Data reduction for optical detection

In an optical detection system, features of interest can be identified from ADC circuitry data prior to inter-circuit communication with downstream object or target processing circuitry. In this manner, a volume of data being transferred to such downstream processing circuitry can be reduced as compared to other approaches, simplifying the receive signal processing chain and providing power savings. First-tier signal processing circuitry to identify features of interest can be located on or within a commonly-shared integrated circuit package with ADC circuitry, and downstream processing circuitry for object processing or range estimation can be fed with a data link meeting less stringent requirements than a link between the ADC circuitry and first-tier signal processing circuitry.

FIELD OF THE DISCLOSURE

This document pertains generally, but not by way of limitation, to apparatus and techniques that can be used for processing electrical signals indicative of a light received from objects in a field-of-view of an optical receiver, such as for performing one or more of object detection or range estimation to an object, and more particularly, to apparatus and techniques for data reduction in such processing.

BACKGROUND

An optical detection system, such as a system for providing light detection and ranging (LIDAR), generally includes a light source (e.g., an illuminator) and an optical receiver. Various schemes can be used to provide illumination of a field-of-regard (FOR), such as a flash technique in which a large portion or an entirety of the field-of-regard is illuminated contemporaneously. In another approach, scanning can be used to selectively illuminate portions of the field-of-regard. Such scanning can include use of one or more mechanical actuators such as rotating or oscillating mirrors or prism structures to orient a beam of light toward specified portions of the field-of-regard. In yet another approach, a non-mechanical beam-steering technique can be used, either alone or in combination with a mechanically-scanned technique.

Detection of scattered or reflected light can be performed using an array of photodetectors, such as to image a field-of-view (FOV) corresponding to the field-of-regard illuminated by the light source. A time-of-flight determination can be made to estimate a distance to an object or target imaged in the field-of-view, or an object or target can otherwise be localized. Optical detection systems such as LIDAR can be applied in vehicular applications, such as to facilitate operation of autonomous or semi-autonomous vehicles, or to otherwise provide enhanced situational awareness to facilitate safe operation of such vehicles. Other applications can include short range sensing for indoor environments, or beam guidance to facilitate communication or tracking of an object, for example.

SUMMARY OF THE DISCLOSURE

As mentioned above, a pulsed (e.g., “flash”) or scanned approach can be used to provide scene illumination, and an optical detection system can determine a range to an object by monitoring a “time-of-flight” of a reflection (e.g., an “echo return”). If analog-to-digital converter (ADC) circuits are used in the receiver, in one approach a “full waveform” processing approach can involve digitizing and digitally-processing an entire return signal at the finest amplitude resolution and finest temporal resolution supported by the ADC. For example, such a full-waveform processing approach can involve determining whether an echo is present (e.g., discrimination), and determining a time at which the echo occurred in the time-domain record. Accordingly, range resolution generally depends on time resolution, and therefore high-sample-rate (e.g., gigasample-per-second) analog-to-digital converter circuits (ADCs) may be used. An amplitude conversion resolution of an ADC may impact a sensitivity of an optical receiver, particularly when operating at or near a range limit or in the presence of interference.

The present inventors have recognized that a challenge can exist when processing is performed for echo discrimination and range estimation on a full-resolution, full-data-rate record from an ADC circuit (e.g., an unabbreviated record having a conversion rate and conversion resolution determined by an input channel of the ADC circuit). For example, amplification and filtering can be performed by analog circuits prior and then such signals can be digitized by an ADC circuit. Such circuitry may be separate from digital processing circuitry used for echo discrimination and range estimation. Accordingly, a data link between and output of one or more ADC circuits and processing circuitry may consume one or more of significant operating power or physical area, and such a scheme may limit an available bandwidth or resolution due to data link capacity limitations. For example, if a low-voltage differential signaling (LVDS) scheme is used for a data link between ADC circuitry and separate processing circuitry, such a serial communication scheme may limit overall data transfer rates to a range of less than one gigabit-per-second. Generally, a data rate required to provide a full-bandwidth, full-resolution record from an ADC circuit is a product of the sample rate times the amplitude resolution (corresponding to a count of bits used to encode the amplitude). As an illustrative example, if a sample rate is 1 gigasample per second, at 16-bit resolution, a single ADC “channel” would require a 16 gigabit-per-second data link to downstream processing circuitry.

The present inventors have also recognized that a significant fraction of the data being transferred between ADC circuitry and downstream processing circuitry is likely unneeded (e.g., background noise or interference), as such a fraction does not correspond to features of interest in the received optical signal and is not relevant for echo discrimination and range estimation. Accordingly, the present inventors have developed, among other things, techniques to identify features of interest from ADC circuitry data prior to inter-circuit communication with downstream object or target processing circuitry. In this manner, a volume of data being transferred to such downstream processing circuitry can be reduced as compared to other approaches, simplifying the receive signal processing chain and providing power savings. First-tier signal processing circuitry to identify features of interest can be located on or within a commonly-shared integrated circuit package with ADC circuitry, and downstream processing circuitry for object processing or range estimation can be fed with a data link meeting less stringent requirements than a link between the ADC circuitry and first-tier signal processing circuitry.

In an example, an optical detection system can include an analog-to-digital conversion (ADC) block comprising at least one input channel, the ADC block configured to receive an electrical signal obtained from a corresponding photosensitive detector and configured to provide an output comprising a digital representation of the input determined by a conversion resolution and a conversion rate defined by the at least one input channel, and a signal processor circuit configured to receive the digital output from the ADC block and to identify at least one temporal window within the ADC output corresponding to a feature, the signal processor circuit comprising a digital output to provide an abbreviated representation corresponding to the at least one temporal window including the feature, the digital output transmitting the abbreviated representation. The feature can include a received echo corresponding to a transmitted optical pulse. The abbreviated representation can maintain at least one of the conversion rate or conversion resolution defined by the at least one input channel, within the temporal window. In an example, the signal processor circuit is configured to identify the feature including comparing amplitude values of samples from the digital representation against a threshold, and the signal processor circuit is configured to establish the abbreviated representation corresponding to the at least one temporal window by preserving samples including samples exceeding the threshold and dropping samples elsewhere. For example, the signal processor circuit can be configured to preserve samples including and adjacent to the samples exceeding the threshold, defining the temporal window spanning a duration before and after the samples that exceed the threshold. In an example, the signal processor circuit is configured to vary the threshold as a function of time with respect to a transmitted optical pulse.

In an example, a technique, such as a machine-implemented method can include performing optical detection, such as including receiving an electrical signal from a photosensitive detector, generating a digital output providing a digital representation of the electrical signal determined by a conversion resolution and a conversion rate defined by at least one input channel of an analog-to-digital converter circuit, identifying at least one temporal window within the digital representation corresponding to a feature, and providing an abbreviated representation corresponding to the at least one temporal window including the feature, including transmitting the abbreviated representation digitally. The abbreviated representation can maintain at least one of the conversion rate or conversion resolution defined by the at least one input channel, within the temporal window. Identifying the feature can include comparing amplitude values of samples from the digital representation against a threshold and establishing the digital representation corresponding to the at least one temporal window by preserving samples including to samples exceeding the threshold and dropping samples elsewhere. In an example, preserving samples can include preserving the samples exceeding the threshold defines the temporal window spanning a duration before and after the samples that exceed the threshold. In an example, the threshold can be varied as a function of time, such as with respect to a transmitted optical pulse.

DETAILED DESCRIPTION

An optical detection system can include an illuminator, such as a laser or other light source to illuminate objects within a field-of-regard, and an optical receiver to detect light from the illuminator that is reflected or scattered by objects with a corresponding field-of-view. Generally, a single photo-sensitive detector or an array of such photo-sensitive detectors is used to detect received light. In optical detection systems, a “readout integrated circuit” (ROIC) can include input channels that can be coupled to corresponding photo-sensitive detectors, and such input channels can provide amplification and digitization of an electrical signal corresponding to a received optical signal from the photo-sensitive detectors. In generally-available systems, such an ROIC generally does not discriminate between echo information in received data versus non-echo information (e.g., noise or interference), and much of the data provided by the ROIC can be discarded. As mentioned above, the present inventors have recognized that apparatus and techniques as described herein can be used to help identify features of interest in such received data, such as providing data reduction and thereby one or more of reducing data link complexity or supporting an increased channel-count using an existing data link topology, as compared to other approaches. Such techniques can be implemented within an ROIC device (e.g., monolithically), as an example. The apparatus and techniques described herein are applicable to optical detection, such as optical ranging or object localization. For example, the apparatus and techniques described herein can be used in a light detection and ranging (LIDAR) system, as an illustrative example.

FIG.1illustrates generally an example comprising an optical detection system100, such as can include an optical receiver signal chain that can provide enhanced data throughput efficiency using a data reduction technique. In the optical detection system100, a field-of-regard (FOR)130can be illuminated by an illumination source102. Such an illumination source102can include one or more solid-state devices, such as a light-emitting diode, a diode laser, or an array of such devices. Illustrative examples of illumination light sources include lasers having a Master Oscillator Power Amplifier (MOPA) architecture, an Edge-Emitting Laser (EEL), or a Diode-Pumped Solid-State Laser (DPSSL). Other device architectures can be used, such as a vertical-cavity surface-emitting laser (VCSEL). In an illustrative example, the illumination source102can generate light in a near infrared (NIR) wavelength band (e.g., from about 750 nanometers to about 1400 nm). Such a range of wavelengths is illustrative, and other wavelength ranges can be used depending on the light source and receiver architecture. According to various examples, a “flash” scheme can be used where a portion or an entirety of the130is illuminated without requiring scanning, and reflections are then detected (e.g., imaged) by an optical receiver. In another approach, a scanned transmit scheme can be used where a spot or a line generated by a transmitter is steered to various locations in the field-of-regard130.

Objects in the field of regard can scatter or reflect the transmitted light, and such scattered or reflected light can be detected by an optical receiver. A time-of-flight can be determined between launch of illumination and an instant corresponding to receipt of scattered or reflected light from the object. Such a time-of-flight can be used to establish a range estimate to the object from the optical detection system100. Generally, an angular field observed by the optical receiver can be referred to as a field-of-view (FOV)140. The FOV140generally overlaps with the FOR130illuminated by the transmitter, but the two need not have the same shape or spatial extent. If the receiver uses a different optical path with respect to the transmitter, the transmit/receive configuration can be referred to as a bistatic configuration. If the receiver uses the same optical path as the transmitter, the transmit/receive configuration can be referred to as a monostatic configuration. If the receiver is arranged to provide an FOV140along an axis that is nearby a corresponding axis of the transmitter field-of-regard, the configuration can be referred to as a biaxial monostatic configuration. The techniques described in this document are applicable to a variety of such transmitter and receiver configurations.

Generally, an array of photodetectors may be used such as to rapidly image a portion or an entirety of an FOV140without requiring scanning. For example, a detector120can receive scattered or reflected light from the FOV140, such as through an optical structure122(e.g., a lens structure or multiple structures such as one or more lenses, filters, or polarizers). The optical structure122can be dedicated to the receive optical path or can be shared with a transmitter optical path. A transmissive optical structure122is shown, but other structures can be used such as reflective optical structures or planar optical devices. The detector120can include an array such as a one-dimensional or two-dimensional array of photodetectors, or even a single photodetector. For example, for detection of wavelengths longer than about 400 nanometers and shorter than 1000 nanometers, a Silicon (Si) photodetector can be used as the optical detector120. Again, such an example of a specific semiconductor detector and a corresponding wavelength range is an illustrative example.

Generally, a photo-sensitive detector generates an electrical signal such as a current in response to incident light. Processing of such electrical signals can be performed an analog-to-digital conversion block in the receiver architecture using respective “channels,” such as defined by one or more amplifiers (e.g., a transimpedance amplifier (TIA)116and a buffer, such as a drive amplifier114) coupled to an analog-to-digital converter (ADC) circuit110. A multiplexer118can be included, such as to route signals from multiple detector elements in the detector120to a single receive channel comprising a TIA116, drive amplifier114, and ADC110. Such multiplexing allows a smaller count of ADC110channels to be shared amongst multiple photo-sensitive detection elements. The order of the analog elements shown inFIG.1is an illustrative example. InFIG.1, the TIA116and drive amplifier114are shown as located post-multiplexing (e.g., downstream from the multiplexer118). In another approach, one or more such amplifiers can be located ahead of the multiplexer118, such as depending on a count of detector120elements or other considerations related to system implementation.

As mentioned elsewhere herein, in one approach, a digital output126of the ADC circuit110can be routed to a separate field-programmable gate area (FPGA) or general-purpose processor circuit106, located “off-chip” with respect the ADC circuit110. The FPGA or general-purpose processor circuit106would then receive a full-resolution, full-data-rate digital representation of a received optical signal as digitized (e.g., discretized and encoded) by the ADC circuit110. However, such an approach can present challenges. As a count of channels increases, and as one or more of a conversion amplitude resolution or conversion (e.g., sampling) rate of the ADC circuit110increases, a volume of data provided to the downstream FPGA or processor circuit can increase dramatically. Most of such data generally corresponds to background noise or interference. IF the ADC circuit110and other upstream circuitry are located in a separate integrated circuit package from the processor circuit106or FPGA, a limited data link capacity between the ADC circuit110and processor circuit106may thereby limit a usable resolution or sampling rate.

For example, assuming a finite data link capacity, a tradeoff can exist between amplitude resolution (corresponding to sensitivity of the optical receiver), sampling rate (corresponding to a range resolution of the optical receiver and detector or channel count (corresponding to a spatial resolution of the optical receiver). Use of a relatively higher-capacity data link also consumes greater power than lower-bandwidth interface circuitry. In one approach, a down-sampling technique can be used to discard samples or a conversion resolution can be reduced, but such approaches can sacrifice one or more of range accuracy or sensitivity.

To address such challenges, the present inventors have recognized that a signal processor circuit108can be co-integrated with at least the ADC circuit110. In this manner, a full-resolution, full-bandwidth signal from the ADC circuit110can be coupled internally to the signal processor circuit108by a digital link126, and the signal processor circuit can perform various techniques (such as shown illustratively in one or more ofFIG.3A,FIG.3B,FIG.3C,FIG.4, orFIG.5, for example) to provide an abbreviated representation via a data link124(e.g., a serial communication interface such as LVDS bus or other interface). The apparatus and techniques described herein need not use a fixed function such as decimation to reduce a volume of sample data to be transferred, but instead identify and separate signals of interest particular to an optical detection system (such as a LIDAR system). As an illustrative example, a buffer such as drive amplifier114, ADC circuit110, digital output126, and signal processor circuit108can be co-integrated in a commonly-shared “read-out integrated circuit” (ROTC) monolithic integrated circuit112. Such an ROIC integrated circuit112can include multiple channels having corresponding ADC and drive amplifier circuits, coupled to the signal processor circuit108. The signal processor circuit108can include one or more digital filter circuits, threshold or comparison circuits, windowing circuits, or other circuitry, such as to identify one or more features of interest in a digital data stream provided by respective ADC outputs (e.g., an ADC circuit110output126). Data not related to the feature of interest can be suppressed (e.g., dropped). As an illustrative example, the signal processor circuit108can include a digital signal processor (DSP) topology or Application Specific integrated Circuit (ASIC).

After processing by the signal processor circuit108, an abbreviated representation of time-domain data corresponding to received optical signals can be provided via data link124to other processing circuit such as the general-purpose processor circuit106or object processing circuitry (e.g., provided by an object processor circuit104). For example, such an object processor circuit104can provide an estimate of one or more of a range to an object corresponding to an identified feature, or a location of such an object as output data. Use of the system100topology shown inFIG.1can provide one or more of lower power consumption as compared to other topologies having a similar count of detector elements in the detector120. In another example, such an approach can provide one or more of enhanced spatial resolution, enhanced temporal resolution, or greater sensitivity by supporting higher-performance ADC circuitry or higher detector element counts, for example.

FIG.2illustrates generally an example comprising a discrete-time representation200of received optical signal, such as provided by an analog-to-digital converter circuit. InFIG.2, a conversion rate is defined as a reciprocal of a time interval between adjacent samples. A conversion resolution is defined by a count of available amplitude quantizing levels, and frequently such a resolution is specified in terms of a count of bits available to encode amplitude samples. InFIG.2, a time-domain representation is shown including various amplitude excursions. It is not clear from the time-domain data ofFIG.2whether one echo, two echoes, or multiple echoes are being received. For example, the portion of the waveform at234could include a single echo return or two echo signals closely-spaced in time. The portion at236could be an echo or might be a transient due to interference or other noise, and the portion at238could be yet another echo signal. In one approach, the entire representation200can be provided to a separate processing circuit, such as located off-chip relative to the analog-to-digital converter circuitry.

As mentioned above, an abbreviated representation of the time-domain data can be established. Such data reduction can include various operations. For example,FIG.3Aillustrates generally an example comprising filtering a received optical signal300, such as using a finite-impulse-response (FIR) digital filter342, to provide a filtered representation360of the received optical signal. Such filtering can assist in removing noise (such as can interfere with threshold determinations or other processing). For example, in the received signal300, a first feature334A is visible, and a second feature338A is visible. The digital filter342can be defined at least in part, by a finite-impulse-response topology having filter coefficients corresponding to a desired impulse response function. For example, the impulse response function can be selected to match a transmitted pulse shape provided by an illumination source. In addition, or instead, the filter coefficients can be established to match (e.g., correlate with) a code sequence, and the illumination source can be controlled to emit a time-domain series of pulses or pulse amplitudes corresponding to the code sequence. In this manner, signal contributions from sources other than the transmitter can be rejected to provide the filtered representation360. InFIG.3A, the filtered feature334B appears more distinct, and a third feature336appears visible between the filtered feature334B and a filtered feature338B.

FIG.3Billustrates generally an example comprising identifying one or more features using a threshold comparison344, such as by applying the thresholding operation to a filtered representation360of a received optical signal300. The threshold comparison can include a simple value comparison in which amplitude values of samples are compared against a static threshold value346, and samples that exceed the threshold value346are preserved, while other samples are dropped. In another approach, multiple thresholds can be used, such as a threshold value346that decreases in magnitude with respect to time. For example, if a time index is measured from a “pulse launch” or other illumination event (e.g., illumination of a particular region being observed by the receiver), samples in the received optical signal that occur later in time relative to the flash or other illumination event can be compared to a threshold that is lower in magnitude than an initial threshold magnitude. In this manner, the threshold comparison takes into consideration a diminishing received signal amplitude corresponding to an increasing range between the reflecting object and the optical receiver. In another example, the thresholding operation can include hysteresis, such as preserving a first sample exceeding a first threshold, and then preserving subsequent samples until a sample value falls below a second threshold that is lesser in magnitude than the first threshold. In the example ofFIG.3B, features334B,336B, and338B are apparent in a filtered representation360. The representation360can be processed using a threshold comparison344, and a resulting abbreviated data set370can be provided, such as including features334C,336C, and338C at full amplitude resolution, and having inter-sample intervals within each feature provided at full temporal resolution, but with samples located elsewhere dropped to condense the data set.

In yet another example of processing the received optical signal to provide abbreviated data,FIG.3Cillustrates generally an example comprising event windowing at348by preserving samples including and adjacent to samples exceeding a threshold corresponding to a feature, such as to define a temporal window spanning a duration before and after the feature. In the example ofFIG.3C, the data set370can include samples corresponding to features334C,336C and338C that exceed a specified threshold. In addition, once sample indices are identified corresponding to such features334C,336C, and338C, samples occurring before the initial threshold-crossing sample in each feature can be preserved. In this manner, a specified count of samples or duration before and after a threshold-crossing sample can be preserved in a data set to provide an event-windowed data set380. As another example, two threshold comparison values can be established, wherein a first threshold is used to define a duration of a temporal window of interest, and second threshold is used to define which samples within the window of interest to preserve. For example, within a temporal window of interest established by the first threshold, samples exceeding the second threshold can be retained, and other samples within the window can be dropped. The data set380may include a greater count of samples in the set360than would be yielded by thresholding alone, but such a data set380is still significantly condensed as compared to a full representation300as shown inFIG.3A, for example. Respective samples in the abbreviated data set380can be tagged with either an absolute or relative time index, such as to preserve an ability to perform a range estimate to an object corresponding to one or more of the features354,356, or358.

FIG.4illustrates generally a technique, such as a method, comprising receiving an electrical signal from a photosensitive detector at405, generating a digital output providing digital representation at410, and at415, identifying at least one temporal window with in the digital representation, the window corresponding to a feature. At410, the generation of the digital output can include use of an analog-to-digital converter (ADC) circuit, and an amplitude resolution and time resolution of the digital representation provided at the digital output can be determined by a conversion resolution and a conversion rate defined by at least one input channel of the ADC circuit. At415, the identification of the temporal window can include use of a signal processor circuit that is located within the same integrated circuit device package as the ADC circuit used at410. At420, an abbreviated (e.g., time-domain digital) representation of the received optical signal can be provided, such as by dropping samples outside one or more temporal windows identified at415. Providing the abbreviated representation can include digitally transmitting the abbreviated representation, such as from a first integrated circuit die housing a signal processing circuit to a second integrated circuit die.

FIG.5illustrates generally a technique, such as a method, comprising comparing values of samples of a received optical signal against a threshold at510, establishing a time-domain digital representation of the optical signal corresponding to a temporal window at515, and optionally including preserving samples prior to a sample that exceeds a threshold at520, and optionally including determining at least one of range to an object or a location of an object corresponding to a received echo defined by the temporal window at525. In an example, the establishment of a temporal window at515or520can be performed using a signal processor circuit that is separate from an object detection or general-purpose processor circuit used for range determination or object localization at525. Providing the abbreviated representation can include digitally transmitting the abbreviated representation, such as from a first integrated circuit die housing a signal processing circuit to a second integrated circuit die including an object detection circuit or generally-purpose processor circuit.

Each of the non-limiting aspects in this document can stand on its own, or can be combined in various permutations or combinations with one or more of the other aspects or other subject matter described in this document.