A directional random laser mediated by transverse Anderson localization in a disordered glass optical fiber is reported. Previous demonstrations of random lasers have found limited applications because of their multi-directionality and chaotic fluctuations in the laser emission. The random laser presented in this paper operates in the Anderson localization regime. The disorder induced localized states form isolated local channels that make the output laser beam highly directional and stabilize its spectrum. The strong transverse disorder and longitudinal invariance result in isolated lasing modes with negligible interaction with their surroundings, traveling back and forth in a Fabry–Perot cavity formed by the air–fiber interfaces. It is shown that if a localized input pump is scanned across the disordered fiber input facet, the output laser signal follows the transverse position of the pump. Moreover, a uniformly distributed pump across the input facet of the disordered fiber generates a laser signal with very low spatial coherence that can be of practical importance in many optical platforms including image transport with fiber bundles.

Anderson localization of light is traditionally described in analogy to electrons in a random potential. Within this description the disorder strength -and hence the localization characteristicsdepends strongly on the wavelength of the incident light. In an alternative description in analogy to sound waves in a material with spatially fluctuating elastic moduli this is not the case. Here, we report on an experimentum crucis in order to investigate the validity of the two conflicting theories using transverse-localized optical devices. We do not find any dependence of the observed localization radii on the light wavelength. We conclude that the modulus-type description is the correct one and not the potential-type one. We corroborate this by showing that in the derivation of the traditional, potential-type theory a term in the wave equation has been tacititly neglected. In our new modulus-type theory the wave equation is exact. We check the consistency of the new theory with our data using a field-theoretical approach (nonlinear sigma model).

We have argued that a high-purity Yb-doped silica glass can potentially be cooled via anti-Stokes fluorescence optical refrigeration. This conclusion is reached by showing, using reasonable assumptions for the host material properties, that the non-radiative decay rate of Yb ions can be made substantially smaller than the radiative decay rate. Therefore, an internal quantum efficiency of near unity can be obtained. Using spectral measurements of the fluorescence emission from a Ybdoped silica optical fiber at different temperatures, we estimate the minimum achievable temperature in Yb-doped silica glass for different values of internal quantum efficiency. arXiv:1810.06165v2 [physics.optics]

Localized states trap waves propagating in a disordered potential and play a crucial role in Anderson localization, which is the absence of diffusion due to disorder. Some localized states are barely coupled with neighbours because of differences in wavelength or small spatial overlap, thus preventing energy leakage to the surroundings. This is the same degree of isolation found in the homogeneous core of a single-mode optical fibre. Here we show that localized states of a disordered optical fibre are single mode: the transmission channels possess a high degree of resilience to perturbation and invariance with respect to the launch conditions. Our experimental approach allows identification and characterization of the single-mode transmission channels in a disordered matrix, demonstrating low losses and densely packed single modes. These disordered and wavelength-sensitive channels may be exploited to de-multiplex different colours at different locations.

We report a detailed formalism aimed at the thermal modeling and heat mitigation in high-power doubleclad fiber amplifiers. Closed form analytical formulas are developed that take into account the spatial profile of the amplified signal and pump in the double-clad geometry, the presence of the amplified spontaneous emission, and the possibility of radiative cooling due to anti-Stokes fluorescence emission. The formalism is applied to a high-power Yb-doped silica fiber amplifier. The contributions to the heat-load from the pump-signal quantum defect, as well as the pump and signal parasitic absorptions are compared to the radiative cooling. It is shown that for realistic cases, the local heat generation in kiloWatt-class fiber amplifiers is either dominated by the quantum defect or the parasitic absorption depending on the pump wavelength. In conventional designs, radiative cooling can be substantial only in properly designed amplifiers, when the pump power is tens of watts or lower, unless the parasitic absorption is reduced compared to the commonly reported values in the literature. We also explore the impact of the non-ideal quantum efficiency of the gain material. The developed formalism can be used to design fiber amplifiers and lasers for optimal heat mitigation, especially due to radiative cooling.