1.3 Selected relevant basic diode laser characteristics
1.3.1 Threshold gain
Light generated in the active region of the laser device propagates along the optical waveguide of the active layer structure and is partially reflected at the mirror facets of the Fabry–Pérot resonator, which contains the laser active material. This propagation and repeated reflection of light cause a loss and gain of light. Loss is formed by two components: first, mirror losses, which are caused by the final mirror reflectivities; and, second, cavity losses, which are due to free-carrier absorption losses in the active layer and cladding layers as well as scattering losses at structural inhomogeneities of the heterointerfaces.
Gain is generated in the stimulated emission process, which is turned on by population-inverted energy levels due to heavy carrier injection. We have to distinguish between material gain, which is the gain of the actual active material, and modal gain, which is determined by the ratio of the transverse dimension of the active layer to that of the cavity mode and depends on the details of the specific laser configuration. Modal gain is always smaller than material gain. This topic will be described in detail in the following sections.
Figure 1.16 illustrates the loss and gain of power of a propagating optical mode in a cavity of length L and with mirror reflectivities R1 and R2. Internal optical losses of the unbounded active material per unit length are expressed by a loss coefficient ...