Laser cavities with increased axial mode separation

Single axial mode operation of gas lasers may be achieved either by reducing the mirror separation, L, until the mode separation, c/2L, is comparable to the oscillation line width, Δ v a , or by using mode suppression or filtering techniques.1,2,3 When the line width exceeds ≍3 gc (as in an argon ion laser for example), L must be less than 5 cm for an unstabilized laser and less than 10 cm for a laser stabilized at line center.∗ Lasers of this length have several disadvantages, the most serious of which (in the case of ion lasers) results from the close proximity of the Brewster windows to the discharge. In the case of the 6328 Ā He-Ne laser, restricting the laser tube length to a few centimeters limits the available single axial mode output power to about 1 mw.