Modes in Microchip Lasers

The transverse modes in microchip lasers (MCL) appear different from the classical mode theory, as the resonator consists of a set of plane mirrors (similar to semiconductor edge-emitting lasers and VCSELs). Then the parabolic potential is absent, and the electromagnetic radiation is confined in the transverse space of the resonator by other mechanisms, essentially by gain guiding. Thermal lensing also can play some role in confining the radiation. Laser engineers use several intuitive assumptions about the number of supported modes in the resonator, relating with the Fresnel number of the resonator. However, despite some attempts to calculate the modes in such resonators and investigate combined gain with index guiding, no clear and straightforward methods were provided to calculate the beam radiation profiles. We aim to fill this gap by providing analytical and numerical treatment of MCL modes in two dimensions. We analyze the transverse modes in microchip laser formed due to both gain guiding and thermal lensing. Analytical and numerical results are compared with the results of experimental measurements. Using the cylindrical pumping profile approach, we provide a simple 2D theory of the gain-guided and thermal-lens induced modes in such plane-mirror resonators. We estimate the mode beam quality factor dependency on pumping strength. The model is versatile and applicable to wider range of optical resonators consisting of plane mirrors with longitudinal pumping. Finally, we compare the experimental measurements of beam quality factor with our theoretical model.