In Chapter 8 we discussed the theory of Fraunhofer diffraction and interference, emphasizing in particular the relevance of Fourier transforms. In this chapter we shall describe the applications of interference to measurement; this is called interferometry. Some of the most accurate dimensional measurements are made by interferometric means using waves of different types, electromagnetic, matter, neutron, acoustic etc. One current highlight of optical interferometry is the development of detectors that can measure dimensional changes as small as 10−19 m, which should be induced by gravitational waves emitted by cataclysmic events in the distant Universe. A picture of one such interferometer, which has two orthogonal arms each 4 km in length, is shown in Fig. 9.1 and the design of this instrument will be discussed in more detail in §9.7.
An enormous variety of interferometric techniques has been developed during the years, and we shall limit ourselves in this chapter to a discussion of examples representing distinctly different principles. There are several monographs on interferometry that discuss practical aspects in greater detail, for example Tolansky (1973), Steel (1983), Hariharan (2003) and Hariharan (2007).
In this chapter we shall learn about:
Young's basic two-slit interferometer and its capabilities;
interference in a reflecting thin film;
diffraction gratings: how they work and how they are made, their resolving power and their efficiency;
two-beam interferometers of several types;