Intracavity heterodyne laser interferometry. Studies of the features of optical Kerr effect in the atmosphere

The features of high-sensitivity frequency and phase measurements in a He–Ne gas laser at a wavelength of 0.63 μm are considered. The single-frequency mode of this laser with a large excess of the lasing threshold is implemented due to longitudinal mode selection using an ultra-thin nickel metal film placed in the standing field node of the cavity. With such an excess of the lasing threshold, the natural linewidth component is very small (∼10⁻³Hz), which was determined from the heterodyne method of measuring natural fluctuations of lasing frequency. This method can be used in combination with the multibeam intracavity interferometry for measuring phase modulation at a level of 10⁻⁹ rad Hz^−1/2. To demonstrate the capabilities of the method, the electro-optical Kerr effect in air at atmospheric pressure in linearly polarized light was studied separately for extraordinary and ordinary waves. The Kerr and the Havelock constants were measured. A deviation from a purely quadratic dependence determined by the fourth power of the amplitude of a low-frequency electric field was found. A possible explanation of the detected deviation from the Kerr effect is the electrostriction.