Image sensor with large dynamic range
Disclosed herein is a system comprising: an avalanche photodiode (APD); a bias source configured to supply a reverse bias to the APD; a current meter configured to measure electric current through the APD; a controller configured to reduce the reverse bias to a value below a breakdown voltage of the APD from a value above the breakdown voltage when an intensity of light incident on the APD is above a threshold, and configured to determine the intensity of the light above the threshold based on the electric current through the APD when the reverse bias is below the breakdown voltage.
1. A system comprising:
an avalanche photodiode (APD);
a bias source configured to supply a reverse bias to the APD;
a current meter configured to measure electric current through the APD;
a controller configured to cause the APD to operate in a linear mode by reducing the reverse bias to a value below a breakdown voltage of the APD from a value above the breakdown voltage when an intensity of light incident on the APD is above a threshold, and configured to determine the intensity of the light above the threshold based on the electric current through the APD when the reverse bias is below the breakdown voltage.
2. The system of claim 1 , wherein the controller is configured to quench the APD after the controller detects a rising edge in the electric current, when the reverse bias is above the breakdown voltage.
3. The system of claim 2 , wherein the controller is configured to increase the reverse bias to above the breakdown voltage after quenching the APD.
4. The system of claim 1 , wherein the controller is configured to determine the intensity of light incident on the APD based on a number of pulses in the electric current in a given amount of time, when the reverse bias is above the breakdown voltage.
5. The system of claim 1 , wherein the threshold is a saturation intensity of the APD operating in a Geiger mode.
6. A method of using an APD, comprising:
(a) applying a first reverse bias above a breakdown voltage of the APD to the APD;
(b) measuring a first intensity of light incident on the APD;
(c) determining whether the first intensity is above a first threshold;
if the first intensity is not above the first threshold, repeating (a)-(c);
if the first intensity is above the first threshold:
(d) applying a second reverse bias below the breakdown voltage to the APD;
(e) measuring a second intensity of light incident on the APD;
(f) determining whether the first intensity is below a second threshold;
if the second intensity is not below the second threshold, repeating (d)-(f);
if the second intensity is below the first threshold, performing (a)-(c).
7. The method of claim 6 , wherein measuring a first intensity comprises counting a number of current pulses through the APD in a giving amount of time.
8. The method of claim 6 , wherein measuring a second intensity comprises measuring an electric current in the APD.
9. The method of claim 6 , wherein the first threshold is a saturation intensity of the APD.
10. The method of claim 6 , wherein the first and second thresholds are the same.