Frequency adaptive single photon detection system

Abstract

The invention provides a frequency adaptive single photon detection system, and relates to the technical field of weak light detection. The frequency self-adaptive single photon detection system comprises a frequency self-adaptive generation circuit which is used for providing gating frequency required by an avalanche diode and comprises a first phase-locked loop unit and a second phase-locked loop unit; the ultra-narrow-band interference circuit is used for extracting an avalanche signal generated by an avalanche diode and adaptively adjusting the center frequency so as to suppress capacitance response caused by a gating signal in the avalanche signal, and the ultra-narrow-band interference circuit comprises a variable capacitance diode used for changing the center frequency of the ultra-narrow-band interference circuit according to the frequency of the gating signal; and the voltage-controlled attenuator is used for balancing the attenuation change of the variable capacitance diode so as to maintain the suppression effect of the ultra-narrow-band interference circuit. Based on an external reference signal, a phase-locked loop technology is used to generate a gating frequency required by an avalanche diode, and the center frequency of the ultra-narrow-band interference circuit is adaptively adjusted to be matched with the gating frequency, so that the ultra-narrow-band interference circuit is kept in an optimal working state.

Description

Technical Field

[0001] The present application relates to the technical field of weak light detection, and in particular to a frequency adaptive single photon detection system. Background Art

[0002] In recent years, due to the need for weak light sensing and detection in multiple fields such as quantum communication, space communication, laser radar, biological imaging and national defense construction, single-photon detectors based on semiconductor InGaAs / InP avalanche photodiodes (APDs) have played an important role. In particular, with the rapid development of quantum information, APD single-photon detectors have become an indispensable module in practical quantum communication networks because of their small size, low cost, low power consumption, and no need for ultra-low temperature refrigeration.

[0003] The APD single-photon detector module based on an extremely narrow-band interference circuit has a high degree of integration and is only 8.8×6×2 cm in size. 3 Although the size of the APD is small, it still faces the problem of requiring an external signal generator to generate a 1.25 GHz gating signal for the APD single-photon detector. This greatly increases the cost of the APD single-photon detector and limits its use scenarios.

[0004] Moreover, in high-speed quantum communication networks, APD single-photon detectors usually use gating signals to achieve fast avalanche and quenching effects, improving detection efficiency and counting rate. However, due to the parasitic capacitance of the avalanche photodiode and the circuit, the capacitive response of the gating signal will mask the avalanche signal caused by the photon. Therefore, it is crucial to effectively eliminate the capacitive response and extract the post-processing circuit of the weak avalanche signal.

[0005] In the prior art, APD single photon detectors generally use post-processing circuits to eliminate capacitance response and extract weak avalanche signals. The post-processing circuits include extremely narrow-band interference circuit technology with fixed delay and extremely narrow-band interference circuit technology with adjustable frequency.

[0006] The existing fixed-delay ultra-narrowband interference circuit technology can effectively eliminate the fundamental frequency and high-order harmonic frequency of the gate signal capacitance response at the APD output end, and is not affected by the working state of the APD device itself. It can stably provide a wide and continuous frequency domain passband to process avalanche narrow pulse signals, and has little effect on the signal-to-noise ratio, pulse waveform and jitter of avalanche narrow pulse signals. It has a significant optimization effect on the detection efficiency, post-pulse and time resolution of single-photon detectors. However, the center frequency of the fixed-delay ultra-narrowband interference circuit technology is not adjustable, and its use is limited.

[0007] The existing frequency-adjustable ultra-narrowband interference circuit technology can passively adjust the center frequency within a certain frequency range, but cannot automatically adjust according to the input external frequency, which affects the suppression effect. When there are frequency differences between different external microwave signal sources, the ultra-narrowband interference circuit cannot work in the best state, reducing the suppression effect on the gate signal and affecting the performance of the entire APD single-photon detector. Summary of the invention

[0008] In order to solve at least one of the above problems, the present application proposes a frequency adaptive single photon detection system.

[0009] According to the first aspect of the present application, at least one embodiment of the present application provides a frequency adaptive single photon detection system, including: a frequency adaptive generation circuit, connected to an avalanche diode, for providing a gating frequency required by the avalanche diode, including: a first phase-locked loop unit, for receiving an external reference signal, and generating a first operating frequency according to the frequency of the external reference signal; a second phase-locked loop unit, connected to the first phase-locked loop unit, for correcting the first operating frequency to generate the gating frequency; an extremely narrowband interference circuit, connected to the avalanche diode, for extracting the avalanche signal generated by the avalanche diode, and adaptively adjusting the center frequency according to the frequency of the gating signal to suppress the capacitive response caused by the gating signal in the avalanche signal, the extremely narrowband interference circuit including: a varactor diode, for changing the center frequency of the extremely narrowband interference circuit according to the frequency of the gating signal; a voltage-controlled attenuator, for balancing the attenuation change of the varactor diode to maintain the suppression effect of the extremely narrowband interference circuit.

[0010] For example, in some embodiments of the present application, the first phase-locked loop unit includes: a first voltage-controlled oscillator, used to generate a first output signal according to the frequency of the external reference signal; a first level conversion circuit, connected to the first voltage-controlled oscillator, used to receive the first output signal, and generate a first feedback signal according to the frequency difference between the first output signal and the external reference signal; a first phase-locked loop, connected to the first level conversion circuit, used to receive the first feedback signal, and generate a first error signal according to the phase difference between the external reference signal and the first feedback signal; a first low-pass filter, connected to the first phase-locked loop and the first voltage-controlled oscillator respectively, used to filter out noise in the first error signal, generate a first error voltage, and output it to the first voltage-controlled oscillator; wherein the first voltage-controlled oscillator is also used to adjust the frequency of the first output signal according to the first error voltage to generate the first operating frequency.

[0011] For example, in some embodiments of the present application, when the frequency difference between the first output signal and the external reference signal is constant, the phase difference between the external reference signal and the first feedback signal is zero, and the first phase-locked loop is locked to keep the frequency difference between the frequency of the first output signal of the first voltage-controlled oscillator and the frequency of the external reference signal constant, thereby generating the first operating frequency.

[0012] For example, in some embodiments of the present application, the first phase-locked loop unit further includes: a first balun, connected between the first phase-locked loop and the first level conversion circuit, and used to transmit the first feedback signal to the first phase-locked loop.

[0013] For example, in some embodiments of the present application, the second phase-locked loop unit includes: a second voltage-controlled oscillator, used to receive the first operating frequency and generate a second output signal; a second level conversion circuit, connected to the second voltage-controlled oscillator, used to receive the second output signal, and generate a second feedback signal according to the difference between the frequency of the second output signal and the first operating frequency; a second phase-locked loop, connected to the second level conversion circuit, used to receive the second feedback signal, and generate a second error signal according to the phase difference between the first operating frequency and the second feedback signal; a second low-pass filter, connected to the second phase-locked loop and the second voltage-controlled oscillator respectively, used to filter out noise in the second error signal, generate a second error voltage, and output it to the second voltage-controlled oscillator; wherein the second voltage-controlled oscillator is also used to adjust the frequency of the second output signal according to the second error voltage to generate a second operating frequency, i.e., the gating frequency.

[0014] For example, in some embodiments of the present application, when the difference between the frequency of the second output signal and the first operating frequency is constant, the phase difference between the first operating frequency and the second feedback signal is zero, and the second phase-locked loop is locked to keep the frequency difference between the frequency of the second output signal of the second voltage-controlled oscillator and the first operating frequency constant, thereby generating the second operating frequency.

[0015] For example, in some embodiments of the present application, the second phase-locked loop unit further includes: a second balun connected between the second phase-locked loop and the second level conversion circuit, and configured to transmit the second feedback signal to the second phase-locked loop.

[0016] For example, in some embodiments of the present application, it also includes: a first single-pole double-throw switch, wherein the input end of the first single-pole double-throw switch is used to receive the external reference signal, and the first output end is connected to the first phase-locked loop unit; a transmission unit, connected to the second output end of the first single-pole double-throw switch, and used to transmit the external reference signal; a second single-pole double-throw switch, wherein the first input end of the second single-pole double-throw switch is connected to the second phase-locked loop unit, and the second input end is connected to the transmission unit.

[0017] For example, in some embodiments of the present application, the first single-pole double-throw switch and the second single-pole double-throw switch are configured as follows: when the external reference signal is less than the required gating frequency of the avalanche diode, the first output end of the first single-pole double-throw switch is connected to the first phase-locked loop unit, and the second output end is disconnected from the transmission unit, and the first input end of the second single-pole double-throw switch is connected to the second phase-locked loop unit, and the second input end is disconnected from the transmission unit; when the external reference signal is equal to the required gating frequency of the avalanche diode, the second output end of the first single-pole double-throw switch is connected to the transmission unit, and the first output end is disconnected from the first phase-locked loop unit, and the second input end of the second single-pole double-throw switch is connected to the transmission unit, and the first input end is disconnected from the second phase-locked loop unit; when the frequency adaptive single-photon detection system is powered off, the first single-pole double-throw switch is disconnected from the first phase-locked loop unit and the transmission unit, and the second single-pole double-throw switch is disconnected from the second phase-locked loop unit and the transmission unit.

[0018] For example, in some embodiments of the present application, the transmission unit includes: a first delay line, connected to the second output end of the first single-pole double-throw switch to receive the external reference signal; and a third balun, respectively connected to the first delay line and the second input end of the second single-pole double-throw switch to transmit the external reference signal.

[0019] For example, in some embodiments of the present application, it also includes: a fourth balun, connected between the first phase-locked loop unit and the second phase-locked loop unit, for transmitting the first operating frequency to the second phase-locked loop unit; a second delay line, connected to the second phase-locked loop unit; a fifth balun, connected to the second delay line and the first input end of the second single-pole double-throw switch, for transmitting the gating frequency.

[0020] For example, in some embodiments of the present application, it also includes: an amplifier, connected to the second single-pole double-throw switch, for amplifying the external reference signal or the second operating frequency; a filter, connected to the amplifier, for filtering out frequencies other than the amplified external reference signal or the second operating frequency to obtain a specific frequency signal.

[0021] For example, in some embodiments of the present application, it also includes: a programmable logic unit, connected to the second phase-locked loop unit, and used to dynamically compile the second phase-locked loop unit to modify the second output frequency.

[0022] Through the above-mentioned example embodiments, a frequency adaptive single-photon detection system provided by the present application uses phase-locked loop technology to generate the gating signal frequency required by the APD single-photon detector, that is, the gating frequency provided in the previous text or the external reference signal. After being input into the avalanche diode APD, an avalanche signal is generated, and the extremely narrowband interference circuit compares the frequency of the avalanche signal with the center frequency of the extremely narrowband interference circuit, and dynamically adjusts the control voltage on the varactor diode in the extremely narrowband interference circuit to adjust the center frequency of the extremely narrowband interference circuit, so that the center frequency of the extremely narrowband interference circuit is aligned with the frequency of the gating signal in the avalanche signal, so that the extremely narrowband interference circuit is always maintained in the optimal working state, effectively eliminating the capacitive response caused by the gating signal, and improving the performance of the APD single-photon detector.

[0023] It should be understood that the foregoing general description and the following detailed description are exemplary only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] By describing in detail exemplary embodiments thereof with reference to the accompanying drawings, the above and other objects, features and advantages of the present application will become more apparent. The accompanying drawings described below are only some embodiments of the present application, and are not intended to limit the present application.

[0025] Figure 1 A schematic diagram of a frequency adaptive single photon detection system is shown according to an exemplary embodiment;

[0026] Figure 2a A waveform diagram showing an input reference signal and an output signal of a frequency adaptive generation circuit according to an exemplary embodiment;

[0027] Figure 2b Another waveform diagram showing an exemplary input reference signal and an output signal of a frequency adaptive generation circuit;

[0028] Figure 3 A schematic diagram of an extremely narrow band interference circuit according to an exemplary embodiment is shown. DETAILED DESCRIPTION

[0029] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this application will be comprehensive and complete and will fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar parts, and thus their repeated description will be omitted.

[0030] The described features, structures or characteristics may be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the technical solutions of the present disclosure may be practiced without one or more of these specific details, or other modes, components, materials, devices, etc. may be adopted. In these cases, known structures, methods, devices, implementations, materials or operations will not be shown or described in detail.

[0031] The flowcharts shown in the accompanying drawings are only exemplary and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps can be decomposed, and some operations / steps can be combined or partially combined, so the actual execution order may change according to actual conditions.

[0032] The terms "first", "second", etc. in the specification and claims of this application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units that are not listed, or optionally includes other steps or units inherent to these processes, methods, products or devices.

[0033] Those skilled in the art will appreciate that the drawings are merely schematic diagrams of example embodiments, and the modules or processes in the drawings are not necessarily necessary for implementing the present application, and therefore cannot be used to limit the scope of protection of the present application.

[0034] Figure 1 A schematic diagram of a frequency adaptive single photon detection system is shown according to an exemplary embodiment.

[0035] like Figure 1As shown, the frequency adaptive single photon detection system includes: a frequency adaptive generation circuit 10, an avalanche diode 20 and an extremely narrow band interference circuit 30. The frequency adaptive generation circuit 10 is connected to the avalanche diode 20 to provide the gating frequency required by the avalanche diode. The extremely narrow band interference circuit 30 is connected to the avalanche diode 20 to extract the avalanche signal generated by the avalanche diode 20, and according to the frequency of the gating signal in the avalanche signal, the center frequency is adaptively adjusted to suppress the capacitive response caused by the gating signal.

[0036] like Figure 1 As shown, the frequency adaptive generation circuit 10 includes: a first phase-locked loop unit 101 and a second phase-locked loop unit 102 .

[0037] The first phase-locked loop unit 101 is used to receive an external reference signal and generate a first operating frequency of the ultra-narrowband interference circuit 101 according to the external reference signal. The second phase-locked loop unit 102 is connected to the first phase-locked loop unit 101 and is used to modify the first operating frequency to generate a second operating frequency, namely, a gated frequency.

[0038] According to an example embodiment, the first phase-locked loop unit 101 includes a first phase-locked loop 1011 , a first low-pass filter 1012 , a first voltage-controlled oscillator 1013 , a first level conversion circuit 1014 , and a first balun 1015 .

[0039] The first phase-locked loop 1011 is used to receive an external reference signal CLK In. The first voltage-controlled oscillator 1013 is used to generate a first output signal according to the frequency of the external reference signal. The first level conversion circuit 1014 is connected to the first voltage-controlled oscillator 1013, and is used to receive the first output signal, and generate a first feedback signal according to the frequency difference between the first output signal and the external reference signal. The first phase-locked loop 1011 is connected to the first level conversion circuit 1014, and is used to receive the first feedback signal, and generate a first error signal according to the phase difference between the external reference signal and the first feedback signal. The first low-pass filter 1012 is connected to the first phase-locked loop 1011 and the first voltage-controlled oscillator 1013, respectively, and is used to filter out high-frequency noise in the first error signal, generate a first error voltage, and output it to the first voltage-controlled oscillator 1013. The first voltage-controlled oscillator 1013 adjusts the frequency of the first output signal according to the first error voltage to generate a first operating frequency. The first balun 1015 is connected between the first phase-locked loop 1011 and the first level conversion circuit 1014 , and is used to transmit the first feedback signal to the first phase-locked loop 1011 .

[0040] According to an example embodiment, when the frequency difference between the first output signal and the external reference signal is constant, the phase difference between the external reference signal and the first feedback signal is zero, and the first phase-locked loop 1011 is locked to keep the frequency difference between the frequency of the first output signal of the first voltage-controlled oscillator 1012 and the frequency of the external reference signal constant, thereby generating a first operating frequency.

[0041] When the external reference signal enters the frequency adaptive generation circuit, initially, the phase-locked loop is in an unlocked state, and the frequency of the output signal generated by the voltage-controlled oscillator is different from the frequency of the external reference signal. Therefore, a feedback signal is generated to the phase-locked loop; the phase-locked loop compares the phase difference between the external reference signal and the feedback signal to generate an error signal, which is filtered out by a low-pass filter to remove high-frequency noise and generate an error voltage; the error voltage adjusts the output frequency through the voltage-controlled oscillator. As long as there is an error signal, the error voltage will change until the output frequency of the voltage-controlled oscillator gradually approaches the frequency of the input signal. At this time, the phase difference tends to zero, the error signal also tends to zero, the phase-locked loop is in a locked state, and the frequency difference between the output frequency of the voltage-controlled oscillator and the frequency of the input signal maintains a stable phase relationship. Once locked, the phase-locked loop will continue to adjust the frequency of the voltage-controlled oscillator to maintain a constant phase difference between the external reference signal and the feedback signal, and can quickly follow even if the frequency of the external reference signal changes.

[0042] According to an example embodiment, the second phase-locked loop unit 102 includes a second phase-locked loop 1021 , a second low-pass filter 1022 , a second voltage-controlled oscillator 1023 , a second level conversion circuit 1024 , and a second balun 1025 .

[0043] The second phase-locked loop 1021 is used to receive the first operating frequency. The second voltage-controlled oscillator 1023 is used to generate a second output signal according to the first operating frequency. The second level conversion circuit 1024 is connected to the second voltage-controlled oscillator 1023, and is used to receive the second output signal, and generate a second feedback signal according to the difference between the frequency of the second output signal and the first operating frequency. The second phase-locked loop 1021 is connected to the second level conversion circuit 1024, and is used to receive the second feedback signal, and generate a second error signal according to the phase difference between the first operating frequency and the second feedback signal. The second low-pass filter 1022 is connected to the second phase-locked loop 1021 and the second voltage-controlled oscillator 1023, respectively, and is used to filter out high-frequency noise in the second error signal, generate a second error voltage, and output it to the second voltage-controlled oscillator 1023. The second voltage-controlled oscillator 1023 adjusts the frequency of the second output signal according to the second error voltage to generate a second operating frequency, that is, a gated frequency. The second balun 1025 is connected between the second phase-locked loop 1021 and the second level conversion circuit 1024 , and is used for transmitting the second feedback signal to the second phase-locked loop 1021 .

[0044] According to an example embodiment, when the difference between the frequency of the second output signal and the first operating frequency is constant, the phase difference between the first operating frequency and the second feedback signal is zero, and the second phase-locked loop 1021 is locked to keep the frequency difference between the frequency of the second output signal of the second voltage-controlled oscillator 1022 and the first operating frequency constant, thereby generating a second operating frequency, i.e., the gated frequency RF Out.

[0045] The present application adopts a two-stage phase-locked loop cascade method, and generates the gate frequency required by the avalanche diode 20 by locking step by step. In order to make the generated frequency more stable, a voltage-controlled crystal oscillator (VCXO) can be used in the first phase-locked loop unit. The voltage-controlled crystal oscillator has a narrower tuning range and can be used in narrower loop bandwidth applications. The narrower loop bandwidth is beneficial to purifying the noise of the reference source. The voltage only needs to be fine-tuned to correct the deviation, and can be used as the reference frequency of the second-stage phase-locked loop; at the same time, the voltage-controlled oscillator has a lower broadband noise floor and can be changed over a wide range.

[0046] Figure 2a The waveform diagram of the input reference signal and the output signal of the frequency adaptive generation circuit of an exemplary embodiment is shown. After the frequency adaptive generation circuit is powered on, it generates an initially set gated frequency signal, and the initial phase difference between the input reference signal and the gated signal is constant and can be further finely adjusted.

[0047] For example, Figure 2a A waveform diagram showing that the phase difference between the input reference signal and the gate signal maintains a first phase difference; Figure 2b A waveform diagram showing that the phase difference between the input reference signal and the gating signal maintains a second phase difference.

[0048] According to an example embodiment, the frequency adaptive generation circuit further includes: a first single-pole double-throw switch 104 , a transmission unit 105 , and a second single-pole double-throw switch 106 .

[0049] The input end of the first single-pole double-throw switch 104 is used to receive an external reference signal, and the first output end is connected to the first phase-locked loop unit 101. The transmission unit 105 is connected to the second output end of the first single-pole double-throw switch 104 for transmitting the external reference signal. The first input end of the second single-pole double-throw switch 106 is connected to the second phase-locked loop unit 102, and the second input end is connected to the transmission unit 105.

[0050] According to an example embodiment, the first single-pole double-throw switch 104 and the second single-pole double-throw switch 106 are configured as:

[0051] When the external reference signal is less than the required gating frequency of the avalanche diode 20, the first output terminal of the first single-pole double-throw switch 104 is connected to the first phase-locked loop unit 101, and the second output terminal is disconnected from the transmission unit 105. The first input terminal of the second single-pole double-throw switch 106 is connected to the second phase-locked loop unit 102, and the second input terminal is disconnected from the transmission unit 105;

[0052] When the external reference signal is equal to the required gating frequency of the avalanche diode 20, the second output terminal of the first single-pole double-throw switch 104 is connected to the transmission unit 105, the first output terminal is disconnected from the first phase-locked loop unit 101, and the second input terminal of the second single-pole double-throw switch 106 is connected to the transmission unit 105, and the first input terminal is disconnected from the second phase-locked loop unit 102;

[0053] When the frequency adaptive generating circuit 10 is powered off, the first SPDT switch 104 is disconnected from the first PLL unit 101 and the transmission unit 105 , and the second SPDT switch 106 is disconnected from the second PLL unit 102 and the transmission unit 105 .

[0054] According to some embodiments, the required gating frequency of the avalanche diode 20 can be set by itself, such as 1.25 GHz, but the present application is not limited thereto.

[0055] According to an example embodiment, the transmission unit 105 includes a first delay line 1051 and a third balun 1052 .

[0056] The first delay line 1051 is connected to the second output terminal of the first SPDT switch 104 to receive an external reference signal. The third balun 1052 is connected to the first delay line 1051 and the second input terminal of the second SPDT switch 106 to transmit an external reference signal.

[0057] According to an example embodiment, the frequency adaptive generation circuit 10 further includes a fourth balun 1071 , a second delay line 1072 , a fifth balun 1073 , an amplifier 1081 , a filter 1082 , and a programmable logic unit 109 .

[0058] The fourth balun 1071 is connected between the first phase-locked loop unit 101 and the second phase-locked loop unit 102, and is used to transmit the first operating frequency to the second phase-locked loop unit. The second delay line 1072 is connected to the second phase-locked loop unit 102. The fifth balun 1073 is connected to the second delay line 1072 and the first input end of the second single-pole double-throw switch 106, respectively, for transmitting the gated frequency.

[0059] The amplifier 1081 is connected to the second SPDT switch 106 and is used to amplify the external reference signal or the second operating frequency. The filter 1082 is connected to the amplifier 1081 and is used to filter out the amplified external reference signal or the frequency other than the second operating frequency to obtain a signal of a specific frequency.

[0060] The programmable logic unit 109 is connected to the second phase-locked loop unit 102 and is used to dynamically compile the second phase-locked loop unit 102 to modify the second output frequency.

[0061] Single photon detectors used in the communication band of 1550nm are mainly based on semiconductor InGaAs / InP materials. In order to suppress the post-pulse noise of APD single photon detectors based on semiconductor InGaAs / InP and improve the photon counting rate of single photon detection devices, gate drive is usually used for avalanche photodiodes to reduce avalanche charge and post-avalanche recovery time. However, due to the parasitic capacitance of avalanche photodiodes and circuits, the capacitive response of the gate signal will mask the avalanche signal caused by photons. Therefore, the capacitive response caused by the gate signal must be effectively eliminated.

[0062] Figure 3 A schematic diagram of an extremely narrow-band interference circuit according to an exemplary embodiment is shown.

[0063] like Figure 3 As shown, the ultra-narrowband interference circuit is used to extract the avalanche signal generated by the avalanche diode, and according to the frequency of the gate signal in the avalanche signal, the center frequency is adaptively adjusted to match it, so as to suppress the capacitive response caused by the gate signal in the avalanche signal. The ultra-narrowband interference circuit includes two 9:1 power couplers (CPL) 3011, 3012, a voltage-controlled attenuator (VCA) 302, a surface acoustic wave filter (SAW) 303, a temperature sensor (DTS) 304, a varactor 305, an inductor 3061, a capacitor 3062 and two 50Ω resistors.

[0064] Among them, the working principle of the ultra-narrowband interference circuit adaptively adjusting the center frequency to match the frequency of the avalanche signal (APD rawsignal) includes: the surface acoustic wave filter (SAW) can filter out the fundamental wave in the avalanche signal, and then respond to the interference signal generated under the gate signal through the transmission capacitor, adjust the control voltage of the varactor diode to achieve real-time compensation for the group delay of the surface acoustic wave filter change, and change the center frequency of the ultra-narrowband interference circuit. And by adjusting the voltage-controlled attenuator on the other interferometer arm to balance the attenuation change of the varactor diode, the suppression effect of the ultra-narrowband interference circuit is maintained.

[0065] The present application is based on an external reference signal and uses phase-locked loop technology to generate the gating signal frequency required by the APD single-photon detector, that is, the gating frequency or external reference signal provided in the previous text; after inputting into the avalanche diode APD, an avalanche signal is generated; the extremely narrowband interference circuit compares the frequency of the gating signal in the avalanche signal with the center frequency of the extremely narrowband interference circuit, and dynamically adjusts the control voltage on the varactor diode in the extremely narrowband interference circuit to adjust the center frequency of the extremely narrowband interference circuit, so as to align the center frequency of the extremely narrowband interference circuit with the frequency of the gating signal in the avalanche signal, so that the extremely narrowband interference circuit is always maintained in the optimal working state, effectively eliminating the capacitive response caused by the gating signal, and improving the performance of the APD single-photon detector.

[0066] This technical solution is applicable to the field of single-photon detection. Based on different input reference signal frequencies, it realizes dynamic adjustment of the operating frequency of the APD single-photon detector post-processing circuit - the ultra-narrowband interferometer circuit, so that the ultra-narrowband interferometer circuit is always in the best suppression state for the capacitive response caused by the gating signal in the avalanche signal of the avalanche diode, thereby further improving the overall performance of the APD single-photon detector.

[0067] It should be clearly understood that the present application describes how to form and use specific examples, but the present application is not limited to any details of these examples. On the contrary, based on the teaching of the content disclosed in the present application, these principles can be applied to many other embodiments.

[0068] In addition, it should be noted that the above figures are only schematic illustrations of the processes included in the method according to the exemplary embodiments of the present application, and are not intended to be limiting. It is easy to understand that the processes shown in the above figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be performed synchronously or asynchronously, for example, in multiple modules.

[0069] The exemplary embodiments of the present application are specifically shown and described above. It should be understood that the present application is not limited to the detailed structures, configurations or implementations described herein; on the contrary, the present application is intended to cover various modifications and equivalent configurations included in the spirit and scope of the appended claims.

Claims

1. A frequency adaptive single photon detection system, characterized in that: include: The frequency adaptive generation circuit is connected to the avalanche diode and is used to provide the gate control frequency required by the avalanche diode, including: A first phase-locked loop unit, configured to receive an external reference signal and generate a first operating frequency according to a frequency of the external reference signal; a second phase-locked loop unit, connected to the first phase-locked loop unit, and configured to modify the first operating frequency to generate the gated frequency; An extremely narrowband interference circuit is connected to the avalanche diode, and is used to extract the avalanche signal generated by the avalanche diode, and adaptively adjust the center frequency according to the frequency of the gate signal in the avalanche signal to suppress the capacitive response caused by the gate signal. The extremely narrowband interference circuit includes: a varactor diode, used for changing the center frequency of the extremely narrow band interference circuit according to the frequency of the gating signal; A voltage-controlled attenuator is used to balance the attenuation variation of the varactor diode to maintain the suppression effect of the extremely narrow-band interference circuit.

2. The frequency adaptive single photon detection system according to claim 1, characterized in that: The first phase-locked loop unit comprises: A first voltage-controlled oscillator, configured to generate a first output signal according to the frequency of the external reference signal; a first level conversion circuit, connected to the first voltage-controlled oscillator, configured to receive the first output signal and generate a first feedback signal according to a frequency difference between the first output signal and the external reference signal; a first phase-locked loop, connected to the first level conversion circuit, configured to receive the first feedback signal and generate a first error signal according to a phase difference between the external reference signal and the first feedback signal; a first low-pass filter, connected to the first phase-locked loop and the first voltage-controlled oscillator respectively, for filtering out noise in the first error signal, generating a first error voltage, and outputting the first error voltage to the first voltage-controlled oscillator; The first voltage-controlled oscillator is further used to adjust the frequency of the first output signal according to the first error voltage to generate the first operating frequency.

3. The frequency adaptive single photon detection system according to claim 2, characterized in that: When the frequency difference between the first output signal and the external reference signal is constant, the phase difference between the external reference signal and the first feedback signal is zero, and the first phase-locked loop is locked to keep the frequency difference between the frequency of the first output signal of the first voltage-controlled oscillator and the frequency of the external reference signal constant, thereby generating the first operating frequency.

4. The frequency adaptive single photon detection system according to claim 2, characterized in that: The first phase-locked loop unit also includes: The first balun is connected between the first phase-locked loop and the first level conversion circuit, and is used for transmitting the first feedback signal to the first phase-locked loop.

5. The frequency adaptive single photon detection system according to claim 1, characterized in that: The second phase-locked loop unit comprises: A second voltage-controlled oscillator, configured to receive the first operating frequency and generate a second output signal; a second level conversion circuit, connected to the second voltage-controlled oscillator, configured to receive the second output signal and generate a second feedback signal according to a difference between a frequency of the second output signal and the first operating frequency; a second phase-locked loop, connected to the second level conversion circuit, configured to receive the second feedback signal and generate a second error signal according to a phase difference between the first operating frequency and the second feedback signal; a second low-pass filter, connected to the second phase-locked loop and the second voltage-controlled oscillator respectively, for filtering out noise in the second error signal, generating a second error voltage, and outputting the second error voltage to the second voltage-controlled oscillator; The second voltage-controlled oscillator is further used to adjust the frequency of the second output signal according to the second error voltage to generate a second operating frequency, namely, the gated frequency.

6. The frequency adaptive single photon detection system according to claim 5, characterized in that: When the difference between the frequency of the second output signal and the first operating frequency is constant, the phase difference between the first operating frequency and the second feedback signal is zero, and the second phase-locked loop is locked to keep the frequency difference between the frequency of the second output signal of the second voltage-controlled oscillator and the first operating frequency constant, thereby generating the second operating frequency.

7. The frequency adaptive single photon detection system according to claim 5, characterized in that: The second phase-locked loop unit also includes: The second balun is connected between the second phase-locked loop and the second level conversion circuit, and is used for transmitting the second feedback signal to the second phase-locked loop.

8. The frequency adaptive single photon detection system according to claim 1, characterized in that: Also includes: A first single-pole double-throw switch, wherein an input end of the first single-pole double-throw switch is used to receive the external reference signal, and a first output end is connected to the first phase-locked loop unit; a transmission unit, connected to the second output end of the first single-pole double-throw switch, and configured to transmit the external reference signal; A second single-pole double-throw switch, wherein a first input end of the second single-pole double-throw switch is connected to the second phase-locked loop unit, and a second input end of the second single-pole double-throw switch is connected to the transmission unit.

9. The frequency adaptive single photon detection system according to claim 8, characterized in that: The first single-pole double-throw switch and the second single-pole double-throw switch are configured as follows: In a case where the external reference signal is less than the required gating frequency of the avalanche diode, the first output end of the first single-pole double-throw switch is connected to the first phase-locked loop unit, the second output end is disconnected from the transmission unit, the first input end of the second single-pole double-throw switch is connected to the second phase-locked loop unit, and the second input end is disconnected from the transmission unit; In a case where the external reference signal is equal to the required gating frequency of the avalanche diode, the second output terminal of the first single-pole double-throw switch is connected to the transmission unit, the first output terminal is disconnected from the first phase-locked loop unit, the second input terminal of the second single-pole double-throw switch is connected to the transmission unit, and the first input terminal is disconnected from the second phase-locked loop unit; When the frequency adaptive single-photon detection system is powered off, the first single-pole double-throw switch is disconnected from the first phase-locked loop unit and the transmission unit, and the second single-pole double-throw switch is disconnected from the second phase-locked loop unit and the transmission unit.

10. The frequency adaptive single photon detection system according to claim 8, characterized in that: The transmission unit comprises: a first delay line connected to the second output terminal of the first single-pole double-throw switch to receive the external reference signal; A third balun is connected to the first delay line and the second input end of the second single-pole double-throw switch respectively to transmit the external reference signal.

11. The frequency adaptive single photon detection system according to claim 8, characterized in that: Also includes: a fourth balun, connected between the first phase-locked loop unit and the second phase-locked loop unit, and configured to transmit the first operating frequency to the second phase-locked loop unit; A second delay line connected to the second phase-locked loop unit; A fifth balun is connected to the second delay line and the first input terminal of the second single-pole double-throw switch, and is used for transmitting the gating frequency.

12. The frequency adaptive single photon detection system according to claim 8, characterized in that: Also includes: an amplifier, connected to the second single-pole double-throw switch, and configured to amplify the external reference signal or the second operating frequency; The filter is connected to the amplifier and is used to filter out the amplified external reference signal or frequencies other than the second operating frequency to obtain a specific frequency signal.

13. The frequency adaptive single photon detection system according to claim 1, characterized in that: Also includes: A programmable logic unit is connected to the second phase-locked loop unit and is used to dynamically compile the second phase-locked loop unit to modify the second output frequency.

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