Factors influencing the relative frequency stability of He--Ne laser structures

6328 Å He–Ne laser structures, made from ultralow expansion materials, suspended in temperature- and pressure-controlled chambers, have shown drift rates in excess of +5 MHz/day. The same structure, evacuated and used as passive Fabry-Perot resonators, suspended in the previous now evacuated chambers, still show a drift rate of the order of +2 MHz/day. Computer simulated mirror model aging shows that relatively large changes of the dielectric constants of the mirror coatings are needed to explain the measured drift. We feel that the dimensional stability of the materials and the properties of the optical contacts used, as well as the behavior of the dielectric mirrors, have to be more carefully investigated.