Maintaining consistent photodetector sensitivity in an optical measurement system

1. An optical measurement system comprising:

a light source configured to emit light directed at a target;

a photodetector configured to detect a photon of the light after the light is scattered by the target; and

a control circuit configured to arm the photodetector by:

applying a bias voltage to a first terminal of the photodetector; and

applying, for a predetermined amount of time using a current source, a current to a second terminal of the photodetector for the predetermined amount of time to produce a predetermined voltage difference different from the bias voltage and higher than a breakdown voltage of the photodetector across the photodetector.

2. The optical measurement system of claim 1 , wherein the control circuit comprises a current mirror coupling the current source to the second terminal of the photodetector.

3. The optical measurement system of claim 2 , wherein:

the first terminal comprises a cathode of the photodetector;

the second terminal comprises an anode of the photodetector; and

the applying the current comprises discharging the anode of the photodetector to produce the predetermined voltage difference across the photodetector.

4. The optical measurement system of claim 2 , wherein:

the first terminal comprises an anode of the photodetector;

the second terminal comprises a cathode of the photodetector; and

the applying the current comprises delivering charge to the cathode of the photodetector to produce the predetermined voltage difference across the photodetector.

5. The optical measurement system of claim 1 , further comprising an array of photodetectors including the photodetector;

wherein the control circuit is configured to arm the array of photodetectors by:

applying the bias voltage to first terminals of the array of photodetectors; and

applying the current using the current source to second terminals of the array of photodetectors.

6. The optical measurement system of claim 5 , wherein the current source is a shared current source configured to apply the current to the second terminals.

7. The optical measurement system of claim 6 , further comprising a plurality of current minors coupling the shared current source to the second terminals of the array of photodetectors.

8. The optical measurement system of claim 7 , wherein the plurality of current mirrors comprise a shared transistor and a plurality of local transistors each corresponding to a different photodetector of the array of photodetectors.

9. The optical measurement system of claim 1 , wherein the 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 , wherein the photodetector is included in a wearable device configured to be worn by a 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 head-mountable component configured to be attached to a head of the user, the head-mountable component comprising a photodetector configured to detect photons from a light pulse after the light pulse reflects off a target within the head; and

a control circuit configured to arm the photodetector by:

applying a bias voltage to a first terminal of the photodetector; and

applying, for a predetermined amount of time using a current source, a current to a second terminal of the photodetector for the predetermined amount of time to produce a predetermined voltage difference different from the bias voltage and higher than a breakdown voltage of the photodetector across the photodetector.

13. The wearable system of claim 12 , wherein the control circuit comprises a current mirror coupling the current source to the second terminal of the photodetector.

14. The wearable system of claim 13 , wherein:

the first terminal comprises a cathode of the photodetector;

the second terminal comprises an anode of the photodetector; and

the applying the current comprises discharging the anode of the photodetector to produce the predetermined voltage difference across the photodetector.

15. The wearable system of claim 13 , wherein:

the first terminal comprises an anode of the photodetector;

the second terminal comprises a cathode of the photodetector; and

the applying the current comprises delivering charge to the cathode of the photodetector to produce the predetermined voltage difference across the photodetector.

16. The wearable system of claim 12 , further comprising an array of photodetectors including the photodetector;

wherein the control circuit is configured to arm the array of photodetectors by:

applying the bias voltage to first terminals of the array of photodetectors; and

applying the current using the current source to second terminals of the array of photodetectors.

17. The wearable system of claim 16 , wherein the current source is a shared current source configured to apply the current to the second terminals.

18. The wearable system of claim 17 , further comprising a plurality of current mirrors coupling the shared current source to the second terminals of the array of photodetectors.

19. The wearable system of claim 18 , wherein the plurality of current minors comprise a shared transistor and a plurality of local transistors each corresponding to a different photodetector of the array of photodetectors.

20. The wearable system of claim 12 , wherein the photodetector comprises:

a single photon avalanche diode (SPAD); and

a fast gating circuit configured to arm and disarm the SPAD.