Totally reflecting laser refractometer

Abstract

Abstract A laser having isotropic polarization characteristics, is operated with a confocal mirror cavity and is adjusted in terms of mirror reflectivity, spacing, and laser active medium to operate at a particular wavelength. A total internal reflection prism is contained inside the laser optical cavity and is rotated to produce an optical path angle slightly greater than the critical angle. For this condition the prism will constrain the laser to oscillate in such a way that adjacent modes have orthogonal polarizations. In the case of two mode operation one mode will be polarized parallel to the plane of incidence at the prism reflecting surface and the other will be perpendicular to the first. The phase shifts produced upon total reflection are different for light polarized parallel and perpendicular to the plane of incidence. Futhermore the difference in the phase shifts for these two polarization directions is a function of the refractive indices of the prism and its facing material as well as the angle of incidence. Since the difference in phase shift is equivalent to a difference in optical cavity length for the two orthogonally polarized modes the frequency of one of the modes is slightly displaced with respect to the other. Measurement of the beat frequency difference between the orthogonal modes will therefore yield information on the refractive index of the facing material provided that prism index and angle of incidence are known. In one embodiment the prism is provided with a calibrating sample of known refractive index placed in close contact with its reflecting face. The prism and laser cavity are then adjusted to produce a stable mode difference beat frequency. Very small changes in refractive index of the material in contact with the prism face can be correlated with changes in beat frequency thereby giving a very sensitive measure of refractive index. If the comparison sample is a fluid confined inside a chamber sealed to the prism face, extremely slight differences in composition of the fluid can be detected. Since the output is read immediately as a frequency, the system is useful in monitoring flowing fluids.

Inventors

• 👤 M White

Bibliographic Details