Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

[Introduction]This white paper is intended to assist in the development of silicon photomultiplier (SiPM) based LiDAR (LiDAR, Light Detection and Ranging) systems. The following sections contain information on the design and implementation of laser, timing, and optical parameters for direct time-of-flight (ToF) rangefinders, as well as a detailed analysis of key aspects that must be considered when integrating SiPMs into such systems.

foreword

LiDAR is a ranging technology that is increasingly being used in applications such as mobile ranging, automotive ADAS (advanced driver assistance systems), gesture recognition, and 3D mapping. Compared to alternative sensor technologies such as avalanche photodiodes (APDs), PIN diodes, and PMTs, the use of silicon photomultipliers (SiPMs) as light sensors has many advantages, especially for mobile and high-volume products.

Onsemi’s SiPM provides:

● Single photon detection from 250 nm to 1100 nm

● Low voltage – easy to implement system requirements

● Low power consumption – lower operating voltage and simple readout electronics enable low power design

● High bandwidth and fast response time – minimizes ranging time

● Ability to take advantage of direct ToF ranging technology with low laser power consumption

● Low noise and high gain—achieves good signal-to-noise ratio (SNR)

● Standard CMOS manufacturing process – low cost, high consistency, scalable production

● Small size SMT package – 1mm sensor available

Migrating to SiPM sensor technology brings a different set of constraints compared to other sensors. This white paper is designed to help users take full advantage of the technology and achieve a working setup with SiPM sensors as quickly as possible.

To this end, ON semiconductor has created three tools to assist users; a MATLAB ranging model for simulation, a ranging demonstrator hardware setup, and this document.

● We created a detailed MATLAB model of the direct ToF system to facilitate the simulation of SiPM-based ranging applications. This model can be used to support the design of ranging systems and can be modified to simulate various applications and implementations.

● A SiPM-based LiDAR demonstration system has been built. This “first-generation” system was measured and used to validate the simulation results of the MATLAB model.

● This document is intended to help new users develop SiPM-based direct ToF ranging systems. It discusses the impact of various system and environmental factors on the resulting signal-to-noise ratio.

Design of Direct ToF Ranging System

The basic components required for a direct ToF ranging system are shown in Figure 1

● a pulsed laser with collimating optics

● a sensor with detection optics

● Timing and data processing electronics

Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

Figure 1. Overview of direct ToF ranging technology

This document focuses on the system design of lasers, sensors, readers, and application environments. The single-point, direct ToF baseline work in this white paper can be extended to more complex scanning and imaging systems. In direct ToF technology, a periodic laser pulse is directed at the target, usually at eye-safe power and wavelength in the infrared region.

The target diffuses and reflects the laser photons, some of which are reflected back to the sensor. The sensor converts the detected laser photons (and some due to noise) into electrical signals, which are then time-stamped by timing electronics. This flight time t can be used to calculate the distance D to the target. The calculation formula is D=ct/2, where c=light speed and t=flight time. The sensor must distinguish the returning laser photons from noise (ambient light). Each laser pulse captures at least one time stamp. This is called a single measurement.

Combining the data from many single measurements to produce one measurement, the signal-to-noise ratio can be greatly improved, from which the detected laser pulse timing can be extracted with high precision. There are several different readout techniques to obtain timing information from the detected pulses of laser photons, summarized as follows:

Ranging readout technology

● LED (leading edge recognition) – involves the detection of the rising edge of a multi-photon signal. The accuracy of the timing is determined by the ability to distinguish the rising edge of the returned optical signal. This technique is not affected by the laser pulse width.

● Full Waveform Digitization – The full waveform is digitized and can be oversampled to improve accuracy. This may be difficult to achieve for short laser pulses or high repetition rate sources.

● TCSPC (Time Correlated Single Photon Counting) – Provides the highest accuracy and maximum ambient light rejection. This technique requires that not one signal photon be detected per laser pulse. This technique is immune to ambient light, but requires short pulse times, high repetition rates, and fast timing electronics for fast and accurate measurements.

● SPSD (Single Photon Simultaneous Detection) – A form of TCSPC that provides high ambient light rejection. The electronics must be designed to deal with the blurring of the range.

Modeling a direct ToF ranging system

We created a MATLAB model of a direct ToF system. A block diagram of the model is shown in Figure 2. Given a set of system parameters similar to those shown in Table 1, the model aims to predict the overall performance of the system. The first step involves analyzing and calculating the illuminance of the sensor (including ambient light and laser), given a selected optical scene, which can be changed by changing the corresponding system parameters. By comparing the calculated illuminance with the saturation limit of the sensor, it is possible to verify that the chosen setting is suitable for ranging. In cases where a particular setting is not suitable for ranging, the improvement of the setting itself can be evaluated by changing the system parameters.

The second part of the model consists of a Monte Carlo simulator in which the random properties of the sensor, mainly photon detection efficiency (PDE) and time jitter, are reproduced. This step allows to obtain the realistic output of the sensor through simulation. Compared to the analysis part, this step takes into account timing information such as acquisition time, repetition rate of the laser, and laser pulse width. The results of the Monte Carlo simulation are passed to a readout model, usually a discriminator, followed by a TDC (time-to-digital converter), which produces a time-stamped histogram from which a range measure can be extracted.

Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

Figure 2. Calculation of illuminance combined with a Monte Carlo simulator, allowing the full system output to be reproduced.

Table 1. Variables in SiPM direct ToF ranging system

Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

Ranging histogram

The acquisition system takes a single measurement each time a laser pulse is emitted. Depending on many factors, including laser power and distance from the target, the number of laser photons detected per pulse can be low. Ideally, each detected photon would be time stamped. But the number of time stamps per single measurement may be limited by the TDC dead time. Typically, many single measurements of time, each containing one or more time stamps, combine to produce a frame. The complete timing data obtained during one frame can be plotted in the form of a histogram, as shown in Figure 3.

The system ranging performance is limited by the data quality of the histogram, which in turn is affected by the system parameters. As can be seen from the analysis of system parameters detailed in the section “Effects of Changing System Variables” on page 7, there are some constraints and some trade-offs that can be made. The ranging histogram used below also provides a visual representation, which is useful for describing the effect of various parameters on the acquired data. The basic histogram signals and timing parameters are described below.

Histogram of signal-to-noise ratio, SNRH, is the ratio of the signal peak to the maximum noise peak. SNRH = signal peak/noise peak.

In the model, the following terms apply to the measurement time: f = laser frequency.

The laser repetition rate limits the maximum ToF that can be measured without distortion, which defines the time per single measurement.

Single measurement time, tss = 1/f.

Frame size refers to the number of single measurements per histogram. Larger frame sizes can improve SNRH, resulting in a better quality histogram. Ranging speed is defined by frame rate: frame rate = number of ranging per second = 1/tacq

Silicon Photomultiplier Tubes for Direct Time-of-Flight Ranging Applications (1): Design of Direct ToF Ranging Systems

Figure 3. Example of a simulated histogram showing signal, noise, and time of flight

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