A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.It is a laser the output wavelength of which can be tuned (i.e., adjusted) (→ wavelength tuning). In some cases, one wants a wide tuning range, i.e., a wide range of accessible wavelengths, while 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, while others can access only discrete wavelengths or at least exhibit mode hops when being tuned over a larger range. 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, one may either spectrally shift the envelope of the frequency comb or the lines in the spectrum. Tunable lasers can continuously change their emission wavelength, or color, in a given spectral range. Tunable lasers are usually operating in a continuous fashion with a small emission bandwidth. Here wavelength tuning occurs.