Abstract
Free space optical communication (FSO) offers a promising solution to the “last mile” bottleneck of high-speed telecommunications. Data is encoded using a modulated laser and optically sent through the air to a receiving photodetector. Although this requires FSO systems to have direct line-of-sight between the emitter and receiver, this is often much easier and more cost-effective than ripping up the streets to lay traditional copper wires or fiber optic cables. Today's cities are full of FSO communications links; however, most of these operate at the conventional fiber optic wavelength of 1.55 μm and face losses due to scattering in the presence of fog, heavy rain, or other inclement weather conditions. By shifting to the mid-wavelength infrared (MWIR) spectrum, the scattering losses can be significantly reduced which would allow a MWIR system to achieve better link uptime. We report the design and realization of a prototype room temperature MWIR FSO system using a room temperature MWIR quantum cascade laser (QCL) and a room temperature, type-II InAs/GaSb superlattice (T2SL) photodiode, both of which have been developed and fabricated at the Center for Quantum Devices (CQD). We designed printed circuit boards for laser modulation and detector signal amplification. We also used field programmable gate arrays (FPGAs) to serve as an interface for encoding and decoding data transmitted between computers and the FSO system. Our system provides an optical link between two computers suitable for ethernet data rates over a range of up to 1 kilometer.