Measurement of small wavelength difference in coherent light using faraday effect

Abstract

An apparatus is provided for determining a target wavelength λ of a target photon beam. The apparatus includes a photon emitter, a pre-selection polarizer, a prism composed of a Faraday medium, a post-selection polarizer, a detector and an analyzer. The photon emitter projects a monochromatic light beam at the target wavelength λ substantially parallel to a magnetic field having strength B. The target wavelength is offset from established wavelength λ' as λ=λ′+Δλ by wavelength difference of Δλ<<λ. The Faraday prism has Verdet value V. After passing through the pre-selection polarizer, the light beam passes through the prism and is incident to an interface surface at incidence angle θto the normal of the surface and exits into a secondary medium as first and second circularly polarized light beams separated by target separation angle δ and having average refraction angle θ. The secondary medium has an index of refraction of n. After passing the post-selection polarizer, the detector measures target pointer rotation angle Abased on the target separation angle δ. The analyzer determines the target wavelength λ by calculating offset pointer rotation angle ΔA=A−A′from calibrated pointer rotation angle A′based on established separation angle δ′ that corresponds to the established wavelength λ′, and by estimating the wavelength difference based on in which ε is an amplification factor. A method is provided incorporating operations described for the apparatus.