Laser at frequency difference
Difference Frequency Generation laser (DFG: Difference Frequency Generation)
The tunable laser diode spectroscopy is a powerful and complementary tool of the IRTF analysis with its many advantages: a very high spectral resolution up to 10-5 cm-1 and a signal to high noise ratio (above 1000) permitting the measurement of fine effects such as line-mixing or the study of line profiles. Moreover, the speed of measurements (minutes) allows the study of unstable species.
As of 2002, to compensate for the lack of commercially available sources, a tunable laser system between 3 to 5.5 µm was developed based on the generation of frequency difference. A Nd Yag laser:at 1064 nm wavelength (Mephisto 800, InnoLight), constituting the source signal, is mixed with a diode laser at ECDL external tunable cavity (Toptica Photonics, DL 100) centered at 850 nm, constituting the pump signal, in a crystal of lithium niobateat (PPLN for "Periodically Poled Lithium Niobate") periodically reverse ferroelectric polarization. It then retrieves in output of the nonlinear crystal signal the idler frequency wi = wp - ws tunable between 2100 and 3000 cm-1. The high fineness of laser sources (1 kHz for Nd: Yag laser and 1 MHz for the ECDL diode) allows to neglect the apparatus function compared to the measured line widths, which is another advantage on the FTIR technique.
Optical device of the DFG laser system
Frequency calibration, essential in laser spectroscopy, is made using a lambdameterBurleigh - WA 1100 (700nm - 1650nm) type giving the scanning wavelength the idler beam with a precision of 10-2 cm-1 and a Fabry-Perot cavity with a free spectral range of 1 GHz and a resolution of 1 MHz.
Thanks to this laser difference-frequency generation system, the phenomenon of collisional interference was highlighted in the nuclear hyperfine structure of HI and intensities of H2O absorption lines were measured by inter-laboratory comparison with those of the HDO isotopologue confirming that the Voigt profile is not suitable for water because of the collisional narrowing.