The changes in the spectrum of photocurrent in two-beam Doppler velocimeter caused by different types of vibration are analyzed. It is shown that, under certain conditions, periodical laser beam scan and surface vibration are producing identical changes in the spectrum of photocurrent. The changes might be quite considerable even with relatively small amplitude of laser beam scan of several arc seconds while monitoring slowly moving surface. The vibrational structure of Doppler and "pedestal" component in photocurrent are studied; it is shown that with vibrational velocity exceeding half of translational velocity the vibrational bands correspondent to "pedestal" and Doppler component are overlapping. The use of short acquisition time, which allows avoiding formation of broadband line structure in the spectrum, is also studied. The impact of bending vibration of translationally moving surface is also considered; due to averaging action of detector receiving aperture the impact of bending vibration is much less pronounced than that of laser beam scan or surface vibration.
The random and bias errors of estimating Doppler frequency from zero counting technique and correlation technique based on correlation function zeros analysis are compared with the accuracy of conventional spectral analysis and ultimate accuracy given by Cramer-Rao lower bound. It is shown that spectral measurement technique has advantage over other techniques in low signal-to-noise ratio region, whereas zero crossing technique based on analysis of probability density ensures lower random and bias errors at high enough SNR. The correlation technique has relatively low random error but shows considerable bias error in the region of low and moderate SNR when information about Doppler frequency is obtained from first zero of correlation function.
The Laser Diode intensity noise as a function of laser current and temperature has been studied in this paper. It is shown that the intensity noise in the current-temperature coordinates forms "islands" of noise amid larger areas of relatively low-noise operation. Although these "islands" of noise exhibit some drift over time, it is generally possible to find operational points characterized by long-term stability and low noise. We show also that the laser noise severely deteriorates the signal-to-noise ratio (SNR) at the receiver --photodiode circuit-- causing random fluctuations of SNR, impairing the performance of interferometric applications. The comparison of noise distributions for isolated laser modules and laser modules incorporated into laser Doppler velocimeters allowed us to reveal design faults contributing to unwanted back reflections deteriorating the overall SNR and operationl stability of the system.
The random and bias errors of Doppler spectral shift measurements, correlation measurements and zero crossing techniques are discussed. It is shown that spectral shift measurement technique has advantage over other techniques in low signal-to-noise ratio region, whereas zero crossing technique based on analysis of probability density ensures lower random and bias errors at high enough SNR. The correlation technique has relatively low random error but shows considerable bias error in the region of low and moderate SNR when information about Doppler frequency is obtained from first zero of correlation function.
Laser diode intensity noise as a function of laser current and temperature is studied. It is shown that the intensity noise in current-temperature coordinates forms "islands" amid larger area of relatively low-noise operation. Although these "islands" of noise are showing some movement in time, generally it is possible to find an operational point characterized by long-term low noise. We show also that laser noise deteriorates signal-to-noise ratio (SNR) in photodiode circuit causing random fluctuations of SNR. The comparison of noise distributions for isolated laser module and the laser incorporated into Doppler velocimeter gauge allowed us to reveal design faults contributing to back reflection and enhancing the noise.
The statistical properties ofphotodetector current in laser Doppler velocimeter are studied. The analysis of zero crossing counting shows that it overestimates by certain amount velocity and displacement measurements. The dispersion of zero counts in the presence of additive shot noise is also studied. The simple analytical expression for dispersion of zero counts is obtained in absence of additive shot noise. It shows that the relative accuracy of velocity or displacement measurement is inversely proportional to the square root ofmeasured distance. The relative dispersion ofzero counts less than 0.005 % can be achieved while measuring 5 mdisplacement if certain requirements concerning signal-to-noise ratio are met.
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