Bernacki B E, Mansuripur M
Appl Opt. 1994 Feb 10;33(5):735-43. doi: 10.1364/AO.33.000735.
High-density magneto-optical recording systems require sensitive and robust focus position sensors that are immune to transient changes in the amplitude and the phase of the light diffracted from pregrooved media during the seek operation. The false focus-error signal produced by track crossing during seeking is termed feedthrough. Total immunity to feedthrough is never achieved, although some focus-error detection methods, notably the obscuration method, perform better in this regard. The astigmatic focus-error detection method is usually operated with a large astigmatic foci separation distance to facilitate detector alignment and to permit push-pull tracking, which increases pattern noise and contributes to its poor resistance to feedthrough. Pattern noise is caused by the projection of the intensity pattern at the exit pupil of the objective lens onto the detector plane, at which it produces false focus-error signals. The obscuration method, a diffraction-limited method of focus-error sensing, evens out his pattern noise and is therefore more resistant to feedthrough. We present numerical modeling results that compare the feedthrough performance of the astigmatic and the obscuration methods of focus-error detection.
高密度磁光记录系统需要灵敏且坚固的聚焦位置传感器，该传感器应不受寻道操作期间从预刻槽介质衍射的光的幅度和相位的瞬态变化影响。寻道期间因轨道交叉产生的虚假聚焦误差信号被称为馈通。尽管某些聚焦误差检测方法，特别是遮光法，在这方面表现更好，但完全消除馈通是无法实现的。像散聚焦误差检测方法通常在较大的像散焦点分离距离下操作，以方便探测器对准并允许推挽跟踪，这会增加图案噪声并导致其对馈通的抵抗力较差。图案噪声是由物镜出射光瞳处的强度图案投影到探测器平面上引起的，在该平面上会产生虚假的聚焦误差信号。遮光法是一种衍射极限的聚焦误差传感方法，它能消除这种图案噪声，因此对馈通的抵抗力更强。我们给出了数值模拟结果，比较了像散聚焦误差检测方法和遮光聚焦误差检测方法的馈通性能。
