Methods, test structures, and test systems for determining a surface characteristic of a chip facet

Abstract

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.

We claim: 
     
         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).