Accurate photo detector measurements for LIDAR
A light ranging system can include a laser device and an imaging device having photosensors. The laser device illuminates a scene with laser pulse radiation that reflects off of objects in the scene. The reflections can vary greatly depending on the reflecting surface shape and reflectivity. The signal measured by photosensors can be filtered with a number of matched filter designed according to profiles of different reflected signals. A best matched filter can be identified, and hence information about the reflecting surface and accurate ranging information can be obtained. The laser pulse radiation can be emitted in coded pulses by allowing weights to different detection intervals. Other enhancements include staggering laser pulses and changing an operational status of photodetectors of a pixel sensor, as well as efficient signal processing using a sensor chip that includes processing circuits and photosensors.
1. A method of using an optical measurement system, the method comprising:
transmitting N pulse trains from a light source as part of an optical measurement, each of the N pulse trains including one or more pulses from the light source and corresponding to a different time interval that is triggered by a start signal, and wherein N is an integer greater than one;
detecting photons of the N pulse trains by a photosensor of the optical measurement system, thereby generating data values at a plurality of time points;
for each of the N pulse trains, assigning a weight to the data values at time points within the time interval corresponding to the pulse train, thereby obtaining weighted values, wherein at least two of the N pulse trains are assigned different weights and have different pulse patterns;
determining a histogram corresponding to the weighted values in a plurality of time bins, wherein determining a counter of the histogram at a particular time bin includes accumulating the weighted values at time points within the particular time bin across a plurality of time intervals; and
detecting a signal corresponding to the N pulse trains within the histogram.
2. The method of claim 1 , further comprising:
determining a received time corresponding to the N pulse trains relative to the start signal.
3. The method of claim 2 , wherein determining the received time includes:
sliding a filter over the histogram to calculate a filtered histogram having counters corresponding to different sliding positions of the filter relative to the histogram, wherein each of the counters of the filtered histogram correspond to an overlap of the filter and the histogram at a particular sliding position; and
identifying a maximum value of the counters of the filtered histogram, the particular sliding position for the maximum value of the counters corresponding to the received time.
4. The method of claim 2 , wherein the optical measurement system is a light ranging system, and wherein the N pulse trains reflect from an object, the method further comprising:
determining a distance to the object using the received time.
5. The method of claim 4 , wherein the distance corresponds to a round trip time between the received time and a start time of the start signal.
6. The method of claim 1 , wherein the at least two of the N pulse trains have different pulse patterns by having pulses at different times relative to the start signal.
7. The method of claim 1 , wherein the weights are vectors in two or more dimensions.
8. The method of claim 1 , wherein a first weight is positive and a second weight is negative.
9. The method of claim 1 , wherein histogram circuitry determines the histogram, and wherein the histogram circuitry and the photosensor are on a same integrated circuit.
10. The method of claim 1 , further comprising:
performing the method of claim 1 for a plurality of channels of light sources and photosensors as part of a plurality of ranging measurements, wherein the plurality of ranging measurements overlap in time.
11. The method of claim 10 , wherein pulse patterns of the N pulse trains of at least two channels of the plurality of channels of light sources are different, thereby causing different histogram patterns for different channels.
12. The method of claim 10 , wherein weights assigned to the N pulse trains of at least two channels of the plurality of channels of light sources are different, thereby causing different histogram patterns for different channels.
13. A light ranging system comprising:
a light source configured to provide pulses;
a sensor integrated circuit that includes:
a plurality of photosensors corresponding to different fields of view, each photosensor of the plurality of photosensors including a plurality of photodetectors that when triggered by photons are configured to output binary signals indicating the photons have been detected;
timing circuitry configured to determine times for when photons are detected based on the binary signals; and
histogram circuitry configured to determine and store counters that each correspond to a number of photodetectors of a photosensor triggered during a time bin for each photosensor of the plurality of photosensors, thereby creating a histogram that is usable to determine a received time of one or more pulses from the light source that reflected from an object to each photosensor of the plurality of photosensors.
14. The light ranging system of claim 13 , wherein plurality of photodetectors comprises single-photon avalanche diodes (SPADs).
15. The light ranging system of claim 13 , wherein the sensor integrated circuit further includes:
a windowing circuit configured to apply one or more matched filters to the histogram to identify a time window within which the received time resides.
16. The light ranging system of claim 15 , wherein the sensor integrated circuit further includes:
an interpolation circuit configured to apply a plurality of interpolation filters to the histogram within the time window, wherein a best matching interpolation filter identifies the received time with an accuracy less than a width of the time bin.
17. The light ranging system of claim 15 , further comprising:
a second integrated circuit that is communicably coupled with the sensor integrated circuit and that includes:
an interpolation circuit configured to apply a plurality of interpolation filters to the histogram within the time window, wherein a best matching interpolation filter identifies the received time with an accuracy less than a width of the time bin.
18. The light ranging system of claim 13 , wherein the timing circuitry and the histogram circuitry are dedicated circuitry during operation of the light ranging system.
19. The light ranging system of claim 18 , wherein the sensor integrated circuit comprises an application-specific integrated circuit (ASIC).
20. The light ranging system of claim 18 , wherein the sensor integrated circuit comprises a field-programmable gate array (FPGA).
21. The light ranging system of claim 13 , further comprising:
a rotation motor that is connected to the light source and the sensor integrated circuit and that rotates the light source and the sensor integrated circuit.
22. The light ranging system of claim 13 , wherein the light source includes a plurality of laser devices on an integrated circuit, and wherein the plurality of laser devices are vertical-cavity surface-emitting lasers (VCSELs).
23. An optical measurement system comprising:
a light source configured to provide pulses;
a photosensor; and
circuitry configured to:
transmit N pulse trains from the light source as part of an optical measurement, each of the N pulse trains including one or more pulses from the light source and corresponding to a different time interval that is triggered by a start signal, and wherein N is an integer greater than one;
detect photons of the N pulse trains by the photosensor of the optical measurement system, thereby generating data values at a plurality of time points;
for each of the N pulse trains, assign a weight to the data values at time points within the time interval corresponding to the pulse train, thereby obtaining weighted values, wherein at least two of the N pulse trains are assigned different weights and have different pulse patterns;
determine a histogram corresponding to the weighted values in a plurality of time bins, wherein a determining counter of the histogram at a particular time bin includes accumulating the weighted values at time points within the particular time bin across a plurality of time intervals; and
detect a signal corresponding to the N pulse trains within the histogram.
24. The optical measurement system of claim 23 , wherein the circuitry is configured to determine a received time corresponding to the N pulse trains relative to the start signal.
25. The optical measurement system of claim 24 , wherein determining the received time includes:
sliding a filter over the histogram to calculate a filtered histogram having counters corresponding to different sliding positions of the filter relative to the histogram, wherein each of the counters of the filtered histogram correspond to an overlap of the filter and the histogram at a particular sliding position; and
identifying a maximum value of the counters of the filtered histogram, the particular sliding position for the maximum value of the counters corresponding to the received time.
26. The optical measurement system of claim 24 , wherein the optical measurement system is a light ranging system, wherein the N pulse trains reflect from an object, and wherein the circuitry is configured to determine a distance to the object using the received time.
27. The optical measurement system of claim 23 , wherein a first weight is positive and a second weight is negative.
28. The optical measurement system of claim 23 , further comprising:
a plurality of channels of light sources and photosensors, wherein the circuitry is configured to detect signals for the plurality of channels of light sources and photosensors as part of a plurality of ranging measurements, and wherein the plurality of ranging measurements overlap in time.
29. The optical measurement system of claim 28 , wherein pulse patterns of the N pulse trains of at least two channels of the plurality of channels of light sources are different, thereby causing different histogram patterns for different channels.
30. The optical measurement system of claim 28 , wherein weights assigned to the N pulse trains of at least two channels of the plurality of channels of light sources are different, thereby causing different histogram patterns for different channels.