Terahertz Spectroscopy with Asynchronous Optical Sampling Using a Compact Bidirectional Mode-locked Fiber Laser

We present a high-speed and broadband time-domain terahertz spectroscopy system based on a single bidirectionally mode-locked fiber laser. This fiber ring laser generates two optical pulse trains having slightly different repetition rates and a slowly varying repetition rate difference in free running mode, eliminating the need for servo locking electronics. Using plasmonics-enhanced photoconductive antennas for terahertz emission and detection, the system generates sufficient output photocurrent on the detector for self-triggering. Acquired spectral data show agreement with the HITRAN water absorption line data up to 1.5 THz. The use of a single compact fiber laser system with simple data acquisition demonstrates the feasibility of a portable time-domain terahertz spectroscopy system with significant reduction in size, weight, and power consumption.