Butt-Jointed DDR Photodiodes for PIC Waveguide 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 capture and integration.

Innovation Solution

The photodiode is configured with a butt joint interface to the optical waveguide, allowing direct light input parallel to the substrate, and includes a dual-depletion region (DDR) design with aligned absorber layers to optimize light absorption and reduce transit time, preserving high responsivity and RF bandwidth.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If conventional DDR photodiodes are used with normal-to-substrate light incidence, then high responsivity and RF bandwidth are achieved, but integration with optical waveguides in PICs is not suitable due to direction mismatch

Engineering Contradiction:

Improveintegration compatibility with PICsVSAvoidlight capture efficiency

Core Design Contradiction:

Adaptability or versatilityVSEase of operation

Solution Approach 1:

The photodiode structure is rotated from normal-to-substrate incidence to parallel-to-substrate incidence, changing the dimensional orientation of light capture to match waveguide propagation direction in PICs

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

Instead of directing light normally to the substrate surface, the design inverts the approach by having light travel parallel to the substrate through the waveguide and enter the photodiode laterally, reversing the conventional light incidence geometry

Inventive Principle:

Principle #13The other way round (Inversion)

2Productivity

If absorber layer thickness is increased to improve light absorption, then responsivity increases, but carrier transit time increases reducing RF bandwidth

Engineering Contradiction:

Improvelight absorption efficiencyVSAvoidcarrier transit time

Core Design Contradiction:

ProductivityVSLoss of time

Solution Approach 1:

The design changes the physical parameters of the absorber layer including thickness, doping concentration, and material composition to optimize the balance between absorption efficiency and carrier transit time for high-speed operation

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The photodiode employs composite material structure with different semiconductor layers (e.g., InGaAs absorber layer on InP substrate) where each material is selected for specific properties: high absorption coefficient for the absorber layer and high electron mobility for the substrate

Inventive Principle:

Principle #40Composite materials

3Speed

If dual-depletion region structure is implemented to reduce carrier transit time, then RF bandwidth improves, but device complexity increases

Engineering Contradiction:

Improvecarrier transit timeVSAvoidphotodiode structure complexity

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The photodiode is segmented into multiple functional layers including p-type cladding, absorber layer, undoped layer, and n-type cladding, with each segment serving a specific function in carrier generation, separation, and collection

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The dual-depletion region structure serves multiple functions simultaneously: it provides efficient carrier separation, reduces capacitance for high-speed operation, and enables optimized absorption in the undoped layer while maintaining structural integration with the waveguide

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 capture and nearly complete absorption with ideal quantum efficiency, maintaining high linearity and reducing carrier transit time, suitable for integration in PICs.

Implementation Method 1

photodiodes that convert the received optical signals into corresponding electrical signals

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

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

Methodology Applied

Scientific EffectLight absorption: Absorption (EM radiation)

Data Source

PatentUS12557430B2Waveguide dual-depletion region (DDR) photodiodes

Publication Date: 2026.02.17 INFINERA CORP

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.