Solid-state LIDAR transmitter with laser control

Abstract

To provide a solid-state LIDAR transmitter with laser control.SOLUTION: A solid state LIDAR transmitter with a matrix-addressable laser drive circuit includes a first electrical bus that provides a first voltage potential to columns of the matrix-addressable laser drive circuit and a second electrical bus that provides a second voltage potential to rows of the matrix-addressable laser drive circuit. A plurality of column switches connects the plurality of columns to the first electrical bus. A plurality of row switches connects the plurality of rows to the second electrical bus. The transmitter includes a plurality of series connected diodes comprising a laser diode in series with another diode, where a respective one of the plurality of series connected diodes is electrically connected between a respective column and row of the matrix-addressable laser drive circuit to form the LIDAR transmitter. At least some of the second diodes increase an overall reverse breakdown voltage of the series connected diodes.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The headings used herein are for organizational purposes only and should not be interpreted in any way as limitations on the subject matter described herein. (Cross-reference of related applications)

[0002] This application is a non-provisional patent application of U.S. Provisional Patent Application No. 62 / 831,668, filed on 9 April 2019, titled "Solid-State LIDAR Transmitter with Laser Control." The entire contents of U.S. Provisional Patent Application No. 62 / 831,668 are incorporated herein by reference. [Background technology]

[0003] Autonomous, self-driving, and semi-autonomous vehicles use a combination of different sensors and technologies, such as radar, image recognition cameras, and ultrasonic transducers, to detect and locate surrounding objects. These sensors enable a range of improvements in driver safety, including collision warning, automatic emergency braking, lane departure warning, lane departure prevention assistance, adaptive cruise control, and pilot driving. Of these sensor technologies, light detection and ranging (LIDAR) systems play a crucial role in enabling real-time, high-resolution 3D mapping of the surrounding environment. [Overview of the project] [Means for solving the problem]

[0004] This instruction will be described in more detail herewith reference to exemplary embodiments thereof, such as those shown in the accompanying drawings. While this instruction will be described in conjunction with various embodiments and examples, it is not intended to be limited to such embodiments. In contrast, this instruction includes various alternatives, modifications, and equivalents, as will be understood by those skilled in the art. Those skilled in the art who have access to the instruction herein will recognize additional implementations, modifications, and embodiments, and other fields of use, which fall within the scope of this disclosure as described herein.

[0005] Any reference in this specification to “one embodiment” or “a particular embodiment” means that certain features, structures, or characteristics described in relation to an embodiment are included in at least one embodiment of this teaching. The phrase “in one embodiment” in various places in this specification does not necessarily refer to the same embodiment in all instances.

[0006] It should be understood that the individual steps of the method in this teaching can be performed in any order and / or simultaneously, as long as this teaching remains operational. Furthermore, it should be understood that the apparatus and method in this teaching can include any number or all of the embodiments described, as long as this teaching remains operational.

[0007] Most commercially available LiDAR systems used for autonomous vehicles today utilize a small number of lasers combined with several methods of mechanically scanning the environment. For current and future autonomous vehicle applications, the use of solid-state semiconductor-based LiDAR systems is highly desirable. Solid-state LiDAR systems, especially those without moving parts, exhibit superior reliability and can operate over a wider environmental operating range compared to current LiDAR systems. Such solid-state systems for use in LiDAR systems can also be physically compact and relatively inexpensive.

[0008] One approach to solid-state LiDAR is to use multiple lasers, each projecting at a unique angle across a desired field of view (FOV), thereby avoiding the need for mechanical scanning. However, electrically connecting driver circuits to multiple lasers while retaining the ability to operate them individually presents a challenge. One solution is to employ a matrix-addressable laser driver circuit that arranges multiple lasers in a 2D matrix and then controls individual and / or group lasers within the array, simultaneously satisfying the need to provide optimal electrical characteristics (e.g., current, voltage, and timing) and activate the lasers. The methods and apparatus of this teaching relate to a laser control method and system architecture that enables individual control of a 2D matrix of lasers while ensuring a low-cost system. The present invention provides, for example, the following: (Item 1) A solid-state light detection and ranging (LIDAR) transmitter with a matrix-addressable laser drive circuit, wherein the LIDAR transmitter is a) A first electric bus that provides a first voltage potential to the row of the matrix-addressable laser drive circuit, b) A second electric bus that provides a second voltage potential to the rows of the matrix-addressable laser drive circuit, c) A plurality of row switches, each of which row switches connects one of the plurality of rows to the first electric bus, d) A plurality of row switches, each of which row switches connects one of the plurality of rows to the second electric bus, e) A plurality of connected diodes comprising a laser diode electrically connected in series with a second diode, wherein each of the plurality of connected diodes is electrically connected between individual columns and individual rows of the matrix addressable laser driver circuit, forming the LIDAR transmitter, and at least some of the second diodes increase the overall reverse breakdown voltage of the plurality of connected diodes, thereby reducing optical crosstalk when the LIDAR transmitter is activated. A LIDAR light transmitter equipped with this feature. (Item 2) The solid-state LIDAR transmitter according to item 1, wherein at least some of the second diodes are equipped with active PN junctions that generate an optical gain that increases the brightness of the associated laser diode. (Item 3) A solid-state LiDAR transmitter as described in item 1, wherein at least some of the second diodes do not generate optical gain. (Item 4) At least some of the second diodes are photodiodes, as described in item 1, for the solid-state LIDAR transmitter. (Item 5) A solid-state LiDAR transmitter as described in item 1, wherein at least some of the second diodes are monolithically integrated with the laser diodes. (Item 6) The solid-state LiDAR transmitter according to item 1, wherein at least some of the second diodes are located on a substrate separate from the substrate of the laser diodes. (Item 7) The solid-state Lidar transmitter according to item 1, wherein at least some of the plurality of connected diodes are configured to have an overall reverse breakdown voltage exceeding the absolute value of the maximum drive voltage provided by the first and second electric buses. (Item 8) The solid-state LIDAR transmitter according to item 1, wherein at least some of the laser diodes have at least two apertures connected in series. (Item 9) The solid-state LIDAR transmitter according to item 1, wherein at least some of the laser diodes have at least two active regions separated by a tunnel junction. (Item 10) The solid-state LIDAR transmitter according to item 1, wherein at least some of the laser diodes are surface-emitting laser diodes. (Item 11) The solid-state LIDAR transmitter according to item 1, wherein at least some of the laser diodes are vertical-cavity surface-emitting laser diodes. (Item 12) The solid-state LiDAR transmitter according to item 1, wherein the first electric bus is configured to supply a positive voltage to the anode of the laser diode. (Item 13) The solid-state Lidar transmitter according to item 1, wherein the second electric bus is configured to provide a ground potential to the cathode of the laser diode. (Item 14) The solid-state LIDAR transmitter according to item 1, wherein at least some of the plurality of column switches and the plurality of row switches are equipped with transistors. (Item 15) The solid-state LIDAR transmitter according to item 1, wherein at least some of the plurality of column switches and the plurality of row switches are equipped with asymmetric on-off driver circuits. (Item 16) The asymmetric on-off driver circuit comprises a GaN FET driver circuit, as described in item 15, for the solid-state LiDAR transmitter. (Item 17) The solid-state LIDAR light transmitter according to item 1, wherein at least some of the plurality of column switches and at least some of the plurality of row switches include extended MOSFET power transistors. (Item 18) The solid-state LIDAR light transmitter according to item 1, wherein at least some of the plurality of column switches and at least some of the plurality of row switches include GaN power transistors. (Item 19) The solid-state LIDAR light transmitter according to item 1, wherein at least some of the laser diodes are configured to emit optical radiation of 830 nm to 1000 nm. (Item 20) The solid-state LIDAR light transmitter according to item 1, wherein the number of the plurality of rows is the same as the number of the plurality of columns. (Item 21) The solid-state LIDAR light transmitter according to item 1, wherein the number of the plurality of rows is not equal to the number of the plurality of columns. (Item 22) The solid-state LIDAR light transmitter according to item 1, further comprising a power supply for generating the first and second voltage potentials, wherein the power supply is configured to enter a reduced power mode when not generating the first and second voltage potentials. (Item 23) A method for generating a light detection and ranging (LIDAR) optical beam, the method comprising: a) providing a two-dimensional array of laser devices comprising a plurality of series-connected diodes each comprising a laser diode electrically connected in series with a second diode, wherein individual ones of the plurality of series-connected diodes are electrically connected between columns and rows of individual matrix-addressable laser driver circuits; b) configuring at least some of the second diodes to increase the overall reverse breakdown voltage of the series-connected diodes so as to reduce optical crosstalk within the LIDAR optical beam; c) Switching the first voltage potential to the selected column of the matrix-addressable laser drive circuit, d) Switching the second voltage potential to a selected row of the matrix-addressable laser driver circuit, thereby sequentially biasing the selected laser diodes in the two-dimensional array of the laser device, causing the series of connected second diodes to emit a LIDAR optical beam with a desired pattern in which optical crosstalk is reduced. Methods that include... (Item 24) A method for generating a LiDAR optical beam as described in item 23, comprising switching the first voltage potential to a selected column of the matrix-addressable laser driver circuit, which includes applying a voltage greater than 10V. (Item 25) A method for generating a LiDAR optical beam according to item 23, comprising switching the first voltage potential to a selected column of the matrix-addressable laser drive circuit over a predetermined period of time to generate a LiDAR optical beam comprising a column of light pulses. (Item 26) A method for generating a LiDAR optical beam according to item 23, further comprising selecting at least one of the first and second voltages such that a two-dimensional array of laser devices causes the LiDAR optical beam to emit light with a peak power of more than 20 watts. (Item 27) A method for generating a LIDAR optical beam as described in item 23, wherein the first voltage potential is switched to a selected column of the matrix-addressable laser drive circuit and the second voltage potential is switched to a selected row of the matrix-addressable laser drive circuit, at a rate that generates pulses having a pulse duration of less than 10 nanoseconds within the column of optical pulses. (Item 28) A method for generating a LiDAR optical beam as described in item 25, wherein the first voltage potential is switched to a selected column of the matrix-addressable laser drive circuit and the second voltage potential is switched to a selected row of the matrix-addressable laser drive circuit, such that power dissipation is reduced when no pulses are being generated. (Item 29) A method for generating a LiDAR optical beam according to item 28, further comprising shutting down the power sources that generate the first and second voltage potentials when no pulses are being generated. (Item 30) A method for generating a LIDAR optical beam according to item 23, comprising switching the first voltage potential to a selected column of the matrix-addressable laser drive circuit for a predetermined time such that the transient temperature rise at the junction of the laser diode is kept below 20°C. (Item 31) A method for generating a LIDAR optical beam as described in item 23, wherein at least some of the laser diodes emit the optical beam at a wavelength of 830 nm to 1,000 nm. (Item 32) A method for generating a LIDAR optical beam as described in item 23, wherein the first voltage potential is above the dielectric breakdown voltage of at least some of the laser diodes. [Brief explanation of the drawing]

[0009] The teachings, in preferred and exemplary embodiments, along with their further advantages, are described in detail below in conjunction with the accompanying drawings. Those skilled in the art will understand that the drawings described below are for illustrative purposes only. The drawings are not necessarily to exact scale, and emphasis is placed, instead, generally to illustrate the principles of the teachings. The drawings are not intended to limit the scope of the applicant's teachings in any way.

[0010] [Figure 1A]Figure 1A shows a schematic diagram of a solid-state LIDAR system.

[0011] [Figure 1B] Figure 1B illustrates the two-dimensional projection of the system field of view (FOV) of the LIDAR system shown in Figure 1A.

[0012] [Figure 2] Figure 2 illustrates a perspective view of a known bottom-emitting vertical-cavity surface-emitting laser (VCSEL) that may be used in a LIDAR system according to this teaching.

[0013] [Figure 3] Figure 3 illustrates a schematic diagram of an embodiment of a two-dimensional (2D) monolithic VCSEL array with 256 separate laser emitters for use in a solid-state LiDAR system, as taught herein.

[0014] [Figure 4] Figure 4 illustrates an exemplary cascaded two-port circuit model for an embodiment of an individual semiconductor laser in a VCSEL array according to this teaching.

[0015] [Figure 5A] Figure 5A shows an electrical schematic diagram of an embodiment of a matrix-addressable laser driver circuit for controlling a two-dimensional laser array with row / column matrix addressing capability, according to one embodiment of this teaching.

[0016] [Figure 5B] Figure 5B illustrates an embodiment of a matrix-addressable laser driver circuit configured as a voltage driver in which a single laser in a 2D laser array is activated using row / column matrix addressing capability, according to this teaching.

[0017] [Figure 5C]Figure 5C illustrates an electrical schematic of a single-matrix addressable laser driver circuit configured as a high-side current driver, which can be used in conjunction with a 2D laser array with row / column addressing capability, according to one embodiment of this teaching.

[0018] [Figure 5D] Figure 5D illustrates an electrical schematic of a single-matrix addressable laser driver circuit configured as a low-side current driver, which can be used in conjunction with a 2D laser array with row / column addressing capability, according to one embodiment of this teaching.

[0019] [Figure 5E] Figure 5E illustrates an electrical schematic of a matrix-addressable laser driver circuit, which can be used in conjunction with a 2D laser array with row / column matrix addressing capability according to one embodiment of this teaching, and which comprises a high-side voltage driver for columns, a low-side voltage driver for rows, and switches that can be used to apply additional voltages to the rows.

[0020] [Figure 5F] Figure 5F illustrates a voltage-potential timing diagram showing one method of operating a matrix-addressable laser driver circuit according to one embodiment of this teaching.

[0021] [Figure 5G] Figure 5G illustrates a schematic electrical diagram of a matrix-addressable laser driver circuit, configured with a high-capacitance discharge circuit in capacitor charging mode, according to one embodiment of this teaching.

[0022] [Figure 5H] Figure 5H illustrates an electrical schematic of a matrix-addressable laser driver circuit, which is described in relation to Figure 5G, according to one embodiment of this teaching, and is configured with a high-side capacitance discharge circuit in capacitor discharge mode for laser diodes 2, 2 (second row and second column).

[0023] [Figure 5I]Figure 5I illustrates a voltage-potential timing diagram showing the voltage potential across capacitor C2 and across laser diodes 2, 2 in the second row and second column, according to one embodiment of this teaching.

[0024] [Figure 5J] Figure 5J illustrates a schematic electrical diagram of a matrix-addressable laser driver circuit, configured with a low-side capacitance discharge circuit in capacitor charging mode, according to one embodiment of this teaching.

[0025] [Figure 5K] Figure 5K illustrates an electrical schematic of a matrix-addressable laser driver circuit, which is described in relation to Figure 5J, according to one embodiment of this teaching, and is configured with a low-side capacitance discharge circuit in capacitor discharge mode for laser diodes 2, 2 (second row and second column).

[0026] [Figure 5L] Figure 5L illustrates a voltage-potential timing diagram showing the voltage potential across capacitor C2 and across laser diodes 2, 2 in the second row and second column, according to one embodiment of this teaching.

[0027] [Figure 6] Figure 6 illustrates an embodiment of a combined high-side and low-side GaN FET driver circuit for electrically driving laser diodes in a matrix-addressable laser driver circuit within a LIDAR system laser array, as taught in this document.

[0028] [Figure 7] Figure 7 illustrates a typical current-voltage curve for an embodiment of a semiconductor diode in a matrix-addressable laser driver circuit according to this teaching.

[0029] [Figure 8]Figure 8 illustrates the voltages induced at nodes in a matrix while activating a single laser in a two-dimensional array with row / column matrix addressing capability, according to an embodiment of the matrix-addressable laser driver circuit controller of this teaching.

[0030] [Figure 9] Figure 9 illustrates an embodiment of a LiDAR system array according to this teaching, in which physical connections to the array allow for a denser layout of associated electronic circuits on a printed circuit board (PCB).

[0031] [Figure 10] Figure 10 illustrates a schematic diagram of an embodiment of a 2x2 laser array with a matrix-addressable laser drive circuit control according to this teaching, showing possible electrical current paths when one laser is activated.

[0032] [Figure 11] Figure 11 shows a schematic diagram of an embodiment of a 2x2 laser array according to this teaching, which includes a laser having a second diode in series with each laser diode in a matrix-addressable laser drive circuit.

[0033] [Figure 12] Figure 12 illustrates an embodiment of a series of diodes comprising a VCSEL array, each laser diode being part of a separate carrier, with an additional diode connected in series with it. [Modes for carrying out the invention]

[0034] Figure 1A illustrates a schematic diagram of a solid-state LiDAR system 100 in which individual lasers in a transmitter array 102 can be fired independently. The system illustrated in Figure 1A does not employ a flash transmitter that illuminates the entire system field of view at once. Instead, each individual laser in the transmitter array 102 can be fired independently, and the optical beam emitted by each laser corresponds to a 3D projection angle that touches only a portion of the total system field of view. One embodiment of such a transmitter is described in detail in U.S. Patent Publication 2017 / 0307736A1, which has been assigned to the assignee. The entire contents of U.S. Patent Publication 2017 / 0307736A1 are incorporated herein by reference.

[0035] The optical beams from the lasers in the laser array 102 share a transmitter optical system 104 that projects the optical beam 106 onto the target 108 in the target plane 110. A portion of the light from the incident optical beam 106 is reflected by the target 108. A portion of the reflected optical beam 112 shares a receiver optical system 114. The detector array 116 receives the reflected light projected by the receiver optical system 114. In various embodiments, the detector array 116 is solid-state and has no moving parts. The detector array 116 may have fewer individual detector elements than the individual lasers in the transmitter array 102.

[0036] The measurement resolution of the LIDAR system 100 is determined not by the size of the detector elements in the detector array 116, but rather by the number of lasers in the light transmitter array 102 and the collimation of the individual optical beams. A processor (not shown) in the LIDAR system 100 performs a time-of-flight (TOF) measurement to determine the distance to the target 108, which will reflect the optical beam 106 from the lasers in the laser array 102 so as to be detected in the detector array 116.

[0037] One feature of the system described herein is that individual lasers and / or groups of lasers within the light transmitter array 102 can be controlled individually. Another feature of the system described herein is that individual detectors and / or groups of detectors within the detector array 116 can be controlled individually. This control provides a variety of desired performance characteristics, including control of the field of view, optical power level, scanning, and / or other features.

[0038] Figure 1B illustrates a two-dimensional projection of the system field of view 150 of the LIDAR system in Figure 1A. The field of view of individual detectors within the detector array is represented by small squares 152. Illuminated measurement points, associated with individual lasers in the light transmitter, are illustrated by circles 154. As an example where one laser element in the array is activated, light from the laser collides with the individual detector field of view within the overall field of view of the LIDAR system in Figure 1A. The detector field of view receiving the light is highlighted by diagonal marks within a specific square 156, and the field of view of measurement points from individual lasers is shown as a specific dark circle 158, which corresponds to a specific individual laser in the laser array.

[0039] From Figure 1B, it can be seen that the measurement points illustrated by the circle 158 correspond to individual detectors, and the field of view of each individual detector is shown by a square 156 with an oblique parallel line pattern for identification. The figure illustrates that the 3D resolution of several embodiments of the LIDAR system is determined by the number of lasers, since each laser corresponds to a specific angular projection angle that produces the size of the circle 154 in the target range and the relative size of the circle 154 and square 152 representing the field of view of the individual detector element. Thus, various fields of view can be established by controlling specific individual or group lasers in a transmitter array for selectively activating and transmitting laser pulses, and / or specific individual or group detectors in a receiver array for converting optical signals received within the detector field of view into electrical signals. One feature of this teaching is an array-driven control system that is capable of providing such selective device control for an array of laser devices capable of illuminating a target.

[0040] In some embodiments, the field of view of individual detectors within a detector array is the active area of ​​the detector. The size of individual detectors within the array is primarily determined by the electrical characteristics of the device. For example, as the size of the active area of ​​an avalanche photodiode (APD) detector increases, the capacitance of the detector increases, reducing the photoelectric bandwidth of the device. The bandwidth of the APD must be kept sufficiently high so as not to attenuate or distort the received signal. Typical values ​​for the optical / electrical (O / E) bandwidth and APD capacitance in a LIDAR system with a laser pulse width <10 nsec and rise / fall time of approximately 1 nsec are less than 350 MHz and less than 2 pF, respectively. In general, an array of detectors must be used to cover the entire field of view of the LIDAR system while maintaining acceptable electrical detector performance. The overall physical size and dimensions of the array are determined by the required field of view and the specifications of the optical lens system of the photodetector.

[0041] Figure 2 illustrates a perspective view of the structure of a known bottom-emitting VCSEL 200 that may be used in a LIDAR system according to this teaching. The area of ​​the emission aperture 202 of the VCSEL 200 typically ranges from a few microns in diameter for mW power operation to more than 100 microns in diameter for 100 mW and above CW power operation. The VCSEL 200 is fabricated on a substrate 204, which may be, for example, GaAs or a number of other semiconductor materials. An n-type distributed Bragg reflector (DBR) layer 206 is positioned on the substrate. An active region 208 is constructed on the n-type DBR layer 206, followed by an aperture, which may be fabricated from an oxide material. A p-type DBR layer 212 is then grown on the active region. Typically, the p-type DBR layer 212 is highly reflective and the n-type DBR layer 206 is partially reflective, resulting in an optical output 214 from the bottom substrate side of the layer structure. The active region 208, oxide aperture 210, and p-type DBR layer 212 are formed within a mesa structure in the shown device. The upper contact 216 and bottom contact 218 are used to supply electrical current to the active region and generate output light. The oxide aperture 210 provides current confinement to the active region 208. The upper contact 216 is p-type, and the bottom contact 218 is n-type.

[0042] The light-emitting aperture 202 is formed within the bottom contact 218, allowing the output light 214 to emerge from the bottom substrate side of the bottom-emitting VCSEL 200. Note that since Figure 2 illustrates only one element of a multi-element VCSEL array, only one light-emitting aperture 202 is shown in Figure 2. This type of VCSEL can be a standalone single element or can be part of a multi-element VCSEL that can be fabricated on the substrate 204 as a one- or two-dimensional array. The VCSEL contacts 216, 218 can be individually addressable and / or they can be electrically connected together in various configurations so that they are addressed to a group of VCSELs using a common electrical input signal. One feature of this teaching is a system and method for controlling the activation of one or more VCSEL 200 devices in an array using appropriate drive signals for LIDAR system applications.

[0043] In some embodiments, the VCSEL array used in the solid-state LIDAR system of this teaching is monolithic, and all lasers share a common substrate on which the lasers are integrated. Various types of common substrates can be used. For example, the common substrate may be made of a semiconductor material. The common substrate may also include a ceramic material. In other embodiments, the 2D VCSEL array is assembled from a 1D bar or even a group of individual dies.

[0044] In some embodiments, the VCSEL is a top-emitting VCSEL device. In other embodiments, the VCSEL device is a bottom-emitting VCSEL. Individual VCSEL devices may have either a single large emission aperture, or individual VCSEL devices may be formed from two or more sub-apertures within a larger effective emission diameter. A group of sub-apertures forming a larger effective emission region is sometimes referred to as a cluster.

[0045] Figure 3 illustrates a schematic diagram of an embodiment of a 2D monolithic VCSEL array 300 with 256 separate laser emitters 302 for use in a solid-state LIDAR system according to this teaching. Each laser emitter 302 has an emission aperture with a diameter "a" 304. The emission from each single laser emitter 302 substantially fills the complete emission aperture. Each laser emitter 302 thus generates a laser beam with an initial diameter "a", which is equal to the diameter "a" 304 of the emission aperture. The laser emitters 302 are uniformly spaced horizontally with a spacing dx 306 and uniformly spaced vertically with a spacing dy 308. The overall size of the array, measured from the center of the outermost laser, is a distance Dx 310 horizontally and a distance Dy 312 vertically. The actual chip size will be slightly larger than the distances Dx 310 and Dy 312. In various embodiments, the emitter 302 can generate beams with various shapes other than a circular emitter shape. For example, oval, square, rectangular, and various special shapes can be realized in various embodiments. In embodiments where the lasers are arranged as a 2D array, the rows and columns of lasers can be electrically driven in a matrix-addressable manner.

[0046] Some embodiments of this teaching utilize bottom-emitting high-power arrays of VCSEL devices with a single large aperture per laser, such as the configuration shown in Figure 3. Other embodiments of this teaching utilize top-emitting or bottom-emitting high-power arrays of VCSELs with an overall emission area having sub-apers. However, those skilled in the art will understand that this teaching is not limited to any particular configuration of top-emitting and bottom-emitting VCSEL devices and associated emission apertures.

[0047] A two-dimensional VCSEL array can be used as a building block for a LiDAR system according to this teaching, establishing a platform that enables a small physical size for the light transmitter. For example, a two-dimensional VCSEL array with 256 high-power individual lasers can be built on a monolithic chip measuring approximately 4 mm × 4 mm. Such a monolithic chip can be used with an optical system selected to keep the physical dimensions as small as possible, for example, through the use of a microlens array, a shared lens with dimensions less than 20 mm, or a diffractive optical system with a maximum dimension of approximately 20 mm.

[0048] However, the LIDAR system described in this instruction imposes certain requirements on the 2D VCSEL array. In particular, it is desirable that the 2D VCSEL array allow for the independent simultaneous control of all VCSEL devices. In some operating modes of the LIDAR system described in this instruction, each VCSEL in the matrix is ​​fired at a different time. With respect to such operation, the VCSEL array needs to operate in a matrix-addressable manner, where the lasers can be fired individually but not always simultaneously.

[0049] Figure 4 illustrates an exemplary cascaded two-port circuit model 400 for an embodiment of an individual semiconductor laser in a VCSEL array according to this teaching. The intrinsic laser junction is represented by a commonly known symbol for diode 402. The active region of the laser, which emits laser light, is sandwiched between the pn junctions of the diodes. Circuit model 400 is shown with voltage V d The circuit model 400 includes the electrical characteristics of the driver connection 404 and the metal contact 406, which supply power. In addition, the circuit model 400 includes a parasitic element 408, which provides a parasitic pad current 408i p and parasitic chip current 410i p This includes pad resistance loss in its form.

[0050] In one embodiment, the solid-state LiDAR system of this teaching uses VCSEL devices assembled using heterogeneous integration techniques. For example, these devices can provide a highly compact method of flip-chip bonding to silicon electronics, connecting to and electrically driving the VCSELs. See, for example, Plant et al., “256-Channel Bidirectional Optical Interconnect Using VCSELs and Photodiodes on CMOS” (IEEE Journal of Lightwave Technology, Vol. 19, No. 8, August 2001). See also U.S. Patent No. 7,702,191, “Electro-Optical Chip Assembly” and U.S. Patent No. 8,675,706, “Optical Illuminator that Fire Devices in Parallel”. However, these known heterogeneous integration techniques are primarily intended for applications in the optical communications market where simultaneous parallel operation of multiple channels is desired as a solution to increase data transmission throughput.

[0051] Figure 5A illustrates an electrical schematic of an embodiment of a matrix-addressable laser driver circuit configured as a 2D laser array 500 with row / column addressing capability, according to one embodiment of this teaching. For convenience, in this and the following figures, the diode symbol 502 is used here to represent a laser, but it should be understood that the laser is more accurately represented by model 400, which is described in relation to Figure 4, and such a model would be used in actual design. Also for convenience, only the 4x4 matrix of diodes is shown in schematic Figure 500, and the voltage and / or current drivers that drive the matrix-addressable laser driver circuit are not shown. However, it should be understood that in practice, the matrix of laser diodes is M x N, where M and N are arbitrary integers greater than or equal to 2, and in some embodiments, M and N are large numbers.

[0052] The matrix-addressable laser driver circuit for the 2D laser array 500 is configured such that the VCSEL device 502 is connected to anodes 504, 504', 504'', 504''''. The rows of VCSEL device 502 are connected by cathodes 506, 506', 506'', 506''''. This anode row and cathode row connection configuration, illustrated in schematic figure 500, allows individual lasers 502 to be turned on / off through row and column operation without the need for individual access to the cathode and anode of a single laser 502.

[0053] Figure 5B illustrates an embodiment of a matrix-addressable laser driver circuit according to this teaching, in which a single laser 502 in a 2D laser array 500 is configured as a voltage driver, activated using row / column matrix addressing capability. Power source 548 applies a voltage potential 550 to ground 552 via an anode contact electric bus 554 electrically connected to the columns of anodes 504, 504', 504'', 504''' using a series of switches 556, 556', 556'', 556''', and a ground bus 552 connected to the rows of cathodes 506, 506', 506'', 506''' using a series of switches 558, 558', 558'', 558'''. Power source 548 applies a voltage potential 550 V with a desired voltage potential waveform. + This generates a current. The row of anodes for its VCSEL, which is activated and turned on, is connected via switch 556' to a voltage potential 550 high enough to sequentially bias the VCSEL diode 550. The row of cathodes 506' containing its VCSEL is connected via switch 558' to the ground bus 552, completing the circuit and thereby enabling the laser to be activated so that current flows through the VCSEL and emits light. The other cathodes and anodes are set to an "open" condition and are not supplied with any specific voltage level by the power source 548 by having open switches 556, 556'', 556''' and open switches 558, 558'', 558''''.

[0054] In an alternative embodiment of the circuit shown in Figure 5B, cathodes 506, 506'', and 506'''' are not "open" but instead connected to a power source 548 (or another power source) which provides a voltage potential waveform with a voltage set to a predetermined voltage level, less than V+, during operation. In other words, the potential on the ground bus 552 shown in Figure 5B cannot be the ground potential but instead can be a voltage level set by the power source 548, less than V+, such that the voltage potential 550 is applied to a predetermined voltage during operation. In these embodiments, switches 556, 556'', 556'''', 558, 558'', and 558'''' would toggle between an anode or cathode voltage potential 550 and a voltage source (not shown) set to the defined voltage level. This alternative embodiment may have performance advantages such as reduced crosstalk. A more detailed circuit implementing this embodiment is described in relation to Figure 5E.

[0055] One feature of the laser array controller in this teaching is that it provides desired laser driving characteristics using various laser driving circuits. In some embodiments, the power source 548 that drives the laser generates high-current, short-duration pulses. In these embodiments, the power source 548 is designed to provide the required high-current, short-duration pulses. The matrix can also be operated by causing the power source 548 to apply a potential waveform with a defined voltage (a so-called voltage driver) or a current waveform with a defined current level (a so-called current driver).

[0056] In some embodiments, the power source 548 is configured to generate a waveform that reduces power dissipation when no pulses are being generated. This can be achieved, for example, by using a circuit configuration that provides a near or complete shutdown of the output of the power source 548 during the pause between the application of short-duration pulses. In one such embodiment, the power source activates the laser driver during a wake-up period before the generation of a short-duration pulse. The power source 548 generates a waveform shutdown over the time between pulses that begins after the pulse is fired. This waveform shutdown period precedes the wake-up period before the generation of another short-duration pulse. Some power sources also have a “lower power” state that is used to further reduce power consumption. For example, in a practical implementation, a controller within the power source or a separate controller can execute a series of commands, namely, (1) putting the laser driver power source into a "low power state", (2) putting the laser driver power source into "wake-up" mode, (3) turning on the laser driver power source output, (4) turning off the laser driver power source output, and (5) returning the laser driver to a "low power state".

[0057] Figure 5C illustrates an electrical schematic of a single-matrix addressable laser driver circuit 570 configured as a high-side current driver, which can be used in conjunction with a 2D laser array with row / column addressing capability, according to one embodiment of this teaching. The high-side, configured as the laser driver circuit 570, includes a field-effect transistor (FET) 572, which has an FET source coupled to the potential V+ of a power source and an FET drain coupled to the anode of a laser diode 574. The drive current for the laser diode 574 is provided by a voltage-controlled current source 576 such that the laser current is proportional to the drive voltage.

[0058] Figure 5D illustrates an electrical schematic of a single-matrix addressable laser driver circuit 580 configured as a low-side current driver, which can be used in conjunction with a 2D laser array with row / column addressing capability, according to one embodiment of this teaching. The laser driver 580 includes a voltage-controlled current source 582 having an input coupled to a power source. The output of the voltage-controlled current source 582 is connected to the anode of a laser diode 584. A field-effect transistor (FET) 586 has a source coupled to the anode of the laser diode 584 and a drain coupled to ground.

[0059] Figure 5E illustrates an electrical schematic of a matrix-addressable laser driver circuit 590, which can be used in conjunction with a 2D laser array with row / column matrix addressing capability according to one embodiment of this teaching. The circuit comprises a high-side voltage driver 591 for columns, a low-side voltage driver 592 for rows, and switches that can be used to apply additional voltages to the rows. A voltage divider circuit 593 is used to set the voltage between rows of the 2D laser array. The voltage divider circuit 593 is controlled by applying a charging signal to its FET gate. A laser diode 594 is shown along with its associated parasitic capacitor.

[0060] The configuration of the matrix-addressable laser driver circuit 590 is similar to the alternative embodiment of the circuit shown in Figure 5B, except that the cathode of the laser diode is at a potential other than ground potential during normal operation. However, in this circuit, the addition of switch 593 allows for more precise control of the voltage applied to the cathode. In the matrix-addressable laser driver circuit 590 configuration shown in Figure 5E, the cathode of the laser diode is at a potential determined by voltage divider 593, which is controlled by a charging signal applied to its FET gate. Operating the matrix-addressable laser driver circuit 590 so that the laser diode is reverse-biased so that the cathode is at a potential other than ground potential can have several performance advantages. One such performance advantage is that crosstalk between laser diodes can be significantly reduced.

[0061] Figure 5F illustrates a voltage timing diagram 599, showing one method of operating the matrix-addressable laser driver circuit 590, which is described in relation to Figure 5E. The waveforms are shown with respect to the column drive signal C2 595 applied to the high-side voltage driver 591, the row drive signal R2 596 applied to the low-side voltage driver 592, and the charge signal 597 applied to the voltage divider 593.

[0062] Optical pulses are generated only when both the column drive signal 595 and the row drive signal 596 are high. The pulse duration of the row drive signal determines the optical pulse width. The duty cycle depends on various operating parameters. For example, in one mode of operation, the duty cycle of the optical pulse is 1%, and the column drive signal 595 pulse is longer than the row drive signal 596 pulse. This prevents conflict between the row pulse and the column pulse.

[0063] One important feature of the methods and apparatus described in this teaching is that various laser driver circuit configurations and operating methods reduce crosstalk and therefore increase performance. Referring to the matrix-addressable laser driver circuit 590 described in relation to Figure 5E, crosstalk can occur when the laser diode is indirectly activated through its associated parasitic capacitor and low-side driver via the electrical path. This undesirable result can be prevented in the configuration shown in Figure 5E by setting the laser diode 594 in reverse bias and charging the parasitic capacitor to voltage +V. Thus, when the desired laser diode is activated, there should be no other laser diodes emitting light. However, a method of biasing the laser diode 594 to a persistent reverse bias condition would result in an increased device failure rate and a decrease in overall device reliability. One solution according to this teaching is to activate the low-side driver for the remainder of the duty cycle when the laser diode is intentionally not activated, thereby discharging the parasitic capacitor of the laser diode. For example, the activation would typically occur over a 1% duty cycle, during an off-duty time of approximately 99%.

[0064] Figure 5G illustrates an electrical schematic of a matrix-addressable laser driver circuit 620, configured with a high-side capacitance discharge circuit 622 in capacitor charging mode. The driver circuit 620 is similar to the driver circuit 500 described in relation to Figure 5A, but includes the high-side capacitance discharge circuit 622. In capacitor charging mode, all high-side switches 624 and all low-side switches 626 are opened, thereby allowing capacitors C1-C3 630 to be charged to a full potential applied to the driver circuit 620, with a certain time constant, which is denoted as -V.

[0065] Figure 5H illustrates an electrical schematic of the matrix-addressable laser driver circuit 620, which is described in relation to Figure 5G, and is configured with a high-side capacitance discharge circuit 622 in capacitor discharge mode for laser diodes 2, 2 (second row and second column). In capacitor discharge mode for laser diodes 2, 2, both the high-side switch 624 and the low-side switch 626 are closed, causing an electrical current to discharge in path 632.

[0066] Figure 5I illustrates a voltage potential timing diagram 635 showing the voltage potential across capacitor C2 and across laser diodes 2, 2 in the second row and second column. Switches LS2 and HS2 are closed until an initial time t0. Before time t0, potential C2+ is at ground potential and potential C2- is at -V potential. At time t0, switches LS2 626 and HS2 624 are closed, transitioning potential C2- to ground potential and charging potential C2+ to +V potential, reaching these potentials at time t1. After the anodes of laser diodes 2, 2 are charged to +V potential, capacitor C2 discharges from potential C2+ through laser diodes 2, 2, thereby generating optical pulses. At time t2, switches LS2 and HS2 are closed, initiating conditions for subsequent pulses. The result of this switching sequence is that the discharge control method generates analog drive pulses in which power consumption is independent of pulse width.

[0067] Figure 5J illustrates an electrical schematic of a matrix-addressable laser driver circuit 640, configured with a low-side capacitance discharge circuit 642 in capacitor charging mode. The driver circuit 640 is similar to the driver circuit 500 described in relation to Figure 5A, but includes the low-side capacitance discharge circuit 642. In capacitor charging mode, all low-side switches 644 and all high-side switches 646 are open, thereby allowing capacitors C1-C3 626 to be charged to a full potential applied to the driver circuit 640, which is represented as a +V potential, with a certain time constant.

[0068] Figure 5K illustrates an electrical schematic of a matrix-addressable laser driver circuit 640, which is described in relation to Figure 5J, and is configured together with a low-side capacitance discharge circuit 642 in capacitor discharge mode for laser diodes 2, 2 (second row and second column). In capacitor discharge mode for laser diodes 2, 2, both the high-side and low-side switches are closed, causing an electrical current to discharge in path 648.

[0069] Figure 5L illustrates a voltage potential timing diagram 650, showing the voltage potential across the second row and second column of laser diodes 2, 2, across capacitor C2. Switches LS2 and HS2 are initially opened. Before time t0, when switches LS2 and HS2 are opened, the C2+ potential on capacitor C2 is at +V potential and the C2- potential is at ground potential. At time t0, switches LS2 and HS2 are closed, driving C2+ to ground and causing C2- to begin charging from ground to -V. At time t1, capacitor C2 begins to discharge through laser diodes 2, 2 with a certain time constant, thereby generating optical pulses in the laser diodes 2, 2. At time t2, switches LS2 and HS2 are closed, thereby initiating conditions for the next pulse. The result of this switching sequence also results in a discharge control method that generates analog drive pulses where power consumption is independent of pulse width.

[0070] Figure 6 illustrates an embodiment of a combined high-side and low-side GaN FET driver circuit 600 for electrically driving laser diodes in a matrix-addressable laser driver circuit within a LIDAR system laser array, according to this teaching. Such a driver circuit is also referred to in the art as an asymmetric on-off driver circuit. In various embodiments of the LIDAR system of this teaching, the driver circuit 600 is connected to each of the column / row anode / cathode connections shown in Figures 5A-B, where the driver circuit 600 is configured as an on-off driver including a high-side drive electrical input 602 and a low-side drive electrical input 604. Referring more specifically to both Figure 5B and Figure 6, the cell 560 in the electrical schematic 500 is configured with the drive circuit 600 in the following way: Transistor 602, Q1 corresponds to switch 556, which connects voltage potential 550 to the laser anode 504. Transistor 604, Q2 corresponds to switch 558, which connects ground 552 to the laser cathode 506. The high-side driver input 606 is electrically connected to the gate of transistor 602. The low-side driver input 608 is electrically connected to the gate of transistor 604.

[0071] The asymmetric on-off driver circuit 600 is suitable for injecting well-controlled, short-duration, high-bias current pulses into the laser junction 610 to activate the laser and cause it to emit light. For pulsed TOF LIDAR systems, the ideal optical power output pulse should be within a duration range of several nanoseconds and should provide high peak output power over that duration. In some embodiments, the asymmetric on-off driver circuit 600 is configured and operated such that the peak output power from the laser is at or below the safe limit for the eye.

[0072] One feature of this teaching is that the array drive control circuit can be configured to optimize the drive based on the characteristics of the laser emitter's current-voltage (IV) curve. Figure 7 illustrates a typical current-voltage curve 700 for an embodiment of a semiconductor diode 702 in a matrix-addressable laser drive circuit according to this teaching. The current-voltage curve schematically represents the relationship between the electrical current flowing through the VCSEL device and the voltage applied across the VCSEL device. As shown in Figure 7, when the laser diode 702 is forward-biased, the voltage at the anode 704 is positive with respect to the cathode 706, and a forward or positive current 708 will flow through the diode 702. The current-voltage characteristics of the diode are nonlinear, and with respect to the positive current 708, the threshold voltage V th After exceeding 710, it increases exponentially from nominally zero.

[0073] When a laser diode is reverse-biased, with the voltage at the cathode being positive relative to the anode, the laser diode blocks current flow, except for a very small leakage current. The laser diode's dielectric breakdown voltage (V) is determined by the reverse voltage across the diode. br The current flow is kept interrupted until it exceeds 712). Once dielectric breakdown is reached, the current increases exponentially in the negative direction, and because the voltage and current are relatively high, the self-power dissipation is also relatively high, and the laser diode will consequently overheat and burn out. Light is generated from the laser under forward bias conditions.

[0074] The current-voltage behavior of each individual laser, combined with the method of controlling the laser drive circuit and activating each individual laser, significantly affects the operational performance and reliability of the laser array. One feature of the matrix-addressable laser drive circuits in this teaching is that they can be configured to minimize harmful effects such as optical crosstalk. Optical crosstalk occurs when other lasers in an array, other than the single laser that is intentionally biased sequentially for activation, are simultaneously biased sequentially due to current and / or voltage leakage from the electric drive circuit supplied to the activated laser. As a result, the other lasers also emit light, but this emission is undesirable. Such optical crosstalk situations have a detrimental effect on the performance of the LIDAR system by illuminating measurement points that were not intended to be illuminated and / or illuminating a larger target area than intended.

[0075] FIG. 8 illustrates the voltage induced at nodes within a matrix while activating a single laser within a two-dimensional array with row / column matrix addressing capabilities, according to an embodiment of the matrix addressable laser drive circuit controller 800 of the present teachings. The matrix addressable laser drive circuit controller 800 provides voltage to all nodes within the matrix in a manner similar to the operation of the embodiment described in relation to FIG. 5B. For example, switch 802 connects the second column to supply voltage 804, and switch 806 connects the second row of the laser to ground 808. This switch configuration induces voltage V'810 at the anodes of all rows except for the row intentionally grounded by the connection of switch 806 to ground 808. Voltage V'810 causes voltage V''812 at the corresponding cathodes of each laser within the array. The exact values of voltages V'810 and V''812 are functions of V+804 and the actual current-voltage curve for the particular laser diode.

[0076] If the value of supply voltage 804 is less than the reverse breakdown voltage of the laser pulse, the forward voltage drop of the laser, i.e., the absolute value of V+, is less than the sum of V br and V th , and the reverse current flowing through the laser diode will be nearly non-existent. Such conditions improve device reliability. Also, if the voltage at the cathode is less than the threshold voltage, i.e., V < V th , the forward current flowing through the diodes on the same row as the active laser 814 will be nearly non-existent.

[0077] Figure 9 illustrates an embodiment of a LiDAR system according to this teaching, in which a VCSEL array chip 900 is mounted on a carrier 902. Physical connections to the array allow for a denser layout of associated electronic circuits on a printed circuit board (PCB) substrate. A 16 × 16 array 904 of emitter clusters 906, each having nine small apertures 908 of addressable VCSEL devices, is shown. The carrier 902 has several electrical edge connectors 910, 910', each connected to wire junctions 912, 912' to the row edges 914 or column edges 914' of the array 904. Connections to the anodes and cathodes alternate the sides of the electrical circuits on which the PCB is connected to the VCSELs. This alternating pattern of connections results in a wider pitch on the PCB between row and column networks, which is GaN The FETs are more closely spaced together in the VCSEL array, resulting in a more compact layout for electrical circuits and reducing the physical footprint.

[0078] As described above, one aspect of the LIDAR system in this teaching is the ability to individually activate each VCSEL located within a 2D matrix-addressable configuration in the laser array using a minimum number of required electrical drivers. When the array is driven in a matrix-addressable manner row by row / column, the minimum number of drivers required is equal to M + N, where M is the number of columns and N is the number of rows. In contrast, if each VCSEL device in the array has its own dedicated driver, the number of drivers will be much greater than M × N. For example, a 16 × 16 element VCSEL array using the matrix addressing described herein requires only 32 drivers, compared to 256 drivers if each VCSEL has its own dedicated driver.

[0079] It should be understood that simultaneous and completely independent operation of all lasers is impossible when using matrix addressing. In other words, only one laser can be activated at a given time. However, this constraint is not significant for the LIDAR systems described herein because, in typical operation, there is no ambiguity that measurement points in space are illuminated around it, and only one laser in a specific monolithic array is activated at a time. Activating only one laser in a specific monolithic array at a time is also useful for maintaining Class I eye safety.

[0080] Furthermore, it should be understood that matrix addressing is well known in the field of electronics. However, the use of aspects of matrix addressing in LIDAR systems requiring short-duration, ultra-high optical power pulses with low duty cycles was previously unknown. A LIDAR system with 256 lasers, as described, operating up to 100m (minimum time of 1μsec between pulses), would have a duty cycle of only 0.002% with pulse durations of 5nsec. Matrix addressing is typically used to activate optically communicating laser devices operating with relatively low peak power (mW compared to W) and relatively long-duration pulses, with a duty cycle of approximately 50%. Under these conditions, the electrical drive requirements are quite different from those of high-power lasers in modern LIDAR applications.

[0081] For example, a pulsed TOF LiDAR system intended for operation beyond 100m range using a 905nm wavelength laser would typically require optical pulses with peak power exceeding 20 watts and pulse durations of less than 10 nanoseconds. The corresponding drive voltages and currents on individual lasers are in the tens of volts and tens of amperes ranges, assuming the laser device has an efficiency of 1 W / A under pulsed conditions. Naturally, with voltages exceeding 10V applied to the matrix-addressable array, there is a significant possibility of undesirable electrical and optical crosstalk. There is also a significant possibility that VCSEL devices in the matrix may be damaged or destroyed when reverse bias conditions are present with such voltages.

[0082] One of the primary factors affecting laser reliability is the device temperature, both mean and transient. Peak current and voltage values ​​can be relatively high, as long as the pulse duration is sufficiently short, provided that the pulse energy is controlled to keep the transient temperature rise of the device sufficiently low. Even under reverse-bias conditions, where thermal runaway is a significant concern, transient reverse current can be acceptable for reliability, as long as the temperature rise near the junction is sufficiently low. For example, assuming certain specific heat and density properties of GaAs, a 1 μJ pulse into a 2-micron thick and 100-micron diameter junction would result in a temperature rise of approximately 9°C relative to that junction. A 20 V / 10 A square pulse with a duration of 5 nsec is equivalent to 1 μJ of energy. The resulting transient temperature rise would be only a few degrees and therefore likely not sufficient to degrade the reliability of the device.

[0083] Figure 10 illustrates a schematic diagram of an embodiment of a 2x2 laser array with a matrix drive control circuit 1000 according to this teaching, showing possible electrical current paths when one laser 1002 is activated. For convenience, only a 2x2 matrix is ​​shown in this schematic diagram. It should be understood that the electrical behavior of a 2x2 matrix can be extended to larger MxN matrices.

[0084] Figure 10 is presented to illustrate the potential problems resulting from the high voltages required for modern LIDAR applications. In Figure 10, the VCSEL device L22 1002 is intentionally biased sequentially and emits light, with column 2 1004 connected to the drive voltage bus 1006, V+, and row 2 1008 connected to the ground bus 1010. The current flow through the VCSEL device L22 is indicated by a solid line 1012 with an arrow in the schematic. Ideally, all other VCSEL devices 1014, 1016, and 1018 in the matrix would be turned off by leaving row 1 1020 and column 1 1022 open and not connecting to either the ground bus 1010 or the V+ bus 1006.

[0085] However, in addition to the primary path shown by solid line 1012, there is a possibility of a second current path. This second current path is indicated by dashed line 1024 with a directional arrow. When V+ on bus 1006 is applied to column 2 1004 by closing switches 1026 and 1028, the VCSEL device 1016 L12 will cause V+ on bus 1006 to be applied to the anode, satisfying the condition that this path is nominally an open circuit through which current cannot flow, and the indicated voltage V'1030 will be induced at the cathode. Note that when the voltage V+ on bus 1006 is first applied to column 2 1004, the voltage V'1030 may initially be zero. When this occurs, transient currents with sufficient forward voltage in the VCSEL devices L12 1016 and L22 1018 may cause them to emit undesirable light, resulting in optical crosstalk. In this situation, crosstalk is additional, undesirable light generated within the field of view, not light produced by the VCSEL device L22 1002.

[0086] As a result of the cathode being connected within a given row, voltage V'1030 is also applied to the cathode of VCSEL device L11 1014, which will immediately place VCSEL device L11 1014 in a reverse bias condition. Voltage V''1032 will be induced at the anode of VCSEL device L11 1014 to satisfy the current / voltage relationship. If voltage V'1030 is less than the reverse breakdown voltage of L11 1014, the current flow will typically be less than 1 μA. The small current flow through L11 1014 will also flow through L21 1018, placing it in a forward bias condition. Voltage V'' will correspond to the forward IV curve of L21 1018. To prevent light from being emitted from L21, the current passing through L21 1018 should be below the laser threshold current, which is expected to be in the range of 10-100mA for LIDAR applications.

[0087] However, if the voltage V'1030 exceeds the dielectric breakdown voltage of VCSEL device L11 1014, a much higher current will flow through the circuit. If this current exceeds the threshold current for L21 1018, light will be emitted from both VCSEL devices L12 1016 and L21 1018, which will result in undesirable optical crosstalk. Therefore, it should be understood that the voltage V'1030 cannot be arbitrarily large, but instead must be constrained so that it is always below the reverse dielectric breakdown voltage of the VCSEL devices, or at least so that the current flow through the corresponding paths is not sufficient to cause light to be emitted from these two VCSEL devices L12 1016 and L21 1018.

[0088] Therefore, one aspect of this teaching is the recognition that, in order to avoid undesirable optical crosstalk, it is desirable to restrict the voltage V'1030 to be below the reverse breakdown voltage for certain VCSEL devices used for LIDAR applications. In addition, persistent current flow under reverse bias conditions is undesirable because, depending on time, the energy associated with the current flow, and the resulting heat rise within the laser diode, among other factors, it can become a potential reliability problem for the laser diode.

[0089] Under operating conditions where voltage V'1030 results in significant transient current flow through devices 1014, 1016, and 1018, the pulse energy should be low enough not to significantly affect reliability, and the transient temperature rise within these devices should be less than 20°C.

[0090] The use of many known VCSEL device structures for LIDAR applications would result in undesirable optical crosstalk, as voltages of 10V to 80V are typically required to generate the high-power optical pulses required for state-of-the-art LIDAR applications, while the reverse breakdown voltage for typical VCSEL devices with a single active region is in the range of 5V to 15V.

[0091] Therefore, another aspect of a LiDAR system that drives a laser array for LiDAR applications using a matrix-addressable control circuit as taught here is the design of the VCSEL device itself, which has desirable operating specifications that are highly reliable while reducing or eliminating optical crosstalk. That is, the VCSEL device as taught here is specifically designed so that the operating conditions prevent undesirable optical crosstalk from affecting system performance. One way to prevent undesirable optical crosstalk is to fabricate a VCSEL device with a laser structure that can achieve relatively high reverse bias operating conditions without entering dielectric breakdown conditions.

[0092] Vth or V br One possible laser structure that can increase both is one that includes multiple junctions in series within a VCSEL device. Laser structures with multiple junctions in series have been demonstrated in devices using tunnel junctions that isolate the active junction. It should be understood that many other similar laser structures with multiple junctions can also be used. Using multiple junctions is advantageous because of the high pulse voltage and current, V th While this may increase, the impact on efficiency and device performance is typically acceptable in this application.

[0093] Figure 11 illustrates a schematic diagram of an embodiment of a 2x2 laser array according to this teaching, comprising lasers having a second diode in series with each laser diode in a matrix-addressable laser driver circuit 1100. Similar to the schematic diagram of the 2x2 laser array 1000 described in relation to Figure 10, the laser array 1100 includes VCSEL devices 1102, 1104, 1106, and 1108 in a matrix. In addition, second diode devices 1110, 1112, 1114, and 1116 are electrically connected in series with the laser diodes 1102, 1104, 1106, and 1108. In some embodiments, the VCSEL devices 1102, 1104, 1106, and 1108 are GaAs laser diodes, and the second diode devices 1110, 1112, 1114, and 1116 are silicon diodes. Similar to the schematic diagram of the 2x2 laser array 1000 described in relation to Figure 10, there is a drive voltage bus 1118, a ground bus 1120, two columns 1122 and 1124, and two rows 1126 and 1128.

[0094] During operation, when the two switches 1132 and 1134 are closed, the laser drive current flows through the path 1130, indicated by the thick line, in the direction indicated by the arrow. The second diodes 1110, 1112, 1114, and 1116 will increase the forward voltage drop between the column and row anode-cathode connections. However, the typical forward voltage drop for GaAs laser diodes is about 2V to 3V, and for silicon diodes it is about 1V to 2V, and since the matrix-addressable laser drive circuit 1100 is designed to generate high optical power from each laser and therefore typically operates at drive voltages exceeding 10V, the additional forward voltage drop is not significant. Therefore, this additional forward voltage drop cannot have a major impact on performance. In some embodiments, more than one additional diode is added in series with the laser diode.

[0095] Different embodiments implement a series-connected second diode 1110, 1112, 1114, 1116 or multiple additional diodes connected in series, using different diode types. For example, some embodiments monolithically stack the second diodes 1110, 1112, 1114, 1116 with individual lasers 1102, 1104, 1106, 1108 on the chip. The chip may be a GaAs chip similar to the one shown in Figure 2, but with an additional layer structure forming one or more series of diodes. In some embodiments, the stacked second diode is another active PN junction that generates optical gain, which is beneficial in increasing the brightness of the VCSEL. In other embodiments, the stacked second diode is not optically active and therefore does not contribute to the light generated. In some embodiments, the stacked second diode is a photodiode. In some embodiments, the implementation of the stacked structure utilizes a tunnel junction to isolate the two stacked diodes in order to keep the overall resistance of the structure relatively low.

[0096] VCSEL devices, which involve multiple diode regions in a stacked or cascaded configuration, are known in the art. See, for example, “Bipolar Cascade VCSEL with 130% Differential Quantum Efficiency” (Annual Report 2000, Optoelectronics Department, University of ULM). Also, multi-diode cascade VCSEL structures have been used to increase overall brightness. See, for example, U.S. Patent Publication US2015 / 0311673A1. Furthermore, VCSELs have been fabricated with integrated photodiodes. See, for example, U.S. Patent No. 6,717,972. However, the prior art has not taught a matrix-addressable laser driver circuit 1100 configured for LIDAR applications using such a structure.

[0097] Another VCSEL device structure according to this teaching, which achieves relatively high reverse bias operating conditions without entering dielectric breakdown conditions, involves connecting two or more VCSEL devices in series within a single laser emitter configuration. This can be accomplished in the chip fabrication process by proper routing of anode and cathode connections.

[0098] High-power VCSEL lasers typically have more than one emitter aperture connected in parallel within a single emitter. For example, the VCSEL array described in relation to Figure 9 is a 16 × 16 array 904 comprising nine small apertures 908 of addressable top-emitting VCSEL devices, where the individual apertures within each single emitter are connected in parallel. A series of connected VCSEL devices have recently been developed for high-power applications, but optical crosstalk and forward voltage drop issues have not been considered. See, for example, U.S. Patent Publication 2019 / 0036308A1, which describes a series of connected single-chip VCSEL devices. Such devices can be configured to reduce optical crosstalk according to this teaching.

[0099] Another VCSEL device structure according to this teaching, which achieves relatively high reverse bias operating conditions without entering dielectric breakdown conditions, incorporates an additional diode into a paired substrate or IC bonded to the VCSEL device. Figure 12 illustrates an embodiment 1200 of multiple diodes configured in series, comprising a VCSEL array 1202, in which an additional diode 1204 is connected in series with each laser diode 1206, which is part of a separate carrier 1208. In some embodiments, the carrier 1208 is an integrated circuit. For example, the integrated circuit can be an inexpensive silicon-based integrated circuit.

[0100] The carrier 1208 can be electrically connected to the array 1202 in various ways. For example, the carrier 1208 can be electrically connected to the array 1202 using a bump junction connector 1210. In the configuration shown in Figure 12, a bottom-emitting VCSEL laser array 1202 is connected to the carrier. For convenience, the schematic shows only a single row of VCSEL emitters sharing a common cathode connection 1212. The anode connection 1214 extends perpendicular to the plane shown in the schematic. Each VCSEL diode 1206 is paired with a diode 1204 on the carrier 1208. It should be understood that additional diodes can be added in series in this and other configurations described herein to further reduce the possibility of optical crosstalk. Using several configurations, more than two diodes have been connected in series to achieve the desired reverse voltage induced in the matrix and to reduce or eliminate optical crosstalk. Equal portions

[0101] While the applicant's teachings are described in conjunction with various embodiments, the applicant's teachings are not intended to be limited to such embodiments. In contrast, the applicant's teachings include various substitutions, modifications, and equivalents that may be made within them without departing from the spirit and scope of the teachings, as will be understood by those skilled in the art.

Claims

1. A solid-state light transmitter for light detection and ranging (LIDAR), wherein the solid-state light transmitter is a) A two-dimensional array of multiple laser devices having multiple columns and multiple rows, wherein the two-dimensional array of multiple laser devices is configured such that multiple anodes of multiple laser devices in a column are connected to a common anode connection, and multiple cathodes of multiple laser devices in a row are connected to a common cathode connection, b) A plurality of first switches electrically connected between the common anode connection of each of the plurality of columns and the supply voltage, wherein the plurality of first switches are configured such that when one of the plurality of first switches is closed, the common anode connection of one of the plurality of columns is coupled to the supply voltage, and the supply voltage is selected such that the absolute value of the supply voltage is less than the dielectric breakdown voltage of each laser device in the two-dimensional array of the plurality of laser devices, c) A plurality of second switches electrically connected between the common cathode connection of each of the plurality of rows and ground, wherein the plurality of second switches are configured such that when one of the plurality of second switches is closed, the common cathode connection of one of the plurality of rows is connected to ground, d) A controller, wherein the controller controls time t １ The controller is configured to instruct one of the plurality of first switches and one of the plurality of second switches to close, thereby causing a laser device connected between the one of the plurality of first switches and the one of the plurality of second switches to emit light, and the controller is configured to prevent adjacent laser devices not connected to the supply voltage from emitting light, thereby reducing crosstalk within the solid-state light transmitter. A solid-state light transmitter equipped with the following features.

2. The solid-state light transmitter according to claim 1, wherein the two-dimensional array of the plurality of laser devices comprises a VCSEL array.

3. The solid-state light transmitter according to claim 1, comprising two or more VCSEL devices connected in series with the plurality of laser devices.

4. The solid-state light transmitter according to claim 1, wherein at least one of the plurality of laser devices comprises a plurality of individual apertures connected in parallel.

5. The solid-state light transmitter according to claim 1, wherein the reverse dielectric breakdown voltage of the laser device is in the range of 5V to 15V.

6. The solid-state light transmitter according to claim 1, wherein the supply voltage is +10 volts or more.

7. The controller controls time t ２ The solid-state light transmitter according to claim 1, further configured to command one of the plurality of first switches and one of the plurality of second switches to open, thereby causing the laser device connected between the one of the plurality of first switches and the one of the plurality of second switches to stop emitting light.

8. The solid-state light transmitter according to claim 1, wherein the controller is further configured such that the peak power of the emitted light is greater than 20 watts.

9. The solid-state light transmitter according to claim 1, wherein the emitted light includes optical pulses of light.

10. The solid-state light transmitter according to claim 9, wherein the duty cycle of the optical pulse of the light is 1%.

11. The solid-state light transmitter according to claim 9, wherein the duty cycle of the optical pulse of the light is 0.002%.

Images