Maintaining consistent photodetector sensitivity in an optical measurement system
An illustrative optical measurement system includes a light source configured to emit a light pulse directed at a target. The optical measurement system further includes a plurality of photodetectors configured to operate in accordance with an input bias voltage. The optical measurement system further includes a control circuit configured to identify a photodetector subset included in the plurality of photodetectors and that detects, while the input bias voltage has a first value, photons of the light pulse after the light pulse is scattered by the target. The control circuit is further configured to determine, based on the identifying of the photodetector subset, an overvoltage associated with the photodetector subset. The control circuit is further configured to update, based on the overvoltage, the input bias voltage for the plurality of photodetectors to have a second value.
1. An optical measurement system comprising:
a light source configured to emit a light pulse directed at a target;
a plurality of photodetectors configured to operate in accordance with an input bias voltage; and
a control circuit configured to:
identify a photodetector subset included in the plurality of photodetectors and that detects, while the input bias voltage has a first value, photons of the light pulse after the light pulse is scattered by the target;
determine, based on the identifying of the photodetector subset, an overvoltage associated with the photodetector subset, the determining the overvoltage comprising measuring a respective overvoltage for each photodetector of the photodetector subset; and
update, based on the overvoltage, the input bias voltage for the plurality of photodetectors to have a second value.
2. The optical measurement system of claim 1 , wherein the determining the overvoltage further comprises abstaining from determining the overvoltage based on an additional photodetector subset included in the plurality of photodetectors and that does not detect photons of the light pulse after the light pulse is scattered by the target.
3. The optical measurement system of claim 1 , wherein:
the light source is further configured to emit, subsequent to emitting the light pulse, an additional light pulse directed at the target; and
the control circuit is further configured to:
identify an additional photodetector subset included in the plurality of photodetectors and that detects, while the input bias voltage has the second value, photons of the additional light pulse after the additional light pulse is scattered by the target;
determine, based on the identified additional photodetector subset, an additional overvoltage associated with the additional photodetector subset; and
update, based on the additional overvoltage, the input bias voltage for the plurality of photodetectors to have a third value.
4. The optical measurement system of claim 1 , wherein the determining the overvoltage associated with the photodetector subset further comprises
determining an average overvoltage based on the respective overvoltage for each photodetector of the photodetector subset.
5. The optical measurement system of claim 1 , wherein the control circuit comprises an analog-to-digital converter (ADC) configured to measure the overvoltage.
6. The optical measurement system of claim 5 , wherein:
the control circuit further comprises a proportional-integral-derivative (PID) controller; and
the updating the input bias voltage comprises setting, using the PID controller, the second value at a value that compensates for a difference between the overvoltage associated with the photodetector subset and a predetermined overvoltage.
7. The optical measurement system of claim 1 , further comprising:
a first detector comprising the plurality of photodetectors;
a second detector comprising an additional plurality of photodetectors, the second detector configured to detect arrival times for additional photons of the light pulse after the light pulse is scattered by the target; and
a processing unit configured to generate, based on the arrival times, histogram data associated with the target.
8. The optical measurement system of claim 1 , wherein the updating the input bias voltage comprises setting the second value to be greater than the first value if the subset of the plurality of photodetectors includes no photodetectors.
9. The optical measurement system of claim 1 , wherein each photodetector of the plurality of photodetectors comprises a single photon avalanche diode (SPAD).
10. The optical measurement system of claim 1 , further comprising a wearable assembly configured to be worn by a user, the wearable assembly configured to house the light source, the plurality of photodetectors, and the control circuit.
11. The optical measurement system of claim 10 , wherein:
the wearable assembly is configured to be worn on a head of the user; and
the target comprises a brain of the user.
12. A wearable system for use by a user comprising:
a component configured to be attached to a body of the user, the component comprising:
a light source configured to emit a light pulse directed at a target within the body, and
a plurality of photodetectors configured to operate in accordance with an input bias voltage; and
a control circuit configured to:
identify a photodetector subset included in the plurality of photodetectors and that detects, while the input bias voltage has a first value, photons of the light pulse after the light pulse is scattered by the target;
determine, based on the identifying of the photodetector subset, an overvoltage associated with the photodetector subset, the determining the overvoltage comprising measuring a respective overvoltage for each photodetector of the photodetector subset; and
update, based on the overvoltage, the input bias voltage for the plurality of photodetectors to have a second value.
13. The wearable system of claim 12 , wherein the determining the overvoltage further comprises abstaining from determining the overvoltage based on an additional photodetector subset included in the plurality of photodetectors and that does not detect photons of the light pulse after the light pulse is scattered by the target.
14. The wearable system of claim 12 , wherein:
the light source is further configured to emit, subsequent to emitting the light pulse, an additional light pulse directed at the target; and
the control circuit is further configured to:
identify an additional photodetector subset included in the plurality of photodetectors and that detects, while the input bias voltage has the second value, photons of the additional light pulse after the additional light pulse is scattered by the target;
determine, based on the identified additional photodetector subset, an additional overvoltage associated with the additional photodetector subset; and
update, based on the additional overvoltage, the input bias voltage for the plurality of photodetectors to have a third value.
15. The wearable system of claim 12 , wherein the determining the overvoltage associated with the photodetector subset further comprises
determining an average overvoltage based on the respective overvoltage for each photodetector of the photodetector subset.
16. The wearable system of claim 12 , wherein the control circuit comprises an analog-to-digital converter (ADC) configured to measure the overvoltage.
17. The wearable system of claim 16 , wherein:
the control circuit further comprises a proportional-integral-derivative (PID) controller; and
the updating the input bias voltage comprises setting, using the PID controller, the second value at a value that compensates for a difference between the overvoltage associated with the photodetector subset and a predetermined overvoltage.
18. A method comprising:
identifying, by a control circuit, a photodetector subset included in a plurality of photodetectors configured to operate in accordance with an input bias voltage, the photodetector subset comprising photodetectors that detect, while the input bias voltage has a first value, photons of a light pulse after the light pulse is scattered by a target;
determining, by the control circuit, based on the identifying of the photodetector subset, an overvoltage associated with the photodetector subset, the determining the overvoltage comprising measuring a respective overvoltage for each photodetector of the photodetector subset; and
updating, by the control circuit, based on the overvoltage, the input bias voltage for the plurality of photodetectors to have a second value.