Multicomponent frequency shifting self-aligning laser velocimeters.
A design using simple lenses and frequency shift by scattering disk has been developed, which allows easy construction, and alignment of one-, two-, and three-orthogonal-component, frequency shifting laser Doppler velocimeters. A series of instruments designed on this basis have proved to be inexpensive to construct, precise and accurate to use, adaptable to a wide range of applications, and have proven particularly advantageous in studies of turbulent phenomena and complex flows. In this paper the design concept is explained, construction and alignment procedures are outlined for the simplest assembly of the one and two component versions, and performance and application examples are presented. The extension of the system to measure the third orthogonal component is discussed.
The frequency shifting of laser light by electro-optic techniques
In velocity measurements by the laser Doppler method, a requirement exists for changing the frequency of one light beam with respect to another derived from the same source. This is particularly useful for the determination of the sign of a velocity. The required frequency shifting or single side-band generation can be accomplished by the use of electro-optical effects and a number of possible arrangements are discussed. Trials were made using Kerr cells and electro-optic crystals and single side-band generation demonstrated by a simple Doppler beating experiment. If a low efficiency can be tolerated as in Doppler beating with a reference beam, the production of a pure frequency shifted beam is not difficult up to frequencies of a few megahertz. The differential Doppler or fringe system however requires shifted and non-shifted beams of equal intensity and hence high efficiency conversion is imperative. Radio-frequency driving power requirements may also be a limitation. The theoretical efficiencies of various multi-cell electro-optic arrangements have been calculated as a function of R.F. excitation. The possible efficiency approaches unity as the number of cells is increased.
Integrated optical units for laser anemometry
The design of one- and two-channel optical units is described and their application to optical anemometry explained. These units permit easy operation of an optical anemometer in fringe, Doppler or reference beam modes and with either forward or backward scattering. The alignment procedures are explained and shown to require no special optical knowledge and little time. The units are robust and can be operated in nonlaboratory environments. The two-channel unit permits measurements of two mutually orthogonal instantaneous velocities and their correlation in a manner similar to that achieved with crossed hot wires. Measurements of mean velocity and one component of fluctuating velocity were obtained with each mode of operation of the single channel unit in a fully developed channel flow. The results demonstrate the ease of use of the unit and the considerable potential of the measuring technique.