# Diffraction Gratings

A diffraction grating is a spatial dispersion element that is commonly used in optics for spectrum analysis of broadband signals [Goodman (2004)]. Let us first consider a complex transmittance function, relating the input and output waveform distributions, of a thin sinusoidal diffraction grating of Fig. 5.3:

where

Now consider a plane wave propagating along the z-axis given by

so that y(x, y, 0-; t) = 1, where the explicit time dependence ejwt is dropped for convenience. The wave just after the grating can then be expressed as:

Considering the lens transmittance function t1(x, y; w), wave output after the lens is given by

where d is the focal length of the lens. The output wave, in the focal plane, is

where h(x,y) is the free-space impulse response. Using the paraxial- wave approximations, the output wave can finally be written as:

|/(x,y; z = d)| = |y(x,y; l+) * h(x,y)|2

where 50 is the Dirac-delta function. This equation consists of three terms where 5(y) corresponds to the 0th-order diffraction and the other two terms, which are symmetric about the origin, are the ±1st- order diffraction terms. It is clear from the equation above that the location of intensity maxima of the ±1st diffraction order depends on the signal wavelength (or the frequency w). Selecting one of these orders, we obtain the frequency-to-space mapping relation of this system where each frequency w is mapped onto a specific point [0, y(w)] on the output plane according to the relation:

Figure 5.3 Thin sinusoidal grating excited with a normally incident plane wave.

This is the 1D frequency scanning along the у-axis using a thin sinusoidal diffraction grating. While a thin sinusoidal grating is taken here as an example, a large variety of diffraction gratings exist, which are used for spectral analysis using their higher-order diffraction patterns.