Juergen Czarske

Measurement of acceleration and multiple velocity components using a laser Doppler velocity profile sensor

For the investigation of turbulent flows, the measurement of Lagrangian acceleration is of great interest as it represents a direct component of the Navier–Stokes equations. The presented sensor is based on the common laser Doppler technique, but offers in addition combined high spatial resolution in the micrometre range and the possibility of measuring the velocity component along the optical axis. As a result of the sensor setup, signals of particles with inclined trajectories show frequency modulation within a single burst similar to the signal of an accelerated particle. A model-based approach to distinguish between both quantities is presented, and a signal processing technique based on the Hilbert transform has been developed. The processing is comparatively fast and showed good agreement with preset values, even for signals of poor quality. The variance of velocity and acceleration measurements nearly reaches the Cramer–Rao lower bound. Experimental verification is done by the measurement of a harmonic oscillator with known parameters and a stagnation flow within a free jet.

In-process shape and roundness measurements at turning machines using a novel laser Doppler profile sensor

We have investigated the application of a laser Doppler profile sensor for in-process shape and roundness measurements at turning machines. This sensor is an extension of a conventional laser Doppler velocimeter (LDV), where two interference fringe systems with contrary fringe spacing gradients are generated inside the same measuring volume using wavelength division multiplexing. Scattering objects passing the measuring volume generate scattered light signals with two different Doppler frequencies, from which the velocity as well as the position of the objects can be determined via a proper calibration function. Hence, the radius and the tangential velocity and, thus, the shape of rotating work pieces and components, e.g. turbine blades or turning parts, can be measured absolutely and with only one single sensor. Two-dimensional and three-dimensional measurements of shape, excentricity, and roundness on quickly rotating cylinders inside a turning machine are presented. The results are compared with tactile measurements conducted with a coordinate measuring machine.

Heterodyne laser Doppler distance sensor with phase coding measuring stationary as well as laterally and axially moving objects

Both in production engineering and process control, multidirectional displacements, deformations and vibrations of moving or rotating components have to be measured dynamically, contactlessly and with high precision. Optical sensors would be predestined for this task, but their measurement rate is often fundamentally limited. Furthermore, almost all conventional sensors measure only one measurand, i.e. either out-of-plane or in-plane distance or velocity. To solve this problem, we present a novel phase coded heterodyne laser Doppler distance sensor (PH-LDDS), which is able to determine out-of-plane (axial) position and in-plane (lateral) velocity of rough solid-state objects simultaneously and independently with a single sensor. Due to the applied heterodyne technique, stationary or purely axially moving objects can also be measured. In addition, it is shown theoretically as well as experimentally that this sensor offers concurrently high temporal resolution and high position resolution since its position uncertainty is in principle independent of the lateral object velocity in contrast to conventional distance sensors. This is a unique feature of the PH-LDDS enabling precise and dynamic position and shape measurements also of fast moving objects. With an optimized sensor setup, an average position resolution of 240 nm was obtained.

Measurement of velocity gradients in boundary layers by a spatially resolving laser Doppler sensor

A novel laser Doppler technique is presented to measure flow fields with micro-scale resolution. The realized laser Doppler sensor provides distributed velocity measurements inside a defined measurement volume. Two laser wavelengths are used to accomplish two Doppler frequency measurements, which determine the position and the velocity of a scattering particle. Repeating of these measurements determines the velocity gradient of the flow. One application area is the evaluation of the velocity variations in boundary layers of flows close to a wall. This contribution presents the principle and experimental results of the spatially resolving laser Doppler velocity sensor. Different concepts on the optical arrangements as well as the signal processing technique are discussed. A scheme for the construction of a miniaturized fiber-coupled sensor is presented, which allows its integration into flow channels.

Undisturbed interferometric sensing through a fluid interface by electrically-tunable lenses and micro mirrors

We have harnessed the power of various programmable photonics devices for an interferometric measurement technique. Distortion-free laser-based velocity measurements through a dynamic gas-liquid interface are enabled by a closed-loop optoelectronic system. We are employing electrically tunable lenses and micro mirrors to correct low-order wavefront distortions effectively. Our work represents a paradigm shift in interferometric velocity measurement techniques from using static to dynamic optical elements.

Nichtinvasives, laseroptisches Messsystem zur hochaufgelösten Vermessung von Strömungsfeldern und Volumenkräften

Zusammenfassung Um den Treibstoffverbrauch von Flugzeugturbinen künftig zu reduzieren, sollen neuartige Plasma-Aktoren integriert werden, um durch Strömungsbeeinflussung den Wirkungsgrad der Turbinen signifikant zu steigern. Jedoch ist die genaue Wirkungsweise derartiger Plasma-Aktoren noch nicht hinreichend verstanden. Um diese Fragestellung zu beantworten wurde ein laserbasiertes Messsystem für die berührungslose, optische Kraftmessung realisiert und eingesetzt. Mit dessen Hilfe konnten weltweit erstmalig die von einem Plasma-Aktor erzeugten Volumenkräfte im Fluid mit einer Zeitauflösung von ca. 7 μs erfasst werden. Die vom Aktor erzeugte, horizontale Gesamtkraft lag dabei bei Fx/L̅ ≈ 17 mN/m, was zur Generierung einer wandparallele Strömung mit einer maximalen Geschwindigkeit von 6.16 m/s führte. Die gewonnen Messergebnisse stellen einen entscheidenden Schritt auf dem Weg zur Optimierung von Plasma-Aktoren und deren Einsatz in Turbinen dar.

Novel adaptive laser measurement techniques

The development of adaptive optics systems is a rapidly growing field. Adaptive optics systems consist of three main components. They include a sensor that captures optical wavefronts (e.g., a Hartmann-Shack camera), an electrical control unit, and a light modulator that corrects the distorted optical wavefronts. Deformable mirrors, segmented micromirror arrays, or liquid crystal arrays are often used as the light modulator. Although adaptive optics systems have so far been mostly used in astronomy applications, progress is now also being made in developing these systems for laser measurement techniques. Astronomy applications of adaptive and active optics have already led to a renaissance of large Earth-based telescopes.1 In these cases, adaptive optics are used to correct the optical distortions of a light path towards an angular resolution that reaches the optical diffraction limit. The distortions are caused by turbulent fluctuations of the light path as it passes through the atmosphere. The planned flagship European Extremely Large Telescope will have a primary mirror diameter of 39m. More than 6000 actuators will be used for the deformable mirror to correct light distortions with a temporal resolution that is better than 1ms. Uses of adaptive optics outside of astronomy include laser systems for free-space communication, laser material processing, lithography, and biomedical microscopy. An additional, intriguing possibility in ophthalmology is to use adaptive optics to resolve retinal cells (cones and rods) based on the correction of aberrations inside the eye,1 which could provide an early detection mechanism for heart attacks and strokes. The development of application-based intelligent photonics systems and adaptive optics has enabled their use in new areas. We have designed two new adaptive measurement techniques that both exhibit unique advantages. First, we use adaptive lenses for confocal microscopy with an agile scanner that does Figure 1. Novel confocal laser scanning microscope. Lenses that can be tuned electrically enable fast and smart scanning. UAL: Voltage.

Fiber optic distributed velocity sensor for precision measurements in microscopic and macroscopic flows

We report about a laser Doppler velocity profile sensor for precision measurements of microfluidic flows. The sensor measures the velocity and position of a tracer particle in the measurement volume, which consists of two superposed interference fringe systems of different wavelength, one with convergent, one with divergent interference fringes. This arrangement was achieved with a special combination of a diffractive and achromatic lenses. Due to fiber-optic beam delivery the sensor can be build compact and offers potential for miniaturization. A spatial resolution in the sub-micrometer range and an uncertainty of the velocity of about 10-4 could be achieved. The sensor offers potential for a variety of applications, like the investigation of shear flows or precise flow rate measurments.

Modulated external-cavity laser for highly resolved distance measurements

An external cavity laser is assembled by using an antireflection coated laser diode together with the surface of a measurement object. The automatic evaluation of the longitudinal modespacing yields the distance between the laser diode and the measurement object. The measurement resolution is increased by utilizing the resonance effect due to synchronous pumping of the laser diode current. Thus, a distance sensor with interesting properties for industrial applications is set up. Nevertheless, a systematic measurement deviation arises as a result of the nonlinear properties of the laser diode. A fundamental understanding of the processes inside the laser diode is necessary for achieving a measurement uncertainty in the micrometer range. Seen applications are in-situ measurements at grinding processes or the focus control at laser material processing.

Ortsaufgelöste Laser-Doppler-Geschwindigkeitsprofilmessung von Grenzschichtströmungen (Spatial-resolved Laser Doppler Velocity Profile Measurement of Boundary Layer Flows)

Abstract Die Erfassung von Scherströmungen erfordert Geschwindigkeitsmessverfahren mit hoher Ortsauflösung. Laser-Doppler-Sensoren eignen sich dafür aufgrund des präzisen, berührungslosen Messprinzips. Die Ortsauflösung ist aber in der Regel durch die Größe des Messvolumens auf typisch 50 μm begrenzt. In diesem Aufsatz wird auf ein neuartiges Messprinzip eingegangen, das eine ortsaufgelöste Geschwindigkeitsmessung innerhalb des Messvolumens ermöglicht. Die Ortsauflösung ist nur durch die Signalqualität begrenzt. Erreicht wurde eine Ortsauflösung von ca. 1,6 μm. Steile Scherschichten können erfasst werden. Die Anwendungen des Laser-Doppler-Geschwindigkeitsprofilsensors liegen z.B. in der Messung von Grenzschichtströmungen sowie von Rohrströmungen.