Ti:sapphire ring laser produces 10-femtosecond pulses
Nearly bandwidth-limited, 10-fs pulses of 790-nm laser light have been produced by researchers at the Max Planck Institute (Munich, Germany) using a Ti:sapphire ring resonator. The pulse-shortening mechanism in a Ti:sapphire resonator is similar to soliton formation in optical fiber. An important difference for short pulses, though, is that in each element of the fiber, negative group-delay dispersion (GDD) and self-phase modulation (SPM) are simultaneously present. In a resonator, these act in an alternating manner. However, in a ring resonator configuration, SPM and GDD are automatically distributed as homogeneously as possible. And, while previous ring cavities contained large amounts of optical material causing significant higher-order dispersion that limited laser performance, the Planck investigators had available very highly doped (absorption coefficient of 6.2 cm-1) Ti:sapphire crystals and chirped mirrors for intercavity dispersion control.
The mirrors compensated for the positive GDD from the laser crystal but only in coarse steps. Thus, thin plates of laser-grade fused quart¥were introduced into the laser cavity at less than Brewster`s angle to eliminate etalon effects, and fine tuning comparable to a prism-controlled reasonator was achieved. The researchers were able to run the reasonator down to zero GDD. Pulses as short as 10 fs were produced with a 1.1-mm-thick quart¥plate in the resonator.