Mid and Long-Wave Infrared Free-Space Optical Communication

Mid-wave (MW) and long-wave infrared (LWIR) spectral bands (3 to 5 mu m and 9 to 14 mu m) are known for their robust transmission characteristics in free-space optical communications (FSOC) under various weather conditions such as haze, fog, rain, and snow. These bands are also expected to be more tolerant to atmospheric turbulence compared to the short-wave IR region (SWIR) near 1.55 mu m. Conversely, low-cost, power efficient laser transmitters (Tx) and receivers (Rx) for the MW-LWIR wavelengths are not as widely available as the 1.55 mu m counterpart especially for high bandwidth. Larger aperture sizes are also likely required for MW-LWIR to maintain acceptable beam divergence and adequate receiver signal-to-noise ratios (SNRs). All of these are challenges for the development of the MW-LWIR FSOC technology. In the framework of ARAP DOC-P program (Applied Research for the Advancement of S&T Priorities Defense Optical Channel Program), CCDC-ARL (Combat Capabilities Development Command Army Research Laboratory) has taken on the challenge to investigate and develop ground-to-space FSOC in the MW-LWIR regions with commensurate comparisons of MW-LWIR and SWIR systems. The effort started with a detailed literature survey on the MWIR and LWIR FSOC experiments and the latest progress. CCDC-ARL has conducted investigations of the state-of-the-art MW-LWIR laser Tx and MW-LWIR photodetectors including in-house development. An FSOC ground testbed employing MW-LWIR COTS quantum cascade laser (QCL) sources is being developed. The Tx will be directly modulated using electronic circuits built in-house. In a collaborative effort with the Naval Research Laboratory (NRL), CCDC-ARL is testing a free-space link emulator based on 1.55 mu m fiber optics components first developed by NRL. CCDC-ARL is also developing atmospheric beam propagation simulation tools based on random phase screens in order to gain insight and compare the performance envelope for MW-LWIR and SWIR.