Abstract
In this thesis, we try to narrow the linewidth of N2O 0001←0000 fundamental band R(15) transition down to 200 kHz by a CW mid-infrared MgO:PPLN-based difference frequency generation (DFG) source pumped by a Ti:Sapphire laser and power boosted Nd:YAG laser. We expect that the electric quadrupole hyperfine structure of N2O 0001←0000 fundamental band R(1) transition can be resolved and could calculate the electric quadrupole constant. According to the mechanism of line broadening, the linewidth of N2O molecular is broadened affected by the Doppler line broadening, collisional line broadening, transit-time broadening and power broadening. Hence, in order to narrow the linewidth down to 200 kHz, we utilize the saturation spectroscopy to remove the Doppler broadening and produce the 3rd derivative of Lamb-dip signal. In order to eliminate the pressure broadening, we utilize a turbo pump to lower the pressure of N2O gas cell to near 6 ~ 8 mtorr. We use beam expander to expand the DFG to reduce the transit-time broadening. In this experiment, we use three kinds of beam expanders to expand the DFG with spot size 0.37 mm, 2.0 mm and 4.17 mm, and we measure the dependence between linewidth and pressure in each spot size. The linewidth can be determined by fitting the 3rd derivative saturation signal in response to the different modulation width. According to the result of experiment, the relation between linewidth versus pressure and spot size are the same as expectation respectively: the lower the pressure is, the narrower the linewidth is; the larger the spot size is, the narrower the linewidth is. Adding the span of the hyperfine structure of fundamental band R(15) 170 kHz and the DFG linewidth is near 300 kHz, this is the limitation of linewidth resolving on R(15). Because the narrowest linewidth we measured is 625 kHz, this means the linewidth can be narrowed further more. We can apply another beam expander with larger beam expanding ratio to investigate the linewidth in the future.