Novel Detectors: SPADs offer possible photodetection solution for ToF LiDAR applications

Driving, especially in an urban environment, is a complex task, requiring a continuous stream of sensory information about the surroundings. A momentary lapse of the driver’s awareness, or lack of the relevant information, can lead to a fatal accident.

Novel Detectors: SPADs offer possible photodetection solution for ToF LiDAR applications
Novel Detectors: SPADs offer possible photodetection solution for ToF LiDAR applications

Novel Detectors: SPADs offer possible photodetection solution for ToF LiDAR applications

A self-driving car, once a technological fantasy, is likely being tested on some road at this moment. It has the potential of being a disruptive innovation—changing the way we live—if science and technology make it safe enough to be adopted by the general public.

Driving, especially in an urban environment, is a complex task, requiring a continuous stream of sensory information about the surroundings. A momentary lapse of the driver’s awareness, or lack of the relevant information, can lead to a fatal accident.

Humans drive instinctively: we do not calculate distances or speeds, we just somehow know what they are. A self-driving car, a machine, has no instinct. All driving “decisions” are based on real-time calculations of sensory inputs, such as distance, speed, color, or shape.

These inputs must be obtained with onboard sensory systems. A high-resolution, three-dimensional spatial view of the car’s surroundings is probably the most critical information needed—light-detection and ranging (LiDAR; also widely known as LiDAR) is a system that can provide this information.1

Light probing

LiDAR probes the surroundings with light. One approach, known as scanning time-of-flight (ToF) LiDAR, uses a laser that emits a pulse of light (see Fig. 1). At the instant of emission, an electronic clock is activated. The beam-steering mechanism directs the pulse in the desired direction. The pulse reflects from the target, and some fraction of the reflected light passes through the detection optics towards the photodetector.

In response, the photodetector, coupled to the frontend electronics, creates an electrical signal that deactivates the clock. The measured time of flight, Δt, allows the calculation of the distance, d, to the reflection point, namely =12t, where c  is the speed of light in the medium where the measurement is being made.

Duration of the pulse and its peak power are two crucial parameters. The first determines the distance resolution, while the second the maximum measurable distance. Simply stated, the ToF concept favors short-duration and high-peak-power pulses. The current designs achieve ~5 ns for the duration and ~100 W for the peak power. Sensing such pulses requires that the detection bandwidth is approximately equal to the inverse of the pulse duration, or ~200 MHz.

Only a tiny fraction of the light emitted by the laser reaches the photodetector. The actual amount depends, among other factors, on the distance to the target, the target’s reflectivity, and atmospheric conditions.2 In addition, the weak light signal is mixed in with information-less background (solar radiation, streetlights, or headlights).

In a basic analog detection system (see Fig. 2), the narrowband optical bandpass filter blocks out, though not completely, the background light. The photodetector, either a linear-mode avalanche photodiode (APD)3 or a silicon photomultiplier (SiPM),4 outputs a current pulse in response to the incident light.

The transimpedance amplifier converts the current pulse to the voltage pulse. If the instantaneous value of the voltage rises above some specified level, the trigger circuit stops the timer, giving the time of flight. This looks simple enough, so why is photodetection in ToF LiDAR challenging?

A noisy problem

The short answer is noise. The weak light signal, which already has an intrinsic noise (photon shot noise), has to compete with noise coming from several sources: unfiltered background, dark current and gain variation of the photodetector, and the amplifier. The measured distance uncertainty, approximately given by:

improves, for a given detection bandwidth (set by the pulse duration), with increasing signal-to-noise ratio, S/N, of the detected light signal.

The ratio must be greater than 1 for the detection to have any useful information, and the higher the ratio, the more accurate the distance measurement. The challenge is to maximize the S/N. Doing so for a given input light level favors a photodetector with a high spectral photosensitivity, high intrinsic gain with a small noise penalty (excess noise factor), low dark current, and small terminal capacitance.

Additional desirable features are a minimal time jitter and high dynamic range. Since no single photodetector exists that satisfies all of the above requirements, an actual engineering design involves numerous tradeoffs.

It is beyond the scope of this article to discuss these intricate compromises. Instead, the remaining sections focus on a photodetector, single-photon avalanche photodiode (SPAD),5 that only recently has found its way into designs of ToF LiDARs.

The structure of a SPAD is similar to that of an APD. Both have a p-n junction within which there is a high-field region where a charge-carrier multiplication, or avalanche, occurs due to impact ionization. The avalanche can be triggered by an injection of a conducting electron or a hole into the high-field region. Prior to the injection, the pair ensued either from a photon absorption, thermal fluctuation, or tunneling event; the latter two constitute dark counts.