A novel frequency estimation method for the evaluation of IJDA burst signals is presented. The new method is based on the measurement of time series of phase angles defined by a pair of quadrature signals (sine/cosine signal pair). Up to now quadrature techniques have preponderantly been used in the two beam interferometry for carrying out direction sensitive and high resolution measurements as well as in radio frequency applications.The quadrature demodulation technique can be considered as a straightforward development of the counter technique with the advantages of a higher resolution for a single burst evaluation and of directional discrimination.Compared with conventional signal processing techniques the application of the quadrature demodulation technique additionally allows a directional discrimination and the analysis of velocity fluctuations within single Doppler signal bursts with a higher accuracy by evaluating the measured time series of phase angle values.
The commonly used techniques for directional discrimination in velocity measurements involve a frequency shift of the LDA light beams which are focussed into the measuring volume. The paper presents a new technique for the generation of a frequency shift by using the optical frequency difference of two current and temperature stabilized monomode laser diodes.By focussing the output beams of the two frequency stabilized laser diodes into the measuring volume it is possible -to save the commonly required components for the beam splitting and frequency shifting and -to double the light intensity in the measuring volume.It will be shown that using conventional laser diodes, the influence of frequency shift fluctuations and large band widths on the measuring information in the carrier frequency signal can be eliminated by appropriate heterodyne signal processing techniques. The concept of the described new frequency shift technique will be discussed critically and first experimental results will be presented.
A novel method for the directional multicomponent laser Doppler velocimetry, LDV, based on the generation of different carrier frequencies, one for each velocity component, is presented. The carrier frequencies are generated by a chirp laser frequency modulation in conjunction with fiber delay lines of different lengths. Since the carrier frequency generation is realized without involving additional frequency shift elements like Bragg cells the LDV arrangement can be significantly simplified. Accurate velocity measurements without influence of carrier frequency fluctuations are accomplished by correlating the generated measuring Doppler signal with reference signals, given by the same carrier frequencies. The employed quadrature demodulation signal processing technique enables the measurement of the momentary Doppler frequency in the baseband. This novel LDV system is demonstrated by a directional measurement of two orthogonal velocity components of fluid flows
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