Repetition frequency locking of a terahertz quantum cascade laser emitting at 4.2 THz.

The electrically-pumped terahertz quantum cascade laser (QCL) is characterized by high power emission, compact, broad frequency coverage, and so on, which shows abilities for frequency comb operations. Although free running QCLs can work as frequency combs by designing the laser structure with small group velocity dispersions and/or inserting mirrors to compensate laser intrinsic dispersions, the ideal comb operation can only be obtained by firmly locking the repetition frequency and carrier frequency of a laser. In this work, we have reported a repetition frequency locking of a terahertz QCL emitting around 4.2 THz. When the 6-mm-long laser is operated in continuous wave mode without any locking techniques, the repetition frequency is measured to be ~6.15 GHz with a linewidth of hundred kilohertz. Once a phase lock loop (PLL) is applied to dynamically control the drive current of the QCL, we have demonstrated a successful repetition frequency locking of the laser with a signal to noise ratio of 80 dB. This technique can be employed for the frequency comb and dual-comb operations of terahertz QCLs for high-resolution applications. [ABSTRACT FROM AUTHOR]