Gas refractive index measurements using dynamic phase-shift keyed gratings

We present a novel experimental technique for measuring optical phase shifts, based on phase-shift keying of reflective holographic gratings. Two holographic gratings are recorded in transmission geometry and simultaneously read out in reflection geometry using a tunable IR laser. We measure the spectral transfer function of the combined gratings, which is extremely sensitive to the phase-shift between the two recorded gratings. As an application of this technique, we demonstrate the measurement of small refractive index variations of gases due to pressure changes of the gases. Using this technique, relative phase shifts can be measured with a resolution of about 0.025 pi. We present experimental investigations as well as a numerical approach to simulate the transfer function for Bragg gratings with a phase shift.