Monemahghdoust Zahra, Montfort Frederic, Cuche Etienne, Emery Yves, Depeursinge Christian, Moser Christophe
Ecole Polytechnique Fédérale de Lausanne, Laboratory of Applied Photonics Devices, CH-1015 Lausanne, Switzerland.
Opt Express. 2013 May 20;21(10):12643-50. doi: 10.1364/OE.21.012643.
In Digital holography Microscopes (DHM) implemented in the so-called "off axis" configuration, the object and reference wave fronts are not co-planar but form an angle of a few degrees. This results into two main drawbacks. First, the contrast of the interference is not uniform spatially when the light source has low coherence. The interference contrast is optimal along a line, but decreases when moving away from it, resulting in a lower image quality. Second, the non-coplanarity between the coherence plane of both wavefronts impacts the coherence vertical scanning measurement mode: when the optical path difference between the signal and the reference beam is changed, the region of maximum interference contrast shifts laterally in the plane of the objective. This results in more complex calculations to extract the topography of the sample and requires scanning over a much larger vertical range, leading to a longer measurement time. We have previously shown that by placing a volume diffractive optical element (VDOE) in the reference arm, the wavefront can be made coplanar with the object wavefront and the image plane of the microscope objective, resulting in a uniform and optimal interferogram. In this paper, we demonstrate a vertical scanning speed improvement by an order of magnitude. Noise in the phase and intensity images caused by scattering and non-uniform diffraction in the VDOE is analyzed quantitatively. Five VDOEs were fabricated with an identical procedure. We observe that VDOEs introduce a small intensity non-uniformity in the reference beam which results in a 20% noise increase in the extracted phase image as compared to the noise in extracted phase image when the VDOE is removed. However, the VDOE has no impact on the temporal noise measured from extracted phase images.
在所谓“离轴”配置的数字全息显微镜（DHM）中，物波前和参考波前并非共面，而是形成几度的夹角。这导致了两个主要缺点。首先，当光源相干性较低时，干涉对比度在空间上不均匀。干涉对比度沿一条线是最佳的，但远离该线时会降低，从而导致图像质量下降。其次，两个波前的相干平面之间的非共面性影响相干垂直扫描测量模式：当信号光束和参考光束之间的光程差改变时，最大干涉对比度区域在物镜平面内横向移动。这导致提取样品形貌的计算更加复杂，并且需要在更大的垂直范围内进行扫描，从而导致测量时间更长。我们之前已经表明，通过在参考臂中放置一个体衍射光学元件（VDOE），可以使波前与物波前和显微镜物镜的像平面共面，从而得到均匀且最佳的干涉图。在本文中，我们展示了垂直扫描速度提高了一个数量级。对由VDOE中的散射和非均匀衍射引起的相位和强度图像中的噪声进行了定量分析。采用相同的工艺制作了五个VDOE。我们观察到，VDOE在参考光束中引入了小的强度不均匀性，与去除VDOE时提取的相位图像中的噪声相比，这导致提取的相位图像中的噪声增加了20%。然而，VDOE对从提取的相位图像中测量的时间噪声没有影响。
