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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 m 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.

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.

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.

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.

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.

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.

Mobile contact-less fingerprint scanners can be very important tools for the forensic investigation of crime scenes. To be admissible in court, data and the collection process must adhere to rules w.r.t. technology and procedures of acquisition, processing and the conclusions drawn from that evidence. Currently, no overall accepted benchmarking methodology is used to support some of the rules regarding the localisation, acquisition and pre-processing using contact-less fingerprint scanners. Benchmarking is seen essential to rate those devices according to their usefulness for investigating crime scenes. Our main contribution is a revised version of our extensible framework for methodological benchmarking of contact-less fingerprint scanners using a collection of extensible categories and items. The suggested main categories describing a contact-less fingerprint scanner are properties of forensic country-specific legal requirements, technical properties, application-related aspects, input sensory technology, pre-processing algorithm, tested object and materials. Using those it is possible to benchmark fingerprint scanners and describe the setup and the resulting data. Additionally, benchmarking profiles for different usage scenarios are defined. First results for all suggested benchmarking properties, which will be presented in detail in the final paper, were gained using an industrial device (FRT MicroProf200) and conducting 18 tests on 10 different materials.

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 m) 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.