Spectral measurement of coherence Stokes parameters of random broadband light beams
A quartz wedge depolarizer modifies the spectral polarization-coherence properties of the beam which are measured using a circular polarizer, digital micromirror device, and a grating. The resulting interference fringes reveal the coherence Stokes parameters as a function of wavelength.
All light sources, natural or man-made, are more or less partially polarized and partially coherent. The extreme cases of fully coherent, fully polarized beams and completely incoherent, unpolarized sources are good approximations, e.g., for single-mode lasers and thermal radiators, respectively. However, several light sources that fall in between these two extremes exist, as exemplified by multimode lasers and LEDs. While the scalar-field treatment is sufficient for uniformly fully polarized field, partial polarization adds a new dimension and the complete coherence characterization necessitates the use of electromagnetic coherence theory and development of methods to measure the related important parameters. The information pertaining to the coherence of vectorial, partially polarized optical beams is conveniently expressed in terms of the two-point (coherence) Stokes parameters introduced relatively recently.
The device introduced by Dr. Henri Partanen from a research group from the University of Eastern Finland in Photonics Research, Volume 7, Issue 6, 2019 (H. Partanen, et al., Spectral measurement of coherence Stokes parameters of random broadband light beams) is a general-purpose device to measure the spectral two-point Stokes parameters of arbitrary broadband light beams. To demonstrate the system a well-defined test source with complicated electromagnetic coherence properties is constructed. This is achieved by modulating a uniformly linearly polarized, spatially partially coherent light from a superluminescent diode with a quartz wedge depolarizer leading to a beam with spatially and spectrally varying electromagnetic coherence structure. In order to verify the beam characteristics experimentally we measure the spectral electromagnetic two-point Stokes-parameter distributions, which is achieved in terms of standard, commonly available devices: a double-pinhole interferometer (digital micromirror device), suitable polarization elements (circular polarizers), and a spectrometer (grating). The setup is flexible as the individual components can be chosen for specific situations, e.g., the interferometer can be replaced with a more light-efficient wavefront folding interferometer. As a secondary result, the coherence and polarization variations as a function of wavelength were found to take place at a subnanometer scale.
Prof. Jari Turunen from the University of Eastern Finland believes that the setup is a valuable tool for characterizing novel light sources such as plasmonic lasers and metamaterial emitters. The method is simple and robust and applies to random electromagnetic beams of any spectral bandwidth and polarization state.
光谱法测量随机宽带光束的相干斯托克斯参数
石英楔形消偏器改变了用圆偏振器、数字微镜器件和光栅测量的光束的光谱偏振相干特性。由此产生的干涉条纹揭示了相干斯托克斯参数与波长的变化关系。
所有光源，不管是天然的还是人造的，都或多或少具有部分偏振和部分相干的特性。极端情况，单模激光器可以认为是完全相干和完全偏振光源的近似；热辐射源可以认为是完全非相干和非偏振光源的近似。当然，也有一些光源介于这两种极端情况之间，例如多模激光器和LED。虽然均匀的完全极化场可以用标量场来处理，但是部分极化的情况将增加新的维度，并且完全相干的表征需要用到电磁相干理论和开发测量相关重要参数的方法。关于矢量，部分偏振光束的相干信息可方便地用相对最近引入的两点（相干）斯托克斯参数表示。
来自东芬兰大学的Henri Partanen博士在Photonics Research第7卷第6期（H. Partanen, et al., Spectral measurement of coherence Stokes parameters of random broadband light beams）中介绍了可测量任意宽带光束的光谱两点斯托克斯参数的通用装置。为了验证该系统，课题组构造了一个具有复杂电磁相干特性的测试源，利用石英楔形消偏器调制来自超辐射发光二极管的均匀线性偏振光和空间部分相干光，从而形成具有空间和光谱变化的电磁相干结构光。为了在实验上验证光束特性，课题组测量了光谱电磁两点斯托克斯参数分布，使用的设备包括：双针孔干涉仪（数字微镜器件）、配套的偏振元件（圆偏振器）和分光仪（光栅）。在实验中可以根据特定情况灵活选择各个组件，例如，干涉仪可以替换为光效率更高的波前折叠干涉仪。另外研究还发现，在亚纳米尺度上相干和偏振会随着波长的变化而变化。
东芬兰大学的Jari Turunen教授认为，该器件是表征等离子体激光器和超材料发射器等新型光源的有效工具。该方法简单、可靠，适用于任意光谱带宽和偏振态的随机电磁波束。