[Paper Review] New-generation silicon photonics beyond the singlemode regime

Long Zhang, Shihan Hong+9 more|arXiv (Cornell University)|Apr 9, 2021

Photonic and Optical Devices47 references6 citations

TL;DR

This paper introduces a new paradigm in silicon photonics by enabling low-loss, low-crosstalk light propagation in multimode waveguides with broadened silicon cores (2–3 μm), achieving an ultra-low propagation loss of ~0.1 dB/cm without special fabrication. The approach leverages standard 220-nm SOI MPW processes to demonstrate a record intrinsic Q-factor of 1.02×10⁷ in a micro-racetrack resonator and an on-chip 100-cm delayline with ~0.14 dB/cm loss, enabling high-performance microwave photonic filters and large-scale photonic integration beyond the singlemode regime.

ABSTRACT

The singlemode condition is one of the most important design rules for optical waveguides in guided-wave optics. The reason following the singlemode condition is that higher-order modes might be excited and thus introduce some undesired mode-mismatching loss as well as inter-mode crosstalk when light propagates along an optical waveguide beyond the singlemode regime. As a result, multimode photonic waveguides are usually not allowed. In this paper, we propose the concept of silicon photonics beyond the singlemode regime, developed with low-loss and low-crosstalk light propagation in multimode photonic waveguides with broadened silicon cores. In particular, silicon photonic waveguides with a broadened core region have shown an ultra-low-loss of ~0.1 dB/cm for the fundamental mode even without any special fabrication process. A micro-racetrack resonator fabricated with standard 220-nm-SOI MPW-foundry processes shows a record intrinsic Q-factor as high as 1.02*107 for the first time, corresponding to ultra-low waveguide propagation loss of only 0.065 dB/cm. A high-performance microwave photonic filter on silicon is then realized with an ultra-narrow 3-dB bandwidth of 20.6 MHz as well as a tuning range of ~20 GHz for the first time. An on-chip 100-cm-long delayline is also demonstrated by using the present broadened SOI photonic waveguides with compact Euler-curve bends, the measured propagation loss is ~0.14 dB/cm. The proposed concept of silicon photonics beyond the singlemode regime helps solve the issue of high propagation loss and also significantly reduces the random phase errors of light due to the random variations of waveguide dimensions. In particularity it enables silicon photonic devices with enhanced performances, which paves the way for new-generation silicon photonics realizing the large-scale photonic integration.

Motivation & Objective

• To overcome the limitations of conventional singlemode silicon photonic waveguides, which suffer from high propagation loss and are incompatible with large-scale integration.
• To address the challenge of high waveguide loss in standard 450-nm-wide SOI waveguides (~1–2 dB/cm), which restricts performance in long-path and high-Q devices.
• To enable ultra-high Q-factor resonators and low-loss long delaylines without requiring non-standard or specialized fabrication processes.
• To demonstrate that broadening the silicon core beyond the singlemode regime can simultaneously reduce propagation loss and minimize random phase errors from fabrication variations.
• To establish a scalable, CMOS-compatible platform for next-generation large-scale photonic integrated circuits (PICs) using standard foundry processes.

Proposed method

• Designing silicon photonic waveguides with core widths of 2–3 μm to operate beyond the singlemode regime, thereby reducing fundamental mode propagation loss.
• Utilizing standard 220-nm SOI MPW foundry processes with deep-UV lithography and inductively-coupled plasma dry-etching for fabrication, avoiding special post-processing steps.
• Implementing compact 90° Euler-curve bends (MWBs) to enable compact, low-loss routing of long delaylines with minimal radiation and bending loss.
• Fabricating a micro-racetrack resonator on a 220-nm SOI platform using standard processes to achieve an intrinsic Q-factor of 1.02×10⁷, corresponding to a waveguide loss of 0.065 dB/cm.
• Integrating the high-Q resonator into a microwave photonic filter with an ultra-narrow 3-dB bandwidth of 20.6 MHz and a tuning range of ~20 GHz.
• Characterizing propagation loss using an ASE source and an optical spectrum analyzer (OSA) on both 100-cm-long and 10-cm-long delaylines for comparison.

Experimental results

Research questions

• RQ1 Can ultra-low propagation loss be achieved in silicon photonic waveguides beyond the singlemode regime without specialized fabrication processes?
• RQ2 To what extent does broadening the silicon core reduce fundamental mode loss and improve Q-factors in micro-resonators?
• RQ3 Can long on-chip delaylines (up to 100 cm) be realized with low loss and compact footprint using standard fabrication processes?
• RQ4 How does the performance of a microwave photonic filter based on a high-Q resonator compare to existing state-of-the-art devices in terms of bandwidth and tunability?
• RQ5 To what extent does operating beyond the singlemode regime reduce random phase errors due to fabrication variations in waveguide dimensions?

Key findings

• A 3-μm-wide silicon photonic waveguide achieved an ultra-low propagation loss of ~0.1 dB/cm without any special fabrication, significantly lower than the typical 1–2 dB/cm in standard 450-nm-wide waveguides.
• A micro-racetrack resonator fabricated with standard 220-nm SOI MPW processes achieved a record intrinsic Q-factor of 1.02×10⁷, corresponding to a waveguide propagation loss of only 0.065 dB/cm.
• An on-chip 100-cm-long delayline with 3-μm-wide waveguides and 90° Euler-curve bends exhibited a measured propagation loss of ~0.14 dB/cm, demonstrating the longest and lowest-loss delayline in uniform strip SOI waveguides.
• A microwave photonic filter based on the high-Q resonator achieved an ultra-narrow 3-dB bandwidth of 20.6 MHz and a tuning range of ~20 GHz, representing the best performance reported to date on a chip.
• The propagation loss in the 100-cm delayline was higher than that estimated from the resonator (0.14 dB/cm vs. 0.065 dB/cm), likely due to cumulative defects and wafer-scale fabrication variations.
• The average propagation loss across more than 10 chips was measured at 0.14 ± 0.025 dB/cm, confirming high process uniformity and reproducibility.