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We demonstrate that chirped pulse up-conversion (CPU), a method routinely used with systems based on 1-kHz Titanium:Sapphire lasers, can be extended to a repetition rate of 100 kHz with an Ytterbium diode-pumped femtosecond amplifier. Individual mid-infrared spectra can thus be measured directly in the near infrared using a fast CMOS linescan camera. After an appropriate Fourier processing, a spectral resolution of 1.1 cm-1 is reported, currently limited by our spectrometer. Additionally, we demonstrate the application of CPU to a pump-probe measurement of the vibrational relaxation in carboxy-hemoglobin, and we show that the combination of fast scanning and fast acquisition enables a straightforward removal of pump scattering interference.
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We demonstrate nonlinear temporal compression of a vortex beam by propagation in a gas-filled capillary. Starting from an ytterbium-based laser delivering 700 µJ 640 fs pulses at a 100 kHz repetition rate, the vortex beam is generated using a spiral phase plate and coupled to a capillary where it excites a set of four modes that have an overlap integral of 97% with a Laguerre-Gauss LG10 mode. Nonlinear propagation of this hybrid, orbital angular momentum (OAM)-carrying mode results in temporal compression down to 74 fs at the output. Beam and pulse characterizations are carried out to determine the spatial profile and temporal duration of compressed pulses. This result in multimode nonlinear optics paves the way towards the generation of OAM-carrying few-cycle pulses, isolated attosecond XUV pulses, and tunable UV pulses through resonant dispersive wave emission.
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Femtosecond laser sources with high repetition rate in the ultraviolet (UV) and vacuum UV (VUV) are fundamental tools enabling tabletop time-resolved and angle-resolved photoemission spectroscopy in solids. We describe a VUV source at 114 nm (10.8 eV) based on an industrial grade ytterbium-doped ultrafast laser, a nonlinear pulse width selection stage, and two cascaded frequency tripling stages, first in crystals, second in xenon. The role of ionization in gas-based perturbative third harmonic generation phase-matching is analyzed using a simple theory, numerical simulations, and experimental data. The source features high photon flux, high repetition rate, and adjustable time resolutions. Thereby, in combination with a state-of-the-art angle-resolved photoemission spectroscopy (ARPES) apparatus it enables the study of the electronic dynamics of the whole Brillouin zone in a large number of materials.
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We demonstrate a table-top source delivering ultra-broadband THz pulses with electric field strength exceeding 100 kV/cm at a repetition rate of 200 kHz. The source is based on optical rectification of 23 fs pulses at 1030 nm delivered by a ytterbium-doped fiber laser followed by a nonlinear temporal compression stage. We generate THz pulses with a conversion efficiency of up to 0.11 % with a spectrum extending to 11 THz using a 1 mm thick GaP crystal and a conversion efficiency of 0.016 % with a spectrum extending to 30 THz using a 30 µm thick GaSe crystal. The essential features of the emitted THz pulse spectra are well captured by simulations of the optical rectification process relying on coupled nonlinear equations. Our ultrafast laser-based source uniquely satisfies an important requirement of nonlinear THz experiments, namely the emission of ultra-broadband THz pulses with high electric field amplitudes at high repetition rates, opening a route towards nonlinear time-resolved THz experiments with high signal-to-noise ratios.
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We report on the nonlinear temporal compression of mJ energy pulses from a Ti:Sa chirped pulse amplifier system in a multipass cell filled with argon. The pulses are compressed from 30 fs down to 5.3 fs, corresponding to two optical cycles. The post-compressed beam exhibits excellent spatial quality and homogeneity. These results provide guidelines for optimizing the compressed pulse quality and further scaling of multipass-cell-based post-compression down to the single-cycle regime.
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Positively chirped femtosecond pulses at 1030 nm are wavelength-converted using spontaneous and stimulated Raman scattering in a potassium gadolinium tungstate crystal inserted inside a multipass cell. Recirculation in the cell and the Raman material allows both a high conversion efficiency and good spatial beam quality for the generated Stokes beams. The converted pulses can be compressed to sub-picosecond duration. Multipass cells could be an appealing alternative to other Raman shifter implementations in terms of thermal effects, control of the Raman cascade, and overall output beam quality.
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We report about a setup for carrier-envelope phase (CEP) control and stabilization in passive systems based on difference frequency generation (DFG). The principle of this approach relies on the amplitude to phase modulation transfer in the white-light generation process. A small modulation of the pump laser intensity is used to obtain a DFG output modulated in CEP. This technique is demonstrated in a CEP-stable system pumped by an Yb-doped fiber amplifier. It is first characterized by measuring CEP modulations produced by applying arbitrary waveforms. The CEP actuator is then used for slow drifts correction in a feedback loop. The results show the capability of this simple approach for OPA/OPCPA CEP-stabilized setups.
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Starting from a femtosecond ytterbium-doped fiber amplifier, we demonstrate the generation of near Fourier transform-limited high peak power picosecond pulses through spectral compression in a nonlinear solid-state-based multipass cell. Input 260 fs pulses negatively chirped to 2.4 ps are spectrally compressed from 6 nm down to 1.1 nm, with an output energy of 13.5 µJ and near transform-limited pulses of 2.1 ps. A pulse shaper included in the femtosecond source provides some control over the output spectral shape, in particular its symmetry. The spatial quality and spatio-spectral homogeneity are conserved in this process. These results show that the use of multipass cells allows energy scaling of spectral compression setups while maintaining the spatial properties of the laser beam.
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Light beams carrying orbital angular momentum (OAM) have led to stunning applications in various fields from quantum information to microscopy. We examine OAM from the recently observed high-harmonic generation (HHG) in semiconductor crystals. HHG from solids could be a valuable approach for integrated high-flux short-wavelength coherent light sources. First, we verify the transfer and conservation of the OAM in the strong-field regime of interaction from the generation laser to the harmonics. Secondly, we create OAM beams by etching a spiral zone structure directly at the surface of a zinc oxide crystal. Such diffractive optics act on the generated harmonics and produces focused optical vortices with sub-micrometric size.
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We demonstrate self-compression of short-wavelength infrared pulses in a multipass cell (MPC) containing a plate of silica. Nonlinear propagation in the cell in the anomalous dispersion regime results in the generation of 14 µJ 22 fs pulses at 125 kHz repetition rate and 1550 nm wavelength. Periodic focusing inside the cell allows us to circumvent catastrophic self-focusing, despite an output peak power of 440 MW well beyond the critical power in silica of 10 MW. This technique allows straightforward energy scaling of self-compression setups and control over the spatial manifestation of Kerr nonlinearity. More generally, MPCs can be used to perform, at higher energy levels, temporal manipulations of pulses that have been previously demonstrated in waveguides.
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Multiphoton microscopy combined with genetically encoded fluorescent indicators is a central tool in biology. Three-photon (3P) microscopy with excitation in the short-wavelength infrared (SWIR) water transparency bands at 1.3 and 1.7 µm opens up new opportunities for deep-tissue imaging. However, novel strategies are needed to enable in-depth multicolor fluorescence imaging and fully develop such an imaging approach. Here, we report on a novel multiband SWIR source that simultaneously emits ultrashort pulses at 1.3 and 1.7 µm that has characteristics optimized for 3P microscopy: sub-70 fs duration, 1.25 MHz repetition rate, and µJ-range pulse energy. In turn, we achieve simultaneous 3P excitation of green fluorescent protein (GFP) and red fluorescent proteins (mRFP, mCherry, tdTomato) along with third-harmonic generation. We demonstrate in-depth dual-color 3P imaging in a fixed mouse brain, chick embryo spinal cord, and live adult zebrafish brain, with an improved signal-to-background ratio compared to multicolor two-photon imaging. This development opens the way towards multiparametric imaging deep within scattering tissues.
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We propose and demonstrate an OPCPA architecture emitting few-cycle pulses at 3070 nm and 1550 nm based on a high-energy femtosecond ytterbium-doped fiber amplifier pump. The short pump pulse duration allows direct seeding by a supercontinuum in the 1.4 - 1.7 µm signal range, generated in bulk YAG. It also allows a simplified dispersion management along the system and broad optical gain bandwidth. The dual output system delivers 20 µJ, 49 fs signal pulses at 1550 nm and 10 µJ, 72 fs idler pulses at 3070 nm. Power scaling limitations due to beam distortion in the last MgO:PPLN-based OPCPA stage are discussed and investigated.
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An error in the rationale presented in the paper "Contradiction within wave optics and its solution within a particle picture" by Altmann [Opt. Express 23, 3731 (2015)10.1364/OE.23.003731] is discussed.
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The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.
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A hybrid ytterbium-doped fiber-bulk laser source allowing the generation of 3 ps, 350 µJ, 116 MW peak power Fourier transform-limited pulses at 50 kHz repetition rate and 1030 nm wavelength is described. Pulse duration tunability is provided by an adjustable spectral compression-based seeder system. Energy scaling capabilities of the architecture by use of the divided-pulse amplification method are investigated. This source provides a robust, compact, and versatile solution for applications such as RF photocathode guns, x- and γ-ray generation by inverse Compton scattering, and optical parametric chirped-pulse amplification pumping.
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We report on the generation of 34 fs and 50 µJ pulses from a high energy fiber amplifier system with nonlinear compression in an air-filled hypocycloid-core Kagome fiber. The unique properties of such fibers allow bridging the gap between solid core fibers-based and hollow capillary-based post-compression setups, thereby operating with pulse energies obtained with current state-of-the-art fiber systems. The overall transmission of the compression setup is over 70%. Together with Yb-doped fiber amplifier technologies, Kagome fibers therefore appear as a promising tool for efficient generation of pulses with durations below 50 fs, energies ranging from 10 to several hundreds of µJ, and high average powers.
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Active coherent beam combination using a 7-non-coupled core, polarization maintaining, air-clad, Yb-doped fiber is demonstrated as a monolithic and compact power-scaling concept for ultrafast fiber lasers. A microlens array matched to the multicore fiber and an active phase controller composed of a spatial light modulator applying a stochastic parallel gradient descent algorithm are utilized to perform coherent combining in the tiled aperture geometry. The mitigation of nonlinear effects at a pulse energy of 8.9 µJ and duration of 860 fs is experimentally verified at a repetition rate of 100 kHz. The experimental combining efficiency results in a far field central lobe carrying 49% of the total power, compared to an ideal value of 76%. This efficiency is primarily limited by group delay differences between cores which is identified as the main drawback of the system. Minimizing these group delay issues, e.g. by using short and straight rod-type multicore fibers, should allow a practical power scaling solution for femtosecond fiber systems.
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We demonstrate a simple and robust passive coherent combining technique for temporal compression of millijoule energy laser pulses down to few-cycle duration in a gas-filled hollow fiber. High combining efficiency is achieved by using carefully oriented calcite plates for temporal pulse division and recombination. Carrier-envelope phase (CEP)-stable, 6-fs, 800-nm pulses with more than 0.6 mJ energy are routinely generated. This method could aid in the energy scaling of CEP-stable hollow-fiber compressor systems.
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We report the measurement of spectral and spatial correlations in supercontinua generated by focusing microjoule pulses from a femtosecond ytterbium-doped fiber amplifier laser in bulk YAG. The measurement is full-bandwidth at a repetition rate of 1 MHz owing to the use of time-stretch dispersive Fourier transform technique. In contrast with fiber-based supercontinuum generation, our results show an excellent stability of the spectral and spatial properties of the output supercontinuum, with an essentially correlated behavior in the 1.4-1.7 µm wavelength range. These results provide strong ground for the development of supercontinuum-seeded ultrafast optical parametric amplifier systems in the mid-infrared using ytterbium lasers as pump sources.
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We report on the generation of 1.1-mJ, 300-fs pulses at 50 kHz by implementing an amplifier architecture whereby four stretched pulse replicas are created in the temporal and spatial domains, allowing pulse energy scaling by the same factor. The whole spatiotemporal coherent combining geometry is passive, avoiding the need for active electronic stabilization loop systems. The combining efficiency remains above 90% at all power levels.
