Measurement technique for refractive index inhomogeneity between plates of a lightguide optical element (LOE)

Patent metadata

Field	Value
Publication number	US-11454590-B2
Application number	US-201917044390-A
Country	US
Kind code	B2
Filing date	Jun 20, 2019
Priority date	Jun 21, 2018
Publication date	Sep 27, 2022
Grant date	Sep 27, 2022

Abstract

A system and method for measuring refractive index inhomogeneity between plates of a Lightguide Optical Element (LOE) uses an innovative measuring technique based on a shearing interferometric technique conventionally used to observe interference and test the collimation of light beams. Another feature of the current implementation is an innovative method for analyzing the characteristics of the generated interferogram to characterize discrepancies between adjacent plates in an LOE.

First claim

What is claimed is:

1. A method for measuring refractive index inhomogeneity comprising:
(a) projecting projected light onto a front surface of a lightguide, said lightguide including:
(i) a first pair of external surfaces parallel to each other, said external surfaces including said front surface and a back surface, and
(ii) a set of coated plates, said set of plates:
(A) parallel to each other,
(B) between said first pair of external surfaces, and
(C) at a non-parallel angle relative to said first pair of external surfaces, and

(iii) each of said plates having a corresponding index of refraction, and

(b) capturing fringes of an interferogram image of an interference pattern to measure the refractive index inhomogeneity between said plates, said interference pattern between refracted/reflected light rays and externally reflected light,
(i) said refracted/reflected light rays being a result of said projected light traversing through difference plates having same or different values of refractive index, reflecting internally from the back surface, and then exiting said lightguide via said front surface and
(ii) said externally reflected light being a result of said projected light reflecting from said front surface.

2. The method of claim 1 wherein said projecting is at an oblique angle relative to said front surface.

3. The method of claim 1 wherein said interferogram image corresponds to refractive index inhomogeneity between said indices of refraction of said plates.

4. The method of claim 1 further including: determining deviation between one or more portions of said fringes and another one or more portions of said fringes, each of said portions corresponding to one or more of said plates, said deviation corresponding to refractive index difference between said indices of refraction of said plates.

5. The method of claim 4 wherein each of said portions of said fringes corresponds to one of said plates.

6. The method of claim 4 wherein said deviation is selected from the group consisting of:
(a) between adjacent said plates,
(b) across multiple said plates, and
(c) across all said plates.

7. The method of claim 4 wherein said determining deviation includes determining how many fringe jumps are in said interferogram.

8. The method of claim 4 further including a step of calculating a pass/fail metric based on said deviation.

9. The method of claim 4 wherein said deviation is determined using at least one technique selected from the group consisting of:
(a) analyzing said interferogram to determine axes of said interferogram,
(b) rotating and/or transforming said interferogram,
(c) splitting said interferogram into N discrete signal arrays orthogonal to said fringes, whereby N is significantly larger than the number of facets in the lightguide, and
(d) calculating a phase of signal arrays n=1 to N, by use of a phase extraction algorithm.

10. The method of claim 9 wherein said at least one technique is said calculating a phase of signal arrays n=1 to N, by use of a phase extraction algorithm, said phase extraction algorithm selected from the group consisting of:
(i) three or four bucket method, and
(ii) wavelet transform.

11. The method of claim 4 wherein a pass/fail metric is derived from said deviation determined over an entirety of said interferogram, and then other pass/fail metrics are derived from deviations between adjacent said plates.

12. The method of claim 4 wherein said deviation is determined by:
(a) splitting said interferogram into N discrete signal arrays orthogonal to said fringes,
(b) calculating a phase of said signal arrays n=1 to N, by use of a phase extraction algorithm, and
(c) plotting said phase as a function of n and determining maximum phase difference between overall maxima and minima in said phase plot.

13. The method of claim 4 wherein said deviation is determined:
(a) using a portion of one of said fringes,
(b) doing a best fit extrapolation to said portion of one of said fringes, and
(c) comparing said extrapolation to another portion of said fringes.

14. The method of claim 4 wherein said deviation is determined by:
(a) extrapolating one of said fringes to generate an ideal fringe,
(b) calculating a departure of an actual one of said fringes from said ideal fringe.

15. The method of claim 4 wherein said projected light is collimated light that is slightly defocused.

16. The method of claim 4 wherein said projected light is monochrome light.

17. The method of claim 4 wherein said projected light is in the visible spectrum.

Assignees

• Lumus Ltd

Inventors

• Gelberg Jonathan
• Rubin Yuval
• Adel Michael

Classifications

• G01M11/0207

  Details of measuring devices · CPC title

• G01N21/45 Primary

  using interferometric methods; using Schlieren methods · CPC title

• G01M11/37 Primary

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

• G01J9/0215

  by shearing interferometric methods · CPC title

• G01J9/02

  by interferometric methods (using interferometers for measuring optically the linear dimensions of objects G01B9/02) · CPC title