FIB-Milled Gold-Coated Singlemode-Multimode- Singlemode Fiber Tip Refractometer

A compact singlemode-multimode-singlemode fiber tip (SMST) refractive index sensor is demonstrated. Focused ion beam milling is exploited to have a clean fiber tip cut which is then coated by a layer of gold to increase reflection. An average sensitivity of 265 nm/RIU and a resolvable index change of 3.77×10
^-5
are obtained experimentally with a ~ 2.94 μm diameter SMST. Because of its compactness, ease of fabrication, linear response, good sensitivity, easy connectivity to other in-fiber optical components, and low cost, this refractometer could find various applications in chemical and biological sensing.