9.7 Originality of Laser Light

The main originality of a laser beam is to have two coherencies, spatial and temporal. Spatial coherency is synonymous with directivity, temporal coherency with monochromaticity. These two properties come directly from the stimulated emission, which produces photons in the same mode of the resonator. It's because of the properties of autooscillation that the optical characteristics (frequency and shape of the wave surfaces) of the emitted wave fit exactly with the resonator.

Figure 9.36: Illustration of spatial and temporal incoherencies. A collimator delivers an almost parallel beam. The narrower slit is, the smaller is the divergence of the beam. When the slit is closed the divergence is only limited by the diameter of the lens, but the intensity is then equal to zero . In the same way, the beam delivered by the collimator is all the more monochromatic as the slit is narrow; a perfectly monochromatic light is obtained when the width of the slit goes to zero, the intensity also goes to zero .

Directivity: The wave surfaces of a laser beam are perfectly defined and don't vary erratically from time to time; their shape is determined by the geometrical characteristics of the mirrors. If the output mirror is plane, the rays are orthogonal to its surface, the difference is only determined by diffraction and the angle is equal to 1.22 ?/ D ( ? is the wavelength and D the diameter of the mirror). With...