Nils Damaschke

Rainbow refractometry with a tailored incoherent semiconductor laser source

The authors demonstrate within a metrology experiment the applicability of a recently proposed temporally incoherent semiconductor laser source which relies on nonlinear dynamics. The realized spectrally broadband emission with an output power of 110mW and a coherence length of only 120μm is used in a rainbow refractometry experiment for sizing of liquid droplets, representing an important problem in industrial processes. The observed emission characteristics are attractive for implementation of modern imaging and metrology techniques which are based on the properties of well-directed, temporally incoherent light.

Time integrated detection of femtosecond laser pulses scattered by small droplets

Scattering of femtosecond laser pulses by small droplets has been measured and compared with predictions, yielding some interesting new applications for time integrated detection of the scattered field. The scattering intensity of integrated detection becomes monotonic with droplet size over large regions of scattering angle and morphology dependent resonances are surpressed, opening the way for particle sizing using the scattered intensity. Furthermore, the ripple structure no longer appears in the rainbow region of scattering, simplifying rainbow refractometry significantly. These scattering proporties of femtosecond laser pulses have been demonstrated in the laboratory using a novel Paul trap for levitating single droplets.

UV LED-based fiber coupled optical sensor for detection of ozone in the ppm and ppb range

A realized novel optical sensor system to measure the concentration of ozone using the emitted light from Light Emitting Diodes in the ultraviolet range is described in this paper. The wavelength selective light interaction of the gas takes place in a fiber coupled and robust reflection cell. The control electronics are separated from the optical sensor head. Therefore it can be used in harsh environment for instance close to discharge plasma in strong electromagnetic fields or at high temperature. The sensor design is potentially low cost, quite small and well suited for a large number of applications. It can be implemented in industrial process control application or in small battery powered hand held devices. This setup is also capable for a parallel and selective measurement of nitrogen dioxide NO2 and sulfur dioxide SO2 - this is already implemented and proved. Further specific LEDs are utilized for this purpose in the setup. For the ozone measurements two different sensor heads where used. A small 4 cm long reflection cell with potential for handheld devices and a 40 cm one thought for high resolution stationary application. The resolution (standard deviation measured at a zero concentration of ozone) of the sensor with the small cell is about 30 ppb at 700 msec measurement time and about 3 ppb at 1,4 sec at the longer cell.

High resolution led-spectroscopy for sensor application in harsh environment

The usage of novel LED-light sources for spectrometric application is shown and described in this paper. The utilization of fiber optical linkage to a simple open path absorption cell and a proper control of the light sources is the key to a robust and high resolved measurement system. A novel optical sensor system based on this approach is realized to measure the concentration of nitrogen dioxide, sulfur dioxide with a resolution below 1 ppm and ozone down to 30 ppb at a 4 cm single reflection cell in a fraction of a second measurement time. In this setup the emitted light from Light Emitting Diodes in the ultraviolet to the visible wavelength range was used. The optoelectronics and the control electronics are separated from the optical sensor head where the pure optical sensor effect takes place. Therefore the sensor can be used in harsh environment for instance in an exhaust tailpipe system or close to discharge plasma in strong electromagnetic fields or at high temperature. Furthermore the sensor design is potentially low cost, quite small, long life and well suited for a large number of applications - from small battery powered hand held devices to industrial process control implementation. An LED-based sensor does not compete to laboratory chemical analytical devices but in many cases it is well suited for high resolved and fast online concentration measurements.

Real time exhaust gas sensor with high resolution for onboard sensing of harmful components

Emission control and reduction of harmful gases produced by combustion engines is a very important task. There is need for a high resolution online detection of harmful gases. This work presents a novel optical sensor system based on newly developed LEDs working in the ultraviolet and visible wavelength range. The developed potentially low cost LED based system is able to individual detect NO2 and SO2 with high resolution (1 ppm), wide measurement range (some 1000 ppm) and temporal dynamic of up to 10 ms (stroke resolution). Fiber optics are applied to a robust free path absorption cell to resist the harsh tailpipe environment and enable the online measurement of hot gases.

Measurement and prediction of the gaussian beam effect in the phase Doppler technique

Under certain circumstances, particle size measurements using a phase Doppler instrument can be erroneous due to the Gaussian beam effect, sometimes referred to as the trajectory effect. This is especially true under extenuating circumstances such as when, for cost reasons, only two detectors are being used, when the choice of detector off-axis and/or elevation angle is limited through the application, when the signal processing has only limited validation possibilities or if a particularly small measurement volume must be employed. All of these factors may be disadvantageous for measuring larger particles.

Optical techniques for the characterization of non-spherical and non-homogeneous particles

Interest in the characterization of non-spherical and/or non-homogeneous particles has increased dramatically in recent years from a wide variety of application areas. For characterization by optical means, several essential elements are identified and briefly discussed, with some indication of their respective state-of-the-art. This article serves as an introduction to the special cluster of articles which follows in this issue, and attempts to put these contributions into perspective with respect to the overall problem.

Light Scattering from Small Particles

In the laser Doppler and phase Doppler techniques a part of the incident laser light is imaged by the particles onto the detectors. It is this scattered light which carries information about the particle velocity and its properties and thus, the light scattered from small particles plays a central role in the basic physics of these measurement techniques. In recognition of this, the following chapter is devoted to the fundamentals of elastic light scattering from small particles. The simplest case of a homogeneous and isotropic sphere is considered.

Optical high-resolution analysis of rotational movement: testing circular spatial filter velocimetry (CSFV) with rotating biological cells

Circular spatial filtering velocimetry (CSFV) was tested during the microscopic registration of the individual rotations of bakers yeast cells. Their frequency-dependent rotation (electrorotation; ER) was induced in rotating electric fields, which were generated in a glass chip chamber with four electrodes (600 μm tip-to-tip distance). The electrodes were driven with sinusoidal quadrature signals of 5 or 8 V PP with frequencies up to 3 MHz. The observed cell rotation was of the order of 1–100 s per revolution. At each measuring frequency, the independent rotations of up to 20 cells were simultaneously recorded with a high-speed camera. CSFV was software-implemented using circular spatial filters with harmonic gratings. ER was proportional to the phase shift between the values of the spatial filtering signal of consecutive frames. ER spectra obtained by CSFV from the rotation velocities at different ER-field frequencies agreed well with manual measurements and theoretical spectra. Oscillations in the rotation velocity of a single cell in the elliptically polarized field near an electrode, which were resolved by CSFV, could not be visually discerned. ER step responses after field-on were recorded at 2500 frames per second. Analysis proved the high temporal resolution of CSFV and revealed a largely linear torque-friction relation during the acceleration phase of ER. Future applications of CSFV will allow for the simple and cheap automated high-resolution analysis of rotational movements where mechanical detection has too low a resolution or is not possible, e.g. in polluted environments or for gas and fluid vortices, microscopic objects, etc.

Laser Doppler Systems

In this chapter the correct choice, layout and adjustment of a laser Doppler system for one, two or three velocity components will be discussed. This requires, among other things, a priori information about the flow field; in particular, which flow quantities are to be measured and their desired measurement accuracy (section 7.1). From this, the necessary dimensions of the measurement volume and the required seed particle concentration follow. Once this information is known, it is necessary to select the components of the optical system to achieve these values in the flow rig considered (sections 7.2, 7.3). Typical examples of such systems and how they can be customized and optimized for particular applications are discussed in section 7.4. For completeness, the laser transit velocimeter (LTV) is presented in section 7.5 as the limiting case of a laser Doppler system with two fringes in the measurement volume.