空间偏振调制器：区分衍射效应和空间偏振调制
Spatial polarization modulators: distinguishing diffraction effects from spatial polarization modulation
摘要：
只有我们吗？在我们寻找地球以外的生命的过程中，我们利用我们自己的星球来开发和验证远程探测生命的新方法和技术。我们的生命特征探测偏振仪（LSDpol）是一种快照全斯托克斯分光偏振仪，将部署在野外和太空中，通过感测反射光的线性和圆偏振状态来寻找地球上生命的信号。这些生物特征的例子是由O2-A带和植被引起的线性偏振，例如红色边缘和绿色凸起，以及由生物分子的同手性引起的圆偏振。LSDpol针对检测圆偏振进行了优化。为此，LSDpol在光谱仪的入口狭缝中采用了空间光调制器，即液晶四分之一波长延迟器，其中快轴作为狭缝位置的函数旋转。LSDpol的原始设计通过将四分之一波片与偏振光栅相结合来实现双光束分光偏振计。不幸的是，这种设计导致显著的线性到循环串扰。此外，它揭示了虚假的偏振调制效应。在这里，我们提出了数值模拟，说明如何菲涅耳衍射效应可以创建这些虚假的调制。我们在实验室中使用100%线性和圆偏振光通过精确的偏振态测量验证了模拟。
英文摘要：
Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump, as well as circular polarization resulting from the homochirality of biotic molecules. LSDpol is optimized for sensing circular polarization. To this end, LSDpol employs a spatial light modulator in the entrance slit of the spectrograph, a liquid-crystal quarter-wave retarder where the fast axis rotates as a function of slit position. The original design of LSDpol implemented a dual-beam spectropolarimeter by combining a quarter-wave plate with a polarization grating. Unfortunately, this design causes significant linear-to-circular cross-talk. In addition, it revealed spurious polarization modulation effects. Here, we present numerical simulations that illustrate how Fresnel diffraction effects can create these spurious modulations. We verified the simulations with accurate polarization state measurements in the lab using 100% linearly and circularly polarized light.
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