**1. Introduction**

Optical interferometry (OI) involves the superposition of electromagnetic waves to form interference fringes where tiny changes in optical path difference cause significant variation in the intensity pattern of the detected light. The precision and flexibility associated with modern-day interferometric techniques have enabled researchers to apply them to a wide range of applications for solving problems related to science and engineering [1–7]. In the recent years, applications have become more sophisticated. For example, optical interferometric methods are now useful for detecting early chemistry, to analyze 3D surface topography, in biosensing, and in several topics related to investigate the matter under extreme conditions [6–10]. The use of OI in astronomy should be mentioned separately due to the volume of research that has been reported over the last few decades [11]. In this chapter, a brief overview on interferometric applications will be presented. The focus will be on the most recent advances from several fronts of science and engineering to note the width of applications where OI techniques have been relevant. The examples are by no means exhaustive due to the limited scope of this chapter. The mentions here serve only pointers for future discussions or an extended review on the state-of-the-art of OI techniques.

In most simple terms, in OI, a coherent light beam is split into two using a beam splitter. The two beams cover different trajectories, commonly known as "paths" in optics before superposing to create interference fringes. The fringes or intensity patterns depend on phase differences between the two beams, resulting from the different lengths of the paths taken by them. Classification of interferometers could be done in many ways, depending on the focus and purpose. If the carrier frequency is of importance, two major groups named homodyne and heterodyne could be mentioned based on if the frequency has changed in the process.

Likewise, interferometers could be classified into major groups. Most OI devices that are being discussed here are double-path interferometers, where the signal and reference beams travel along distinct paths before recombining. A common path interferometer is, as their name suggests, example where the two beams travel the same paths.
