Sonja Unger

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.

High-power cw neodymium-doped fiber laser operating at 9.2 W with high beam quality

We report on a 9.2-W cw, two-times diffraction-limited, neodymium-doped fiber laser operating in the 1.06-microm region. For a silica-based double-clad fiber, slope efficiencies of more than 25% are observed for pumping by a diode-laser system operating near 810 nm.

High-average-power picosecond Yb-doped fiber amplifier.

We report on a cladding-pumped, ytterbium-doped large-core-area fiber amplifier that is capable of generating 51.2 W of average power at a 1064-nm center wavelength, an 80-MHz repetition rate, and a 10-ps pulse duration. In an ytterbium-doped large-mode-area fiber these pulses could be amplified up to 43.2 W with diffraction-limited beam quality (M(2)~1.3) . Power scaling limitations that arise from nonlinear distortions such as self-phase modulation and stimulated Raman scattering are discussed.

Single-frequency master-oscillator fiber power amplifier system emitting 20 W of power

We report a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring laser as the master oscillator and a Yb-doped large-mode-area double-clad fiber as the power amplifier. The system emits up to 20.1 W of single-frequency radiation at a wavelength of 1064 nm with diffraction-limited beam quality (M(2)</=1.3) . The optical emission spectrum and amplitude-noise behavior are investigated. Furthermore, the power-scaling possibilities are discussed.

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.

Multifilament-core fibers for high energy pulse amplification at 1.5 μm with excellent beam quality

We report on what we believe to be the first demonstration of an erbium-ytterbium-doped multifilament-core (MFC) fiber for single-mode amplification of narrow linewidth high peak power pulses. A master-oscillator-power-fiber-amplifier laser source has been demonstrated using a 37-filament MFC fiber in the last amplification stage. Pulses with 750 microJ (940 W peak power) and laser linewidth<1 MHz have beam generated with M2 approximately 1.3. This value is close to the theoretical value M2 approximately 1.5.

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).

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.

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.

A highly efficient Yb-doped silica laser fiber prepared by gas phase doping technology

In this paper we report on an alternative technique for the preparation of ytterbium (Yb)-doped silica fibers and their characteristics compared to the conventional modified chemical vapor deposition (MCVD) process in combination with solution doping and powder sinter technology (REPUSIL). In the case of the technique applied here, the active core diameter in the preform can be significantly increased via the deposition of Yb and the most important codopant, aluminum (Al), in the gas phase through the high-temperature evaporation of the Yb chelate compound and Al chloride in the MCVD process. The prepared preform shows a homogenous distribution of the refractive index and dopant concentration. The background loss of the drawn fiber was measured to be 25 dB km−1 at 1200 nm. Efficient lasing up to 200 W, showing a slope efficiency of about 80%, was demonstrated, which is comparable to fibers made via MCVD/solution doping and the REPUSIL technique.