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All-optical switching of 75 fs pulses centered at 1560 nm, driven by 270 fs, 1030 nm pulses in a dual-core optical fiber exhibiting high index contrast is presented. The fiber is made of a thermally matched pair of lead silicate and borosilicate glasses used as core and cladding material, respectively. The novel switching approach is based on nonlinear balancing of dual-core asymmetry, by control pulse intensity induced group velocity reduction of the fast fiber channel. Due to the fiber core made of soft glass with high nonlinearity high switching contrast exceeding 20 dB is reached by using control pulses of only few nanojoule energy. The optimum fiber length is 14 mm, which closely match the calculated coupling length at the signal wavelength. The results express significant progress in comparison to similar studies based on self-switching of solitonic pulses in dual-core fibers and represent high application potential.

We investigate experimentally and theoretically effects of the inter-core propagation mismatch on nonlinear switching in dual-core high-index-contrast soft-glass optical fibers. Incident femtosecond pulses of various energy are fed into a single ("straight") core, to identify transitions between different dynamical regimes, viz., inter-core oscillations, self-trapping in the cross core, and retaining the pulse in the straight core. The transfer between channels, which has solitonic character, is controlled by the pulse's energy. A model based on the system of coupled nonlinear Schr\"{o}dinger equations reveals the effect of the mismatch parameter and pulse duration on the diagram of the various energy dependent dynamical regimes. Optimal values of the mismatch and pulse width, which ensure stable performance of the nonlinear switching, are identified. The theoretical predictions are in agreement with experimental findings., Comment: 25 pages, 9 figures. To be published in "Chaos, Solitons & Fractals"

We present a complex study of pulse-energy-controlled solitonic self-switching of femtosecond pulses at wavelengths of 1700 and 1560 nm in two nonlinear high-index contrast dual-core fibers having different levels of slight asymmetry. In the case of the fiber with higher dual-core asymmetry excited by 1700 nm pulses, the highest switching contrast of 20.8 dB at 40 mm fiber length was demonstrated. It was accompanied by multiple exchanges of the dominant core at the fiber output, which is a strong signature of the soliton-based switching process. In the case of the fiber with lower dual-core asymmetry, excited by 1560 nm pulses, the highest switching contrast of 21.4 dB at 35 mm fiber length was achieved with a broadband character of the switching in the spectral range of 1450–1650 nm. Both demonstrations represent progress in all-optical switching studies at these particular wavelengths thanks to a comparison between their results, which reveals the requirement of a higher level of dual-core symmetry for applicable C-band operation.

Due to numerous unique properties, such as the capability of resonantly exciting crystal lattice vibrations, molecular rotations and other low energy transitions in solids, liquids and gases, terahertz radiation belongs to one of the most interesting but yet less explored spectral regions. Recently, a considerable progress has been demonstrated in THz generation by optical rectification (OR) in organic crystals, conventionally driven by near-IR fs-pulses centred at the telecommunication wavelength of 1.5 µm. Although high optical- to THz conversion efficiencies of 1-3% with pulse energies of up to 0.9 mJ [1] can be achieved, the conversion efficiency is nevertheless deteriorated by multiphoton absorption and is fundamentally limited by the damage threshold of the crystal. This restricts the applicable pump fluence to nothing more than 20 mJ/cm 2 .

A pair of sub-mJ femtosecond pulses separated by 110 ns, generated in a single cw-pumped Yb regenerative amplifier, is employed as a frontend for mid-IR OPCPA operating in the atmospheric transparency window.

We demonstrate a pulse energy-governed nonlinear self-switching of femtosecond pulses at the wavelengths of 1700 nm and 1560 nm in a highly nonlinear high index contrast dual-core fiber. The improved technology process allowed the first-time registration of the switching effect in the communication C-band.

All-optical switching of 77 fs pulses centered at 1550 nm, driven by 270 fs, 1030 nm pulses in dual-core optical fibre exhibiting high index contrast is presented. The switching is achieved by nonlinear balancing of dual-core asymmetry. High switching contrast exceeding 20 dB is attained with driving pulses of only few nanojoule energy, which is due to the fibre core made of soft glass possessing high nonlinearity.

We report on efficient THz generation in DAST, driven by mid-infrared pump sources. Due to the suppression of multi-photon absorption, high pump fluences can be applied and outstanding optical to THz conversion efficiencies of more than 4% can be reached.

We report on unexpected efficient THz generation in organic crystals (DAST and DSTMS) by optical rectification (OR) of intense mid-infrared pulses. The crystals experience superior damage threshold and an ultra-broadband THz spectrum reaching 10 THz.

In this paper, we demonstrate a pulse energy-governed nonlinear self-switching of femtosecond pulses at wavelength of 1700 nm in a highly nonlinear high index contrast dual-core fibre. The fibre structure is composed of two microdimension cores and a solid cladding made of two thermally matched soft glasses developed in-house. A double switching behaviour under monotonic increase of the input pulse energy from 1 nJ to 3 nJ was observed at 35 mm fibre length in its anomalous dispersion region. A spatial intensity distribution measurement identified a switching contrast of 16.7 dB, and the registered spectra of the two output channels revealed a broadband switching behaviour within 150 nm. The obtained results indicate a novel type of solitonic switching and represent a significant progress in comparison to the previous experimental works and has a high application potential.

Ultrafast laser sources operating in the mid-infrared spectral region, where many molecules and their functional groups have vibrational and rotational resonances, are very important tools for studying novel regimes of non-linear optics and strong field physics. Next to optical parametric amplifiers (OPAs) based on ZnGeP 2 (ZGP) crystals which, on the one hand, allow generation of energetic ultrashort mid-IR pulses at a mJ energy level, but, on the other, rely on complex and immature holmium pump lasers [1], a difference frequency generation (DFG) in AGS or GaSe crystals starting from a single broadband near-infrared (NIR) pulse [2] or via cascaded two-stage parametric down-conversion [3] is an alternative way to generate mid-IR pulses above 4-μm wavelength. In the later case more conventional Ytterbium, Neodymium or Ti:Saphire lasers are used to pump the frequency conversion stages. The use of a longer-wavelength 1.03 μm Ytterbium laser as compared to Ti:Sapphire pump sources yields an improved phase matching bandwidth, lower group velocity miss-match, as well as a lack of three-photon absorption, which reduces detrimental photo-darkening in AGS.

We generate fully controllable fs multimillijoule pulse bursts with the energy handling, throughput efficiency and frequency resolution substantially exceeding that achievable in spatial-light-modulator and interferometric techniques. The demonstrated proof-of-concept experiments include coherent control of nitrogen-ion emission via multiple-pulse excitation and generation of tunable narrowband THz pulses via optical rectification.

A robust Ho:YAG chirped pulse amplifier with a simple dispersion management was developed for material processing applications. The amplifier produces 3.2 ps pulses centered at 2.09 μm with energies up to 1.6 mJ. At 10 kHz repetition rate this results in 16 W of average power. Both pulse stretching and compression in the amplifier is realized within a single chirped volume Bragg grating. With the developed amplifier we study laser ablation of a gold layer through a carrier substrate and, particularly, the effect of pulse duration on the process. A balance between nonlinear effects within the carrier and interaction with the thin ablation layer makes few picosecond pulses most suitable for non-thermal ablation. Furthermore, since only a few-microjoules of pulse energy is required for de-bonding, the developed 16 W amplifier can be potentially operated at MHz repetition rates for high throughput.

Burst-mode regenerative amplifiers permit substantial energy and average-power scaling of pump lasers. We demonstrate advantages of parametric burst-to-single-pulse up- and down-conversion in the generation of optical harmonics and in multicolor pumping of long-wave parametric amplifiers.

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 Schrodinger 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.

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.

Pulse burst generation has important applications in material processing [1], seeding of free electron lasers [2] and coherent pulse stacking [3]. In particular, coherent pulse stacking technique allows further scaling of fiber and solid-state amplifiers by splitting the seed pulse into multiple replicas and stacking them into a single dominant pulse after the amplification. For this task, a pulse burst consisting of 4–90 pulses, with fully controllable amplitudes and phases is required. In this contribution 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. Unlike in the case of pulse burst formation from a GHz repetition rate oscillator [4], the pulse burst formation using the Vernier effect allows very high repetition rate (much higher than the driving oscillator frequency) pulse burst formation with controllable amplitudes and phases.

We demonstrate a scheme for LWIR pulse amplification based on spatial and spectral de-multiplexing of a pulse burst from a 1-m laser amplifier. This method holds promise for energy scaling of 5-10-m OP(CP)A using Joule-class Nd and Yb pump lasers.

We propose and demonstrate a scheme for LWIR pulse amplification based on spatial and spectral de-multiplexing of a pulse burst from a 1-μm laser amplifier. This method enables multi-color multi-beam pumping of an OPA without complications intrinsic to coherent pulse combining techniques.

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 Schr\"odinger 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., Comment: 19 pages, 9 figures

High-peak-power longwave OPAs operating above the wavelength of 5μm [1] require high-energy picosecond pump pulses >2 μm. These can be generated directly in laser amplifiers, e.g. Ho-doped [1, 2] or via cascaded OPAs [3]. Burst-mode pump lasers offer great advantages over single-pulse amplifiers because they help overcoming the challenges of chirped-pulse as well as direct picosecond pulse amplifiers operating in the singlepulse amplification mode. The techniques of coherent pulse combining [4] and coherent pulse stacking of pulse bursts [5] offer an opportunity, at the expense of interferometric phase stabilization, for a multifold energy increase in a single pulse without modifying the laser amplifier itself. Simple non-interferometric combining techniques, such as energy combination via frequency doubling in a non-collinearly phase-matched nonlinear crystal also exist, but they are not suitable for pumping LWIR OPAs.

A novel type dual-core fiber (DCF), composed of two in-house synthesized glasses with high index contrast enabling significant anomalous dispersion [1] and cm range coupling length was optimized and studied in the near infrared. Following promising results of numerical simulations [2], we performed an experimental study in the 1500–1900 nm spectral range by using a tunable ultrafast laser source generating 70-fs pulses at a few μJ energy level. Continuous control of attenuation and polarization direction of the input field was ensured under separate excitation of a single fiber core, made of the low index glass with an effective mode area of 1.73 μm2 at 1500 nm. The dual-core cladding is made of high index glass including the 1.4-μm gap between the two cores. The series of camera pictures, imaging the output facet of the dual-core fiber at different input pulse energies, indicated nonlinear switching at distinct excitation conditions. A change in the dominancy of the output core of 32 mm long DCF was registered when 1700 nm, 70 fs pulses with increasing energies in range of 100–300 pJ (measured before in-coupling optics) were launched into the fiber. Output spectra were registered separately for the two cores, when the nonlinear switching was observed according to the camera registrations. The spectra were processed by plotting the curves representing a ratio R(λ) of the spectral intensities of the two cores: R(λ) = S t (λ)/S b (λ) − 1, where S t (λ) represent the spectra registered from the top core (dominant at low pulse energies) and S b (λ) are the spectra originating from the bottom core (Fig. 1a). In Fig. 1b, the excitation energy dependence of R(λ) is depicted, with positive and negative values representing the cases of the dominancy of the top and the bottom cores, respectively. The spectra reflect the similar energy dependence as the series of camera images, shown in the insets of Fig. 1b. In the case of low excitation energy, the spectral intensity ratio curves reveal a dominancy of the top core, while at higher pulse energies a majority of launched pulse energy is located in the bottom core. According to our knowledge, the presented simultaneously recorded spatial and spectral study is the first experimental demonstration of the nonlinear switching of a whole ultrashort pulse in a DCF in a solitonic propagation regime. Furthermore, in contrast to previously reported cases, the switching identified here is accompanied by rather moderate spectral broadening at pulse energies in the sub-nJ range.

Ultrafast lasers are of interest for applications in medicine, material processing [1] and as drivers for mid-IR optical parametric frequency converters [2]. Recently a sub-picosecond operation of Ho:YAG chirped pulse amplification (CPA) system was realized by extending the amplified pulse spectrum over two emission peaks centered at 2090 nm and 2096 nm [3], which required strong adaptive spectral shaping of the seed pulse to equalize spectral amplitude gain. In addition, dispersion management of the CPA was complicated due to the resonant spectral phase distortion and could only be accomplished by using an acousto-optic programmable dispersive filter (DAZZLER). The most intense emission line of Ho:YAG is centered at 2090 nm, and, without compensation of gain narrowing, supports amplified bandwidth of >1.5 nm corresponding to

We study an effect of pulse duration of 2.09-pm laser on the laser de-bonding of gold from silicon wafers. Developed picosecond Ho:YAG laser with a simple dispersion management is found to be optimal for the process.

Here we present a coherent pulse stacking approach for up-scaling energy of a solid-state femtosecond chirped pulse amplifier. We demonstrate pulse splitting into four replicas, amplification in a specially designed regenerative amplifier cavity, and phase-locked stacking into one dominant pulse. An optimized regenerative amplifier design for non-overlapping pulse train amplification is presented.

Gigawatt-peak-power radiation, tunable in the 5-9 µm spectral window is generated as a difference frequency between the signal and idler pulses, originating from a KTA optical parametric amplifier pumped by 100-mJ, 200-fs Yb laser.

Dual-core optical fibres (DCFs) represent devices for a simple ultrafast all-optical signal processing, in particular for cross-switching tasks [1] , [2] . Recently, we have developed a highly nonlinear, all-solid DCF, scanning electron micrograph (SEM) of which is shown in inset of Fig. 1a ). High contrast of refractive index of the DCF enabled the efficient self-switching of the femtosecond solitons in the C-band at sub-nJ switching pulse energies [3] . Here, we present an improved approach: using the same DCF with a slight dual-core (DC) asymmetry, we perform a two-colour control-signal switching. This permits to achieve higher switching contrast and preserves better pulse shape.

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].