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

We present a high-speed and broadband timedomain 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 selftriggering. 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.