Fraunhofer Institute for Laser Technology ILT
The cross-sectional intensity distribution of the laser radiation field is not homogenous: the intensity is modulated transverse to the propagation direction and decreases steadily at the edges rather than abruptly. This is due to diffraction which always occurs during propagation and is related to the wave-like nature of light.
If one assumes a homogenous intensity distribution on one of the laser mirrors, the intensity distribution will change during propagation from one mirror to the other due to diffraction. After numerous passes back and forth, the intensity distribution takes shape and is reproduced with each pass. These marked intensity distributions represent the intrinsic solution of the optical resonator of which there are, in fact, several. One of these intrinsic solutions is, however, of particular importance: the so-called fundamental mode. In many cases, the fundamental mode approximates the so-called Gaussian beam. The Gaussian beam has a radial intensity distribution with a Gaussian profile.
A significant parameter involved in designing optical resonators is the Fresnel number which is defined as the radius of the resonator mirror squared divided by the product of the wavelength of the laser radiation and the distance to the laser mirror. Fundamental mode radiation is associated with Fresnel numbers of around 1. Diffraction losses increase with smaller Fresnel numbers (see below); at larger Fresnel numbers, the radiation field modes arise which causes a decline in beam quality. The beam quality determines, for instance, how small the beam cross-section in the focal plane of a lens can be - a crucial factor for many applications. In the fundamental operating mode, the minimal focal radius is solely limited by diffraction - it physically cannot get any smaller.
The effect of diffraction on the transverse beam distribution can be simulated in this applet. For simplicity, the diffraction problem will be considered one-dimensionally based on a strip resonator. The middle section corresponds to the mirror and the radiation intensity to the left and right of the mirror is not re-reflected and is lost. This is known as diffraction losses. The red line represents the intensity of the n-1th pass; the black line represents the nth pass applied over the coordinates transverse to the propagation direction.