Audio communications with a mid-IR laser

Abstract

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.

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