Selectively filled dual-core photonic crystal fiber sensors interrogated by low coherence interferometer for temperature measurement

In this work, we propose and demonstrate an optical fiber sensing system consists of fiber sensors made by selectively infiltrated dual-core photonic crystal fiber and demodulation system based on low coherence interferometry. The sensor used in the system is made by selectively filling about 1/3 area of air holes at the cladding of photonic crystal fiber with deionized water. So the two cores of the fiber surrounded by different materials have different effective-refractive indices, which provide a phase delay between two lights propagating in the two cores. The phase delay of the sensor can be affected by surrounding temperature, strain, refractive index et al. By tracking the value of phase delay, the change of the measurand can be detected. A Mach-Zehnder interferometer with a scanning optical tunable delay line in one arm is adopted to demodulate the phase delay from the sensor, namely temporal interrogation. The demodulation system can multiplex several sensors with different phase delays to form a quasi-distributed sensing system with quasi-real time measurement. Temperature measurement is carried out to testify the system performance. This work provides a new method to demodulate self-interfering photonic crystal fiber sensor based on low coherence interferometry. The proposed all-fiber sensing system, with the merits of cost-effective, stability, and flexibility, can demodulate the self-interfering fiber sensor signals well. Further improvements such as better sensitivity, larger measurement range and higher multiplexing capacity can be realized by tailoring the PCF sensor's structure.

I. Introduction

Photonic crystal fibers (PCF) have attracted much research interest in various fields, such as optical fiber sensor, opto-fludic, optical fiber laser etc. Generally the core of PCF is surrounded by a periodic lattice of air-holes running along the propagation axis, so the air-silica structure as fiber cladding exhibits lower effective-refractive index than that of silica-core, making the confinement of light possible. The optical properties of PCFs strongly depends on the geometry of the photonic crystal. Therefore it is available for tailoring them by adjusting the cladding structure. The selectively filling technique of PCF air holes is a direct method to tailor the structure of the fiber cross-section, which is widely applied for novel fiber sensors [1]–[3]