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

Abstract

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.

Metal-Coated Fiber-Optic Platforms for Surface Plasmon Polariton Generation and Interconnection

“…We have assumed that the refractive index of the top layer material can change by simply putting a new material on top of the FIFO-MCAFF platform. As typical examples, we choose four difference cases having refractive indices near unity, which are very hard to distinguish and identify with conventional techniques other than delicate interferometry techniques [24]: vacuum, liquid helium, and two types of silica aerogel, the refractive indices of which are given by 1, 1.025, 1.05, and 1.08, respectively. The solid and dashed lines denote the corresponding results for t 3 = 15 nm and t 3 = 20 nm, respectively.…”

Alternative phase-shifting technique for measuring full-field refractive index

“…Currently, some methods have been proposed to measure full-field refractive index, such as the interferometric methods, [1][2][3][4][5][6] digital holographic microscopy, 7 and ellipsometric methods. 8 In the interferometric methods, the phase difference is relative to the refractive index.…”

“…To obtain the phase difference distribution to measure full-field refractive index, the phase-shifting technique is an important and effective method that features simple structures, easy operation, and rapid measurement. Generally, the phase-shifting methods introduce successive phase steps into interferometric signals with various phase-shifting strategies, such as piezoelectric transducer, 1,2 holographic grating, 3 and tunable wavelength. 4 Aforementioned methods could suffer from mechanical perturbations, and their costs are high.…”