Introduction to Lasers

In the context of measurements, lasers fall in the category of light sources with certain helpful properties. Unlike a material probe (say, a thermocouple), lasers are often called photon probes . The measurement of a flow property may, however, rely on the wave-like nature of light.

Though light propagates as a wave, generation of light from a material is a quantum-mechanical phenomenon. In fact, the subject is one of emission of electromagnetic radiation, with light being EM radiation in the visible range (400-700 nm, wavelength). In a conventional light source, for example a tungsten filament lamp, the metallic wire is electrically heated to a high enough temperature. Under a thermal stimulus, electrons undergo transition to higher energy levels. As they return to the ground state, they emit photons. Transitions occuring closer to the surface to the material result in a net emission of radiation to the environment. If the filament temperature is high enough, photons would be energetic and radiation can fall in the visible range. Unfortunately, such a light source has limited utility in measurements.

The tungsten filament lamp is often called a conventional light source , to contrast its behaviour with a laser. Characteristically, the filament emisison is random in time, spatially distributed, and photon energies are distributed over several wavelengths, thus giving rise to polychromatic radiation. The randomness in time ensures that the phases of the wave packets leaving the filament are practically uncorrelated. Further, the emisison is in all directions and intensity diminishes with distance. In contrast, a laser output is

1. monochromatic,

2. intense,

3. directional, and

4. coherent.