Laser the output wavelengths of which can be tuned.
A tunable Laser is a laser the output Wavelength of which can be tuned (i.e., adjusted). In some cases, a particularly wide tuning Range is desired, 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, it is possible either to spectrally shift the envelope of the Frequency comb or the lines in the spectrum.