太赫兹量子级联激光器光束特性分析

用有限元法计算太赫兹量子级联激光器激光模式的阈值增益.结果表明:接触层厚度和掺杂浓度对阈值增益的影响远远大于波导宽度和激射波长;接触层厚度较小(大)和掺杂浓度较低(高)时,TM₁(TM₀)模的阈值增益较小.在此基础上,用矢量衍射理论分析岀射光束的远场特性,得到光束的远场光斑基本是椭圆;x方向的远场散射角随波导宽度或激射波长的增加分别线性减小或增加,尽管对应的接触层厚度和掺杂浓度不同,但TM₀和TM₁模x方向的远场散射角相同;另外,还得到y方向远场散射角不受波导宽度或少受激射波长的影响.在阈值增益和光束质量方面,TM₁模都优于TM₀模.     

含间隔层的增益导引-折射率反导引平面波导激光器中高阶模式抑制研究

为了抑制高功率波导激光器中的高阶模式,改善其光束质量,基于增益导引-折射率反导引理论,在对称分层波导结构中添加了一层间隔层,进一步加大了高阶模式与基模之间阈值增益系数的差异,利用这种模式竞争抑制高阶模式.同时从波动方程出发,推导出各模式的本征方程,并给出了基模和高阶模的场分布.引入间隔层之后的高低阶模式损耗计算结果表明,通过合理地选择间隔层参数,可以有效地抑制高阶模式.     

The interaction of high- and low-frequency waves in a free electronlaser

When a free electron laser (FEL) interaction occurs inside a waveguide, there are two possible operating frequencies at the two intersections of the beam line with the waveguide dispersion curve. Using a one-dimensional model valid for a low-gain oscillator, the mode competition between the high- and low-frequency waves is studied. It is found that the low-frequency wave can be nonlinearly suppressed, even when its linear growth rate is positive. When the frequency ratio is two, phase locking occurs and the nonlinear gain of the low-frequency wave can exceed its linear growth rate     

Circular partially coherent flattened Gaussian beam

An alternative theoretical model called circular partially coherent flattened Gaussian beam (FGB) is developed to describe a circular partially coherent beam with a flat-topped spatial profile. Explicit expression for the propagation factor of a circular partially coherent FGB is derived. We drive the analytical formulae for the cross-spectral density and mean-squared beam width of a circular partially coherent FGB propagating through a paraxial ABCD optical system based on the generalized Collins formula. The intensity, spreading and directionality properties of a circular partially coherent FGB propagating in free space are studied as numerical examples. The propagation properties of a circular partially coherent FGB agree well with those of a partially coherent flat-topped beam reported in the literature. Thus, our model provides an alternative but reliable model for describing a circular partially coherent beam with flat-topped profile.     

An off-axis elliptical flat-topped beam and it’s propagation

A new kind of laser beam named off-axis elliptical flat-topped beam is proposed. This beam is expressed as a finite series of off-axis elliptical Gaussian beams with different beam parameters. Analytical propagation formulas for the off-axis elliptical flat-topped beam through aligned and misaligned optical systems are derived. As a numerical example, the propagation properties of the off-axis elliptical flat-topped beam in free space are calculated and discussed.     

Transverse Writing of Multimode Interference Waveguides inside Silica Glass by Femtosecond Laser Pulses

Multi-mode interference waveguides are fabricated inside silica glass by transverse writing geometry with femtosecond laser pulses. The influences of several writing and reading factors on the output mode are systematically studied. The experimental results of straight waveguides are in good agreement with the simulations by the beam propagation method. By integrating a straight waveguide with a bent waveguide, a 1× 2 multi-mode splitter is formed and 2×3 lobes are observed in the output mode.     

Enhanced sensitivity of Z-scan technique by use of flat-topped beam

Based on the Huygens–Fresnel diffraction integral method, we report a theoretical investigation on the closed-aperture Z-scan technique by using the flat-topped beam. The sensitivity of the flat-topped beam Z-scan technique, which can be enhanced with the increase of the flatness order N for the flat-topped beam, is greatly higher than of the Gaussian beam. Some salient characteristics of the flat-topped beam Z-scan traces are addressed. The flat-topped beam Z-scan technique for characterizing the instantaneous nonlinearity is also presented.     

Waveguide properties and optical switching of prism-coupled Au:SiO2 nanocomposite films

In this study, we present theoretical and experimental analyses on the waveguide mode properties of prism-coupled Au:SiO₂ nanocomposite films with the near infrared laser of 1550 nm wavelength where the optical absorption diminished enough for the generation of guided mode. The evolution of guided mode in the nanocomposite waveguide and its propagation properties were also evaluated. As an effective way of utilizing the surface plasmon resonance properties for the application to optical switching devices, we employed an attenuated total internal reflection type optical switch geometry and tested its effectiveness for the absorptive opto-functional materials system using a cross-modulation technique with 532-nm pump and 1550-nm probe beams. The index change probe beam experiences was found to be purely refractive in nature and negative in sign, presumably due to the photo-thermal effect induced in the nanocomposite film by the irradiation of pump beam.     

Vectorial vortex mode transformation for a hollow waveguide using Pancharatnam-Berry phase optical elements

Transformation and inverse transformation between a free-space linearly polarized beam and the vectorial vortex mode of a circular hollow waveguide by use of Pancharatnam-Berry phase optical elements is proposed. Demonstration was achieved by fabricating GaAs subwavelength gratings and utilizing a 300 microm diameter hollow metallic waveguide for 10.6 microm wavelength CO(2) laser radiation. The mode transformations and the excitation of a single vectorial mode inside the hollow waveguide were verified by full polarization measurements. In addition, the inverse mode transformation of the single vectorial mode excitation in the waveguide enabled us to experimentally obtain a linearly polarized bright spot with a high central lobe.     

LDA侧面Zigzag泵浦多边形薄片激光放大器模拟与实验研究

介绍了一种激光二极管叠阵（LDA）侧面Zigzag泵浦多边形薄片激光放大器构型,采用三维光线追迹方法进行了详细的模拟仿真,优化设计了此放大器系统的泵浦耦合结构,主要研究了多边形增益介质的掺杂离子浓度与侧面切角对介质内部泵浦光分布的影响。在晶体厚度1.5 mm、端面口径16 mm的条件下,侧面切角在35°～65°,Nd3+掺杂浓度为0.20 at.%～0.30 at.%时,模拟仿真中Nd:YAG多边形薄片对泵浦光的吸收分布较均匀,泵浦光分布均匀性均优于0.1,同时在实验中得到了平顶的荧光分布和增益分布。介质内储能的均匀平顶分布有利于实现高功率高光束质量的激光输出,为侧面Zigzag泵浦多边形薄片激光器系统的设计与进一步实验提供了重要参考。     

Focus shaping of cylindrically polarized vortex beams by a high numerical-aperture lens

The focus-shaping technique of a cylindrically polarized vortex beam by a high numerical-aperture lens is reported. Such a polarized vortex beam is decomposed into radial and azimuthal polarization. It is shown that the total intensity distribution in the focal region is dependent not only on the numerical-aperture maximal angle and the polarization rotation angle but also on the topological charge. By choosing the proper combination of parameters, the adjustably confined flat-topped focus and focal hole can be obtained. The focus-shaping technique may find wide applications, such as optical tweezers, laser printing and material processing.     

Modeling of large flattened mode area fiber lasers

Potentialities of independent tailoring the index and gain profiles in fiber laser aiming to achieve a strong modal discrimination are theoretically examined. It is demonstrated by numerical simulations existence of fiber amplifier constructions which have the flattened fundamental mode profile in the gain region. It is shown that the fundamental mode retain the largest modal gain in comparison with modal gains of higher-order-modes for any depletion of the gain by the fundamental mode. The particular design is presented with the flattened fundamental mode area 6360 μm squared.     

Analytical formulas for a circular or non-circular flat-topped beam propagating through an apertured paraxial optical system

Propagation of a flat-topped beam of circular or non-circular (rectangular or elliptical) symmetry through an apertured optical system is investigated. By expanding the hard aperture function as a finite sum of complex Gaussian functions, some approximate analytical propagation formulas are derived for a flat-topped beam of circular or non-circular (rectangular or elliptical) symmetry propagating through an apertured paraxial general astigmatic (GA) optical system or an apertured paraxial misaligned stigmatic (ST) optical system. The derived formulas are very fast to compute. The results obtained by using the approximate analytical expressions are in a good agreement with those obtained by direct numerical integration. The present analytical formulas provide a convenient and effective way for studying the propagation and transformation of a circular or non-circular flat-topped beam through an apertured general optical system.     

太赫兹波段折叠波导行波管的线性分析和计算

给出了太赫兹波段折叠波导行波管中考虑管壁欧姆损耗的带电子束扰动的色散方程。求解该方程可以得到器件中小信号增益的值。通过编制Matlab程序,分析计算了不同工作点、不同折叠波导结构参数和不同工作频段时小信号增益的变化特性。计算结果表明:适当选取工作点和结构参数,可以获得最佳增益值;考虑损耗后,除了前向增长的辐射场外,还会出现反向传输的静态波;随着工作频段向高频方向延伸,前向波的增益显著降低,而反向波强度则增大。因此,工作频率提高时,为了达到一定大小的增益,需要的折叠波导的周期数也相应增加。     

Performance Characteristics of Power Build-Up Cavity for Raman Spectroscopic Measurement

Performance characteristics of power build-up cavity (PBC) as the light source of a Raman spectroscopy based gas sensor were studied. The key parameter to optimize stable and high intra-cavity power operation was beam diameter of the back reflected beam from external cavity to diode laser. The optimum diameter determined by an appropriate distance between the cavity and diode laser was found to be comparable with the waveguide cross section of diode laser for the effective spatial filtering, where inevitable cavity coupling loss caused by slight spatial mode mismatching existed. A PBC with a finesse of ∼10300 achieved a stable TEM₀₀ mode in excess of intra-cavity power of 80 watts pumped by a 10 milliwatts diode laser. Simultaneously, the PBC wavelength is found to be passively locked effectively at 670 +/− 0.15 nm where the center of the gain region exists. A Raman spectrum of nitrogen measurement was demonstrated.     

Focusing properties of a general-type beam in turbulent atmosphere

Based on the generalized Huygens–Fresnel integral, analytical propagation formulas for a general-type beam propagating through aligned or misaligned ABCD optical systems in turbulent atmosphere are derived. The derived formulas provide a convenient way for studying the focusing properties of a variety of laser beams, such as Gaussian, cos-Gaussian, cosh-Gaussian, sine-Gaussian, sinh-Gaussian, flat-topped, Hermite-cosh-Gaussian, Hermite-sine-Gaussian, higher-order annular Gaussian, Hermite-sinh-Gaussian and Hermite-cos-Gaussian beams in turbulent atmosphere. As an application example, the focused intensities of cos-Gaussian, Hermite-sine-Gaussian and flat-topped beams in turbulent atmosphere are studied numerically. Focal shift of a flat-topped beam in turbulent atmosphere is investigated. Effect of the misalignment of the thin lens on the focusing properties of a cos-Gaussian beam is also explored. Our results will be useful for the applications of the general-type beam in LIDAR systems and remote sensing operating in turbulent atmosphere, where optical elements such as aligned or misaligned thin lens are commonly encountered.     

Optimum design of a distributed Bragg-reflector laser with optical loss in the corrugated waveguide

The distributed Bragg-reflector (DBR) laser with optical loss in the corrugated waveguide is analyzed by using the coupling mode theory. The threshold gain and the mode selectivity are given as a function of the optical loss, the coupling coefficient and the pumped length. These results show that the DBR laser has an optimum condition by considering the threshold gain and the mode selectivity.     

Mode-selective amplification in a waveguide free electron laserwith a two-sectioned wiggler

One important issue in waveguide free electron lasers (FELs) involves an interaction of the electron beam with one waveguide mode at two different resonant frequencies. Since the low-frequency mode often has a higher pain, the usually preferred high-frequency mode is suppressed as a result of mode competition. In this paper, possible control of this mode competition is considered using a nonstandard wiggler magnet consisting of two cascaded wiggler sections with different periods and field strengths. It is demonstrated that with an appropriate differentiation between the two wiggler sections the high-frequency mode may be amplified preferentially. This mode-selective amplification may be used to suppress the low-frequency mode. A small signal gain formulation is developed for a waveguide FEL with such a two-sectioned wiggler arrangement and numerical examples are used to demonstrate its applicability to mode control in waveguide FELs. Effects of wiggler field errors and electron energy spread are also considered. It is shown that the requirement for wiggler field errors and electron energy spread in the two-sectioned wiggler arrangement is similar to that in the usual straight wiggler configuration     

Laser diode integrated with a low radiation loss thickness-tapered beam expander for highly efficient fiber coupling

The optimal structure of a laser diode monolithically integrated with a thickness-tapered beam expander waveguide is demonstrated by analyzing the relationship between fiber coupling efficiencies and radiation losses. It is also found that mode conversion loss is lowered in a ridge waveguide structure than in a buried hetero structure under equivalent fiber coupling. A fabricated ridge waveguide device based on this design shows threshold current as low as 16 mA and narrow beam divergences of 13° and 12°.