Patent Application
20230176196
The present disclosure relates to a submount for a transmitter of an optical sensing system that includes co-packaged off-the-shelf (OTS) laser bars aligned with semiconductor-level precision on a ceramic substrate.
Optical sensing systems, e.g., such as LiDAR systems, have been widely used in advanced navigation technologies, such as to aid autonomous driving or to generate high-definition maps. For example, a typical LiDAR system measures the distance to a target by illuminating the target with pulsed laser light beams that are steered towards an object in the far field using a scanning mirror, and then measuring the reflected pulses with a sensor. Differences in laser light return times, wavelengths, and/or phases (also referred to as “time-of-flight (ToF) measurements”) 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 high-definition map surveys.
To scan the narrow laser beam across a broad field-of-view (FOV) in two-dimensions (2D), conventional systems mount two separate one-axis scanning mirrors on separate actuators, which drive the scanning mirrors to rotate around their respective axes to scan the two dimensions, respectively. Rotation about one axis provides a fast sweep of the surrounding environment in one dimension and the other axis provides a slow sweeps in an orthogonal dimension. Using these sweeps, a digital 3D image (e.g., 3D point cloud) of the far-field can be constructed. The slow axis (or horizontal-axis) is typically implemented using mechanical actuator (e.g., a galvanometer) and the fast axis (or vertical-axis) can be implemented by a mechanical or solid-state actuator. Thus, the galvanometer may be configured to drive the scanning mirror to rotate about the horizontal-axis, and electrostatic drive combs drive the scanning mirror to rotate about the vertical-axis, for example.
Due to laser beam spreading, a high-resolution 3D point cloud may not be resolved without sub-pixelization. One way to achieve a high-resolution or “camera-like” LiDAR system, is with the use of a specific scanning pattern, laser array, and a detector array. For example, to achieve a resolution of 0.025 ° in both scanning directions, the vertical-axis scanner may operate at a scanning frequency of 1.7 kHz. With the horizontal-axis scanner operating at a scanning frequency of 10 Hz and a scanning angle of 60 °, the vertical-axis scanner needs to scan horizontally at a step of 0.4 ° per period. To accommodate these settings, an edge-emitting laser with non-uniformly spaced laser channels could be used. For example, eight channels of lasers need to be fired sequentially, and eventually incident on eight detector arrays. However, no such edge-emitting lasers are available off-the-shelf (OTS) and having one custom designed comes at great expense. Moreover, the length of a custom designed edge-emitting laser may be unsuitable for use in a LiDAR system.
Thus, there exists a need for an edge-emitting laser with a non-uniform distribution of laser channels that can be realized without the expense or size that comes with custom designing an edge-emitting laser.
Embodiments of the disclosure provide for a submount for a transmitter of an optical sensing system. The submount may include a substrate. The submount may also include a set of alignment fiducials formed using semiconductor lithography and coupled to the substrate. Still further, the submount may include at least one laser bar coupled to the substrate based on the set of alignment fiducials.
Embodiments of the disclosure also provide for a transmitter_of an optical sensing system. The transmitter may include a submount. The submount may include a substrate. The submount may also include a set of alignment fiducials formed using semiconductor lithography and coupled to the substrate. Still further, the submount may include at least one laser bar coupled to the substrate based on the set of alignment fiducials. The transmitter may also include a scanner. The scanner may be configured to steer a light beam emitted by the at least one laser bar towards an object.
Embodiments of the disclosure further provide for method of fabricating a transmitter submount for an optical sensing system. The method may include forming a substrate. The method may also include forming a set of alignment fiducials using semiconductor lithography. The method may further include coupling the set of alignment fiducials to the substrate. Still further, the method may include coupling at least one laser bar to the substrate based on the set of alignment fiducials.
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.
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.
LiDAR is an optical sensing technology that enables autonomous vehicles to “see” the surrounding world, creating a virtual model of the environment to facilitate decision-making and navigation. An optical sensor (e.g., LiDAR transmitter and receiver) creates a 3D map of the surrounding environment using laser beams and time-of-flight (ToF) distance measurements. ToF, which is one of LiDAR’s operational principles, provides distance information by measuring the travel time of a collimated laser beam to reflect off an object and return to the sensor. Reflected light signals are measured and processed at the vehicle to detect, identify, and decide how to interact with or avoid objects.
As mentioned above in the BACKGROUND section, there is currently no easy and inexpensive way to produce a laser emitter with non-uniformly distributed laser channels. The high-resolution specifications mentioned above in the BACKGROUND section can be achieved using an edge-emitting laser with adjacent laser channels spaced 400 µm apart, except for the middle two laser channels, which are spaced 600 µm apart. The difference in spacing is adopted to enable the region in the 3D point cloud scanned between channel 4 and channel 5 to include high-resolution features. However, edge-emitting lasers with a non-uniform distribution of laser channels are not available OTS, and to custom design such a laser emitter would be both expensive and challenging due to the length (>3 mm) of the final semiconductor chip.
Ideally, such a laser emitter could be fabricated by co-packaging a pair of OTS laser bars onto a substrate such that the non-uniform spacing associated with the center laser channels is achieved. However, even a small misalignment of one of the laser bars in either transverse direction may significantly impact system precision to levels unacceptable for autonomous navigation. This is because a transverse misalignment on the substrate equates to an error in the direction the laser beams are steered towards the far-field.
One way to achieve a high-level of alignment precision is through semiconductor lithography. However, due to the heat generated by edge-emitting lasers, the substrate to which the laser bars are co-packaged cannot be a semiconductor or conductor, while still maintaining thermal safety limits. Instead, the substrate in such a system needs to be ceramic, which cannot be etched using semiconductor lithography.
To overcome these challenges, the present disclosure provides a co-packaging technique that enables relatively inexpensive OTS laser bars to be co-packaged onto a ceramic substrate with a high-degree of alignment accuracy. The exemplary co-packaging technique may include, for example, 1) forming a set of alignment fiducials from a semiconductor wafer using semiconductor lithography, 2) transferring and coupling the set of alignment fiducials to a ceramic substrate, and 3) coupling a pair of laser bars to the ceramic substrate based on the set of alignment fiducials. Thus, the co-packaging technique of the present disclosure can enable OTS laser bars to be co-packaged onto a ceramic substrate with the level of accuracy needed for the high-resolution LiDAR systems.
Some exemplary embodiments are described below with reference to a receiver used in LiDAR system(s), but the application of the emitter array disclosed by the present disclosure is not limited to the LiDAR system. Rather, one of ordinary skill would understand that the following description, embodiments, and techniques may apply to any type of optical sensing system (e.g., biomedical imaging, 3D scanning, tracking and targeting, free-space optical communications (FSOC), and telecommunications, just to name a few) known in the art without departing from the scope of the present disclosure.
Moreover, some exemplary embodiments are described below in connection to co-packaged laser bars that each respectively include four laser emitters for use in a system that operates at a vertical-axis scanning frequency of 1.7 kHz. However, the exemplary co-packaging technique is not limited to two laser bars, each with four laser emitters. Rather, the number of laser bars and the number of laser emitters per laser bar may be different depending on the vertical-axis scanning frequency used and the intended degree of resolution. For example, in a system that uses 3.4 kHz as the vertical-axis scanning frequency, two laser bars, each respectively including two laser emitters, may be co-packaged onto a ceramic substrate without departing from the scope of the present disclosure. The numbers of laser emitters in the laser bars do not necessarily have to be the same either. Also, depending on the number of laser bars and/or laser emitters per laser bar, the non-uniform spacing between the laser emitters may be different that those dimensions described below in connection with a two laser bar/four laser emitter per laser bar system.
Transmitter 102 can sequentially emit a stream of pulsed laser beams in different directions within a scan range (e.g., a range of scanning angles in angular degrees), as illustrated in
In some embodiments of the present disclosure, laser source 106 may include a pair of laser bars, which are edge-emitting semiconductor lasers, and aligned along an edge of a ceramic substrate of submount 150. Each laser bar may include a plurality of laser emitters equally spaced apart. For example, adjacent laser emitters of a single laser bar may be separated by a first distance (e.g., 100 µm, 200 µm, 400 µm, etc.). However, the pair of laser bars may be spaced apart on the substrate such that adjacent laser emitters respectively located in the two laser bars (e.g., the center two laser channels) are separated by a second distance (e.g., 200 µm, 300 µm, 600 µm, etc.), which is greater than the first distance. In one exemplary embodiment, laser channels 1 and 2 may be separated by 400 µm, laser channels 2 and 3 may be separated by 400 µm, laser channels 3 and 4 may be separated by 400 µm, laser channels 4 and 5 may be separated by 600 µm, laser channels 5 and 6 may be separated by 400 µm, and laser channels 7 and 8 may be separated by 400 µm. By increasing the distance between the center two laser channels, the high-resolution scanning procedure of the present disclosure (see
Each of the laser emitters may be formed as, e.g., a pulsed laser diode (PLD), a vertical-cavity surface-emitting laser (VCSEL), a fiber laser, etc. For example, a PLD may be a semiconductor device similar to a light-emitting diode (LED) in which the laser beam is created at the diode’s junction. In some embodiments of the present disclosure, a PLD includes a PIN diode in which the active region is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into the active region from the N and P regions, respectively. Depending on the semiconductor materials, the wavelength of incident laser beam 107 provided by a PLD may be greater than 700 nm, such as 760 nm, 785 nm, 808 nm, 848 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 edge-emitting laser bar may be used as laser source 106 for emitting laser beam 107.
Scanner 108 may be configured to steer a laser beam 109 towards an object 112 (e.g., stationary objects, moving objects, people, animals, trees, fallen branches, debris, metallic objects, non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds and even single molecules, just to name a few) in a direction within a range of scanning angles. In some embodiments consistent with the present disclosure, scanner 108 may include a micromachined mirror assembly, e.g., such as scanning mirror 110. Scanning mirror 110 may be a microelectricalmechanical (MEMS) mirror. Scanning mirror 110 may be configured to resonate during the scanning procedure. Although not shown in
In some embodiments, receiver 104 may be configured to detect a returned laser beam 111 returned from object 112. Returned laser beam 111 may be returned from object 112 and have the same wavelength as laser beam 109. Returned laser beam 111 may be in a different direction from laser beam 109. Receiver 104 can collect laser beams returned from object 112 and output electrical signals reflecting the intensity of the returned laser beams. Upon contact, laser beam 109 can be reflected by object 112 via backscattering, e.g., such as Raman scattering and/or fluorescence.
As illustrated in
LiDAR system 100 may also include one or more signal processor 124. Signal processor 124 may receive electrical signal 119 generated by photodetector array 120. Signal processor 124 may process electrical signal 119 to determine, for example, distance information carried by electrical signal 119. Signal processor 124 may construct a point cloud based on the processed information. The point cloud may include a frame, which is an image of the far-field at a particular point in time. In this context, a frame is the data/image captured of the far field environment within the 2D FOV (horizontal FOV and vertical FOV). Signal processor 124 may include a microprocessor, a microcontroller, a central processing unit (CPU), a graphical processing unit (GPU), a digital signal processor (DSP), or other suitable data processing devices.
The exemplary scanning procedure depicted in
Furthermore, the scanning frequency in the vertical direction may determine the number of vertical lines scanned per frame. Hence, the number of laser channels associated with a vertical-axis scanning frequency of 1.7 kHz is greater than the number of laser channel associated with 3.4 kHz. For example, eight laser channels (shown in
Referring to
To accommodate a vertical-axis scanning frequency of 1.7 kHz, while avoiding the manufacturing expense and chip size of custom designed laser bars, the present disclosure provides a co-packaging technique to form the side-by-side laser bars with additional spacing between the center laser channels (e.g., channels 4 and 5), as shown in
However, as mentioned above, even a small misalignment of the first laser bar 202a and/or the second laser bar 202b along either the x-axis (first transverse direction) or the y-axis (second transverse direction) of the substrate can cause significant errors that reduce overall accuracy to levels unacceptable for autonomous navigation. This is because any transverse misalignment of the laser bars generates the same amount of angular misalignment in the 3D point cloud. For example, a 1 milliradian (mrad) directional error in either yaw, pitch, or roll would cause the same amount of directional error. The maximum amount of misalignment that can be tolerated in either transverse direction may be different depending on the system and number of laser channels used. By way of example, a transverse misalignment of +/- 5 µm may cause the following angular misalignments in the final point cloud: 1) 0.5 mrad in the x- direction, 2) 0.1 mrad in the y- direction, and 3) 0.5 mrad in the z-direction. Angular misalignments greater than those listed above may be unacceptable from a precision and safety standpoint. Thus, in some embodiments, the maximum transverse misalignment may be +/- 5 µm, which may also be referred to as the “misalignment tolerance threshold.”
Semiconductor lithography can achieve this level of alignment precision by etching (into a semiconductor wafer) alignment fiducials, which can then be used for placement of first and second laser bars 202a, 202b. However, because first and second laser bars 202a, 202b generate a certain amount of heat during operation, it is beneficial for substrate 220 to include a ceramic rather than semiconductor material. Using a ceramic material for substrate 220 may enable thermal safety limits within transmitter 102 to be maintained, for example. However, ceramic materials cannot be etched using semiconductor-lithography techniques.
Thus, according to the exemplary co-packaging technique of the present disclosure, a set of alignment fiducials may be etched on a semiconductor wafer using semiconductor-lithography, and then transferred and coupled to a ceramic substrate, e.g., substrate 220. The precise alignment of the set of alignment fiducials may then be used for placement of the pair of laser bars.
For example, referring to
Thus, using the co-packaging technique described above in connection with
Referring to
At S304, a set of alignment fiducials may be formed using semiconductor-lithography. For example, referring to
At S306, the set of alignment fiducials may be coupled to the substrate. For example, referring to
At S308, at least one laser bar may be coupled to the substate based on the set of alignment fiducials. For example, referring to
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.
