复宗量厄米-高斯光束的相干模表示

基于激光的相干模分解理论，讨论了复宗量厄米_高斯(EHG)光束的模相关和相干模表示，推导出EHG光束的模相关系数的解析表达式，并给出了一种只包含有限个模式的相干模表示. 在此基础上，根据EHG光束的模结构，分析了EHG光束和常规厄米_高斯(SHG)光束M²因子的差异. 关键词： 复宗量厄米_高斯(EHG)光束 相干模分解 2因子')" href="#">M²因子 模相关系数(MCCs)     

Propagation properties of partially coherent modified Bessel–Gauss beams

Starting from the propagation law of partially coherent light, the analytical propagation equations of partially coherent modified Bessel–Gauss beams (MBGBs) through a paraxial optical ABCD system are derived and illustrated with typical application examples. Furthermore, by using the intensity moments method and integral transformation technique, the important characteristic parameters, including the beam width, far-field divergence angle, M² factor and kurtosis parameter of partially coherent MBGBs, are expressed in a closed and simple form. As a result, some basic properties of MBGBs and the dependence of the M² factor and kurtosis parameter on the spectral degree of coherence and beam order are illustrated both analytically and numerically.     

多模激光的模相关和相干模表示

对多模激光的模相关和相干模表示作了深入的理论研究.利用部分相干光理论,将与高斯-谢尔模型光束等效的多模激光表示为空间相干,并且彼此可相关模式的叠加.给出了直角坐标系下模相关系数的解析表达式.基于M²因子概念,建立了在考虑模式相关的一般情况下的多模激光模分解理论. 关键词：     

Dye laser model with pump and quantum fluctuations; White noise

We discuss a single mode dye laser model with two stochastically fluctuating forces representing pump and quantum fluctuations. We investigate the different influences of white pump and quantum fluctuations on the statistical properties of the laser light intensity. The corresponding Fokker-Planck equations are solved by means of scalar continued fractions. Stationary as well as instationary properties such as distribution functions, stationary moments, correlation functions, correlation times and transient moments are presented.     

Spatial correlation properties and the spectral intensity distributions of focused Gaussian Schell-model array beams

The spatial correlation properties and the spectral intensity distributions of focused Gaussian Schell-model (GSM) array beams are studied in detail. The closed-form expressions for the spectral degree of coherence and the spectral intensity of focused GSM array beams are derived. It is shown that the spectral degree of coherence of focused GSM array beams is the same as that of focused GSM beams in the focal plane. On the other hand, it is found that, in the focal plane the spectral intensity distribution of focused GSM array beams is the fringe pattern when the value of the coherence length is small. However, it becomes one peak located at the center as the value of the coherence length is large enough. In the focal plane, the spectral intensity maximum increases and the width of the normalized spectral intensity distribution decreases as the beam number increases. In general, for GSM array beams, the width of the modulus of the spectral degree of coherence in the focal plane always exceeds that of the normalized spectral intensity distribution, which is different from the behavior of focused GSM beams. In addition, the power in the bucket (PIB) and the beam propagation factor (M² factor) are also discussed. The main results are explained physically.     

Solution of a laser Fokker-Planck equation for intensity and inversion

In the commonly used laser model (two levels, homogeneously broadened, one running mode, no detuning) the polarization can be eliminated adiabatically, if the dephasing time is much shorter than the other time constants. We thus arrive at a Fokker-Planck equation for inversion and light intensity, which does not fulfill the detailed balance condition. By a suitable expansion of the distribution function it is shown that the Fokker-Planck equation is equivalent to a tridiagonal recurrence relation for the coefficient vectors, which can be solved by matrix continued fractions. In this way all moments and two-time correlations can be obtained. Especially the correlation coefficients between inversion and intensity, the photon counting coefficient [<(n–<n>)²>–<n>]/<n>² for the stationary state as well as the eigenvalues of the Fokker-Planck equation are presented in the threshold region.     

Effect of driving laser intensity fluctuations on optical bistability of a system with large permanent-dipole molecules

The statistical properties of an optical bistable system consisting of two-level atoms with large permanent dipole moments contained in a Fabry-Perot cavity are investigated assuming the external laser intensity fluctuations to be the main source of noise. The probability transformation rule is applied to obtain the probability distribution for the transmitted intensity whereas the probability distribution for the incident intensity is assumed to be that of the partially coherent laser light. It turns out that in the optical bistability region two values of fluctuation parameters are possible for a given value of input intensity, which produces interesting hysteresis effects.     

二次谐波中的饱和效应和乘性噪声

研究了单模激光中产生的二次谐波的统计性质,用全饱和激光理论导出了二次谐波光子计数ｍ的几率分布函数、二次阶乘矩和三次阶矩的表达式,同实验值比较,理论与实验在阈值附近符合得很好。研究结果表明,激光场基波和二次谐波的涨落随空腔衰减常数的增大而减小;当激光场中含有乘性噪声时,乘性噪声增强基波的涨落,同时抑制二次谐波的涨落。     

Spectral linewidth of inversionless and Raman lasers in V-type three-level systems

A theoretical analysis of the spectral linewidth of V-type inversionless and Raman lasers is presented. First, we examine the effects of the atomic coherence between dressed states and the Autler-Townes splitting on the linewidth. It is demonstrated that near above threshold, the V inversionless laser has a narrower linewidth than that of the two-level laser. Instead of the dressed coherence, it is the Autler-Townes splitting that is responsible for the linewidth reduction though the dressed coherence determines the laser gain. Next, we explore the effects of the generated laser intensity on the linewidth. It is shown that the linewidths of the V inversionless and Raman lasers follow the usual 1/I decrease for smaller laser intensity I, but a slower decrease than 1/I for larger laser intensity. For the V Raman laser, even more surprisingly, with the laser intensity increasing, the linewidth appreciably increases as well. As a result, well above threshold, the V inversionless and Raman lasers may have a larger linewidth than that of the two-level laser. Finally, a comparison is made between the V lasers and the Λ lasers. It is found that the linewidth of the Λ inversionless laser shows a fast 1/I ² decay under optimum conditions. Received 25 October 1999 and Received in final form 10 March 2000     

Propagation of the off-axis superposition of partially coherent beams through atmospheric turbulence

The propagation properties of the off-axis superposition of partially coherent beams through atmospheric turbulence and their beam quality in terms of the mean-squared beam width w(z) and the power in the bucket (PIB) are studied in detail, where the effects of partial coherence, off-axis beam superposition and atmospheric turbulence are considered. The analytical expressions for the intensity, the beam width and the PIB are derived, and illustrative examples are given numerically. It is shown that the maximum intensity I_max and the PIB decrease and w(z) increases as the refraction index structure constant C_n² increases. Therefore, the turbulence results in a degradation of the beam quality. However, the resulting partially coherent beam with a smaller value of spatial correlation parameter γ and larger values of separate distance x_d and beam number M is less affected by the turbulence than that with a larger value of γ and smaller values of x_d and M. The main results obtained in this paper are explained physically.     

Holographic Image Denoising using Random Laser Illumination

In holographic applications, coherent lasers are indispensable source of illumination. Despite high intensity from coherent light sources, they fail in full-field image projection and illustrate speckle images due to high spatial coherence. This article demonstrates speckle-free high contrast computer-generated holographic image projection upon illumination with a perovskite–polystyrene 10 wt%-based random laser. Solvent-engineered efficient and durable perovskite and perovskite–polystyrene 10 wt%-based random lasers are fabricated. Optical characterizations are elucidated and controlled coherence random lasing operation is achieved under room temperature upon addition of polystyrene concentration 10 wt% on perovskite thin film. The addition of 10 wt% polystyrene concentration results in a low far-field divergence angle of ≈10⁰. The controlled coherence in random lasers is necessary to produce a stable interference pattern and to retain the depth of field in holograms. Additionally, the holographic image projection using random lasers reduces diffraction noise, and exhibits high spatial resolution with full-field imaging. Moreover, this study is clear evidence of an effective strategy to achieve high-performance, indigenous designed-controlled coherence in disordered random lasing media and its application to novel holographic image projection.     

Experimental investigation and numerical simulation of the spatio-temporal dynamics of nanosecond pulses in Q-switched Nd:YAG lasers

This paper reports on an experimental investigation and numerical simulation of the spatio-temporal dynamics of the pulse formation in flashlamp-pumped Q-switched Nd:YAG lasers. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD camera. The measurements are performed for two lasers with different cavity configurations. A laser with an optically stable resonator and an internal mode aperture generated pulses with a spatial intensity distribution which is Gaussian at all times during the 10-ns-long pulse. During the pulse evolution the value of the beam-quality factor M²remains below 1.3. In a laser with a positive-branch unstable resonator the laser pulse also starts with a Gaussian intensity distribution, but becomes rapidly non-Gaussian. The corresponding M²values increase from about 1 at the beginning of the formation of the pulse to more than 12 in the tail of the pulse. The measurements are compared with the results of a numerical simulation which takes the laser amplification, the properties of the laser cavity, and the diffraction of the beam in the laser cavity into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results.PACS 42.60.Jf     

Propagation of flat-topped beams

The propagation of flat-topped beams passing through paraxial ABCD optical system is investigated based on the propagation formulas of Gaussian beam. The focal shift of focused coherent flat-topped beam is also studied in detail. Analytical expressions of the M² factor and the far-field intensity distribution for flat-topped beams are derived on the basis of second-order moments.     

Radiation efficiency of planar Gaussian Schell-model sources

A general expression is derived for the ratio of the radiated power and the source-integrated intensity for any planar gaussian Schell-model source. The behavior of this quantity, known as the radiation efficiency of the source, is displayed graphically as a function of the rms width of the intensity profile and the spatial coherence length of the light distribution across the source. Some limiting cases are discussed and it is shown that a gaussian-correlated quasi-homogeneous source may have higher radiation efficiency than a fully coherent Schell-model source with a gaussian intensity profile (e.g. a single mode laser).     

Statistical properties in the chaotic regime of the Maxwell-Bloch equations

The statistical properties of the electric field solution of the Maxwell-Bloch equations describing a single mode, homogeneously broadened laser in the mean field limit are investigated in the strange attractor regime. The electric field distribution sis calculated and it is found that the low order intensity moments are somewhat higher than those for thermal-chaotic light whilst the higher order moments are substantially lower. The field and intensity correlation functions are also calculated and found to exhibit radically different behaviour. The results are interpreted in terms of iterative map which is dederived using multiple time-scale perturbation theory. It is shown that a simple random phasor model is compatible with the numerical data.     

部分相干源实验研究

 在无阶梯诱导空间非相干光束平滑技术中，用作前端的部分相干源是重要的组成部分。介绍了利用准分子激光进行的部分相干源实验，发现聚四氟乙烯是一种较理想的散射材料。研究了聚四氟乙烯散射源对光束的散射性能，给出了光束均匀性、空间相干性和能量转换效率等参数。     

Moments of intensity distribution of super-Gauss beam in turbulent atmosphere

Moment of intensity distribution is one of the most important parameters in studying the propagation of a beam in turbulent atmosphere. With the help of Wigner transform and the expansion of mutual coherence function (MCF) of turbulence, a relation between the moment of intensity distribution and the moment of power spectrum of refractive index fluctuations is found. The study shows that the jth-order moment of beam intensity distribution is only affected by the moment of power spectrum of refractive index fluctuations which order is not larger than j. Meanwhile, the expressions for the moment of beam intensity distribution, in which order is not larger than the fourth order are given. As a special case, the moments of the super-Gaussian beam (SGB) in turbulence are studied. The evolution of the kurtosis parameters of SGB under different condition is investigated in detail.     

Experimental generation of a partially coherent Laguerre–Gaussian beam

We report experimental generation of a partially coherent Laguerre–Gaussian beam with mode orders 0 and l (i.e., LG_0l beam) using a rotating ground-glass plate and a spatial light modulator. Focusing properties of the generated beam are studied both theoretically and experimentally. It is found that the focused intensity of the LG_0l beam is shaped through varying the initial spatial coherence, and our experimental results agree well with the theoretical predictions. Our results will be useful in particle trapping and free-space optical communications.     

Multi-wavelength,high spatial brightness operation of a phase-locked stripe-array diode laser

Stable continuous wave multi-wavelength operation of a stripe-array diode laser with an externalcavity spectral beam combining geometry is presented. In this setup each emitter of the stripe-array is forced to operate at a different wavelength, which leads to a decoupling between the usually phase-locked emitters. With a reflective diffraction grating with a period of 300 lines per mm, 33 equidistant laser lines around a center wavelength of 978 nm were realized, spanning a spectral range of 26 nm. With this novel approach near-diffraction limited emission with a beam quality of M ² < 1.2 and an output power of 450 mW was achieved. This laser light source can be used for applications requiring low temporal but high spatial coherence.     

Coherent fluorescence emission by using hybrid photonic–plasmonic crystals

The spatial and temporal coherence of the fluorescence emission controlled by a quasi‐two‐dimensional hybrid photonic–plasmonic crystal structure covered with a thin fluorescent‐molecular‐doped dielectric film is investigated experimentally. A simple theoretical model to describe how a confined quasi‐two‐dimensional optical mode may induce coherent fluorescence emission is also presented. Concerning the spatial coherence, it is experimentally observed that the coherence area in the plane of the light source is in excess of 49 μm², which results in enhanced directional fluorescence emission. Concerning temporal coherence, the obtained coherence time is 4 times longer than that of the normal fluorescence emission in vacuum. Moreover, a Young's double‐slit interference experiment is performed to directly confirm the spatially coherent emission. This smoking gun proof of spatial coherence is reported here for the first time for the optical‐mode‐modified emission.