Bias voltage generation in an optical measurement system
An exemplary optical measurement system described herein includes a control circuit configured to output a global bias voltage and a module communicatively coupled to the control circuit. The module includes a light source configured to emit light directed at a target. The module further includes a plurality of detectors configured to detect arrival times for photons of the light after the light is scattered by the target. The module further includes a module control circuit configured to receive the global bias voltage and output a plurality of detector bias voltages based on the global bias voltage. The plurality of detector bias voltages include a respective detector bias voltage for each detector of the plurality of detectors.
1. An optical measurement system comprising:
a control circuit configured to output a global bias voltage; and
a module communicatively coupled to the control circuit, the module comprising:
a light source configured to emit light directed at a target,
a plurality of detectors configured to detect arrival times for photons of the light after the light is scattered by the target, and
a module control circuit configured to:
receive the global bias voltage,
output a plurality of detector bias voltages based on the global bias voltage and including a respective detector bias voltage for each detector of the plurality of detectors, wherein the global bias voltage is a voltage level higher than the plurality of detector bias voltages and the outputting the plurality of detector bias voltages comprises stepping down the global bias voltage for each respective detector bias voltage;
monitor the plurality of detector bias voltages; and
adjust, based on a highest voltage level of the plurality of detector bias voltages, the global bias voltage.
2. The optical measurement system of claim 1 , wherein:
the module control circuit is further configured to measure a respective optimal detector bias voltage for each detector of the plurality of detectors; and
the outputting the plurality of detector bias voltages is further based on the respective optimal detector bias voltage for each detector.
3. The optical measurement system of claim 1 , wherein:
the module control circuit is further configured to measure a respective optimal detector bias voltage for each detector of the plurality of detectors; and
the monitoring the plurality of detector bias voltages comprises receiving, from the module control circuit, the highest voltage level of the plurality of detector bias voltages based on the respective optimal detector bias voltage for each detector.
4. The optical measurement system of claim 1 , wherein:
the global bias voltage is a voltage level lower than the plurality of detector bias voltages; and
the outputting the plurality of detector bias voltages comprises stepping up the global bias voltage for each respective detector bias voltage.
5. The optical measurement system of claim 4 , wherein the module control circuit comprises a charge pump configured to step up the global bias voltage.
6. The optical measurement system of claim 1 , further comprising an additional module communicatively coupled to the control circuit, the additional module comprising:
an additional light source configured to emit additional light directed at the target,
an additional plurality of detectors configured to detect arrival times for photons of the additional light after the additional light is scattered by the target, and
an additional module control circuit configured to:
receive the global bias voltage, and
output an additional plurality of detector bias voltages based on the global bias voltage and including a respective detector bias voltage for each detector of the additional plurality of detectors.
7. The optical measurement system of claim 6 , wherein the control circuit is configured to output the global bias voltage to the module control circuit and the additional module control circuit via a single communication bus.
8. The optical measurement system of claim 1 , wherein each detector comprises a plurality of photodetectors.
9. The optical measurement system of claim 8 , wherein each photodetector comprises:
a single photon avalanche diode (SPAD); and
a fast gating circuit configured to arm and disarm the SPAD.
10. The optical measurement system of claim 1 , further comprising a wearable device configured to be worn by a user, wherein:
the wearable device includes the module; and
the target includes tissue within the user.
11. The optical measurement system of claim 10 , wherein the wearable device includes a head-mountable component configured to be worn on a head of a user.
12. A wearable system for use by a user comprising:
a control circuit configured to output a global bias voltage; and
a head-mountable component configured to be attached to a head of the user, the head-mountable component comprising a module communicatively coupled to the control circuit, the module comprising:
a light source configured to emit light directed at a target,
a plurality of detectors configured to detect arrival times for photons of the light after the light is scattered by the target, and
a module control circuit configured to:
receive the global bias voltage, and
output a plurality of detector bias voltages based on the global bias voltage and including a respective detector bias voltage for each detector of the plurality of detectors, wherein the global bias voltage is a voltage level higher than the plurality of detector bias voltages and the outputting the plurality of detector bias voltages comprises stepping down the global bias voltage for each respective detector bias voltage;
monitor the plurality of detector bias voltages; and
adjust, based on a highest voltage level of the plurality of detector bias voltages, the global bias voltage.
13. The wearable system of claim 12 , wherein:
the module control circuit is further configured to measure a respective optimal detector bias voltage for each detector of the plurality of detectors; and
the outputting the plurality of detector bias voltages is further based on the respective optimal detector bias voltage for each detector.
14. The wearable system of claim 12 , wherein:
the module control circuit is further configured to measure a respective optimal detector bias voltage for each detector of the plurality of detectors; and
the monitoring the plurality of detector bias voltages comprises receiving, from the module control circuit, the highest voltage level of the plurality of detector bias voltages based on the respective optimal detector bias voltage for each detector.
15. The wearable system of claim 12 , wherein:
the global bias voltage is a voltage level lower than the plurality of detector bias voltages; and
the outputting the plurality of detector bias voltages comprises stepping up the global bias voltage for each respective detector bias voltage.
16. The wearable system of claim 15 , wherein the module control circuit comprises a charge pump configured to step up the global bias voltage.
17. The wearable system of claim 12 , wherein the head-mountable component further comprises an additional module communicatively coupled to the control circuit, the additional module comprising:
an additional light source configured to emit additional light directed at the target,
an additional plurality of detectors configured to detect arrival times for photons of the additional light after the additional light is scattered by the target, and
an additional module control circuit configured to:
receive the global bias voltage, and
output an additional plurality of detector bias voltages based on the global bias voltage and including a respective detector bias voltage for each detector of the additional plurality of detectors.
18. The wearable system of claim 17 , wherein the control circuit is configured to output the global bias voltage to the module control circuit and the additional module control circuit via a single communication bus.
19. The wearable system of claim 12 , wherein each detector comprises a plurality of photodetectors.
20. The wearable system of claim 19 , wherein each photodetector comprises:
a single photon avalanche diode (SPAD); and
a fast gating circuit configured to arm and disarm the SPAD.
21. The wearable system of claim 12 , wherein the control circuit is housed in the head-mountable component.
22. A method comprising:
receiving, by a module control circuit of a module of an optical measurement system, a global bias voltage from a control circuit of the optical measurement system; and
outputting, by the module control circuit, a plurality of detector bias voltages based on the global bias voltage and including a respective detector bias voltage for each detector of a plurality of detectors of the module, wherein the global bias voltage is a voltage level higher than the plurality of detector bias voltages and the outputting the plurality of detector bias voltages comprises stepping down the global bias voltage for each respective detector bias voltage;
monitoring the plurality of detector bias voltages; and
adjusting, based on a highest voltage level of the plurality of detector bias voltages, the global bias voltage.