Florian Just

Highly efficient Yb-doped silica fibers prepared by powder sinter technology

We report on the characteristics of an active fiber with core material made by sintering of Yb-doped silica powders as an alternative to a conventional modified chemical vapor deposition (MCVD) technique. This material provides the possibility to design very large and homogenously rare-earth doped active fiber cores. We have determined a fiber background attenuation of 20 dB/km and measured a slope efficiency of 80%. These values are comparable to established fibers made by MCVD technology.

Multi-kW single fiber laser based on an extra large mode area fiber design

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.

Design evolution, long term performance and application tests of extra large mode area (XLMA) fiber lasers

XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber laser.

Ytterbium-Doping Related Stresses in Preforms for High-Power Fiber Lasers

Frozen-in mechanical stresses can significantly influence the optical and mechanical properties of optical fibers, especially in laser fibers for high-power operation. In the following, we will report on the polarimetric measurement of stresses induced by the spatially varying doping composition in fiber preforms. We investigated the effect of rare-earth laser ions and found that the dopant ytterbium generates higher stresses than other common dopants in lightwave technology such as phosphorus or aluminum. The stress-induced index change relevant for the guiding properties is derived from the stress data. Especially in large-mode-area laser fibers with low numerical apertures, such stresses can significantly modify the index profile and thereby influence the propagation behavior.

New developments in high power fiber lasers based on alternative 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 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.

Development, manufacturing and lasing behavior of Yb-doped ultra large mode area fibers based on Yb-doped fused bulk silica

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.

Preparation and characterization of microstructured silica holey fibers filled with high-index glasses

Silica based microstructured holey fibers offer the possibility for filling with unconventional fiber materials. Of special interest are chalcogenide glasses due to their high refractive index and their nonlinear optical properties. We demonstrate two types of fibers: an index guiding fiber type with high-index glass core and silica cladding and a fiber with silica core surrounded by a periodic, hexagonal high-index glass structure giving antiresonant guiding properties. We prepared such fibers filled with arsenic sulphide glass and arsenic selenide glass by a pressurized infiltration technique. The manufacturing process is modelled on the basis of viscous glass flow parameters and is compared with experimental results obtained from the filled fibers. The propagation and spectral transmission properties of such fibers are measured and discussed.

All-fiber optical parametric oscillator for bio-medical imaging applications

Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

Birefringence optimization in PM fibers by specifically influencing the draw induced intrinsic stresses

Intrinsic stress in optical fibers significantly influences the mechanical properties of glass and modifies the refractive index. This can have an impact on fiber stability and modal behavior. Corresponding effects are often undesired but in some cases essential for certain applications, e.g. in polarization maintaining (PM) fibers. Stresses in optical fibers are caused by various reasons like differences in the thermal expansion coefficient of differently doped fiber regions or the mechanical force applied during the fiber drawing process. Understanding the reasons for stress generation and thus birefringence yields the opportunity to optimize this special property of PM fibers.

The influence of the drawing process on the intrinsic stress in optical fibers and the arising possibility to optimize the birefringence of PM fibers

The properties of optical fibers can significantly be influenced by intrinsic stress. It is well known that these stresses are caused by various reasons, e.g. the variations in the thermal expansion coecient of the differently doped regions in the fiber. The so called thermal stresses are only dependent on the composition of the fiber and not on its preparation history. Another main reason for stress in the final fiber is the mechanical force that is applied during the fiber drawing process. It generates so called mechanical stress that depends on the composition of the fiber and the thermal history of the fiber fabrication process. Using a non-destructive polarimetric system, we are able to measure the intrinsic stress state in optical fibers as well as in their preforms. Knowing on the one hand the thermal induced stresses in the preform of a fiber and on the other hand the final stress state in the fiber itself, we are able to differentiate between the two kinds of stress. In this paper we present results of stress measurements on optical ber preforms and fibers. We show, that the measured stress profile in the preform matches the theoretically assumed stress profile for thermal stress very well. Moreover we used this preform to draw fibers under different drawing conditions represented in a large difference in the applied force during the fiber drawing. We present the stress results for these differently fabricated fibers and show how huge the effect of the drawing tension can be. We find that for high drawing forces, the stress state can be reversed in comparison to the thermal stresses that are induced by the material composition. Due to the fact that stress on the one hand has a strong effect on the mechanical properties of glass and modifies the refractive index, this can lead to signicant effects on the fiber stability and modal behaviour. Finally, we present a way to compensate the additionally induced mechanical stress, which is for example a very good possibility to increase the stress birefringence in polarization maintaining (PM) fibers with panda structure. We compare the mechanical stress states of such Panda Fibers after their fabrication with the state after an additional high temperature step. We clearly find that it is possible to improve the birefringence of these fibers using appropriate preparation steps.