Strong Saturation Research Articles

Phenomenological Model for the Optical Properties of Semiconductor‐Doped Glasses

AbstractThe optical properties of semiconductor‐doped glasses are determined by semiconductor crystals with diameters of 3 to 5 nm embedded in a glass matrix. The confinement of charge carriers leads to discrete energy states. The motion of the carriers is described by a Ritz ansatz for the wave function combining properties of free particles in a box with those of bulk excitons. In a first approximation the dynamical behaviour of the system can be described in analogy to a two‐level system. Since each energy level can contain only two pairs strong saturation of absorption occurs. Additionally the size distribution of the crystals is taken into account. It results in an inhomogeneous broadening of the spectral lines that can be described by convolution of the spectrum with the diameter distribution of the crystals.

Iodine stabilized HeNe laser at 612 nm with composite resonator

The properties of HeNe/I 2 lasers at 612 nm with composite resonator are investigated. The strong absorption saturation is completely eliminated by use of the composite resonator. The long term stability (τ=100 s) and reproducibility are equal to ∼1.2×10 −13 and 5×10 −12, respectively, which substantially exceeds the frequency stability and reproducibility of the HeNe/I 2 laser with a Fabry-Perot resonator at 633 nm and 612 nm.

Elemental analysis by doubly resonant four-wave mixing in Na-seeded flames

Doubly resonant four-wave mixing has been used with two lasers set for resonance with the two Na D-line transitions to perform elemental analysis with a Na-seeded flame. The method is analogous to phase-conjugation methods where all lasers have the same frequency. Strong saturation effects and dynamic Stark splittings are observed and used to optimize the mixing efficiency. A nonperturbative theory is presented and shown to describe the experimental results.

Theory of rotational depolarization of laser radiation emitted by dye solutions

The rate equations for the populations of states and the intracavity radiative transfer equations are used, in an analytic description of rotational depolarization of the output radiation of cw dye-solution lasers under strong saturation conditions. Expressions are obtained for the energy and the degree of polarization of radiation. It is shown that under such conditions the degree of polarization depends linearly on the velocity of Brownian rotation of molecules.

Exciton-related optical nonlinearities in semiconductors and semiconductor microcrystallites

We report on large optical nonlinearities due to specific many-body effects in high-density electron-hole pair systems. On bulk CdS we study the effect of free-carrier screening on the absorption and refraction in the vicinity of the band-gap at room temperature. Steady-state saturable absorption at mW-power levels and four-wave mixing with first-order efficiencies as large as 2% of the incoming light are demonstrated on a 1μm slab. On CuBr microcrystallites embedded in glass we investigate the changes of the exciton absorption caused by many-exciton effects both at cryo and room temperature. In contrast to bulk semiconductors, we observe a blue shift of the exciton peak at resonant optical excitation. In addition, strong saturation of absorption with very large contrast shows up. We find Lorentzian saturation intensities in the 100 kW/cm2 range.

Nonlinear amplification in the case of resonant optical pumping of a three-level medium

The results are given of analytic and numerical investigations of the amplification of laser radiation under conditions of strong saturation of resonant transitions induced by optical pumping. Wave-like solutions of propagation of light pulses in a three-level medium are found. Characteristic dependences of the radiation conversion length on the parameters of the radiation and medium are determined.

Statistical analysis of nonlinear optical amplifier in high saturation

In the work, analytic expressions describing the behaviour of a strongly saturated nonlinear optical amplifier are deduced. These expressions make possible the calculation of the mean and variance of the amplified light photon number. Results are compared with those obtained by the numerical resolution of the photon density matrix equation, and good accordance is observed in the strong saturation regime. The results numerically obtained enable us to make conclusions about the statistical fluctuations of the optical power at the nonlinear optical amplifer output.

Travelling wave treatment of mode-locked semiconductor lasers

Travelling effect of the pulses inside the amplifying medium has been included in the rate equations to study the actively mode-locked semiconductor laser. For the modulation freqency and cavity configuration specified in this work two photon equations have been used to treat the counter-propagating pulses circulating inside the resonator. The pulse has been dealt with the product of two functions, one stands for the amplitude of the pulse and the other for the envelope of the pulse. Each optical component inside the cavity has been considered separately. Taking into account the strong saturation effect of the injection carriers due to the interaction between pulses and the gain medium, self reproducing of the system after each cycle of the pump has led to the evolution of the pulse amplitude. As a result, expressions for the output powers of the mode-locked laser have been derived. In addition, some other parameters defining the oscillator can also be evaluated.

Zircon in charnockitic rocks from rogaland (southwest Norway): Petrogenetic implications

Zircons separated from monzonoritic rocks and quartz mangerites (igneous and metamorphic) have been studied for various mineralogical, chemical and physical properties (colour, zoning, presence of core, U distribution, morphology, lattice parameters, thermoluminescence) to determine the relationships with chemical composition of the host rocks and geological setting. The zircons grade from nearly perfectly crystalline to slightly metamict types. The crystal morphology shows prismatic habits and a striking lack of well-developed pyramidal faces, possibly indicating growth in a dry formation environment. The zircons can be divided into two groups. The first one comprises zircons from monzonoritic rocks. They are unzoned, devoid of brownish core, with a uniform U distribution, and low U content (except in Hidra). The second group comprises zircons from igneous and metamorphic quartz mangerites; they contain brownish, zoned, U-rich cores. In rocks where temperature of crystallization can be determined, Watson and Harrison's (1983) experimental data indicate strong zircon saturation. The cores are considered as inherited, a fact that can be accounted for either by dry anatexis or bulk contamination. In the Bjerkreim-Sokndal layered lopolith, previous isotopic data are confirmed, which indicate that the acidic upper part of the massif is not comagmatic with the anorthosito-noritic lower part and the monzonoritic rocks.

Nonlinear absorption in AlGaAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition

We report the study of growth conditions for achieving the sharp exciton resonances and low intensity saturation of these resonances in AlGaAs-GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. Low growth temperature is necessary to observe this sharp resonance feature at room temperature. The optimal growth conditions are a tradeoff between the high temperatures required for high quality AlGaAs and low temperatures required for high-purity GaAs. A strong optical saturation of the excitonic absorption has been observed. A saturation density as low as 250 W/cm2 is reported.

Magneto-spatial dispersional effect in a diamagnetic molecular system in d.c. electric field

The theory of magneto-spatial dispersional variations in refractive index and absorption index is extended to systems composed of diamagnetic molecules acted on by a d.c. electric field. The variations are discussed in their dependence on the electrical pure reorientation process and electric distortional-reorientational cross-process for molecules with the point group symmetries 3, 4, 6, ∞, 32, 422 and 622. In particular, an expression is derived for the magneto-chiral birefringence and dichroism in the presence of weak as well as very strong (saturation) electric molecular reorientation. At saturation reorientation the two effects are shown to be described by a lesser number of multipolar polarizability tensor components than in the case of weak molecular reorientation or in the absence of a d.c. electric field.

Line shapes in high-resolution optoacoustic spectroscopy

The line shapes in intermodulated optoacoustic spectroscopy are experimentally investigated by a laser-Stark technique. A stabilized CO2 laser is used to study some infrared transitions of NH3 under different conditions of gas pressure and laser power. In the presence of strong saturation the experimental line shapes are appreciably different from simple Lorentzian curves. We prove that the main reason for this distortion is the inhomogeneous saturation caused by the Gaussian profile of the beam intensity. Similar effects are also indicated in Doppler-limited spectra by an additional power-dependent broadening and are explained by a same model. At strong saturation, a line distortion of this type may also be present in conventional and nonconventional detection methods of high-resolution spectroscopy.

Specific heat in a mean-field-like solid: KDP above the critical electric field

Measurements of the specific heat in KDP (KH2PO4) at ambient pressure and electric fields above the critical field Ec are presented and analysed. The specific heat at E>Ec shows a remarkably strong anomaly which is shifted upwards in temperature and reduced in sharpness with increasing electric field. The results can be satisfactorily accounted for by Landau theory provided terms are included in the free-energy expansion to describe the strong saturation behaviour at large polarisation. This use of mean-field theory is fully consistent with theoretical analysis which has shown that KDP has an upper critical dimensionality below three.

Simple analytic approximation to continuous-wave on-resonance beam reshaping

We present a simple, analytically tractable, generalized aperture model for treating the stripping of a laser beam in a medium with saturable absorption. The model departs from previous treatments in allowing a soft transition from transparency to opacity, which yields a physically realistic diffraction-free amplitude encoding for the laser beam and thereby reduces unphysical fringes in subsequent beam evolution. The model is applied to the problem of cw exactly resonant self-focusing in a vapor of two-state atoms. It is confirmed that for strong beam saturation and absorption that is not too strong, beam propagation can be well modeled as a single stripping–diffraction sequence. A simple interpretation of the on-axis enhancement features is given.

Color samples for colorimetric fidelity testing in television

AbstractThe case for testing television (TV) colorimetric reproduction performance for groups of colors is discussed. The groups consist of colors which have some characteristic in common, like strong or moderate saturation. Such groups actually contain a large number of discernible colors but these are for testing purposes represented by a set with a limited number of reference samples. In recommended methods the evaluation of the performance for the group is based on the average of the individual color differences for the samples in the set. This average is here based on the CIE 1976 color difference ΔEuv* and designated ΔEa*. The question investigated is what properties the reference samples in the set should have to make D̊Ea* the best estimator of performance for the whole group. It is shown that the samples should be chosen for evenness in distribution and uniformity in lightness, preferably representative of average scene lightness. These points seem to have been overlooked for some presently recommended sets as illustrated by an example. Experimental data for TV cameras indicate that a collection of 10 color samples might be satisfactory for colorimetric acceptance testing.

Scattering of atoms by coherent interaction with light

A theoretical and experimental study is presented of the effect of stimulated radiation pressure (SRP) under the conditions of strong saturation of the resonance transition when the interaction times between the atom and the field are short (shorter than the radiative lifetime). An experimental method for studying the scattering of an atomic beam by the field of a standing light wave is described. The measured dependence of the scattering efficiency on the electric field and the detuning from resonance are reported. Experimental relationships are compared with theoretical calculations based on the quasi-classical description of the motion of atoms in the field of a light wave, using an effective potential. It is shown that the efficiency of scattering of atoms by SRP forces is of the order of unity in relatively weak (100–1000-V/cm) laser fields.

Two-dimensional transverse solitary waves as asymptotic states of the field in a bistable optical resonator

The dynamical evolution of a two transverse dimensional laser beam profile is monitored as it switches from a low to a high transmission state in a bistable optical resonator. Sharp gradients appearing at the edge of the switched-on central portion of the beam initiate a train of solitary waves which grow inwards towards the beam center. In a high finesse resonator, strong saturation of the beam in the on state tends to stabilize the solitary waves as concentric rings. Unlike earlier one transverse dimensional studies, the central part of the beam undergoes a slow periodic oscillation, but the shapes of the outer rings are consistent with earlier one transverse dimensional predictions. Numerical evidence is provided for filamentation of lower amplitude rings.

Absorption and fluorescence in frequency-modulated fields under conditions of strong modulation and saturation.

The dynamical behavior of a two-level system in an intense frequency-modulated field is obtained without recourse to the weak-modulation assumption. Solutions to the optical Bloch equations are expressed in terms of continued fractions involving 2\ifmmode\times\else\texttimes\fi{}2 matrices. Such matrix-continued fractions are numerically evaluated to obtain a wide variety of absorption and fluorescence spectra in frequency-modulated fields. Results in various special cases involving combinations of weak fields, weak modulation, strong modulation, and strong fields are given. Analytical results for weak fields but with an arbitrary degree of modulation are also presented. The spectra exhibit additional resonances for strong modulation. The main resonance shifts and broadens considerably due to strong modulation and saturation effects.

Generation of picosecond pulses in dye lasers pumped by single pulses

The dynamics of the generation of picosecond radiation pulses by dye lasers operating in the strong gain saturation regime is investigated theoretically and experimentally. Systems are described which are characterized by reliability of operation and simplicity of construction. Generation of bandwidth-limited 3.5 psec duration pulses is reported.

Experimental Study of the Effect of Saturation on the Steady State Operation of a Salient Pole Microalternator

ABSTRACT The direct and quadrature axis inductances of a salient pole micro alternator have been measured by low frequency, standstill tests. Existence of a strong cross saturation between the two axes has been shown. The application of the results to the P, Q curve of the microalternator for given supply voltage and field current has shown the importance of saturation and cross saturation on the maximum value of P. The choice of a value of the unsaturated leakage reactance of the machine appears as a difficult problem.