  A new scheme of a highly efficient hybrid laser cavity is proposed and
experimentally demonstrated utilizing a hot cesium (Cs) vapor cell as an
optical gain medium. The laser cavity consists of a macroscopic concave
reflecting mirror (>99% reflectivity) and a 4% Fresnel-reflecting
perpendicularly cleaved facet of a single mode fiber (SMF). The cylindrical
cesium gain cell is located between these two reflectors. The SMF serves
multiple roles: 1) a passive mode-matching component to approximate the pump
beam diameter to that of the laser cavity mode within the cesium cell, 2) an
output coupler with low reflectivity, and 3) a high beam-quality laser delivery
with a low loss. Optimizing the pump beam waist diameter and the cesium vapor
cell temperature, a high slope efficiency of 86% and continuous wave power of
419 mW were obtained in the pump power range of 400 to 600 mW, with an
optical-to-optical conversion efficiency of 71%. The unique multi-functional
role of the SMF in the hybrid cavity is fully described, and it can also be
applied to other phases of high optical gain media.
