High sensitivity refractive index sensor based on a stack of layers at the tip of an optical fiber

Refractive Index (RI) sensors are important for different bio sensing applications and there are several ways to implement them. From these fiber optical sensors are very interesting since can be miniaturized and integrated into a simple and small device. The fiber optic sensors can be based on Bragg gratings, long period gratings, interferometers and tapers. Among these the Fabry-Perot interferometer (FPI) is a very popular option since it can be fabricated in different ways at the tip of a single mode fiber, which has a diameter of 125 μm. These sensors measure the refractive index of the medium surrounding the optical fiber, which can be either gas or liquid. Therefore, it can be convenient for some applications to have the RI sensor at the tip of an optical fiber since this can be easily introduced into the sample. Moreover, typical FPI formed at the tip of the optical fiber has a reflection spectrum with fringes reaching amplitude of 4% in air, and this amplitude is varied as the RI of the sample is changed. In this work a high sensitivity refractive index sensor based on a novel FPI, formed by a stack of 3 layers at the tip of a fiber optic, it is presented. Here, our FPI generates a reflectivity spectrum with fringes reaching amplitude up to 60%, which is considerably higher than the typically obtained with FPI formed at the optical fibers. Nominally, this increment in reflectivity helps to enhance the sensing capabilities of the sensor, particularly the resolution and the sensitivity. The sensor can measure, ideally, a refractive index from 1 to 3.4.