InGaAs/InP chip-based mid-infrared optical phased arrays for communications and sensing

The mid-infrared atmospheric transmission windows (wavelengths ~ 3–5 and 8–14 µm) provide a unique opportunity for low-loss transmission in areas such as thermal imaging, lidar, infrared homing, and line-of-sight communications. Additionally, in comparison to traditional operation in the near-infrared, the lower background solar irradiation of the mid-infrared permits higher signal-to-noise detection. In spite of these inherent benefits, the advance of applications in the mid-infrared has lagged their near-infrared counterparts, due to the lack of maturity in material properties, light sources, and detectors specific to the region. This research seeks to advance the state of the art in on-chip photonics operating in the mid-infrared transmission windows. In particular, in this work I build and demonstrate operation of optical phased arrays (OPAs) operating at a wavelength of 4.6 µm. On-chip OPAs provide non-mechanical beam formation and steering in a compact form factor—an enabling technology for the next generation of miniaturized mid-infrared systems that will be incorporated into, e.g., unmanned aerial vehicles. The work is carried out in an InP-based platform in consideration of the fact that InP-based quantum cascade lasers are now the preeminent mid-infrared light source, therefore conceiving the potential for a fully monolithic integrated OPA solution. In the first phase of this research, I developed and demonstrated a 1D periodic OPA, that is to say, a 1D periodic array of emitting elements, permitting beam steering in one dimension. The thermo-optic effect was employed for phase tuning and surface gratings were used for emitting elements. An unambiguous steering field-of-view (FOV) of 23° with 38 resolvable points was achieved. In the second phase of this research, the OPA was revamped to improve performance. First, heat-isolation trenches were incorporated to improve phase tuning efficiency. Second, the emitting elements were changed to total-internal-reflection mirrors, whose small area permitted distribution in a 2D array. Third, the array was made aperiodic—the emitter positions being optimized to maximize the number of resolvable points. Within an observed 23°-diameter-FOV >500 resolvable points were achieved, with an estimate of >1000 resolvable points within the full device FOV.