Shaping pulses for fusion

Abstract

Two approaches to the creating of pulse-shaping techniques that meet the requirements associated with operation of a large fusion laser are examined. These techniques are passive pulse-shaping with double-etalone devices that coherently add or stack short pulses from a modelocked oscillator, and active pulse-shaping with pockels cells to carve a shaped pulse from a much longer pulse generated by a q-switched oscillator. The passive pulse shaper consists of two high-reflectivity mirrors and a wedge with partially transmitting coatings and produces a train of pulses from a single input pulse. The principal advantage of the passive approach is that timing, pulse shape and attenuation depend solely on the device geometry and coatings. Pockels cells have been used for many years to slice a variable-width pulse from a longer q-switched pulse. Pulse-shaping applications require optical-pulse rise times under 200 ps. Measurements of optical rise times in cylindrical ring-electrode pockels cells of potassium-dideuterium phosphate indicate that 200- to 300-ps rise times can be reached in existing devices and somewhat shorter rise times can be achieved in the future. The pockels-cell technique provides greater flexibility and lower insertion loss for some pulse shapes than passive pulse shaping.