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采用图2所示的新型结构来模拟点阵结构光投影,新型结构的参数设置如表1所示。输出光为基模拉盖尔-高斯光束,
$ {w_0} = 2\;{\text{μm}} $ ,$ {\textit{z}} = 505\;{\text{μm}} $ ,$ L = 500\;{\text{μm}} $ ,DOE的采样间隔为:Parameter Value Wavelength,λ/nm 980 Length of target light field,Lx/m 0.01 Width of target light field,Ly/m 0.01 Waist radius of VCSEL,w0/μm 2 Distance from oxide aperture to DOE,z/μm 505 Thickness of substrate,L/μm 500 Refractive index of GaAs,n 3.52 Horizontal sampling ratio,M - Longitudinal sampling ratio,N - Table 1. Structural parameters used in structure-2
式中:
${{\textit{z}}_1}$ 为经过DOE后的衍射距离。DOE的刻蚀深度为[14]:文中采用的点阵编码如图3(a)所示。由于大尺寸目标光场的编码点数太多,计算量非常大,只选取了点阵编码中心位置的5×5个编码点(图3(b))进行验证。为了增加设计自由度和抑制场内噪声,对5×5的编码区域外补0处理,补0后的目标光场边长是原来的1.8倍,形成新的9×9编码图案(图3(c))。设每个编码点的采样倍数为3×3,即整个编码区域的采样点数为
$ {\left( {9 \times 3} \right)^2} $ ,图4(a)为计算得出的DOE相位分布图,白色部分表示相位为$ {\text{π }} $ ,黑色部分表示相位为0,图4(b)为衍射面处的输出光光强分布。同样,将编码点的采样倍数改变为5×5和7×7后,再分别计算得到DOE相位分布和输出光光强分布,如图4(c)~(f)所示。若将编码点的采样倍数增加,从图4中可以看到相位分布会更加细化和复杂,相应的相位计算量也更大。通过仿真计算可知,当采样倍数为7×7时,最终的输出光光强与目标光强最为吻合,虽然存在一定的误差和畸变,但不影响特征点匹配,而采样倍数为3×3时,输出光畸变较大,影响特征点匹配。理论上,在一定范围内采样倍数越大,还原编码图时的误差畸变越小,也就是实际出光的光强分布和目标光强分布差别越小。
为了比较结构1和结构2对最终输出光光强分布的差异,对结构1也进行了相同计算,衍射全角、DOE的采样间隔、衍射距离、补0区域等都保持不变。采样倍数分别设置为3×3、5×5和7×7,对补0后的9×9编码点进行计算,结果如图5所示。比较图4和图5的出射光光强分布可知,结构1得到的输出光光强分布不均匀,这是由于在计算DOE相位时为了简化计算复杂度,将经过准直后的高斯光束近似为平面波,而实际情况下高斯光在经过准直镜准直后依旧为高斯光,那么计算得到的DOE相位分布并不精确,因此结构1在实际出光时会有较强的零级衍射。此外,在实际应用中,高斯光束经过准直透镜时会发生反射和吸收,损失一部分入射光,降低入射光利用率。
Design of on-chip dot array structured light projector using bottom-emitting VCSEL
doi: 10.3788/IRLA20210640
- Received Date: 2021-09-03
- Rev Recd Date: 2022-01-14
- Publish Date: 2022-07-05
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Key words:
- diffractive optics /
- on-chip structured light /
- Rayleigh-Sommerfeld diffraction integral /
- diffractive optical element /
- Gerchberg-Saxton algorithm /
- bottom-emitting VCSEL
Abstract: To solve the problem that the collimating lens in the existing dot array structured light projector will result in dramatical zero-order diffraction and uneven intensity distribution of the projected dot, an on-chip dot array structured light projector based on bottom-emitting vertical-cavity surface-emitting laser was proposed as well as the design of the corresponding diffractive optical element. In the design, the target intensity distribution was firstly modified with intensity adjustment and coordinate transformation. Then, the improved Gerchberg-Saxton algorithm based on the Rayleigh-Sommerfeld diffraction integral was applied to obtain the phase distribution of the on-chip diffractive optical element of the projector without collimating lens. Eventually, evaluations of the projected dot was conducted. Simulation results show that the projector can suppress the zero-order diffraction better and obtain more uniformly-distributed intensity when the gaussian beam is used as the light source. In addition, the structure can omit the installation of the collimating lens, thus, reducing the size of the projector and realizing the integration of the light source and the diffractive optical element through chip manufacturing process.