This PDF file contains the front matter associated with SPIE Proceedings Volume 11985, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

We report a mid-infrared (MIR) source emitting at 3 μm, employing a novel χ⁽³⁾/χ⁽²⁾ cascaded nonlinear conversion architecture. Picosecond pulses from a 1.064 μm mode-locked Yb:fiber pump laser are used to generate 1.65 μm signal pulses through χ⁽³⁾ based four-wave mixing in photonic crystal fiber (PCF). The output of the PCF is then directly focused into a periodically poled lithium niobate crystal to generate idler radiation around 3 μm through χ⁽²⁾ based three-wave mixing between the pump and signal pulses.

We report on an ultra-broadband tunable mid-IR source delivering 110 nJ, 64 fs-long pulses at 250 kHz repetition rate. The experiment starts with a Yb-doped fiber amplifier system delivering 200 μJ, 300 fs-long pulses, followed by a 70% high-efficiency dual-stage nonlinear compression based on a multipass cell and a 1 m-long capillary filled with argon. The 9.5 fs-long resulting pulses drive intrapulse difference-frequency generation (iDFG) in a 1 mm-thick LiGaS₂ crystal and a specially designed waveplate is used prior to this crystal to further enhance the iDFG yield. This source paves the way for new experiments in 2D ultrafast spectroscopy.

Graded-index fibers enable efficient Raman conversion of multimode radiation into a Stokes beam with improved beam quality offering new approach for high-brightness diode-pumped all-fiber tunable lasers. Here we study an opportunity to broaden the operating wavelength range via cascaded Raman lasing in such scheme. Highly-multimode 940-nm laserdiode radiation is serially converted into the 1st(976nm), 2nd(1019nm) and 3rd(1065 nm) order Stokes beam. At the conversion the beam quality is greatly improved approaching diffraction limit (M²<1.4). Linear and half-open cavities are compared showing that the conversion efficiency is higher for half-open cavity, whereas the threshold is lower for linear cavity that was used for the 3rd -order generation. Generation of 3rd Stokes order is accompanied by unstable pulsations with higher (4th and 5th) orders involved in the lasing with total power of ~4 W, while its beam quality worsens to M²~2.

We present updated Sellmeier equations for CdGa₂S₄ that reproduce well the phase-matching angles for Yb:KGd(WO₄)₂ and Cr:forsterite femtosecond-amplifier-pumped Hg_0.35Cd_0.65Ga₂S₄ optical parametric amplifiers (OPAs) and a Ti:Al₂O₃ femtosecond-amplifier-pumped Hg_0.51Cd_0.49Ga₂S₄ OPA in the 5.6–11.5 μm range, when combined with our previously derived Sellmeier equations for HgGa₂S₄.

We perform transmission measurements in the 0.3-25 μm spectral range on thin (0.2-0.4-mm) GaAsP layers grown by Hydride Vapor Phase Epitaxy (HVPE) on plain (100) GaAs substrates. The purpose is to evaluate the evolution of the cut-off wavelengths in the visible (VIS) and in the mid-IR spectral range. The former is important in order to assess the potential of such mixed ternary quasi-phase-matching structures for pumping by intermediate laser systems (compared to pure GaP and GaAs), such as Cr⁴⁺ and Er³⁺ lasers emitting in the 1.5-1.6 μm spectral range. The latter is essential to see how the presence of P shortens the mid-IR clear transparency cut-off wavelength of GaAs due to phonon absorption. In all cases the substrates have been completely removed by polishing prior to the measurements. The multiple-reflection effect, which is considerable for such high refractive index materials has been taken into account for the incoherent light source of the spectrophotometer both in the region of clear transparency and in the presence of absorption. The actual sample thickness is derived from the observed interference fringes and the refractive index which is interpolated using available data on GaAs and GaP. With this information we calculate the wavelength dependent absorption coefficient. For some compositions, e.g. 33% P, we observe nice clear transmission plateau with almost vanishing absorption/scatter losses. Further increase of the P content shifts the VIS cut-off limit to shorter wavelengths but has negligible effect on the clear transparency upper limit which is around 13 μm in the mid-IR.

Traditional processes for the design of metamaterial structures are often computational heavy, time-consuming, and occasionally does not lead to the desired optical response. Deep learning can quickly optimize structures through inverse design, and create new geometries for devices. This research uses a deep learning framework for the inverse design of an optimal plasmonic structure to maximize the second-order nonlinear response from a nonlinear metamaterial. The thinfilm nonlinear metamaterial employed is a nanolaminate, and the optimal plasmonic structure is fabricated to establish the validity of the deep learning algorithm.

This work presents a commercial webcam CMOS (Complemented-Metal-Oxide-Semiconductor) implemented as a spectrometer for femtosecond pulses characterization at the Near-Infrared region (NIR, 1.1 - 1.6 μm), applying spectral interferometry. The spectral interferometry setup consists of a collinear Michelson interferometer in which two femtosecond pulses replicas, generated from a home-made Optical Parametric Oscillator (fs-OPO), are relatively delayed with respect to each other. A reflecting grating disperses the pulse replicas and then, the modulated spectrum is generated in a 2-Fourier setup, using a single lens, with the CMOS sensor located at the Fourier plane. The NIR CMOS response is produced through the Two-Photon Absorption (TPA) effect, capable of generating the nonlinear spectral intensity and the corresponding modulated spectrum (spectral interferometry signal). The cost-effective TPAspectrometer is capable of measuring the interferogram, with a high resolution of 0.72nm and very high sensitivity of few 𝜇W average power or few fJ per pulse. Finally, we calculate the spectral phase difference using a phase retrieval algorithm from the nonlinear spectral interferometry signal.

We control the spectrum of supercontinuum generation by phase shaping the driving pulse, greatly increasing power at wavelengths of interest. Previous work generally optimized one or two wavelength ranges, with spectral broadening below an octave, using spatial light modulators. We optimize a strongly nonlinear octave-spanning supercontinuum for six wavelengths, and we use much a more practical fully fiberized system, using temperature-controlled chirped fiber Bragg gratings. Our particular application is optical frequency transfer with an Er fiber frequency comb. We aim to replace multiple supercontinuum branches for f-2f interferometry and clock wavelengths with a single shaped branch that provides all the needed wavelengths, removing multiple path phase noise. We generated an optimized spectrum with strong outputs at the half harmonics of Yb, Ca, and Sr transitions: 1157, 1315, and 1397 nm, 1050 and 2100 nm for f-2f interferometry, and 1556 nm for c-band referencing. Without optimization, some target lines might be tens of nW, but we increase them to about 100 nW or more. Optimizing only one or two clock lines can further increase output by hundreds of nW. We confirmed the coherence of the shaped spectrum by interference with a second, unshaped supercontinuum branch with a 70 MHz acousto-optical frequency shift. The interference had strong contrast, and the beat notes had over 40 dB signal to noise at 100 kHz resolution bandwidth, showing suitability for high precision frequency comparison and locking.

We report on the experimental demonstration of nonlinear spectroscopy of crystalline quartz in the terahertz regime. Using accumulated time shift method in the time domain, we observe that with increasing the THz pulse intensity, the experienced delay increases. At higher field intensities, the delay increases with a smaller rate, demonstrating a phase saturation. Analysing the frequency response, we estimate a nonlinear refractive index of the order of 10⁻¹³ m²/W which exceeds its value in the visible range by seven orders of magnitude. Furthermore, a negative fifth-order susceptibility of the order of 10⁻³⁰ m⁴/V⁴ is obtained.

We recently fabricated a large-aperture quasi-phase matching stack of 53 GaAs plates using the room-temperature bonding. Although 29 times higher second-harmonic power was obtained than from a 9-plate stacked device, its surface profile was degraded and the positions with high transmittance were limited. By putting the GaAs plates on YAG crystals polished to laser grade instead of directly putting on metallic stages, we confirmed that the surface flatness as well as the transmittance were improved. We are now developing additional processes, which we expect realizes 100-plate or more stacked QPM structures with low loss.

We theoretically investigate noncritical phasematching in thin-film, periodically poled lithium niobate (LN) waveguides. Noncritical phasematching relaxes fabrication tolerances and is needed for long devices or when ideal tuning curves are required. Geometries exist for noncritical phasematching with respect to waveguide width, but we could not identify geometries noncritically phasematched with respect to LN thickness (the least well-control geometrical parameter). Our calculations showed that thicker waveguides had less sensitivity to thickness variation. We present a model of how geometrical variations affect the nonlinear tuning curves. We estimate limits on the acceptable thickness variation and discuss the limits scale with device length.

A design study is presented for difference-frequency generation (DFG) in aluminum gallium arsenide (AlGaAs)- on-insulator waveguides. AlGaAs is a mature technology platform with large optical nonlinearities, a high refractive index contrast, and the presence of second-order susceptibility, making it interesting for chip-based frequency conversion. This work targets to efficiently down-convert single-photons to the telecom C-band by DFG. Modal phase-matching (PhM) is used, where the waveguide dimensions are optimized for an efficient and robust conversion of single-photons at 930 nm to around 1550 nm. This paves the way for a single-photon converter that can be integrated on a chip-platform with a single-photon emitters, along other photonic components with standard fabrication techniques. Furthermore, a thorough revision of the DFG theory provides insight into the particular case of a low-powered pump, which is relevant for quantum applications. Finally, a comparison is made with state of the art devices in periodically poled thin film lithium niobate (PPLN). This is, to the best of our knowledge, the first design of a single-photon converter operating in the telecom band that is realized with a III-V material.

Sub-picosecond green-pumped OPG/OPA using non-critical phase matching in LBO was investigated at high average powers, utilizing a novel high-power ultrafast fiber laser providing 300W of 1ps pulses at 515nm and 1.4GHz pulse repetition rate. The 4-stage collinear OPA produced about 100W of combined Signal plus Idler output powers when tuning the Signal wavelength between 740nm and 940nm. An extended spectral tuning range between 300nm and 1800nm with average power between 10W and 70W was demonstrated by using SFG of Signal plus Pump and SHG of Signal and of Idler. The OPA output pulse durations were ~0.5ps long.

We present our activities on the development of narrow linewidth tunable optical parametric sources and their integration in lidar systems. In particular, we present different implementations of the nested cavity optical parametric oscillator (NesCOPO) that enables tunable single-frequency emission from the SWIR to the LWIR, when pumped by a fixed or a tunable wavelength laser beam. We show how to amplify the output energy and while preserving the spectral linewidth to perform standoff detection of greenhouse gases and toxic chemicals with direct detection lidars.

We present a narrowband, non-resonant optical parametric oscillator based on 5-mm thick Rb-doped periodically-poled KTiOPO₄ (PPKTP) operating in the high-energy/low repetition-rate regime. An uncoated volume Bragg grating (VBG) is employed as one of the cavity mirrors reflecting only the signal whereas the other cavity mirror is reflecting only the idler. Pumping by a Nd:YAG laser at 1.0642 μm in a double-pass, the signal plus idler output energy reached almost 5 mJ at a repetition rate of 100 Hz corresponding to a conversion efficiency of ⁓26%. Both signal and idler are narrowband with full width at half maximum (FWHM) of 0.5 nm at 1942 nm and 0.76 nm at 2355 nm, respectively.

A polarization-maintaining (PM) pulsed three-stage master oscillator power amplifier (MOPA) emitting at 2047 nm is reported, generating 19.8W of output power (396 μJ pulse energy) for a 50 ns pulse width at a repetition rate of 50 kHz. The output signal is linearly polarized and a diffraction limited beam quality is achieved. This MOPA laser is used to pump a doubly resonant ZnGeP₂ (ZGP) optical parametric oscillator (OPO) in a linear cavity. A mid-IR output power of 8.1W, accordingly 162 μJ of pulse energy, and a conversion efficiency of 44 % are obtained in the 3-5 μm band.

We present a versatile high repetition rate, optical-parametric chirped-pulse amplifier system (OPCPA) in combination with a high-harmonic-generation (HHG) source. Tuning of the fundamental OPCPA driver wavelength allows for high harmonic generation within the full range between 25 and 50 eV. All energies between two adjacent odd harmonics can be addressed, making the system a powerful, gaplessly tunable extreme-ultraviolet (XUV) light source for spectroscopy.

We report on research into the properties of supercontinuum (SC) generated from low-coherence bursts of different duration in a 1-km long P₂O₅ fibre. It was found out that SC with a spectral width of ~135–150 nm is formed within the ~900–1200-nm range mostly due to cascaded Raman scattering of noise-like pulses with the variable envelope duration of 36–153 ps (sub-pulse duration was ~ 300 fs) and average power of 560 mW at 1080 nm. It was discovered that the spectral width of SC is predominantly affected by the duration of interaction between the pumping and Raman pulses.