Robert Scarmozzino

Mixed-level optical simulations of light-emitting diodes based on a combination of rigorous electromagnetic solvers and Monte Carlo ray-tracing methods

Abstract. Over the last two decades, extensive research has been done to improve light-emitting diodes (LEDs) designs. Increasingly complex designs have necessitated the use of computational simulations which have provided numerous insights for improving LED performance. Depending upon the focus of the design and the scale of the problem, simulations are carried out using rigorous electromagnetic (EM) wave optics-based techniques, such as finite-difference time-domain and rigorous coupled wave analysis, or through ray optics-based techniques such as Monte Carlo ray-tracing (RT). The former are typically used for modeling nanostructures on the LED die, and the latter for modeling encapsulating structures, die placement, back-reflection, and phosphor downconversion. This paper presents the use of a mixed-level simulation approach that unifies the use of EM wave-level and ray-level tools. This approach uses rigorous EM wave-based tools to characterize the nanostructured die and generates both a bidirectional scattering distribution function and a far-field angular intensity distribution. These characteristics are then incorporated into the RT simulator to obtain the overall performance. Such a mixed-level approach allows for comprehensive modeling of the optical characteristic of LEDs, including polarization effects, and can potentially lead to a more accurate performance than that from individual modeling tools alone.

Bi-directional beam propagation method for problems with multiple dielectric interfaces

Conventional beam propagation methods (BPM) only deal with the forward propagating optical field, and, thus, they are useful only if the structure has a sufficiently slowly-varying index along the propagation direction such that the accumulated reflections are negligible. However, many practical guided-wave optical devices involve junctions of different waveguides, laser facets, grating structures, anti-reflection or high-reflection coatings, etc. In all these structures there exists a refractive index discontinuity or variation along the direction of propagation of the optical field, hence consideration of coupling the forward and backward waves must be included for accurate modeling. We present an iterative approach to dealing with problems involving multiple dielectric interfaces, which has significantly reduced memory requirements.

Accounting for coherent effects in the ray-tracing of light-emitting diodes with interface gratings via mixed-level simulation

Abstract. Ray-tracing (RT) has long been the workhorse technique for analyzing light-emitting diode (LED) dies and packages and has led to significant improvements in extraction efficiency and beam shaping. However, to achieve further enhancements, nano-/microscale features such as patterned substrates and surface textures have been explored. The coherent effects arising from these near/subwavelength features are difficult to include in the RT of the packaged device. We show that under certain conditions these effects can have a significant impact on LED performance, especially if back-reflectors are present. Furthermore, we demonstrate that coherence must be accounted for even in structures that would otherwise be considered as having relatively large feature sizes, such as gratings with periods many times the wavelength. We present comparisons between the optical responses of prototypical periodically patterned substrates modeled with RT alone and with a mixed-level approach that combines RT and rigorous electromagnetic simulation, such as rigorous coupled wave analysis and finite-difference time-domain. Several examples with varying lateral periods are computed with both methods. It is shown that these results may differ, and that these differences can be significant if back reflection is present. We conclude that a mixed-level approach is an efficient and accurate method to model light extraction in modern LEDs.

Multimode system simulation as an alternative to spreadsheet analysis for the study of Gb/s optical communication systems

This paper describes the disadvantages of spreadsheet-based analysis for high-speed multimode systems and presents multimode system simulation as an alternative. An example is depicted in which the system power margin calculated by the spreadsheet model was overly pessimistic relative to that predicted by system simulation. The higher degree of accuracy and flexibility afforded by multimode system simulation should enable component tolerances to be relaxed, resulting in an overall reduction in system cost.

Complex propagators for evanescent waves in bidirectional beam propagation method

Existing algorithms for bidirectional optical beam propagation proposed to simulate reflective integrated photonic devices do not propagate evanescent fields correctly. Thus inaccuracy and instability problems can arise when fields have significant evanescent character. We propose complex representations of the propagation operator by choosing either a complex reference wave number or a complex representation of Pade approximation to address this issue. Therefore correct evolution of both propagating waves and evanescent waves can be simultaneously realized, significantly reducing the inaccuracy and instability problems. Both test problems and practical problems are presented for demonstration.

Broadband 3D optical modeling of HgCdTe infrared focal plane arrays

We present a modeling technique for the efficient broadband simulation of infrared HgCdTe-based focal plane arrays. The approach performs a single broadband FullWAVETM FDTD optical simulation and a series of discrete Fourier transforms for obtaining absorbed photon density profiles on a set of frequencies. These distributions are then aggregated, through a weighted average, and imported as a generation term into a single Sentaurus Device electrical simulation, which calculates the corresponding photocurrent and inter-pixel crosstalk. This technique can save an order of magnitude in memory and computation time compared to performing multiple monochromatic optical and electrical simulations.

Using system simulation to evaluate design choices for automotive ethernet over plastic optical fiber

The growing bandwidth demands of advanced driver assistance systems (ADAS) and infotainment technologies make Gigabit Ethernet over plastic optical fiber (POF) a natural choice for next-generation automotive data networks, especially in light of the recent approval of the IEEE 802.3bv standard for Gigabit Ethernet transmission over POF. POF-based transmission provides the advantages of low cost, light weight, easy termination, durability, and immunity to electromagnetic interference (EMI), while Gigabit Ethernet extends the current maximum data rate of 150 Mb/s provided by Media Oriented Systems Transport (MOST). Thus, we examine important design choices that impact the performance of POF-based automotive data links for data rates up to and beyond 1 Gb/s and different choices of modulation format, including NRZ and PAM-n. Because simulation is an efficient and cost-effective solution for studying the complex interplay of multiple design choices without requiring physical prototypes, we base our analysis on a comprehensive modeling framework for optical communication systems incorporating large-core step-index fiber and fiber-to-fiber connectors. We study anticipated system performance in terms of bandwidth and BER for different choices of link length and connector count, including the IEEE 802.3bv targets of 15 meters with four connectors and 40 meters with no connectors. In addition, we consider the impact of connector misalignments (both lateral and longitudinal) and source launch profile (measured in terms of its encircled angular flux, or EAF), which also directly affect link bandwidth.

Non-Monochromatic 3D Optical Simulation of HgCdTe Focal Plane Arrays

Combined optical and electrical simulations of infrared HgCdTe-based focal plane arrays under broadband, non-monochromatic illumination are critically relevant to the design of small-volume detectors with sub-wavelength pixel pitches. We present an efficient technique, based on a single finite-difference time-domain electromagnetic simulation, that provides the photogeneration rate profile due to realistic, broadband optical sources, avoiding multiple monochromatic simulations. This technique is applied to assess the effects of the temperature of blackbody optical sources on quantum efficiency and inter-pixel crosstalk of planar LWIR arrays.

Mixed-level simulation of opto-electronic devices

Presented here are two important devices that cannot be modeled accurately and/or tractably by a single simulation technique. Simulation flows to address each device are presented. The first is a patterned Light Emitting Diode (LED), the optical modeling of which requires a mixed-level simulation approach combining FDTD (or RCWA) and Ray Tracing. The second is a CMOS Image Sensors (CIS), which requires process, optical and electrical simulation techniques.

Mixed-level and mixed-domain modeling and simulation for optical interconnect design

The physical-layer design of optical interconnects for box-to-box, backplane, intra-board, and intra-chip applications must address a number of considerations including system performance, manufacturing yield, reliability, power consumption, thermal properties, and cost. Also, there are multiple levels of abstraction and domains over which optical interconnect designs must be addressed. For a complete design solution, modeling and simulation across the multiple levels of abstraction in both electronic and optical domains is required. We present novel modeling and simulation approaches that address this challenge.