Measurements and analysis of antireflection coatings reflectivity related to external cavity lasers

Facets of semiconductor optical amplifiers (SOA) designed for external cavity lasers must be coated with an antireflection (AR) film of high quality and extremely low reflectance. Therefore measurements of facet reflectance play a crucial role in the fabrication of such AR coating. The reflectance can be estimated by studying the optical power reflected from the Fabry-Perot cavity formed for that purpose by the examined SOA facet and the end of a single-mode fibre. We have made analysis of practical suitability of such measurement method. Theoretical calculations show that, for the low reflectance coatings, losses due to light coupling into optical fibre cannot be omitted in the analysis of the experimental results. To verify this conclusion a theoretical model was tested for a low reflectance surface and we have found that the relative error of the measurements supported by the theoretical model is on the order of 8%.     

Vectorial whispering gallery mode solvers based on straight waveguide modes

In this paper, whispering gallery mode solvers based on a set of corresponding straight waveguide modes are proposed. The solvers project whispering gallery modes onto a linear combination of straight waveguide modes through a cavity residual operator eigensolution formalism and direct straight waveguide expansion procedures. With these implementations, a perfectly matched layer can be imposed at the cavity computation window edges automatically and optical properties of plain and metal coated silica microtoroids can be analyzed from a novel viewpoint. Additionally, the techniques can be employed in cavity mode matching methods for the modelling of cavities that strongly scatter light.     

Modelling the excitation field of an optical resonator

Assuming the paraxial approximation, we derive efficient recursive expressions for the projection coefficients of a Gaussian beam over the Gauss--Hermite transverse electro-magnetic (TEM) modes of an optical cavity. While previous studies considered cavities with cylindrical symmetry, our derivation accounts for “simple” astigmatism and ellipticity, which allows to deal with more realistic optical systems. The resulting expansion of the Gaussian beam over the cavity TEM modes provides accurate simulation of the excitation field distribution inside the cavity, in transmission, and in reflection. In particular, this requires including counter-propagating TEM modes, usually neglected in textbooks. As an illustrative application to a complex case, we simulate reentrant cavity configurations where Herriott spots are obtained at cavity output. We show that the case of an astigmatic cavity is also easily modelled. To our knowledge, such relevant applications are usually treated under the simplified geometrical optics approximation, or using heavier numerical methods.     

High Q vertical surface emitting photonic crystal surface-mode cavities with periodic perturbation

We introduce a periodical perturbation to realize photonic crystal vertical cavity surface emitting laser cavities based on surface modes (PCSM-VCSEL) and to enhance their quality factor. Different from the early reported direct periodical modulation to the cavity surface, a periodical perturbation under the surface can also result in the vertical emission due to the beaming effect. It is more important that this perturbation causes less energy diffraction loss or light output because the perturbation is imposed on not the center but the tail of the field of surface modes. Therefore, this modulation method greatly improves the quality factor of such PCSM-VCSEL cavities, which is favorable to their realization in practice. The numerical results demonstrate their characteristics and good performances in the application as light sources.     

高速850 nm垂直腔面发射激光器的优化设计与外延生长

利用传输矩阵理论和TFCalc薄膜设计软件分析了分布布拉格反射镜和垂直腔面发射激光器(VCSEL)的反射率谱特性,对比了从谐振腔入射与从表面入射时反射率谱的差异,为白光反射谱表征VCSEL外延片提供了依据.利用Crosslight软件模拟了InGaAs/AlGaAs应变量子阱的增益谱随温度的变化特性及VCSEL器件内部温度分布,设计了增益-腔模调谐的VCSEL.采用金属有机物化学气相淀积设备外延生长了顶发射VCSEL,制作了氧化孔径为7.5μm的氧化限制型VCSEL器件,测试了器件的直流特性、光谱特性和眼图特性;6 mA,2.5 V偏置条件下输出光功率达5 mW,4级脉冲幅度调制传输速率达50 Gbit/s.     

由光子晶体局域模对称关系分析其简并性

使用平面波展开法计算了二维正方格子介质柱光子晶体在改变中央介质柱半径的情况下谐振腔的模式，根据所计算的局域模式场分布的对称关系，研究了二维光子晶体谐振腔模式的简并性.结果表明：具有C4v对称群光子晶体谐振腔，存在着二重简并偶极模和二重简并六极模，同时也存在非简并单极模和四极模，二重简并模的90°旋转态与其形成简并对.模式的简并与非简并可以直观地通过考察模式场分布的对称关系而得到.     

Vertical cavity-surface emitting photonic crystal surface-mode lasers

We present the research of a type of vertical cavity-surface emitting photonic crystal laser with a periodically modulated surface. The proper modulation to the cavity surface changes nonradiative surface modes into radiative ones and the constructive interference of the radiative waves results in the vertical surface emission. The numerical investigation of such cavities shows that, for a cavity with a definite length, resonant frequencies and their quality factors mainly depend on the magnitude of the corrugated surface. Furthermore, the simulation of the lasing processes demonstrates the features of this type of laser, such as vertical surface emitting, flat wavefront and single mode operation which is robust.     

Design and simulation of a new type of 500 MHz single-cell superconducting RF cavity

This paper illustrates the design and simulation of a unique 500 MHz single-cell superconducting radio frequency cavity with a fluted beam pipe and a coaxial-type fundamental power coupler. The simulation results show that the cavity has a high r/Q value, a low peak surface field and a large beam aperture, so it can be a candidate cavity for high current accelerators. With the help of a fluted beam tube, almost all the higher order modes can propagate out of the cavity, especially the first two dipole modes, TE₁₁₁ and TM₁₁₀, and the first higher monopole mode, TM₀₁₁. The external quality factor of the coaxial fundamental power coupler is optimized to 1.2×10⁵, which will be useful when it is applied in the light source storage ring.     

HOMs simulation and measurement results of IHEP02 cavity

In accelerator RF cavities, there exists not only the fundamental mode which is used to accelerate the beam, but also higher order modes(HOMs). The higher order modes excited by the beam can seriously affect beam quality, especially for the higher R/Q modes. 1.3 GHz low-loss 9-cell superconducting cavity as a candidate for ILC high gradient cavity, the properties of higher order mode has not been studied carefully. IHEP based on existing low loss cavity, designed and developed a large grain size 1.3 GHz low-loss 9-cell superconducting cavity(IHEP02cavity). The higher order mode coupler of IHEP02 used TESLA coupler's design. As a result of the limitation of the mechanical design, the distance between higher order mode coupler and end cell is larger than TESLA cavity.This paper reports on measured results of higher order modes in the IHEP02 1.3 GHz low-loss 9-cell superconducting cavity. Using different methods, Q e of the dangerous modes passbands have been obtained. The results are compared with TESLA cavity results. R/Q of the first three passbands have also been obtained by simulation and compared with the results of the TESLA cavity.     

光学微球腔的壳层结构研究

使用时域有限差分法,对带表面壳层的光学微球腔进行模拟计算,分析了微腔内回音壁模式的能量密度分布,总结了壳层厚度对微球腔谐振性能的影响,并由此探讨对微球腔进行调谐和模式控制的方法.采用这种壳层结构,微球腔的谐振性能得到了有效提高（Q值提高了30％以上,模式体积减小了60％）,为光学微球腔后续的结构设计和实际应用提供了一个新的优化思路.     

Surface combustion of gas lean mixture in a slot cavity

Critical conditions for the transition of surface combustion into “blue flame” mode for lean mixtures on an open permeable matrix and in a slot cavity were experimentally and theoretically investigated. Temperatures of the matrix surface and combustion products were computed depending on the power density of combustion. The boundary of the region of steady-state surface combustion was determined. The role of a radiation shield in the expansion of the given region was shown. The limiting modes of surface combustion in the slot cavity were experimentally investigated in an arrangement over a matrix of a heat-insulated radiation shield with a smooth or rough surface. The use of the rough shield was shown to result in a substantial expansion of the region of steady-state combustion. Surface combustion of the lean mixtures was found to be possible at an excess air ratio of 2 and a limiting power density of ～20 W/cm². With an increase in the power to 60 W/cm², the excess air ratio decreases to 1.6. The results of experiments are compared with computations and show satisfactory coincidence with them.     

Geometrical perturbation of an inclined wall on decay times of acoustic modes in a trapezoidal cavity with an impedance surface

Decay times of acoustic modes of a trapezoidal cavity (TC modes) with an inclined wall are studied. Each cavity wall is successively assigned an impedance surface and the other five walls are rigid. The decay times are obtained from the coupling between rigid-walled modes of the rectangular cavity (RC modes) that bounds the trapezoidal cavity. Two coupling mechanisms are identified, namely, the damping coupling and the geometrical coupling. The former is related to the coupling of RC modes at the impedance surface, while the latter is related to the coupling of RC modes at the inclined wall. Both mechanisms include the same volume coupling where RC modes couple throughout the trapezoidal cavity. When the impedance surface is at either of the two trapezoidal walls, the grouping of TC modes with same decay times and the decay time variation with the wall inclination are determined only by the damping coupling. When the surface is at any of the other rectangular walls, both the damping and geometrical couplings are at work. This paper provides an understanding of how the inclined wall and the impedance surface location affect the TC-mode grouping, and what determines the decay time variation with the inclination.     

Optical surface modes of photonic crystals for dual-polarization waveguide

In this study, the design of a polarization-independent (dual-polarization) waveguide is presented by utilizing surface modes of photonic crystals. The waveguide structure operates in a frequency interval that is commonly shared by both transverse-electric (TE) and transverse-magnetic (TM) polarizations. The numerical calculations based on plane wave expansion and finite-difference time-domain methods are carried out to design and demonstrate a surface mode waveguide that provides confinement and guiding for both TE and TM modes. Once the relevant modes are properly excited, the high transmission efficiency of the photonic crystal surface waveguide is ensured. The demand to have polarization-insensitive devices makes our proposed design an important component for the photonic integrated circuit applications. Finally, we also propose a broadband surface mode photonic crystal waveguide with a bandwidth value of 28% for only TE polarization.     

Lateral shift dependence of spontaneous emission in a planar cavity with perfect conducting cladding

We consider the effect of lateral shift of the “beams" resulting from the atom emissions on spontaneous decay of an excited atom embedded in a planar cavity with perfect conducting cladding. It is found that the spontaneous decay could be enhanced or suppressed owing to the positive and negative lateral shift at the interfaces of the cavity mirror. Compared with the emission between two perfect conducting plates, the surface guided modes could exist and for very thin planar cavity, the density of surface guided modes may be obviously larger than those of the propagating guided modes.     

Different optical properties in different periodic slot cavity geometrical morphologies

In this paper,optical properties of two-dimensional periodic annular slot cavity arrays in hexagonal close-packing on a silica substrate are theoretically characterized by finite difference time domain(FDTD) simulation method.By simulating reflectance spectra,electric field distribution,and charge distribution,we confirm that multiple cylindrical surface plasmon resonances can be excited in annular inclined slot cavities by linearly polarized light,in which the four reflectance dips are attributed to Fabry–Perot cavity resonances in the coaxial cavity.A coaxial waveguide mode TE11 will exist in these annular cavities,and the wavelengths of these reflectance dips are effectively tailored by changing the geometrical pattern of slot cavity and the dielectric materials filled in the cavities.These resonant wavelengths are localized in annular cavities with large electric field enhancement and dissipate gradually due to metal loss.The formation of an absorption peak can be explained from the aspect of phase matching conditions.We observed that the proposed structure can be tuned over the broad spectral range of 600–4000 nm by changing the outer and inner radii of the annular gaps,gap surface topography.Meanwhile,different lengths of the cavity may cause the shift of resonance dips.Also,we study the field enhancement at different vertical locations of the slit.In addition,dielectric materials filling in the annular gaps will result in a shift of the resonance wavelengths,which make the annular cavities good candidates for refractive index sensors.The refractive index sensitivity of annular cavities can also be tuned by the geometry size and the media around the cavity.Annular cavities with novel applications can be implied as surface enhanced Raman spectra substrates,refractive index sensors,nano-lasers,and optical trappers.     

Coupling of cavity modes and guiding modes in two-dimensional phononic crystals

Cavity modes for the single-cavity and the double-cavities in two-dimensional phononic crystal are studied by using a plane waves expansion supercell method. We show that the single-cavity behaves like an atom while the double-cavities behave like a molecule. Couplings of cavity modes with the waveguide modes are investigated for typical configurations. Suitably designed waveguides with introduced cavities might be used as acoustic filter in waveguide applications.     

Surface-structure-assisted chaotic mode lasing in vertical cavity surface emitting lasers

We demonstrate chaotic mode lasing in vertical cavity surface emitting lasers at room temperature, with an open cavity confined laterally by the native oxide layer. Instead of introducing any defect mode, we show that suppression of lower-order cavity modes can be achieved by destroying vertical reflectors with a surface microstructure. Lasing on chaotic modes is observed directly through collecting near-field radiation patterns. Various vertical emission transverse modes are identified by the spectrum in experiments as well as numerical simulations in real and phase spaces.     

Fiber taper coupled high-quality-factor planar microdisk

A fiber taper can evanescently couple to whispering gallery modes (WGMs) in a planar silica microdisk for observing the optical properties of the microdisk cavity. It is revealed that WGMs have very high quality (Q) factors by controlling the air gap between the taper and the microdisk. The best coupling efficiency from the taper to the microdisk is as high as 98%, and can be continuously adjusted from the under-coupling, critical-coupling to over-coupling regimes. The influence of the laboratory circumstance such as surface contamination on the microdisk is also discussed. It is experimentally shown that the high-Q-factor (105) modes can be kept for a long period in a general laboratory circumstance.     

水平腔面发射半导体激光器研究进展

近年来,水平腔面发射半导体激光器具有高功率、高光束质量及易封装集成等优良性能,已成为激光器领域的研究热点。本文详细阐述了几种水平腔面发射半导体激光器的结构设计、工作原理以及激光输出特性,并对该激光器国内外最新研究进展与发展现状进行了总结和论述。在此基础上,对该激光器的研究方向和发展趋势进行了分析与展望。目前,水平腔面发射半导体激光器的激光输出功率可达瓦级,美国Alfalight公司引入曲线形光栅的单一发射器输出功率可达73 W。随着应用领域的不断拓展,中远红外波段水平腔面发射激光器将成为未来的研究焦点。     

Network-theoretical model of the gyrotron oscillator part I: Empty cavity oscillation modes

A network-theoretical model of the gyrotron has been elaborated which is both conspicious and rigorous. The problem of self-consistently representing the field in the presence of the electron beam is attacked by expansion into the empty structure oscillation modes. In this first part, a method is presented to determine the empty gyrotron cavity oscillation modes, i.e. theQ factors, resonance frequencies, and field distributions in axisymmetric, but otherwise quite general cavities. It is based on the field representation in terms of local normal waveguide modes. Matrix equations in these modes for different type of cavity sections (building blocks) are formulated. Along with the open resonator boundary conditions, these equations form a nonstandard eigenvalue problem; the eigenvalues are the complex eigenfrequencies, the eigenvectors correspond to the field distributions. By way of example, the method is applied to a simple and a complex cavity.