I. Introduction
High-resolution cross-sectional noninvasive and real-time optical imaging has been used to measure depth-resolved images to differentiate between the cancer and normal tissues in esophagus, colon, and coronary [1], [2]. Particularly, for neuromorphic (structural) and functional imaging and image-guided surgery, there have also been great efforts to early diagnose various kinds of tumors and cancers such as glioma and melanoma, as well as detecting the vein in which information directly inside the tissue if is necessary [3], [4]. However, there is only limited space within biological object so that most of conventional scanning probes are limited for their endo/microscopic in vivo imaging of internal micro tissues [5]–[7]. Thus, to meet the above mentioned concerns for a specific circumferential visualization, various kinds of side-viewing micro-optic probes have been suggested in a variety of sensing and imaging modalities [8]–[14]. Especially, in the previous report in [15], feasibility of the bare-fiber side-viewing probe was demonstrated with inserting a separate external microlens that can focus the output beam to have a better lateral resolution than that of normal diverging bare fiber-optic probes without beam concentration. Furthermore, it has failed to achieve an appropriate strong level of self-contained interferometric reference that can practically work due to the small reflectance at the polished fiber interface so that an external partial reflector has been employed instead to accomplish that purpose only for free-space standing specimens (it usually requires a air gap between the imaging probe and the sample for having a reference signal). Those external bulk optic components eventually make the probing imaging sensor to be greater in overall size, as well as embrace alignment issues in between. In this work, a true integrated micro side-viewing fiber-optic probe (m) has been fabricated that can be inserted directly into the brain for profiling depth images coated with a semitransparent thin gold (Au) film that allows a self-incorporated reference plane, an essential part for optical reflectometry-based interferometric imaging, from the distal end of the probe without using additional external semitransparent and/or partial reflectors for achieving references or other bulk optics. This technique, in principle, is similar to the conventional electrical or optical time-domain reflectometry (TDR, OTDR), where it localizes and determines the discontinuity or faults from the reflected waveforms for the specimen with a finer dead zone or resolution by using the mixing interference of a broadband light source. Additionally, gold coating which is a biomedically safe material provides and optical protection by avoiding the effect of refractive index dependence of an interfacial contact medium on the reference amplitude whether the probe works in a free-space standing or aqueous environment, even in contact with the specimen. Moreover, in order to achieve beam focusing, a micro dome-shape lens has been fabricated which enables a simple integration without adapting external focusing elements by a high-energy melting process. The angle polished fiber aims the beam for circumferential view by strong interface internal reflection on the gold-coated layer in which one-way directional beam is suitable for the integration with conventional medical hypodermic needles having asymmetrical shapes where one side can be considered as an optical biopsy window. The fiber-optic probe has been characterized as well as its signal-to-noise ratio (SNR) performance and its ability for depth imaging for various specimens including a brain sample of a rat for the potential usage in noninvasive optical biopsy and diagnosis as well as cross-sectional subsurface image profiler.