We report on a new source able to provide probe pulses in the UV visible range and on the demonstration of its
application to hyperspectral (fluorescence lifetime) imaging measurements. The source is able to generate UV (down to
300 nm) and blue light exploiting high-order mode propagation in a microstructured fiber pumped by a Ti:Sapphire laser.
We believe that further optimization of pump wavelength, fiber length and fiber zero-dispersion wavelength could
generate light well below 300 nm using a simple and stable set-up and become a useful tool for biomedical imaging. We
demonstrated its versatility using the source for FLIM-FRET measurement a 460 nm and hyperspectral FRET-FLIM
In this paper we report imaged neuronal rat cells in a confocal laser scanning microscope by simultaneous generation of
the three requested wavelengths obtained by a UV-extended supercontinuum source. This is to the best of our
knowledge that such a measure was performed using a microstructured fiber pumped by a standard Ti:Sapphire laser.
We observed efficient UV light generation when a novel pumping scheme was used. The pump wavelength is close to
the zero-dispersion wavelength of the fiber first high-order mode and offset axial pumping is used. By tuning the pump
wavelength and power level we were able to generate mW-power levels in the visible wavelength interval down and of
about hundreds of microwatt in the UV wavelength interval down to 300 nm. The pump alignment was very simple and
very stable. We believe that further optimization of pump wavelength, fiber length and fiber zero-dispersion wavelength
could generate light well below 300 nm using a simple and stable set-up. To demonstrate the potentiality of this
technique we imaged neuronal rat cells in a confocal laser scanning microscope by simultaneous generation of the three
We report on generation of blue light exploiting high-order mode propagation in a microstructured fiber pumped by a
Ti:Sapphire close to the zero-dispersion wavelength of the first high-order mode. An new interesting regime was
observed with axial offset pump. With 230 mW of incident pump power we generated over 3 mW in the 450-510 nm
window achieving 50 μW/nm power density. In a final round of measurements we were able to show generation of a
peak at 350 nm. This complex regime has still to be fully investigated but we believe an optimized fiber design will
allow to efficiently extend the operation of Ti:Sapphire laser to UV/blue wavelength region.
We filled a refractive index matching liquid into the air holes of a Ge-doped solid-core microstructured optical fiber
(MOF) with a fiber Bragg grating (FBG) to investigate its switching functions. A type of thermo-optic in-fiber switch
based on the tunable bandgap effect was demonstrated in the fluid-filled FBG at the Bragg wavelength of 830nm, and its
extinction ratio depends strongly on the reflectivity of the FBG. Another type of optical switch with an extinction ratio of
30 dB was developed in the fluid-filled MOF at a long wavelength of 1200 or 1400nm, attributing to the absorption of
the filled liquid. Such two types of switches can turn on/off the light transmission via a small temperature adjustment of
±5 or ±10ºC, respectively, and will find useful applications in all-fiber optical communication systems.
Important progress in the development of rare earth doped high power fiber lasers was possible by large-mode-area
(LMA) fibers with increased core diameters and reduced core apertures as low as 0.05. In this way, the excellent beam
quality is maintained, but the power density can be reduced below critical values despite of very high output powers
beyond 1 kW. Sophisticated concepts had to be developed in order to maintain the low NA in the case of high doping,
e.g. the codoping by index-decreasing components as boron or fluorine.
Here we report on the progress in the preparation of microstructured LMA laser fibers, the core area of which is
composed of parts with high doping and parts with refractive index lower than the silica pump cladding. In contrast to
the direct codoping, in this way the atomic environment of the active atoms can be tailored and optimized independently
on the mean refractive index of the core. The preparation was carried out by stacking different rods in a multistep
process, leading to cores with up to more than hundred single elements. Both for ytterbium and erbium/ytterbium doped
fibers, good optical properties concerning basic attenuation and rare earth fluorescence could be reached by introducing
additional purification steps. Different fiber structures were characterized concerning mode field distribution, pump
power absorption and laser behavior.
In the last years rare earth doped double-clad fibers have been developed to high-power laser sources. Important progress was possible by increasing numerical aperture of the pump cladding and decreasing numerical aperture of the laser core. The high NA of the pump cladding enables the acceptance of large pump intensities whereas the low NA of the laser core makes possible to increase the core diameter and to decrease the laser power density retaining high beam quality. Here, actual challanges are discussed and possibilities are demonstrated to use microstructures for improved fiber designs which are realized by stacking and drawing of capillaries and rods. The rare earth doped parts are prepared by modified chemical vapor deposition and solution doping, but other routes of powder technology are also studied. Concerning the currently most important laser and amplifier types - Yb doped at 1.1 μm wavelength and Er/Yb doped at 1.55 μm wavelength -, the question of a high pump aperture is similar, but the limitations concerning a low core aperture are fairly different, because an efficient Er/Yb laser demands high phosphorus co-doping which naturally increases the core NA. The applied microstructures comprise "holey" fiber cross sections in form of "air clads" for the pump light and multiple hole ring structures for laser core and inner cladding. Moreover, microstructured cores made from solid parts yield new possibilities and parameters to compensate the high refractive index of the active material and to optimize the large mode area design.
We have doped the core of Ytterbium (Yb) laser fibers additionally with Neodymium (Nd) to exploit wavelength-multiplexed high-power pump systems, which are commercially available. By pumping such a Nd:Yb-codoped fiber with the 808/940/978 nm-diode system, we could demonstrate CW output powers of more than 1 kW with high laser slope efficiency. In order to get a better understanding of this laser medium, we studied the fluorescence and the laser behavior of Nd:Yb fibers with different rare earth concentrations in comparison to a fiber doped solely with Nd. A theoretical model for the calculation of the fluorescence decay curve and spectrum as well as the laser characteristic and wavelength was developed, that takes the energy transfer process from Nd to Yb ions into account. Comparing the experimental and theoretical results, the behavior of the Nd:Yb high power fiber laser is understood as a collective emission of both ion types within the same wavelength region. These investigations contribute to the optimization of high power fiber lasers under the viewpoint of thermal load.
A new type of multi-clad rare-earth doped silica fiber was designed, prepared and tested for the power scaling of high power fiber lasers in the 1 .1 tm wavelength region. By means of a dedicated laboratory setup a maximum output power of more than 1 .300 watts with excellent spectral and beam behavior was achieved. The fundamental investigation of the energy transfer processes and of the fluorescence lifetimes of different Nd:Yb co-doped has been studied.
Such fiber-lasers were tested in the laboratory at several materials (plastics, metals, glass) in the fields of material processing and micro-marking, respectively.
In conventional optical fibers the light guiding and other optical properties are mainly controlled by the index profile as given by the composition of the doping materials. In this case technological and material properties limit the design flexibility of the optical properties. For example, it is not possible to shift the region of anomalous dispersion to wavelengths shorter than the zero dispersion value of silica. In contrast to these conventional fibers, for microstructured or photonic crystal fibers (PCF) the optical properties are determined by the size, distribution and geometry of the air holes running the entire length of the fiber. Variations of the geometrical parameters offer a wide flexibility in designing different optical properties. It is possible, for example, to modify numerical aperture and mode field area over wide ranges or to shift the zero dispersion wavelength down to the visible spectral region. In the case of hollow core fibers (photonic bandgap fibers) the interaction of the guided light with the glass material is reduced and allows the transmission of higher intensities with reduced nonlinear effects.
The optical loss behavior of index guiding PC fibers made from high purity silica, was investigated with regard to the preform preparation steps and drawing procedure. Loss effects in the 1.4 µm region are caused mostly by incorporation of hydroxide groups during PC preform preparation. Typical sources are flame heat treatment procedures. However, hydroxide based absorption by water permeation into the holey structure was not observed, not even by storage in humid atmosphere over days. PCFs show additional NIR attenuation, possibly caused by drawing induced atomic defects in the pure silica material. By advanced PC preform preparation the minimum attentuation in the NIR range can be depressed down to 2.9 dB/km at 1.3 μm. PCFs have a reduced tensile strength in comparison with compact silica fibers. The mechanical stability increases with the cross section area of the solid outer cladding. This resembles the behavior of single capillary fibers without inner holey or cobweb structure. The tensile strength of PCFs decreases after a few days of hole contamination with condensed water.
We report on applications of Yb3+-doped fiber-laser systems in the micro-processing and the markign of different materials. We have demonstrated the drilling of small holes in glass slides with a thickness of 1 mm using a fiber-laser system wtih an otuput-power of about 35 watts. Furthermore, we have used a narrowband fiber-laser system with an output-power of a few watts for the marking of thin plastic films and samples of anodized aluminum. These applications are based on the experience of more than 10 years in the design, development and preparation of rare-earth doped double-clad silica fibers at the IPHT e.V. Jena. We systematically have optimized the composition and the geometry of these specialty fibers. So we could manufacture very efficient samples which were tested in several fiber-laser setups with output-powers of up to almost 200 watts. Especialy, in the high-power region we could solve some thermal problems using new concepts for the double-clad fiber cross section. We successfully designed, prepared and tested so-called Large-Mode-Area fibers as well as fibers with two or mroe refractive indices in the pump-core. Additionally, we increased the available pump-power of the one-end-pumped fiber-laser systems using a wavelength-multiplexed diode-laser fiber-coupled system.