Jun Long Lim

Long period grating cascaded to photonic crystal fiber modal interferometer for simultaneous measurement of temperature and refractive index

We propose and demonstrate a novel and simple dual-parameter measurement scheme based on a cascaded optical fiber device of long-period grating (LPG) and photonic crystal fiber (PCF) modal interferometer. The temperature and refractive index (RI) can be measured simultaneously by monitoring the spectral characteristics of the device. The implemented sensor shows distinctive spectral sensitivities of -30.82  nm/RIU (refractive index unit) and 47.4  pm/°C by the LPG, and 171.96  nm/RIU and 10.4  pm/°C by the PCF modal interferometer. The simultaneous measurement of the temperature and external RI is experimentally demonstrated by the sensor. The temperature shift and RI shift calculated by the sensor matrix agree well with the actual temperature and RI change in the experiment.

Photonic crystal fiber tip interferometer for refractive index sensing

In this paper we present an interferometer based on photonic crystal fiber (PCF) tip ended with a solid silica-sphere for refractive index sensing. The sensor is fabricated by splicing one end of the holey PCF to a single mode fiber (SMF) and applying arc at the other end to form a solid sphere. The sensor has been experimentally tested for refractive index and temperature sensing by monitoring its wavelength shift. Measurement results show that the sensor has the resolution of the order of 8.7×10(-4) over the refractive index range of 1.33-1.40, and temperature sensitivity of the order of 10 pm/°C in the range of 20-100 °C.

Fabrication and Characterization of a Highly Temperature Sensitive Device Based on Nematic Liquid Crystal-Filled Photonic Crystal Fiber

We report on the fabrication and characterization of a highly sensitive temperature sensor by selectively filling the nematic liquid crystal (NLC) 6CHBT into a single void within the photonic crystal fiber (PCF) structure. The temperature response of the device is experimentally characterized, showing good linearity, repeatability, and sensitivity at around -3.90 nm/°C within the temperature range from 44°C to 53°C. The mode properties of the device are theoretically investigated, confirming the mode coupling principle and the temperature sensitivity of the device.

Novel Miniaturized Fabry–Perot Refractometer Based on a Simplified Hollow-Core Fiber With a Hollow Silica Sphere Tip

In this paper, we report and demonstrate a novel miniaturized intrinsic Fabry-Perot interferometer (IFPI) based on a simplified hollow core fiber ended with a hollow silica sphere tip. The hollow core fiber is spliced to single mode fiber and subsequently applied with fusion arc at the end, forming a hollow sphere with a thin silica wall. The reflection spectrum is modulated by the environment of the sensor head such as refractive index and temperature. By monitoring and measuring the fringe visibility, the measurement of refractive index of the calibrated solution is carried out experimentally. The achievable refractive index (RI) resolution is about 6.2 × 10-5 by measuring the fringe visibility. This sensor offers key features and advantages of IFPI including easy fabrication, low loss, low cost, and good fringe visibility. Furthermore, the sensor head is made of single material and therefore has the potential for measurements in harsh environments. Measurements at high temperatures up to ~ 1000°C are carried out. Results reveal that the sensor has low temperature.

Photonic Crystal Fiber-Based Interferometric Biosensor for Streptavidin and Biotin Detection

We report the fabrication of a highly sensitive refractometer based on photonic crystal fiber (PCF) interferometry. Using fusion arc splicing technique, a short length of PCF (∼3 mm) was spliced to conventional single-mode fibers on both sides. The voids of the PCF were completely collapsed in the splice region, which acted as mode splitter and combiner on both sides. Microbubbles were formed in the splice region and acted as diverging lens for exciting cladding modes efficiently. The transmission spectrum exhibited detectable “dips” for refractive index (RI) sensing. The bulk liquid sensitivity can reach up to 320 nm/RIU (RI unit) based on the linear curve fit for RI ranging from 1.33 to 1.34. Biosensing experiment was carried out using streptavidin and biotin as respective analyte and receptor. To the best of authors’ knowledge, this is the first time of reporting bioaffinity test on the external surface of PCF using modal interferometry.

A Compact and Temperature-Sensitive Directional Coupler Based on Photonic Crystal Fiber Filled With Liquid Crystal 6CHBT

A directional coupler structure formed by a nematic liquid crystal (NLC)-filled photonic crystal fiber (PCF) represents a promising configuration in sensing applications. Because of large refractive index difference between the NLC and silica material, the mode coupling between the NLC waveguide and the silica core is more complicated than the situation of coupling between two fundamental modes of the waveguides. Therefore, it is necessary to perform a theoretical investigation of the mode properties associated with the experimental studies of the coupling characteristics. In this paper, we present a thorough analysis, both theoretically and experimentally, of the directional coupler structure, including the mode properties, coupling characteristics, and thermal sensing properties. The temperature response of the device is experimentally measured, showing a polynomial curve in nematic phase and a linear curve in isotropic phase. The nonlinearity of the temperature response of the device in nematic phase and the linearity in isotropic phase are attributed to the temperature dependence of the refractive index of the NLC. Specifically, the sensitivity is -3.86 nm/°C in isotropic phase of the 6CHBT with good linearity and shows good agreement with simulation results.

Cascaded Photonic Crystal Fiber Interferometers for Refractive Index Sensing

We present a cascaded fiber device based on photonic crystal fiber (PCF) interferometers for refractive index (RI) sensing. The PCF modal interferometers have microbubbles at both sides of the splice regions. The microbubbles act as thick diverging lens that scatter light for efficient excitation of higher order cladding modes and attenuation of the core modes in the transmitted spectrum. Three resonance wavelengths are monitored, and their corresponding RI sensitivities are found to be 252, 187, and 207 nm/refractive index unit (RIU). The results, to our best knowledge, demonstrate that PCF interferometers with microbubbles are repeatable for high RI sensitivity, and the best crosstalk achievable is 0.295 nm/RIU.

Design and Analysis of Microfluidic Optical Fiber Device for Refractive Index Sensing

In this letter, the microfluidic optical fiber devices composed of microchannel are modeled and thoroughly analyzed. The microchannel intersecting the fiber core allows direct infusion of surrounding substances for detection. The simulation results suggest that the dimension of the microchannel can significantly affect the detection sensitivity of the device. In addition, the off-center position of the microchannel also plays an important role in shaping the device performance. The optimized microfluidic device can be used for sensitive refractive index sensing and biosensing applications.

Miniaturized photonic crystal fiber tip sensor for refractive index sensing

We report on the fabrication of a miniaturized photonic crystal fiber (PCF) tip sensor for sensitive liquid refractive index measurement. The PCF tip sensor has a hybrid configuration of Fabry-Perot (FP) and modal interferometer. A short length (less than 1 mm) of PCF is spliced to a standard single mode fiber. An air bubble is created in the splice region forming the FP cavity. The air holes are fully collapsed on both sides of the PCF. The collapsed sections act as multimode fibers which excite and recombine higher order modes experiencing optical path difference (OPD) than that of the core mode and forming modal interference. The end of the PCF is modified to a solid sphere by fusion arc splicer. Experimental results show that the sensitivity of the probe can reach up to 21.4 nm/RIU (refractive index unit) to detect bulk liquid with refractive indices from 1.33 to 1.4.

Temperature response of polarization-maintaining photonic crystal fiber based interferometer

Photonic crystal fibers (PCFs) offer innovative design and optical properties. In this paper, we measured the temperature dependence of a polarization-maintaining PCF based interferometer. The sensor head was fabricated by splicing PMPCF to single mode fiber (SMF) and temperature sensitivity of 15.4 pm/°C was obtained. The temperature measurements were carried out up to 1000°C.