METHODS, TEST STRUCTURES, AND TEST SYSTEMS FOR DETERMINING A SURFACE CHARACTERISTIC OF A CHIP FACET
A test system for determining a surface characteristic of a chip facet comprises a chip, which has a facet and includes a waveguide, a detector, and a processor. The on-chip waveguide is configured to direct test light towards the facet, where a portion of the test light is reflected and a portion of the test light is transmitted. The detector is configured to measure an amount of the reflected portion or the transmitted portion, and the processor is configured to determine a surface characteristic of the facet, such as a facet angle, a facet curvature, and/or a facet roughness, on the basis of the measured amount.
1 . A method for determining a surface characteristic of a facet of a photonic chip, the method comprising:
using a first waveguide defined in a wafer to direct test light toward a sidewall etched in the wafer, the sidewall forming the facet of the photonic chip;
measuring at least one of: a reflected portion of the test light that is reflected by the sidewall, or a transmitted portion of the test light that is transmitted through the sidewall; and,
determining a surface characteristic of the facet based upon at least one of the reflected portion or the transmitted portion.
2 . The method of claim 1 comprising selectively etching the wafer to form the sidewall.
3 . The method of claim 1 wherein the measuring is performed before the photonic chip is separated from the wafer.
4 . The method of claim 1 wherein the measuring is performed after the photonic chip is separated from the wafer.
5 . The method of claim 1 comprising coupling the test light into the first waveguide using a first vertical coupler.
6 . The method of claim 1 comprising coupling the reflected portion of the test light to a first photodetector (PD).
7 . The method of claim 6 wherein the first PD is separate from the photonic chip, and wherein the coupling of the reflected portion of the test light to the first PD comprises using a second vertical coupler.
8 . The method of claim 6 wherein the first PD is integrated with the photonic chip.
9 . The method of claim 6 comprising coupling the reflected portion of the test light into a second waveguide that is monolithically integrated with the first waveguide, for guiding at least a fraction thereof into the first PD.
10 . The method of claim 9 first waveguide, and collecting the reflected portion of the test light upon a reflection thereof from the facet using an end section of the second waveguide that is inclined toward the first waveguide.
11 . The method of claim 6 comprising collecting the reflected portion of the test light into an end section of the first waveguide that is proximate to the facet.
12 . The method of claim 10 comprising coupling at least a fraction of the reflected portion of the test light from the first waveguide into a second waveguide for guiding to the first PD.
13 . The method of claim 12 comprising using an on-chip Michelson interferometer to measure the reflected portion of the test light.
14 . The method of claim 13 comprising using the first waveguide as a test arm of the on-chip Michelson interferometer, using the second waveguide as a reference arm of the on-chip Michelson interferometer, using an integrated 2×2 optical coupler to optically couple the first waveguide to the second waveguide, and an integrated reflector disposed at one end of the second waveguide to reflect a fraction of the test light back toward the integrated 2×2 optical coupler as reference light for mixing with the reflected portion of the test light prior to coupling to the first PD.
15 . The method of claim 14 comprising using an integrated balanced photodetector comprising the first PD coupled to the second waveguide and a second PD coupled to the first waveguide.
16 . The method of claim 14 comprising adjusting an optical phase of the reference light using an integrated phase shifter.
17 . The method of claim 8 wherein the first PD is disposed above or below the first waveguide relative to a plane of the wafer.
18 . The method of claim 1 wherein the measuring comprises measuring the transmitted portion of the test light.
19 . The method of claim 18 wherein the measuring is performed on-wafer using an optical fiber to collect the transmitted portion of the test light into a photodetector (PD).