Transfer of the Useful Pump Radiation Emitted by the Pump Source to the Gain Medium

The transfer of flashlamp pump radiation to the laser medium is accomplished by means of a completely enclosed reflective chamber or pump cavity. The radiation transfer efficiency ηt can be defined as
P_e = η_tP_λ, (1)
where P_λ is the useful pump radiation emitted by the source and P_e is the fraction of this radiation transferred into the laser material. The factor ηt is a combination of the capture efficiency, defined by the fraction of rays leaving the source and intersecting the laser rod, and the transmission efficiency. The former is based on the geometrical shape of the pump cavity, diameter and separation of the pump source, and laser rod. The latter is a function of the reflectivity of the walls of the pump cavity, reflection losses at the rod surface and coolant jacket, absorption losses in the coolant fluid, and radiation losses through the clearance holes at the side walls of the pump cavity. For close-coupled cavities, typical values are η_t = 0.3–0.6.
In diode-pumped lasers the radiation transfer is much simpler. In so-called endpumped lasers, the transfer system usually consists of lenses for the collection and focusing of diode radiation into the laser crystal. Furthermore, in side-pumped systems, the laser diodes are mounted in close proximity to the laser crystal without the use of any intervening optics. If we express reflection losses and spill-over losses at the optics or active medium by the parameter R, we can write
η_t = (1 − R). (2)
Since the laser crystal and optical components are all antireflection coated, the radiation transfer losses are very small in these systems. Values for the radiation transfer efficiency are typically η_t = 0.85–0.98.