Light detection and ranging (LIDAR) is rapidly emerging as a key technology for depth sensing applications. LIDAR acts as an auxiliary ranging technique, such as radar, thermal image ranging, image recognition, and time-of-flight (TOF) sensor. As a special type of direct time-of-flight (D-TOF) system, this work presents the system design and the hardware implementation of a one dimension time-of-flight (1D-TOF) sensor that uses LIDAR with single-photon avalanche diodes (SPADs). The system is designed for consumer electronics, serving for short-distance target ranging in proximity and face detection.
A typical 1D-TOF LIDAR system is composed of a pulsed laser which emits photons, and a sensor which measures the photons that are reflected back from the target. In 1D-TOF LIDAR systems, the following elements are of great importance: SPAD, time-to-digital converter (TDC), TOF histogram, and depth estimation algorithm.
SPAD is the key component of the system, which is designed to detect a single photon. The target ranging can be estimated by measuring the travel time of the photons. With the help of SPADs, the detection of the photons is converted into a response of the SPAD circuity, in which the optical signal is converted to the electrical signal. And, we can measure the moment in which the electrical signal is active to estimate the travel time of the photons.
TDC is a tool for time measurement. In a 1D-TOF LIDAR system, it measures the exact moment, when a SPAD is triggered by a single photon. The measured time is called the timestamp of the photon.
A TOF histogram accumulates the timestamps of the photons at every time interval. In a TOF histogram, we can observe the time distribution of the photons. In essence, the photons come from two sources, noise and signal. With the help of algorithms, we can derive signal information and reject noise information from a TOF histogram. Also, the algorithm enables us to retrieve the TOF information of the histogram, in which the distance between the target and the sensor lies within.
In this work, the basic concepts of the 1D-TOF LIDAR system is described in chapter 1, and a prototype architecture is discussed in chapter 2. Next, the design is decomposed into stages, including system modeling in chapter 3, system trade-off analysis in chapter 4, algorithm design in chapter 5 and hardware implementation in chapter 6. Moreover, we also propose novel approaches both in system level modeling and algorithm design based on artificial intelligence methods.