A weak light signal heterodyne coherent detection system based on wavelet transform

Abstract

The application discloses a weak light signal heterodyne coherent detection system based on wavelet transform and belongs to the technical field of microwave photonics. The detection system comprises a distributed feedback laser, a 90 / 10 coupler, a 50 / 50 coupler, an optical attenuator, an acousto-optic modulator, an arbitrary signal generator, a photodetector, an amplifier, a band-pass filter, an oscilloscope and a data processing unit. The application utilizes the low-noise amplification characteristics of the coherent detection technology, realizes an ideal signal-to-noise ratio through the interference effect of signal light and local oscillator light, and gets rid of the dependence of single-photon detection on an ultralow-temperature environment. Meanwhile, a data processing method combining variational mode decomposition, continuous wavelet transform, cross-correlation analysis and phase analysis is adopted to extract single-photon signals from noisy signals, so that the weak signals reaching the single-photon level can be efficiently detected at room temperature, and the application range of single-photon detection devices is expanded.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonics technology, specifically relating to a coherent heterodyne detection system for weak optical signals based on wavelet transform. This system can be applied to fields such as coherent optical communication, quantum optics, and lidar. Background Technology

[0002] The room-temperature operating single-photon level weak optical signal detection system based on heterodyne coherent detection technology has advantages such as low cost, strong photon number resolution, strong anti-interference ability, room temperature operation, high detection efficiency, and no dead time. It has broad application prospects and great application value in the fields of lidar, quantum optics, coherent optical communication, and microwave photonics.

[0003] Traditional photodetectors are limited by electronic noise, making single-photon detection difficult. Single-photon detectors, however, push the sensitivity of photodetectors to the quantum limit, possessing significant application value in coherent optical communication and ultra-long-range lidar systems. They are also crucial tools for developing new research directions and exploring new physical phenomena in numerous scientific fields. Currently, single-photon detectors can be categorized into traditional and novel types. Traditional single-photon detection devices mainly include photomultiplier tubes (PMTs) and avalanche photodiodes (APDs). Currently, the detection efficiency of traditional single-photon detectors is generally low. However, novel single-photon detectors, represented by superconducting nanowire single-photon detectors (SNSPDs), offer a fundamental improvement in overall performance compared to traditional single-photon detectors. The core of this technology is a nanowire made of ultrathin, low-temperature superconducting material. When the energy of an incident photon is absorbed by the nanowire, a state change occurs from superconducting to resistive, generating a voltage pulse across the nanowire and enabling high-efficiency single-photon detection. However, low-temperature single-photon detectors, such as SNSPDs, require operation in near-absolute-zero environments, necessitating expensive, bulky, and energy-intensive cooling equipment, which limits their application scope to some extent. Furthermore, SNSPDs achieve photon number resolution by increasing the number of nanowires and employing various multiplexing techniques, significantly increasing the complexity and cost of fabricating the detector. Summary of the Invention

[0004] The purpose of this invention is to address the problems existing in the background technology by proposing a wavelet transform-based heterodyne coherent detection system for weak optical signals. This invention utilizes the low-noise amplification characteristics of coherent detection technology and, through the interference effect of the signal light and the local oscillator light, eliminates the dependence of single-photon detection on ultra-low temperature environments, thereby significantly reducing the system's size and power consumption, and achieving high-efficiency single-photon detection at room temperature. Simultaneously, this invention determines the number of photons contained in the pulse by measuring and analyzing the amplitude of the beat frequency signal generated after the interference of the signal light and the local oscillator light, thus possessing photon count resolution capability itself, greatly reducing the complexity and cost of fabricating the detection device.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A wavelet transform-based heterodyne coherent detection system for weak optical signals includes a distributed feedback laser, a 90 / 10 coupler, a 50 / 50 coupler, a first optical attenuator, a second optical attenuator, a third optical attenuator, a fourth optical attenuator, an acousto-optic modulator, an arbitrary signal generator, a first photodetector, a second photodetector, a first amplifier, a second amplifier, a third amplifier, a fourth amplifier, a first bandpass filter, a second bandpass filter, an oscilloscope, and a data processing unit.

[0007] The optical output port of the distributed feedback laser is connected to the input port of a 90 / 10 coupler. The 90% output port of the 90 / 10 coupler is connected to the input port of a fourth optical attenuator. After being attenuated to an appropriate local oscillator power by the fourth optical attenuator, it is connected to one input port of a 50 / 50 coupler as the local oscillator light. The 10% output port of the 90 / 10 coupler is connected to the input port of an acousto-optic modulator. An arbitrary signal generator generates a pulse signal, which is pulse-modulated by the acousto-optic modulator. The signal then passes through a first optical attenuator, a second optical attenuator, and a third optical attenuator in sequence. By adjusting the attenuation of the attenuators, the power value of the optical pulse signal is controlled, so that the signal power reaches the single-photon level.

[0008] The output port of the third optical attenuator is connected to the other input port of the 50 / 50 coupler. The signal light and the local oscillator light interfere at the 50 / 50 coupler to generate a beat frequency signal. The two output ports of the 50 / 50 coupler are connected to the input ports of the first photodetector and the second photodetector, respectively, to perform photoelectric conversion on the signal. The output electrical signal of the first photodetector is amplified by the first amplifier and the second amplifier, filtered by the first bandpass filter, acquired by an oscilloscope, and input to the data processing unit. The output electrical signal of the second photodetector is amplified by the third amplifier and the fourth amplifier, filtered by the second bandpass filter, acquired by an oscilloscope, and input to the data processing unit. The data processing unit processes the received two signals and extracts the single-photon signal.

[0009] The present invention provides a heterodyne coherent detection system for weak optical signals based on wavelet transform, the working principle of which is as follows:

[0010] The continuous laser generated by the distributed feedback laser is input to a 90 / 10 coupler for beam splitting. 90% of the output port of the 90 / 10 coupler is connected to the fourth optical attenuator in the local oscillator branch. After being attenuated to an appropriate local oscillator power by the fourth optical attenuator, it is connected to one input port of a 50 / 50 coupler as the local oscillator light.

[0011] The 10% output port of the 90 / 10 coupler is connected to the RF input port of the acousto-optic modulator in the signal optical branch. An arbitrary signal generator generates a pulse signal, which is then pulse-modulated by the acousto-optic modulator and sequentially input to the first optical attenuator, the second optical attenuator, and the third optical attenuator. By adjusting the attenuation of the first optical attenuator, the second optical attenuator, and the third optical attenuator, the signal optical power value reaches the single-photon level.

[0012] The output port of the third optical attenuator is connected to another input port of the 50 / 50 coupler as the signal light; the local oscillator light and the signal light reaching the single-photon level interfere at the 50 / 50 coupler to generate a beat frequency signal; the two output ports of the 50 / 50 coupler are connected to the first photodetector and the second photodetector respectively to perform photoelectric conversion on the signal;

[0013] Taking the upper branch of the 50 / 50 coupler as an example, the output port of the first photodetector is connected sequentially to the first amplifier, the second amplifier, and the first bandpass filter. The signal output from the first bandpass filter is connected to a spectrum analyzer. The amplification factor of the amplifier and the filtering bandwidth of the bandpass filter are adjusted, and the spectrum analyzer is observed until the system signal-to-noise ratio reaches a stable state, thus achieving high signal-to-noise ratio single-photon level heterodyne coherent detection. The lower branch of the 50 / 50 coupler is adjusted in the same way as the upper branch.

[0014] Replace the spectrum analyzer with an oscilloscope to acquire the output signals of the first and second bandpass filters, and transmit the acquired data to the data processing unit for digital signal processing.

[0015] Furthermore, the arbitrary signal generator produces a Gaussian pulse signal with a repetition frequency of 2.4 MHz and a pulse width of 49 ns.

[0016] Furthermore, the oscilloscope in the detection system is replaced with a spectrum analyzer. The spectrum analyzer measures electronic noise and shot noise, and the attenuation of the fourth optical attenuator is adjusted to change the power value of the local oscillator branch so that the shot noise is at least 10 dB greater than the electronic noise.

[0017] Furthermore, the data processing unit's processing procedure is as follows:

[0018] (1) The output signal of the first bandpass filter is subjected to variational mode decomposition and divided into intrinsic mode function components containing different frequency bands;

[0019] (2) Perform continuous wavelet transform on each intrinsic mode function component to obtain the wavelet coefficient matrix; select "CMOR(f b -f c Let f be a wavelet basis, CMOR be a complex-valued wavelet function, and f be a wavelet function. c f is the center frequency factor. b Bandwidth factor;

[0020] (3) Select the VisuShrink threshold as the threshold for wavelet transformation, and select a soft threshold function with good continuity of wavelet coefficients as the threshold function;

[0021] Each element in the wavelet coefficient matrix is ​​compared with the threshold. If it is greater than or equal to the threshold, the corresponding element is substituted into the threshold function for calculation and then the corresponding element is updated. If it is less than the threshold, the corresponding element is set to 0. The updated wavelet coefficient matrix is ​​obtained.

[0022] (4) Perform inverse continuous wavelet transform on the updated wavelet coefficient matrix to obtain the processed intrinsic mode function components; select "CMOR(f b -f c Let f be a wavelet basis, CMOR be a complex-valued wavelet function, and f be a wavelet function. c f is the center frequency factor. b Bandwidth factor;

[0023] (5) The components of each intrinsic mode function after step (4) are added together to obtain the signal to be processed; the repetition period is calculated according to the repetition frequency of the pulse signal generated by any signal generator, and the signal to be processed is segmented according to the repetition period; and each segment of the signal is cross-correlated with the target signal to obtain the cross-correlation coefficient.

[0024] Set a cross-correlation coefficient threshold, compare the obtained cross-correlation coefficient with the cross-correlation coefficient threshold, if it is greater than or equal to the cross-correlation coefficient threshold, retain the signal of the corresponding segment; if it is less than the cross-correlation coefficient threshold, set the signal of the corresponding segment to zero.

[0025] Among them, the target signal The expression is:

[0026] ;

[0027] In the formula, R is the input impedance of the receiving device oscilloscope, and R is the response coefficient of the photodetector. The optical power of a single-photon signal. The optical power of the local oscillator. To modulate the Gaussian pulse signal of continuous laser, and These are the angular frequency difference and initial phase difference between the signal light and the local oscillator light, respectively.

[0028] (6) Perform phase analysis on the signal processed in step (5);

[0029] Calculate the phase of each segment of the signal after processing in step (5), and draw a phase distribution diagram based on the calculated phase.

[0030] Select a region of phase concentration as the region where single-photon signals may exist, and the remaining region as the noise region;

[0031] The size of the region where a single-photon signal may exist is used as the phase window. The noise region is divided according to the phase window, and the number of segmented signals in each phase window is counted. The number of segmented signals in each phase window in the noise region is added together and divided by the number of phase windows in the noise region to obtain the average noise quantity M.

[0032] The N segments of signal in the region where the single photon signal may exist are arranged in descending order according to the cross-correlation coefficient of each segment, and the first (NM) segments are selected as the single photon signal output.

[0033] (7) Repeat steps (1) to (6) to extract the single-photon signal from the output signal of the second bandpass filter 2, and use it together with the single-photon signal obtained in step (6) as the final single-photon signal output.

[0034] Furthermore, in step (6), the size of the region where the single-photon signal may exist is π / 6 to π / 2.

[0035] Furthermore, by adjusting the bandwidth and repetition frequency of the pulse signal, the amplification factor of the amplifier, and the passband bandwidth of the bandpass filter, the signal-to-noise ratio of the beat frequency signal can be optimized, thereby achieving high-efficiency detection of signal light at the single-photon level.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] 1. The present invention proposes a wavelet transform-based heterodyne coherent detection system for weak optical signals, which uses a Gaussian pulse signal with a small time-bandwidth product to modulate the signal light, thereby achieving high-efficiency single-photon detection under room temperature conditions and further reducing the detection sensitivity of the heterodyne coherent detection system.

[0038] 2. The present invention proposes a wavelet transform-based heterodyne coherent detection system for weak optical signals, which achieves photon number-resolvable detection at the single-photon level. This greatly reduces the manufacturing complexity and cost of single-photon detection devices and is of great value for improving the performance of quantum optical systems.

[0039] 3. The present invention proposes a weak optical signal heterodyne coherent detection system based on wavelet transform. It adopts a data processing method that combines variational mode decomposition, continuous wavelet transform, cross-correlation analysis and phase analysis to process and analyze the beat frequency signal and obtain the accurate single photon arrival time. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of a weak optical signal heterodyne coherent detection system based on wavelet transform proposed in this invention;

[0041] Figure 2 The waveform of the beat frequency signal generated by the interference of an unattenuated Gaussian pulse signal light and a local oscillator light on an oscilloscope.

[0042] Figure 3 This is a flowchart of the data processing process of the data processing unit in the weak optical signal heterodyne coherent detection system based on wavelet transform of the present invention. Detailed Implementation

[0043] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0044] Example

[0045] Figure 1 This is a schematic diagram of the structure of a weak optical signal heterodyne coherent detection system based on wavelet transform proposed in this invention. It includes a distributed feedback laser, a 90 / 10 coupler, a 50 / 50 coupler, a first optical attenuator 1, a second optical attenuator 2, a third optical attenuator 3, a fourth optical attenuator 4, an acousto-optic modulator, an arbitrary signal generator, a first photodetector 1, a second photodetector 2, a first amplifier 1, a second amplifier 2, a third amplifier 3, a fourth amplifier 4, a first bandpass filter 1, a second bandpass filter 2, an oscilloscope (ADC), and a data processing unit.

[0046] The optical output port of the distributed feedback laser is connected to the input port of a 90 / 10 coupler. 90% of the output port of the 90 / 10 coupler is connected to the input port of the fourth optical attenuator 4. After being attenuated to an appropriate local oscillator power by the fourth optical attenuator 4, the light is connected to one input port of a 50 / 50 coupler as the local oscillator light. 10% of the output port of the 90 / 10 coupler is connected to the input port of an acousto-optic modulator. An arbitrary signal generator generates a pulse signal, which is then pulse-modulated by the acousto-optic modulator. The signal then sequentially passes through the first optical attenuator 1, the second optical attenuator 2, and the third optical attenuator 3. By adjusting the attenuation of the attenuators, the power value of the optical pulse signal is controlled, enabling the signal power to reach the single-photon level.

[0047] The output port of the third optical attenuator 3 is connected to the other input port of the 50 / 50 coupler. The signal light and the local oscillator light interfere at the 50 / 50 coupler to generate a beat frequency signal. The two output ports of the 50 / 50 coupler are connected to the input ports of the first photodetector 1 and the second photodetector 2, respectively, to perform photoelectric conversion on the signal. The output electrical signal of the first photodetector 1 is amplified by the first amplifier 1 and the second amplifier 2, filtered by the first bandpass filter 1, acquired by an oscilloscope, and then input to the data processing unit. The output electrical signal of the second photodetector 2 is amplified by the third amplifier 3 and the fourth amplifier 4, filtered by the second bandpass filter 2, acquired by an oscilloscope, and then input to the data processing unit. The data processing unit processes the received two signals and extracts the single-photon signal.

[0048] The present invention provides a heterodyne coherent detection system for weak optical signals based on wavelet transform, the working principle of which is as follows:

[0049] The continuous laser generated by the distributed feedback laser is input to a 90 / 10 coupler for beam splitting. 90% of the output port of the 90 / 10 coupler is connected to the fourth optical attenuator in the local oscillator branch. After being attenuated to an appropriate local oscillator power by the fourth optical attenuator 4, it is connected to one input port of a 50 / 50 coupler as the local oscillator light.

[0050] The 10% output port of the 90 / 10 coupler is connected to the RF input port of the acousto-optic modulator in the signal optical branch. An arbitrary signal generator generates a pulse signal, which is then pulse-modulated by the acousto-optic modulator and sequentially input to the first optical attenuator, the second optical attenuator, and the third optical attenuator. By adjusting the attenuation of the first optical attenuator, the second optical attenuator, and the third optical attenuator, the signal optical power value reaches the single-photon level.

[0051] The output port of the third optical attenuator 3 is connected to another input port of the 50 / 50 coupler as the signal light; the local oscillator light and the signal light reaching the single-photon level interfere at the 50 / 50 coupler to generate a beat frequency signal; the two output ports of the 50 / 50 coupler are connected to the first photodetector and the second photodetector respectively to perform photoelectric conversion on the signal;

[0052] Taking the upper branch of a 50 / 50 coupler as an example, the output port of the first photodetector 1 is sequentially connected to the first amplifier 1, the second amplifier 2, and the first bandpass filter 1. The signal output from the first bandpass filter is connected to a spectrum analyzer. The amplification factor of the amplifier and the filtering bandwidth of the bandpass filter are adjusted, and the spectrum analyzer is observed until the system signal-to-noise ratio reaches a stable state, achieving high signal-to-noise ratio single-photon level heterodyne coherent detection. The lower branch of the 50 / 50 coupler is adjusted in the same way as the upper branch.

[0053] Replace the spectrum analyzer with an oscilloscope to acquire the output signals of the first bandpass filter 1 and the second bandpass filter 2. Set the sampling rate to 500 MHz and the sampling depth to 10 μm. Transmit the acquired data to the data processing unit for digital signal processing.

[0054] The arbitrary signal generator produces a Gaussian pulse signal with a repetition frequency of 2.4 MHz and a pulse width of 49 ns.

[0055] The detection system described in this invention needs to operate under conditions where shot noise is dominant, meaning that shot noise is much greater than thermal noise. The oscilloscope in the detection system is replaced with a spectrum analyzer. By measuring electronic noise and shot noise using the spectrum analyzer, the attenuation of the fourth optical attenuator 4 is adjusted, changing the power value of the local oscillator branch so that the shot noise is at least 10 dB greater than the electronic noise. In this embodiment, the power value of the local oscillator branch is 7 mW, which satisfies the condition of shot noise dominance.

[0056] In this embodiment, the frequency of the beat frequency signal is 53 MHz, the quantum efficiency of the first and second photodetectors is about 96%, the amplifiers are both 20 dB power amplifiers, and the passband frequency range of the bandpass filter is 43 MHz-63 MHz.

[0057] The data processing unit's processing procedure is as follows: Figure 3 As shown, the specific steps include:

[0058] (1) The output signal of the first bandpass filter 1 is subjected to variational mode decomposition and divided into intrinsic mode function components containing different frequency bands. By optimizing the number of intrinsic mode function components and the second-order penalty factor, the effect of digital signal processing can be improved. In this embodiment, the number of intrinsic mode function components is 3 and the second-order penalty factor is 5000.

[0059] (2) Perform continuous wavelet transform on each intrinsic mode function component to obtain the wavelet coefficient matrix; select "CMOR(f b -f c Let f be a wavelet basis, CMOR be a complex-valued wavelet function, and f be a wavelet function. c f is the center frequency factor. b Bandwidth factor;

[0060] (3) Select the VisuShrink threshold as the threshold for wavelet transformation, and select a soft threshold function with good continuity of wavelet coefficients as the threshold function;

[0061] Each element in the wavelet coefficient matrix is ​​compared with the threshold. If it is greater than or equal to the threshold, the corresponding element is substituted into the threshold function for calculation and then the corresponding element is updated. If it is less than the threshold, the corresponding element is set to 0. The updated wavelet coefficient matrix is ​​obtained.

[0062] (4) Perform inverse continuous wavelet transform on the updated wavelet coefficient matrix to obtain the processed intrinsic mode function components; select "CMOR(f b -f c Let f be a wavelet basis, CMOR be a complex-valued wavelet function, and f be a wavelet function. c f is the center frequency factor. b Bandwidth factor;

[0063] (5) The components of each intrinsic mode function after step (4) are added together to obtain the signal to be processed; the repetition period is calculated according to the repetition frequency of the pulse signal generated by any signal generator, and the signal to be processed is segmented according to the repetition period; and each segment of the signal is cross-correlated with the target signal to obtain the cross-correlation coefficient.

[0064] Set a cross-correlation coefficient threshold, compare the obtained cross-correlation coefficient with the cross-correlation coefficient threshold, if it is greater than or equal to the cross-correlation coefficient threshold, retain the signal of the corresponding segment; if it is less than the cross-correlation coefficient threshold, set the signal of the corresponding segment to zero.

[0065] Among them, the target signal The expression is:

[0066] ;

[0067] In the formula, R is the input impedance of the receiving device oscilloscope, and R is the response coefficient of the photodetector. The optical power of a single-photon signal. The optical power of the local oscillator. To modulate the Gaussian pulse signal of continuous laser, and These are the angular frequency difference and initial phase difference between the signal light and the local oscillator light, respectively.

[0068] The target signal, namely the beat frequency signal generated by the interference of the unattenuated Gaussian pulse signal light and the local oscillator light, is shown in the waveform on the oscilloscope as follows: Figure 2 As shown.

[0069] (6) Perform phase analysis on the signal processed in step (5);

[0070] Calculate the phase of each segment of the signal after processing in step (5), and draw a phase distribution diagram based on the calculated phase.

[0071] Select a region of phase concentration as the region where single-photon signals may exist, and the remaining region as the noise region;

[0072] The size of the region where a single-photon signal may exist is used as the phase window. The noise region is divided according to the phase window, and the number of segmented signals in each phase window is counted. The number of segmented signals in each phase window in the noise region is added together and divided by the number of phase windows in the noise region to obtain the average noise quantity M.

[0073] The N segments of signal in the region where the single photon signal may exist are arranged in descending order according to the cross-correlation coefficient of each segment, and the first (NM) segments are selected as the single photon signal output.

[0074] (7) Repeat steps (1) to (6) to extract the single-photon signal from the output signal of the second bandpass filter 2, and use it together with the single-photon signal obtained in step (6) as the final single-photon signal output.

[0075] In step (6), the size of the region where the single-photon signal may exist is π / 6 to π / 2.

[0076] Specifically, the phase difference distribution of a single-photon sequence is an integer multiple of 2π, while the phase of noise is randomly distributed. This invention selects a region with the most concentrated phase as the area where single-photon signals may exist, while the remaining regions are considered noise regions. Furthermore, the selected phase-concentrated region contains noise events at a level comparable to those in noise regions with the same phase window size.

[0077] This invention utilizes the low-noise amplification characteristics of coherent detection technology. Through the interference effect of the signal light and the local oscillator light, a relatively ideal signal-to-noise ratio can be achieved, eliminating the dependence of single-photon detection on ultra-low temperature environments, thereby significantly reducing the system's size, power consumption, and cost. Simultaneously, this invention also proposes a data processing method combining variational mode decomposition, continuous wavelet transform, cross-correlation analysis, and phase analysis. This method can extract single-photon signals from noisy signals, enabling high-efficiency detection of weak signals at the single-photon level under room temperature conditions, thus expanding the applicability of single-photon detection devices.

Claims

1. A heterodyne coherent detection system for weak optical signals based on wavelet transform, characterized in that, It includes a distributed feedback laser, a 90 / 10 coupler, a 50 / 50 coupler, a first optical attenuator, a second optical attenuator, a third optical attenuator, a fourth optical attenuator, an acousto-optic modulator, an arbitrary signal generator, a first photodetector, a second photodetector, a first amplifier, a second amplifier, a third amplifier, a fourth amplifier, a first bandpass filter, a second bandpass filter, an oscilloscope, and a data processing unit. The optical output port of the distributed feedback laser is connected to the input port of a 90 / 10 coupler. The 90% output port of the 90 / 10 coupler is connected to the input port of a fourth optical attenuator. After attenuation by the fourth optical attenuator, the light is connected to one input port of a 50 / 50 coupler as the local oscillator. The 10% output port of the 90 / 10 coupler is connected to the input port of an acousto-optic modulator. An arbitrary signal generator generates a pulse signal, which is pulse-modulated by the acousto-optic modulator. The pulse signal then passes through the first, second, and third optical attenuators in sequence to control the power value of the optical pulse signal, so that the signal power reaches the single-photon level. The output port of the third optical attenuator is connected to another input port of the 50 / 50 coupler. The signal light and the local oscillator light interfere at the 50 / 50 coupler to generate a beat frequency signal. The two output ports of the 50 / 50 coupler are connected to the input ports of the first photodetector and the second photodetector, respectively, to perform photoelectric conversion on the signal. The output electrical signal of the first photodetector is amplified by the first amplifier and the second amplifier, filtered by the first bandpass filter, acquired by an oscilloscope, and input to the data processing unit; the output electrical signal of the second photodetector is amplified by the third amplifier and the fourth amplifier, filtered by the second bandpass filter, acquired by an oscilloscope, and input to the data processing unit; the data processing unit processes the received two signals and extracts the single-photon signal.

2. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 1, characterized in that, Replace the oscilloscope in the detection system with a spectrum analyzer. Measure electronic noise and shot noise using the spectrum analyzer, adjust the attenuation of the fourth optical attenuator, and change the power value of the local oscillator branch so that the shot noise is at least 10 dB greater than the electronic noise.

3. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 1, characterized in that, Replace the oscilloscope in the detection system with a spectrum analyzer, and adjust the amplifier's gain and the bandpass filter's bandwidth until the system's signal-to-noise ratio reaches a stable state.

4. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 1, characterized in that, The data processing procedure of the data processing unit is as follows: (1) The output signal of the first bandpass filter is subjected to variational mode decomposition and divided into intrinsic mode function components containing different frequency bands; (2) Perform continuous wavelet transform on each intrinsic mode function component to obtain the wavelet coefficient matrix; (3) Compare each element in the wavelet coefficient matrix with the threshold. If it is greater than or equal to the threshold, then substitute the corresponding element into the threshold function for calculation and update the corresponding element. If the value is less than the threshold, the corresponding element is set to 0; the updated wavelet coefficient matrix is ​​obtained. (4) Perform inverse continuous wavelet transform on the updated wavelet coefficient matrix to obtain the processed intrinsic mode function components; (5) The components of each intrinsic mode function after step (4) are added together to obtain the signal to be processed; the signal to be processed is segmented according to the repetition frequency of the pulse signal generated by any signal generator; and each segment of the signal is cross-correlated with the target signal to obtain the cross-correlation coefficient. Set a cross-correlation coefficient threshold, compare the obtained cross-correlation coefficient with the cross-correlation coefficient threshold, if it is greater than or equal to the cross-correlation coefficient threshold, retain the signal of the corresponding segment; if it is less than the cross-correlation coefficient threshold, set the signal of the corresponding segment to zero. (6) Calculate the phase of each segment of the signal after processing in step (5), and draw a phase distribution diagram based on the calculated phase. Select a region of phase concentration as the region where single-photon signals may exist, and the remaining region as the noise region; The size of the region where a single-photon signal may exist is used as the phase window. The noise region is divided according to the phase window, and the number of segmented signals in each phase window is counted. The number of segmented signals in each phase window in the noise region is added together and divided by the number of phase windows in the noise region to obtain the average noise quantity M. The N segments of signal in the region where the single photon signal may exist are arranged in descending order according to the cross-correlation coefficient of each segment, and the first (NM) segments are selected as the single photon signal output. (7) Repeat steps (1) to (6) to extract the single-photon signal from the output signal of the second bandpass filter and use it together with the single-photon signal obtained in step (6) as the final single-photon signal output.

5. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 4, characterized in that, In step (3), the threshold is the VisuShrink threshold, and the threshold function is the soft threshold function.

6. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 4, characterized in that, In steps (2) and (4), select "CMOR(f b -f c Let f be a wavelet basis, where CMOR is a complex-valued wavelet function, and f c f is the center frequency factor. b This is the bandwidth factor.

7. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 4, characterized in that, In step (5), the target signal is: ； In the formula, R is the input impedance of the oscilloscope, and R is the response coefficient of the photodetector. The optical power of a single-photon signal. The optical power of the local oscillator. To modulate the Gaussian pulse signal of continuous laser, and These represent the angular frequency difference and initial phase difference between the signal light and the local oscillator light, respectively.

8. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 4, characterized in that, In step (6), the size of the region where the single-photon signal may exist is π / 6 to π / 2.

9. The weak optical signal heterodyne coherent detection system based on wavelet transform according to claim 4, characterized in that, By adjusting the bandwidth and repetition frequency of the pulse signal, the amplification factor of the amplifier, and the passband bandwidth of the bandpass filter, the signal-to-noise ratio of the beat frequency signal can be optimized, thereby achieving high-efficiency detection of signal light at the single-photon level.

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