The concept essential to a laser is the emission of light which is spatially coherent and spectrally coherent :
Spatial coherence means that the emission is contained in the form of a beam of light that diverges little as it propagates. In a well designed laser this beam divergence obeys the limit imposed by quantum mechanics known as the uncertainty principle.
Spectral coherence means that the color of the emission is pure or extremely pure. The purity of the emission is quantified by the bandwidth or linewidth, that is the spread, of the color. The narrower the bandwidth, or linewidth, the higher the spectral coherence.
The radiation described above is emitted by a laser. A laser can be a macroscopic, or classical, device that emits quantum mechanical radiation. A laser uses two basic components, the gain medium, and the laser cavity:
Gain medium: this is an atomic, or molecular, medium that can be excited either optically or electrically. Once excited this medium emits light. This medium can be in the gas, liquid, or solid state.
Laser cavity: the gain medium is confined within a laser cavity that also includes optical components such as mirrors, prisms, and gratings. The simplest laser cavity is comprised by two mirrors. More sophisticated laser cavities include mirrors, prisms, and gratings. Generally these more sophisticated laser cavities are known as lasers resonators or laser oscillators.
This is an optimized tunable laser oscillator yielding extremely pure emission. The schematics show a gain medium, a tuning grating, a multiple-prism beam expander, and an output mirror which is also a polarizer. For this particular laser (Duarte, 1999 ) the emission is tunable in the visible spectrum in the 550-603 nm range (that is, mainly in the yellow-orange portion of the spectrum). The linewith of this high-power laser oscillator, Δλ ≈ 0.0004 nm, is only limited by Heisenberg's uncertainty principle.
With these basic ideas in mind the initial reader should now be more familiar with the concepts described in our
laser tutorial. Tunable Laser Optics explains and explores further these exciting ideas and provides a simple introduction to the quantum physics behind the laser. Tunable lasers are very widely applied (from astronomy to medicine) and, for those interested, Tunable Laser Applications provides an ample authoritative survey also suitable for professionals and university students.