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LWPT系统由激光发射模块、接收模块以及其他辅助模块组成。发射端输出光束与接收端电池的匹配优化设计可提高LWPT效率,选用激光器的主要考量因素包括激光器的输出功率、电光效率、中心波长等。激光在大气中传输存在由吸收、散射、折射等引起的损耗,在大气窗口中LWPT的常用波长主要有808 nm、1064 nm[11]。不同材料的光伏电池在不同波长的激光照射下的光电转换效率不同[12],以GaAs材料为体系的新型光电池与中心波长为808 nm的激光相匹配,其光电转换效率远大于传统Si材料体系光电池(1064 nm)。此外,中心波长为808 nm、条宽为350 μm的半导体激光单管的效率可高达65%,功率接近20 W[13-14]。半导体激光光源快轴和慢轴方向的光束质量差值大,需要通过合束来平衡光束质量、提高输出功率。半导体激光的合束方法主要有空间合束、偏振合束和光谱合束。空间合束将多个子光束在空间上进行叠加来提高功率,其合束效率在三种合束方法中最高。
通过前期研究表明,优异的接收端光斑均匀度可以有效提升光电池的光电转换效率,接收端光斑强度分布不均匀将导致光电池转换效率和负载传输效率降低[11]。通常对于光场的不均匀度以光场各点的标准差和平均值的比值
$\Delta $ E作为其衡量标准[15]:$$ \Delta E = \dfrac{{\sqrt {{{{{\displaystyle\sum\limits_{j = 1}^n {\left( {{\chi _j} - \overline X } \right)} }^2}} / n}} }}{{\overline X }} $$ (2) 式中:n表示光斑分为n个辐照度点;χj为光斑各点的辐照度值;
$\overline X $ 为光斑n个点的辐照度均值。为提高电池阵布片率,简化电路,将接收端光电池设计成矩形,电池尺寸为接收端光斑的有效窗口。以高斯光束为例,如图1(a)所示,分别对峰值功率10%、30%、50%处光束的宽度所对应的矩形窗口内的光束不均匀度进行分析,其对应功率占比分别为92%、74%、56%,光斑不均匀度分别为0.853、0.458、0.281。文中也同样会采用以上评判方式对指定光束的有效窗口内的不均匀度进行分析。
Design of high efficiency diode laser module for wireless power transmission
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摘要: 激光无线能量传输在无人机、卫星空间站和探月机器人供电等方面具有潜在应用前景,其系统效率成为了其应用的关键瓶颈。为了提高激光无线能量传输系统发射端激光器的电光转换效率、接收端光斑均匀性和有效窗口收光比,提出了用于激光无线能量传输发射端的高效率半导体激光器设计方案。基于合束效率较高的空间合束设计了一套高功率高效率半导体激光系统,接收端光斑不均匀度可优化至0.207,有效窗口内收光比大于94%。搭建了千瓦级激光无线能量传输实验装置,发射端半导体激光系统直接输出矩形光斑,与矩形光电池匹配,提高了电池阵布片率。利用多光束指向性可调节特点,优化了接收端光斑均匀度,有利于提高接收端电池的转换效率及简化电源管理。该设计与研究为激光无线能量传输的实际应用提供了借鉴意义。Abstract: Laser wireless power transmission in unmanned aerial vehicle, satellite station and lunar robot power supply has potential application prospect, whose efficiency becomes the key to the application bottleneck. In order to improve the electro-optic conversion efficiency of the transmitter, the spot uniformity and the effective power ratio in window at the receiver, a design scheme of high efficiency diode laser for the transmitter of laser wireless power transmission was proposed. A high power and high efficiency diode laser system was designed, which is based on space combination with high efficiency, what’s more, the unevenness of the intensity at the receiving end can be optimized to 0.207, and the power ratio in the effective window is more than 94%. A laser wireless power transmission experimental device was built, whose power is over kilowatt, and the rectangular spot was outputted directly by diode laser system, which is matched with rectangular photovoltaic cell, thus the battery array layout rate was improved. The feature of multi-beam modulation was used to improve uniformity of the intensity, which is beneficial to improve the optic–electro conversion efficiency of battery and simplify the power management at the receiving end. The design and research provide reference for the practical application of laser wireless power transmission.
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图 4 角度偏转对接收端光场分布的影响。(a) 8个子光束分别以不同偏转角度进行优化组合得到的不均匀度分布;(b) 光斑不均匀度达到最小值时的光场分布;(c) 光斑不均匀度达到最大值时的光场分布
Figure 4. The effect of angle deflection on the optical field distribution at the receiver. (a) The unevenness distribution was obtained by optimizing the combination of 8 modules with deflection angles respectively; (b) Optical field distribution when the spot unevenness reaches the minimum; (c) Optical field distribution when the spot unevenness reaches the maximum
图 5 FA柱透镜离焦距离对接收端光场分布的影响。(a)光斑不均匀度和功率占比随透镜移动距离的变化;(b)透镜沿光束传输相反方向移动至出光口(−0.08 m)时的光场分布图,其不均匀度为0.227;(c)光斑不均匀度达到最小值0.218时的光场分布;(d)透镜沿光束传输方向移动距离为0.08 m时的光场分布,不均匀度为0.237
Figure 5. The effect of defocusing of FA cylindrical lens on optical field distribution at receiver. (a) The relationship between the unevenness and the moving distance of lens; (b) The optical field distribution when the lens moves in the opposite direction of beam propagation to the beam outlet (−0.08 m), and the unevenness is 0.227; (c) The unevenness reaches the minimum which is 0.218; (d) The optical field distribution of the lens with a distance of 0.08 m in the direction of beam transmission, and the unevenness is 0.237
图 6 各个模块功率变化对接收端光场分布的影响。(a) 8个模块分别以不同输出功率进行优化组合得到的不均匀度分布;(b) 光斑不均匀度达到最小值时的光场分布;(c) 光斑不均匀度达到最大值时的光场分布
Figure 6. The influence of the power variation of each module on the optical field distribution of the receiver. (a) The unevenness distribution was obtained by optimizing the combination of 8 modules with different output power respectively; (b) Optical field distribution when the spot unevenness reaches the minimum; (c) Optical field distribution when the spot unevenness reaches the maximum
图 7 不同光束的远场光斑和光强分布曲线。(a)短距离LWPT激光系统输出光束在接收端的光强分布曲线;(b) 短距离LWPT激光系统输出光束在接收端的光斑
Figure 7. The far-field spots and intensity distribution curve of different beams. (a) The far-field intensity distribution curve of the output beam from diode laser system for short distance LWPT at the receiver; (b) The far-field spots of the output beam from diode laser system for short distance LWPT at the receiver
图 9 工作电流下激光系统输出光束传输20 m时有效窗口内的光场分布。(a) 8个子模块输出光束传输20 m的光场分布;(b) 激光系统输出光束传输20 m的光场分布,光斑尺寸为0.44 m×0.49 m
Figure 9. Measure of optical field distribution which is transmitted beyond 20 m under working current. (a) Optical field distribution of 20 m transmitted by 8 sub-beams; (b) The optical field distribution of laser system, which is transmitted beyond 20 m, and the spot size is 0.44 m×0.49 m
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