Factors influencing the long-term frequency stability of 6328A He-Ne lasers

Independent, free running 6328Å He-Ne lasers have shown a relatively good short term frequency stability, ie., a beat frequency spectral width of the order of 100 Hz measured over a period of 0.1 seconds. However, the same lasers, made from ultra low expansion materials show a disappointing drift rate of several MHz per day over periods of months despite careful temperature and pressure control. This drift, always in the direction of increasing frequency is due to changes in the optical and mechanical path lengths, since the contributions of He-Ne pressure changes and H2and O2impurities can only explain a total frequency shift of the order of 20 MHz over the life span of the laser.