驱动场相位扩散对三能级梯型系统无反转激光的影响

在旋波、慢变振幅近似下,求解考虑了驱动场相位扩散后的系统密度矩阵运动方程,并给出了这个三能级梯型系统稳态线性解析解.利用得到的稳态线性解析解分析驱动场相位扩散是如何影响该系统输出无反转激光的.对稳态线性解析解数值计算的结果显示:由于驱动场相位扩散会导致无反转激光增益减小;即使由于驱动场相位扩散引起的线宽足够大,在该系统中仍能够获得无反转激光;线宽往往是破坏无反转激光产生和折射率的提高;因驱动场相位扩散导致无反转激光增益的减小,并不是总能够通过增大驱动场的Rabi频率得到补偿.     

Nonlinear theory of lasing with or without inversion in a simple three-level atomic system

We investigate, using a nonlinear theory of laser physics, for a simple three-level atomic system the steady-state intensity behavior due to the effects of quantum coherence and decoherence. We find that a change from a noninversion laser to an inversion laser action can occur as the Rabi frequency of the driving field increases. The steady-state intensity of the laser field can arrive at a maximal value for a certain Rabi frequency of the driving field. We also find that the linear gain and the laser intensity tend to decrease for the linewidth of the driving field.     

Coherence and Decoherence Effects on Lasing with o without Population Inversion in a Four-level Atomic System

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开放V型无反转激光系统相位涨落的影响

通过对从开放的V型双泵浦系统的定态解析解得到的数值计算结果的分析表明:1)驱动场相位将涨落导致一个有限线宽;一般说来,被探测场耦合的上下两能级间的粒子数差随线宽的增加而减小,当线宽增加到一定值时,系统将发生从传统的反转激光到无反转激光的转化;线宽的增加将使无反转激光增益降低;当线宽不为零时,仍可产生无吸收高色散(折射率)2)线宽变化对增益、色散和粒子数差影响的性质和大小与系统其它参量的取值密切相关,只有适当选择线宽和系统其它参量的值才能获得最大的无反转激光增益和无吸收高色散(折射率).     

Effects of phase fluctuation in an open four-level inversionless lasing system

A steady analytical solution of an open four-level inversionless lasing system with a driving field having the phase fluctuation has been given, and the effects of the finite width due to the phase fluctuation on the gain, dispersion, and population difference have been analyzed by using the numerical simulation from the steady analytical solution. It is found that: with the linewidth increasing, the gain decreases and the absolute value of population difference between levels coupled by the probe field increases, but the variation of the linewidth cannot change the properties of the inversionless lasing and refractive index increase of the system; when the linewidth does not equal to zero, the system can still get a high refractive index with zero absorption, and these conclusions have very obvious difference from those obtained in other inversionless lasing systems.     

LINEWIDTH REDUCTION IN AN INVERSIONLESS LASER WITH PUMP PHASE FLUCTUATIONS

The effect of the pump phase diffusion on the linewidth of a laser without inversion is studied. It is shown that the linewidth at equal intensity may be reduced below that for a coherent driving, enen if the net effect of the pump bandwidth leads to an increase in the linewidth. The physical mechanism is analyzed in the dressed-state picture. Furthermore, the effects of the pump phase noise on the linewidths of an inversionleess laser and a Raman laser are compared with each other.     

带有多孔二氧化硅间隔层的导模共振光栅实现染料激光器发射增强

导模共振光栅是一种典型的平面波导共振结构,可在光栅表面或波导层内形成较强的局域电场,能增强光与物质的相互作用.本文在导模共振结构的光栅层和基底层之间,引入低折射率的多孔二氧化硅间隔层,显著增强了局域电场与增益介质的接触度.结果表明,引入多孔二氧化硅后,共振产生的电场增强区域上移至激光染料层,增加了激光染料与电场的相互作用,实现了激光出射增强.本文基于时域有限差分法,对结构参数进行分析优化,研究了820 nm共振波长激发下的出射激光特性,得到了连续的激光出射,其能量阈值约为2.5 mJ/cm^2,线宽约为0.3 nm.本文提出的结构实现了对表面局域电场的有效调控,增强了激发光与增益介质的相互作用,不但可应用于激光器,还为其它发光器件的设计提供了参考.     

Refractive index measurements of solid parahydrogen

Solid para-H2 is a promising gain medium for stimulated Raman scattering, due to its high number density and narrow Raman linewidth. In preparation for the design of a cw solid hydrogen Raman laser, we have made the first measurements, to our knowledge, of the index of refraction of a solid para-H2 crystal, in the wavelength range of 430-1100?nm. For a crystal stabilized at 4.4?K, this refractive index is measured to be n(p-H2)=1.130±0.001 at 514?nm. A slight, but significant, dependence on the final crystal-growth temperature is observed, with higher n(p-H2) at higher crystal-growth temperatures. Once a crystal is grown, it can be heated up to 10?K with no change in n(p-H2). The refractive index varies only slightly over the observed wavelength range, and no significant birefringence was observed.     

Phase control in an open Λ-type system with spontaneously generated coherence

This paper investigates the control role of the relative phase between the probe and driving fields on the gain, dispersion and populations in an open A system with spontaneously generated coherence （SGC）. It shows that by adjusting the value of the relative phase, a change from lasing with inversion to lasing without inversion can be realized; the values and frequency spectrum regions of the inversionless gain and dispersion can be obviously varied; high refractive index with zero absorption and electromagnetically induced transparency can be achieved. It is also found that when the driving field is resonant, the shapes of the dispersion and the gain curves versus the probe detuning are very similar if the relative phase of the dispersion lags π/2 than that of the gain, however for the off-resonant driving field the similarity will disappear; the gain, dispersion and populations are periodical functions of the relative phase, the modulation period is always 2π; the contribution of SGC to the inversionless gain and dispersion is much larger than that of the dynamically induced coherence.     

开放四能级原子系统粒子数反转或无反转激光研究

在封闭的简单四能级原子系统模型的基础上提出了开放的四能级原子系统模型.通过在电偶极和旋转波近似下,解此系统的半经典密度矩阵运动方程得其稳态线性解析解.对该稳态解的数值模拟显示:随着驱动场的Rabi频率的连续增加系统会发生由输出的无粒子数反转激光到输出粒子数反转激光的转变;随着系统退出速率或者注入速率比的连续增大,系统会发生由输出的粒子数反转激光到输出无粒子数反转激光的转变;该系统能获得无吸收高色散;在稳态,探测激光上下能级粒子数差随探测场失谐的变化曲线呈弱矩形波.     

基于低散射和高增益全息液晶/聚合物光栅的分布反馈式激光器

利用低官能度的丙烯酸酯单体进行全息液晶/聚合物光栅的制备, 获得了具有聚合物支撑形貌的光栅结构. 由于这种光栅内部不存在液晶微滴, 当作为分布反馈式激光器的谐振腔时, 可以有效降低光栅内部的散射损失(<4%), 降低激光腔损耗. 此外, 选用的高折射率单体提升了光栅的折射率调制量, 增强了光栅的反馈增益. 在以上两种因素的共同作用下, 采用染料DCM为激光工作物质, 以532 nm的Nd:YAG脉冲激光器作为抽运光源, 最终获得了中心波长为635 nm, 转化效率为1.2%的高性能激光, 在以阈值能量0.8 μJ/pulse抽运下获得激光线宽0.3 nm, 较之国内外同类激光器的报道, 在阈值、线宽、转化效率三方面均有不同程度提升.     

Anomalous Effects of Driving Field Linewidth on a One-Atom Dressed-State Laser

We examine the effects of driving field linewidth on a one-atom dressed state laser. Unexpectedly, the linewidth leads to anomalous effects on the cavity field. The mean photon number of the cavity field is raised or the normalized variance is reduced to a certain degree as the linewidth increases for an appropriate range of parameters. The responsible mechanism is attributed to the fluctuation-induced modification of the electromagnetic reservoir where the atom stays.     

QUANTUM LANGEVIN THEORY OF WHISPERING-GALLERY-MODE MICROSPHERE LASER

A quantum Langevin theory of whispering-gallery-mode microsphere laser theory is developed. The linear and nonlinear analysis are made for laser operation below and above the threshold. In these analysis, corresponding to the specific property of microsphere, the effect of inversion fluctuation is treated. The coherence functions of laser field are calculated, and the intensity, the amplitude fluctuation and the linewidth of the field are obtained, which are connected with the enhancement factor of whispering-gallery-mode microsphere. It is shown that the strong couple and strong pumping are useful for the amplification of intensity and the decrease of linewidth below the threshold. It is also shown that, for the laser action above threshold, the variances of photon number and the linewidth of internal field are related to the enhancement factor and the square of the enhancement factor, respectively.     

Structural changes in BK7 glass upon exposure to femtosecond laser pulses

A general picture of refractive index change mechanisms in glass modified by a femtosecond laser has proven elusive. In this paper, Raman microscopy was used in conjunction with refractive near‐field profilometry to analyse the structure of borosilicate glass (Schott BK7) modified by a femtosecond laser and determine the mechanism of the observed refractive index change. For a pulse repetition rate of 1 kHz, it was determined that the refractive index change was due to an elevated population of non‐bridging oxygen atoms, resulting in more ionic bonds forming within the glass network and increasing the molar refractivity of the glass. For a pulse repetition rate of 5.1 MHz, the dominant mechanism of refractive index change was densification and rarefaction of the glass network. Different refractive index change mechanisms were attributed to different thermal conditions imparted to the glass under different pulse repetition rates. Implications for device fabrication are also discussed. These findings constitute an important step toward a complete overview of femtosecond‐laser‐induced refractive index change in glass. Copyright © 2010 John Wiley & Sons, Ltd.     

光场的线宽对无粒子数反转光放大增益的影响

针对拉曼驱动场下的四能级理论模型，讨论了实现无吸收、高折射率的条件，并分析了探测场及相干激发场的线宽对介质的折射率及增益的影响。计算结果表明，改变相干激发场 场强，或改变相干场的频率与原子能级间的失谐均可获得介质的无吸收，高折射率。激发扬线宽破坏了原子的相干性，使使介质的增闪下降并改变了折射率的大小，探测场的线宽使介质的探测增益谱及折射率谱的分辨率下降。     

Nonlinear optics in radiatively cooled vapors

We consider the use of a radiatively cooled resonant vapor as a media for 3rd order nonlinear optics processes. The 3rd order nonlinear refractive index coefficient due to both steady state saturation of the atomic polarizability and electrostrictive induced density changes are computed for a radiatively cooled vapor with sodium as an example. For pump detuning on the order of the natural linewidth the electrostrictive and saturation induced nonlinear refractive index coefficients are comparable and several orders of magnitude greater than the highest values yet reported. At pump detunings larger than the natural linewidth the electrostrictive mechanism is shown to dominate in the steady state.     

Single-qubit lasing and cooling at the Rabi frequency

For a superconducting qubit driven to perform Rabi oscillations and coupled to a slow electromagnetic or nanomechanical oscillator we describe previously unexplored quantum optics effects. When the Rabi frequency is tuned to resonance with the oscillator, the latter can be driven far from equilibrium. Blue detuned driving leads to a population inversion in the qubit and a bistability with lasing behavior of the oscillator; for red detuning the qubit cools the oscillator. This behavior persists at the symmetry point where the qubit-oscillator coupling is quadratic and decoherence effects are minimized. There the system realizes a "single-atom-two-photon laser."     

Study on measurement of refractive index profile of GI-POF by light scattering

This paper is devoted to the study on measurement of refractive index profile of graded-index polymer optical fiber (GI-POF) by light scattering. Using Generalized Airy theory and Debye series of an inhomogeneous cylinder, the scattering intensity distributions are obtained of Airy structure of rainbows for different refractive index profile. The results show that positions of Airy peaks depend closely on refractive index profile of GI-POF. Since each order of rainbow penetrates it to different depths, such methods could be used to provide information of the refractive index profile of GI-POF. For GI-POF with given diameter, positions of Airy peaks of rainbows are simulated as a function of refractive index profile, which can be used to inverse unknown parameters of refractive index profile. The least square method is used in inversion of refractive index profile with the given refractive index of the cladding. The results obtained agree with theoretical values with high precision. The method has the advantages of non-instructive and on-line measurement, and can be used for the measurement of other inhomogeneous droplets.     

Intense laser field effects on the linear and nonlinear intersubband optical properties of a semi-parabolic quantum well

By using the compact-density matrix approach, the effect of a nonresonant intense laser field on the linear and nonlinear optical absorptions based on intersubband transitions and the refractive index changes in an asymmetric semiconductor quantum well have been presented. Our results show that the peak position of the absorption coefficient is sensitive to intense laser field, the absorption maximum shifts towards lower energies for increasing intense laser field value. Also we observe as the intense laser field strength increases, the total refractive index change has been increased in magnitude and also shifted towards lower energies. The results indicate that linear and nonlinear optical properties of the low dimensional semiconductor heterostructures can be adjusted in a desired energy range by using intense laser field.     

Transmission characteristics of graded-index fibers

The use of graded-index fibers for optical communication systems is now under consideration for numerous civil and military applications, and in these either LED or laser sources can be developed. The bandwidth of the system depends on the source linewidth, the dispersion of refractive index in the fiber, the strength of excitation of the modes by the source, and the extent of mode mixing caused, by example, by microbending. In fibers with refractive-index profiles that have a nearly parabolic dependence upon radius, pulse broadening is a slight, but small departures from an optimum profile can cause a dramatic decrease in bandwidth. It is of considerable importance to predict the bandwidth of fibers from a knowledge of the refractive-index profile and source geometry. The purpose of the first part of this paper is to compare two computational methods by which the propagation characteristics of a fiber, hence the impulse response and system bandwidth, may be determined. In the second part, we shall describe a computer program that determines the excitation of modes in an arbitrarily graded refractive index fiber for a model of laser source.