Showing papers in "Journal of The Optical Society of America B-optical Physics in 2002"

Photonic-crystal slow-light enhancement of nonlinear phase sensitivity

Abstract: We demonstrate how slow group velocities of light, which are readily achievable in photonic-crystal systems, can dramatically increase the induced phase shifts caused by small changes in the index of refraction. Such increased phase sensitivity may be used to decrease the sizes of many devices, including switches, routers, all-optical logical gates, wavelength converters, and others. At the same time a low group velocity greatly decreases the power requirements needed to operate these devices. We show how these advantages can be used to design switches smaller than 20 µm×200 µm in size by using readily available materials and at modest levels of power. With this approach, one could have ∼105 such devices on a surface that is 2 cm×2 cm, making it an important step towards large-scale all-optical integration.

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers

Abstract: Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources.

Supercontinuum generation in air–silica microstructured fibers with nanosecond and femtosecond pulse pumping

Abstract: We study the generation of supercontinua in air–silica microstructured fibers by both nanosecond and femtosecond pulse excitation. In the nanosecond experiments, a 300-nm broadband visible continuum was generated in a 1.8-m length of fiber pumped at 532 nm by 0.8-ns pulses from a frequency-doubled passively Q-switched Nd:YAG microchip laser. At this wavelength, the dominant mode excited under the conditions of continuum generation is the LP11 mode, and, with nanosecond pumping, self-phase modulation is negligible and the continuum generation is dominated by the interplay of Raman and parametric effects. The spectral extent of the continuum is well explained by calculations of the parametric gain curves for four-wave mixing about the zero-dispersion wavelength of the LP11 mode. In the femtosecond experiments, an 800-nm broadband visible and near-infrared continuum has been generated in a 1-m length of fiber pumped at 780 nm by 100-fs pulses from a Kerr-lens model-locked Ti:sapphire laser. At this wavelength, excitation and continuum generation occur in the LP01 mode, and the spectral width of the observed continuum is shown to be consistent with the phase-matching bandwidth for parametric processes calculated for this fiber mode. In addition, numerical simulations based on an extended nonlinear Schrodinger equation were used to model supercontinuum generation in the femtosecond regime, with the simulation results reproducing the major features of the experimentally observed spectrum.

Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source

Abstract: Broadband continua extending from 400 to 1600 nm are generated in photonic crystal fibers and in tapered conventional optical fibers. The continuum is generated in the fundamental fiber mode. Femtosecond pulses from an unamplified Ti:sapphire laser with energies up to 4 nJ are used, and the resultant spectra from several photonic crystal fibers and taper structures are compared and analyzed.

Theoretical and experimental characterization of coherent anti-Stokes Raman scattering microscopy

Abstract: We present a systematic characterization of coherent anti-Stokes Raman scattering (CARS) microscopy. CARS signal generation in a heterogeneous sample under a tight-focusing condition is formulated by the Green’s function method. The CARS radiation pattern and the forward- and backward-detected CARS signals from a three-dimensional Raman scatterer are calculated. The coherent nature of CARS image formation and its consequences for image contrast and spatial resolution are investigated. Experimental implementations of CARS microscopy with collinearly copropagating and counterpropagating excitation beams, forward and backward data collection, and polarization-sensitive detection are described. Finally, CARS images of unstained live cells with forward detection, epidetection, and polarization-sensitive detection are presented and compared.

Study of femtosecond-laser-written waveguides in glasses

Abstract: Mechanisms of the femtosecond-laser-induced refractive-index change (Δn) were investigated for fused silica and a borosilicate glass. Experiments were based on two exposure situations: (a) high repetition rate and low pulse energy and (b) low repetition rate and high pulse energy. The calculated temperature rise based on model (b) was above 1000 °C, whereas for situation (a) it was negligible. The results do not support a thermal origin of the induced Δn; rather, heat may limit the magnitude of the change. Correlation between color-center formation and Δn in both glasses suggests that defects contribute substantially to the index increase. However, annealing studies have shown that the induced Δn persisted beyond the disappearance of the color centers. Analysis of the induced stress showed that densification plays a small role in this change.

Self-similar propagation of parabolic pulses in normal-dispersion fiber amplifiers

Abstract: Pulse propagation in high-gain optical fiber amplifiers with normal group-velocity dispersion has been studied by self-similarity analysis of the nonlinear Schrodinger equation with gain. For an amplifier with a constant distributed gain, an exact asymptotic solution has been found that corresponds to a linearly chirped parabolic pulse that propagates self-similarly in the amplifier, subject to simple scaling rules. The evolution of an arbitrary input pulse to an asymptotic solution is associated with the development of low-amplitude wings on the parabolic pulse whose functional form has also been found by means of self-similarity analysis. These theoretical results have been confirmed with numerical simulations. A series of guidelines for the practical design of fiber amplifiers to operate in the asymptotic parabolic pulse regime has also been developed.

Multiple-vibrational-mode model for fiber-optic Raman gain spectrum and response function

Abstract: We describe a model for the stimulated Raman gain spectrum and the Raman response function in silica fiber using multiple vibrational modes. We base the model on previous spectroscopic data [J. Opt. Soc. Am. B1, 652 (1984) and J. Opt. Soc. Am. B6, 1159 (1989)] and extend an earlier proposed model [Appl. Opt.21, 359 (1982)] by making use of an inhomogeneous distribution of damped oscillators. The model provides a simple analytical expression for the Raman response function and an expression for the Raman gain spectrum that is easy to evaluate numerically.

Multipole method for microstructured optical fibers. II. Implementation and results

Abstract: We describe the numerical verifications of a multipole formulation for calculating the electromagnetic properties of the modes that propagate in microstructured optical fibers. We illustrate the application of this formulation to calculating both the real and the imaginary parts of the propagation constant. We compare its predictions with the results of recent measurements of a low-loss microstructured fiber and investigate the variations in fiber dispersion with geometrical parameters. We also show that the formulation obeys appropriate symmetry rules and that these rules may be used to improve computational speed.

Analysis and optimization of optical parametric chirped pulse amplification

Abstract: Optical parametric amplification can give particularly high values for gain, gain bandwidth, energy, efficiency, and wave-front quality. In combination with chirped pulse amplification, in a technique we call optical parametric chirped pulse amplification, it offers the prospect of generating peak powers up to 100 PW and intensities greater than 1024 W/cm2 with existing technology. Here we study the technique in detail using both analytical and computational techniques, and the limit of validity of the analytical approach is identified. The effects of spectral phase, energy extraction, signal pulse chirp, and pump pulse nonuniformity are analyzed, and optimization techniques are proposed and discussed.

Stable (2+1)-dimensional solitons in a layered medium with sign-alternating Kerr nonlinearity

Abstract: Transverse beam propagation is considered in a layered structure in which Kerr nonlinearity alternates between self-focusing and self-defocusing, which makes it possible to prevent collapse. A structure composed of alternating self-focusing layers with strongly different values of the Kerr coefficient is considered too. By means of both a variational approximation (which is implemented in a completely analytical form, including the stability analysis) and direct simulations, it is demonstrated that stable quasi-stationary (2+1)-dimensional soliton beams exist in these media (direct simulations demonstrate stable propagation over a distance exceeding 100 diffraction lengths of the beam). Quasi-stationary cylindrical solitons with intrinsic vorticity exist too, but they all are unstable, splitting into separating zero-vorticity beams.

Supercontinuum generation, four-wave mixing, and fission of higher-order solitons in photonic-crystal fibers

Abstract: The nonlinear propagation of femtosecond pulses in photonic-crystal fibers is investigated theoretically without the use of the slowly varying envelope approximation. Low-intensity supercontinuum generation caused by fission of higher-order solitons into red-shifted fundamental solitons and blue-shifted nonsolitonic radiation is studied in a large range of fiber and pulse parameters. It is shown that phase matching of degenerate four-wave mixing can be achieved in an extremely broad frequency range from the IR to the UV. Spontaneous generation of new frequency components and parametric amplification by four-wave mixing as well as its possible overlap with soliton fission are studied in detail.

Nonlinear transmission and light localization in photonic-crystal waveguides

Abstract: We study light transmission in two-dimensional photonic-crystal waveguides with embedded nonlinear defects. First, we derive effective discrete equations with long-range interaction for describing the waveguide modes and demonstrate that they provide a highly accurate generalization of the familiar tight-binding models that are employed, e.g., for the study of coupled-resonator optical waveguides. Using these equations, we investigate the properties of straight waveguides and waveguide bends with embedded linear and nonlinear defects. We emphasize the role of evanescent modes in the transmission properties of such waveguides and demonstrate the possibility of the nonlinearity-induced bistable (all-optical switcher) and unidirectional (optical diode) transmission. Additionally, we demonstrate adaptability of our approach for investigation of multimode waveguides by the example of the bound states in their constrictions.

Growth, spectroscopic characterization, and laser performance of Nd:LuVO 4 , a new infrared laser material that is suitable for diode pumping

Abstract: The spectroscopic properties and laser operation of a new neodymium-doped vanadate crystal, Nd:LuVO4, grown by the flux technique are reported. Polarized absorption and emission spectra were recorded at low and room temperatures, excited-state absorption was measured near 1060 and 1340 nm, and laser emission at 1066 nm was obtained after pumping near 809 and 880 nm.

SCISSOR solitons and other novel propagation effects in microresonator-modified waveguides

Abstract: We consider the linear and nonlinear optical properties of an optical waveguide consisting of a side-coupled integrated spaced sequence of resonators (SCISSOR). This fully transmissive system possesses large and controllable dispersion because the phase shift imparted by each resonator is strongly frequency dependent. Additionally, near resonance, the circulating power in each resonator can greatly exceed the power carried by the waveguide, leading to greatly enhanced nonlinear effects. We show that the effects of nonlinearity and dispersion can be balanced to create temporal solitons and that many other novel and useful pulse propagation effects can occur over short propagation distances in such a structure.

Thermal-Lens Model Compared with the Sheik-Bahae Formalism in Interpreting Z-Scan Experiments on Lyotropic Liquid Crystals

Abstract: We carried out Z-scan measurements on lyotropic liquid crystals and on lyotropic liquid crystals doped with ferrofluid. In these experiments, the sample is translated through the focal region of a focused Gaussian laser beam. The dependence of the far-field intensity on sample position due to intensity-dependent optical nonlinearities has been analyzed on the basis of the thermal-lens model of Gordon et al. and the Gaussian decomposition analysis of Sheik-Bahae et al. The thermal-lens model is nonlocal in space and time, whereas the Gaussian decomposition is predicated on a strictly local response. We compare the goodness of fit of the predictions of these models to experimental data, and we discuss limitations of these models in describing Z-scan experiments on systems with nonlocal response.

Determination of laser parameters of ytterbium-doped oxide crystalline materials

Abstract: Spectroscopic and laser properties at ∼1 µm of seven new ytterbium-doped crystalline materials—silicates Y2SiO5,Ca2Al2SiO7,SrY4(SiO4)3O, borates Ca3Y2(BO3)4,Ca3Gd2(BO3)4,Sr3Y(BO3)3, and Ba3Lu(BO3)3— are investigated Absorption and emission cross sections are strongly dependent on the crystalline host Good agreement is obtained between emission cross sections calculated by the reciprocity method and by the Fuchtbauer–Ladenburg method The wavelength dependence of the gain cross sections are also determined Except for Ba3Lu(BO3)3, which is of poor crystalline quality, laser oscillations in a plano-concave cavity are obtained for all the presented hosts From optical spectroscopy and laser results, large tunability of the emission is expected for these materials Slope efficiencies are in the range 40%–60% with very low threshold values

Green’s function formulation for third-harmonic generation microscopy

Abstract: We report a theoretical study of third-harmonic generation (THG) microscopy by use of a Green’s function formulation. The third-harmonic signal under a tight-focusing condition is calculated for samples with various shapes and sizes. Our results show that THG signals can be efficiently generated at a sizable interface perpendicular or parallel to the optical axis or from a small object with a size comparable to the width of the axial excitation intensity profile. The signal-generation mechanism of THG microscopy is explained by a modified phase-matching condition, |k3-3(k1+Δkg)|l≪π, where Δkg is the wave vector mismatch induced by the Gouy phase shift of the focused excitation field. The relation of the THG power and radiation pattern to the orientation of an interface is investigated. A comparison between signal generation in THG microscopy and that in coherent anti-Stokes Raman scattering microscopy is given.

Third- and fifth-order optical nonlinearities in a new stilbazolium derivative

Abstract: The third- and fifth-order optical nonlinear properties of a new stilbazolium derivative, trans-1-[p-(p-dimethylaminobenzyl-azo)-benzyl]-2-(N-methyl-4-pyridinium)-ethene iodide (DABAZO-PEI), were investigated with a Q-switched and mode-locked pulse laser with a duration of 35 ps at 1064 nm. Its linear optical properties were also discussed. DABAZO-PEI exhibits two-photon absorption and strong excited-state absorption induced by two-photon absorption. A positive third-order nonlinear refraction caused by bound electrons and negative excited-state refraction caused by the two-photon absorption-generated excited states were also observed. The magnitude and the sign of the real (refractive) and the imaginary (two-photon absorption) parts of the molecular third-order polarizability, and the excited-state absorption and refraction cross sections, were measured. A feature worth noting is the values of excited-state absorption and refraction cross sections. They are all as high as 10-17 cm2. Based on the excited-state absorption induced by the two-photon-absorption mechanism, a highly efficient optical-limiting behavior for incident fluence in the range of 0–10 J/cm2 was demonstrated in a 1-cm-path quartz cell containing DABAZO-PEI solution in dimethyl sulfoxide. The optical-limiting threshold was estimated to be as low as 5 mJ/cm2.

Influence of substrate-lens design in terahertz time-domain spectroscopy

Abstract: We describe measurements of the angular radiation patterns from lens-coupled terahertz antennas fabricated on photoconductive substrates. These measurements were performed with a novel terahertz (THz) time-domain spectrometer in which the femtosecond optical pulses used to gate the emitter and receiver antennas were delivered by optical fiber. We used this system to perform a comparison between the two substrate-lens designs commonly used in THz time-domain spectrometers. We measured both E-plane and H-plane emission patterns for a 90° bow-tie antenna. By comparing these experimental results with simulations based on Fresnel–Kirchoff diffraction, we find that the choice of substrate-lens design is important in determining not only the directivity of the emitted beam but also the spectral bandwidth. These results emphasize the significance of this crucial component in the design of broadband THz spectrometers.

All-optical analog-to-digital converter by use of self-frequency shifting in fiber and a pulse-shaping technique

Abstract: We propose a new, to our knowledge, method for an all-optical analog-to-digital converter by using self-frequency shifting in a fiber and a pulse-shaping technique. Preliminary experimental results show that various digitized temporal bit signals can be generated by the variation of the power of an ultrashort analog input pulse of less than 1 ps.

Soliton compression of femtosecond pulses in quadratic media

Abstract: We describe efficient soliton compression of femtosecond pulses by use of cascade quadratic nonlinearity and normal dispersion in quadratic media. Pulse compression by a factor of ∼3 is achieved in ∼30-mm-long beta-barium borate at a wavelength of 800 nm. We investigate the dependence of compression performance on phase mismatch, input intensity, and propagation length. The compressed pulses are fully characterized by use of the frequency-resolved optical gating method.

Formation of thermally stable chemical composition gratings in optical fibers

Abstract: Experimental results and a discussion of possible chemical pathways in the formation of thermally stable chemical composition gratings in optical fibers are presented. Gratings are formed through high-temperature treatment of UV-exposed hydrogen-loaded fibers. The final refractive-index modulation is ascribed to variations in fluorine concentration attained by periodically increased diffusion of fluorine. The mechanism behind this increase is the formation of mobile hydrogen fluoride from chemical reactions of fluorine and UV-induced hydroxyl, which occur with the spatial periodicity of the UV pattern. A hydroxyl-assisted increase in fluorine diffusion has been verified by time-of-flight secondary-ion mass spectroscopy. Formation of ultrastable grating by periodic variation of oxygen concentration through diffusion of molecular water is also discussed.

Characterization of the laser crystal Nd:GdVO 4

Abstract: Nd:GdVO4 crystal was grown by the Czochralski method. The absorption and fluorescence spectra of the crystal were measured at room temperature. The thermal expansion and the specific heat of the crystal were also measured. Laser outputs at 1.06 and 1.34 µm were achieved when a Nd:GdVO4 crystal sample with a high Nd doping concentration was pumped by a low-power laser diode (LD) at 808.5 nm, and visible green and red laser outputs of intracavity frequency doubling at 0.53 and 0.67 µm were also achieved when nonlinear KTiOPO4 and LiB3O5 crystal, respectively, were used. The highly Nd-doped Nd:GdVO4 crystal was pumped by a high-power LD, and a greater than 5-W laser output power at 1.06 µm was obtained. A low-Nd-doping concentration Nd:GdVO4 crystal sample was pumped by a high-power LD, yielding laser output powers at 1.06 and 0.53 µm; 0.53-µm green laser output was obtained when a KTiOPO4 crystal was used, and the output beam’s values were M2=1.76 at an output power of 14.3 W at 1.06 µm and M2=1.55 at an output power of 3.3 W at 0.53 µm. Acousto-optical Q-switched laser outputs at 1.06 and 0.53 µm were also achieved. A thermal lens made from a Nd:GdVO4 crystal was measured; it was weaker than that of a Nd:YVO4 crystal. Some important material parameters, such as temperature-induced changes in refractive index, material constant, thermal-stress resistance figure of merit, and power per unit length at the stress fracture limit, have been estimated.

High power and good beam quality at 980 nm from a vertical external-cavity surface-emitting laser

Abstract: We demonstrate 1.5-W continuous-wave output power from a vertical external-cavity surface-emitting laser (VECSEL) based on InGaAs quantum wells as the gain medium. The VECSEL is pumped by the output of a single-bar diode-laser array at 814 nm and produces an optical-to-optical efficiency of 19%. The high output power is made possible by the use of a sapphire window optically contacted to the intracavity semiconductor surface for heat removal. We demonstrate the good beam quality of the VECSEL output by obtaining 1-W output from a single-mode fiber for 1.5 W launched with simple lenses. Pulsed operation produces a maximum peak power of ∼4.4 W and maximum average power of ∼2 W.

Entangled-photon Fourier optics

Abstract: Entangled photons, generated by spontaneous parametric downconversion from a second-order nonlinear crystal, present a rich potential for imaging and image-processing applications. Since this source is an example of a three-wave mixing process, there is more flexibility in the choices of illumination and detection wavelengths and in the placement of object(s) to be imaged. Moreover, this source is entangled, a fact that allows for imaging configurations and capabilities that cannot be achieved by use of classical sources of light. We examine a number of imaging and image-processing configurations that can be realized with this source. The formalism that we utilize facilitates the determination of the dependence of imaging resolution on the physical parameters of the optical arrangement.

Infrared femtosecond light filaments in air: simulations and experiments

Abstract: Experiments in which several-meter-long infrared ultrashort laser pulses are guided in air are compared with numerical simulations. During a first self-focusing stage that is affected by modulational instability, the beam is shown to break up into two channels of light that finally coalesce into a narrow filament that is able to propagate over several Rayleigh lengths. The filament propagation is associated with the generation of an electron plasma, whose density is greater than 1016 cm−3. Electron generation persists well beyond the focal region. The simulations restore the global dynamics of the pulse, including the main stages of Kerr focusing, light guiding driven by ionization, and the ultimate diffraction of the beam.

Nonlinear phase shifts induced by semiconductor optical amplifiers with control pulses at repetition frequencies in the 40–160-GHz range for use in ultrahigh-speed all-optical signal processing

Abstract: In a semiconductor optical amplifier (SOA) with copropagating optical pump pulses, the application of a nonlinear phase shift to optical signals provides the driving force for all-optical interferometric switching. We study, both analytically and experimentally, the dependencies of the nonlinear phase shift on the driving frequency (42–168 GHz) and on the SOA parameters. We have found that the nonlinear phase shift (ΔΦNL) decreases with the driving frequency but that this decrease is only linear, i.e., ΔΦNL∝f-1. We have also found that the nonlinear phase shift in the SOA linearly increases with the injection current (Iop), i.e., ΔΦNL∝Iop, even in this ultrahigh-frequency range.

Multipolar interband absorption in a semiconductor quantum dot. I. Electric quadrupole enhancement

Abstract: We present a theoretical investigation of a semiconductor quantum dot interacting with a strongly localized optical field as encountered in high-resolution near-field optical microscopy. The strong gradients of these localized fields suggest that higher-order multipolar interactions will affect the standard electric dipole transition rates and selection rules. For a semiconductor quantum dot in the strong confinement limit we calculated the interband electric quadrupole absorption rate and the associated selection rules. We found that the electric quadrupole absorption rate is comparable with the absorption rate calculated in the electric dipole approximation. This implies that near-field optical techniques can extend the range of spectroscopic measurements beyond the standard dipole approximation. However, we also show that spatial resolution cannot be improved by the selective excitation of electric quadrupole transitions.

Luminescence of dye molecules on oxidized silicon and fluorescence interference contrast microscopy of biomembranes

Abstract: The luminescence of dye molecules depends on their position in a layered optical system. Conversely, the luminescence can be applied to measure the position of dye molecules above an interface. We formulate the electromagnetic theory of stationary fluorescence in a layered optical system—of light absorption, light detection, and fluorescence lifetime—computing the angular dependence of dipole interaction with all plane waves by a classical Sommerfeld approach. The theory is checked by experiments with stained lipid membranes on silicon with 256 terraces of silicon dioxide. We apply the electromagnetic theory to fluorescence micrographs of living cells on oxidized silicon chips and evaluate distances between the cell membrane and the substrate in a range of 1–150 nm.