High power (60mW) single frequency erbium:ytterbium codoped fiber laser

The characteristics of a high power Er3+:Yb3+ single frequency fiber laser pumped at 980nm are reported. The device gives 60mW output power with RIN 10MHz and linewidth 500kHz. At low output powers (< 30mW) the slope efficiency is as high as 25%, falling to 12% at higher powers, the saturation behaviour is related to a bottleneck effect due to the finite Yb-Er transfer rate. Improved performance can be obtained using new fibers with an increased rare-earth concentration which show negligible signs of erbium clustering

“God Was With Us:” Child Labor in Colonial Kenya, 1922 - 1950s

Contentious debates about the allowable minimum age of child laborers informed the discourse of child labor in colonial Kenya between 1922 and the 1950s. Beginning with the Harry Thuku Uprising of 1922 that instigated the discussion over labor policy concerning juvenile wage laborers and heightened the tension between the British colonial administration and African adult workers, the British government in Kenya struggled to forge coherent labor policies concerning the ages of African child workers. Frequent changes in labor laws made it easier for labor recruiters and employers to manipulate the system by recruiting younger children for work thus drawing them into the orbit of an alien labor force that often interfered with their childhood. The uncertainty surrounding the minimum age engendered acrimonious debate between white employers and anti-child labor advocates over who among them had the moral authority to speak for the children and act as their moral guardians and avuncular figures. This article discusses child labor in colonial Kenya. Focusing almost exclusively on African boys as actors in child labor, the article analyzes labor history that highlights changes in the meaning of minimum age in an economic system that promised African children prosperity. It frames age as a category of analysis that explains the intersection of colonial labor laws and juvenile workers. The close analysis of African children and their labor situation also reveals a mosaic of everyday life in colonial Kenya that brings children into an acknowledged circulation of imperial ideas and imagination

Deterministic creation, pinning, and manipulation of quantized vortices in a Bose-Einstein condensate

We experimentally and numerically demonstrate deterministic creation and manipulation of a pair of oppositely charged singly quantized vortices in a highly oblate Bose-Einstein condensate (BEC). Two identical blue-detuned, focused Gaussian laser beams that pierce the BEC serve as repulsive obstacles for the superfluid atomic gas; by controlling the positions of the beams within the plane of the BEC, superfluid flow is deterministically established around each beam such that two vortices of opposite circulation are generated by the motion of the beams, with each vortex pinned to the \emph{in situ} position of a laser beam. We study the vortex creation process, and show that the vortices can be moved about within the BEC by translating the positions of the laser beams. This technique can serve as a building block in future experimental techniques to create, on-demand, deterministic arrangements of few or many vortices within a BEC for precise studies of vortex dynamics and vortex interactions.Comment: 9 pages, 7 figure