1.College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China 2.Beijing Institute of Space Mechanics and Electricity, China Academy of Space Technology, Beijing 100094, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61775102)
Received Date:27 May 2019
Accepted Date:15 July 2019
Available Online:01 October 2019
Published Online:05 October 2019
Abstract:The angular resolution of optical system is limited by the ratio of the wavelength to the aperture of the entrance pupil, indicating that the optical system with large aperture has a high spatial resolution. Sparse aperture imaging is one of the effective solutions to the problem that the telescope is bulky, heavy and difficult to manufacture. According to the self-similarity and multi-scale characteristics of fractal configuration, we propose a sparse aperture array and analyze its performance for synthetic aperture imaging system. In the array Golay-3 is used as a structural unit to expand a multi-layered fractal configuration in a self-similar manner. Given the analytical expression of the pupil function which is reduced by dimensionless parameters, we calculate the modulation transfer functions (MTFs), the practical cut-off frequencies and the middle spatial frequency characteristics of the fractal configuration under different fill factors and different outer layer rotational angles. We analyze both the MTF values and the performance parameters of the fractal structure for the cases of N = 3, 9, and 18, respectively. The results show that the decrease of fill factor does not significantly change the MTF curve nor the practical cutoff frequency in a range of fill factor between 0.0952 and 0.2246. The outer layer rotational angle has a periodicity, and the change in the angle has no large influence on the practical cutoff frequency. When the reduced aperture parameter is $ {d_0} = 1$ and the fill factor is 22.46%, the middle spatial frequency of N = 18 array is more stable and the practical cut-off frequency is higher. Using the fractal self-similarity, the aperture of the system can be expanded effectively while maintaining the middle spatial frequency characteristics. The computing results are of scale invariance due to the adoption of the reduced aperture parameter. Keywords:fractal configuration/ sparse aperture array/ pupil function/ modulation transfer function
图2(a)是子孔径直径${d_0}$与填充因子F的关系图. 由图可见, 随着${d_0}$的增大, 填充因子F单调增大. 图 2 结构特征 (a)子孔径直径与填充因子曲线图; (b)结构层数与包围圆半径关系 Figure2. Configurationcharacteristics: (a) Sub-aperture diameter and fill factor curve; (b) the relationship of the number of fractal configuration and the radius of aperture.
表1分形阵列在不同填充因子下的特性指标 Table1.Characteristics of fractal array with different fill factors.
23.3.外层旋转角对MTF的影响 -->
3.3.外层旋转角对MTF的影响
当${\theta _1} = 0,{\theta _2} = 0$时, 第三层旋转角${\theta _3} = \theta $称为外层旋转. 外层旋转角度的变化会对MTF出现周期性影响, 变化周期为 ${{{\text{π}}} / 3}$. 取${d_0} = 1$的情况. 图5为分形孔径阵列外环分别旋转0°, 15°, 30°和45°时, 旋转前后MTF沿${f_x}$和${f_y}$方向的截面图. 稀疏孔径系统的MTF随空间频率的增大在中低频部分下降比较快, 在中高频比较平坦, 但有适当起伏; 在${f_x}$和${f_y}$方向, 分形结构MTF在截止频率内无零点, 且比较平缓. 外层旋转会导致系统的MTF沿不同方向的分布产生变化. 图 5 分形阵列随外环旋转角度变化MTF曲线 (a)沿fx归一化频率方向; (b)沿fy归一化频率方向 Figure5. MTF curves of fractal array with different outer layer rotational angles: (a) Normalized frequency along fx - axis; (b) normalized frequency along fy - axis.
图6给出实际截止频率随外层旋转角的变化曲线. 此时子孔径直径${d_0} = 1$, 填充因子为22.46%. 可以看出, 随着外环旋转, 系统实际截止频率虽有下降趋势, 但总体没有太大变化, 影响甚小. 总之, 外层旋转对MTF和实际截止频率的影响不是很大. 图 6 实际截止频率随外层旋转角的变化曲线 Figure6. The curve of the practical frequency with outer layer rotational angles.
23.4.自相似结构效应 -->
3.4.自相似结构效应
为了进一步分析自相似结构效应, 对N = 3, N = 9, N = 18时的阵列进行比较, N = 3, N = 9时阵列结构如图7所示. 采用填充因子F = 0.2246的情况, 三种阵列的MTF曲线如图8所示. 在归一化频率fx方向, N = 3时分形阵列结构在归一化频率为0.28处出现零值, N = 9时分形阵列结构在0.31处出现一个低谷. 在整个频率范围内有较大波动, 随频率呈起伏式下降. N = 18时结构MTF曲线平缓且截止频率高, 在MTF截止频率范围内没有零点. 在归一化频率fy方向, N = 3, N = 9时结构的MTF曲线差别不大, 出现较多的频率缺失. N = 18时结构的MTF有些起伏但在中高频趋于平稳, 在0.64处趋于零值. 图 7N = 3, N = 9阵列结构 Figure7.N = 3, N = 9 array configuration.
图 8 3种阵列的MTF曲线(F = 22.46%) (a)沿fx归一化频率方向; (b)沿fy归一化频率方向 Figure8. MTF curves of three kinds of array configuration (F = 22.46%): (a) Normalized frequency along fx - axis; (b) normalized frequency along fy - axis.
由实际截止频率定义和(11)式计算分析可得3种阵列的特性指数, 如表2所示. N = 3, N = 9时阵列结构实际截止空间频率很低. 综合分析三种阵列结构的特性指数, 在F不变的条件下, 随着分形阵列结构孔径数的增加, 实际截止频率和中频特性数值都显著增加. 在相同的填充因子情况下, 增加子孔径数可以改善中高频平稳性, 提高系统的实际截止频率.
阵列结构
N = 3
N = 9
N = 18
实际截止频率
0.2778
0.2778
0.6382
中频特性
0.1515
0.0571
0.0632
表23种阵列的特性指数 Table2.Characteristics of three kinds of array configuration.