This PDF file contains the front matter associated with SPIE Proceedings Volume 11264 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Pushing the boundaries of cavity-enhanced spectroscopy experiments in the mid-IR is strongly tied to the availability of suitable mid-IR frequency combs and mirror coatings with well-characterized properties. Recently, substrate-transferred crystalline coatings (e.g. epitaxial GaAs/AlGaAs multilayers bonded on silicon substrates) have emerged as a groundbreaking new concept for the fabrication of high-performance thin-film interference coatings in the mid-IR, circumventing limitations of established material systems and physical vapor deposition technology. In this presentation, I will talk about state of the art, mid-IR frequency combs and present a detailed characterization of substrate-transferred crystalline mirrors centered at a wavelength of 4.55µm

A high-power Er:Fiber amplifier design allows for generation of <10 fs pulses with 0.5 MW peak power from a 1.5 micron frequency comb using robust telecom technology in a turn-key package. Chi-2 and Chi-3 nonlinear optical processes result in milliwatt and multi-milliwatt infrared generation through intrapulse difference frequency generation, high contrast few-cycle pulse generation through cross phase modulation, and harmonic generation through cascaded quadratic nonlinear processes. Applications such as dual-comb electric field sampled become more accessible with the milliwatt mid-infrared radiation from our source.

We generate about 2 mW of midinfrared in orientation-patterned gallium phosphide pumped by an erbium fiber laser. The midinfrared spectrum has a strong component around 8 to 10 μm wavelength, with a broad background from about 7 to 12 μm. The pump laser has 5 W, single mode output at 96 MHz compressed to 65 fs, with a peak power of up to about 0.6 MW. Before midinfrared generation, we further broaden and recompress the pulses in a small length of fiber, obtaining 3 W of 17 fs pulses with peak power near 1 MW but broader bandwidth to generate midinfrared.

High repetition rate pulsed lasers are used for applications such as highspeed optical communications, nonlinear optics and optical sampling. Conventionally, mode locked lasers are used as pulsed sources. However, they suffer from low repetition rate that is not tunable. Electro-optic modulation allows generation of frequency combs with tunable high repetition rate. These combs can be compressed to generate pulses, but the pulse widths are large owing to the limited bandwidth of electro-optic frequency combs. Though, modulators can be cascaded to scale the bandwidth, it is only a linear enhancement and requires additional RF components. Spectral broadening of these combs in nonlinear fibers results in marginally improved bandwidths. The broadening achieved at a given optical power can be enhanced several times by suitably modifying the temporal profile of the comb before spectral broadening with a pulse-shaper. In this work, electro-optic intensity and phase modulators are driven at 25GHz to generate an initial comb. A pulse-shaper adaptively optimises the temporal profile of the electro-optic frequency comb to enhance the spectral broadening in highly nonlinear fiber (HNLF). The optimised comb is power scaled in an Er-Yb co-doped fiber amplifier before HNLF. The adaptively optimised comb is compressed with single mode fiber and is characterized with zero delay implementation of spectral shearing interferometry to obtain the spectral phase and temporal profile of the pulses. The output pulses have 1 dB bandwidth of ~0.6 ps and 3 dB bandwidth of ~ 1 ps with high repetition rate of 25GHz. This adaptive technique is shown to be immune to drifts and changes in modulator drive conditions.

We report a technique for generation of ultra-broadband coherent femtosecond continua in the infrared. The laser architecture is based on the Cr:ZnS–GaSe and Cr:ZnS–ZGP tandem arrangements that enable simultaneous amplification of ultrashort middle IR pulses and augmentation of pulses’ spectrum via a chain of intrapulse three-wave mixings. The first part of the tandems is based on a single-pass polycrystalline Cr:ZnS amplifier, which is optically pumped by off-the-shelf continuous wave Er-doped fiber laser and outputs 2-cycle pulses with multi-Watt average power at 80 MHz repetition rate, at the central wavelength 2.5 μm. The second stage of the tandems comprises a GaSe or ZGP crystals configured for intrapulse difference frequency generation. The Cr:ZnS–GaSe tandem has allowed us to achieve multi-octave 2–20 μm continuum with 2 W power in the range 2–3 μm and power in excess of 20 mW in the important range 3–20 μm. On the other hand, Cr:ZnS–ZGP tandem features long-wave infrared (6–12 μm) output pulses with record braking sub-Watt power level. Last but not least, Cr:ZnS–GaSe and Cr:ZnS–ZGP IR sources have small footprints and are easily convertible to the optical frequency combs with low carrier-to-envelope timing jitter.

Supercontinuum (SC) generation towards the mid-infrared (MIR) range is an active field of research and development motivated by a wide range of applications including optical coherence tomography (OCT), material processing, optical sensing and absorption spectroscopy. In this work, we investigate mid-IR SC generation in a cascaded silica and soft-glass fiber system directly pumped with a commercially-available picosecond fiber laser operating in the telecommunications window at 1.55 μm. This all-fiber system is shown to generate a flat broadband mid-IR-SC covering the entire range from 2 to 10 μm with several tens of mW of output power. This technique paves the way for practical and robust broadband SC sources in the mid-IR without the requirement of mid-infrared pump sources or Thulium-doped fiber amplifiers. We also describe a fully-realistic numerical model used to simulate the nonlinear pulse propagation through the cascaded fiber system and use our numerical results to discuss the physical processes underlying the spectral broadening in the cascaded system.

The mid-IR supercontinuum generation has attracted much attention during the recent years because many unique molecular absorption bands of most of the molecules exist in this domain. Additionally, mid-IR supercontinuum light sources are expected to have potential applications including astro-photonics, bio-photonic diagnostics, nonlinear spectroscopy, infrared imaging and sensing. For high spatial resolution imaging a spatially coherent supercontinuum light source is desirable. The soft-glass optical fiber is the promising medium for the design and development of a high spatially coherent mid-IR light source with the high brightness. Earlier, the broadband mid-IR supercontinuum generation has been reported using the optical fibers in different materials including tellurite, and chalcogenide, but, its coherence property has not been demonstrated extensively. In this work, we experimentally demonstrate the mid-IR supercontinuum spectrum spanning ⁓1.6 μm to 3.7 μm using a 3 cm long tapered chalcogenide step-index optical fiber pumped with femtosecond laser pulses at 2.6 μm. To justify the experimentally obtained results, a numerical simulation also carried out for the same fiber and pulse parameters. The measured supercontinuum spectrum matches well with the simulated spectrum and generated supercontinuum spectrum is highly coherent within the whole spectral range of the supercontinuum generation.

For as widely used a tool as nonlinear optical frequency conversion is for both science and industry, it remains widely limited in efficiency and bandwidth (and ultimately also in cost) due to the fundamental problem of backconversion in the nonlinear evolution dynamics. This review paper covers new developments and capabilities in frequency conversion devices, including optical up- and down-converters and amplifiers, based on nonlinear evolution dynamics in which back-conversion is suppressed. One such approach is adiabatic frequency conversion, in which the dynamics of rapid adiabatic passage replace the regular cyclic conversion evolution in phase-matched sum- and difference-frequency generation. This approach has enabled devices far surpassing the conventional efficiency-bandwidth trade-off. For example, in chirped quasi-phase matched quadratic crystals, microjouleenergy single-cycle mid-infrared pulses were generated with arbitrary pulse shaping capability, presenting a source with unique features for nonlinear spectroscopy and strong-field physics applications. We review new developments in the use of optical fibers as a cubic nonlinear platform for the same concept, utilizing a tapered core diameter or a pressure gradient to allow up- and down-conversion with ultra-wide bandwidth and high efficiency. We also review a newly introduced concept for high efficiency optical parametric amplification, via a novel approach for suppressing back-conversion in optical parametric amplification by simultaneously phasematching the idler wave for second harmonic generation.

Raman microscopy is a key technique for biological imaging since it can provide valuable information about the chemical constituents of a sample without any labels. However, because two wavelengths are required for either CARS or SRS to occur, most Raman imaging set ups use multiple lasers with complicated synchronization requirements. In this presentation, we discuss the design and performance of a tunable Ytterbium-based fiber laser and an optical parametric oscillator for Raman microscopy. Our system uses a single laser that creates both pump and probe beams via nonlinear optical effects. Due to its reasonable high peak power, this laser system is a suitable light source for multimodal microscopy using both Raman and multiphoton imaging functionalities.

We demonstrated an electro-optically tunable ultra-broadband singly resonant ns-optical parametric oscillator (OPO) based on a periodically poled lithium niobate crystal (PPLN) with a three-segment structure. The ultra-broadband parametric gain was obtained by precise selection of the pump wavelength and the poling period to match the group velocities of the signal and idler. Selected wavelengths in the broadband parametric signal and idler gain spectrum generated by the first segment was amplified in the third segment. This selection could be electro-optically controlled by applying a variable dc electric field in the middle single-domain segment. A singly resonant OPO cavity was then constructed and the output signal wavelength could be tuned from 1.373 to 1.749 μm (corresponding idler tuning range from 2.863 to 1.972 μm) by changing the electric field from -6.5 to +5.0 kV/mm. Electro-optic tuning facilitated fast wavelength switching for the OPO to span the entire near-infrared spectral region. This is the largest electro-optic tuning range observed to our knowledge.

With diode-pumped Yb laser technology reaching maturity, average power scaling of multi-GW, few-cycle, short-wave and mid-infrared (MIR) optical parametric amplifiers (OPA’s) to the 100-W level has become a reality. Well established, commercially available oxide crystals offer a relatively straightforward solution in the 1.4-4-μm spectral range. Extension of the spectral coverage of high-power OPA’s beyond 5 μm may be enabled by novel, wide-bandgap non-oxide crystals with growth processes still under major development and optimization. Here, we present our results on the nonlinear optical properties of oxide (LiNbO₃, KTiOAsO₄) and non-oxide (LiGaS₂, BaGa₄S₇) crystals and the resulting 100-kHz, ultrafast infrared OPA’s based on these materials. The reported data provide design parameters and guidelines for high-average-power MIR OPA’s pumped by Yb lasers both below and above 5 μm.

Generation of tunable middle infrared radiation is important for a number of applications ranging from biomedical imaging to remote sensing and imaging. Short laser pulses provide an opportunity to generate high-intensity radiation to facilitate nonlinear optical interaction. In this report, we discuss the use of ZnSe material to generate tunable infrared picosecond pulses.

We report on the first polarization-maintaining (PM) Thulium (Tm³⁺):Holmium (Ho³⁺)-codoped triple-clad fiber (THTF) laser. First fundamental studies were done in a CW regime and showed highly promising results as a high power pump source for frequency conversion in ZGP crystals. For an unpolarized output, the fiber laser delivered up to 145 W. Switching to a polarized operation, up to 140 W of optical output power with a slope efficiency of 36.3 %, an optical-to-optical efficiency of 32 %, and a beam propagation factor of M² _x,y < 1:9 was obtained with a degree of polarization > 99:8 %. The laser output wavelength was tunable from 2022 nm to 2068 nm. Operating in Q-switched regime at a repetition rate of 140 kHz, the pulses of the THTF laser had a pulse width of 96 ns, a pulse energy of 275 µJ, and a peak power of 2.84 kW. The emission spectrum was centered at ~2050 nm with a linewidth of 60 pm.

We present a MgO:PPLN- and AgGaSe2-based, picosecond OPG/OPA scheme, enabling the generation of mid-to-long-infrared continuously tunable, microjoule energy pulses with a narrow spectral bandwidth of sub-8 cm-1 over the full tuning range. The approach combines a 1-µm-pumped, double-pass OPG setup with a 2-µm-pumped parametric booster amplifier stage. As pump laser is applied a CPA-free Ho:YLF laser delivering 32 µJ at 2051 nm wavelength and 20 kHz repetition rate. The OPG/OPA system specifications of µJ-level pulse energies in combination with narrowband and widely tunable, mid-to-long-IR spectra enable numerous possibilities for mid-IR spectroscopy and wavelength-specific mid-IR material processing.

We report on a monolithic photonic chip including a microdisk for chi(2) optical interactions which is evanescently coupled with two suspended waveguides used for pump injection and nonlinear signal collection: one for critical coupling at λω ≈ 1600 nm and the other for critical coupling at λ2ω ≈ 800 nm. Our sources are CW tunable lasers: an external-cavity laser diode at ω and a Ti:Sapphire laser at 2ω, both connected to a microlensed fiber. Thanks to critical coupling at input/output wavelengths, we achieved 11% W−1 SHG efficiency. The same chip enabled us to demonstrate spontaneous parametric down-conversion (SPDC).

Materials with vanishing real part of permittivity, also known as epsilon-near-zero, have emerged as a new paradigm to obtain large optical nonlinearities. In this region, light-matter interaction enhances significantly, which gives rise to an unprecedentedly large nonlinear phase shift. The phase sensitivity enhancement is due to the strong dispersion in this spectral region. We present nonlinear optical measurements via the Beam Deflection technique, to directly measure the spectral dependence of the ultrafast nonlinear phase shift of an Indium Tin Oxide thin film. We present simultaneously cross-phase modulation induced frequency changes that depend on how fast the nonlinear phase changes.

Terahertz (THz) nonlinear optics is an emerging field thanks to recent developments in the generation of intense THz pulses. THz radiation interacts with the vibrational/rotational resonances of molecules. For example, water molecules in the atmosphere show a very rich absorption spectrum with hundreds of sharp resonances below 3 THz. We report on the first experimental demonstration of the nonlinear interaction of THz pulses with water vapor. We observed a strong nonlinear response of the vibration/rotation resonances to the THz pulses. The frequency response of the nonlinear Kerr coefficient is extracted from the experimental results.

Nonlinear frequency conversion processes depend on the polarization state of interacting beams. On the other hand, vector-vortex beams have space-variant polarization in beam transverse plane. In light of these two points, it is challenging to do nonlinear frequency conversion of vector vortex beam in single-pass geometry and retain the polarization characteristics of the beam. Here, we report an experimental scheme for single-pass second harmonic generation (SHG) of vector-vortex beams. Using two contiguous bismuth borate crystals with optic axis orthogonal to each other, we have frequency-doubled the near-IR vector-vortex beam into visible vector-vortex beams with order as high as lsh=24.

A concept for nonlinear pulse compression is presented, which relies on alternating the dispersion sign by repeatedly splicing segments of normal to anomalous dispersion fibers. This approach accomplishes to limit temporal defocusing by repetitive recompression of the broadened bandwidth, resulting in few-cycle pulses without the need of soliton formation or free-space compressors. We show the compression of 80 fs pulses down to 25 fs pulses, which are nearly bandwidth-limited by using a special dispersion compensating fiber. Besides further scaling of pulse duration, we show a stability and scaling analysis regarding the input conditions and the highly coherent few-cycle output.

We performed direct measurements of phase-matching conditions of Second-Harmonic Generation (SHG) and Difference-Frequency Generation using the sphere method to determine reliable Sellmeier’s equations valid in the 1-12 microns transparency range of the new BGSe monoclinic nonlinear crystal. We also recorded SHG conversion efficiencies under and out-of phase-matching conditions to determine without the magnitudes and relative signs of the associated non zero quadratic nonlinear coefficients of BGSe. By combining all these data, we were able to calculate pump wavelengths enabling the generation of a widely tunable light in the infrared range from phase-matched Optical Parametric Generation.

Polycrystalline Cr:ZnSe and Cr:ZnS have greatly advanced as mid-IR laser host materials in the last 20 years. These materials are also highly attractive as passive q-switches (PSQs) for 1.5- and 2-micron lasers, enabling greatly-simplified laser architectures for compact, multi-watt, pulsed 2-micron lasers and mid-infrared optical parametric oscillators (OPOs). Evidence suggests that power scaling of lasers and PSQs can be achieved by using single crystal Cr:ZnSe and Cr:ZnS. Here we report successful growth of high purity single crystal ZnS and ZnSe by CVT and PVT respectively, and Cr-doping achieved by iodine-assisted vapor transport of CrSe located in a separate temperature-controlled zone.

We discuss low-pressure HVPE growth of ZnSe on GaAs substrates (~350 µm thick) and on OP-GaAs templates (~115 µm thick) that achieved single-crystalline quality ZnSe layers which will be used to develop OP-ZnSe QPM structures for nonlinear frequency conversion devices. Material characterization techniques including SEM, HR-XRD, XTEM, and PL have been used to verify that the ZnSe grown by HVPE has a superior quality to the commercially available ZnSe substrates. Current focus is to obtain thicknesses beyond 500 µm using plain and OP templates for frequency conversion in the MLWIR.

Laser damage thresholds of CdSiP₂ have previously been measured at wavelengths of 1064 nm and 2090 nm using nanosecond durations lasers^1,2 and at 1940 nm using a continuous wave laser³. In the continuous wave measurement attempted in the past³, the CdSiP₂ sample was found to withstand an irradiance of 150 kW/cm² for over 60 seconds without any damage to the sample, whereas earlier grown samples of CdSiP₂ exposed to the same irradiance level damaged in 5 seconds or less. In that work, or in any other work to our knowledge, the samples were not exposed to millisecond or longer duration lasers at 1064 nm or 1550 nm. Because of the importance of CdSiP₂ in nonlinear frequency conversion of lasers in the 1000 to 2000 nm spectral range, this study was performed to measure the damage thresholds at wavelengths in this spectral regime. Results of the damage threshold at different laser spot sizes will be presented.

We investigate the nonlinear optical properties of transparent optical materials using ultrashort midwave infrared laser pulses between 3 and 4 microns. Random quasi-phase matching in polycrystalline materials generates multiple frequency harmonics of both odd and even orders throughout the transmission window of the target. We also investigate single crystal and amorphous materials and demonstrate a range of frequency conversion and pulse broadening. Simulations using a nonlinear polarization model enhanced with ionization and experimentally measured n₂ values provide good qualitative agreement with experimental data.

We report on efficient, two stage single-pass second harmonic generation of ultrafast Cr2+:ZnS laser with spectral bandwidth of 138 nm centered at ~2360 nm and pulse width of ~43 fs at a repetition rate of 80 MHz into tunable yellow radiation across 577 - 589 nm in multi-grating MgO:PPLN crystals. A maximum average output power ~940 mW at 589 nm wavelength with a single-pass conversion efficiency as high as 41% was achieved. The yellow radiation has a spectral bandwidth of 2 nm and pulse-width of ~913 fs in absence of any pulse compression with a time-bandwidth product of 1.58.

High-efficiency single-pass CW UV 355nm generation using PPLN has been studied under focusing optimization concept. Using a 25mm (1st order, SHG) PPLN crystal cascaded with a 10mm (3rd order, SFG) PPLN crystal and 3.8W 1064nm input pump, we have successfully generated >100mW CW 355nm output with good beam quality and power stability. As compared to previous publications and to other kind of nonlinear materials, maximum CW UV 355nm power generated from PPLN has been enhanced by a factor of 2~3, and the single pass conversion efficiency at 4W 1064nm input level has been enhanced by a factor of ~4.

Shaping and compression of short laser pulses in the course of a second harmonic (2H) generation is a well verified technique in the case of Nd:glass laser radiation converted in KTP crystal cut for type II phase-matching. The laser pulse, few ps in duration, is divided into two orthogonally polarized pulses and one is delayed in respect to the other. Both of them enter the nonlinear optical crystal, in which they propagate with different group velocities. As they are approaching each other, short 2H pulse is generated at their temporal overlap. Shaping of the 2H pulse is achieved by changing the predelay and input pulse intensities. For current high power picosecond laser sources of choice, Yb:YAG lasers, borate crystals have to be used for efficient harmonic generation because of their convenient thermal properties and long lifetime. Although the requirement on group velocities is not fulfilled for the LBO and BBO crystals, numerical calculations have revealed conditions where pulse shaping and even significant pulse compression occur. Fundamental input pulse of 1.8 ps was compressed down to 0.8 ps (FWHM) by 2H generation in LBO crystal. In this work, details about the calculations and measurements of shaping of 2H picosecond Yb:YAG laser pulses in LBO and BBO crystals in dependence on input intensities and predelay will be presented. The shaped 2H pulse measurements performed by an autocorrelator agree well with the calculations, achieving almost 3-fold compression. For the most interesting cases, 2H pulses are characterized by FROG in detail.

We present the development of a high-power laser source operating at 532 nm produced by frequency doubling a Ybdoped fiber amplifier. The fiber amplifier has a multistage design, and uses large mode area Yb-doped fibers as the gain medium to produce > 2 kW of laser power at 1064 nm. The amplifier design is optimized to reduce non-linear effects, and operates at linewidths as narrow as 45 GHz. By focusing the fiber amplifier output into an LBO crystal, more than 1 kW of 532 nm light is produced. Single pass conversion efficiencies as high as 54% are achieved providing a unique combination of high power and high quality 532 nm laser source. The 532 nm laser is fiber coupled, making it an ideal source for industrial applications.

A dual-channel, high-power laser system with gap-less tuning from 250-1300nm at 30-50 femtoseconds pulse duration is presented as the ideal tool for time-resolved photo-emission microscopy and spectroscopy experiments at repetition rates up to 4MHz.

Despite the popularity and ubiquitousness of the tilted-pulse-front technique for single-cycle terahertz (THz) generation, optimization of the experimental setup remains complex and difficult due to the sensitive dependence on and coupling between the optical pulse parameters, including fluence, beam size, angular dispersion and temporal compression. Here we present a systematic and robust method to tune the tilted pulse-front setup, based on use of selected multi-dimensional scans, which enables a straight-forward and accurate determination of optimum parameter values. Our methodology not only allows us to determine parameter sensitivities and achieve a robust optimum in the performance, but also enables a verification of certain physical properties of the lithium niobate prism, including the THz refractive index. The detailed step-by-step procedure is discussed and applied to a tilted-pulse-front THz setup at both room temperature and cryogenic temperatures. The procedure can be applied to any setup based on the tilted-pulse-front geometry and is important for the construction of high energy THz sources required for strong field terahertz applications such as novel particle acceleration schemes or beam manipulators.

Various electro-optic nonlinear organic crystals have been recently developed and successfully used as very efficient materials for the generation and detection of broadband terahertz (THz) waves due to their large second-order optical nonlinearities and excellent phase matching characteristics. In the present talk, some of newly developed highly nonlinear organic crystals, which can be applied for efficient ultra-broadband THz wave generation up to ~10 THz by near-infrared pumping, are introduced. Additionally, two approaches for suppressing phonon-mode absorption, leading to strong modulations of the THz spectra generated in most organic crystals, are discussed.

We identified eight nonlinear crystals enabling THz emission from quadratic phase-matched Difference-Frequency-Generation: YCOB, BNA, LBO, CSP, AGS, CdSe, ZnO and GaP. For all these crystals, we performed Time-Domain Spectroscopy in the same conditions to determine their absorption spectra in polarized light as well as their principal refractive indices as a function of wavelength in the 0.5-2.0 THz range. By combining previous data with the Sellmeier equations valid in their visible and infrared transparency ranges, we calculated the coherence length of Difference-Frequency-Generation associated to all possible configurations of polarization and found interesting and complementary phase-matching conditions in the eight studied crystals.

The atmospheric window at 3 to 5 μm is one of the most important spectral regions for molecular spectroscopy. This region accommodates strong fundamental vibrational spectra of several interesting molecules, including species relevant for air quality monitoring, medical diagnostics, and fundamental research. These applications require excellent spectroscopic sensitivity and selectivity. For example, atmospheric research often needs precise quantification of trace gas fractions of down to the parts-per-trillion level (10^-12), with the capability of resolving individual spectral features of different molecular compounds. This sets stringent requirements for the light source of the spectrometer in terms of output power, noise, and linewidth. In addition, the wavelength tuning range of the light source needs to be large, preferably over the entire atmospheric window, in order to enable measurements of molecular fingerprints of several compounds. Continuous-wave optical parametric oscillators (CW-OPOs) are one of the few light sources that have the potential of combining all these favorable characteristics. This contribution summarizes our progress in the development of CW-OPOs, with an emphasis on precise frequency control methods for high-resolution molecular spectroscopy. Examples of new applications enabled by the advanced CW-OPO technologies will be presented. These examples include a demonstration of world-record detection sensitivity in trace gas analysis, as well as the first characterization of infrared spectrum of radioactive methane ¹⁴CH₄.

Upconversion imaging using a χ⁽²⁾ material can conveniently be viewed as an optical filter known from Fourier optics. First part discusses the solution to Helmholtz Equation with a nonlinear source term representing the χ⁽²⁾ interaction process. Assuming non-depleted interaction, an explicit solution can be found using Greens Function. In the far-field the solution is found in terms of a simple 3D Fourier integral. We will analyze a 4f-setup, with the nonlinear crystal situated in the Fourier plane, for upconversion imaging. While the results resembles the linear case known from standard imaging systems, χ⁽²⁾ imaging has an additional phase match term, dictated by the dispersion and birefringence amongst the three interacting fields. Birefringent crystals can be implemented as the nonlinear medium to ensure phase matching, i.e. efficient conversion of the mid-IR signal to the visible. When interaction takes place in the fs regime, group velocity mismatch will be included. The main features of the theory is presented, including applications.

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). We developed an indirect phase control approach based on the phase matching relation intrinsic to efficient nonlinear processes. Previously, we demonstrated coherent combining of difference frequency generation through real time active control of the phases of the pump waves, using high bandwidth fibered electro-optic phase modulators. The straightforward follow-up is the application of such process to OPOs, higher efficiency frequency converters when compared to DFGs. In this paper, we present an experimental demonstration of coherent OPOs emitting tunable idler wave in the mid-infrared. We present the architectures of continuous wave OPOs we are working on, their pros and cons and threshold properties, and the first results of coherent combining. We detail how the cavity modes of the OPOs are overlapped and how the active phase control used for DFG combining can be implemented in this case.

This paper reports a high-average-power mid-infrared source based on an orientation-patterned gallium arsenide (OP-GaAs) optical parametric oscillator (OPO) with wide wavelength tunability. An average power of 4.8 W of signal (3093 nm) and 3.5 W of idler (5598 nm) was achieved at a pump power of 25.5 W. Tuning ranges of 2895 nm-3342 nm (signal) and 4935 nm-6389 nm (idler) were obtained. The idler-resonant OPO offered good beam quality of the mid-infrared idler waves with an M2 of 1.1. High-average-power induced thermal effects for the OP-GaAs OPO were observed.

Mid-infrared spectroscopy is one of the most important techniques in chemical analysis. However, the detectors for the mid-infrared range suffer from lower specific detectivities in comparison to their visible counterparts, cost more and often require cryogenic cooling. Nonlinear interferometers allow measuring mid-infrared spectra by detecting only visible light using the induced coherence effect. In our work, we realize a nonlinear interferometer designed for broadband mid-infrared spectra with high resolution, which is easily tunable, and in analogy to classical Fourier transform infrared (FTIR) spectrometers requires no additional spectral selection.

We investigate both theoretically and experimentally the polarization properties of Brillouin light scattering in silica optical nanofibers. Our results show that while all hybrid acoustic waves scatter light without altering the state of polarization, one of the surface acoustic wave generates a depolarized Stokes light. Because of the slight ellipticity of the nanofiber, the surface wave is actually split into two torso-radial modes which give rise to polarization scrambling of the backward Brillouin Stokes signal. Our model also predicts that the polarization of the scattered light can be restored for one specific pump polarization.

We report a successful combination of stimulated Raman spectroscopy (SRS) and surface-enhanced Raman scattering (SERS) using cw laser sources and gold/silica nanoparticles with embedded reporter molecules. We describe the preparation method for our gold/silica nanoparticles as well as the effect of probe wavelength, pump and probe power, polarization and sample concentration on the cwSESRS signal. Altogether, a stable 10-12 orders of magnitude enhancement in the stimulated Raman signal is achieved because of the amplification of both pump and probe beams, leading us to detect pico-molar nanoparticle concentrations. The coherent Raman spectra matches the incoherent conventional Raman spectra of the reporter molecules. Unlike conventional incoherent SERS, this approach offers several advantages including improved trace analyte detection, the low cost accessibility of cw sources, and a coherent stimulated signal of microwatt intensities for applications such as Molecular Holography.

We demonstrate an external-cavity KGd(WO₄)₂ (KGW) Raman laser, pumped by an actively Q-switch Tm:YLF MOPA. The fundamental spectral line emitting at 1881 nm allowed the KGW bi-axial crystal to lase at two separate output spectral lines, 2198 and 2265 nm, depending on the seed polarization axis relative to the KGW's axis. The Tm:YLF seed was amplified using a double-pass Tm:YLF crystal based MOPA setup. After amplification, the seed achieved an output power of 9.15 W, and an energy pulse of 4.57 mJ, a pulse duration of 43 ns at a repetition rate of 2 kHz. The max output average power achieved for the 2265 nm was 1.85 W, with a pulse energy of 0.923 mJ at a repetition rate of 2 kHz implying a conversion efficiency of ~20.5%. We noticed a very low conversion efficiency of the shorter KGW spectral shift (at 2198 nm). The reason for this efficiency drop was validated to be the 2^nd stokes forming and thus consuming the 1^st stokes energy. In favor of the KGW inherent properties and according to the aforementioned results, this crystal appears to be suitable for power scaling as well as for improvement of the Raman conversion efficiency in this spectral range. The KGW crystal is well known for its use in shorter spectral wavelengths. To the best of our knowledge, it is the highest average power achieved by lasing in the 2 μm region using SRS with KGW.

Graded-index fibers with special in-fiber Bragg gratings are shown to convert efficiently highly multimode (M² ~30) laser diode pump radiation in all-fiber Raman laser configuration into a high-quality Stokes beam at 954 and 976 nm (M² =1.6-2.6 in different configurations). The beam quality improvement is provided by well-known Raman beam cleanup effect defined by Raman gain profile, and additional impact of transverse mode-selective properties of cavity feedback provided by special femtosecond-pulse inscribed fiber Bragg gratings and/or random Rayleigh backscattering distributed along the fiber. Here we study frequency doubling of the diode-pumped Raman fiber laser in a simple singlepass scheme with 5-mm PPLN crystal demonstrating efficient second harmonics generation at 477 and 488 nm with high beam quality after elimination of interfering effects.

CW (continuous wave) coherent anti-Stoke Raman spectroscopy (CARS) of gases using a hollow-core photonic crystal fiber (HC-PCF) is demonstrated with gaseous CO₂ as an example. Raman transition is pumped by a tunable dye laser. The seeding stokes light is provided by a home-made ECDL. Both lasers operate in single longitudinal mode. The output radiation from the fiber is filtered and coupled into an imaging spectrometer. Anti-stokes signal is observed with 10 mW pump power and 1 mW stokes power. The peak anti-Stokes signal of ν₁ band of 6 atm CO2 is 59000 CCD counts with the exposure time set to 5 ms. The v₁ and 2v₂ components of the fermi dyad of CO₂ is resolved.

We demonstrate, build and optimize evanescent Raman converters at the sub-nanosecond regime based on a silica nanofiber immersed in ethanol. Two different standard silica fibers (SMF28, 460HP) are tested and compared. The converters are pumped at 532 nm and deliver pulses at 630 nm, which is the first Stokes order wavelength of ethanol. They present highly reproductible performances. A maximum output Stokes energy of 0.29 μJ is usually reached with an external conversion efficiency of 60%. Lowering the Raman threshold and pushing up the nanofiber breakdown allow a higher conversion operating range, that is the conception key of these converters.

Narrow linewidth fiber lasers find widespread applications in beam combining, frequency conversion and remote detection. Power scaling of these lasers is mainly limited by Stimulated Brillouin scattering (SBS). Currently, SBS is mitigated through linewidth broadening and/or fibers with enhanced mode area. The latter suffers from problems of beam degradation and modal instability making line broadening the primary technique for SBS suppression. Line broadening can be achieved with phase modulation of lasers using white noise, pseudo-random bit streams or arbitrary waveform generators. The simplest implementation is with white noise source with the latter two requiring greater resources. We recently demonstrated a 10GHz linewidth 0.5kW polarization maintaining fiber laser, where it was observed that the SBS threshold did not directly scale with linewidth. This effect was identified as arising from the slow roll-off of the spectrum in white-noise modulated spectra which seeds the SBS process. The seeding is due to the reflections from the fiber end facet at these broadened linewidths where the spectrum has appreciable power at the Stokes wavelength. This is anticipated to be fundamental limiter for power scaling of narrow linewidth fiber lasers. In this work we overcome these drawbacks through a simple phase modulation scheme that incorporates noise waveforms together with sinusoidal modulation. This enables the spectrum to have sharp roll-off with flatter central region resulting in substantial reduction in seeding of SBS from end facet. With this simple architecture, we demonstrate scaling of SBS limited power by more than 1.5 times over pure noise modulation.

This paper reports on the new thermo-optic dispersion formula for AgGaSe₂ that provides a good reproduction of the temperature-dependent phase-matching conditions for second-harmonic generation (SHG) and sum-frequency generation (SFG) of a CO₂ laser and a Nd:YAG laser-pumped KTP optical parametric oscillator (OPO) in the 1.5558 - 10.5910 μm range as well as those for difference-frequency generation (DFG) between the signal and idler outputs of a Nd:YAG laserpumped 90° phase-matched CsTiOAsO₄ OPO at λ_i = 7.5190μm when combined with the modified Sellmeier equations of the present authors [E. Takaoka and K. Kato, Appl. Opt., 37, 561-564 (1998)].

In this work we present a novel concept of compact broadband high resolution sum frequency generation spectroscopy system. Multiple channel picosecond fiber laser was used as a seed for narrowband (~1.5 cm^-1 ) and broadband ultrafast radiation sources. In order to achieve >500 cm^-1 linewidth widely tunable microjoule-level pulses in MIR spectral region (2 - 10 μm) broadband femtosecond source optimization was performed. Numerical simulations of various schemes with different nonlinear crystals and experimental results were presented and compared.

We have developed a periodically poled LiNbO3 (PPLN) wavelength converter with a buried waveguide to improve a mode overlap between a fundamental light and a converted light. We formed a periodically poling by applying highvoltage and a buried waveguide by mechanical processes and a burying process. We designed quasi-phase-matching for second harmonic generation (SHG) in a telecommunication wavelength. We achieved a larger overlap between them in the PPLN buried waveguide than that in a ridge waveguide and confirmed improvement of the SHG conversion efficiency. We also demonstrated wavelength conversion based on cascaded SHG and difference frequency generation (DFG) in a single PPLN waveguide device for fiber-optic communication systems. Additionally, we found out a signal wavelength conversion bandwidth of the cascaded SHG and DFG covering most of C and L bands.

Supercontinuum (SC) has been applied in many applications such as optical coherence tomography, high-precision spectroscopy and frequency metrology [1]. SC can be generated in highly nonlinear fibers by launching high intensity laser pulses into these fibers. The dynamics of SC generation (SCG) closely relates to the chromatic dispersion of the fibers [2]. In the normal dispersion regime, the spectrum broadening dynamics is mainly based on self phase modulation and optical wave breaking which are self-seeded processes. Thus, the output SC preserves its high coherence. Highly coherent and broadband mid-infrared SCG in the all-normal dispersion regime was demonstrated by pumping at 8, 10, and 12 μm [3]. However, only few laser sources are able to provide such long wavelengths. Moving the pumping laser wavelengths to around 1.5 or 2 μm will be more attractive because many commercial fiber lasers are available. In this report, we propose a novel tellurite fiber for supercontinuum generation with a pumping laser at 2 μm. The fiber is obtained by adding six solid rods around the core of a step-index fiber. Such fiber is called all-solid hybrid microstructured optical fiber (ASHMOF). The fiber possesses flattened chromatic dispersion from 2 to 4 μm. Successful fabrication of the ASHMOF was done with an in-house drawing tower. Using a laser pumping at 2 μm into the ASHMOF, highly coherent and high spectral flatness supercontinuum spanning a range from 1.4 to 3.0 μm at - 20 dB level was experimentally generated. Such broad and highly coherent SC will be valuable for applications as optical coherence tomography, ultrafast transient absorption spectroscopy, etc.

Fiber optical parametric amplifier (FOPA), which is based on the four-wave mixing (FWM) effect in optical fibers, is an important amplifier in fiber-based communication systems. To date, FOPAs have extensively studied in variety of single mode fibers. Recently, few-mode fiber (FMF) has attracted much attention because of its potential for providing further increase in per-fiber transmission capacity via mode-division multiplexing (MDM) technology. To amplify the signal of MDM system, few-mode FOPA (FM-FOPA) with high gain and large bandwidth are required. So far, a lot of efforts have been made on proposing the structure and design of FMFs for simultaneously amplifying the telecom band signals in different spatial modes via FWM in FMFs, however, the experimental demonstration has not been carried out yet. In this work, using 90-m-long homemade few-mode dispersion-shifted fiber, we demonstrate the first experimental realization of FM-FOPA and study its gain dependence on polarization and spatial mode. The gain spectra of the intramodal FWMs in LP01 and LP11 modes are in the telecom C and S bands, respectively. When the average powers of pulsed pump in LP01 and LP11 modes are 7 mW and 10 mW, the measured gains are about 24.5 dB and 7 dB, respectively. Moreover, we show that the gain equalized amplification can be realized for 1535 nm seed injection in LP01 and LP11 mode, respectively. Our investigation has potential application in developing low noise amplifier for MDM communication systems.

This paper reports on the phase-matching properties of LiIn(S_xSe_1-x)2 for type-2 sum-frequency generation (SFG) between the fundamental and second-harmonic of a CO₂ laser at 10.5910 μm. The calculations based on the Sellmeier and thermo-optic dispersion formulas of the present authors [Appl. Opt. 53, 7998 (2014) / 53, 1063 (2014)] for LiInS₂ and LiInSe₂ revealed that LiIn(S_0.8Se_0.2) is nearly 90° phase-matchable for this process along the y (= a) axis with Δθ_ext·ℓ^1/2 = 33.4 deg·cm^1/2, Δφ_ext·ℓ^1/2 = 13.9 deg·cm^1/2, and ΔΤ·ℓ = 117°C·cm at 20°C.

The suspended-core fluoride fiber was fabricated by rod-in-tube method associated with casting method. Visible dispersive wave (DW) was obtained and tunable Raman soliton pulses from 1680 nm to 1960 nm were generated in the suspended-core ZBLAN fiber pumped by a 1560 nm femtosecond fiber laser. 0.685 W supercontinuum (SC) generation from 350 to 2400 nm is obtained with a launched pump power of 2.2 W. The dispersion-engineered ZBLAN fibers are excellent hosts for ultraviolet to MIR nonlinear optics and SC generation.

The optical properties of the guided modes in the core of photonic crystal fibers (PCFs) can be easily manipulated by changing the air-hole structure in the cladding. A two-dimensional photonic crystal with a silicon dioxide central core and a hexagonal arrangement of air holes is an established method for achieving endless single-mode properties. In addition to guiding the light in the core, various cladding modes exist. The structural parameters of the fiber can also be used to achieve very defined guidance properties in the cladding. We investigated the fiber parameters in the core and cladding modes through both measurements and calculations. The calculated group velocity dispersion (GVD) of different cladding modes based on the measurement of the fiber structure parameters, the hole diameter and the pitch of a hexagonal arrangement of all holes of a real PCF. Based on the scanning electron image, a calculation of the optical guiding properties of the microstructured cladding was performed. The results of the calculations were compared with a wavelength-dependent time delay measurement method using a white light interferometer. The light of a picosecond fiber laser was selectively coupled into the core and cladding of the PCF and a comparison of the conversion properties of both microstructures was made. The supercontinuum based on cladding modes enabled the power limit based on the damage threshold of the core to be exceeded. In the visible range, significantly higher spectral power densities were seen in the cladding over those of the core generated spectral distributions. Subsequently, we investigated the beam quality of the generated supercontinua by measuring the beam propagation factor with a wavefront sensor.

Nonlinear effects are well-known to limit the power scalability of continuous-wave (CW) high power fiber lasers (HPFL). In addition to nonlinear effects than act over the fundamental mode, intermodal nonlinear effects are currently drawing the interest of researchers. Specifically, intermodal nonlinear effects in CW-HPFLs such as degenerate intermodal four-wave mixing (IM-FWM) and stimulated Raman scattering(SRS)-induced intermodal wave-mixing (IM-WM) have been investigated recently; the former generates Stokes and idler at different modes, and the latter transfers power from the fundamental mode to the Raman shifted high-order modes. Here, we report a model that encompasses the aforementioned intermodal couplings in CW-HPFLs and simulate them. More specifically, a model based on multimode generalized nonlinear Schrodinger equations is developed and used to simulate the intermodal couplings in 25/400μm Yb-doped large-mode area fiber amplifiers. Based on the phase-matching condition of IM-FWM, the relation between degenerate IM-FWM frequency shift, modal group velocities, and modal group velocity dispersions is found and applied in the model. By using this model, degenerate IM-FWM and SRS-induced IM-WM, which are intermodal phenomena recently discussed in literatures of CW-HPFLs, are successfully simulated. In addition, a novel intermodal phenomenon is found and discussed, which is the high-order mode second-ordered Stokes resulting from the joint effect of degenerate IM-FWM and SRS-induced intermodal wave-mixing (IM-WM). To the best of our knowledge, this model is the first to include degenerate IM-FWM in the context of CW-HPFLs and reveal the joint effect of the aforementioned intermodal couplings. The result also gives insight of the conditions leading to intermodal couplings.

Lithium triborate crystals LiB₃O₅ (LBO) considered as a high efficiency nonlinear material that is applied to transform laser beam frequency. The most effective application of the crystal is the transformation of neodymium based laser with 1064nm wavelength to the second and to the third harmonic generation. The high damage threshold and operation characteristics of LBO are the reasons to use it in wide-aperture high-power laser systems with aperture diameters up to several cm. Experimentally it was verified that improved crystal growth technique allows to grow the crystals that are given the opportunity to enlarge the dimensions of nonlinear elements by modifying the ratio of the grown crystal dimensions, i.e. habitus.

In this study, the nonlinear orthogonal rotation of a linear polarized optical wave propagating through a nanorods-based hyperbolic metamaterial (NRHMM) was investigated numerically. This process is described by degenerate four-wave mixing (DFWM) of three strong linearly polarized pump waves and a weak generated orthogonal polarized wave, sometime called nonlinear-cross polarized wave (XPW) generation. The efficient nonlinear cross-polarization generation was created by optimal design of NRHMM structure, which made of two-dimensional periodically arrangement of subwavelength-sized indium tin-oxide (ITO) nanorods immersed in barium difluoride (BaF₂) host dielectric material. Numerical results showed that the field intensity of nonlinear XPW conversion are dependent on the incident angle and the intensities of input pumping wave. By optimizing the radius and the lattice formation of ITO nanorods arrangement, the nearly perfect phase-matched condition for the nonlinear process based on hyperbolic phase-matching (HPM) method was achieved implicitly and exhibited by the intersection point of isofrequency contour of each interacting waves in wave-vector space. The intersection points would exhibit the optimal value of incident angle of pumping waves, which satisfy the phase-matched condition. Finally, the maximum conversion efficiencies at various pumping levels were obtained at this condition.

We have demonstrated a simple technique to make continuous wave (CW), tunable visible lasers. In this experiment, the output from a CW, tunable Raman fiber laser (RFL) is frequency doubled using LBO crystal in a single pass configuration. By tuning the output wavelength of RFL from 1.1-1.3um visible light in green to red wavelengths are generated. The efficiency and output power can be substantially improved by placing the crystal within a resonator cavity and pumping using a polarized RFL with narrower linewidth. Also, by tuning the operating wavelength of Yb laser (1060-1100 nm) broadband visible wavelength tunability can be achieved.

Femtosecond optical parametric oscillator (OPO) pumped by second harmonic of Yb: KGW solid state oscillator was investigated. Comparison between LBO and BBO crystals was carried out. The phase mismatch induces cascaded nonlinearity which acts as effective nonlinear refractive index. Negative group delay dispersion (GDD) of the resonator and effective nonlinear refractive index allows linear phase and nonlinear phase compensation leading to soliton like pulse formation. Z-scan experiment was used to determine nonlinear refractive indices under conditions identical to those in the oscillator cavity. Surprisingly, the signs of nonlinear refraction as measured by Z-scan and indirectly observed in optical parametric oscillator were opposite.