Compact THz time-domain spectroscopy based on pre-chirped pulses from mode-locked Yb-doped fiber laser

We report on a compact terahertz (THz) time-domain spectroscopy (TDS) system based on fiber-integrated log-spiral low-temperature-grown InGaAs photo-conductive antenna (PCA) modules pumped by pre-chirped pulses from a 1.03-μm mode-locked Yb-doped fiber laser. A mode-locked Yb-doped fiber laser based on a birefringent spectral filter was produced with 240 mW average output power and 2.68-ps pulse duration. The output pulse had high chirp because the mode-locked fiber laser was implemented in the normal dispersion region. Therefore, the 733-fs output pulse was pre-chirped using two parallel diffraction gratings to facilitate its use as an optical pump source with THz radiation. After propagating in the single-mode fiber, the pulse-width of the pre-chirped pulse was compressed to 331 fs and injected into fiber-integrated PCA modules to generate a THz wave. We successfully obtained a THz emission bandwidth of over 2.5 THz for 120 mm in free space using the compact THz measurement setup. THz radiation was achieved by easily aligning two fiber integrated PCA modules. Absorption lines for water vapor were observed at room temperature. We also observed absorption lines from a 30% α-lactose sample. We confirmed that the fiber-integrated PCA modules and mode-locked Yb-doped fiber laser have potential applications in compact and portable THz-TDS systems.