This PDF file contains the front matter associated with SPIE Proceedings Volume 9347, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

Absorption spectroscopy with frequency combs in the molecular fingerprint portion of the spectrum (2-10 μm) has great potential for trace molecular detection and in particular for such applications as monitoring of the atmosphere and medical breath analysis. Especially attractive is dual-comb Fourier transform spectroscopy where full advantage is taken of temporal and spatial coherence of frequency combs as well as of their broadband nature. The promise is high speed, broad spectral coverage, superior sensitivity, high spectral resolution, and the possibility of absolute frequency calibration of molecular resonances. Here we report a system suitable for performing dual-comb spectroscopy in the range of 2.5-6 μm and beyond. Broadband mid-IR frequency combs are obtained via a doubly-resonant near-degenerate synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned GaAs (OP-GaAs) pumped by a femtosecond Tm-fiber laser at 2-μm wavelength. Low pump threshold (7 mW), high coherence, and broad instantaneous spectral coverage make this system extremely promising for spectroscopic studies.

Kilowatt-level narrow-linewidth SM ytterbium fiber laser operating in high-repetition-rate QCW regime was used to obtain 700 W average power at 532 nm with single-mode beam quality and wall-plug efficiency of over 23 %. To the best of our knowledge, this is ~60 % higher power than previously reported for single-mode green lasers based on other platforms, and also is ~30 % increase comparing to the previous result obtained by our group on the base of similar fiber laser platform. We have also experimentally proved that the same type of fiber laser can be used for generating of world-record levels of power at other wavelengths of visible and UV spectral ranges by employing cascaded non-linear frequency conversion. Thus, utilizing frequency tripling in 2 LBO crystals, we achieved over 160 W average power of nearly single-mode UV light at 355 nm with THG efficiency of more than 25 %. As far as we know, this is the highest output power ever reported for UV laser with nearly diffraction limited beam quality. We also conducted some preliminary experiments to demonstrate suitability of our approach for generating longer wavelengths of the visible spectrum. By pre-shifting fundamental emission wavelength in fiber Raman converter, followed by frequency doubling in NCPM LBO, we obtained average powers of 36 W at 589 nm and 27 W at 615 nm. These proof-of-concept experiments were performed with low-power pump laser and were not fully optimized with respect to frequency conversion. Our analysis indicates that employing kW-level QCW ytterbium laser with optimized SRS and SHG converters we can achieve hundreds of Watts of average power in red and orange color with single-mode beam quality.

More than 200mW of CW 229nm for Cd atom cooling application was generated by the 4^th harmonic of a single frequency optically pumped semiconductor laser using a 10-mm long, Brewster-cut BBO crystal in an external cavity. With 650mW of 458nm input, 216mW of 229nm power was observed. Conversion efficiency from 458nm to 229nm was more than 33%.

Here we report on the generation of ten deep blue to cyan laser emission lines using an intracavity frequency converted Raman laser. The fundamental laser field of the intracavity Raman laser is based on the 3 level transition of a Nd:YLF laser crystal, providing a short wavelength at 903 or 908 nm. When combined with generation of a Stokes shifted field via intracavity stimulated Raman scattering (SRS) by a KGW Raman crystal, enables generation of laser emission in the deep blue to cyan wavelength regime via additional nonlinear frequency conversion. Output at several blue-green wavelengths was achieved, with quasi continuous wave (qcw) output powers of up to 1W. A detailed study of the spectral behavior of the underlying Raman laser processes revealed strong spectral broadening of the fundamental laser line at 908 nm to a width of up to 4 nm. The effect of the spectral broadening on the overall laser efficiency is analyzed.

In this work, we demonstrate frequency doubling of a DBR tapered diode laser operating around 1064 nm in a nonlinear bulk crystal enhanced by a multi-pass cavity resonant for the generated green light. This novel approach to generate visible laser radiation is characterized by an increased conversion efficiency in comparison to a single-pass configuration. Through the proper choice of the standing wave plane-parallel cavity parameters, the introduced concept requires no impedance matching and frequency locking. A maximum second-harmonic power of 1 W at a conversion efficiency of 20 % is achieved.

For second-order nonlinear-optical processes in the ultraviolet, appropriate materials with a sufficiently large band-gap typically exhibit smaller nonlinear coefficients than materials with comparably smaller band-gap. Whispering gallery resonators, with their outstanding quality factors, provide field enhancement and can compensate for these small coefficients. We report on the successful fabrication of a whispering gallery resonator made of lithium tetraborate, a suitable material for ultraviolet applications with a small nonlinear coefficient of d₃₁ = 0:073 pm/V. Quality factors of the order of 108 are observed from the ultraviolet to the near-infrared spectrum. The inferred absorption coefficients of lithium tetraborate are below 0.2 m^-1 in the visible and near-infrared. Continuous-wave second harmonic generation from 490 nm light to 245 nm is observed with conversion efficiencies up to 2.2 %.

Second harmonic generation (SHG) is a ubiquitous technique for extending the spectral coverage of laser sources into regions that would otherwise be technologically challenging to access. SHG schemes typically rely on the use of bulk optical components, resulting in systems with large footprints requiring precise optical alignment. Integration of the SHG components into a single unit facilitates the implementation of compact, robust and turn-key sources, suitable for applications in biophotonic imaging, amongst others. We report on the development of fiber-coupled frequency doubling modules and their application to novel fiberintegrated picosecond pulse sources in the visible and near-visible. The modules employ a simple, single-pass configuration using a periodically-poled lithium niobate (PPLN) crystal as the nonlinear conversion medium. They are readily adaptable for different fiber pump laser configurations and are configurable with either fiber-coupled or collimated free-space outputs. Two sources using the modules are presented, operating at 780 nm and 560 nm. The 780 nm source utilizes an erbium master oscillator power fiber amplifier (MOPFA) scheme. SHG was performed in a 35 mm long crystal, generating 3.5 W of 780 nm radiation with a pulse duration of 410 ps at 50 MHz and conversion efficiencies exceeding 20%. Results of this source being used for parametric wavelength conversion in photonic crystal fiber are discussed. The 560 nm source was based on SHG of a Raman amplified CW diode pumped by a pulsed ytterbium-fiber MOPFA. This source generated 450 mW of average power with conversion efficiencies greater than 20%.

A single frequency fiber Bragg grating (FBG) stabilized laser at 972 nm is coupled into a doubling ring cavity with an optical length of 138 mm, a 91% input coupler, a 30 mm long Brewster cut magnesium doped periodically poled lithium tantalate (PPMgO:SLT) crystal and a high reflector. The cavity buildup is 37 and loss is 0.63%. The cavity is monitored, controlled and locked with a single chip processor. With IR power of 572 mW in the input fiber, 466 mW blue output is obtained, giving 81.5% net efficiency. The blue and IR beams are separated by refraction at the crystal’s Brewster surface with negligible loss and without the need for dichroic optics.

Tunable continuous wave (CW) green light generation between 517 nm and 538 nm at room-temperature has been demonstrated from a frequency-doubled broadly tunable quantum well (QW) external-cavity fiber-coupled diode laser by use of an uncoated periodically poled potassium titanyl phosphate (PPKTP) crystal waveguide crystal. Green light at 530 nm with maximum conversion efficiency of 14.8% and output power of 12.88 mW has been generated using a PPKTP crystal waveguide with the cross-sectional area of 3x5μm². The possibility of tunable second harmonic generation in the PPKTP crystal waveguides with the cross-sectional areas of 4x4μm² and 2x6μm² was also investigated.

This investigation represents a deep and advanced analysis of exploiting lithium niobate (LiNbO3) crystals for the collinear acousto-optical tunable filter (AOTF) in violet and near ultraviolet ranges. The selection of this material is motivated by its high birefringence, which is a key parameter for improving the resolution of AOTF. For this matter, we take into account all the important factors that can deteriorate the resolution in order to find extreme conditions for the best performances. In concrete, we analyze the well- known photorefraction effect accompanied by the light induced absorption in those ranges for the LiNbO3 crystals doped by selected materials. The best observed results have been obtained with magnesium (Mg) dopant in the congruent melt of LiNbO3, which also shifts the absorption edge far into the middle UV-range. This analysis had made it possible to formulate the physical criterion determining the enlarged practical limitations of the incident light power density. Together with previously studied non-uniformity and dispersion of the birefringence along the length of acousto-optical interaction in a crystal, we exploit the recently discovered and experimentally confirmed acousto-optical nonlinearity, which can improve the transmission function inherent in the collinear interaction via applying the acoustic waves of finite amplitude in the AOTF. As a result, the obtained spectral resolution is the best available for any collinear AOTF to our knowledge.

Ultrafast transport of electrons in semiconductors lies at the heart of high-speed electronics, electro-optics and fundamental solid-state physics. Intense phase-locked terahertz (THz) pulses at photon energies far below electronic interband resonances may serve as a precisely adjustable alternating bias, strongly exceeding d.c. breakdown voltages. Here, we exploit the near-field enhancement in gold metamaterial structures on undoped bulk GaAs, driven by few-cycle THz transients centered at 1 THz, to bias the semiconductor substrate with field amplitudes exceeding 12 MV/cm. Such fields correspond to a potential drop of the bandgap energy over a distance of only two unit cells. In this extremely off-resonant scenario characterized by a Keldysh parameter of γK ≈ 0.02, massive interband Zener tunneling injects a sizeable carrier density exceeding 10¹⁹ cm^-3, and strong photoluminescence results. At a center frequency of 30 THz, THz transients with peak fields of 72 MV/cm analogously excite carriers in a bulk, semiconducting GaSe crystal, without metamaterial. Here, in contrast, we are able to drive coherent interband polarization and furthermore dynamical Bloch oscillations of electrons in the conduction band, on femtosecond time scales. The dynamics entail the generation of absolutely phase-stable high-harmonic transients containing spectral components up to the 22^nd order of the fundamental frequency, spanning 12.7 optical octaves throughout the entire terahertz-to-visible domain between 0.1 and 675 THz. Our experiments establish a new field of light-wave electronics exploring coherent charge transport at optical clock rates and bring picosecond-scale electric circuitry at the interface of THz optics and electronics into reach.

Recent development in high intensity laser-based Terahertz (THz) sources has enabled novel experiments on nonlinear THz phenomena. We will briefly review current state of the art sources of high-field THz pulses and discuss their applications in condensed matter physics and nonlinear THz spectroscopy.

The discovery of high temperature superconductors (HTS) and the expected applications in the field of ultrafast opto-electronics has created a unique opportunity where the technology has the potential to bridge the frequency gap from infrared to microwave. A pulsed ultrafast laser impinging on a HTS thin film grown using yttrium barium copper oxide (YBCO) excites transient electron dynamics to generate radiation that spans from the terahertz to the microwave regime. The radiation phenomena were demonstrated by making transient photo-excitation measurements using an ultrafast laser to induce non-equilibrium quasi-particle dynamics. The photo-response from a laser of an average power of 1 W and a pulse duration greater than 120 fs (808 nm wavelength) incident on charged YBa₂Cu₂O_7-δ (YBCO) thin film at superconductive temperatures was measured using a series of microwave antennas. From the observed nanosecond response time of the transient pulse, we were able to extract frequency band structure in the GHz regime that was dependent on the incident beam diameter, pulse duration, power, and the physical structure of the YBCO thin film. The electron-phonon energy relaxation time is known to be on the order of a picosecond. However, by manipulating the resistive and kinetic inductive response of the material we demonstrate the ability to generate wideband microwave frequencies with a transient response on the order of the nanosecond time scale. Quasi-particle dynamics and the temporal response were analyzed using the Rothwarf-Taylor rate equations.

Orientation patterned gallium phosphide (OP-GaP) is a new quasi-phase-matched (QPM) nonlinear optical (NLO) semiconductor for mid-infrared frequency generation. It overcomes several limitations of ZGP, the current NLO crystal of choice for 2-μm-pumped optical parametric oscillators (OPOs): OP-GaP exhibits lower 2-μm absorption loss, higher thermal conductivity, noncritical phase matching via quasi-phase matching (QPM), and a larger band gap that allows for pumping at 1064 nm. Here we report the first OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterned, reactive ion etched, and regrown by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). A Tm-fiber-pumped Ho:YAG pump laser was line narrowed with a volume Bragg grating (2090nm, 20W, 20kHz, 12 ns) and linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm³ OP-GaP crystal (QPM layer = 800 μm thick, grating period = 92.7 μm) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump spot size at the crystal face was 250 μm. The observed OPO threshold was 3.1 W (44 MW/cm²) with a slope efficiency of 16% and a maximum output power of 350 mW until surface damage occurred at 1.25 to 1.5 J/cm². The signal (3.54 μm) and idler (5.1 μm) output wavelengths agreed well with sellmeier predictions.

Orientation patterned gallium phosphide (OP-GaP) is a new nonlinear optical (NLO) crystal which exhibits the highest nonlinear coefficient (d₁₄=70.6 pm/V) and the longest infrared cut-off (12.5 μm) of any quasi-phase-matched (QPM) material that can be pumped at 1-μm without significant two-photon absorption. Here we report the first 1064nm-pumped OPO based on bulk OP-GaP. Multi-grating OP-GaP QPM structures were grown by producing an inverted GaP layer by polar-on-nonpolar molecular beam epitaxy (MBE), lithographically patterning, reactive ion etching, and regrowing by MBE to yield templates for subsequent bulk growth by low-pressure hydride vapor phase epitaxy (LP-HVPE). The pump source was a diode-end-pumped Nd:YVO₄ monoblock laser with an RTP high-voltage Q-switch (1064 nm, 1W, 10kHz, 3.3 ns) which was linearly polarized along the <100> orientation of the AR-coated 16.5 x 6.3 x 1.1 mm³ OP-GaP crystal (800-μm thick HVPE layer, 20.8 μm grating period only 150 μm thick) mounted on a copper blocked maintained at 20°C by a thermo-electric cooler. The OPO cavity was a linear resonator with 10-cm ROC mirrors coated for DRO operation (85%R at signal, 55%R at idler). The pump 4σ-diameter at the crystal face was 175 μm. The observed OPO signal (idler) threshold was 533 mW (508 mW) with a slope efficiency of 4% (1%) and maximum output power 15 mW (4 mW). The signal (1342 nm) and idler (4624 nm) output wavelengths agreed well with sellemier predictions. Orange parasitic output at 601.7nm corresponded to 9^th order QPM sum frequency mixing of the 1064-nm pump and the 1385-nm signal.

We report the first demonstration of frequency conversion in an orientation patterned GaAs (OPGaAs) crystal with a fan-out grating configuration. Room temperature frequency doubling of a continuous-wave carbon dioxide laser tuned from 9.26 to 10.65 micrometers was obtained using the large-aperture OPGaAs crystal (>3 mm thick, >7.5 mm wide and 40 mm long) with fan-out grating periods ranging between 212.5 μm and 220 μm.

By pumping a yellow phase, 1.4-cm-long, θ=9.2°(Φ=0°) cut BaGa4S7 crystal with a Nd:YAG laser-pumped KTP and AgGaS2 optical parametric oscillators (OPOs) and second-harmonic generation (SHG) of a frequency-doubled CO2 laser, we have achieved the 90°phase-matched type-1 SHG in the 0.8390-2.7750 μm range along the y(=a) and z(=b) axes by heating the crystal from 25ºC to 130ºC.

In addition, by mixing the signal outputs of a Nd:YAG laser-pumped KTP/OPO and its pump source, we have achieved the 90°phase-matched type-1 difference-frequency generation (DFG) in the 5.341-7.506μm range along the z axis at a crystal temperature of 25-185ºC. The Sellmeier and thermo-optic dispersion formulas that reproduce well these experimental results are presented.

6.5 W of average power have been generated by a mid-infrared ZnGeP₂ (ZGP) optical parametric oscillator (OPO) pumped directly by a Q-switched Tm3+-doped single-oscillator fiber laser. The Tm³⁺-fiber pump laser based on a silica polarization-maintaining (PM) double-clad fiber provided average powers of up to 23 W at pulse widths of 65 ns at 40 kHz repetition rate. The ZnGeP₂ OPO produces 45 ns mid-IR pulses. The OPO slope efficiency was 40% and the optical-to-optical conversion efficiency 32%.

Optical parametric oscillators (OPOs) producing longwave output from a much shorter pump wavelength suffer from low conversion efficiency into the idler due to the large quantum defect compared with similar devices operating in the 3 – 5 μm regime. One method to increase pump to idler conversion efficiency is to recycle the undesired and higher energy signal photons into additional idler photons in a second nonlinear stage. We present numerical simulation results showing the improvement in efficiency that can be obtained in a linear, two stage, cascaded orientation patterned gallium arsenide (OPGaAs) nanosecond OPO. It includes diffraction, crystal loss, phase mismatch, pump depletion, and back conversion; and it assumes monochromatic waves but it neglects group velocity dispersion. For a singly resonant oscillator (SRO) pumped by a 2.054 μm Tm:Ho,YLF laser with 45 ns pulse widths, the addition of the second crystal in the cavity increases idler generation by overall factor of two and exceeds the quantum defect limit. The model has been validated by comparison with SNLO for the case of a single-stage OPO, and suggests crystal and resonator parameters that will lead to an optimized cascaded OPO.

We present an efficient coherent source widely tunable in the mid-infrared spectral range consisting of a commercial picosecond Yb-fiber laser operating at 80 MHz repetition rate, a synchronously-pumped OPO (SPOPO) and differencefrequency generation (DFG) in AgGaSe₂. With an average input pump power of 7.8 W at 1032 nm and at 80 MHz, the SPOPO outputs are tunable from 1380 to 1980 nm (Signal) and from 2.1 to ~4 μm (Idler) with pulse durations between 2.1 and 2.6 ps over the entire tuning range. After temporally overlapping Signal and Idler through a delay line, the two beams are spatially recombined with a dichroic mirror (reflecting for the Signal in s-polarization and transmitting for the Idler in p-polarization), and focused by a 150 mm CaF₂ lens to a common focus. For DFG we employ an AR-coated 10- mm thick AgGaSe₂ nonlinear crystal cut for type-I interaction at θ =52°. The generated mid-infrared picosecond pulses are continuously tunable between 5 and 18 μm with average power up to 130 mW at 6 μm and more than 1 mW at 18 μm. Their spectra and autocorrelation traces are measured up to 15 μm and 11 μm, respectively, and indicate that the input spectral bandwidth and pulse duration are maintained to a great extent in the nonlinear frequency conversion processes. The pulse duration slightly decreases from 2.1 to 1.9 ps at 6.7 μm while the spectral bandwidth supports ~1.5 ps (~10 cm^-1)durations across the entire mid-infrared tuning range. For the first time narrow-band mid-infrared pulses with energy exceeding 1 nJ are generated at such high repetition rates.

Quasi-phase matching (QPM) can be used to increase the conversion efficiency of the high harmonic generation (HHG) process. We observed QPM with an improved dual-gas foil target with a 1 kHz, 10 mJ, 30 fs laser system. Phase tuning and enhancement were possible within a spectral range from 17 nm to 30 nm. Furthermore analytical calculations and numerical simulations were carried out to distinguish QPM from other effects, such as the influence of adjacent jets on each other or the laser gas interaction. The simulations were performed with a 3 dimensional code to investigate the phase matching of the short and long trajectories individually over a large spectral range.

Nonlinear optical properties of coaxial InGaN/GaN multiple quantum well (MQWs) submicron hetero-structures were investigated using a tunable femtosecond laser at room temperature. Co-axial InGaN/GaN MQW hetero-structures were fabricated by depositing InGaN/GaN layers on the side walls of GaN submicron tubes on top of GaN micro-pyramids. Excitation and signal collection from a single micro-structure was achieved using multi-photon spectroscopy. Two photon excited photoluminescence (TPEL) was observed at around 390 nm independent of excitation wavelength. In addition to TPEL, observation of second harmonic signal of the excitation laser is also presented.

Second harmonic generation generated by an obliquely incident fundamental wave in a nonlinear photonic bandgap structure is analyzed by applying the transfer matrix method, where multiple reflection and interference effects are taken into account. The radiation of fundamental and second harmonic waves from the exit plane of the nonlinear photonic bandgap structure, and the distribution of the fields within the structure are discussed. Under the non-depleted pump wave assumption, the conversion efficiency of the second harmonic wave versus the incident angle of the fundamental is studied in detail.

Second harmonic generation (SHG) of a random laser operating at 1308 nm via Rayleigh backscattering in a phosphosilicate fiber with Raman gain has been studied. SHG power of < 100 mW at 654 nm has been obtained in PPLN pumped by ~7 W laser power. The SHG beam is stable, herewith its spectrum has no mode structure within ~0.5 nm bandwidth that is different from conventional Raman fiber laser with linear cavity. Direct comparison of SHG in two laser configurations demonstrates better parameters of the random laser having great potential for imaging and other applications requiring low-coherent visible light.

Hyper-Rayleigh scattering (HRS) is an incoherent variant of second harmonic generation. The theory involves terms of increasing order of optical nonlinearity: for molecules or unit cells that are centrosymmetric, and which accordingly lack even-order susceptibilities, HRS is often regarded as formally forbidden. However, for the three-photon interaction, theory based on the standard electric dipole approximation, represented as E1³, does not include the detail required to describe what is observed experimentally, in the absence of a static field. New results emerge upon extending the theory to include E1²E2 and E1²M1, incorporating one electric quadrupolar or magnetic dipolar interaction respectively. Both additional interactions require the deployment of higher orders in the multipole expansion to govern these processes, with the E1²E2 interaction analogous in rank and parity to a four-wave susceptibility. A key feature of the present work is its foundation upon a formal tensor derivation which does not oversimplify the molecular components, yet leads to results whose interpretation can be correlated with experimental observations. Results are summarized for the perpendicular detection of both parallel and perpendicular polarizations. Using such methods to investigate molecular systems that might have useful nonlinear characteristics, HRS therefore provides a route to data with direct physical interpretation, to enable more sophisticated design of molecules with sought optical properties.

We study the nonlinear processes involved in the generation of broadband spectra with selectable spectral width. The continuum spectra were generated in short piece of SMF-28 fiber as nonlinear media pumped by a microchip laser at 1064 nm. Bending effects in the spectra were observed through wrapping the fibers on a cylindrical tube. We show that the spectral characteristics directly depend on the properties of the nonlinear medium, the excited nonlinear effects, and the location of the wrapped section which can produce a filtering effect on the SC spectrum. Finally, the main advantages of the proposed scheme are to obtain an adjustable bandwidth for a supercontinuum source through an easy and flexible control, and to provide a low-cost configuration in which photonic crystal fibers are not used.

Photonic crystal fiber (PCF) with two closely spaced zero dispersion wavelengths (TZDW) offers a unique route to efficient energy transfer to two spectrally localized continua beyond either side of the ZDWs, which we have employed in previous work for mid-IR difference frequency generation and speckle-free red-green-blue generation. In this manuscript, we report the interferometric coherence characterization and radio frequency (RF) noise measurements of the Stokes side TZDW component. With a custom-built 1.3 W, 1035 nm, 40 MHz, 240 fs Yb:fiber chirped pulse amplifier as the pump source, we use 12 cm of commercially available TZDW PCF to excite the dual narrow-band continua from which the Stokes pulse is filtered out with a 1180 nm long wave pass filter. We achieve 0.8 to 3 nJ of narrow-band pulses within the spectral range of 1200 – 1315 nm at an average power conversion efficiency of 33%. Employing an un-balanced Michelson interferometer, measured mutual spectral coherence of the Stokes pulse is in excess of 0.76 with pump Soliton order as high as N ~70. Its measured RF noise spectrum at the first harmonic of the laser repetition rate shows less than 8 dBc/Hz increase in relative intensity noise (RIN) compared to that of the power amplifier, which is consistent with reported studies employing sub-100 fs pulses from relatively low noise oscillators. In contrast to the broadband continuum from a single ZDW PCF wherein severe de-coherence is found with pumping at high soliton order and longer pump pulse width, the reported TZDW fiber source shows preservation of intensity stability and phase coherence against variation in pump pulse parameters, which not only attests to the stability of our reported method for mid-IR generation, but also shows promising potential towards an all-fiber, efficient and low noise ultrafast source that can be helpful for applications such as biomedical deep-tissue imaging.

In this work we demonstrate for the first time, to the best of our knowledge, quasi-continuous wave (qcw) laser operation of a diode-side-pumped Nd:YVO₄ self-Raman laser operating at 1176 nm. The double beam mode controlling (DBMC) technique used in this work allows fundamental mode laser oscillation, resulting in a beam quality M² of 2.42 and 2.18 in the horizontal and vertical directions, respectively. More than 3.5 W of peak output power at 1176 nm was achieved with TEM₀₀ laser mode, corresponding to an optical conversion efficiency of 5.4%.With multimode operation, more than 8W of peak output power was achieved, corresponding to 11.7% optical conversion efficiency.

We show that the stimulated Brillouin scattering (SBS) is enhanced in photonic crystal fibres (PCF) with elliptical air holes characterized by air hole ellipticity η which is the ratio of major to minor axis of the elliptical hole. The study of SBS in elliptical photonic crystal fibres (EPCF) is based on a full modal analysis of the optical and acoustic properties using the finite element method. We investigate the influence of the shape of the air holes on Brillouin gain (gB) spectrum. We find that the interaction between optical and acoustic modes is enhanced in EPCF with a multi-peak behavior of the Brillouin gain spectrum. We also find that by squeezing the lattice of the air holes, the interaction between optical and acoustic waves is enhanced and the gB coefficient is increased.

Widely tunable mid infrared radiation achievable using quantum cascade lasers (QCLs) often requires external cavities and several QCL chips to cover a large bandwidth similar to the range reported here (~ 1000s nm). The cost and mechanical stability of these designs leaves room for alternative more rugged approaches, which require no cavities to achieve very broad band tunability. While difference frequency generation (DFG) will unlikely match the power levels achievable from QCLs, it can provide spectral brightness and extremely wide tunablity, which can be valuable for numerous applications. Recently, we have demonstrated that dispersion engineering techniques can be used for phase matching of second order nonlinearities near the bandgap in monolithic waveguides. In this work we demonstrate an extremely simple structure to grow and fabricate, which utilizes dispersion engineering not only to achieve phase matching but also to expand the tuning range of the frequency conversion achieved in a waveguide through difference frequency generation. Frequency conversion in monolithic AlGaAs single-sided Bragg reflection waveguides using χ⁽²⁾ nonlinearities produced widely tuneable, coherent infrared radiation between 2-3 μm and 7-9 μm. The broad tunability afforded by dispersion engineering and possible current injection, waveguide width chirping and temperature tuning makes it possible to produce a single multi-layer substrate to generate mid-IR signals that span μms in wavelength.

Materials grown by vapor phase techniques such as chemical vapor deposition or hydride vapor phase epitaxy (HVPE) often exhibit very low losses which are difficult to quantify by simple transmission measurements. The measurement of extremely low absorption coefficients can be carried out by laser calorimetric or thermal rise techniques, which determine the absorption coefficients by measuring the temperature increase caused by the absorbed laser radiation. We report here on results of measuring absorption coefficients of bulk HVPE-grown orientation-patterned GaAs (OP-GaAs) and GaP (OP-GaP) crystals using one of the methods of laser calorimetry, called transient calorimetry. In our setup, the sample under test is attached to a high-conductivity copper holder and placed in a vacuum chamber. A 2-micron cw laser beam is transmitted through the sample and the temperature rise in the sample is measured and, through the calorimeter calibration process, related to the power absorbed in the sample. The absorbed power, P_a, is a function of the total attenuation coefficient α_tot , the length of the sample, and the laser power P_o, defined as P_a = P_o exp (-α_tot l), where total attenuation α_tot is the sum of absorption and scattering: α_tot = α_abs + α_scat. Since scattered light does not cause heating, the calorimetric technique is only applicable to determining αabs. By this technique we have measured 2-micron absorption coefficients in OP-GaAs and OP-GaP as low as 0.007 cm^-1.

The thermo-optic coefficient of lithium niobate (LiNbO₃) has been measured in the temperature range from 10 to 160 °C using an interferometric setup. Undoped and magnesium-doped congruently melting LiNbO₃ and undoped stoichiometric LiNbO₃ were studied over a wide wavelength range in the visible and near infrared (450 – 600 nm and 900 – 1130 nm) using a frequency-doubled cw optical parametric oscillator. Experimental results for congruently grown lithium niobate were aggregated using a Schott equation to describe the wavelength and temperature dependence of the thermo-optic coefficient.

The absorption coefficient of undoped, congruently grown lithium niobate (LiNbO₃) for ordinarily and extraordinarily polarized light is measured in the wavelength range from 390 to 2600 nm using whispering gallery resonators (WGRs). These monolithic cavities guide light by total internal reflection. Their high Q-factor provides several hundred meters of propagation for the coupled light in millimetre size resonators allowing for the measurement of absorption coefficients below 10^-2 cm^-1, where standard methods such as Fourier-transform or grating spectroscopy meet their limit. In this work the lowest measured value is 10^-4 cm^-1 at 1700 nm wavelength. Furthermore, the known OH- overtone at 1470 nm wavelength can be resolved clearly.

Tunable optical solitons due to intra-pulse stimulated Raman scattering are well-known for their ultrashort pulse width, superb pulse quality and broadband tunability. Consequently they are suitable for a variety of applications, especially multi-photon microscopy (MPM). Recent progress in MPM demands two- or multi-color excitation to match the absorption peaks of multiple fluorophores. Here we propose a new scheme to generate two-color solitons with each wavelength individually tunable. The wavelength of the most energetic soliton can be easily tuned through energy tuning, while stretching the input pulse width narrows the wavelength separation between the most and the second most energetic solitons. This tunable two-color source may find application in various modalities of MPM.

AgGaGeS₄ compound (AGGS) is a promising nonlinear material for mid-IR applications. The different steps of this materials processing are presented. The chemical synthesis of polycrystals and the single crystal growth process are described. Compounds volatility can induce stoichiometry deviation and reduce the quality of obtained single crystals. Nevertheless, 28 mm diameter and 70 mm length single crystals have been grown by Bridgman-Stockbarger method, cut and polished AGGS crystal is obtained. The crystal has good homogeneity and absorption coefficient of less than 0.1 cm-1 in the 0.5-11.5 μm range which make it usable in nonlinear devices.

ZnSe doped with Cr²⁺ was analyzed by EDS, XPS and Micro-Raman spectroscopy techniques. EDS and XPS analysis revealed that chromium concentration is more than 2% and there are additional impurities, Ga, Ti, and Ta. EDS measurements did not reveal any variation in chromium concentration when a line scan was performed over a 200 μm distance. XPS analysis indicated that the sample surface is inhomogeneous. Photoluminescence was acquired by exciting the sample with 325 nm laser beam. Photoluminescence revealed charge transfer bands. Micro-Raman study revealed the LO, TO and 2TA modes at 252, 205 and 140 cm^-1. Under 488 or 514.5 nm excitation background luminescence was predominant due to excitation of Cr²⁺ electrons into the conduction band. However, 632.8 nm laser excitation revealed, strong Raman signals. Raman data were acquired by exciting the sample on the grain boundary and inside the domain. The ratio of LO and TO peak intensities changed randomly when data were acquired from different points on the grain boundary indicating the presence of random strain in the material. When Raman data were acquired from different points on the sample surface for comparison, it revealed that the LO mode was distorted as well as broadened whereas the TO mode intensity increased. This was due to the presence of local modes induced by the sample inhomogeneity and the interaction of the holes with the LO mode.

The pump laser was a cw-diode-pumped, acousto-optically Q-switched Nd:YAG laser. The laser had a pulse width of ~85 ns when operating at 10 kHz repetition rates. For infrared output of 2300 nm, we used 35-mm-long PPMgSLT which has a grating period of 32.7 μm for the first-order quasi-phase matching, resulting in the signal wavelength of 1980 nm at the crystal temperature of 76.5oC. Our optical parametric oscillator (OPO) was of a simple linear extra-cavity structure, formed by two flat dichroic mirrors with a separation of ~45 mm. The input coupling mirror had a high transmission of 98% for the pump, high reflectance of 98% at the signal and idler wavelengths, whereas the output coupler had a high reflectance of 98% at the pump wavelength. Hence, the OPO can be considered as singly resonant with double-pass pumping. In order to find an optimum reflectance for the efficient generation of infrared radiation of 2300 nm, we used the three different output mirrors whose reflectivity are ranging from 90% to 38% at the signal wavelength. We measured the signal and idler power as a function of the pumping power of Nd:YAG laser for three different output couplers. A maximum extraction efficiency with an optimum reflectance of output mirror was 27% for the idler, corresponding to 5.6 W of average output power. The fluctuations in the idler root-mean-square output power were measured to be below 1.5%. Our result is comparable with the recent one based on PPLN even with a simple cavity.

Third harmonic 355nm picosecond pulses are generated by sum frequency mixing in a periodically poled magnesium doped stoichiometric lithium tantalate (PPMgSLT) crystal. The third harmonic generation is based on the 1064nm radiation of a gain-switched distributed feedback (DFB) diode laser which is amplified by a two-stage fiber amplifier. The diode laser is freely triggerable at variable repetition rates up to 80MHz and provides optical pulses of 65 ps FWHM duration and pulse energies in the range of 5 pJ. The 355nm third harmonic generation is realized in a two-step conversion process. First, the 1064nm fundamental radiation is frequency-doubled to 532 nm, afterwards both frequencies are mixed in the PPMgSLT crystal to 355 nm. The UV-radiation shows a pulse width of 60 ps, a good beam profile and stable pulse energy over a wide range of repetition rates by proprietary pump power management. At 355nm a pulse peak power of 5.3W was achieved with 192W pulse peak power of the fundamental radiation.

Chalcogenide suspended core fibers are a valuable solution to obtain supercontinuum generation of light in the mid-infrared, thanks to glass high transparency, high index contrast, small core diameter and widely-tunable dispersion. In this work the dispersion and nonlinear properties of several chalcogenide suspended core microstructured fibers are numerically evaluated, and the effects of all the structural parameters are investigated. Optimization of the design is carried out to provide a fiber suitable for wide-band supercontinuum generation in the mid-infrared.

Germania doping is commonly used in the core of optical fiber due to its advantages compared to other materials such as superior transparency in near-infrared telecommunication wavelength region. During fiber preform manufacturing using the outside vapor deposition (OVD) process, Ge is doped into a silica soot preform by chemical vapor deposition. Since the Ge doping concentration profile is directly correlated with the fiber refractive index profile, its characterization is critical for the fiber industry. Electron probe micro-analyzer (EPMA) is a conventional analysis method for characterizing the Ge concentration profile. However, it requires extensive sample preparation and lengthy measurement. In this paper, a multiphoton microscopy technique is utilized to measure the Ge doping profile based on the multiphoton fluorescence intensity of the soot layers. Two samples, one with ramped and another with stepped Ge doping profiles were prepared for measurements. Measured results show that the technique is capable of distinguishing ramped and stepped Ge doping profiles with good accuracy. In the ramped soot sample, a sharp increment of doping level was observed in about 2 mm range from soot edge followed by a relative slow gradient doping accretion. As for the stepped doping sample, step sizes ranging from around 1 mm (at soot edge) to 3 mm (at soot center) were observed. All the measured profiles are in close agreement with that of the EPMA measurements. In addition, both multiphoton fluorescence (around 420 nm) and sharp second harmonic generations (at 532 nm) were observed, which indicates the co-existence of crystal and amorphous GeO₂.

We investigate the components of dissipative multi-wave solitons in the form of three-wave weakly coupled states originating within the collinear acousto-optical interaction due to acoustic waves of finite amplitude. This investigation is carried out in a square-law nonlinear birefringent medium with linear optical losses, theoretically and experimentally. Theoretically, we study the three-wave collinear acousto-optical interaction using several acoustic pulse profiles, with the cases of infinite support (when the acoustic pulse envelope is gradually vanishing on the boundaries) and compact support (when acoustic pulse envelope is cut down on the boundaries), and consider the appropriate boundary conditions in a quasi-stationary regime with the phase mismatch. As a theoretical result, one has found that the system can be described; in particular, by the cnoidal Jacobi elliptic functions whose limiting cases lead to hyperbolic and trigonometric solutions. The experiments dedicated to examine these theoretical results have been done with a X-ray irradiated α-quartz crystalline cell enabling the collinear acousto-optical interaction. The cell used the pure longitudinal acoustic wave with the frequency mismatch along the 6 cm interaction length. Two types of acoustic pulses had been generated, namely, hyperbolic-secant pulse (infinite support) and a bounded rectangular pulse (compact support). During these experiments one had observed the optical components peculiar to the mismatched weakly coupled states. Rather well agreement between the theoretical model, simulated numerically, and the obtained data of measurements for the frequency detuning, acoustic power density, and efficiency of the coupled states localization have been achieved.

This paper reports the high-accuracy Sellmeier and thermo-optic dispersion formulas for the ordinary ray of a 5mol.% MgO-doped congruent LiNbO3 that provide an excellent reproduction of the temperature-dependent birefringent and quasi-phase-matching conditions with oo-e and oo-o interactions coupled with our Sellmeier and thermo-optic dispersion formulas for the extraordinary ray of this material. We believe that these equations could be highly useful for practically designing the frequency conversion systems based on the periodically poled 5mol.% MgO-doped congruent LiNbO_3.

This paper reports the temperature-dependent phase-matching properties of 1.3mol% MgO doped stoichiometric LiNbO₃ (1.3mol%MgO:SLN) together with the new Sellmeier equations, which reproduce well our experimental results for the birefringent phase-matching data for second-harmonic generation (SHG) and sum-frequency generation (SFG) in the 0.41-2.03μm range. It was found that a set of our index formula are useful for predicting the temperature-dependent phase-matching conditions of 1.3mol%MgO:SLN coupled with the thermo-optic dispersion formula for a 5mol% MgO doped congruent LiNbO₃ (MgO:CLN) published in our papers. In addition, the temperature-tuned 90° phase-matched SHG and SFG in the undoped stoichiometric LiNbO3 fabricated by vapor transport equilibration (VTE:SLN) are also presented.

A new proposal towards the polarization tunable Goos-Hänchen (GH) and Imbert-Fedorov (IF) spatial and angular shifts is explored analytically in a four layer Kretschmann-Raether geometry comprising a ZnSe prism, a dielectric layer of PMMA-DR1 (Polymethylmthacryalate-Disperse red) and two metal layers of silver having thicknesses of 50 nm and 200 nm respectively. Observations from the different graphical representations reveal that in correspondence of the long range surface plasmon (LRSP) resonant angle both spatial and angular GH shifts get appreciably enhanced in case of p polarized light whereas negligible or very less amplification of spatial and angular GH shifts are obtained for s polarized light. With the switching of polarization of the incident light beam on the proposed configuration through the half wave plate, the spatial and angular GH shift is tuned from -17.35 μm to 0.105 μm and -0.631 μrad to 4.28 nrad respectively and the spatial and angular IF shift is tuned from 94.16 μm to -53.58 μm and about 7.774 μrad to -11.17 μrad respectively. To the best of our knowledge, several articles have been devoted for depicting the GH shift without considering the IF shift, whereas the exact beam position of the output beam can only be identified with the composite effect of GH and IF spatial and angular shifts. The above new proposal can be implemented in the field of fine tuning of optical switching at the μm ranges with varying polarization, optical sensors applications and serves interesting opportunities to make atomic mirrors.

Nanocomposite of metal oxide semiconductors are multifunctional and one such nanocomposite ZnO/ Mn₃O₄ (2:1) was synthesized ZMA4 (ZnO: Mn₃O₄ using ammonia), ZMH4 (ZnO: Mn₃O₄ using hexamine) by the coprecipitation route. In addition pure ZnO was prepared using ammonia (ZA) and hexamine (ZH). XRD reveals the crystal structure of ZnO/Mn₃O₄ and the average crystallite size is estimated as 43, 12, 42 and 18 nm for ZA, ZMA4, ZH and ZMH4 respectively. FTIR bands at 440-490 and 616-621cm^-1 are due to Zn-O and Mn-O vibrational bands. Presence of manganese in nanocomposites is confirmed by EDS. SEM micrographs indicate the formation of nanoparticles (ZA and ZMA4) and nanorods (ZH-98 nm length, 63 nm dia). Excitonic absorption peaks at 370 and 290 nm in the UV-Vis spectra are attributed to ZnO/Mn₃O₄ nanocomposite. The bandgap is estimated as 2.7, 2.3, 2.6 and 3.0 eV for ZA, ZMA4, ZH and ZMH4 respectively. FL spectra of ZA, ZMA4 expose the emission at 366 and 396 nm owing to the near band edge (NBE) and zinc interstitial at 468 nm. ZH nanorods show the emission at 386, 468 and 558 nm which are attributed to NBE, zinc interstitial and oxygen vacancy respectively. The reduction of oxygen vacancy is observed in ZMH4 as manganese effectively changes the morphology from nanorod to nanoparticle. The second harmonic generation efficiency measured for ZA and ZH is 0.6 and 0.9 times KDP using Q - switched Nd: YAG laser (1064nm, 10 Hz, 9 ns).

This work discusses the results of research into comparative efficiency of non-linear transformation of picosecond single- and double-scale fibre laser pulses in generation of Raman-dominated supercontinuum (SC) due to cascaded stimulated Raman scattering of these pulses in a long extra-cavity fibre. We demonstrate properties of SC formed by superposition of spectra of several Stokes components when pumped with pulses of different structure. More efficient Raman transformation of double-scale pulses was identified, leading to broader SC spectra. The work presents features of temporal distributions of radiation intensity of single- and double-scale pulses-induced SC at different average input powers.

The enhancement of nonlinear characteristics of a tapered fiber, which is covered with a thin layer of highly nonlinear glass, is theoretically studied in this work. The generation of a flat, octave spanning supercontinuum extending from 750 nm to 2750nm (at -20dB) has been numerically presented by considering the coupling of the proposed structure with a typical Yb laser able to emit ultrashort pulses. The physical processes behind the SC formation are described in the anomalous dispersion regions.