We analyze the polarization-dependent performance and the loss performance of volume grating couplers using a leaky-mode approach in conjunction with rigorous coupled-wave analysis for two configurations: the volume grating in the cover layer and the volume grating in the waveguide. The angular dependence of TE and TM polarization coupling efficiency is studied, and designs for polarization-dependent and polarization-independent couplers are presented for both configurations. Polarization-dependent couplers are obtained with an outcoupling angle close to normal. Polarization-independent couplers are obtained with outcoupling angles away from normal, 46.7 deg in the case of a volume grating in the cover layer and 54.4 deg in the case of a volume grating in the waveguide. The effect of loss on coupler performance is also analyzed. It is found that, for cases of practical importance, the effect of lossy coupler materials is small. The estimated loss for a commercially available material is 5 dB/cm. For TE-polarized light and the volume grating in the waveguide, a loss of this magnitude reduces the coupling efficiency by less than 3%, whereas in the case of the volume grating in the cover layer, it reduces the coupling efficiency by less than 0.3%.
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机译：我们使用泄漏模式方法和严格的耦合波分析，对两种配置：覆盖层中的体光栅和波导中的体光栅，分析了体光栅耦合器的偏振相关性能和损耗性能。研究了TE和TM偏振耦合效率的角度依赖性，并针对两种配置提出了偏振相关和偏振无关耦合器的设计。偏振相关耦合器的输出耦合角接近法线。获得与偏振无关的耦合器，其出耦合角与法线背离，在覆盖层中为体积光栅时为46.7度，在波导中为体积光栅时为54.4度。还分析了损耗对耦合器性能的影响。已经发现，对于具有实际重要性的情况，有损耦合器材料的影响很小。市售材料的估计损耗为5 dB / cm。对于TE偏振光和波导中的体积光栅，此量级的损耗使耦合效率降低了不到3％，而在覆盖层中的体积光栅的情况下，耦合效率降低了小于0.3 ％。
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