CAD tools speed complex component design

Wei-Ping Huang

In optical networking, as with any other industry, market share and de facto standards are routinely determined by the first to market. At the same time, optical component manufacturers will admit it`s difficult to bring innovation to the marketplace quickly and cost effectively, much less bring products to market first. The reasons range from a shortage of experienced designers to the exorbitant cost of prototype development.

Computer-aided design (CAD) has long been one way companies attempt to keep pace, but traditional CAD software focuses on only parts and pieces of the circuit, and most still require an advanced knowledge of optical design. As competition in the market continues to grow fierce, parts and pieces no longer meet industry needs. To design next-generation optical devices with the speed, accuracy, and cost efficiencies necessary to satisfy market demand, product designers need coherent, end-to-end CAD systems with intrinsic knowledge of basic optical foundations.

A new breed of optical CAD tools is giving designers more control than ever before. However, careful evaluation of capabilities is necessary to determine whether these tools are accurate, user friendly, and offer the features necessary to meet the needs of product designers.

Integrated modeling

Integration of device and circuit modeling within a hierarchical framework is the most direct and efficient approach to designing an end-to-end circuit. With a common database capable of sharing information among modules, circuits can be created vertically instead of horizontally. This integration is accomplished by segmenting the four core steps into modules across a common platform, making device design more logical and coherent. The core steps include material design, waveguide design, device design, and circuit design.

By ordering the project design within a logical hierarchal order, information can be exported between levels, giving designers more speed and flexibility to work. For example, designers working together, whether in the same room or across the country, can build on a common foundation without repetition. A truly comprehensive optical CAD system will provide tools for all of these steps.

Material design

Material design lays the groundwork for the end product. To rapidly develop new design ideas, designers must have fingertip access to the full range of material models used in optical design, from standard Sellmeier formulas to complicated models that incorporate the effects of temperature and pressure within material design components. An on-board knowledge base is also important because it can provide ready access to the characteristics of materials such as indium phosphide or silica and how they perform and function under a range of circumstances.

Ready access to commonly used design calculations reduces repetitive input and the opportunity for error. Still, room for invention is mandatory. More experienced or creative designers want to experiment with their own models through a table/data file or analytical expressions, or through the calibration of the standard models (see Fig. 1).

Waveguide design

The next logical step in the design of an optical networking product is waveguide design. Building on the material design, designers can move quickly to the next level to process subjective waveguide geometry and random refractive index profiles. A comprehensive waveguide suite will enable designers to process waveguide geometry and refractive index profiles and must be able to accurately compute basic and higher-order modes, guided and leaky modes, semi-vector and full-vector modes. The system is even more complete if it includes a predefined waveguide library of waveguide structures.

Waveguide design also requires the computation of basic and higher-order modes, guided and leaky modes, semi-vector and full-vector modes. Other key capabilities to consider in evaluating waveguide photonic CAD tools include a wide range of simulation methods, from analytical to numerical, confinement factor, far-field pattern, overlap fields, alignment, and facet calculation and equivalent index (see Fig. 2).

Device design

Device design is the third step in the development of an optical device. This critical step is frequently found as a separate CAD tool; however, by using a stand-alone tool, designers may lose speed and accuracy as they try to integrate information with other tools for additional testing and modeling. By building on the material design and waveguide designs created specifically for that product, designers gain continuity, accuracy, and speed during the device design phase.

Design templates and knowledge-based libraries are perhaps the most valuable tool available in developing optical devices because they eliminate the blank page as a starting point. With a template, all the designer must supply is the idea, and then quickly and easily customize a predefined structure from a predetermined performance level.

Built-in libraries for multimode interference devices, Y-branch, direct couplers, gratings, arrayed waveguide gratings (AWG), bends, facets, and gaps offer easy starting points for novice designers. However, as designers gain experience, the same system must give them a simple way to control complex photonic structures through user-defined structures and computations.

Other design performance features to consider include advanced wave propagation solvers for device simulation, and analytical methods for fast simulation and reflection analysis. The design system should also provide all information about the simulated device, such as transmission and radiation loss, polarization-dependent loss, group delay, dispersion losses, power-splitter ratio, and phase difference between ports (see Fig. 3).

Circuit design

The last step in the design of an optical networking product is to create photonic integrated circuits. Even though circuit design photonic CAD tools are rare, this is a critical step within the entire design process and should be included in any evaluation of an optical CAD system. The first consideration in evaluating circuit design tools is whether circuit simulation and rigorous device modeling are seamlessly integrated.

Another factor to consider is how well the connector setup uses symbolic devices in the physical circuit layout so that designs are easily visualized, particularly multiple connectors based on device ports, including S-bends, rings, and mirrors (see Fig. 4).

As a final stage of simulation in photonic CAD, and the most important part in manufacturing of a circuit, packaging of the circuit should be simulated.

The use of neural network technology also should be considered because it facilitates abstraction transition from the low level to the high level of the model hierarchy, creating connections between processing elements rather than just manipulating zeros and ones. This function is particularly useful when the simulation can draw from a large database of prior examples.

Simulation speed, particularly with large, complicated circuits, also is an important characteristic to consider because it can shrink the testing time frame from several months with traditional prototype methods to hours. Not only does this factor weigh heavily in a company`s race to market, but it also affects the cost efficiencies of the design. Products that can be accurately simulated via computer models are simpler and more economical to develop than products that must be tested through physical trial and error.

FIGURE 4. A circuit design module for photonic integrated circuits (PICs) integrates circuit simulation and device modeling, and delivers a new level of design capabilities. This module can differentiate devices and connectors.