Mode-locked diode lasers for THz asynchronous optical sampling

Abstract

Even though Terahertz Time Domain Spectroscopy (TDS) setups have been available for decades in laboratories worldwide and possible applications have been shown in many research papers, a breakthrough of applications actively used on an industrial scale is yet to come. Apart from the THz sources, such as photoconductive antennas (PCAs), a conventional TDS system consists of an ultrafast laser source, a mechanical delay line used for the sampling, and a data acquisition system. While the femtosecond laser makes up for the majority of the system cost, the mechanical delay is accountable for the long acquisition time. In order to push pulsed THz systems one step further towards industrial applications, this work addresses both: the optical source and the sampling mechanism. To overcome the necessity of the delay line an asynchronous optical sampling (ASOPS) approach is chosen. Here, the sampling mechanism is obtained by operating two femtosecond lasers with slightly different repetition rates uf resulting in an inherent sampling of the THz transient. While this was shown with ultrafast solid state lasers such as Ti:Sapphire or fiber lasers, we use edge emitting mode locked semiconductor quantum well lasers operating in the 830nm wavelength regime. In a first step, two laser diodes operated in compact external cavity configurations are hybridly modelocked at repetition rates around 390MHz with a RF synthesizer each in order to obtain a stable pulse scanning. In a second step, we evaluate hybridly modelocked monolithic edge emitting laser diodes at 12:8 GHz for THz TDS ASOPS.