JPH0731096B2 - Optical fiber group delay time difference measuring method and measuring apparatus - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method and apparatus for measuring group delay characteristics of an optical signal propagating in an optical fiber.

[Conventional technology]

The group delay characteristics of the optical signal propagating in the optical fiber include the group delay characteristics (chromatic dispersion characteristics) due to the change of the wavelength of the optical signal, the group delay characteristics due to the tensile stress or the compressive stress in the length direction and the optical fiber. There are group delay characteristics (group delay difference characteristics) between orthogonal polarization modes of an optical fiber in which birefringence exists in the cross section.

In order to obtain these group delay characteristics, the group delay time difference is measured. As a method of measuring the group delay time difference,
A pulse method that measures in the time domain, a phase method or a frequency sweep method that measures in the frequency domain, and an interferometry method that detects the position where the sharpness of the interference fringes of light are maximum are used.

[Problems to be solved by the invention]

The measurement of the group delay time difference by the pulse method, the phase method, or the frequency sweep method has a drawback that a small time difference cannot be measured due to the time resolution of the measurement system and the limitation of the frequency band.
For example, the group delay difference that can be measured by the pulse method is limited to several tens of picoseconds.

In addition, the interferometry that utilizes the interference of light has a measurable group delay time difference of about 0.1 picosecond, and can realize extremely high time resolution. However, the frequencies of light used for interference are the same, and it is necessary to measure the definition of interference fringes as a function of optical path difference. For this reason, it is necessary to successively measure at a large number of sampling points, and there is a drawback that the measurement cannot be performed automatically in a short time.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide a method and apparatus for measuring a group delay time difference of an optical fiber, which has a high time resolution and is capable of performing automatic measurement.

[Means for solving problems]

The optical fiber group delay time difference measuring method of the present invention spatially separates light emitted from a light source into two light beams, and frequency shifts at least one of the two light beams to generate a frequency difference between the two light beams. A difference is given, and at least one of the two light beams having this frequency difference is propagated to the optical fiber to be measured, and after being propagated to this optical fiber, the two light beams are caused to interfere with each other,
By measuring the intensity of the beat signal due to this interference, and obtaining the group delay time difference of the optical fiber under measurement by utilizing the fact that the intensity of the beat signal obtained by interfering two light beams with different frequencies is proportional to the degree of coherence. Is characterized by.

Specifically, two light beams having a frequency difference are propagated to the optical fiber under measurement with their polarization directions being substantially orthogonal to each other, and the two light beams emitted from the light source and propagating through the optical fiber under measurement are interfered with each other. The first optical path difference that maximizes the intensity of the beat signal is obtained by changing the optical path length of at least one of the light fluxes, and the two light fluxes with the frequency difference are made to have their polarization directions substantially coincident with each other, and the optical fiber under test is measured. The optical path length of at least one of the two luminous fluxes that are emitted from the light source and propagated through the optical fiber under measurement and then interfere with each other to obtain the second optical path difference that maximizes the intensity of the beat signal. The group delay time difference due to the polarization dispersion is obtained from the difference between the optical path difference of 1 and the second optical path difference.

In addition, one of the two light beams having a frequency difference is propagated to the measured optical fiber, the other of the two light beams is propagated in the air, and the light beam output from the measured optical fiber and the light beam propagated in the air To change the optical path lengths of the two light beams emitted from the light source and passing through different optical paths and then interfering with each other, and the beat signals are emitted to the two different wavelengths λ ₁ and λ ₂ emitted from the light source. It is also possible to obtain the group delay time difference from the maximum difference in optical path length.

The optical fiber group delay time difference measuring method of the present invention spatially separates one light source and the light emitted from this light source into two light fluxes, and at least one of the two light fluxes is frequency-shifted to obtain the two light fluxes. Means for giving a frequency difference to two light fluxes, means for causing at least one of the two light fluxes having this frequency difference to propagate to the optical fiber to be measured, and means for interfering the two light fluxes, and means for causing the light flux to interfere with each other And a means for changing the optical path length of at least one of the two luminous fluxes to measure the optical path difference at which the intensity of the beat signal is maximized.

The means for interfering controls at least one polarization direction so that the polarization directions of the two light beams having a frequency difference are switched between a direction in which they are substantially coincident with each other and a direction in which they are substantially orthogonal to each other. The measuring means includes means for combining two light beams whose polarization directions are controlled and making them incident on the optical fiber to be measured, and the measuring means is such that the polarization directions of the two light beams incident on the optical fiber to be measured are substantially the same. It is preferable to include a means for repeating the measurement in each of the case where the measurement is performed and the case where the measurement is substantially orthogonal. In addition to this, the means for causing interference causes one of the two light beams having a frequency difference to enter one end of the optical fiber to be measured and the other light beam having a frequency difference to propagate in the air. And a means for combining the light flux emitted from the other end of the optical fiber to be measured with the light flux propagated in the air by the means for propagating, and the measuring means includes two different light emitted from the light source. Means may be included for repeating the measurements for wavelengths λ ₁ and λ ₂ .

[Action]

The optical fiber group delay time difference measuring method of the present invention, one of two spatially separated light fluxes is frequency-shifted, at least one of the two light fluxes is propagated to the optical fiber to be measured, and then two Interfere the light flux. The group delay time difference can be measured by observing the intensity of interference with respect to the optical path length.

In the present invention, the intensity of the beat signal obtained by causing two light beams having different frequencies to interfere with each other is proportional to the coherence degree γ, and the coherence degree γ has a maximum value when the propagation time difference τ ₁₂ of the two light fluxes is zero. Using the physical phenomenon of taking, the group delay time difference of the optical fiber is obtained. Regarding the characteristics of the coherence degree γ, Reference 1: Max Born and Emil Wolf, Principles of Optics
(Fourth edition), pp.498-505, Pergamon Press (197
0) Reference 2: Robert J. Collier et al., Optical Holography, p.
p140-145, Academic Press (1971).

〔Example〕

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

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

The light output from the light source 1 passes through the polarizer 2 and is split into two light beams by the light branching element 3. One of the two light beams is reflected by the fixed mirror 4, passes through the semitransparent mirror 7, and enters the polarization maintaining optical fiber 8. The other of the two light beams passes through the semi-transparent mirror 7 and the quarter-wave plate 5, is reflected by the movable mirror 6, passes through the quarter-wave plate 5 again, is reflected by the semi-transparent mirror 7, and is polarization-maintained light. It is incident on the fiber 8. The light flux transmitted through the polarization maintaining optical fiber 8 passes through the polarizer 9 and enters the photodetector 10. The photodetector 10 is connected to the selective level meter 11. The selection level meter 11 is connected to the recording device 12.

The coherent light output from the light source 1 becomes linearly polarized light whose polarization direction coincides with the main axis of the light branching element 3 by the polarizer 2. The light branching element 3 can perform frequency shift due to an acousto-optic effect or the like. The light branching element 3 spatially separates the linearly polarized light transmitted through the polarizer 2 into a first light beam having a frequency shift and a second light beam having no frequency shift.

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

The second light flux passes through the semi-transparent mirror 7, the quarter-wave plate, and is reflected by the movable mirror 6. The movable mirror 6 is fixed to a supporting table, and the supporting table can be moved by a pulse motor or a step motor. The light beam reflected by the movable mirror 6 again passes through the quarter-wave plate 5, is reflected by the semitransparent mirror 7, and enters the polarization maintaining optical fiber 8.

The two light beams described above are incident on the polarization maintaining optical fiber 8 in the polarization directions that coincide with the birefringence axis direction of the polarization maintaining optical fiber 8.

The polarizer 9 adjusts its main axis direction so that the two light beams emitted from the polarization maintaining optical fiber 8 interfere with each other.
The photodetector 10 is composed of a silicon avalanche photodiode, a germanium avalanche photodiode, or the like, and detects a beat signal generated by interference between two light beams. The selection level meter 11 quantitatively measures the strength of this beat signal. The recording device 12 records this measurement value. As the recording device 12, for example, an XY recorder or the like is used.

The group delay time difference is obtained by measuring the intensity of the beat signal with respect to the optical path difference between the two light beams.

First, the axis of the quarter-wave plate 5 is rotated so that the polarization directions of the two light beams incident on the polarization maintaining optical fiber 8 are orthogonal to each other. The polarization directions of these two orthogonal light beams coincide with the two birefringence axis directions of the polarization maintaining optical fiber 8. Therefore, the two light fluxes cause a group delay time difference (polarization dispersion) while propagating in the polarization maintaining optical fiber 8. The two light fluxes having the difference in group delay time are caused to interfere with each other, and the intensity of the beat signal generated thereby is sequentially recorded. This measurement is performed by the movable mirror 6
Repeat while moving the position of.

Next, the axis of the quarter-wave plate 5 is rotated to match the polarization directions of the two light beams. That is, the main axis of the quarter wave plate 5 and
The polarization direction of the light beam incident on the / 4 wavelength plate 5 is made to match.
As a result, the two light beams propagate as a polarization mode 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 can be measured, and the group delay time difference can be obtained.

This measurement method will be described using equations.

The light intensity I detected by the photodetector 10 is I = I ₁ + I ₂ + 2 × (I ₁ I ₂ ) ^1/2 │γ (τ ₁₂ ) │ × cosδ ₁₂ cosΩ (1) (j ² = -1) is expressed as Here, I _1, I ₂ is the light intensity of the two light fluxes, | γ (τ ₁₂₎ | is a coherence factor determined by the light source tau ₁₂ functions, tau ₁₂ is a light source 1 of the two light bundles
To the photodetector 10, δ ₁₂ is the phase difference between the two light beams, and Ω is the angle between the planes of polarization between the two light beams. γ (τ ₁₂ ) is a known function, and it is known to take a maximum value when τ ₁₂ = 0.

If the complex amplitudes of the two light fluxes are E ₁ and E ₂ , then E ₁ = a ₁ exp j [2πft + φ ₁ ] ... (2) E ₂ = a ₂ exp j × [2π (f + Δf) t + φ ₂ ] ... ( 3) can be expressed as Where a ₁ and a ₂ are complex amplitudes E ₁ and E ₂
, F is the frequency of light, Δf is the amount of frequency deviation, t is time, and φ ₁ and φ ₂ are respective phases.

From the expressions (2) and (3), the phase difference δ _{12 in the} expression (1) is δ ₁₂ = 2πΔft + (φ ₂ −φ ₁ ) (4). Here, I ₁ = αI ₀ I ₂ = (1-α) I ₀ (where 0 <α <1) and the AC component I of the equation (1)
(T) is I (t) = 2 × I ₀ | γ (τ ₁₂ ) | × {α (1-α)} ^1/2 cosΩ × cos [2πΔft + (φ ₂ −φ ₁ )] ... (5 ). By adjusting the arrangement of each part and setting α = 0.5 cos Ω = 1, the equation (5) is given by: I (t) = I ₀ | γ (τ ₁₂ ) | ² × cos [2πΔft + (φ ₂ −φ ₁ )] … (6) This equation represents a beat signal amplitude-modulated with the degree of coherence | γ (τ ₁₂ ) |.

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 polarization signal does not occur (the polarization directions of the two light beams match) By calculating 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 calculated. That is, if the difference between these positions is 2dmax, the speed of light in free space is c, and the length of the polarization-maintaining optical fiber 8 is L, the group delay time difference τp due to polarization dispersion is Can be calculated as

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

The upper part of FIG. 2 shows the beat signal strength obtained by the interference between the same polarization modes, and the lower part shows the beat signal strength obtained by the interference between the orthogonal polarization modes. The optical path length difference that maximizes the beat signal is 22.13 mm, and the measured group delay time difference is τpL = 73.76 [picoseconds] according to equation (7).

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

The present embodiment is an example of a method of measuring the group delay time difference between a light beam propagating in air whose 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 the two light beams enters the optical fiber 13 to be measured, propagates through the optical fiber 13 to be measured, passes through the semitransparent mirror 17 and the polarizer 9, and enters the photodetector 10.

The other light beam is reflected by the fixed mirrors 14, 15 and 16,
The light passes through the semi-transparent mirror 17 and is reflected by the movable mirror 18.
Is reflected by the light, is transmitted through the polarizer 9, and is incident on the photodetector 10.

The photodetector 10 is connected to the selective level meter 11. The selection level meter 11 is connected to the recording device 12.

The light branching element 3 performs frequency shift due to an acousto-optic effect or the like.

In the present embodiment, two adjacent wavelengths are set to λ ₁ and λ _2, and the movable mirror that maximizes the beat signal for the two wavelengths.
Assuming that the movement amount of 18 is 2d ₁₂ , 2d ₁₂ can be substituted for 2dmax in the equation (7) to obtain the group delay time difference. further,
By this difference in group delay time, the chromatic dispersion D is Can be asked as

In this embodiment, the wavelength is fixed and the measured optical fiber 13
It is possible to measure the change in the group delay time difference due to the temperature change by putting in a high temperature tank and changing the temperature. In addition, use a tensile tester to measure the optical fiber to be measured.
It is also possible to apply tensile stress or compressive stress to 13 and measure the change in the group delay time difference due to these stresses.

〔The invention's effect〕

As described above, according to the optical fiber group delay time difference measuring method of the present invention, the time resolution is extremely small, on the order of sub-picoseconds, and it is possible to measure the group velocity with high accuracy regardless of the length of the optical fiber to be measured. . The measuring method of the present invention is
It is possible to measure various group delay time differences.

[Brief description of drawings]

FIG. 1 is a block diagram of a group delay time difference measuring device for an optical fiber according to a first embodiment of the present invention. FIG. 2 is a diagram showing measured values of the beat signal strength with respect to the optical path length. FIG. 3 is a block diagram of a group delay time difference measuring device for an optical fiber according to a second embodiment of the present invention. 1 ... Light source, 2 ... Polarizer, 3 ... Optical branching element, 4 ...
Fixed mirror, 5 ... 1/4 wave plate, 6 ... Movable mirror, 7 ... Semi-transparent mirror, 8 ... Polarization maintaining optical fiber, 9 ... Polarizer, 10 ...
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 (6)

[Claims] 1. A light emitted from a light source is spatially separated into two light beams, and at least one of the two light beams is frequency-shifted to give a frequency difference to the two light beams. At least one of the two light beams is propagated to the optical fiber to be measured, and after propagating to this optical fiber, the two light beams are interfered with each other, and the intensity of the beat signal due to this interference is measured. An optical fiber group delay time difference measuring method for obtaining the group delay time difference of the optical fiber under test by utilizing the fact that the intensity of the beat signal obtained by interfering with each other is proportional to the coherence degree. 2. The two light beams having the frequency difference are propagated to the optical fiber under measurement with their polarization directions being substantially orthogonal to each other, and are emitted from the light source and propagated through the optical fiber under measurement. The optical path length of at least one of the two interfering light beams is changed to obtain the first optical path difference that maximizes the intensity of the beat signal, and the two light beams having the above frequency difference are made to have their polarization directions substantially coincident with each other. And propagates to the optical fiber to be measured, changes the optical path length of at least one of the two light beams emitted from the light source and propagating through the optical fiber to be measured, and the intensity of the beat signal becomes maximum. The method for measuring a group delay time difference of an optical fiber according to claim 1, wherein a group delay time difference due to polarization dispersion is calculated based on a difference between the first optical path difference and the second optical path difference. 3. One of the two light beams having the frequency difference is propagated to an optical fiber to be measured, the other of the two light beams is propagated to the air, and the light beam and the air outputted from the optical fiber to be measured. Two light beams having different wavelengths λ ₁ , emitted from the light source, which interfere with a light beam propagating in the light source, change the optical path lengths of two light beams emitted from the light source and then interfere with each other,
The method for measuring a group delay time difference of an optical fiber according to claim 1, wherein the group delay time difference is obtained by a difference in optical path length that maximizes the beat signal with respect to λ ₂ . 4. A light source, and means for spatially separating the light emitted from this light source into two light fluxes, and at least one of the two light fluxes is frequency-shifted to give a frequency difference to the two light fluxes. And a means for causing at least one of the two light beams having this frequency difference to propagate to the optical fiber to be measured and then causing the two light beams to interfere with each other, and two light fluxes emitted from the light source and reaching the interference means. An optical fiber group delay time difference measuring device comprising: a means for changing at least one optical path length to measure an optical path difference that maximizes the intensity of a beat signal. 5. The interfering means sets at least one polarization direction so that the polarization directions of the two light beams having the frequency difference are switched between a direction in which they substantially coincide with each other and a direction in which they are substantially orthogonal to each other. A means for controlling, and a means for synthesizing two light fluxes in which at least one of the polarization directions is controlled to be incident on the optical fiber to be measured, wherein the means for measuring comprises two light fluxes incident on the optical fiber to be measured. The optical fiber group delay time difference measuring device according to claim 4, further comprising means for repeating the measurement when the polarization directions of the luminous fluxes are substantially coincident and when they are substantially orthogonal to each other. 6. The means for interfering causes one of the two light beams having the frequency difference to be incident on one end of the optical fiber to be measured, and the other of the two light beams having the frequency difference in the air. And a means for combining the light flux emitted from the other end of the optical fiber to be measured with the light flux propagated in the air by the means for propagating, the means for measuring is from the light source. The optical fiber group delay time difference measuring device according to claim 4, further comprising means for repeating the measurement for each of two different wavelengths λ ₁ and λ ₂ emitted.