Laser based missile defence systems (DIRCM) are being increasingly employed on aircraft. In certain circumstances the
laser must pass through the exhaust gases of the aircraft engine. In order to predict the degree of divergence and
dispersion of the laser, an understanding of the exhaust gas structure and its influence is required. Specifically the effect
of parameters such as temperature, carbon dioxide, turbulence intensity and length scales as well as the laser beam
wavelength and beam diameter. A parametric study under laboratory controlled conditions was undertaken to examine
these effects. The results of beam propagation through high temperature turbulent flows and for various CO2 and H2O
concentrations are presented in this paper for wavelengths 632.8 nm and 4.67 μm. It was found that the beam
displacement showed an approximate inverse square relationship to temperature. At high combustion temperatures the
632.8 nm beam was significantly broken-up and dispersed. Displacement of both beams appeared to be asymptotic above
600°C. Carbon dioxide absorption effects were found not to significantly influence the beam displacement at the
wavelengths and temperatures studied. Quantifying these effects at high temperatures will assist with the development of
a parametrically based laser beam propagation model.
Laser based missile defence systems (DIRCM) are being increasingly employed on aircraft. In certain circumstances the
laser must pass through the exhaust plume of the aircraft engines. In order to predict the degree of divergence and
dispersion of the laser beam an understanding of the exhaust gas structure and its influence is required. Specifically the
effect of parameters such as temperature, carbon dioxide, turbulent intensity and turbulence scale sizes within the flow
are of interest, in addition to the laser beam wavelength and beam size. A parametric study under controlled laboratory
conditions was undertaken to examine these effects. The results of beam propagation through a high temperature
turbulent flow at various turbulence intensity levels are presented in this paper. Beams with wavelengths 632.8 nm and
4.67 μm at various beam diameters are used to study laser beam interaction with various turbulence intensity and eddy
scales. The effect of the relative size of the beam diameter with respect to the turbulence scale is also reported. It was
found that the beam displacement was strongly related to turbulence intensity and beam diameter. Scale lengths, path
length and turbulence intensity were found to influence beam displacement. Quantifying these parametric effects at high
temperatures will assist with development of a parametrically based laser beam propagation model.
We have demonstrated audio communications with a mid-IR laser. The laser is a frequency doubled Q-switched CO2
system producing approximately 12ns pulses at 4.6μm. 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 1.5μs, 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.
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