The Tunability of Laser Radiation

Nowadays, tunable coherent radiation has become a fundamental tool in many areas (remote sensing, isotope separation, photochemistry, etc.), with particular relevance in optical spectroscopy. 

Tunable coherent light sources can be realized in several ways. One possibility is to make use of lasers that offer a large spectral gain profile. In this case, wavelength-selecting elements inside the laser resonator restrict the laser oscillation to a narrow spectral interval and the laser wavelength may be continuously tuned across the gain profile. Examples of this type of tunable laser are the dye lasers were treated in the previous section. 

Another possibility for laser wavelength tuning is based on the shift of the energy levels in the active medium by external perturbations, which can cause a corresponding spectral shift in the gain profile and, therefore, in the laser wavelength. For instance, this shift may be caused by a temperature variation, as in the aforementioned case of semiconductor lasers. 

A third way of generating continuously tunable coherent radiation uses more complicated systems based on the principle of optical parametric oscillation (and amplification). Since the gain in these systems is not originated by stimulated emission, but by means of a nonlinear frequency conversion process, we will treat them in a separate section. 

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