Depth of focus extension by filtering in the frequency domain in laser frequency-shifted feedback imaging

The depth of focus extension in optical imaging is of considerable interest. In this paper, a laser frequency-shifted feedback scanning imaging configuration is demonstrated whose depth of focus is greatly extended through numerical filtering. The transmission characteristics of the system are studied. The original image is acquired through a two-dimensional scanning point by point with the target placed on a defocused plane. Filtered in the frequency domain, images on any oriented plane can be refocused. The superior performances are presented by imaging a three-dimensional target, and the process of gradual refocusing is demonstrated. To obtain the maximum extension in the depth of defocus, a series of numerical experiments has been carried out, which reveals its depth of focus is capable of being extended to four times the length of the objective focus length. The fabulous performances can motivate three-dimensional surface profile measurement.