Tomographic refractive index profile evaluation of non-symmetrical glass fiber preforms and fibers themselves

Patent metadata

Field	Value
Publication number	US-12422359-B2
Application number	US-202318214156-A
Country	US
Kind code	B2
Filing date	Jun 26, 2023
Priority date	Jun 26, 2023
Publication date	Sep 23, 2025
Grant date	Sep 23, 2025

Abstract

A method for determining the refractive index profile of a preform when the RIP is not substantially symmetrical. (i) The preform is scanned, starting with a first projection angle, and raw data are created representing the object through measured data. (ii) Optionally, the object is rotated and step (i) repeated iteratively until all projection angles have been scanned and all measured data have been created. (iii) The measured data are processed to form a sinogram and, if the optional step (ii) has been completed, the method proceeds to step (v). (iv) The object is rotated and steps (i) and (iii) are repeated iteratively until all projection angles have been scanned. (v) A 2D RIP is calculated. (vi) A line section of interest is selected within the 2D RIP. (vii) A fitting procedure is applied to the line section of interest. (viii) Finally, refractive index steps/gradients and dimensions are determined.

First claim

What is claimed:

1. A method for determining a refractive index profile (RIP) of an object, the method comprising:
(a) providing the object, the object including an optical object which has a longitudinal axis around which at least one layer extends outwardly wherein the at least one layer lacks a complete rotational symmetry in cross section;
(b) scanning the object with a light beam, starting with a first projection angle, and creating raw data representing the object through measured data;
(c) optionally rotating the object and repeating step (b) iteratively until all projection angles have been scanned and all measured data have been created;
(d) processing the measured data to form a sinogram and, if the optional step (c) has been completed, proceeding to step (f);
(e) rotating the object and repeating steps (b) and (d) iteratively until all projection angles have been scanned;
(f) calculating a 2D RIP from the sinogram;
(g) determining whether an RIP measurement artifact is present in the 2D RIP;
(h) selecting a line section of interest within the 2D RIP to correct the RIP measurement artifact;
(i) applying a fitting procedure to the line section of interest; and
(j) determining refractive index steps/gradients and dimensions.

2. The method according to claim 1 , wherein the step (b) of scanning the object includes directing an entry beam at an entry point into the cylindrical optical object in a direction transverse to the cylinder longitudinal axis, wherein a 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.

3. The method according to claim 1 , wherein the step of calculating the 2D RIP is carried out on the basis of an inverse Radon transformation.

4. The method according to claim 1 , wherein the step of processing the measured data to form a sinogram includes calculating a deflection function, calculating a phase-shift diagram, and stacking the phase-shift diagram to form the sinogram.

5. The method according to claim 1 , wherein the raw data includes a raw image and the method further comprises the step of improving the quality of the raw image by one or more of using a scanning camera that has an “Image ROI” feature, gathering several images with different exposures, using a scanning camera with HDR, and removing OVD-diffractions using a filtering algorithm.

6. The method according to claim 1 , further comprising the step of improving the quality of the sinogram by one or more of applying a radii-run-out correction and interpolating the sinogram to a fine mesh.

7. The method according to claim 1 , wherein the object provided in step (a) is a fiber preform.

8. The method according to claim 7 , further comprising the step of using the determined refractive index steps/gradients and dimensions to characterize the fiber preform and adapt a preform manufacturing process.

9. The method according to claim 1 , further comprising the step, before the step (f), of determining whether all projection angles have been scanned.

10. The method according to claim 1 , further comprising if not if no RIP measurement artifact is present in the 2D RIP, determining geometries and refractive index steps and concluding the method.

11. The method according to claim 1 , wherein the step (i) of applying the fitting procedure to the selected line section of interest includes extracting start parameters for the fitting procedure from the line section, defining a first fitting region, and determining whether an actual layer is step-like.

12. The method according to claim 11 , wherein the fitting procedure further includes, if the actual layer is step-like, generating an analytical deflection curve, transforming the analytical deflection curve to a deflection RIP, and calculating the accuracy of the deflection RIP compared with the initial line section.

13. The method according to claim 1 , further comprising, after the step (j), the step (k) of determining whether all line sections of interest within the 2D RIP have been characterized.

14. The method according to claim 13 , further comprising repeating steps (h), (i), (j), and (k) iteratively until all line sections of interest within the 2D RIP have been characterized.

15. The method according to claim 14 , further comprising calculating a measurement artifact compensated refractive index profile for the object.

Assignees

• Heraeus Quartz North America Llc
• Heraeus Quarzglas

Inventors

• Schmitt Maximilian
• Ponader Carl

Classifications

• G01N2201/104

  Mechano-optical scan, i.e. object and beam moving · CPC title

• G01N2201/0813

  Arrangement of collimator tubes, glass or empty · CPC title

• G01N2201/0633

  Directed, collimated illumination · CPC title

• G01N2021/8887

  based on image processing techniques · CPC title

• G01N2021/4193

  using a PSD · CPC title