Origin of thermal modal instabilities in large mode area fiber amplifiers.

We present a dynamic model of thermal modal instability in large mode area fiber amplifiers. This model allows the pump and signal optical intensity distributions to apply a time-varying heat load distribution within the fiber. This influences the temperature distribution that modifies the optical distributions through the thermo-optic effect thus creating a feedback loop that gives rise to time-dependent modal instability. We describe different regimes of operation for a representative fiber design. We find qualitative agreement between simulation results and experimental results obtained with a different fiber including the time-dependent behavior of the instability and the effects of different cooling configurations on the threshold. We describe the physical processes responsible for the onset of the instability and suggest possible mitigation approaches.

Frequency-doubling of a CW fiber laser using PPKTP, PPMgSLT, and PPMgLN.

Second-harmonic generation (SHG) in PPKTP, PPMgSLT, and PPMgLN crystals is analyzed by frequency-doubling CW light from a 1064 nm fiber laser over a range of powers up to 10 W. Data for fundamental powers less than 3 W is used to determine the effects of the fundamental laser linewidth on SHG and to identify imperfections in the periodicity and boundary sharpness of the crystals' poled domains which can reduce SHG. Data for fundamental powers greater than 3 W is used to diagnose and model limiting effects on SHG such as pump depletion and thermal dephasing. Thermal dephasing was found to reduce second-harmonic power by 25% or more for input fundamental powers approaching 10 W.

Planar geometry thin-film all-optical programmable switch.

We describe an all-optical programmable switch that can perform logic gate functions. This switch consists of a planar geometry germanium-doped silica waveguide, a Q-switched and mode-locked Nd:YAG laser, and the means of coupling laser light into different waveguiding modes of the thin film at the fundamental and second-harmonic frequencies. By the application of the appropriate optical programming sequence, the film-generated second-harmonic light can be made to perform the functionalities of various gates. In particular, a single waveguide was optically programmed to perform the Or function and was then made to perform the And function with little change to the experimental arrangement.