JPS6147533A - Method and apparatus for measuring group delay time difference of optical fiber - Google Patents

Abstract

PURPOSE:To enhance time resolving power and to enable automatic measurement, by spacially separating light emitted from a light source into two luminous fluxes and interfering two luminous fluxes to measure the intensity of a beat signal while measuring group delay time difference to the difference between two light path lengths. CONSTITUTION:The shaft of an 1/4 wavelength plate 5 is rotated so that the light polarizing direction of two luminous fluxes incident to a polarized wave maintaining optical fiber 8 cross at right angles. These light polarizing directions coincide with two double refraction axes of the polarized wave maintaining optical fiber 8 and group delay time difference (polarized wave dispersion) is generated and two luminous fluxes generating said group delay time difference are interfered while the number of generated beat signals are recorded and this measurement is repeated while the position of a movable mirror 6 is moved. Next, the shaft of the 1/4 wavelength plate 5 is rotated to allow the main shaft of the 1/4 wavelength plate 5 to coincide with the light polarizing direction of the luminous flux incident to said 1/4 wavelength plate 5. In this case, no group delay time difference is generated and the measurement of the beat signal at this time is repeated while the position of the movable mirror 6 is moved. In two measurements as mentioned above, the light path difference at a position, where the beat signal becomes max., is measured to calculate group delay time difference.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method and apparatus for measuring group delay characteristics of an optical signal propagating through an optical fiber.

[Conventional technology]

The group delay characteristics of an optical signal propagating within an optical fiber include group delay characteristics due to changes in the wavelength of the optical signal (color molecule ik characteristics),
Group delay characteristics caused by temperature, longitudinal tensile or compressive stress, and group delay characteristics between orthogonal polarization modes of optical fibers where birefringence exists within the optical fiber cross section (group delay time difference)
and so on.

In order to obtain these group delay characteristics, group delay time differences are measured. The method to measure the group delay time difference is as follows:
The pulse method that measures in the time domain, the phase method or frequency sweep method that measures in the frequency domain, and the interferometry that detects the position where the clarity of the interference fringes of light is maximum are used.

[Problem that the invention seeks to solve]

Measuring group delay time differences using the pulse method, phase method, or frequency sweep method has the disadvantage that small time differences cannot be measured due to limitations in the time resolution and frequency band of the measurement system.

For example, the group delay difference that can be measured by the pulse method is limited to several tens of picoseconds.

Further, in the interferometry method that uses optical interference, the measurable group delay time difference is about 0.1 picosecond, and extremely high time resolution can be achieved. However, the frequencies of the lights used for interference are the same, and it is necessary to measure the clarity of the interference fringes as a function of the optical path difference. For this reason, it is necessary to sequentially perform measurements at a large number of sampling points, and there is a drawback that measurements cannot be performed automatically in a short period of time.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide a method and apparatus for measuring group delay time differences of optical fibers, which have high time resolution and are capable of automatic measurement.

[Means for solving problems]

The optical fiber group delay time difference measurement method of the present invention spatially separates light emitted from a light source into two light beams, and shifts the frequency of at least one of these two light beams to create a frequency difference between the two light beams. is given, and one of the two beams with this frequency difference is propagated into the optical fiber to be measured,
After propagating through this optical fiber, the above two light beams are made to interfere, the intensity of the beat signal due to this interference is measured, and the measurement is repeated by changing the optical path length of at least one of the above two light beams. Measure the group delay time difference for the length difference.

Further, the optical fiber group delay time difference measuring device of the present invention includes:
A light source, a means for spatially separating the light emitted from this light source into two light beams, and a means for giving a frequency difference to the two light beams by shifting the frequency of one of the two light beams; means for making at least one of the two light beams given a frequency enter one end of an optical fiber to be measured; means for interfering the two light beams at the other end of the optical fiber; and measuring the intensity of the beat signal caused by this interference. means and
It is characterized by comprising means for changing the optical path length of at least one of the two light beams.

[Effect]

The optical fiber group delay time difference measurement method of the present invention frequency-shifts one of two spatially separated light beams, propagates at least one of the two light beams to the optical fiber to be measured, and then Interfering with light flux. By observing the intensity of interference with respect to the optical path length, the group delay time difference can be measured.

〔Example〕

FIG. 1 is a block diagram of an optical fiber group delay time difference measuring device according to a first embodiment of the present invention.

This example is an example of a method for measuring the group delay time difference between orthogonal polarization modes of the polarization-maintaining optical fiber 8.

The light output from light a, 1 passes through a polarizer 2, and is split into two light beams by a light branching element 3. One of these two light beams is reflected by the fixed mirror 4, passes through the semi-transparent mirror 7, and enters the polarization-maintaining optical fiber 8. The other of the two beams passes through the semi-transparent mirror 7 and the A-wave plate 5, is reflected by the movable mirror 6, passes through the A-wave plate 5 again, is reflected by the semi-transparent mirror 7, and enters the polarization-maintaining optical fiber 8. do. The light beam transmitted through the polarization-maintaining optical fiber 8 passes through a polarizer 9 and enters a photodetector 10 . Photodetector 10 is connected to a selection level meter 11 . The selection level meter 11 is connected to the recording device 12.

The coherent light outputted by the light H,1 is polarized by the polarizer 2,
It becomes linearly polarized light whose polarization direction coincides with the main axis of the optical branching element 3. The optical branching element 3 can shift the frequency by an acousto-optic effect or the like. This light branching element 3 spatially separates the linearly polarized light that has passed through the polarizer 2 into a first light beam that has undergone a frequency shift and a second light beam that has not undergone a frequency shift.

A fixed mirror 4 is arranged on the optical path of the first light beam, and the first light beam is reflected by this fixed mirror 4. The first light beam reflected by the fixed mirror 4 passes through the semi-transparent mirror 7 and enters the polarization-maintaining optical fiber 8 . Here, the polarization direction of the first light beam coincides with one of the two birefringence axes of the polarization-maintaining optical fiber 8.

The second light beam passes through the semi-transparent mirror 7, passes through the A wavelength plate,
It is reflected by the movable mirror 6. The movable mirror 6 is fixed to a support base, and this support base can be moved by a pulse motor or a step motor. The light beam reflected by the movable mirror 6 passes through the A wavelength plate 5 again, is reflected by the semi-transparent mirror 7, and enters the C1 front wave holding optical fiber 8.

The above-mentioned two light beams are incident on the polarization maintainer 1 blue light lover 8 with the polarization direction coinciding with the direction of the birefringence axis of the polarization maintaining optical fiber 8.

By adjusting the direction of its principal axis, the polarizer 9 causes the two beams emitted from the polarization-maintaining optical fiber 8 to interfere with each other. The photodetector 10 is composed of a silicon anno\'-Ranchet photodiode or a germanium anno-rancier photodiode, and detects a beat signal generated by interference between two light beams.

The selection level meter 11 quantitatively measures the number of B1-signals. The recording device 12 records this measured value. The recording device 12 uses, for example, an X-Y recorder.

The group delay time difference is determined by measuring the number of beat signals for the optical path difference G2 between two light beams.

First, the axis of the A-wavelength manual pressure 5 is rotated so that the polarization directions of the two light beams incident on the polarization-maintaining optical fiber 8 are orthogonal. The polarization directions of these two orthogonal beams coincide with the two birefringence axes directions of the polarization-maintaining optical fiber 8.

Therefore, a group delay time difference (polarization dispersion) occurs between the two light beams while propagating within the polarization-maintaining optical fiber 8. The two light beams with the difference in group delay time are caused to interfere, and the number of beat signals generated thereby is recorded. This measurement is repeated while moving the position of the movable mirror 6.

Next, the axis of the A wavelength plate 5 is rotated to match the polarization directions of the two light beams. That is, the main axis of the % wavelength plate 5 and the polarization direction of the light beam incident on the two-wave plate 5 are made to be the same. As a result, the two light beams are propagated as polarization modes in the same birefringence axis direction of the polarization-maintaining optical fiber 8. therefore,
In this case, no group delay time difference occurs. The measurement of the beat signal at this time is repeated while moving the position of the movable mirror 6.

In the above two measurements, the optical path difference at the position where the beat signal is maximum is measured, and thereby the group delay time difference can be determined.

This measurement method will be explained using equations.

The light intensity ■ detected by the photodetector 10 is I=11+rz +2X, (II Iz )"" lγ(τ+2)
1x cosδIzcosΩ −=−(
11(j" = -1). Here, II and Iz are the light intensities of the two luminous fluxes, 1γ (τ, 2)1 is the degree of coherence determined by the light source, and is a function of τ1□, τ12 is the propagation time difference between the two light beams from the light source 1 to the photodetector 10, δ1□
is the phase difference between the two beams, and Ω is the angle between the planes of polarization between the two beams.

Letting the complex amplitudes of the two luminous fluxes be E+ and Ez, E, =
a, eXp j (2πft+φ1.1 −(2
1B2=az exp j × [2π(f+Δf) t+φ2) −...=(3)
It can be expressed as. Here, alzaz is the complex amplitude E
+ and Ez, f is the frequency of the light, Δf is the amount of frequency deviation, L is time, and φ1 and φ2 are the respective phases.

From equations (2) and (3), the phase difference δ1□ of (1) 2 is δ, 2=2πΔft+(φ2−φ,) ・−−−−−
-(4). Here, ■1 = αI.

I2=(1-α)■.

(where o x α x 1), then the AC component 1 of equation (1)
(t) is 1(t)=2XI. 1 γ (τ, 2) 1× (α (
1-α) ) I/Z cosΩ×cos
[2πΔft+(φ2−φr)) −r5+. By adjusting the arrangement of each part to α-0,5 cosΩw1, equation (5) becomes 1(tl=Io lrc t-+z) l”xc
os(27CΔft+(φ2−φ+)) −−(6
). This formula has a coherence degree of 1γ (τ1□)
1 indicates an amplitude modulated beat signal.

Therefore, the position where the beat signal is maximum when polarization dispersion occurs (the polarization directions of the two light beams are orthogonal) and the position where the beat signal is maximum when polarization dispersion does not occur (the polarization directions of the two light beams are the same) By determining the difference from the position where the intensity of the beat signal is maximum, the group delay time difference due to polarization dispersion can be determined. That is, if this positional difference is 2d,,X, the speed of light in free space is C, and the length of the polarization-maintaining optical fiber 8 is L, then the group delay time difference due to polarization dispersion can be calculated as τ2. can.

FIG. 2 is a diagram showing the beat signal intensity with respect to the optical path length actually measured in this example.

The upper part of FIG. 2 shows the beat signal intensity obtained by interference between the same polarization modes, and the lower part shows the beat signal intensity obtained by interference between orthogonal polarization modes.

The optical path length difference at which the beat signal is maximum is 22.13 mm, and the measured group delay time difference is τ, L = 7 according to equation (7).
3.76 (picoseconds).

FIG. 3 is a block diagram of an optical fiber group delay time difference measuring device according to a second embodiment of the present invention.

This example is an example of a method for measuring the group delay time difference between a light beam propagating in the air where dispersion can be ignored and a light beam propagating in the optical fiber 13 to be measured.

The light output from the light source 1 is split into two light beams by the light branching element 3.

One of these two light beams is incident on the optical fiber 13 to be measured, propagates through the optical fiber 13 to be measured, passes through the semi-transparent mirror 17 and the polarizer 9, and enters the photodetector 10.

The other beam is reflected by the fixed mirror 14, 15, 16, transmitted through the semi-transparent mirror 17, reflected by the movable mirror 18, reflected by the semi-transparent v1.17, transmitted through the polarizer 9, and detected by light. The light enters the vessel 10.

Photodetector 10 is connected to a selection level meter 11 .

The selection level meter 11 is connected to a recording device 12.

The optical branching element 3 performs frequency shift using an acousto-optic effect or the like.

In this embodiment, if two adjacent wavelengths are λ1 and λ2, and the amount of movement of the movable mirror 18 that maximizes the beat signal for the two wavelengths is 2d+z, then 2d-8 of equation (7) is obtained.
By substituting 2a+□ into , the group delay time difference can be obtained. Furthermore, the chromatic dispersion can be obtained from this group delay time difference as follows.

In this example, the wavelength is fixed and the optical fiber to be measured 1 is
3 in a high-temperature bath and changing the temperature, it is possible to measure changes in the group delay time difference due to temperature changes. Furthermore, it is also possible to apply tensile stress or compressive stress to the optical fiber 13 to be measured using a tensile tester or the like, and measure changes in the group delay time difference due to these stresses.

〔Effect of the invention〕

As explained above, the optical fiber group delay time difference measurement method of the present invention has an extremely small time resolution of about sub-picoseconds, and has the effect of being able to measure the group velocity with high precision regardless of the length of the optical fiber to be measured. . The measurement method of the present invention includes:
It can be carried out to measure various group delay time differences.

i

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an optical fiber group delay time difference measuring device according to a first embodiment of the present invention. FIG. 2 is a diagram showing actual measured values of beat signal intensity with respect to optical path length. FIG. 3 is a block configuration diagram of an optical fiber group delay time difference measuring device according to a second embodiment of the present invention. 1... Light source, 2... Polarizer, 3... Light branching element,
4... Fixed mirror, 5... Noise wave plate, 6... Movable mirror,
7... Semi-transparent mirror, 8... Polarization maintaining optical fiber, 9...
- Polarizer, lO... photodetector, 11... selection level meter, 12... recording device, 13... optical fiber to be measured, 14.15.16... fixed mirror, 17...・Semi-transparent mirror, 18...Movable mirror.

Claims (2)

[Claims] (1) Split the light emitted from the light source into two light beams spatially, shift the frequency of at least one of these two light beams to give a frequency difference between the two light beams, and At least one of the two light beams is propagated into the optical fiber to be measured, and after propagating through the optical fiber, the two light beams are made to interfere with each other, the intensity of the beat signal due to this interference is measured, and the optical path of at least one of the two light beams is determined. A method for measuring a group delay time difference of an optical fiber, which measures a group delay time difference with respect to the difference between the two optical path lengths by repeating the measurement while changing the length of the optical fiber. (2) one light source; means for spatially separating the light emitted from this light source into two light beams, and frequency-shifting at least one of the two light beams to give a frequency difference between the two light beams; A means for making at least one of the two light beams given this frequency enter one end of the optical fiber to be measured, a means for interfering the two light beams at the other end of the optical fiber, and measuring the intensity of the beat signal due to this interference. An apparatus for measuring a group delay time difference of an optical fiber, comprising: means for changing the optical path length of at least one of the two light beams.