Comprehensive investigation of fundamental mode profiles in monolithic nonplanar ring oscillators

Nonplanar ring oscillators (NPROs) are building blocks for high-performance single-frequency lasers and ring-laser gyroscopes that have profoundly improved state-of-the-art laser technologies, fundamental research, and precision measurements. However, a comprehensive investigation of fundamental mode profiles in monolithic NPROs has been a missing part, even though they affect the performance of the lasers or ring-laser gyroscopes. Here, we present a comprehensive finite-element modeling of the output beam profiles of monolithic NPROs by combining the ABCD transmission matrix and generalized Huygens-Fresnel integral. We theoretically investigate the effects of geometric parameters of monolithic NPROs on their output mode profiles. In particular, we focus on the thermal effect inside the monolithic NPRO and calculate the equivalent focal length of the thermal lens by using a ray-tracing method. Furthermore, we experimentally characterize the output laser beam profile, reconstruct the beam profile at the A-facet of the monolithic NPRO, and compare the experimental results with the simulation, thereby validating the accuracy and reliability of our model. The investigation may facilitate monolithic NPRO design and subsequently improve the performances of NPRO lasers and gyroscopes in the future.