Some factors affecting the pumping efficiency of optically pumped lasers.
As a preliminary to the calculation and discussion of pumping efficiency, the results of an investigation into the properties of xenon flash tubes are given. Time resolved spectroscopy showed that the radiation during the flash is essentially that of a blackbody. The small range of measured blackbody temperatures (6000 degrees K to 7000 degrees K) cannot account for the variation in radiated power during the flash. However, emissivity variations (deduced from published absorption data) explain the observed time dependence exactly. An ionic mechanism is outlined which explains the spectral distribution of the radiation, the electrical properties of the discharge, and the emissivity variations. Possible reasons for the low radiative efficiency of 28% during the flash are discussed. The fraction of the electrical energy supplied to the tube reaching the laser crystal is calculated, taking into account the cavity geometry and wall reflectivity, the flash tube radiative efficiency and spectral distribution, the crystal ze and sorption, and the partial reconversion by the flash tube of light not absorbed in the crystal which returns to the tube. The predicted laser oscillation threshold agrees well with experiment. It is concluded that, while reconversion can significantly increase the effective radiative efficiency of the tube, the over-all efficiency of the laser is still limited by the low intrinsic radiative efficiency. Numerical examples are given to show that this parameter exercises the biggest influence on the present pumping efficiencies of about 0.5%. Over-all efficiencies of up to 9% could be obtained if this radiative efficiency could be raised to about 90%. Success in this area could give greater benefits than attempts to improve the spectral distribution of the tube radiation.
Edwards
J G
JG
The General Electric Company Limited,Central Research Laboratories, Hirst Research Centre, Wembley.