Method of enhancing contrast in prism coupling measurements of stress

Patent metadata

Field	Value
Publication number	US-9696207-B2
Application number	US-201514691778-A
Country	US
Kind code	B2
Filing date	Apr 21, 2015
Priority date	Apr 23, 2014
Publication date	Jul 4, 2017
Grant date	Jul 4, 2017

Abstract

A method and apparatus for improving contrast in prism coupling measurements of waveguide mode spectra, wherein the measured waveguide sample has a surface region of rapidly decreasing index, characterized with normalized slope



λ
n

⁢

ⅆ
n

ⅆ
z



>

0.0004
.

An opaque light-blocking element is placed in the portion of the light beam closest to the plane of the contact between prism and measured sample, on the input side, output side or both sides of the prism. The light blocking element prevents light from the light source to reach a portion of the length of the prism-sample coupling interface along the optical path, prevents light reflected from a portion of the aforementioned length to reach the detector, or both when input and output light-blocking elements are used.

First claim

The invention claimed is:

1. A method of measuring waveguide mode spectra, the method comprising:
forming a coupling interface between a coupling surface of a prism and a surface of a waveguide, the waveguide having a surface region of decreasing index with normalized slope



λ
n

⁢

ⅆ
n

ⅆ
z



>
0.0004

,

where n=n(z) is a refractive index profile of the waveguide as a function of a distance z into the waveguide from the waveguide surface and λ is a measurement wavelength of a light beam from a light source, the surface region extending from a surface into the waveguide sample, and the prism having an input surface and an output surface;
disposing an opaque light-blocking element spaced apart from both the input surface and the output surface in a portion of a path of the light beam emanating from the light source, the portion being closest to a plane of contact between the coupling surface and the waveguide sample;
at least one of:
blocking at least a portion of the light beam with the opaque light-blocking, element, wherein the portion of the light beam is prevented from reaching a portion of the coupling surface along the path of the light beam; and
blocking a first portion of light reflected from the coupling interface with the opaque light-blocking element, wherein the first portion is prevented from reaching a detector;

allowing a second portion of light reflected from the coupling interface to reach the detector; and
detecting the second portion with the detector at a position corresponding to as lowest-order mode, and is less than or equal to 7 times the maximum possible effective coupling length of the lowest-order mode or 7 mm, whichever is less.

2. The method of claim 1 , wherein the position corresponding to the lowest-order mode is less than or equal to 5 times the maximum effective coupling length.

3. The method of claim 2 , wherein the at least one light-blocking element is located within 30 mm of an edge of the prism.

4. The method of claim 2 , wherein the at least one light-blocking element is located within 7 mm of an edge of the prism.

5. The method of claim 2 , wherein the waveguide further includes a deep region of decreasing index, the deep region penetrating to a depth of at least 170λ/n from the surface of the waveguide, and having normalized slope



λ
n

⁢

ⅆ
n

ⅆ
z



<

0.00007
.

6. The method of claim 5 , wherein the depth of the deep region is at least 200λ/n.

7. The method of claim 6 , wherein at the least one light-blocking element is located at least 30 mm from a plurality of edges of the prism, and provides a substantial difference in illuminated interaction length or detected interaction length between the lowest-order mode and the highest-order mode.

8. The method of claim 7 , wherein the light-blocking element is located less than or equal to 100 mm from each of the edges of the prism.

9. A method of measuring waveguide mode spectra, the method comprising:
forming a coupling interface between a coupling surface of a prism and a surface of a waveguide, the waveguide having a surface region of decreasing index with normalized slope



λ
n

⁢

ⅆ
n

ⅆ
z



>
0.0004

,

where n=n(z) is a refractive index profile of the waveguide as a function of a distance z into the waveguide from the waveguide surface and λ is a measurement wavelength of a light beam from a light source, the surface region extending from a surface into the waveguide sample, and the prism having an input surface and an output surface;
disposing an opaque light-blocking element spaced apart from both the input surface and the output surface in a portion of a path of the light beam emanating from the light source, the portion being closest to a plane of contact between the coupling surface and the waveguide sample, wherein the light-blocking element has a transmission, the transmission being a variable function of distance from the plane of contact with the transmission increasing as the distance from the plane of contact increases, and wherein the light-blocking element having the variable transmission is not a circular aperture;
at least one of:
blocking at least a portion of the light beam with the opaque light-blocking element, wherein the portion of the light beam is prevented from reaching a portion of the coupling surface along the path of the light beam; and
blocking a first portion of light reflected from the coupling interface with the opaque light-blocking element, wherein the first portion is prevented from reaching a detector;

allowing a second portion of light reflected from the coupling, interface to reach the detector; and
detecting the second port at with the detector at a position corresponding to a lowest-order mode, and is less than or equal to 7 times the maximum possible effective coupling length of the lowest-order mode or 7 mm, whichever is less.

10. The method of claim 9 , wherein the light-blocking element is a plate having spatially varying absorption.

11. The method of claim 9 , wherein the light-blocking element has a shape with more open space near a first side that is distal from the prism-sample coupling plane.

12. The method of claim 11 , wherein the light-blocking element is made of essentially opaque material.

13. The method of claim 11 , wherein the variation in open space from the first side to the prism-sample coupling, plane is achieved by a V-shaped cutout.

14. The method of claim 13 , wherein of the V-shaped cutout forms an angle γ of between about 30 degrees and about 50 degrees.

15. The method of claim 9 , wherein the light-blocking element is positioned up to 100 mm from at least one edge of the prism.

16. The method of claim 9 , wherein the light-blocking element is positioned up to 10 mm from

Assignees

• Corning Inc

Inventors

• Roussev Rostislav Vatchev
• Young Emily Elizabeth

Classifications

• G01N21/552 Primary

  Attenuated total reflection · CPC title

• G01L1/242

  the material being an optical fibre · CPC title

• G01L1/241

  by photoelastic stress analysis · CPC title

• G01J3/42 Primary

  Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry (beam switching arrangements G01J3/08) · CPC title

• G01N21/4133

  Refractometers, e.g. differential · CPC title