Free space optical communication and sensing integrated system based on intensity modulation direct detection and time domain cross-correlation

Abstract

The invention discloses a free space optical communication and sensing integrated system based on intensity modulation direct detection and time domain cross-correlation, and belongs to the field of optical communication, the system adopts an intensity modulation direct detection architecture, a transmitter uses a differential pulse position modulation signal, a DPIM signal is loaded to an optical carrier through a Mach-Zehnder modulator, and the DPIM signal is transmitted to the optical carrier through the Mach-Zehnder modulator; shaping the light beam and coupling the light beam into a free space; and the receiver couples the spatial optical signal into the optical fiber and demodulates the spatial optical signal after photoelectric conversion, so that a communication function is realized. And the transmitter receives the echo optical signal reflected by the receiver, performs analog-to-digital conversion on the echo optical signal, and then obtains the time delay of the echo signal compared with the transmitting signal by combining with a time domain cross-correlation algorithm, thereby obtaining the distance information of the two communication parties. According to the system, the radar ranging function is seamlessly integrated into a free space optical communication system, the communication and ranging functions can be simultaneously realized without additional hardware, the two functions can be simultaneously performed without mutual interference, and the system has the advantages of simple structure, low cost and high ranging precision.

Description

Technical Field

[0001] This invention belongs to the field of optical communication, and more specifically, relates to a free-space optical communication and sensing integrated system based on intensity modulation direct detection and time-domain cross-correlation. Background Technology

[0002] Integrated Sensing and Communication (ISAC) is an important technological direction for future space-air-ground networks and 6G systems. Compared with traditional wireless ISAC, the integration of optical communication and optical sensing can leverage the advantages of abundant optical spectrum resources, strong resistance to electromagnetic interference, and excellent beam directivity to provide higher precision and higher bandwidth integrated communication-sensing capabilities for scenarios such as inter-satellite optical links, unmanned system navigation, and intelligent driving perception.

[0003] In free-space optical communication (FSOC) systems, the transmitter typically consists of a signal source, laser, modulator, and collimating optical module; while the transmitter of a time-of-flight (ToF) lidar generally includes a signal drive unit and a directly modulated laser source. The two systems share a high degree of similarity in hardware structure, and the FSOC transmitter can functionally cover the main transmission requirements of lidar, providing a hardware foundation for the integration of the two types of systems.

[0004] However, traditional Time-of-Flight (ToF) based lidar relies on periodic pulse signals, which significantly conflicts with continuously high-speed modulated communication signals. Existing integrated solutions mainly include: (1) Time-sharing hardware architecture: Communication and ranging share hardware but work in turn, making it impossible to achieve synchronous tasks and resulting in low system utilization; (2) Waveform multiplexing scheme: Inserting a ranging frame into the communication sequence can achieve simultaneous function, but communication and ranging interfere with each other significantly, resulting in reduced communication rate and decreased ranging accuracy.

[0005] Therefore, there is an urgent need for an integrated optical communication-sensing solution that can achieve high-precision distance measurement without interrupting high-speed communication, and should have good compatibility with existing IMDD (Intensity-Modulation Direct Detection)-FSOC systems to facilitate engineering deployment. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a free-space optical communication and sensing integrated system based on intensity modulation direct detection and temporal cross-correlation, thereby solving the problems of communication interruption, functional conflict, and severe mutual interference in existing optical communication and laser ranging integrated systems.

[0007] To achieve the above objectives, according to a first aspect of the present invention, a free-space optical communication and sensing integrated system based on intensity modulation direct detection and temporal cross-correlation is provided, including a first and a second integrated sensing terminal; The first integrated sensing terminal includes: A communication signal generation unit is used to output the DPIM signal to be transmitted; The DAC module is used to perform digital-to-analog conversion on the DPIM signal to be transmitted; A laser used to output an optical carrier signal; A Mach-Zehnder modulator is used to load the DPIM signal to be transmitted onto an optical carrier signal and output a modulated optical signal. A 1:1 optical coupler is used to split a modulated optical signal into a first modulated optical signal and a second modulated optical signal. A transmitting optical antenna is used to transmit the first modulated optical signal as a forward communication signal; The forward communication signal is received by the second receiving optical antenna of the second integrated sensing terminal, and at the same time, the forward communication signal is reflected by the reflective surface of the second integrated sensing terminal to become a ranging echo signal. A first receiving optical antenna is used to receive the ranging echo signal; The second receiving optical antenna and the reflector are respectively used to receive the backward communication signal and reflect the backward communication signal when the transmitting optical antenna of the second integrated communication terminal transmits the backward communication signal; The first and second photodetectors and the avalanche photodetector are used to perform photoelectric conversion on the second modulated optical signal, the backward communication signal and the ranging echo signal, respectively. The ADC module is used to perform analog-to-digital conversion on the second modulated optical signal after photoelectric conversion, the backward communication signal, and the ranging echo signal. The cross-correlation ranging unit is used to perform cross-correlation calculations on the second modulated optical signal after analog-to-digital conversion and the ranging echo signal to obtain the communication distance between the first and second integrated sensing terminals. The communication signal demodulation unit is used to demodulate the backward communication signal after analog-to-digital conversion; The first and second integrated sensing terminals have the same structure.

[0008] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: The system provided by this invention employs a direct intensity modulation (DIIM) detection architecture. The transmitter uses a differential pulse position modulation (DPIM) signal, which is loaded onto an optical carrier via a Mach-Zehnder modulator. The beam is shaped and coupled into free space using a fiber optic collimator. The receiver uses a fiber optic collimator to couple the spatial light signal into the optical fiber and uses a photodiode for detection, thereby achieving communication. The transmitter uses a high-sensitivity avalanche diode photodetector to receive the weak echo light signal reflected from the receiver, and a high-precision analog-to-digital converter converts the analog signal into a digital signal. A time-domain cross-correlation algorithm is used to obtain the time delay of the echo signal relative to the transmitted signal, thus determining the distance information between the communicating parties. This system has the following advantages: 1. Uninterrupted integrated solution: Ranging relies solely on correlation processing, without altering the communication waveform and having no impact on communication performance.

[0009] 2. High hardware reusability and strong compatibility: The ranging module can directly reuse the IMDD-FSOC transmitter without the need for an additional laser or pulse source, making engineering implementation simple.

[0010] 3. High ranging accuracy: The time difference of flight is extracted by analyzing the temporal cross-correlation between the local reference signal and the echo ranging signal, thereby achieving high-precision ranging; ranging based on temporal cross-correlation can achieve sub-sampling level accuracy far higher than the sampling rate, which is suitable for medium and long distance FSO links.

[0011] 4. Scalable to bidirectional and full-duplex scenarios: Bidirectional communication and full-duplex operation can be achieved by deploying homogeneous hardware on both sides. It can achieve high-precision distance measurement between the two parties while maintaining the continuous operation of the high-speed IMDD communication link, without changing the communication signal format or inserting additional ranging frames.

[0012] 5. The communication optical signal and the ranging optical signal share the transmitting optical antenna and the receiving optical antenna, so that the system forms an integrated optical channel, thereby reducing the hardware size and improving the optical path collinearity.

[0013] 6. Communication modulation adopts IMDD technology, which enables communication rates to reach Gbps or higher.

[0014] In summary, the system provided by this invention seamlessly integrates radar ranging functionality into a free-space optical communication system, enabling simultaneous communication and ranging functions without additional hardware. The two functions can be performed concurrently without interference, offering advantages such as simple structure, low cost, and high ranging accuracy. Attached Figure Description

[0015] Figure 1This is a structural diagram of a free-space optical communication and sensing integrated system based on intensity modulation direct detection and temporal cross-correlation provided in an embodiment of the present invention. Figure 2 The signal spectrum diagram received by the communication signal receiving optical antenna provided in the embodiment of the present invention; Figure 3 The signal waveform diagram received by the communication signal receiving optical antenna provided in the embodiment of the present invention; Figure 4 This is a diagram showing the cross-correlation ranging results provided in an embodiment of the present invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0017] This invention provides a free-space optical communication and sensing integrated system based on intensity modulation direct detection and temporal cross-correlation, such as... Figure 1 As shown, it includes the first and second integrated sensing terminals; The first integrated sensing terminal includes: The communication signal generation unit is used to output the DPIM (Differential Pulse Interval Modulation) signal to be transmitted; The DAC module is used to perform digital-to-analog conversion on the DPIM signal to be transmitted; A laser used to output an optical carrier signal; A Mach-Zehnder modulator (MZM) is used to load the DPIM signal to be transmitted onto an optical carrier signal and output a modulated optical signal. A 1:1 optical coupler is used to split a modulated optical signal into a first modulated optical signal and a second modulated optical signal. The transmitting optical antenna (also known as the communication signal transmitting optical antenna) is used to transmit the first modulated optical signal as a forward communication signal; The forward communication signal is received by the second receiving optical antenna of the second integrated sensing terminal, and at the same time, the forward communication signal is reflected by the reflective surface of the second integrated sensing terminal to become a ranging echo signal. The first receiving optical antenna (also known as the ranging echo receiving optical antenna) is used to receive the ranging echo signal; The second receiving optical antenna (also known as the communication signal receiving optical antenna) is used to receive the backward communication signal when the transmitting optical antenna of the second integrated communication terminal transmits the backward communication signal; The reflective surface is used to reflect the backward communication signal when the transmitting optical antenna of the second integrated sensing terminal transmits the backward communication signal; The first and second photodetectors and the avalanche photodetector are used to perform photoelectric conversion on the second modulated optical signal, the backward communication signal and the ranging echo signal, respectively. The ADC module is used to perform analog-to-digital conversion on the second modulated optical signal after photoelectric conversion, the backward communication signal, and the ranging echo signal. The cross-correlation ranging unit is used to perform cross-correlation calculations on the second modulated optical signal after analog-to-digital conversion and the ranging echo signal to obtain the communication distance between the first and second integrated sensing terminals. The communication signal demodulation unit is used to demodulate the backward communication signal after analog-to-digital conversion; The first and second integrated sensing terminals have the same structure.

[0018] Preferably, the communication distance between the first and second integrated sensing terminals is... The sampling point offset corresponding to the peak position of the cross-correlation function between the second modulated optical signal after analog-to-digital conversion and the ranging echo signal. Satisfying the relation ;in, For ADC sampling rate, The speed of light in air.

[0019] Preferably, it also includes an electrical amplifier EA disposed between the DAC module and the Mach-Zehnder modulator.

[0020] Preferably, the system further includes an optical amplifier disposed between the 1:1 optical coupler and the transmitting optical antenna.

[0021] Preferably, the optical transmitting antenna is a fiber optic collimating lens.

[0022] Preferably, the system further includes an automatic alignment module, which is used to adjust the pointing of the transmitting optical antenna, the first receiving optical antenna and the second receiving optical antenna in real time according to the changes in the received light intensity, so as to ensure both communication quality and ranging accuracy.

[0023] Preferably, to improve coupling efficiency, the optical fiber connecting the second receiving optical antenna and the second photodetector is an OM3 multimode fiber, so as to utilize its larger numerical aperture to achieve higher light collection capability.

[0024] Specifically, the system provided by this invention includes two identical and symmetrical integrated sensing terminals (first and second integrated sensing terminals), enabling bidirectional full-duplex real-time communication and bidirectional high-precision distance measurement. Each terminal includes key components such as a modulation module, amplifier, digital-to-analog / analog-to-digital converter circuit, photodetector, and optical antenna, and can simultaneously perform functions such as generating and transmitting communication signals, receiving and demodulating communication signals, and receiving and cross-correlation calculation of ranging echo signals.

[0025] The following explanation uses the one-way processing flow of the first integrated sensor terminal for distance measurement as an example.

[0026] (1) Optical carrier signal generation: The laser generates an optical carrier, which is then input into the Mach-Zehnder modulator after polarization control.

[0027] (2) Modulated optical signal generation: The communication signal generation unit outputs a digital baseband signal (i.e., DPIM signal), which is converted into an analog signal by a digital-to-analog converter (DAC), amplified by an EA amplifier, and then loaded onto the MZM RF port. The MZM is biased at the linear operating point ( This is used to achieve light field intensity modulation and obtain modulated light signals.

[0028] (3) Optical beam splitting and local reference signal generation: The signal light modulated by MZM is split into two signals by a 3dB coupler, which are called the first modulated optical signal. Second modulated optical signal The two remain completely consistent; among them, As a forward communication signal, it is amplified by an optical amplifier before being transmitted. It serves as a local reference signal. The local reference signal is used for subsequent time-domain cross-correlation calculations.

[0029] (4) Beam shaping and collimation: In steps (1)-(3), the optical signal is transmitted in the optical waveguide. In order to couple it into free space, it is necessary to use an optical collimator to shape and expand the beam. That is, the optical collimator is used as the optical transmitting antenna for communication signals to transmit the forward communication signal and output it to free space.

[0030] (5) Free space channel transmission: The expanded optical signal (i.e. the forward communication signal) is propagated to the other end (i.e. the second integrated sensing terminal) in the form of a Gaussian beam. It is received by the communication signal receiving optical antenna of the second integrated sensing terminal. At the same time, it is reflected by the reflective surface of the second integrated sensing terminal to form a backward ranging echo signal. This backward ranging echo signal is received by the ranging echo receiving optical antenna of the first integrated sensing terminal.

[0031] Let the distance between the two communicating parties be... The speed of light in air is The forward communication signal travels a distance in free space. Then it reaches the second integrated sensor terminal, that is, the one-way delay is The reflective surface of the second integrated sensing terminal reflects the forward communication signal back to the first integrated sensing terminal, i.e., the distance the backward ranging echo signal propagates again. After returning to the first integrated sensor terminal, the total latency (i.e., round-trip latency) is... Ignoring amplitude fluctuations caused by turbulence, the forward communication signal received by the optical antenna of the second integrated sensing terminal is: The backward ranging echo signal received by the ranging echo receiving optical antenna of the first integrated sensing terminal is: .

[0032] (6) Photoelectric conversion and communication processing of forward communication signals: On the second integrated sensing terminal side, the beam of the forward communication signal is coupled to the optical fiber through the collimating lens and then converted into an electrical signal by the second photodetector. The signal is then sampled by the ADC and processed in the digital domain to complete the recovery and decision of the communication signal, thereby realizing high-speed communication.

[0033] (7) Reception and photoelectric conversion of backward ranging echo signal: On the first integrated sensing terminal side, the local reference light The signal is converted into an electrical signal by the first photodetector, and then used for backward ranging echo signal. After being coupled to an avalanche photodiode (APD) via a focusing lens, the signal is also converted into an electrical signal. The two signals are then simultaneously sampled by a high-speed ADC.

[0034] (8) Ranging based on time-domain cross-correlation: due to and Carrying identical communication information, their time difference is determined solely by the optical path length. By performing time-domain cross-correlation calculations on the two digital signals and extracting their cross-correlation peak values, the delay corresponding to the cross-correlation peak value is the round-trip propagation delay. This allows us to calculate the distance between the two communicating parties.

[0035] It is understandable that the above process describes the unidirectional processing of distance measurement by the first integrated sensor terminal. The processing flow for distance measurement by the second integrated sensor terminal is similar and will not be repeated here.

[0036] To verify system performance, a 100m free-space optical communication link was built based on the above structure, and ranging tests were conducted at 11 locations within the 0–100m range. The first and second integrated sensing terminals have identical structures and configurations: the laser used is a 1550 nm continuous-wave laser, the MZM model is FTM7937EZ200, the driving voltage is 1.8V, the communication signal generation unit outputs a 1GHz bandwidth DPIM signal, the optical amplifier is an EDFA, the forward communication signal is amplified to 15dBm by the EDFA, the communication signal transmitting optical antenna uses a Thorlabs C80APC-C fiber collimator, and the second photodetector is a Guilin Guangyi DET30C-5G with multimode fiber coupling input capability, an analog bandwidth of 5 GHz, and an ADC sampling frequency of 6GHz.

[0037] The two sampled digital signals are input into the cross-correlation ranging unit. The cross-correlation function is obtained through time-domain cross-correlation calculation, and the peak position corresponds to the sampling point offset. The formula for calculating the distance between the two terminals is: .in This represents the ADC sampling rate. At this sampling rate, the theoretical ranging error can be controlled within ±5 cm. The ranging results for 11 test points within the 0–100 m range are shown below. Figure 4 As shown, except for Loc1 and Loc11 which are used for hardware delay calibration, the ranging error of the other test points is within ±5 cm.

[0038] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A free-space optical communication and sensing integrated system based on intensity modulation direct detection and temporal cross-correlation, characterized in that, Including the first and second integrated sensor terminals; The first integrated sensing terminal includes: A communication signal generation unit is used to output the DPIM signal to be transmitted; The DAC module is used to perform digital-to-analog conversion on the DPIM signal to be transmitted; A laser used to output an optical carrier signal; A Mach-Zehnder modulator is used to load the DPIM signal to be transmitted onto an optical carrier signal and output a modulated optical signal. A 1:1 optical coupler is used to split a modulated optical signal into a first modulated optical signal and a second modulated optical signal. A transmitting optical antenna is used to transmit the first modulated optical signal as a forward communication signal; The forward communication signal is received by the second receiving optical antenna of the second integrated sensing terminal, and at the same time, the forward communication signal is reflected by the reflective surface of the second integrated sensing terminal to become a ranging echo signal. A first receiving optical antenna is used to receive the ranging echo signal; The second receiving optical antenna and the reflector are respectively used to receive the backward communication signal and reflect the backward communication signal when the transmitting optical antenna of the second integrated communication terminal transmits the backward communication signal; The first and second photodetectors and the avalanche photodetector are used to perform photoelectric conversion on the second modulated optical signal, the backward communication signal and the ranging echo signal, respectively. The ADC module is used to perform analog-to-digital conversion on the second modulated optical signal after photoelectric conversion, the backward communication signal, and the ranging echo signal. The cross-correlation ranging unit is used to perform cross-correlation calculations on the second modulated optical signal after analog-to-digital conversion and the ranging echo signal to obtain the communication distance between the first and second integrated sensing terminals. The communication signal demodulation unit is used to demodulate the backward communication signal after analog-to-digital conversion; The first and second integrated sensing terminals have the same structure.

2. The system as described in claim 1, characterized in that, Communication distance between the first and second integrated sensing terminals The sampling point offset corresponding to the peak position of the cross-correlation function between the second modulated optical signal after analog-to-digital conversion and the ranging echo signal. Satisfying the relation ;in, For ADC sampling rate, The speed of light in air.

3. The system as described in claim 1 or 2, characterized in that, It also includes an electrical amplifier EA located between the DAC module and the Mach-Zehnder modulator.

4. The system as described in claim 1 or 2, characterized in that, It also includes an optical amplifier positioned between the 1:1 optical coupler and the transmitting optical antenna.

5. The system as described in claim 1 or 2, characterized in that, The optical transmitting antenna is a fiber optic collimating lens.

6. The system as described in claim 1 or 2, characterized in that, It also includes an automatic alignment module, which is used to adjust the orientation of the transmitting optical antenna, the first receiving optical antenna, and the second receiving optical antenna in real time according to changes in the received light intensity.

7. The system as described in claim 1 or 2, characterized in that, The optical fiber connecting the second receiving optical antenna and the second photodetector is an OM3 multimode optical fiber.

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