Role of Ce in Yb/Al laser fibers: prevention of photodarkening and thermal effects.

We report on detailed investigations of ytterbium (Yb) and aluminum (Al) doped silica fiber and preform samples co-doped with cerium (Ce). The prevention of pump-induced photodarkening (PD) by temporary oxidation of Ce3+ to Ce4+ (or rather Ce3++) was proved by observed modifications in the ultraviolet (UV) spectra of transient absorption during near-infrared (NIR) pumping of thin preform slices. Only a small part of available Ce3+ ions (< 4%) was found to be involved in this process despite Yb inversions of up to 0.28. The modifications in the UV absorption spectra disappeared completely when the pump power was switched-off. From these observations we conclude that the recombination to Ce3+ takes place very fast thereby enabling these ions to capture liberated holes h+ perpetually during further pumping. We found a concentration ratio of Ce/Yb ≈0.5 to be sufficient to reduce PD loss to 10% in comparison to Ce-free fibers. Thus, the thermal load caused by absorption of PD color centers at pump (and laser) wavelength is expected to be also reduced. Unfortunately, new heat sources arise with the presence of Ce which cannot be explained by the absorption of Ce ions at the pump wavelength but must be attributed to the interaction with excited Yb ions. Fiber temperature increase of more than 200 K was observed if both, Yb2O3 and Ce2O3 concentration exceed 0.4 mol%.

Advanced attenuation and fluorescence measurement methods in the investigation of photodarkening and related properties of ytterbium-doped fibers.

Considering the continuous development of ytterbium (Yb)-doped fibers for fiber lasers and amplifiers, thorough characterization of the features and quality of such fibers is necessary now more than ever. In particular, the evaluation of the very strong core attenuation that results from Yb absorption and the losses accompanying photodarkening (PD) processes in Yb-doped optical fibers is still of immense interest. Keeping that in mind, the potential of a method using pinhole fibers will be demonstrated here. Moreover, the investigation of fluorescence properties of Yb3+ is also important; it can be affected by phenomena such as reabsorption, inhomogeneous pump intensity distribution, amplified spontaneous emission, lasing, and, lastly, PD. As these phenomena occur while measuring the fluorescence properties of Yb3+ in optical fibers, they can distort the fluorescence spectra and fluorescence decay significantly. We suggest a twice-perpendicular measurement method and strict time management to avoid these interfering phenomena during the fluorescence measurements.

Evidence of Tm impact in low-photodarkening Yb-doped fibers.

In contrast to Yb/Al-doped fibers, the influence of very low Tm(2)O(3) concentrations (≥ 0.1 mol-ppm) on photodarkening (PD) is clearly detectable in Yb/P-doped fibers that are known to show little degradation effects. For Tm(2)O(3) additions of more than 50 mol-ppm, the measured PD loss is even similar to Yb/Al-doped fibers with comparable rare earth concentrations. Our work reveals the risk of color center generation by pumping at wavelengths of 915 nm or 976 nm even in Al-free Yb-doped fibers and emphasizes the importance of high purity of raw materials for the preparation of Yb laser fibers with expected very low PD.

Measurement of higher-order mode propagation losses in effectively single mode fibers.

We present a direct and nondestructive measurement of the propagation loss of higher-order modes (HOMs) in effectively single-mode fibers. Lossy HOMs are excited by applying local stress at various points alongside a straight single mode fiber. The change of the HOM power as a function of the propagation distance is recorded at the fiber end by performing a modal decomposition with a correlation filter. The results for the HOM propagation loss are compared to simulations yielding very good agreement.

Multicore fiber with integrated fiber Bragg gratings for background-free Raman sensing.

In the last years a variety of fiber optic Raman probes emerged, which are only partly suited for in vivo applications. The in vivo capability is often limited by the bulkiness of the probes. The size is associated with the required filtering of the probes, which is necessary due to Raman scattering inside the fibers. We employed in-line fiber Bragg gratings (FBG) as notch filter for the collection path and integrated them in a novel type of Raman probe. Multicore singlemode fibers (MCSMF) were designed and drawn integrating 19 singlemode cores to achieve better collection efficiency. A Raman probe was assembled with one excitation fiber and six MCSMF with inscribed FBGs as collection fibers. The probe was characterized regarding Raman background suppression, collection efficiency, and distance dependence. First Raman measurements on brain tissue are presented.

Multicorelike guidance in a triangular-core hollow optical fiber and spectral evolution of its eigenmode degeneracy.

A unique multicorelike guidance was achieved in a microstructured optical fiber composed of a circular air hole at the center surrounded by a high-index triangular core. Unique spectral evolution of the degeneracy was theoretically investigated using a vectorial finite element method to find a threefold degeneracy in both the fundamental and the first excited modes in the visible range, which evolved to twofold degeneracy as the wavelength increased to IR. Experimental measurements also confirmed evolution of the modal intensity from three spatially isolated patterns confined to individual corners into a supermode combining the three corners.

Photodarkening kinetics as a function of Yb concentration and the role of Al codoping.

We investigated the photodarkening (PD) kinetics of two fiber series with variation of the Yb content for constant Al concentration or constant ratio of Al/Yb, respectively. The results show the outstanding importance of the absolute value of Al concentration also in the case of fibers with strongly reduced Yb content. An Al/Yb ratio of 5 to 6 is not sufficient to mitigate PD loss. Moreover, a model to describe PD loss and rate constant as functions of Yb concentration and excitation is suggested that links measurements of PD in single fibers of the same type (variation of Yb inversion) and in fiber series (constant Yb inversion).

Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers.

We investigated photodarkening (PD) parameters of Yb/Al-doped silica fibers as a function of the concentration of additional rare earth ions like Tm or Er. It was found that both Tm and Er cause a decrease in Yb inversion followed by a reduction of PD in the case of Er, whereas Tm-codoping with more than 10 mol-ppm can strongly accelerate the process and also increase the PD loss. However, contrary to [1], we conclude that the typical PD behavior of Yb/Al fibers is an intrinsic feature of this fiber type and not caused by trace impurities of Tm (< 1 mol-ppm) unintentionally incorporated by the raw materials during fiber preparation.

A micro-structured aperture made of a hollow triangular-core fiber for novel beam shaping.

We demonstrate a micro-structured aperture made of a unique hollow triangular-core fiber (HTCF) that consists of a central air hole, a high-index hollow triangular core, and silica cladding for all-fiber novel beam shaping. Detailed fabrication processes to embed a hollow triangular structure into a cylindrical optical fiber are described and unique diffraction patterns out of the HTCF for monochromatic light are analyzed both experimentally and theoretically. Fourier-optic analysis combined with guided mode calculation was pursued to interpret experimental patterns in terms of the beam propagation distance.

Non-isothermal bleaching of photodarkened Yb-doped fibers.

We report on the thermal treatment of photodarkened Yb-doped fiber samples. The method of non-isothermal bleaching at different temperature ramp rates can be used to determine the thermal energy distribution of photodarkening induced color centers. A distributed activation energy with a mean value of about 1.3 eV and a FWHM of 0.5 eV was found. Spectral changes during thermal treatment were observed and could be interpreted, e.g. as an enhancement of the absorption cross section.

Efficient Yb laser fibers with low photodarkening by optimization of the core composition.

We report on photodarkening (PD) investigations at Yb doped fibers with specific variation of the concentrations of the codopants aluminum and phosphorus, measured during cladding pumping at 915 nm. A core composition with equal content of Al and P is most promising to achieve Yb fibers with low PD, high laser efficiency and low numerical aperture of the laser core despite of high codoping. A laser output power of more than 100 W was demonstrated on such a fiber with a slope efficiency of 72%. The correlation of the PD loss with the NIR-excited cooperative luminescence encourages the supposition that cooperative energy transfer from excited Yb(3+) ions to the atomic defect precursors in the core glass enables the formation of color centers in the pump-induced PD process.

Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power.

Photodarkening in Yb doped fibers was examined at 633 nm in-situ during cladding pumping at 915 nm with varying pump powers and with no indication of an onset threshold. For the first time, the partial bleaching of the photodarkening loss by the pump power itself was observed. We found the relaxation to well-defined equilibrium states of the core excess loss, depending on the Yb inversion. From the dependence of the measured rate constant on the density of excited Yb ions we conclude, that on average 3 to 4 excited Yb ions create or bleach one color center responsible for the core excess loss.

Raman-on-chip device and detection fibres with fibre Bragg grating for analysis of solutions and particles.

An all-fibre based Raman-on-chip setup is introduced which enables analysis of solutions and trapped particles without microscopes or objectives. Beside the novel quartz microfluidic chip, innovative multi-core single-mode fibres with integrated fibre Bragg gratings are used for detection. The limit of quantitation is 7.5 mM for urea and 2.5 mM for nicotine with linear Raman spectroscopy. This is an improvement of more than two orders of magnitude compared with previous fibre-based microfluidic Raman detection schemes. Furthermore, our device was combined with optical traps to collect Raman-on-chip spectra of spherical polymer beads.

Single-pulse fiber Bragg gratings and specific coatings for use at elevated temperatures.

The technique of recording fiber Bragg gratings (FBGs) with single exposure pulses during the fiber drawing process allows production of such gratings in complex array structures, with high mechanical strength of the fiber and in a simple and cost-efficient way. This is of special interest for the growing field of fiber sensor applications with FBGs. A general advantage of fiber sensor systems is their ability to be used also at elevated temperatures compared with conventional electric or electronic sensors. For this purpose, the fiber itself as well as the grating structure and the fiber coating should be stable under such elevated temperature conditions. We have investigated different coating materials and possibilities of making temperature-stable FBGs of types I and II in the range of 100 degrees C-1000 degrees C with good reflection efficiency by single-pulse exposure during the fiber drawing process.