In this work we demonstrate low-threshold oscillation in the mid-IR spectral region with periodically poled KTiOPO₄ (PPKTP) pumped at 1064 nm by a single-frequency Nd:YAG laser. THe compact pump laser had a diode-pumped passively Q-switched monolithic ring cavity configuration and could generate up to 70 (mu) J, 2 ns pulses. With two PPKTP crystals having ferroelectric domain inversion periods of 37.8 micrometers , the temperature tuning bands for the signal of 1720-1750 nm and 1850-1920 nm have been obtained. The lowest oscillation threshold of 8.3 (mu) J was obtained in a cavity using two mirrors reflecting 99 percent at the signal and the idler wavelengths. The maximum OPO output power of 6 mW and the pump depletion of 39 percent was achieved by driving cavity close to stability limit for the idler field. The output power was substantially increases to 36 mW by using output mirror with 90 percent reflectivity only in the signal band and optimizing pump focusing conditions. The external OPO efficiency in this case reached 21 percent.

We describe the wavelength tunability and conversion efficiency of 532-nm pulse pumped optical parametric oscillators (OPOs) using periodically pulsed lithium tantalate (PPLT). The OPOs reported here used PPLT crystal lengths of 1,2 and 4 cm with signal wavelengths between 660 and 950 nm. These OPO's were pumped with a pump pulse energy of 100 (mu) J at 1 kHz yielding internal slope efficiencies of almost 90 percent and pump depletions of over 70 percent. Average power scaling experiments were also performed with a pump pulse energy of 66 (mu) J at 33 kHz yielding internal slope efficiencies of 50 percent.

We used elliptical beams to demonstrate aperture scaling effects in nanosecond single- and multi-grating periodically poled lithium niobate (PPLN) monolithic optical parametric oscillators (OPOs) and generators (OPGs). Increasing the cavity Fresnel number in single-grating crystals broadened both the beam divergence and the spectral bandwidth. Both effects are explained in terms of the phase matching geometry. These effects are suppressed when using a multi- grating PPLN crystal since the individual gratings provide small effective subapertures. A flood-pumped multi-grating OPG displayed a low output beam divergence, and contained nineteen pairs of signal and idler frequencies. We also demonstrated beam quality improvement in a pulsed periodically poled lithium niobate monolithic optical parametric oscillator (OPO) by operating at degeneracy, which eliminated the off-axis non-collinear phase-matched interactions. Using a 10:1 elliptical beam, we obtained 110 mJ of degenerate output when pumping with 175 mJ. We observed narrowing of the degenerate OPO beam divergence and output spectrum when temperature tuning beyond the collinear degenerate phase-matching condition.

We report on the first realization of an all-optical frequency-by-three divider. The frequency to be divided is generated by a single-frequency diode master-oscillator power-amplifier (MOPA) system emitting at a wavelength of 812 nm. By-three-division is achieved with a continuous-wave (CW) optical parametric oscillator (OPO) based on a periodically pulsed LiNbO₃ crystal, which carries tow sections with different poling periods. The first section provides quasi-phase matching (QPM) for parametric oscillation at 1218 nm and 2436 nm such that the idler frequency is approximately one-third of the pump frequency. The other section provides QPM for second harmonic generation (SHG) of the idler frequency which gives an additional field at a wavelength of 1218 nm. By fine tuning the OPO output wavelengths, the frequency difference between the signal and the idler-SHG is brought close to zero. This results in a mutual injection-locking of the idler and signal waves, giving phase-coherent oscillator of both waves with the pump at a frequency ratio of exactly 1 to 2 to 3. The measured locking-range of 0.5 to 1 MHz shows good agreement with the theoretical value of 0.8 MHz obtained form numerically solving the coupled wave equations for the OPO with resonator internal idler-SHG.

Here we report on a noncollinear optical parametric oscillator (OPO) in periodically poled KTiOPO₄. The noncollinear OPO cavity consisted of a 10 mm-long periodically poled KTiOPO₄ crystal placed between two flat mirrors and pumped at 532nm by a frequency doubled Q- switched Nd:YAG. The OPO threshold for the collinear configuration was reached at the pump pulse energy of 10 (mu) J and increased monotonically when increasing the noncollinear interaction angle. The efficiency of the OPO with PPKTP reached 50 percent in the collinear configuration and about 40 percent in the noncollinear configuration. The noncollinear OPO generated two signal-idler pairs. Only one of the waves in each pair was resonated in the OPO cavity, while the complementary waves were generated at different angels as required by momentum conservation. The OPO could be tuned over a range of about 290 nm around 1064 nm wavelength by adjusting the cavity angle and the PPKTP temperature. Narrowing of the OPO spectrum was observed for small noncollinear interaction angles.

We present 2 experiments on intracavity pumping of a KTiOPO₄ (KTP) optical parametric oscillator (OPO) within a high power 1064nm Nd:YAG laser cavity producing multiwatt level 2-micron outputs. In such high power regime, the Nd:YAG rod laser suffers significant thermally-induced birefringence loss when it is linearly polarized. Hence in the first experiment, we present our intracavity KTP OPO pumped within the simple cavity of an unpolarized Nd:YAG laser. This simple configuration, with 1-micron high reflectors forming the Nd:YAG laser cavity and R equals 75 percent and 100 percent 2-micron mirrors forming the short flat-flat OPO cavity, delivered 6.5W of 2-micron output power at 3kHz Q-switched operation. Next we pumped our intracavity OPO within a more complex polarized Nd:YAG laser cavity. In this second experiment, we compensated for the thermally-induced birefringence loss in the Nd:YAG laser by using a re-entrant laser cavity with a Faraday rotator, and the OPO was pumped within one arm of this set-up. In this case, we also obtained approximately 6.5W of 2-micron output. FInally, studies of the temporal profiles of the 1 and 2-micron laser beams also revealed interesting multiple pulse features in such intracavity OPO output.

A KTiOPO₄ (KTP) OPO operating at an average power level of 26.3 W was demonstrated. To our knowledge this is the highest average power that a KTP OPO has ben operated at to date. The OPO used three 1 sq. cm aperture KTP crystal cut for non-critical phase matching at 1.064 micrometers . The crystals were 10 by 10 by 20 mm in size, cut from a large crystal, which was grown from a modified self-flux by pulling on an x-oriented seeded. The pump laser operated at 100 pps repetition rate at an average power of just over 71 watts, and pump power densities above 60 MW/sq. cm. All of the elements were AR coated for 1.06 micrometers and 1.576 micrometers . Signal wavelength conversion efficiency of 37 percent was achieved in this configuration. Idler output was not measured, but no thermo-lensing was observed due to the enhanced transmission of our crystals at the idler wavelength of 3277 nm. Some crystal damage was observed over time, apparently due to pump laser instability. The result are especially interesting since past experience had shown that KTP elements fail rapidly under these high average power conditions. Future work will compare the performance of a single long KTP OPO element with that of multiple short elements, in the same resonator.

We present a simple analytical solution for singly resonant OPO. The present analysis permits calculating the depletion efficiency of the OPO even when the resonated signal suffers from strong intracavity losses. To the best of our knowledge, this is the first model, which yields an analytical formula in the depleted signal case. The model can be a useful tool to design cavity mirror reflectivity for a given pump intensity and intracavity losses.

SNLO is public domain software developed at Sandia National Labs. It is intended to assist in the selection of the best nonlinear crystal for a particular application, and in predicting its performance. This paper briefly describes its functions and how to use them.

The aim of this survey is to compare the performance of different nonlinear crystals used as optical parametric oscillator material when pumped at 1064nm with short pulse and low repetition rate Nd:YAG laser. Signal and Idler wavelengths have been monitored and slope efficiency viewgraphs as well as other relevant parameters displayed. Some experiments with KTP used as monolithic OPO have been performed and discussed in this paper.

It is known that compositional modification of KTP can be used to tailor its linear and nonlinear properties. Substitution of Ta or Nb onto Ti sites in KTP increases the birefringence to potentially allow SHG phase matching into the technologically important blue region.

The quasi phase-matched Optical Parametric Amplification (OPA) gain formula, within the undepleted pump approximation, is developed with the aid of the Rotating Wave Approximation (RWA). THe RWA keeps only the nearly phase-matched terms in the coupled Maxwell's equations for the signal and idler beam. The basis for dripping all the other terms is that they are rapidly oscillating on the length scale over which the nearly phase-matched terms vary appreciably and therefore will average to zero. This permits a closed from solution for the gain formula to be obtained. From the gain formula, important quantities such as the spectral, temperature, and angular bandwidth can be calculated. The RWA solution is compared with exact numerical solution.

We discuss here the feasibility of an optical parametric oscillator integrated on a GaAs chip, after reviewing the recent frequency conversion experiments using from birefringence in GaAs/oxidized-AlAs (Alox) waveguides. Recently, phase-matching has been demonstrated for the first time in a GaAs-based waveguide, using form birefringence in multilayer heterostructures GaAs/Alox. Birefringence n(TE)- n(TM) from 0.15 to 0.2 have been measured for different GaAs/Alox waveguides, which is sufficient to phase match mid-IR generation between 3 micrometers and 10 micrometers by difference frequency generation form two near-IR beams. A second step was the observation of parametric fluorescence. Results on parametric fluorescence at 2.1 micrometers will be described, in an oxidized AlGaAs form-birefringent waveguide, consisting of a high-index, strongly birefringent GaAs-Alox core embedded in an AlGaAs cladding. One of the most existing perspectives opened with this new type of nonlinear material is the realization of an optical parametric oscillator on a GaAs chip. To this aim, minimization of losses is the most crucial point. A typical calculated value of this threshold is less than 70 mW for 1 cm^-1 losses, and with 90 percent reflection coefficients. The level of losses has been reduced from 2 cm^-1 in ridges obtained by a standard reactive ion etching technique, to less than 0.5 cm^-1 in ridges realized with a more refined reactive ion etching process, using a 'three layer' mask. There is still a need for an improvement of the waveguide fabrication process, before reaching the oscillation threshold.

Large and good optical quality crystal of YCOB and GdCOB were grown from the melt by Czochralski technique. Typically they are 50 mm in diameter and 150 long. We have used the Fresnel formula to calculate GdCOB and YCOB phase matching angle loci for SHG of 1064 nm radiation, in type I and type II configurations. We have used femtosecond broadband pulses to measure the phase matching angels in type I SHG for the three principal planes of YCOB and compared them to the computed values. Different crystals of YCOB and GdCOB were cut and polished in principal plane configurations for type I and type II interactions. Moreover, crystal were also cut by using some SHG configurations selected out of principal planes. All these crystal were used to evaluate in the same conditions, the SHG conversion efficiency of Q-switched Nd:YAG laser in relation with the angular acceptance, walk- off angle and the non-linear coefficients which were also determined. The type II configurations give the highest angular acceptance. The high damage threshold and the relatively large angular acceptance of YCOB and GdCOB led to efficiency greater than 50 percent.

We present the optimization design of aperiodic optical superlattices (AOSs) realized by inverting poled ferroelectric domains in sample. This design problem belongs to solving an inverse source problem in nonlinear optics. The optical design of the AOS can be achieved with use of the simulated annealing method. The constructed AOSs can implement multiple wavelength second-harmonic generation and the coupled third-harmonic generation with an identical effective nonlinear coefficient, at the preassigned wavelengths. The simulations show that the harmonic generations in the constructed AOSs can approach the prescribed goal better than those with the Fibonacci optical superlattice. The effective nonlinear coefficients vs the optical wave propagating distance from the impinging surface of incident light in samples exhibit monotonically increasing behavior. This clearly infers that the contribution form every block to the otpical parametric processes is with each other in the constructive interference state. It is expected that this new design method may provide an effective and useful technique for flexibly constructing nonlinear optical material to achieve the desired functions and match various practical applications.

Stimulated Raman scattering of picosecond pulses was investigated in KGd(WO₄)₂, and BaWO₄ tungstate crystals. BaWO₄ crystal was previously found to be a new very efficient material for pumping with nanosecond pulses. In our experiment, we used the second harmonic output of a mode-locked Nd:YAG laser system, which generated single pulses of 28 ps duration. The length of the crystal was 40 mm for KGW, 23 mm for KYW and 29 mm for BaWO₄. The SRS threshold, conversion efficiency, and time dependence of the first Stokes with picosecond resolution were measured. Picosecond Raman gains at a 532 nm wavelength were found to be correspondingly 11.5, 18.7, and 14.4 cm/GW for KGW, KYW, and BaWO₄. The Raman shifting up to the third Stokes component was detected, and the first Stokes conversion efficiency reached 30 percent in BaWO₄ and 18 percent in KGd(WO₄)₂. Measurements of a temporal pulse length have shown that the first Stokes pulse was twice shorter than the pump pulse. The new BaWO₄ crystal occurs to be an efficient solid state Raman material for a wide variety of pump pulse durations.

A novel phase-matching technique for coherent THz-wave generation was proposed. This approach uses cross- Reststrahlen band dispersion compensation phase-matching in a collinear optical mixing technique using isotropic, semiconductor nonlinear crystals. The pump and signal sources are in the near-IR transmission window of the nonlinear crystal and the generated idler wave is in the far-IR transmission window, on the other side of the crystal's Reststrahlen band. The choice of the nonlinear crystal for frequency conversion from the near-IR to the far-IR is highly dependent on the optical properties of the nonlinear crystal being used as the frequency converter element. We evaluate the use of the proposed phase-matching technique with a variety of III-V and II-VI compounds taking into account their optical dispersion and their phase- matching properties. Theoretical projections of the device performance such as perfect phase-matching conditions, coherence length and wavelength tuning range for different nonlinear crystals are compared.

We have modeled the passive Q-switch performance of divalent cobalt and its spectroscopic parameters in various host media for the Er:Yb:Glass laser that operates near 1.534 micrometers . Our method involves the use of rate equations that assume a three-level gain medium and a four-level absorber medium including excited-state absorption. Numerical integration techniques are used where analytical functions are unobtainable to describe the dynamics within the systems that we have examined. Input into the rate equations is obtained from experimental data that include Co²⁺ ion concentrations, cross-sections, and lifetimes obtained by time-resolved spectroscopy. The calculated laser output in terms of pulse energies and pulsewidths in ns is compared with experimental results based on different Co²⁺ absorber host matricies and different cavity designs.

The dynamics of the transversal structure of the reflected light from stimulated Brillouin scattering (SBS) is examined experimentally. The setup allows to observe the transversal structure of the backscattered pulses with a time resolution of 2 ns. Two different SBS-materials, SF₆ and methanol with significant different phonon lifetimes, were investigated. The results show a rapid growth of the beam size at the onset of the SBS. Generally, a different behavior in the development of the transversal structure between transient and stationary SBS was observed.

THe laser oscillator with a cavity completed by the refractive index grating induced by generating beams in a nematic liquid crystal cell was studied experimentally and theoretically. It is shown that both the thermal and orientational nonlinearity of the liquid crystal can provide the holographic mirror formation and the self-starting condition in the laser oscillator. The self-starting laser with the nonlinear mirror demonstrates single-longitudinal- mode generation with good quality of the beam.

We present the results of theoretical and experimental studies on creation of nonlinear optical systems on a base of fullerene-containing media: power radiation limiters, photorefractive media for dynamic hologram recording, and devices for controlling spatial and temporal parameters of radiation.

We consider theoretically spatial pattern formation processes in a unidirectional ring cavity with thin layer of Kerr-type nonlinear medium. Our method is based on studying of two coupled equations. The first is a partial differential equation for temporal dynamics of phase modulation of light wave in the medium. It describes nonlinear interaction in the Kerr-type lice. The second is a free propagation equation for the intracavity field complex amplitude. It involves diffraction effects of light wave in the cavity.

We present digital photograph images demonstrating real-time visualization of domain formation in periodically-poled lithium niobate (PPLN). This in-situ, non-destructive technique provides important visual information concerning the global quality and dynamics of domain patterning during the fabrication of PPLN.

We review our recent results on efficient generation of coherent blue/green light based on different configurations and structures. We have used second-harmonic generation with forward or backward configuration in short-period periodically-poled bulk and waveguide KTP to generate blue light. We have used backward second-harmonic generation to characterize periodically-segmented sub-micron KTP waveguides. By cascading second-harmonic generation and subsequent sum-frequency generation, we have generated coherent blue light in KTP and/or Ce:KTP crystals. We have also studied damage threshold of bulk KTP; we hope that we can eventually use it towards efficient generation of CW coherent green light.

In this paper, we describe a red OPO device that is synchronously pumped by a mode-locked laser. The pump source is a diode pumped mode-locked Nd:YAG laser. The motivation for this approach is that the high peak power of the mode- locked pulses allows the use of shorter PPLN crystals that reduces the impact of refractive index perturbations. We expect that eh mode-locked implementation will not exhibit the power instabilities that limited the previous continuous-wave version, while the high repetition rate of the mode-locked format will be comparable to true cw in most applications.

We use computer simulation to illustrate how thermally induced phase mismatch affect deep UV harmonic generation. A multicrystal harmonic generator that compensates for thermally include phase mismatch is then presented. We have tested this multicrystal design with a Nd:YAG lasers 4th harmonic generator based on two pieces of (beta) -BaB₂O₄ crystals, and our results demonstrate that it compensates for the thermally include phase mismatch, effectively increasing the interaction length of nonlinear optical crystals during harmonic generation under high loading.

We measured the two-photon absorption (TPA) cross sections inside (beta) -BBO crystal during UV harmonic generation. We found that the 2-photon absorption is dominating the absorption effect inside the BBO crystal during UV harmonic generation. Both 2 UV photons and 1 UV photon + 1 fundamental photon absorption cross sections are significant. Possible explanations are presented, and compared with other nonlinear otpical crystals. Thermal profiles inside the crystal as a result of the strong absorption processes are discussed through a computer program that simulates the heat dissipation process. We conclude that TPA is the significant factor in high power scaling of UV harmonic generation inside nonlinear optical crystals.

In this letter we introduce a new nonlinear organometallic complex CdHe(SCN)₄ crystal, which is used to double the frequency of the 808nm laser diodes. The blue-violet light output of 11.8 mw, and the SHG conversion efficiency of 0.60 percent are obtained.

Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Columbic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.

Nonlinear changes in the spectra of 100-picosecond optical pulses at 1061 nm transmitted through Nd³⁺ and Er³⁺ doped optical silica fibers were investigated both experimentally and theoretically. Generation of the Stokes and anti-Stokes frequency satellites was measured in Er³⁺ fibers and identified with the modulation instability due to four-wave mixing and stimulated Raman scattering. The generation of the Stokes Raman component was measured for single-core-mode double-clad Nd³⁺ fibers.

We analyze the properties of vortex solitons generated in Kerr and non-Kerr nonlinear optical media, including the vortex drift and rotation into a diffracting Gaussian beam. We also present our recent analytical and experimental results on the vortex-induced break-up of a dark-soliton stripe, and discuss a connection of this phenomenon with the well-known Aharonov-Bohm effect in solids.

Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air are presented. Our model, based on the nonlinear Schroedinger equation for the vector field, incorporates the main effects present in air, including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The field evolution is coupled to a model that describes the plasma density evolution. Intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air are simulated to study turbulence-induced filament formation, their mutual interaction via a low-intensity background, dynamics of the field polarization, and evolution of the polarization patterns along the propagation direction.

The advent of the laser as an intense, coherent light source gave birth to nonlinear optics, which now plays an important role in many areas of science and technology. One of the first applications of nonlinear optics was the production of coherent light of a new frequency by multi-wave mixing of several optical fields in a nonlinear medium. Until the experimental realization of Bose-Einstein Condensation (BEC) there had been no intense coherent source of matter-waves analogous to the optical laser. BEC has already been exploited to produce a matter-wave 'laser' atom optics was reported: the observation of coherent four wave mixing in which three sodium matter waves mix to produce a fourth. The experiment utilized light pulses to create two high-momentum wavepackets via Bragg diffraction from a stationary Bose- Einstein condensate. The high-momentum components and the remaining zero momentum condensate component interact to form a new momentum component due to the nonlinear self- interaction of the bosonic atoms. We develop a quantum mechanical description, based on the slowly-varying-envelope approximation to the time-dependent nonlinear Schroedinger equation, to describe four-wave mixing in Bose-Einstein condensates and apply this description to understand the experimental observations and to make new predictions. We examine the role of phase-modulation, momentum and energy conservation, and particle number conservation in four-wave mixing of matter waves.

We simulate pulse compression mechanism based on a near-two- photon-resonance (NTPR) transition contribution to the nonlinear refractive index of atomic Noble gas filled hollow wave-guides. The negative refractive index contribution in the normal dispersive gas waveguide, plays a similar role as in the case of soliton compression with positive Kerr non- linearity and anomalous dispersion in optical fibers. The self pulse compression to approximately 15 fsec can be achieved at moderate peak powers for 100 fsec pulses in the spectral range 100-245 nm. We present simulated data concerning pulse and spectral shapes for xenon as a case study. The total throughput of the propagated pulse energy is > 90 percent, mostly determined by the linear attenuation of the hollow wave-guide propagation mode while two photon absorption and the corresponding enhanced three photon photo-ionization does not significantly reduce the pulse energy.

We present a brief overview of the physics of vector optical spatial solitons formed by an incoherent interaction of two optical beams in a medium with saturable nonlinearity and describe the families of two-mode vector spatial solitons, which appears via bifurcations of one-component solitons, and their bound states. We report the results of the experimental observations of bound states formed by two vector spatial solitons due to a force balance between the soliton components, and also demonstrate a link between such bound states and earlier reported multihump multi-mode optical solitons.

There is considered formation and propagation of shock electromagnetic waves (SEW) of visible spectral range as possible nonlinear optical phenomenon taking place at laser intensities characteristic of femtosecond laser interaction with transparent solids. Main regularities of SHEW formation are studied on the basis of 1D model of plane-wave propagation in isotropic dielectric with nonlinear optical response. Special attention is paid to influence of color dispersion and absorption on SEW formation and propagation. Necessary conditions for appearing of SHEW are obtained, in particular, threshold amplitude is estimated. There is presented a model for numerical simulation of SHEW formation and propagation influenced by dispersion of linear and nonlinear parts of refractive index. Using the simulation, we studied dynamics of SHEW formation on several first optical cycles of femtosecond laser pulse in transparent medium. Important observed features of SHEW of optical frequency are discussed. Obtained results are considered from the viewpoint of experiments on femtosecond laser interaction, in particular, laser-induced damage.