This PDF file contains the front matter associated with SPIE Proceedings Volume 10516, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We are developing a 100-W level DUV light source using a solid-state seed laser and an ArF amplifier. Seed pulse at 193nm is generated by the sum frequency mixing of the pulse at 221nm and the pulse at 1553nm using CLBO crystals. To realize a compact and robust seed laser, we improved the conversion efficiency of DUV light generation. In case of the 221nm pulse generation, the conversion efficiency was increased from 22% to nearly 50% by optimizing the conversion process which enables the reduction of amplifier unit of fundamental light source to obtain the 1W seed pulse at 193nm.

We report on results of fifth harmonic generation in Cesium Lithium Borate (CLBO) using a three-crystal cascaded frequency conversion scheme designed to study the energy balance of the final sum frequency generation stage. The experimental setup independently combines the first and fourth harmonic of a Nd:Glass laser in a 5mm thick CLBO crystal. Energy balance between the incoming and output energy is close to unity when the CLBO is out of phase matching and approximately 80% when the crystal is in phase matching. A detailed analysis of the residual fundamental and fourth harmonic energy indicates 5^th harmonic light is being generated but only 26% is unaccounted for. We attribute the missing light to linear transmission loss in the CLBO oven. The ratio of the output to input energy is unity when the missing 5^th harmonic is incorporated into the calculations. Two-dimensional plane wave mixing simulations show agreement with the results at lower intensities.

A wide range of laser medical treatments are based on coagulation of blood by absorption of the laser radiation. It has, therefore, always been a goal of these treatments to maximize the ratio of absorption in the blood to that in the surrounding tissue. For this purpose lasers at 577 nm are ideal since this wavelength is at the peak of the absorption in oxygenated hemoglobin. Furthermore, 577 nm has a lower absorption in melanin when compared to green wavelengths (515 − 532 nm), giving it an advantage when treating at greater penetration depth. Here we present a laser system based on frequency doubling of an 1154 nm Distributed Bragg Reflector (DBR) tapered diode laser, emitting 1.1 W of single frequency and diffraction limited yellow light at 577 nm, corresponding to a conversion efficiency of 30.5%. The frequency doubling is performed in a single pass configuration using a cascade of two bulk non-linear crystals. The system is power stabilized over 10 hours with a standard deviation of 0.13% and the relative intensity noise is measured to be 0.064 % rms.

We report several non-linear process that occur when two-photon pumping the cesium 6D states. Cesium vapor possess some of the largest two-photon pump cross sections in nature. Pumping these cross sections leads to strong amplified spontaneous emission that we observe on over 17 lasing lines. These new fields are strong enough to couple with the pump to create additional tunable lines. We use a heat pipe with cesium densities of 10¹⁴ to 10¹⁶ cm^-3 and 0 to 5 Torr of helium buffer gas. The cesium 6D States are interrogated by both high energy pulses and low power CW sources. We observe four-wave mixing, six-wave mixing, potential two-photon lasing, other unknown nonlinear processes, and the persistence of some processes at low thresholds. This system is also uniquely qualified to support two-photon lasing under the proper conditions.

We report a simple robust and broadly spectral-adjustable source generating near fully compressed 1053 nm 62 fs pulses directly out of a highly-nonlinear photonic crystal fiber. A dispersion-nonlinearity balance of 800 nm Ti:Sa 20 fs pulses was obtained initially by negative pre-chirping and then launching the pulses into the fibers' normal dispersion regime. Following a self-phase modulation spectral broadening, some energy that leaked below the zero dispersion point formed a soliton whose central wavelength could be tuned by Self-Frequency-Raman-Shift effect. Contrary to a common approach of power, or, fiber-length control over the shift, here we continuously varied the state of polarization, exploiting the Raman and Kerr nonlinearities responsivity for state of polarization. We obtained soliton pulses with central wavelength tuned over 150 nm, spanning from well below 1000 to over 1150 nm, of which we could select stable pulses around the 1 μm vicinity. With linewidth of > 20 nm FWHM Gaussian-like temporal-shape pulses with ∼ 62 fs duration and near flat phase structure we confirmed high quality pulse source. We believe such scheme can be used for high energy or high power glass lasers systems, such as Nd or Yb ion-doped amplifiers and systems.

We demonstrate optical frequency comb generation in a continuously pumped optical parametric oscillator, in the parametric region around half of the pump frequency. We also model the dynamics of such quadratic combs using a single time-domain mean-field equation, and obtain simulation results that are in good agreement with experimentally observed spectra. Moreover, we numerically investigate the coherence properties of simulated combs, showing the existence of correlated and phase-locked combs. Our work could pave the way for a new class of frequency comb sources, which may enable straightforward access to new spectral regions and stimulate novel applications of frequency combs.

We report a supercontinuum of high polarization extinction ratio (PER) generated from a birefringent photonic crystal fiber (PCF). The zero dispersion wavelength of the fiber is calculated to be around 1.06μm. The pump source is a linear polarized femtosecond ytterbium-doped fiber laser. The SC performance with the pump pulse linear polarized both along the fast and slow axis of the PCF is studied. The SC ranging from 470nm to 2040nm has an all-spectral PER of -15.5dB, and the corresponding maximum output power is ~1W.

Highly nonlinear fibers (HNLF) is one of the most preferred optical nonlinear mediums for efficient supercontinuum (SC) generation due to its property of good laser beam confinement in a small core area and high nonlinearity. Till date, SC generation in HNLF is explored using very long length fibers, typically sub-kilometre. In this work, for the first time to our knowledge, the physical length of the HNLF used to generate broad SC is reduced by considering the nonlinearity length. This is also first-time demonstration of a very flat and broad SC generation in a short-length of HNLF using carbon-nanotube (CNT) based passively mode-locked erbium-doped femtosecond fiber laser. The CNT based modelocking has achieved a pulse width of 620 fs with a repetition rate of 18 MHz at a centre wavelength of 1565 nm. A single stage amplification is employed on the mode-locked pulses and the spectral evolution of pulse inside the HNLF at various input power levels is studied. A flat and broad SC covering a bandwidth of ~1500 nm ranging from 1000 nm to 2500 nm has been demonstrated successfully. The spectrum throughout the ~1500 nm band is smooth with a flatness of <5 dB. The effect of input pulse polarization on SC spectrum smoothness has been studied and maintained proper polarization state to achieve spectral flatness of <5 dB.

Here we propose a simple and effective system for conversion of 200-fs, 1030 nm pulses into few-optical-cycles pulses in the 3-6 um spectral range. The scheme works as follows. Small part of pump radiation is converted into supercontinuum and the radiation from the 700-900 nm spectral range is used as a seed in the first OPA. The remaining pump beam passes through a BBO crystal which converts less than 3% of the pulse energy into the second harmonic (515 nm). The green beam is used as a pump in first OPA based on 1 mm-think BBO crystal. Collinear geometry is used to generate idler pulses in the 1.2-1.6 um range without angular chirp. The idler beam is then used as a seed in the second collinear OPA, which is pumped by the unconverted 1030 nm beam. The resulting idler pulses in 3-6 um range are slightly chirped depending on the crystal used (LGS, GS, AGS, LiIO3) and are compressed in the bulk material (silicon or germanium). Such scheme provides a passive stabilization of the carrier-to-envelope phase of the resulting pulses. Because of the lower group velocity mismatch (GVM) between interacting pulses in nonlinear infrared crystals, as compared to Ti:sapphire based pump lasers, bandwidths supporting few-optical-cycles pulses in the 3-6 um spectral range are easily obtained. In the experiment we have used 200 uJ pulses (1030 nm) at repetition rate 1-10 kHz and the total quantum conversion efficiency 25-35% was reached. The resulting mid-infrared pulses were fully characterized.

We report on the first demonstration of a linearly polarized cascaded Raman fiber laser based on a simple half-open cavity with a broadband composite reflector and random distributed feedback in a polarization maintaining phosphosilicate fiber operating beyond zero dispersion wavelength (~1400 nm). With increasing pump power from a Yb-doped fiber laser at 1080 nm, the random laser generates subsequently 8 W at 1262 nm and 9 W at 1515 nm with polarization extinction ratio of 27 dB. The generation linewidths amount to about 1 nm and 3 nm, respectively, being almost independent of power, in correspondence with the theory of a cascaded random lasing.

Microresonator frequency combs, or microcombs, provide a new opportunity for ultra-compact and chip-integrated optical and microwave frequency synthesis. A high-Q dielectric microresonator pumped by a continuous wave laser generates a frequency comb (typically 10-1000 GHz mode spacing) via parametric four‑wave mixing. Due to the tight waveguide confinement and high nonlinearity of the integrated-photonics platform, comb generation occurs with only milliwatts of input power. Additionally, precise geometric control of the waveguide dispersion has led to ultra‑broad bandwidth microcombs suitable for f-2f self-referencing. We have leveraged these advances to demonstrate a dual microcomb optical frequency synthesizer that employs inter-locked silicon nitride and silica microcombs. A silicon nitride resonator with 1 THz mode spacing provides an octave-spanning spectra for self-referencing and connection of the 200 THz pump laser to the silicon nitride repetition rate. The mode-spacing of this comb is subsequently phase-locked with a 22 GHz silica comb that provides the countable repetition rate and final link between microwave and optical domains. This dual microcomb system thus forms a backbone in the telecom C-band against which a tunable chip-integrated laser is phase-locked with user-defined frequency at 1 Hz resolution and absolute accuracy. The core elements of this system are fully reversible, and can also be used to generate low-noise microwaves on both short and long timescales.

The tunability of cw optical parametric oscillators can be extended in the mid-infrared spectral range by using whispering gallery resonators (WGRs) made of non-oxide crystals. We demonstrate the fabrication of WGRs from silver gallium selenide, cadmium silicon phosphide and orientation-patterned gallium phosphide with quality factors above 10⁶ . The resonators made of the first two provide optical parametric oscillation in the mid-infrared pumped by a compact laser diode at 1.57 μm wavelength. The wavelength tunability of the device based on silver gallium selenide provides a tuning range beyond 8 μm wavelength. These achievements are considered as the first steps of cw optical parametric oscillators into this important spectral region.

The generation of THz-frequency radiation via nonlinear parametric frequency down-conversion has long been driven by the spectroscopy and imaging communities. As a result, little efforts have been undertaken toward the generation of high energy THz-frequency pulses. THz-frequency radiation has however recently been identified has a promising driver for strong-field physics and an emerging generation of compact particle accelerators. These accelerators require THzfrequency pulses with energies in the multi-millijoule range therefore demanding orders of magnitude improvements from the current state-of-the-art.

Much can be gained by improving the intrinsically low efficiency of the down-conversion process while still resorting to existing state-of-the-art lasers. However, the fundamental Manley-Rowe limit caps the efficiency of parametric downconversion from 1-μm wavelength lasers to sub-THz frequency to the sub-percent range.

We present methods that promise boosting the THz radiation yield obtained via parametric down-conversion beyond the Manley-Rowe limit. Our method relies on cascaded nonlinear three-wave mixing between two spectrally neighboring laser pulses in periodically poled Lithium Niobate. Owing to favorable phase-matching, the down-conversion process avalanches, resulting in spectral broadening in the optical domain. This allows in-situ coherent multiplexing of multiple parametric down-conversion stages within a single device and boosting the efficiency of the process beyond the ManleyRowe limit. We experimentally demonstrated the concept using either broadband, spectrally chirped optical pulses from a Joule-class laser or using two narrowband lasers with neighboring wavelengths. Experimental results are backed by numerical simulations that predict conversion efficiencies from 1 μm to sub-THz radiation in the multi-percent range.

A compact and flexible dual-wavelength laser with combined two laser crystals (a-cut and c-cut Nd:YLF) as the gain media under coaxially laser-diode (LD) end-pumping configuration was demonstrated and μW-level THz wave was generated based on difference frequency generation (DFG) in a GaSe crystal. The dynamics of coaxial pumping dualwavelength laser was theoretically investigated, showing that the power ratio and pulse interval for both wavelengths could be tuned by balancing the gains at both wavelengths via tuning pump focal position. Synchronized orthogonal 1047/1053 nm laser pulses were obtained and optimal power ratio was realized with the total output power of 2.92W at 5 kHz pumped by 10-W LD power. With an 8-mm-long GaSe crystal, 0.93 μW THz wave at 1.64 THz (182 μm) was generated. Such coaxially LD end-pumped lasers can be extended to various combinations of neodymium doped laser media to produce different THz wavelengths for costless and portable applications.

A simple scheme for nonlinear refractive index measurement via FROG reconstruction is presented. Samples are measured in weakly focused beam of uJ-level 200 fs pulses. Computing intensity profile and temporal phase for a situation with and without the sample in the beam path yields SPM-induced phase contribution. Through fitting of that phase to intensity profile, n2 of a measured material can be directly calculated. Different optical materials have been measured (fused silica, calcite, YVO­4, BiBO, YAG, CaF2) and are shown to agree well with literature.

Recently some of us have shown that the use of a correct ab initio approach to nonlinear pulse propagation simulations during nonlinear optical device designing can result in significant efficiency increase with respect to the efficiency of existing solutions [1]. Herewith, we present the results of expanding our model to treatment of noncolinear optical configurations. To the best of our knowledge this is the first propagation model that enables noncolinear configurations while in minimal assumptions regime: unidirectionality and paraxial approximation. The noncolinear propagation is required when interaction of two or more beams is considered. In this case the use of not necessarily slowly varying envelope for each interacting beam is justified from both physical and computational reasons. In case of 3D noncolinear propagation the above conclusion leads to the concept of reference wavevector. Our model is based on unidirectional pulse propagation equation (UPPE) in a rotated frame of reference. Different rotation has to be performed for each of the interacting beams. Finally the initial conditions (rotated optical pulses) can be prepared through arbitrary 3D rotation through Fourier Transform shear operations. The idea, realization and advantages of the above mentioned, novel concepts: reference wavevector, rotated UPPE and arbitrary Fourier rotation will be discussed in the presentation. It will be supported by examples of simulation of chirped pulse noncolinear optical parametric amplifier (OPCPA), fluorescence up-conversion and four-wave mixing. 1. T. M. Kardaś, M. Nejbauer, P. Wnuk, B. Resan, C. Radzewicz, and P. Wasylczyk, Scientific Reports 7, 42889 (2017).

Calcium fluoride, BK7 and fused silica are common optical materials used in lenses and windows. In this report, we discuss supercontinuum generation using tunable femtosecond mid-IR laser pulses with wavelengths ranging from 2.7 μm to 7.0 μm and pulse energies between 3 and 18 microjoules. We observed harmonic generation in fused silica and BK7, but not supercontinuum generation. Other borosilicate targets generated supercontinuum in both visible and near infrared regions of the spectrum. The visible supercontinuum was, in some instances, strong enough to be observed directly by the human eye. These results contribute to ongoing work being done to refine eye safety standards for femtosecond lasers.

Filamentation in air is a profound effect caused by high energy photons. While it has been studied in a wide-range of laser systems, there still exist wavelength regimes where filamentation hasn’t been created, due to lack of sources. Using a tunable near-infrared femtosecond laser, we generated filamentation in air by wavelengths from 1.2 to 2.5 µm. The observed filaments produced harmonic and continuum generation well into the visible spectrum; a rainbow of colors.

In this work, we report and analyse the surprising observation of a rainbow of visible colors, spanning 390nm to 620nm, in silica-based, Near Infrared, continuous-wave, cascaded Raman fiber lasers. The cascaded Raman laser is pumped at 1117nm at around 200W and at full power we obtain ∼100 W at 1480nm. With increasing pump power at 1117nm, the fiber constituting the Raman laser glows in various hues along its length. From spectroscopic analysis of the emitted visible light, it was identified to be harmonic and sum-frequency components of various locally propagating wavelength components. In addition to third harmonic components, surprisingly, even 2^nd harmonic components were observed. Despite being a continuous-wave laser, we expect the phase-matching occurring between the core-propagating NIR light with the cladding-propagating visible wavelengths and the intensity fluctuations characteristic of Raman lasers to have played a major role in generation of visible light. In addition, this surprising generation of visible light provides us a powerful non-contact method to deduce the spectrum of light propagating in the fiber. Using static images of the fiber captured by a standard visible camera such as a DSLR, we demonstrate novel, image-processing based techniques to deduce the wavelength component propagating in the fiber at any given spatial location. This provides a powerful diagnostic tool for both length and power resolved spectral analysis in Raman fiber lasers. This helps accurate prediction of the optimal length of fiber required for complete and efficient conversion to a given Stokes wavelength.

The heavy fluoride fiber, ZBLAN (ZrF₄-BaF₂-LaF₃-AlF₃-NaF) is the commonly used fiber to generate supercontinuum (SC) in mid-IR region due to its transparency, high glass stability and technology maturity. Multi-stage mid-IR SC generation that involves multi-stage SC generation and multi-stage amplification is adopted widely. The system is generally complex, and suffers poor stability. Here, we reduced the complexity and improved stability and compactness of the system by using single-stage SC generation approach. For the first time to our knowledge, we demonstrate a carbon-nanotube (CNT) femtosecond passively mode-locked erbium-doped fiber laser based SC generation in ZBLAN fiber. The CNT based mode-locking has achieved a pulse width of 620 fs with a repetition rate of 18 MHz at a centre wavelength of 1565 nm. A single stage amplification is employed on the mode-locked pulses and the trend of spectral broadening inside the ZBLAN fiber is studied experimentally at different input power levels. The study of the SC generation inside the ZBLAN fiber at high input powers is limited by the OSA range. We successfully demonstrated an SC generation inside ZBLAN fiber covering a bandwidth of over 2300 nm extending from 900 nm to beyond 3200 nm at an input power of 200 mW. We achieved a smooth spectrum with spectral flatness of less than 8 dB. The relationship between the generated SC bandwidth and input power to the ZBLAN fiber is observed to be almost linear in log scale, with ~110 nm bandwidth increments per unit dBm.

We report on a metrological-grade mid-IR source with a 10^–14 short-term instability for high-precision spectroscopy. Our source is based on the combination of a quantum cascade laser and a coherent radiation obtained by difference-frequency generation in an orientation-patterned gallium phosphide (OP-GaP) crystal. The pump and signal lasers are locked to an optical frequency comb referenced to the primary frequency standard via an optical fiber link. We demonstrate the robustness of the apparatus by measuring a vibrational transition around 6 μm on a metastable state of CO molecuels with 11 digits of precision.

Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process. Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.

In the past two decades, ultrashort pulse lasers oscillators and amplifiers became common equipment in the fundamental scientific exploration as well as in handful of industrial applications. Those sources, which by their nature are broadband and coherent, allow exploring processes and dynamics in nature at ultrafast time scale. Due to the extremely high peak power, nonlinear optics in the ultrashort regime results in efficient frequency conversion generation processes. Among the various nonlinear conversion processes, three wave mixing and especially second harmonic generation (SHG) became widely used. Yet, frequency conversion in the ultrashort regime usually exhibits a tradeoff between the conversion bandwidth and the conversion efficiency due to the phase mismatch between the interacting waves. In the last decade, adiabatic frequency conversion method has overcome the tradeoff between conversion efficiency and bandwidth for sum frequency generation, difference frequency generation, Optical parametric amplification and recently in SHG processes. Here, we experimentally demonstrate that an adiabatic design is capable of extremely robust and efficient SHG at power levels characteristic of high-rep-rate femtosecond oscillators. We show that with pulse peak energies of nJ, one can achieve above 50% of energy conversion efficiency for 70fs Ti-Sapphire pulses. Furthermore, the flat conversion response of the presented design allows performing broadband pulse shaping manipulations before the nonlinear optical conversion. More specifically, using a spatial light modulator in a 4-f configuration, we present a tunable pump-probe based on a varying spectral phase profile in the frequency domain. Additionally, we show that by applying a π-step spectral phase, coherent control of the SHG spectrum can be achieved.

Frequency conversion in orientation-patterned (OP) materials is a leading approach for generating mid- and long-wave IR radiation. Although several phase-matching and quasi-phase-matching (QPM) materials have been investigated to date none of these have met all requirements for power, tunability and frequency range of the pursuit applications. We present an original approach that successfully combines in a QPM heterostructure two of the most promising materials, GaP—a material with lower two and three-photon absorption than GaAs, and GaAs—a material with a mature process for fabrication of high quality OP templates. Up to 300 µm thick OPGaP with excellent domain fidelity has been repeatedly grown with 100 µm/h by hydride vapor phase epitaxy on the robust and high quality OPGaAs templates. Some simplifications of both template fabrication and growth process are also reported. The samples, characterized by AFM, SEM, XRD, EDS and TEM, showed smooth surface morphology and high crystalline quality. Special attention was paid to the interface and especially to the mechanism of forming an intermediate ternary transition layer. This led to determining certain criteria that indicate, which other heteroepitaxial cases would be also successful. Thick growths of GaAsP and GaP on other alternative substrate materials by combining a-close-to-equilibrium with a-far-from-equilibrium processes were also performed. Efforts to develop heterostructures in horizontal and vertical direction have been also made. The success with one less favorable (in point of view of lattice mismatch) case, presented here, indicates that we should have even better results in other cases with closer lattice matches.

Two new mid-IR NLO materials - barium thiogallate (BaGa4S7) and barium sellenogallate (BaGa4Se7) – have recently been reported to exhibit notably wide band gaps (3.54 eV and 2.64 eV repectively), deep infrared transparency (cut-off at 13.7 m and 18 m respectively), moderately high nonlinear coefficients (5 pm/V and 20 pm/V), and favorable growth of high-optical-quality crystals from stoichiometric melts. To evaluate this final claim, and to establish a viable commercial source for these materials, we investigated the growth of these compounds in transparent furnaces using the Horizontal Gradient Freeze (HGF) technique. The compounds were synthesized from high-purity elemental starting materials by direct synthesis in carbon-coated boats in fused-silica ampoules. Self-seeded, crack-free single crystals (up to 19-mm diameter x 50-mm long) were in fact grown in a single attempt with good optical quality. FTIR and VIS/NIR spectral measurements confirmed the low losses and wide transparency range of these materials. Orientation and fabrication of device crystals (aided by facets on the as-grown boules) is in progress.

CdSiP2 (CSP) is a nonlinear optical chalcopyrite semiconductor developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation with widely-available 1- and 1.55-micron pump laser sources. CSP has an even higher nonlinear coefficient (d36=84.5 pm/V) than ZGP (d36=79 pm/V), and its lower thermal conductivity (13.6 W/mK vs 35 W/mK for ZGP) is more than offset by nearly 10-fold lower absorption losses in the 1.06- to 2.1-micron wavelength range, making CSP an attractive alternative to ZGP even for power-scaling 2-micron-pumped OPOs. CSP growth presents significant crystal growth challenges compared to ZGP including: a higher melting point and vapor pressure that push the limits of fused silica based growth technology, a higher reactivity with boat materials and fused silica ampoules, an increased incidence of twin formation, and a negative c-axis thermal expansion coefficient (which makes it prone to cracking). Despite these difficulties, recent advances in crystal growth from stoichiometric melts using the horizontal gradient freeze (HGF) technique have resulted in scaling boule diameters from 19 to 28 millimeters. Improved refractive index data was recently measured at AFRL over a wide temperature range from 85K through 450K and fit to a temperature-dependent Sellmeier equation. More accurate thermal conductivity measurements along the major crystallographic axes are also reported. Finally, we report the first multi-watt CSP OPO: over 5 Watts of average power output (near-degenerate signal plus idler) were achieved at a slope efficiency of 60% from tandem, walk-off-compensated CSP crystals in a linear resonator pumped by a q-switched 32-nanosecond 1.94-micron Tm:YAP laser.

Orientation-patterned gallium phosphide (OP-GaP) is a recently developed nonlinear material with wide transparency across 0.8-12 μm and high nonlinearity (d₁₄∼70 pm/V), which is a promising candidate material for mid-infrared generation. Here we report the full performance characterization of a tunable single-pass nanosecond difference frequency generation (DFG) source based on OP-GaP by mixing the output of a Q-switched Nd:YAG laser at 1.064 μm with the signal from a pulsed MgO:PPLN OPO pumped by the same laser. Using the longest OP-GaP crystal (40 mm) deployed to date, the DFG source provides up to ∼14 mW of average output power at 2719 nm at 80 kHz repetition rate, with >6 mW across 2492-2782 nm, in TEM₀₀ spatial profile. By performing relevant measurements, detrimental issues such as residual absorption and thermal effects have been studied and confirmed. The temperature and spectral acceptance bandwidths for DFG in the 40-mm-log OP-GaP are measured to be 18 °C and 17 nm, respectively, at 1766 nm. The DFG beam exhibits passive power stability better than 1.7% rms over 1.4 hours at 2774 nm, compared to 1.6% and 0.1% rms for the signal and pump, respectively. The polarization dependence of the input beams on the DFG power has also been systematically investigated, for the first time to our knowledge. Further, we have measured the damage threshold of the OP-GaP crystal to be 0.8 J/cm² at 1064 nm.

Since the early 1990’s, a substantial effort has been devoted to the development of quasi-phased-matched (QPM) nonlinear devices, not only in ferroelectric oxides like LiNbO₃, LiTaO₃ and KTiOPO₄ (KTP), but also in semiconductors as GaAs, and GaP. The technology to implement QPM structures in ferroelectric oxides has by now matured enough to satisfy the most basic frequency-conversion schemes without substantial modification of the poling procedures. Here, we present a qualitative leap in periodic poling techniques that allows us to demonstrate devices and frequency conversion schemes that were deemed unfeasible just a few years ago. Thanks to our short-pulse poling and coercive-field engineering techniques, we are able to demonstrate large aperture (5 mm) periodically poled Rb-doped KTP devices with a highly-uniform conversion efficiency over the whole aperture. These devices allow parametric conversion with energies larger than 60 mJ. Moreover, by employing our coercive-field engineering technique we fabricate highlyefficient sub-µm periodically poled devices, with periodicities as short as 500 nm, uniform over 1 mm-thick crystals, which allow us to realize mirrorless optical parametric oscillators with counter-propagating signal and idler waves. These novel devices present unique spectral and tuning properties, superior to those of conventional OPOs. Furthermore, our techniques are compatible with KTA, a KTP isomorph with extended transparency in the mid-IR range. We demonstrate that our highly-efficient PPKTA is superior both for mid-IR and for green light generation – as a result of improved transmission properties in the visible range. Our KTP-isomorph poling techniques leading to highly-efficient QPM devices will be presented. Their optical performance and attractive damage thresholds will be discussed.

Ultrafast optical parametric sources for the mid-infrared (mid-IR) based on the new birefringent nonlinear crystal, cadmium silicon phosphide, CdSiP₂, using well-established Nd-based solid-state and Yb-based fiber pump lasers near 1 μm are presented. Optical parametric generators (OPGs) and optical parametric oscillators (OPOs) providing high-energy picosecond pulses, as well as high-repetition-rate femtosecond pulses at high average powers across the 6-7 μm spectral range in the mid-IR are reviewed. Various pump laser technologies suitable for ultrafast femtosecond OPOs based on CdSiP₂ are discussed. These results represent the highest output pulse energy and record average power in the ultrafast picosecond and femtosecond time-scales across the 6-7 μm spectral range.

Cadmium silicon phosphide, CdSiP2 or CSP, exhibits the highest nonlinear Figure of Merit (d2/n3) among all bulk birefringent crystals that can be pumped at wavelengths ≤ 2 microns - and the highest among all nonlinear optical (NLO) crystals at any wavelength with practically low absorption losses. Although a 2-micron pumped optical parametric oscillator (OPO) and numerous 1-micron-pumped CSP OPOs have been demonstrated previously, here we report the first 1.5-micron-pumped CSP OPO. The pump laser for this experiment was a non-critically phase-matched (NCPM) KTP singly-resonant OPO generating 6.3 ns, 10 mj pulses at 1.57 microns, which was in turn pumped by a 1.064-m 5-Hz, 7-ns, 20-mJ Nd:YAG laser. The 1.57-m signal from the KTP OPO pumped a 5x5 mm2 x 5.5-mm long CSP crystal cut for type I phase matching at 63°) inside a linear flat-flat cavity. The measured absorption coefficient at 1570 nm was approximately 0.015cm-1. The input mirror was highly transmitting (97%T) at 1.57 microns and highly reflective from 2.5 to 3.8 microns. The best results were achieved at signal and idler wavelengths of 2.67 microns (50%R output coupling) and 3.83 microns (30%R output coupling) respectively, yielding 2.24 mJ of combined signal and idler output energy for a total optical conversion efficiency of 32%. The slope efficiency was 42%. At 66 MW/cm2 pump peak intensity the OPO was running nearly 4 times over threshold with no signs of roll-over. The CSP crystal was angle-tuned to produce output wavelengths ranging from 2.2 microns to 5.05 microns.

We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared (deep-IR) based on the nonlinear optical crystal, CdSiP₂ (CSP), pumped directly by a Ti:sapphire laser, for the first time. By pumping CSP at <1 μm, we have achieved practical output powers at the longest wavelengths generated by any Ti:sapphire-pumped OPO. Using a combination of pump wavelength tuning, type-I critical phase-matching, and cavity delay tuning, we have generated continuously tunable radiation across 6654−8373 nm (1194−1503 cm^-1) at 80.5 MHz repetition rate, providing up to 20 mW of average power at 7314 nm and <7 mW beyond 8000 nm, with idler spectra exhibiting bandwidths of 140−180 nm across the tuning range. Moreover, the near-IR signal is tunable across 1127−1192 nm, providing up to 37 mW of average power at 1150 nm. Signal pulses, characterised using intensity autocorrelation, have durations of ∼260–320 fs, with corresponding time-bandwidth product of ∆υ∆τ∼1. The idler and signal output exhibit a TEM00 spatial profile with single-peak Gaussian distribution. With an equivalent spectral brightness of ∼6.68×10²⁰ photons s^-1 mm^-2 sr^-1 0.1% BW^-1, this OPO represents a viable table-top alternative to synchrotron and supercontinuum sources for deep-IR applications in spectroscopy, metrology and medical diagnostics.

We employed mercury thiogallate (HgGa2S4 or HGS) to achieve the highest average output power from a 1 µm synchronously pumped optical parametric oscillator (SPOPO) generating femtosecond idler pulses continuously tunable in the mid-IR. HGS is one of the most promising wide band-gap non-oxide nonlinear crystals without two-photon absorption (TPA) losses at such pump wavelengths. The SPOPO was pumped by an 80 MHz mode-locked Yb-fiber laser with 6.5 W of average power at 1.035 µm and a pulse duration of 220 fs. Continuous tuning of the non-resonated idler was achieved from 4.4 to beyond 12 µm with an average output power of over 350 mW at 5.2 µm (>2 W at 1.25 µm for the signal) and tens of mW at the edges of the idler tuning range (hundreds of mW for the signal). The quantum efficiency of the device exceeded 20 % up to 8 µm (> 30% in the maximum). The output of the OPO was diffraction limited with a beam quality factor (M²) below 1.1. No crystal damage was observed at average power densities as high as 60 kW / cm² although using lower power densities was necessary to achieve the best possible beam quality. The only other crystals that were successfully employed in lower power SPOPOs in the past, CdSiP2 (CSP) and OPGaP, lack either the tuning range due to limited mid-IR transparency or the capability for continuous tuning.

PPLN based optical parametric oscillators pumped by high power lasers around 1 µm are well established sources for generating light in the 3-5 µm spectral region, of interest for a wide range of scientific, commercial and military applications. We have been investigating optical parametric amplification (OPA), or difference-frequency generation (DFG), single-pass alternatives to conventional resonant OPOs. This avoids the need for a cavity and the corresponding design constraints that this can impose; such as, fixed repetition rates, sensitive alignment and/or poor output beam qualities at high average power levels. In this paper, we review recent results on high average power ( > 6 W) nanosecond pulse generation in the 3.3-3.5 μm region at MHz repetition rates, employing Yb:fibre and Er:fibre master oscillator power amplifiers (MOPA) systems pumping PPLN OPAs. We use focused Gaussian beam theory to validate the experimental results. We will also discuss spectral extension further into the mid-infrared, using different nonlinear crystal and alternative rare-earth doped fibre MOPA and Raman shifted fibre laser combinations. Ongoing work aimed at the power scaling of the mid-infrared light in both the nanosecond pulsed and continuous wave regimes will be presented.

Optical Parametric Oscillator (OPO) is a well-known solution when wide tunability in the mid-infrared is needed. A specific design called NesCOPO (Nested Cavity doubly resonant OPO) is currently integrated in the X-FLR8 portable gas analyzer from Blue Industry and Science. Thanks to its low threshold this OPO can be pumped by a micro-chip nanosecond YAG (4 kHz repetition rate and a 30 GHz bandwidth). To achieve very high resolution spectra (10 pm of resolution or better), the emitted wavelength has to be finely controlled. Commercial Wavemeter do not meet price and compactness required in the context of an affordable and portable gas analyzer. To overcome this issue, Blue first integrated an active wavelength controller using multiple tunable Fabry-Perot (FP) interferometers. The required resolution was achieved at a 10 Hz measurement rate. We now present an enhanced Wavemeter architecture, based on fixed FP etalons, that is 100 times faster and 2 times smaller. We avoid having FP ‘blind zones’ thanks to one source characteristic: the knowledge of the FSR (Free Spectral Range) of the OPO source and thus, the fact that only discrete wavelengths can be emitted. First results are displayed showing faster measurement for spectroscopic application, and potential future improvement of the device are discussed.

We demonstrate an optical parametric oscillator (OPO) based on random phase matching in a polycrystalline χ⁽²⁾ material, ZnSe. The subharmonic OPO utilized a 1.5-mm-long polished ZnSe ceramic sample placed at the Brewster's angle and was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a central wavelength of 2.35 μm, a pulse duration of 62 fs, and a repetition frequency of 79 MHz. The OPO had a 90-mW pump threshold, and produced an ultrabroadband spectrum spanning 3-7.5 μm. The observed pump depletion was as high as 79%. The key to success in achieving the OPO action was choosing the average grain size of the ZnSe ceramic to be close to the coherence length (~ 100 μm) for our 3-wave interaction. This is the first OPO that uses random polycrystalline material with quadratic nonlinearity and the first OPO based on ZnSe. Very likely, random phase matching in ZnSe and similar random polycrystalline materials (ZnS, CdS, CdSe, GaP) represents a viable route for generating few-cycle pulses and multi-octave frequency combs, thanks to a very broadband nonlinear response.

The last couple of years have witnessed an unprecedented boost in nonlinear nanophotonics, allowed both by constant technological evolution in the fabrication of integrated micro- and nano-structures, and by a deeper understanding of multipolar magnetic and electric response of all-dielectric sub-wavelength nanoparticles. Besides reviewing the recent progress in the hybrid and monolithic semiconductor platforms that are being employed to demonstrate parametric sources on chip, we will focus on high-contrast nonlinear nanophotonic sources in the near-infrared spectral region, where plasmonic devices fall short of broad applicability. To confine photons at the nanoscale by total internal reflection, metal-less devices rely on a high-refractive-index core surrounded by low-index cladding in one or more dimensions, and therefore they typically consist of semiconductor nanostructures that either lie on an oxide substrate or are suspended in air. We will report on our results on χ(2) AlGaAs suspended nanowires [1] and AlGaAs-on-AlOx nanoantennas [2], with a perspective view on collective nonlinear effects in 1D and 2D arrays. Finally we will comment on the most sought applications of integrated parametric sources, including optical frequency comb generators and optical parametric oscillators on-chip with sub-milliwatt threshold. [1] N. Morais, I. Roland, M. Ravaro, I. Favero, A. Lemaître, S. Wabnitz, M. De Rosa, and G. Leo, SPIE Nanophotonics Australasia 2017, Melbourne 10-13/12/2017. [2] V. F. Gili, L. Carletti, A. Locatelli, D. Rocco, M. Finazzi, L. Ghirardini, I. Favero, C. Gomez, A. Lemaître, M. Celebrano, C. De Angelis, and G. Leo, Opt. Express 24, 15965 (2016).

This paper reports the 90°phase-matched Hg_0.35Cd_0.65Ga₂S₄ optical parametric oscillator (OPO) pumped by a Nd:YAG laser at 1.0642 μm in the 8.24-9.40 μm range. The resulting data for the temperature tuning curves of the two idler wavelengths were found to agree well with the theoretical values calculated with the Sellmeier and thermo-optic dispersion formulas constructed from those of HgGa₂S₄ and CdGa₂S₄ presented in our previous paper (K. Kato et al. Opt. Commun. 386, 49-52, (2017)). The OPO threshold was ~ 0.3 W, which is 57% lower than ~ 0.7 W observed for the 8-mm-long HgGa₂S₄ crystal.

We discuss results on a novel fabrication scheme for optical ridge waveguides in KTP by combination of ridge definition using a diamond blade dicing saw and Rb/Ba ion exchange. In contrast to conventional waveguides in KTP, our samples were prepared in substrates that have their z-axis lying in the surface plane, perpendicular to the waveguide. The ridge waveguides were ion-exchanged through their cut flanks followed by further annealing treatment in air. This results in a homogeneous refractive index increase across the rectangular waveguide cross section. Since Rb ions are only mobile along the z-axis, they cannot diffuse into the depth. Therefore, the fabricated waveguides can withstand elevated temperatures. We determined propagation losses to be only 0.3dB/cm (0.4dB/cm) for TE (TM) polarization at ∼1060nm. These novel waveguides can allow for improved mode overlap in frequency conversion processes and better coupling of light from a fiber to the waveguide.

This paper reports on the phase-matching properties of LiGaS₂ for upconverting a Nd:YAG laser-pumped KTP and AgGaS₂ optical parametric oscillator (OPO) at mid-IR to the near-IR by mixing with its pump source together with the new Sellmeier equations that provide a good reproduction of the present experimental results as well as the published data points of second-harmonic generation (SHG) and sum-frequency generation (SFG) of a CO₂ laser, a Ti:Al₂O₃ laser-pumped optical parametric amplifier (OPA), and a Nd:YAG laser-pumped OPO in the mid-IR.

This index formula gives the important information that group velocity mismatch (GVM) (Δ_sp = 1/υ_s - 1/υ_p) of LiGaS₂ in the 4 - 11 μm range is 12 ~ 27 fs/mm lower than that of the widely used LiInS₂, which makes it ideal for the upconversion of the mid-IR femtosecond pulses having large spectral bandwidths to the near-IR.

Mid-IR wavelength range (between 2 and 8 μm) offers perspective applications, such as minimally-invasive neurosurgery, gas sensing, or plastic and polymer processing. Maturity of high average power near-IR lasers is beneficial for powerful mid-IR generation by optical parametric conversion. We utilize in-house developed Yb:YAG thin-disk laser of 100 W average power at 77 kHz repetition rate, wavelength of 1030 nm, and about 2 ps pulse width for pumping of a ten-watt level picosecond mid-IR source. Seed beam is obtained by optical parametric generation in a double-pass 10 mm long PPLN crystal pumped by a part of the fundamental near-IR beam. Tunability of the signal wavelength between 1.46 μm and 1.95 μm was achieved with power of several tens of miliwatts. Main part of the fundamental beam pumps an optical parametric amplification stage, which includes a walk-off compensating pair of 10 mm long KTP crystals. We already demonstrated the OPA output signal and idler beam tunability between 1.70-1.95 μm and 2.18-2.62 μm, respectively. The signal and idler beams were amplified up to 8.5 W and 5 W, respectively, at 42 W pump without evidence of strong saturation. Thus, increase in signal and idler output power is expected for pump power increase.

Photonic circuits suffer from thermal drift of device performance, which is a key obstacle to the development of commercial optoelectronic products. Temperature-insensitive integrated waveguides and resonators have been demonstrated by using materials with a negative TOC at a single wavelength, which are not suitable for WDM devices and wideband nonlinear devices. Here, we propose a waveguide structure with temperature-insensitivity over a bandwidth of 780 nm (1280 to 2060 nm) with an ultra-small effective TOC within ±1×10^-6/K. Uniquely, the waveguide has small anomalous dispersion (from 66 to 329 ps/nm/km) over the same band and is suitable for frequency comb generation without being affected by intra-cavity thermal dynamics.

The present work proposes and studies approaches for development of new modified non-linear amplifying loop mirror (NALM) allowing flexible and dynamic control of their non-linear properties within a relatively broad range of radiation powers. Using two independently pumped active media in the loop reflector constitutes one of the most promising approaches to development of better NALM with nonlinear properties controllable independently of the intra-cavity radiation power. This work reports on experimental and theoretical studies of the proposed redesigned NALM allowing both a higher level of energy parameters of output generated by mode-locked fibre oscillators and new possibilities of switching among different mode-locked regimes.

We focused on wavelength conversion of simple and compact CW Yb-Doped fiber laser based on FBGs with wavelength adjustable function. By controlling temperatures of FBGs in fiber laser, it was possible to tune oscillated wavelength from 1064.101 nm to 1064.414 nm with more than 20 W in CW operation mode. Based on this fundamental light, frequency converted light (SHG and THG) were generated by utilizing two PP:Mg-SLT devises. We obtained more than 3 W of SHG light with tuning range of 150 pm and more than 35 mW of THG with tuning range of 100 pm. By selecting FBG grating and QPM grating properly, we can realize adjustable wavelength laser with the same scheme from 1040 nm to 1090 nm and their SHG/THG. With this combination of FBG based fiber laser and QPM devices, it is possible to tune the wavelength just by temperature tuning without any changes of beam shape and beam pointing.

Demand for bandwidth in optical communications necessitates the development of scalable transceivers that cater to these needs. For this, in DWDM systems with/without Superchannels, the optical source needs to provide a large number of optical carriers. The conventional method of utilizing separate lasers makes the system bulky and inefficient. A multi-wavelength source which spans the entire C-band with sufficient power is needed to replace individual lasers. In addition, multi-wavelength sources at high repetition rates are necessary in various applications such as spectroscopy, astronomical spectrograph calibration, microwave photonics and arbitrary waveform generation. Here, we demonstrate a novel technique for equalized, multi-wavelength source generation which generates over 160 lines at 25GHz repetition rate, spanning the entire C-band with total power >700mW. A 25GHz Comb with 16 lines is generated around 1550nm starting with two individual lasers using a system of directly driven, cascaded intensity and phase modulators. This is then amplified to >1W using an optimized, Erbium-Ytterbium co-doped fiber amplifier. Subsequently, they are passed through Highly NonLinear Fiber at its zero-dispersion wavelength. Through cascaded Four Wave Mixing, a ten-fold increase in the number of lines is demonstrated. A bandwidth of 4.32 THz (174 lines, SNR>15 dB), covering the entire C-band is generated. Enhanced spectral broadening is enabled by two key aspects - Dual laser input provides the optimal temporal profile for spectral broadening while the comb generation prior to amplification enables greater power scaling by suppression of Brillouin scattering. The multi-wavelength source is extremely agile with tunable center frequency and repetition rate.

We present an experimental study of liquid refractive index effects into Ytterbium ring fiber laser cavity configuration. The laser is operated using a bi-tapered optical fiber immersed in water-alcohol concentrations. When the tapered fiber is dipped into a distilled water, a single lasing line with a peak power centered at 1025 nm is achieved. Afterward, by changing the polarization state into the cavity the lasing line can be switched. Moreover, by modifying the refractive index liquid surrounding media the lasing lines can be controlled and special liquid provide nonlinear response. The laser offers compactness, low effective cost and good stability.

Multilayered hyperbolic metamaterials (MHM) is proposed to create phase matching of fundamental-frequency (FF) and third-harmonic (TH) field components with a unique dispersion provided by the engineered MHM structure for third-harmonic generation in ultra-short pulse regime. In this work, we analytically study the ensuing possibilities and demonstrate that a birefringent phase-matching can be alternatively achieved with a wide range of involved material parameters and optimal engineering of MHM structure. When the phase-matched conditions is satisfied by birefringent phase-matching method, the growth rate of the TH intensity generating as a function of the nonlinear-optical interaction length to be obviously increased. This method opens new ways of improving the conversion efficiency of frequency tripling regardless of the coherence length in the bulk of a nonlinear material.

Beta-Barium Borate (β-BBO) crystal is commonly used in nonlinear frequency conversion from visible to deep ultraviolet (DUV). However, in a single crystal BBO, its large spatial walk-off effect will reduce spatial overlap of ordinary and extraordinary beam, and thus degrade the conversion efficiency. To overcome the restrictions in current DUV conversion systems, Onyx applies adhesive-free bonding technique to replace the single crystal BBO with a spatial Walk-off Compensated (WOC) BBO stack, which is capable of correcting the spatial walk-off while retaining a constant nonlinear coefficient in the adjacent bonding layers. As a result, the β-BBO stack will provide good beam quality, high conversion efficiency, and broader acceptance angle and spectral linewidth, when compared with a single crystal of BBO.

In this work, we report on performance of a spatial walk-off compensated β-BBO stack with adhesive-free bonding technique, for efficiently converting from the visible to DUV range. The physics behind the WOC BBO stack are demonstrated, followed by simulation of DUV conversion efficiency in an external resonance cavity. We also demonstrate experimentally the beam quality improvement in a 4-layer WOC BBO stack over a single BBO crystal.

In the last years, the Mid Infrared (MIR) spectral region has attracted the attention of many areas of science and technology, opening the way to important applications, such as molecular imaging, remote sensing, free- space communication and environmental monitoring. However, the development of new sources of light, such as quantum cascade laser, was not followed by an adequate improvement in the MIR detection system, able to exceed the current challenges. Here we demonstrate the single-photon counting capability of a new detection system, based on efficient up-converter modules, by proving the correlated nature of twin photons pairs at about 3.1μm, opening the way to the extension of quantum optics experiments in the MIR.