Pumping geometries

Four-level lasers offer a distinct advantage over tlieir tliree-level counterjiarts, (figure C2.15.5). The Nd YAG system is an excellent example of a four-level laser. Here tlie tenninal level for tlie laser transition, 2), is unoccupied tlius resulting in an inverted state as soon as any atom is pumped to state 3. Solid-state systems based on tliis pumping geometry dominate tlie marketplace for high-power laser devices. 

As before, let us first look at pulsed Ti sapphire lasers pumped by g-switched Nd YAG lasers. The overall resonator components are very much the same as for the tuneable dye laser. However, because of the much longer lifetime of the upper laser level (a few microseconds opposed to a few nanoseconds for laser dyes), the pumping geometry is now longitudinal rather than transverse. The pumping geometry is... 

Figure 4.13 Longitudinal pumping geometry for pulsed Ti sapphire laser...

Various pumping geometries and resonator designs have been proposed or demonstrated [5.151]. In transverse pumping (Fig. 5.83), the pump laser beam is focused by a cylindrical lens into the dye cell. Since the absorption coefficient for the pump radiation is large, the pump beam is strongly attenuated... 

For sub-Doppler spectroscopy, single-mode cw dye lasers represent the most important laser types besides cw tunable solid-state lasers. Great efforts have therefore been undertaken in many laboratories to increase the output power, tuning range, and frequency stability. Various resonator configurations, pump geometries, and designs of the dye flow system have been successfully tried... 

Fig.5.92a-c. Three possible standing-wave resonator configurations used for cw dye lasers (a) collinear pumping geometry (b) folded astigmatically compensated resonator of the Kogelnik type [5.143] with a Brewster prism for separation of pump beam and dye-lascr beam and (c) the pump beam is focused by an extra pump mirror into the dye jet and is tilted against the resonator axis... 

A sinusoidally driven vibrator described by Philippoff also belongs in this category, although under most circumstances its inertia and elastance can be neglected. It employs annular pumping geometry for liquid samples, and other geometries for different kinds of viscoelastic materials which will be mentioned... 

With a suitable mechanical drive, small angular oscillating deformations can be imposed on steady rotation at various shear rates, using either coaxial cylinder or cone and plate geometry, and the dynamic mechanical properties can be measured under conditions of non-Newtonian flow. By use of annular pumping geometry with steady rotation of the outer cylinder, the viscoelastic properties under conditions of non-Newtonian flow perpendicular to the oscillatory motion can be measured. ... 

Sect. 5.7.5). A collinear pump geometry allows optimum overlap between the pump beam and the waist of the fundamental resonator mode in the crystal. The mode-matching parameter (i.e., the ratio of pump-beam waist to resonator-mode waist) can be chosen by appropriate mirror curvatures. The optical density of the active medium, which depends on the preparation of the Fa centers [5.146], has to be carefully adjusted to achieve optimum absorption of the pump wavelength. The crystal is mounted on a cold finger cooled with liquid nitrogen in order to achieve a high quantum efficiency t]. 

Chen, F. Gindre, D. Nunzi, J. M. First order distributed feedback dye laser effect in reflection pumping geometry for nonlinear optical measurements. Proc. SPIE-Int. Soc. Opt. Eng. 2007, 6653, 665304/1-665304/5. 

---