| As the dimensions of a microlaser approach the wavelength of the laser radiation and the resonator is constructed so the fraction, {dollar}beta{dollar}, of spontaneous emission into the laser mode is appreciable, the lasing threshold as a function of pump power can begin to vanish. In a case when {dollar}beta{dollar} is exactly unity such lasers can become thresholdless, where the linear region of input/outputlight curve shifts toward the origin, and there is no threshold knee in the light curve. Initially low-threshold atomic and diode lasers are treated semiclassically by rate equations. The diode system is typically absorbing until a certain carrier number at transparency No is pumped into the gain medium. Any intracavity noise from spontaneous emission of photons is properly modelled quantum-mechanically. We employ Scully-Lamb theory to treat both the atomic and diode laser systems. Our photon-statistical results closely compare to the original experimental results obtained at an industrial laboratory, A.T.&T. Bell Labs, Murray Hills, NJ. We then use Scully-Lamb theory to obtain the phase-diffusion coefficient of the laser using a vector-kick model of the random walk nature of the electric field associated with the laser field. The laser linewidth is directly proportional to the phase diffusion coefficient D, which is treated here as a time-independent constant, and hence is an approximate solution. We have applied this linewidth theory to the microlasers and calculations including the absorptive action of such gain media is reported. The evolution to low-{dollar}beta{dollar} lasers has improved the noise properties of such coherent sources which has culminated in a low-threshold diode-laser that operates on micro-ampere injection currents. |
