自由空间光通信中的波前畸变补偿方法研究

发布时间：2018-05-17 11:32

本文选题：自由空间光通信 + 自适应光学　；参考：《吉林大学》2017年硕士论文

【摘要】：随着全球通信业务的增长,通信容量不断增加,无线信道频谱资源日益紧张,人们将通信频段不断向高频频段扩展,因此光通信以通信频率高、频带宽、传输速率高、保密性好的特点,成为重要的通信手段。其中自由空间光通信(Free Space Optical Communication,简称FSOC)因其频谱资源丰富、组网灵活的特点,在卫星通信、应急通信和接入网等方面发挥着重要作用。但在FSOC中,光信号在大气中传播,受到大气湍流的干扰,相位发生扰动,产生像差,导致波前畸变,从而增大误码率,降低通信质量。因此,大气湍流导致的波前畸变问题始终是一个重要课题。作为补偿大气湍流导致的波前畸变的一种实用有效方法,自适应光学技术利用波前传感器或相机探测光束波前的信息,然后将这些数据传给波前控制器,利用一定的算法计算出控制波前校正器的电压,使其产生与畸变波前相共轭的波前,抵消原来的畸变。本文的研究重点是控制波前校正器的算法。在总结现有利用自适应光学进行波前畸变校正算法的基础上,确定了一种组合补偿算法,用于控制波前校正器产生与畸变波前共轭的波前,抵消原来的畸变。组合补偿算法将直接斜率法和随机并行梯度下降算法相结合。由于直接斜率法无需波前重构、计算量小但是对高阶像差的校正能力较差,随机并行梯度下降算法无需波前探测、机制简单但是对畸变严重的像差校正的收敛速度慢,所以直接斜率法校正较低阶像差,随机并行梯度下降算法校正较高阶像差,从而提高整体校正效果,减少迭代次数,加快收敛速度。为了验证组合补偿算法的校正效果,本文利用Zernike多项式分别模拟了不同精度的多组畸变波前和不同湍流强度下的多组畸变波前,以斯特列尔比(Strehl Ratio,简称SR)作为系统性能指标,对组合补偿算法进行仿真。仿真结果表明,组合补偿算法能够使这两种算法互相取长补短,得到明显优于直接斜率法和随机并行梯度算法的校正效果,即更高的SR,达到相同的SR需要的迭代次数明显减少,尤其是对波前畸变更严重的波前的校正效果更明显,校正速度的改善也更加明显。组合补偿算法能够有效提高SR,使更多的光能量耦合到接收光纤中,从而降低误码率,提高通信质量,符合FSOC系统对实时性的要求。在进行理论研究的同时,本文还进行了自适应光学系统实验,包括平台的搭建、校准和波前畸变校正的实验验证。该自适应光学平台经校准后原始波前峰谷值小于1μm,引入波前畸变后,能够校正畸变,使波前峰谷值下降到1μm,具有较好的校正效果,因此为FSOC系统的实现打下了坚实的基础。
[Abstract]:With the growth of global communication services, the communication capacity is increasing, and the spectrum resources of wireless channels are increasingly tight. People expand the communication frequency band to the high frequency band, so the optical communication has high communication frequency, high frequency bandwidth and high transmission rate. Confidentiality of good characteristics, become an important means of communication. Free Space Optical Communication, plays an important role in satellite communication, emergency communication and access network because of its rich spectrum resources and flexible networking. However, in FSOC, the optical signal propagates in the atmosphere, which is disturbed by atmospheric turbulence, and the phase is disturbed, resulting in aberration, which leads to the distortion of the wavefront, which increases the bit error rate and reduces the communication quality. Therefore, the wavefront distortion caused by atmospheric turbulence is always an important subject. As a practical and effective method for compensating wavefront distortion caused by atmospheric turbulence, adaptive optics uses wavefront sensors or cameras to detect beam wavefront information, which is then transmitted to the wavefront controller. The voltage of the control wavefront corrector is calculated by using a certain algorithm to produce the wavefront conjugated with the distorted wavefront to cancel the original distortion. The emphasis of this paper is on the algorithm of controlling wavefront corrector. On the basis of summarizing the existing wavefront distortion correction algorithms using adaptive optics, a combined compensation algorithm is proposed, which is used to control the wavefront corrector to produce wavefront conjugated with the distorted wavefront and cancel out the original distortion. The combined compensation algorithm combines the direct slope method and the stochastic parallel gradient descent algorithm. Because the direct slope method does not require wavefront reconstruction, the computation is small but the correction ability for high order aberrations is poor, the random parallel gradient descent algorithm does not need wavefront detection, the mechanism is simple but the convergence rate for aberration correction with serious distortion is slow. So the direct slope method corrects the lower order aberrations, and the random parallel gradient descent algorithm corrects the higher order aberrations, which improves the overall correction effect, reduces the number of iterations, and accelerates the convergence speed. In order to verify the correction effect of the combined compensation algorithm, the Zernike polynomial is used to simulate the multi-group distortion wavefront with different accuracy and the multi-group distortion wave front under different turbulence intensity respectively. The Strehl Ratio (short for short) is taken as the system performance index. The combined compensation algorithm is simulated. The simulation results show that the combined compensation algorithm can make the two algorithms learn from each other's weaknesses, and get a better correction effect than the direct slope method and the stochastic parallel gradient algorithm, that is, higher SRs, and the number of iterations needed to achieve the same SR are obviously reduced. Especially, the correction effect of the wavefront with more serious wavefront distortion is more obvious, and the improvement of correction velocity is more obvious. The combined compensation algorithm can effectively improve the SRS and make more optical energy coupled to the receiving fiber, thus reducing the bit error rate, improving the communication quality and meeting the real-time requirements of the FSOC system. At the same time, the experiments of adaptive optics system are carried out, including platform construction, calibration and wavefront distortion correction. After calibration, the peak and valley value of the original wave front is less than 1 渭 m. After introducing the wavefront distortion, the distortion can be corrected, and the peak and valley value of the wave front can be reduced to 1 渭 m, which has a good correction effect, so it lays a solid foundation for the realization of FSOC system.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TN929.1

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