Terahertz differential absorption spectroscopy using multifurcated subnanosecond microchip laser

摘要

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.