This PDF file contains the front matter associated with SPIE Proceedings Volume 10088, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Semiconductor lasers are ideal sources for efficient electrical-to-optical power conversion and for many applications where their small size and potential for low cost are required to meet market demands. Yellow lasers find use in a variety of bio-related applications, such as photocoagulation, imaging, flow cytometry, and cancer treatment. However, direct generation of yellow light from semiconductors with sufficient beam quality and power has so far eluded researchers. Meanwhile, tapered semiconductor lasers at near-infrared wavelengths have recently become able to provide neardiffraction- limited, single frequency operation with output powers up to 8 W near 1120 nm.

We present a 1.9 W single frequency laser system at 562 nm, based on single pass cascaded frequency doubling of such a tapered laser diode. The laser diode is a monolithic device consisting of two sections: a ridge waveguide with a distributed Bragg reflector, and a tapered amplifier. Using single-pass cascaded frequency doubling in two periodically poled lithium niobate crystals, 1.93 W of diffraction-limited light at 562 nm is generated from 5.8 W continuous-wave infrared light. When turned on from cold, the laser system reaches full power in just 60 seconds. An advantage of using a single pass configuration, rather than an external cavity configuration, is increased stability towards external perturbations. For example, stability to fluctuating case temperature over a 30 K temperature span has been demonstrated. The combination of high stability, compactness and watt-level power range means this technology is of great interest for a wide range of biological and biomedical applications.

Multi-Watt efficient compact green laser sources are required for a number of applications e.g. within biophotonics, laser pumping and laser displays. We present generation of 3.5 W of diffraction-limited green light at 515 nm by second harmonic generation (SHG) of a tapered diode laser, itself yielding more than 9 W at 1030 nm. SHG is performed in single pass through a cascade of two nonlinear crystals with re-focusing and dispersion compensating optics between the two nonlinear crystals. The laser is single-frequency and the output power is stabilized to better than ±0.4%.

Narrow band light sources in the vacuum ultraviolet (VUV) region are attractive for photo lithography and high resolution photoelectron spectroscopy. Phase matching is essential to generate high power VUV lights by using a narrow band, low peak intensity and nanosecond pump source. In this research, sum frequency mixing has been demonstrated below 150 nm in KBe₂BO₃F₂ by using the fundamental with its fourth harmonic of a 6 kHz Ti:sapphire laser. The laser system we have developed in this research, consists of a Ti:sapphire laser system and a frequency conversion stage. We generated 149.8-nm radiation, which is the shortest wavelength ever obtained to our knowledge by phase matching in nonlinear crystals. The fifth harmonic output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 nm to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The optical transmission spectra of some KBBF crystals were measured by the spectrophotometer. The transmittance near the absorption edge supports the generation of coherent radiation below 150 nm. The improvement of a prism-coupled device contributed to the generation of coherent radiation below 150 nm. Another reason for the present break through to the shorter wavelength is the use of the short pulse driving source compared with our previous research.

Raman-gas filled HC-PCF has proved to be an outstanding Raman-convertor, as illustrated by the generation of more than 5 octaves wide Raman comb using a hydrogen-filled Kagome HC-PCF pumped with high power picosecond-laser, or the generation of multiline Raman-source in the UV-Vis using a very compact system pumped with micro-chip laser. Whilst these demonstrations are promising, a principal challenge for the industrialization of such a Raman source is its lifetime as the H₂ diffusion through silica is high enough to leak out from the fiber within only a few months. Here, we report on a HC-PCF based Raman multiline source with a very long life-span. The system consists of hydrogen filled ultra-low loss HC-PCF contained in highly sealed box, coined CombBox, and pumped with a 532 nm micro-chip laser. This combination is a turnkey multiline Raman-source with a "shoe box" size. The CombBox is a robust and compact component that can be integrated and pumped with any common pulsed laser. When pumped with a 32 mW average power and 1 ns frequency-doubled Nd:Yag microchip laser, this Raman-source generates 24 lines spanning from 355 to 745 nm, and a peak power density per line of 260 mW/nm for the strongest lines. Both the output power and the spectrum remained constant over its monitoring duration of more than six months. The spectrum of this multiline laser superimposes with no less than 17 absorption peaks of fluorescent dyes from the Alexa Fluor family used as biological markers.

We report a single frequency, linearly polarized, near diffraction-limited, pulsed laser source at 775 nm by frequency doubling a single frequency nanosecond pulsed all fiber based master oscillator-power amplifier, seeded by a fiber coupled semiconductor DFB laser diode at 1550 nm. The laser diode was driven by a pulsed laser driver to generate ~ 5 ns laser pulses at 260 Hz repetition rate with ~ 50 pJ pulse energy. The pulse energy was boosted to ~ 200 μJ using two stages of core-pumped fiber amplifiers and two stages of cladding-pumped fiber amplifiers. The multi-stage synchronous pulse pumping technique was adopted in the four stages of fiber amplifiers to mitigate the ASE. The frequency doubling is implemented in a single pass configuration using a periodically poled lithium niobate (PPLN) crystal. The crystal is 3 mm long, 1.4 mm wide, 1 mm thick, with a 19.36 μm domain period chosen for quasi-phase matching at 33°C. It was AR coated at both 1550 nm and 775 nm. The maximum pulse energy of ~ 97 μJ was achieved when ~ 189 μJ fundamental laser was launched. The corresponding conversion efficiency is about 51.3%. The pulse duration was measured to be 4.8 ns. So the peak power of the generated 775 nm laser pulses reached ~20 kW. To the best of our knowledge, this is the first demonstration of a 100 μJ-level, tens of kilowatts-peak-power-level single frequency linearly polarized 775 nm laser based on the frequency doubling of the fiber lasers.

Visible lasers have a wide range of applications in imaging, spectroscopy and displays. Unfortunately, they suffer from coherent artifacts such as speckle. Various compounding techniques have been developed to remove speckle, but these methods usually involve mechanically moving parts and require long acquisition times. A different approach to prevent speckle formation is developing lasers with low spatial coherence. A careful design of the laser cavity can facilitate lasing in many spatial modes with distinct emission pattern. The total emission from those mutually incoherent lasing modes has low spatial coherence. To date, several types of such lasers have been developed, but most of them have emission beyond the visible spectrum, making them unsuitable for imaging or display applications that require visible light.

An alternative way of making visible sources, especially of green color, is frequency doubling of infrared (IR) lasers. We develop a green light source with low spatial coherence via intracavity frequency doubling of a solid-state degenerate laser. The second harmonic emission is distributed over a few thousands independent transverse modes, and exhibits low spatial coherence. A strong suppression of speckle formation is demonstrated for both fundamental and second harmonic beams. Using the green emission for fluorescence excitation, we show the coherent artifacts are removed from the full-field fluorescence images. The achievable high power, low spatial coherence, and good directionality make the green degenerate laser an attractive illumination source for parallel imaging and projection display.

A miniaturized picosecond pulsed semiconductor laser source in the spectral range around 560nm is realized by integrating a frequency doubled distributed Bragg reflector ridge waveguide laser (DBR-RWL) into a micromodule. Such compact laser sources are suitable for mobile application, e.g. in microscopes. The picosecond optical pulses are generated by gain-switching which allows for arbitrary pulse repetition frequencies. For frequency conversion a periodically poled magnesium doped lithium niobate ridge waveguide crystal (PPLN) is used to provide high conversion efficiency with single-pass second harmonic generation (SHG). The coupling of the pulsed radiation into the PPLN crystal is realized by a GRIN-lens. Such types of lenses collect the divergent laser radiation and focus it into the crystal waveguide providing high coupling efficiency at a minimum of space compared to the usage of fast axis collimator(FAC)/slow axis collimator (SAC) lens combinations. The frequency doubled output pulses show a pulse width of about 60 ps FWHM and a spectral width around 0.06nm FWHM at a central wavelength of 557nm at 15‰. The pulse peak power could be determined to be more than 300mW at a repetition frequency of 40 MHz.

Degenerate (subharmonic) optical parametric oscillators (OPO) show great promise for the generation of broadband mid-infrared (MIR) frequency combs. Their main features are low pump threshold, dramatic extension of the spectrum of the pump laser, and phase locking to the pump frequency comb. Here we report on obtaining instantaneous spectrum ranging from 2.85 to 8.40 μm at -40 dB level from a subharmonic OPO pumped by an ultrafast Cr²⁺:ZnS laser. Our experimental setup includes a free running Kerr lens mode locked 2.35 μm Cr²⁺:ZnS laser, with 62-fs time-bandwidth limited pulse duration, 630-mW average power, and 79 MHz repetition rate that synchronously pumps a ring-cavity orientation-patterned (OP-GaAs) based OPO. A 0.5-mm-long OP-GaAs crystal has a quasi-phase-matching (QPM) period of 88 μm and is designed to provide a broadband parametric gain at OPO degeneracy. A 0.3-mm-thick ZnSe wedge inside the cavity was used to minimize group velocity dispersion. Spectral span of 1.56 octaves in the MIR that we achieved can be further improved by fabricating an in-coupling dielectric mirror with (i) broader reflectivity range and (ii) with compensation of the residual group velocity dispersion. The broad spectrum achieved, 2.85 - 8.40 μm (2320 cm^-1 wide instantaneous span), overlaps with a plethora of fundamental molecular IR resonances and can be used for frequency comb spectroscopic detection applied to such fields as remote sensing, study of fast combustion dynamics and medical diagnostics, to name a few.

We experimentally investigate mid-infrared (MIR) supercontinuum (SC) generation in chalcogenide step-index optical fibers. The pump source is generated by the difference frequency generation with a pulse width of ~170 fs, a repetition rate of ~1000 Hz, and a wavelength range tunable from 2.5 to 11 μm. The wide MIR SC will be applied in sensing, medical and biological imaging areas.

We experimentally demonstrated 10 GHz frequency comb spectral broadening in an AlGaAsOI nano-waveguide with the peak power of only several watts. The spectral broadened 10 GHz frequency comb has high optical signal to noise ratio (OSNR) at the output of the nano-waveguide. As far as we know, it is the first photonic chip based frequency comb, relying on spectral broadening of a 10 GHz mode-locked laser comb in an AlGaAsOI nano-waveguide, with a sufficient comb output power to support several hundred Tbit/s optical data.

Supercontinuum light sources provide a high power spectral density with a high spatial coherence. Coherent octavespanning supercontinuum can be generated in photonic crystal fibers (PCFs) by launching short pulses into the fiber. In the field of optical metrology, these light sources are very interesting. For most applications, only a small part of the entire spectrum can be utilized. In biological tissue scattering, absorption and fluorescence limits the usable spectral range. Therefore, an increase of the spectral power density in limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the spectral power density of supercontinuum sources by amplifying the excitation wavelength inside a nonlinear photonic crystal fiber (PCF). An all-fiber-based setup enables higher output power and power stability. An ytterbium-doped photonic crystal fiber was manufactured by a nanopowder process (drawn by the fiberware GmbH, Germany) and used in a fiber amplifier setup as the nonlinear fiber medium. In order to characterize the fiber’s optimum operational characteristics, group-velocity dispersion (GVD) measurements were performed. The performance of the fiber-based setup was compared with a free space setup. Finally, the system as a whole was characterized in reference to common solid state-laser-based supercontinuum light sources. An improvement of the power density was observed in the spectral range between 1100 nm to 1400 nm.

We present mid-infrared supercontinuum sources based on chalcogenide, tellurite, and liquid-filled capillary fibers and sub-picosecond oscillator pumping. Depending on the fiber geometry and material, the experimentally achieved spectral bandwidths and output powers vary significantly. In As₂S₃ chalcogenide step-index fibers we achieve a maximum output power of 550 mW at a spectral width of 2 μm, covering the important transparent atmospheric window between 3 and 5 μm. In tellurite step-index fibers we attain an ultra-broadband spectrum ranging from 1.3 to 5.3 μm with an average power of 150 mW. The spectral behavior of the supercontinua is investigated by changing the pump wavelength, core diameter, fiber length, and pump power. As pump source we use high repetition rate (42 MHz) optical parametric oscillators/amplifiers which deliver Watt-level pulses tunable between 1.4 – 4.1 μm. These supercontinuum sources promise to be excellent laboratory tools for high resolution spectroscopy owing to their high brilliance and near TEM00 spatial beam profiles.

We have demonstrated Raman frequency conversion and supercontinuum light generation in a hollow core Kagomé fiber filled with air at atmospheric pressure, and developed a numerical model able to explain the results with good accuracy. A solid-state disk laser was used to launch short pulses (~6ps) at 1030nm into an in-house fabricated hollow core Kagomé fiber with negative core curvature and both ends were open to the atmosphere. The fiber had a 150 THz wide transmission window and a record low loss of ~12 dB/km at the pump wavelength. By gradually increasing the pulse energy up to 250 μJ, we observed the onset of different Kerr and Raman based optical nonlinear processes, resulting in a supercontinuum spanning from 850 to 1600 nm at maximum input power. In order to study the pulse propagation dynamics of the experiment, we used a generalized nonlinear Schrödinger equation (GNLSE). Our simulations showed that the use of a conventional damping oscillator model for the time-dependent response of the rotational Raman component of air was not accurate enough at such high intensities and large pulse widths. Therefore, we adopted a semiquantum Raman model for air, which included the full rotational and vibrational response, and their temperature-induced broadening. With this, our GNLSE results matched well the experimental data, which allowed us to clearly identify the nonlinear phenomena involved in the process. Aside from the technological interest in the high spectral density of the supercontinuum demonstrated, the validated numerical model can provide a valuable optimization tool for gas based nonlinear processes in air-filled fibers.

We report on high power Raman-converter frequency stage based on hydrogen-filled inhibited-coupling hollow-core photonic crystal fibers pumped by an Yb-fiber picosecond laser. This fiber Raman-convertor can operate in two SRS emission regimes by simply controlling the fiber length or the gas pressure. It can set to either generate favorably single laser line or to generate an extremely wide Raman comb. Based on this we demonstrate a pico-second pulse Raman source of 9.3 W average-power at 1.8 μm, and an ultra-wide Raman comb spanning over more than five octaves from UV to mid-infrared, containing around 70 laser lines.

Spectral imaging in the 6 to 8μm range has great potential for medical diagnostics. Here a novel technique based on frequency upconversion of the infrared images to the near visible for subsequent acquisition using a Si-CCD camera is investigated. The upconversion unit consists of an AgGaS₂ crystal and a 1064nm diode pumped solid state laser. A globar is used as mid-infrared illumination source. Acquired images contain both spectral and spatial information. Angle tuning of the nonlinear crystal is exploited to scan the phase match condition, which allows to cover the full spectral range of interest for the full field of view. Simulated images are created and compared with the measured images.

We present a novel mid-infrared imaging system born from the combination of an all-fiber mid-IR supercontinuum source developed at NKT with ultra-sensitive upconversion detection technology from DTU Fotonik. The source delivers 100 mW of average power and its spectrum extends up to 4.5 μm. The infrared signal is passed through a sample and then focused into a bulk AgGaS₂ crystal and subsequently mixed with a synchronous mixing signal at 1550 nm extracted from the pump laser of the supercontinuum. Through sum frequency generation, an upconverted signal ranging from 1030 nm to 1155 nm is generated and acquired using an InGaAs camera.

Imaging and spectroscopy in the mid-infrared (Mid-IR) wavelength region have received considerable attention in recent years. The reason is the high Mid-IR spectral specificity of many gases and complex molecules. In this pilot study we focus on picosecond upconversion imaging exploiting the χ⁽²⁾ nonlinearity of a bulk lithium niobate crystal as a means to convert the optical Mid-IR signal into the visible wavelength region, thus allowing the use of fast and sensitive silicon based CCD cameras. The picosecond upconversion system is synchronously pumped in order to increase the quantum efficiency, hence allowing for upconversion of faint pulsed Mid-IR light.

We extend our recently developed Beam-Deflection (BD) method which we used to determine the sign, magnitude, and ultrafast dynamics of bound electronic and nuclear nonlinear optical responses, to measure elements of the third-order nonlinear optical susceptibility tensor with longitudinal field components. In these measurements, in contrast to the conventional BD technique where excitation and probe beams are nearly collinear, the interaction geometry involves orthogonal propagation of excitation and probe beams. This cross-propagating BD method enables probing with the electric field polarized parallel to the wavevector of the excitation beam as well as conventional parallel and perpendicular polarizations as in most other experimental methods. This technique may be of use in detecting possible magneto-electric contributions to the nonlinear susceptibility. The ratio between parallel and perpendicular bound-electronic responses in this method is shown to be larger than the factor of 3 predicted by theory and measured in the conventional configuration.

New approaches for generating coherent light are of great interest due to the increasing demand for unique sources tailored to specific applications. In fields like microscopy, remote sensing, and soft X-ray/XUV lasers this interest is especially pronounced, as novel laser sources would have a profound impact on those technologies. Here, we evaluate the possibility for generating coherent light by inducing collective coherence in the gain medium.

Orientation patterned GaAs and GaP crystals are increasingly being used for frequency conversion of laser beams into desired wavelengths. The ternary alloy InGaAs has smaller band gap energy compared to GaAs and is expected to have a nonlinear optical coefficient that is higher than that of GaAs. We will present the results of measurements of the second order nonlinear optical coefficient of InGaAs, along with recent measurements of GaAs and GaP obtained through second harmonic generation of a tunable carbon dioxide laser in the 9.2 to 10.6 μm range. The results will be compared to available values obtained at other wavelengths [1,2]

Keywords: nonlinear optics, frequency conversion, second harmonic generation

REFERENCES

[1] T. Skauli, K. L. Vodopyanov, T. J. Pinguet, A. Schober, O. Levi, L. A. Eyres, M. M. Fejer, J. S. Harris, B. Gerard, L. Becouarn, and E. Lallier, G. Arisholm, “Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation”, Optics Letters , Vol. 27, No. 8, 628, (2002)

[2] I. Shoji, T. Kondo, A. Kitamoto, M. Shirane, and R. Ito, “Absolute scale of second-order nonlinear-optical coefficients”, J. Opt. Soc. Am. B, Vol. 14, No. 9, 2268, 1997

Thick hydride vapor phase epitaxially grown orientation-patterned gallium phosphide (OPGaP) is a leading material for quasi-phase matching (QPM) frequency conversion in the mid- and longwave infrared (IR). This is due to its negligible two-photon absorption (2PA) in the convenient pumping range 1 – 1.7 μm, compared with the 2PA of some traditional QPM materials, such as GaAs. In this paper, we describe homo- and heteroepitaxial growth techniques aimed to produce hundreds of microns thick OPGaP on: 1) OPGaAs templates fabricated using an improved wafer-fusion process; 2) OPGaAs templates fabricated by using a molecular beam epitaxy (MBE) for sublattice polarity inversion, but one with and one without MBE regrowth after the inversion. Some of the advantages of the heteroepitaxial growth of OPGaP on OPGaAs templates include: 1) achieving good domain fidelity as a result of the significantly higher OPGaAs template quality; 2) eliminating the needs of using the poor quality commercially available GaP in the production of thick OPGaP material, and 3) suppression of the additional absorption band between 2 – 4 μm (which is due to incorporation of n-type impurities) and, in general, improvement of the IR transmittance in the entire IR region. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, will accelerate the development of high power, broadly tunable laser sources in the IR which, in addition, will be offered with higher device quality and at a reasonably lower unit cost.

We generate 52 mW of broadband pulses at 8 μm wavelength by difference frequency generation (DFG) in orientation patterned GaP (OP-GaP). Pumping by fiber lasers is possible because of the wide transparency range of GaP. An Er fiber oscillator at 1.5 μm wavelength and 90 MHz repetition rate provides pulses for direct amplification and shifting to 1.9 μm followed by amplification in Tm doped fiber. The two beams are mixed in OP-GaP with up to 20% photon conversion efficiency. The wavelength is tunable from 5 to 11 μm by changing the crystal period or the degree of wavelength shifting. With coherent broadening, we can also generate midinfrared frequency combs with tunable carrier envelope offset frequencies.

Orientation-patterned gallium phosphide (OP-GaP) crystals are used here for the first time for the generation of continuous-wave coherent mid-infrared radiation around 5.85 μm by difference frequency generation (DFG) of a Nd:YAG laser at 1064 nm and a diode-laser at 1301 nm. We provide the first characterization of the linear, thermo-optic and nonlinear properties of OP-GaP in a DFG configuration, and we derive an effective nonlinear coefficient d_eff = 17 pm/V (3) for first-order quasi-phase-matched OP-GaP. This novel nonlinear material can be used to referenced the mid-IR light to a frequency standard by locking the pump and signal laser to a near-IR optical frequency comb.

We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation patterned gallium phosphide (OP-GaP) and being the first example of a broadband OPO operating across the molecular fingerprint region. OP-GaP crystals with lengths of 1 mm and several patterning periods were diced, polished, and antireflection (AR) coated for near- to mid-infrared wavelengths. We configured a synchronously pumped OP-GaP OPO in a 101.2-MHz resonator with high reflectivity from 1.15–1.35 μm, pumped with 150-fs pulses from a 1040-nm femtosecond laser (Chromacity Spark). The coating of one spherical mirror was optimized for transmission at the pump wavelength of 1040 nm and for high reflectivity at the resonant signal wavelength in a range from 1.15–1.35 μm, while the other spherical mirror collimated the idler beam emerging from the OP-GaP crystal and was silver coated to provide high reflectivity for all idler wavelengths. This collimated idler beam was output-coupled from the cavity by transmission through a plane mirror coated with high transmission for the idler wavelengths (5–12 μm) and high reflectivity for the signal wavelengths (1.15–1.35 μm) on an infrared-transparent ZnSe substrate. Idler spectra centered from 5.4–11.8 μm and extending to 12.5 μm were collected. The maximum average power was 55 mW at 5.4 μm with 7.5 mW being recorded at 11.8 μm. Details of Fourier transform spectroscopy using water vapor and a polystyrene reference standard are presented.

We report on the first single-frequency nanosecond optical parametric oscillator (OPO) emitting in the longwave infrared, and use it to perform standoff detection of ammonia vapor by differential spectrometry. The OPO is based on orientation-patterned GaAs (OP-GaAs) pumped by a pulsed single-frequency Tm:YAP microlaser. Single-longitudinal mode emission is obtained owing to a nested cavity OPO (NesCOPO) scheme. The OPO is tuned over 700 nm around 10.4 μm, allowing to measure the absorption spectrum of ammonia across several lines at atmospheric pressure. The potential of this OPO for standoff detection of hazardous gases is also discussed.

A singly-resonant OPO (SRO) based on AgGaSe₂ (AGSe) intracavity pumped at ~1.85 μm by the signal pulses of a Rb:PPKTP doubly-resonant OPO (DRO) provided extremely broad tuning (5.8 to ~18 μm) for the non-resonated idler. In a similar set-up with the same nonlinear crystals, we studied intracavity difference-frequency generation (DFG). Both AGSe and the new monoclinic crystal BaGa₄Se₇ (BGSe) generated single pulse energies of ~0.7 mJ near 7 μm at an overall conversion efficiency from the 1.064 μm pump of 1.2%. The main advantage of BGSe is its damage resistivity up to the maximum pump levels applied at 100 Hz.

Half-harmonic generation is the reverse of second harmonic generation that happens in optical parametric oscillators (OPOs) at degeneracy. It is an intrinsically phase-locked down-conversion process, which can be used to efficiently transfer well-developed near-IR frequency combs to the mid-IR.

We overview recent experimental progress in cascading multiple stages of half-harmonic generation of femtosecond frequency combs starting from a 1-μm pump. We have achieved stable operation with efficiencies as high as ~64%, pulses as short as three optical cycles at 4 μm, and output powers as high as 2.6 W at 2 μm. Our recent numerical and analytical studies of nonlinear dynamics and different operation regimes of femtosecond OPOs indicate a path toward achieving even higher efficiencies and shorter pulses.

We describe a large-scale degenerate optical parametric oscillator (DOPO) network for a coherent Ising machine that solves combinatorial optimization problems. By pumping a fiber-based phase-sensitive amplifier placed in a 1-km fiber cavity at a 2-GHz repetition frequency, we generated more than 10,000 DOPOs multiplexed in the time domain. We confirmed that the DOPO phases were discretized to {0, π} indicating that they could be used as stable artificial spins. We also implemented a one-dimensional Ising model by optically coupling adjacent DOPOs, and confirmed that the DOPOs well simulated the behavior of low-temperature spins.

Structured coherent optical beams including optical vortices, hollow Gaussian beam, and Airy beam have found wide range of applications in variety of fields in science and technology. All existing techniques used to date to generate such beams suffer from different limitations including lower output power and restricted wavelength coverage. On the other hand, interactions of such beams with nonlinear media are mostly unexplored. We have been involved in the study of nonlinear interaction of optical beams with different spatial structures. Here, we discuss our recent results on nonlinear generation of various structured laser beams including optical vortices, perfect vortices, Airy beam, and hollow Gaussian beam and some of their applications

We report on a widely tunable synchronously-pumped picosecond OPO combining an aperiodically poled MgOdoped LiNbO₃ crystal as a broadband gain medium and an intracavity axially chirped volume Bragg (VBG). Owing to the high dispersion induced by the chirped VBG, only a narrow spectral band, corresponding to a thin slice of the VBG, satisfies the synchronous-pumping condition. At a fixed position, the VBG is thus a narrow-band filtering element; variation of its position along the cavity axis enables to tune the idler wavelength over 215nm around 3.82 μm. Rapid continuous tuning over 150nm in 100 ms is also demonstrated.

We report the operation of an optical parametric amplifier (OPA) capable of producing gigawatt peak-power laser pulses with tunable wavelength in either the visible or near-infrared spectrum. The OPA has two distinct operation modes (i) generation of > 350 μJ, sub 100 fs pulses, tunable between 1250 - 1550 nm; (ii) generation of > 170 μJ, sub 150 fs pulses tunable between 490 - 530 nm. We have recorded high-order harmonic spectra over a wide range of driving wavelengths. This flexible source of femtosecond pulses presents a useful tool for exploring the wavelength-dependence of strong-field phenomena, in both the multi-photon and tunnel ionization regimes.

In this paper we study, both experimentally and by theory, a new nonlinear dynamic in multimode optical fiber where intermodal four-wave mixing (IM-FWM) and Kerr-induced nonlinear mode coupling (NLC) are strongly interacting. We show that presence of phase-matched IM-FWM and strong nonlinear mode coupling results in an unprecedented saturable mode conversion in the FWM anti-Stokes beam. We investigate the evolution of modal conversion as a function of fiber length and show that the result of this novel nonlinear dynamic is a controllable, length independent mode conversion. Our theoretical calculations suggest that the converted anti-Stokes beam can also be amplified via IM-FWM. We observe a mode conversion with an efficiency as high as 90 percent.

The use of visible lasers for medical treatments is on the rise, and together with this comes higher expectations for the laser systems. For many medical treatments, such as ophthalmology, doctors require pulse on demand operation together with a complete extinction of the light between pulses. We have demonstrated power modulation from 0.1 Hz to 10 kHz at 532 nm with a modulation depth above 97% by wavelength detuning of the laser diode. The laser diode is a 1064 nm monolithic device with a distributed feedback (DFB) laser as the master oscillator (MO), and a tapered power amplifier (PA). The MO and PA have separate electrical contacts and the modulation is achieved with wavelength tuning by adjusting the current through the MO 40 mA.