These remarks imply that optical pulses which are intended for studying the microcosm need to be made up of just a few oscillation cycles of the electromagnetic wave. At this limit, i.e., when the pulse bandwidth is a considerable fraction of the optical carrier frequency, the relative phase difference between the peak of the pulse envelope and the oscillations of the wave, termed the carrier-envelope phase, becomes a relevant measure. Indeed, the shape of a pulse comprising only a few optical cycles will depend strongly on the relative phase of these cycles. In turn, the carrier-envelope phase will then affect the atomic scale motion of the electrons controlled by the light field. The problem of carrier-envelope phase stability becomes ever more critical when the pulses propagate in a medium where the group and phase velocities are not the same. Such is the case when the generation of the few-cycle pulses relies on strong nonlinearities arising from their propagation in a gas-filled hollow core fibre. The intense electromagnetic field experiences spectral broadening through the effects of self-phase modulation in the nonlinear gas; once a sufficiently broad spectrum is generated, the ultra-short pulses are obtained through linear pulse compression. Any fluctuations either in the intensity of the laser system or the gas flow inside the hollow fibre will result in instabilities of the carrier-envelope phase of the resultant ultra-short pulses. It is therefore important to understand how the method of filling the fibre with the nonlinear gas may affect the system performance.
The paper by W.A. Okell et al. is a systematic study that compares the carrier-envelope phase stability of a few-cycle pulse generation system based on spectral broadening in a hollow-core fibre, when this operates in either of three commonly used modes. The study gives emphasis to the case of differentially pumping neon gas at the two ends of a hollow core fibre, which in certain instances might exhibit advantages with respect to the stability of the system over the more conventional static filling. The study has found that the maximum output pulse energy can be increased in differentially pumped fibres as compared to their statically filled counterparts.
Furthermore, the carrier-envelope phase stability of a differentially pumped system is not compromised and is suitable for attosecond pulse experiments. This work will surely constitute an invaluable reference to those designing pulse compressors based on hollow core fibres. It is a very well presented and accessible paper, highly recommended for both expert researchers in ultra-fast optics and students studying the mechanisms behind stable, short pulse, high energy systems.
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