High-speed Resonant-Cavity Infrared Detector Arrays and Narrow-Linewidth Quantum Cascade Lasers for Free Space Communication Links

We propose to develop transmitter (TX) and receiver (RX) components that will enable up to 40 Gb/s communication over a free-space communication link (FSCL) operating in the mid-wave infrared (MWIR). Such components when commercialized will enable lower latency than satellite links, greater security than radio-frequency (RF), longer range than mm-wave, and greater availability in haze and turbulence relative to near-IR systems. To maximize the spectral efficiency, and therefore the data throughput, of a limited-bandwidth free-space link, we propose to develop linear E/O and O/E transducers at MWIR (and eventually LWIR) wavelengths. These linear E/O and O/E modules will enable software-defined-radio methods to maximize the data throughput based on the available optical channel. The buried-heterostructure (BH) distributed feedback (DFB) quantum cascade laser (QCL) designed in the Phase 1 program is expected to deliver a stable, single- mode output ( 5 GHz 3-dB bandwidth with usable bandwidth exceeding 15 GHz. Further optimization of the lasers, such as our proprietary high-power QCL concept that would achieve > 1 W stable, single- mode average power for shorter cavity lengths, is expected to double these bandwidth values. The detector design we have developed is based on the original resonant-cavity infrared-detector (RCID) structure but with major improvements: rapid wavelength tunability, carrier collection time of ~ 2 ps, and drastically reduced device capacitance. Even with a 5-GHz modulation bandwidth, we can achieve > 20 Gb/s using 16 QAM modulation based on our expected link dynamic range. If atmospheric conditions reduce the available dynamic range, the proposed link architecture can switch to robust QPSK or BPSK modulation instantaneously. The transducers may also include multiple elements to enable multiple-input multiple-output (MIMO) or single-input multiple output (SIMO) methods. For example, on the receive side, light received by multiple apertures is independently detected and then combined judiciously to maximize the resulting signal to noise ratio (SNR) in the presence of turbulence and beam wander.  The proposed program will conclude with a link demonstration showing the capacity and robustness of the approach.