Self-mixing interference in a thin-slice solid-state laser with few feedback photons per observation period

Formation of a peculiar lasing pattern leading to a TEM₀₀-like mode accompanied by a nonzero bright outer-ring emission (namely, a modified TEM₀₀ mode) was observed in a 300-μm-thick LiNdP₄O₁₂ laser with reflective flat end mirrors that was pumped with a laser diode whose focused spot size was larger than the lasing-beam spot size determined by the thermal lens effect. The photon lifetimes of the laser oscillations were found to be greatly shortened to τₚ=6.740.28em0exps as compared with those in pure TEM₀₀ mode operation, while the fluorescence lifetime was τ=1300.28em0exμs. The formation of the peculiar transverse mode was theoretically treated in terms of the pump-intensity-dependent thermal lens effect, which yields the lasing-beam spot sizes in the resultant optical cavity. In addition, transverse spatial hole burning of population inversions due to the preceding transverse eigenmode intensity and the associated antiguidance of the pure TEM₀₀ field leading to formation of the TEM₀₀ with an emergent outer-ring emission was shown to appear with increasing pump power through the electric lens (i.e., population lens) effect, in which almost all of the population inversion contributes to lasing with a pronounced effective modal gain. Self-mixing laser Doppler velocimetry (LDV) experiments and numerical simulations revealed that spontaneous emission factors in the peculiar mode oscillations were greatly decreased in comparison with those in pure TEM₀₀ mode operation. The present thin-slice solid-state laser possessing a large fluorescence-to-photon lifetime and decreased spontaneous emission factor enabled us to detect LDV signals in the regime of a few feedback photons per observation period.