Waveguide-Integrated DDR Photodiodes With Butt-Joint Coupling
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Solution Overview
Problem
Conventional DDR photodiodes are not suitable for integration in photonic integrated circuits (PICs) due to their orientation being normal to the substrate, which is different from the direction of optical signal propagation in optical waveguides within PICs, leading to inefficiencies in light absorption and non-linear high-speed photocurrent response.
Innovation Solution
The integration of DDR photodiodes with optical waveguides using a butt-joint configuration, where the photodiode is aligned with the waveguide to receive optical signals propagating in a non-orthogonal angle, allowing direct absorption of light without evanescent coupling, and incorporating band smoothing regions to enhance carrier transit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional DDR photodiodes are oriented normal to the substrate, then they can achieve high speed photocurrent response, but they cannot efficiently receive optical signals propagating parallel to the substrate in PICs
Solution Approach 1:
The photodiode structure is rotated from the conventional normal orientation to a tilted orientation matching the waveguide propagation direction. This dimensional change in orientation allows the photodiode to receive optical signals efficiently while maintaining the dual-depletion region structure that enables high-speed response.
Solution Approach 2:
The photodiode structure employs asymmetric orientation where the absorber layer and depletion regions are tilted relative to the substrate normal. This asymmetric configuration matches the oblique propagation path of optical signals in waveguides, enabling efficient light absorption while preserving the high-speed photocurrent response characteristic of DDR photodiodes.
2Productivity
If the photodiode is tilted to match waveguide propagation, then light absorption efficiency improves, but the dual-depletion region structure may be compromised
Solution Approach 1:
The photodiode structure incorporates local quality variations through the dual-depletion region configuration, where the absorber layer and undoped layers are strategically positioned to create separate depletion regions. This local structuring maintains the functionality of both depletion regions even when the overall photodiode is tilted to match waveguide propagation.
Solution Approach 2:
The photodiode is segmented into distinct functional layers including the absorber layer, first undoped layer, and second undoped layer, each contributing to forming separate depletion regions. This segmentation allows the structure to maintain dual-depletion region functionality while being tilted to optimize light absorption from waveguides.
3Productivity
If a thick absorber layer is used to absorb more light, then responsivity increases, but the high speed photocurrent response becomes non-linear
Solution Approach 1:
The photodiode structure optimizes the thickness and positioning of the absorber layer and undoped layers to control the depletion region characteristics. By carefully adjusting these parameters, the structure achieves high responsivity while maintaining linear high-speed photocurrent response, avoiding the non-linearity that would result from a excessively thick absorber layer.
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 enables efficient light absorption, nearly complete quantum efficiency, and improved linearity with reduced carrier transit time, enhancing the RF bandwidth and responsivity of the photodiodes.
Implementation Method 1
an absorber layer provided between the p-type cladding layer and the n-type cladding layer... to receive, via the interface, an optical signal propagating in the optical waveguide
Implementation Method 2
The depletion region extends from above the absorber layer to below the lower undoped layer. Since electron mobility is much higher than hole mobility in InP and related materials, the total transit time of such holes and electrons from the absorber to their respective contacts is comparable or minimized.
Data Source
AI summary
Consistent with the present disclosure, a DDR photodiode is provided on a substrate adjacent to a passive waveguide. In order to efficiently capture light output from the waveguide, the photodiode is coupled to the waveguide with a butt-joint. As a result, the photodiode and the waveguide abut one another such that the dominant mode of light propagating in the waveguide parallel to the substrate is supplied directly to a side of the absorber layer of the photodiode without, in one example, evanescent coupling, nor is a resonant coupler required to supply light to the photodiode. Thus, light is absorbed more efficiently in the photodiode such that the photodiode may have a shorter length. In addition, since substantially all light is input to the photodiode, nearly complete absorption and nearly ideal quantum efficiency can be achieved in a relatively short length. Further, the improved linearity associated with DDR photodiodes is preserved with the exemplary butt joint configurations disclosed herein.