Design and measurement of an ultrafast absorber for monolithically integrated InGaAsP/InP mode-locked laser diodes

We discuss the simulation of monolithic mode-locked semiconductor laser diodes (MLLD) and the design and measurement of its key element, the saturable absorber. We demonstrate the necessity of a fast absorber for sub ps pulse generation by distributed time domain simulations of the pulse formation. As a result a uni-travelling-carrier (UTC) structure is proposed as a necessary absorber for fast carrier sweep-out and fast saturation recovery. Design and modelling of the UTC-absorber with the commercial software DESSIS (ISE®) is discussed and pump-probe measurements of the fabricated absorber device are presented. Fast absorption recovery of less than 1 ps is achieved. Further absorber material structure optimisation is required to control the saturation energy E_Sat.