Yole: What is the main benefit of SiPMs relative to other sensor technologies when applied to LiDAR?
SensL: Historically, LiDAR systems have been built with a wide variety of sensor types ranging from a simple PiN photodiode to vacuum-tube PMTs. With increasing performance demands from applications such as ADAS and drone systems, the number of available sensors that actually fulfill the requirements is limited. Generally, each sensor type will have implementation challenges, but in some cases those challenges can be met with practical system solutions.
A simple PiN photodiode has no internal gain and so requires external amplification that limits the SNR (signal to noise ratio) and bandwidth. Avalanche photodiodes (APDs) also require amplification and, in addition, suffer from sensor-to-sensor non-uniformity. Therefore SNR limits the APD and to a greater extent PiN photodiodes to only being suitable for short-range LiDAR.
SiPMs have very high internal gain and therefore do not require external amplification that would contribute additional noise to the signal. Therefore the SiPM will have a better SNR than the APD in many use cases.
Ambient light rejection needs to be carefully considered to successfully implement an SiPM based LIDAR system in order to avoid the sensor being saturated by sunlight due to it’s single photon sensitivity at 905nm wavelength. SensL has worked to solve this at a system level by using a combination of techniques. Firstly, by limiting the aperture so that the sensor field-of-view, secondly, by using a bandpass optical filter to block ambient light outside the wavelengths of interest and finally, by using a short laser pulse to provide a higher optical peak power while still being within eye-safe limits.
To illustrate the benefits of the SiPM versus an APD, SensL has taken the example of a long-range LiDAR system operating in bright light conditions. We have modeled an ADAS system at >100m with 100klux background, which corresponds to bright daylight and target reflectivity down to 5%. 10cm resolution was achieved at 100m in these conditions using an eye-safe laser. We modeled the same situation with an APD and it was only possible to range to 60m.
We think this limitation of APDs for long-distance ranging is becoming clear to the automotive companies now as well. The high gain of the SiPM when compared to an APD, and hence it’s single-photon sensitivity, is really the key factor that makes it uniquely possible to achieve long-distance ranging in a real-world situation.