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We report on the fluorine incorporation in powder based materials for the fabrication of Al and Al/Yb co-doped silica glasses. The achieved maximum Fluorine concentration of 1.55 mol% SiF 4 corresponds to a refractive index decrease of -8 x 10 -3 . Simultaneously, the Tg of the doped material is reduced by about 200 K compared to pure silica. Moreover, the fluorine doping is also eminently suitable for the direct refractive index adjustment in active doped silica glass materials (e.g. Al/Yb or Al/Tm). The index matching with pure silica is possible to date up to 2.7 mol% Al 2 O 3 and 0.1 mol% Yb 2 O 3 . The additional influence on the blue shift of the UV transmission will also be discussed.

In this paper an early vision tracking algorithm particularly adapted to the tracking of road users in video image sequences is presented. The algorithm is an enhanced version of the regression based motion estimator in Lucas-Kanade style. Robust regression algorithms work in the presence of outliers, while one distinct property of the proposed algorithm is that it can handle with datasets including 90% outliers. Robust regression involves finding the global minimum of a cost function, where the cost function measures if the motion model is conform with the measured data. The minimization task can be addressed with the graduated non convexity (GNC) heuristics. GNC is a scale space analysis of the cost function in parameter space. Although the approach is elegant and reasonable, several attempts to use GNC for solving robust regression tasks known from literature failed in the past. The main improvement of the proposed method compared with prior approaches is the use of a preconditioning technique to avoid GNC from getting stuck in a local minimum. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We report on the development of large-core Yb-doped fibers with up to 100 μm core diameter and present first experimental results for high peak power amplification. The material for core and pump cladding was fabricated by Powder Sinter Technology. Using a high Al concentration we achieved a numerical aperture (NA) of 0.21 of the pump cladding and a core NA below 0.1. The rod-type fiber exhibits high pump absorption. Using a 0.55 m short fiber sample as the main amplifier in a 3-stage ns pulsed fiber Master Oscillator Power Amplifier system we achieved 3 ns output pulses with 360 kW peak power and 2 mJ pulse energy. We observed suppressed Stimulated Raman Scattering with respect to the signal pulses, which offers the possibility of further power scaling of such fiber amplifier systems.

We demonstrate a spatially distributed measurement technique to observe temperature changes along a pumped Ybdoped fiber. The technique is based on an array of fiber Bragg gratings acting as temperature sensor line. The Bragg gratings are inscribed directly into the actively doped fiber core using high intensity laser pulses and an interferometric setup. We studied the temperature evolution in differently doped fibers under pumping conditions and find different effects contributing to the observed temperature increase. To avoid an additional heating caused by pump-induced photodarkening, we measure the spatially resolved temperature profile during pumping along a photodarkening-reduced, cerium co-doped fiber.

The active core diameter in silica preforms can be significantly increased by the deposition of ytterbium (Yb) and the most important codopant aluminum (Al) in the gas phase through the high-temperature evaporation of an Yb chelate compound and Al chloride in the Modified Chemical Vapor Deposition (MCVD) process. Here, we report on systematic investigations of the incorporation of Yb and Al into silica by gas phase doping technique. Preforms and fibers were prepared in a wide range of Yb and Al concentrations. The samples were characterized concerning the radial distribution of the refractive index and dopant concentrations, the efficiency of the deposition, and the absorption and emission properties in the NIR region. First laser experiments have demonstrated a slope efficiency of 80%, which is comparable to fibers made by MCVD/ solution doping and powder sinter technology.

We demonstrate three-level laser operation at 976 nm of a large-core Yb-doped aluminosilicate fiber, which is fabricated by powder-sinter technology and shows a very homogeneous refractive index profile. The investigated fiber has a core diameter of 126 μm and a numerical aperture of 0.18, well-matched to standard fiber coupled pump diodes. The core composition has been optimized to reduce photodarkening effects. Multimode and single mode operation with multiple Watts output power is presented for this fiber making it useful for the realization of high brightness fiber coupled pump sources.

We report on the inscription of FBGs in rare earth doped optical fibers, the reduction of inherent absorption effects in the FBGs and the FBG-based temperature measurement within the core of actively doped fiber samples during core pumping. Besides a temperature increase due to the quantum defect of Yb-ions a change in temperature during pumping was observed and fits qualitatively well to the parallel measured photodarkening evolution.

The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser systems based on a single extra large multimode laser fiber (XLMA fiber). When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the characterization of Yb-doped fibers with different core compositions. We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first experimental welding results of steel sheets achieved with such a laser.

The preventive application of automated latent fingerprint acquisition devices can enhance the Homeland Defence, e.g. by improving the border security. Here, contact-less optical acquisition techniques for the capture of traces are subject to research; chromatic white light sensors allow for multi-mode operation using coarse or detailed scans. The presence of potential fingerprints could be detected using fast coarse scans. Those Regions-of- Interest can be acquired afterwards with high-resolution detailed scans to allow for a verification or identification of individuals. An acquisition and analysis of fingerprint traces on different objects that are imported or pass borders might be a great enhancement for security. Additionally, if suspicious objects require a further investigation, an initial securing of potential fingerprints could be very useful. In this paper we show current research results for the coarse detection of fingerprints to prepare the detailed acquisition from various surface materials that are relevant for preventive applications.

Since decades, the age determination of latent fingerprint traces left at crime scenes is a challenge to forensic investigators, since fingerprint traces can often only be used in a lawsuit if they can be assigned to the specific time interval of a crime taking place. In this paper, we suggest a six-step framework on how an age determination scheme might be developed for a given application scenario using optical and non-invasive image sensory. We explain and discuss each step of such framework, using three different aging features (the binary pixel feature based on prior work as well as the novel aging features corrosion blob size and corrosion blob amount), which are based on the loss of contrast of a fingerprint on a hard disk platter surface as well as on corrosion properties of fingerprint residue applied to a copper coin surface. We furthermore evaluate different aspects of the scheme in practical experiments, to show its feasibility and conclude, that the steps 1, 3, 5 and 6 of the framework are comparatively easy to be realized, whereas the steps 2 (developing new aging features) and 4 (determining all significant influences on a given aging feature) are challenging, yet possible.

Latent fingerprints provide vital information in modern crime scene investigation. On frequently touched surfaces the fingerprints may overlap which poses a major problem for forensic analysis. In order to make such overlapping fingerprints available for analysis, they have to be separated. An additional evaluation of the sequence in which the fingerprints were brought onto the surface can help to reconstruct the progression of events. Advances in both tasks can considerably aid crime investigation agencies and are the subject of this work. Here, a statistical approach, initially devised for the separation of overlapping text patterns by Tonazzini et al.,1 is employed to separate overlapping fingerprints. The method involves a maximum a posteriori estimation of the single fingerprints and the mixing coefficients, computed by an expectation-maximization algorithm. A fingerprint age determination feature based on corrosion is evaluated for sequence estimation. The approaches are evaluated using 30 samples of overlapping latent fingerprints on two different substrates. The fingerprint images are acquired with a non-destructive chromatic white light surface measurement device, each sample containing exactly two fingerprints that overlap in the center of the image. Since forensic investigations rely on manual assessment of acquired fingerprints by forensics experts, a subjective scale ranging from 0 to 8 is used to rate the separation results. Our results indicate that the chosen method can separate overlapped fingerprints which exhibit strong differences in contrast, since results gradually improve with the growing contrast difference of the overlapping fingerprints. Investigating the effects of corrosion leads to a reliable determination of the fingerprints' sequence as the timespan between their leaving increases.

A machine-assisted analysis of traces from crime scenes might be possible with the advent of new high-resolution non-destructive contact-less acquisition techniques for latent fingerprints. This requires reliable techniques for the automatic extraction of fingerprint features from latent and exemplar fingerprints for matching purposes using pattern recognition approaches. Therefore, we evaluate the NIST Biometric Image Software for the feature extraction and verification of contact-lessly acquired latent fingerprints to determine potential error rates. Our exemplary test setup includes 30 latent fingerprints from 5 people in two test sets that are acquired from different surfaces using a chromatic white light sensor. The first test set includes 20 fingerprints on two different surfaces. It is used to determine the feature extraction performance. The second test set includes one latent fingerprint on 10 different surfaces and an exemplar fingerprint to determine the verification performance. This utilized sensing technique does not require a physical or chemical visibility enhancement of the fingerprint residue, thus the original trace remains unaltered for further investigations. No particular feature extraction and verification techniques have been applied to such data, yet. Hence, we see the need for appropriate algorithms that are suitable to support forensic investigations.

Fiber Bragg gratings (FBGs) are attractive as reflector elements in fully integrated all-fiber laser systems. Furthermore, FBGs made with femtosecond laser technology allow to reduce splice connections in the fiber, since no special photosensitive fibers are required. Fiber Bragg grating inscription with deep ultraviolet femtosecond laser (267 nm) and two beam interferometry allows to target germanium-free and non-photosensitive fibers while maintaining versatility in the choice of the output wavelength of the fiber laser. This concept offers the potential of gratings with high spatial resolution, great flexibility and good homogeneity and complements the methods of point-by-point inscription at 800 nm or of phase-mask inscription with 400 nm femtosecond laser exposure. We report on the application of the interferometric fiber Bragg grating inscription technology to build a grating-stabilized fiber laser with high beam purity. Output powers up 160 W have been achieved.

Sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT and Heraeus Quarzglas and it produces very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By using a newly developed laser induced deflection (LID) technique we are able to pre-characterize directly the material absorption properties of the bulk material prior to the laser fiber production. The bulk absorption results measured by LID are without scattering effects and they are typically in good agreement with the total attenuation measured in the fibers. We achieved a fiber background loss of 20 dB/km. Furthermore, we present detailed studies of the refractive index homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk silica. Multimode double cladding laser fibers with an extra large mode area XLMA fiber design (core diameter up to 100 μm) have been produced from the Yb-doped bulk silica rods by two different techniques. One is a classical jacketing method; the other employs the stacking of un-doped, Yb- and F-doped rods and F-doped tubes. Different fiber types have been tested in different fiber laser setups. The influence of the fiber end cap properties on the fiber laser focus shift is discussed in detail. We have achieved fiber laser output powers up to 1.925 kW, limited only by the pump power. We also investigated the long term laser stability at different power levels.

Non-destructive latent fingerprint acquisition is an emerging field of research, which, unlike traditional methods, makes latent fingerprints available for additional verification or further analysis like tests for substance abuse or age estimation. In this paper a series of tests is performed to investigate the overall suitability of a high resolution off-the-shelf chromatic white light sensor for the contact-less and non-destructive latent fingerprint acquisition. Our paper focuses on scanning previously determined regions with exemplary acquisition parameter settings. 3D height field and reflection data of five different latent fingerprints on six different types of surfaces (HDD platter, brushed metal, painted car body (metallic and non-metallic finish), blued metal, veneered plywood) are experimentally studied. Pre-processing is performed by removing low-frequency gradients. The quality of the results is assessed subjectively; no automated feature extraction is performed. Additionally, the degradation of the fingerprint during the acquisition period is observed. While the quality of the acquired data is highly dependent on surface structure, the sensor is capable of detecting the fingerprint on all sample surfaces. On blued metal the residual material is detected; however, the ridge line structure dissolves within minutes after fingerprint placement.

The recently developed dual-view touch screens, which are announced to be installed in cars in a near future, give rise to completely new challenges in human-machine interaction. The automotive system should be able to identify if the driver or the passenger is currently interacting with the touch screen to provide a correct response to the touch. The optical devices, due to availability, acceptance by the users and multifunctional usage, approved to be the most appropriate sensing technology for driver/passenger discrimination. In this work the prototypic optical user discrimination system is implemented in the car simulator and evaluated in the laboratory environment with entirely controlled illumination. Three tests were done for this research. One of them examined if the near-infrared illumination should be switched on around the clock, the second one if there is a difference in discrimination performance between day, twilight and night conditions, and the third one examined how the intensive directional lighting influences the performance of the implemented user discrimination algorithm. Despite the high error rates, the evaluation results show that very simple computer vision algorithms are able to solve complicated user discrimination task. The average error rate of 10.42% (daytime with near-infrared illumination) is a very promising result for optical systems.

According to the European Commission around 200,000 counterfeit Euro coins are removed from circulation every year. While approaches exist to automatically detect these coins, satisfying error rates are usually only reached for low quality forgeries, so-called "local classes". High-quality minted forgeries ("common classes") pose a problem for these methods as well as for trained humans. This paper presents a first approach for statistical analysis of coins based on high resolution 3D data acquired with a chromatic white light sensor. The goal of this analysis is to determine whether two coins are of common origin. The test set for these first and new investigations consists of 62 coins from not more than five different sources. The analysis is based on the assumption that, apart from markings caused by wear such as scratches and residue consisting of grease and dust, coins from equal origin have a more similar height field than coins from different mints. First results suggest that the selected approach is heavily affected by influences of wear like dents and scratches and the further research is required the eliminate this influence. A course for future work is outlined.

We report on the characterization of photodarkening (PD) kinetics at Yb-doped fiber samples, inducing the PD loss by core-pumping at 975 nm with respect to different fiber temperatures in the range of 77 to 770 K. The thermal dependency of important PD parameters is presented. Additionally, we introduce a phenomenological model to include thermal and recovery effects in the description of the PD loss evolution and to improve the understanding of the PD process.

At the Photonics West 2008 we presented our rare earth doped fused bulk silica for fiber laser applications [1]. This approach overcame the typical geometrical limitations of other well known production methods for rare earth doped silica materials. Our unique production technique is based on the sintering of Yb-doped granulates of high-purity SiO2 particles. We have processed our Yb-doped bulk silica rods into ultra large mode area (XLMA) multi-mode double cladding laser fibers with an active core diameter in the range of 40 μm to 100 μm (depending on the core doping level). In the XLMA fiber the active core is surrounded by a so-called 2D- or 4D-shaped pure silica pump cladding (with diameter between 850 μm and 900 μm) and an F-doped outer silica cladding with an outer diameter of 1000 μm. We have investigated the refractive index and the intrinsic stress profiles of different XLMA laser fibers and their preforms to visualize interface effects. The fiber cross section designs, the quality of all interfaces and the material composition are important factors for the laser fiber performance. The laser properties of these fibers have been investigated in detail. In addition, the preparation of the fiber end-face is important to reduce heat effects and we have developed concepts to mitigate such thermal load at the fiber end face.

In the last years, photodarkening in ytterbium doped silica based laser fibers turned out to be a critical factor for high power laser action. Several investigations have been carried out in order to characterize the time dependent increase of the fiber loss and to understand and model the complex optical phenomenon. Despite of progress in this field, there is still a lack of data concerning the detailed influence of fiber composition and preparation process parameters as well as concerning the role of atomic defects in the core glass. Here we report on investigations about the photodarkening in dependence on the glass composition of the fiber laser core. By MCVD, fibers with different codopants (additional to the active ytterbium doping) have been prepared in a well-defined manner, regarding process parameters and glass composition, and comprehensively characterized. In addition to the photodarkening measurements, further optical properties have been measured on the fibers and fiber performs, which are related to the photodarkening effect: intensity and spectral behaviour of the Yb 3+ absorption and emission in the NIR, cooperative visible fluorescence, UV absorption and UV excited visible emission. The concentration of codopants which are commonly used for active and passive lightguide fibers (aluminium, germanium, phosphorus) was systematically varied and correlated with the optical properties.

Ytterbium-doped high-power fiber lasers with high beam quality are promising devices for a variety of applications. Extreme power load and complicated fiber structures make great demands on material properties and preparation technology. Recently, with the further increase of output power, the problem of photodarkening has been identified as a critical issue for fiber laser devices. Detailed knowledge of optical properties of materials and fibers are needed for the successful development of laser fibers, aimed at increasing efficiency and power. It is well known that the properties of silica based rare earth doped fibers can be influenced and remarkably improved by the incorporation of further dopants. Here, the influence of combined aluminium-phosphorus codoping on the optical properties of Yb doped laser fibers was investigated. Preforms and fibers were prepared by MCVD and solution doping both with phosphorus and aluminium excess. The samples were characterized concerning the radial distribution of refractive index and dopant concentrations and the absorption and emission properties in the UV/VIS/NIR region. The observed spectral effects and active properties (laser efficiency, photodarkening) were correlated with changes in the fiber composition. It could be shown, that the combined doping leads to effects which deviate from a simple additivity and which can be beneficially utilized for the improvement of the laser fiber performance.

We have developed a production process for rare earth doped bulk silica to fulfill the demand of such material for fiber laser applications. In contrast to the standard techniques such as a combination of MCVD (modified chemical vapor deposition) and solution doping, our novel technology is based on a granulate process that enables novel ultra large mode area fiber designs (XLMA) with active core diameters above 100 μm as well as larger batch sizes. Several Yb-doped fibers with two different fiber designs were manufactured and successfully tested in both side- and end-pumped fiber laser setups. Both fiber designs have been compared to similar MCVD fibers. The influence of the material composition on the photodarkening properties has been investigated.

Several devices for use in high speed fiberoptical transmission systems are discussed. Beside two types of distributed amplifiers, a photoreiver with a tremendous conversion gain bandwidth product is described.

Flip-chip-mounting of integrated circuits has been shown as an effective way to connect integrated circuits with substrates providing highest bandwidths. This technique also has been used to connect chips directly without causing significant parasitics. In this paper, hybrid integrated photoreceivers with large bandwidths for fiber-optic data transmission will be discussed. Several photoreceivers with bandwidths of more than 60 GHz have been fabricated by combining photodiodes and amplifiers, which have been optimised in separated wafer runs. Key features of this type of photoreceivers are a waveguide photodiode with a high responsivity (0.8 A/W) and a traveling wave amplifier with low input impedance as well as high amplification to a 50 Ω output termination up to 65 GHz. Further improvement of bandwidth is expected by using metamorphic HEMTs instead of pseudomorphic ones in the amplifier.

This paper briefly describes the principle of operation and science goals of the AMANDA high energy neutrino telescope located at the South Pole, Antarctica. Results from an earlier phase of the telescope, called AMANDA-B10, demonstrate both reliable operation and the broad astrophysical reach of this device, which includes searches for a variety of sources of ultrahigh energy neutrinos: generic point sources, Gamma-Ray Bursts and diffuse sources. The predicted sensitivity and angular resolution of the telescope were confirmed by studies of atmospheric muon and neutrino backgrounds. We also report on the status of the analysis from AMANDA-II, a larger version with far greater capabilities. At this stage of analysis, details of the ice properties and other systematic uncertainties of the AMANDA-II telescope are under study, but we have made progress toward critical science objectives. In particular, we present the first preliminary flux limits from AMANDA-II on the search for continuous emission from astrophysical point sources, and report on the search for correlated neutrino emission from Gamma Ray Bursts detected by BATSE before decommissioning in May 2000. During the next two years, we expect to exploit the full potential of AMANDA-II with the installation of a new data acquisition system that records full waveforms from the in-ice optical sensors., 14 pages, 14 figures, Slightly updated from manuscript submitted to SPIE Conference (Hawaii, 2002). Sky plot and preliminary limits given for point sources using AMANDA-II. First report on GRB search using AMANDA-II. Updated acknowledgements, references and table entries