Question

A He-Ne laser has the following characteristics: (1) A resonator with 97% and 100% mirror reflectances and negligible internal losses; (2) a Doppler –broadened atomic transition with Doppler linewidth ΔνD = 1.5 GHz; and (3) a small – signal peak gain coefficient γ0(ν0) = 2.5 x 10-3 cm-1 . While the laser is running, the frequencies of its longitudinal modes drift with time as a result of small thermally induced changes in the length of the resonator. Find the allowable range of resonator lengths such that the laser will always oscillate in one or two (but not more) longitudinal modes. The refractive index n=1.

Answer 1