Method for determining the refractive index profile of a cylindrical optical object, particularly a preform for an optical fiber

Patent metadata

Field	Value
Publication number	US-10508973-B2
Application number	US-201715785690-A
Country	US
Kind code	B2
Filing date	Oct 17, 2017
Priority date	Oct 26, 2016
Publication date	Dec 17, 2019
Grant date	Dec 17, 2019

Abstract

A method for determining the refractive index profile of a preform is provided. The method involves: preparing the measured deflection angle distribution, including an extreme value determination of the deflection angle distribution, to obtain a prepared deflection angle distribution; transforming the prepared deflection angle distribution into a prepared refractive-index profile; evaluating the prepared refractive-index profile for the fixation of orientation values for the layer radius and for the layer refractive index of a hypothetical refractive index profile; generating a simulated deflection angle distribution on the basis of the hypothetical refractive-index profile with the orientation values, and transforming the deflection angle distribution into a simulated refractive-index profile; fitting the simulated refractive index profile to the prepared refractive-index profile by iterative adaptation of parameters to obtain a fitted, simulated refractive-index profile which is defined by adapted parameters, and obtaining the refractive index profile as the hypothetical refractive-index profile with the adapted parameters.

First claim

We claim:

1. Method for determining a radial refractive-index profile of a cylindrical optical object which has a cylinder longitudinal axis around which at least one layer k with a layer radius r k and with a layer refractive index n k extends radially symmetrically, the method comprising:
measuring a deflection angle distribution Ψ(y), measuring of the deflection angle distribution Ψ(y) comprising directing an entry beam at an entry point into the cylindrical optical object in a direction transverse to the cylinder longitudinal axis, wherein deflection angle Ψ is defined as the angle between an exit beam relative to the entry beam, and y is the distance between the cylinder longitudinal axis and the entry point of the entry beam in a Cartesian coordinate system; and
reconstructing the refractive index profile therefrom on the basis of a model, the model comprising the following measures:
(a) adjusting the measured deflection angle distribution Ψ(y), including an extreme value determination of the deflection angle distribution and including regions of a refractive index step, wherein an adjusted deflection angle distribution Ψ′(y) is obtained and wherein the adjusting of the measured deflection angle distribution comprises a correction in which the origin of the deflection angle distribution is adjusted,
(b) transforming the adjusted deflection angle distribution Ψ′(y) into an adjusted refractive-index profile n′(r),
(c) evaluating the adjusted refractive-index profile n′(r) for fixation of orientation values, the orientation values comprising an orientation value r* k for the layer radius and an orientation value n* k for the layer refractive index of a hypothetical refractive-index profile n*(r),
(d) creating a simulated refractive-index profile n″(r) by generating a simulated deflection angle distribution Ψ″(y) on the basis of the hypothetical refractive-index profile n*(r) with the orientation values r* k and n* k , and transforming said deflection angle distribution into the simulated refractive-index profile n″(r),
(e) fitting the simulated refractive index profile n″(r) to the adjusted refractive-index profile n′(r) by iterative adaptation of the orientation values r* k and n* k , wherein a fitted, simulated refractive-index profile n*(r) fit is obtained which is defined by adapted parameters r* k,fit and n* k,fit , and
(f) obtaining the refractive index profile as the hypothetical refractive-index profile with the adapted parameters r* k,fit and n* k,fit .

2. The method according to claim 1 , wherein the extreme value determination according to measure (a) is a smoothing of the measured deflection angle distribution by means of a spline function using several different smoothing parameters.

3. The method according to claim 2 , wherein in the extreme value determination, an innermost right extreme value y k,right and an innermost left extreme value y k,left are determined.

4. The method according to claim 3 , wherein the preparation of the measured deflection angle distribution comprises a correction in which the origin of the deflection angle distribution is adjusted and wherein the adjustment of the origin of the deflection angle distribution comprises a shifting in the direction of a y-axis of the coordinate system into the middle between innermost right extreme value y k,right and innermost left extreme value y k,left of the refractive index profile.

5. The method according to claim 3 , wherein in the evaluation of the adjusted refractive-index profile n′(r) according to measure (c), the determined extreme values y k,right and y k,left are used for the fixation of the orientation value r* k .

6. The method according to claim 5 , wherein the adjustment of the origin of the deflection angle distribution comprises a shifting in the direction of a y-axis of the coordinate system into the middle between innermost right extreme value y k,right and innermost left extreme value y k,left of the refractive index profile.

7. The method according to claim 3 , wherein the determined extreme values y k,right and y k,left are converted into layer radii r k,right and into r k,left , respectively, and the layer radii are used for the fixation of the orientation value r* k .

8. The method according to claim 7 , wherein the preparation of the measured deflection angle distribution comprises a correction in which the origin of the deflection angle distribution is adjusted and wherein the adjustment of the origin of the deflection angle distribution comprises a shifting in the direction of a y-axis of the coordinate system into the middle between innermost right extreme value y k,right and innermost left extreme value y k,left of the refractive index profile.

9. The method according to claim 3 , wherein the preparation of the measured deflection angle distribution comprises a correction in which the origin of the deflection angle distribution is adjusted.

10. The method according to claim 9 , wherein the adjustment of the origin of the deflection angle distribution comprises a shifting about an offset in the direction of a z-axis of the coordinate system, wherein the offset is calculated as a positional difference between the zero line of the coordinate system and a straight line which with the help of the sum of least squares method is fitted to the middle between innermost right extreme value y k,right and innermost left extreme value y k,left .

11. The method according to claim 1 , wherein the transformation of the prepared refractive-index profile n′(r) according to measure (b) is carried out on the basis of an Abel transform.

12. The method according to claim 1 , wherein a fitted, simulated refractive-index profile n*(r) fit is present when the deviation between the simulated refractive-index profile n″(r) and the adjusted refractive-index profile n′(r), as calculated on the basis of the “least absolute residuals” or on the basis of the “least squares method,” is below a predetermined threshold value.

13. The method according to claim 1 , wherein the parameters determined in the reconstruction of the real refractive-index profile n(r), in particular the adapted parameters r* k,fit and n* k,fit , are used for the adaptation of a preform manufacturing process.

Assignees

• Heraeus Quarzglas

Inventors

• Schmitt Maximilian
• Kühn Bodo

Classifications

• G02B6/0365

  arranged - - + · CPC title

• G02B27/0012 Primary

  Optical design, e.g. procedures, algorithms, optimisation routines · CPC title

• G01M11/0228

  by measuring refractive power · CPC title

• G01M11/35 Primary

  in which light is transversely coupled into or out of the fibre or waveguide, e.g. using integrating spheres (G01M11/31 takes precedence) · CPC title

• G01M11/37

  in which light is projected perpendicularly to the axis of the fibre or waveguide for monitoring a section thereof · CPC title