Comprehensive reading Solid - State Lasers

The laser, by definition, is a device that amplifies light by means of stimulated emission of radiation. The major properties of laser radiation are high intensity, narrow width, directionality and coherence.

In practice a laser is generally used as a source or generator of radiation.

The working element of the ruby laser is a cylinder of pink ruby containing 0.05 per cent chromium. The cylinder is usually between 0.1 to 2 cm in diameter and 2 to 23 cm long; the end faces are plane and parallel to a high degree of accuracy (Fig.1).

In the commonly used laser configuration a ruby rod is surrounded by the coils of a helical flashlamp operated usually for a few milliseconds with an input energy of 1000 to 2000 joules.

Lasers require some type of resonator for the radiation field. A resonator provides for a stronger coupling between the radiation and the excited atoms.

flashtube

trigger ruby

electrode

Fig. 1. Construction of ruby laser.

The resonator most commonly used for laser action is composed of two small mirrors facing each other. When the ruby crystal is illuminated by short, intense bursts of white light from a flash tube, a red light beam of enormous power starts to bounce back and forth between the mirrors increasing in strength each time it passes through the ruby. One of the mirrors is partially transparent and from this mirror emerges the intense coherent radiation.

The intensity of this radiation exceeds that of the spontaneous radiation by several orders of magnitude, and spectral range of the induced radiation is considerably narrower than that of fluorescence. The narrowing of the line-width is due to effect of the resonant cavity formed by the mirrors.

Solid-state lasers generally operate in the pulsed condition. The reasons for this are mostly technical. First it is difficult to provide a powerful source of exciting light capable of continuous operation; second, a great deal of heat evolves within the laser which must be dissipated. Ordinary ruby lasers are excited for periods of a few milliseconds, the length of the period being determined by the duration of the exciting flash.