Patent Description:
Autonomous, self-driving, and semi-autonomous automobiles use a combination of different sensors and technologies such as radar, image-recognition cameras, and sonar for detection and location of surrounding objects. These sensors enable a host of improvements in driver safety including collision warning, automatic-emergency braking, lane-departure warning, lane-keeping assistance, adaptive cruise control, and piloted driving. Among these sensor technologies, light detection and ranging (LiDAR) systems take a critical role, enabling real-time, high-resolution 3D mapping of the surrounding environment.

Most current LiDAR systems used for autonomous vehicles today utilize a small number of lasers, combined with some method of mechanically scanning the environment. Some state-of-the-art LiDAR systems use two-dimensional Vertical Cavity Surface Emitting Lasers (VCSEL) arrays as the illumination source and various types of solid-state detector arrays in the receiver. It is highly desired that future autonomous cars utilize solid-state semiconductor-based LiDAR systems with high reliability and wide environmental operating ranges. These solid-state LiDAR systems are advantageous because they use solid state technology that has no moving parts. However, currently state-of-the-art LiDAR systems have many practical limitations and new systems and methods are needed to improve performance.

<CIT> discloses a ceilometer operating according to a gating method that integrates output signals from a light receiver. <CIT> discloses a VCSEL assembly including a VCSEL, an optical element, and an optical detector.

According to a first aspect of the invention, a light detection and ranging transmitter with optical power monitoring is provided according to claim <NUM>. Optional features of the first aspect of the invention are provided in dependent claims <NUM>-<NUM>.

According to a second aspect of the invention, a method of light detection and ranging with optical power monitoring is provided according to claim <NUM>. Optional features of the second aspect of the invention are provided in dependent claims <NUM>-<NUM>.

The present teaching, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description, taken in conjunction with the accompanying drawings. The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not necessarily to scale; emphasis instead generally being placed upon illustrating principles of the teaching. The drawings are not intended to limit the scope of the Applicant's teaching in any way.

The present teaching will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teaching is described in conjunction with various embodiments and examples, it is not intended that the present teaching be limited to such embodiments. On the contrary, the present teaching encompasses various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teaching herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein.

Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the teaching.

It should be understood that the individual steps of the method of the present teaching can be performed in any order and/or simultaneously as long as the teaching remains operable. Furthermore, it should be understood that the apparatus and method of the present teaching can include any number or all of the described embodiments as long as the teaching remains operable.

The present teaching relates generally to Light Detection and Ranging (LiDAR), which is a remote sensing method that uses laser light to measure distances (ranges) to objects. LiDAR systems generally measure distances to various objects or targets that reflect and/or scatter light. Autonomous vehicles make use of LiDAR systems to generate a highly accurate 3D map of the surrounding environment with fine resolution. The systems and methods described herein are directed towards providing a solid-state, pulsed time-of-flight (TOF) LiDAR system with high levels of reliability, while also maintaining long measurement range as well as low cost.

Some embodiments of LiDAR systems according to the present teaching use a laser transmitter that includes a laser array. In some specific embodiments, the laser array comprises Vertical Cavity Surface Emitting Laser (VCSEL) devices. These may include top-emitting VCSELs, bottom-emitting VCSELs, and various types of high-power VCSELs. The VCSEL arrays may be monolithic. The laser emitters may all share a common substrate, including semiconductor substrates or ceramic substrates.

In various embodiments, individual lasers and/or groups of lasers using one or more transmitter arrays can be individually controlled. Each individual emitter in the transmitter array can be fired independently. The optical beam emitted by each laser emitter corresponds to a 3D projection angle subtending only a portion of the total system field-of-view. One example of such a LiDAR system is described in <CIT>. In addition, the number of pulses fired by an individual laser, or group of lasers, can be controlled based on a desired performance objective of the LiDAR system. The duration and timing of this sequence can also be controlled to achieve various performance goals.

Some embodiments of LiDAR systems according to the present teaching use detectors and/or groups of detectors in a detector array that can also be individually controlled. See, for example, U. Patent Publication No. <CIT>, entitled "Eye-Safe Long-Range Solid-State LiDAR System". This independent control over the individual lasers and/or groups of lasers in the transmitter array and/or over the detectors and/or groups of detectors in a detector array provide for various desirable operating features including control of the system field-of-view, optical power levels, and scanning pattern.

<CIT> discloses a device for measuring height of clouds, the device including a light transmitter consisting of an array of laser diodes. A transmitter control unit monitors the light output power of the laser diode array by means of a photodetector and regulates the amplitude of current pulses driving the laser diode array so as to keep the light output power constant.

<CIT> discloses a VCSEL assembly which may include multiple VCSELs arranged in an array, a prism having an indentation, and an optical detector. In an embodiment, a portion of the output light of the VCSEL array is reflected by the indentation towards the optical detector to be used for power monitoring. The optical detector is arranged at the same substrate of the VCSEL and used to detect the optical power of the small portion reflected by indentation.

The optical power level(s) emitted by the LiDAR system transmitter is an important parameter that factors into numerous performance metrics of the LiDAR system. This includes, for example, distance, field-of-view, resolution, speed and frame rate and eye safety, among other performance metrics. As such, systems and methods that monitor transmit power for LiDAR systems are desirable. It is desirable that the monitor systems be compact, low-cost and produce a desired precision and accuracy of the monitored parameter.

One feature of the LiDAR systems of the present teaching is the inclusion of optical performance monitoring directly in the LiDAR transmit module. Optical performance monitoring within a LiDAR module may be important for a variety of reasons. For example, incorporating optical power monitoring inside the illuminator assembly can improve calibration, performance, and reliability monitoring. Lasers degrade with lifetime and so it can be useful to monitor the laser output power within the projector assembly itself. For example, by monitoring the light as is exiting the projector, rather than just relying on the received optical signal after the light has been reflected from an external object, it is possible to monitor generated power more accurately and quickly. Also, it is possible to monitor the temperature proximate to the VCSEL lasers. Such a feature is useful to improve the reliability and performance. The optional monitoring of both the temperature and the power can be used not only for diagnostics, but also for controlling the lasers during operation to improve performance and/or lifetime of the system.

Another feature of the present teaching is that the power monitoring elements are configured to monitor light reflected off of, or directed from, various optical devices within the LiDAR transmitter that are providing other functions. The reflected or directed light detected within the transmitter can be used not just for passive monitoring purposes, but also to provide additional active control of the lasers and detectors in the transmitter.

Some embodiments of the performance monitor for LiDAR system transmitters of the present teaching monitor one or more parameters of the light generated by the LiDAR system transmitter itself. For example, the light generated by the transmitters can be monitored for laser wavelength, optical power, pulse timing, and pulse frequency. The wavelength of the generated light can be detected by using a power monitor including a receiver that is not simply a photodiode, but instead comprises a more complicated set of optics that allows detection of wavelength as well as optical power.

In a LiDAR design where multiple wavelengths are used, particularly if the wavelengths are close in absolute value, it may be desired to monitor their absolute or relative values in order to ensure that the system parameters are as intended. There are various known methods of monitoring either absolute wavelength of light generated by the laser, or the relative offset between light generated by the lasers of different wavelength. For example, an etalon-based device could be used as a wavelength monitor.

Embodiments of the systems and method of the present teaching that use multi-wavelength power monitoring also can improve the system robustness for detecting whether a fault is caused by laser degradation or shifts in optical performance metrics. Multi-wavelength power monitoring can also provide redundancy if one set of wavelengths should fail in the transmitter. A partial or full failure in operation of one set of wavelengths in the transmitter would still allow the ability for partial operation of the system using the other set of wavelengths in the transmitter if the optical monitoring for each wavelength is independent.

Another feature of the method and system of the present teaching is that multielement, multi-wavelength optical power monitoring can be realized. One or more directing elements can be positioned to direct light to one or more monitors so that collective and individual powers from the one or more laser elements at one or more wavelengths can be monitored. For example, in one embodiment, the multiple reflection elements can be partial mirrors. In other embodiments, the multiple reflection elements can be configured to project the beam. In some embodiments, the monitors include photodetectors which are each sensitive to only one particular wavelength band of light. This configuration allows monitoring optical power of one or more wavelengths independently, which improves the system capabilities. Multi-wavelength power monitoring according to the present teaching can be configured to monitor for multiple parameters, such as laser wavelength, including absolute wavelength and/or relative wavelength, optical power, pulse timing, and pulse frequency.

Multi-wavelength power monitoring according to the present teaching also improves the LiDAR system's robustness for detecting whether a fault is caused by laser degradation or shifts in optical performance. Multi-wavelength power monitoring is useful, for example, to provide redundancy if one set of wavelengths generated by the transmitter should fail. A partial or full failure in the transmitter's generation of one set of wavelengths would still allow the ability for partial operation of the LiDAR system using the other set of wavelengths if the system is configured so that the optical monitoring for each wavelength band is independent.

It is known that degradation in the performance of the optical transmitter can be determined by monitoring the optical transmitter's laser output power, and then comparing the measured optical power to an expected reference value. The degradation in the performance of the optical transmitter can be caused by either or both of the laser itself or caused by various aspects of the opto-mechanical assembly. The degradation in the performance of the optical transmitter can then be analyzed. For example, U. Patent Application Publication No. <CIT>, entitled "System and Method for Monitoring Optical Subsystem Performance in Cloud LiDAR Systems" describes the benefits of laser output power monitoring for a LiDAR system designed for cloud measurements.

Measurements of the optical signal generated by the LiDAR transmitter can also be used in a passive monitoring system. In addition, the optical signal from the LiDAR transmitter can be used for active control of the laser bias current driving the semiconductor laser. A laser diode operates over a range of operating bias currents. Many types of semiconductor laser diode systems are operated in closed loop fashion where a received photodiode current from a monitor photodiode is used as an input to a bias control feedback loop. By monitoring and maintaining the monitor photodiode current at a constant value, which is a largely linear function of the incident power, the output power of the semiconductor laser can be maintained at a near constant value. This condition enables the system to react to environmental changes, such as temperature and mechanical movements, to achieve improved output power stability. Also, monitoring the optical power, and controlling the laser bias in response to the monitored optical power, can be used to accommodate degradation of the laser efficiency over its lifetime, without loss of optical power at system level.

In various embodiments, the transmitter optical signal can be monitored for numerous parameters, including, for example, laser wavelength, optical power, pulse timing, pulse frequency, and pulse duration among other parameters. The laser wavelength can be detected using a power monitor, which is not simply a photodiode or other optical detector, but instead, it is an optical system that allows detection of wavelength as well as optical power. In a LiDAR system design where multiple wavelengths are used, particularly if the wavelengths are close in absolute value, it may be desired to monitor their absolute or relative values in order to ensure that the system parameters are as intended. Various methods of monitoring either absolute wavelength, or the relative offset between lasers of different wavelength, are known within the art. For example, an etalon-based device could be used as a wavelength monitor.

It is desirable for some LiDAR systems to perform a calibration at the beginning of life (BOL) to provide a reference during the lifetime operation of the system. By calibration, we mean the characterization of the initial laser bias, temperature, and output power of the device, and then the subsequent tuning of laser bias and output power as a function of temperature to meet the required performance specifications with suitable margins at BOL. Often this process is performed as part of the manufacturing process for the LiDAR system. The performance parameters, such as the laser bias and the measured optical power, obtained during the calibration process, will often be stored as a function of temperature in the LiDAR system memory as a reference to be used for various operations. Various monitors of the present teaching can provide measured optical power for some of these calibration processes.

During operation of LiDAR systems, the actual temperature can be monitored and used in conjunction with the reference values stored in memory in some form of a look-up table to determine the optical laser bias set point. Alternatively, in combination with an optical power monitor, the actual values of output power, laser bias, and temperature during operation can be compared to the reference values in a look-up table to identify any significant change or degradation in the system which can indicate a potential reliability issue. In various implementations of practical systems, a LiDAR system detecting such changes could then communicate with the overall monitoring system in an automotive other vehicle to identify a need for potential service or repair.

<FIG> illustrates an embodiment of a monitored transmitter <NUM> comprising a reflective directing element <NUM> for a LiDAR system of the present teaching. The monitored transmitter <NUM> uses a 2D VCSEL array <NUM> for the laser source combined with a set of optics for projecting the laser beams along a direction of transmission <NUM>. A monitor photodiode (MPD) <NUM> is shown mounted on the same substrate <NUM> as the VCSEL array <NUM>. The monitor photodiode <NUM> or VCSEL <NUM> could also be mounted on separate carriers, or in separate packages. In some embodiments, the monitor photodiode <NUM> and the VCSEL array <NUM> are nominally positioned in the same plane. In some embodiments, the monitor photodiode <NUM> and the VCSEL array <NUM> have the normal projections to their respective surfaces having the same orientation. In some embodiments, this orientation is the same orientation as the direction of transmission <NUM>. A directing element <NUM>, which is located between a first lens <NUM> and a second lens <NUM>, directs a portion of optical beams generated by the VCSEL array <NUM> back to the monitor photodiode <NUM>. In some embodiments, the directing element <NUM> comprises a partial mirror located between a first lens <NUM> and a second lens <NUM> that reflects a portion of optical beams generated by the VCSEL array <NUM> back to the monitor photodiode <NUM>. The first lens <NUM>, second lens <NUM>, and directing element <NUM> can be mounted in or on a common housing <NUM> that is secured to substrate <NUM>. The direction of transmission in some embodiments is along the optical axis of the first lens <NUM> and/or the second lens <NUM>.

The monitored transmitter <NUM> is described using lenses <NUM>, <NUM> to project the light in the optical beams generated by the VCSEL array <NUM>. However, one skilled in the art will appreciate that numerous other projecting optical elements can also be used. Numerous known implementations of projecting elements can be used in the monitored transmitter <NUM> of the present teaching. In various embodiments, these projecting elements serve to shape and project the optical beams toward a target to achieve, for example, a desired field-of-view, target range, resolution, etc. These projecting elements can be configured to produce a common point for the optical beams within the transmitter footprint that allows the directing element to generate an illumination region at the monitor plane such that light from each of the optical beams can be monitored.

It will be understood to those of skill in the art that some embodiments of the present teaching do not require all of the generated optical beams from the VCSEL array <NUM> to appear in the illumination region. Instead, in some embodiments, only a subset of the optical beams that share the common point for the optical beams are provided in the illumination region and sampled by the monitor. The relative positions of the projecting elements, VCSEL array <NUM> and directing element serve to determine a desired subset of optical beams that share the common point and therefore are provided in the illumination region.

The directing element <NUM> of the monitored transmitter <NUM> embodiment of <FIG> is illustrated as being inclined towards the monitor photodiode <NUM>. In other embodiments, the reflective mirror is not inclined towards the photodiode. Instead, the directing element <NUM> orientation might be such that the normal to the surface of the directing element <NUM> is parallel to an optical axis of the lens system in order to maintain rotational symmetry. This orientation can be advantageous in some cases for ease of assembly/manufacturing. Also, the directing element <NUM> is shown in <FIG> as a separate optical element, but it should be understood that in some embodiments, one or more of the surfaces of one or more of the two lenses <NUM>, <NUM> or some other optical elements in the transmitter path, can function to provide the required reflection or direction.

In some embodiments, the optical components including one or more of the first lens <NUM>, second lens <NUM>, and directing element <NUM>, have an optical coating on a surface that is designed to control reflectance. In embodiments, where no monitoring is needed, the optical reflectance is often set as low as possible to maximize the output power from the transmitter <NUM> in the direction of transmission <NUM>. In some embodiments, an optical coating on the directing element <NUM> might have a reflectance of up to <NUM>%. In some embodiments, the directing element <NUM> is not a separate discrete element but instead is an optical coating or set of coatings on one or more of the optical lens surfaces, where the reflectance of the coating has been selected to optimize the measured optical signal.

A feature of the present teaching is that the directing element <NUM> can be placed within the path of the optical beams being generated by the laser at a common point so that every laser within the transmitter array has some portion of light that can be reflected. See, for example, <CIT>, which describes a LiDAR transmitter that includes multiple lenses to achieve a small angular divergence of the transmitted optical beam.

In some embodiments, the directing element <NUM> is a diffractive optic element. In other embodiments, the directing element <NUM> is both a partially reflective mirror and an optical filter which blocks a portion of the visible spectrum. In one specific embodiments, the directing element is a prism. In some embodiments, the directing element is a holographic element. Numerous known lens configurations can be utilized to achieve the desired lens power, size and relative positions of the first lens <NUM> and second lens <NUM> and laser array <NUM>.

In some embodiments, the monitor photodiode <NUM> is manufactured monolithically with the VCSEL array <NUM>. The monitor photodiode <NUM> can be fabricated in numerous configurations. For example, in some embodiments, the monitor photodiode <NUM> is a single photodiode, which receives light from all the individual lasers within the 2D array. In other embodiments, more than one photodiode is used where the outputs of the multiple photodiodes are combined in some fashion to provide a common signal output at some point in the signal chain.

In some embodiments, the monitor photodiode <NUM> includes an optical element positioned so that an input receives the light propagating from the monitor photodiode <NUM>. For example, the monitor photodiode <NUM> can include a light guide or a prism positioned to receive light and configured to direct that collected light to a photodiode. The optical element can take all the light in the monitoring aperture, or only a portion of the light from different spatial regions of the monitoring aperture, and then direct that light to one or more photodiodes. For example, the optical element may be a lightpipe as described herein.

<FIG> illustrates an expanded view <NUM> with additional detail of a portion of the monitored transmitter <NUM> for a LiDAR system of <FIG>. <FIG> shows an optical ray trace diagram <NUM> illustrating light emitted from the VCSEL array <NUM> that passes through lens <NUM><NUM>, and then reflects off the partially reflective mirror <NUM> back towards the monitor photodiode <NUM> and VCSEL array <NUM>. The monitor photodiode <NUM> is shown as photodiode positioned in the same plane <NUM> as the VCSEL array <NUM>. The optical beams illustrated in the ray trace diagram <NUM> are shown as they pass to the directing element <NUM> where they are redirected to propagate to the second lens (not shown) and then out of the LiDAR transmitter <NUM> in the direction of transmission <NUM>.

<FIG> illustrates a cross-sectional view <NUM> of the optical ray trace <NUM> shown in <FIG>. Referring to <FIG>, <FIG>, the cross-sectional view <NUM> shows the illuminated area <NUM> encompassing the reflections off of the directing element <NUM> from different lasers within the VCSEL array <NUM>. The illuminated area <NUM> of each beam is illustrated mapped over the area <NUM> of the monitor photodiode <NUM> and the area <NUM> of the VCELS array <NUM>. This is because the directing element <NUM> projects light from a common point of the optical beams generated by the VCSEL, so the illumination region <NUM> includes at least some light from all the beams. That is, the location of the directing element <NUM> and the incline angle of the mirror are chosen, so that no matter the location of a particular VCSEL element within the VCSEL array <NUM>, the reflected areas all overlap in the proximity of the monitor photodiode <NUM>. This ensures that the emitted power from every laser within the VCSEL array <NUM> can be monitored. The monitor photodiode <NUM> location is often constrained by other electronic components within the LiDAR system.

In some configurations, it is not possible to place the monitor photodiode <NUM> immediately adjacent to the VCSEL array <NUM>. Instead, for easier manufacturing the monitor photodiode <NUM> may need to be some distance away from the VCSEL array <NUM>, yet still close enough that it falls within the illuminated area of the reflected optical beams. Thus, the inclination and position of the directing element <NUM> must be properly chosen such that light from multiple elements in the array <NUM> overlaps at the monitor photodiode <NUM>.

<FIG> illustrates a portion <NUM> of an embodiment of a monitored transmitter comprising a monitor having a lightpipe <NUM> for a LiDAR system of the present teaching. The embodiments shown in <FIG> and <FIG> share many features. A VCSEL array <NUM> and monitor photodiode <NUM> are positioned in a same plane <NUM>. A first lens <NUM> directs light beams from the array <NUM> toward a directing element <NUM>, which may be a partially reflecting mirror, and then to a second lens (not shown) and out the transmitter along a direction of transmission <NUM>. <FIG> shows an optical ray trace diagram <NUM> illustrating light emitted from the VCSEL array <NUM> that passes through lens <NUM><NUM>, and then reflects off the partially reflective mirror <NUM> back towards the lightpipe <NUM> and the VCSEL array <NUM>.

<FIG> illustrates a cross-sectional view <NUM> of the optical ray trace <NUM> shown in <FIG>. Referring to <FIG>, <FIG>, the cross-sectional view <NUM> shows the illuminated area <NUM> encompassing the reflections off of the directing element <NUM> from different lasers within the VCSEL array <NUM>. The illuminated area <NUM> of each beam is illustrated mapped over the area <NUM> of the monitor area that includes the collection area of a lightpipe <NUM> and the area <NUM> of the VCELS array footprint. This lightpipe <NUM> is shown connected to a remotely located monitor photodiode <NUM>. The illuminated area <NUM> includes light from each of the optical beams generated by the VCSEL array <NUM> because the directing element <NUM> directs light from a common point of the optical beams generated by the VCSEL array <NUM> to the monitor <NUM> located at the monitor plane.

In this embodiment, the monitor <NUM> includes the lightpipe <NUM>. A lightpipe is a device that acts as a waveguide that retains the light internally through total internal reflection as the light propagates along the length of the lightpipe. The lightpipe <NUM> can be made of glass or plastic, or can be a hollow waveguide with internal mirrored surfaces. Typically, the dimensions of the cross-section of the lightpipe <NUM> perpendicular to the axis of propagation are much smaller than the propagation distance. The lightpipe <NUM> can be constructed with fixed or flexible bends to direct the light as desired provided that the bend radius is sufficiently large to maintain total internal reflection. A common example of a lightpipe is a fiber optic cable.

In a lightpipe configuration, instead of the reflected light being measured directly by a photodiode located within the illuminated area as in the embodiment of <FIG>, the lightpipe <NUM> is used to capture the light and provide the collected light to a photodiode (not shown). Thus, the lightpipe <NUM> captures a portion of the light from the illuminated area and redirects it to a monitor photodiode (not shown) that is physically distant from the VCSEL array <NUM> and also outside the illuminated area.

Embodiments of the monitored transmitter that use a lightpipe do have an additional component, which can increase cost. For example, the lightpipe might incorporate various optical elements including lenses, and mirrors. However, lightpipe embodiments have several potential advantages compared to other embodiments that do not use a lightpipe. One advantage of a lightpipe is that the size of the monitor photodiode for the active area is often restricted in physical size, which limits the optical signal that can be produced. Particularly, if the actual pulse shape, that is often nanoseconds in duration, needs to be measured, the photodiode active area should be small to have good dynamic response. In this case, a lightpipe can be designed with a larger collection area that allows measured light to be condensed/focused onto a smaller photodiode active area, thereby improving the SNR. Because the lightpipe is purely a passive optic component, it can be positioned on top of elements which may be physically close to the VCSEL array <NUM>, such as the electrical drive components. Closer placement of the lightpipe to the VCSEL can also improve the optical efficiency of the monitoring function.

Another potential advantage of using a lightpipe is the ability to locate the monitor photodiode relatively far from the VCSEL array and the VCSEL drive circuits. This feature is important because the drive currents/voltages used to operate the VCSEL can electrically couple and be a source of noise within the optical monitor circuit when the monitor diode is placed physically close to those components. It is highly desirable for the noise level be kept as low as possible in the optical monitor circuit in order to provide high SNR and any false readings.

<FIG> shows a portion <NUM> of an embodiment of monitored transmitter comprising a transmissive directing element <NUM> for a LiDAR system of the present teaching. A VCSEL array <NUM> generates a plurality of optical beams that are directed to a first and second lens <NUM>, <NUM>. In this embodiment, the light is not reflected back towards the VCSEL array <NUM>. The light impinges on a directing element <NUM>, which in this embodiment is a transmissive element that is a mounting plate. In this configuration, the directing element <NUM> transmits, or passes, the light at the common point of passage of the optical beams toward the monitor <NUM>. For example, in some embodiments, the monitor <NUM> is a microprism. The directing element <NUM> resides at a position with a common point of passage of the optical beams generated by the laser array that defines an illumination region that contains light from each laser. As such, the directing element <NUM> passes light from each of the optical beams being generated by the laser so that every laser within the VCSEL array <NUM> has some portion of light provided at an illumination region so that the light can be sampled by the monitor <NUM> placed within the illumination region. A small portion of the transmitted light in the illumination region is reflected by the monitor <NUM> in a direction largely perpendicular to the optical axis of the transmitter using a small micro-prism, which can be a diffractive optical element.

The micro-prism of the monitor <NUM> is shown attached to the directing element <NUM>, which is a mounting plate that in some embodiments is a transparent optical window, and in other embodiments is an optical filter. In some configurations, the directing element <NUM> is an optical element in the LiDAR transmitter that would be required even without the monitor <NUM>. Thus, the directing element <NUM> provides two functions, one of which is securing the micro-prism of the monitor <NUM> and the other is optical in nature. In some configurations, the directing element <NUM> protects the LiDAR system from the outside environment. The micro-prism in the monitor <NUM> is shown coupling the light into an optical fiber <NUM> that has a core that is large enough to maintain the required optical signal level. The optical fiber <NUM> then directs the light to a monitor photodiode (not shown in the diagram).

<FIG> illustrates a cross-sectional view <NUM> of the optical ray trace shown in <FIG>. The cross-sectional view <NUM> shows the illumination region <NUM> encompassing the illumination from all of the different lasers within the VCSEL array <NUM> at the plane of the directing element <NUM>. The illumination region <NUM> that includes light from each beam is illustrated mapped over the collection area <NUM> of the sample prism area. The directing element <NUM> is placed within the path of the optical beams being generated by the laser at a common point so that every laser within the transmitter array has some portion of light that can be sampled by the monitor <NUM> that includes a microprism.

The embodiment of a monitored transmitter with sampling prism shown in <FIG> has some advantages. One feature of this embodiment is that by removing the reflecting plate from within the lens system, the manufacturing and assembly of that lens system is simplified, and rotational symmetry of the lens system is maintained.

The embodiment of a monitored transmitter with optical fiber <NUM> shown in <FIG> also has some advantages. One feature of this embodiment is that the fiber <NUM> allows the monitor photodiode to be placed physically distant from the VCSEL array <NUM> and the VCSEL drive circuitry. In some embodiments, the monitor photodiode is not even on the same circuit board as the VCSEL array <NUM> drive circuitry. This configuration largely eliminates the possibility of the VCSEL array <NUM> drive circuit resulting in any unwanted noise or spurious signals being present in the optical monitor signal. Eliminating the physical constraints associated with the monitor photodiode being on a common circuit board with the VCSEL array <NUM> also enables the size and type of optical monitor to be very flexible, which can be advantageous for many reasons including reducing the size and/or complexity of the transmitter or the manufacturing process.

Another feature of the monitored transmitter of the present teaching is that the common point where the light is directed to from the illumination region can be positioned at various points in the optical transmitter. The common point can be determined by the positions of the projecting elements and the laser device. For example, the common point can be positioned before the last optical lens surface in the transmitter optical system. Alternatively, or in addition, the common point can be positioned after the last optical lens surface in the transmitter optical system. In some embodiments, the optical monitor can be attached to an optical window which is the last optical element in the transmitter path and protects the LiDAR system from the outside environment. In other embodiments, the optical monitor can be attached to one of the optical lens surfaces. In yet other embodiments, the optical monitor can be attached to an optical filter element which blocks some portion of the visible spectrum.

It should be understood that the monitored transmitter of the present teaching has been described in connection with particular configurations. These embodiments are only illustrative and not intended to limit the scope of the present teaching. It should be understood that various aspects of the different embodiments can be used in different combinations to achieve the advantages of the method and system of the present teaching.

<FIG> illustrates a block diagram of an embodiment of a LiDAR system <NUM> that includes a monitored transmitter according to the present teaching. The LiDAR system <NUM> has six main components: (<NUM>) controller and interface electronics <NUM>; (<NUM>) transmit electronics including the laser driver <NUM>; (<NUM>) the laser array <NUM>; (<NUM>) receive and time-of-flight computation electronics <NUM>; (<NUM>) detector array <NUM>; and the (<NUM>) monitor <NUM>. The controller and interface electronics <NUM> controls the overall function of the LiDAR system <NUM> and provides the digital communication to the host system processor <NUM>. The transmit electronics <NUM> controls the operation of the laser array <NUM> and, in some embodiments, sets the pattern and/or power of laser firing of individual elements in the array <NUM>. The receive and time-of-flight computation electronics <NUM> receives the electrical detection signals from the detector array <NUM> and then processes these electrical detection signals to compute the range distance through time-of-flight calculations.

The monitor <NUM> is connected to one or both of the controller and interface electronics <NUM> and the transmit electronics including laser driver <NUM>. The monitor <NUM> provides information on the detected signal power, and in combination with processing in one or both of the controller and interface electronics <NUM> and the transmit electronics including laser driver <NUM>, provides information about laser wavelength, optical power, pulse timing, pulse frequency, and/or pulse duration among other parameters. In some embodiments, the controller and interface electronics <NUM> directly controls and gets information from the monitor <NUM>. The embodiment of <FIG> shows a partial mirror <NUM> directing light to the monitor <NUM>. However, as is clear to those skilled in the art, the LiDAR system <NUM> can operate with any of the configuration of the monitor <NUM> described herein and known variations of these configurations.

In some embodiments, the lasers in the array <NUM> are operated in closed loop configuration using one or both of the controller and interface electronics <NUM> and the transmit electronics including laser driver <NUM> that respond to a received photodiode current from a monitor photodiode that serves as an input to a bias control loop. This configuration can allow the transmitter optical power including the power from some or all of the optical beams generated by the laser array <NUM> to be maintained at a near constant value. This allows the system to be more stable against temperature and/or mechanical shifts. Also, using a control loop via one or both of the controller and interface electronics <NUM> and transmit electronics including laser driver <NUM> that includes monitoring of the optical power and control of the laser bias can accommodate some amount of degradation of the laser efficiency over its lifetime, without loss of optical power at output of the LiDAR system <NUM>.

In some embodiments, the controller and interface electronics <NUM> calculates an object reflectivity using an optical power reading generated by the monitor <NUM>. The monitored optical power can be used as a reference and then, based on actual photon counting/intensity and pre-calibration, improved reflectivity data can be achieved. This improved reflectivity data can be utilized in systems that are used for various known LiDAR applications that relate to perception.

Another feature of the method and apparatus of the present teaching is that these monitor photodiode implementations can address functional safety of the LiDAR system itself. For example, a control loop via the controller and interface electronics <NUM> and/or the transmit electronics including laser driver <NUM>, including the power monitor <NUM> can be used to indicate that the LiDAR transmitter is faulting, for example if the measured optical power is below or above a certain threshold. For example, a control loop using one or both of the controller and interface electronics <NUM> and transmit electronics including laser driver <NUM> and including the power monitor <NUM> can be used to indicate the LiDAR transmitter is operating beyond an eye safety threshold if optical power is above a certain threshold.

The monitored transmitter for a LiDAR system of the present teaching has been described in connection with various embodiments that use a single VCSEL array. It should be understood that the present teaching can be extended to LiDAR transmitters that include more than one VCSEL array. In these embodiments including more than one VCSEL array, the VCSEL arrays are positioned such that the illumination region that covers the monitor has a distinct region for each VCSEL array. In these embodiments, the monitor photodiode can be configured such that it includes more than one photodiode and a separate monitor photodiode can be used for each of the separate illumination regions.

Claim 1:
A light detection and ranging transmitter (<NUM>) with optical power monitoring, the transmitter (<NUM>) comprising:
a) a laser array (<NUM>; <NUM>; <NUM>; <NUM>) positioned in a first plane (<NUM>; <NUM>), the laser array (<NUM>; <NUM>; <NUM>; <NUM>) being configured to generate a plurality of optical beams that propagate along an optical path in response to an electrical signal provided at an input;
b) a first projecting optical element (<NUM>; <NUM>; <NUM>) positioned in the optical path that is configured to project the plurality of optical beams such that the plurality of optical beams at least partially overlaps at a common point;
c) a second projecting optical element (<NUM>; <NUM>) positioned in the optical path of the plurality of optical beams after the first projecting optical element (<NUM>; <NUM>; <NUM>), and configured to project light from the first projecting optical element (<NUM>; <NUM>; <NUM>) in a direction of transmission;
d) a directing optical element (<NUM>; <NUM>; <NUM>) positioned at the common point in the optical path of the plurality of beams, the directing optical element (<NUM>; <NUM>; <NUM>) being configured to produce an illumination region (<NUM>; <NUM>; <NUM>) comprising at least some light from each of the plurality of beams in a second plane (<NUM>; <NUM>);
e) a monitor (<NUM>; <NUM>; <NUM>; <NUM>) positioned within the illumination region (<NUM>; <NUM>; <NUM>) in the second plane (<NUM>; <NUM>) that is configured to collect at least some light from each of the plurality of beams, the monitor (<NUM>; <NUM>; <NUM>; <NUM>) comprising a photodiode that is configured to generate a detected signal at an output in response to the collected light; and
f) a controller (<NUM>) with an input connected to an output of the monitor (<NUM>; <NUM>; <NUM>; <NUM>) and an output connected to the input of the laser array (<NUM>; <NUM>; <NUM>; <NUM>), the controller (<NUM>) being configured to generate the electrical signal in response to the detected signal that controls the laser to achieve a desired operation of the light detection and ranging system transmitter (<NUM>).