Finite-aperture waveguide-laser resonators

A general theory of finite-aperture waveguide-laser resonators is developed which represents the external reflectors by matrices which couple linearly polarized waveguide modes having the same azimuthal symmetry. The theory allows the determination of resonator efficiency, resonator frequencies, and laser near- and far-field patterns. Computations of the coupling loss for the fundamental waveguide mode as a function of mirror curvature, separation, and aperture are in good agreement with recent infinite-aperture calculations in the limit of large apertures and indicate three low-loss configurations: 1) large radius of curvature mirrors close to the guide; 2) "large" radius of curvature mirrors centered at the guide entrance: and 3) generally smaller curvature mirrors separated by half their curvature from the guide entrance. The importance of higher order waveguide modes in determining laser output power and far-field patterns is demonstrated experimentally and compared to theoretical predictions. Design guidelines for the construction of high-efficiency CO2, CO, and He-Ne waveguide-laser resonators are summarized in tabular form.     

Cold tests and modeling of low-Q quasi-optical resonators

Measurements are reported on low-Q open resonators at 85 GHz with a large centered coupling hole in each mirror. This is the first study of open resonators with large diffraction losses and efficient input coupling at millimeter wavelengths. The input resonator for a quasi-optical gyroklystron is described with quality factor Q=2000 and 13% round-trip losses where approximately 65% of the incident power in the waveguide is coupled to the fundamental TEM₀₀ mode. Two models have been developed to calculate the coupling from the rectangular waveguide through a circular aperture into the open resonator. Both models properly account for the increased diffraction loss of the resonator when coupling apertures are present. Measured results are in reasonably good agreement with values calculated using both scalar diffraction theory and an equivalent-circuit model     

A broad-band waveguide grating coupler with a subwavelength grating mirror

A new diffractive device for light coupling between a planar optical waveguide and free space is proposed. The device utilizes a second-order waveguide grating to diffract the fundamental waveguide mode into two free propagating beams and a subwavelength grating (SWG) mirror to combine the two free propagating beams into a single beam. The finite-difference time-domain (FDTD) simulations show that the SWG mirror improves the coupling efficiency of the waveguide fundamental mode into the single out-coupled beam from about 30% to 92%. A high efficiency (>90%) is predicted for a broad wavelength range of 1520-1580nm. The proposed device is compact (/spl sim/80 /spl mu/m in length) and it eliminates the need for blazing the waveguide grating.     

一种提高波导CO_2激光器支线分辨能力的方法

一、前言 目前,波导CO_2激光器大多采用红外金属光栅(光栅常数d～1/(150))进行选支,光栅分辨本领不高往往要限制最大可调谐频率范围。光栅分辨本领正比于光斑所复盖的光栅条纹数N。波导激光器光斑较小,因而分辨本领不高,改变腔长容易出现支线跳变,从而也就限制频率调谐范围。通常是采用图1方法来解决这一问题。即将焦距f=b的凸透镜置于Z=f=b处,光栅紧挨透镜放置。其中     

Analysis of integrated optical corner reflectors using a finite-difference beam propagation method

Integrated optical corner reflectors in III-V semiconductors are analyzed employing a finite-difference beam propagation method and propagating the beam in parallel with the etched semiconductor-air interface. For this choice of propagation direction, the effects of mirror roughness, rotation, and displacement of the mirror surface from its ideal position can be assessed very easily. The integrated reflector whose mode size is larger shows less dependence on the mirror displacement error. The loss due to mirror surface roughness depends weakly on the mode size and strongly on the mode polarization, being larger for the quasi-transverse-electric polarization. The loss due to rotational errors of the mirror surface is not a strong function of polarization, but increases as the waveguide width increases. However, for a rotation error smaller than 0.1 degrees , which should be achieved easily, the excess loss is smaller than 0.2 dB at 1.3 % mu m regardless of the waveguide width.<>     

方形波导激光器的反射耦合研究

针对方形波导激光器的反射耦合问题提出了一种采用柱面反射镜的新型耦合方式。应用衍射理论详细推导了柱面反射镜和球面反射镜对方形波导激光器的耦合效率。通过数值计算分别得到采用球面反射镜时,EH11模和EH21模的耦合效率对反射镜曲率半径R的关系曲线,以及采用柱面反射镜时EH11模,EH21模和EH12模的耦合效率曲线。通过对比,得到波导口到反射镜距离d较小的时候,柱面反射镜对EH11模具有与球面反射镜接近的耦合效率。同时相对球面反射镜,柱面反射镜对EH21模和EH12模具有更强的抑制作用。在d较大时,相对球面反射镜柱面反射镜对EH21模的抑制作用将会变得更强,而对EH12模的抑制作用将会减弱。     

聚合物PMMA/DR1条型单模光波导传输损耗的测量

本文用光漂白法制备出了有机聚合物PMMA/DR1条形单模光波导 ,分析了波导损耗产生的各种来源。采用了一种简便的优化端面耦合法来消除入射波与导波模式间失配引起的损耗 ,以此来测量波导的其它损耗     

The Design of Evanescent Mode Waveguide Bandpass Filters for a Prescribed Insertion Loss Characteristic

A design theory for evanescent dominant mode waveguide filters is presented. The cutoff guide is represented by its equivalent /spl pi/-section network, the elements of which closely approximate lumped inductances. Resonators may be formed by introducing appropriate capacitive obstacles at suitable intervals along the guide. The filter that results is a microwave analog of lumped inductance filters with series inductance coupling, the magnitude of the series "inductance" being controlled by the separation between capacitive obstacles. The theory derives the equivalent ladder network from the low-pass prototype and is accurate up to at least 20 percent band-widths. Filters may be designed to couple into other cutoff waveguide components using the same basic principles, a propagating guide, and coaxial terminations. The filters are normally constructed in standard production waveguide and are simple and cheap to manufacture. A considerable size and weight reduction, compared with orthodox waveguide filters, is possible at the cost of a small increase in insertion loss.     

Generation of millimeter wave Gaussian beams and their propagation in a dielectric waveguide

Linearly polarized fundamental mode Gaussian beams were generated and coupled to a hollow circular oversized dielectric waveguide by placing the waist of the beam at the guide entrance. The transmission properties of the waveguide were characterized as a function of frequency for a variety of coupling conditions. These conditions included changes in the input beam waist radius, angle of incidence, and displacement perpendicular and parallel to the guide axis. It has been found that: 1.) power transmission is maximized when the waist of the input beam is centered at the guide input, injected normally, and has a radius of 0.43 times the waveguide radius, 2.) power transmission decreases rapidly with increasing angle of incidence and the rate of that loss increases with frequency, 3.) the waveguide preserves the linear polarization of the input beam, 4.) power transmission in the fundamental waveguide mode is not greatly affected by moderate displacements in the input beam position, and 5.) upon exit from the waveguide the launched EH₁₁ mode propagates as a fundamental mode Gaussian beam in the quasi-far field. The results compare favorably to the transmission theory of Belland and Crenn and approximately to the near and far field mode pattern theory of Degnan.     

Excitation of an open resonator coupled with a waveguide via a slot in a mirror

A technique and results of the solution of the problem on excitation of an open mirror resonator coupled with a waveguide via a slot in one of the mirrors are presented. The effect of the coupling slot on the radiation loss is analyzed. The results obtained with the help of a rigorous approach and approximate techniques are compared.     

Design of Dielectric Ridge Waveguides for Millimeter-Wave Integrated Circuits

All-dielectric ridge waveguides may be useful as elements of millimeter- and submillimeter-wave integrated circuits; A planar metallic V-coupler can be used to couple energy between the guide and small circuit elements such as diodes. Desirable characteristics in such a guide/coupler system are a) quasi-single mode propagation; b) low radiation loss in bendy c) low coupling loss between guide and devices and d) adequate physical strength. In this paper, we discuss the general problem of designing guides and couplers to obtain the desired characteristics. The principal method used is simulation in the range 2-7 GHz. We find that with good compromise designs, typical coupling loss between waveguide and a small device is about 1.4 dB, exclusive of dielectric loss and ohmic loss in the coupler.     

Coupled-mode theory of unstable resonators

Although there has been a sustained interest in unstable open resonators for high-power laser applications, no satisfactory theory has previously been developed to explain the intricate behavior of the complex eigenvalues obtained by numerical solution of the resonator integral equation. The present study proposes a remarkably simple model that predicts the basic features of the published results. In this model, a two-dimensional resonator with finite convex mirrors is regarded as an open-ended waveguide with hyperbolic boundaries. Through use of the modal fields in the waveguide region and modal reflection and coupling coefficients due to the open end, one may construct a resonance equation for the eigenvalues of the free resonant fields (eigenmodes) in the unstable resonator. From a study of the resonance equation as a function of mirror size, it may be concluded that a single waveguide mode is primarily responsible for the eigenmode behavior near a loss minimum, and that two selectively coupled waveguide modes provide the behavior in the intervals between loss minima. The selection of the appropriate waveguide modes is accomplished with the aid of a resonance chart for the simpler problem wherein mode coupling at the open waveguide end is omitted. Results are presented and compared with the earlier literature.     

Reflection and transmission of a dielectric waveguide mirror

A numerical analysis of a generalized dielectric waveguide mirror of arbitrary structure angle is presented. A plane wave expansion technique is used to calculate output angle, power transmission, inter-waveguide and intra-waveguide coupling, far-field emission profile and losses of the waveguide mirror. In order to evaluate the effect of divergence, two methods are used and the result indicate that the effect is minimal. Furthermore, the introduction of a tilt in the mirror surface is examined and reveals a reduction in the inter-waveguide coupling as the tilt is increased. However, this coupling remains symmetric in tilt despite the asymmetry of the geometry due to the tilt. Results of the output angle and the FWHM (full-width half-maximum) of the emission profile are compared to experimental values obtained for a semiconductor triangular ring laser     

The Comparison Between the Graded Photonic Crystal Coupler and Various Couplers

The coupling between a dielectric waveguide and a photonic crystal waveguide is studied theoretically and experimentally. The coupling methods studied in this work include graded photonic crystal coupler, photonic crystal taper, waveguide taper, and parabolic mirror coupler. The properties of each coupler are discussed and compared.      

Analysis of a multiple-beam waveguide for free-space transmissionof microwaves

The transmission of several quasi-optical beams via one system of oversized focusing mirrors is discussed. The analysis of this multiple-beam waveguide shows that such a system can substantially reduce the total mirror surface compared to the corresponding number of individual lines. With the help of analytic coupling coefficients for Gaussian beam modes deduced from the coupling integrals, configurations of four mirrors are found, which give low losses due to the reconversion of spurious modes generated at the different mirrors. Optimization of the mirror surfaces as a function of system parameters is performed and expressions for the loss as a function of the number of transmission channels are deduced. The results are confirmed by diffraction calculations and laboratory scale experiments. The application to a high-power system promises a very efficient transmission with high mode purity     

3kW射频板条CO2激光器抛物面非稳波导腔研究

为了提高传统球面镜腔3kW射频板条CO2激光器光束质量,设计了新型抛物面镜负支非稳-波导混合腔;结合标量光束的瑞利-索末菲衍射理论与特征向量法,研究了大菲涅耳数下镜面类型对非稳波导混合腔光场传输特性的影响;利用矩形波导理论、1维近似分析得出了波导传输损耗及偏振情况。结果表明,将非稳方向输出镜和尾镜改为与光轴交点处曲率半径为R1=951.32mm和R2=1088.68mm的抛物镜面,能够避免尾镜漏光,可有效改善模式鉴别特性;常用波导电极镀膜材料中,镀铝电极内波导方向传输损耗最小,传输光场为x偏振的EH1模。采用电极镀铝的抛物面非稳波导腔能够降低损耗、提升光束质量。     

多孔耦合型太赫兹波导定向耦合器的设计

设计了一种结构紧凑、工作频带较宽、耦合平稳、高方向性的十字形多孔耦合的太赫兹波导定向耦合器。基于多孔耦合原理,利用HFSS软件对太赫兹波导定向耦合器进行了模型仿真和结构优化。仿真结果表明:在325~475 GHz带宽范围内,该多孔耦合太赫兹波导定向耦合器耦合度达到7.50.8 dB,隔离度达到30 dB,即方向性优于20 dB,各端口回波损耗小于-20 dB。通过对该波导定向耦合器进行高温高压模拟仿真,确定了使用负性光刻胶SU-8作为结构材料的可行性,提出应用MEMS工艺在硅衬底上进行加工,将牺牲层工艺应用到波导腔结构的制作中。利用光刻在直通波导和耦合波导公共宽壁上形成的十字形等间距排列耦合孔结构,可以实现较宽的带宽和良好的耦合平坦度。该方法提高了耦合孔尺寸和位置的精度,减小了反射损耗,为太赫兹波导结构的加工提供了新思路。     

Fully Embedded Board-Level Optical Interconnects From Waveguide Fabrication to Device Integration

This paper presents the latest progress toward fully embedded board-level optical interconnects in the aspect of waveguide fabrication and device integration. A one-step pattern transfer method is used to form a large cross-section multimode waveguide array with 45deg micromirrors by silicon hard molding method. Optimized by a novel spin-coating surface smoothing method for the master mold, the waveguide propagation loss is reduced to 0.09 dB/cm. The coupling efficiency of the metal-coated reflecting mirror, which is embedded in the thin-film waveguide, is simulated by an M² factor revised Gaussian beam method and is experimentally measured to be 85%. The active optoelectronic devices, vertical surface emitter lasers and p-i-n photodiodes, are integrated with the mirror-ended waveguide array and successfully demonstrate a 10 Gbps signal transmission over the embeddable optical layer.     

Regrowth-free waveguide-integrated photodetector with efficienttotal-internal-reflection coupling

A novel technique utilizing an etched total-internal-reflection mirror for vertical waveguide-to-photodetector coupling is described. An 83% quantum efficiency and 5-GHz bandwidth are achieved at λ=0.84 μ in a GaAs MSM photodetector illuminated by a low-loss (0.5 dB/cm) AlGaAs double-heterostructure (DH) waveguide. A significant advantage of total-internal-reflection coupling is its compatibility with the integration of low-loss active DH waveguide components without requiring epitaxial regrowth. This coupling technique is independent of both optical wavelength and polarization, and permits efficient waveguide coupling to extremely small-area photodetectors     

Coupling of an open resonator and a rectangular waveguide through a smooth waveguide transition

Under investigation is a coupling element for coupling an open resonator to a rectangular waveguide and consisting of a smoothly irregular waveguide section conjugate to a narrow slot in a mirror, A technique for the determination of the coupling Q-factor is proposed and good agreement between calculations and experimental results is obtained. Based on such a coupling element, an open resonator has been well matched with a waveguide and made retunable in a wide frequency range without coupling adjustment..