Consider that both ends of ruby laser rod of 5 cm length are coated to have a reflectance of R=0.9 each. what is the minimum fraction of excited Cr+3 ions achieving the threshold condition of oscillation? Assume that the concentration of Cr+3 ions is N=10^19 /cm^3, the induced-emission cross-section is o =2x10^-20 cm^2, and the effective loss constant of the rod is 0.011/cm