Active area selection for LIDAR receivers

Techniques provided herein are directed toward providing an optical sensor that reduces noise from sources of light other than the LIDAR transmitter by changing the active area of the sensor of a LIDAR receiver. The optical sensor may include a two dimensional array of single photon avalanche devices (SPADs) with row-select and column-select transistors, where rows and columns are selected based on a predicted spot size and angle of reflected laser light detected at the LIDAR receiver. Among other things, this can eliminate or reduce the need for moving parts within the LIDAR receiver.

BACKGROUND

Light Detection And Ranging (LIDAR) is a surveying technology that measures distance by illuminating a target with a laser light and reading a pulse corresponding to the reflected laser light. LIDAR is often utilized to determine the topology of a landscape, and LIDAR is commonly used in modern vehicles to help determine distances between the vehicles and objects in their surroundings. However, because the sensor of a LIDAR receiver can be so sensitive, it is vulnerable to noise from light from sources of light other than the LIDAR transmitter.

SUMMARY

Techniques provided herein are directed toward providing an optical sensor that reduces noise from sources of light other than the LIDAR transmitter by changing the active area of the sensor of a LIDAR receiver. The optical sensor may include a two dimensional array of single photon avalanche devices (SPADs) with row-select and column-select transistors, where rows and columns are selected based on a predicted spot size and angle of reflected laser light detected at the LIDAR receiver. Among other things, this can eliminate or reduce the need for moving parts within the LIDAR receiver.

An example optical sensor, according to the disclosure, comprises and array of SPADs having a plurality of rows and a plurality of columns. Each SPAD may have a resistive element, a capacitive element, and a photo detection element, and further comprises a first input, a second input, and an output. Each row of the plurality of rows may have a corresponding row-select transistor that, when activated, causes the first input of each SPAD in the row to receive a bias voltage. Each column of the plurality of rows may have a corresponding column-select transistor connected with the second input of each SPAD in the column. For each column, the output of each SPAD in the column may be connected with a column output for that column.

Embodiments of the optical sensor can include one or more of the following features. The photo detection element of each SPAD may comprise an avalanche photodiode (APD). The row-select transistors and column-select transistors may comprise bipolar junction transistors (BJTs). The row-select transistors and column-select transistors may comprise field-effect transistors (FETs). Each SPAD may comprise a resistor coupled between the capacitive element and the output of the SPAD. Each column output may be further connected to an input of a trans impedance amplifier.

An example method of activating a portion of an array of single photon avalanche devices (SPADs) having a plurality of rows and a plurality of columns, according to the disclosure, comprises activating the portion of the array of SPADs by providing a bias voltage to each SPAD in a subset of the plurality of rows and a subset of the plurality of columns, and reading an output of each of the plurality of columns in the array of SPADs.

The method may further comprise one or more of the following features. Each SPAD may comprise an avalanche photodiode (APD). The method further may comprise using row-select bipolar junction transistors (BJTs) and column-select BJTs to provide the bias voltage to each SPAD in the subset of the plurality of rows and the subset of the plurality of columns. The method further may comprise using row-select field-effect transistors (FETs) and column-select FETs to provide the bias voltage to each SPAD in the subset of the plurality of rows and the subset of the plurality of columns. Each SPAD may comprise a resistor coupled between a capacitive element and the output of the SPAD. Reading the output of each of the plurality of columns in the array of SPADs may comprise amplifying the output of each of the plurality of columns in the array of SPADs with a trans impedance amplifier. The method may further comprise using a processing unit to provide the bias voltage to each SPAD in the subset of the plurality of rows and the subset of the plurality of columns by activating row-select and column-select transistors.

An example apparatus, according to the disclosure, comprises means for activating a portion of an array of single photon avalanche devices (SPADs) having a plurality of rows and a plurality of columns SPADs by providing a bias voltage to each SPAD in a subset of the plurality of rows and a subset of the plurality of columns, and means for reading an output of each of the plurality of columns in the array of SPADs.

The apparatus further may comprise one or more of the following features. Each SPAD may comprise an avalanche photodiode (APD). The means for activating the portion of an array of SPADs may comprise row-select bipolar junction transistors (BJTs) and column-select BJTs. The means for activating the portion of an array of SPADs may comprise row-select field-effect transistors (FETs) and column-select FETs. Each SPAD may comprise means for providing an electrical resistance coupled between a capacitive means and the output of the SPAD. The means for reading the output of each of the plurality of columns in the array of SPADs further may comprise means for amplifying the output of each of the plurality of columns in the array of SPADs. The means for activating the portion of an array of SPADs may comprise processing means for providing the bias voltage to each SPAD in the subset of the plurality of rows and the subset of the plurality of columns by activating row-select and column-select transistors.

DETAILED DESCRIPTION

The ensuing description provides embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the embodiment(s) will provide those skilled in the art with an enabling description for implementing an embodiment. It is understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of this disclosure.

This disclosure is generally related toward enabling a selectable area and a Light Detection And Ranging (LIDAR) receiver to provide for more favorable signal-to-noise ratios, although embodiments are not so limited. It will be understood that the techniques provided herein can be utilized in other applications (e.g., 3D and/or medical imaging), and for other desired results.

LIDAR is a surveying technology that measures distance by illuminating a target with a laser light (e.g., one or more laser beams from a LIDAR transmitter) and reading a pulse corresponding to the reflected laser light. LIDAR is often utilized to determine the topology of a landscape, and LIDAR is commonly used in modern vehicles (e.g., to implement self-driving and/or other features) to help determine distances between the vehicles and objects in their surroundings. In some implementations, for example, the pulse can be on the order of 300 ps.

FIG. 1is a simplified block diagram of an embodiment of a LIDAR system, which illustrates the basics functionality of a LIDAR system. As illustrated, a LIDAR system100can comprise a LIDAR transmitter130(which includes a laser135and beam-steering optics133), a LIDAR receiver120(which includes filtering optics122, focusing optics124, and a sensor126), and a processing unit110. A person of ordinary skill in the art will recognize that alternative embodiments of a LIDAR system100may include additional or alternative components to those shown inFIG. 1. For example, components may be added, removed, combined, or separated, depending on desired functionality, manufacturing concerns, and/or other factors. In some embodiments, for example, the LIDAR receiver and the LIDAR transmitter may have separate processing units or other circuitry controlling the operation thereof.

In general, the operation of the LIDAR system100is as follows. The processing unit110causes the laser135to generate a laser beam137that is fed to the beam-steering optics. The beam-steering optics133adjusts the direction and/or spot size of the laser beam137(using, for example, a Risley prism pair, micro electromechanical systems (MEMS) reflectors, and/or other means) to create a transmitted laser beam140that scans a field of view (FOV) of the LIDAR system100. (It can be noted that the direction of the transmitted laser beam140may be measured by an angle170from an axis of the beam-steering optics133. It can further be noted that the axis may arbitrarily be determined, and that multiple angles may be used to extend this idea in two (or more dimensions.) In so doing, the transmitted laser beam140reflects off an object150within the FOV, creating a reflected laser beam160that is detected by the LIDAR receiver120. The filtering optics122can be used to filter out unwanted light (e.g., wavelength of light other than the wavelength(s) generated by the laser135), and the focusing optics124can be used to project to the reflected laser beam160onto a light-sensing surface of the sensor126. The sensor126can then provide information to the processing unit110that enables the processing unit110to determine a distance of the object. As the beam-steering optics133scans the entire FOV of the LIDAR system100, reflected laser light is received by the LIDAR receiver, and the processing unit110is able to determine the distance of many objects within the entire FOV of the LIDAR system100.

The sensor126of the LIDAR receiver may comprise a high speed sensor such as an avalanche photo diode (APD) or silicon photomultiplier. The signal-to-noise ratio (SNR) degrades as light from sources other than the LIDAR transmitter falls onto the photodetector. For example, light from the sun creates shot noise. For good SNR, the filtering optics122and the focusing optics124can be designed to put as much light that originated from the LIDAR transmitter as possible onto the sensor126, while minimizing light from other sources.

The sensor126can comprise a silicon photomultiplier (SiPM) which includes an array of single photon avalanche devices (SPADs, also referred to herein as “SPAD microcells” or “SPAD cells”) where a single photon can trigger a SPAD for a brief period of time. During the time a SPAD is triggered, may no longer be sensitive to light. That is, when a photon is detected by a SPAD, the SPAD goes into saturation and remains there until the capacitive element of the SPAD is discharged, thereby creating a spike at the output (that is measured by downstream circuitry such as the processing unit110to determine, for example, a measured distance). If the SPADs are made small and connected together with quenching resistors, the output of the SiPM is a function of the light reaching the photodetector. It will be appreciated that, although embodiments described herein utilize SPADs, other embodiments may utilize other photon detection means.

Because SiPMs can be so sensitive, the sensor126can be vulnerable to noise (i.e., light other than the reflected laser beam160). And although filtering optics122help reduce the amount of this noise, it does not eliminate it. Furthermore, in situations where other LIDAR systems are operating in the vicinity of the LIDAR system100(such as when multiple car-based LIDAR systems are in use in high-density traffic), filtering optics122may not filter out light generated from the LIDAR transmitters of other LIDAR systems100.

According to the techniques presented herein, SNR may be further increased by selectively activating only a portion of the sensor126that is expected to be illuminated by the reflected laser beam160. In other words, because the beam-steering optics133determines the angle170and the spot size of the transmitted laser beam140at any given time during the operation of the LIDAR system100, the LIDAR system100can further determine the expected angle and spot size of the reflected laser beam160. With this information, the LIDAR system100can then determine which SPADs of the sensor126are likely to be illuminated by the reflected laser light. As such, and according to techniques disclosed herein, the SNR for the sensor126can be further improved by utilizing a sensor126having a SiPM capable of dynamically changing its active area in both location and size. Among other things, this can eliminate or reduce the need for moving parts within the LIDAR receiver.

FIG. 2is a simplified schematic diagram of a SiPM200, according to one embodiment. As mentioned above, the SiPM200may be Incorporated into the sensor126ofFIG. 1. Here, the SiPM comprises an array of SPADs, wherein each SPAD210comprises resistive, capacitive, and photo sensitive elements, such as a resistor, a capacitor, and a photodiode, respectively. A person of ordinary skill in the art will appreciate typical values and/or other properties of these elements, which may vary depending on desired functionality. In some embodiments, for example, values of capacitance may be in the femtofarad range, and the resistance used in the implementation could be over a wide range of values, depending on how the array design is optimized. For example, in some embodiments, the RC time constant of the capacitance acting against the quench resistor may be 10 nS or less. In these embodiments, the resistor in series with each capacitor inFIG. 3could be anywhere from zero (e.g., the resistor is omitted) to any value producing an RC time constant shorter than 10 nS. Other embodiments may have values above and/or below the values of this example.

It will be understood that the size of the array can vary, depending on desired functionality. That is, although the SiPM200illustrated inFIG. 2has only 25 SPADs illustrated, embodiments may have a larger or smaller number of SPADs, depending on desired functionality. Some embodiments may include, for example, hundreds, thousands, or millions of SPADs, or more. In some embodiments, the entire SiPM (and optionally some of the additional circuitry shown inFIG. 2) may be implemented on a single semiconductor die. A person of ordinary skill in the art will appreciate the fact that various modifications can be made to the basic layout shown inFIG. 2. It can be further noted that, although the array is described as a SiPM200, some embodiments may utilize photomultiplier arrays that utilize materials in addition or as an alternative to silicon.

In this embodiment, the SiPM200utilizes a plurality of row-select transistors220, as well as a plurality of column-select transistors230, enabling controlling circuitry (such as a microprocessor or other processing circuit) to, by activating these transistors, select which SPADs in the array to use for sensing at a particular moment. In other words, these row-select transistors220and column-select transistors230can, when activated, enable the controlling circuitry to “activate” a certain subset of the SiPM200. Depending on desired functionality, manufacturing concerns, and/or other factors, these transistors could comprise NPN, PNP, N- or P-channel MOSFETs, or other types. Generally speaking, the size of the transistor may be big enough to have a on resistance between drain and source (RDS (on)) that is substantially smaller than all the quench resistors in parallel driven by that transistor, or small enough to drive the worst-case current needed by the row or column without more than a couple volt drop. In some embodiments, the voltage rating may be 30V or possibly more. However, this might vary depending on the photodetector used.

A SPAD and APD are both sensitive to the amount of bias voltage, which is set by bias voltage240. Up to the breakdown voltage of the sensor, the gain of the SPAD and APD may be up to one electron per photon. Due to the efficiency of the sensor, the actual sensitivity may be substantially less than one electron per photon. But as the bias voltage increases to the breakdown voltage and beyond, the sensitivity of the APD and SPAD increases. For the arrangement of the SiPM SPAD microcells into a matrix with the cathodes connected to rows and anodes connected to columns (as shown inFIG. 2), an area of high gain cells can be achieved by driving bias voltage into rows and columns to select an active area with high gain. The unselected cells without the high reverse bias have a very low gain while the active area (selected using row- and column-select transistors) has a gain of 20^5 or more. By selection of multiple adjacent rows and multiple adjacent columns, a desired square or rectangular area can be selected. Alternatively, some embodiments may be modified to allow a non-rectangular area may be selected.

The grouping of the SPAD cells can be column by column, row by row, or an arbitrary grouping of SPAD cells without regard to all the cells falling on the same row or column. It can be noted that the structure illustrated inFIG. 2(as well as the structure ofFIG. 3below) will work if the anodes and cathodes are swapped, or rows and columns are interchanged, as long as the polarity of drive at the SPAD cells is reverse biased.

The signal level from each SPAD may be quite high. This value depends on how many SPAD cells are connected together. For example, a single SPAD cell on its own may produce 5V, but if it is connected together in the same column with 1000 other SPAD cells, the loading of the other cells would drop that voltage to 5 mV. other embodiments may include higher or lower signal levels, depending on desired functionality. In some embodiments, for smaller arrays of SPADs, all of the capacitive coupled cell outputs could be tied together to form a single output. Unbiased SPAD cells would create minimal response to being hit by a photon, while biased cells will have very high gain in terms of electrons per photon. As the array size gets larger, the capacitive coupled outputs may need to be broken into groups of microcells and connected to a multiplexer or peak detector to prevent the signal level from getting too small due to voltage division amongst the connected cells. According to some embodiments, if a set of switches or multiplexer is used, it may be at the output of each row inFIG. 3(after the amplifier). Alternative embodiments may connect rows before each amplifier, but the signal from the active cells would be divided by the total number of cells. For extremely large arrays (e.g., arrays having several hundred microcells or more) the microcell groups could be amplified before being summed or multiplexed. This would further improve the signal to noise ratio by preventing the signal from becoming too weak before amplification.

It can be noted that, inFIG. 2, combining circuitry (such as multiplexers and/or amplifiers) is not shown, but may be used depending on desired functionality. The columns of the SiPM200each have an output (labeled “OUT” inFIG. 2) that is utilized to detect an output of the SiPM200. If combining circuitry is utilized, it could combine one or more of these column outputs in a manner similar to the embodiment illustrated inFIG. 3.

FIG. 3is a simplified schematic diagram of a SiPM300, according to a different embodiment. Here, the row-select transistors320and the column select transistors330are field-effect transistors (FETs). Other embodiments may additionally or alternatively include other types of transistors, depending on manufacturing concerns, desired functionality, and/or other factors. Each SPAD cell310differs slightly from the SPAD cells210ofFIG. 2in that the SPAD cells310ofFIG. 3include an additional output resistor element to reduce the effects of the capacitances of other SPADs. The lower-drive-strength signal is then amplified by a layer of trans impedance amplifiers (TIAs)350to increase drive strength before being passed on for detection. It can be noted that broadband amplifiers may be utilized in addition or as an alternative to the TIAs350. It can be further noted that the SiPM200ofFIG. 2may utilize TIAs350in a similar manner.

It should be noted, however, that such output resistive elements are optional. Embodiments utilizing bipolar junction transistors (BJTs) and/or FETs (such as those illustrated inFIGS. 2 and 3respectively) may choose to include or omit such additional resistive elements, depending on desired functionality. The presence and/or value of such additional resistive elements can determine how an output pulse is shaped. A person having ordinary skill in the art will appreciate the factors that may be utilized in determining whether to include such additional resistive elements and what values they may be. In some embodiments, these resistive elements may simply be parasitic resistive elements.

As withFIG. 2, embodiments may employ a larger or smaller number of SPADs than is illustrated in the SiPM300illustrated inFIG. 3, depending on desired functionality. Again, embodiments may include, for example, hundreds, thousands, or millions of SPADs (or more).

As indicated previously, bias voltages240and340utilized inFIGS. 2 and 3, respectively, can be chosen to determine the sensitivity of the SPADs, when activated (via row- and column-select transistors). In some embodiments, these bias voltages may be dynamic, enabling the SiPM to provide a variably-sensitive output depending on, for example, lighting conditions and/or other factors. Adjusting the bias voltage of the SPAD can change the sensitivity. The value of bias would likely be the same for all of the cells that are active (with the inactive cells unbiased). In other embodiments, these bias voltages may be static and/or factory set.

In a LIDAR application, the active rows and columns can be selected a microsecond or more before the laser fires. As indicated previously, the row- and/or column-select signals used to activate the row-select transistors220,320and column-select transistors230,330can be generated by a processing unit or other logic (e.g., processing unit110ofFIG. 1) which may be in communication with and/or control of LIDAR transmission circuitry. As such, the processing unit or other logic may know the angle and/or spot size at which the LIDAR transmission circuitry will transmit a laser beam, and can select the rows and columns of the SPADs in the SiPM corresponding to the known angle and/or spot size. With the corresponding SPADs of the SiPM activated when the LIDAR transmission circuitry fires the laser beam (and the rest of the SPADs remaining inactive), this will enable the SiPM to provide a higher SNR, resulting in more accurate LIDAR functionality.

FIG. 4is a schematic diagram of a single SPAD, according to an embodiment. Here, the SPAD includes a current-limiting or bias/quench resistor410, a high-speed capacitive structure420, an optional output resistor430, and a photodiode440. As indicated above, the input bias voltage can determine the sensitivity of the photo detection. A value for the current-limiting or bias/quench resistor410can be determined to limit the amount of current drawn when the photodiode440detects light. Values for the capacitive structure420and the optional output (decoupling) resistor430may be determined by known techniques based on requirements for the circuitry utilized to combine and/or measure the output of the SPADs. According to some embodiments, all components of the SPAD (and indeed the entire SiPM and optionally supporting circuitry, such as TIAs) may be implemented on a single semiconductor die. Such a die may be packaged in an integrated circuit (IC) package and utilized in larger LIDAR or other imaging devices.

FIG. 5is a flow diagram of a method500of enabling the activation of a portion of a SiPM, according to some embodiments. It will be understood that alternative embodiments may utilize additional or alternative functions from those illustrated inFIG. 5.

At block510, in an array of SPADs having a plurality of rows and a plurality of columns, a portion of the array of SPADs is activated by providing a bias voltage to each SPAD in a subset of the rows and a subset of the columns. As illustrated in the embodiments shown inFIGS. 2-3, the bias voltage may be provided to the inputs of each SPAD by activating corresponding row-select and/or column-select transistors. In some embodiments, each SPAD comprises resistive, capacitive, and photo detection elements electrically connected with, a first input, a second input, and an output, each row of the plurality of rows has a corresponding row-select transistor that, when activated, causes the first inputs of each SPAD in the row to receive a bias voltage, each column of the plurality of rows has a corresponding column-select transistor connected with the second inputs of each SPAD in the column, and for each column, the outputs of each SPAD in the column are connected with a column output for that column. As described previously, the activation of such transistors can be done with a processing unit (such as a microprocessor) and/or other logic configured to generate signals to activate the transistors. The rows and columns selected for activation may correspond with those SPADs that are expected to receive reflected laser light (this determination may also be made by the processing unit and/or other logic).

At block520, and output of each of the columns of the array of SPADs is read. As indicated previously, this may be done by any of a variety of types of circuitry. In some embodiments, to ensure these outputs are read accurately, a multiplexor may be utilized to read only the outputs of the columns of SPADs selected at block510. Additionally or alternatively, these outputs may be amplified by TIAs and/or other types of amplifiers. In some embodiments, the outputs may be provided to a processing unit (such as the processing unit110ofFIG. 1) and/or other processing circuitry.

As with other embodiments described herein, the method500illustrated inFIG. 5may vary, depending on desired functionality. For example, transistors utilized in block510may comprise bipolar junction transistors (BJTs) and/or field-effect transistors (FETs). Each SPAD may include an output resistor coupled with an output of the SPAD.