Spurious frequency spectra caused by the scattered lights from transparent channel wall in the laser doppler velocimetry

Spurious frequency spectra, SFS, in laser Doppler velocimetry, caused by a superposition of the scattered light from a stationary transparent wall of a thin flow channel with the incident light are discussed and systematically analyzed. The observed SFS as well as the amplitude of SFS changed with the position and aperture size of the receiving optics. They were entirely different from the so-called frequency broadening and the characteristics indicated that the scattered light served as reference beams on the heterodyne detection of the Doppler signals. A theoretical analysis of the SFS, undertaken by using a simple model, is compared with experimental results; both were in good agreement. A rejection method of SFS is also discussed.