Frequency stabilisation of visible output laser diodes

Laser interferometry uses the wavelength of light as the unit of measurement, and therefore the absolute wavelength of the source must be accurately known and must be stable. The technique of normal absorption was used to stabilise the frequency of the laser diode output. A molecular iodine vapour having a detailed absorption spectrum was chosen for the stabilisation of visible 670 nm laser diodes. This vapour has a number of absorption features in the 600-700 nm spectrum range which enabled the frequency stabilisation of several visible output laser diodes. At 670 nm a relative frequency stability of ±42 MHz was achieved over a period of 20 min with an Allan variance of between 1.5 MHz and 200 kHz over an averaging time between 2.5 and 250 ms