Oscillation Threshold Research Articles

High-power, continuous-wave, singly resonant optical parametric oscillator based on MgO:sPPLT

We report a high-power, widely tunable, cw singly resonant optical parametric oscillator (OPO) based on MgO:sPPLT. The OPO is pumped in the green by a cw diode-pumped Nd:YVO(4) laser at 532 nm and can provide continuously tunable output across 848-1430 nm. Using a 30 mm crystal and double-pass pumping, an oscillation threshold of 2.88 W has been obtained, and single-pass idler powers in excess of 1.51 W have been generated over 1104-1430 nm for 6W of pump power at an extraction efficiency of 25.2% and photon conversion efficiency of 56.7%.

Pulsed optical parametric oscillators with intracavity optical parametric amplification: a critical study

It is known that the idler conversion efficiency of optical parametric oscillators (OPOs) can be increased by adding a second nonlinear crystal in the cavity. This crystal is pumped by the signal and acts as an optical parametric amplifier (OPA) for the idler. However, this technique unavoidably increases the oscillation threshold because of additional losses and increased build-up time due to cavity lengthening. In this paper, we investigate both theoretically and experimentally the benefits and drawbacks of this so called OPO–OPA configuration versus the singly resonant OPO (SRO) configuration. Calculations are found to be in agreement with an experimental study of a SRO and an OPO–OPA operating near 3.4 μm both pumped by a 90-mJ 27-ns Nd:YAG laser. Our study reveals that the OPO–OPA needs to be driven at least two times above threshold to produce more idler energy than the SRO. In addition, near 3 μm the OPO–OPA is particularly efficient given that the difference frequency wave generated in the second crystal is also output coupled.

Microbubble shape oscillations excited through an ultrasound‐driven parametric instability

An ultrasonically driven air bubble can become shape‐unstable through a parametric instability. Here, we report time‐resolved optical observations of shape oscillations (mode n=2 to 6) of micron‐sized single air bubbles for a range of acoustic pressures. The observed mode number n was found to be linearly related to the resting radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which as expected is minimum at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small, nonspherical linear perturbation into a Rayleigh‐Plesset‐type equation model which includes a physical thermal damping mechanism describing heat and mass transport in the thin boundary layer at the bubble‐to‐water interface. Indeed, a parametric instability is responsible for the shape oscillations, and the Rayleigh‐Plesset‐type equation captures the experimental observations in great detail.

Spindle Oscillations during Asymmetric Cell Division Require a Threshold Number of Active Cortical Force Generators

Asymmetric division of the C. elegans zygote is due to the posterior-directed movement of the mitotic spindle during metaphase and anaphase. During this movement along the anterior-posterior axis, the spindle oscillates transversely. These motions are thought to be driven by a force-generating complex-possibly containing the motor protein cytoplasmic dynein-that is located at the cell cortex and pulls on microtubules growing out from the spindle poles. A theoretical analysis indicates that the oscillations might arise from mechanical coordination of the force-generating motors, and this coordination is mediated by the load dependence of the motors' detachment from the microtubules. The model predicts that the motor activity must exceed a threshold for oscillations to occur. We have tested the existence of a threshold by using RNA interference to gradually reduce the levels of dynein light intermediate chain as well as GPR-1 and GPR-2 that are involved in the G protein-mediated regulation of the force generators. We found an abrupt cessation of oscillations as expected if the motor activity dropped below a threshold. Furthermore, we can account for the complex choreography of the mitotic spindle-the precise temporal coordination of the buildup and die-down of the transverse oscillations with the posterior displacement-by a gradual increase in the processivity of a single type of motor machinery during metaphase and anaphase. The agreement between our results and modeling suggests that the force generators themselves have the intrinsic capability of generating oscillations when opposing forces exceed a threshold.

3.77-5.05-μm tunable solid-state lasers based on Fe/sup 2+/-doped ZnSe crystals operating at low and room temperatures

Spectroscopic properties and lasing of Fe:ZnSe and co-doped Fe:Cr:ZnSe crystals in the mid-infrared spectral range were studied at room and low temperatures. Using a free-running Er:YAG laser as a pump source, the output energy of the thermoelectrically cooled Fe:ZnSe laser was 142 mJ with 30% slope efficiency at T=220 K. Passive Q-switched oscillation of Er:YAG laser with Fe:ZnSe crystal was demonstrated and used as a pump source for a Fe:ZnSe laser system. Room-temperature (RT) gain-switched lasing of Fe:ZnSe was achieved in microchip and selective cavity configurations using Q-switched Er:YAG and Raman-shifted Nd:YAG lasers as pump sources. The microchip laser threshold of 100 mJ/cm2 was demonstrated using a Fe:ZnSe crystal without any reflection coatings. A slope efficiency of 13%, oscillation threshold of 1.3 mJ, and tunable oscillation of Fe:ZnSe laser systems over 3.95-5.05 mum spectral range were realized at RT

Generalized derivation of equations for four-wave interaction in photorefractive media

The emergence of optical waves out of optical noise (fanning) in a photorefractive medium is considered in an unconventional manner, i.e., using the Lorentz lemma and ensemble averaging. The derivation of equations for four-wave interaction in the scheme of a double incoherent phase-conjugate mirror is described in detail. Numerical calculation based on the resulting system of equations provides insight into the effect of noise on the oscillation threshold and makes it possible to discover the “higher threshold” effect in saturation-free noisy materials.

Transition of absolute instability from global to local modes in a gyrotron traveling-wave amplifier

The gyrotron traveling-wave amplifier employing the distributed-loss scheme is capable of very high gain and effective in suppressing the global absolute instabilities. This study systematically characterizes the local absolute instabilities and their transitional behavior. The local absolute instabilities are analyzed using a model that incorporates the penetration of the field from the copper section into the lossy section. The axial modes were characterized from the perspective of beam-wave interaction and were found to share many characteristics with the global modes. The transition from global modes to local modes as the distributed loss increases was demonstrated. The electron transit angle in the copper section, which determines the feedback criterion, governs the survivability of an oscillation. In addition, the oscillation thresholds predicted using this model are more accurate than those obtained using a simplified model.

Intra-cavity power effects in singly resonant cw OPOs

A continuous wave singly resonant optical parametric oscillator (cw SRO) has been developed that can be pumped at a level of up to 15 times its oscillation threshold by a single frequency fiber laser pump source. We report the observation of extremely high intra-cavity circulating powers of over 1 kW at the signal wavelength, and associated effects including heating of the nonlinear crystal, and beam quality degradation. Furthermore we show that these effects may be mitigated by implementing output coupling, while also extracting multi-Watt single frequency power output at both signal and idler wavelengths.

Effective Aperture in Periodically Poled Mg-Doped Stoichiometric LiTaO3 for Quasi-Phase-Matched Optical Parametric Oscillation

A two-dimensional mapping method was employed to investigate the homogeneity of optical parametric oscillator (OPO) performance, such as output power distribution, slope efficiency, and oscillation threshold, based on periodically poled 1 mol % Mg-doped stoichiometric LiTaO3 devices fabricated by single and multi-pulse methods. The output power decreased from the electrode-patterned face to the opposite face at low pump intensities and became homogenous over the aperture at high pump intensities. The devices fabricated by the two methods showed no large differences in OPO properties, except for better homogeneity in depth for multi-pulse devices. A large effective aperture (fluctuation < 10%) of 1.4×1.3 mm2 was obtained in both types of devices.

Theoretical analysis of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator

The basic theory of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator consisting of two two-cavity klystron amplifiers reversely coupled through input/output slots is theoretically investigated. Application of Kirchhoff’s laws to the coupled equivalent RLC circuit model of the device provides four nonlinear coupled equations, which are the first-order time-delayed differential equations. Analytical solutions obtained through linearization of the equations provide oscillation frequencies and thresholds of four fundamental eigenstates, symmetric/antisymmetric 0∕π modes. Time-dependent output signals are numerically analyzed with variation of the beam current, and a self-modulation mechanism and transition to chaos scenario are examined. The oscillator shows a much stronger multistability compared to a delayed feedback klystron oscillator owing to the competitions among more diverse eigenmodes. A fully developed chaos region also appears at a relatively lower beam current, ∼3.5Ist, compared to typical vacuum tube oscillators (10-100Ist), where Ist is a start-oscillation current.

Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source

An oscillation threshold of 780mW has been demonstrated in a singly-resonant, continuous-wave optical parametric oscillator (CW SRO) using a fiber-amplified, distributed feedback (DFB) fiber laser as pump source. A linewidth of 1MHz was measured, and the idler frequency was fine-tuned by up to 130GHz by tuning the pump laser. To our knowledge, this is the first example of a single frequency CW SRO pumped by an all-fiber pump source, a reduction in threshold by a factor of three over previous 1- microm-pumped CW SROs, and a reduction by two orders of magnitude in the linewidth of CW SROs pumped by fiber pump sources.

Ring-down spectroscopy for studying properties of CW Raman lasers

We describe theoretically a simple technique for analyzing the conversion efficiency and the oscillation threshold of all-resonant intracavity Raman lasers. The method is based on a dependence of the ring-down time of the pump cavity mode on the energy accumulated in the cavity.

An analytical prediction of the oscillation and extinction thresholds of a clarinet

This paper investigates the dynamic range of the clarinet from the oscillation threshold to the extinction at high pressure level. The use of an elementary model for the reed-mouthpiece valve effect combined with a simplified model of the pipe assuming frequency independent losses (Raman's model) allows an analytical calculation of the oscillations and their stability analysis. The different thresholds are shown to depend on parameters related to embouchure parameters and to the absorption coefficient in the pipe. Their values determine the dynamic range of the fundamental oscillations and the bifurcation scheme at the extinction.

Phonation thresholds as a function of laryngeal size in a two-mass model of the vocal folds

This letter analyzes the oscillation onset-offset conditions of the vocal folds as a function of laryngeal size. A version of the two-mass model of the vocal folds is used, coupled to a two-tube approximation of the vocal tract in configuration for the vowel /a/. The standard male configurations of the laryngeal and vocal tract models are used as reference, and their dimensions are scaled using a single factor. Simulations of the vocal fold oscillation and oral output are produced for varying values of the scaling factor. The results show that the oscillation threshold conditions become more restricted for smaller laryngeal sizes, such as those appropriate for females and children.

Characterization of a high-thermal-stability spiroanthracenefluorene-based blue-light-emitting polymer optical gain medium

We report the solid-state optical gain characteristics of a spiroanthracenefluorene polymer, namely, poly(9-spiro(10,10-bis(2-ethylhexyl)-10H-anthracene)fluorene) (PEHSAF), specifically designed for thermal stability. An efficient stimulated emission occurs at λ=445nm under amplified spontaneous emission conditions for asymmetric slab waveguide structures. The modal gain and propagation loss coefficients were found to be g⩽38cm−1 and α=0.8cm−1, respectively. The PEHSAF stimulated emission characteristics are shown to be thermally stable in vacuo for temperatures up to 250 °C. Surface-emitting distributed feedback lasers have been fabricated by spin-coating PEHSAF onto one-dimensional grating structures. The lasers operate in the blue spectral region and exhibit low oscillation thresholds (⩾18nJ) and relatively high slope efficiencies (⩽5%). Varying the PEHSAF film thickness allows the tuning of the emission wavelength within a 16 nm window.

Enhancement of Available Conversion Efficiency of OpticalParametric Amplifier in a Cascaded Photonic Crystal Structure

Using the cascaded structure of a linear and a second-order nonlinearphotonic crystals, we realize a high-efficiency optical parametricamplifier in the case of exact phase matching. This proposal isverified using the slow-envelope nonlinear finite difference time domainnumerical method. Compared with the case of the individual nonlinearphotonic crystal structure, the oscillation threshold is decreasedobviously, and the peak power amplification factor of the transmittedsignal is enhanced more than 20 times.

Spontaneous microcavity-polariton coherence across the parametric threshold: Quantum Monte Carlo studies

We investigate the appearance of spontaneous coherence in the parametric emission from planar semiconductor microcavities in the strong coupling regime. Calculations are performed by means of a Quantum Monte Carlo technique based on the Wigner representation of the coupled exciton and cavity-photon fields. The numerical results are interpreted in terms of a non-equilibrium phase transition occurring at the parametric oscillation threshold: below the threshold, the signal emission is incoherent, and both the first and the second-order coherence functions have a finite correlation length which becomes macroscopic as the threshold is approached. Above the threshold, the emission is instead phase-coherent over the whole two-dimensional sample and intensity fluctuations are suppressed. Similar calculations for quasi-one-dimensional microcavities show that in this case the phase-coherence of the signal emission has a finite extension even above the threshold, while intensity fluctuations are suppressed.

Cross-resonant optical parametric oscillators: study of and application to difference-frequency generation

We revisit both theoretically and experimentally the study of a two-crystal optical parametric oscillator (OPO) for which the signal and the idler beams are totally and exclusively output coupled after the first and the second crystals, respectively. This geometry, referred as cross-resonant OPO, is useful for applications that require the production of two beams that can be independently adjusted. A theoretical analysis is carried out by use of a plane-wave semianalytical rate-equation approach that completely includes pump depletion and reconstruction effects. We also report on an experimental investigation of a 1.064 µm pump-pulsed KTP cross-resonant OPO whose performance is compared with that of a singly resonant OPO with a similar oscillation threshold. To illustrate the practical advantages of such a configuration, we performed difference-frequency generation in a CdSe crystal by mixing the signal and idler beams of the cross-resonant OPO to produce mid-infrared radiation over the 8-12 µm range.

Doublet dynamics of magnetizable particles under frequency modulated rotating fields

We study experimentally the dynamics of an isolated pair of paramagnetic micro-particles when applying rotating and frequency modulated magnetic fields. In the case of purely rotating field, the inter-particle distance performs radial oscillations above a given threshold. The oscillation threshold depends on the viscosity and the amplitude of the magnetic field. When the data are represented in terms of the relative distance to the critical Mason number all curves collapse nicely. The master curve obtained shows the same features that are obtained in numerical simulations. For frequency modulated rotating fields, complex and irregular dynamics is obtained. This dynamics is again in qualitative agreement with numerical simulations.

Evolution of a Small Distortion of the Spherical Shape of a Gas Bubble under Strong Expansion-Compression

The evolution of a small distortion of the spherical shape of a gas bubble which undergoes strong radial expansion-compression upon a single oscillation of the ambient liquid pressure under a harmonic law are analyzed by numerical experiments. It is assumed that the distortions of the spherical bubble shape are axisymmetric and have the form of individual spherical surface harmonics with numbers of 2–5. Bubble-shape oscillations prior to the beginning of expansion are taken into account. Generally, the distortion value during bubble expansion-compression depends on the phase of bubble-shape oscillation at the beginning of the expansion (initial phase). Emphasis is placed on the dependence of the maximum distortions in the initial phase at certain characteristic times of bubble expansion-compression on the amplitude of the external excitation, liquid viscosity, and distortion mode (harmonic number). The parameters of the problem are typical of the stable periodic sonolumiescence of an individual air bubble in water at room temperature. An exception is the liquid pressure oscillation amplitude, which is varied up to values that are five times the static pressure. That large excitation amplitudes are beyond the stability threshold of periodic oscillations of spherical bubbles. Their consideration is of interest from the point of view of increasing the compression ratio of the bubble gas, i. e., increasing the maximum temperature and density achievable in the final compression stage.