HIGH PRECISION FREQUENCY MEASUREMENT ON A CO MOLECULAR BEAM USING A 6 UM COHERENT SOURCE REFERENCED TO A FIBER DELIVERED ABSOLUTE STANDARD

The vibrational transition $|v=1, J=1, - angle leftarrow |v=0, J=1, + angle$ in the a$^3Pi_1$ state of CO is measured at the kHz level as 51399115447(3) kHz. The measurement is done on a pulsed, supersonic molecular beam. The achieved relative accuracy is $6cdot 10^{-11}$, more than four orders of magnitude better than previous measurements. The thesis consists of two parts. The first part treats the generation of coherent radiation in the mid-infrared by difference frequency generation of two near-infrared lasers in a custom-made orientation-patterned gallium phosphide (OP-GaP) crystal. This novel nonlinear material is extensively characterized. This crystal is particularly interesting for its wide transparency range, its high nonlinear coefficient, and for the possibility of quasi phase-matching with the widely used 1064-nm lasers. Then, it is explained how the accuracy and the stability of the frequency standard at the national metrological institute (INRIM) is transferred to the 6-micron radiation using a 642-km long fiber link and an optical frequency comb. The mid-IR source is characterized in terms of phase noise power spectral density. The second part of the thesis deals with the experimental setup and the spectroscopic measurement. After a brief discussion of the carbon monoxide energy levels and experimental apparatus, the high precision measurements are presented. The effects of stray fields, mainly magnetic, and of the Doppler shift are extensively discussed.