High-performance, adiabatically nanotapered fiber-chip couplers in silicon at 2 microns wavelength.

Fiber optic technology connects the world through the Internet, enables remote sensing, and connects disparate functional optical devices. Highly confined silicon photonics promises extreme scale and functional integration. However, the optical modes of silicon nanowire waveguides and optical fibers are very different, making efficient fiber-chip coupling a challenge. Vertical grating couplers, the dominant coupling method today, have limited optical bandwidth and are naturally out-of-plane. Here we demonstrate a new method that is low-loss, broadband, manufacturable, and naturally planar. We adiabatically couple a tapering silicon nanowire waveguide to a conic nanotapered optical fiber, measuring transmission between 2.0 µm and 2.2 µm wavelength. The silicon chip is fabricated at a commercial foundry and then post-processed to release the tapering nanowires. We estimate an optimal per-coupler transmission of -0.48 dB (maximum; 95% confidence interval [+0.46, -1.68] dB) and a 1-dB bandwidth of at least 295 nm. With automated measurements, we quantify the device tolerance to lateral misalignment, measuring a flat response within ±0.968 µm. This new design can enable low-loss modular systems of integrated photonics irrespective of material and waveband.