Refractive-index measurement of gases with a phase-shift keyed interferometer

The presented new type of interferometer combines the principle of two-beam interferometry and the technique of phase-shift keying of holographic gratings. On the basis of the phase-shift keying technique, the interferometer employs two different geometries for the recording and the readout process. Two holographic Bragg gratings are recorded in transmission geometry and simultaneously read out in reflection geometry using a tunable IR laser. Both gratings have the same grating period but a relative phase shift. The wavelength of the readout beam is fitted to the Bragg condition for the gratings. Using a tunable IR laser for the readout process, we can measure the spectral transfer function of both combined gratings. The shape of the measured transfer function is extremely sensitive to the phase shift between the two gratings. We demonstrate an application of this method by the measurement of refractive-index variations of gases due to pressure changes of the gases. The achieved resolution with respect to the measurement of phase shifts is approximately 1/40 pi. We present experimental investigations on two kinds of gas (an inert gas and a gas composition) as well as an efficient numerical approach to simulate the transfer function for Bragg gratings with a phase shift. Furthermore, we present a method to increase the resolution based on the controlled manipulation of the transfer function.