New laser illumination method for Kerr microscopy

Abstract

Many useful materials such as Permalloy exhibiting a very small magneto-optic Kerr-effect (MOKE), are in a class called weak phase objects (WPOs). Polarized light microscopy converts a WPO’s phase distribution into a viewable and recordable intensity map. Nearly crossing the polars is necessary to convert a WPO to an amplitude image. Therefore, digital image processing procedures are usually necessary to enhance MOKE images. In order to increase efficiency and throughput, three methods to increase light intensity are described and demonstrated. All three methods conduct laser light to the conventional source spot in the microscope by imaging the output face of a multimode optical fiber onto the objective’s rear focal plane. Dithering the fiber or its image more rapidly than video scan rates, average illumination nonuniformities caused by fiber modes and diminishes short-range artifacts due to laser coherence. With a third method the long-range nonuniformity is avoided by dithering the location of the fiber-face image. Time-averaged distributions of fiber-face output and corresponding patterns of illumination on the air bearing surface of a recording head, are presented together with polar Kerr contrast images as evidence of the quality, resolution, and stability of the improved method. A combination of all three methods synchronized at a multiple of the video scan rate, produced the most uniform, steady and incoherent source, making it suitable for efficient, real-time, Kerr-effect video microscopy of magnetic materials and devices.