A laser Doppler interferometer

A laser anemometer has been developed to measure the time history of particles or aerosols carried in near isotropic turbulent air flows under a variety of laboratory conditions. The resulting instrument is of a form not previously used, and can be viewed as an adaptation of the Mach-Zehnder interferometer in which both beams are scattered, on a central rotation axis. Wavefronts are corrected to achieve efficient use of scattered laser light. The interferometer produces visible fringes in the scattered light and can be visually aligned. The angles of scattering and of yaw to the flow can be changed with the instrument in alignment, allowing frequency tuning by geometric adjustments. It is portable and can traverse flows up to 0.5 m wide and is powered with a 1 W argon ion laser. A new tracking filter specially developed for turbulent flows is used to produce a frequency proportional voltage signal which is recorded on magnetic tape and analysed on a Marconi Elliot Myriad computer.