Zero dead time single photon counting system based on time division multiplexing

Abstract

The invention discloses a zero-dead-time single photon counting system based on time division multiplexing, and belongs to the technical field of laser radars. The system comprises an optical multiplexer, N detection channels, N delay modules, a shift register and a counting circuit, according to a time division multiplexing mechanism, a time axis is divided into a plurality of independent time slots, and in different time slots, incident signal light is input into different detection channels according to a preset sequence, so that zero-dead-time single photon counting is realized. According to the invention, the dead time problem of a traditional single-photon detector is solved, the photon counting pulse and the dark current pulse are effectively distinguished through a multi-parameter pulse discrimination technology, the signal-to-noise ratio of the system is significantly improved, and a reliable solution is provided for application scenes requiring high-sensitivity detection, such as laser radar and quantum communication.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser radar, and in particular relates to a zero dead time single photon counting system based on time division multiplexing. Background Art

[0002] In the field of LiDAR (LiDAR), single-photon detectors (SPDs) are widely used in low-light detection scenarios such as long-range laser ranging, three-dimensional imaging, and quantum communication due to their extremely high sensitivity. SPDs primarily include photomultiplier tubes (PMTs), Geiger-mode avalanche photodiodes (GM-APDs), and superconducting nanowire single-photon detectors (SNSPDs). GM-APDs operate in Geiger mode, exceeding their breakdown voltage to achieve extremely high internal gain, enabling detection of single-photon signals. However, conventional GM-APDs suffer from a significant drawback: the "dead time" problem. When a GM-APD detects a single photon, the internal avalanche effect triggers a large number of carriers, and the junction capacitance and readout circuitry undergo a discharge and reset process. During this period, the detector cannot effectively respond to subsequent photons, resulting in a brief detection blind spot known as dead time. The existence of this time window limits the maximum counting rate of the detector, making it very easy for photon missed detection to occur in high-speed photon flow scenarios, which in turn affects the imaging resolution of the lidar and the stability of quantum communication. The current solution is to "compress" multiple signals into a single detector for processing through optical fiber delay. In essence, it is still a single-channel system and there is a risk of accumulated dark counts. In addition, dark counts will increase significantly under high temperature or high bias conditions, further reducing the system's signal-to-noise ratio and ranging accuracy. Therefore, there is an urgent need for a new photon detection system that can completely eliminate dead time and effectively suppress dark counts to meet the application requirements of high-speed dynamic scenes and high-precision lidar. Summary of the Invention

[0003] To address the dead-time limitations and dark-count interference issues associated with GM-APDs in existing lidar systems, this paper proposes a zero-dead-time single-photon counting system based on time-division multiplexing. By leveraging multi-channel dynamic switching technology, this system achieves efficient photon detection without detection time blind spots, while significantly improving the system's signal-to-noise ratio and environmental adaptability.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A zero-dead-time single-photon counting system based on time-division multiplexing (TDDM) is disclosed. The system divides the time axis into multiple independent time slots according to the TDDM mechanism. Within different time slots, the incident signal light is input into different detection channels in a preset order to achieve zero-dead-time single-photon counting.

[0006] Preferably, the system comprises: an optical multiplexer, N detection channels, N delay modules, a shift register, and a counting circuit;

[0007] The shift register generates N output signals based on a synchronous clock pulse, and at any time, only one output signal is at a high level. Each high-level signal output is used for two control channels simultaneously. One channel drives the optical multiplexer to switch the optical path, and the other channel is adjusted by the delay module to control the opening of the corresponding gate switch.

[0008] The optical multiplexer is used to divide the time axis into N independent time slots based on the time division multiplexing mechanism, and input the incident signal light into the corresponding detection channel according to the preset time sequence;

[0009] The detection channel includes a GM-APD detector, a gate switch, and a pulse discriminator; wherein the gate switch is used to control the operating state of the GM-APD detector, so that the GM-APD detector is in an operating state only during the allocated time slot and is completely closed at other times to prepare for the next activation time slot; the GM-APD detector is used to detect the photon signal of the incident signal light and transmit it to the pulse discriminator; the pulse discriminator uses multi-parameter pulse discrimination technology to distinguish the input photon signal into a true photon signal and a noise signal based on the time domain characteristics of the avalanche pulse, and transmits the true photon signal to the counting circuit;

[0010] The counting circuit receives and counts the real photon signals transmitted by the multiple detection channels to obtain a single photon count value of the incident signal light;

[0011] The delay module configures the delay amount according to the time deviation between the optical path switching of the optical multiplexer and the gate opening of the GM-APD detector, thereby achieving synchronization between the optical path switching and the gate opening.

[0012] Preferably, the number of detection channels N is determined by the following formula:

[0013]

[0014] Among them, t dead is the dead time of a single GM-APD detector; t slot =max(t switch ,t rise )+max(t dwell ,t gm) is the time slot period occupied by a single detection channel; t switch is the optical multiplexer switching transient time, t rise is the transient time of the detector opening. In the optimal case, t switch =t rise , that is, the optical path and the detector start and complete the state switching at the same time; t dwell is the time window during which the optical path is open so that the incident photon resides on the detector, t gm is the time window in which the detector is in Geiger mode. In the optimal case, t dwell =t gm , meaning the effective operating time windows of the optical path and detector completely overlap in time, opening and closing synchronously. This relationship ensures that by the end of a detection cycle (N time slots), the GM-APD detector has completed dead-time recovery from the first triggering photon, enabling continuous and efficient detection.

[0015] Preferably, the delay amount t of the delay module delay According to the optical path switching time configuration, set it to t delay =t switch -t rise , in order to compensate for the delay of the incident light signal during transmission and the device response process, and ensure the precise synchronization of the optical path switching of the optical multiplexer and the gating opening of the GM-APD detector.

[0016] The present invention adopts a time-division multiplexing mechanism to achieve efficient detection of single photons through the coordinated work of an optical multiplexer, a GM-APD detector array, a gate switch group, a shift register, and a pulse discriminator. In terms of system composition, the optical multiplexer serves as the core of optical path distribution, and divides the time axis into multiple independent time slots based on the time-division multiplexing mechanism. In different time slots, the optical multiplexer distributes the incident signal light to the photosensitive surface of each GM-APD detector according to a predetermined timing sequence, and at the same time triggers the corresponding gate switch to turn on, so that the corresponding detector enters the working state. Each GM-APD detector is equipped with an independent gate switch, and its working state is precisely controlled by the parallel output signal of the shift register. The pulse discriminator adopts multi-parameter pulse discrimination technology to achieve effective distinction between real photon signals and noise signals by analyzing the time domain characteristics of the avalanche pulse.

[0017] The core of the system is achieving strict synchronization between optical path switching and detector gate switching. A shift register generates two output signals based on a clock pulse: one directly drives the optical multiplexer to switch the optical paths; the other, adjusted by a delay module, precisely controls the activation of the corresponding GM-APD detector gate switch. To ensure precise synchronization, a specially designed delay module configures the delay based on the optical multiplexer's response delay and the time deviation caused by optical path switching, thereby achieving synchronization between optical path switching and gate bias activation.

[0018] During operation, the system periodically switches and activates each detection channel according to a preset timing sequence. The length of each detection channel's activation time slot is determined by the overlap between the optical multiplexer's conduction time in that channel and the opening time of the channel's gate switch. This design ensures that each incident signal photon reaches the detector's photosensitive surface in a specific channel with maximum optical efficiency, is efficiently converted into a macroscopic electrical pulse, and quickly suppresses any possible afterpulses after the optical multiplexer closes the optical path to that detector. Each GM-APD detector is active only during its assigned time slot and is completely shut down the rest of the time to prepare for the next activation time slot. This operating mode offers multiple advantages: First, only one channel is active at any given time, avoiding possible thermal noise afterpulse blockage caused by simultaneous activation of multiple channels; second, when the current channel enters its dead time, the system quickly switches to the next channel, minimizing the dead time for a single incident photon (ideally, close to zero); and third, incident photons only enter and are received by the detector in the active channel, maximizing the probability of detection.

[0019] The beneficial effects of the present invention are:

[0020] It not only solves the dead time problem of traditional single-photon detectors, but also effectively distinguishes photon counting pulses from dark current pulses through multi-parameter pulse discrimination technology, significantly improving the system's signal-to-noise ratio and providing a reliable solution for application scenarios such as lidar and quantum communication that require high-sensitivity detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of a zero-dead-time single-photon counting system based on time-division multiplexing.

[0022] Figure 2 This is a timing diagram of the optical path switching and detector gating for multi-channel time division multiplexing. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and implementation examples. It should be understood that the specific examples described herein are only used to explain the present invention and are not intended to limit the present invention. For those skilled in the art, other specific embodiments can be easily proposed when adopting the core technical ideas of the present invention.

[0024] See also Figure 1 As shown, a zero dead time single photon counting system based on time division multiplexing of this embodiment includes: an optical multiplexer, four detection channels, four delay modules, a shift register, and a counting circuit;

[0025] The number of detection channels N is determined by the following formula:

[0026]

[0027] Among them, t dead is the dead time of a single GM-APD detector; t slot =max(t switch ,t rise )+max(t dwell ,t gm ) is the time slot period occupied by a single detection channel; t switch is the optical multiplexer switching transient time, t rise is the transient time t when the detector opens the door dwell is the time window during which the optical path is open so that the incident photon resides on the detector, t gm is the time window during which the detector is in Geiger mode. This relationship ensures that at the end of a detection cycle (4 time slots), the GM-APD detector of the first triggering photon has completed dead time recovery, thereby achieving continuous and efficient detection. In this embodiment, the GM-APD detector dead time t dead The optical multiplexer switching transient time t is 80ns. switch The transient time t of the detector opening is 25ns. rise The time window t during which the light path is opened so that the incident photon stays on the detector is 10 ns. dwell and the time window t during which the detector is in Geiger mode gm , therefore, N is 4.

[0028] The shift register generates four output signals based on a synchronous clock pulse, and at any time, only one output signal is at a high level. Each output signal is further divided into two paths, one of which drives the optical multiplexer to switch the optical path, and the other is adjusted by the delay module to control the opening of the corresponding gate switch.

[0029] The optical multiplexer is used to divide the time axis into four independent time slots based on a time-division multiplexing mechanism, and input the incident signal light into the corresponding detection channels according to a preset timing sequence. Specifically, the incident signal light is connected to the input of the optical multiplexer via optical fiber, and the output of the optical multiplexer is connected to the photosensitive surfaces of the four GM-APD detectors via optical fiber, thereby achieving optical path distribution.

[0030] The detection channel includes a GM-APD detector, a gate switch, and a pulse discriminator. The negative electrode of the GM-APD detector is connected to a bias voltage via a gate switch, while the positive electrode is grounded via a load resistor. A signal is also drawn out to connect to the pulse discriminator. The gate switch is used to control the operating state of the GM-APD detector, ensuring that it operates only during the allocated time slot and is completely shut down at all other times to prepare for the next activation time slot. The GM-APD detector is used to detect the photon signal of the incident signal light and transmit it to the pulse discriminator. The pulse discriminator uses multi-parameter pulse discrimination technology to distinguish the input photon signal from a noise signal based on the time domain characteristics of the avalanche pulse, and transmits the real photon signal to the counting circuit.

[0031] The counting circuit receives the real photon signals transmitted by the multiple detection channels and performs counting processing, and finally obtains the single photon counting value of the incident signal light.

[0032] The delay module configures the delay amount according to the time deviation between the switching transient time of the optical multiplexer and the detector door opening transient time, thereby achieving strict synchronization in time between the optical path switching and the opening of the gate switch.

[0033] In terms of specific device selection, the optical multiplexer uses an electro-optical deflection modulator, characterized by fast response speed and high switching precision, capable of meeting the time-division multiplexing mechanism's demand for rapid optical path switching. The gate switch uses an NMOS transistor, whose low on-resistance and high switching speed enable efficient control of the GM-APD detector's operating state. The pulse discriminator uses a constant-ratio timing discriminator. With its high-precision threshold setting, fast response speed, and excellent anti-interference capabilities, it can accurately discriminate the GM-APD detector's output signal, effectively distinguishing true photon signals from noise signals, and providing the system with reliable single-photon counting data.

[0034] All four detection channels have the same connection configuration. Taking the jth channel (j∈[1,4]) as an example, its specific connection is: NMOS tube N j The drain of the high voltage power supply V H connected to provide bias voltage for GM-APD detector; NMOS tube N j The source of the GM-APD is connected to the cathode of the GM-APD detector j; the anode of the GM-APD detector j is connected to the resistor Rx on one hand, and the other end of the resistor Rx is grounded for passive quenching of the GM-APD, and is connected to the pulse discriminator on the other hand; the NMOS tube N jThe gate of the shift register is connected to the j-th output pin Q1 of the shift register. In addition, the shift register is composed of four shift register units connected in series. The serial output pin QTR of the shift register unit No. 1 is connected to the serial input pin SER of the shift register unit No. 2. The serial output pin QTR of the shift register unit No. 2 is connected to the serial input pin SER of the shift register unit of the next stage. Similarly, the serial output pin QTR of the shift register unit No. 3 is connected to the serial input pin SER of the shift register unit No. 4. The serial output pin QTR of the shift register unit No. 4 is fed back to the serial input pin SER of the shift register unit No. 1, forming a ring connection structure to ensure that the four shift register units can cyclically output high-level signals according to a preset timing.

[0035] The workflow of the multi-detector zero dead time single photon counting system based on time division multiplexing of the present invention is as follows:

[0036] When the system starts, it first completes the comprehensive initialization configuration. In this stage, the activation time slot length t of each channel is preset. slot =max(t switch ,t rise )+max(t dwell ,t gm ), which is the time slot period occupied by a single channel. Figure 2 As shown, t slot It includes the time for a single channel to establish the state and the effective door opening state time. The time for establishing the state is determined by the transient time t switch and the transient time t of the detector opening rise In the best case, t switch =t rise , that is, the light path and the detector state are established and completed synchronously. The effective door open state time is the time window t in which the light path is opened so that the incident photon stays on the detector. dwell and the time window t during which the detector is in Geiger mode gm The larger value of dwell =t gm At the same time, the delay module's delay t delay It will be configured according to the optical path switching time, usually set to t delay =t switch -t rise , in order to compensate for the delay in signal transmission and device response, and ensure the precise synchronization between the optical path switching of the optical multiplexer and the gating opening of the GM-APD detector.

[0037] After initialization, the shift register is reset, activating the first detection channel by default. The electro-optical deflection modulator (EODM) initially points to the first detection channel, ensuring that incoming photons precisely strike the GM-APD photosensor in the corresponding channel. Simultaneously, after timing calibration by the delay module, the calibrated signal activates the GM-APD detector in the first channel at the instant the EODM completes optical path switching. At this point, when an incoming photon reaches the currently activated GM-APD detector, it is captured by the detector's photosensor and rapidly converted into an avalanche pulse signal. This avalanche pulse signal is then transmitted to a pulse discriminator, where multi-parameter analysis effectively distinguishes true photon signals from noise. A pulse discriminator typically consists of a signal conditioning circuit, a threshold comparator, and an output driver circuit. Its operating principle is based on the difference in time-domain characteristics between real photon signals and noise signals. When the GM-APD detector's output signal enters the discriminator, the signal conditioning circuit first amplifies, shapes, and filters it to enhance the useful signal and suppress some noise. A threshold comparison unit then compares the processed signal with a preset threshold and analyzes characteristic parameters such as rise time and pulse width. Only when the signal meets both the amplitude threshold and characteristic parameter ranges does the output driver circuit generate a standard logic pulse, marking it as a valid photon event. Otherwise, it is treated as noise and rejected. At the end of the time slot in a detection channel, the system automatically switches to the next detection channel to continue single-photon detection. The entire workflow loops according to a preset timing sequence. After completing detection on the last channel, the system restarts the detection cycle on the first channel, achieving seamless, continuous single-photon detection. This design achieves zero-dead-time detection, significantly improving single-photon detection efficiency and system performance, providing solid and reliable technical support for high-precision lidar and quantum communication applications.

[0038] Although the above embodiments describe the present invention, the present invention is not limited to the above specific implementation methods. The above specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many variations without departing from the purpose of the present invention, and these are all protected by the present invention.

Claims

1. A zero dead time single photon counting system based on time division multiplexing, characterized in that: The system divides the time axis into multiple independent time slots based on the time division multiplexing mechanism. In different time slots, the incident signal light is input into different detection channels according to a preset timing, achieving zero dead time single photon counting.

2. A zero dead time single photon counting system based on time division multiplexing according to claim 1, characterized in that: The system includes: an optical multiplexer, N detection channels, N delay modules, a shift register, and a counting circuit; The shift register generates N output signals based on a synchronous clock pulse, and at any time, only one output signal is at a high level. Each high-level signal output is used for two control channels simultaneously. One channel drives the optical multiplexer to switch the optical path, and the other channel is adjusted by the delay module to control the opening of the corresponding gate switch. The optical multiplexer is used to divide the time axis into N independent time slots based on the time division multiplexing mechanism, and input the incident signal light into the corresponding detection channel according to the preset time sequence; The detection channel includes a GM-APD detector, a gate switch, and a pulse discriminator; wherein the gate switch is used to control the working state of the GM-APD detector, so that the GM-APD detector is in an operating state only during the allocated time slot and is completely closed at other times to prepare for the next activation time slot; the GM-APD detector is used to detect the photon signal of the incident signal light and transmit it to the pulse discriminator; the pulse discriminator is used to distinguish the input photon signal into a real photon signal or a noise signal, and transmit the real photon signal to the counting circuit; The counting circuit receives and counts the real photon signals transmitted by the multiple detection channels to obtain a single photon count value of the incident signal light; The delay module configures the delay amount according to the time deviation between the optical path switching of the optical multiplexer and the gate opening of the GM-APD detector, thereby achieving synchronization between the optical path switching and the gate opening.

3. The zero dead time single photon counting system based on time division multiplexing according to claim 2, characterized in that: The number of detection channels N is determined by the following formula: Among them, t dead is the dead time of a single GM-APD detector; t slot =max(t switch ,t rise )+max(t dwell ,t gm ) is the time slot period occupied by a single detection channel; t switch is the optical multiplexer switching transient time, t rise is the transient time of the detector opening; t dwell is the time window during which the optical path is open so that the incident photon resides on the detector, t gm is the time window that the detector is in Geiger mode.

4. A zero dead time single photon counting system based on time division multiplexing as claimed in claim 3, characterized in that: The delay amount t of the delay module delay According to the optical path switching time configuration, set it to t delay =t switch -t rise , in order to compensate for the delay of the incident light signal during transmission and the device response process, and ensure the precise synchronization of the optical path switching of the optical multiplexer and the gating opening of the GM-APD detector.

5. The zero dead time single photon counting system based on time division multiplexing according to claim 4, characterized in that: The pulse discriminator adopts a multi-parameter pulse discrimination technology to distinguish the input photon signal into a real photon signal or a noise signal based on the time domain characteristics of the avalanche pulse.

Images