Parametric amplification of 100 fs mid-infrared pulses in ZnGeP2 driven by a Ho:YAG chirped-pulse amplifier

Nonlinear compression of few-cycle multi-mJ 5 µm pulses in ZnSe around zero-dispersion

We present a compact nonlinear compression scheme for the generation of millijoule few-cycle pulses beyond 4 µm wavelength. For this purpose 95 fs pulses at 5 µm from a 1 kHz midwave-IR optical parametric chirped pulse amplifier (OPCPA) are spectrally broadened due to a self-phase modulation in ZnSe. The subsequent compression in a bulk material yields 53 fs pulses with 1.9 mJ energy. The compression succeeds efficiently with only slight beam distortions and an energy throughput of 85%, which results in a peak power of 34 GW. The nonlinear refractive index of ZnSe was derived from the nonlinear compression and self-focusing measurements. Furthermore, we explore to which extent multiphoton absorption affects the nonlinear compression regime.

Highly efficient octave-spanning long-wavelength infrared generation with a 74% quantum efficiency in a χ(2) waveguide

The realization of compact and efficient broadband mid-infrared (MIR) lasers has enormous impacts in promoting MIR spectroscopy for various important applications. A number of well-designed waveguide platforms have been demonstrated for MIR supercontinuum and frequency comb generations based on cubic nonlinearities, but unfortunately third-order nonlinear response is inherently weak. Here, we propose and demonstrate for the first time a χ(2) micrometer waveguide platform based on birefringence phase matching for long-wavelength infrared (LWIR) laser generation with a high quantum efficiency. In a ZnGeP2-based waveguide platform, an octave-spanning spectrum covering 5-11 μm is generated through optical parametric generation (OPG). A quantum conversion efficiency of 74% as a new record in LWIR single-pass parametric processes is achieved. The threshold energy is measured as ~616 pJ, reduced by more than 1-order of magnitude as compared to those of MIR OPGs in bulk media. Our prototype micro-waveguide platform could be extended to other χ(2) birefringence crystals and trigger new frontiers of MIR integrated nonlinear photonics.

Mid-infrared frequency domain optical parametric amplifier

We report on an optical architecture delivering sub-120 femtosecond laser pulses of 20 µJ tunable from 5.5 µm to 13 µm in the mid-infrared range (mid-IR). The system is based on a dual-band frequency domain optical parametric amplifier (FOPA) optically pumped by a Ti:Sapphire laser and amplifying 2 synchronized femtosecond pulses each with a widely tunable wavelength around 1.6 and 1.9 µm respectively. These amplified pulses are then combined in a GaSe crystal to produce the mid-IR few-cycle pulses by means of difference frequency generation (DFG). The architecture provides a passively stabilized carrier-envelope phase (CEP) whose fluctuations has been characterized to 370 mrad RMS.

Mid-infrared quasi-parametric chirped-pulse amplification based on Sm:LGN crystals

We numerically demonstrate highly efficient mid-infrared quasi-parametric chirped-pulse amplification (QPCPA) based on a recently developed Sm³⁺-doped La₃Ga_5.5Nb_0.5O₁₄ (Sm:LGN) crystal. At pump wavelength around 1 µm, the broadband absorption of Sm³⁺ on idler pulses can enable QPCPA for femtosecond signal pulses centered at 3.5 or 5 µm, with a conversion efficiency approaching the quantum limit. Due to suppression of back conversion, such mid-infrared QPCPA exhibits robustness against phase-mismatch and pump-intensity variation. The Sm:LGN-based QPCPA will provide an efficient approach for converting currently well-developed intense laser pulses at 1 µm to mid-infrared ultrashort pulses.

Adaptive optics system for a short wavelength mid-IR laser based on a Shack-Hartmann wavefront sensor and analysis of thermal noise impacts

We present an adaptive optics (AO) system for a 1.94-µm laser source. Our system consists of a home-made Shack-Hartmann wavefront sensor and silver-coated bimorph deformable mirror operating in a closed-loop control scheme. The wavefront sensor used an uncooled vapor phase deposition PbSe focal-plane array for the actual light sensing. An effect of thermal afterimage was found to be reducing the centroid detection precision significantly. The effect was analyzed in detail and finally has been dealt with by updating the background calibration. System stability was increased by reduction of control modes. The system functionality and stability were demonstrated by improved focal spot quality. By replacing some of the used optics, the range of the demonstrated mid-IR AOS could be extended to cover the spectral range of 1-5 µm. To the best of our knowledge, it is the first AO system built specifically for mid-IR laser wavefront correction.

Ultra-broadband long-wave-infrared pulse production using a chirped-pulse difference-frequency generation

We present a broadband light source based on near-infrared chirped-pulse difference-frequency mixing that is suitable for seeding long-wave-infrared (LWIR) optical parametric chirped-pulse amplification (OPCPA). A nitrocellulose pellicle is used in a Ti:sapphire regenerative amplifier to generate dual-frequency output pulses, which are subsequently mixed in a 0.4-mm thick AgGaS₂ crystal. LWIR pulses with ∼1 µm full width at half maximum (FWHM) bandwidth centered at 10.5 µm are generated by mixing transform-limited pulses. Assisted by genetic algorithm optimization, the bandwidth is broadened to ∼3 µm FWHM within the 8-12 µm atmospheric transmission window. The seed source paves the path towards tabletop ultrafast terawatt-class passively carrier-envelope-phase stabilized OPCPA in the LWIR region.

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm

We present a setup for generating broadband (up to 1050 cm^-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Simultaneously Wavelength- and Temperature-Insensitive Mid-Infrared Optical Parametric Amplification with LiGaS2 Crystal

Ultrafast mid-infrared (mid-IR) lasers with a high pulse repetition rate are in great demand in various fields, including attosecond science and strong-field physics. Due to the lack of suitable mid-IR laser gain medium, optical parametric amplifiers (OPAs) are used to generate an ultrafast mid-IR laser. However, the efficiency of OPA is sensitive to phase mismatches induced by wavelength and temperature deviations from the preset points, which thus limits the pulse duration and the average power of the mid-IR laser. Here, we exploited a noncollinear phase-matching configuration to achieve simultaneously wavelength- and temperature-insensitive mid-IR OPA with a LiGaS2 crystal. The noncollinearity can cancel the first-order dependence of phase matching on both wavelength and temperature. Benefitting from the thermal property of the LiGaS2 crystal, some collinear phase-matching solutions derived from the first-order and even third-order wavelength insensitivity have comparatively large temperature bandwidths and can be regarded as approximate solutions with simultaneous wavelength and temperature insensitivity. These simultaneously wavelength- and temperature-insensitive phase-matching designs are verified through numerical simulations in order to generate few-cycle, high-power mid-IR pulses.

Millijoule 265 fs Tm:YAP regenerative amplifier for driving ultrabroad band collinear mid-infrared optical parametric amplifiers

Generation of 265-fs millijoule pulses at 1940 nm from a solid-state regenerative amplifier has been demonstrated. The amplification chain consists of a thulium-doped fluoride (Tm:ZBLAN) fiber oscillator, a two stage Tm:ZBLAN fiber preamplifier, and a regenerative amplifier with a thermoelectrically cooled thulium-doped yttrium aluminium perovskite crystal. The newly developed light source is used for pumping an ultra broadband mid-infrared optical parametric amplifier based on a gallium selenide crystal. The 2.5-4 µm range of a multioctave supercontinuum, generated in a polarization-maintaining ZBALN fiber, is used as the MIR seed. The amplified signal in combination with the corresponding idler pulses spread from 2.5 to 10 µm in a collinear geometry.

High-Power Solid-State Near- and Mid-IR Ultrafast Laser Sources for Strong-Field Science

This review highlights the development of ultrafast sources in the near- and middle-IR range, developed in the laboratory of Nonlinear Optics and Superstrong Laser Fields at Lomonosov Moscow State University. The design of laser systems is based on a powerful ultrafast Cr:Forsterite system as a front-end and the subsequent nonlinear conversion of radiation into the mid-IR, THz, and UV spectral range. Various schemes of optical parametric amplifiers based on oxide and non-oxide crystals pumped with Cr:Forsterite laser can receive pulses in the range of 4–6 µm with gigawatt peak power. Alternative sources of mid-IR ultrashort laser pulses at a relatively high (MHz) repetition rate are also proposed as difference frequency generators and as a femtosecond mode-locked oscillator based on an Fe:ZnSe crystal. Iron ion-doped chalcogenides (Fe:ZnSe and Fe:CdSe) are shown to be effective gain media for broadband high-peak power mid-IR pulses in this spectral range. The developed sources pave the way for advanced research in strong-field science.

Few-cycle pulses tunable from 3 to 7 µm via intrapulse difference-frequency generation in oxide LGN crystals

An ultrashort mid-infrared (IR) source beyond 5 µm is crucial for a plethora of existing and emerging applications in spectroscopy, medical diagnostics, and high-field physics. Nonlinear generation of such sources from well-developed near-IR lasers, however, remains a challenge due to the limitation of mid-IR crystals. Based on oxide La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystals, here we report the generation of femtosecond pulses tunable from 3 to 7 µm by intrapulse difference-frequency generation of 7.5 fs, 800 nm pulses. The efficiency and bandwidth dependences on pump polarization and crystal length are studied for both Type-I and Type-II phase-matching configurations. Maximum pulse energy of ∼10nJ is generated at 5.2 µm with a conversion efficiency of ∼0.14%. Because of the few-cycle pump pulse duration, the generated mid-IR pulses are as short as about three cycles. These results, to the best of our knowledge, represent the first experimental demonstration of LGN in generating mid-IR ultrashort pulses.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP₂-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼_x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 µm

We report experimental demonstration of graphene mode-locked operation of ${{\rm Tm}^{3 + }}\!:\!{{\rm YLiF}_4}$Tm³⁺:YLiF₄ (YLF) and ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm³⁺:KY₃F₁₀ (KYF) lasers near 2.3 µm. To scale up the intracavity pulse energy, the cavity was extended, and double-end pumping was employed with a continuous-wave, tunable ${{\rm Ti}^{3 + }}\!:\!{\rm sapphire}$Ti³⁺:sapphire laser delivering up to 1 W near 780 nm. The extended ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser cavity was purged with dry nitrogen to eliminate pulsing instabilities due to atmospheric absorption lines, but this was not needed in the case of the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser, 921 fs pulses were generated at a repetition rate of 17.2 MHz at 2304 nm. 739 fs pulses were obtained at the repetition rate of 54 MHz from the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser, and 1.2 nJ and 1.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

Generation and characterization of mid-infrared supercontinuum in polarization maintained ZBLAN fibers

We present mid-infrared (MIR) supercontinuum generation in polarization-maintained ZBLAN fibers pumped by 2 µm femtosecond pulses from a Tm:YAP regenerative amplifier. A stable supercontinuum that spreads from 380 nm to 4 µm was generated by coupling only 0.5  µJ pulse energy into an elliptical core ZBLAN fiber. The supercontinuum was characterized using cross-correlation frequency-resolved optical gating (XFROG). The complex structure of the XFROG trace due to the pulse-to-pulse spectrum instability have been fixed by reducing the length of the applied fibers or improving the quality of the incident pulse spectrum.

9 μm few-cycle optical parametric chirped-pulse amplifier based on LiGaS2

We report a long-wavelength mid-infrared (mid-IR), few-cycle optical parametric chirped-pulse amplifier (OPCPA) based on LiGaS₂ crystals, pumped by a 1 μm Yb:YAG laser, at a 10 kHz repetition rate. The mid-IR OPCPA system generates pulses centered at 9 μm, with 1 4 μJ pulse energy and 140 mW average power. A 142 fs pulse width, which corresponds to less than 5 optical cycles at 9 μm, is measured by an interferometric autocorrelator. This is, to the best of our knowledge, the first long-wavelength mid-IR OPCPA pumped at 1 μm wavelength. It paves the way for the energy and power scaling of the ultrafast long-wavelength mid-IR lasers by utilizing advanced high-energy, high-power 1 μm pump lasers.

Multimicrojoule GaSe-based midinfrared optical parametric amplifier with an ultrabroad idler spectrum covering 4.2-16 μm

We report a multimicrojoule, ultrabroadband midinfrared optical parametric amplifier based on a GaSe nonlinear crystal pumped at ∼2  μm. The generated idler pulse has a flat spectrum spanning from 4.5 to 13.3 μm at -3  dB and 4.2 to 16 μm in the full spectral range, with a central wavelength of 8.8 μm. The proposed scheme supports a subcycle Fourier-transform-limited pulse width. A (2+1)-dimensional numerical simulation is employed to reproduce the obtained idler spectrum. To our best knowledge, this is the broadest -3  dB spectrum ever obtained by optical parametric amplifiers in this spectral region. The idler pulse energy is ∼3.4  μJ with a conversion efficiency of ∼2% from the ∼2  μm pump to the idler pulse.

Eye safety implications of high harmonic generation in zinc selenide

Polycrystalline zinc selenide (ZnSe) has been the subject of many nonlinear optics studies for wavelengths under 4.0 µm including sum/difference frequency generation, harmonic generation, and filamentation. In this report, the conversion efficiency of high harmonic generation (HHG) in ZnSe is quantified for mid-infrared wavelengths ranging from 2.7 µm to 8.0 µm. By increasing the fundamental wavelength, we demonstrate that HHG in thick ZnSe targets is limited by the band gap. The high conversion efficiency of mid-infrared to near-infrared light in ZnSe raises concerns of a nonlinear retinal hazard. We contrast the HHG behavior of ZnSe against the observed harmonic generation of calcium fluoride, BK7, and fused silica over the same wavelengths.

Millijoule femtosecond pulses at 1937 nm from a diode-pumped ring cavity Tm:YAP regenerative amplifier

We present an infrared source operating at 1937 nm center wavelength capable of generating 1.35 mJ pulse energies with 1 kHz repetition rate and 2 GW peak power based on a diode-pumped Tm:YAP regenerative amplifier. The obtained pulses after 45 round trips have been compressed down to 360 fs. Using only a small portion (15 μJ) of the output of the system we managed to generate a white light continuum in a 3 mm YAG window that exhibits the viability of the system as a suitable candidate for a pumping source of a mid-infrared optical parametric amplifier.

Multi-octave supercontinuum generation in YAG pumped by mid-infrared, multi-picosecond pulses

High-energy, multi-octave supercontinuum (SC) generation in bulk media pumped with picosecond pulses in the mid-infrared, though pivotal in a myriad of applications, poses severe constraints due to wavelength scaling of the critical power criterion and the propensity to induce avalanche-ionization-seeded breakdown mechanisms. Here, we demonstrate a simple experimental geometry, relying on a very low numerical aperture for the pump pulse, and a crystal length commensurate with the Rayleigh length of the focusing geometry, generating a multi-octave, stable SC in yttrium aluminum garnet (YAG). The SC ranges from 500 nm to 3.5 μm (measured at -30  dB with spectral components at wavelengths up to 4.5 μm) when pumped by a 3 ps pulse centered at 2.05 μm in the anomalous dispersion regime. We also investigate the dynamics of filament formation in this interaction regime by monitoring the spectral and temporal evolution of the pulse during its propagation through the length of the crystal.

Multi-watt, multi-octave, mid-infrared femtosecond source

Spectroscopy in the wavelength range from 2 to 11 μm (900 to 5000 cm^-1) implies a multitude of applications in fundamental physics, chemistry, as well as environmental and life sciences. The related vibrational transitions, which all infrared-active small molecules, the most common functional groups, as well as biomolecules like proteins, lipids, nucleic acids, and carbohydrates exhibit, reveal information about molecular structure and composition. However, light sources and detectors in the mid-infrared have been inferior to those in the visible or near-infrared, in terms of power, bandwidth, and sensitivity, severely limiting the performance of infrared experimental techniques. This article demonstrates the generation of femtosecond radiation with up to 5 W at 4.1 μm and 1.3 W at 8.5 μm, corresponding to an order-of-magnitude average power increase for ultrafast light sources operating at wavelengths longer than 5 μm. The presented concept is based on power-scalable near-infrared lasers emitting at a wavelength near 1 μm, which pump optical parametric amplifiers. In addition, both wavelength tunability and supercontinuum generation are reported, resulting in spectral coverage from 1.6 to 10.2 μm with power densities exceeding state-of-the-art synchrotron sources over the entire range. The flexible frequency conversion scheme is highly attractive for both up-conversion and frequency comb spectroscopy, as well as for a variety of time-domain applications.

Highly efficient optical parametric amplifier tunable from near- to mid-IR for driving extreme nonlinear optics in solids

We have developed a robust optical parametric amplifier (OPA) based on three-AgGaS₂-crystal pumping by a Cr:Forsterite GW femtosecond laser system, generating 150 fs pulses in dual bands of 1.6-2.0 μm (signal wave) and 3.5-5.5 μm (idler wave). By introducing a negative prechirp to the pump, a combined efficiency in two waves of greater than 10% was achieved, with signal energy up to 110 μJ and idler energy up to 43 μJ. Operation parameters of the system (intensity up to 90  TW/cm²) make OPA a promising tool for driving nonlinear optical phenomena, including generation of optical harmonics and laser-induced extreme states of matter in solids and liquids. As a proof of principle, we generated harmonics up to the sixth order and sum frequencies in 5 mm thick polycrystalline ZnSe.

Kerr-lens mode-locked 2.3-μm Tm3+:YLF laser as a source of femtosecond pulses in the mid-infrared

We report what is to our knowledge a new source of femtosecond pulses in the mid-infrared, based on a Kerr-lens mode-locked (KLM) Tm³⁺:YLF laser at 2303 nm. An undoped ZnSe substrate was included in the resonator to provide enhanced nonlinear phase modulation during KLM operation. The Tm³⁺:YLF laser was end-pumped with a continuous-wave Ti³⁺ : sapphire laser at 780 nm. With 880 mW of pump power, we generated 514-fs pulses at a pulse repetition rate of 41.5 MHz with an average power of 14.4 mW. The spectral width (full width at half-maximum) was measured as 15.4 nm, giving a time-bandwidth product of 0.44. We foresee that the wide availability of this gain medium, as well as the straightforward pumping scheme near 800 nm, will make 2.3-μm, mode-locked Tm³⁺:YLF lasers versatile sources of ultrashort pulses in the mid-infrared.

5 μm few-cycle pulses with multi-gigawatt peak power at a 1 kHz repetition rate

A mid-infrared (mid-IR) optical parametric chirped pulse amplification (OPCPA) system generating few-cycle pulses with multi-gigawatt peak power at a 1 kHz repetition rate is reported. The system is pumped by a highly stable 2 μm picosecond chirped pulse amplifier based on Ho:YLF gain media to exploit the high nonlinearity of ZnGeP₂ (ZGP) crystals for parametric amplification. The ZGP optical parametric amplification (OPA) is characterized by a high conversion efficiency of >10 %, resulting in 1.3 mJ idler pulses at a center wavelength of 5.1 μm. Employing a dispersion management scheme based only on bulk materials, pulses as short as 160 fs are obtained. By adding a spatial light modulator in the OPCPA setup, the pulses are further recompressed to 75 fs duration which corresponds to less than five optical cycles. Taking into account the pulse energy of 0.65 mJ in this configuration, it translates into a peak power of 7.7 GW. The achieved pulse durations and peak powers, to the best of our knowledge, represent record values for high-energy mid-IR OPCPAs beyond 4 μm.