A space optical communication system and method based on a single-photon level detection array

Abstract

The application belongs to the technical field of space optical communication, and discloses a space optical communication system and method based on a single-photon-level detection array, wherein a transmitting end comprises a laser, a coded signal generation module, a modulation module, a transmitting light path and a transmitting end tracking and aiming module; a receiving end comprises a receiving light path, a receiving end tracking and aiming module, a fiber array, an array balance detection module and a data processing module; the tracking and aiming modules of the transmitting end and the receiving end are used for coarse tracking and aiming, and fine tracking and aiming is performed according to the distribution position of a response pixel of the array balance detection module. Compared with the prior art, the application improves communication sensitivity; high-precision local oscillator light and a complex heterodyne circuit are not needed, fine tracking and aiming and detection share an array detector, an extra fine tracking and aiming CCD is saved, system complexity is reduced; the array detection covers a multi-pixel area, even if laser slightly deviates, part of pixels still receive signals, and the detection efficiency can be improved through real-time tracking and aiming adjustment, so that communication interruption is avoided.

Description

Technical Field

[0001] This invention relates to the field of space optical communication technology, and in particular to a space optical communication system and method based on a single-photon level detector array. Background Technology

[0002] Space laser communication boasts advantages such as high communication speed, strong anti-interference capability, small size, and low power consumption, and has broad application prospects in satellite communication, deep space exploration, and space-to-ground communication. Space optical communication, with its high bandwidth and anti-interference advantages, has become an important direction for next-generation space communication; however, current technologies face numerous bottlenecks.

[0003] 1. Limitations of ordinary photodetectors: Traditional space optical communication receivers mostly use ordinary single-point photodetectors, which have low detection sensitivity and weak received signal strength, resulting in limited communication distance and making it difficult to meet the long-distance communication needs of deep space exploration, etc.

[0004] 2. Conventional coherent detection is highly complex: Although coherent detection can improve sensitivity, it has strict requirements on the linewidth and frequency stability of the local oscillator light, and heterodyne detection requires complex optical path alignment and signal processing circuits, which increases system cost and failure rate.

[0005] 3. Poor single-point detection efficiency: A single-point detector can only receive the light signal from a single location. When the laser signal is slightly offset due to spatial interference, the amplitude of the received signal drops sharply, resulting in low detection efficiency and easy communication interruption.

[0006] 4. Redundancy in tracking and aiming system: The existing system requires separate CCDs for coarse tracking and aiming and fine tracking and aiming, which not only increases hardware costs and system size, but also makes the coordinated control of dual CCDs complex. In addition, ordinary CCDs have low sensitivity, and tracking and aiming accuracy is easily affected in low light environments. Summary of the Invention

[0007] To address the aforementioned shortcomings of existing technologies, this invention proposes a space optical communication system based on a single-photon level detector array, comprising a transmitter and a receiver.

[0008] The transmitting end includes: a laser for generating optical signals in the communication band; an encoding signal generation module for generating an encoded signal to be transmitted; a modulation module for amplitude encoding modulation of the optical signal according to the encoded signal; a transmitting optical path for expanding the encoded optical signal and transmitting it to the receiving end; and a transmitting end tracking module.

[0009] The receiving end includes: a receiving optical path for collecting coded optical signals emitted by the transmitting end; an optical fiber array for transmitting the coded optical signals collected by the receiving optical path to an array balanced detection module; an array balanced detection module for detecting the optical signals transmitted by the optical fiber array and generating local oscillator light; a data processing module for filtering, decoding, and error correction of the output signal of the array balanced detection module, wherein the symbol duration of the coded optical signal is not less than twice the difference frequency period between the local oscillator light and the signal light; and a receiving end tracking module.

[0010] The transmitting end tracking module and the receiving end tracking module are used for coarse tracking and fine tracking by responding to the distribution position of the pixels in the array balance detection module.

[0011] Preferably, the array balanced detection module includes multiple pixel units distributed in an array for detecting the optical signal transmitted by the fiber optic array; wherein each pixel unit uses one local oscillator light and one optical signal from the fiber optic array for balanced detection, and demodulates the signal light through a predetermined signal processing method.

[0012] Preferably, the array balanced detection module is a first array balanced detector, comprising a laser chip LC integrated on the same substrate and N×M balanced detection units distributed in an array, where N and M are both positive integers. The laser chip LC is connected to each balanced detection unit via a beam-splitting waveguide.

[0013] The laser chip LC is used to output local oscillator light, which is coupled to the beam splitter waveguide through a lens;

[0014] The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit.

[0015] Each balanced detection unit is used to detect single-photon level optical signals, including a multimode interference coupler (MMI), two photodetectors (PD), and a signal processing module (SP). One input port of the MMI is used to input one local oscillator light, and the other input port is coupled to the polarization-maintaining fiber (PMF) through a mode-spot converter (MSC) to transmit one signal light from the fiber array.

[0016] SP is used to perform digital filtering, signal rectification, and low-pass filtering on the differential signals of two PDs, and output the envelope amplitude.

[0017] Preferably, the array balanced detection module includes a second array balanced detector and a local oscillator laser, wherein the second array balanced detector includes N×M balanced detection units integrated on the same substrate and distributed in an array, where N and M are both positive integers.

[0018] The local oscillator laser is used to output local oscillator light, which is coupled to the waveguide of the second array balanced detector through a polarization-maintaining fiber PMF and a mode converter MSC.

[0019] The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit.

[0020] Each balanced detection unit can detect single-photon level optical signals, including a directional coupler DC, two photodetectors PD and a signal processing module SP. One input port of the DC is used to input one local oscillator light, and the other input port is connected to a grating coupler GC to couple one signal light of the fiber array.

[0021] SP is used to rectify and detect peaks in the differential signals of two PDs, and compare the average of the peak values ​​with a predetermined threshold.

[0022] Preferably, the amplitude encoding adopts the OOK encoding method. When the encoded signal is "1", a high amplitude level signal is output, and when the encoded signal is "0", a low amplitude level signal or a zero level signal is output.

[0023] Preferably, the transmitting optical path includes an optical fiber amplifier and a transmitting telescope, used to amplify the encoded optical signal to a predetermined power and then expand the beam through the transmitting telescope; the receiving optical path is a receiving telescope.

[0024] Preferably, the modulation module includes an intensity modulator, a beam splitter, and a bias control module.

[0025] The intensity modulator is used to perform intensity modulation encoding on the optical signal;

[0026] The beam splitter is used to split the intensity-modulated optical signal into one path and transmit it to the bias control module as a feedback signal.

[0027] The bias control module is used to control the bias voltage of the intensity modulator according to the feedback signal to maintain a stable operating point.

[0028] Preferably, the modulation module includes a phase modulator and a polarization-maintaining beam splitter. The two output ports of the polarization-maintaining beam splitter are connected through the phase modulator to form a Sagnac loop, which is used to achieve stable intensity modulation through phase modulation.

[0029] Preferably, the specific method for determining the incident deviation of the laser signal when the two tracking modules perform precise tracking is as follows: when the laser signal is accurately incident on the central region of the array detector, the pixel units in the central region produce a high-amplitude response, while the pixel units in the edge region have a weak or no response; when the laser signal deviates from the central region, the response amplitude of the pixel units in the deviation direction increases, while the response amplitude of the pixel units in the central region decreases; the tracking module calculates the deviation direction and deviation amount of the laser signal based on the distribution difference of the response amplitude of each pixel unit, and then generates tracking control commands.

[0030] This invention also discloses a space optical communication method based on a single-photon level detector array, comprising the following steps:

[0031] S1: Transmitter signal processing and transmission: The encoding signal generation module generates an encoded signal, and the modulation module uses the OOK encoding method to encode the amplitude of the optical signal generated by the laser. After beam expansion through the transmission optical path, it is transmitted into space.

[0032] S2: Laser signal reception at the receiving end: The receiving optical path collects the optical signal, which enters each pixel unit of the array balanced detection module through the fiber array for detection, and converts the optical signal into the corresponding electrical signal;

[0033] S3: Precise tracking and aiming adjustment: The tracking and aiming module of the receiving end obtains the electrical signal response of each pixel unit, judges the incident deviation of the light signal based on the distribution of the response pixels, and generates control commands to adjust the attitude or position of the array detector so that the light signal is always accurately incident on the effective area of ​​the array detector.

[0034] S4: Signal superposition processing: The signal superposition module superimposes the electrical signals output by each pixel unit to obtain the total electrical signal;

[0035] S5: Amplitude Demodulation and Restoration: The amplitude demodulation module demodulates the total electrical signal, extracts amplitude change information, restores the original baseband signal, and completes space laser communication.

[0036] Preferably, in step S3, after the receiving end tracking control module determines the incident deviation, if the deviation of the optical signal is less than a preset threshold, no tracking adjustment is performed; if the deviation is greater than or equal to the preset threshold, tracking adjustment is started until the deviation is less than the preset threshold.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] This invention proposes a space optical communication system and method based on a single-photon-level detection array. By employing a single-photon-level balanced detection array, sensitivity and communication distance can be improved. It eliminates the need for high-precision local oscillator light and complex heterodyne circuitry using coherent detection. Furthermore, the precision tracking and detection share the same array detector, saving the need for an additional precision tracking CCD and reducing system complexity. The array detection covers a multi-pixel area, ensuring that even with slight laser deviation, some pixels still receive signals. Combined with real-time tracking adjustments, this improves detection efficiency and avoids communication interruptions. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the space optical communication system based on a single-photon level detector array according to the present invention.

[0040] Figure 2 This is a schematic diagram of a first embodiment of the space optical communication system based on a single-photon level detector array according to the present invention;

[0041] Figure 3 This is a schematic diagram of the first array balanced detector and each pixel in Embodiment 1 of the present invention;

[0042] Figure 4 This is a schematic diagram of a second embodiment of the space optical communication system based on a single-photon level detector array according to the present invention.

[0043] Figure 5 This is a schematic diagram of the structure of the second array balanced detector and each pixel in Embodiment 2 of the present invention. Detailed Implementation

[0044] The present invention will now be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention.

[0045] like Figure 1 As shown, a space optical communication system based on a single-photon level detector array includes a transmitter and a receiver.

[0046] The transmitter includes a laser, an encoded signal generation module, a modulation module, an optical transmission path, and a transmitter tracking module;

[0047] The laser is used to generate optical signals in the communication band.

[0048] The encoded signal generation module is used to generate the encoded signal to be transmitted;

[0049] The modulation module is used to perform amplitude encoding modulation on the optical signal according to the encoded signal;

[0050] The transmitting optical path is used to expand the encoded optical signal and transmit it to the receiving end;

[0051] The receiving end includes a receiving optical path, a receiving end tracking module, an optical fiber array, an array balanced detection module, and a data processing module;

[0052] The receiving optical path is used to collect the coded optical signals emitted by the transmitting end;

[0053] The fiber optic array is used to transmit the coded optical signals collected by the receiving optical path to the array balanced detection module;

[0054] The array balanced detection module can be composed of multiple pixel units distributed in an array, used to detect the optical signal transmitted by the fiber optic array; wherein each pixel unit uses one local oscillator light and one optical signal from the fiber optic array for balanced detection, and demodulates the signal light through a predetermined signal processing method;

[0055] The data processing module is used to filter, decode, and correct the output signal of the array balance detection module;

[0056] The duration of the symbol in the encoded optical signal is not less than twice the difference frequency period between the local oscillator light and the signal light;

[0057] The transmitting end tracking module and the receiving end tracking module are used for coarse tracking and fine tracking by responding to the distribution position of the pixels in the array balance detection module.

[0058] The specific work process is as follows:

[0059] First, the system performs coarse tracking using tracking modules at both the transmitter and receiver to ensure that the coded optical signal emitted by the transmitter roughly falls within the range of the receiving optical path. Then, a fine tracking process is performed.

[0060] At the transmitting end, the encoding signal generation module generates an encoded signal, and the modulation module uses the OOK encoding method to encode the amplitude of the optical signal generated by the laser. After beam expansion through the transmission optical path, it is transmitted into space.

[0061] At the receiving end, the receiving optical path collects optical signals, which are then detected by each pixel unit of the array balanced detection module through the fiber optic array, and the optical signals are converted into corresponding electrical signals.

[0062] For each pixel unit of the array balanced detection module, the local oscillator light generated by the local oscillator laser can be written as:

[0063] ,

[0064] in, , respectively, represent the amplitude, frequency, and initial phase of the local oscillator light, t is the period time, and j is the imaginary number.

[0065] Assuming the signal light received at the receiver after transmission through a spatial channel is very weak, on the order of a single photon, and exhibits quantum effects, it can be written as follows:

[0066] ,

[0067] in, The signal light frequency and initial phase are given by the following parameters: canonical coordinate X and canonical momentum P. , This represents the amplitude of the signal light.

[0068] The signal light interferes with the local oscillator light, producing two interference outputs. Each output is then detected by a photodiode, generating two photocurrents, which are then differentially divided. The resulting differential photon count operator is...

[0069] ,

[0070] in, These are the photon count operators for the two photodiodes. To generate operators for photons, For photon annihilation operator, , The difference frequency between the local oscillator light and the signal light. This represents the phase difference between the local oscillator light and the signal light.

[0071] Due to the presence of vacuum shot noise, the corresponding differential current can be written as:

[0072] ,

[0073] Where R is the detector's response coefficient. This refers to the intermediate frequency signal. Vacuum shot noise.

[0074] ,

[0075] The phase difference between the vacuum state and the local oscillator light is given. The vacuum state has a random phase θ.

[0076] Since the pulse duration of the signal light is not less than twice the difference frequency variation period between the local oscillator signal and the signal light, the intermediate frequency signal will traverse the range [0, 2π) within one encoded pulse duration of the signal light. Therefore, the amplitude of the signal light can be obtained when the amplitude reaches its maximum value.

[0077] ,

[0078] The variance of signal current and the variance of vacuum shot noise are respectively

[0079] ,

[0080] ,

[0081] Therefore, the signal-to-noise ratio can be obtained. , Let be Planck's constant. The frequency of the optical signal is denoted as SNR. When SNR=1, it represents the sensitivity of the balanced detector, meaning it can detect one photon.

[0082] The signal processing module processes the signal output from the high-speed sampling module and demodulates the signal light using a predetermined method to obtain the amplitude of the signal received by each pixel of the array balanced detection module. The tracking module acquires the electrical signal response of each pixel unit, determines the incident deviation of the laser signal based on the distribution of the responding pixels, and generates control commands to adjust the attitude or position of the array detector, ensuring that the laser signal is always accurately incident on the effective area of ​​the array detector, thus completing the precision tracking process.

[0083] The specific method by which the receiver tracking module determines the incident deviation of the laser signal during precise tracking is as follows: when the laser signal is accurately incident on the central region of the array detector, the pixel units in the central region produce a high-amplitude response, while the pixel units in the edge region have a weak or no response; when the laser signal deviates from the central region, the response amplitude of the pixel units in the deviation direction increases, while the response amplitude of the pixel units in the central region decreases; the receiver tracking module calculates the deviation direction and deviation amount of the laser signal based on the distribution differences of the response amplitude of each pixel unit, and then generates tracking control commands.

[0084] After establishing a stable space communication link through tracking, optical communication can proceed. Assuming the optical signal is encoded using OOK (Out of Memory) encoding, the optical signal of each pixel unit can be detected by an array balanced detection module, and the detection signal can be filtered, decoded, and error-corrected by a data processing module to complete the space optical communication process.

[0085] like Figure 2 As shown, Example 1:

[0086] The array balanced detection module is the first array balanced detector, which includes a laser chip LC integrated on the same substrate and N×M balanced detection units distributed in an array. The laser chip LC is connected to each balanced detection unit through a beam-splitting waveguide.

[0087] The laser chip LC is used to output local oscillator light, which is coupled to the beam splitter waveguide through a lens;

[0088] The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit.

[0089] Each balanced detection unit can detect single-photon level optical signals, including a multimode interference coupler (MMI), two photodetectors (PD), and a signal processing module (SP). One input port of the MMI is used to input one local oscillator light, and the other input port is coupled to the polarization-maintaining fiber (PMF) through a mode-spot converter (MSC) to transmit one signal light of the fiber array.

[0090] SP is used to perform digital filtering, signal rectification, and low-pass filtering on the differential signals of two PDs, and output the envelope amplitude.

[0091] The amplitude encoding adopts the OOK encoding method. When the encoded signal is "1", a high amplitude level signal is output, and when the encoded signal is "0", a low amplitude level signal (or zero level signal) is output.

[0092] The transmitting optical path includes an optical fiber amplifier and a transmitting telescope, used to amplify the encoded optical signal to a predetermined power and then expand the beam through the transmitting telescope; the receiving optical path is a receiving telescope.

[0093] The modulation module includes an intensity modulator, a beam splitter, and a bias control module.

[0094] The intensity modulator is used to perform intensity modulation encoding on the optical signal;

[0095] The beam splitter is used to split the intensity-modulated optical signal into one path and transmit it to the bias control module as a feedback signal.

[0096] The bias control module is used to control the bias voltage of the intensity modulator according to the feedback signal to maintain a stable operating point.

[0097] The specific principle is as follows:

[0098] First, the system performs coarse tracking using tracking modules at both the transmitter and receiver to ensure that the coded optical signal emitted by the transmitter roughly falls within the range of the receiving optical path. Then, a fine tracking process is performed.

[0099] At the transmitting end, the encoding signal generation module generates an encoded signal. The intensity modulator uses OOK encoding to encode the amplitude of the optical signal generated by the laser. Since the operating point of the intensity modulator drifts slowly over time, affecting the encoded signal, a beam splitter separates the intensity-modulated optical signal into one path and transmits it to the bias control module as a feedback signal. The latter controls the bias of the intensity modulator based on the feedback signal to maintain a stable operating point. Subsequently, the encoded optical signal is beam-expanded through the transmission optical path and emitted into space.

[0100] At the receiving end, the receiving optical path collects the optical signal, which is then transmitted through the fiber optic array to each pixel unit of the first array balanced detector for detection, and the optical signal is converted into a corresponding electrical signal. A schematic diagram of the first array balanced detector and each pixel is shown below. Figure 3 As shown.

[0101] For each pixel unit of the first array balanced detector, the laser chip acts as a local oscillator laser, and the generated local oscillator light can be written as:

[0102] ,

[0103] in, , respectively, represent the amplitude, frequency, and initial phase of the local oscillator light, t is the period time, and j is the imaginary number.

[0104] Assuming the signal light received at the receiver after transmission through a spatial channel is very weak, on the order of a single photon, and exhibits quantum effects, it can be written as follows:

[0105] ,

[0106] in, The signal light frequency and initial phase are given by the following parameters: canonical coordinate X and canonical momentum P. , This represents the amplitude of the signal light.

[0107] The signal light interferes with the local oscillator light, producing two interference outputs. Each output is then detected by a photodiode, generating two photocurrents, which are then differentially divided. The resulting differential photon count operator is...

[0108] ,

[0109] in, These are the photon count operators for the two photodiodes. To generate operators for photons, For photon annihilation operator, , The difference frequency between the local oscillator light and the signal light. This represents the phase difference between the local oscillator light and the signal light.

[0110] Due to the presence of vacuum shot noise, the corresponding differential current can be written as:

[0111] ,

[0112] Where R is the detector's response coefficient. This refers to the intermediate frequency signal. Vacuum shot noise.

[0113] ,

[0114] The phase difference between the vacuum state and the local oscillator light is given. The vacuum state has a random phase θ.

[0115] Since the pulse duration of the signal light is not less than twice the difference frequency variation period between the local oscillator signal and the signal light, the intermediate frequency signal will traverse the range [0, 2π) within one encoded pulse duration of the signal light. Therefore, the amplitude of the signal light can be obtained when the amplitude reaches its maximum value.

[0116] ,

[0117] The variance of signal current and the variance of vacuum shot noise are respectively

[0118] ,

[0119] ,

[0120] Therefore, the signal-to-noise ratio can be obtained. , Let be Planck's constant. The frequency of the optical signal is denoted as SNR. When SNR=1, it represents the sensitivity of the balanced detector, meaning it can detect one photon.

[0121] The raw electrical signal obtained from the signal light is first digitally filtered by selecting a bandpass filter centered on the difference frequency between the local oscillator signal and the signal light to filter out high-frequency and low-frequency signals.

[0122] Then the signal is rectified to obtain the absolute value of the signal.

[0123] ,

[0124] The maximum value is .

[0125] The rectified signal is then low-pass filtered to obtain the signal envelope amplitude, which is then detected according to a predetermined threshold. The threshold is set to... The portion greater than the threshold is bit 1, and the portion less than the threshold is bit 0.

[0126] The tracking module acquires the electrical signal response of each pixel unit, determines the incident deviation of the laser signal based on the distribution of the responding pixels, generates control commands to adjust the attitude or position of the array detector, so that the laser signal is always accurately incident on the effective area of ​​the array detector, and completes the precision tracking process.

[0127] The specific method by which the tracking module judges the incident deviation of the laser signal during precise tracking is as follows: when the laser signal is accurately incident on the central region of the array detector, the pixel units in the central region produce a high-amplitude response, while the pixel units in the edge region have a weak or no response; when the laser signal deviates from the central region, the response amplitude of the pixel units in the deviation direction increases, while the response amplitude of the pixel units in the central region decreases; the tracking module calculates the deviation direction and deviation amount of the laser signal based on the distribution differences of the response amplitude of each pixel unit, and then generates tracking control commands.

[0128] After establishing a stable space communication link through tracking, optical communication can proceed. Assuming the optical signal is encoded using OOK (Out of Memory) encoding, the optical signal of each pixel unit can be detected by an array balanced detection module, and the detection signal can be filtered, decoded, and error-corrected by a data processing module to complete the space optical communication process.

[0129] like Figure 4 As shown, Example 2:

[0130] The array balanced detection module includes a second array balanced detector and a local oscillator laser, wherein the second array balanced detector comprises N×M balanced detection units integrated on the same substrate and distributed in an array.

[0131] The local oscillator laser is used to output local oscillator light, which is coupled to the waveguide of the second array balanced detector through a polarization-maintaining fiber PMF and a mode converter MSC.

[0132] The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit.

[0133] Each balanced detection unit can detect single-photon level optical signals, including a directional coupler DC, two photodetectors PD and a signal processing module SP. One input port of the DC is used to input one local oscillator light, and the other input port is connected to a grating coupler GC to couple one signal light of the fiber array.

[0134] SP is used to rectify and detect peaks in the differential signals of two PDs, and compare the average of the peak values ​​with a predetermined threshold.

[0135] The amplitude encoding adopts the OOK encoding method. When the encoded signal is "1", a high amplitude level signal is output, and when the encoded signal is "0", a low amplitude level signal (or zero level signal) is output.

[0136] The transmitting optical path includes an optical fiber amplifier and a transmitting telescope, used to amplify the encoded optical signal to a predetermined power and then expand the beam through the transmitting telescope; the receiving optical path is a receiving telescope.

[0137] The modulation module includes a phase modulator and a polarization-maintaining beam splitter. The two output ports of the polarization-maintaining beam splitter are connected through the phase modulator to form a Sagnac loop, which is used to achieve stable intensity modulation through phase modulation.

[0138] The specific principle is as follows:

[0139] First, the system performs coarse tracking using tracking modules at both the transmitter and receiver to ensure that the coded optical signal emitted by the transmitter roughly falls within the range of the receiving optical path. Then, a fine tracking process is performed.

[0140] At the transmitting end, the encoding signal generation module generates an encoded signal, employing OOK encoding to encode the amplitude of the laser-generated optical signal. A Sagnac ring structure, consisting of a phase modulator and a polarization-maintaining beam splitter, is used for amplitude modulation of the optical signal. Since the two optical signal components generated by the polarization-maintaining beam splitter propagate along the clockwise and counterclockwise directions of the Sagnac ring respectively, they travel the same optical path. However, when they pass through the phase modulator, they modulate different phases. The phase difference is only related to the modulation phase difference; therefore, the operating point does not drift over time, enabling encoding. Subsequently, the encoded optical signal is beam-expanded through the transmitting optical path and emitted into space.

[0141] At the receiving end, the receiving optical path collects the optical signal, which is then transmitted through the fiber optic array to each pixel unit of the second array balanced detector for detection, and the optical signal is converted into a corresponding electrical signal. A schematic diagram of the second array balanced detector and each pixel is shown below. Figure 5 As shown.

[0142] For each pixel unit of the second array balanced detector, the local oscillator light generated by the local oscillator laser can be written as:

[0143] ,

[0144] in, , respectively, represent the amplitude, frequency, and initial phase of the local oscillator light, t is the period time, and j is the imaginary number.

[0145] Assuming the signal light received at the receiver after transmission through a spatial channel is very weak, on the order of a single photon, and exhibits quantum effects, it can be written as follows:

[0146] ,

[0147] in, The signal light frequency and initial phase are given by the following parameters: canonical coordinate X and canonical momentum P. , This represents the amplitude of the signal light.

[0148] The signal light interferes with the local oscillator light, producing two interference outputs. Each output is then detected by a photodiode, generating two photocurrents, which are then differentially divided. The resulting differential photon count operator is...

[0149] ,

[0150] in, These are the photon count operators for the two photodiodes. To generate operators for photons, For photon annihilation operator, , The difference frequency between the local oscillator light and the signal light. This represents the phase difference between the local oscillator light and the signal light.

[0151] Due to the presence of vacuum shot noise, the corresponding differential current can be written as:

[0152] ,

[0153] Where R is the detector's response coefficient. This refers to the intermediate frequency signal. Vacuum shot noise.

[0154] ,

[0155] The phase difference between the vacuum state and the local oscillator light is given. The vacuum state has a random phase θ.

[0156] Since the pulse duration of the signal light is not less than twice the difference frequency variation period between the local oscillator signal and the signal light, the intermediate frequency signal will traverse the range [0, 2π) within one encoded pulse duration of the signal light. Therefore, the amplitude of the signal light can be obtained when the amplitude reaches its maximum value.

[0157] ,

[0158] The variance of signal current and the variance of vacuum shot noise are respectively ,

[0159] ,

[0160] Therefore, the signal-to-noise ratio can be obtained. , Let be Planck's constant. The frequency of the optical signal is denoted as SNR. When SNR=1, it represents the sensitivity of the balanced detector, meaning it can detect one photon.

[0161] The raw electrical signal obtained from the signal light is first digitally filtered by selecting a bandpass filter centered on the difference frequency between the local oscillator signal and the signal light to filter out high-frequency and low-frequency signals.

[0162] The raw electrical signal obtained from the signal light is rectified to obtain the absolute value of the signal.

[0163] ,

[0164] The maximum value is Therefore, based on the M peak values ​​that appear in N sampled data... The average value of the peak value is obtained. .

[0165] For an OOK modulated signal, bit 0 corresponds to a signal optical amplitude of 0, and bit 1 corresponds to a signal optical amplitude of... The system noise is vacuum shot noise. Then a threshold can be predetermined. The average of the peak values With the predetermined threshold The signal light is compared, and if it is greater than the threshold, it is bit 1; otherwise, it is bit 0. This completes the demodulation of the signal light.

[0166] The tracking module acquires the electrical signal response of each pixel unit, determines the incident deviation of the laser signal based on the distribution of the responding pixels, generates control commands to adjust the attitude or position of the array detector, so that the laser signal is always accurately incident on the effective area of ​​the array detector, and completes the precision tracking process.

[0167] The specific method by which the tracking module judges the incident deviation of the laser signal during precise tracking is as follows: when the laser signal is accurately incident on the central region of the array detector, the pixel units in the central region produce a high-amplitude response, while the pixel units in the edge region have a weak or no response; when the laser signal deviates from the central region, the response amplitude of the pixel units in the deviation direction increases, while the response amplitude of the pixel units in the central region decreases; the tracking module calculates the deviation direction and deviation amount of the laser signal based on the distribution differences of the response amplitude of each pixel unit, and then generates tracking control commands.

[0168] After establishing a stable space communication link through tracking, optical communication can proceed. Assuming the optical signal is encoded using OOK (Out of Memory) encoding, the optical signal of each pixel unit can be detected by an array balanced detection module, and the detection signal can be filtered, decoded, and error-corrected by a data processing module to complete the space optical communication process.

[0169] As can be seen from the various embodiments of the present invention, the present invention proposes a space optical communication system and method based on a single-photon-level detection array. By employing a single-photon-level balanced detection array, sensitivity and communication distance can be improved; there is no need to use high-precision local oscillator light and complex heterodyne circuits for coherent detection; at the same time, the precision tracking and detection share the same array detector, eliminating the need for an additional precision tracking CCD and reducing system complexity; the array detection covers a multi-pixel area, and even if the laser is slightly deviated, some pixels still receive signals. With real-time tracking adjustment, detection efficiency can be improved and communication interruption can be avoided.

Claims

1. A space optical communication system based on a single-photon-level detector array, characterized in that, Includes the transmitter and receiver; The transmitting end includes: a laser for generating optical signals in the communication band; an encoding signal generation module for generating an encoded signal to be transmitted; a modulation module for amplitude encoding modulation of the optical signal according to the encoded signal; a transmitting optical path for expanding the encoded optical signal and transmitting it to the receiving end; and a transmitting end tracking module. The receiving end includes: a receiving optical path for collecting coded optical signals emitted by the transmitting end; an optical fiber array for transmitting the coded optical signals collected by the receiving optical path to an array balanced detection module; an array balanced detection module for detecting the optical signals transmitted by the optical fiber array and generating local oscillator light; a data processing module for processing the output signal of the array balanced detection module, wherein the symbol duration of the coded optical signal is not less than twice the difference frequency corresponding period between the local oscillator light and the signal light; and a receiving end tracking module. The transmitting end tracking module and the receiving end tracking module are used for coarse tracking, and for fine tracking by adjusting the pixel distribution positions based on the response of the array balance detection module. The array-balanced detection module includes multiple pixel units arranged in an array for detecting optical signals transmitted through the fiber optic array. Each pixel unit uses one local oscillator beam and one optical signal from the fiber optic array for balanced detection, and demodulates the signal light using a predetermined signal processing method. The specific method by which the two tracking modules determine the incident deviation of the laser signal during precise tracking is as follows: when the laser signal is accurately incident on the central region of the array detector, the pixel units in the central region produce a high-amplitude response, while the pixel units in the edge region have a weak or no response; when the laser signal deviates from the central region, the response amplitude of the pixel units in the deviation direction increases, while the response amplitude of the pixel units in the central region decreases; the tracking module calculates the deviation direction and deviation amount of the laser signal based on the distribution differences of the response amplitude of each pixel unit, and then generates tracking control commands.

2. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The array balanced detection module is a first array balanced detector, comprising a laser chip integrated on the same substrate and N×M balanced detection units distributed in an array, where N and M are both positive integers. The laser chip is connected to each balanced detection unit via a beam-splitting waveguide. The laser chip is used to output local oscillator light, which is coupled to a beam splitter waveguide through a lens; The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit. Each balanced detection unit is used to detect single-photon level optical signals and includes a multimode interference coupler, two photodetectors and a signal processing module. One input port of the multimode interference coupler is used to input one local oscillator light, and the other input port is coupled to the polarization-maintaining fiber PMF through a mode-spot converter to transmit one signal light of the fiber array. The signal processing module is used to perform digital filtering, signal rectification, and low-pass filtering on the differential signals from the two photodetectors, and output the envelope amplitude.

3. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The array balanced detection module includes a second array balanced detector and a local oscillator laser. The second array balanced detector comprises N×M balanced detection units integrated on the same substrate and distributed in an array, where N and M are both positive integers. The local oscillator laser is used to output local oscillator light, which is coupled to the waveguide of the second array balanced detector through polarization-maintaining fiber and mode converter. The local oscillator light is evenly distributed through a beam-splitting waveguide and transmitted to each balanced detection unit. Each balanced detection unit is used to detect single-photon level optical signals and includes a directional coupler, two photodetectors and a signal processing module. One input port of the directional coupler is used to input one local oscillator light, and the other input port is connected to a grating coupler to couple one signal light from the fiber array. The signal processing module is used to rectify and detect peaks in the differential signals from the two photodetectors, and compare the average value of the peaks with a predetermined threshold.

4. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The amplitude encoding adopts the OOK encoding method. When the encoded signal is "1", a high amplitude level signal is output, and when the encoded signal is "0", a low amplitude level signal or a zero level signal is output.

5. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The transmitting optical path includes an optical fiber amplifier and a transmitting telescope, used to amplify the encoded optical signal to a predetermined power and then expand the beam through the transmitting telescope; the receiving optical path is a receiving telescope.

6. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The modulation module includes an intensity modulator, a beam splitter, and a bias control module. The intensity modulator is used to perform intensity modulation encoding on the optical signal; The beam splitter is used to split the intensity-modulated optical signal into one path and transmit it to the bias control module as a feedback signal. The bias control module is used to control the bias voltage of the intensity modulator based on the feedback signal.

7. The space optical communication system based on a single-photon-level detector array as described in claim 1, characterized in that, The modulation module includes a phase modulator and a polarization-maintaining beam splitter. The two output ports of the polarization-maintaining beam splitter are connected through the phase modulator to form a Sagnac loop, which is used to achieve stable intensity modulation through phase modulation.

8. A space optical communication method based on a single-photon level detector array, implemented using the system described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Transmitter signal processing and transmission: The encoding signal generation module generates an encoded signal, the modulation module performs amplitude encoding on the optical signal generated by the laser, and transmits it into space after beam expansion through the transmission optical path; S2: Laser signal reception at the receiving end: The receiving optical path collects the optical signal, which enters each pixel unit of the array balanced detection module through the fiber array for detection, and converts the optical signal into the corresponding electrical signal; S3: Precise tracking and aiming adjustment: The tracking and aiming module of the receiving end obtains the electrical signal response of each pixel unit, judges the incident deviation of the light signal based on the distribution of the response pixels, and generates control commands to adjust the attitude or position of the array detector so that the light signal is always accurately incident on the effective area of ​​the array detector. S4: Signal superposition processing: The signal superposition module superimposes the electrical signals output by each pixel unit to obtain the total electrical signal; S5: Amplitude Demodulation and Restoration: The amplitude demodulation module demodulates the total electrical signal, extracts amplitude change information, restores the original baseband signal, and completes space laser communication.

9. The space optical communication method based on a single-photon-level detector array as described in claim 8, characterized in that, In step S3, after the receiver tracking control module determines the incident deviation, if the deviation of the light signal is less than the preset threshold, no tracking adjustment will be performed. If the deviation is greater than or equal to the preset threshold, then tracking adjustment will be initiated until the deviation is less than the preset threshold.

10. The space optical communication method based on a single-photon-level detector array as described in claim 8, characterized in that, The modulation module uses OOK encoding to encode the amplitude of the optical signal generated by the laser.

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