High-speed transient effects in quarternary lasers

Measurements of the modulation characteristics of semiconductor lasers under typical systems-operating conditions are discussed. Slow components in the transient response, which limit the speed of three different types of device, are found to be structure-dependent. On a device with a built-in lateral waveguide but no lateral current confinement, a slow tail is observed on the trailing edge of the light pulse. Calculations using a model which includes lateral diffusion suggest that charge storage coupled with diffusion are responsible for the tail. Devices with both a built-in lateral waveguide and lateral current confinement (through the use of reverse-biased blocking junctions) are found to contain significant parallel capacitance in their impedance. An equivalent electrical circuit, based on impedance measurements, is used in conjunction with the model to calculate the modulation response. It is found that the speed of these devices is limited by the parallel capacitive elements, which are attributed to reverse-bias blocking junctions used to give good DC laser characteristics.