Color Centers and Ruby-Laser Output-Energy Degradation

Abstract

The use of lasers in rangefinding and other applications requires that the laser be capable of being fired many times without its output characteristics changing substantially. A gradual reduction in ruby (Cr³⁺ in Al₂O₃) laser output energy has been observed during repeated firing of the laser. This degradation is found to occur with varying degrees of severity in all ruby rods tested. We have traced this to the formation of color centers within or on the surface of the ruby rod: defects, impurities, etc., present before exposure can be converted into color centers by the blue and ultraviolet content of the pumplight. Conversion of Cr³⁺ to other valence states is very small, amounting to less than 1% of the total Cr³⁺ density (the sensitivity of the measurement) for most samples and to 3% for one vintage of ruby. The radiation induced absorption which was found agrees well with x‐ray induced color‐center absorption in undoped α‐Al₂O₃ rather than absorption by Cr²⁺ or Cr⁴⁺. We have shown that the subsequent pump light absorption by these color centers, which serves to deplete the pumping energy internally in the rod and to cause bulk absorption loss at 6943 Å, fully explains the degradation observed to within the limits of experimental error. A reliable indicator of color center formation is the orange‐brown appearance which develops in the ruby; only a partial restoration of output energy can be achieved by heating the rods at 900°C for 24 h; the use of pyrex to filter the ultraviolet from the pump light can reduce but not eliminate the rate of energy decay. The contribution of this work is that we have determined the principal mechanisms out of several possible ones which explains the energy decay observed in U.S. grown ruby samples. The complete solution to this problem will be known when it is understood how to prevent color center formation in Al₂O₃ upon ultraviolet irradiation.