Modal Properties and Spontaneous Emission Factor in Unstable-Resonator Semiconductor Lasers
An experimental study has been carried out on the spectral, modal, and dynamical properties of a monolithic unstable-resonator semiconductor laser. Due to the large loss and the non-power-orthogonal nature of the transverse and longitudinal modes in the unstable resonator, the spontaneous emission factor, defined as the ratio of the power of the spontaneous emission entering the lasing mode to the total power of the spontaneous emission, is much larger than that of a regular Fabry-Perot cavity of the same dimension. The large spontaneous emission factor has profound effects on the lasing properties. For a 100-{dollar}\mu{dollar}m-wide and 500-{dollar}\mu{dollar}m-long stripe-geometry unstable resonator with a convex mirror of 2.2-mm radius of curvature, the enhancement factor is measured to be 500. The large spontaneous emission factor results in a less well-defined lasing threshold, multi-longitudinal mode operation above the threshold, and strongly damped relaxation oscillation. In addition, the unstable-resonator is sensitive to the thermal lensing effect that normally occurs when the laser in under CW pumping. In the laser device used in this study, the thermal lensing effects can make the resonator more "stable", resulting in a drastic reduction in the spontaneous emission factor and nearly single longitudinal mode operation. A numerical modeling for the eigenmodes of unstable-resonator semiconductor laser with a lateral gain guiding is carried out using the wave-propagation method. The result of numerical calculation is consistent with the experimental observations. The calculation also provides a general description of the relationship between the spontaneous emission enhancement factor and the cavity parameters such as, cavity round-trip diffraction loss, the cavity length, strip-width and the radius of curvature of the end-mirror.
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