Quantum-noise-induced order in lasers placed in chaotic oscillation by frequency-shifted feedback

A kind of chaotic oscillations featuring random switching between sustained relaxation oscillations (RO) and spiking oscillations (SO) has been demonstrated in lasers with frequency-shifted feedback. The presence of stochastic frequency locking between two periodicities of RO and SO motions and selective quantum-noise-induced ordering of chaotic spiking oscillations is demonstrated theoretically and experimentally.     

Compact free electron lasers: From Cerenkov to waveguide free electron lasers

The basic principles of free electron generators of coherent radiation and the practical considerations related to the development of far infrared and sub-millimeter wave sources are discussed in this paper. The attention is focused on compact devices, which reduce the size and cost issue. A review of the main results obtained at the Frascati FEL Facility is presented together with the planned experiments.     

Effects of grating order and tooth rounding in laterally coupled distributed feedback lasers

We demonstrate the results of an analysis of laterally coupled distributed feedback (LC-DFB) lasers with higher order gratings, including the effects of radiating partial waves. For a given fabrication resolution, first-order gratings, if they can be reliably manufactured, always provide the strongest coupling. However, at resolutions requiring higher order gratings, if duty cycles of >0.5 are used, the lowest grating order is not always the one with the strongest coupling. An analysis of the rounding of the grating teeth showed that the required threshold gain was increased by nearly 20% when the rectangular grating was rounded in fabrication.     

Thermal lens measurement technique in end-pumped solid state lasers: Application to diode-pumped microchip lasers

A simple derivation, which relates the thermal lens focal length in solid state lasers to pump power and a method for direct estimation of thermal lens focal length, is reported. This method is applicable to any type of stable resonator. The method is used for the measurement of the thermal lens focal length with an accuracy of 8% in an axially pumped microchip laser. The variation of focal length with pump power is also measured.     

Fluorescence-lifetime measurements in atmospheric-pressure flames using nanosecond-pulsed lasers

Measurements of fluorescence lifetimes are needed to quantify concentration measurements when using linear laser-induced fluorescence. However, lifetimes are only a few nanoseconds for many important species at atmospheric pressure. When using a typical Q-switched laser with a pulse width of about 10 ns, the fluorescence follows the shape of the laser pulse and the lifetime cannot be easily measured. In this paper, a technique is described for experimentally determining the fluorescence lifetime in atmospheric-pressure flames using a nanosecond-pulsed laser; that is, measurement of a lifetime an order-of-magnitude faster than the laser pulse itself. This technique relies on an observable temporal shift in the fluorescence signal as a function of the lifetime. Simulations show the efficacy of this approach, and data in liquid samples and in an atmospheric-pressure flame show excellent agreement with prior picosecond measurements. This technique is successful because only the temporal shift is examined and details of the fluorescence profile are ignored. Received: 23 August 2001 / Revised version: 19 November 2001 / Published online: 17 January 2002     

Generation of ultrasound in materials using continuous-wave lasers

Generating and detecting ultrasound is a standard method of nondestructive evaluation of materials. Pulsed lasers are used to generate ultrasound remotely in situations that prohibit the use of contact transducers. The scanning rate is limited by the repetition rates of the pulsed lasers, ranging between 10 and 100 Hz for lasers with sufficient pulse widths and energies. Alternately, a high-power continuous-wave laser can be scanned across the surface, creating an ultrasonic wavefront. Since generation is continuous, the scanning rate can be as much as 4 orders of magnitude higher than with pulsed lasers. This paper introduces the concept, comparing the theoretical scanning speed with generation by pulsed laser.     

Tunable fir lasers in semiconductors using hot holes

It is shown that active systems based on inter-subband transitions of hot holes could provide tunable FIR sources.Liquid helium cooled Ge is the most promising system—and some results are quoted—while for higher temperatures (with higher E and H fields) Si is the most likely material.     

Beyond k-space: spectral localization using higher order gradients

Chemical shift imaging (CSI) often suffers from the inconvenient shape of its spatial response function (SRF), which affects both localization and signal-to-noise ratio. Replacing the magnetic field gradients for phase encoding by higher order magnetic fields allows a better adjustment of the SRF to the structures in the sample. We combined this principle with the SLOOP (spectral localization with optimal pointspread function) technique to simultaneously obtain spectra from several arbitrarily shaped compartments within a sample. Linear combinations of the fields of the shim coils are used to generate the pulsed fields for phase encoding. Their shapes are matched to the given sample geometry by numerical optimization. Using this method, spectra from a phantom were obtained that show a higher signal-to-noise ratio and a strongly reduced contamination compared to an equivalent CSI experiment.     

Magneto-spectroscopy of photo-excited states in semiconductors using far-infrared lasers

The photo-excited states in pure and doped germanium samples, i.e. excitons, electronhole drops (EHD) and bound excitons (BE), have been examined by using far-i.r. lasers of H₂O, DCN and HCN and the scanning magnetic field. The apparatus for photo-excitation and far-i.r. measurements is explained. From the temperature and excitation dependence of the absorption spectrum, the correspondence of the spectral lines and these photo-excited events has been clearly established. The abrupt change of excitons to EHD with temperature has been observed. The Zeeman effect of excitons has been also studied and experimentally it has been demonstrated that the adiabatic approximation for the Coulomb potential is valid for the fields above several 10 kG. In doped germanium, new absorption lines due to BE have been found.     

Electroreflectance vibrational spectroscopy: A new surface analysis technique using diode lasers

A new infrared technique to measure the vibrational spectra of adsorbed molecules is introduced. An Infrared active mode of an oriented molecule has a first order Stark effect. Therefore, infrared absorption by oriented adsorbed molecules is modulated by an alternating electric field. The spectra is obtained by measuring this using a diode laser. The advantages and limitations of Electroreflectance Vibrational Spectroscopy (EVS) are discussed. We calculate the Stark tuning rate and consequent signal amplitude for a number of diatomic molecules. Measurements of diode laser Intensity noise are used to estimate the signal to noise ratio for comparison with other techniques.     

Sixtieth order harmonic mixing to 4.25 THz using a Schottky diode

Harmonic mixing of mm-waves with radiation of 3.7 THz with different Schottky diodes is reported. The highest mixing order is 60 producing a beat between 72 GHz and 4.25 THz radiation. Such high frequency mixing has so far only been possible using cryogenic Josephson mixers. The present result permits substantial simplification of measurements of optical frequencies.     

Analysis of three-nucleon scattering using Faddeev equations up to third order

The Faddeev equations for three nucleons are iterated up to third order using separable interactions: the Tabakin potential for the first order and the Yamaguchi potential for the other orders. This method is applied to the quasi-free scattering ²H(p, 2p)n and ²H(s, np)p. Apparent convergence of the rescattering series is obtained for incident lab energies $\text{E}{}_{\mathrm{<mtext>lab</mtext>}}\text{? 40}\text{MeV}$ and the shape and magnitude of the experimental results are thus reproduced.     

Line selection in carbon dioxide waveguide lasers using diffraction gratings

This paper treats theoretically the coupling losses which arise in square dielectric waveguide CO₂ lasers when one element of the optical resonator is a diffraction grating. These losses are due to imperfect coupling of the radiation which is launched from the end of the guide to the grating and returned from the grating back to the guide. The results of the calculations show that the coupling efficiency is a sensitive function of radiation wavelength. This result is then used to investigate the ability of a 150 lines/mm diffraction grating to resolve adjacent rotational-vibrational transitions in the CO₂ emission. It is shown that “line-hopping” places a limitation on the maximum tunability which can be obtained and is a result of the poor discrimination at the grating surface especially in the case of waveguide lasers having much reduced apertures. Computations based on the scaling laws are employed to maximise the tunability as a function of waveguide aperture taking into account limitations imposed by “line-hopping”, cut-off and the optical resonator frequency offset condition of c/4L. Comparisons are made between theory and experiment, and design guidelines for the construction of widely tunable CO₂ waveguide lasers are presented graphically.     

Repeatedly using the decomposition product of struvite by ultrasound stripping to remove ammonia nitrogen from landfill leachate

In this study, the decomposition of struvite by ultrasound stripping and the recycle use of the decomposition product for the treatment of landfill leachate were investigated. The results indicated that when the decomposition of struvite by ultrasound stripping was performed at 55 °C for 40 min, the ammonium in the struvite could be almost completely eliminated from the solution system. The characterization analysis showed that magnesium phosphate and the dissolved phosphate ions were the main active derivatives. Approximately 90% of the total ammonia nitrogen (TAN) in landfill leachate can be removed by reusing the decomposition product at pH 9 for 60 min. Repeated use of the struvite decomposition product revealed that the TAN removal efficiency decreased with an increase in the number of recycles. However, in the process of multiple recycling, about 90% of TAN removal could be maintained by supplementing a certain amount of the preformed struvite to the solution for every recycle. An economic analysis demonstrated that 79.3% of the treatment cost could be saved by the proposed process compared to the non-recycling process.     

飞秒激光放大的奠基性发明及其能力边界的突破

2018年度诺贝尔物理学奖的一半授予了法国科学家莫罗(G. Mourou)和加拿大科学家斯特里克兰(D. Strickland),以表彰他们对超短超强激光领域的奠基性贡献。他们发明的啁啾脉冲放大(CPA)方法解决了飞秒激光放大的问题,使得超快走向超强,诞生了超短超强激光新技术,也改变了人类的生活。文章将从新的视角解读飞秒激光放大的CPA方法,分析该领域的研究现状,讨论其未来创新发展趋势,深度思考如何拓展超短超强激光的能力边界。     

Mode-coupling control in resonant devices: Application to solid-state ring lasers

We report the theoretical and experimental investigation of the effects of mode coupling in a resonant macroscopic quantum device, in the case of a solid-state ring laser. This is achieved by introducing an additional coupling source whose interplay with the already-existing nonlinear effects ensures the coexistence of two counterpropagating cavity modes yielding a rotation-sensitive beat note. The determination of the condition for rotation sensing, both theoretically and experimentally, allows a quantitative study of the role of various mode-coupling mechanisms, in particular, the gain-induced mode coupling. We point out the connection between our work and the theoretical work on mode coupling in superfluid devices. This work opens up the possibility of new types of active rotation sensors.