Why Automotive LiDAR Uses APDs and SiPMs for Pulse Detection

Automotive LiDAR systems rely on high-gain photodetectors—specifically Avalanche Photodiodes (APDs) and Silicon Photomultipliers (SiPMs)—to detect extremely weak reflected laser pulses and enable safe autonomous driving. This article explains the physics of signal attenuation over long distances, how strict eye-safety regulations limit initial laser power, and why the internal gain mechanisms of APDs and SiPMs are required to overcome electronic noise and bright ambient sunlight to detect low-reflectivity obstacles.

The Challenge of Signal Attenuation

Autonomous vehicles must detect obstacles up to 200 or 300 meters away to allow enough time for braking or evasive maneuvers at highway speeds. LiDAR systems measure this distance by emitting a laser pulse and calculating the time it takes for the light to bounce off an object and return to the receiver.

As the laser pulse travels through the air, it experiences severe signal loss. According to the inverse-square law, the intensity of the light drops exponentially both on its way to the target and on its way back to the sensor. Furthermore, adverse weather conditions like fog, rain, and dust scatter the light, further degrading the signal. By the time a laser pulse returns from a distant object, only a microscopic fraction of the original light reaches the LiDAR receiver.

Eye-Safety Regulations Limit Laser Power

The simplest way to get a stronger return signal would be to increase the power of the transmitting laser. However, automotive LiDAR systems operate in public spaces and must adhere to strict Class 1 eye-safety standards. The laser cannot damage the naked human eye under any circumstances. Because engineers cannot simply crank up the laser power at the source, the burden falls entirely on the LiDAR receiver to be sensitive enough to detect the faintest return signals.

Detecting Low-Reflectivity Targets

Not all objects reflect light equally. While retroreflective road signs send a strong signal back to the LiDAR, dark objects absorb most of the laser energy. A black car, a pedestrian wearing dark clothing, or a rubber tire sitting on black asphalt may reflect less than 5% to 10% of the light that hits it. When a low-power, eye-safe laser hits a low-reflectivity object 200 meters away, the returning signal may consist of just a few individual photons.

How APDs and SiPMs Amplify Weak Signals

Standard PIN photodiodes cannot detect a handful of photons because the tiny electrical current they generate gets completely drowned out by the internal thermal noise of the receiving electronics. To solve this, automotive LiDAR receivers use high-gain photodetectors that amplify the signal before it hits the noisy amplification circuitry.

Avalanche Photodiodes (APDs) use a high reverse-bias voltage to create a strong internal electric field. When a returning photon strikes the silicon, it generates a free electron. The electric field accelerates this electron so rapidly that it collides with the silicon lattice, knocking loose additional electrons. This chain reaction, known as impact ionization or the "avalanche effect," multiplies a single photon into hundreds of electrons, creating a readable electrical current.

Silicon Photomultipliers (SiPMs) take this concept further. A SiPM is a dense array of thousands of microscopic APDs operating in "Geiger mode," meaning they are biased above their breakdown voltage. These micro-cells are called Single-Photon Avalanche Diodes (SPADs). When even a single photon hits one of these cells, it triggers a massive, self-sustaining avalanche that generates millions of electrons. By summing the output of thousands of these cells, a SiPM can accurately detect and measure light down to the single-photon level.

Overcoming Ambient Sunlight Noise

In addition to internal electronic noise, LiDAR receivers must deal with external noise from the sun. Bright sunlight floods the environment with infrared photons that constantly strike the LiDAR receiver.

Because APDs and SiPMs have high internal gain and extremely fast response times (measured in picoseconds or nanoseconds), the LiDAR system can isolate the tightly packed, ultra-short laser pulse from the continuous background glare of the sun. Without this high-speed internal amplification, the weak reflected laser pulses would be completely invisible against the backdrop of daytime driving conditions, rendering the autonomous vehicle blind to distant hazards.