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 control circuit configured to drive the light source with a current pulse comprising a non-linear rise, and a decline from a maximum output to zero having a duration within a threshold percentage of a total pulse duration of the current pulse.
1. An optical measurement system comprising:
a light source configured to emit a light pulse directed at a target; and
a control circuit configured to drive the light source with a current pulse comprising:
a non-linear rise, and
a decline from a maximum output to zero having a duration within a threshold percentage of a total pulse duration of the current pulse, wherein the threshold percentage is twenty percent.
2. The optical measurement system of claim 1 , wherein the non-linear rise comprises an exponential rise.
3. The optical measurement system of claim 1 , wherein the current pulse is configured to be within a threshold variance from a pulse defined by
I
(
t
)
=
eVN
th
τ
N
sinh
(
T
/
τ
N
)
exp
(
t
/
τ
N
)
,
where e is an elementary charge constant, V is a volume of an active region of the light source, N th is a lasing threshold carrier density of the light source, T is the total pulse duration, and τ N is a growth rate constant of the light source.
4. The optical measurement system of claim 1 , wherein the control circuit comprises a driver circuit comprising:
a rectangular pulse voltage source;
a pull-up transistor; and
a pull-down transistor.
5. The optical measurement system of claim 4 , wherein the control circuit further comprises a capacitor configured to be coupled in parallel with the light source.
6. The optical measurement system of claim 4 , wherein the driver circuit further comprises:
a first resistor coupled to the rectangular pulse voltage source and the pull-down transistor;
a diode coupled in parallel to the first resistor;
a second resistor coupled to the rectangular pulse voltage source and the pull-up transistor; and
a capacitor coupled in parallel with the pull-down transistor.
7. The optical measurement system of claim 4 , wherein the control circuit further comprises:
a reverse diode coupled in parallel to the light source;
a resonant circuit comprising an inductor and a capacitor coupled to the driver circuit; and
at least one additional resonant circuit comprising an additional inductor and an additional capacitor coupled in parallel to the resonant circuit.
8. The optical measurement system of claim 4 , wherein the control circuit further comprises a nonlinear inductor.
9. The optical measurement system of claim 1 , further comprising:
at least one detector configured to detect arrival times of 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 of the photons at the at least one detector, histogram data associated with the target; and
determine, based on the histogram data, a property of the target.
10. The optical measurement system of claim 9 , wherein the at least one detector comprises a single photon avalanche diode (SPAD).
11. The optical measurement system of claim 9 , wherein the at least one detector is included in a wearable device configured to be worn by a user.
12. The optical measurement system of claim 11 , wherein the wearable device includes a head-mountable component configured to be worn on a head of the user.
13. An optical measurement system comprising:
a light source configured to emit a light pulse directed at a target; and
a control circuit configured to drive the light source, the control circuit comprising a nonlinear inductor and a driver circuit comprising:
a rectangular pulse voltage source;
a pull-up transistor; and
a pull-down transistor.
14. The optical measurement system of claim 13 , wherein the control circuit further comprises a capacitor configured to be coupled in parallel with the light source.
15. The optical measurement system of claim 13 , wherein the driver circuit further comprises:
a first resistor coupled to the rectangular pulse voltage source and the pull-down transistor;
a diode coupled in parallel to the first resistor;
a second resistor coupled to the rectangular pulse voltage source and the pull-up transistor; and
a capacitor coupled in parallel with the pull-down transistor.
16. The optical measurement system of claim 13 , wherein the control circuit further comprises:
a reverse diode coupled in parallel to the light source; and
a resonant circuit comprising an inductor and a capacitor coupled to the driver circuit; and
at least one additional resonant circuit comprising an additional inductor and an additional capacitor coupled in parallel to the resonant circuit.
17. The optical measurement system of claim 13 , wherein the control circuit is configured to drive the light source with a current pulse comprising:
a non-linear rise, and
a decline from a maximum output to zero having a duration within a threshold percentage of a total pulse duration of the current pulse.
18. The optical measurement system of claim 17 , wherein the threshold percentage is twenty percent.
19. The optical measurement system of claim 17 , wherein the non-linear rise comprises an exponential rise.
20. The optical measurement system of claim 17 , wherein the current pulse is configured to be within a threshold of a pulse defined by
I
(
t
)
=
eVN
th
τ
N
sinh
(
T
/
τ
N
)
exp
(
t
/
τ
N
)
,
where e is an elementary charge constant, V is a volume of an active region of the light source, N th is a lasing threshold carrier density of the light source, T is the total pulse duration, and τ N is a growth rate constant of the light source.
21. The optical measurement system of claim 13 , further comprising:
at least one detector configured to detect arrival times of 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 of the photons at the at least one detector, histogram data associated with the target; and
determine, based on the histogram data, a property of the target.
22. The optical measurement system of claim 21 , wherein the at least one detector comprises a single photon avalanche diode (SPAD).
23. The optical measurement system of claim 21 , wherein the at least one detector is included in a wearable device configured to be worn by a user.
24. The optical measurement system of claim 23 , wherein the wearable device includes a head-mountable component configured to be worn on a head of the user.
25. A wearable system for use by a user comprising:
a module unit configured to be attached to a body of the user, the module unit comprising:
a light source configured to emit a light pulse directed at a target within the body, and
at least one detector configured to detect photons of light after the light pulse is scattered by the target; and
a control circuit configured to drive the light source with a current pulse comprising:
a non-linear rise, and
a decline from a maximum output to zero having a duration within a threshold percentage of a total pulse duration, wherein the threshold percentage is twenty percent.
26. The wearable system of claim 25 , wherein the non-linear rise comprises an exponential rise.
27. The wearable system of claim 25 , wherein the current pulse is configured to be within a threshold variance from a pulse defined by
I
(
t
)
=
e
V
N
t
h
τ
N
sinh
(
T
/
τ
N
)
exp
(
t
/
τ
N
)
,
where e is an elementary charge constant, V is a volume of an active region of the light source, N th is a lasing threshold carrier density of the light source, T is the total pulse duration, and τ N is a growth rate constant of the light source.
28. The wearable system of claim 25 , wherein the control circuit comprises a driver circuit comprising:
a rectangular pulse voltage source;
a pull-up transistor; and
a pull-down transistor.
29. The wearable system of claim 28 , wherein the control circuit further comprises a capacitor configured to be coupled in parallel with the light source.
30. The wearable system of claim 28 , wherein the driver circuit further comprises:
a first resistor coupled to the rectangular pulse voltage source and the pull-down transistor;
a diode coupled in parallel to the first resistor;
a second resistor coupled to the rectangular pulse voltage source and the pull-up transistor; and
a capacitor coupled in parallel with the pull-down transistor.
31. The wearable system of claim 28 , wherein the control circuit further comprises:
a reverse diode coupled in parallel to the light source; and
a resonant circuit comprising an inductor and a capacitor coupled to the driver circuit;
at least one additional resonant circuit comprising an additional inductor and an additional capacitor coupled in parallel to the resonant circuit.
32. The wearable system of claim 28 , wherein the control circuit further comprises a nonlinear inductor.
33. The wearable system of claim 25 , wherein:
the at least one detector is further configured to detect arrival times of the photons of the light after the light pulse is scattered by the target; and
the wearable system further comprises a processing unit configured to:
generate, based on the arrival times of the photons at the at least one detector, histogram data associated with the target; and
determine, based on the histogram data, a property of the target.
34. The wearable system of claim 25 , wherein the at least one detector comprises a single photon avalanche diode (SPAD).
35. The wearable system of claim 25 , wherein the module unit is configured to be worn on a head of the user.