  Quantum cascade laser-pumped molecular lasers (QPMLs) have recently been
introduced as a new source of powerful (> 1 mW), tunable (> 1 THz), narrow-band
(< 10 kHz), continuous-wave terahertz radiation. The performance of these
lasers depends critically on molecular collision physics, pump saturation, and
on the design of the laser cavity. Using a validated three-level model that
captures the essential collision and saturation behaviors of the QPML gas
nitrous oxide (N2O), we explore how threshold pump power and output terahertz
power depend on pump power, gas pressure, as well as on the diameter, length,
and output-coupler transmissivity of a cylindrical cavity. The analysis
indicates that maximum power occurs as pump saturation is minimized in a manner
that depends much more sensitively on pressure than on cell diameter, length,
or transmissivity. A compact laser cavity with 10-100 times smaller volume than
the nominally optimal geometry is able to achieve at least 95% of the maximum
laser performance.
