Continuous wavelength tuning

The next chapter discusses in more detail the basic properties and techniques which make the laser such an attractive spectroscopic light source. The important questions of wavelength stabilization and continuous wavelength tuning are treated, and experimental realizations of single-mode tunable lasers are presented. 

Fig. 5.89. Continuous mechanical tuning of the dye laser wavelength without mode hops by tilting mirror M2 around an axis through the intersection of two planes through the grating surface and the surface of mirror M2...

In spectroscopy using continuous laser tuning it is very important to be able to monitor the change in wavelength. For this purpose, a stable multipass interferometer (Sect.6.2.3) can be used from which the fringes are recorded together with the spectrum [9.43]. 

The common liquid lasers utilize a flowing dye as the active medium and are pumped by a flash lamp or another laser. These are typically more complex systems requiring more maintenance. They can he operated as either CW (continuous wave) or pulsed. One advantage liquid lasers have is they can be tuned for different wavelengths over a 100-nm range. 

Since our main objective was to remove all the chlorine and hydrogen atoms from the polymer chain, C-PVC films were further exposed to the UV radiation of the medium pressure mercury-lamp. This led to a dark brown material w.hich was found to be unable to carry an electrical current, even after extended irradiation time. Therefore we turned to a powerful laser source, a 15 W argon ion laser tuned to its continuous emission at 488.1 nm. At that wavelength, the degraded polymer film absorbs about 30 % of the incident laser photons. The sample was placed on a X-Y stage and exposed to the laser beam at scanning rates in the range of 1 to 50 cm s, in the presence of air. 

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. 

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