Tunable lasers

We often tend to associate the term "tunable lasers" with laser media whose spectral width exceeds several tens of nanometers (a few THz in frequency). However, tunability can be defined more generally as the ability of an optical oscillator to change its emission wavelength. Depending on the nature of the amplifying media and the type of devices used to control the spectrum, the spectral range can extend from a few Hz to a hundred THz. Tunability can be stepwise or continuous. It can be made definitive, or it can give the laser a certain frequency agility. Are all laser media "tunable"? The question arises for atoms, molecules or ions with simple energy diagrams and perfectly defined quantized levels, corresponding to discrete wavelengths. In fact, the levels are always broadened by a multitude of physical effects, the most fundamental of which is the lifetime of the levels, implying spectral broadening by Fourier transform. All light-amplifying media using stimulated emission are therefore tunable, it's just a question of how much.

We can go even further, using media that are not based on stimulated emission but on nonlinear effects. With second-order nonlinear crystals, we can create optical parametric oscillators capable of competing with or even surpassing tunable sources based on stimulated emission. By using the 3rd-order nonlinear effect in photonic fibers, we can generate a continuum of light over a very wide range of wavelengths and filter it to obtain tunable light.

The purpose of this article is to review the principles and performance of "tunable lasers" in the most general sense. Paragraph 1 describes tunable amplifying media, both lasers and nonlinear, and explains the physical origin of the width of spectra depending on the media considered. Section 2 gives the main principles of tunability, which in the vast majority of cases is achieved using an optical cavity containing the...