Abstract

Very-low-threshold lasing may be obtained¹ from a small single-mode-waveguide cavity of length L with an active-medium gain bandwidth Δv_k, less than the longitudinal mode spacing Δv_PP = c/(ZnL). The light is then emitted almost entirely into a single resonant mode. As a coarse model of a typical cw-activu medium in a soil'd forming the waveguide, we have studied a three- level ladder configuration with a radiative upper 3→2 transition and a nonradiative lower 2 →1 transition, which has to provide depopulation of level 2 at a rate γ₂₁ faster than the total decay from level 3 if lasing is to start at low pump rales. We investigate the dependence of the quantum-noise limit on cavity parameters (optical length n x L, mirror power transmission T), on free-space decay rate γ₂₂ of the laser transition and on γ₂₁, which is also a lower limit to Δv_x, by a continuous-mode quantum calculation valid for arbitrary parameter values. The cavity-induced effects we find (see Fig. 1) apply to a simplified system summarizing the quantum-noise sources, which are inevitable if low-threshold cw laser operation is desired. Within this model all noise properties are contained in the total Hamiltonian.

© 1994 Optical Society of America