Ignas Astrauskas

Nonlinear performance of asymmetric coupler based on dual-core photonic crystal fiber: Towards sub-nanojoule solitonic ultrafast all-optical switching

We demonstrate ultrafast soliton-based nonlinear balancing of dual-core asymmetry in highly nonlinear photonic crystal fiber at sub-nanojoule pulse energy level. The effect of fiber asymmetry was studied experimentally by selective excitation and monitoring of individual fiber cores at different wavelengths between 1500 nm and 1800 nm. Higher energy transfer rate to non-excited core was observed in the case of fast core excitation due to nonlinear asymmetry balancing of temporal solitons, which was confirmed by the dedicated numerical simulations based on the coupled generalized nonlinear Schrodinger equations. Moreover, the simulation results correspond qualitatively with the experimentally acquired dependences of the output dual-core extinction Ratio on excitation energy and wavelength. In the case of 1800 nm fast core excitation, narrow band spectral intensity switching between the output channels was registered with contrast of 23 dB. The switching was achieved by the change of the excitation pulse energy in sub-nanojoule region. The performed detailed analysis of the nonlinear balancing of dual-core asymmetry in solitonic propagation regime opens new perspectives for the development of ultrafast nonlinear all-optical switching devices.

High-energy pulse stacking via regenerative pulse-burst amplification

Here we present a coherent pulse stacking approach for upscaling the energy of a solid-state femtosecond chirped pulse amplifier. We demonstrate pulse splitting into four replicas, amplification in a burst-mode regenerative Yb:CaF2 amplifier, designed to overcome intracavity optical damage by colliding pulse replicas, and coherent combining into a single millijoule level pulse. The thresholds of pulse-burst-induced damage of optical elements are experimentally investigated. The scheme allows achieving an enhancement factor of 2.62 using a single-stage stacker cavity and, potentially, much higher enhancement factors using cascaded stacking.

Towards ultrafast sub-nanojoule solitonic nonlinear directional coupler based on soft glass dual-core photonics crystal fibers

We demonstrate narrow band spectral intensity switching in dual-core photonic crystal fibers made of highly nonlinear glass under femtosecond excitation. The fibers expressed dual-core asymmetry, thus the slow and fast fiber cores were unambiguously distinguished according to their dispersion profiles. The asymmetry effect on the dual-core propagation in anomalous dispersion region was studied both experimentally and numerically. The experimental study was carried out using femtosecond laser amplifier system providing tunable pulses in range of 1500 nm - 1800 nm. The obtained results unveiled, that it is possible to improve nonlinearly the coupling between the two waveguides by excitation of the fast fiber core. The results were obtained in regime of high-order soliton propagation and were verified numerically by the coupled generalized nonlinear Schrödinger equations model. The spectral analysis of the radiation transferred to the non-excited core revealed the role of effects such as third order dispersion, soliton compression and spectral dependence of the coupling efficiency. The simulation results provide reasonable agreement with the experimentally observed spectral evolutions in the both fiber cores. Under 1800 nm excitation, narrow band spectral intensity switching was registered with contrast of 23 dB at 10 mm fiber length by changing the excitation pulse energy in sub-nanojoule range.

Generation of a single UV pulse from a near-IR pulse burst

High harmonic generation (HHG) requires high peak-power laser sources. Most of the well-known high peak power lasers are operating in the near-IR wavelength region. Recently it was demonstrated that HHG can be effectively phase matched in the soft X-ray region by using very high intensity UV lasers and multiply charged ions [1]. High average and high peak power UV sources operating around and below 280 nm are required for many other applications, such as ablation in ophthalmology, materials processing and photoelectron spectroscopy. Due to lack of ultrafast high peak power lasers operating in UV, generation of ultrashort UV pulses is possible by up-converting frequency of near-IR laser. This can be done by cascaded harmonic generation in nonlinear crystals with efficiency higher than 40% [2]. However, to obtain high pulse energies in UV region, high energy IR pump is necessary. This becomes increasingly difficult for femtosecond laser pulses because of the optical damage problem in CPA systems. Very high pulse stretching rates in the CPA become unfeasible due to the limited size of dispersive optics. Alternatively, the intensity in the laser cavity can be decreased by using a pulse burst which effectively increases the pulse duration. Therefore, this approach is also suitable for increasing energy throughput in fiber delivery and fiber post compression schemes [3].

High repetition rate fs pulse burst generation using the Vernier effect

We demonstrate pulse burst generation method based on the Vernier effect. The pulse burst with controllable amplitudes and phases is formed using a femtosecond oscillator and regenerative amplifier cavity that have slightly different round trip times. This operation mode can be used for the purposes of coherent pulse stacking, rapid material miroprocessing and rapid scan spectroscopy.