Optimized Compensation of High Third Order Dispersion of Few-Meters-Long Commercially Available Hollow Core Fibres

In recent years, photonic bandgap hollow core fibres (PB-HCF) have been studied as a solution to implement fibre delivered lasers, capable of delivering high output powers at long distances from the actual source. Application of this technology to ultrafast lasers exhibits some challenges, such as achieving good polarization maintenance, high output powers, insensitivity to bends, and low pulse durations, all while still achieving transmission over significant distances. Regarding these last two aspects, most commercially available, easily reproducible PB-HCF display high amounts of positive third order dispersion (TOD) and low, negative second order dispersion (GDD) (Figure 1a) [1], [2]. This high TOD/GDD ratio requires an appropriate design of the overall dispersion of the laser system, to allow the use of longer PB-HCF sections while still achieving high quality pulse shapes. To do this, it is critical to ensure that dispersion compensation can be achieved with feasible compressor geometries, which typically require both gratings and prisms to approach the transform limited (TL) pulse duration. In this work, we discuss the impact of dispersion from HC-920-PM and HC-1060 fibres from NKT Photonics, demonstrating optimized dispersion compensation for a 3.25 m long PB-HCF section. To do this, we have first performed Fiberdesk simulations of the system under consideration, obtaining spectra and phases matching those experimentally observed. We then added the dispersion of a 3.25 m section of the respective PB-HCFs, determining the overall phases of the systems, and we simulated different compressor configurations, including with and without prism, estimating the overall impact of the fibres on the pulse shapes and the minimum achievable pulse durations. After determining the optimal compressor configuration, we performed various tests with real folded grating-prism compressors, using sources delivering 1W, 80 MHz train of pulses centred at 920 nm and 1064 nm respectively. The outputs were first sent into the compressors, and then coupled into the respective 3.25-m-long PB-HCF section, minimizing the pulse duration while monitoring the autocorrelation (AC) trace obtained from the pulses. After optimizing the compressors, the pulses displayed a FWHMs of 134 fs and 127 fs under the Scch<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> assumption for 920 and 1064 nm respectively, just 3 and 2 fs longer than the calculated TL durations from the associated spectra (Figure 1b). Further evidence of the pulse quality was obtained by calculating the Strehl ratio of the pulse, defined as the ratio between the peak of the measured AC trace, and the peak of a calculated AC trace assuming Gaussian pulses and having same FWHM and area as the measured one. For TL pulses, the Strehl ratio was close 0.99, whereas for poorly compressed ones the value progressively lowers, as side lobes/tails appear and the pulse's peak intensity gets lower. The Strehl ratio of the compressed pulses was equal to 0.95, indicating very good compression with minimal energy located in the pulses' tails. These results confirm the possibility of using long PB-HCF sections in fibre delivered lasers without significant pulse quality degradation. This is particularly important for applications such as two-photon absorption microscopy, where optimizing the pulse quality reduces the amount of unusable background signal and leads to significant improvements in terms of absorption efficiency [3].