The impact of CMP and underlying back-end topographical features on losses in deposited dielectric waveguides

The bending losses in rectangular dielectric waveguides deposited on a chemical mechanical polished (CMP) surface above the metal interconnect/interlayer dielectric stack of a processed Si wafer are modeled and estimated. CMP efficiently removes local topography and microroughness, but leaves long-range surface profile undulations due to variations in the metal pattern density. These surface undulations are then transferred to the waveguides deposited on this surface. A beam propagation method (BPM) waveguide simulation program and an equation for bending loss developed by Marcuse have been used to examine the bending losses seen by waveguides deposited on such a surface. In order to simplify the simulation of the bending losses of the waveguides, the undulations are modeled as a series of arcs, which is shown to be a good approximation. It was determined that under typical conditions, the bending losses may be ignored as they are less than 0.1 dB/cm, which is below the range of typical propagation losses for a straight guide.     

GaAs/GaAlAs curved rib waveguides

Curved dielectric optical waveguides suffer from radiation loss due to bending. To minimize the bending loss and reduce the radius of curvature, it is necessary to fabricate guides which provide strong optical confinement. This paper gives a brief review of curved waveguide analysis and presents some experimentally measured loss values for GaAs/GaAlAs curved rib waveguides. The rib waveguides, fabricated using ion beam milling, have a large rib height and are tightly guided structures. When corrected for reflection losses and input coupling efficiency, a minimum loss of approximately 3 dB has been achieved for a multimode 90° curved guide with a radius of curvature of 300 μm, and 8.5 dB for a single-mode curved guide with a radius of curvature of 400 μm. It is believed that most of this residual loss is not radiation loss due to bending, but rather scattering loss due to rib wall imperfections.     

Scattering loss of antiresonant reflecting optical waveguides

The scattering loss of two-dimensional antiresonant reflecting optical waveguides (ARROW) and of ARROW-B, which has a similar structure to ARROW and less polarization dependence, are analyzed by the first-order perturbation theory. Calculated results are compared with those of conventional three layer waveguides. Optimum design for the reduction of scattering loss of these ARROW-type waveguides is discussed. It was found that the scattering loss of ARROW-type waveguides is no larger than that of a conventional waveguide having a relative refractive-index difference, Δ of 2.5%, despite each interface of ARROW-type waveguides having a large Δ, normally larger than 20%. The optimum design for the reduction of essential radiation loss of ARROW is also optimum for the reduction of scattering loss     

Study and Design of Spiral Bent Waveguide Configuration

A new version of the scalar transverse electric（TE） wave equation in the bent waveguide is introduced. Then. TE polarized field in curved single-mode waveguides is analyzed by using the finite- difference beam propagation method（FD-BPM）. The bending loss in bent waveguides is gotten for the optical fields obtained from BPM and comparisons are made among losses of the waveguides with various curvature radiuses, refractive index differences and cross sections. Based on the results, the design of spiral bent waveguide configuration is proposed as follows： refractive index difference being of 0. 007, both width and thickness of waveguides being of 6 μm, the curvature radius in the spiral centre being of 4 mm, and the bending loss coefficient of the designed spiral bent waveguide being of 0. 302 3 dB/cm.     

Estimation of rib waveguide radiation losses

An estimative procedure given recently for approximating radiation losses from indiffused waveguides is adapted to rib waveguides and compared with a recent Huygens-type solution for bending loss. The implications for the wider application of the estimative procedure are favourable     

Amorphous silicon rich silicon nitride optical waveguides for high density integrated optics

Amorphous silicon rich silicon nitride optical waveguides clad in silica are presented as a high-index contrast platform for high-density integrated optics. Performance of different cross-sectional geometries have been measured and are presented with regards to bending loss and insertion loss. A sample double ring add-drop filter is presented.     

High-silica single-mode optical reflection bending and intersecting waveguides

Single-mode high-silica optical reflection bending and intersecting waveguides are examined under conditions of large bending and intersecting angles (5?90 degrees). The reflection bending waveguide shows a loss of 2 dB at the bending angle of less than 50 degrees. When the intersecting angle was 30 degrees or more, a small intersecting loss (less than 0.1 dB) and a low crosstalk (less than ?20 dB) were attained.     

GaAs spiral optical waveguides for delay-line applications

Spiral optical waveguides can be used to provide moderately long delays for a variety of microwave applications. A brief analysis of the performance limitations of spiral delay lines is presented, together with loss measurement data for spirals having air-equivalent lengths as long as 46 cm. It is shown that the present limitation to longer delay lengths is the straight-guide loss of GaAs/AlGaAs waveguides, and not bending or intersectional losses.<>     

基于贝塞尔函数数值求解平面光波导的弯曲损耗

为了数值求解平面光波导的弯曲损耗,首先给出了平面弯曲光波导的传输模型。基于这种模型,使用基于求解贝塞尔函数的方法数值求解出平面光波导的弯曲损耗。为了验证该方法的准确性,将计算得到的弯曲损耗结果与以往的实验结果相比较,结果表明,在弯曲半径较大的情况下两者基本吻合。     

Dispersion and radiation loss characteristics of antiresonantreflecting optical waveguides-numerical results and analyticalexpressions

Propagation characteristics of novel single-mode slab waveguides, ARROW (antiresonant reflecting optical waveguides), ARROW-B, and their modified configurations, are investigated. The dispersion and radiation loss characteristics, field profile, confinement factor, and spot size of these ARROW-type waveguides are analyzed in detail by a mode analysis method using the system interference matrix. Approximate expressions for these characteristics are obtained in simple forms, which provide useful suggestions for the design of these waveguides. Possible combinations of materials and design conditions of ARROW-type waveguides are discussed on the basis of these results     

两种不同Y分支光波导的弯曲损耗研究

针对低损耗S型弯曲Y分支的分析与设计,分别用弯曲损耗理论和有限差分光束传输法分析了两种不同的S型弯曲Y分支的损耗特性,比较了不同的波导宽度、分支间距和分支张角对波导输出损耗特性的影响,得出了一组最优化的Y分支波导设计参数.     

Analysis of N×N passive optical starcoupler based on the normal modes of N input waveguides

An integrated passive N×N optical star coupler on silicon wafer is described. Antiresonant reflecting optical waveguides (ARROWs) are analyzed and utilized as the input and output waveguides of the N×N coupler. Combining the exact solutions of the slab ARROW waveguide with the effective index method, a 5×5 coupler is analyzed. In the slab waveguide analysis, the input waveguides are coupled to their neighbors. The interaction of the waveguides is described in terms of the normal modes of propagation. The resultant field distribution is then diffracted into the free space region which separates the input and output sections. The radiation illuminates the receiving aperture from which the receiving N waveguides branch out, each output element obtaining equal power levels. Different types of loss such as spillover loss and mismatch loss were analyzed and estimated for N=5. A 5×5 star coupler with a transmission efficiency of 56% at a wavelength of 1.3 μm is achievable     

A novel hybrid coupler based on antiresonant reflecting opticalwaveguides

A novel hybrid coupler based on the antiresonant reflecting optical waveguides (ARROWs) is presented. This device consists of two parallel antiresonant reflecting optical waveguides with a tapered outermost cladding layer. Such a device can provide advantageous features of low radiation loss, low crosstalk at the output end, and compatible core dimensions and indexes with single-mode optical fibers. The staircase approximation and the eigenmode expansion analysis are used to analyze and optimize this device. In our design on a Si-substrate, radiation losses below 0.38 dB and extinction ratios below -20 dB for two waveguide channels are achieved. The beam propagation method (BPM) is also used to verify our design and analysis results     

Scalar BPM analyses of TE and TM polarized fields in bent waveguides

We have applied the effective index method to reduce the two-dimensional (2-D) refractive index profile into the 1-D refractive index structure and modified the wave equations to obtain the paraxial wave equations. Then, transverse electric (TE) and transverse magnetic (TM) polarized fields in the curved single-mode planar waveguides are analyzed by using the scalar beam-propagation method (BPM) employing the finite-difference method with a slab structure. The bending loss in bent waveguides is analyzed for optical fields obtained from the BPM and comparisons are made between the loss for the waveguides with various radius of curvature and refractive index difference. The outward shift of the optical field, which is generated at the connection between a straight and a bent waveguide, is obtained from the results of calculation of location of the maximum optical intensity. The transition loss can be reduced by introducing an optimized inward offset at a straight-to-bend junction. The birefringence for TE and TM polarized fields in bent waveguides is calculated from the phase difference of the optical fields. The wavelength shift due to the birefringence of TE and TM polarized fields in bent waveguides is also calculated.     

Optical loss measurements on GaAs/GaAlAs single-mode waveguideY-junctions and waveguide bends

The propagation losses in GaAs/GaAlAs double heterostructure single-mode waveguide symmetrical Y-junctions were measured as a function of the full angle α of the Y-junction. It was found that it was possible experimentally to identify whether the Y-junction waveguide was single mode. The bending loss in bent waveguides, due to the change in the axial direction of the waveguide, was measured as a function of the bending angle α/2. It was also experimentally confirmed that the bending loss was significantly reduced by increasing the lateral optical field confinement, i.e., by increasing the etch depth of the rib waveguide     

A novel design and analysis of low loss abrupt bends of dielectricslab waveguides

On the basis of the well-known laws of reflection and refraction of the plane electromagnetic waves, a concept for designing very low loss nonadiabatic junctions between tilted straight sections of dielectric waveguides is investigated. Combining the ray representation of the propagating modes with the equivalent source formulation of the dielectric waveguide excitation problem, a simple and accurate 2-D theory for treating such bends in slab waveguides is developed. Numerical results for typical cases are explained along with some new junctions for achieving large angle redirection of guided waves with very low bending loss     

Superluminescent diodes with bent waveguide

Superluminescent diodes are fabricated with a bent waveguide to prevent optical feedback. The bent ridge waveguide is formed with one end normal to the cleaved facet and the other angled at 7/spl deg/ from the facet normal. The spectral ripple of these devices is around 10% at 6 mW output power. The farfield pattern emitted from the angled facet is crescent-shaped. As the device is set up in an external-cavity configuration, lasing operation is observed regardless of the additional 10.8 dB bending loss introduced by the bent waveguide.     

Loss reduction in curved dielectric optical slab waveguide

Formulas of the pure bending loss and the transition loss valid for TM modes in dielectric optical slab waveguides are presented. The characteristic of the total transmission loss of a step-index slab waveguide containing a circular bend having an asymmetric waveguide structure are analysed numerically by applying derived loss formulas. It is found that the features of the total transmission loss of TM modes are closely analogous to those of TE modes except for small discrepancies in the numerical values.     

Three-Dimensional Tapered Spot-Size Converter Based on (111) Silicon-on-Insulator

A novel method is presented to realize a three- dimensional (3-D) spot-size converter (SSC) by standard silicon micromachining techniques, for efficient coupling from single-mode fiber to silicon photonic chip. The SSC is comprised of input–output waveguides and a 3-D tapered coupler on silicon-on-insulator (SOI) substrate. The dimensions are decreased linearly in both vertical and horizontal directions from input facet to output facet. The slope in the vertical direction is originated from the angle between the surface of (111) SOI wafer and the real (111) crystal plane. Fabrication of the device has large process tolerance and its effectiveness was conformed by optical loss measurement.      

半矢量模式求解在光波导弯曲损耗计算中的应用

以柱坐标下的半矢量波动方程为基础,采用基于完美匹配层(PML)边界条件的有限差分方法,对弯曲波导进行模式求解,进而得到波导弯曲引起的辐射损耗.基于计算得到的直弯波导的模场分布,采用二维重叠积分法计算了两者连接时的过渡损耗.计算结果与已有实验结果符合较好.采用该方法,研究了SOI脊型波导的弯曲损耗与波导结构参数之间的关系,并对直弯波导的连接进行了优化.