Measurements using Optic and RF Waves

Chapter 4Low Coherence Interferometry¹

 

 

 

4.1. Introduction

Optical wave frequencies are very high, the eye and other detectors respond to light intensity only, in other words to the time average of the electric field amplitude squared. For this reason, we almost totally miss the sinusoidal wave character of light in our daily life. In order to get full access to the phase of a lightwave experimentally, it is necessary to use interferometric techniques. Two centuries after Young and Fresnel's experiments, interferometry remains a very active domain of research: more precisely, the definition of new measurement systems. The reason for this vivid activity is the fact that the phase of a light wave is a real goldmine of information about the media through which this wave has been propagating since it is proportional firstly to the propagation distance inside the media, and secondly to their refractive index. Therefore, any change in the propagation distance of a wavelength fraction can be detected in the phase, and we can for this reason proceed to very precise measurements of small displacements. As far as the refractive index is concerned and bound to the structure of a material, any external strain (heat, pressure, electric field, etc.) modifying this structure also modifies the refractive index, and therefore the phase. If we then have a relevant theory connecting the phase with the constraint and successful inverse methods, it is possible to find the constraint applied by ...