, transmission There is a device that evaluates the power coupling between propagation modes in a transmission path, which evaluates the strength of the propagation signal at the end of the transmission path.However, in this conventional device, the amount of power coupling is integrated over the length of the transmission path. As a result, the dependence in the longitudinal direction of the transmission path could not be obtained and the power coupling could not be evaluated appropriately.As another conventional example, optical pulses were used to determine the dependence in the longitudinal direction. There is a method that evaluates mode coupling from backscattered light that is incident on the transmission path and propagates in the opposite direction that occurs in the transmission path (M, Nakazaw
a etal, ” Measurellen
ts of polarization mode
couplings atongp. Iarization-Ilaintaining
s ingl -1ode 0ptic
al fibers”, J, 0f)t
, SOc, Am, A. vat-1, pp285-292.1984). However, in a device that uses this backscattered light,
First, the intensity of the backscattered light is 30 to 4 compared to the intensity of the incident light.
It will drop by 0dB. It becomes necessary to increase the intensity of this diagonal incident light. In addition, the resolution in the longitudinal direction using a light pulse as the incident light is approximately the same as the pulse width, and as a typical numerical example, when the pulse width is 1 μs, it is approximately 100 rn
It becomes. In addition, the pulse height (intensity) is required to be about several kW due to the light receiving sensitivity limit of the photodetector, and the extinction ratio of the optical fiber to be measured is limited to about 20 dB/km or less, and mode coupling is required. The dynamic range becomes narrower depending on the degree of . Furthermore, when the intensity of the optical pulse exceeds several kW, stimulated Raman scattering and Prillouin scattering are induced in the transmission path, and these scattered lights are detected as noise. Therefore, correction is required. (Problems to be Solved by the Invention) In the conventional example in which the strength of the propagation signal is evaluated at the end of the transmission line, the dependence in the longitudinal direction of the transmission line cannot be obtained, and it is difficult to properly perform mode coupling with high resolution. There was a problem that it could not be evaluated. In addition, in the conventional example in which a light pulse is used as the incident light and mode coupling is evaluated from the backscattered light, the intensity of the backscattered light decreases, so the intensity of the incident light is reduced to several k.
There were problems in that it was necessary to increase the distance above W, the distance resolution was low, and the dynamic range with respect to the degree of mode coupling was narrow. This invention was made based on the above circumstances, and it is possible to quantitatively measure the distribution of the degree of mode coupling in the longitudinal direction with high resolution, and it is possible to achieve a mode coupling that allows a large dynamic range for the degree of mode coupling. The purpose is to provide an evaluation device. [Configuration of the Invention (Means for Solving the Problems) In order to solve the above problems, the present invention includes a light source that emits coherent light, and a light source that emits coherent light, and a method that combines the light emitted from the light source into two.
A demultiplexing means for splitting, a means for variably setting a required optical path length difference between the two demultiplexed lights, and a means for combining the two lights transmitted through each optical path without setting the required optical path length difference. means for making the light incident on a transmission line under test having two propagation modes; means for extracting light transmitted in one of the two propagation modes in the transmission line under test; and means for photoelectrically converting the extracted light. The object of the present invention is to include means for detecting the amplitude of the interference signal of the persimmon light as a function of the optical path length difference. (Operation) Coherent light emitted from a light source is split into two, and after the split lights are transmitted through each optical path set to the required optical path length difference, they are combined into two light beams. The signal is input to a transmission line under test having a propagation mode. The multiplexed light is transmitted through the transmission line under test in each propagation mode. Then, of the transmitted light, only the light transmitted in one propagation mode is extracted and photoelectrically converted, and the amplitude of the interference signal that interfered in the transmission path to be measured is detected as a function of the optical path length. Using the light that has passed through the transmission path under test in this way, interference signals due to both propagation mode components are detected, and the degree of mode coupling is detected temporally separated by the group delay time difference between the two propagation modes. Ru. Thus, the longitudinal distribution of the degree of mode coupling is quantitatively measured with high resolution, and the dynamic range of the degree of mode coupling is expanded. (Embodiment) First, an embodiment of the present invention will be conceptually explained using FIG. In FIG. 1, 1 is a light source that emits coherent light waves;
2 is a variable optical path length device, 3 is a transmission line (waveguide), and 4 is a photodetector. The coherent light waves emitted from the light source 1 enter the transmission line 3 and are propagated as modes 1 and 2. At this time, the optical path length variable device 2 controls mode 1.
The difference between the optical path lengths of mode 2 and mode 2 is adjusted to be ΔL, as will be described later. Now, if the distance of the transmission line 3 is divided into N and the power coupling coefficient between both modes at the position of the distance Zi is hi, then the time average light intensity Is detected by the photodetector 4 can be expressed as follows. be able to. Is-ku1φ1212>+<lφ22+2>+2Re
(<φ12 'φ” 22 > ) −(1) f(:
A (ω) expJ ((βjiβ2)Zi+β2L−
ωtldω ... (2) ... (3) Here, ω is the angular frequency of light, and φ12 and φn are the electric field converted from mode 1 to mode 2 and the electric field propagated without conversion from mode 2, respectively. and Re represents its real part. β1 and β2 are propagation constants in mode 1 and mode 2, C is the speed of light in free space, and ΔL is the optical path difference given by the optical FI & length variable device. A(ω) is the amplitude spectrum of the light source 1, and the oscillation power spectrum S(ω) of the light source 1 is <A(ω)・A*
As a general approximation, (ω)〉〉 is a Gaussian distribution with a half width Δω (ω)= (Io /F11Δω)−e xp (
-((A)-(+)O) 2/2Δω2]...(4) It is assumed that. (4) In the formula, IO is the light intensity of light source 1,
ω0 is the oscillation center frequency. At this time, the amplitude I of the interference term, which is the third term in equation (1), is
is calculated as follows. -exp(-Δ(L)2Ri2/2)...(
5) ...(6) From equations (5) and (6) above, the amplitude of the interference signal is Ri
The maximum value is taken at −〇. That is, for an optical path where power coupling occurs at position 2 Z i , the interference intensity becomes maximum when the following is satisfied. In this case, the position where the interference intensity takes the maximum value and the point where the maximum value is 1
The difference ΔZ (coherence distance of light source 1 from the position where / e is
and hi can be considered as a coupling coefficient around 2△Z. Equation (8) above is expressed as the coherence time of light source 1 τc=r7Δ
It can be expressed as a product of ω14. Next, in order to normalize the coupling coefficient hiO value, if the light beam emitted from light source 1 and split into two is set so that both propagate in the same mode, the above equation (2) becomes (3)
Similar to the formula, it becomes: As a result, the interference signal becomes . , exp(−Δ(&)2 Ri 2/2 > −(I
I) The interval between the positions Zi where mode coupling occurs is resolved,
When the function ΔZ is sufficiently larger than the function ΔZ, the IR at the position Zi can be approximated by the following equation. IRi= hi 1 hi ・
...0 From formula (12), the degree of coupling coefficient is 11 for h
i = I Ri 2/ (1+ I R12)
・・・It is given by ■. Furthermore, since the extinction ratio η between two propagation modes is defined as the value obtained by integrating the coupling coefficient converted into power with respect to the length of the transmission path, η can be approximated as N=L/2ΔZ. Next, one embodiment of the present invention will be described based on FIGS. 2 and 3. Note that in FIG. 2 and FIG. 4, which will be described later, the same or equivalent components as those in FIG. First, to explain the configuration, in FIG. 2, 5 is a frequency shift device, and this frequency shift device 5 is used to split the coherent light from the light source 1 into two light waves that differ by a certain frequency difference. A demultiplexing means is configured. As the optical path length variable device 2, for example, a movable reflection device as shown in FIG. f! 20 and two fixed reflecting mirrors 21 is used. 6 is a reflecting mirror, 7 is a retarder, 8.9 is a polarizer, and 11 is a semi-transparent mirror. The semi-transparent mirror 11 constitutes means for recombining the two demultiplexed and unpolarized light waves. 12 is an optical fiber to be measured (transmission line to be measured) having two propagation modes; 13 is an analyzer; the analyzer 13 constitutes a means for extracting a light component of only one of the two propagation modes; be done. 14 is an intermediate frequency filter (band pass filter), 15 is an amplifier, and 16 is a waveform recording device, and means for detecting (detecting) the amplitude of the interference signal as a function of the optical path length difference by the intermediate frequency filter 14 and the waveform recording device 16. is configured. Next, the effect will be explained. Coherent light emitted from the light source 1 passes through the frequency shift device 5 and is split into two light waves that differ by a constant frequency difference Δν. One of the split light waves passes through the phase shifter 7 and the polarizer 8 and enters the semi-transparent mirror 11, and the other light wave passes through the optical path length mask I12, the reflector 6 and the polarizer 9, and enters the semi-transparent mirror. 1
1. Then, the two divided and polarized light waves are combined again through the semi-transparent 1all and input into the optical fiber to be measured 12 having birefringence. At this time, the opening directions of the respective divided light waves are set to be orthogonal to each other and coincide with the birefringence axis of the optical fiber 12 to be measured. The two light waves propagate in the optical fiber 12 to be measured as HEM and HEN modes, respectively, and after exiting the optical fiber 12 to be measured, only the component in the direction of one birefringence axis is extracted by the analyzer 13. and is photoelectrically detected by the photodetector 4. At this time, HE generated in the optical fiber 12 under test
Depending on the degree of power coupling between the i-mode and the HE blue mode, as explained using FIG.
It is possible to obtain an interference intensity corresponding to the 2 higher order TIZ. However, in this embodiment, a constant angular frequency difference Δωb (Δωb(Δω)) is given between the two interfering light waves,
The interference signal is observed as the amplitude of a beat signal that oscillates at Δωb. The intermediate frequency filter 14 is used to extract the Δωb component, and the amplitude value of the Δωb component is detected by the waveform recording device 1.
Recorded at 6. Next, FIG. 4 shows another embodiment of the present invention. This embodiment allows evaluation of changes in the degree of mode coupling relative to distance dependence. In this embodiment, the retarder and polarizer in the previous embodiment (FIG. 2) are omitted, and a mode filter 17 is provided in place of the analyzer. The coherent light waves emitted from the light source 1 are
The light passes through the frequency shift device 5 and is split into two light waves that differ by a constant frequency difference Δ. One of the divided light waves is directly incident on the semi-transparent mirror 11, and the other light wave is incident on the semi-transparent mirror 11 via the variable optical path length device 2 and the reflection 3A6. The two split light waves are then combined again by the semi-transparent mirror 11 to the optical fiber 1 to be measured.
2. The two light waves each travel through the optical fiber 12 under test.
After propagating as the Po+ mode and the LP+1 mode and exiting the optical fiber 12 to be measured, only one mode light passes through the mode filter 17 and is photoelectrically detected by the photodetector 4. The normalized value of the interference signal detected by the photodetector 4 is
Similar to the above equation (12), it is expressed by the following equation. ■ ふ＞ =Cbi 1 hi...(
16) Here, C is the interference efficiency deterioration coefficient based on the difference in the spatial electric field distribution of the LPG mode and the L P , mode, and its value differs because it takes a constant value for the same optical fiber. A relative change in the mode coupling coefficient with respect to the position W Z i can be observed. Next, FIGS. 5A to 5D show specific examples of measurement results based on the measurement system of FIG. 2 described above. The optical fiber 12 to be measured has a total length of 100 m, and the spectral width of the light source 1 used is approximately 500 GHz. (A>~(C) in Figure 5 are interference signals (optical beat signals)
The horizontal axis is the optical path length difference ΔL″C″. The horizontal axes of FIGS. 5(A), (B), and (C) are 8.2 and 4.1 mm/div, respectively, based on actual measurements. ': &' The vertical axis is the gA width of the optical beat signal and is a linear scale. First, (A) in FIG. 5 shows the dependence of the beat amplitude on ΔL when the two light waves in FIG. , which corresponds to the temporal coherence degree curve of light source 1 (Reference 2N.
ientS of Potarisation
Mode D 1spersion by 0p
ticalHeterodyne Detection
n”, E 1f3ctron Lett, 20
．． pp1055-1057.1984). 5(A) represents the propagation mode of the three beams that generate the interference signal. FIG. 5(B) shows the beat amplitude when the binary wave in FIG. 2 is propagated as HBr and HE blue modes in the optical fiber 12 under test, and only the HE mode is transmitted through the analyzer 13. It shows ΔL dependence. HBrl all over the country
-” means HEM mode to HEN in the optical fiber.
The vertical axis, which represents the propagation mode components converted into modes, has been expanded 18 times compared to FIG. 5(A). The two peaks visible at both left and right ends are due to a mismatch between the birefringence axis of the optical fiber 12 to be measured and the polarization direction of the incident light when the optical fiber 12 to be measured is excited. In the example of the 0 figure corresponding to the position, the distance between the peaks is 3
The difference in group delay between the two modes at 9.0 mm is (=L/δV
g) Equivalent to 130Ps. The amplitude level between the peaks in FIG. 5(B) is -35 dB compared to the peak level in FIG. 5(A).
From this, it can be seen that the degree of coupling in the longitudinal direction of the optical fiber 12 to be measured is approximately constant. The distance resolution in this measurement is given by the above equation (8), and from the actual measurement value τc (ho, 67 ps) and the group velocity difference δVg, the distance resolution ΔZ is approximately 0.5 m. (C) and (D> in FIG. 5 are 36 mm from the input end of the optical fiber 12 under test under the same conditions as in FIG. 5 (B) above.
At the positions of m and 37 m, the optical fiber to be measured 12
The beat amplitude is shown when external pressure is applied to cause mode coupling. Two peaks indicating the external pressure application area can be seen in the center of FIG. 5(C). In Figure 5 (DJ) is an enlarged view of the horizontal axis of these two peaks. From this figure, two points 1 m apart where mode coupling is occurring are detected, and the height of each peak shows that the coupling coefficient hi
can be planned using equation (13). FIG. 5 (in the example D>, the height of the left peak, which corresponds to IrH in formula (13)) was about 4×10 −2 , and as a result, a coupling coefficient of 1.6×10 −3 was obtained. As described above, it can be seen that in this example, the degree of mode coupling in the longitudinal direction of the optical fiber can be measured with a distance resolution of about 0.5 m, and the coupling coefficient can be quantitatively evaluated. [Effects of the Invention] As explained above, according to the configuration of the present invention, only the light transmitted in one propagation mode among the light transmitted in the two propagation modes through the transmission line under test is extracted. The amplitude of the interference signal that has been photoelectrically converted and interfered in the transmission line to be measured is detected as a function of the optical path length difference set by the optical path length difference setting means. In this way, the interference signal due to both propagation mode components is detected using the light transmitted through the transmission line under test, and the degree of mode coupling is detected temporally separated by the group delay time difference between the two propagation modes. There is an advantage that the longitudinal distribution of the degree of mode coupling can be quantitatively measured with high resolution, and the dynamic range of the degree of mode coupling can be expanded. 4. Brief description of the drawings Fig. 1 is a block diagram for conceptually explaining the mode coupling evaluation device according to the present invention, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is the same as above. FIG. 4 is a block diagram showing another embodiment of the present invention, and FIG. 5 is a diagram showing the beat amplitude with respect to the advantageous optical path length difference in the same embodiment. It is a figure which shows the example of a measured value. 1: Light source, 2: Optical path length variable device, 4: Photodetector, 5: Frequency shift device (demultiplexing means), 8.9: (ii photon, 11: Semi-transparent if!A (combining means) , 12: Optical fiber to be measured (transmission line to be measured), 13: Analyzer (means for extracting one propagation mode component), 14: Intermediate frequency filter, 16: Together with the intermediate frequency filter, the amplitude of the interference signal is calculated based on the optical path length difference. 17: Mode filter (means for extracting one propagation mode component) Optical path length difference ΔL Fig. 5 (A) 1% optical path difference ΔL Figure 6 (B)

Claims (1)

[Claims] A light source that emits coherent light; a demultiplexer that divides the light emitted from the light source into two; and a variable required optical path length difference between the two demultiplexed lights. means for combining the two lights transmitted through each optical path set to a required optical path length difference and inputting the two lights into a transmission line under test having two propagation modes; means for extracting light transmitted in one of the two propagation modes; photoelectrically converting the extracted light and detecting the amplitude of the interference signal that interfered in the transmission path to be measured as a function of the optical path length difference; A mode coupling evaluation device characterized in that it has means for.