Cascaded synchronous terahertz optical parametric oscillations in a single MgO:PPLN crystal

We present a synchronously pumped optical parametric oscillator (OPO) based on a single MgO-doped periodically poled lithium niobate crystal (MgO:PPLN) delivering high-repetition-rate picosecond idler output in the mid-infrared. At high power levels, cascaded optical parametric oscillations are observed, from which the forward and backward idler waves are generated in the terahertz (THz) spectral region. We demonstrate the cascaded processes involving THz-wave generation and make explanations for the highorder cascaded optical parametric processes. The cascaded terahertz optical parametric oscillations in a synchronously pumped optical parametric oscillator are reported for the first time to the best of our knowledge.     

Second order parametric processes in nonlinear silica microspheres

We analyze second order parametric processes in a silica microsphere coated with radially aligned nonlinear optical molecules. In a high-Q nonlinear microsphere, we discover that it is possible to achieve ultralow threshold parametric oscillation that obeys the rule of angular momentum conservation. Based on symmetry considerations, one can also implement parametric processes that naturally generate quantum entangled photon pairs. Practical issues regarding implementation of the nonlinear microsphere are also discussed.     

Statistical properties of four-mode quantum states in coupled optical parametric down-conversions

We study the quantum properties of four coupled parametric down-conversion processes which can be realized in an aperiodical nonlinear photonic crystal. In the interaction under consideration, each mode is created by two parametric processes. The photon statistics of particular modes and correlation functions for photon numbers and their fluctuations are studied in detail. It is shown that photon number correlations are nonclassical at small interaction lengths. We calculate the covariance matrix for continuous quantum variables of the generated modes and make use of the criterion of separability based on the partial scaling transform to reveal the entanglement between the variables. It is established that cluster entangled states are created in these coupled parametric processes.     

Fiber parametric oscillator for the 2 μm wavelength range based on narrowband optical parametric amplification

We describe a widely tunable synchronously pumped coherent source based on the process of narrowband parametric amplification in a dispersion-shifted fiber. Using an experimental fiber with a zero-dispersion wavelength of 1590 nm and pump wavelengths of 1530 to 1570 nm yields oscillations at 1970 to 2140 nm-the longest reported wavelength for a fiber parametric oscillator. The long-wavelength oscillations are accompanied by simultaneous short-wavelength oscillations at 1200 to 1290 nm. The parametric gain is coupled to stimulated Raman scattering. For parametric oscillations close to the Raman gain peak, the two gain processes must be discriminated from each other. We devised two configurations that achieve this discrimination: one is based on the exploitation of the difference in group delay between the wavelengths where Raman and parametric gain peak, and the other uses intracavity polarization tuning.     

A Review of Shannon and Differential Entropy Rate Estimation

In this paper, we present a review of Shannon and differential entropy rate estimation techniques. Entropy rate, which measures the average information gain from a stochastic process, is a measure of uncertainty and complexity of a stochastic process. We discuss the estimation of entropy rate from empirical data, and review both parametric and non-parametric techniques. We look at many different assumptions on properties of the processes for parametric processes, in particular focussing on Markov and Gaussian assumptions. Non-parametric estimation relies on limit theorems which involve the entropy rate from observations, and to discuss these, we introduce some theory and the practical implementations of estimators of this type.     

Origin of dark holographic scattering patterns in photorefractive crystals

We explain the origin of recently reported dark parametric holographic scattering patterns observed in photorefractive strontium–barium niobate (SBN) and barium–calcium titanate (BCT) crystals. Taking into account the model for parametric four-wave-mixing processes in photorefractive crystals, exponential gain factors are presented for the scattering patterns and compared with the gain for the scattering background. We interpret the dark scattering patterns as a result of counteracting energy-transfer processes. PACS 42.65.Hw; 42.70.Nq     

Soliton algebra by vortex-beam splitting

We experimentally demonstrate the possibility of breaking up intense vortex light beams into stable and controllable sets of parametric solitons. We report observations performed in seeded second-harmonic generation, but the scheme can be extended to all parametric processes. The number of generated solitons is shown to be determined by a robust arithmetic rule.     

Dynamical signatures of self-phase-locking in a triply resonant optical parametric oscillator

We report on specific signatures of self-phase-locking in a cw frequency divide-by-three optical parametric oscillator subject to two competing chi(2) nonlinear processes that couple the signal and idler subharmonic waves. The self-phase-locked pair appears as a broad fringe dip within the mode-pair cluster. We have also observed Hopf instabilities of the zero-detuning case at approximately 4 x the pump threshold. These results open the path to experimental investigation of quantum entanglement and phase-locked transverse mode structures in this novel class of parametric oscillators.     

Multistep cascading in periodically poled directional couplers

We suggest a novel scheme for multistep cascaded parametric processes based on a periodically poled nonlinear directional coupler and derive the conditions for simultaneous phase matching of the second-harmonic generation and sum-frequency mixing processes. We predict an efficient wavelength conversion by the cascaded third-harmonic generation from the third optical communication window (1.53-1.56 microm) to visible in a LiNbO3 poled waveguide coupler.     

On the generation of picosecond light continua depending on various nonlinear processes

The generation of picosecond light continua depends on various nonlinear processes such as parametric four-photon interaction, self-phase modulation, self-focusing, and avalanche ionization. In order to systematize the influence of these processes we have indicated both the threshold for continuum generation measured by various authors and the threshold for the nonlinear processes in a power diameter diagram characterizing the laser beam. The result is that all the thresholds for continuum measured under very different experimental conditions are situated nearly on one curve given by the nonlinear process with the lowest threshold. We have carried out measurements on the spectral energy distribution and on the angular distribution of the continuum generated in water and alkali halides by a mode-locked single-pulse Nd-glass laser. Under our experimental conditions both self-phase modulation and parametric four-photon interaction contribute to continuum generation.     

Group-velocity-matched multistep cascading in nonlinear photonic crystals

We demonstrate that simultaneous phase- and group-velocity-matched cascaded parametric processes can be achieved in two-dimensional quadratic nonlinear photonic crystals by proper matching of noncollinear processes with front tilting of the fundamental pulse. We present examples of cascaded third- and fourth-harmonic generation in a poled LiNbO3 planar structure.     

Parametric oscillator based on a single-layer resonant metamaterial

We perform numerical simulations demonstrating parametric generation and oscillation processes in a single-layer of metamaterial composed of split ring resonators (SRRs). In general, a parametric resonance is achieved by introducing a time-modulation of one of the energy-storing parameters of a resonant system. An individual SRR is a resonant system that can be modeled as an RLC-circuit inductively coupled to an applied time-varying magnetic field. The relatively simple circuit model can be employed to establish the range of parameters providing the growth of parametric oscillations within the SRR medium. We relate the numerically predicted circuit parameters that ensure the buildup of parametric oscillations to those parameters expected to be available in a modified SRR medium, in which lumped element varactors integrated into the SRRs are used as modulable capacitance elements.     

Third-harmonic generation via nonlinear Raman-Nath diffraction in nonlinear photonic crystal

We report on the observation of multiple third-harmonic conical waves generated in an annular periodically poled nonlinear photonic crystal. We show that the conical beams are formed as a result of the cascading effect involving two parametric processes that satisfy either the transverse and/or longitudinal phase-patching conditions. This is the first experimental observation of third-harmonic generation based on nonlinear Raman-Nath diffraction.     

Phase-matched self-doubling optical parametric oscillator

We report a synchronously pumped intracavity frequency-doubled optical parametric oscillator that employs a single KTiOPO(4) crystal for both parametric generation and frequency doubling. Both nonlinear processes are phase matched for the same direction of propagation in the crystal. The parametric oscillator, pumped by a femtosecond Ti:sapphire laser at a wavelength of 745 nm, generates a green output beam at 540 nm with a 29% power conversion efficiency. Angle tuning in conjunction with pump wavelength tuning provides output tunability in the 530-585-nm range.     

Thermo-optical effect and saturation of nonlinear absorption induced by gray tracking in a 532-nm-pumped KTP optical parametric oscillator

We present experiments that show that gray tracking modifies the parametric gain and the generated wavelengths of a KTP optical parametric oscillator pumped at 532 nm near degeneracy. These perturbations occur over a limited range of pump intensity. We propose a satisfactory model that takes into account photochromic damage, the thermo-optical effect, and the combined processes of creation and saturation of a two-photon absorber at 532 nm. The temperature dependence of Sellmeier equations of KTP is also established at 20-200 degrees C.     

Polariton parametric amplifier pump dynamics in the coherent regime

We studied the pump coherent dynamics in a II-VI microcavity parametric amplifier, using angle-resolved four-wave mixing. The polariton parametric amplification is found to result in a strong quenching and saturation of the pump coherence lifetime above the threshold. For the polariton scattering processes that remain below the amplification threshold, we find an angle-dependent collision broadening associated with the efficiency of the polariton scattering towards the excitonic reservoir.     

Quantum statistical properties of higher-order non-linear optical processes

We study the quantum statistical properties of radiation in higher-order parametric processes and higher harmonic generation. Adopting the Heisenberg picture, we use the short-time approximation to solve the Heisenberg equations and to calculate the quantum characteristic functions and quasi-distributions. The lossy mechanism is included with or without rotating terms. We show that the statistical properties of radiation involved in these non-linear optical processes are described by the model of the superposition of coherent and chaotic fields with correlated components in this approximation. Generalizations of the well-known conservation laws for the number operators are derived. We show for parametric processes that the pumping mode has tendency to be coherent while the signal modes are obtaining a noise. In the higher-harmonic generation, the basic radiation is losing its coherence proportionally to its intensity while the generated higher-harmonic radiation has tendency to be coherent.The part of this paper devoted to thek-th harmonic generation describes a portion of the Diploma Work.     

利用微扰法分析两个相互耦合的参量四波混频过程

利用微扰法在旋转波近似下分析了铷原子中两个独立的参量四波混频过程和一个耦合的参量四波混频过程.当用超短激光脉冲相干激发这两个参量四波混频过程时,两个过程就会同时出现并且存在相互耦合作用.微扰分析表明,耦合的参量四波混频信号中可以观察到量子拍,量子拍幅度的大小不仅取决于超短脉冲的特性,而且依赖于两个耦合的参量四波混频过程中的相位匹配.     

Multiple conversion and optical limiting in a subharmonic-pumped parametric oscillator

We demonstrate that multiple coexisting frequency-conversion processes can occur in an externally resonant second-harmonic generator under suitable conditions. Besides the generation of signal and idler waves by subharmonic-pumped parametric oscillation, sum-frequency mixing among the resonant subharmonic (1064-nm), signal, and idler waves was observed, leading to additional emission wavelengths around the harmonic wavelength (532 nm). The output waves both exhibit high frequency stability, with as long as 4 h of mode-hop-free parametric oscillation, and are continuously tunable over 2 GHz. Near degeneracy the parametric oscillator operates as an optical limiter for the harmonic wave.     

Multiple wavelength generation using aperiodically poled lithium niobate

We present a simple method to design aperiodically poled crystals to obtain multiple nonlinear optical processes simultaneously. We use this technique to design and make an aperiodically poled lithium niobate crystal with which we obtained eight distinct sum-frequency generation processes, and another sample with which we obtained four distinct optical parametric generation processes. We also show that the peak effective nonlinearities of aperiodic crystals designed with this technique are larger and their widths are narrower than what can be obtained with a crystal of the same length with separate sections with well-defined poling periodicities.