Reference link: https://www.iqoqi-vienna.at/de/detail/news/a-simple-and-robust-method-for-estimating-afterpulsing-in-single-photon-detectors 

Technical Nature of Afterpulsing Phenomena

In the performance evaluation of SPAD devices, afterpulsing phenomena represent a fundamental physical limitation. When avalanche events occur, numerous charge carriers move at high speeds under strong electric field driving, with some carriers being captured by various defect traps in the semiconductor lattice.

These captured carriers do not remain permanently, but are randomly released under thermal excitation, re-triggering the avalanche process and forming false signals unrelated to the initial photon events.

The temporal characteristics of this phenomenon exhibit complex multi-exponential decay distributions, with the mathematical expression P_AP(t) = Σ_i A_i × exp(-t/τ_i). In actual SPAD devices, deep-level traps typically have release time constants of 1 to 100 microseconds, mainly originating from metallic impurities and contributing 40% to 60% of total afterpulsing. Shallow-level traps have shorter release times, ranging from 10 to 1000 nanoseconds, primarily from donor-acceptor impurities.

Additionally, interface state traps at p-n junction interfaces have moderate release time constants, typically ranging from 100 nanoseconds to 10 microseconds.

Environmental parameters have decisive influence on afterpulsing behavior.

Temperature effects are particularly significant, with AP values exhibiting exponential growth relationships with temperature, with typical temperature coefficients of approximately 0.078 K^-1. Taking silicon-based SPADs as an example, when operating temperature increases from 25°C to 50°C, AP values may surge from 10% to 17%.

Increased excess bias also deteriorates afterpulsing performance, as higher electric field intensity enhances the interaction probability between carriers and traps. Experimental observations show that when excess bias increases from 2V to 4V, AP values typically rise from 8% to 18%.

Dead time design provides an effective suppression method. When dead time extends from 100 nanoseconds to 1 microsecond, up to 85% of afterpulsing events can be suppressed, but this improvement comes at the cost of sacrificing maximum count rates.

Performance Constraints in Application Systems

In quantum communication systems, afterpulsing phenomena pose direct threats to security. Each afterpulsing event increases quantum bit error rates by approximately AP/2. When AP reaches 4%, system QBER may exceed 2%, approaching the critical point for quantum communication security.

More seriously, attackers may exploit the temporal correlation of afterpulsing to launch side-channel attacks, inferring key information by analyzing afterpulsing patterns.

Therefore, high-end quantum communication systems typically require AP values below 1% and need to operate in low-temperature environments at -10°C to suppress the activity of carrier traps.

The challenges faced by LiDAR systems are manifested in the deterioration of ranging accuracy. Timing jitter introduced by afterpulsing directly affects the accuracy of time-of-flight measurements, while false echo signals generated by afterpulsing create “ghost” targets in environmental perception, misleading path planning algorithms.

In autonomous driving applications, such interference can lead to catastrophic consequences. Typical automotive LiDAR systems can tolerate AP values below 5%, but need to maintain stable performance across a wide temperature range from -40°C to 85°C, imposing stringent requirements on device design and system control.

The afterpulsing impact in medical imaging systems is more subtle but equally critical. In fluorescence lifetime imaging applications, afterpulsing events mix with true fluorescence signals, distorting the shape of fluorescence decay curves.

When AP values reach 12%, the accuracy of fluorescence lifetime measurements may deteriorate by 2.4 times, seriously affecting the early diagnostic capability for diseases such as cancer.

Since fluorescence lifetime differences in biological tissues are typically only a few hundred picoseconds, medical imaging systems have extremely strict requirements for AP values, usually needing to be controlled below 3%.

Bottlenecks in Traditional Measurement Techniques

Existing afterpulsing measurement methods each have their inherent limitations. Oscilloscope-based measurement methods, while having widespread equipment availability and relatively simple operation, have time resolution of 1 to 5 nanoseconds that is far insufficient to accurately capture the release characteristics of fast carrier traps.

More importantly, oscilloscope methods cannot effectively distinguish afterpulsing events from thermal noise or electromagnetic interference, leading to systematically inflated measurement results.

Time-to-digital converter methods show significant improvement in accuracy, providing sub-nanosecond time resolution and good statistical characteristics.

However, this method requires expensive professional equipment, typically exceeding $100,000 investment threshold, with high operational complexity requiring professional technicians to perform tedious calibration procedures. Single complete measurements often take several hours, severely limiting testing efficiency.

Counter methods, while lower in cost, suffer from extremely severe information loss. This method can only indirectly estimate AP values by changing dead time, unable to obtain detailed information about time distributions or distinguish contributions from different types of traps.

Their measurement accuracy typically only reaches ±5% to 10% levels, far from meeting the requirements of modern SPAD device development.

Technical Innovation of the SPD2200 System

Facing these measurement challenges, Enlitech’s SPD2200 system achieves breakthrough technological innovation. The system employs advanced multi-channel time-to-digital conversion technology, improving time resolution to 25 picoseconds RMS, nearly 20 times better than traditional methods. This ultra-high precision enables the system to accurately distinguish carrier trap release processes with different time constants, providing unprecedented tools for deep understanding of device physical characteristics.

The system’s measurement uncertainty is strictly controlled within ±2%, an indicator that meets requirements even in the most demanding quantum communication applications. The SPD2200 can process over ten million statistical events, ensuring measurement results have extremely high statistical reliability. Its measurement range extends from 1 picosecond to 1 second, completely covering all possible trap release time constants in SPAD devices.

In algorithm innovation, the SPD2200 integrates multi-dimensional correlation analysis technology, capable of precisely distinguishing true afterpulsing events from other types of false counts through comprehensive judgment of temporal, statistical, and physical characteristics.

The system features adaptive parameter optimization capability, automatically adjusting key measurement parameters such as excitation power and count rate control based on the characteristics of the device under test. The built-in multi-exponential fitting algorithm can automatically identify the most suitable number of exponential components and provide fitting quality assessment, ensuring R² values consistently greater than 0.995.

Environmental control is another technical advantage of the SPD2200. The system’s precision temperature control module achieves ±0.01°C stability with operating range covering -30°C to +80°C, meeting all requirements from research experiments to industrial applications. The electromagnetic shielding system provides over 120 dB shielding effectiveness, effectively eliminating external interference.

The integrated pulsed laser features pulse widths less than 50 picoseconds and power stability better than 0.1%, providing reliable excitation sources for precise measurements.

Technical Value and Future Prospects

The SPD2200 system provides strong support for comprehensive development of SPAD technology. In the device development phase, engineers can deeply understand the physical characteristics of carrier traps through precise AP measurements, guiding optimization of material selection and structural design. In production quality control, standardized testing procedures ensure consistency and traceability in mass production. In system application validation, precise performance data supports accurate performance prediction and operating condition optimization.

As an important component of technical services, Enlitech provides comprehensive support systems for the SPD2200. The system adopts modular design, supporting broad spectral wavelength ranges from 400 to 1700 nanometers, with up to 8-channel parallel testing capability, significantly improving testing efficiency. Professional technical training and application consulting services ensure users can fully leverage the system’s technical potential.

As single photon detection technology finds deeper applications in quantum information, autonomous driving, precision medicine, and other fields, precise measurement of afterpulsing parameters becomes increasingly important. Through its breakthrough technological innovations, the SPD2200 system not only solves current measurement challenges but also establishes a solid foundation for future SPAD technology development. Precision measurement capability, as an important driving force for technological innovation, will continue to propel the entire photon detection industry toward higher performance and higher reliability goals.

Contact Information

Technical consultation contact: qeservice@enli.com.tw

Product details: https://enlitechnology.com/product/spd2200/