Analysis of output characteristics of the actively Q-switched intracavity optical parametric oscillator considering Gaussian distribution of intracavity photon densities

By considering the Gaussian spatial distribution of the initial population-inversion density and the intracavity photon densities of the fundamental, the signal and the idle lights, the rate equations of a LD-pumped actively Q-switched intracavity optical parametric oscillator (IOPO) are derived. The influence of the pump rate, the thermal effect in the gain medium and the change of the photon density along the cavity axis have also been taken account in the equations. By numerically solving these coupled equations, the dependences of pulse width, pulse energy and peak power on incident pump power for the generated-signal light are obtained. In the experiment, a laser-diode-pumped actively Q-switched Nd:GdVO₄/KTP IOPO with an acousto-optic (AO) modulator is realized and the experimental results agree with the theoretical calculations.     

Optical Amplification and Spontaneous Emission in an Ar + Discharge

The relationship between optical amplification and spontaneous emission noise has been studied in an argon ion travelling wave amplifier by beat spectrum analysis. Different optical arrangements have been examined with both single and double pass amplification. The results establish that when coherent light travels through a gain tube, the amplified beam acquires intensity fluctuation noise which is greater than that of a Poissonian distribution (shot noise). The noise power is shown to be the same as that of an equivalent unamplified beam mixed with the amplified spontaneous emission from the gain tube. It is shown that such measurements provide a practical technique for the otherwise difficult determination of the degree of population inversion in the gain medium. They also give insight into the potential of optical amplifiers for enhancement of lidar and system performance.     

高功率双包层Er3+/Yb3+共掺光纤放大器的动态响应

基于速率方程的离散算法,分析了双包层Er3 /Yb3 共掺光纤放大器的动态响应,显示了输出功率和增益的动态特征。当单个脉冲注入放大器时,输出脉冲的峰值功率不仅依赖于输入脉冲的峰值功率,而且依赖于泵浦功率;当脉冲序列注入时,输出脉冲的功率和增益最终将收敛于它们的稳态值。在双信道情况下,输入脉冲重叠时的输出功率和增益变得更陡峭。在连续波泵浦下,反向自发辐射输出功率(ASE-)首先快速地增加到峰值功率,然后单调下降到稳态值;在脉冲波泵浦下,反向自发辐射输出功率(ASE )与光纤长度成反比,而ASE-与光纤长度成正比。     

抑制噪声的宽带碲基掺铒光纤放大器性能分析

设计了一个带光隔离器的复合型宽带碲基掺铒光纤放大器（EDTFA）,通过对该结构模型下的速率方程和光功率传输方程组的数值模拟,理论研究了EDTFA在插入光隔离器后的性能变化。研究表明,通过插入光隔离器抑制反向传输的放大自发辐射（ASE）噪声,可以有效地改善宽带EDTFA的信号增益和噪声特性。在光纤激活长度240cm、铒掺杂浓度2000ppm和前向泵浦功率200mW下,光隔离器插入在最佳位置处时,1540nm～1600nm波段内16路信道小信号增益分别提高了0.8dB～5．9dB,噪声系数降低了0.2dB～2．2dB。研究结果对于新型宽带EDTFA应用于WDM通信系统中具有理论指导意义。     

宽带铋基掺铒光纤放大器增益及噪声特性研究

利用Er3 离子四能级结构速率方程组和光功率传输方程组,数值模拟了铋基掺铒光纤放大器(EDFA)的增益及噪声特性,模拟结果与实验报道结果取得了很好的一致。同时,详尽地分析了增益及噪声特性与光纤长度、泵浦功率和输入信号功率的关系,优化了放大器的性能,从理论上得出一个20dB增益带宽达76nm、噪声系数接近4dB的铋基EDFA。研究表明了铋基掺铒光纤放大器适合用作于现代DWDM系统中C L波段的光纤放大器。     

Compression of pulse duration in a laser-diode, end-pumped, double Q-switched laser

By considering the Guassian transversal distribution of intracavity photon density and the longitudinal distribution of photon density along the cavity axis as well as the influence of the turn-off time of the acousto-optic Q switch, we introduce the coupled rate equations of a laser-diode, end-pumped, double Q-switched laser with an acousto-optic modulator and GaAs saturable absorber. In addition the thermal effect of the gain medium is taken into account. These coupled rate equations are solved numerically, and the dependence of pulse width on incident pump power at different pulse repetition rates is obtained for the generated output pulses. It is shown that the pulse duration is obviously shorter in contrast to the actively Q-switched Nd:YVO4 laser with an acoustic-optic modulator, and the maximum compression ratio of the pulse width is more than 60%. In the experiment a laser-diode, end-pumped, double Q-switched Nd:YVO4 laser with both an acousto-optic modulator and GaAs is realized, and the experimental results agree with the numerical solutions.     

Thermal Resonance at the Microscale in AC Scanning Thermal Microscopy with a Thermal-Resistive Probe

Investigation of the thermal-resistive probe response in an AC scanning thermal microscope (SThM) as a function of the distance probe–material surface under ambient conditions and the current excitation frequency is presented. The analysis of temperature experimental results points out a phenomenon which can be interpreted as a thermal wave resonance. The modeling of the thermal response with the finite element method considering the surrounding medium as a thermally conducting medium tends to confirm this. This phenomenon is independent of the sample nature and is related to the thermal diffusion length of the air medium between the probe and the sample. An equivalent resonance factor is defined: it shows a linear dependence with the characteristic distance for which the thermal resonance phenomenon is an extremum. The system—probe/sample surface—behaves as a resonant cavity at the microscale. This configuration is not specific to a SThM and can occur within electronic devices.     

Boundary elements approach for solving stochastic nonlinear problems with fractional Laplacian terms

Models involving stochastic diffusion equations are utilized for describing the evolution of a number of natural phenomena and are widely discussed in the open literature. In recent years, these models have been revisited in light of experimental observations in which “anomalous” diffusion processes were identified, such as in the propagation of acoustic waves in random media. In this context, a critical characteristic of the theoretical models is the introduction of fractional derivative operators in the associated governing equations. Specifically, anomalous diffusion involves a fractional Laplacian operator replacing the classical Laplacian. Currently, solutions to equations with fractional Laplacians are available for a quite limited numbers of cases. Further, to the authors’ knowledge, no solutions are available for nonlinear equations involving fractional Laplacians. This fact creates the need of developing adequate numerical methods for estimating the response of this kind of systems. This paper proposes a Boundary Element Method (BEM)-based approach to determine the response of a system governed by a nonlinear fractional diffusion equation involving a random excitation. The approach is constructed by utilizing the integral representation of the classical Poisson equation solution, in which unknown constants are determined by the BEM. Then, based on a recently proposed representation of the fractional Laplacian operator, the value of the fractional Laplacian of the response is updated progressively by matrix transformation of these constants. Numerical results pertaining to a system exposed to white noise are presented to elucidate the mechanization of the approach. Further, parameter studies are done for examining the influence of the fractional Laplacian order on the system response.     

提高脉冲LD测距系统测距能力的一种方法

从激光测距机的噪声构成分析入手,寻求提高脉冲 LD 测距机测距能力的方法。脉冲 LD测距系统的噪声主要包括热噪声、散粒噪声、背景噪声、暗电流噪声及放大器噪声。在一定的光功率、暗电流情况下,LD 测距的测距能力更容易受背景噪声的影响。研究表明,LD 测距系统中探测器偏压的变化只补偿温度变化,由此提出,在背景光照射较强时让背景噪声参与偏压调整从而抑制背景噪声的方案。实验结果证实,强背景下的信噪比提高 1 倍以上,测距能力提高 1 倍。     

Tunable four-pass narrow spectral bandwidth amplifier for use at approximately 508 nm

We report the implementation of a tunable, narrow-spectral-bandwidth, pulsed, four-pass dye-laser amplifier with strongly reduced amplified spontaneous emission. We present temporal pulse profiles, pulse spectra, and gain measurements of the amplifier output for the case of Coumarin 307 dye as the gain medium, seeded at wavelengths of approximately 508 nm and pumped at 355 nm.     

Passively Q-switched quasi-three-level laser and its intracavity frequency doubling

A passively Q-switched quasi-three-level Nd:YAG laser is intracavity frequency doubled to generate a blue laser. The 473-nm blue laser has a peak power of 37 W and a pulse width of 23 ns at a pumping power of 1.6 W. To model this laser numerically, we developed rate equations by taking into consideration both the quasi-three-level nature of the gain medium and the four-level nature of the saturable absorber. Good agreement was achieved between experimental and simulated results for both the fundamental and the second-harmonic output. The reabsorption loss of the gain medium is estimated under pulsed operation.     

Analysis of a diode-pumped passively Q-switched Nd:GdVO4 self-stimulating Raman laser

This paper presents the theoretical and experimental results of a laser diode-pumped passively Q-switched Nd:GdVO₄ self-Raman laser in detail. In the experiment, by using two Cr⁴⁺:YAG crystals with different initial transmissions as the saturable absorber, using Nd:GdVO₄ as the gain medium and Raman medium simultaneously, the passively Q-switched operation of the Nd:GdVO₄ self-Raman laser at 1176 nm was investigated. The average power, pulse energy, pulse width and pulse repetition rate with respect to the incident pump power were measured. The obtained maximum output average power is 244.6 mW with respect to incident power of 5.7 W and the corresponding conversion efficiency is 4.3%. In the theoretical part, we used the rate equations to obtain the theoretical results. In the rate equations, the Gaussian distributions of the intracavity photon densities of the fundamental and Raman lasers and the initial population-inversion density were taken into account, and the ground-state population density of the saturable absorber at t = 0 is assumed to be uniform. The obtained theoretical results were in agreement with the experimental results on the whole.     

Ultrafast generation of acoustic waves in copper

The ultrafast generation of acoustic waves in copper films is investigated with a femtosecond optical pump and probe technique. By studying the generation at times before the electrons and the lattice come into equilibrium, the strength of their interaction can be measured and the dynamics of ultrafast electron diffusion can be studied. The acoustic strain pulses observed are bipolar in shape with exponential tails that are much broader than expected from simple thermoelastic stress generation. This can be explained by the supersonic diffusion of electrons over distances larger than the optical skin depth. The nonequilibrium diffusion equations governing stress generation are nonlinear, and are solved numerically. Using a linearized formulation, we also solve them analytically to a good approximation. The acoustic strain profile provides a `snapshot' of the initial spatial temperature distribution of the lattice, thus allowing a sensitive probe of the nonequilibrium dynamics of the diffusion. The electron-phonon coupling constant can be estimated directly from the acoustic pulse duration, provided that the sound velocity and thermal conductivity are known. In general, the relaxation and diffusion of carriers is specific to the sample in question, whether metal or semiconductor, suggesting the use of this method for thin film characterization     

Demonstration of a White Laser with V2C MXene‐Based Quantum Dots

Multicolor photoluminescence over the full visible color spectrum is critical in many modern science and techniques, such as full‐color lighting, displays, biological and chemical monitoring, multiband communication, etc., but the ultimate white lasing especially on the nanoscale is still a challenge due to its exacting requirements in the balance of the gain and optical feedback at different wavelengths. Recently, 2D transition metal carbides (MXenes) have emerged, with some superior chemical, physical, and environmental properties distinguishing them from traditional 2D materials. Here, a white laser with V₂C MXene quantum dots (MQDs) is originally demonstrated by constructing a broadband nonlinear random scattering system with enhanced gain. The excitation‐dependent photoluminescence of V₂C MQDs is enhanced by passivation and characterized, and their localized nonlinear random scattering is realized by the generation of excitation‐power‐dependent solvent bubbles. With the optimized excitation, the blue, green, yellow, and red light is amplified and simultaneously lased. This work not only provides a kind of promising material for white lasers, but also a design strategy of novel photonics for further applications.     

Linear systems driven by martingale noise

A method is developed for calculating second moment properties and moments of order three and higher of the state X of a linear filter driven by martingale noise. The martingale noise is interpreted as the formal derivative of a square integrable martingale with continuous samples. The Gaussian white noise is an example of a martingale noise. It is shown that the differential equations of the mean and correlation functions of the state X developed in the paper resemble the corresponding equations of the classical linear random vibration and coincide with these equations if the input is a Gaussian white noise. The moment equations are derived by (1) the Itô formula for semimartingales and (2) the classical Itô formula applied to a diffusion process whose coordinates include X. An advantage of the second method is use of more familiar concepts. However, this method requires to calculate unnecessary moments and can be applied only for a class of martingale noise processes. Examples are presented to illustrate and evaluate the two methods for calculating moments of X and demonstrate the use of these methods in linear random vibration.     

Optimization of a laser satellite communication system with an optical preamplifier

We derive a model that optimizes the performance of a laser satellite communication link with an optical preamplifier in the presence of random jitter in the transmitter-receiver line of sight. The system utilizes a transceiver containing a single telescope with a circulator. The telescope is used for both transmitting and receiving and thus reduces communication terminal dimensions and weight. The optimization model was derived under the assumption that the dominant noise source was amplifier spontaneous-emission noise. It is shown that, given the required bit-error rate (BER) and the rms random pointing jitter, an optimal transceiver gain exists that minimizes transmitted power. We investigate the effect of the amplifier spontaneous-emission noise on the optimal transmitted power and gain by performing an optimization procedure for various combinations of amplifier gain and noise figure. We demonstrate that the amplifier noise figure determines the optimal transmitted power needed to achieve the desired BER but does not affect the optimal transceiver telescope gain. Our numerical example shows that for a BER of 10(-9), doubling the amplifier noise figure results in an 80% increase in minimal transmitted power for a rms pointing jitter of 0.44 microrad.     

Subluminal and superluminal light propagation via electromagnetically induced transparency in radiatively and inhomogeneously broadened media

Contrary to popular belief, we demonstrate the feasibility of generating superluminal (and subluminal) probe (and signal) light via electromagnetically induced transparency in a medium comprising coupled double-ladder systems. This scheme can be realized in both homogeneously (radiative) as well as in inhomogeneously (Doppler) broadened atomic systems. Unlike more intricate earlier schemes, our scheme is based simply on steady-state propagation dynamics resulting from compensation of the inevitable absorption losses by large nonlinear gain generated through appropriate choice of the pump and coupling fields. We show how easily in this scheme the speed of weak probe (and signal) fields can be switched from subluminal to superluminal by simply varying the strengths of the coherent pump and coupling fields. Furthermore, it is shown that under these conditions both the signal and probe fields are intensity matched and both propagate with the same subluminal (or superluminal) group velocity without suffering loss or gain for long distance in the medium.     

Thermal Diffusion Through Multilayer Coatings: Theory and Experiment

Thermal diffusion through aluminum oxide/molybdenum multilayers (10, 20, and 40 bilayers on molybdenum substrates) was studied through thermal pulse experiments. In these experiments, a thermal pulse (<50 ns) was delivered by a laser to the front surface of the specimen and the resulting temperature transient on the rear surface, typically several degrees, was recorded. An integral solution for the equations governing heat transfer was used to relate experimental results to the thermal resistance of the interfaces. Experimental studies were conducted at temperatures from 950 to 1150°C, relevant to thermal barrier applications. Significant densification of the oxide layers was observed during the experiments at all but the lowest temperature. As a result of these experiments, an upper bound has been placed on the interface thermal resistance.     

Spectrum-preserving Amplification of Ultra-broadband Optical Pulses

Abstract

An amplifier scheme which allows control of the spectrum of the amplified pulse is proposed. The spectral components of the initial pulse are sent into laterally shifted regions of the amplifying medium, which enables control of the gain of particular frequency components. By the use of a properly formed pump, spectral narrowing during amplification can be avoided. A ‘spectrum preserving’ amplification of ultra-broadband (f.w.h.m. of about 30 nm) nanosecond pulses is demonstrated.     

Evaluation of an ultrabroadband high-gain amplification technique for chirped pulse amplification facilities

Recently, an amplification technique for ultrashort pulses was explored in detail in a theoretical paper by Ross et al. [Opt. Commun. 144, 125 (1997)]. The technique, based on nonlinear optics, is called optical parametric chirped pulse amplification. It has a number of features that, in principle, make it highly attractive. It primarily offers extremely large gains simultaneously with extremely large bandwidths. Additional attractions are virtually no spatial and temporal phase distortion of the amplified pulse, high efficiencies and a low thermal loading, reduced amplified spontaneous emission levels, small optical material lengths, and an inherent simplicity of implementation. We present an evaluation of the technique as a front end amplifier for the ultrashort pulse amplification chain of the Vulcan laser system. Such a device could replace some of the existing amplification in Nd:glass and additionally have a wider effect as a direct replacement of Ti:sapphire regenerative amplifiers on large-scale chirped pulse amplification scale facilities.