Light Sources

Conventional light sources emit radiation by a series of successive spatially distributed random phenomena. These involve atomic excitation followed by emission as electrons in the valence band jump to lower energy levels. This downward transition follows predetermined rules that finally govern the wavelength of the emitted photon. Note that energy emitted and wavelength are related as where is Planck's constant. The average life of a radiating atom is and so the average length of a single train of waves is m. Coherence length of a tungsten filament is at best a few mm. In contrast to this a laser produces a beam whose coherence length is several mm. Further a laser beam is thin and approximates a point source.

We describe briefly the principles involved in the operation of a laser. Laser is an acronym for light amplification by stimulated emission of radiation. Its operation employs the following ideas.

a. Metastable states: Normally valence electrons of an atom can be excited to a higher energy level from which they return to the ground state by emitting a photon. However, for certain materials there exist energy levels beyond the ground state from which the return of electrons to the ground state is considerably delayed (Figure 3.1). This return can, however, occur in the event of a collision between an electron and a photon. The average life of a normally excited electron is ; in the metastable state it is around s.

b. Optical Pumping: It is possible to raise electrons to metastable states by light absorption. This is called optical pumping.

c. Fluorescence: Emission of light when an electron jumps from a metastable state to the ground state is called fluorescence. In light sources, the gas pressure is kept very low to minimize the possibility of collision between particles, increase the particle life in the metastable state and minimize the production of thermal energy. Hence the resulting electronic transitions mainly classify as fluorescence.