According to the uncertainty relation, the product of a pulse’s temporal width and that of its frequency spectrum is fundamentally required to be larger than a certain numerical constant, meaning that the generation of a shorter pulse in time demands a broader spectral bandwidth. Therefore, to achieve possibly the shortest pulses with a given spectral bandwidth, finding the best possible coherent combination of spectral components (i.e. no frequency chirp) is the only way to go. Previously, in 2012, by only selecting a spectral range with almost no frequency chirp, the authors generated a blue-violet 200-fs pulse with a dispersion-compensated GaInN MLLD (mode-locked laser diode). Unfortunately, this filtering decreased the output power of the MLLD by 90%, potentially making the shortest possible pulse duration broaden due to the narrowed spectral bandwidth. In this new Optics Express
article, Shunsuke Kono and collaborators cleverly avoid these two losses by first mapping the group velocity dispersion (GVD) of the MLLD in the whole frequency range, and then compensating phase dispersion up to the third order using the mapped GVD with an external nonlinear pulse compressor that consists of a spatial light modulator and gratings. With this compensation, the authors successfully demonstrate 140-fs duration and 60 W peak power blue-violet optical pulses, currently the shortest optical pulse created from an MLLD. As the authors note, this nonlinear compression technique is potentially a very important tool for generating high peak power fs optical pulses with MLLDs, since the higher order phase dispersion induced from high power semiconductor optical amplifiers can also be easily compensated.
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