Physics
Single-Photon Avalanche Diode Arrays for Time-of-Flight LiDAR
Quick fact
A SPAD can detect a single photon with a timing jitter of only a few tens of picoseconds, enabling distance measurements with centimeter-level accuracy over distances of hundreds of meters.
Why this is interesting
Imagine being able to 'see' a single photon of light as it bounces off an object and returns to you. What if such a tiny signal was the key to self-driving cars and robot vision?
Read the full explanation
Understanding Single-Photon Avalanche Diode Arrays for Time-of-Flight LiDAR
Think of a SPAD as a super-sensitive pixel that can 'catch' a single photon. It's a semiconductor diode biased above its breakdown voltage, so when a photon hits it, it triggers a small avalanche of current—a controlled electrical landslide inside the silicon. Because this avalanche happens incredibly fast, the SPAD provides a precise signal that marks the exact moment a photon arrived. In a ToF LiDAR system, we emit a short laser pulse and use an array of these SPADs to detect the reflected light. By measuring the time between emission and detection, we compute the distance to the object using the speed of light. SPAD arrays are essentially a grid of these tiny detectors, each with its own timing electronics. This allows the system to capture depth information for many points simultaneously, building a detailed 3D map of the scene.
A deeper explanation
The core mechanism that makes a SPAD special is Geiger-mode operation: the photodiode is biased above its breakdown voltage, so a single absorbed photon can trigger a self-sustaining avalanche. This avalanche needs to be stopped (quenched) and reset for the next detection, typically using active or passive quenching circuits. The key advantage for LiDAR is the exceptional timing accuracy: because the avalanche is triggered immediately upon photon absorption, the electrical pulse is tightly synchronized with the photon arrival. ToF LiDAR exploits this by measuring the round-trip time of laser pulses: distance = (c × Δt) / 2. SPAD arrays combine many such detectors on a single chip, enabling compact, high-resolution depth sensors that work in low-light conditions and even at long ranges. This is why they're increasingly used in autonomous vehicles, drones, and robotics, where precise 3D perception is critical.