Low-earth orbit satellite laser communication system and method

Abstract

This application relates to a low-speed satellite laser communication system and method, belonging to the field of laser communication technology. The low-speed satellite laser communication system includes an optomechanical unit and an electronic unit. The transceiver module of the optomechanical unit adopts a common aperture design. A single quadrant avalanche photodiode is placed at the focal plane of the common aperture optical antenna in the receiving optical path to convert the optical signal into four electrical signals. The signal detection module of the electronic unit is connected to the quadrant avalanche photodiode to preprocess the four electrical signals, separating the position detection signal and the communication signal. The main control communication module processes the communication signal to recover data and generates tracking commands based on the position detection signal. The fast-reflecting mirror in the optomechanical unit is controlled by a fast-reflecting mirror drive module. This application achieves both communication reception and precise tracking functions simultaneously using a single QAPD detector, significantly reducing the terminal size, weight, and power consumption, making it suitable for microsatellite platforms.

Description

Technical Field

[0001] This application relates to the field of laser communication technology, and in particular to a low-speed satellite laser communication system and communication method. Background Technology

[0002] Satellite laser communication, with its advantages of high bandwidth, strong security, and no need for spectrum licensing, has become a core technology for high-speed inter-satellite data transmission. With the large-scale deployment of low-Earth orbit satellite constellations, extremely stringent requirements are placed on laser communication terminals mounted on satellite platforms. Stable and reliable laser link establishment and data communication must be achieved within extremely limited size, weight, and power consumption constraints (such as the standard 2U CubeSat size). Therefore, developing highly integrated, lightweight, and miniaturized low-speed laser communication systems is of crucial practical significance for reducing satellite payload costs and realizing large-scale network applications.

[0003] To meet miniaturization requirements, existing technologies typically employ a shared-aperture optical design for both transmit and receive to save space, and seek integration at the signal reception and processing levels. For example, patent CN115483974A discloses a lightweight, compact laser communication optical transceiver with a large field of view and shared-aperture, which uses two quadrant detectors (QAPDs) working in tandem: the first is used for large-area scanning and acquisition, reducing the uncertainty area; the second is used for subsequent fine tracking and communication signal reception. Its output signal needs to be split for calculating the spot position and demodulating the communication data. Existing technologies mostly aim to reuse parts of the optical path to achieve functional integration.

[0004] Although the aforementioned existing technologies have promoted the miniaturization of terminals to some extent, they typically require multiple independent photoelectric sensors and their corresponding processing circuits, which directly leads to an increase in the number of internal components, complex structure, large space occupation, and increased power consumption. Furthermore, they present challenges in optical axis calibration and signal synchronization among multiple detectors, making it difficult to meet the current satellite platform's urgent needs for payload miniaturization, high integration, and low power consumption. Summary of the Invention

[0005] The purpose of this application is to provide a low-speed satellite laser communication system and communication method to solve the problems existing in the prior art.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows: This application provides a low-speed satellite laser communication system, comprising: The optomechanical unit includes a common aperture optical antenna, a fast reflector module, and a transceiver module. The transceiver module includes a transmitting optical path and a receiving optical path. The receiving optical path includes a single quadrant avalanche photodiode located at the focal plane of the common aperture optical antenna. The quadrant avalanche photodiode is used to convert the received optical signal into four independent electrical signals. The electronic unit includes a signal detection module, a main control communication module, and a fast-reflecting mirror drive module. The signal detection module is connected to the quadrant avalanche photodiode and is used to preprocess the four electrical signals and separate the position detection signal and the communication signal. The main control communication module is connected to the signal detection module and is used to process the communication signal to recover the communication data and generate tracking commands based on the position detection signal. The fast-reflecting mirror drive module is connected to the main control communication module and the fast-reflecting mirror module and is used to receive the tracking commands and drive the fast-reflecting mirror module to move.

[0007] Furthermore, the signal detection module includes: Four parallel amplification channels are connected to the four quadrants of the quadrant avalanche photodiode, respectively, for transimpedance amplification and subsequent amplification of the electrical signals in each quadrant. An adder whose input is connected to the output of four amplification channels, used to add the four amplified signals together to output a communication signal; Four integration channels, whose inputs are connected to the outputs of four amplification channels respectively, are used to integrate the amplified signals from each channel to output position detection signals. The bias circuit is connected to the quadrant avalanche photodiode and provides the bias voltage.

[0008] Furthermore, the integration channel includes an integration circuit, which includes an operational amplifier, an input resistor, and a feedback capacitor. The input resistor is connected in series between the input terminal of the integration circuit and the non-inverting input terminal of the operational amplifier. The feedback capacitor is connected between the non-inverting input terminal and the output terminal of the operational amplifier. The integration circuit also includes a DC feedback resistor connected in parallel with the feedback capacitor.

[0009] Furthermore, the main control communication module includes an analog-to-digital converter and a digital signal processor. The analog-to-digital converter is used to synchronously sample the four electrical signals corresponding to the communication signal to obtain four digital signals. The digital signal processor is used to perform time synchronization calibration and amplitude gain calibration on the four digital signals in sequence, and merge the calibrated four digital signals into one high signal-to-noise ratio signal. Then, bit synchronization, frame synchronization and decoding operations are performed on the high signal-to-noise ratio signal to restore the communication data.

[0010] Furthermore, the electronic unit includes a power supply and laser drive module, which is connected to the main control communication module to receive communication data and provide modulated drive current. The transmitting optical path includes a laser and a transmitting optical fiber. The laser is connected to a power supply and a laser driving module. It generates a communication laser signal that directly modulates the communication data onto itself through a driving current. The communication laser signal is transmitted to the transmitting optical path through the transmitting optical fiber.

[0011] Furthermore, the laser operates at a wavelength of 800 nm.

[0012] Furthermore, the transceiver module includes a beam splitter, which is positioned between the outgoing optical path of the transmitting fiber and the receiving optical path of the common aperture optical antenna. The beam splitter is used to transmit the emitted laser to the common aperture optical antenna for emission and to reflect the external signal light collected by the common aperture optical antenna to the quadrant avalanche photodiode. The common aperture optical antenna, the fast reflector module, and the beam splitter together constitute the transceiver common aperture optical path.

[0013] Furthermore, the common aperture optical antenna includes a folding telescope with an aperture of 30 mm.

[0014] Furthermore, the incident light path of the telescope is equipped with a solar filter at the front end.

[0015] Furthermore, the system includes an optical engine housing and a processing housing. The optical engine unit is integrated within the optical engine housing, and the circuit boards of the various modules of the electronic unit are stacked in multiple layers within the processing housing. The optical engine housing and the processing housing are an integrated structure stacked vertically, so that the whole system can be accommodated within the standard envelope of a 2U CubeSat.

[0016] This application also provides a communication method based on a low-speed satellite laser communication system according to any one of the above, comprising the following steps: S1. Transmit modulated communication laser signals through the transmitting optical path; S2: Receives optical signals from the other end through a common aperture optical antenna and fast reflector module, and focuses them onto a quadrant avalanche photodiode; S3: Converts optical signals into four independent electrical signals using quadrant avalanche photodiodes; S4: The signal detection module processes the four electrical signals to separate the position detection signal and the communication signal; S5: Processes communication signals through the main control communication module to recover communication data; S6: Simultaneously, the main control communication module generates tracking commands based on the position detection signals, and controls the movement of the fast-reflecting mirror module through the fast-reflecting mirror drive module to achieve closed-loop stable tracking of the optical link.

[0017] Furthermore, S4 includes the following steps: S41. The four electrical signals output by the object-limited avalanche photodiode are respectively amplified by transimpedance and then amplified by the subsequent stage. S42: Integrate the amplified four signals separately to obtain four integrated signals; S43: Add the amplified four signals together to obtain the communication signal; S44: Calculate and generate a position detection signal based on the amplitude difference between the four integrated signals.

[0018] Furthermore, S5 includes the following steps: S51. Synchronously sample the four electrical signals corresponding to the communication signal to obtain four digital signals; S52. Perform time synchronization calibration and amplitude gain calibration on the four digital signals obtained by sampling in sequence to eliminate the deviation between channels; S53. Combine the calibrated four digital signals into one high signal-to-noise ratio signal; S54. Perform bit synchronization, frame synchronization and decoding operations on high signal-to-noise ratio signals to finally restore communication data.

[0019] The beneficial effects of the technical solution provided in this application include at least the following: (1) This application uses a single QAPD as both a communication and fine tracking detector, and achieves signal separation through a signal detection module, which effectively reduces the overall size, weight and power consumption of the terminal.

[0020] (2) The single QAPD detector in this application effectively avoids the precise calibration of the optical axis between multiple detectors, simplifies the assembly and adjustment process, and simulates the front-end separation scheme to physically separate the communication and tracking signals at the earliest stage, reducing the real-time and computing power requirements of the back-end digital processor and improving the reliability and response speed of the system. Attached Figure Description

[0021] The accompanying drawings are provided to further illustrate the present application and form part of the specification. They are used together with the embodiments of the present application to explain the application and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the frame structure of the low-speed satellite laser communication system in one embodiment of the present invention; Figure 2 This is a schematic diagram of the photosensitive surface of a quadrant avalanche photodiode in one embodiment of the present invention; Figure 3 This is a schematic diagram of the working process of the signal detection module in one embodiment of the present invention; Figure 4 This is a circuit diagram of an integrating circuit in one embodiment of the present invention; Figure 5 This is a flowchart illustrating the process of the main control communication module processing communication signals in one embodiment of the present invention; Figure 6 This is a schematic diagram of the structure of the optical engine housing and the processing unit housing in one embodiment of the present invention; Figure 7 This is a schematic diagram of the structure of the optomechanical unit integrated into the optomechanical housing in one embodiment of the present invention; Figure 8This is a schematic diagram of the structure of the electronic unit integrated into the processor housing in one embodiment of the present invention.

[0022] Explanation of key figure labels: 10. Optomechanical unit; 110. Common aperture optical antenna; 111. Solar filter; 120. Fast reflector module; 130. Transceiver module; 131. Quadrant avalanche photodiode; 132. Transmitting fiber; 133. Beam splitter; 20. Electronic unit; 210. Signal detection module; 211. Amplification channel; 212. Adder; 213. Integrator channel; 214. Bias circuit; 220. Main control communication module; 230. Fast reflector drive module; 240. Power supply and laser drive module; 30. Optomechanical housing; 40. Processor housing. Detailed Implementation

[0023] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] In this specification, identical components are represented by the same reference numerals. It should be noted that the terms "front," "rear," "left," "right," "upper," and "lower" used in the following description refer to directions in the accompanying drawings, while the terms "bottom surface," "top surface," "inner," and "outer" refer to directions towards or away from a specific component, respectively. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this specification, "multiple" means two or more. Example 1

[0025] Please refer to Figures 1-8A low-speed satellite laser communication system includes an optomechanical unit 10 and an electronic unit 20. The optomechanical unit 10 includes a common-aperture optical antenna 110, a fast-reflecting mirror module 120, and a transceiver module 130. The transceiver module 130 includes a transmitting optical path and a receiving optical path. The receiving optical path includes a single quadrant avalanche photodiode 131 located at the focal plane of the common-aperture optical antenna 110. The quadrant avalanche photodiode 131 is used to convert the received optical signal into four independent electrical signals. The electronic unit 20 includes a signal detection module 210 and a main control communication module 220. Module 220 and fast-reflecting mirror drive module 230, signal detection module 210 connected to quadrant avalanche photodiode 131, are used to preprocess four electrical signals and separate position detection signals and communication signals. Main control communication module 220 connected to signal detection module 210 is used to process communication signals to recover communication data and generate tracking commands based on position detection signals. Fast-reflecting mirror drive module 230 is connected to main control communication module 220 and fast-reflecting mirror module 120, and is used to receive tracking commands and drive fast-reflecting mirror module 120 to move.

[0026] In this embodiment, as Figure 1 , Figure 2 As shown, the optomechanical unit 10 adopts a common aperture structure for transmitting and receiving. The common aperture optical antenna 110 serves as both a collimator for transmitting signals and a collector for receiving signals. The fast reflector module 120 uses a fast reflector (FSM) driven by a two-dimensional voice coil motor, with a deflection range of not less than ±3mrad. The fast reflector is precisely calibrated and placed between the common aperture optical antenna 110 and the back-end optical path, located on the common path of the entire common aperture optical path. The transceiver module 130 in the optomechanical unit 10 includes a transmitting optical path and a receiving optical path. The transmitting optical path can transmit communication laser signals. The quadrant avalanche photodiode 131 (QAPD) in the receiving optical path is located at the focal plane of the common aperture optical antenna 110. The photosensitive surface of the quadrant avalanche photodiode 131 is uniformly divided into four independent sector quadrants (A, B, C, D) by a cross-shaped channel. The distant signal light collected by the common aperture optical antenna 110 is precisely focused onto the photosensitive surface of the QAPD, so that the light spot size occupies 50%-80% of the size of the photosensitive surface. The quadrant avalanche photodiode converts the optical signals received in the four quadrants into four independent electrical signals and transmits them to the signal detection module 210.

[0027] Electronic unit 20 is responsible for signal processing, calculation and control, including signal detection module 210, main control communication module 220 and fast reflector drive module 230. Signal detection module 210 is a dedicated analog signal processing circuit board, which is directly connected to the electrodes of the four quadrants of quadrant avalanche photodiode 131. It receives four independent electrical signals from quadrant avalanche photodiode, preprocesses the four electrical signals, and can separate the received electrical signals into position detection signals and communication signals, and transmit the separated signals to main control communication module 220. The main control communication module 220 can be an FPGA circuit board, which is connected to the signal detection module 210. It receives the position detection signal and communication signal separated by the signal detection module 210 and processes them separately. For the communication signal, the main control communication module 220 performs recovery processing to recover the original communication data from the transmitting end. For the position detection signal, the main control communication module 220 calculates the position deviation of the light spot relative to the center on the QAPD in real time based on the position detection signal, and generates a closed-loop control command (i.e., a tracking command) based on this deviation, which is then transmitted to the fast reflector drive module 230. The fast reflector drive module 230 receives the tracking command from the main control communication module 220, converts it into an analog drive voltage via a high-precision DAC, amplifies it, and drives the voice coil motor of the fast reflector module 120 (FSM) to control the deflection of the reflector, thereby compensating for the position offset of the received light spot and achieving stable tracking.

[0028] The aforementioned structure employs a single QAPD as both a communication and fine-tracking detector, and separates the signals through a signal detection module. This eliminates the need for separate communication detectors (such as APDs) or multiple detectors used for coarse and fine tracking in traditional solutions, significantly reducing the overall terminal size, weight, and power consumption. Furthermore, the single QAPD detector configuration effectively avoids the need for precise optical axis calibration between multiple detectors, simplifying the assembly and adjustment process. The simulated front-end separation scheme physically separates communication and tracking signals at the earliest stage, reducing the real-time performance and computing power requirements of the back-end digital processor and improving system reliability and response speed. The avalanche multiplication effect of the QAPD provides high detection sensitivity, ensuring sufficient link margin for low-speed communication. As a communication detector, the QAPD has a large receiving field of view. Communication between CubeSats is generally inter-orbit communication with a fixed lead angle, thus eliminating the need for additional lead pointing units; the tracking point can be set to a fixed offset.

[0029] In the specific structure of the signal detection module 210, the signal detection module 210 includes four parallel amplification channels 211, an adder 212, four integration channels 213, and a bias circuit 214. The four amplification channels 211 are respectively connected to the four quadrants of the quadrant avalanche photodiode 131, and are used to perform transimpedance amplification and subsequent amplification of the electrical signals in each quadrant. The input terminal of the adder 212 is connected to the output terminal of the four amplification channels 211, and is used to add the four amplified signals to output a communication signal. The input terminals of the four integration channels 213 are respectively connected to the output terminals of the four amplification channels 211, and are used to perform integration processing on the amplified signals to output a position detection signal. The bias circuit 214 is connected to the quadrant avalanche photodiode and provides a bias voltage.

[0030] In this embodiment, as Figure 3 As shown, the core function of the signal detection module 210 is to receive four raw current signals from the quadrant avalanche photodiode 131 (QAPD) and perform preliminary conditioning and functional separation. The bias circuit 214 generates a stable high-voltage DC power supply (+36V), which is applied to the common cathode (or anode, depending on the device structure) of the QAPD through a high-resistance resistor network. This reverse bias voltage is used to operate the QAPD in the avalanche multiplication region, thereby increasing its internal gain by hundreds of times and greatly improving the module's sensitivity to weak light signals.

[0031] The four parallel amplification channels 211 are completely symmetrical in circuit layout and component parameters to ensure consistent processing. Each channel corresponds to a quadrant (A, B, C, D) of the QAPD and includes a transimpedance amplifier (TIA) and a subsequent voltage amplifier in sequence. The transimpedance amplifier, as the first stage of amplification, has its inverting input directly connected to the output electrode of the corresponding quadrant of the QAPD. It converts the nanoampere to microampere photocurrent generated in the quadrant into a voltage signal, simultaneously performing current-to-voltage conversion and preliminary amplification, and exhibiting low noise characteristics. The subsequent voltage amplifier, connected after the TIA, is typically a non-inverting amplifier. Its function is to further amplify the voltage signal output by the TIA to an amplitude range suitable for subsequent circuit processing and to provide sufficient drive capability. The output of the subsequent amplifier of each channel outputs two signals.

[0032] Adder 212 is a summing circuit. Its four input terminals are respectively connected to the first output terminal of the voltage amplifier in the four amplification channels 211. Adder 212 performs the operation Vsum = VA + VB + VC + VD. Its output Vsum is the communication signal. This signal contains the communication information carried by all the optical power incident on the QAPD and has the highest signal-to-noise ratio.

[0033] The four integration channels 213 consist of four independent integration circuits. The input of each integration circuit is connected to the second output of the voltage amplifier in the corresponding amplification channel 211 to realize the integration function. The output signal is proportional to the time integral of the input voltage, thereby effectively extracting the low-frequency or DC component in the signal (corresponding to the static or slowly changing position information of the light spot). At the same time, it provides a DC feedback path for the circuit to prevent the amplifier from saturating due to input offset or extremely low frequency signals. The outputs VintA, VintB, VintC, and VintD of the four integration channels 213 are the four position detection signals (or tracking component signals).

[0034] In the above structure, a single QAPD detector, combined with the unique separation and processing architecture of the signal detection module 210 (i.e., each amplification channel 211 outputs two paths, supplying the adder 212 and the integrator respectively), achieves physical separation and preprocessing of the communication signal and the tracking signal at the analog circuit level and simultaneously. Amplification, summation, and integration are all performed at the analog front end, effectively reducing processing latency.

[0035] In the specific structure of the integration channel 213, the integration channel 213 includes an integration circuit, which includes an operational amplifier, an input resistor, and a feedback capacitor. The input resistor is connected in series between the input terminal of the integration circuit and the non-inverting input terminal of the operational amplifier. The feedback capacitor is connected between the non-inverting input terminal and the output terminal of the operational amplifier. The integration circuit also includes a DC feedback resistor connected in parallel with the feedback capacitor.

[0036] In this embodiment, as Figure 4 As shown, the integrator circuit consists of operational amplifier U1, input resistor R1, and feedback capacitor C3. Input resistor R1 is connected in series at the input of operational amplifier U1, serving as the input current-limiting resistor. Feedback capacitor C3 is connected between IN+ and signal_OUT of operational amplifier U1. When processing low-frequency or DC signals, feedback capacitor C3 can block current, leading to reduced feedback and output voltage saturation. To avoid this, a DC feedback resistor R2 is typically connected in parallel in the circuit. This resistor provides necessary support for the feedback path, offering a stable DC operating point and limiting the lower frequency range used to perform the integration function.

[0037] When the input signal signal_IN is first connected to the input terminal of operational amplifier U1, the feedback capacitor C3 is not charged, and its resistance is close to zero, similar to a short circuit. At this time, the current in the input resistor R1 reaches its maximum. According to the characteristics of operational amplifier U1, no current flows into the amplifier, causing IN+ to appear as a virtual ground, resulting in a zero output voltage. Due to the extremely low impedance of the capacitor, the gain ratio Xc / R1 is also very small, so the total voltage gain approaches 1, similar to a voltage follower circuit. As the input signal is applied, the feedback capacitor C3 begins to charge, and the potential voltage across the feedback capacitor C3 gradually increases, while the charging current decreases accordingly. As the capacitor impedance increases, Xc / R1 gradually increases, resulting in a linearly increasing ramp output voltage. This process continues until the feedback capacitor C3 is fully charged. At the moment the feedback capacitor C3 is fully charged, it is equivalent to an open circuit, hindering the further flow of DC current. At this time, the ratio Xc / R1 of the feedback capacitor C3 to the input resistor R1 becomes infinitely large, providing extremely high gain to operational amplifier U1, similar to the open-loop gain of an operational amplifier. The integrator circuit after the QAPD detector amplifies and integrates the raw signal acquired by the QAPD. The integrated signal is then input into the main control board as the system tracking parameter (i.e., position detection signal) for processing.

[0038] In the specific structure of the main control communication module 220, the main control communication module 220 includes an analog-to-digital converter and a digital signal processor. The analog-to-digital converter is used to synchronously sample the four electrical signals corresponding to the communication signal to obtain four digital signals. The digital signal processor is used to perform time synchronization calibration and amplitude gain calibration on the four digital signals in sequence, and merge the calibrated four digital signals into one high signal-to-noise ratio signal. Then, bit synchronization, frame synchronization and decoding operations are performed on the high signal-to-noise ratio signal to restore the communication data.

[0039] In this embodiment, as Figure 5 As shown, the main control communication module 220 is a digital processing unit with a digital signal processor (DSP) or field programmable gate array (FPGA) as its core. Its main task is to perform coordinated processing of multiple signals derived from QAPD from the signal detection module 210 in order to accurately recover communication data.

[0040] The analog-to-digital converter (ADC) employs a high-precision ADC chip with four-channel synchronous sampling capability, or is composed of four single-channel ADCs with strictly synchronized clocks. Its four analog input terminals are respectively connected to the output terminals of the four parallel amplification channels 211 in the signal detection module 210, directly digitizing the original amplified four-quadrant voltage signals VA, VB, VC, and VD. The synchronous sampling mechanism ensures the capture of the four signals at the same time, preserving accurate phase relationships for subsequent processing.

[0041] The digital signal processor (DSP / FPGA) is the core of the module, and it internally implements a dedicated digital signal processing flow, which mainly includes: Time Synchronization Calibration Unit: This unit receives four digital sequences sampled by the ADC. Due to differences in PCB trace length, slight clock skew of the sample-and-hold circuits of each ADC channel, or inconsistencies in the response delays of each quadrant of the QAPD, these four signals may have a slight time offset of several sampling periods. This unit estimates the time delay of each signal relative to the reference channel through cross-correlation analysis or a preset calibration sequence, and uses digital delay lines or fractional delay filters to interpolate and compensate for the corresponding sequences, ensuring that the four signals are perfectly aligned on the time axis.

[0042] Amplitude Gain Calibration Unit: After time synchronization, this unit is responsible for correcting gain differences between channels. These differences stem from the feedback resistor of the transimpedance amplifier, the gain of the subsequent amplifier, and subtle inconsistencies between the channels of the ADC itself. In calibration mode, or using known uniform light spot illumination, this unit calculates the scaling factor of the four signal amplitudes and performs digital multiplication scaling on each digital signal during normal operation, ensuring that the amplitudes of the four digital signals are consistent for the same input light intensity.

[0043] The signal combining unit directly adds the four clean digital signals, after time and amplitude calibration, thereby reconstructing the total optical power signal theoretically received by an ideal single large-area detector in the digital domain, and outputting a single high signal-to-noise ratio digital signal. This process effectively suppresses uncorrelated noise in each channel and maximizes signal power.

[0044] Communication demodulation and decoding chain: This chain performs subsequent processing on the merged high signal-to-noise ratio signal. Bit synchronization: Extract the symbol clock from the high signal-to-noise ratio signal to determine the decision time for each data bit.

[0045] Frame synchronization: Identify specific frame header patterns in the data stream, determine the starting boundary of the data frame, and achieve alignment of bytes or data packets.

[0046] Demodulation and decoding: Demodulate according to the modulation method used by the system (such as OOK, PPM) and decode the channel code (such as RS code) to correct the bit errors that may occur during transmission and finally restore the original communication data of the sending end.

[0047] In addition, the electronic unit 20 includes a power supply and laser drive module 240, which is connected to the main control communication module 220 and is used to receive communication data and provide modulated drive current. The transmitting optical path includes a laser and a transmitting fiber 132. The laser is connected to the power supply and laser drive module 240 and is driven by the drive current to generate a communication laser signal that directly modulates the communication data onto itself. The communication laser signal is transmitted to the transmitting optical path via the transmitting fiber 132. Specifically, the operating wavelength of the laser is 800nm.

[0048] In this embodiment, the power supply and laser drive module 240 is a highly integrated mixed-signal circuit board that combines power supply and communication modulation functions, including: The interface and data processing unit receives raw communication data streams and transmit enable control signals from the main control communication module 220.

[0049] A direct modulation driver converts the input communication data stream into a corresponding analog drive current in real time. This method of directly loading communication data onto the laser pump current is called direct modulation.

[0050] A precision constant current source and protection circuitry provide a stable, low-noise DC bias for the direct modulation driver, ensuring the laser operates near its optimal operating point. Simultaneously, integrated overcurrent, overtemperature, and electrostatic discharge protection circuitry guarantees long-term reliable laser operation.

[0051] The system power management unit provides the required DC voltages for the entire terminal (including itself, the main control communication module 220, the signal detection module 210, etc.).

[0052] The laser employs a semiconductor laser diode (LD) with a center wavelength of 800nm. Its anode and cathode are directly connected to the power supply and the modulation current output terminal of the laser drive module 240. The optical output characteristics of the laser are directly linearly related to the injected drive current. Therefore, after the drive current is modulated by data, its output optical power is also synchronously modulated, thereby generating a communication laser signal. The 800nm ​​band was chosen to balance device maturity, high efficiency, and atmospheric transmission characteristics.

[0053] The transmitting fiber 132 is a short single-mode or multimode fiber. One end of it is aligned and fixed with the output surface of the laser through a precision fiber coupler to receive the modulated light generated by the laser. The other end is connected to the inside of the optical head to guide the optical signal to the subsequent collimation and common aperture optical system.

[0054] In the above structure, direct modulation technology is used, which eliminates the need for independent external modulators (such as electro-optic modulators) that are large in size and power consumption commonly used in traditional high-speed laser communication. The modulation function is integrated into the driving circuit and the electro-optic conversion is completed by the laser itself, making the entire transmitter simpler and further reducing the number of components.

[0055] In the transceiver module 130, the transceiver module 130 also includes a beam splitter 133. The beam splitter 133 is disposed between the outgoing optical path of the transmitting fiber 132 and the receiving optical path of the common aperture optical antenna 110. It is used to transmit the emitted laser to the common aperture optical antenna 110 for emission and to reflect the external signal light collected by the common aperture optical antenna 110 to the quadrant avalanche photodiode 131. The common aperture optical antenna 110, the fast reflector module 120, and the beam splitter 133 together constitute the transceiver common aperture optical path. Specifically, the common aperture optical antenna 110 includes a folding telescope with an aperture of 30 mm. More specifically, a solar filter 111 is provided at the front end of the incident optical path of the folding telescope.

[0056] In this embodiment, as Figure 1 As shown, the common-aperture optical antenna 110 employs a folding telescope (specifically a Cassegrain type). Its primary mirror (parabolic) has an effective aperture of 30mm, and the secondary mirror (hyperboloid) is located before the focal point of the primary mirror, folding the optical path. This structure allows for an effective focal length exceeding 300mm within a physical telescope tube length of approximately 150mm, significantly compressing axial space and perfectly meeting the height limitations of the 2U unit. A solar filter 111 is integrated into the telescope's entrance window. This filter is designed for high transmittance at the system's communication wavelengths while exhibiting extremely high suppression of solar background light in the visible and near-infrared bands. It is used to pre-filter out strong solar stray light during reception, protecting the downstream detector and improving the signal-to-noise ratio.

[0057] The beam splitter 133 is a wavelength-selective optical plate, fixed inside the optical head at an angle of approximately 45 degrees, located between the output optical path of the transmitting fiber 132 and the image-side receiving optical path of the folding telescope. Its coating characteristics are optimized for the system's operating wavelength: high transmittance for the emitted laser wavelength (e.g., 808nm); and high reflectivity for the received signal wavelength.

[0058] The reflective surface of the fast-reflecting mirror module 120 is placed in the common parallel optical path between the beam splitter 133 and the secondary mirror of the telescope. Whether it is the emitted beam or the received beam, it must pass through the deflection control of this mirror before entering or leaving the main aperture of the telescope.

[0059] The telescope, fast-reflecting mirror module 120, and beam splitter 133 are connected in series to form a common-aperture optical path for transmission and reception. All space light signals are transmitted and collected through a single 30mm aperture telescope antenna, achieving complete aperture multiplexing.

[0060] By using beam splitter 133 and a common fast-reflecting mirror, the complete transmitting and receiving optical paths are combined within a single optical aperture, making the terminal structure more compact, reducing the number of moving parts and independent optics, lowering the assembly and adjustment complexity, and improving the mechanical stability and environmental adaptability of the entire optical system.

[0061] In addition, the system includes an optical engine housing 30 and a processor housing 40. The optical engine unit 10 is integrated in the optical engine housing 30, and the circuit boards of each module of the electronic unit 20 are stacked in multiple layers in the processor housing 40. The optical engine housing 30 and the processor housing 40 are an integrated structure stacked vertically, so that the whole can be accommodated within the standard envelope of a 2U CubeSat.

[0062] In this embodiment, as Figure 6 , Figure 7 , Figure 8 As shown, the communication system terminal adopts a highly integrated modular, layered stacked structure, mainly composed of two large rigid boxes joined together: The upper part of the optical engine housing 30 is a sealed metal shell with an optical window, which precisely integrates all the components of the entire optical engine unit 10. A common aperture optical antenna 110 is fixedly mounted on one side of the housing, with its optical axis perpendicular to the mounting surface. A solar filter 111 is glued and encapsulated at the optical window at the front of the telescope. Inside the housing, the drive mechanism and mirror of the fast reflector module 120 are precisely mounted sequentially along the optical axis. At the optical path bend, a dedicated mounting position is designed for the quadrant avalanche photodiode 131 (QAPD) and its pre-amplifier circuit, ensuring that its photosensitive surface is strictly located on the focal plane of the telescope. The end connector of the transmitting fiber 132 is also fixed in the corresponding position inside the housing and aligned with the beam splitter 133.

[0063] The processor housing 40 (lower part) is a sealed metal enclosure with a multi-layered internal cavity structure, specifically designed for the integration of the electronic unit 20. All circuit boards are mounted vertically in a multi-layered stack within the housing. All circuit boards are secured using guide rails, posts, or studs, with pre-drilled ventilation channels or thermal pads installed between the boards. The side walls of the housing are equipped with rectangular electrical connectors conforming to spaceborne standards for power supply, communication with the satellite platform, and thermal control interfaces.

[0064] The optical engine housing 30 and the processor housing 40 are rigidly connected by locating pins and fastening screws, forming a stable stack with the optical head on top and the processor on the bottom. Electrical and signal connections between the two housings are achieved through internal wiring harnesses or board-to-board connectors passing through their mating surfaces, with no exposed external cables. The overall shape of the stack is strictly controlled within a rectangular enclosure of 100mm long, 100mm wide, and 200mm high, fully conforming to the standard load mounting dimensions and interface specifications of a 2U CubeSat.

[0065] In the above structure, the optical engine box 30 and the processor box 40 are vertically stacked, with the relatively concentrated mass and structural rigidity of the optical engine box 30 placed at the top, while the mass body with relatively uniform mass distribution and containing most of the circuitry is placed at the bottom, which helps to form a lower and more stable system center of gravity. Example 2

[0066] A communication method based on a low-speed satellite laser communication system includes the following steps: S1. Transmit modulated communication laser signals through the transmitting optical path; S2: Receive the optical signal from the other end through the common aperture optical antenna 110 and the fast reflector module 120, and converge it onto the quadrant avalanche photodiode 131; S3: The optical signal is converted into four independent electrical signals by the quadrant avalanche photodiode 131; S4: The signal detection module 210 processes the four electrical signals to separate the position detection signal and the communication signal; S5: The main control communication module 220 processes the communication signals to recover the communication data; S6: At the same time, the main control communication module 220 generates a tracking command based on the position detection signal, and controls the movement of the fast-reflecting mirror module 120 through the fast-reflecting mirror drive module 230 to achieve closed-loop stable tracking of the optical link.

[0067] In this embodiment, the communication data to be transmitted is encoded (such as RS encoding) and modulated in the main control communication module 220, thereby controlling the laser driving circuit to drive the laser in a direct modulation manner. The generated modulated laser is transmitted through the transmitting fiber 132, initially collimated by the collimating lens group, reflected by the beam splitter 133, reflected by the fast reflector module 120 (FSM), and finally enters the telescope and is collimated into a parallel beam and emitted into space, pointing towards the target satellite.

[0068] Laser signals from the other satellite are captured and collected by the local telescope. After passing through a fast reflector module 120 (FSM) and a beam splitter 133, they converge onto the photosensitive surface of the QAPD, forming a tiny light spot. The QAPD converts the optical signal into four photocurrents proportional to the received light intensity in each quadrant. The signal detection module 210 performs synchronous operations: the four currents are amplified by transimpedance and then by a post-amplification stage. The amplified four voltage signals are fed in parallel into adder 212 for summation, resulting in a communication signal containing communication information. Simultaneously, the amplified four voltage signals are fed in parallel into four integrators to obtain the position detection signal.

[0069] The communication signal is sent to the main control communication module 220, where it undergoes a series of digital signal processing steps, including synchronization calibration, merging, demodulation, and decoding, to ultimately reconstruct the valid communication data sent by the other party and output it via the on-board bus. Simultaneously, four position detection signals are sent to the main control communication module 220 to calculate the position error of the light spot in the X and Y directions in real time. This error data is processed by a control algorithm (such as PID) to generate tracking commands to drive the fast reflector module 120. These commands are converted into driving voltages by the fast reflector drive module 230, causing the FSM to deflect accordingly and dynamically adjusting the receiving optical path to keep the light spot locked near the center of the QAPD, thus maintaining the stability of the laser link. This tracking closed loop operates completely in parallel with the communication demodulation process, without interference.

[0070] Specifically, S4 includes the following steps: S41, the four electrical signals output by the object-limited avalanche photodiode 131 are respectively amplified by transimpedance and then amplified by the subsequent stage; S42: Integrate the amplified four signals separately to obtain four integrated signals; S43: Add the amplified four signals together to obtain the communication signal; S44: Calculate and generate a position detection signal based on the amplitude difference between the four integrated signals.

[0071] In this embodiment, the four quadrants of the QAPD convert the received light intensity into photocurrent, and these four current signals are immediately sent to the corresponding four parallel amplification channels 211. Within each channel, the TIA first converts it into a voltage, which is then amplified to a preset level by a subsequent voltage amplifier. The first voltage signal output from the four channel amplifiers is synchronously fed into adder 212. Adder 212 instantly performs a four-way summation and outputs a communication signal. This signal directly reflects the instantaneous change in total optical power and contains modulated communication data, which is then sent to the main control communication module 220 for digitization and demodulation. The second voltage signal output from the four channel amplifiers is synchronously fed into their respective integration channels 213. The integrator performs continuous integration of the input signal. As the light spot moves on the QAPD, the light intensity received in each quadrant changes, causing the voltage output by the integrator to rise or fall at different slopes. The relative magnitudes of these four voltages directly encode the precise positional information of the light spot's deviation from the center in the X and Y directions. The final module outputs two sets of grid signals in parallel: one is a communication signal from adder 212; the other is a position detection signal from the four integrators. These two sets of signals are completely synchronized in time, but carry different information, and are provided to the main control communication module 220 for data recovery and tracking error calculation, respectively.

[0072] Additionally, S5 includes the following steps: S51. Synchronously sample the four electrical signals corresponding to the communication signal to obtain four digital signals; S52. Perform time synchronization calibration and amplitude gain calibration on the four digital signals obtained by sampling in sequence to eliminate the deviation between channels; S53. Combine the calibrated four digital signals into one high signal-to-noise ratio signal; S54. Perform bit synchronization, frame synchronization and decoding operations on high signal-to-noise ratio signals to finally restore communication data.

[0073] In this embodiment, under the control of a unified master clock, the ADC synchronously and at high speed samples the voltage signals from the four QAPD quadrants that have been amplified by the analog front end, and converts them into four synchronous digital sequence streams.

[0074] First, the time synchronization calibration unit analyzes the four digital sequences, calculates the relative delay between them using an algorithm, and performs digital preprocessing on the sequences with larger delays to eliminate time offsets caused by different physical paths, ensuring that signal components from the same optical pulse are at the same index position in the four sequences.

[0075] Next, the amplitude gain calibration unit performs amplitude normalization on the time-aligned signal, multiplies it by a pre-stored or real-time calculated calibration coefficient, eliminates amplitude differences, and ensures that the contribution weight of each quadrant to the total signal is accurate.

[0076] The signal combining unit sums the four perfectly matched digital signals after correction. Since the signal components are coherent, the signal amplitude increases by 4 times (power by 16 times) after summing, while the noise of each channel is largely uncorrelated, and the noise power only increases by about 4 times after summing. This theoretically improves the signal-to-noise ratio (SNR) by about 6 dB, producing a high SNR signal with significantly optimized quality.

[0077] The high signal-to-noise ratio signal is sent into the standard communication processing chain. The bit synchronization circuit first locks the bit clock, the frame synchronization module then finds the start position of the data packet, and finally the demodulation and decoding unit recovers the correct bit stream according to the set communication protocol, completes channel decoding and error correction, and outputs error-free raw communication data to be delivered to the on-board data system.

[0078] In the embodiments disclosed in this application, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments disclosed in this application according to the specific circumstances.

[0079] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A low-speed satellite laser communication system, characterized in that, include: The optomechanical unit includes a common aperture optical antenna, a fast reflector module, and a transceiver module. The transceiver module includes a transmitting optical path and a receiving optical path. The receiving optical path includes a single quadrant avalanche photodiode located at the focal plane of the common aperture optical antenna. The quadrant avalanche photodiode is used to convert the received optical signal into four independent electrical signals. The electronic unit includes a signal detection module, a main control communication module, and a fast-reflecting mirror drive module. The signal detection module is connected to the quadrant avalanche photodiode and is used to preprocess the four electrical signals and separate the position detection signal and the communication signal. The main control communication module is connected to the signal detection module and is used to process the communication signal to recover the communication data and generate a tracking command based on the position detection signal. The fast-reflecting mirror drive module is connected to the main control communication module and the fast-reflecting mirror module and is used to receive the tracking command and drive the fast-reflecting mirror module to move.

2. The low-speed satellite laser communication system according to claim 1, characterized in that, The signal detection module includes: Four parallel amplification channels are connected to the four quadrants of the quadrant avalanche photodiode, respectively, for transimpedance amplification and subsequent amplification of the electrical signals in each quadrant; An adder, whose input is connected to the output of the four amplification channels, is used to add the four amplified signals together and output the communication signal. Four integration channels, whose input terminals are respectively connected to the output terminals of the four amplification channels, are used to integrate the amplified signals of each channel and output the position detection signal. A bias circuit is connected to the quadrant avalanche photodiode and provides a bias voltage.

3. The low-speed satellite laser communication system according to claim 2, characterized in that, The integration channel includes an integration circuit, which includes an operational amplifier, an input resistor, and a feedback capacitor. The input resistor is connected in series between the input terminal of the integration circuit and the non-inverting input terminal of the operational amplifier. The feedback capacitor is connected between the non-inverting input terminal and the output terminal of the operational amplifier. The integration circuit also includes a DC feedback resistor connected in parallel with the feedback capacitor.

4. The low-speed satellite laser communication system according to claim 1, characterized in that, The main control communication module includes an analog-to-digital converter and a digital signal processor. The analog-to-digital converter is used to synchronously sample the four electrical signals corresponding to the communication signal to obtain four digital signals. The digital signal processor is used to sequentially perform time synchronization calibration and amplitude gain calibration on the four digital signals, and merge the calibrated four digital signals into one high signal-to-noise ratio signal. Then, bit synchronization, frame synchronization and decoding operations are performed on the high signal-to-noise ratio signal to restore the communication data.

5. The low-speed satellite laser communication system according to claim 1, characterized in that, The electronic unit includes a power supply and a laser drive module, which is connected to the main control communication module and is used to receive the communication data and provide modulated drive current. The transmitting optical path includes a laser and a transmitting optical fiber. The laser is connected to the power supply and laser driving module. It is driven by the driving current to generate a communication laser signal that directly modulates the communication data onto itself. The communication laser signal is transmitted to the transmitting optical path through the transmitting optical fiber. The laser operates at a wavelength of 800 nm.

6. The low-speed satellite laser communication system according to claim 5, characterized in that, The transceiver module includes a beam splitter, which is disposed between the outgoing optical path of the transmitting fiber and the receiving optical path of the common aperture optical antenna. The beam splitter is used to transmit the emitted laser to the common aperture optical antenna for emission and to reflect the external signal light collected by the common aperture optical antenna to the quadrant avalanche photodiode. The common aperture optical antenna, the fast reflector module and the beam splitter together constitute a transceiver common aperture optical path. The common aperture optical antenna includes a folding telescope with an aperture of 30 mm. The telescope is equipped with a solar filter at the front end of the incident optical path.

7. The low-speed satellite laser communication system according to any one of claims 1-6, characterized in that, It includes an optical engine housing and a processing housing. The optical engine unit is integrated in the optical engine housing, and the circuit boards of each module of the electronic unit are stacked in multiple layers and integrated in the processing housing. The optical engine housing and the processing housing are an integrated structure stacked vertically, so that the whole can be accommodated within the standard envelope of a 2U CubeSat.

8. A communication method based on a low-speed satellite laser communication system according to any one of claims 1-7, characterized in that, Includes the following steps: S1. Transmit the modulated communication laser signal through the transmitted optical path; S2: Receive the optical signal from the opposite end through the common aperture optical antenna and the fast reflector module, and converge it onto the quadrant avalanche photodiode; S3: The optical signal is converted into four independent electrical signals through the quadrant avalanche photodiode; S4: The signal detection module processes the four electrical signals to separate the position detection signal and the communication signal; S5: The communication signal is processed by the main control communication module to recover the communication data; S6: Simultaneously, the main control communication module generates the tracking command based on the position detection signal, and the fast-reflecting mirror drive module controls the movement of the fast-reflecting mirror module to achieve closed-loop stable tracking of the optical link.

9. The communication method according to claim 8, characterized in that, The S4 includes the following steps: S41. The four electrical signals output by the quadrant avalanche photodiode are respectively amplified by transimpedance and then amplified by a subsequent stage. S42: Integrate the amplified four signals separately to obtain four integrated signals; S43: Add the amplified four signals together to obtain the communication signal; S44: Calculate and generate the position detection signal based on the amplitude difference between the four integrated signals.

10. The communication method according to claim 8, characterized in that, The S5 includes the following steps: S51. The four electrical signals corresponding to the communication signal are synchronously sampled to obtain four digital signals; S52. Perform time synchronization calibration and amplitude gain calibration on the four digital signals obtained by sampling in sequence to eliminate the deviation between channels; S53. Combine the four calibrated digital signals into one high signal-to-noise ratio signal; S54. Perform bit synchronization, frame synchronization and decoding operations on the high signal-to-noise ratio signal to finally restore the communication data.

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