Characteristics of a single-frequency Michelson-type He-Ne gas laser

A new mode suppression configuration is described for producing a single-frequency output from gaseous lasers. This configuration has the form of a modified Michelson interferometer where the basic modification is in the introduction of a third feedback mirror for coupling the two branches of the Michelson. Small path length differences between the two branches of the Michelson are responsible for the mode suppression properties of the device which come about through interference phenomena at the beam splitter. This results in a frequency-dependent power loss out of the beam splitter which can provide for large amounts of mode discrimination. The device is made into an oscillator by introducing gain into each of the two branches of the Michelson. A detailed analysis of the device is given based on the scattering matrix formalism. The results of the analysis give the oscillation frequencies, the degree of mode discrimination, the internal laser radiation intensity, and equations for the output power. Experimentally, a 10 mW single-mode 6328 Å He-Ne Michelson-type gas laser is described. Data on the frequency spectrum, output power, and amplitude stability of a free running laser are given and correlated with theory. While it is shown that single-frequency operation is easily obtained, the device in its present form does not include a means for stabilizing the frequency.