Microwave modulation of a ruby laser output by absorption

Ruby exhibits the Zeeman effect in the energy levels responsible for the absorption of the R1line. When a ruby laser is directed to pass through a section of unexcited ruby rod, the intensity of the transmitted beam is strongly dependent upon the magnetic field applied to the absorber. This effect was used to modulate the amplitude of a pulsed ruby laser in the microwave frequency region. The experimental modulator consisted of a 0.2 inch long ruby absorber located at one end of an S-band TE111mode cylindrical cavity pumped by a 3.2 Gc microwave source. A d.c. bias magnetic field was also applied to the absorber. When both the laser and the absorber rods were cooled to 80° K, the modulated beam attained maximum modulation index with a bias field of 2.5 kOe. This and other experimental results agree well with calculations made from a small-signal analysis of this modulation technique, and indicate that the various known relaxation mechanisms in the spin system of ruby do not impose a frequency limit to this method of modulation. Comparison between this technique and other conventional methods, as well as criteria for achieving high modulation efficiency, will be discussed in light of the small-signal analysis.