Mechanisms for inducing chaos in broad and narrow linewidth external-cavity laser diodes

Abstract

Optical power spectra of two chaotic external-cavity diode lasers with different single mode linewidths are compared. We report what we believe is the first observation of a novel mechanism for the onset of chaos in broad linewidth external-cavity laser diodes.     

Singly resonant sum-frequency generation of 520-nm laser via a variable input-coupling transmission cavity

We experimentally present a three-mirror folded singly resonant sum-frequency generation (SFG) cavity with an adjustable input coupling, which has been applied to 520-nm single-frequency laser generation via 780-nm laser and 1560-nm laser frequency mixing in a periodically poled KTiOPO₄ crystal (PPKTP). A continuous variation in the input coupling reflectivity from 81.4 to 96.1% for 780-nm resonant laser is achieved by tilting the input coupler, and the impedance matching of the resonator can be optimized. Up to 268 mW of SFG output power at 520-nm is obtained with 6.8 W of the 1560-nm laser input and 1.5 W of 780-nm laser input.     

IR-stimulated second harmonic generation in Sb2Te2 Se-BaF2-PbCl2 glasses

Abstract

Theoretically predicted and experimentally observed infraredinduced second-harmonic generation of glasses in the mid-infrared spectral region can be described by fifth-order nonlinear optical susceptibility. The effect is observed in the mid-IR region when the value of the electronic energy gap is comparable to the energies of actual phonons participating in the anharmonic (non-centrosymmetric) electron-phonon interactions. As subjects for investigation, chalcohalide Sb₂Te₂Se-BaF₂-PbCl₂ glasses were chosen. They are transparent, over a spectral range of 1.1–10.9 μm. The second-harmonic generation (SHG) output signal within the 1.5–4.8 μm spectral range has significant spectral dependence. Correlation of the SHG spectral maxima positions with spectral positions of anharmonic phonon frequencies confirms that the fifth-order steady-state process occurs due to cascading processes and IR-induced charge density non-centrosymmetry. A maximum value of the SHG is achieved at a pump-probe delay time of 18–36 ps, which is typical for anharmonic electron-phonon interactions. As temperature rises, the values of the photoinduced SHG signal susceptibility increases up to 3 × 10^?39 m⁴/V⁴. The SHG signal reaches a saturation point for the IR-pump power densities of about 0.8 GW/cm², which corresponds to the output SHG intensities of about 6 × 10^?4 with respect to fundamental one. The values of the diagonal fifth-order tensor component χ^(2ω) _xxxxx are at least one order of magnitude larger than the off-diagonal tensor components.     

Nonlinear optics in materials: integral transformation of optical four-wave mixing spectra

We analyze the estimation of the third-order susceptibility χ⁽³⁾ of nonlinear optical (NLO) materials by means of a Hilbert transformation applied to their optical four-wave mixing spectra. In particular, the coherent anti-Stokes Raman scattering (CARS) spectrum is considered. In this case, the integral line-shape processing enables the determination of the excitation profile for resonance CARS (RCARS), or the ratio χ_R/χ_EL between the Raman susceptibility and the electronic susceptibility, if nonresonance CARS is performed. As an illustration, we use this procedure to evaluate χ_EL for a NLO polymer in solution.     

Oscillation characteristics analysis of transversely and coaxially pumped Ti-diffused Nd:MgO:LiNbO3 waveguide lasers

Abstract

A theoretical analysis of oscillation characteristics is developed for Ti-diffused Nd:MgO:LiNbO₃ waveguide lasers by using the formalism of mode overlap. Simple and explicit expressions in terms of mode sizes are derived for effective mode volume and coupling efficiency between pump mode and laser mode, which are directly proportional to the threshold pump power and slope efficiency, respectively, both for transverse and coaxial pumping considering the spatial distribution of the pump beam and the laser beam. A comparison for the two pumping configuration has shown that the transverse pumping requires much higher input pump power and has much lower slope efficiency than the coaxial pumping. The fundamental transverse mode (TM ₀) oscillation conditions relative to the cavity loss are also discussed for the two pumping configurations.

Indexing terms: Ti-diffused Nd:MgO:LiNbO ₃ waveguide laser, effective mode volume and coupling efficiency, threshold pump power and slope efficiency, transverse and coaxial pumping, single mode oscillation condition.     

Third-order nonlinear optical responses of nanoparticulate Co3O4 films

Third-order nonlinear optical responses of nanoparticulate Co₃O₄ films, prepared by sputtering and pyrolysis, were investigated. The sputtered Co₃O₄ film showed a large third-order nonlinear susceptibility (|χ⁽³⁾|) of 3.2×10⁻⁸ esu, determined by degenerate four-wave mixing (DFWM). This |χ⁽³⁾| is 7.1 times greater than that of the pyrolyzed Co₃O₄ film, attributable to the greater density and refractive index of the sputtered film. Time-resolved DFWM experiments show a slowly decaying component with a lifetime of approximately 100 ps existing for both films in addition to a fast decaying component. Nonlinear transmission experiments revealed both films exhibiting significant nonlinear absorption. The imaginary part of the susceptibility (Im[χ⁽³⁾]) found for each film was of the same order of magnitude as the |χ⁽³⁾| found by the DFWM method, showing that the major part of the fast decaying component of |χ⁽³⁾| comes from the nonlinear absorption.     

Linewidth-determining Processes in Distributed Feedback Dye Lasers

Abstract

Processes determining the linewidths of distributed feedback dye lasers (DFDL) have been investigated. Time resolution of the frequency of the output pulse shows that the linewidth, averaged over a pulse, arises predominantly from a dynamic sweeping of the laser frequency during the course of the pulse. This sweeping results from refractive-index changes in the dye over the duration of the pumping pulse; either through thermal effects or dispersion associated with the saturated gain. Thermal effects may be minimized by suitable choice of solvent but the dispersive sweep is inherent in this type of laser. The magnitude of the dispersive sweep changes across the tuning range of the laser. By judicious choice of dye solvent and dye parameters we have developed a narrow linewidth DFDL of 140 MHz for τ = 3·2 ns pulses, which is close to the transform limit.     

Order-parameter collective modes in 3He-A and their effect on nuclear magnetic resonance (NMR) and ultrasonic attenuation

A generalization of our previous microscopic theory is used to derive the spin and density correlation functions for the Anderson-Brinkman-Morel (ABM) state. The coupling of the 18 fluctuation components of the order parameter due to the nuclear dipole interaction is taken into account rigorously. The results are valid for arbitrary temperature, frequency, wave number (with q parallel to the anisotropy axis), and magnetic field. Damping by pair-breaking processes is calculated explicitly; however, quasiparticle collisions are neglected. The frequencies of the clapping modes that couple to spin, and the frequencies of the flapping modes that couple to density, are found to split in a magnetic field. Near T _c the linewidth of the clapping mode and, at low T, the linewidth of the normal-flapping mode become small while the linewidth of the super-flapping mode is broad at all temperatures. The linewidth of the transverse NMR and the ultrasound attenuation coefficient, considered as functions of temperature, exhibit pronounced peaks at low temperatures. These arise from a coupling, induced by the dipole interaction, between spin or density fluctuations and the normal-flapping mode. Our results for the linewidth and line shift of the longitudinal NMR at low temperatures due to orbital effects are in agreement with results of Combescot.     

An Optically Narrowed Diode Laser for Rb Saturation Spectroscopy

Abstract

Diode laser linewidths have been reduced by a factor of up to 20 000 using the technique of resonant optical feedback. In this method, the laser frequency also locks optically to a confocal cavity, greatly improving the passive frequency stability. For the first time, such an optically narrowed diode laser has been used to obtain high-resolution Doppler-free spectra of the D₂ line of ⁸⁵Rb and ⁸⁷Rb at 780 nm.     

Electron spin resonance linewidths of Cr3+ in magnesium oxide

Electron spin resonance linewidths of Cr³⁺ in single crystal MgO at 9 GHz and 35 GHz were examined experimentally and theoretically for a range of Cr³⁺ concentrations. In contrast to the behaviour expected from dipolar broadening the experimental peak-to-peak linewidth for the 1/2 to–1/2 transition, which had a value of about 0.5 mT at 293 K, was independent of both polar angle and concentration over the range from 800 p.p.m. Cr to 15 100 p.p.m. Cr. The calculated dipolar linewidths exceeded those observed by factors of over one hundred; the ratio of momentsM ₄ ^1/2 /M ₂ ^1/2 derived from the experimental data lay between 1.33 and 1.39 and the lineshapes were markedly Lorentzian. The data suggested that Cr³⁺ entered the lattice substitutionally, occupying magnesium sites, that the linewidths were determined by exchange narrowing over the whole concentration range examined and that the exchange energy, whose values lay between 4 GHz and 100 GHz, varied linearly with concentration.     

Fabrication and characterization of a third-order nonlinear organic-polymer composite glass waveguide: a self-phase modulator

Composite organic-polymer glass optical waveguides in which coupling to the nonlinear organic-polymer layers was achieved by excitement of the underlying ion-exchanged glass waveguide and coupling of the light to the organic-polymer layer were fabricated and measured. A picosecond pulsed color center laser (λ = 1.5 μm) was used to measure the third-order optical susceptibility χ((3))(-w; w, -w, w) in an organic-dye-polymer composite glass waveguide with a Mach-Zehnder interferometer. For a squaryliumdye-doped poly(methyl methacrylate)-styrene-acrylonitrile matrix polymer layer, a composite χ((3)) of roughly 90, in units of (χLiNbO)(3)((3)), was measured.     

Nonlinear Optical Characteristics of C60 Thin Films

Abstract

The results of investigation of the real and imaginary parts of third‐order nonlinear susceptibility (χ⁽³⁾) of C₆₀ thin films (～100 nm) at the wavelength of Nd: YAG laser radiation (532 nm, τ = 55 ps) are presented using Z‐scan technique. Our studies show that the sign of Reχ⁽³⁾ changes from negative, at pulse repetition rate of 2 Hz to positive, at 0.5 Hz. Sign variations of the real part of the third‐order susceptibility were attributed to the influence of the thermal lens.     

Scattering of Light from Two Interacting Spherical Particles

Abstract

Linear optical properties of two spherical particles interacting via their dipole fields are studied. The dipole susceptibility and cross-sections of extinction, scattering and dissipation are found as functions of susceptibility χ₀ of an isolated particle. The case of arbitrary distance between particles is considered (which include interaction in near-zone, transitional-zone and far-zone). It is shown, that radiative energy losses of an oscillating dipole give rise to a finite phase shift between oscillations of the dipole and it's electromagnetic field in the near-zone. Application of this fact to the problem of two interacting dipoles leads to appearance of two additional resonances of susceptibility of the pair with radiative half-width tending to be zero when r₁₂→0 as (r₁₂/λ)², where r ₁₂ is the distance between particles.     

Quantitative Polarization-Resolved Second-Harmonic-Generation Microscopy of Glycine Microneedles

Second-harmonic generation (SHG) is a nonlinear optical process that can provide disease diagnosis through characterization of biological building blocks such as amino acids, peptides, and proteins. The second-order nonlinear susceptibility tensor χ⁽²⁾ of a material characterizes its tendency to cause SHG. Here, a method for finding the χ⁽²⁾ elements from polarization-resolved SHG microscopy in transmission mode is presented. The quantitative framework and analytical approach that corrects for micrometer-scale morphology and birefringence enable the determination and comparison of the SHG susceptibility tensors of β- and γ-phase glycine microneedles. The maximum nonlinear susceptibility coefficients are d₃₃ = 15 pm V⁻¹ for the β and d₃₃ = 5.9 pm V⁻¹ for the γ phase. The results demonstrate glycine as a useful biocompatible nonlinear material. This combination of the analytical model and polarization-resolved SHG transmission microscopy is broadly applicable for quantitative SHG material characterization and diagnostic imaging.     

Electron spin resonance linewidths of Co2+ in magnesium oxide

Electron spin resonance linewidths of Co²⁺ in single-crystal MgO at 9 GHz were examined experimentally and theoretically for a range of Co²⁺ concentration from 310 to 9900 ppm. In contrast to the behaviour expected from dipolar broadening, the experimental peak-to-peak linewidth for the octed hyperfine lines were about twelve to fifty times less than the calculated dipolar linewidths according to temperature and concentrations. The peak-to-peak linewidths as determined experimentally increase from 0.8 mT at 310 ppm Co²⁺ to 4.0 mT at 9900 ppm Co²⁺ at a polar angle _H= O° and at 20 K. Peak-to-peak linewidths independent of polar angle but strongly dependent on temperature and slightly dependent on concentrations were observed. The ratio of moments M ₄ ^1/4 /M ₂ ^1/2 derived from the experimental data lay between 1.35 to 1.44 and the lineshapes were markedly Lorentzian in the range of temperature measured. The data suggest that Co²⁺ entered the lattice substitutionally, occupying magnesium sites, that the linewidths were determined, after exchange energy, over the whole concentration range examined and that the exchange energy whose values lay between 6 and 77 GHz, varied linearly with concentration.     

Fabrication parameters optimization of Ti-diffused Nd:LiNbO 3 channel waveguide lasers

Abstract

The variations of lower-order TM mode size and corresponding effective refractive index, effective pump area and coupling efficiency between pump and laser modes with the controllable fabrication parameters, i.e. initial Ti-strip width W, diffusion temperature T and initial Ti-strip thickness H in c-cut Ti-diffused Nd:LiNbO₃ waveguide lasers have been calculated by using a variational method, taking into account both laser wavelength, 1085 nm, and pump wavelength, 815 nm. The main features of these curves were summarized and discussed in detail; consequently, the single-mode, fundamental mode cutoff and multimode conditions were presented and compared with the published experimental data. The effective pump area, directly proportional to threshold pump power, reveals a weak dependence on W in the considered range 4–16 μm and strong dependences on T and H in, respectively, the ranges studied 950–1100°C and 40–160 nm, while the coupling efficiency, directly proportional to slope efficiency, is hardly changed (with the values 0.82–0.85) with these parameters. Finally, appropriate diffusion parameters were proposed for the fabrication of a more efficient laser.     

Laser direct writing of tin oxide patterns

Tin oxide pattern generation by laser deposition from SnCl₄ · 5H₂O in isopropanol is reported. Smooth, even stripes of thicknesses ranging from 20 to 120 nm with sharp, well defined edges and cross-section are deposited by scanning Ar⁺ laser beam (λ = 514.5 nm) focused onto the substrate—solution interface with a constant speed of 1 mm s⁻¹. The linewidth linearly increases from 26 to 42 μm with increasing the power from 40 to 120 mW. The reproducibility of pattern generation is extremely good as revealed by SEM-EDXI and μRBS. The minimum DC resistivity of 1.7×10⁻² Ωcm, measured without any process optimization, favourably compares with those reported for films prepared by other techniques. The chemical composition of the film material is SnO_x with 1.1 < x < 1.5 as determined by an XPS-XAES study.     

On the longitudinal static and dynamic susceptibility of spin-1/2 Kondo systems

The impurity spin polarization, static susceptibility, and longitudinal impurity spin relaxation rate are calculated for thes-d model as function of temperature and magnetic field for ferromagnetic and antiferromagnetic exchange coupling. The thermodynamic functions and the dynamical susceptibility are obtained from the impurity relaxation spectrum, which is approximated by taking into account the infrared-like singularities. For antiferromagnetic coupling the zero-field susceptibility obeys a Curie-Weiss law1/χ～4.6(T+θ) for high and intermediate temperatures and it approaches the finite value1/χ～3.8θ for zero temperature. The zero-field relaxation rate is much larger than the Korringa value; it decreases with temperature and approaches the nonzero value1/T ₁～1.2θ for zero temperature. The relaxation rate decreases with increasing field. The results for the spin polarization agree well with the experimental data for the Cu:Fe alloy.     

Optical properties of binary composite materials with two nonlinear components

Abstract

A new formulation is presented for the calculation of effective dielectric magnitudes of two-component composites in which both components (the host matrix particles and the embedded particles) exhibit nonlinear behaviour of the Kerr type. It is predicted that, under certain conditions, two nonlinear component composites can exhibit optical bistable behaviour as a function of the shape and concentration of the embedded particles, the dielectric contants of the components, the intensity of the external electric field (power density) and the intrinsic third-order nonlinear optical susceptibilities χ⁽³⁾ _p and χ⁽³⁾ _m of the nonlinear components. It is also deduced that, as the power density increases, the effective third-order nonlinear optical susceptibility χ⁽³⁾ of the composite exhibits a clear transition from values close to χ⁽³⁾ _p (low power density) to χ⁽³⁾ _m (high power density). Therefore, it is shown that the optical response of binary composites dramatically changes at moderate and high power densities. A comparison is performed between the optical response of a real two nonlinear component composite and that of a composite with a single nonlinear component.     

Electron spin resonance linewidths of Fe3+ in magnesium oxide

Electron spin resonance linewidths of Fe³⁺ in single crystal MgO at 9 GHz were examined experimentally and theoretically for a range of Fe³⁺ concentration. In contrast to the behaviour expected from dipolar broadening the experimental derivative peak-to-peak linewidth for the 1/2 ↔ −1/2 transition, about 0.6 mT at 77 K and a polar angle of 0°, was independent of concentration from 140 to 8500 ppm. The calculated dipolar linewidths greatly exceeded those observed and values of the ratio of moments M ₄ ^1/4 /M ₂ ^1/2 derived from the experimental data lay between 1.33 and 1.48. Optical examination, coupled with heat-treatment experiments, confirmed that the predominant valency state present was Fe³⁺. The data suggested that Fe³⁺ entered the lattice substitutionally, occupying magnesium sites, and that the linewidths were determined by exchange narrowing over the whole concentration range examined.