This PDF file contains the front matter associated with SPIE Proceedings Volume 11670, including the Title Page, Copyright information, and Table of Contents.

Vibrational spectroscopy is widely used in various fields where non-invasive molecular diagnosis is required, but conventional infrared and Raman spectroscopy techniques suffer from low measurement speed and therefore are used mainly for measuring static samples. Recent advancement of nonlinear frequency generation and conversion techniques allow us to develop sophisticated high-speed measurement techniques that make possible to measure dynamic phenomena with high temporal resolutions. In this talk, I introduce newly developed infrared absorption and Raman scattering spectroscopy techniques enabled by light sources with nonlinear optical techniques, including phase-controlled Fourier-transform infrared spectroscopy, rapid-scan Fourier-transform coherent Raman scattering spectroscopy, complementary vibrational spectroscopy and time-stretch infrared spectroscopy.

Microresonator frequency combs have seen remarkable progress in the past decade. In this work, we investigate optical parametric oscillation in a mm-sized cadmium silicon phosphide (CdSiP₂) whispering gallery resonator for X⁽²⁾ frequency comb generation in the mid-infrared region. The comb formation starts with optical parametric oscillation (OPO) near degeneracy followed by a series of internally pumped second-order nonlinear processes. The first step of OPO has been achieved with thresholds below 100 μW, efficiencies of up to 17 % and wavelength tunability from 2.3 to 5 μm. The control of the wavelength tuning is hampered when the OPO is operated close to degeneracy. Nevertheless, we have observed sideband generation around the pump wavelength, indicating that frequency combs can be generated using this scheme. However, improved control of the wavelength tuning near degeneracy is needed to increase the number of sidebands, leading to broadband frequency combs in the mid-infrared.

A dual frequency comb spectrometer is realized by electro-optic modulation of a 1550 nm laser and subsequent conversion to the mid-infrared by difference-frequency generation (DFG). Using an optical parametric oscillator for the DFG the combs can be tuned from 3 μm to 4.7 μm with 440 comb modes covering 220 GHz (< 6 cm^-1). Trace gas detection of nitrous oxide, carbon dioxide and methane is demonstrated with a 7.2-m-multi-pass cell while a sufficiently low noise-equivalent absorbance is reached in already 1 s. The bandwidth normalized noise-equivalent-absorption coefficient is consistently below 2.8 × 10-6 Hz^-1/2 cm^-1 while the precision of the determined concentrations is better 2 % Hz^-1/2.

We demonstrate a compact mid-IR system using difference frequency generation based on a femtosecond Ytterbium fiber laser system with a repetition rate of 100 MHz. Based on the fundamental 1040 nm beam, we use frequency conversion in a nonlinear fiber to generate a tuneable signal beam for mid-IR generation based on difference frequency generation. We investigate different nonlinear fibers for signal generation between 1100 nm and 1600 nm for optimum mid-IR generation between 3 µm and 10 µm, for which power levels of more than 200 mW have been observed at an emission wavelength of 4 µm.

CdSiP2 (CSP) is a nonlinear optical chalcopyrite semiconductor developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. Two laser architectures were explored to pump CSP crystals at 2 microns. The first was a ring OPO with two CSP crystals that produced 27 W of average power, demonstrating the viability of CSP as a material capable of producing high average power output. The second architecture was an OPO seeded OPA train that was used to directly compare the thermal lenses generated by pumping either CSP or ZGP with high average power 2 micron light. The CSP crystals demonstrated significantly less thermal lensing than the ZGP crystals.

We report a CdSiP₂ (CSP) based seeded optical parametric generator (OPG), emitting sub-nanosecond duration, 3 MHz repetition rate, wavelength tunable mid-infrared (MIR) light at 4.2-4.6 μm. We generate up to 0.25 W at 4.2 μm with a total pump conversion efficiency of 42%. The OPG is pumped by a 1.24 μm Raman fiber amplifier system. This is the first demonstration of pumping CSP with a Raman fiber source in this region, and we show that Raman fiber sources in the near-infrared (NIR) are ideal pump sources for non-critically phasematched (NCPM) CSP devices. Pumping CSP at 1.24 μm permits the use of NCPM whilst decreasing the negative effects of both two-photon absorption and linear absorption losses, when compared to conventional 1 μm pumping. This offers a potential advantage for MIR power scaling of CSP parametric devices due to a reduced thermal load in the crystal from residual pump absorption. The OPG is seeded with a continuous-wave fiber supercontinuum source emitting radiation in the 1.7 μm region, to lower the threshold pump intensity required for efficient conversion. NCPM and temperature tuning of the crystal allow for simple wavelength tuning of the idler radiation. We report on laser damage induced by elevated crystal temperatures, which we propose is linked to the decrease in CSP bandgap energy with increasing temperature.

Mid-infrared (MIR) laser sources are used in a number of applications such as remote sensing, air pollution monitoring, combustion diagnostics, and molecular spectroscopy. Here, we present our work on the development of a MIR laser source based on the difference frequency generation (DFG) process between an external-cavity quantum-cascade-laser tunable over 1750–1835 cm^–1 (pump source) and a CO₂ gas laser tunable over 921–1083 cm^–1 (signal source). The DFG process was realized in a nonlinear, orientation-patterned GaAs crystal, and resulted in an idler spectral range between 667–865 cm^–1 with a linewidth of ~2.3 MHz and an output power of up to ~31 μW. Exploiting the fine tunability of our DFG laser source, we performed high-resolution absorption measurements of ethylene (C₂H₄) and acetylene (C₂H₂).

A diode-pumped actively Q-switched Tm³⁺-doped fiber laser is reported generating pulse energies of 800 μJ, pulse widths of 43 ns and peak powers of 17.5 kW. By using the single-oscillator as a pump source for nonlinear frequency conversion, mid-IR pulse energies of 230 μJ are extracted from a ZnGeP₂ (ZGP) optical parametric oscillator (OPO).

We experimentally and theoretically investigate the nonlinear frequency conversion of transparent chalcogenide optical materials using ultrashort midwave infrared laser pulses at 3.6 microns. Evidence of the structure of second through sixth harmonic generation demonstrates different levels of filamentation related to laser intensity, sample thickness, and sample position. Simulations using a (3+1)D model with experimentally measured n₂ values and random quasi phase matching provide good qualitative agreement with experimental data. Together, the data suggests that focusing geometry and material structure play a significant role in harmonic generation in these materials.

We report on an actively Q-switched high-pulse-energy Ho³⁺:YAG laser in-band pumped by a Tm³⁺-doped fiber laser, both operated at room temperature. The Ho³⁺:YAG active medium inside a plane-plane cavity is pumped using a commercial Tm³⁺-doped fiber laser at 1908 nm from one side. In continuous operation a maximum power of 20.1 W with a slope efficiency of 45.1%, central wavelength of 2090 nm and a beam quality factor M² below 1.5 were achieved. Q-switched operation was achieved using a Brewster-cut acousto-optic modulator (AOM) based on crystalline Quartz. During Q-switching the incident power was kept stable at 47 W to obtain an M² of 1.3 and a stable thermal lens inside the laser crystal. With the variation of the repetition frequency a lower limit of the quasi-continuous pulsed regime was investigated and measured to be approximately 3 kHz. The maximum pulse energy in Q-switching operation was achieved with a repetition frequency of 700 Hz leading to an energy of 15 mJ at 12.1 ns pulse width, corresponding to a peak power of 1.2 MW. The laser showed no sign for a loss of performance during many hours of testing. Using this laser as a pump source for a double resonant OPO, a maximum mid-infrared output power of 6.3 W could be achieved at a repetition frequency of 2 kHz, accompanied by a low threshold power of 1.6 W and a slope efficiency of 49.2%.

We report on current advances in polarization-maintaining (PM) Thulium (Tm³⁺):Holmium (Ho³⁺)-codoped triple-clad fiber (THTF) laser. First fundamental studies were performed in a continuous-wave (CW) regime. A fiber laser with a 7 m active fiber delivered high output powers of up to 180 W for an emission wavelength centered at ~2050 nm. In addition, a setup with a 5 m active fiber was investigated with a slope efficiency of 41.6 % and a close to diffraction limited beam propagation with an M² _x,y < 1:1. Operating in Q-switched regime at a repetition rate of 63 kHz, the pulses of the THTF laser had a pulse width of 45.6 ns and a pulse energy of 760 μJ, resulting in a peak power of 15.7 kW with an average output power of 48 W. With an M² _x,y < 1:2 and a FWHM of 290 pm for the emission spectrum (compared to 54 pm in CW) the fiber laser shows a good basis for efficient frequency conversion.

Using nonlinear interferometers is highly attractive for accessing spectral regions in which the photon detection is technically challenging. Especially for terahertz radiation this is a long-lasting problem, as their photon energy is in the range of a few meV. Therefore, the properties of terahertz photons after interaction with a sample are transferred to visible photons, whose detection can be realized by widely available imaging sensors. We demonstrated this concept with terahertz photons propagating in free space, determining the thickness of polytetrafluoroethylene pla tes by only detecting visible photons. We utilized a nonlinear interferometer with one periodically poled lithium niobate crystal driven by a 660-nm pump source. In the crystal, pairs of visible signal and correlated terahertz photons are created and separated into different paths of the interferometer afterwards. The terahertz photons pass the sample and gain information that can be transferred to the visible photons. To detect the signal photons with an uncooled sCMOS camera, the pump photons are filtered out by narrowband volume Bragg gratings. As the signal photons are mainly generated by down - and up-conversion of thermal photons besidesspontaneous parametric down-conversion, the frequency-angular spectra show nonlinear interference in the Stokes and anti-Stokes regions. This interference can be used to assess information of coatings that are mainly transparent in the terahertz frequency range. Establishing on this first demonstration of a nonlinear interferomete r with terahertz photons, we improved the visibility by a factor of 3 by modifying the experimental setup, helping this concept on its way towards industrial applications.

Second harmonic amplification - a hybridization of optical parametric amplification and second harmonic generation - is a route to ultra-efficient parametric amplification. Requiring the simultaneous phase matching of two parametric wave-mixing processes, it has limited frequency coverage in the collinear geometry in bulk media. Here we show that noncollinear birefringent phase matching can provide wide frequency tunability of second harmonic amplification across the near- and mid-infrared in the materials ZnGeP₂, CdSiP₂, LiNbO₃, β - BaB₂O₄, and KD₂PO₄ in applications designed for accommodating high-energy picosecond pulses generated by solid state lasers. We discuss practical limitations including acceptance angle, phase-matching bandwidth, spatial walk off, and parasitic processes.

Previous demonstrations of coherent combination of continuous-wave difference frequency generators (DFG) emitting at 3.4 μm have proved that coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical parametric oscillators (OPOs). We developed an experiment using indirect phase control to demonstrate coherent combining of mid-infrared optical parametric oscillators. The main improvement from DFG combining is the enhancement in generated power and in conversion efficiency brought by the OPO cavity. But there’s also an additional challenge: to be able to control the cavity modes of the OPO accurately to perform simultaneously real-time wavelength and phase control. In this paper, we present the difficulties encountered to perform efficient coherent combining of mid-infrared emitting OPOs. We detail the different OPO cavity configurations we tested and compare theoretical assessment of their threshold with measurements. The first results of coherent combining of continuous-wave OPOs are then given as well as the methods we use to couple the wavelength and phase control feedback loops. Future work on the combining of pulsed OPOs is finally introduced as well as a comparison between pulsed laser combining and pulsed OPO combining limitations.

The nonlocal response associated with the collective oscillation of conduction electrons (plasmons) in atomically thin films plays a significant role in the optical response. Here we exploit the use of surface plasmons to characterize the nonlinear optical behavior of few atom-thick films. The results show beneficial use of thinner films that are computed employing a full quantum mechanical model that incorporates details of the crystallographic orientation allowing us to exploit the facets (111) and (100) in the random phase approximation. These results facilitate the fabrication of nanophotonic devices based on crystalline metals films which offer lower losses than their amorphous counterparts.

As shown recently, highly multimode (M2~30) radiation of laser diodes may be converted in graded-index fibers into high-quality (M2≤2) Raman beam at Stokes-shifted wavelength. Here we experimentally study output beam profiles for the transmitted pump and generated Stokes beams and develop an analytical balance model enabling theoretical beam profiles both near the Raman threshold and well above the threshold where significant depletion of the pump beam occurs due to the Raman conversion. Comparison of the theory and experiment allows for the identification of main mechanisms of the Raman beam clean up in this system, including linear and nonlinear effects.

As KLTN undergoes its room-temperature transition from a cubic to a tetragonal phase, the emerging ferroelectric domains spontaneously lock into a three-dimensional highly organized lattice formed by 3D vortex-like structures. The result is a material, a ferroelectric supercrystal, with unexpected broadband optical properties, such as giant refraction. I will describe our explorations of enhanced nonlinear optical properties of the supercrystal, with emphasis on second-harmonic generation. Experiments indicate the emergence of a constraint-free wavelength conversion in the form of non-linear Cherenkov emission characterized by a broad angular and spectral acceptance, absence of chromatic walk-off, and negligible diffraction.

Here we report for the first time nonlinear frequency conversion in OP-GaP layers grown by hydride vapor-phase epitaxy on OP-GaAs templates. Multi-grating 3-inch wafer design enabled discrete wavelength tuning via stepped gratings, continuous tuning via fan gratings, and bandwidth engineering via chirped gratings, with 14- 35.2-micron periods that propagated up to 300 microns in a 1.2-mm-thick layer before breakdown. Polished, AR-coated, 3-mm-long OPO crystals were fabricated and pumped at 1040 nm (5.5W, 100 MHz, 2.5 ps) with a Chromacity Yb-fiber laser, yielding output powers of 140, 90, and 60 mW at idler wavelengths of 5.6, 7.8, and 10.7 microns respectively

We present GaAsxP1-x as an attractive ternary material that combines the properties of GaAs and GaP for nonlinear optical applications as it combines the higher nonlinear susceptibility of GaAs with the lower 2PA of GaP for a given x-composition. We discuss the HVPE growth results of GaAsP on plain substrates and on orientation patterned (OP) templates fabricated by the conventional MBE assisted polarity inversion technique with and without the MBE regrowth step. Along with the growth results showing in excess of 500 µm thick growth, we also present the structural and optical properties of the ternary material.

BaGa4Se7 is a promising new nonlinear optical material with a reported bandgap of 2.64 eV, and a broad spectral range out to 18 microns. Our experimental investigations use a variety of light sources from 325 – 442 nm below 160 K revealing the presence of an electron paramagnetic resonance (EPR) spectrum that is tentatively assigned to singly ionized selenium vacancies in the bulk single crystals, while 633 nm light is shown to remove the photoinduced signal. We correlate EPR results with optical data obtained using temperature-dependent absorption, thermoluminescence, and photoluminescence to investigate and characterize a broad absorption band resulting from illumination.

Chalcopyrite and chalcogenide crystals are investigated for optical rectification and electro-optic sampling. Broadband and narrowband terahertz radiation is produced using ZnGeP₂, CdSiP₂, and AgGaSe₂ chalcopyrite crystals, as well as a BaGa₄Se₇ chalcogenide crystal. Broadband terahertz radiation is detected using a ZnGeP₂ chalcopyrite crystal. We envision these crystal classes being used to generate and detect terahertz radiation in research and commercial applications.

Temperature- and wavelength-dependent values of the ordinary (n_o) and extra-ordinary refractive index (n_e) of GaN and 4H-SiC were measured over wavelength ranges of 1.9 to 7 μm and 1.9 – 5.5 μm, respectively, and over a temperature range of 79 to 400 K. Temperature-dependent Sellmeier equations for both GaN and SiC were obtained and thermooptic coefficients determined.

We report on the design of OP-GaAs rib waveguides for frequency conversion in the mid-infrared and explore their performances for parametric generation. The samples used are between 10 and 25 mm long and exhibit quasi-phasematched (QPM) periods from 85 to 100 μm. The waveguides are pumped by a femtosecond erbium-doped fluoride fiber laser combined with a soliton self-frequency shift converter delivering sub-300 fs pulses at a wavelength tunable between 2.8 and 3.3 μm. By adjusting the pump wavelength, our OP-GaAs platform can produce ultrashort pulses widely tunable around 4 and 12 μm for the signal and idler, respectively. These results fit quite well our calculations of QPM curves.

Silicon nitride waveguides offer a high nonlinear refractive index and tight mode confinement, ideal for efficient four-wave mixing (FWM) processes. We present a light source for broadband as well as narrowband coherent anti-Stokes Raman scattering (CARS), with the potential to be set up as an all-integrated device, based on FWM in silicon nitride waveguides. Signal and idler pulses are generated via FWM with only 4 nJ input pulse energy and stimulated using a tunable continuous-wave seed source, such that the idler and residual pump pulses can be used for CARS measurements, enabling chemically-selective label-free imaging across the entire fingerprint region.

We report what we believe to be the first octave-spanning supercontinuum generation from all-normal dispersion waveguide on a silicon-based chip in the mid-infrared. Here we use 205 fs pulses at 4 μm coupled to low loss, 5.0 μm x 2.7 μm cross-section, silicon germanium-on-silicon waveguide operating in tha all-normal dispersion regime. Generated supercontinuum spans more than an octave between 2.8 and 5.7 μm. Besides, simulations show a high degree of coherence across the entire -30 dB spectrum. Such supercontinuum provides an ideal light probe for broadband mid-infrared molecular absorption spectroscopy.

Simulations predict perfect phasematching for difference frequency generation (DFG) in a nonlinear ridge waveguide. It makes use of InGaAsP lattice-matched to InP as a nonlinear ridge waveguide. Both crystals possess high second-order susceptibility χ⁽²⁾ and low loss, making them ideal for second-order nonlinear effects. The design allows two lasers in the telecom spectrum to interact in the nonlinear waveguide and emit in the mid-infrared (mid-IR). The InGaAsP ridge waveguide heterogeneously integrated on silicon-rich silicon nitride achieves phase-matching, resulting in a conversion efficiency η = 4.5 %/W.

Converting single photons from one wavelength to another is of fundamental interest for future quantum communication and computing. Using commercially available lasers and a multimode PPLN waveguide a DFG scheme was set up. Phase-matching was shown in the fundamental transverse mode of the waveguide for wavelengths between 851 nm and 862 nm. The setup is capable of converting up to 87% of photons from 856 nm to 1526 nm in transverse fundamental mode. Simulations were performed showing that the quantum conversion efficiency at 856 nm is representative for powers down to thousands of photons per second.

Nonlinear vibrational and strong-field spectroscopies can tremendously benefit from high-power few-cycle mid-infrared (MIR) laser pulses beyond 5 μm at repetition rates << 10 kHz. Here, we explore the potential of a novel non-oxide crystal, BaGa₄S₇ (BGS), for the direct coverage of the MIR beyond 5 µm starting from Yb-laser sources and report on a μJ-scale, sub-4-cycle, 100-kHz optical parametric amplifier (OPA) at 10 μm. We compare BGS with LiGaS₂ (LGS) with respect to performance in ultrafast MIR OPAs. The unique properties of BGS indicate great potential for nonlinear optical applications, especially due to the much faster development of its manufacturing processes and the larger achievable single-crystal sizes compared to LGS.

High power mid-infrared (MID-IR) laser systems delivering ultrashort pulses at high repetition rates are of considerable interest for vibrational spectroscopy, label-free microscopy, ultrafast dynamics and HHG studies. To meet these requirements, we developed an optical parametric chirped-pulse amplifier (OPCPA) with a difference frequency generation (DFG) stage to provide an efficient platform to down-convert a 200 W Yb-YAG pump laser into the MID-IR range. The MID-IR pulses are tunable from 4.2-11 μm, with a maximum pulse energy around 9 μm with 2.2 μJ at 200 kHz supporting a Fourier limit of 50 fs. In addition, thermal parameters of various nonlinear crystals are reviewed for high power MID-IR OPCPAs.

High power and high repetition rate femtosecond lasers at 1.45–2.40 μm wavelength are critical for many applications in the physical, chemical, and biological sciences, such as microchip electron accelerators and soft-X-ray coherent diffractive imaging. Previously, such systems have been realized by optical parametric amplification from Ti:Sapphire lasers at 800 nm with limited power levels. A novel optical parametric chirped-pulse amplifier (OPCPA), pumped by high-power Yb-doped solid state laser, and combined with bulk crystal white-lightgeneration seeding (WLG) is demonstrated here. The laser system features tunable and broadband operation in the 1.45–2.40 μm spectral range, requiring no complex cooling with a compact footprint. Such systems have recently become commercially available from Class 5 Photonics and allow for scalability up to millijoule pulse energies at 100 W average power.

Active carrier envelope phase (CEP) stabilization in the few-cycle regime is essential for most attosecond experiments, e.g. studying the coherent evolution of electronic structure and dynamics in solids or complex many body phenomena crystals. Here, we present a dual-channel optical parametric chirped-pulse amplifier (OPCPA) design providing CEP stable, sub 9 fs pulses around 800 nm center wavelength as a high-harmonic driver for attosecond experiments. Additionally, a second 1.7 - 2 μm CEP stable outlet is available. Two OPCPA designs (a) high repetition rate and (b) a high pulse-energy system will be demonstrated.

Nonlinear interferometers based on non-degenerate spontaneous parametric down-conversion (SPDC) create a link between separate spectral ranges. This allows for measurements in remote spectral regions while detecting light in easily accessible wavelengths. In our work, we use periodically poled lithium niobate to create correlated signal (visible or near-infrared) and idler (mid-infrared) photon pairs. Using a nonlinear interferometer in Michelson geometry, we obtain broadband mid-infrared spectra from light detected with a silicon avalanche photodiode. Combining the nonlinear interferometer with a measurement scheme in close analogy to classical Fourier-transform infrared spectroscopy allows for sub-wavenumber spectral resolution, which opens up possibilities for applications such as precise spectroscopic gas analysis.

We report on efficient mid-IR difference-frequency generation (DFG) at ~8 μm in orientation-patterned GaAs (OPGaAs), by mixing the signal and idler fields inside a nanosecond, singly-resonant, periodically-poled MgO-doped LiNbO₃ optical parametric oscillator (OPO). The temperature and spectral acceptance bandwidths as well as the DFG output performance are compared for two OP-GaAs samples with different lengths. Temperature tuning of the DFG is studied by implementing a transversely chirped Volume Bragg Grating (VBG) as one of the OPO cavity mirrors for the signal wave. The maximum DFG average power amounts to 215 mW at 8.15 μm for a pulse repetition rate of 35 kHz. The corresponding overall optical conversion efficiency from 1 to 8 μm is ~1.1%.

NASA demands the special laser transmitter for the lidar system to detect water-ice on the Moon and other planetary bodies. Based on the data from the Moon Mineralogy Mapper (M³) instrument, the water ice was on the Moon has been claimed, but such a claim was disputed because OH- and/or H₂O-bearing materials share the absorption line at the wavelength range of 2.8-3 μm. Lunar Flashlight, another mission to explore the surface of Moon (the launch date delays to this year), allows scientists map the minerals on dark side of the Moon, but it still has difficult to resolve the ambiguity mentioned above. The absorption line around 6.08 μm uniquely associated with the bending resonance of H₂O has not any comparable vibration in confounding OH-bearing materials. However, a 6.08 μm laser in the gap between the atmospheric windows, middle-wave infrared (3-5 μm) and long-wave infrared (8-12 μm), has not been commercially available. Our approach is a Q-switched Ho:YLF laser pumped the orientation-pattern Gallium Arsenide optical parametric oscillator (OP-GaAs OPO) for generating high-energy laser pulses at the wavelength of 6.08 m. In the current design, a 1.94 μm Tm:fiber is used as the pump source of Ho:YLF laser. In the compact design, a 1.94 μm laser diode will replace the Tm:fiber laser as the pump source. The combination of proposed 6.08 μm laser and the latest HgCdTe avalanche photodiode (APD) array allow us to design a lidar capable of unambiguously identifying water ice on the Moon and Mars from their respective orbits, enabling novel science and in-situ resource utilization. Our instrument is an enabling technology for the Artemis program and future missions.

We report on the development of a pulsed MOPA laser as a pump source for nonlinear fibers, for supercontinuum generation, or optical parametric generation in the mid-IR. The master oscillator is a directly modulated semiconductor laser, while the power amplifier is a two-stage polarization-maintaining fiber amplifier. Such a laser setup allows a flexible output pulse. We investigate the amplification of pulses from 5 to 50 ns long with pulse frequencies from 10 kHz to 1 MHz at a signal wavelength of 2050 or 2090 nm with the goal of kW peak power level for a rectangular output pulse shape.

We discuss principles, design challenges, performance highlights as well as current limitations of state-of-the-art widely tunable continuous-wave optical parametric oscillators sought to be practical for implementation as turn-key systems. Employing a flexible two-stage design concept that can be adapted to several single-frequency laser pump sources, we demonstrate how a wavelength range from 450 nm up to 3500 nm can be covered almost seamlessly. Emerging keyapplications in the realm of quantum technology, like fundamental studies of novel color centers in diamond, are presented in an illustrative manner.

COVID-19 pandemic is calling for new methods of remote chemical sensing, which can potentially detect small number of viral particles in air at safe distances. Raman spectroscopy is a chemically specific spectroscopic technique; however, it is often difficult to apply it for remote sensing due to the non-directional scattering. We propose and demonstrate a novel way to achieve directional backward emission through nonlinear Raman interactions.

We report efficient amplification of supercontinuum pulses in a stimulated Raman amplifier. The Yb:YAG pump laser produced 1.2 ps transform-limited pulses with an energy of 20 mJ at 100 Hz. Supercontinuum pulses in the wavelength range of 1050 – 2500 nm were obtained in a 15 mm YAG crystal. A larger portion of the laser energy was used to pump a two-stage stimulated Raman amplifier based on Np-cut KGW crystals with the main Stokes shifts of 768 and 901 cm^-1 . Spectral broadening to ~16 nm of amplified pulses at 901 cm^-1, conversion efficiency of 55%, and pulse width of ~145 fs after compression were demonstrated. Spectral synthesis of Stokes components at 768 and 901 cm^-1 provides an even wider spectrum up to ~38 nm, which corresponds to ~50 fs transform-limited pulse. The differences between collinear and non-collinear TSRCPA configurations are investigated in terms of conversion efficiency and gain bandwidth.

Beam self-imaging of ultrashort pulses in nonlinear graded-index (GRIN) multimode optical fibers is of interest for many applications, including spatiotemporal mode-locking in fiber lasers. We obtained a new analytical description for the nonlinear evolution of a laser beam of arbitrary transverse shape propagating in a GRIN fiber. The longitudinal beam evolution could be directly visualized by means of femtosecond laser pulses, propagating in the anomalous or in the normal dispersion regime, leading to light scattering out of the fiber core via the emission of blue photo-luminescence. As the critical power for self-focusing is approached and even surpassed, a host of previously undisclosed nonlinear effects is revealed, including strong multiphoton absorption by oxygen-deficiency center defects and Germanium inclusions, splitting and shifting of the self-imaging period, filamentation, and conical emission of the guided light bullets. We discovered that nonlinear loss has a profound influence on the process of high-order spatiotemporal soliton fission. The beam energy carried by the fiber is clamped to a fixed value, and nonlinear bullet attractors with suppressed Raman frequency shift and fixed temporal duration are generated, leading to highly efficient frequency conversion of the input near-infrared femtosecond pulses into mid-infrared multimode solitons.

Supercontinuum generation in bulk media is not normally observed at the nJ-level pulse energies available from high-repetition-rate femtosecond oscillators. Here, we present results demonstrating how a visible supercontinuum can be produced in bulk orientation-patterned gallium phosphide from 100-MHz 1040-nm femtosecond pulses with energies of up to 32 nJ. High-order parametric gain near 550 nm, seeded by self-phase-modulated spectral sidebands, underpins this new and simple supercontinuum process which yields an output spectrum spanning from the blue/green to the red.

Broadband femtosecond supercontinuum sources find applications in fields such as Optical Coherence Tomography, fluorescence lifetime imaging, and frequency metrology. A mechanism to achieve the required spectral bandwidth is to broaden the output of a femtosecond laser source in nonlinear media such as highly nonlinear fibers (HNLF) utilizing a combination of nonlinear effects such as self-phase modulation (SPM) and four-wave mixing (FWM). However, conventional spectral broadening often suffers from supercontinua with degraded spectral flatness. The profile of the broadened spectrum depends on the properties of the medium, as well as the power and the temporal profile of the input pulse. The pulse can be shaped before broadening to improve the supercontinuum spectrum. However, the envelope is highly sensitive to the pulse spectral phase, potentially time-varying, resulting in a sub-optimal performance with any single pass optimization approach. Here, we overcome this by adaptively optimizing the input pulse by perturbing the spectral phase in an automated closed-control loop. A Fourier pulse shaper modifies the C-band sub-picosecond pulses from a mode-locked fiber laser before spectral broadening in HNLF. An evolutionary strategy algorithm is used to process the measured spectrum and adaptively optimize the spectral phase to realize a smooth supercontinuum with a broad Gaussian spectrum iteratively. We allowed the spectral phase to evolve with multiple variables across the pulse. We achieved a 4X bandwidth enhancement of the input pulse with high fidelity between the supercontinuum spectra and the target Gaussian shape. Spectral fluctuations were <3dB across the bandwidth of the generated supercontinuum.

This presentation recorded for SPIE Photonics West LASE, 2021

A single-frequency blue laser at 461 nm is generated by frequency doubling an amplified diode laser operating at 922 nm via a LBO crystal placed in an ultra-compact resonant linear cavity. The best optical conversion efficiency achieved by the setup is 87% which gives more than 1 W of power in the blue. The frequency-converted beam is characterized in terms of long-term power stability, residual intensity noise, hysteresis induced by the input power stability, and geometrical shape. The generated 461 nm radiation can be used to obtain a magneto-optical trap on strontium 5s^{2 1}S₀ – 5p¹ P₁ transition.

We present updated Sellmeier equations for β-BaB₂O₄ based on measurements of the phase-matching angles for second-harmonic (SHG) and sum-frequency generation (SFG) of a Nd:YAG laser-pumped 90° phase-matched RbTiOAsO₄ optical parametric oscillator (OPO) and a Nd:YAG laser in the 0.6407 - 3.1392 μm range. The phase-matching angles predicted by this index formula agree well with the recently published data points of SFG between a Ti:Al₂O₃ laser and Nd:YAG laser-pumped KTiOAsO₄ OPO in the 0.40 -5.3 μm range {G. Tamosauskas et al., Opt. Mater. Express 8, 1410 (2018)] as wellas those for SFG below 0.2048 μm thus far reported in the literature.

Ultrashort laser pulses in the deep ultraviolet (DUV) based on nonlinear-optical conversion of NIR solid state sources promise advantages for the machining of wide-bandgap materials such as compound semiconductors like indium / aluminum / gallium nitride (InAlGaN, AlN, GaN), silicon carbide (SiC), zinc sulfide (ZnS), or boron nitride (BN). We present systems providing few-picosecond pulses at 257.5 nm with high average power exceeding 20 W of DUV obtained by frequency quadrupling of our proven TruMicro Series of industrial ultrafast amplifiers. High beam quality and decent lifetime of the DUV optics are demonstrated.

The e-ray spatial walk-off effect is intrinsic with all anisotropic crystals but is of special concern for nonlinear optical (NLO) crystals in critical phase matching harmonic conversions. It presents one of the fundamental limitations for obtaining optimum conversion efficiency with NLO crystals. The principle of walk-off compensation has long been recognized and various schemes for overcoming walk-off has been discussed in the literature. A key prerequisite in this process for compensation is the ability to determine the deviation from the theoretical cut angle in starting NLO crystals and its subsequent correction. Our approach is to combine phase-angle-corrected NLO crystals by Adhesive-Free Bond (AFB®) into a stack of components for WOC by periodically inverting stack components to minimize walk-off and allow the generated power to grow proportionally with the length of the stack. We have implemented a measuring and correction system that consists of a precision six axes hexapod of a 0.0003° angular scanning resolution and an in-situ detection system with feedback of the generated power level as a function of angular position. As-received nonlinear crystals are oriented within ≤±.05° accuracy. We report on critical parameters of stack formation and improvement of a BBO (BaB₂O₄) quadruplet for conversion efficiency of 532 nm to 266 nm over a single BBO crystal and considerations for bonding LBO (LiB₃O₅) stacks.

Alternating dual-wavelength second harmonic generation at 532 nm using a 1064 nm Y-branch distributed Bragg reflector diode laser will be presented. The light source is based on single-pass frequency conversion in a periodically poled lithium niobate waveguide crystal with superimposed poling periods. All components, including the diode laser, optics for beam shaping and the nonlinear crystal, are mounted on a 5 x 25 mm² aluminum nitride micro-optical bench soldered on a 25 x 25 mm² conduction cooled package mount. Separated electrical contacts for the diode laser allow for alternating, wavelength stabilized dual-wavelength laser emission with spectral widths of 0.02 nm (0.2 cm^-1). Heater elements implemented above the DBR gratings enable wavelength tuning, which is utilized for phase-matching at both wavelengths. At a heat sink temperature of 25°C, optical output powers of 5.6 mW at 532.45 nm and 6.7 mW at 531.85 nm with a spectral width of 0.01 nm (0.35 cm^-1) are obtained. The developed light source is suitable for demanding applications such as Raman spectroscopy and shifted excitation Raman difference spectroscopy.

Spontaneous parametric down-conversion can produce pairs of entangled photons with very different wavelengths. Using SPDC, the scheme of Imaging with Undetected Photons promises to be a versatile tool to facilitate imaging in various spectral ranges and various interferometer designs and geometries. Here, we tackle the task of Imaging with Undetected Photons with a Mach-Zehnder-type Interferometer in MIR. With imaging achieved in a preliminary setup, we investigate the limitations set by nonlinear conversion efficiencies, optical resolution, laser power, and fluorescence of optics on our way to Imaging with Undetected Photons in MIR.

This paper presents refined Sellmeier equations for AgGaSe2 that provide a good reproduction of the phase-matching angles for optical parametric oscillation and difference-frequency generation processes in the 1.85–18 μm range thus far reported in the literature. In addition, these equations are found to reproduce the experimental results for second-harmonic and sum-frequency generation of CO2 laser radiation in the 1.7652–10.5910 μm range.

This paper reports on the updated Sellmeier equations for AgGaS₂ that provide an excellent reproduction of the 90° phasematching conditions for up-conversion of a CO₂ laser to the green spectral range (0.565–0.566 μm) as well as those for difference-frequency generation using wavelengths below 0.7 μm.

Lasers emitting in the ultraviolet C-band (UVC) have recently attracted considerable attention for germicidal purposes. Combining diode lasers with nonlinear crystals used for second-harmonic generation (SHG) is a promising approach thanks to their relatively low cost, small footprint and long lifetime. The output power in the UVC is limited by the output power of the diode lasers and by the conversion efficiency in the nonlinear crystal. This work compares the SHG conversion efficiency using a bulk approach to values expected using guided modes in waveguides. It discusses the phase-matching (PhM) condition for different input polarizations, the effective nonlinearity, and the Poynting vector walk-off. This last effect is particularly detrimental as it reduces the effective length for the nonlinear interaction in bulk, which ultimately limits the conversion efficiency towards the UVC. Values for the walk-off angle are computed for barium borate (BBO), and a comparison is provided with other nonlinear crystals.