450kW, all fibre, picosecond Chirped Pulse Amplification system based on a large mode area fibre Bragg grating compressor

Abstract

Chirped pulse amplification (CPA) is a well known technique for producing high power short optical pulses suitable for nonlinear optical experiments. Using such techniques with bulk amplifier systems it is possible to produce femtosecond optical pulses with terawatts of peak power. In a CPA system a short transform limited optical pulse is first chirped by a highly dispersive element and thereby temporally stretched by factors of up to 10000. The pulse is then amplified to a high energy before being recompressed back to its initial duration at the system output by a device of opposite dispersive characteristics to the pulse stretcher. In this manner high peak powers are avoided, and thereby the limiting effects of nonlinearity within the amplifier, thus massively extending the peak powers attainable. Pulse stretching and compression is usually performed using bulk optic devices such as a diffraction grating pair. However in order to obtain the temporal stretching factors required grating separations of order 1m are required, making such systems large and cumbersome to use.
Recently the CPA technique has been applied to pulse systems based on erbium doped fibre amplifiers opening up the possibility of the development of robust, compact CPA systems based on linearly chirped fibre Bragg gratings (FBGs). The main advantage of FBGs is that they are highly dispersive and can thus provide suitable time-delays in only centimetre lengths of fibre. Using a 12 cm grating Galvanauskas et al. recently demonstrated a CPA scheme capable of producing 2 ps picosecond pulses with 300 nJ of energy and peak powers of order 150 kW. One drawback to the use of FBGs is that they are intrinsically nonlinear and at high powers this nonlinearity will degrade the pulse quality. In Galvanauskas experiment the limiting factor to producing µJ pulses was the nonlinear phase-shift caused by pulse propagation in the fibre preceding the grating. This length can clearly be reduced but then one runs into the problem of nonlinearity inside the grating itself. As has been recently shown nonlinear effects in gratings can become significant at intensities as low as 5GW/cm². The power level corresponding to this threshold clearly depends on the mode area of the fibre. Hence it is advantageous to use fibres with large mode areas but which are still singlemoded at the operating wavelength. Recently such a fibre was developed for use in high power amplifiers in order to reduce the effect of the nonlinearity as well as allowing for more efficient energy extraction. By slightly modifying the composition of this fibre we are able to fabricate a photosensitive fibre suitable for grating writing. These gratings were then used as the compressor for a CPA system based around an erbium fibre amplifier chain to produce an all-fibre based source of high power (> 400 kW) short pulses.

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ORCID for D.J. Richardson: orcid.org/0000-0002-7751-1058

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