Chirped Pulse Amplification
Foreword
The FERMI team and collaborators have already demonstrated the feasibility of recompressing chirped free-electron (FEL) laser pulses in the extreme ultraviolet range.
In optical conventional lasers, chirped pulse amplification (CPA) became, in the early 80’s, a revolutionary technique, allowing the generation of extremely powerful femtosecond pulses in the infrared and visible spectral ranges.
Nowadays, CPA is the basis for the worldwide operation of laser systems delivering very ultrashort pulses, in the few femtosecond (quasi single cycle) regime, and carrying peak powers up to the Petawatt scale.
The experimental implementation of CPA on a seeded FEL in the extreme-ultraviolet has been driven by the following considerations:
- For any given free-electron laser setup, a limit presently exists in the generation of ultra-short pulses, as the output pulse energy is limited by the reduced number of electrons participating in the amplification process.
- In all cases, the pulse shortening is constrained by the FEL gain bandwidth.
- Stretching the seed pulse in time by means of a linear frequency chirp before amplification allows one to extract energy from the whole electron bunch, substantially enhancing the FEL pulse energy at saturation.
- The bandwidth of a seeded FEL operated in CPA mode increases with the harmonic number hn. Indeed, although in standard lasers the spectral content of the generated light is fundamentally identical to that of the input pulse, in seeded FELs the bandwidth of the output emission can be significantly larger than that of the seed. This allows obtaining, after compression, a FEL pulse shorter than that which is generated when the FEL is operated in standard (i.e., no-CPA) mode.
Results
The CPA scheme was successfully tested at the FERMI FEL, on the FEL-1 branch, in 2016. The FERMI team and collaborators obtained, at a wavelength of 37.3 nm, corresponding to hn =7, a significant shortening of the FEL pulse with respect to the no-CPA case.
The latter, according to the theory and to the experimental results, was measured to be 90 fs, starting from a quasi transform-limited (time-bandwidth product = 1.2 x transform limit) seed pulse at 261 nm of 170 fs.
Then, to enable the CPA regime, a positive linear frequency chirp in the seed pulse was induced (Group Delay Dispersion, GDD ≈ 8500 fs2), stretching it up to 290 fs.
The interaction of the chirped pulse with an increased portion of electrons (due to the time-energy correlation in the bunch) led to a broadened FEL bandwidth by a factor 1.9. This led to the creation of a chirped amplified final pulse, which was compressed by means of a two-grating compressor developed in- house. The resulting pulse length was ≈51 fs, giving a Time Bandwidth Product = 1.1 x Transform Limited.
This remarkable result shows that the CPA technique is also able partially to compensate some unwanted residual phase errors generated by other sources, such as the seed transport and the quadratic curvature of the electron-beam energy profile.
Layout of the Chirped Pulse Amplification (CPA) experiment. In CPA regime, the FEL is seeded with a chirped Gaussian laser pulse. Under proper conditions, the frequency chirp of the seed is transmitted to the FEL and can then be compensated by an optical compressor with two gratings (G1 and G2) in classical diffraction geometry. At the compressor exit, the FEL beam is directed towards the experimental chamber of a beamline where a cross-correlation scheme is used, for measuring the FEL pulse duration. In the insets are shown the cross correlation curves and the reconstructed laser pulse (left) and the behaviour of the pulse duration versus the compressor geometry (right): The minimum is the shortest pulse achieved, below the “normal” FEL pulse value and very close to the theoretical CPA minimum.
Perspectives
In the mid-term, the FERMI CPA collaboration team (an open entity to which interested colleagues from abroad can participate) is willing to setup a new in-house experiment, to be performed at a higher harmonic order (from hn =12 to hn =20) on the FEL-2 branch.
This has manifold purposes:
- To demonstrate the capability of transposing the chirp induced by the seed onto the electron bunch through a two-stage cascade, preserving the feautres along the delay line.
- To start with a shorter seed (nominally 50 fs transform-limited), with an appropriate induced chirp, for getting a final FEL pulse length below 10 fs.
- To pave the way towards a longer-term goal: to generate coherent and phase-tailored few-femtosecond pulses with gigawatt peak power in the sub-10 nm spectral range. For instance, 5-fs 5nm, 5 microjoules pulses focused on an interaction region of 5 microns x 5microns means a radiation intensity of 4 x 1015 W/cm2.
