Investigation of a High-Power Low-Threshold Single-Mode Microsphere Laser Using a Serially Coupled Double Microsphere Structure

A high-power narrow-linewidth low-threshold single-mode microsphere laser is investigated by coupling an undoped microsphere with a different size to a single-microsphere laser. A model is proposed for microsphere lasers and it is verified by comparing its results to the experimental results published in the literature for a thulium-doped tellurite glass single 12.5-μm microsphere laser. It is illustrated that by coupling an 11.5-μm undoped microsphere to a multimode 50-μm thulium-doped tellurite glass microsphere laser, a single-mode operation is obtained, and despite the relatively high output power, the quality factor also remains high. For the proposed structure, an output power of 208 μW at 30-mW pump power is achieved. Its threshold power and full width at half maximum linewidth are 0.06 mW and 0.53 fm (40 kHz), respectively, which are significantly enhanced compared to the single 12.5-μm microsphere laser.