Thorsten A. Goebel

Atomistic Characterisation of Li+ Mobility and Conductivity in Li7−xPS6−xIx Argyrodites from Molecular Dynamics Simulations, Solid‐State NMR, and Impedance Spectroscopy

The atomistic mechanisms of Li(+) ion mobility/conductivity in Li(7-x)PS(6-x)I(x) argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid-solid phase transition, which was characterised by low-temperature differential scanning calorimetry, (7)Li and (127)I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177+/-2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li(+) ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS(4) tetrahedra. From connectivity analyses and free-energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X-ray diffraction experiments indicates a homogeneity range for Li(7-x)PS(6-x)I(x) with 0.97 < or = x < or = 1.00. Within this range, molecular dynamics simulations predict Li(+) conductivity at ambient conditions to vary considerably.

Variable wavefront tuning with a SLM for tailored femtosecond fiber Bragg grating inscription

We report on the inscription of fiber Bragg gratings using femtosecond laser pulses and the phase-mask technique. The wavefront of the inscription laser is variably tuned with a spatial light modulator (SLM). By applying Fresnel lenses with different focal lengths, the period of the fiber Bragg gratings could be shifted. A linear change of the grating period for a FBG inscribed with a third-order deformed wavefront and a quadratic-period behavior for a fourth-order wavefront could be verified experimentally for the first time.

Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings

The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.

Minimizing residual spectral drift in laser diode bars using femtosecond-written volume Bragg gratings in fused silica

Ultrashort laser pulses are used to inscribe volume Bragg gratings (VBGs) into fused silica. These VBGs demonstrate excellent performance for the external stabilization of laser diode bars. The stabilized system emits at a wavelength of 969 nm with a signal width (FWHM) of 100 pm and shows a spectral drift as low as 24 pm for a change in output power of 45 W for a grating surface area of 10  mm2.

Fs-written fiber Bragg gratings in multicore fibers for astrophotonic applications (Conference Presentation)

For astronomy, the search of exo-planets and spectral analysis of galaxies and stars with ground based telescopes is an ongoing topic especially with the future planned telescopes with even larger mirror diameters. But for the ground based observation the atmosphere is a limiting factor. Besides of air fluctuations also spectral noise is influencing the observation. A forest of emission lines by OH relaxations that are orders of magnitudes stronger than the stars and galaxies appear in the night sky. These lines vary in intensity but are fixed in wavelength. Therefore, fiber Bragg gratings (FBG) are a perfect tool for the suppression of these emission lines. FBGs provide a high filter quality by filtering out magnitudes of intensity in a narrow wavelength window of below 0.5nm bandwidth. Because fibers are already widely used in telescopes to deliver light into spectrographs, the FBGs could be inscribed directly into the fibers. But for astronomy mostly multimode fibers are used where FBGs do not work as needed because of the different propagation constants of higher modes. The solution is the transition to single mode fibers. In terms of compactness and robustness a multicore fiber would be the optimal solution. But the homogeneous modification of a multicore fiber is a challenging task. We report on the ultrashort pulse laser inscription of FBGs into a multicore fiber consisting of 7 cores. Furthermore, investigations on the homogeneity of the inscribed modifications as well as the spectral properties are presented. I would like to participate in the Student Competition.

Ultrashort pulse point-by-point written aperiodic fiber Bragg gratings for suppression of OH-emission lines

We demonstrate the direct inscription of aperiodic fiber Bragg gratings (AFBGs) for their use as in-fiber filter elements. The modifications are induced by focusing ultrashort laser pulses with an oil-immersion objective into the fiber core. We apply an advanced point-by-point inscription technique for flexible period adaptation. The fabricated AFBGs are targeted on the suppression of 10 lines in a single grating and simulations based on the specific design show excellent agreement. Furthermore, we discuss the application in astronomy as filters for the suppression of OH emission lines.

Femtosecond laser pulse written long period gratings in large mode area fibers

The control of light propagation inside optical fibers is essential for a broad variety of applications from sensing to high-power fiber lasers. As an example, long period gratings (LPG) enable the defined coupling of modes guided in the fiber core into co-propagating cladding modes [1]. Thus, they can be used as efficient spectral filter elements e.g. for the suppression of stimulated Raman scattering in high-power fiber laser applications [2]. Fig. 1a shows an example of the transmission spectrum of a LPG written into a single mode fiber (9μm core, 125 μm cladding diameter) by means of ultrashort laser pulses. The spectrum shows distinct and strong resonance peaks with simultaneously minimal out of band losses.

Wavelength tuning of through-coating-written fiber Bragg gratings

Fiber Bragg gratings (FBGs) are key elements for fiber lasers or sensors to measure strain or temperature. Using femtosecond laser pulses for inscription provides high temperature stable modifications [1]. Point by point inscription enables very flexible control of the Bragg wavelength however at the expense of coupling to cladding modes [2, 3]. In contrast a phase mask technique, where the interference behind the phase mask produces a grating [4], allows to suppress coupling to cladding modes by scanning fiber and phase mask with respect to the beam [5]. This results in high reproducibility however at the cost of a fixed period. Several approaches exist to overcome this limitation, e.g. by tuning the wavefront [6], all having certain drawbacks.

Thermal shift and residual absorption in ultrashort pulse written Volume-Bragg-Gratings

When inscribing Volume-Bragg-Gratings into fused silica using ultrashort laser pulses residual absorption increases. We present annealing experiments, correlate results to activation energies and investigate influence on grating resonance shift.