Longitudinal pumping

Three-photon active (SPA) materials have been studied extensively over the last few years owing to their potential applications in the fields of telecommunications and biophotonics [26-28, 30], Two major advantages of these materials—longer excitation wavelengths and much better spatial confinement—make them attractive in comparison with two-photon absorption (2PA) based materials [29], One of the most important applications of SPA materials is three-photon pumped frequency-upconversion cavity-less lasing [26, 30], Short infra-red (IR) pulses induce the ASE process via 3-photon absorption followed by fast non-radiative decay to a long-lived state which collects population. Conventional experiments with a pulsed longitudinal pump [27, 28, 30] show that stimulated emission occurs in both forward and backward directions with respect to the pump pulse. 

The essential physics is illustrated with just these four terms, in the time-independent, plane-wave, longitudinal pumping case. (The pump light propagates along the same axis as the dye laser beam.) The gain constant is given by... 

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

Fig. 5.84. Possible resonator designs for longitudinal pumping of dye lasers [5.151]...

For reliable single-mode operation of the Littman laser longitudinal pumping is better than transverse pumping, because the dye cell is shorter and inhomogenities of the refractive index caused by the pump process are less severe [5.183J. 

Fig. 5.86. Littman laser with grazing incidence grating and Littrow grating using longitudinal pumping...

An intracavity LMR spectrometer based on an optically pumped laser is shown in Figure 2B. The only difference is the change of the CO (or CO2) laser in Figure 2A by an optically pumped laser. The pump radiation is multiply reflected between two metallic-coated flats parallel to the far-infrared laser axis. Longitudinal pumping in which the pump radiation is introduced along the laser axis through a hole in one of the far-infrared laser mirrors is also employed. The cavity is divided by a beam splitter (e.g. thin polypropylene). 

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