A tunable laser is a laser the output wavelength of which can be tuned (i.e. adjusted) (→ wavelength tuning). In some cases, a particularly wide tuning range is desired, i.e. a wide range of accessible wavelengths, whereas in other cases it is sufficient that the laser wavelength can be tuned (factory-set) to a certain value. Some single-frequency lasers can be continuously tuned over a certain range, whereas others can access only discrete wavelengths or at least exhibit mode hops when being tuned over a larger range. Lasers are sometimes called wavelength agile or frequency agile when the tuning can be done with high speed.
Tunable lasers are usually operating in a continuous fashion with a small emission bandwidth, although some Q-switched and mode-locked lasers can also be wavelength tuned. In the latter case, it is possible to shift either the envelope of the frequency comb or the lines in the optical spectrum.