Photon-echo novelty filter for measuring a sudden change in index of refraction

An optical novelty filter with phase-conjugate photon echoes is constructed to measure a sudden change in the index of refraction of a transparent medium. This novelty filter is set up as a Michelson interferometer and employs a europium-doped crystal as a phase-conjugate mirror. The filter is sensitive to the sudden change in index of refraction when the phase-conjugate echo experiences a different path length from the first laser excitation pulse in a photon-echo experiment. Using a Pockels cell as a pure phase modulator, we demonstrate that the filter can resolve a sudden phase change occurring on a nanosecond time scale with an accuracy and resolution close to 5°.