Light bullets in waveguide arrays: spacetime-coupling, spectral symmetry breaking and superluminal decay [Invited].

We investigate the effects of the space-time coupling (STC) on the nonlinear formation and propagation of Light Bullets, spatiotemporal solitons in which dispersion and diffraction along all dimensions are balanced by nonlinearity, through periodic media with a weak transverse modulation of the refractive index, i.e. waveguide arrays. The STC arises from wavelength dependence of the strength of inter-waveguide coupling and can be tuned by variation of the array geometry. We show experimentally and numerically that the STC breaks the spectral symmetry of Light Bullets to a considerable degree and modifies their group velocity, leading to superluminal propagation when the Light Bullets decay.

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.

Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers.

The description of the photodarkening (PD) kinetics of Yb-doped silica fibers with a stretched exponential function is analyzed in more detail, revealing a dependence of the determined PD parameter values on the measurement time. Thereby, serious uncertainties may occur in advanced interpretations of the results, e.g., the discussion of the rate constant as a function of the Yb inversion, if the measurement time is too short. A method to circumvent the problems of the fitting procedure is suggested, taking advantage of the self-similarity of the PD curves for different inversions. An exponent of 4.3 +/- 0.5 in the power-law dependence of the PD rate constant tau(-1) on the Yb inversion was found for a typical aluminium-codoped fiber.

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.