Asymmetric Rib-Waveguide Couplers for Large-FSR Ring Resonators
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Solution Overview
Problem
Existing silicon-photonics devices face challenges in achieving a high coupling coefficient and large free-spectral range (FSR) in ring resonators due to the difficulty in fabricating beta-matched directional couplers with small bend radii, leading to excess loss and increased radiation loss.
Innovation Solution
The use of an asymmetric rib-waveguide-based directional coupler with an asymmetric slab configuration, where the bus waveguide has a shared inner slab portion and a thinner or absent outer slab portion, enabling strong optical confinement and large coupling gaps while maintaining beta matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If a ring resonator has a small diameter to achieve large free-spectral range, then the FSR is improved, but the bend radius becomes too small causing increased bending loss and radiation loss
Solution Approach 1:
The waveguide structure is designed with different properties in different regions: the ring waveguide has a standard rib structure, while the bus waveguide has an asymmetric slab configuration where the slab is removed on the inner side and reduced on the outer side. This local structural differentiation allows the bus waveguide to have enhanced mode confinement specifically in the bending region, enabling tight bends with small radius while maintaining low loss, thus resolving the contradiction between small diameter (for large FSR) and bending loss.
2Reliability
If the bus waveguide is made narrower to achieve beta matching with the ring waveguide, then the coupling coefficient is improved, but the radiation loss increases dramatically
Solution Approach 1:
The bus waveguide employs an asymmetric slab configuration where the slab is removed on the inner side and reduced on the outer side of the bend. This creates different effective widths at different locations: narrower effective width on the inner side for beta matching and coupling, while the outer side maintains sufficient width for mode confinement and reduced radiation loss. This local structural variation resolves the contradiction between achieving high coupling coefficient through narrowing and minimizing radiation loss.
Solution Approach 2:
The bus waveguide is designed with asymmetric slab distribution - completely removed on the inner side adjacent to the ring waveguide and reduced thickness on the outer side. This asymmetric configuration enables the waveguide to simultaneously achieve narrow effective width for beta matching (improving coupling coefficient) and sufficient physical width for mode confinement (reducing radiation loss), resolving the contradiction between these two requirements.
3Reliability
If advanced lithography technology is used to fabricate narrow coupling gaps for interior-ridge waveguides, then the coupling coefficient is improved, but the fabrication cost becomes prohibitive
Solution Approach 1:
Instead of using interior-ridge waveguides that require narrow coupling gaps fabricated with advanced lithography, this invention uses a rib-waveguide-based structure with asymmetric slab configuration. The coupling is achieved through the asymmetric slab design rather than narrow gaps, allowing the use of standard lithography technology while maintaining high coupling coefficient, thus resolving the contradiction between coupling performance and fabrication cost.
Solution Approach 2:
The asymmetric slab configuration of the bus waveguide (removed on inner side, reduced on outer side) provides an alternative mechanism for achieving high coupling coefficient without relying on narrow coupling gaps. This asymmetric structural approach enables effective coupling through standard fabrication processes, resolving the contradiction between high coupling coefficient and low fabrication cost.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration allows for low-cost fabrication of photonic devices with high mode confinement, large FSR, and reduced bending losses, suitable for applications like optical modulators and spectral filters.
Implementation Method 1
its optical energy can evanescently couple between the ring and bus waveguide
Data Source
AI summary
The present disclosure is directed toward photonic elements comprising rib-waveguide-based ring resonators having high coupling efficiency between their bus and ring waveguides within the coupling region of the ring resonator, as well as operability over a wide spectral range. Embodiments disclosed herein employ a small-diameter ring waveguide and a bus waveguide that collectively define an asymmetrical coupler having a coupling region at which the optical confinement of the bus waveguide is stronger on side of the bus waveguide distal to the ring waveguide than on the side of the bus waveguide that is proximal to the ring waveguide. In some embodiments, in the coupling region, the bus waveguide has ridge and an inner bus-slab portion that is shared with the ring waveguide, while the outer bus-slab portion is at least partially removed to give rise to stronger optical confinement at the outer edge of the ridge of the bus waveguide.