Measurement method of non-circularity of core optical fiber base material and apparatus therefor

1. A measurement method for measuring non-circularity of a core part of an optical fiber base material having the core part and a clad part, the method comprising:

immersing the optical fiber base material in liquid having a refractive index substantially equal to that of the clad part of the optical fiber base material;

irradiating parallel light from a side face of the optical fiber base material to measure an intensity distribution of transmitted light;

measuring a width between two change points, at which two bright spaces corresponding to the clad part are changed to a dark space and which correspond to two boundaries between the clad part and the core part, on the intensity distribution to obtain a relative value for a core diameter;

rotating the optical fiber base material to further obtain the relative value for the core diameter at plural points for a circumferential direction; and

obtaining non-circularity of the core part based on the obtained plurality of relative values for the core diameter.

2. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein the non-circularity of the core part is obtained by dividing a difference between a maximum value and a minimum value for the relative values for the core diameter measured from the plurality of circumferential directions by a mean value for the relative values for the core diameter.

3. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein the measurement of non-circularity of the core part is performed by vertically arranging the optical fiber base material.

4. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein a portion of a vessel accommodating liquid, which is passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, consists of a material having a refractive index substantially equal to that of the clad part.

5. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein a portion of a vessel accommodating liquid, which is passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, consists of a material equal to that of the clad part.

6. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein the liquid in a vessel is regulated to constant temperature.

7. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein temperature of an atmosphere in which a core non-circularity measuring apparatus is provided is substantially constantly regulated.

8. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein the width between the change points is measured by a parallel light projection type diameter measurement device.

9. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein the relative value D c (φ) for the core diameter measured from the plurality of circumferential directions is fitted to D c (φ)=A+Bsin2φ, and the non-circularity of the core part is set to 2B/A.

10. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 1 , wherein a vessel accommodating liquid has a parallel outer surface opposite to a portion passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, and a cylindrical hole is provided in a center of the vessel.

11. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 8 , wherein the parallel light projection type diameter measurement device can adjust a detection threshold value.

12. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 9 , wherein the fitting uses Fourier analysis or high-speed Fourier analysis.

13. The measurement method of non-circularity of a core part of an optical fiber base material as claimed in claim 10 , wherein the parallel outer surface and an inner surface of the cylindrical hole facing each other are polished.

14. A measurement apparatus for measuring non-circularity of a core part of an optical fiber base material having the core part and a clad part, the apparatus comprising:

means for immersing the optical fiber base material in liquid having a refractive index substantially equal to that of the clad part of the optical fiber base material;

means for irradiating parallel light toward a side face of the optical fiber base material immersed in the liquid;

means for measuring an intensity distribution of transmitted light passing through the optical fiber base material;

means for measuring a width between two change points, at which two bright spaces corresponding to the clad part are changed to a dark space and which correspond to two boundaries between the clad part and the core part on the intensity distribution to obtain a relative value for a core diameter; and

means for rotating the optical fiber base material to measure relative values for the core diameter at plural points for a circumferential direction of the optical fiber base material, wherein the non-circularity of the core part is obtained based on the plurality of relative values for the core diameter.

15. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , further comprising means for vertically supporting the optical fiber base material.

16. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , wherein a portion of a vessel accommodating liquid, which is passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, consists of a material having a refractive index substantially equal to that of the clad part.

17. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , wherein a portion of a vessel accommodating liquid, which is passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, consists of a material equal to that of the clad part.

18. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , further comprising a preform analyzer.

19. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , further comprising a control and arithmetic unit for performing control and arithmetic processing for each means.

20. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , wherein said means for obtaining a relative value for a core diameter is a parallel light projection type diameter measurement device.

21. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 14 , wherein a vessel accommodating liquid has a parallel outer surface opposite to a portion passed through by parallel light and the transmitted light at least irradiated on the optical fiber base material, and a cylindrical hole is provided in a center of the vessel.

22. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 15 , wherein the parallel light projection type diameter measurement device can adjust a detection threshold value.

23. The measurement apparatus of non-circularity of a core part of an optical fiber base material as claimed in claim 21 , wherein the parallel outer surface and an inner surface of the cylindrical hole facing each other are polished.