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Modulated High Power and Narrow Pulse Width Laser Drive Circuit for Lidar System electronics Article Modulated High Power and Narrow Pulse Width Laser Drive Circuit for Lidar System Zhiwen Chen, Yingying Yan, Jun Shu, Kefu Liu * and Jian Qiu * School of Information Science and Technology, Fudan University, Shanghai 200433, China; [email protected] (Z.C.); [email protected] (Y.Y.); [email protected] (J.S.) * Correspondence: kfl[email protected] (K.L.); [email protected] (J.Q.) Abstract: This manuscript introduces a laser drive circuit for a light detection and ranging (Lidar) system. A Lidar system usually requires its drive circuit to provide laser pulses with nanosecond pulse width, >100 W peak power and high repetition frequency. However, the existing research results show difficulties in meeting these requirements. In order to reduce the pulse width and increase the peak power of laser pulses, special circuit design and component selection are used to optimize the parasitic parameters of the drive circuit, and GaN devices are used to increase the switching speed. The characteristics of laser pulses are tested under different input voltage, pulse per second and switch conducting time. Meanwhile, the reasons for the changes in these characteristics are analyzed and explained. In order to meet the requirements of the Lidar system to detect targets at different distances, a modulation method to change the peak power of the laser pulse is proposed. In our experiment, ideally, the peak power of the laser pulse reaches 135 W, and the pulse width is less than 2 ns at a pulse per second rate of 400 kHz. Keywords: laser drive circuit; TOF Lidar system; GaN devices; power modulation Citation: Chen, Z.; Yan, Y.; Shu, J.; Liu, K.; Qiu, J. Modulated High Power and Narrow Pulse Width Laser Drive Circuit for Lidar System. 1. Introduction Electronics 2021, 10, 823. A light detection and ranging (Lidar) system is a remote sensing method to measure https://doi.org/10.3390/ target ranges by using laser beams. It has great prospects for application in autonomous electronics10070823 driving, 3D imaging, face recognition and many other fields [1–3]. Academic Editor: Alexander Barkalov Most of the Lidar systems are based on time-of-flight (TOF) depth sensing. The TOF Lidar system calculates the time difference between the transmitted laser pulse and the echo Received: 26 February 2021 signal pulse received by the photodetector to obtain the distance of the target object [4]. Accepted: 27 March 2021 In a TOF Lidar system, the higher the emission peak power, the farther the detection Published: 30 March 2021 range and the shorter the laser pulse width, the more better the detection accuracy. The detection range of the Lidar system used for autonomous vehicles must be greater than Publisher’s Note: MDPI stays neutral 200 m with the measurement error less than 5 cm. According to the distance equation of with regard to jurisdictional claims in Lidar system [5], the peak power of the laser pulse should be higher than 100 W and the published maps and institutional affil- ideal pulse width should be of the order of a few nanoseconds. Both the pulse width and iations. peak power of the laser pulse are determined by the laser drive circuit, so it is necessary to design a high power and narrow pulse width laser drive circuit for Lidar system. At the same time, it is also important to ensure that the laser pulse is safe for human eyes. However, due to the influence of switching speed and parasitic parameters in the Copyright: © 2021 by the authors. circuit, the pulse current rise rate is limited, and the current of hundreds of amps cannot be Licensee MDPI, Basel, Switzerland. reached within a few nanoseconds. Therefore, in the same laser drive circuit, it is difficult This article is an open access article to meet the requirements of the narrow pulse width and high peak power at the same time. distributed under the terms and For example, the laser diode SPL LL90_3 from OSRAM can reach 70 W peak power at the conditions of the Creative Commons recommended voltage with a pulse width about 30 ns, but if a 10 ns pulse width is to be Attribution (CC BY) license (https:// achieved by changing the design of the drive circuit, the maximum laser pulse peak power creativecommons.org/licenses/by/ will be reduced to 50 W [6]. 4.0/). Electronics 2021, 10, 823. https://doi.org/10.3390/electronics10070823 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 823 2 of 15 In the existing studies on pulsed semiconductor laser drive circuits for Lidar systems, high peak power and narrow pulse width are still very important research considerations and have not been solved yet. In recent years, many researchers have been conducting research on semiconductor laser drive circuits, and Table1 shows some of the research results. The laser pulse generated by the existing laser drive circuit shows difficulty in meeting the requirements of the Lidar system for the peak power, pulse width and pulse per second of the laser pulse at the same time. For comparison, in the last row of Table1 , the parameters of the laser pulse generated by the drive circuit in this manuscript are listed. Table 1. Recent research results of semiconductor laser drive circuits. Reference Peak Power Pulse Width Pulse per Second [7] 70 W 20 ns 30 kHz [8] 100 W 10 ns 10 kHz [9] 50 W 8 ns 10 kHz [10] 150 W 14 ns 1 kHz Drive circuit in 135 W 2 ns 400 kHz this manuscript Low peak power will limit the detection range of the Lidar system, and a wider pulse width will also affect the measurement accuracy of the Lidar system. In addition, in order to adapt to different application scenarios of the Lidar system, there are also many requirements for the circuit volume, pulse per second (PPS), power modulation, etc. Therefore, it is necessary to design a special laser drive circuit to meet the requirements of a Lidar system. In this manuscript, a laser drive circuit for a TOF Lidar system is proposed. The influence of parasitic parameters on the laser pulse is analyzed in detail, which provides a reference for TOF Lidar system laser drive circuit design. GaN devices are used to reduce the switching time and further reducing the pulse width. Meanwhile, in order to meet the needs of different detection ranges of a Lidar system, the drive circuit can also realize the function of adjustable output power by modulating the switch. In our experiment, the laser pulse can achieve a peak power of 135 W and a pulse width less than 2 ns at 400 kHz PPS. Compared with existing research results, these parameters have been improved. 2. Drive Circuit Structure and Analysis 2.1. Circuit Structure Design The common semiconductor laser drive circuit structure is shown in Figure1; it mainly includes power supply, current limiting unit, energy storage unit, laser diode, switching unit and current sampling unit. Figure 1. Laser drive circuit structure. The current limiting unit can generally use two methods: resistance current limiting and inductor current limiting. When the resistance current limiting method is used, many unnecessary energy losses will be generated when the current passes through the current limiting resistor, and when the circuit works at a very high number of pulses per second, Electronics 2021, 10, 823 3 of 15 the current limiting resistor loses more energy and generates serious heat. However, the inductor current limiting method will not have a lot of energy loss problems. The energy in the current limiting process will be stored in the inductor and play an important role in the charging stage. The effect of the inductor current limiting method on the charging process and power modulation will be discussed in detail in Sections 2.2 and 4.3. The main problem of the inductance current limiting method is that the current limiting effect exists for a relatively short time, but in our short pulse width circuit, this does not affect the operation of the circuit. Based on the above analysis, the inductance current limiting method is chosen in the drive circuit we designed. The energy storage unit is usually a capacitor or several capacitors connected in parallel. In the pulse laser drive circuit, there are two options for the value of the energy storage capacitor. One is to take a large capacitance value. During a pulse discharge period, the voltage on the energy storage capacitor is almost unchanged, and the laser can be controlled by turning the switch on and off. However, this scheme is limited by the switching speed and the pulse width of the control signal, and the ultra-short pulse width control signal is very difficult to obtain through an external FPGA. The signal distortion and pulse width changes will occur during signal transmission. The other option is to select a small capacitance value. In a pulse discharge period, the capacitor is fully discharged to generate a laser pulse. The peak power and pulse width of the laser pulse are greatly affected by the value of the capacitor, and a suitable capacitor value can be selected to achieve an ultra-short pulse width of the laser pulse. Here, in order to obtain a nanosecond narrow pulse width, it is better to choose a small capacitance value. The switching unit needs a high-power fast switching device, such as Insulated Gate Bipolar Transistor (IGBT), Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), and Gallium Nitride Field-Effect Transistor (GaN FET). However, IGBT is far from meet- ing the requirements of Lidar system on switching time. MOSFET is a relatively mature switching device, and many laser drive circuits of Lidar systems also use MOSFET as the switching unit, but it is difficult to achieve a switching time of a few hundred picosec- onds or shorter.
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