文章：Dirk Lorenser, Xiaojie Yang, and David D. Sampson, "Ultrathin fiber probes with extended depth of focus for optical coherence tomography," Opt. Lett. 37, 1616-1618 (2012)
实现extend depth of focus的原理：the light travels through two sections of GRIN fibers with different apertures to create two different focal positions, which results in a extended depth of focus.
存在的问题：The peak SNR is reduced (reported), and the image quality might be degraded due to the increased chromatic aberration (not explicitly mentioned in the paper).
Comments: It is a simple method to increase the depth of focus at the cost of reduced efficiency and potentially degraded image quality. According to the paper, the depth of focus can be extended by 2 times. Whether this method can be practical depends on the real applications, for example, the power is not a problem in certain scenarios.