Tunneling-induced high gain and narrow linewidth of a cavity with an asymmetric quantum-well system

A scheme is proposed for obtaining high gain and narrow linewidth of a cavity with an asymmetric quantum-well system. Due to resonant tunneling, destructive interference for linear absorption leads to a tunneling-induced transparency window which compresses the cavity linewidth; moreover, constructive interference for cross-nonlinear susceptibility occurs, which introduces high gain and large dispersion, and the cavity linewidth is much compressed. In the latter case, the intensity of cavity transmission could be enhanced one order of magnitude larger than that of input field, and its linewidth could be one-seventieth of the empty cavity.     

Two-step Optical Excitation for Doppler Linewidth Reduction and Motion Study of Ions Stored in a Paul Trap

Abstract

Atomic thorium ions were stored in an r.f. quadrupole trap and excited by tunable laser light. One-step and two-step excitation schemes were applied; a four to eightfold reduction of the Doppler linewidth is obtained for the two-step scheme with essentially no loss in signal strength. The Doppler-free two-photon transition was also observed. The lineshapes were calculated using a simple model; the observed profiles were well reproduced proving that the velocity and acceleration features of stored ions are properly accounted for. A further reduction of the Doppler linewidth is expected for the two-step method if a short lived intermediate level is used.     

Analytic theory of coherent cross-gain modulation in semiconductor travelling-wave amplifiers

Abstract

We report a simple analytic solution to the problem of cross amplitude-phase modulation in nonlinear two-wave mixing, under the approximation of constant saturating power. The results are applied to travelling-wave semiconductor laser amplifiers where the linewidth enhancement factor inevitably leads to amplitude-phase coupling and asymmetric energy transfer. Our solution is used to obtain the transmission gain of each beam, the spectral profile and the amplifier input-output characteristics. Limits are established as to the applicability of the analytic solution, by comparison with numerical solutions to the related full four-wave mixing problem.     

Spectrum of a Micromaser with Kerr Nonlinearity

Abstract

We analyse the spectrum of a micromaser with a Kerr-type nonlinearity in its cavity. We show that the intracavity Kerr nonlinearity has significant effects on the spectrum leading to narrowing of the subnatural linewidth and frequency-shifts.     

Doppler-enhanced gain in an open ladder inversionless lasing system

Abstract

An analysis is made of the effect of Doppler broadening on the gain in an open ladder inversionless lasing system. It is shown that for co-propagating probe and driving fields, the gain does not monotonously decrease or increase with increasing Doppler width, and at a suitable Doppler width one can obtain a maximum value much larger than that without Doppler broadening. For counterpropagating probe and driving fields, when the Doppler width is large enough, gain oscillation occurs, and the oscillation amplitude and region increase with increasing Doppler width. This conclusion is very different from that obtained in the corresponding closed system.     

Doppler-induced dynamics of fields in fabry-perot cavities with suspended mirrors

The Doppler effect in Fabry-Perot cavities with suspended mirrors is analyzed. The Doppler shift, which is intrinsically small, accumulates in the cavity and becomes comparable with or greater than the linewidth of the cavity if the cavity's finesse is high or its length is large. As a result, damped oscillations of the cavity field occur when one of the mirrors passes a resonance position. A formula for this transient is derived. It is shown that the frequency of the oscillations is equal to the accumulated Doppler shift and that the relaxation time of the oscillations is equal to the storage time of the cavity. Comparison of the predicted and the measured Doppler shifts is discussed, and application of the analytical solution for measurement of the mirror velocity is described.     

Disturbance decoupling of nonlinear systems by high gain feedback

Abstract

For a smooth, affine nonlinear system of uniform relative degree one, it is shown that the asymptotic disturbance decoupling of full order system by high gain feedback can be achieved if the slow reduced subsystem can be exactly disturbance decoupled. The analysis is made in a properly selected coordinate system and the system in such a coordinate system is in a canonical form suitable for singular perturbation techniques.     

Ultra-stable Hg+

Abstract

We report the development of a fieldable frequency standard based on ¹⁹⁹Hg⁺ ions confined in a hybrid r.f./dc linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the r.f. confining fields. A 160 mHz wide atomic resonance line for the 40·5 GHz clock transition is used to steer the output of a 5 MHz crystal oscillator to obtain a stability of 2 × 10^?15 for 24 000 s averaging times. For longer averaging intervals, measurements are limited by instabilities in available hydrogen maser frequency standards. Measurements with 37 mHz linewidth for the Hg⁺ clock transition demonstrate that the inherent stability for this frequency standard is at least as good as 1 × 10^?15.     

Solution‐Grown CsPbBr3/Cs4PbBr6 Perovskite Nanocomposites: Toward Temperature‐Insensitive Optical Gain

With regards to developing miniaturized coherent light sources, the temperature‐insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature‐insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II–VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands‐free CsPbBr₃ nanocrystals in a wider band gap Cs₄PbBr₆ matrix by solution‐phase synthesis. The unique electronic structures of CsPbBr₃ nanocrystals enable temperature‐insensitive gain spectrum while the lack of ligands and protection from Cs₄PbBr₆ matrix ensure the thermal stability and high temperature operation. Specifically, a color drift‐free stimulated emission irrespective of temperature change (20–150 °C) upon two‐photon pumping is presented and the characteristic temperature is determined to be as high as ≈260 K. The superior gain properties of the CsPbBr₃/Cs₄PbBr₆ perovskite nanocomposites are directly validated by a vertical cavity surface emitting laser operating at temperature as high as 100 °C. The results shed light on manipulating optical gain from the advantageous CsPbBr₃ nanocrystals and represent a significant step toward the temperature‐insensitive frequency‐upconverted lasers.     

Inhomogeneous broadening of electronic transitions in a liquid helium bubble: The role of shape fluctuations

Recent experiments of Grimeset al. [Phys. Rev. B 41, 6366 (1990)] and Parshinet al. [JETP,74, 68 (1992)] demonstrate a substatial broadening in the 1s-1p transition of a single electron trapped in a liquid helium bubble (bubblonium) compared to theoretical predictions based on natural radiative linewidth. We show that the larger observed linewidth can be explained by inhomogeneus broadening due to quantum quadrupole fluctuations in the bubble shape. A simple adiabaticity rule for the bubblonium transitions similar to the Franck-Condon principle for molecular transitions is established. Quantitative estimates of the additional inhomogeneous linewidth atT=0 and 2.2 K are provided. The full theoretical linewidth, due to inhomogeneous and homogeneous broadening, has a Voigt-profile shape, and accounts for the data reasonably well.     

On chip optical isolator based on non-linear silicon photonic crystal by using asymmetric engineering waveguide

We propose an optical isolator formed in a nanoscale structure based on non-linear silicon photonic crystal that can be easily realized on an optical integrated chip. The structure comprises an engineered waveguide that is coupled to a L₂ cavity. By using a passive ultra-compact cavity-based isolator, without changing incident characteristics such as mode or frequency in outputs, an admirable transmission contrast of 20.5 dB with a small insertion loss (in the forward direction) of ?14.8 dB is achieved. The isolator attains a broad isolating linewidth operation of 0.9 nm without bistability response that is outstanding in comparison with the currently proposed cavity-based isolators. The non-linear Fano resonances that are created by the interplay between the non-linearity and spatial asymmetry notion in the structure play a critical role in the isolator efficiency. In this study, the finite-difference time-domain and finite element methods are used for simulations.     

Stabilization of an 852 nm extended cavity diode laser using the Zeeman effect

Abstract

We demonstrate the use of the Zeeman effect in Cs vapour to stabilize an ultra-compact extended cavity diode laser (ECDL) operating at 852 nm. We investigate the expected laser stabilization error signal for a range of magnetic fields and are able to tune the locked ECDL by variation of the magnetic field. We also study in detail the tuning of the laser frequency using optical methods. The ECDL has a linewidth of 520kHz and the drift, when locked, is of the order of 5 MHzh^?1.     

Fast fabrication of silicon based microstructures using 355 nm UV laser

Abstract

In this study, a 355 nm UV Nd:YAG laser is used to process silicon wafers. In order to obtain microstructures with high aspect ratio, a dual prism optical system is set up to control the cutting linewidth of the UV laser beam. During the laser beam propagation through the prisms, the two prisms are rotated with the same angular velocity, which results in the focal spot of the laser beam moving in a circular path on the silicon substrates. When the laser beam moves relative to the holder (workstation), a laser cutting process can be carried out. With this laser system, the cutting linewidth is controllable ranging from 10 μm to 1 mm by adjusting the initial phase difference in the two prisms. The experimental results show that arbitrary shaped silicon based microstructures with high aspect ratio can be fabricated by this 355 nm UV laser system, and the aspect ratio over 10 can be obtained.     

Optimization of an optical filter with a square-shaped passband based on coupled microcavities

Abstract

An approach using simple analytical theory has been developed for the design of multilayered spectral optical filters based on coupled microcavities. The method has been applied to find the parameters of the optimal structure, providing a near square-shaped transmission band. The case of normal and oblique incident light has been considered. It is shown that the situation corresponding to the weak coupling-strong coupling threshold for coupled cavity modes is optimal for achieving a square-shaped transmission band.     

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.     

Effects of a highly dispersive atomic medium inside an optical ring cavity

Atomic media inside an optical cavity can significantly alter the spectral response of the cavity. Both theoretical and experimental examinations are made of the cavity transmission with a highly dispersive intracavity multilevel atomic medium. It is found, owing to the reduced absorption and steep dispersion change accompanying electromagnetically induced transparency in such a multi-level atomic medium, that the cavity linewidth can be made much narrower than the empty cavity linewidth. Cavity linewidth narrowing is measured as a function of both the coupling beam power and the atomic density. These experimental results are in good agreement with the theoretical predictions.     

Current‐gain degradation and failure‐rate prediction in bipolar junction transistors

Abstract

Although dc current gain has been widely treated, its relation to device reliability is still unavailable. We show here that the variation of current gain of bipolar junction transistors depends upon device operating time and may be used as a failure indicator in case no failure occurs in a test. From the accelerated test data with zero failure, a method for the computation of device age is illustrated and compared with traditional methods. Owing to the composite nature of current gain, we obtain an empirical formula which is useful in providing us a quantitative anticipation of device life. A current gain degradation mechanism is also discussed.     

Bragg Cell Diffraction in Resonant Cavities

Abstract

Steier et al. [1] have shown that the diffraction efficiency of a stationary grating may be improved by placing the grating in a resonant cavity. In this paper we describe the diffraction of light by a Bragg cell in a resonant cavity. Large improvements in diffraction efficiency are shown to be possible if scattering losses on the surface of the cell can be kept small. The effects of introducing gain into the cavity are calculated and the changes in bandwidth, resolution and dynamic range are discussed for the system when used as a radiofrequency spectrum analyser. A multichannel fibre optic system has been designed and is described.     

Frequency modulation response due to the intensity modulation of fiber-grating Fabry–Perot lasers

A comprehensive study on the small-signal frequency modulation (FM) characteristics of a fiber-grating Fabry–Perot (FGFP) laser is numerically conducted. Fiber Bragg grating (FBG) is used as a wavelength selective element to control the properties of the laser output by controlling the external optical feedback (OFB) level. In addition to the external OFB level, the effect of other parameters such as temperature, injection current, cavity volume, nonlinear gain compression factor, linewidth enhancement factor, and fiber-grating (FG) parameters on FM characteristics of the laser are investigated. The study is performed by modifying a set of rate equations that are solved by considering the effects of external OFB and ambient temperature (T) variations. The temperature dependence (TD) of FM characteristics is calculated according to TD of laser parameters instead of using well-known Pankove relationship. Results show that the optimum external fiber length (L_ext) is 3.1?cm and the optimum range of working temperature for FGFP laser is within?±?2?°C from the FBG reference temperature (T_o). Also, it is shown that antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the FM response. The FM spectra peak amplitude is less than 5?dB with 5?mW output power. Good temperature stability is also obtained.