Fabry-Perot-Insensitive Edge Coupling for Robust PIC Characterization

Abstract: In this work, we investigate the impact of Fabry-Perot (FP) cavities formed between a fiber array and a silicon chip during edge-coupled testing of photonic integrated circuits (PICs). Our results show that subwavelength variations in the distance between the fiber array and the device under test induce coupling efficiency modulations of several tenths of a decibel - challenging current alignment precision and undermining measurement repeatability in wafer-level probing. To address these FP-induced fluctuations, we introduce a complementary dual-measurement technique that shifts the chip-fiber separation by a quarter wavelength, thereby generating phase-opposite modulation artifacts that cancel upon averaging. This approach allows a higher tolerance in probe-to-PIC positioning by substantially eliminating the FP modulation. We also demonstrate how it can be leveraged to enhance repeatability and accurate positioning during alignment, reducing the insertion loss variability between multiple measurements from 0.34 to 0.04 dB. Such precision is essential for emerging applications like quantum photonics, silicon photonic sensors, and high-bandwidth optical interconnects where measurement accuracy directly impacts device performance characterization. Ultimately, our method offers a robust and accurate solution for high-throughput PIC characterization without requiring index-matching gel, which is impractical for wafer-level testing.