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The central feature of this chapter is the phenomenon of birefringence, also known as double refraction, which occurs in crystals that are optically anisotropic. Given that birefringence is a linear optical effect, why is the whole of Chapter 3 being devoted to it? Firstly, most nonlinear crystals are birefringent, and so one naturally needs to know how light propagates in these media. Secondly, several important nonlinear optical techniques (the most obvious being phase matching) exploit birefringence to achieve their goal. Lastly, the material in this chapter provides essential background for the following chapter on the nonlinear optics of crystals.

Section 3.2 is a brief tutorial on crystal symmetry. Crystallography is something of a world on it own, and many people find it a complete mystery. Although the summary offered here is very basic, it should provide everything needed for what comes later.

Section 3.3 discusses the propagation of EM waves in optically anisotropic media, and contains a fairly detailed analysis of birefringence (double refraction), ordinary and extraordinary waves, and associated topics. The treatment is mainly centred on uniaxial media because of their relative simplicity and the fact that most nonlinear crystals are of this type.

Section 3.4 describes how birefringence can be exploited in the construction of wave plates, while Section 3.5 is reserved for a brief mention of biaxial media in which the propagation characteristics are considerably more complicated.