来源：ACS Publications
Single-mode lasers are of great importance due to their superior stability, low noise characteristic, high beam quality, and high spectral purity. Here, we employ thulium (Tm) and erbium (Er) codoped microcavities to demonstrate a mode suppression strategy in which Tm acts as a direct loss modificator for Er, suppressing low-gain competing modes by bringing them much higher absorption losses and then achieving lasing from a single high-gain mode. As a result, Er–Tm codoped multimode microcavities exhibit intrinsic single-mode lasing in the telecom band with a side-mode suppression ratio exceeding 50 dB without mode hopping. Our demonstration indicates a simple and robust mechanism for realizing intrinsic single-mode lasing in rare-earth-based microlasers. This strategy is further verified in Rhodamine-doped multimode microcavities, indicating that it is universal for different gain media and cavity structures.
We have demonstrated intrinsic single-mode laser in Er–Tm codoped microsphere cavities, which implies gain-material-manipulation-based loss engineering for mode suppression. From the aspect of the wavelength dependence of the absorption cross-section and the spatial overlap between the pump mode and the competing modes, Tm imposes a higher absorption loss on the low-gain competing modes than that on a certain high-gain mode. As a result, the low-gain competing modes are suppressed, achieving intrinsic single-mode laser from the high-gain mode. Saturated absorption of Tm is resisted by the further Tm-based lasing onset, maintaining its mode suppression effect at high pump power. The Tm-based laser in the 1824 nm band will not influence the practical applications of the Er-based laser. As the doping concentration of Tm is up to 25% of that of Er, all twenty resonant-mode-abundant microsphere cavities achieve high-SMSR (exceeding 50 dB) single-mode lasing without mode hopping. We believe that our demonstration provides a simple, flexible, robust, and high-performance strategy for realizing single-mode laser in various types of rare-earth-based microlasers, which can be expanded to microlasers based on different gain media with appropriate loss modificator.