Waveguide Photodiode Shared Bias Pad for High-Speed Response
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Solution Overview
Problem
High-frequency response of waveguide photodiodes is hindered by increased parasitic capacitance due to a thinned absorption layer, which is necessary to shorten transit time in optical receivers for high-speed optical communication systems.
Innovation Solution
A functional optical device monolithically integrating a coupling unit, optical waveguides, light-receiving elements, and pads on a semiconductor substrate, where the substrate configuration includes signal pads between bias and ground pads to reduce parasitic capacitance and enhance high-frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the absorption layer is thinned to shorten transit time, then the response speed is improved, but the parasitic capacitance increases which deteriorates the high frequency response
Solution Approach 1:
The absorption layer is divided into a first absorption layer and a second absorption layer with different thicknesses. The first absorption layer has a smaller thickness to reduce transit time and improve response speed, while the second absorption layer has a larger thickness to maintain adequate light absorption. This segmentation allows the device to achieve both fast response and sufficient absorption efficiency.
Solution Approach 2:
Different regions of the absorption layer are given different thicknesses to optimize local functions. The first absorption layer region is made thinner to improve response speed in that local area, while the second absorption layer region is made thicker to ensure adequate light absorption. This local quality differentiation resolves the contradiction between speed and capacitance by optimizing each region for its specific function.
2Loss of time
If the absorption layer is thinned to improve high frequency response, then the transit time is reduced, but the signal degradation increases due to higher parasitic capacitance
Solution Approach 1:
The absorption layer is segmented into two layers with different thicknesses. The first absorption layer is optimized for fast carrier transit (smaller thickness), while the second absorption layer compensates for signal loss through adequate light absorption (larger thickness). This segmentation simultaneously reduces transit time and maintains signal quality.
Solution Approach 2:
The absorption layer uses a composite structure of two different thickness regions, effectively combining the advantages of thin layers (fast response) and thick layers (adequate absorption). This composite approach mitigates signal degradation while maintaining reduced transit time.
3Productivity
If a thinned absorption layer is used to enhance response, then the transit time decreases, but the device complexity increases due to additional electrode structures
Solution Approach 1:
The substrate is designed to serve multiple functions: it acts as both the structural support and as the common electrode (anode or cathode) for both waveguide photodiodes. This eliminates the need for separate electrode structures for each photodiode, reducing device complexity while maintaining the benefits of the thinned absorption layer for enhanced response efficiency.
Solution Approach 2:
The substrate and the electrode functions are merged into a single component. The substrate simultaneously provides mechanical support and electrical connection, eliminating redundant electrode structures. This merging simplifies the device architecture while preserving the high response efficiency achieved through the thinned absorption 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
The configuration effectively widens the frequency bandwidth and improves response speed of the optical receiver by stabilizing ground potential and reducing signal degradation, particularly for high-frequency components.
Implementation Method 1
The coupling unit generates a pair of optical signals complementary to each other by preforming interference between signal light and local light
Implementation Method 2
The light-receiving elements generate a pair of photocurrents complementary to each other based on the optical signals carried on the optical waveguides
Data Source
AI summary
A functional optical device applicable to a coherent optical communication system as a front end device is disclosed. The functional optical device includes a pair of light-receiving elements of a type of waveguide photodiode (PD), a pair of signal pads, a pair of ground pads, a bias pad, and a substrate that monolithically integrates those elements thereon. The light-receiving elements generate a photocurrent complementary to each other in respective anodes thereof; while, receive biases through the bias pad common to the light-receiving elements. Those pads are disposed along an edge of the substrate such that the signal pads put the bias pads therebetween, and the ground pads put the signal pads and the bias pad therebetween.