OPTICAL DOPPLER MEASUREMENTS

Light from a low-cost He-Ne laser is filtered to provide a single-mode beam of long coherence length. Beat frequencies in the 100- to 500-kH z range are produced by superimposing laser radiation and waves which are Doppler shifted by a moving retroreflector. Varying speeds of a mo del train mounted reflector are independently measured with a photogat e and allow a check of the Doppler predicted speed, or the photogate d etermined speed can be used to measure the laser wavelength by a fit o f data points with a resulting standard deviation of less than 0.5% of lambda. A frequency to voltage converter allows direct velocity plots for constant acceleration or simple harmonic motion. (C) 1997 America n Association of Physics Teachers.