Discretely Tunable (2594, 2629, 2670 nm) GaSb/Si₃N₄ Hybrid Laser for Multiwavelength Spectroscopy

A discrete, tunable photonic integrated laser is showcased for multiwavelength spectroscopy of CO₂, H₂S, and H₂O. The laser combines an AlGaInAsSb/GaSb type-I quantum well-reflective semiconductor optical amplifier with a Si₃N₄ photonic integrated circuit (PIC). Operating at room temperature, the laser emits at 2670.42, 2629.12, and 2594.27 nm, with mW-level average powers. The PIC employs a novel approach for achieving switching between three distinct emission wavelengths by using two cascaded tunable Mach–Zehnder interferometers (MZIs) that are connected to three spiral-shaped narrow-band distributed Bragg reflectors (DBRs). Phase tuning the arms of the MZIs allows the laser emission wavelength to be switched between the DBRs, while the DBRs themselves offer fine-tuning of the emission wavelength. This design enables a simpler tuning mechanism with fewer control variables compared to hybrid PIC-based lasers using Vernier architectures or a combination of multiple semiconductor amplifiers. Low-loss and broadband MZIs are achieved by employing two interconnected directional couplers, which utilize asymmetric waveguides in the coupling region to effectively control the phase. Besides achieving state-of-the-art performance for Si₃N₄-based integrated lasers at 2.6 µm wavelength region, the demonstrated performance of passive components, including MZIs and spiral DBRs, opens new possibilities for mid-infrared PIC technology.