Pulse stretching utilizing two-photon-induced light absorption

The effect of inserting an element exhibiting induced absorption into a
Q
-switched laser cavity is investigated theoretically and experimentally. The rate equations are solved assuming two types of nonlinear loss, one being proportional to the square of the laser intensity and the other being proportional to the product of laser intensity and density of excited electrons in the nonlinear absorber. Experiments are performed with a rotating-prism ruby laser with a CdS crystal in its cavity. It is established that two-photon absorption takes place, and as predicted by the theory that the output intensity and output energy both decrease and pulse length increases as compared with the normal
Q
-switched case. In addition, the output pulse has an oscillatory behavior, and it is suggested that this is caused by loss due to the excited electrons, which are found to have a lifetime of approximately 24 ns.