Narrow laser beams directed from aircraft may at times pass through the exhaust plume of the engines and potentially degrade some of the laser beam characteristics. This paper reports on controlled studies of laser beam deviation arising from propagation through turbulent hot gases, in a well-characterized laboratory burner, with conditions of relevance to aircraft engine exhaust plumes. The impact of the temperature, laser wavelength, and turbulence length scale on the beam deviation has been investigated. It was found that the laser beam displacement increases with the turbulent integral length scale. The effect of temperature on the laser beam angular deviation, σ, using two different laser wavelengths, namely 4.67 μm and 632.8 nm, was recorded. It was found that the beam deviation for both wavelengths may be semiempirically modeled using a single function of the form, σ=a(b+(1/T)(2))(-1), with two parameters only, a and b, where σ is in microradians and T is the temperature in °C.

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We have demonstrated audio communications with a mid-IR laser. The laser is a frequency doubled Q-switched CO 2 system producing approximately 12ns pulses at 4.6 m. The audio signal was encoded on the beam by means of pulse frequency modulation (PFM) with a carrier frequency of 37kHz. A 1mm diameter, low noise thermoelectrically cooled IR photovoltaic detector with electrical bandwidth 250MHz was used to detect the laser beam. A custom-built circuit stretched the resultant electrical pulses to approximately 1.5 s, before being demodulated. High quality audio signals were received and recorded, and still images were successfully transmitted using slow scan television techniques.The demonstration was conducted at the Defence Science & Technology Organisation's laser range at Edinburgh, South Australia in July 2008. The distance was 1.5km, with a slant path (8m to 1.5m). The maximum range using this system is estimated to be tens of kilometres.