Pr3+-doped YPO4 nanocrystal embedded into an optical fiber.

Optical fiber with YPO4:Pr3+ nanocrystals (NCs) is presented for the first time using the glass powder-NCs doping method. The method's advantage is separate preparation of NCs and glass to preserve luminescent and optical properties of NCs once they are incorporated into optical fiber. The YPO4:Pr3+ nanocrystals were synthesized by the co-precipitation and hydrothermal methods, optimized for size (< 100 nm), shape, Pr3+ ions concentration (0.2 mol%), and emission lifetime. The core glass was selected from the non-silica P2O5-containing system with refractive index (n = 1.788) close to the NCs (no = 1.657, ne = 1.838). Optical fiber was drawn by modified powder-in-tube method after pre-sintering of glass powder-YPO4:Pr3+ (wt 3%) mixture to form optical fiber preform. Luminescent properties of YPO4:Pr3+ and optical fiber showed their excellent agreement, including sharp Pr3+ emission at 600 nm (1D2-3H4) and 1D2 level lifetime (τ = 156 ± 5 µs) under 488 nm excitation. The distribution of the YPO4:Pr3+ NCs in optical fiber were analyzed by TEM-EDS in the core region (FIB-SEM-prepared). The successful usage of glass powder-NCs doping method was discussed in the aspect of promising properties of the first YPO4:Pr3+ doped optical fiber as a new way to develop active materials for lasing applications, among others.

2 MW peak power generation in fluorine co-doped Yb fiber prepared by powder-sinter technology.

We report on the first, to the best of our knowledge, implementation of a fluorine co-doped large-mode-area REPUSIL fiber for high peak power amplification in an ultrashort-pulse master oscillator power amplifier. The core material of the investigated step-index fiber with high Yb-doping level, 52 µm core and high core-to-clad ratio of 1:4.2 was fabricated by means of the REPUSIL powder-sinter technology. The core numerical aperture was adjusted by fluorine codoping to 0.088. For achieving high beam quality and for ensuring a monolithic seed path, the LMA fiber is locally tapered. We demonstrate an Yb fiber amplifier with near-diffraction-limited beam quality of M2=1.3, which remains constant up to a peak power of 2 MW. This is a record for a tapered single core fiber.

Yb-doped large mode area fiber for beam quality improvement using local adiabatic tapers with reduced dopant diffusion.

A newly designed all-solid step-index Yb-doped aluminosilicate large mode area fiber for achieving high peak power at near diffraction limited beam quality with local adiabatic tapering is presented. The 45µm diameter fiber core and pump cladding consist of active/passively doped aluminosilicate glass produced by powder sinter technology (REPUSIL). A deliberate combination of innovative cladding and core materials was aspired to achieve low processing temperature reducing dopant diffusion during fiber fabrication, tapering and splicing. By developing a short adiabatic taper, robust seed coupling is achieved by using this Yb-doped LMA fiber as final stage of a nanosecond fiber Master Oscillator Power Amplifier (MOPA) system while maintaining near diffraction limited beam quality by preferential excitation of the fundamental mode. After application of a fiber-based endcap, the peak power could be scaled up to 375 kW with high beam quality and a measured M2 value of 1.3~1.7.

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%.

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.

In situ FBG inscription during fiber laser operation.

We demonstrate the inscription of a 266 nm UV femtosecond pulse-induced fiber Bragg grating (FBG) in an Yb-doped fiber during optical pumping at 976 nm and the initiation of lasing with increasing grating reflectivity. Output spectra show the emission of the pumped fiber changing from the broad-ranged amplified spontaneous emission in the nonlasing case to the narrow-range laser operation due to the enhancement of FBG reflectivity during inscription. The proposed technique enables the direct characterization and control of FBG performance in fiber lasers. After FBG fabrication, we investigate the spectral characteristics of the fiber laser for different laser powers and study the influence of a thermal treatment of the FBG.

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.

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.

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.