Fig. 1: Single-mode laser platform by integrating a multi-mode Fabry–Perot (FP) interband cascade laser (ICL) laser with a high quality factor (Q) microresonator.

a Illustration of a single-frequency mid-IR source, consisting of a multi- mode ICL laser and a silicon microring cavity. b Microscope image of the silicon chip with a 150-μm radius high-Q microring (coupling gap of 510 nm), a Sagnac reflector, and aluminum microheaters. c Schematic of the single-mode laser. A multi-mode ICL around 3.4 μm with an anti-reflection coating on the front facet. The Sagnac reflector at the drop port provides frequency-selective optical feedback and form an external cavity with the ICL back facet. Microheaters are placed on top of the bus waveguide and the microring to control the phase of the external cavity and the ring resonance, respectively. d Simulated reflection spectrum of the Sagnac loop mirror over wavelengths from 3 μm to 4 μm using Finite-Difference Eigenmode solver. The Sagnac loop mirror reflects about 99% at the drop port at 3.4 μm. A coupler of the Sagnac loop mirror at the drop port consists of a 400 nm gap and a 115 μm length.