闪烁条件下相位误差的测量
Measuring Phase Errors in the Presence of Scintillation
摘要：
强湍流条件通过近场衍射效应产生振幅像差。当在长光程长度上积分时，振幅像差（被视为闪烁）可以使相干光束的记录图像中的局部区域无效，从而使波前重建过程复杂化。为了估计望远镜光束控制系统在强湍流环境下的相位畸变，自适应光学的波前传感器（WFS）必须对闪烁具有鲁棒性。我们已经设计并建造了一个WFS，我们称之为“菲涅耳传感器”，它使用近场衍射来测量中等到强湍流条件下的相位误差。系统的研究其灵敏度进行了实验室实验，使用点源信标。然后将结果与Shack-Hartmann WFS（SHWFS）进行比较。当SHWFS在存在中等湍流的情况下经历辐照度衰减时，菲涅耳WFS继续常规地提取相位信息。对于闪烁指数为$S = 0.55$，我们表明，菲涅耳WFS提供了一个因素的$9\times$增益的灵敏度超过SHWFS。我们发现，菲涅尔WFS是能够与极低的光水平，对应于一个信噪比只有$\mbox{SNR}\approx 2-3$每像素的操作。这种器件非常适合于相干光束传播、激光通信、遥感以及涉及长光程、沿地平线的位置线和微弱信号的应用。
英文摘要：
Strong turbulence conditions create amplitude aberrations through the effects of near-field diffraction. When integrated over long optical path lengths, amplitude aberrations (seen as scintillation) can nullify local areas in the recorded image of a coherent beam, complicating the wavefront reconstruction process. To estimate phase aberrations experienced by a telescope beam control system in the presence of strong turbulence, the wavefront sensor (WFS) of an adaptive optics must be robust to scintillation. We have designed and built a WFS, which we refer to as a "Fresnel sensor," that uses near-field diffraction to measure phase errors under moderate to strong turbulent conditions. Systematic studies of its sensitivity were performed with laboratory experiments using a point source beacon. The results were then compared to a Shack-Hartmann WFS (SHWFS). When the SHWFS experiences irradiance fade in the presence of moderate turbulence, the Fresnel WFS continues to routinely extract phase information. For a scintillation index of $S = 0.55$, we show that the Fresnel WFS offers a factor of $9\times$ gain in sensitivity over the SHWFS. We find that the Fresnel WFS is capable of operating with extremely low light levels, corresponding to a signal-to-noise ratio of only $\mbox{SNR}\approx 2-3$ per pixel. Such a device is well-suited for coherent beam propagation, laser communications, remote sensing, and applications involving long optical path-lengths, site-lines along the horizon, and faint signals.
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