A small and light laser communication optical terminal with large field of view and common aperture for transmission and reception

Abstract

The invention discloses a small and light laser communication optical terminal for transmitting and receiving with a common aperture and a large field of view, belonging to the technical field of optical communication terminals; it comprises a laser emitting unit, a MEMS reflector scanning unit and a tracking and receiving communication unit; the MEMS reflector scanning unit performs light beam scanning through a two-axis quasi-static MEMS reflector; the collimated laser enters the MEMS reflector, enters the positive lens group after one reflection, and the beam aperture is compressed and enters the negative lens group, and finally is emitted in the form of parallel light; wherein the positive lens group is a double cemented lens, and the negative lens group is a single double concave lens. The invention has the advantages of small size, light weight, low power consumption, high response speed, high scanning frequency, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of communication optical terminals, and in particular to a light and small laser communication optical terminal for transmitting and receiving with a common aperture and a large field of view. Background Art

[0002] Wireless laser communication has the characteristics of high carrier frequency, fast communication speed, no need to apply for communication frequency, direct signal pointing through a specific path, good confidentiality, not susceptible to interference, and applicable to complex electromagnetic environments. It has received extensive attention and application in military and civilian fields. In recent years, wireless optical communication technology has developed rapidly, and the communication rate and application scenarios have been continuously enriched, such as satellite-to-ground communication, high-altitude and high-speed air-based communication, light and small drone-to-ground communication, networking, indoor visible light (Li-Fi) Internet of Things, etc.

[0003] Wireless optical communication uses the atmosphere as a channel or transmits in a vacuum without using optical fiber guidance, and it is necessary to ensure that the communication link is maintained stably. To solve this problem, the laser carrier carrying the signal can be emitted at a large angle, and the receiving antenna can also receive at a large angle, thereby overcoming the link interruption problem when the two communicating parties move relative to each other or the terminal vibrates, such as Li-Fi technology and underwater laser communication technology. However, the laser power density will drop sharply with the increase of its divergence angle. Under this technical system, the laser emission power limits its communication distance. Under the technical system of narrow beam communication, due to the good directivity of the laser, the power density can also be guaranteed at a long distance, but the stable maintenance of the link requires a tracking servo mechanism. Satellite-to-ground, inter-satellite, air-to-air, and air-to-ground signal search and tracking generally use a planar array detector with a large-range steering antenna and a large target surface small-angle fast reflection mirror to perform coarse and fine tracking of beacon light to ensure link maintenance. The two-stage beacon light tracking system has high tracking accuracy, but it is heavy and bulky, and has complex structure and control, which is not conducive to the miniaturization of laser communication terminals. Summary of the invention

[0004] Based on the technical problems raised above, the present invention provides a lightweight laser communication optical terminal with a common aperture, large field of view, and a small size for transmitting and receiving. The optical terminal adopts an antenna system and a two-axis quasi-static MEMS reflector instead of a mechanical structure for light beam scanning. The terminal has the advantages of small size, light weight, low power consumption, high response speed, and high scanning frequency.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A small and light laser communication optical terminal for transmitting and receiving with a common aperture and a large field of view, comprising a laser transmitting unit, a MEMS reflector scanning unit, and a tracking and receiving communication unit;

[0007] The laser emitting unit is used to emit modulated optical signals, and the user data is modulated into laser signals through the driving circuit; the laser light source is emitted with constant power, and then the laser signal is modulated by the signal modulator, and finally the signal power is amplified by the optical fiber amplifier EDFA, so as to realize high-power and high-speed signal transmission;

[0008] The MEMS reflector scanning unit performs light beam scanning through a two-axis quasi-static MEMS reflector; the collimated laser is incident on the MEMS reflector, and after one reflection, enters the positive lens group, and the beam aperture is compressed and then enters the negative lens group, and finally emitted in the form of parallel light; wherein the positive lens group is a double cemented lens, and the negative lens group is a single double concave lens;

[0009] The tracking and receiving communication unit includes two four-quadrant detectors; the first four-quadrant detector is used for primary scanning capture, converts the optical signal into an electrical signal and then amplifies it to extract the centroid of the target light spot and reduce the uncertainty area; the second four-quadrant detector is used for secondary scanning capture and communication, converts the received optical signal into an electrical signal and then amplifies it and divides it into two paths, one signal is demodulated and decoded to restore the user data signal; the other signal is used to extract the centroid of the target light spot, and then the laser signal is captured and tracked through the scanning capture tracking control module.

[0010] Furthermore, the laser emitting unit is mainly composed of a laser and a driving circuit; the driving circuit includes a random number generation module, an RS encoding and decoding module and a modulation and demodulation module; the random number generation module is used to simulate the data transmitted by laser communication, and the RS encoding and decoding technology is used to enhance the anti-interference ability of the system, and the modulation and demodulation module is used to realize the modulation and demodulation of the signal.

[0011] Furthermore, the MEMS reflector scanning unit is a transmit-receive common aperture multiplexing unit composed of a MEMS reflector and a Galilean telescope.

[0012] Furthermore, the tracking and receiving communication unit includes a four-quadrant detector, a photoelectric detection unit, a servo control unit and a communication demodulation unit, and completes the communication and tracking functions respectively through electronic processing means mainly based on addition and subtraction circuits.

[0013] Furthermore, the tracking and receiving communication unit uses a four-quadrant detector to simultaneously complete tracking and communication multiplexing technology as follows:

[0014] After the signal light is modulated, it is emitted by the optical antenna. After being transmitted in the space link, the energy is received by the optical antenna at the receiving end and coupled to the photosensitive surface of the four-quadrant detector through the internal relay optical path. After mutual impedance amplification and secondary amplification, the signals generated by the four quadrants enter the X-axis offset, Y-axis offset solution and signal summation solution units respectively. The signals after data processing are collected by the high-speed A / D conversion module and sent to the microcontroller. The microcontroller is responsible for calculating the execution amount of the fast-tilt galvanometer, converting it into analog voltage through D / A and finally driving the fast-tilt galvanometer to deflect and complete real-time tracking of the light spot. During the light spot tracking, the data of the summed signal is restored to the communication data through the communication processing unit, and the tracking of the signal light by the four-quadrant detector and the multiplexing of the communication function are completed at the same time, realizing the multi-task execution of a single detector, reducing the size and power consumption of the optical terminal, and realizing the lightweight and miniaturization of the optical terminal.

[0015] The beneficial effects of the above technical solution of the present invention are:

[0016] 1) A two-axis quasi-static MEMS mirror is used instead of a mechanical structure for beam scanning, which greatly widens the search angle while reducing the volume, mass, and power consumption of the system. By reasonably setting the speed difference and initial phase difference of the double optical wedges, the advantages of high response speed and high scanning frequency can be achieved within the maximum beam deflection angle range.

[0017] 2) The laser emission and signal reception based on MEMS reflectors adopt a common aperture system. The signal receiving optical path can be obtained by inverting the emission optical path, which can expand the pointing range by 3 times and greatly reduce the full field of view distortion, saving system space, reducing system weight and power consumption, and reducing the difficulty of system installation and adjustment as well as the difficulty of optical axis parallelism detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a principle block diagram of a small laser communication terminal according to an embodiment of the present invention.

[0019] Figure 2 The invention discloses a scanning range amplification principle of a Galileo telescope based on a combination of a MEMS reflector and positive and negative lenses.

[0020] Figure 3 It is a schematic diagram of the initial structural optimization result of the positive and negative lens combination of the present invention.

[0021] Figure 4 This is a schematic diagram of the optimization result of the positive and negative lens combination after adding the cone coefficient of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0024] MEMS (Micro-Electro-Mechanical) mirrors are small and light in size, and can achieve a significant reduction in SWaP (Size, Weight and Power) when used in search and tracking systems. MEMS mirrors are generally in chip form, and the millimeter-caliber reflective surface packaged on the chip is driven by electrostatic, electromagnetic, electrothermal or piezoelectric actuators to achieve two-axis deflection. It has two working modes. The resonant MEMS mirror swings at a high frequency (KHz level) within a certain angle range to scan the entire area, and is widely used in the field of laser radar. In addition, some MEMS mirrors can also work in a quasi-static state. After scanning the entire area, a target in the area is tracked in a closed loop through the feedback signal. This type of device can be used in search and tracking systems for wireless optical communications.

[0025] In summary, in order to realize lightweight and miniaturized wireless optical communication terminals, MEMS reflectors and four-quadrant detectors are used to realize a large-scale search and communication link system. An inverted Galilean telescope is used as an antenna to greatly widen the search angle. With low SWaP as its main feature, it is a relatively unique form of wireless laser communication terminal at present and has multi-platform application prospects.

[0026] The principle diagram of capture and alignment of a small laser communication terminal is as follows: Figure 1 As shown, the laser transmitting module modulates the user data signal into the laser signal through the modulation driving circuit. The scanning is mainly to scan the transmitted laser signal in a large range of 40°×60° through a high-bandwidth MEMS reflector and an optical antenna, which is convenient for the receiving end to capture. The signal light receiving and transmitting circuit integrates the three functions of signal light transmission, signal light reception and capture tracking, and adjusts the direction of the optical axis through a micro-reflector. The optical antenna couples the received optical signal to two four-quadrant detectors. The four-quadrant detector 1 is used for the first-level scanning capture. After converting the optical signal into an electrical signal, it is amplified first to extract the centroid of the target light spot and reduce the uncertainty area; the four-quadrant detector 2 is used for the second-level scanning capture and communication. Therefore, the received optical signal is converted into an electrical signal and amplified and then divided into two paths. One signal is demodulated, decoded and processed to restore the user data signal, and the other signal is used to extract the centroid of the target light spot, and then the laser signal is captured and tracked through the scanning capture and tracking control module. The whole process is divided into the following steps:

[0027] (1) The uncertainty area captured by the first-level scanning is 40°×60°. The four-quadrant detector 1 is used as the receiving device. The field of view of the four-quadrant detector is 36° (half field of view of the object side). The master end uses a spiral grating scanning method with a step size of 0.16° to scan the entire uncertainty area. The slave end keeps staring and adjusts the optical axis to point to the master end after detecting the signal light of the master end.

[0028] (2) After the master device completes scanning, it returns to the center zero position. The slave device scans the entire uncertain area in the same way. The master device keeps staring, and after detecting the signal light from the slave device, it adjusts the optical axis to point to the slave device.

[0029] (3) After the master and slave devices complete scanning respectively, the uncertainty area is reduced. The size of the uncertainty area after scanning is completed depends on the subdivision capability of the four-quadrant detector, and enters the second-level scanning capture.

[0030] (4) In the secondary scanning capture, the four-quadrant detector is still used as the receiving device. The master end scans the entire 4°×6° secondary scanning capture field of view with a step size of 0.16°. The slave end keeps staring, and after detecting the signal light of the master end, it further adjusts the optical axis to accurately point to the master end.

[0031] (5) After the master device completes the scan, the slave device scans the entire secondary scanning capture field of view in the same way. The master device keeps staring, and after detecting the signal light from the slave device, it further adjusts the optical axis to accurately point to the slave device.

[0032] (6) After completing the two-level scan, enter the tracking and communication stage.

[0033] Here is a more specific description:

[0034] This system is a duplex unit, which consists of three parts: laser emission unit, MEMS reflector scanning unit, and tracking and receiving communication unit. The specific implementation plan of the three parts is as follows: Figure 1 As shown in the figure, the optical components of the MEMS reflector scanning unit focus on the optical system design, the servo component focuses on the device selection, and the laser emission unit and the tracking and receiving communication unit focus on the selection and design of the solution.

[0035] 1. MEMS mirror scanning unit

[0036] The present invention uses a two-axis quasi-static MEMS reflector to replace a mechanical structure for light beam scanning. The scanning module has the advantages of small size, light weight, low power consumption, fast response speed, high scanning frequency, etc.

[0037] The mechanical deflection range of the two axes of the MEMS reflector is ±5°, and the optical deflection range of the reflected light when tilted at 45° is ±10°; the angular resolution of the two axes is 14 bits, which means that the minimum angular step of the reflected light of the MEMS reflector is 0.0012°. The use of an optical telescope system can expand the scanning range to 40°×60°. There are two commonly used structures for telescope systems, namely Keplerian and Galilean. The Keplerian telescope system consists of two positive lens groups, and the Galilean system consists of a positive lens group and a negative lens group. The angular magnification capabilities of these two systems are determined by the focal length multiples of the two lens groups, and the total length of the system is approximately the sum of the focal lengths of the two lens groups. Under the condition of the same angular magnification capability, the Galilean system of the positive and negative lens combination has a shorter total length, which is conducive to miniaturization and lightweight overall design. Therefore, the Galilean telescope system is used in the present invention, and its magnification principle is as follows Figure 2 shown.

[0038] The collimated laser enters the MEMS reflector, and after one reflection, it enters the positive lens group. The beam aperture is compressed and then enters the negative lens group, and finally emitted in the form of parallel light. The focal length of the positive lens group is three times that of the negative lens, thereby achieving a 3x angular magnification.

[0039] Reference Figure 3 and Figure 4 In the initial structure, the positive lens group is a double cemented lens, and the negative lens group is a single biconcave lens, whose front surface curvature is large and the rear surface curvature is small. Using the AMAG operand to control the angular magnification and perform a simple optimization, we get Figure 2 The results are shown. For the double cemented lens, spherical aberration and coma are basically all corrected, and the residual field curvature, astigmatism, and distortion are also small; in the negative group, due to the large field angle, both the front and rear surfaces will produce a large pincushion distortion.

[0040] On the one hand, the generation of distortion will affect the search range of the wireless optical communication terminal, especially barrel distortion, which will cause the search range to become smaller; on the other hand, distortion will cause positioning errors, making it difficult to search for signal light. In order to correct the distortion, cone coefficients are added to the front and back surfaces of the negative lens group, and the DIST operand is used to further optimize the distortion, and further optimization is completed. In the re-optimized structure, the aberrations in the positive and negative groups are self-corrected. The equivalent aperture remains unchanged before and after further optimization. In the point diagram, the black circle represents the diffraction limit of 2mrad. The geometric aberrations of the output light except for the distortion have been completely corrected to within the diffraction limit, and the maximum distortion is -0.7%, which is greatly improved.

[0041] 2. Laser emission unit

[0042] The laser communication transmitting unit is used to generate laser with the required frequency and energy, and radiate light energy into space in the form of a light beam, forming the required detection field of view in space, and at the same time giving communication and ranging signals.

[0043] The laser communication transmitting unit consists of a laser and a driving circuit. The laser light source transmits at a constant power, and then modulates the laser signal through a signal modulator. Finally, the signal power is amplified through an optical fiber amplifier EDFA to achieve high-power and high-speed signal transmission.

[0044] The laser achieves power control by controlling the fiber amplifier. The constant power light output by the laser source is modulated by the signal modulator, amplified by the fiber amplifier and then emitted. Communication optical power control mainly drives and controls the strength of the pump laser current to achieve the power amplification of the fiber amplifier.

[0045] The drive circuit controls the parameters of the communication optical system in real time by sending control commands to the single-chip microcomputer through the monitoring host, and its temperature control part is also automatically monitored and controlled. The drive circuit includes a random number generation module, a coding and decoding module, and a modulation and demodulation module.

[0046] In an actual communication system, the information sent by the transmitter is basically random. The present invention uses an M sequence as the transmitted information, which is a binary PN sequence commonly used in communication and can well verify the performance of the communication system. The M sequence is also called a maximum length linear shift register sequence, which is generated by a linear feedback n-stage shift register.

[0047] In order to enhance the anti-interference capability of the system, the present invention uses RS coding technology to encode and decode the transmitted data stream before modulating it. The basic idea of ​​RS coding is to select a suitable generating polynomial g(x), and make the codeword polynomial calculated for each information segment a multiple of g(x), that is, the remainder obtained by dividing the codeword polynomial by the generating polynomial is zero. The RS decoding module finds the position and error value of the codeword error according to the system receiving codeword polynomial, and then removes the error value from the received codeword to obtain the codeword sent by the transmitter, and finally realizes the error correction function.

[0048] In the modulation and demodulation of information, the present invention adopts the PPM modulation format. Each code element of PPM contains two bits of information, so there are four values ​​in total. Each code element needs to be represented by four pulse time slots. Among the four time slots, there is only one time slot signal of "1", corresponding to the LED lighting, and the LED does not light up in other time slots. In PPM modulation, the original serial data is first converted into 2-bit parallel data according to 2 times the clock frequency, and it is stored in the latch in sequence. After each 4 bits of data are stored, the value in the latch is judged, and the corresponding PPM signal sequence is output according to the corresponding relationship in the figure below. The implementation of PPM demodulation first needs to extract the bit synchronization clock signal and frame synchronization clock signal of the PPM signal to determine the pulse position. After obtaining the pulse position, the signal is decoded by using a latch and parallel-to-serial conversion. After obtaining the bit synchronization clock and frame synchronization clock, the synchronization clock is counted by a counter. When the received signal is at a high level, the value of the counter is output, and then the PPM inverse mapping can be realized by using parallel-to-serial conversion.

[0049] 3. Tracking and receiving communication units

[0050] The present invention uses a four-quadrant detector as the core light processing element of the receiving unit. The photosensitive surface of the four-quadrant detector is generally a PIN type or APD type structure, and the circular photosensitive surface is evenly divided into four independent fan-shaped areas, with a cross-line dead zone in the middle; when the target light beam forms a light spot distribution on the four-quadrant photosensitive surface through the receiving lens, the light spot is divided into four parts, and the light intensity of each part is converted into an electrical signal output after being converted by the corresponding quadrant; the four-way signal can determine the light intensity center of the target light spot after passing through the sum-difference circuit or the direct difference circuit.

[0051] The four-quadrant detector tracks the light spot by detecting the energy change of the light spot on the photosensitive surface. The geometric coordinates of the four-quadrant detector are parallel to the coordinates of the optical system. The center of the detector is the coordinate origin. The incident light spot is divided into four parts by the four-quadrant detector. The areas of each part on the detector are I1, I2, I3, and I4 respectively. If the energy of the light spot obeys uniform distribution, the received light power on each quadrant is proportional to the distribution area of ​​the light spot on each quadrant. Therefore, when the center of the light spot is offset relative to the center of the detector, the offset is a function of physical parameters such as the area on each quadrant, the light power on each quadrant, and the photocurrent generated on each quadrant. The angular component obtained after the solution is converted into an analog signal after conversion to drive the rapid tilt galvanometer, so that it can control the pitch and azimuth deflection angles of the signal light, thereby realizing the tracking of the light spot.

[0052] Based on the overall composition of the tracking and communication integrated unit of the quadrant detector, when the signal light carrying the modulation information is coupled to the photosensitive surface of the four-quadrant detector through the optical system, each quadrant channel of the detector will respond to the optical signal, and the photocurrent of each channel will be converted into a voltage signal through the mutual impedance amplifier. After secondary amplification, it enters the adder to obtain the sum signal, the purpose is to maximize the energy generated by the detector and improve the detection sensitivity. Then the signal dynamic range is adjusted through the single-ended differential circuit and the limiting amplifier, and finally the clock and data are recovered after entering the data clock recovery signal and sent to the subsequent decision circuit to complete the signal demodulation.

[0053] The specific parameters of this embodiment are shown in Table 1:

[0054] Table 1 Overall parameters of large field of view scanning capture optical antenna

[0055]

[0056]

[0057] In summary, the process of the multiplexing technology of using a four-quadrant detector to simultaneously complete tracking and communication is as follows: after the signal light is modulated, it is emitted by the optical antenna, and after being transmitted in the space link, the energy is received by the optical antenna at the receiving end, and coupled to the photosensitive surface of the four-quadrant detector through the internal relay optical path. After mutual impedance amplification and secondary amplification, the signals generated by the four quadrants enter the X-axis offset, Y-axis offset solution and signal summation solution unit respectively. The signals after data processing are collected by the high-speed A / D conversion module and sent to the microcontroller. The microcontroller is responsible for calculating the execution amount of the fast tilt galvanometer, converting it into an analog voltage through D / A and finally driving the fast tilt galvanometer to deflect to complete the real-time tracking of the light spot. During the light spot tracking, the data of the summed signal is restored to the communication data through the communication processing unit, and the tracking of the signal light and the multiplexing of the communication function of the four-quadrant detector are completed at the same time, realizing the multi-task execution of a single detector, reducing the size and power consumption of the optical terminal, and realizing the light and small size of the optical terminal.

Claims

1. A small and light laser communication optical terminal with a large field of view and a common aperture for transmission and reception, characterized in that: It includes a laser emitting unit, a MEMS reflector scanning unit, and a tracking and receiving communication unit; The laser emitting unit is used to emit modulated optical signals, and the user data is modulated into laser signals through the driving circuit; the laser light source is emitted at a constant power, and then the laser signal is modulated by the signal modulator, and finally the signal power is amplified by the optical fiber amplifier EDFA, so as to realize high-power and high-speed signal transmission; The MEMS reflector scanning unit performs light beam scanning through a two-axis quasi-static MEMS reflector; The collimated laser enters the MEMS reflector, enters the positive lens group after one reflection, and then enters the negative lens group after the beam aperture is compressed and finally emitted in the form of parallel light; the positive lens group is a double cemented lens, and the negative lens group is a single double concave lens; The tracking and receiving communication unit includes two four-quadrant detectors; the first four-quadrant detector is used for primary scanning capture, converts the optical signal into an electrical signal and then amplifies it to extract the centroid of the target light spot and reduce the uncertainty area; the second four-quadrant detector is used for secondary scanning capture and communication, converts the received optical signal into an electrical signal and then amplifies it and divides it into two paths, one signal is demodulated and decoded to restore the user data signal; the other signal is used to extract the centroid of the target light spot, and then the laser signal is captured and tracked through the scanning capture tracking control module.

2. According to claim 1, a small and light laser communication optical terminal with a common aperture and a large field of view for transmitting and receiving, characterized in that: The laser emitting unit is mainly composed of a laser and a driving circuit; the driving circuit includes a random number generation module, an RS encoding and decoding module and a modulation and demodulation module; the random number generation module is used to simulate the data transmitted by laser communication, the RS encoding and decoding technology is used to enhance the anti-interference ability of the system, and the modulation and demodulation module is used to realize the modulation and demodulation of the signal.

3. According to claim 1, a small and light laser communication optical terminal with a common aperture and a large field of view for transmitting and receiving, characterized in that: The MEMS reflector scanning unit is a transmitting and receiving common aperture multiplexing unit composed of a MEMS reflector and a Galileo telescope.

4. According to claim 1, a small and light laser communication optical terminal with a common aperture and a large field of view for transmitting and receiving, characterized in that: The tracking and receiving communication unit comprises a four-quadrant detector, a photoelectric detection unit, a servo control unit and a communication demodulation unit, and completes the communication and tracking functions respectively through electronic processing means mainly based on addition and subtraction circuits.

5. According to claim 1, a small and light laser communication optical terminal with a common aperture and a large field of view for transmitting and receiving, characterized in that: The tracking and receiving communication unit uses a four-quadrant detector to simultaneously complete the tracking and communication multiplexing technology process as follows: After the signal light is modulated, it is emitted by the optical antenna. After being transmitted in the space link, the energy is received by the optical antenna at the receiving end and coupled to the photosensitive surface of the four-quadrant detector through the internal relay optical path. After mutual impedance amplification and secondary amplification, the signals generated by the four quadrants enter the X-axis offset, Y-axis offset solution and signal summation solution units respectively. The signals after data processing are collected by the high-speed A / D conversion module and sent to the microcontroller. The microcontroller is responsible for calculating the execution amount of the fast-tilt galvanometer, converting it into analog voltage through D / A and finally driving the fast-tilt galvanometer to deflect and complete real-time tracking of the light spot. During the light spot tracking, the data of the summed signal is restored to the communication data through the communication processing unit, and the tracking of the signal light by the four-quadrant detector and the multiplexing of the communication function are completed at the same time, realizing the multi-task execution of a single detector, reducing the size and power consumption of the optical terminal, and realizing the lightweight and miniaturization of the optical terminal.

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