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Few-cycle source development



Fig. 1: Photograph of the few-cycle source: The fibre can be seen lit up on the left of the photo, with the output pulses propagating off a set of chirped mirrors on the right of the image.

A few-cycle laser source based on hollow-fibre pulse compression has been constructed Imperial College and will serve as the drive laser for the attosecond source (Fig. 1). We have generated pulses of ~6.5fs in duration with output energies of 0.36mJ after compression. The system uses a differentially pumped hollow fibre which provides a number of advantages over the conventional static fill method. These advantages include a decreased dependency of energy transmission on the input laser pulse and gas parameters, improved shot-to-shot output energy fluctuations. Most significantly, this setup allows for smooth tuning of the duration of near transform-limited pulses in the range ~6.5-30fs, with constant energy and alignment, by simply adjusting the gas pressure in the fibre.

Hollow fibre pulse compression

The generation of few-cycle pulses at a centre wavelength of 800nm requires bandwidths on the order of 200nm. These pulses can be generated with a few nJ of energy directly from a laser, using broad-band gain media such as Ti:Sapphire. However, to produce high energies suitable for the study of strong-field interactions, the pulses must be amplified which results in narrowing of the spectrum and an increase in the transform-limited duration of the compressed pulse.

To generate few-cycle pulses at high energy, amplified pulses can be spectrally broadened and recompressed to shorter durations. One technique for achieving this is based on the use of self-phase-modulation (SPM) in a gas-filled hollow fibre, which generates additional frequency components as the intense pulse propagates along the fibre. We have implemented a slightly modified version of this process in which the fibre is differentially pumped to produce a pressure gradient along the fibre, rather than filling the fibre with a fixed pressure along its length.

A differentially pumped hollow fibre

Fig. 2: Photograph of the amplifier in the 30fs drive laser system.

Our system uses input pulses of 30fs, ~700μJ at 1kHz repetition rate (Fig. 2). These pulses are focused into a 250μm inner diameter, neon-filled, hollow, fused-silica fibre, which is 60cm in length. Two small chambers are sealed to each end of the fibre to allow gas to be fed into the fibre at the exit end and pumped away at the input end, resulting in a vacuum at the entrance and increasing pressure along the fibre towards the exit. This removes the pressure dependence on coupling efficiency that we found in a statically filled fibre, as shown in Fig. 3, by removing the effect of ionisation defocusing at the fibre entrance, and also results in greater energy stability at the fibre output.

The constant energy transmission in the differentially pumped fibre allows the spectral bandwidth to increase linearly with pressure, in contrast to the static case where the drop in coupled energy results in a more complex relation between pressure and bandwidth (Fig. 4). There is no need to move the fibre to reoptimise transmission and broadening, as can be the case in a statically filled fibre, resulting in a greatly simplified setup.

Pressure tuning of pulse duration

The simple relationship between gas pressure and bandwidth in the differentially pumped fibre allows for a simple way of smoothly altering the pulse duration, whilst maintaining near-transform-limited pulses of constant energy and alignment. If the compression stage is setup to compress the largest bandwidth (i.e. largest pressure), pulses produced at lower pressures will also be well compressed, as the contribution to the spectral phase of the pulse comes largely from material dispersion rather than SPM, so the spectral phase is not significantly affected by the change in pressure. The pulses therefore remain close to the transform limit, indeed, the longer pulses can be compressed closer to the transform limit, as the higher order phase terms become less significant for the reduced bandwidth.

In contrast, increasing the pulse duration by altering the compression at a fixed bandwidth (as is usually the case in CPA systems e.g. varying the compressor grating separation) results in a departure from the transform limit proportional to the increase in the pulse duration. For modulated spectra, such as those produced from SPM, this generally results in significant structure in the pulse, which introduces undesirable complexities in experimental interactions. Even if the pulse profile remains unstructured, the undesirable effects from the chirp on the pulse still remain. Our pressure tuning technique avoids these problems and opens up experimental opportunities, where processes can be studied as a function of pulse duration without the presence of chirp effects.

Fig. 3:Energy transmission as a function of pressure in a 1m long fibre. Differential pumping of the fibre is compared to a static fill and is found to give constant transmission for neon pressures ranging from 0-3bar.
Fig. 4:Spectral broadening as a function of pressure in a 1m long fibre. The bandwidth from the differentially pumped fibre increases linearly with increasing pressure compared to the static fill in which the broadening is adversely affected by the reduced energy transmission shown in Fig. 3. The input spectrum is shown with the grey curve over the 0.5bar spectra.
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