# US 8,772,700 B2 · Photon detection system and method of photon detection for determining the number of photons received at a photon detector > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,772,700 B2 - Document ID: US8772700B2 - Status: Granted - Kind: B2 - Application number: 12/829,446 - Filed: 2010-07-02 - Published: 2010-11-25 - Granted: 2014-07-08 - Earliest claimed: 2008-01-03 - Assignees: Kabushiki Kaisha Toshiba - Applicants: Beata Ewa Kardynal; Zhiliang Yuan; Andrew James Shields - Inventors: Beata Ewa Kardynal; Zhiliang Yuan; Andrew James Shields - Web page: https://patentgrep.com/patent/US8772700B2 ## CPC - H10F 30/225: the potential barrier working in avalanche mode, e.g. avalanche photodiodes - G01J 1/44: Electric circuits {(for command of an exposure part G03B7/02)} - H10F: INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) ## Abstract According to one embodiment, a photon detection system determines the number of detected photons. The detection system includes an avalanche photodiode and a measuring unit. The measuring unit measures an avalanche signal induced by illumination before the avalanche current through the device has saturated. ## Claims ### Claim 1 A photon detection system configured to determine a number of detected photons in excess of a single photon in a single light pulse, the detection system comprising an avalanche photodiode and a measuring unit configured to measure an avalanche signal induced by illumination by the single light pulse before an avalanche current through the avalanche photodiode has saturated, the avalanche signal being output from the avalanche photodiode, the avalanche signal being measured for a time less than the time which an avalanche takes to saturate, the number of detected photons in excess of the single photon in the single light pulse being determined based on an amplitude of a peak formed by the avalanche signal; wherein the measuring unit comprises an applying unit configured to apply a bias larger than the breakdown voltage of said photodiode across said photodiode for a time duration, said time duration being shorter than the time required for the avalanche current through the avalanche photodiode to saturate after illumination of the avalanche photodiode and a measuring unit configured to measure the size of the avalanche signal. ### Claim 2 (depends on 1) A system according to claim 1, further comprising an isolating unit configured to isolate a first part and a second part from the signal due to the avalanche. ### Claim 3 (depends on 1) A system according to claim 1, further comprising an applying unit configured to apply a gating signal to said avalanche photodiode. ### Claim 4 (depends on 1) A system according to claim 1, wherein the measuring unit is configured to apply bias above the breakdown for a time which allows said avalanche to saturate and said avalanche signal is measured for a time less than the time which the avalanche takes to saturate. ### Claim 5 (depends on 1) A system according to claim 1, wherein the avalanche charge or avalanche current is measured to determine the number of photons. ### Claim 6 (depends on 2) A system according to claim 2, wherein the isolating unit comprises an applying unit configured to apply signal which compensates for the response of said photodiode in the absence of illumination. ### Claim 7 (depends on 2) A system according to claim 2, wherein the isolating unit comprises a signal divider to divide the output signal of the photodiode into the first part and the second part, where the first part is substantially identical to the second part, a delaying unit configured to delay the second part with respect to the first part and a combiner for combining the first and delayed second parts of the signal such that the delayed second part is used to cancel periodic variations in the first part of the output signal. ### Claim 8 (depends on 3) A system according to claim 3, wherein the gating signal is a rectangular gating signal. ### Claim 9 (depends on 3) A system according to claim 3, wherein the gating signal has a frequency in excess of 50 MHz. ### Claim 10 (depends on 7) A system according to claim 7, wherein the delaying unit is configured to delay the second part of the signal by an integer multiple of said period. ### Claim 11 (depends on 7) A system according to claim 7, further comprising a balancing unit configured to balance the amplitudes of the two signals arriving at the combiner. ### Claim 12 (depends on 7) A system according to claim 7, further comprising an inverting unit configured to invert one part of the signal with respect to the other part of the signal. ### Claim 13 (depends on 4) A system according to claim 4, further comprising an operating unit configured to operate on the signal measured from the APD and block the part of the signal where the avalanche current has saturated. ### Claim 14 A circuit for determining a number of photons in excess of a single photon in a single light pulse detected by an avalanche photodiode, the circuit comprising a measuring unit configured to measure an avalanche signal induced by illumination by the single light pulse before an avalanche current through the avalanche photodiode has saturated, the avalanche signal being output from the avalanche photodiode, the avalanche signal being measured for a time less than the time which an avalanche takes to saturate, the number of detected photons in excess of the single photon in the single light pulse being determined based on an amplitude of a peak formed by the avalanche signal; wherein the measuring unit comprises an applying unit configured to apply a bias larger than the breakdown voltage of said photodiode across said photodiode for a time duration, said time duration being shorter than the time required for the avalanche current through the avalanche photodiode to saturate after illumination of the avalanche photodiode and a measuring unit configured to measure the size of the avalanche signal. ### Claim 15 A method for determining a number of photons in excess of a single photon in a single light pulse detected by an avalanche photodiode comprising measuring an avalanche signal induced by illumination by the single light pulse in the avalanche photodiode before an avalanche current through the avalanche photodiode has saturated, the avalanche signal being output from the avalanche photodiode, the avalanche signal being measured for a time less than the time which an avalanche takes to saturate, the number of detected photons in excess of the single photon in the single light pulse being determined based on an amplitude of a peak formed by the avalanche signal; applying a bias above the breakdown voltage of said photodiode across said photodiode for a time duration, said time duration being shorter than the time required for the avalanche current through the avalanche photodiode to saturate after illumination of the avalanche photodiode and measuring the size of the avalanche signal. ### Claim 16 (depends on 15) A method according to claim 15, comprising applying a bias above the breakdown voltage of said photodiode across said photodiode for a time which allows said avalanche to saturate and measuring said avalanche signal for a time less than the time which the avalanche takes to saturate. ### Claim 17 (depends on 15) A method according to claim 15, wherein the source of illumination is synchronised such that the avalanche signal is measured when photons arrive at the avalanche photodiode. ### Claim 18 (depends on 15) A method according to claim 15, wherein the size of the avalanche signal is compared with one of more predetermined levels to determine the number of photons which have been detected.