Etching-enabled extreme miniaturization of graded-index fiber-based optical coherence tomography probes

We introduced and validated a method to miniaturize graded-index (GRIN) fiber-based optical coherence tomography (OCT) probes down to 70  μm in diameter. The probes consist in an assembly of single-mode (SM), coreless (CL), and graded-index (GRIN) fibers. We opted for a probe design enabling controlled size reduction by hydrogen fluoride etching. The fabrication approach prevents nonuniform etching for both the GRIN and SM fiber components, while it requires no probe polishing postetching. We found that the miniaturized probes present insignificant loss of sensitivity (∼1  dB) compared to their thicker (125  μm) counterparts. We also showed that their focusing capabilities remain tunable and highly predictable. The fabrication process is simple and can be carried out by using inexpensive telecom equipment. Both the fabrication process and the developed probes can benefit the prototyping of minimally invasive endoscopic tools.