SPAD Photodetector Array With Waveguide Readout for Fast Photon Detection
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Solution Overview
Problem
High-speed free-space optical communication systems face limitations in data rate due to the large capacitance of SPAD arrays, which restricts their integration into compact consumer devices like smartphones and tablets, and existing solutions fail to optimize sensitivity and speed simultaneously.
Innovation Solution
A semiconductor photodetector array integrated with optical waveguides, where each SPAD cell is connected to the waveguide structure through a coupling mechanism, allowing for independent optimization of SPAD and waveguide performance to achieve high-speed and high-sensitivity detection without thermal noise issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the total area of the SPAD active region is increased to capture more signal power, then the sensitivity is improved, but the capacitance increases linearly with area resulting in slower response time
Solution Approach 1:
The patent divides the large-area photodetection function into multiple small SPAD cells arranged in an array. Each cell has small capacitance and fast response, while the array collectively provides large aperture for high sensitivity. The segmentation allows the system to achieve both high sensitivity (large total area) and fast response (small individual cell capacitance).
Solution Approach 2:
The patent combines multiple small SPAD cells into a unified array structure with shared readout circuitry. By merging the detection function across multiple cells while maintaining their individual fast-response characteristics, the system achieves both high sensitivity (through large total aperture) and high speed (through low individual cell capacitance).
2Measurement precision
If a larger aperture lens is used to gather more incoming power, then the sensitivity is improved, but the optical system size increases in both width and depth
Solution Approach 1:
The patent replaces the traditional mechanical lens-based light gathering system with an electrical/electronic approach using multiple SPAD cells in an array. Instead of using a large physical lens to focus light, the system uses multiple small detectors arranged to collectively capture photons across a large aperture, eliminating the need for bulky optical components while maintaining high sensitivity.
Solution Approach 2:
The patent transitions from a single-point detection approach (requiring large lenses to gather light) to a distributed array approach across two dimensions. By arranging SPAD cells in a planar array, the system achieves large effective aperture without requiring large optical components, as the light gathering capability is distributed across the array area rather than focused by a single lens.
3Measurement precision
If more photodetectors are used in an array to capture larger signal power, then the sensitivity is improved, but thermal noise accumulates limiting the gain in BER
Solution Approach 1:
The patent replaces conventional photodetectors that are susceptible to thermal noise with SPAD cells that operate in avalanche breakdown mode. This substitution fundamentally changes the detection mechanism from thermal-noise-limited operation to photon-counting operation, where each detected photon generates a standardized electrical pulse. The SPAD's digital response eliminates thermal noise accumulation that plagues analog photodetector arrays, enabling high sensitivity without proportional noise increase.
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 high-speed and high-sensitivity detection of optical signals, overcoming the limitations of large capacitance and dead time in SPAD arrays, allowing for faster data rates and integration into compact consumer devices.
Implementation Method 1
the coupling structure being configured to connect at least one of the plurality of semiconductor photodetector cells and the optical waveguide structure to convert a photodetection event at the at least one of the plurality of semiconductor photodetector cells to an optical signal in the optical waveguide structure
Implementation Method 2
an optical waveguide structure with an optical waveguide, the optical waveguide being connected electrically to the semiconductor photodetector array
Implementation Method 3
a semiconductor photodetector array with a plurality of semiconductor photodetector cells
Data Source
AI summary
An optical signal to be detected enters single photon avalanche diode cells (SPAD) in a SPAD array and triggers photo event currents. Optical waveguides in the sensor carry an internal optical signal and a 1xN splitter divides the optical power into waveguide branches 206, 206′, . . . 206N-1. At the coupling structure, comprising 207, 207′, . . . 207N-1 positive-intrinsic-negative diode photodetector/waveguide structures, photo event currents from SPAD cells are converted to a change in the internal optical signal, by modifying internal optical signals 208, 208′, . . . 208N-1. A waveguide combiner further integrates the modified internal optical signals resulting from the photo event currents from all the cells in the sub-array. After all waveguide branches' signals are combined, a photodetector detects the internal optical signal and outputs an electrical signal. The SPAD circuit may include additional capacitive coupling structures positioned between the SPAD cells and the photodetector/waveguide coupling structures.