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Abstract
Traditional free-space laser communication systems use beacon and signal lights for target detection and alignment. However, these approaches are inaccurate owing to signal dispersion errors. To overcome this difficulty, we propose a new method using transient radio frequency (RF) signals to achieve highly accurate target detection and alignment. To validate the feasibility of our proposed method, we built an experimental multi-target space-laser communication system based on a rotating double prism and applied it to achieve multi-target space-laser communication. The results demonstrate the efficiency of the proposed method to capture multi-target positions in the field of view using wireless RF signals and a rotating double prism. In addition, we show that the system is capable of rapid scanning and accurate pointing as well as establishing a one-way stable communication with multiple targets. When the target is 36 cm away, the pointing accuracy of the system motor is less than 0.8°, the pointing time is 1.2 s, and the average pointing lateral error is 0.666 mm.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical communication has abundant bandwidth resources, which meet the demand for large-capacity, high-speed, and real-time data transmission, as well as promote the vigorous development of free-space laser communication [1]. Compared to traditional radio frequency (RF) communication, optical communication offers important advantages, such as high security and confidentiality, small emission aperture, strong anti-interference, high stability, and license-free spectrum usage [2,3]. Extensive research has been carried out on the applications of space-laser communication (SLC) among ground terminals, deep space, geosynchronous, and low-orbit satellites [4–7]. Nowadays, the development of FSO systems has been very mature, especially in Japan, the United States and Europe [8–12]. Fast and accurate optical link establishment is one of the most important steps in a free-space laser communication system. A typical optical link establishment method uses a beacon light to determine exact communication terminal positions [13,14]. In this process, key issues such as the beacon beam width, beacon optical power, uncertain area size, and capture probability need to be considered. The system has a high degree of integration and the algorithm is relatively complex [15–18]. However, the difference in wavelengths between the beacon and signal lights leads to dispersion errors, which is a major problem during optical link establishment [19]. Jiang et al. [19] proposed the use of an achromatic prism to solve the dispersion error issue. They designed an achromatic double-wedge scanner for free-space optical communication with wavelengths of 800 and 1550 nm. Researchers have designed achromatic prisms for different wavelength bands from the perspective of prism material properties to solve the dispersion error issue, such approaches can only reduce dispersion errors to a certain extent over a given wavelength band [20–22]. It is difficult to detect the beacon light at the receiver level as it weakens owing to atmospheric turbulence and scattering [23–25]. A correlation beacon detection scheme under strong background interference conditions was proposed by Han et al. [24] as an alternative to overcome the weak beacon light detection problem.
Laser scanning and pointing technology is critical to the optical link establishment in optical communication systems [26]. An operating system can provide a large field of view (FOV), fast and accurate beam pointing and position detection. A rotating double prism is an extended development of the Risley-prism scanning technology, which is one of the best-known methods to perform beam scanning and precise pointing in free-space laser communication [27–30]. Conceptually, a rotating double prism is composed of a matched Risley-prism pair capable of coaxial and independent rotation, in which the incident beam can be steered to any point within a cone by adjusting the rotation angles of two prisms [31,32]. The rotating double-prism system presents many excellent scanning characteristics and precise pointing performance, including a large FOV, rapid response, and superior precision [33,34]. Therefore, rotating double prisms have been widely used for free-space optical (FSO) communication [35,36] and other applications [37–39].
In this study, a novel scheme using RF signals as the beacon is proposed to address the issues with both dispersion and beacon light detection. The proposed approach comprises the triggering of the RF signal by the received light signal. Then, the transmitter determines whether the target alignment is accurate from the RF signal sent by the target. Additionally, we built a point-to-multi-point laser communication system based on a rotating double wedge, applying the rotating double-prism method. The proposed system uses an RF signal instead of the beacon light to avoid the dispersion error caused by the different light wavelengths between the beacon and signal lights. In addition, the double-wedge beam system can realize a large FOV, as well as fast and accurate beam scanning and pointing.
The remainder of this paper is organized as follows. Section 2 expounds upon communication optical link establishment methods and the structures of the multi-target free-space laser communication system. The experiments and data analysis in terms of system accuracy, speed, and stability are described in Section 3. Finally, the main conclusions are drawn in Section 4.
2. Methods and systems
2.1 Communication optical link establishment method
A block diagram of a traditional FSO communication system with beacon light is shown in Fig. 1. A traditional FSO communication system consists of an information source, information sink, laser diodes, avalanche photodiodes (APD), double optical prism, and receiving lens. In a traditional method using beacon light for optical link establishment, the alignment process is as follows. First, the source sends the beacon light toward the sink. Upon capturing the beacon light, the sink sends out another beam of beacon light toward the source. The latter must complete the corresponding capture process after receiving the beacon light sent by the sink. Thus, the optical link between the source and sink is established. Second, the source switches the signal light pointing it toward the sink for optical communication. However, beacon and signal lights have different wavelengths, which inevitably introduces a dispersion error into the process of pointing the signal light to the target, directly impacting the accuracy of the optical link establishment.
Figure 2 illustrates the problem of inaccurate beacon and signal light pointing due to dispersion errors in the alignment process of traditional FSO communication. In the figure, λ1 and λ2 correspond to the signal and beacon light wavelengths, respectively, while ρ1 and ρ2 respectively represent the deviation angles of the emergent beam vectors with respect to the positive direction for the two wavelengths after passing through the double prism. The dispersion error angle Δρ represents the deviation angle error due to the different wavelengths of the two lights, and Δd represents the dispersion error distance formed on the target plane. The separation angle between the two prisms is denoted as Δθ. The difference in wavelength between the signal and beacon lights causes different refractions through the double prisms and results in a deflection angle between the two light beams when Δθ is kept constant. Even a small deflection angle between the beams, at the output of the double prism, would induce large dispersion pointing errors. This is mainly owing to Δd enlargement for long-distance laser communication. Inevitably, the pointing accuracy in these cases is considerably low.
The deflection angle ρ of the outgoing beam (from the double prisms) is expressed as follows [33]:
To overcome the problem of pointing inaccuracy due to dispersion error, we use the signal light to indirectly generate a beacon signal to achieve target alignment. The schematic of this method is shown in Fig. 3. Compared to the traditional beacon light alignment method, our proposed system mainly adds wireless transceiver antennas and omits the beacon light transceiver unit. The transient wireless RF signal is used instead of the beacon light to achieve accurate alignment with the communication target. The wireless signal is used to indicate whether the laser source is aimed at the sink target during optical link establishment, whereas the wireless RF signal transmission is triggered by the received laser signal. The optical link establishment process in this case is as follows. First, the source sends the signal light. When the signal light is captured by the sink, it immediately sends the RF signal. After the source captures the RF signal, it records the target position and then rotates the double prism to the corresponding position according to the target position information. Accordingly, the accurate establishment of the optical link is completed.
Finally, we introduced the wireless signal instead of the beacon light in free-space laser communication, which avoided the dispersion error of the beacon and signal lights and made the optical link establishment reliable, accurate, and stable. At the same time, the use of a wireless signal instead of beacon light not only increases the usable optical communication wavelength, but also saves the beacon light, reduces the waste of resources, ensures the communication bandwidth, and retains the advantages of laser communication.
2.2. System structure
Figure 4 shows a block diagram of the entire proposed system, which comprises three main parts: information source, rotating double-prism scanner, and information sink. The information source consists of a modulation circuit, pulse laser transmitter, and pulse laser diode. According to the instructions of the source master controller, the modulation circuit will generate different pulse sequence signals when employed in scanning and communication modes. It will transmit the pulse sequence signals to the pulse laser emission circuit. The laser emission circuit drives the pulse laser diode to emit different pulse sequences according to the received pulse sequence signal. The rotating double-prism scanner is mainly composed of a double prism, DC brushless motor, wireless modules, angle-absolute-value magnetic encoders, and corresponding control circuits. The DC brushless motor and double prism form a rotating double-prism system. Through the coaxial independent rotation of the double prism, the pulse laser generated by the pulse laser diode can be deflected at different angles to achieve FOV scanning and beam pointing. The angle absolute values of the magnetic encoders are mainly used to record the position of the motors, which will be utilized as the target location information. The function of the wireless module is equivalent to beacon light, i.e., to indicate whether the information sink receives the laser signal. The information sink consists of an avalanche photo diode (APD), pulse laser receiving circuit, wireless signal trigger, and wireless module. When the APD receives the pulse laser, the receiving circuit immediately converts the optical signal into an electrical signal and transmits it to the wireless signal trigger. After receiving the electrical signal, the wireless signal trigger drives the wireless module to send the RF signal to the information source. Following the reception of this wireless signal, the source immediately generates an interrupt to record the encoder angle value as the target position information of the sink.
The specific working process of the system is as follows. First, the system works in scanning mode. The master controller controls the motor to perform the coaxial independent reverse rotation of the double prism through the motor control circuit. In parallel, the source sends a 5 kHz pulse laser signal. The laser signal deflects the beam to scan the FOV by rotating the double prism. When the information sink APD receives the pulse laser signal, it converts the optical signal into an electrical signal through the pulse laser receiving circuit and sends it to the wireless signal trigger. Then, the wireless signal trigger drives the wireless module to send the RF signal to the source. Upon reception of this signal, the wireless module immediately generates an interrupt signal. The master controller enters the interrupt and reads the two angle absolute values of the magnetic encoder through the detection circuit, which will be recorded as the position information of the sink target. After collecting the location information of three targets (see Fig. 5), the system enters a waiting state. Then, we can use it to point toward a selected target to complete the optical communication link establishment.
As shown in Fig. 5, the system is mainly composed of a field programmable gate array (FPGA) modulation circuit, laser transmitting circuit, pulse laser receiving circuit, DC brushless motor, double prisms, and angle-absolute-value magnetic encoder. In addition, it also includes separate transmitter and receiver lenses. The system is installed on an optical platform where the positions of three targets can be randomly changed according to the experiments. The distances to the source and heights of the three targets in the FOV are different. An oscilloscope is used to observe the communication signal waveform. The Figs. 5(b)–(d) show photographs of the main chips used for the system circuitry. A 32-bit MCU (STM32H743IIT6, STMicroelectronics, Switzerland) is used as the main control chip, the modulation subsystem uses an FPGA (cyclone IV, EP4CE30F23C8N, Altera, USA), and the wireless module uses a 2.4-GHz RF module (NRF24L01, Nordic Semiconductor, Norway).
3. Experimental verification and data analysis
3.1. Dispersion error measurement
To verify the influence of the dispersion error on the communication system, first, we performed a simulation of the deflection angle ρ for the emergent beam of the double prism according to Eq. (1). We set the apex angle α to 18°9’ and select N-BK7 as a material for the prisms, which is consistent with the actual wedge parameters. The simulation results are shown in Fig. 6. We find that the deflection angle ρ of the emergent beam reaches a maximum when the separation angle between the two prisms is zero. It decreases to zero when the separation angle becomes 180°. Thus, the beam can be steered to any point within a cone with a half-angle equal to the maximal deflection angle by adjusting the rotation angles of the two prisms. Figure 6(a) shows that, as the wavelength of the laser increases, ρ becomes smaller for the same separation angle Δ θ between the two prisms. Furthermore, we compare the ρ curves of the common laser wavelengths in Fig. 6(b). The inset shows the changes in ρ for a Δ θ between +60° and –60°. The results indicate that the dispersion error between different wavelengths increases as ρ becomes larger. The relationship between the dispersion error and ρ is non-linear, and the value of dispersion angle is approximately 0.5° when ρ is maximum. Therefore, the dispersion error is a challenge for large-angle, high-range, and long-distance beam scanning and pointing in a free-space laser communication system.
Fig. 6. Simulation results of the dispersion angle in the double prism. a) Full and b) common wavelength simulation results.
Meanwhile, to further analyze the influence of the dispersion error in the traditional free-space laser communication method, we use 905 and 1550 nm lasers to conduct dispersion error tests. During the experiment, one prism was fixed and the other was rotated to point the beam to different positions. A light screen, placed 13 cm away from the double prisms, was used to observe the position of the outgoing beam spot. The position of the outgoing beam spot for the two wavelengths was recorded using a CCD camera (Gobi-640-GigE, Xenics, Leuven, Belgium) under the same separation angle Δ θ between the two prisms. We use the angle-absolute-value magnetic encoder to record the position of the optical prism and rotate it by 10°. The experimental test obtained 36 different positions for the two wavelength spot position pictures. The circle fitting algorithm is used to calculate the spot center in the beam spot photo. The spot position coordinates are shown in Fig. 7(a), which corresponds to the distribution diagrams of the beam spots for the two beams at 36 different positions. The distances between the center of the two wavelength spots under the same separation angle Δ θ between the two prisms are shown in Fig. 7(b) for the 36 different positions. In this case, the maximum and minimum dispersion error distances are 4.79 and 0.62 mm, respectively. These values will reach 36.8 and 4.7 m, respectively, when the light screen is placed at a distance of 1 km. Therefore, in large-distance laser communication, the dispersion error between the signal and beacon lights will result in pointing inaccuracies, making it difficult to establish the optical link, which in turn affects the accuracy and reliability of the laser communication.
Fig. 7. a) Spot center distribution at 36 different positions of the two wavelengths. b) Distance between the center of the two wavelength spots.
3.2. System characteristics
After setting the target position information, we let the system start from a random position and point to the target position. This operation is repeated 100 times. The distance between the double prisms and light screen is set to 36 cm. The pointing error (show in Fig. 8) and alignment time (show in Fig. 9) of the system were measured. Figure 8(a) shows that the pointing error for the 905 nm laser after 100 measurements ranges between 0.087–1.243 mm. Furthermore, the dispersion pointing error between the 905 and 1550 nm lasers is 2.242 mm. The statistical distribution of the pointing error is shown in Fig. 8(b). The mean and root mean square (RMS) values are 0.666 and 0.711 mm, respectively. Figure 8(c) shows the spot center position distribution for the outgoing beam. The coordinate center is the spot center position on the target position obtained when scanning with a 905 nm laser. The blue spot is the spot center position of the 1550 nm laser at the same target position obtained by 905 nm. We can see that the 1550 nm laser spot (blue point, dispersion pointing error: 2.242 mm) center is farther from the coordinate center than the 905 nm laser center (red points, the maximum dispersion pointing error: 1.243 mm). The pointing error of the system is much smaller than that caused by the different beacon and signal light wavelengths. Theoretically, the pointing error of our method will not be affected by the communication distance, but will be affected by the beam diameter of the signal light. The smaller the beam diameter, the smaller the pointing error, which is also one of the advantages of our proposed method.
Fig. 8. a) Pointing error of the system. b) Statistical distribution of the pointing error. c) Spot center position distribution of the outgoing beam.
Fig. 9. a) Pointing alignment time of the system. b) Statistical distribution of the pointing alignment time.
The alignment time required for the system to point to the target position is distributed between 0.92–1.46 s, as shown in Fig. 9(a). Further statistical distribution is shown in Fig. 9(b). The mean and RMS values were found to be 1.211 and 1.215 s, respectively. It is observed that the system can quickly and accurately point to the target position within 1.5 s after obtaining the target position information.
Additionally, to verify the performance of the rotating double-prism pointing system, the three targets in the FOV were scanned 100 times, and the positions were determined by recording the angle value of the magnetic encoder for each target. Here, we use the angle value of the magnetic encoder to characterize the target position and the beam direction. By comparing the angle values during scanning and pointing, we obtain the error distribution diagram of beams that point target-A, -B, and -C, as shown in Figs. 10 (a)–(c), respectively. The pointing accuracy is found to be within ±2°.
The mathematical analysis results of the scanning and pointing data are shown in Table 1. The RMS of the motor pointing error is found to be less than 0.8°. Therefore, the demand for accurate pointing could be met during the experiment, and a stable communication link was successfully established.
Table 1. Mathematical analysis of the scanning and pointing data
The waveforms corresponding to the transmitted and received signals were recorded using an oscilloscope when communication with different target respectively, as shown in Fig. 11 (a), (b), and (c). Interestingly, the results show that when communicating with a single target, other targets do not receive a signal, and no error is noted in the comparison between the signal received by each target and the transmitted signal. The enlarged portions on the right side of each figure show the time delay between the transmitted and received signals. From this, the distances between different targets and the transmitting end could be distinguished.
Fig. 11. Waveforms of the transmitted and received signals of a) target-A, b) target-B, and c) target-C.
Finally, the system design achieves the initial goals of multi-target scanning, pointing, and establishing a stable communication optical link with wireless RF signals, which verifies the feasibility of our proposed method. The system avoids the pointing error of signal and beacon lights and is not required to consider the detection problem of weak beacon light, which improves the accuracy, speed, and stability of the establish communication optical link. Despite its many advantages, this system fails to complete the modulation and demodulation of the communication; however, this is not the focus of this study and should be addressed in future work. Here, we aim at providing a feasible method and system for establishing communication optical links for FSO.
4. Conclusions
This study utilizes a rotating double wedge in free-space laser communication and uses wireless RF signals instead of beacon light to address the problem of dispersion error caused by the beacon and signal lights in traditional SLC link establishment. When the target is 36 cm away, the average pointing lateral error is 0.666 mm (RMS: 0.711 mm) and the dispersion error is 2.242 mm. The proposed system performs the scanning and pointing of multiple communication targets in the FOV and can achieve stable communication with a single target, providing a new multi-target communication system solution for FSO communication. The average pointing alignment time is 1.2 s and motor pointing accuracy is less than 0.8°. In general, the system achieves a highly accurate, fast, and stable communication link establishment. We believe that the multi-objective FSO communication system proposed in this study can provide a reference for the development of satellite-to-ground and inter-satellite space optical communication.
Funding
National Natural Science Foundation of China (,61934003, 62090053, 62090054); Major Scientific and Technological Program of Jilin Province (20200501007GX); Major Science and Technology Special Project Subjects of Jilin Province and Changchun City (20210301014GX); Program for JLU Science and Technology Innovative Research Team (2021TD-39).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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