Transverse-mode selection in apertured super-Gaussian resonators: an experimental and numerical investigation for a pulsed CO_2 Doppler lidar transmitter

Abstract

A representative prototype of a high-energy, long-pulse, and narrow-bandwidth pulsed CO(2) laser suitable for a spaceborne Doppler wind lidar application has been developed. We obtained 10 J of output energy at greater than 8% efficiency in long, narrow-bandwidth, single-longitudinal, and transverse-mode pulses. We used a positive branch unstable resonator with a fourth-order super-Gaussian mirror as the output coupler. Experiments were carried out to assess the effect of intracavity hard apertures of different diameters that induce diffractive perturbation of the theoretical field and reduce the transverse-mode selectivity of the cavity. An upper limit to the choice of the mirror soft radius has been found, which allows optimization of the trade-off between laser efficiency and beam quality. We determined experimentally that a value of 0.75-0.8 for the ratio between the exp(-1) diameter of the beam intensity and the laser clear aperture gave a single-transverse-mode operation without significant loss of efficiency.