A Intracavity Polarimeter and Measurements of the Kerr Effect in Helium, Neon, Molecular Hydrogen and Molecular Nitrogen

In this thesis we report on the construction and use of a laser based, intracavity polarimeter. Two orthogonally polarized modes were produced by optically pumping an Na _2 ring laser. The change in the beat frequency between the two modes is a measure of any extra birefringence inserted into the cavity, provided the axes of the birefringence are aligned with the polarization modes of the laser. The sensitivity obtained, to changes in the birefringence, was 13 nrad/sqrt{ rm Hz}. We used the polarimeter to measure the Kerr constant in several gases. The beat frequency was modulated by applying an ac electric field to an intracavity Kerr cell. The experiments were performed at the minimum in the static beat frequency to eliminate effects due to electrostriction. There remained a small systematic bias to the measurements due to frequency pulling by the gain medium. A correction factor of the order of 2% was determined from a combination of spectroscopic and laser measurements. We measured the Kerr effect in He, Ne, H _2 and N_2 as a function of pressure with this new type of polarimeter. From the portion depending linearly on density, we determined the Kerr hyperpolarizability of the free molecules with a precision of 1%. For the electronic part of the hyperpolarizability our results agree, at the 95% confidence level, with very recent Kerr measurements and theoretical calculations. This is important since accurate Kerr measurements are difficult and it is essential, for confirming the calculations, that two totally different types of measurements yield results that agree with each other and with theory. For the rotational contribution to the hyperpolarizability, our results for H_2 taken with those of others indicate that further modeling or theoretical work is required in this area.