A semiconductor laser-Doppler-anemometer for applications in aerodynamic research

A miniature LDA (laser Doppler anemometer) system based on a semiconductor laser and an avalanche photodiode has been developed for aerodynamic measurements during flight tests. Extensive laboratory tests, guided by a generalized theory of frequency broadening, have led to an optimal design of the LDA system which is now ready for intended application. This device consumes significantly less power than a conventional system and can easily be operated with the electrical power available in an aircraft during flight. It is very small in size and can be conveniently transported and mounted at places where space is limited, such as an aircraft wing. This device is less expensive than a conventional system, which allows one to use several of such units simultaneously to cover a large volume of the flow field. The hardware of the unit is described here. A theory for predicting the measuring accuracy is presented and applied to the system under consideration. The theoretical predictions have been experimentally verified. Measurements in boundary layers are presented to demonstrate that a well-optimized semiconductor LDA system is as reliable as a conventional system based on a gas laser and a photomultiplier tube