High-energy Femtosecond Pulses Research Articles

Investigation of Multimode Self-Similar Pulse Compression in Tapered Photonic Crystal Fibers

In this paper, we theoretically investigate the multimode self-similar pulse compression (MM-SSPC) in tapered photonic crystal fibers. To expand the self-similar theory beyond single-mode scenarios, we have discovered through analytical analysis that the width of the input pulse has a significant impact on perturbing the MM-SSPC process. We proceed to solve the multimode generalized nonlinear Schrödinger equation using numerical methods for various input pulse widths, which leads to the observation of three distinct types of multimode pulse compression. MM-SSPC only takes place when the input pulse width is comparatively longer. Our numerical results are consistent with the predictions of analytical calculations. Our results demonstrate that by using input pulses of 2.5 ps in duration, three modes can be compressed in a self-similar manner to 200 fs, which corresponds to a compression factor of 12.5. Finally, We establish the minimum input pulse width required to enable the MM-SSPC to function for the three specific modes of interest. Our findings provide an effective guidance to design pulse compressor for the generation of pedestal-free, high-energy femtosecond pulses.

Microjoule‐Level Mid‐Infrared Femtosecond Pulse Generation in Hollow‐Core Fibers (Laser Photonics Rev. 17(6)/2023)

Mid-Infrared Femtosecond Pulse Generation in Hollow-Core Fibers In article number 2200882, Ang Deng, Wonkeun Chang, and colleagues present a novel approach to generate high-energy femtosecond pulses in mid-infrared from a gas-filled hollow-core fiber. The efficient conversion of near-infrared pump to mid-infrared is facilitated by engineering the cross-sectional geometry of cladding elements in the hollow-core fiber. It promises new opportunities in molecular spectroscopy.

Synchronized triple-wavelength fiber lasers at 1, 1.55, and 1.9 µm.

Synchronized lasers working at different wavelengths are of great significance for numerous applications, such as high-energy femtosecond pulse emission, Raman microscopy, and precise timing distribution. Here, we report synchronized triple-wavelength fiber lasers working at 1, 1.55, and 1.9 µm, respectively, by combining the coupling and injection configurations. The laser system consists of three fiber resonators gained by ytterbium-doped fiber, erbium-doped fiber, and thulium-doped fiber, respectively. Ultrafast optical pulses formed in these resonators are obtained by passive mode-locking with the use of a carbon-nanotube saturable absorber. A maximum cavity mismatch of ∼1.4 mm is reached by the synchronized triple-wavelength fiber lasers in the synchronization regime by finely tuning the variable optical delay lines incorporated in the fiber cavities. In addition, we investigate the synchronization characteristics of a non-polarization-maintaining fiber laser in an injection configuration. Our results provide a new, to the best of our knowledge, perspective on multi-color synchronized ultrafast lasers with broad spectral coverage, high compactness, and a tunable repetition rate.

Spectrally combined four-diode-pumped femtosecond Ti:sapphire laser with 16.3 nJ pulse and its application to video-rate coherent anti-Stokes Raman scattering spectro-microscopy.

We demonstrate the generation of high-energy femtosecond pulses by a spectrally combined four-diode-pumped Ti:sapphire laser with semi-conductor saturable absorber mirror mode locking. To achieve energy scaling, the laser cavity was extended using a design based on the Herriott multipass cell. The laser operates at a 17.5 MHz repetition rate and generates pulses with energies as high as 16.3 nJ and 80 fs in duration. The signal-to-noise ratio at the fundamental frequency showed an extinction ratio of >50 dB relative to the carrier. This compact single laser was applied to video-rate forward and backward coherent anti-Stokes Raman scattering spectro-microscopy with a pixel dwell time of 122 ns, which is the lowest dwell time ever achieved, to the best of our knowledge.

Few-cycle 65-µJ pulses at 11.4 µm for ultrafast nonlinear longwave-infrared spectroscopy

Low-energy excitations can provide insight into the basic ultrafast nonequilibrium dynamics of condensed matter. High-energy femtosecond pulses in the long-wavelength infrared are required to induce such processes, and can be generated in an optical parametric chirped pulse amplification (OPCPA) system comprising three GaSe stages. A femtosecond Cr:ZnS laser serves as the front-end, providing the seed for the 2.0-µm pump and the 2.4-µm signal pulses without nonlinear conversion processes. The OPCPA system is pumped at 2.05 µm by a picosecond Ho:YLF regenerative amplifier at a 1-kHz repetition rate. The recompressed idler pulses at 11.4 µm have a duration of 185 fs and an unprecedented energy of 65 µJ, corresponding to a pump-to-idler conversion efficiency of 1.2%. Nonlinear transmission experiments in the range of the L2 infrared band of liquid water demonstrate the potential of the pulses for nonlinear vibrational spectroscopy of liquids and solids.

Fingerprint multiplex CARS at high speed based on supercontinuum generation in bulk media and deep learning spectral denoising.

We introduce a broadband coherent anti-Stokes Raman scattering (CARS) microscope based on a 2-MHz repetition rate ytterbium laser generating 1035-nm high-energy (≈µJ level) femtosecond pulses. These features of the driving laser allow producing broadband red-shifted Stokes pulses, covering the whole fingerprint region (400-1800 cm-1), employing supercontinuum generation in a bulk crystal. Our system reaches state-of-the-art acquisition speed (<1 ms/pixel) and unprecedented sensitivity of ≈14.1 mmol/L when detecting dimethyl sulfoxide in water. To further improve the performance of the system and to enhance the signal-to-noise ratio of the CARS spectra, we designed a convolutional neural network for spectral denoising, coupled with a post-processing pipeline to distinguish different chemical species of biological tissues.

中红外可调谐大能量飞秒脉冲激光产生

中红外可调谐大能量飞秒脉冲激光产生

Towards high power broad-band OPCPA at 3000 nm.

High-energy femtosecond laser pulses in the mid-infrared (MIR) wavelength range are essential for a wide range of applications from strong-field physics to selectively pump and probe low energy excitations in condensed matter and molecular vibrations. Here we report a four stage optical parametric chirped pulse amplifier (OPCPA) which generates ultrashort pulses at a central wavelength of 3000 nm with 430 μJ energy per pulse at a bandwidth of 490 nm. Broadband emission of a Ti:sapphire oscillator seeds both the four stage OPCPA 800 nm and the pump line at 1030 nm. The first stage amplifies the 800 nm pulses in BBO using a non-collinear configuration. The second stage converts the wavelength to 1560 nm using difference frequency generation in BBO in a collinear geometry. The third stage amplifies this frequency non-collinearly in KTA. Finally, the fourth stage generates the 3000 nm radiation in a collinear configuration in LiIO 3 due to the broad amplification bandwidth this crystal provides. We compress these pulses to 65 fs by transmission through sapphire. Quantitative calculations of the individual non-linear processes in all stages verify that our OPCPA architecture operates close to optimum efficiency. Low absorption losses suggest that this particular design is very suitable for operation at high average power and multi kHz repetition rates.

High-energy mid-infrared femtosecond pulses at 3.3 μm directly generated by dual-chirped optical parametric amplification

By employing a dual-chirped optical parametric amplification (DC-OPA) using MgO:LiNbO3 crystals, we generate 31 mJ mid-infrared (MIR) pulses at 3.3 um with a repetition rate of 10 Hz. After passing through a CaF2 bulk compressor which has 70% throughput efficiency, these MIR pulses are compressed to 70 fs (6.3 optical cycles), which is close to the transform-limited duration of 66 fs. Thus, the peak power is evaluated to be 0.3 TW. Our results present notable progress in the generation of high-energy MIR pulses and prove that DC-OPA is a superior method for efficiently generating MIR pulses with few-cycle duration and TW-class peak power.

Direct time-domain shaping of high-energy femtosecond pulses at THz burst frequencies

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.

Fiber-Based High-Energy Femtosecond Pulses Tunable From 920 to 1030 nm for Two-Photon Microscopy

We demonstrate a fiber-based femtosecond laser source, which can deliver 37 MHz, ~100 fs laser pulses tunable from 920 nm to 1030 nm with up to 10 nJ pulse energy. Pumped by an ultrafast Yb-doped fiber laser, a short piece of tapered photonic crystal fiber with lower dispersion was used to generate self-phase-modulation filtered for the tunable pulses. We believe that this new type of high-energy ultrafast fiber laser source can be an ideal solution for the two-photon fluorescence microscopy applications.

High-energy femtosecond fiber laser system and pulse selection based on high-repetition rate KTiOPO4 Pockels cell

A fiber chirped-pulse amplification system with pulse energy as high as 105 μJ is achieved at 200-kHz repetition rate using the rod-type photonic crystal fiber. The whole system’s nonlinearity accumulated in the fiber amplification is effectively suppressed, and the compressed pulse duration of 808 fs is obtained. A 500-kHz high-repetition rate KTiOPO4 Pockels cell is also applied to make the ultrafast laser pulse selection for generating pulse trains with controllable pulse number and pulse splitting without changing the pulse energy. The demonstrated pulse selection and splitting method are useful for processing of different materials and parallel processing. The pulse selection efficiency of the Pockels cell is as high as 96%.

Generation of high-energy femtosecond pulses by use of spectral broadening effects in Yb:YAG thin-disk regenerative amplifiers

As a result of experimentally verified simulations of Yb:YAG regenerative amplifiers utilizing spectral broadening effects due to self-phase modulation, we provide a map for the optimized choice of initial chirp and internal group-delay dispersion (GDD) within the amplifier. We show that the shortest pulse durations can be obtained with negative internal GDD, being, however, limited by the damage threshold of the optical components due to increased peak powers and by a breakup of pulses comparable to the propagation of higher-order solitons. We propose that by usage of increasing amounts of positive internal GDD, along with a corresponding amount of negative initial GDD, the obtainable peak powers after compression can be scaled far beyond the usual gain-bandwidth limitation.

High-power femtosecond Raman frequency shifter

We report on the generation of broadband, high-energy femtosecond pulses centered at 1.28 μm by stimulated Raman scattering in a pressurized hydrogen cell. Stimulated Raman scattering is performed by two chirped and delayed pulses originating from a multi-mJ Ti:sapphire amplifier. The Stokes pulse carries record-high energy of 4.4 mJ and is recompressed down to 66 fs by a reflective grating pair. We characterized the short-wavelength mid-infrared source in view of energy stability, beam profile, and conversion efficiency at repetition rates of 100 and 10 Hz. The demonstrated high-energy frequency shifter will benefit intense THz sources based on highly nonlinear organic crystals.

High-energy multicolor femtosecond pulses in the deep-ultraviolet generated through four-wave mixing induced by three-color pulses

Multicolor deep-ultraviolet femtosecond pulses are generated in a spectral range of 220–300 nm with pulse energies exceeding 1 μJ. Pulses with shorter wavelengths are also generated with the shortest wavelength of 185 nm. These pulses are generated through cascaded four-wave mixing induced in hydrogen gas by use of three-color femtosecond pump pulses at 1200, 800, and 267 nm. The third pulse dramatically enhances the intensities of the multicolor deep-ultraviolet pulses. The cross-phase modulation induced by the two intense near-infrared pulses broadens the spectral width of each multicolor emission, resulting in a spectral width supporting a transform-limited duration shorter than 10 fs.

Effect of geometrical focusing on femtosecond laser filamentation with low pressure

The influence of geometrical focusing on the filamentation of femtosecond laser pulses at various low pressures (<1 atm) in air has been numerically demonstrated. The main peculiarities, such as filamentation dynamics, spatial-temporal evolution and supercontinuum generation manipulated by external geometrical focusing in the low atmospheric pressure regime, are analyzed by numerically solving a spatial-temporal equation for femtosecond laser pulse propagation in air. The results show that those important characteristics are more sensitive to the focal length than the variation of atmospheric pressure. It indicates that suitable design of the focal length will result in further amplitude uniformity and a lack of temporal aberrations in the compressed pulse. This theoretical modelling of pulse shaping optimization is a step to realization of high-energy femtosecond pulse delivery from the Earth’s surface to altitudes of several kilometers up into the atmosphere.

50 nJ 250 fs all-fibre Raman-free dissipative soliton oscillator

The regime of highly chirped dissipative solitons is a powerful technique for generating high-energy femtosecond pulses. In this letter we demonstrate a successful scaling of the pulse energy in the all-normal-dispersion all-fibre configuration by increasing the cavity length and the mode-field diameter of the fibre simultaneously. Using a PM fibre of 10 mkm core diameter, it appears possible to lengthen the cavity up to 40 m, thus generating highly chirped pulses with energy above 50 nJ, whereas further growth is limited by the Raman effect. We have also found that the Raman threshold and compressed pulse duration strongly depend on the bandwidth of the intracavity spectral filter. The threshold length is increased by a reduction of the filter bandwidth at the expense of larger compressed pulse duration (250 fs instead of 150 fs). Further energy upscaling by means of core enlargement is shown to be possible, but for that one should waive the all-fiber design, or use custom-made fibre components.

Monolithic polarization maintaining fiber chirped pulse amplification (CPA) system for high energy femtosecond pulse generation at 1.03 µm.

A monolithic polarization maintaining fiber chirped pulse amplification system with 25 cm Yb(3+)-doped high efficiency media fiber that generates 62 µJ sub-400 fs pulses with 25 W at 1.03 µm has recently been demonstrated.

Multicolour nonlinearly bound chirped dissipative solitons.

The dissipative soliton regime is one of the most advanced ways to generate high-energy femtosecond pulses in mode-locked lasers. On the other hand, the stimulated Raman scattering in a fibre laser may convert the excess energy out of the coherent dissipative soliton to a noisy Raman pulse, thus limiting its energy. Here we demonstrate that intracavity feedback provided by re-injection of a Raman pulse into the laser cavity leads to formation of a coherent Raman dissipative soliton. Together, a dissipative soliton and a Raman dissipative soliton (of the first and second orders) form a two (three)-colour stable complex with higher total energy and broader spectrum than those of the dissipative soliton alone. Numerous applications can benefit from this approach, including frequency comb spectroscopy, transmission lines, seeding femtosecond parametric amplifiers, enhancement cavities and multiphoton fluorescence microscopy.

1 kHz, 1.6 μm high energy femtosecond pulses with self-stabilized carrier-envelope phases

This paper reports a passive carrier-envelope phase (CEP) stabilization scheme to generate a 1 kHz, 90 fs, 0.4 mJ, 1.6 μm CEP-stabilized femtosecond pulse train through three optical parametric amplifiers with a total gain of ∼2 × 106. The CEP fluctuations are measured within 240 mrad (rms) for 5 min. The CEP-stabilized pulses are generated by the nonlinear difference frequency between the second harmonic (SH) of the output from the Ti:S femtosecond laser system and the super-continuum (SC) around 533 nm excited by the SH pulses. Our design does not need octave-spanning SC, which is beneficial for improving the passive CEP-stabilization of ultrashort pulses.