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Microcavity-feedback fiber-loop laser gives a promise for single-mode and tunable lasing. Here, a whispering gallery mode(WGM) microcavity-feedback fiber-loop laser is constructed and dynamic lasing mechanisms are investigated by rate equation models. For the single-mode lasing, the results show that the lasing efficiency can be maximized by tuning the coupling of waveguide and WGM microcavity, and two critical coupling rates exist for a fixed internal coupling coefficient of clockwise and counter-clockwise WGMs. By simply increasing the total length of fiber-loop laser, multi-mode lasing originated from one WGM feedback of a silica microsphere is explored, the lasing behaviors are well predicted by the rate equations and the WGM resonance linewidth can be effectively evaluated from the beat notes bandwidth of multiple lasings. We wish the results demonstrated here can be used for optimization on the lasing output of a fiber-loop laser with an external feedback mirror.

Microsquare resonators laterally confined by SiO2/Au/air multilayer structure are investigated by light ray method with reflection phase-shift of the multiple layers and two-dimensional (2-D) finite-difference time-domain (FDTD) technique. The reflectivity and phase shift of the mode light ray on the sides of the square resonator with the semiconductor/SiO2/Au/air multilayer structure are calculated for TE and TM modes by transfer matrix method. Based on the reflection phase shift and the reflectivity, the mode wavelength and Q factor are calculated by the resonant condition and the mirror loss, which are in agreement well with that obtained by the FDTD simulation. We find that the mode Q factor increases greatly with the increase of the SiO2 layer thickness d , especially as d < 0.3 ?m . For the square resonator with side length 2 ?m and refractive index 3.2, anticrossing mode couplings are found for confined TE modes at wavelength about 1.6 ?m at d = 0.11 ?m, and confined TM modes at d = 0.71 ?m, respectively.