Iodine Stabilized Laser Strainmeter

A new type of laser strainmeter has been operated in Queensbury tunnel, Yorkshire, England. It has design features which possess fundamental and practical advantages over other laser strainmeters which have been constructed. The system uses two helium-neon lasers which operate at 633 nm. One illuminates a Michelson interferometer with a 54 m long arm in which strain is to be measured. The laser frequency is controlled to keep a particular fringe at the outputs of the Michelson interferometer as the path difference in the interferometer alters. The other laser is stabilized by saturated absorption in iodine vapour, and has extremely good frequency stability over very long periods. The frequency difference between the lasers is measured to give a signal directly proportional to the strain in the interferometer with very high resolution and linearity. Twelve weeks of tidal data have been produced by the instrument.