Research On Beamforming Technology Based On Few-Mode Fiber Bragg Grating

Beamforming is the key technology of array signal processing and smart antenna system,which is the core unit to build radar systems and wireless communication systems.It has a wide range of applications in mobile communication,satellite communication,deep sea and deep space explorations,medical image diagnosis,and electronic interference reconnaissance.The beamforming system based on optical true time delay(OTTD)technology has become a research hotspot in academia and industry through the optical method of microwave photonic phase shifting,which can solve the problem of limited system bandwidth due to the aperture effect with the advantages of low loss and anti-electromagnetic interference.At present,the mainstream technologies of optical control beamforming systems are switching matrix and dispersion.Both of them combine with wavelength division multiplexing technology to realize the time delay of signals.However,the system requires a large-scale laser array,which results in high cost.In addition,the advanced optical true time delay beamforming system should have two-dimensional(2-D)scanning characteristics.This system needs two-stage delay units to realize the scanning of elevation and azimuth angles,which increases the structural complexity and system volume.Therefore,the study on the 2-D optical controlled beamforming system with low cost and lightweight has important application value and practical significance.In this dissertation,we closely focus on the problem of the cost and lightweight of the beamforming system,and we propose a system based on the few-mode fiber Bragg grating(FM-FBG)and mode division multiplexing technology to solve it.Based on an in-depth analysis of the coupled-mode theory of the few-mode fiber Bragg grating,a few-mode fiber Bragg grating with a high coupling coefficient is designed and fabricated.Then,on the basis of analyzing two theories of mode quadrature and differential mode group delay(DMGD),a new two-dimensional OTTD beamforming system involving mode and FM-FBG is investigated in detail.The main works and contributions of this dissertation are summarized as follows.1.A second-order parallel integrated few-mode fiber Bragg grating is proposed and realized using the femtosecond laser point-by-point method.First,the self-coupling reflection model of the FM-FBG is established by COMSOL software.Then,according to the simulation results,FM-FBG with the same parameters are written three times at different positions in the radial direction of the fiber core region,which will increase the overlaps between the refractive index modulation area and the mode field intensity distribution area,so that the high-order mode obtains a higher self-coupling coefficient.The performance of FM-FBG is tested after our fabrication.The results show that the self-coupling coefficient of the non-centrosymmetric mode of the parallel integrated FM-FBG is significantly higher than that of the ordinary grating.Finally,the effects of grating length,grating position,laser energy,and grating order on the parallel integrated FM-FBG are further investigated.The results show that all four factors have obvious effects on the self-coupling reflectivity of different modes.2.We proposed a circle delay line structure based on FM-FBG and a single 2×2 optical switch by differentiating modes.First,the DMGD between different modes is tested,and according to the measurement results the FM-FBG is written into the few-mode fiber core in different positions to adjust the time delay of different modes.Second,the response time of the 2×2 optical switch is tested,and the length of the few-mode fiber circle is designed based on the test results and this circle is fabricated.With the cooperation of the cutter,the length of the fabricated delay line is monitored online with a vector network analyzer until it met the design requirements.Finally,the preparation and testing of the delay line of the few-mode fiber circle are completed.The results show that the operating state of the switch is controlled by a personal computer to cycle the signals of the different modes from 0 to 5 times in the circle delay line,which leads to six equal-difference time delay distributions between the modes.3.We propose a 2-D beamforming system based on FM-FBG and photonic lantern.Under single-wavelength conditions,the photonic lantern is used to excite the spatial mode,and the equal-difference time-delay regulation between modes and branches is realized by adjusting the position of FM-FBGs with different periods.Therefore,the two-dimensional delay can be realized by a single-stage OTTD module to achieve 2-D beam scanning,which greatly simplifies the system structure and reduces the system complexity,volume,and cost.In addition,the effect of different types of photonic lanterns on the 2-D beamforming system is investigated.The experimental results show that the system with non-mode-selective photonic lanterns is better in inter-channel amplitude jitter,beam width,and sidelobe level than those in the system based on mode-selective photonic lanterns.The effect of the polarization state of light on the output power in different mode channels is tested to the problem of low transmit power of the system.The experimental results illustrate that the transmit power,in other words,the gain of the system can be increased by 2.27 dB when adjusting the polarization state of light.Under the optimal condition of the polarization state of light,a 2-D beamforming system with a 3×3 array based on FM-FBG and non-mode-selective photonic lanterns is successfully built,with the scan range of elevation and azimuth angles up to 0°～60°,0°～35°,respectively.