Improving ultrashort optical pulse compression
Most femtosecond lasers emit pulses with near-IR wavelengths and, due to the limited gain bandwidth of laser amplifiers, these pulses have durations over many optical cycles when amplified. It is therefore desirable to efficiently compress them to few-cycle duration. Moreover, currently available laser technologies cannot directly deliver mid-IR femtosecond pulses. Instead near-IR pumped optical parametric amplifiers can frequency-convert the near-IR pulses to the mid-IR. However, this requires overcoming frequency conversion bandwidth limitations to achieve mid-IR few-cycle pulses. The EU-funded project 'Cascaded optical pulse compressor' (COPULCO) project investigated a method where longer, multi-cycle near-IR pulses could be efficiently compressed to few-cycle duration. This may happen in a carefully designed two-stage nonlinear crystal. At the same time energetic mid-IR few-cycle pulses can be generated through optical Cherenkov radiation. This pulse compressor is based on cascaded quadratic nonlinearities and uses solitons to compress the pulses. The mid-IR radiation originates from a near-IR soliton formed in the crystal, and upon propagation it is perturbed by high-order dispersion. The radiation stems from a resonant phase-matching condition with the soliton. The novelty of this particular soliton-Cherenkov wave phase-matching condition is that it occurs at mid-IR wavelengths, and is therefore a very important alternative for efficient and broadband mid-IR frequency conversion. The project also investigated the necessary conditions for dispersive-wave generation accompanied with the few-cycle pulse compression. The results promise a new route for high-efficiency and broadband wavelength conversion to the mid-IR. The scientists used chirped quasi-phase matching crystals to achieve soliton self-compression to few-cycle duration with increased efficiency and quality.. By using a multi-section structure of the nonlinear crystal, they managed to create engineered cascaded quadratic nonlinearities in sections, thus increasing the pulse quality significantly. Part of the work was geared toward understanding and accurately modelling ultra-fast and ultra-broadband interaction in cascaded quadratic nonlinear media. The team developed a new nonlinear wave equation in frequency domain as a platform for investigating soliton compression to few-cycle duration. COPULCO also studied the anisotropic nature of the Kerr nonlinear response in a specific nonlinear crystal, so as to better model the ultrafast interaction caused by the crystal’s anisotropic nonlinear coefficients. Focus was placed on determining the cubic tensor components that affect interactions of cascaded second-harmonic generation. The Kerr nonlinear tensor component competing with the self-phase modulation caused by cascading was found to be considerably larger than what had historically been used, but once the historical measurements were corrected for deterministic errors the agreement was restored. The impact of using such a cubic anisotropic response in ultra-fast cascading experiments was also evaluated. The project findings should help consolidate Europe as a leader in ultra-fast femtosecond processes.
