Silicon photonics offers the ability to fabricate and integrate photonic and electronic components using existing integrated circuit fabrication infrastructure. Recent work seeks to understand the impact of IC process variations on performance of photonic components. In particular, methods for analysis that identify sensitivity of photonic components to process variations are crucial to enable viable design and manufacturing of silicon photonic systems.
We present two different and complementary methods for understanding the impact of geometric process variations on photonics components: ensemble statistical virtual fabrication simulations, and adjoint methods. These are utilized to identify the most sensitive regions of a Y-splitter photonic component to line edge roughness (LER) due to inherent lithography and etch process variations. In the ensemble approach, we simulate multiple instantiations with random LER applied to specific sections of the Y-splitter. This enables localization and quantification of LER impact on transmission, phase imbalance, and excess losses. These evaluations, however, come at the cost of many simulations. In adjoint sensitivity evaluation, only one or two simulations can identify regions most sensitive to LER. While first-order linear sensitivity is extracted, the adjoint has challenges in quantifying mean variation impacts. Both methods reveal that the Y-splitter is most sensitive to LER in the input taper, accounting for over 95% of the imbalance transmission. These two methods can be combined to quantify mean, variance, and sensitivity of photonic device components in the face of statistical variations. Incorporated into future photonic process design kits (PDKs), these analysis methods will help designers predict and optimize photonic component performance and yield.
Silicon photonics is rapidly emerging as a promising technology to enable higher bandwidth, lower energy, and lower latency communication and information processing, and other applications. In silicon photonics, existing CMOS manufacturing infrastructure and techniques are leveraged. However, a key challenge for silicon photonics is the lack of mature models that take into account known CMOS process variations and their effect on photonic component behavior. A key factor for the adoption of silicon photonics into high-yield manufacturing is to extend process design kits (PDKs) to include photonic process variability models that are aware of variations that may occur during the fabrication process. <p> </p>We study the effect of a well-known random process variation, line edge roughness (LER), present in the lithography and etch process, on the performance of a fundamental component, the Y-branch, through virtual fabrication simulations. Ideally, the Y-branch transmits the input power equally to its two output ports. However, imbalanced transmission between the two output ports is observed when LER is imposed on the Y-branch, depending on the statistical nature (amplitude and correlation length) of the LER. The imbalance can be as low as 1% for small LER amplitudes, and reach up to 15% for large LER amplitudes. In addition, LER increases the excess loss compared to the nominal (smooth) case. Ensemble statistical virtual fabrication and FDTD photonic simulations across a range of LER amplitude and correlation lengths are reported. These results can be captured as worst-case corner models and included in variation-aware photonic compact models.
Cities and towns around the world are becoming more condensed due to the shrinking amount of buildable areas, which significantly reduces the amount of light that occupants have access to. This lack of natural lighting results in health, safety and quality of life degradation. This paper presents a new technique of transmitting sunlight downward into narrow alleys and streets, by using a daylighting guiding acrylic panel that is capable of changing the direction and distribution of the incident light. The core of the proposed daylight guidance system is made up of light transmission panels with high quality. The corrugations have sine wave shaped cross-section so that the panel functions as an optical diffuser perpendicular to the direction of sunlight propagation. The day lighting system consists of the corrugated panels and a lattice frame, which supports the panel. The proposed system is to be mounted on the building roof facing the sun so as to redirect the incident sunlight downward into the narrow alleys or streets. Since building sizes and orientations are different the frame is arranged such that substantially deep light penetration and high luminance level can be achieved. Simulation results show that the proposed panel improves the illuminance values by more than 200% and 400% in autumn and winter, respectively, provides fan-out angle that exceeds 80° for certain solar altitudes and the transmitted power percentage varies from 40% to 90% as the solar altitude varies from 10° to 80°. Experimental results are in a good agreement with the simulations.