If a laser is tunable, its wavelength can be changed within a defined range in a controlled manner. In some cases, this tuning range is in excess of several hundred nanometers. The wavelength resolution differs from system to system. Other important variables are tuning speed and prevention of mode hopping.
Solid state lasers based on titanium-sapphire or chromium-doped zinc selenide/zinc sulfite ceramics (Cr:ZnSe/ Cr:ZnS), which enable wide tuning ranges in the near and mid-infrared range, are commonly used. Dye lasers and lasers with rare earths (especially ytterbium) and diode lasers are available in tunable versions, although with the latter, wavelength changes are achieved with temperature control. Quantum cascade lasers also provide good tunability.
These lasers are used in spectroscopy, microscopy, and interferometry, and, increasingly, in the area of quantum technology, where tunable lasers are used for selective cooling of atoms in the generation of Bose-Einstein condensates, for ionization, or to analyze quantum dots.
A freely available brochure from Newport subsidiary New Focus gives an overview of the different tuning methods.