Christoph Leithold

Axial scanning in confocal microscopy employing adaptive lenses (CAL)

In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ~2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results.

Interferometric velocity measurements through a fluctuating gas-liquid interface employing adaptive optics

Optical transmission through fluctuating interfaces of mediums with different refractive indexes is limited by the occurring distortions. Temporal fluctuations of such distortions deteriorate optical measurements. In order to overcome this shortcoming we propose the use of adaptive optics. For the first time, an interferometric velocity measurement technique with embedded adaptive optics is presented for flow velocity measurements through a fluctuating air-water interface. A low order distortion correction technique using a fast deformable mirror and a Hartmann-Shack camera with high frame rate is employed. The obtained high control bandwidth enables precise measurements also at fast fluctuating media interfaces. This methodology paves the way for several kinds of optical flow measurements in various complex environments.

Advancement of an Interferometric Flow Velocity Measurement Technique by Adaptive Optics

Flow measurements often take place under difficult conditions. Optical flow measurement techniques are affected by variations of the refractive index, caused e.g., by temperature, concentration, or pressure gradients. This will give rise to an increased measurement uncertainty or cause the measurement to fail. To overcome these limitations, we propose the employment of adaptive optics. In this contribution we present interferometric flow velocity measurements through a fluctuating air-water interface by the use of adaptive optics. Using the adaptive optics, the rate of valid measurement signals can be improved from 28% to 83%. The results are promising to enable measurements in difficult environments affected by refractive index variations which were not accessible so far.

Optical dynamic deformation measurements at translucent materials

Due to their high stiffness-to-weight ratio, glass fiber-reinforced polymers are an attractive material for rotors, e.g., in the aerospace industry. A fundamental understanding of the material behavior requires non-contact, in-situ dynamic deformation measurements. The high surface speeds and particularly the translucence of the material limit the usability of conventional optical measurement techniques. We demonstrate that the laser Doppler distance sensor provides a powerful and reliable tool for monitoring radial expansion at fast rotating translucent materials. We find that backscattering in material volume does not lead to secondary signals as surface scattering results in degradation of the measurement volume inside the translucent medium. This ensures that the acquired signal contains information of the rotor surface only, as long as the sample surface is rough enough. Dynamic deformation measurements of fast-rotating fiber-reinforced polymer composite rotors with surface speeds of more than 300 m/s underline the potential of the laser Doppler sensor.

Characterization of two ultrashort laser pulses using interferometric imaging of self-diffraction

Noncollinear pulse characterization methods can be applied to over-octave spanning waveforms, but geometrical effects in the nonlinear medium such as beam smearing and critical sensitivity to beam alignment hinder their accurate application. Here, a method is introduced for the temporal and spatial characterization of two pulses by interferometric, spectrally resolved imaging of self-diffraction. Geometrical effects are resolved by the method and, therefore, do not limit the accuracy. Two methods for quantitative pulse retrieval are presented. One method is analytical and very fast; the other method is iterative and more robust if applied to noisy data.

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.

In-process deformation measurements of translucent high speed fibre-reinforced disc rotors

The high stiffness to weight ratio of glass fibre-reinforced polymers (GFRP) makes them an attractive material for rotors e.g. in the aerospace industry. We report on recent developments towards non-contact, in-situ deformation measurements with temporal resolution up to 200 µs and micron measurement uncertainty. We determine the starting point of damage evolution inside the rotor material through radial expansion measurements. This leads to a better understanding of dynamic material behaviour regarding damage evolution and the prediction of damage initiation and propagation. The measurements are conducted using a novel multi-sensor system consisting of four laser Doppler distance (LDD) sensors. The LDD sensor, a two-wavelength Mach-Zehnder interferometer was already successfully applied for dynamic deformation measurements at metallic rotors. While translucency of the GFRP rotor material limits the applicability of most optical measurement techniques due to speckles from both surface and volume of the rotor, the LDD profits from speckles and is not disturbed by backscattered laser light from the rotor volume. The LDD sensor evaluates only signals from the rotor surface. The anisotropic glass fibre-reinforcement results in a rotationally asymmetric dynamic deformation. A novel signal processing algorithm is applied for the combination of the single sensor signals to obtain the shape of the investigated rotors. In conclusion, the applied multi-sensor system allows high temporal resolution dynamic deformation measurements. First investigations regarding damage evolution inside GFRP are presented as an important step towards a fundamental understanding of the material behaviour and the prediction of damage initiation and propagation.

Einsatz eines adaptiven Optiksystems in der Strömungsmesstechnik

Zusammenfassung Die adaptive Optik stellt ein heterogenes, dynamisches, optomechatronisches System dar. Aufgrund der rasanten Fortentwicklung erobert sie sich neben der Astronomie und der Ophthalmologie zunehmend neue Anwendungsfelder. In diesem Beitrag wird ein interferometrisches Geschwindigkeitsmessverfahren präsentiert, das ein adaptives Optiksystem für die Korrektur von Störungen durch eine fluktuierende Phasengrenzschicht nutzt. Es wird der Fortschritt für die Messtechnik dargestellt.

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.

Undisturbed imaging through a dynamic gas-liquid boundary by wavefront manipulation

We present a technique for distortion-free laser-based measurements through a dynamic gas-liquid interface. Suitable components, methods and applications of the adaptive interferometric technique will be discussed by accomplished experiments and simulations.