Ultrasensitive refractive index sensor based on twin-core photonic bandgap fibers

Abstract

ABSTRACT We have theoretically investigated twin-core all-solid phot onic bandgap fibers (PBGFs) for evanescent wave sensing of refractive index within one single microflu idic analyte channel centered between the two cores. The sensor can achieve ultrahigh sensitivity by detecting the change in transmission. We find novel features in the sensing characteristics: the sensitivity is higher at the short wavelength edge of a bandga p than at the long wavelength edge, the effective index of the odd supermode (n odd ) is more sensitive to ambient refractive index change compared with that of the even supermode (n even ). Keywords: fiber sensor, photonic bandgap fiber, directional coupler 1. INTRODUCTION There is a growing research interest in optical fiber refractive index sensors and biosensors using the principle of evanescent wave sensing. Most effort has concentrated on increasing the sensitivity of fiber sensors to detect small refractive index changes. In this context, photonic crystal fibe rs (PCFs) have received consid erable attention recently, as PCF allows infiltration of the analyte into the air holes, thereby maximizing the interaction between the probing electromagnetic field and ambient analyte [1, 2, 3]. Furthermore, due to the flexibility of design and ease of fabrication, multi-core PCF is advantageous for applications in optical communication and sensing [4, 5, 6]. However, fiber sensors based on either conventional single mode fiber (SMF) or PCFs currently cannot compete with some other photonic sensors, e.g., surface plasmon resonance (S PR) sensors. The performance of fiber se nsors is currently limited either by the detection limit of around 10