Side-pumped WGM Milled Microstub Resonator Laser

Whispering Gallery Mode (WGM) resonators are promising candidates for realization of ultra-small micro lasers. Conventional 3D resonators such as micro toroids [1], micro spheres [2, 3] and microbottles [4] are well studied as both passive and active devices. The excitation and signal collection are mostly done using evanescently coupled micro-sized fibers [5], integrated waveguides [3] and\or collecting the scattered light [6]. As the spectrum and coupling efficiency highly depend on the excitation position, the coupling system requires precise alignment, and packaging is complex. Here, we demonstrate a completely new micro laser based on WGM generated in an Yb3+-doped stub resonator side-pumped at 976nm wavelength. The device is fabricated based on material ablation by a pulsed CO2 laser. Micro grooves are initially milled by a CO2 laser around a 240μm-thick fiber with 200μm-thick doped core to form the desired length of the stub resonator, then the structure is fire-polished by electrical arcs in a fusion splicer in order to improve the surface quality. The quality factor of the fabricated resonator exceeds 106. As illustrated in Fig. 1a, the device is separated in three regions where the first section shapes the pump mode before entering the resonator acting as a lens or multimode interferometer, then, WGMs are generated in the resonator section, and finally, pump residuals and generated signals are coupled out through the fiber stem. The efficiency and selection of WGMs as well as the output coupling can be controlled by precisely engineering the micro grooves shape. This is an advantage over our previous work [7] which involved time consuming cleaving and splicing separate piece of fibers to form the cavity. The pump, from a single mode fiber, is coupled through free space into the resonator, hence, it is possible to optimize the coupling condition in order to reach a wide range of azimuthal WGMs by offsetting the launch fiber along the doped fiber facet. Figure 1b shows the scattered light when the device is pumped almost from the center of the coupling section. The scattered light shows the formation of WGMs close to the surface of the resonator. The spectra of the collected signals guided through the fiber stem are plotted in Fig. 1c. Lasing peaks corresponding to WGMs of the resonator are observed at 1060nm-1110nm wavelength range. The threshold, efficiency and spectra of the laser can be further enhanced and controlled by varying the length of the resonator.