A comparative study of 3.6μm and 1.55μm atmospheric transmission

The use of 1.55μm lasers for free space optical links has become well established. While one expects there to be advantages to operating at 3.6μm, namely higher transmission through water vapor and reduced scintillation, the availability of both lasers and detectors at the mid IR wavelength is not as mature as those available at the telecomm choice of 1.55μm. However, there are potential schemes for using frequency conversion to probe the atmosphere in the mid-ir but to detect back in the near-ir. A sequence of experiments has been conducted, over a 16km one-way link across the Chesapeake Bay, to directly compare the intensity variances and the power spectrum imposed by the atmosphere at the two wavelengths. An interband cascade laser was used which operated at a wavelength of 3.6μm and had an output power of 100mW. The 1.55μm system used standard telecomm parts. Data were recorded simultaneously from both systems at 4kHz and were digitized using a 16-bit card. A telescope measured the angle-of-arrival variance of the 1.55mm beam in order to give a measure of the atmospheric structure constant C_n². In addition a visibility monitor and weather station were usually operational at each end of the link.