Spatiotemporal supermodes: Rapid reduction of spatial coherence in highly multimode lasers
Spatiotemporal supermodes: Rapid reduction of spatial coherence in highly multimode lasers
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DOI：
10.1103/physreva.98.023812
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发表时间：
2018-08-07
影响因子：
2.9
通讯作者：
Davidson, Nir
中科院分区：
文献类型：
--
作者：
Chriki, Ronen;Mahler, Simon;Davidson, Nir
空间相干性量化了空间场相关性，是光的基本属性之一。在这里，我们研究了短时间尺度下高度多模激光器的空间相干性。与直觉相反，我们表明在这种情况下，时间（纵向）模式在空间相干性降低中发挥着至关重要的作用。为了评估空间相干性，我们测量了具有超过 10(5) 个激光空间（横向）模式的高度多模激光器产生的散斑场的时间动态，并检查了散斑对比度对检测设备曝光时间的依赖性。我们证明，在短时间尺度的情况下，空间和时间模式相互作用形成时空超模，从而将空间自由度编码到时间模式上。结果，散斑对比度根据时间模式的数量被抑制，并且空间相干度降低。此外，空间相干性的函数形式显示出具有双峰分布。在长时间尺度的情况下，超模不再是激光模态结构的有效表示。因此，空间相干性与时间模式无关，并且获得了根据空间模式的数量抑制散斑对比度的经典结果。由于这种时空机制，高度多模激光器可用于高速全场成像应用中的散斑抑制，正如我们在这里演示的快速移动物体的成像一样。
Spatial coherence quantifies spatial field correlations and is one of the fundamental properties of light. Here we investigate the spatial coherence of highly multimode lasers in the regime of short timescales. Counterintuitively, we show that in this regime, the temporal (longitudinal) modes play a crucial role in spatial coherence reduction. To evaluate the spatial coherence we measured the temporal dynamics of speckle fields generated by a highly multimode laser with over 10(5) lasing spatial (transverse) modes and examined the dependence of speckle contrast on the exposure time of the detecting device. We show that in the regime of short timescales, the spatial and temporal modes interact to form spatiotemporal supermodes, such that the spatial degrees of freedom are encoded onto the temporal modes. As a result, the speckle contrast is suppressed according to the number of temporal modes, and the degree of spatial coherence is reduced. Moreover, the functional form of the spatial coherence is shown to have a bimodal distribution. In the regime of long timescales, the supermodes are no longer a valid representation of the laser modal structure. Consequently, the spatial coherence is independent of the temporal modes, and the classical result, where the speckle contrast is suppressed according to the number of spatial modes, is obtained. Due to this spatiotemporal mechanism, highly multimode lasers can be used for speckle suppression in high-speed full-field imaging applications, as we demonstrate here for imaging of a fast moving object.