Photodetector systems with low-power time-to-digital converter architectures to determine an arrival time of photon at a photodetector based on event detection time window

1. A wearable system for use by a user, comprising:

a head-mountable component configured to be worn on a head of the user, the head-mountable component comprising a time-to-digital converter (TDC) configured to:

receive, during a predetermined event detection time window that commences in response to an application of a light pulse to a target, a signal triggered by an event in which a photodetector detects a photon of the light pulse after the light pulse reflects from the target, the signal configured to enable a gated ring oscillator (GRO) of the TDC; and

measure, using the GRO, a time interval between when the event occurred and an end of the predetermined event detection time window; and

a processor configured to determine, based on the time interval and the predetermined event detection time window, an arrival time of the photon at the photodetector.

2. The wearable system of claim 1 , wherein the signal is further configured to:

enable the GRO of the TDC while the signal is being received by the TDC; and

disable the GRO of the TDC upon a stopping of the signal.

3. The wearable system of claim 1 , wherein the TDC is further configured to receive, subsequent to the predetermined event detection time window, a second signal configured to disable the GRO of the TDC.

4. The wearable system of claim 3 , wherein the second signal is received upon completion of the predetermined event detection time window.

5. The wearable system of claim 1 , wherein:

the GRO includes one or more pairs of cross-coupled inverters configured to store a state of the GRO when the GRO is disabled; and

the measuring the time interval includes decoding the stored state of the GRO.

6. The wearable system of claim 5 , wherein:

the TDC receives a voltage from a phase-locked loop (PLL) or a delay-locked loop (DLL) that provides an external reference clock; and

at least one of the enabling the GRO and the measuring the time interval is based additionally on the received voltage.

7. The wearable system of claim 1 , wherein:

the TDC is further configured to measure an additional time interval subsequent to the time interval;

the processor is further configured to calibrate the TDC based on the measured additional time interval; and

the determining the arrival time of the photon is further based on the calibrating.

8. The wearable system of claim 7 , wherein the additional time interval is a same or substantially similar length as the predetermined event detection time window.

9. The wearable system of claim 7 , further comprising a calibration circuit configured to provide calculations for the calibrating the TDC.

10. The wearable system of claim 7 , further comprising a lookup table configured to provide calculations for the calibrating the TDC.

11. The wearable system of claim 7 , wherein the TDC is further configured to receive, subsequent to the additional time interval, a second signal configured to disable the GRO of the TDC.

12. The wearable system of claim 1 , wherein:

the TDC is further configured to receive an event window signal specifying a starting time of the predetermined event detection time window; and

the determining the arrival time of the photon is further based on the starting time of the predetermined event detection time window.

13. The wearable 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.

14. The wearable system of claim 1 , wherein the head-mountable component is implemented by a non-invasive wearable brain interface system.

15. The wearable system of claim 1 , further comprising a wearable battery configured to provide power to the head-mountable component.

16. A system comprising:

a time-to-digital converter (TDC) configured to:

receive, during a predetermined event detection time window that commences in response to an application of a light pulse to a target, a signal triggered by an event in which a photodetector detects a photon of the light pulse after the light pulse reflects from the target, the signal configured to enable a gated ring oscillator (GRO) of the TDC; and

measure, using the GRO, a time interval between when the event occurred and an end of the predetermined event detection time window; and

a processor configured to determine, based on the time interval and the predetermined event detection time window, an arrival time of the photon at the photodetector.

17. The system of claim 16 , wherein the signal is further configured to:

enable the GRO of the TDC while the signal is being received by the TDC; and

disable the GRO of the TDC upon a stopping of the signal.

18. The system of claim 16 , wherein the TDC is further configured to receive, subsequent to the predetermined event detection time window, a second signal configured to disable the GRO of the TDC.

19. The system of claim 16 , wherein:

the GRO includes one or more pairs of cross-coupled inverters configured to store a state of the GRO when the GRO is disabled; and

the measuring the time interval includes decoding the stored state of the GRO.

20. A method comprising:

receiving, by a time-to-digital converter (TDC), during a predetermined event detection time window that commences in response to an application of a light pulse to a target, a signal triggered by an event in which a photodetector detects a photon of the light pulse after the light pulse reflects from the target, the signal configured to enable a gated ring oscillator (GRO) of the TDC;

measuring, by the TDC and using the GRO, a time interval between when the event occurred and an end of the predetermined event detection time window; and

determining, by a processor, based on the time interval and the predetermined event detection time window, an arrival time of the photon at the photodetector.