Compact FIR ring laser with open resonator and variable output coupling

We describe the design, construction and first experimental results with an open triangle FIR ring resonator with Michelson interferometer output coupling, which pre combined with a Brewster-windowless 10W CO2 pump laser to a compact laser package of Im length. The achieved stability of power and frequency of this system is very high, typically on the order of +/-0.01% over several hours and <20kHz per hour at 800GHz. For the design, we investigated the balance of all laser processes and determined the optimum dimensions for the open resonator cavity. This analysis is based on a radially homogenous overall laser model, including commonly accepted simplified rate equations together with molecular parameters for weak FIR laser lines. In accordance with the modelling the efficiency on the very most prominent lines is better than with conventional waveguide lasers, giving up to approximate to 4 limes better efficiencies especially on lines with low absorption pump transitions. Several laser Lines, however, have substantially enhanced pump thresholds on purely copropagating emission, suppressing laser operation below about 10W pump power threshold. We show that this can not be due to effects from the interferometer.