Yuma Takida, Toshiyuki Ikeo, Kouji Nawata, Yoshio Wada, Yasuhiro Higashi, Hiroaki Minamide
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| Schematic experimental setup. VCSEL: vertical cavity surface emitting laser; ECDL: external cavity diode laser; YDFA: Yb-doped fiber amplifier; is-TPG: injection-seeded THz-wave parametric generator; SBD: Schottky-barrier-diode receiver; and ADC: analog-to-digital converter. |
https://aip.scitation.org/doi/abs/10.1063/1.5119411
Using spectral multifurcated oscillations in a passively Q-switched microchip laser, we demonstrate frequency-domain differential absorption spectroscopy in the terahertz (THz) frequency region. Within a single quasi-continuous-wave (QCW) excitation, a microchip laser comprising a 7-mm-long Nd:YAG/Cr:YAG composite ceramic provides up to three subnanosecond pulses with a 1064-nm wavelength and a 50-Hz QCW repetition rate. We have observed that the longitudinal mode of double and triple pulses shows stable bifurcation and trifurcation, respectively, induced by a spatial hole burning effect within the laser cavity. These pulses are directly used to drive an injection-seeded THz-wave parametric generator based on a MgO-doped LiNbO3 crystal, thereby generating up to three monochromatic, self-frequency-switched THz-wave pulses separated from each other in frequency by a free spectral range of the laser cavity. By precisely tuning one of the THz-wave frequencies to the gas absorption line, multifurcated THz-wave pulses facilitate the measurement of differential absorption signals every 20 ms without any active frequency modulation. We also show that first- and second-order derivative spectra of gas absorption can be derived from a single frequency sweep of multifurcated pulses without a reference spectrum and computational derivation. Our approach paves the way toward realization of a THz differential absorption lidar for use in fast gas sensing applications.
This work was supported in part by Japan Science and Technology Agency (JST), Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP), Grant No. VP30118067340. The authors would like to thank Professor H. Ito of RIKEN/Tohoku Univ., Professor M. Kumano of Tohoku Univ., and Mr. T. Suzudo of Ricoh Company, Ltd. for fruitful discussions.
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