JPH0361898B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method and apparatus for measuring mode birefringence, which is the most important factor in understanding the transmission characteristics of polarization-maintaining optical fibers. In this conventional measurement method, the polarization-maintaining optical fiber to be measured is cut in 1 mm increments, and the degree of polarization is measured each time. Since it can be expressed as , the beat length L was determined, and the mode birefringence B was determined. P=|COS(2π・B・l/λ)| (1) In equation (1), λ is the measurement wavelength, and the relationship between the beat length L and the mode birefringence B is L・B=λ (2) becomes. With this measurement method, it is necessary to cut the polarization-maintaining optical fiber to be measured several dozen times in order to obtain at least the beat length with high accuracy, and it is extremely difficult to cut a length of about 1 mm without failure. This is a difficult technique. In addition, the degree of polarization must be measured every time cutting is performed, resulting in poor workability. Furthermore, in order to measure the mode birefringence by cutting the fiber once, there is also a method of detecting and measuring the phase difference between both modes with respect to the cutting length ΔL. FIG. 1 is an explanatory diagram of this measurement method, in which the light source 1
The light emitted from the polarizer 2 is linearly polarized by the polarizer 2, and is input to the polarization-maintaining optical fiber 3 to be measured. At this time, the light is incident at an angle of 45° to the birefringence axis of the optical fiber. The light propagated through the optical fiber becomes an appropriate elliptically polarized wave depending on the length l of the fiber and the mode birefringence B and is emitted. This elliptically polarized light is converted into linearly polarized light by passing through a quarter-wave plate 4, and the plane of polarization is detected by an optical power meter 6 using an analyzer 5. Now, if the optical fiber is cut slightly and the angular difference φ between the principal axes of the polarizer and analyzer changes to φ' due to this cutting, the mode birefringence B is B=2. It is given by (φ−φ′)/KΔL (3). In equation (3), K is 2π/λ. In this method, what is important in measurement is that when the beat length L is unknown, 2ΔL>L may hold, and it is necessary to measure by changing ΔL several times. Therefore, this method also has the same measurement difficulties as the conventional method described above, and when the cut optical fiber is put back together, the optical fiber may be twisted or the cut end surface may be irregular. φ' changes and requires considerable skill to make accurate measurements. The present invention solves these drawbacks by changing the wavelength of the light source and utilizing reflection without changing the length of the fiber to be measured.
It is characterized by measuring mode birefringence, and its purpose is to shorten measurement time and improve measurement accuracy. FIG. 2 is a configuration diagram of an embodiment of the present invention,
7 is a wavelength variable light source, 8 is a lens for parallelizing light, 9
is an optical chopper, 10 is a photodetector, 11 is a condensing lens, 12 is reflected light having an electric field component perpendicular to the incident light, 13 is a condensing lens, and 14 is a straight line by a polarizer (rotation prism) 2. polarized incident light and light reflected from the polarization-maintaining optical fiber 3;
15 is light from a parallel light source, 16 is a light intensity signal, 17 is a synchronization signal from an optical chopper, 18
is a lock-in amplifier, and 19 is a recorder. In the operation shown in FIG. 2, first, the light from the variable wavelength light source 7 is made into parallel light by the lens 8, and then made into intermittent light by using the optical chopper 9. The reason for using the optical chopper 9 is to improve the S/N ratio and improve measurement accuracy when the level of detection light is weak. Now, the parallel light is linearly polarized by a polarizer (rotation prism) 2, and is incident on the polarization-maintaining optical fiber 3 to be measured through a condenser lens 13. The light reflected at the end face of the condenser lens 13 and the input end of the optical fiber 3 has the same plane of polarization as the incident light, so it passes through the polarizer 2 as it is, but is reflected at the output end of the optical fiber 3. Since the light is elliptically polarized, it has an electric field component perpendicular to the incident light. When this reflected light also passes through the polarizer 2, its optical path is deflected and guided to the photodetector 10. If the birefringence axis of the optical fiber 3 matches the polarization plane of the incident light, the polarization plane will be the same as that of the reflected light, and the detected light will be minimum. Therefore, the polarization plane of the incident light is made to be incident at an angle of approximately 45° with respect to the birefringence axis of the optical fiber 3, so that the light reception and power of the detector are maximized. On the other hand, when the length of the optical fiber happens to be an integral multiple of 1/2 of the beat length L between propagation modes,
The light reflected from the optical fiber 3 may also always be linearly polarized. Therefore, in this measurement method, a halogen lamp and a spectrometer are used as the variable wavelength light source 7, and the spectral width of the light source 7 can also be changed by changing the slit of the spectrometer, and the incident angle can be set as described above. In this case, the spectral width of the light source 7 is expanded so that the reflected light from the output end of the optical fiber 3 is straight except when the polarization plane of the incident light coincides with the birefringence axis of the polarization-maintaining optical fiber 3. I try to avoid polarization. Therefore, the rotational position of the birefringence axis of the optical fiber 3 at which the received light power at the detector 10 is maximum is approximately equal to the polarization plane of the incident light.
It becomes 45°. Next, the spectral width of the light source 7 is narrowed and the wavelength is changed, and the received light power at that time is synchronously detected by the lock-in amplifier 18 and recorded by the recorder 19. FIG. 3 shows the change in received light power with respect to wavelength measured using the measurement system shown in FIG. 2. First, since the optical fiber to be measured was manufactured for a wavelength band of 1.3 μm, when reading the change in received light power around 1.3 μm, in Fig. 3, λ _A = 1.30054 μm,
λ _B =1.30374 μm. Furthermore, the length of the fiber at this time is 1.001 m. The following calculation is used to determine the mode birefringence B of the fiber to be measured from the measurement results shown in FIG. The beat length of the polarization-maintaining optical fiber is L _b , the length of the fiber is l, the wavelength at which the received light power is the lowest is λ _A , and the wavelength of the lowest part next to the long wavelength side of λ _A is λ _B
Then, the following relationship is obtained. nL _b (λ _A ) = 2l, (n-1) L _b (λ _B ) = 2l (4) Equation (4) is expressed as , which means that the polarization plane of the incident light and the polarization plane of the reflected light match, and the received light power is minimized. Furthermore, from the function of equation (2), equation (4) becomes nλ _A /B (λ _A ) = 2l, (n-1)λ _B /B (λ _B ) = 2l (5), and B (λ _A ) {λ _B −λ _A B(λ _B )/B(λ _A )}=λ _A λ _B
/2l(6) is obtained. On the other hand, the mode birefringence B is under the condition that the normalized frequency V of the optical fiber is V>1.8.
Since the mode birefringence _BSO at _the center of the core matches, _BBSO = _C (σ The photoelastic constant difference C and the stress difference (σ _X0 −
σ _y0 ). In equation (7), C has wavelength dependence, and C is expressed by the following equation (Reference NKSinha; “Normalised dispersion
of birefringence of quartz and stress optical
coefficient of fused silica and plate glass”,
Physics and Chemistry of Glasses, Vol.19,
No. 4, Aug. 1978). C(λ)=C( _λ0 )n( _λ0 )/n( ^λ )・^λ2 / _λ02・( _λ02 ⁻ _λ12 )/( ^λ2 ₋ ^{λ12 )}・( ^λ2 −λ ₂ ² )/
(λ ₀ ² −λ ₂ ² )(8) λ ₀ =0.5461μm λ ₁ =0.31215μm λ ₂ =6.9μm C(λ ₀ )−3.489868×10 ^-5 mm ² /Kg・w n(λ)=[ 0.6961663λ ² /λ ² −(0.0684043) ² +0.407
9426λ ² /λ ² −(0.1162414) ² +0.8974794λ ² /λ ² −(9.896161) ² +1〕 ^1/2 (Reference IHmalitson; “Interspecimen
Comparison of the Refractive Index of
“Fused Silica”, Journal of the Optical
Society of America, Vol.55, No.10,
Oct.1965). Therefore, from equations (7) and (8),

[Table] is obtained, and from equations (6) and (9), B (λ _A ) = λ _A λ _B /2l・1/λ _B −δ・λ _A (10) Mode birefringence B (λ )B( _λA , equation (10)
The mode birefringence can be determined from B(λ _A ). Here, δ represents the ratio of the difference in photoelastic constants between λ _A and λ _B. Using equation (10) to find the mode birefringence B of the polarization-maintaining optical fiber used in the measurements in Figure 3, B=
It becomes 2.45×10 ^-4 . The mode birefringence determined using the conventional fiber cutting method is B = 2.2 × 10 ^-4 ,
The method of the present invention is superior in terms of number of significant digits and accuracy. Note that the mode birefringence is expressed as the sum of the birefringence due to core shape and the birefringence due to stress, but the birefringence due to changes in core shape is extremely small compared to the birefringence due to stress. It is ignored in the above calculation. On the other hand, when the birefringence due to changes in core shape cannot be ignored, the following calculation method is used. Measurement wavelengths λ _A , λ _B , and _{λ C are} taken so that the target wavelength λ satisfies (λ A +λ B )/2<λ<( _{λ B} +λ _C )/2 as shown in FIG. First, λ _A ~ _{λ B}
Assuming that B(λ _A )=B(λ _B )=B ₁ , B ₁ is obtained from equation (6). B ₁ = λ _A・λ _B / {2l (λ _B − λ _A )} (11) Similarly, in λ _B to λ _C , B (λ _B ) = B (λ _C ) = B ₂
Assuming that, B ₂ is obtained from equation (6). B ₂ = λ _B・λ _C / {2l (λ _C − λ _B )} (12) From equations (11) and (12), the mode birefringence B(λ) can be found using the interpolation method. can. B(λ)=1/2l{2l−λ _A −λ _B /λ _C −λ _A (λ _B λ _C /λ _C −λ _B −λ _A λ _B /λ _B −λ _A )+λ _A λ _B /
λ _B −λ _A } (13) In this way, by using equation (13), the mode birefringence of a polarization-maintaining optical fiber whose core shape is not circular can also be determined. As explained above, the present invention has the advantage that measurement can be performed quickly by simply changing the wavelength without cutting the polarization-maintaining optical fiber to be measured. Furthermore, while the conventional measuring method could only provide a measurement accuracy of 1 to 2 digits, the present invention has the advantage of obtaining a measurement accuracy of at least 3 digits.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a conventional method for measuring mode birefringence, FIG. 2 is a configuration diagram of an embodiment of the present invention, and FIG.
This figure is a diagram showing changes in received light power with respect to wavelength measured using the measurement system shown in FIG. 2. 1... Light source, 2... Polarizer, 3... Polarization maintaining optical fiber, 4... 1/4 wavelength plate, 5... Analyzer, 6
...Optical power meter, 7...Wavelength variable light source, 8...
...Lens for parallel light, 9...Lens for parallel light, 10
...Photodetector, 11...Condensing lens, 12...Reflected light having an electric field component perpendicular to the incident light, 13...
Condensing lens, 14... Incident light linearly polarized by polarizer 2 and light reflected from polarization-maintaining optical fiber 3, 15... Light from a light source made into parallel light, 16... Optical intensity signal, 17... Synchronization signal from optical chopper, 18... Lock-in amplifier, 1
9...Recorder.

Claims (1)

[Claims] 1. A method for measuring the modal birefringence at wavelength λ of a polarization-maintaining optical fiber, in which a light source capable of changing the wavelength is used as a light source, the light from the light source is made into linearly polarized light, and the With the birefringence axis of the polarization-maintaining optical fiber to be measured tilted with respect to the polarization plane of the polarized light, the polarized light is incident on the optical fiber and propagated, and reflected at the output end of the optical fiber, While detecting the electric field component perpendicular to the incident light of the light returning to the input end, periodic changes in the power of the detected light with respect to changes in the wavelength of the light source are measured, and the core shape of the optical fiber is determined. If the birefringence caused by changes can be ignored, measure the wavelength λ _A corresponding to the valley of the period closest to the desired wavelength λ, and then move from λ _A to the longer wavelength side by 1.
When the wavelength of the period is λ _B , the ratio of the photoelastic constant difference between λ _A and λ _B is δ, the length of the polarization-maintaining optical fiber to be measured is l, and the mode birefringence at λ _A is B ( λ _A ), find the mode birefringence B(λ) from the following formula B(λ)B(λ _A )=λ _A λ _B /2l・1/λ _B −δ・λ _A , or If the birefringence due to changes in the fiber core shape cannot be ignored, the wavelength corresponding to the trough of the period is set to λ _A , λ _B and λ _C sequentially in each period to the longer wavelength side, and (λ _A + λ _B ) /2
λ _A , λ _B , and λ _C are measured so that the relationship <λ < (λ _B + λ _C )/2 holds, and the following formula B(λ)=1/2l{2λ−λ _A −λ _B /λ _C −λ _A (λ _B λ _C /λ _C −λ _B −λ _A λ _B /λ _B −λ _A )+λ _A λ _B /
A method for measuring the mode birefringence of a polarization-maintaining optical fiber, characterized by determining the mode birefringence B(λ) from λ _B −λ _A }. 2. A wavelength tunable light source, a lens system that introduces the light from the light source into the polarization-maintaining optical fiber to be measured, and a lens system that converts the light from the light source into linearly polarized light provided in the middle of the lens system, and introduces the light from the light source into the polarization-maintaining optical fiber to be measured. A polarization-maintaining optical fiber comprising a polarization device that spatially separates light having an electric field component perpendicular to the incident light in reflected light from the fiber, and a photodetector that receives the separated light. Modal birefringence measuring device.