Uniform focusing with an extended depth range and increased working distance for optical coherence tomography by an ultrathin monolith fiber probe.

Abstract

It is difficult to maintain high transverse resolution over an increased depth range using miniature probes for optical coherence tomography (OCT) due to the rapid divergence of light and the space limitation. To solve this problem, we introduce a fiber-based filter in the proposed probe to manipulate its output beam. Significant mode interference (MI) is exploited to enhance the depth of focus (DOF), and the mode phase difference is tuned to achieve a uniform axial intensity within the DOF. The magnified MI field instead of the diffracted one is adopted as the final pupil filter in the probe to increase its working distance (WD). The probe is fabricated with a diameter of 125 µm and a total length of 2.6 mm for its distal fiber optics. Compared to the conventional probe with similar minimal lateral resolution of better than 4.4 µm, the proposed probe achieves two times that of the DOF gain and 1.7 times that of the WD. Improvements in performance of the probe are demonstrated by OCT imaging using a fresh lemon and human skin. With merits of enhanced imaging quality and easy fabrication, the proposed probe poses great potential for important applications, especially for endoscopic imaging of human internal organs in vivo.