High-sensitivity detection of magnetically induced birefringence

We developed a technique for the high-sensitivity detection of the magnetically induced birefringence of gases. Elliptically polarized light from a GaAlAs diode laser was used to probe the change in ellipticity caused by the presence of oxygen gas in a transverse magnetic field. The change in ellipticity was analysed with a polarization analyser. By using high-quality optical elements with small optical anisotropies, we improved the light-extinction ratio of the polarization-analysis system to 1 × 10 -9 so that the noise caused by intensity fluctuations of the light source was reduced to a level below that of shot noise. The minimum value that could be detected for the change in ellipticity was 6 × 10 -9 rad, which was larger by a factor of 1.4 than the value expected at the shot-noise limit. The experimental value of the signal-to-noise ratio agreed with the theoretical prediction taking account both of the shot noise and of the residual noise caused by light-source-intensity fluctuations.