Self-mixing Laser Doppler Velocimeter Research Articles

Quadratic detection of an enhanced self-mixing laser Doppler signal with two-photon absorption

A new detection scheme is proposed for an enhanced self-mixing laser Doppler velocimeter (SLDV) using two-photon absorption (TPA), which is based on the TPA-induced quadratic photoconductivity dependence of a photodiode on the incident radiance at 1550 nm with self-mixing interference (SMI). The harmonics of the SMI signal in the frequency spectrum are enhanced simultaneously, as for a fiber-Bragg-grating-based enhanced SLDV. The quadratic response induced by TPA can suppress the undesirable harmonics, making their amplitude into the same level as the noise. From the experimental results presented in this paper, the TPA effect in a commercial Si photodiode implements a sort of ultra-wide bandwidth filter without passively adjusting the cutoff frequencies for an unknown Doppler frequency, which simplifies Doppler signal processing. The signal-to-noise ratio (SNR) is also improved to a considerable level of 15 dB for a Doppler frequency of 4 kHz due to the amplification of an erbium-doped fiber amplifier. This paper provides guidance for the design of the enhanced SLDV with optical filtration.

Laser Doppler velocimeter and its calibration system

A high-precision calibration system for laser Doppler velocimeter (LDV) is designed to calibrate the reference-beam LDV. Larger velocity range, lower velocity uncertainty and smaller alignment error can be provided in calibration process by employing a rotary cylinder with 600 mm radius in the calibration system. Experiments are carried out for detailed analysis. The results show that the relative measurement precision of the LDV under calibration is 0.1% (1σ) in the velocity range from 0 to 200 km/h. The main sources of uncertainty in the calibration system are discussed. The expanded uncertainty of the proposed system is 0.03%. The proposed calibrating method can be used in the calibration process for the reference-beam LDV, the dual-beam differential LDV, the self-mixing LDV and the spatial filtering velocimeter.

Observation of motion of colloidal particles undergoing flowing Brownian motion using self-mixing laser velocimetry with a thin-slice solid-state laser.

We achieved a highly sensitive method for observing the motion of colloidal particles in a flowing suspension using a self-mixing laser Doppler velocimeter (LDV) comprising a laser-diode-pumped thin-slice solid-state laser and a simple photodiode. We describe the measurement method and the optical system of the self-mixing LDV for real-time measurements of the motion of colloidal particles. For a condensed solution, when the light scattered from the particles is reinjected into the solid-state laser, the laser output is modulated in intensity by the reinjected laser light. Thus, we can capture the motion of colloidal particles from the spectrum of the modulated laser output. For a diluted solution, when the relaxation oscillation frequency coincides with the Doppler shift frequency, fd, which is related to the average velocity of the particles, the spectrum reflecting the motion of the colloidal particles is enhanced by the resonant excitation of relaxation oscillations. Then, the spectral peak reflecting the motion of colloidal particles appears at 2×fd. The spectrum reflecting the motion of colloidal particles in a flowing diluted solution can be measured with high sensitivity, owing to the enhancement of the spectrum by the thin-slice solid-state laser.

Measurements of liquid surface fluctuations using a self-mixing solid-state laser

We propose a highly sensitive method for measurements of liquid surface fluctuations with nanometer-order displacement using a self-mixing laser Doppler velocimeter (LDV) comprising a laser-diode-pumped thin-slice solid-state laser and a simple photodiode. In this measurement system, the scattered light from the liquid surface is reinjected into the solid-state laser, and then the laser output from the solid-state laser is modulated by this reinjected laser light. The time dependence of the displacement of liquid surface fluctuations can be obtained from the analysis of the modulated laser output. We describe various measuring methods and the optical system of the self-mixing LDV for real-time measurements of liquid surface fluctuations. Real-time measurement of liquid surface fluctuations can be performed with high amplitude and frequency resolutions by the analysis of the modulated laser output.

Easy measurement and analysis method of zeta potential and electrophoretic mobility of water-dispersed colloidal particles by using a self-mixing solid-state laser

We describe a highly sensitive method of measuring electrophoretic mobility and zeta potential of water-dispersed colloidal particles by using a self-mixing laser Doppler velocimeter with a laser-diode-pumped, thin-slice solid-state laser with extremely high optical sensitivity. The power spectra of laser output modulated by reinjected laser light scattered by the electrophoretic particles were observed. The power spectrum cannot be described by the well-known formula for translational motion or flowing Brownian motion, i.e., a combination of Doppler shift, diffusion, and translation. The power spectra shape is found to reflect the velocity distribution of electrophoretic particles in a capillary tube due to the electro-osmotic flow contribution. Not only evaluation of the electrophoretic mobility and zeta potential but also the particle diameter undergoing electrophoretic motion can be performed from the shape of the power spectrum.

Effect of angle of incidence on self-mixing laser Doppler velocimeter and optimization of the system

Based on the theory of speckle and self-mixing interference in laser-diode, three-facet cavity model is introduced to analyze laser Doppler effect based on self-mixing interference in the case of a rough surface, and numerical solution of the signal is obtained. Simulation results of speckle-modulated Doppler signal based on self-mixing effect and tracking accuracy at different incident angles are given using parameters employed in the experiment. Simulation results indicate incident angle of around 30° is most suitable when both tracking accuracy and signal amplitude are considered. Experimental waveforms agree well with simulation results, and similar conclusions as simulation predictions about changing trend of tracking accuracies of the system at different incident angles can be made. Combining with difference frequency analog phase-lock loop (PLL) technique and appropriate sampling time, a laser Doppler velocimeter with tracking accuracy better than 1.3% in the range of 10–470mm/s is realized.

Detection of small particles in fluid flow using a self-mixing laser

We describe a highly sensitive, real-time method of detecting small particles in a fluid flow by self-mixing laser Doppler measurement with a laser-diode-pumped, thin-slice solid-state laser with extremely high optical sensitivity. Asymmetric power spectra of the laser output modulated by the re-injected scattered light from the small particles moving in a dilute sample-flow, through a small-diameter glass pipe, were observed. The observed power spectra are shown to reflect the velocity distribution of the fluid flow, which obeys Poiseuille's law. Quick measurements of flow rate and kinetic viscosities of water-glycerol mixtures were also performed successfully. Measurable low-concentration limits for 262-nm polystyrene latex spheres and 3-mum red blood cells in a fluid flow were below 1 and 10 ppm, respectively, in the present self-mixing laser Doppler velocimeter system.

Two-Dimensional Velocity Measurements With Self-Mixing Technique in Diode-Pumped Yb : Er Glass Laser

Self-mixing laser Doppler velocimeter based on a two beam geometry scheme is investigated using a single-frequency Yb:Er-doped phosphate glass laser. The geometry allows the observation of a beat mode with optical crosstalk between the two optical paths, giving two major improvements: 1) a precise positioning of the optical head compared to the scattering target; 2) the measurement of both the absolute value of the velocity and the orientation of the speed.