Prism-coupling systems and methods with improved intensity transition position detection and tilt compensation

A method implemented by a prism coupling system of estimating at least one stress-related characteristic of an ion-exchange (IOX) article having a waveguide region, comprising:

• illuminating the IOX article through a coupling prism to generate transverse magnetic (TM) and transverse electric (TE) guided modes in the waveguide region of the IOX article;
• capturing a digital mode spectrum comprising mode lines of a TM mode spectrum and a TE mode spectrum representative of the TM and TE guided modes of the waveguide region;
• calculating at least one of a TE derivative spectrum or a TM derivative spectrum from the corresponding mode spectrum comprising derivative intensity transitions corresponding to intensity transitions in the corresponding mode spectrum;
• for each derivative intensity transition, determining a fitted line representative of a position of the derivative intensity transition, to define a set of fitted lines having respective positions in at least one of the TE mode spectrum or the TM mode spectrum;
• performing error checking on each fitted line in the set of fitted lines to assess whether the associated derivative intensity transition corresponds to a mode line or to noise in the mode spectrum and discarding any of the fitted lines found to be due to noise, thereby establishing at least one of a set of checked fitted lines for the TE mode spectrum or a set of checked fitted lines for the TM mode spectrum;
• defining estimated positions of the intensity transitions in at least one of the TE mode spectrum or the TM mode spectrum as the respective positions of the set of checked fitted lines to define at least one of a corrected TE mode spectrum or a corrected TM mode spectrum, respectively;
• and
• estimating the at least one stress-related characteristic of the IOX article using at least one of the corrected TM mode spectrum or the corrected TE mode spectrum.

A prism coupling system for measuring at least one stress-related characteristic of an ion-exchanged (IOX) article having a waveguide region, comprising:

• a light source system configured to generate input light along an input optical axis;
• a coupling prism configured to interface with the IOX article to define a coupling interface, wherein the input light incident upon the coupling interface reflects therefrom to form reflected light that exits the coupling prism and travels along an output optical axis, the reflected light comprises a guided mode spectrum comprising mode lines of transverse magnetic (TM) and transverse electric (TE) guided mode spectra with positions as defined by the waveguide region;
• a digital sensor comprising pixels and configured to detect the TM and TE guided mode spectra and form therefrom a two dimensional (2D) digital image of the TM and TE guided mode spectra;
• a controller configured to receive the 2D digital image of the TM and TE guided mode spectra, the controller having instructions stored in a non-transitory computer-readable medium that cause the controller to carry out the following acts:
• i) establishing for each mode line an amount of tilt;
• ii) establishing from the amounts of tilt for the mode lines an average amount of tilt and a tilt variance;
• iii) subtracting the average amount of tilt from the mode spectrum to define a tilt-compensated mode spectrum that has a reduced amount of tilt for each of the mode lines;
• and
• iv) estimating the at least one stress-related characteristic of the IOX article using the tilt-compensated mode spectrum.