Laser Doppler velocity profile sensor using a two-wavelength technique and diffractive optics

A measuring system based on a differential laser-Doppler anemometer has been extended to determine one-dimensional velocity profiles of shear flows with a spatial resolution in the micrometer range. The principle of the realised velocity profile sensor is based on two-wavelength fringe systems with different gradients of their fringe spacings. After wavelength-sensitive detection two Doppler frequencies are determined, which yield the position as well as the velocity component of individual tracer particles. For fluid flow investigations with high spatial resolution and high data rate, laser Doppler anemometers (LDA) are often applied. An LDA set-up can be considered a Mach-Zehnder interferometer operating by beam splitting and subsequent superposition of the partial beams. In the volume of intersection an interference pattern consisting of parallel fringes develops, which defines the measurement volume of the LDA. The velocity of small particles passing the measurement volume is determined from the modulation frequency of the scattered light, the Doppler frequency. The spatial resolution is generally limited by the size of the measurement volume which is usually about 1/spl middot/0.1/spl middot/0.1 mm/sup 3/. But in many applications, especially for turbulence research, highly spatially resolved velocity measurements with a resolution down to e.g. 10 /spl mu/m (Kolmogorov scale) are required. We have developed a laser Doppler velocity profile sensor, which enables velocity measurements with a spatial resolution inside the measurement volume with nearly 1 /spl mu/m accuracy. A Mach-Zehnder configuration is used with two wavelengths each of which generates a fringe pattern. A two-wavelength laser was used, which emits an upconversion laser line and the pump light. The beam waist of the different wavelengths are spatially separated by a diffractive lens offering a great chromatic dispersion. Due to the wavefront curvature of the employed Gaussian beam a convergent an- - d a divergent interference pattern results. The ratio of the fringe spacing functions is monotonously varying along the optical axis and used as a calibration function for the determination of the position. From particles passing the measurement volume and scattering a bi-chromatic light two Doppler frequencies can be calculated, the ratio of which yields together with the calibration curve the position. In conclusion a differential laser Doppler profile sensor for spatial resolved velocity measurements of shear flows was realised. The advantages of this profile sensor can be outlined as follows: (i) A spatial resolution of about 1.6 /spl mu/m, (ii) a relative velocity measurement error of about 1.4/spl times/10-4 and (iii) distributed velocity measurements without traversing the sensor were achieved. The drawbacks mentioned above concerning conventional point-wise LDA systems, such as strongly limited spatial resolution, systematic velocity measurement errors due to a non-correctable varying fringe spacing and the need for scanning the velocity profile are overcome.