When a wave strikes an obstacle, the edge of this obstruction affects the further propagation of the field. For obstructions very much larger than the wavelength, the result is a shadow, well known to us from our everyday experience. If we look in detail at the edges of a shadow, or consider the transmission of light around obstructions that have size scales of the order of the wavelength, we see variations in the intensity, owing to interference effects at the edge of the opaque region. This phenomenon is known as diffraction, and plays a very important role in micro-optics.

For transmission of light through a small aperture, diffraction effects can strongly affect the intensity distribution of the transmitted optical field, and these effects increase in relevance the smaller the apertures become. The definition of a diffraction-limited lens, which we saw in Section 7.2.3, results from these considerations, and we will examine the physical origins and applications of diffraction in this chapter. As we saw in Section 6.4.3 for micromirrors, and will see again for microlenses below, diffraction can limit performance in micro-optical systems, but is more often a source of optical functionality of great utility for a wide variety of novel components (Turunen and Wyrowski, 1997; Kress and Meyrueis, 2000).

We will begin with an overview of diffraction and its manifestation in various simple configurations, including rectangular and circular apertures.