Patent Publication Number: US-2022221557-A1

Title: Systems and methods for controlling laser power in light detection and ranging (lidar) systems

Description:
TECHNICAL FIELD 
     The present disclosure relates to Light Detection and Ranging (LiDAR) systems, and more particularly to, systems and methods for controlling power of laser pulses in the LiDAR systems. 
     BACKGROUND 
     Optical sensing systems such as LiDAR systems have been widely used in autonomous driving and producing high-definition maps. For example, a typical LiDAR system measures the distance to a target by illuminating the target with pulsed laser light beams and measuring the reflected pulses with a sensor such as a photodetector. Differences in laser light return times, wavelengths, and/or phases can then be used to construct digital three-dimensional (3D) representations of the target. Because using a narrow laser beam as the incident light can map physical features with very high resolution, a LiDAR system is particularly suitable for applications such as sensing in autonomous driving and/or high-definition map surveys. 
     A LiDAR system can use a transmitter to transmits a signal (e.g., pulsed laser light) into the surroundings, and use a receiver to collect the returned signal (e.g., laser light reflected by an object in the surroundings). The LiDAR system can then calculate parameters such as the distance between the object and the LiDAR system based on, e.g., the speed of light and the time the signal travels (e.g., the duration of time between the time the signal is transmitted and the time the returned signal is received) and use the parameters to construct 3D maps and/or models of the surroundings. To improve the detection range and the signal-to-noise-ratio (SNR), higher energy of the laser light is often needed. On the other hand, however, the energy of the signal also needs to be limited to avoid potential harm to human eyes. Therefore, it is challenging to balance the performance demands and regulatory safety mandate in LiDAR system development. 
     Embodiments of the disclosure address the above challenges by systems and methods for controlling power of laser pulses used in LiDAR systems. 
     SUMMARY 
     Embodiments of the disclosure provide a system for controlling laser pulses emitted by an optical sensing device. The system may include a laser emitter configured to emit a plurality of laser pulses, a power source coupled to the laser emitter and configured to deliver electrical currents to the laser emitter for emitting the plurality of laser pulses, and a control circuit configured to deliver electrical currents from the power source to the laser emitter. The control circuit may include a first control path configured to deliver a first electrical current rising at a first rate from the power source to the laser emitter to emit a first laser pulse. The control circuit may also include a second control path configured to deliver a second electrical current rising at a second rate from the power source to the laser emitter to emit a second laser pulse following the first laser pulse. The second rate may be higher than the first rate. 
     Embodiments of the disclosure also provide a method for controlling laser pulses emitted by an optical sensing device. The method may include providing, by a controller of the optical sensing device, a first control signal to a first control path to deliver a first electrical current rising at a first rate from a power source to a laser emitter to emit a first laser pulse to an environment surrounding the optical sensing device. The method may also include determining, by the controller, whether an object is present in the environment based on feedback received from the environment resulting from the emission of the first laser pulse. After it is determined that no object is present in the environment, the method may further include, providing, by the controller, a second control signal to a second control path to deliver a second electrical current rising at a second rate from the power source to the laser emitter to emit a second laser pulse to the environment. The second rate may be higher than the first rate. 
     Embodiments of the disclosure also provide an optical sensing system. The optical sensing system may include a laser emitter configured to emit a plurality of laser pulses, a control circuit coupled to the laser emitter and configured to deliver electrical currents from a power source to the laser emitter to emit the plurality of laser pulses, and a controller coupled to the control circuit and configured to provide control signals to the control circuit to control delivery of the electrical currents. The control circuit may include a first control path configured to deliver a first electrical current rising at a first rate from the power source to the laser emitter to emit a first laser pulse to an environment surrounding the optical sensing system. The controller may be configured to determine whether an object is present in the environment based on feedback from the environment resulting from the emission of the first laser pulse. The control circuit may further include a second control path configured to deliver a second electrical current rising at a second rate from the power source to the laser emitter to emit a second laser pulse following a determination that no object is present in the environment. The second rate may be higher than the first rate. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of an exemplary vehicle equipped with a LiDAR system, according to embodiments of the disclosure. 
         FIG. 2  illustrates a block diagram of an exemplary LiDAR system, according to embodiments of the disclosure. 
         FIG. 3  illustrates a block diagram of an exemplary controller for controlling the power of laser pulses in a LiDAR system, according to embodiments of the disclosure. 
         FIG. 4  illustrates an exemplary two-pulse laser emission method, according to embodiments of the disclosure. 
         FIG. 5  illustrates an exemplary control system for a LiDAR system, according to embodiments of the disclosure. 
         FIG. 6  illustrates another exemplary control system for a LiDAR system, according to embodiments of the disclosure. 
         FIG. 7  illustrates exemplary control signal curves used and corresponding electrical current curves generated in a LiDAR system, according to embodiments of the disclosure. 
         FIG. 8  illustrates a flowchart of an exemplary method for controlling laser pulses emitted by a LiDAR system, according to embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a schematic diagram of an exemplary vehicle  100  equipped with a LiDAR system  102 , according to embodiments of the disclosure. Consistent with some embodiments, vehicle  100  may be a survey vehicle configured for acquiring data for constructing a high-definition map or 3-D buildings and city modeling. Vehicle  100  may also be an autonomous driving vehicle. 
     As illustrated in  FIG. 1 , vehicle  100  may be equipped with an optical sensing system (e.g., a LiDAR system)  102  (also referred to as “LiDAR system  102 ” hereinafter) mounted to a body  104  via a mounting structure  108 . Mounting structure  108  may be an electro-mechanical device installed or otherwise attached to body  104  of vehicle  100 . In some embodiments of the present disclosure, mounting structure  108  may use screws, adhesives, or another mounting mechanism. In some embodiments, LiDAR system  102  may be integrated with body  104  of vehicle  100  without using mounting structure  108 . It is contemplated that the manners in which LiDAR system  102  can be equipped on vehicle  100  are not limited by the example shown in  FIG. 1  and may be modified depending on the types of LiDAR system  102  and/or vehicle  100  to achieve desirable 3D sensing performance. 
     Consistent with some embodiments, LiDAR system  102  may be configured to capture data as vehicle  100  moves along a trajectory. For example, a transmitter of LiDAR system  102  may be configured to scan the surrounding environment. LiDAR system  102  measures distance to a target by illuminating the target with laser beams and measuring the reflected/scattered laser signals with a receiver. The laser beams used for LiDAR system  102  may be ultraviolet, visible, or near-infrared, and may be pulsed or continuous wave laser beams. In some embodiments, LiDAR system  102  may capture point clouds including depth information of the objects in the surrounding environment, which may be used for constructing a high-definition map or 3-D buildings and city modeling. As vehicle  100  moves along the trajectory, LiDAR system  102  may continuously capture data including the depth information of the surrounding objects (such as moving vehicles, buildings, road signs, pedestrians, etc.) for a map, building, or city modeling construction. 
     Consistent with the present disclosure, a controller may be included for processing and/or analyzing collected data for various operations. For example, the controller may process received signals and control any operations based on the processed signals. The controller may also communicate with a remote computing device, such as a server (or any suitable cloud computing system) for operations of LiDAR system  102 . Components of the controller may be in an integrated device or distributed at different locations but communicate with one another through a network. In some embodiments, the controller may be located entirely within LiDAR system  102 . In some embodiments, one or more components of the controller may be located in LiDAR system  102 , inside vehicle  100 , or may be alternatively in a mobile device, in the cloud, or another remote location. 
     In some embodiments, the controller may process the received signal locally. In some alternative embodiments, the controller is connected to a server for processing the received signal. For example, the controller may stream the received signal to the server for data processing and receive the processed data from the server. 
       FIG. 2  illustrates a block diagram of an exemplary LiDAR system, according to embodiments of the disclosure. As illustrated, LiDAR system  102  may include a transmitter  202 , a receiver  204 , and a controller  206  coupled to transmitter  202  and receiver  204 . Transmitter  202  may emit optical beams (e.g., pulsed laser beams, continuous wave (CW) beams, frequency modulated continuous wave (FMCW) beams, etc.) along multiple directions. Transmitter  202  may include one or more laser sources  208  for emitting laser beams and one or more scanners  210  for directing the emitted laser beams into multiple directions. According to one example, transmitter  202  may sequentially emit a stream of laser beams in different directions within a scan filed-of-view (FOV) (e.g., a range in angular degrees), as illustrated by dotted-dashed lines in  FIG. 2 . 
     Laser source  208  may be configured to emit laser beams  207  (also referred to as “native laser beams”) to scanner  210 . For instance, laser source  208  may generate laser beams in the ultraviolet, visible, or near-infrared wavelength range, and provide the generated laser beams to scanner  210 . In some embodiments, depending on underlying laser technology used for generating laser beams, laser source  208  may include one or more of a double heterostructure (DH) laser emitter, a quantum well laser emitter, a quantum cascade laser emitter, an interband cascade (ICL) laser emitter, a separate confinement heterostructure (SCH) laser emitter, a distributed Bragg reflector (DBR) laser emitter, a distributed feedback (DFB) laser emitter, a vertical-cavity surface-emitting laser (VCSEL) emitter, a vertical-external-cavity surface-emitting laser (VECSEL) emitter, an extern-cavity diode laser emitter, etc., or any combination thereof. Depending on the number of laser emitting units included in laser source  208 , laser source  208  may include a single emitter containing a single light-emitting unit, a multi-emitter unit containing multiple single emitters packaged in a single chip, an emitter array or laser diode bar containing multiple (e.g., 10, 20, 30, 40, 50, etc.) single emitters in a single substrate, an emitter stack containing multiple laser diode bars or emitter arrays vertically and/or horizontally built up in a single package, etc., or any combination thereof. Depending on the operating time, laser source  208  may include one or more of a pulsed laser diode (PLD), a CW laser diode, a Quasi-CW laser diode, etc., or any combination thereof. Depending on the semiconductor materials of diodes in laser source  208 , the wavelength of laser beams  207  may be at different values, such as 405 nm, between 445 nm and 465 nm, between 510 nm and 525 nm, 532 nm, 635 nm, between 650 nm and 660 nm, 670 nm, 760 nm, 785 nm, 808 nm, 848 nm, 870 nm, 905 nm, 940 nm, 980 nm, 1064 nm, 1083 nm, 1310 nm, 1370 nm, 1480 nm, 1512 nm, 1550 nm, 1625 nm, 1654 nm, 1877 nm, 1940 nm, 2000 nm, etc. It is understood that any suitable laser source may be used as laser source  208  for emitting laser beams  207  at a proper wavelength. 
     Transmitter  202  may include one or more optical components (not shown) such as lenses, mirrors, etc. that can shape the laser light (e.g., laser beams  207 ), e.g., collimate the laser light into a narrow laser beam  209 , to increase scan resolution and/or scan range. Scanner  210  may be configured to alter the emission angle of laser beams  209  to scan the FOV of transmitter  202  to detect an object  212  in the surrounding environment. 
     Object  212  may be made of a wide range of materials including, for example, non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds, and even single molecules. The wavelength of laser beams  209  may be adjusted based on the composition of object  212 . In some embodiments, at each time point during the scan, scanner  210  may emit laser beams  209  to object  212  in a direction within a range of scanning angles by rotating a deflector, such as a micromachined mirror assembly. 
     Receiver  204  may be configured to detect returned laser beams  211  returned from object  212 . Upon contact with object  212 , laser light can be reflected/scattered by object  212  via backscattering, such as Rayleigh scattering, Mie scattering, Raman scattering, and fluorescence. Returned laser beams  211  may be in a same or different direction from laser beams  209 . For instance, in some embodiments, receiver  204  may be disposed in close proximity to transmitter  202  to detect returned laser beams  211  that may be in a same direction (albeit opposite) as laser beams  209 . On the other hand, receiver  204  may be located elsewhere to detect the returned laser beams  211  that may be in a different direction from laser beams  209 . In some embodiments, after receiving laser beams  211  returned from object  212 , receiver  204  may process the received laser beams and output signals reflecting the intensity of returned laser beams  211 . 
     In some embodiments, receiver  204  may include a lens  214 , a photodetector  216 , and a readout circuit  218 . Lens  214  may be configured to collect light from a respective direction in a receiver FOV (shown by dotted-dashed lines) and converge the returned laser beams  211  to focus on photodetector  216 . At each time point during the scan, returned laser beams  211  may be collected by lens  214 . Laser signals in returned laser beams  211  may have the same waveform (e.g., bandwidth and wavelength) as those in laser beams  209 . 
     Photodetector  216  may be configured to detect returned laser beams  211  returned from object  212  and converged by lens  214 . In some embodiments, photodetector  216  may convert the laser light (e.g., returned laser beams  211 ) converged by lens  214  into an electrical signal  213  (e.g., a current or a voltage signal). Electrical signal  213  may be an analog signal, or even a digital signal in some embodiments, which is generated when photons are absorbed in a photosensor included in photodetector  216 . In some embodiments, photodetector  216  may include a PIN detector, an avalanche photodiode (APD) detector, a single-photon avalanche diode (SPAD) detector, a silicon photo multiplier (SiPM) detector, or the like. 
     Readout circuit  218  may be configured to integrate, amplify, filter, and/or multiplex signals detected by photodetector  216  and transfer the integrated, amplified, filtered, and/or multiplexed signal  215  to controller  206  for further processing. In some embodiments, readout circuit  218  may act as an interface between photodetector  216  and a signal processing unit (e.g., controller  206 ). Depending on the configuration, readout circuit  218  may include one or more of a transimpedance amplifier (TIA), an analog-to-digital converter (ADC), a time-to-digital converter (TDC), or the like. 
     Controller  206  may be configured to control transmitter  202  and/or receiver  204  to perform optical signal sensing/detection operations. For instance, controller  206  may control laser source  208  to emit laser beams  207 , or control scanner  210  to direct laser beams  209  into multiple directions. In some embodiments, controller  206  may also be implemented to perform data acquisition and analysis functions. For instance, controller  206  may collect digitalized signal information from readout circuit  218 , determine the distance of object  212  from LiDAR system  102  according to the travel time of laser beams, and construct a high-definition map or 3-D buildings or city modeling surrounding LiDAR system  102  based on the distance information of object(s)  212 . 
     In some embodiments, partial or full functions of controller  206  may be distributed to a similar component (e.g., a microcontroller) located in transmitter  202 , receiver  204 , or distributed to another local or remote computing device. 
       FIG. 3  shows an exemplary implementation of controller  206 , according to embodiments of the disclosure. As shown in  FIG. 3 , controller  206  may include a communication interface  340 , a processor  310 , a memory  320 , and a storage  330 . In some embodiments, controller  206  may have different modules in a single device, such as an integrated circuit (IC) chip (implemented as, for example, an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA)), or separate devices with dedicated functions. In some embodiments, one or more components of controller  206  may be located in a cloud, or may be alternatively in a single location (such as inside vehicle  100  or a mobile device) or distributed locations. Components of controller  206  may be in an integrated device, or distributed at different locations but communicate with each other through a network. 
     Communication interface  340  may send data to and receive data from components such as receiver  204  and transmitter  202  via communication cables, a Wireless Local Area Network (WLAN), a Wide Area Network (WAN), wireless communication links such as radio waves, a cellular network, and/or a local or short-range wireless network (e.g., Bluetooth™), or other communication methods. In some embodiments, communication interface  340  can be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection. As another example, communication interface  340  can be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links can also be implemented by communication interface  340 . In such an implementation, communication interface  340  can send and receive electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Processor  310  may include any appropriate type of general-purpose or special-purpose microprocessor, digital signal processor, or microcontroller. Processor  310  may be configured as a stand-alone processor module dedicated to analyzing signals and/or controlling the emission of laser pulses. Alternatively, processor  310  may be configured as a shared processor module for performing other functions unrelated to signal analysis/laser pulse emission control. 
     Memory  320  and storage  330  may include any appropriate type of mass storage provided to store any type of information that processor  310  may need to operate. Memory  320  and/or storage  330  may be volatile or non-volatile, magnetic, semiconductor-based, tape-based, optical, removable, non-removable, or other type of storage device or tangible (i.e., non-transitory) computer-readable medium including, but not limited to, a ROM, a flash memory, a dynamic RAM, a static RAM, a hard disk, an SSD, an optical disk, etc. Memory  320  and/or storage  330  may be configured to store one or more computer programs that may be executed by processor  310  to perform functions disclosed herein. For example, memory  320  and/or storage  330  may be configured to store program(s) that may be executed by processor  310  to analyze LiDAR signals and/or control laser pulse emission. 
     In a LiDAR system, the detection distance and image resolution highly depend on the power level of laser output, e.g., power of laser beam  209 . For example, the power of laser beam  209  needs to be sufficiently high for LiDAR system  102  to detect object  212  from a desired distance. However, in many LiDAR systems, laser power is strictly regulated due to its potential damage to human eyes. Traditionally, the energy or power of each laser pulse is set to be within the regulation/safety limit, based on the assumption or worst case scenario that all pulses emitted towards a 7 mm pupil-size aperture is absorbed by the aperture. For high resolution and long-range LiDAR systems, this becomes a fundamental challenge. First, high resolution LiDAR may transmit more pulses with small spatial step size, which means 10 or more pulses would be inside the 7 mm pupil area at close distance, forcing each pulse to be lower energy. Also, long distance LiDAR requires high energy pulses, which is difficult or even impossible to achieve with the power limit set by the regulation. 
     To reach longer detection distance without violating the eye-safety rules, a two-pulse laser emission scheme has been developed, in which the first laser pulse having low power is used to detect whether there is any object present in the near-field. If no object in the near-field is detected, the second laser pulse having much higher power is then emitted for far-field, long-distance detection. To maintain relevancy of the detection result of the first laser pulse, the second laser pulse needs to be emitted shortly after the first laser pulse. For example, the interval between the two pulses may be in the order of nano seconds, e.g., less than 100 nano seconds, less than 10 nano seconds, or even a few nano seconds. Within such a small interval, it is difficult to generate two pulses having a large power ratio, because the same laser emitter is usually used to generate both pulses and the typical driving circuit of the laser emitter cannot be reconfigured fast enough. 
     The present disclosure provides systems and methods for achieving high power ratio between the second laser pulse and the first laser pulse by increasing the driving current rising rate of the second laser pulse so that the power of the second laser pulse builds up much faster than that of the first laser pulse. Details of the embodiments are described in greater detail as follows. 
       FIG. 4  illustrates an exemplary two-pulse laser emission method, according to embodiments of the disclosure. As shown in  FIG. 4 , an environment surrounding LiDAR system  102  can be schematically divided into a near-field region (e.g., within distance D 1  from LiDAR system  102 ) and a far-field region (e.g., outside distance D 1 , such as at a distance D 2 &gt;D 1 ). It is noted that the division between near- and far-field regions can be system dependent, and may not have a clear boundary as shown in  FIG. 4 . The purpose of the two-pulse laser emission method is to ensure that no high-power laser pulse is emitted along a direction in which an object is present in the near-field region. Such an object can be detected by a low-power laser pulse (also referred to as a pilot laser pulse or the first laser pulse) before any high-power laser pulse (also referred to as a follow-up laser pulse or the second laser pulse) is emitted along the same direction as the lower-power laser pulse. For example, as shown in  FIG. 4 , transmitter  202  may first emit a low-power laser pulse  410  into the environment along a scanning direction. The power of laser pulse  410  may be under the power threshold set by the eye-safety regulations. If an object  402  is present along the scanning direction, laser pulse  410  is reflected back as a reflected laser pulse  412  and detected by receiver  204 . Because an object is detected in the near-field region, no follow-up high-power laser pulse will be emitted. In another example, transmitter  202  may emit another low-power laser pulse  430  along another scanning direction. This time, no object is present, so no laser pulse is reflected back, or any returned laser pulse is not strong enough to indicate the presence of an object. Transmitter  202  then emit a high-power laser pulse  440  shortly after low-power laser pulse  430  along the same direction of low-power laser pulse  430  to reach the far-field region. In this way, the detection range of LiDAR system  102  can be increased. 
       FIGS. 5 and 6  illustrate two exemplary control systems for controlling laser pulse emission in a LiDAR system.  FIG. 7  illustrates exemplary control signal curves used and corresponding electrical current curves generated in the LiDAR system equipped with the control systems shown in  FIG. 5 or 6 .  FIG. 8  illustrates a flowchart of an exemplary method for controlling laser pulses emitted by the LiDAR system equipped with the control systems shown in  FIG. 5 or 6 .  FIGS. 5-8  will be described together below. 
     Referring to  FIG. 5 , an exemplary control system  500  may include a laser emitter  510  configured to emit a plurality of laser pulses. Laser emitter  510  may include any laser emitting units of laser source  208  describe above. For simplicity, a laser emitting diode is used to represent laser emitter  510 . Laser emitter  510  may be coupled to a power source  512 . Stray inductance can be represented by an inductor  518  between laser emitter  510  and power source  512 . In addition, a clamp diode  516  is connected in parallel to laser emitter  510  to limit the voltage drop across laser emitter  510 . A decoupling capacitor  514  is connected across power source  512  to stabilize the power supply voltage level. Power source  512  may include any suitable power sources and is represented by a voltage source in  FIG. 5 . Power source  512  may be configured to deliver electrical currents to laser emitter  510  for emitting the plurality of laser pulses. The delivery of electrical currents from power source  512  to laser emitter  510  can be controlled by a control circuit including a first control path  520  and a second control path  530 . 
     Control path  520  may include a first switching device  526  coupled to laser emitter  510 . Switching device  526  may include any suitable switches such as a field-effect transistor (FET). For example, switching device  526  may include a metal-oxide-semiconductor field-effect transistor (MOSFET), a gallium nitride field-effect transistor (GaNFET), etc. Switching device  526  may be controlled by a first control signal applied to its gate terminal. In  FIG. 5 , the first control signal is shown to be generated by a first trigger generator  522  and applied to the gate terminal of switching device  526  through a first operational amplifier (opamp)  524 . In some embodiments, trigger generator  522  and/or opamp  524  may be part of controller  206 , or controller  206  may generate the first control signal and apply the first control signal to the gate terminal of switching device  526  to control the switching operations of switching device  526 . In some embodiments, control path  520  may include a resistor  528  coupled to switching device  526 . In other embodiments, resistor  528  may be omitted. 
     Control path  530  may include a second switching device  536  coupled to laser emitter  510 . Similar to switching device  526 , switching device  536  may include any suitable switches, such as MOSFET, GaNFET, etc. Switching device  536  may be controlled by a second control signal applied to its gate terminal. In  FIG. 5 , the second control signal is shown to be generated by a second trigger generator  532  and applied to the gate terminal of switching device  536  through a second operational amplifier (opamp)  534 . In some embodiments, trigger generator  532  and/or opamp  534  may be part of controller  206 , or controller  206  may generate the second control signal and apply the second control signal to the gate terminal of switching device  536  to control the switching operations of switching device  536 . 
       FIG. 6  shows another exemplary control system  600 , which is similar to control system  500  except the control circuit portion. As shown in  FIG. 6 , first and second control paths  620  and  630  share the same switching device  626 , opamp  624 , and trigger generator  622 . A variable resistor  628  is coupled between laser emitter  510  and switching device  626  and configured to be set to different resistance values for first and second control paths. Variable resistor  628  may be implemented in various ways. For example, variable resistor  628  may include a resistor  644  and a resistance controller  642  coupled to resistor  644 . Resistance controller  642  may be implemented by a FET device and configured to short-circuit resistor  644  when the FET device is switched on, thereby altering the resistance of variable resistor  628 . 
     An example method  800  for controlling laser pulse emission is shown in  FIG. 8 . Method  800  may be performed by LiDAR system  102  and may include steps  810 - 860 . It is noted that in some embodiments, one or more steps may be omitted. In addition, the steps may be performed in a different order than that shown in  FIG. 8 , and multiple steps may be performed simultaneously. 
     In step  810 , controller  206  may provide a first control signal to a first control path (e.g., control path  520 / 620 ) to deliver a first electrical current rising at a first rate from a power source (e.g., power source  512 ) to a laser emitter (e.g., laser emitter  510 ). Referring back to  FIG. 5 , controller  206  (e.g., represented by trigger generator  522 ) may provide the first control signal to the gate terminal of switching device  526  through opamp  524  to switch on switching device  526 . This operation is also shown in  FIG. 7 , in which the first control signal is shown as a square wave trigger CS 1  in dashed line spanning from T 0  to T 1 . The first control signal CS 1  has a voltage level V TR , which is sufficiently high to switch on switching device  526 . Once switching device  526  is switched on, a current loop is formed, from power source  512  through inductor  518 , laser emitter  510 , resistor  528  (if present), switching device  526 , to the ground. As a result, a first electrical current starts to flow from power source  512  to laser emitter  510 . The first electrical current is shown in  FIG. 7 , first as a rising ramp starting from T 0 , reaching to the peak of I PK1  at T 1 , and then ramping down to zero at T 3 . The rising rate from T 0  to T 1  is denoted as RT 1 . Mathematically, the first electrical current i LASER  during T 0  to T 1  can be written as a function of time as: 
     
       
         
           
             
               
                 
                   
                     
                       
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     when resistor  528  is not present, where V supply  is the supply voltage of power source  512 , V LD  is the operation voltage of laser emitter  510 , and L stray  is the inductance of stray inductor  518 . When resistor  528  is present, the first electrical current i LASER  during T 0  to T 1  can be written as a function of time as: 
     
       
         
           
             
               
                 
                   
                     
                       
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                   ) 
                 
               
             
           
         
       
     
     where R atten  is the resistance of resistor  528 . The rising rate of the first electrical current can be written as the first derivative of i LASER : RT 1 =di LASER /dt. Rising rate RT 1  can be approximated using the angle formed by the rising portion of the first electrical current and the time axis, as shown in  FIG. 7 . 
     Referring back to  FIG. 8 , as the first electrical current is flowing through laser emitter  510 , laser emitter  510  may emit a first laser pulse to an environment surrounding LiDAR system  102  (step  820 ). In some embodiments, the first electrical current is controlled to be small enough so that the power of the emitted laser pulse meets the eye-safety requirements. For example, laser pulse  410  in  FIG. 4  may represent the first laser pulse. In step  830 , receiver  204  may receive feedback from the environment resulting from the emission of the first laser pulse. For example, photodetector  216  may detect any returned laser pulse resulting from the first laser pulse being reflected by an object in the near-field region of the environment. In step  840 , controller  205  may determine whether an object is present in the environment based on the feedback. For example, when a reflected laser pulse is detected by photodetector  216 , or the detected signal is larger than a threshold, controller  206  may determine that an object is present (e.g., the scenario of receiving reflected laser pulse  412  in  FIG. 4 ). Otherwise, controller  206  may determine that no object is present (e.g., the scenario of receiving no reflected laser pulse after emitting laser pulse  430  in  FIG. 4 ). Based on the determination, a selection is made in step  850 . If an object is present in the environment (YES branch of step  850 ), method  800  loops back to step  810  to start another emission cycle of emitting a low-power pilot pulse. If no object is present (NO branch of step  850 ), method  800  proceeds to step  860 , in which controller  206  may provide a second control signal to a second control path (e.g.,  530 / 630 ) to deliver a second electrical current rising at a second rate from the power source to the laser emitter. The emission of the second laser pulse (shown in  FIG. 4  as laser pulse  440 ) is described in more detail below. 
     Referring back to  FIG. 5 , control path  530  is used to deliver a second electrical current to laser emitter  510  to emit the second laser pulse. Similar to the case of control path  520 , switching device  536  of control path  530  can be switched on by a second control signal provided by controller  206  (represented by trigger generator  532  in  FIG. 5 ) and applied to the gate terminal of switching device  536  through opamp  534 . In  FIG. 7 , this switching on process is shown from T 4  to T 5 , a duration the second control signal (e.g., a trigger voltage CS 2 ) lasts. During this time period, the second electrical current ramps up rising at a second rate RT 2 , similarly represented by equation (1) and approximated by the angle formed by the rising portion of the second electrical current and the time axis. The second electrical current reaches its peak I PK2  at time point T 5 , then ramps down to zero at time point T 6 . 
     One objective of the present application is to generate a relatively large power ratio between the first and second laser pulses. Conventional two-pulse systems fail to achieve this because RT 1  and RT 2  are the same in those systems. As a result, the second electrical current rises at the same rate as the first electrical current during the switching on period, and the only way to generate a second pulse having a higher power is to prolong the trigger duration (e.g., T 5 −T 4 ), resulting in larger pulse width. However, larger pulse width will degrade the performance of LiDAR detection. Embodiments of the present application address this problem by making RT 2 &gt;RT 1 . Therefore, the second electrical current rises faster than the first electrical current, thereby reach a higher peak during the same trigger duration. In this way, a higher power second laser pulse can be emitted. 
     There are several ways to achieve a larger RT 2  than RT 1 . In one example, as shown in  FIG. 5 , resistor  528  may be used to increase the resistance of the first control path  520 . As a result, the first electrical current is represented by equation (2). In the second control path  530 , no resistor is used. Therefore, the second electrical current is represented by equation (1). Comparing equations (1) and (2), the introduction of resistor  528  reduces the rising rate of the electrical current used to drive laser emitter  510 , thereby achieving RT 1 &lt;RT 2 . 
     In another example, different kinds of devices may be used as switching devices  526  and  536 . To keep the rising rate of the first electrical current slow, switching device  526  may have a slower switching speed than switching device  536 . For instance, a MOSFET may be used as switching device  526 , while a faster GaNFET may be used as switching device  536 . 
     Using slower and faster switching devices in the first and second control paths, respectively, and introducing a resistor into the first control path can each independently achieve RT 1 &lt;RT 2 . In some embodiments, these two methods can also be combined. 
     In a further example, as shown in  FIG. 6 , instead of using two separate switching devices, a single switching device  626  may be shared by the first and second control paths  620  and  630 . Here, control paths  620  and  630  refers to electrical current passages at different operation durations (e.g., control path  620  is from T 0  to T 3  and control path  630  is from T 4  to T 6 ). The difference between control paths  620  and  630  is achieved by variable resistor  628 , which increase its resistance during the emission of the first laser pulse, or decreases its resistance during the emission of the second laser pulse. As discussed above, variable resistor  628  may be implemented using resistor  644  and switch  642  connected in parallel, such that switch  642  when switched on short-circuit resistor  644 , reducing the resistance of variable resistor  628 . In this way, RT 1 &lt;RT 2  can also be achieved. 
     Another aspect of the disclosure is directed to a non-transitory computer-readable medium storing instructions which, when executed, cause one or more processors to perform the methods, as discussed above. The computer-readable medium may include volatile or non-volatile, magnetic, semiconductor, tape, optical, removable, non-removable, or other types of computer-readable medium or computer-readable storage devices. For example, the computer-readable medium may be the storage device or the memory module having the computer instructions stored thereon, as disclosed. In some embodiments, the computer-readable medium may be a disc or a flash drive having the computer instructions stored thereon. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed system and related methods. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed system and related methods. 
     It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.