Photonic Crystal Fiber Based Refractive Index Sensor

K. Nazeri, V. Ahsani, F. Ahmed, P.C. Lee, M.B.G. Jun
Purdue University,
United States

Keywords: photonic crystal fiber, fiber optic sensor, interference

Summary:

Photonic Crystal Fiber (PCF) is an optical fiber which owes its waveguide properties to an arbitrary arrangement of closely-spaced and tiny air holes that go through the whole length of fiber. Much attention has been paid through recent years to unique properties of PCF and researchers have investigated various properties such as frequency range of single-mode operation, large mode area, and high birefringence. The modal and light guidance properties of PCFs are appealing for ambient refractive index sensing. McPherson e al. demonstrated a Mach-Zehnder intereferenc using PCF in 2001. Since then, PCF based fiber optic MZI sensors in diverse structural configurations have been used in many sensing applications. MZI sensor in SMF-PCF-SMF configuration with sensing length of 3.5 to 5 cm has been demonstrated for RI sensor [6]. It was proposed by Li et al. that PCF based MZI sensor with taper waist diameter down to 30 ┬Ám and sensing length of ~2.4 cm can achieve RI sensitivity of 1600 nm/RIU. Too much reduction in taper waist may reduce rigidity of the sensor and in point sensing applications sensor with shorter length is desirable. To keep the sensor length considerably small, acid etching of the PCF also has been reported. Chemical etching using hydrofluoric acid is probably not the best method for many laboratory setups. So, alternative approaches are required to achieve miniature MZI that has considerable sensitivity for ambient RI sensing. In this paper, we present PCF based refractive index sensors employing different configurations. A sharply tapered miniature PCF based MZI sensors that show high ambient RI sensitivity. The sensors are built by splicing small stubs of PCF between SMFs. The PCF is then tapered using a custom made tapering machine. Also, in SME-PCF-SMF configurations, varying fiber lengths at SMF-PCF interfaces are investigated for sensitivity increase as well as filling the air holes in the PCF.