倾斜可控阵列光场的二维扫描技术研究

Two-Dimensional Scanning Technique Based on Light Field in Tilting Phase-Controlled Arrays

以19路、127路和919路激光相干阵列为模型,通过倾斜波前调控技术对阵列光场的子阵元施加倾斜相位控制,对倾斜可控阵列光场的大角度二维扫描特性进行了理论和实验研究。结果表明,所提出的方法可实现大视场范围内任意位置的单点扫描,同时得到能量分布均匀的二维连续扫描路径,实现特殊光场图案定制。此外,该方法具有扫描范围不受限制、扫描模式准连续以及衍射效率高等特点,在提升输出功率和扫描精度等方面具有优势。

Objective Laser scanning technologies are applied into numerous fields,such as free space optical communication,LIDAR,laser processing,and remote imaging.Therefore,it is important in military equipment and industrial manufacturing.Currently,most laser scanning technologies are realized by silicon-based photo-electronics,liquid crystal spatial light modulator,micro electro mechanical system(MEMS),and coherent laser arrays,etc.Among them,coherent laser arrays are proved to be an efficient method for generating laser with high power,brightness,and beam quality.Owing to designable coherent laser arrays and flexible phase controlling algorithm,remarkable progress in scanning technologies has been realized.In 2022,Zhou et al.utilized constant piston phase differences between-πandπto control the positions of far-field light spots,with limited maximum scanning angle and relatively low diffraction efficiency.In 2021,construction of quasi-continuous scanning system with the combination of micro-lens arrays and adaptive fiber optics collimators(AFOCs)was proposed.Such a system realizes controllable tilting-phase mainly by AFOCs.However,only experimental results of one-dimensional quasi-continuous scanning patterns are provided.Thus,it is urgent to study more possibilities in the customization of any light field patterns and determine the detailed scanning characteristics under the condition of huge coherent laser array to satisfy additional application requirements.Methods The method of two-dimensional continuous scanning is mainly based on the regular hexagonal arrangement of coherent laser arrays.Then,the phase modulation mode is set as the sawtooth titling phase corresponding to maximum optical path differences,which belongs to a type of blazed grating phase-controlling mode.When the maximum optical path differences of adjacent subapertures increase,their phase differences increase,i.e.,the tilting phase in single sub-aperture can sustain periodic change compared to constant piston phase.Therefore,the beam will deflect an angle ofθduring transmission as the wavefront iso-phase surface tilts at a certain angle ofθ.Results and Discussions Using typical coherent laser arrays with 19,127,and 919 sub-apertures shown in Fig.6,simulated results of single scanning point locating atγ=0,γ=π/2,γ=π/4,andγ=-π/4 are displayed in Figs.7‒9,respectively.Utilizing these single scanning points,two-dimensional quasi-continuous scanning can be realized along x,y,y=x,and y=-x axes,as illustrated in Fig.10.All patterns show clear outlines,evenly distributed energy,and a smooth curved effect.Owing to the advantages of this tilting phase-controlling model,specific scanning patterns(S,B,and W)are constructed by switching the distributed phase calculated in advance(Fig.11).Barring the scanning patterns achieved by coherent laser arrays,spatial scanning characteristics are further studied.Owing to the linear relationships between the tilting phase and the scanning angles,the steering angles of far-field beams continuously increase as the tilting phase experiences more periods.Thus,the scanning angles have no limitations under ideal conditions.Moreover,the scanning straight lines with average distributed energy indicate that near-unity diffraction efficiency can be achieved by tilting phase-controlled coherent laser arrays.Most importantly,the number of sub-apertures shows no influence on the diffraction efficiency,energy distribution,and scanning scope.With increasing number of sub-apertures,the scanning precision is improved owing to the larger caliber of coherent laser arrays.Although the grating lobes exist near the central bright spots,the increased sub-apertures can avoid the interferences to some extent because of long distances among them.Notably,the focused energy of far-field spots is higher as the number of sub-apertures increases,which is beneficial in obtaining scanning pattens with better performance.Conclusions With the regulation of tilting wavefront,coherent laser arrays can realize periodical phase change within a single subaperture to achieve single scanning points at any position,quasi-continuous scanning,and customized specific optical field patterns in a two-dimensional plane.Compared to the piston phase-controlling model,the scanning characteristics of coherent laser array with the controlled tilting phase are optimized.First,the diffraction efficiency can reach one theoretically.Second,the scanning range is not limited under the ideal condition.Last,the far-field spot energy and scanning precision can be further improved by increasing the number of sub-apertures.This work can provide significant guidance in terms of fast optical field coverage and target tracking,and scanning technologies will develop towards direction of non-mechanical mode,large steering angle,high precision,and antiinterference.In future,more studies will be performed in this regard,including reducing the influence of gate lobe,realizing the customization of arbitrary optical field pattern,and expanding the function of coherent laser arrays.