Progress on intra-pulse difference frequency generation in femtosecond laser

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.

In optical experiments involving variably-delayed ultrashort pulses, the precise calibration of the delay axis is of paramount importance. This is particularly challenging if interferometric delay tracking (with an auxiliary, monochromatic laser [1] ) cannot readily be implemented, such as in the case when the pulses originate from separate modelocked oscillators with detuned repetition frequencies. In the new scheme of electro-optic delay tracking [2] (i) an optical-phase-stable mid-infrared (MIR) waveform is obtained via intrapulse difference frequency generation (IPDFG) driven by an ultrashort near-infrared pulse and (ii) electro-optically sampled (EOS) using a second (independent) near-infrared gate pulse. IPDFG grants (sub-)attosecond lock of the intensity envelope of the driving near-infrared pulse to resulting MIR waveform [3] , while both IPDFG and EOS exhibit (multi-)percent quantum efficiencies [4] being 2 nd -order nonlinear processes. Here, we demonstrate the elongation of a MIR pulse obtained by IPDFG via a narrowband, Fabry-Perot type multilayer optical filter, providing a monochromatic MIR waveform for electro-optic delay tracking which is robust against variations of the initial MIR pulse. The zero crossings of the resulting MIR waveform are reproducible down to a few tens of attoseconds over a time window of 8 ps, which is sufficient for most condensed-phase measurements.

Intrapulse difference-frequency generation (IPDFG) is a relatively simple technique to produce few-cycle mid-infrared (MIR) radiations. The conversion efficiency of IPDFG could be potentially improved by using the long driving wavelength to reduce the quantum defect. In this paper, we report a high-energy MIR IPDFG source with a record-high conversion efficiency of up to 5.3%, driven by 3 µm, 35 fs, 10 kHz pulses. The IPDFG output has a 5 µJ pulse energy and 50 mW average power. It spans over a spectral range from 6 to 13.2 µm. A 68 fs of IPDFG pulse width is measured, corresponding to 2.1 cycles, centered at 9.7 µm. The high-energy, two-cycle IPDFG pulses are used to produce a 3-octave supercontinuum in a KRS-5 crystal, spanning from 2 to 16 µm, with a 2.4 µJ pulse energy and a 24 mW average power.

We report on the generation of optical pulses with a nearly one octave-spanning spectrum ranging from 1300 nm to 2500 nm at 1 kHz repetition rate, which are based on intra-pulse difference frequency generation (DFG) in β-barium borate crystal (β-BBO) and passively carrier-envelope-phase (CEP) stabilized. The DFG is induced by few-cycle pulses initiated from spectral broadening in multiple thin plates driven by a Ti: sapphire chirped-pulse amplifier. Furthermore, a numerical simulation is developed to estimate the conversion efficiency and output spectrum of the DFG. Our results show that the pulses from the DFG have the potential for seeding intense mid-infrared (MIR) laser generation and amplification to study strong-field physics and attosecond science.

The development of high-power, ultrafast coherent light sources in the mid-infrared (MIR) spectral region is subject to intensive research, due to the promise of improving both time- and frequency-domain spectroscopic methods, in particular for background-free field-resolved spectroscopy (FRS) [1,2]. In FRS, the molecular vibrational response of a sample upon impulsive, resonant excitation is measured in the time domain, e.g., via electro-optic sampling (EOS). Recent efforts have identified frequency down-conversion via intra-pulse difference frequency generation (IPDFG) as a viable route for producing high-power, waveform-stable MIR pulses for FRS [2,3]. With 2-μm driving sources, broader bandwidths [4,5] and higher efficiencies [5] can be achieved compared to 1-μm and 1.5-μm, in part due to a wider variety of nonlinear crystals with a high nonlinearity. Here, we present IPDFG driven by a 2-μm, thulium-fibre chirped-pulse amplification (CPA) system. This source uniquely combines broad bandwidth (6–18 μm), high average power (0.5 W), waveform stability, and a high repetition rate (50 MHz). In addition, we demonstrate for the first time EOS of the MIR using a short, high-power 2-μm sampling pulse.

In recent years, the development of high-power, near-infrared (NIR) femtosecond lasers has enabled a new class of broadband mid-infrared (MIR) radiation sources that combine the spectral coverage of the molecular fingerprint region, a brilliance exceeding even that of 3rd-generation synchrotrons, and outstanding temporal coherence [1–3]. One of the challenges that arises in the development of these sources is the need to separate a tens-of-Watts NIR beam from the broadband MIR beam arising out of a parametric process such as intra-pulse difference frequency generation (IPDFG) in a suitable nonlinear crystal. Conventional coated optics face the issues of adding significant amount of dispersion to a temporally well-compressed pulse and the linear and nonlinear absorption restrict the usable NIR power in many MIR-transparent materials, such as germanium.

Intra-pulse difference frequency generation (IDFG) has been widely applied for generating few-cycle carrier-envelope-phase (CEP) stable mid-infrared (MIR) pulses, whose maximal spectral bandwidth reaches around one octave, limited by the phase-matching bandwidth of crystals. Here, we propose and demonstrate a chirp-optimized cascaded IDFG (CC-IDFG) scheme that overcomes the phase-matching bandwidth limitation of nonlinear crystals. By carefully manipulating the chirp of a supercontinuum pulse in 0.5-1 µm, an ultra-broadband pulse in 1-4 µm is generated by CC-IDFG employing cascaded BBO, MgO:LiNbO3, and BIBO crystals, which supports a sub-cycle MIR pulse. Our study presents an attractive method for generating ultra-broadband pulses, which is significantly helpful for applications such as arbitrary waveform tailoring, isolated attosecond pulse generation, and ultrafast electron dynamics control in materials.

We report a microjoule, 10.3 μm, mid-infrared (MIR) intrapulse difference frequency generation (IPDFG) in GaSe, driven by a 3 μm optical parametric chirped-pulse amplifier. The generated IPDFG pulse spanning from 7 to 15 μm in spectrum is measured with a pulse width of 60 fs, corresponding to 1.8 optical cycles. We experimentally confirmed that the spectral broadening of the driving pulses caused by the self-phase modulation in GaSe plays a crucial role in the process of IPDFG. To the best of our knowledge, this is the first exploration of IPDFG driven at 3 μm wavelength. It provides an approach to generate few-cycle long-wavelength MIR pulses, with the recent rapid development of the 3 μm driving sources.

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. Although IDFG systems pumped with 1.5- or 2-µm ultrashort pulsed lasers have been extensively developed, few systems have been demonstrated with 1-µm lasers, which use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-µm-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm-1 in the fingerprint region (760-1240 cm-1, 8.1-13.1 µm). The 1-µm-based IDFG system allows for additional generations of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 50-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm−1) spectral point spacing and a full bandwidth of >5 THz (>166 cm−1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources.

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ(2) nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive wave generation in the 2.5-3μm region and intrapulse difference frequency generation in the 4-5μm region. By engineering the quasi-phase-matched grating profiles, tunable, narrowband MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment-and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

Coherent mid-infrared (MIR) light has a plethora of important applications ranging from life-science to industrial processes. Simultaneous coverage of this region will enable the parallel detection of various chemicals and enhance the specificity of their detection [1]. One of the most popular broadband infrared generation methods is nonlinear down-conversion from the near-infrared. An effective conversion can be achieved by using phase-matching and quasi-phase-matching in birefringent crystals and crystals with periodically poled structure respectively. Random quasi-phase-matching (RQPM) in poly-crystals is an alternative method that has recently shown great promise [2,3], which results in a gradual growth of the generated signal linear to the propagation length. Compared to generic phase-matching schemes, RQPM offers an unparalleled phase-matching bandwidth that is insensitive to incident angle. In addition, unlike single-crystals, poly-crystals can easily be grown into larger dimensions to enable longer interaction lengths. Here we describe the generation of octave-spanning MIR continuum at over 20 mW of average power based on RQPM driven by a Ho:YAG thin-disk oscillator at 2.1 μm [4]. To the best of our knowledge, this is the first time RQPM has been implemented for intra-pulse difference-frequency generation (DFG). A 1 μm laser system based on a Yb:YAG thin-disk oscillator [5] was also tested as the driving source in this scheme.

We characterize the intensity noise of two mid-infrared (MIR) ultrafast tunable (3.5-11 μm) sources based on difference frequency generation (DFG). While both sources are pumped by a high repetition rate Yb-doped amplifier delivering 200 μJ 300 fs at a central wavelength of 1030 nm, the first is based on intrapulse DFG (intraDFG), and the second on DFG at the output of an optical parametric amplifier (OPA). The noise properties are assessed through measurement of the relative intensity noise (RIN) power spectral density and pulse-to-pulse stability. The noise transfer mechanisms from the pump to the MIR beam is empirically demonstrated. As an example, improving the pump laser noise performance allows reduction of the integrated RIN (IRIN) of one of the MIR source from 2.7% RMS down to 0.4% RMS. The intensity noise is also measured at various stages and in several wavelength ranges in both laser system architectures, allowing us to identify the physical origin of their variation. This study presents numerical values for the pulse to pulse stability, and analyze the frequency content of the RINs of particular importance for the design of low-noise high repetition rate tunable MIR sources and future high performance time-resolved molecular spectroscopy experiments.

We present a mid-infrared (MIR) source based on intra-pulse difference-frequency generation under the random quasi-phase-matching condition. The scheme enables the use of non-birefringent materials whose crystal orientations are not perfectly and periodically poled, widening the choice of media for nonlinear frequency conversion. With a 2μm driving source based on a Ho:YAG thin-disk laser, together with a polycrystalline ZnSe element, an octave-spanning MIR continuum (2.7-20μm) was generated. At over 20mW, the average power is comparable to regular phase-matching in birefringent crystals. A 1μm laser system based on a Yb:YAG thin-disk laser was also tested as a driving source in this scheme. The new approach provides a simplified way for generating coherent MIR radiation with an ultrabroad bandwidth at reasonable efficiency.

Mid-infrared (MIR) ultrashort laser pulses have a wide range of applications in the fields of environmental monitoring, laser medicine, food quality control, strong-field physics, attosecond science, and some other aspects. Recent years have seen great developments in MIR laser technologies. Traditional solid-state and fiber lasers focus on the research of the short-wavelength MIR region. However, due to the limitation of the gain medium, they still cannot cover the long-wavelength region from 8 to 20 µm. This paper summarizes the developments of 8–20 μm MIR ultrafast laser generation via difference frequency generation (DFG) and reviews related theoretical models. Finally, the feasibility of MIR power scaling by nonlinear-amplification DFG and methods for measuring the power of DFG-based MIR are analyzed from the author’s perspective.