The HD absorption spectrum is investigated near 2.3 µm with the help of a newly developed Cavity Ring Down Spectrometer (CRDS) using a VECSEL (Vertical External Cavity Surface Emitting Laser) as light source. The HD CRDS spectra were recorded for a series of ten pressure values in the range 50-650 Torr. The sensitivity of the recordings-noise equivalent absorption of the spectra on the order of min 5×10-10 cm-1-has allowed for the first detection of the S(3) quadrupole electric transition of the HD fundamental band, at 4359.940 cm-1. The line center determined with an uncertainty of 0.002 cm-1 agrees with the most recent theoretical calculations. The retrieved value of the line intensity (2.5×10-27 cm/molecule at 296 K) agrees within 12% with the ab initio values included in the HITRAN spectroscopic database. We take the opportunity of this contribution to provide an exhaustive review of seventy three HD absorption lines previously detected up to 20000 cm-1. From the pressure dependence of the baseline of the CRDS spectra, the binary absorption coefficient of the HD collision induced absorption band is determined to be 1.17(4)×10-6 cm-1 amagat-2 at 4360 cm-1 .
We report a continuous-wave highly-coherent and tunable dual-frequency laser emitting at two frequencies separated by 30 GHz to 3 THz, based on compact III-V diode-pumped quantum-well surface-emitting semiconductor laser technology. The concept is based on the stable simultaneous operation of two Laguerre-Gauss transverse modes in a single-axis short cavity, using an integrated sub-wavelength-thick metallic mask. Simultaneous operation is demonstrated theoretically and experimentally by recording intensity noises and beat frequency, and time-resolved optical spectra. We demonstrated a >80 mW output power, diffraction-limited beam, narrow linewidth of <300 kHz, linear polarization state (>45 dB), and low intensity noise class-A dynamics of <0.3% rms, thus opening the path to a compact low-cost coherent GHz to THz source development.
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