A Miniature Fiber Optic Refractive Index Sensor Built in a MEMS-Based Microchannel

Ye Tian,Nan Wu,Charles Guthy,Xiaotian Zou,Xingwei Wang,Wenhui Wang

doi:10.3390/s110101078

Abstract

A small, highly sensitive, and electromagnetic interference (EMI)-immune refractive index (RI) sensor based on the Fabry-Perot (FP) interferometer is presented. The sensor’s FP cavity was fabricated by aligning two metal-deposited, single-mode optical fiber endfaces inside a microchannel on a silicon chip. The mirrors on the fiber endfaces were made of thermal-deposited metal films, which provided the high finesse necessary to produce a highly sensitive sensor. Microelectromechanical systems (MEMS) fabrication techniques, specifically photolithography and deep dry etching, were used to precisely control the profile and depth of the microchannel on the silicon chip with an accuracy of 2 μm. The RI change within the FP cavity was determined by demodulating the transmission spectrum phase shift. The sensitivity and finesse of the transmission spectrum were controlled by adjusting the cavity length and the thickness of the deposited metal. Our experimental results showed that the sensor’s sensitivity was 665.90 nm/RIU (RI Unit), and the limit of detection was 6 × 10−6 RIU. Using MEMS fabrication techniques to fabricate these sensors could make high yield mass production a real possibility. Multiple sensors could be integrated on a single small silicon chip to simultaneously measure RI, temperature, and biomolecule targets.

Highlights

Optical refractive index (RI) sensors are a very attractive sensing option for a wide variety of environmental, chemical, and biomedical sensing applications because of their high sensitivity, fast response, ease of fabrication, immunity to electromagnetic interference, and survivability in harsh environments
Grating-based RI sensors have high sensitivity and dynamic range, but their fabrication costs prohibit their use in many applications, and the relatively long length of the gratings limit their applications as point sensors [2,3]
This paper presents a simple extrinsic FP fiber sensor that measures the RI of a liquid in a silicon microchannel

Summary

Introduction

Optical refractive index (RI) sensors are a very attractive sensing option for a wide variety of environmental, chemical, and biomedical sensing applications because of their high sensitivity, fast response, ease of fabrication, immunity to electromagnetic interference, and survivability in harsh environments. Tapered fiber RI sensors’ tiny structures are suitable as point sensors, but their unstable high-order mode interference and extreme sensitivity to different kinds of environmental changes, such as temperature, humidity, and mechanical stress, can negatively affect their measurement accuracy [4,5,6]. In intrinsic FP sensors, the sensing cavity is between two dielectric mirrors inside the fiber [13]. In extrinsic FP sensors, the sensing cavity is between two cleaved fiber endfaces that are aligned and bonded within a channel [14]. For both kinds of FP sensors, the reflections at the dielectric mirrors or endfaces of the cavity form an interference pattern whose optical path is determined by the length and RI of the cavity. Multiple sensors can be integrated in a small chip for simultaneous measurements of RI, temperature, and biomolecule targets

Materials and Fabrication Method

Sensor Spectrum

Refractive Index Test

Temperature Effect

Discussion

Conclusions