We report an additive-pulse mode-locked (APM) thulium-doped fiber ring laser producing 350–500 fs pulses tunable from 1798 to 1902 nm. The laser operates in the soliton regime, where periodic perturbations cause predictable sidebands and modulation in the optical spectrum.

Stretched-pulse additive pulse mode-locked fiber lasers use fibers with opposite signs of group velocity dispersion to introduce large changes of pulse width per pass and avoid excessive nonlinearity. We have investigated these intracavity pulse dynamics. Optimal points for output coupling and for the placement of the artificial absorber element in the cavity are determined for a fiber ring laser. The dispersion balance is important for the generation of clean sub-100 fs pulses, but the net cavity length is found to be relatively uncritical.

The spin of a circularly polarized wave in vacuo is clarified if one acknowledges the finite transverse extent of the wave. This is done self-consistently in the paraxial wave approximation.

An alternative derivation of the radiation field of a point charge is presented. It starts with the Fourier components of the current produced by the moving charge. The electric field is found from the vector wave equation. Each step in the integration permits physical interpretation. The retarded time appears very naturally in this derivation. The interpretation of the present derivation is that a charge at constant velocity v̄(‖v̄‖<c) does not radiate, not because it is unaccelerated, but because it has no Fourier components synchronous with waves traveling at the speed of light. Of course, Cherenkov radiation in a medium, in which the velocity of electromagnetic propagation is less than c, is the classic example of radiation by a charge moving at constant velocity.

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