Fiber Optic Tutorials
Rate Equations for Semiconductor Lasers
This is a continuation from the previous tutorial - laser arrays. Since the electromagnetic field inside the laser cavity satisfies Maxwell's equations, the starting point to obtain the field rate equation is the wave equation (2-2-12) [refer to the Maxwell's equations for semiconductor lasers tutorial]. In general, the dynamics of the semiconductor material should be taken into account to obtain the induced polarization \(\pmb{\mathscr{P}}\). However, the material response governed by the intraband scattering processes is relatively fast (~ 0.1 ps) compared to other time scales of interest such as the photon lifetime and the carrier recombination time. Considerable simplification...
Laser Arrays
This is a continuation from the previous tutorial - strongly index-guided lasers. InGaAsP semiconductor laser structures described in the previous tutorials have been developed for low-power (5-10 mW/facet) applications, such as providing a source in a lightwave communication system. With a proper design, these lasers can generally emit continuously up to powers in the range of 30-60 mW/facet near room temperature. This limitation in the output power mainly arises from the leakage current, which increases with an increase in the applied current. Semiconductor laser diodes for high-power operation have been extensively studied using the AlGaAs material system. High-power lasers...
Strongly Index-Guided Lasers
This is a continuation from the previous tutorial - weakly index-guided lasers. The lateral mode control in injection lasers can be achieved using index guiding alone the junction plane. The mode control is necessary for improving the light-current linearity and the modulation response of injection lasers. In strongly index-guided lasers, the active region is buried in higher band-gap layers (e.g., InP) on all sides. For this reason, these lasers are called buried-heterostructure lasers. The lateral index step along the junction plane is ~ 0.2 in these laser structures and is about two orders of magnitude larger than the carrier-induced...
Weakly Index-Guided Lasers
This is a continuation from the previous tutorial - gain-guide lasers. The lateral mode in gain-guided lasers is determined by the distribution of optical gain along the junction plane. The optical gain is determined by the carrier distribution that is influenced by both current spreading (due to the sheet resistance of the p cladding layer) and carrier diffusion in the active region. As discussed in previous tutorials, gain-guided InGaAsP lasers have undesirable characteristics, such as a high threshold current and a low differential quantum efficiency, as a consequence of the carrier-induced index reduction leading to index antiguiding. The effective...
Gain-Guided Lasers
This is a continuation from the previous tutorial - broad-area lasers. Most injection lasers intended for commercial applications have a built-in feature that restricts current injection to a small region along the junction plane. This restriction serves several purposes: It allows continuous-wave (CW) operation with reasonably low threshold currents (10-100 mA) compared with an unacceptably high value (~ 1 A) for broad-area lasers; It can allow fundamental-mode operation along the junction plane, which is necessary for applications where the light is coupled into an optical fiber The requirements for heat sinking are considerably less severe than those for a...