Chien-Hsing Chen

A Multi-D-Shaped Optical Fiber for Refractive Index Sensing

A novel class of multi-D-shaped optical fiber suited for refractive index measurements is presented. The multi-D-shaped optical fiber was constructed by forming several D-sections in a multimode optical fiber at localized regions with femtosecond laser pulses. The total number of D-shaped zones fabricated could range from three to seven. Each D-shaped zone covered a sensor volume of 100 μm depth, 250 μm width, and 1 mm length. The mean roughness of the core surface obtained by the AFM images was 231.7 nm, which is relatively smooth. Results of the tensile test indicated that the fibers have sufficient mechanical strength to resist damage from further processing. The multi-D-shaped optical fiber as a high sensitive refractive-index sensor to detect changes in the surrounding refractive index was studied. The results for different concentrations of sucrose solution show that a resolution of 1.27 × 10−3–3.13 × 10−4 RIU is achieved for refractive indices in the range of 1.333 to 1.403, suggesting that the multi-D-shaped fibers are attractive for chemical, biological, and biochemical sensing with aqueous solutions.

Self-referencing fiber optic particle plasmon resonance sensing system for real-time biological monitoring.

We present the design and experimental verification of a self-referencing dual-channel fiber optic particle plasmon resonance (FOPPR) sensing system for compensation of thermal and bulk-composition effects as well as nonspecific adsorption in real-time biosensing of complex samples. A theoretical model is first proposed and then a systematic experimental approach is used to verify the model. The sensing system comprises an analysis fiber sensor and a reference fiber sensor in a single microfluidic chip, where the analysis fiber is functionalized with a recognition molecule. The compensation still works even if the surface coverages of gold nanoparticles on the reference and analysis fibers are not exactly the same. The potential of this approach is illustrated by a model biosensing experiment in which the detection of anti-biotin is compensated for bulk refractive index change, nonspecific adsorption and/or color interference, in various sample media. The percent recovery is 103.2% under both the effects of bulk refractive index change and nonspecific adsorption and is 93.9% under both the effects of color interference and nonspecific adsorption, suggesting that the compensation is effective.

Fabrication quality analysis of a fiber optic refractive index sensor created by CO2 laser machining.

This study investigates the CO2 laser-stripped partial cladding of silica-based optic fibers with a core diameter of 400 μm, which enables them to sense the refractive index of the surrounding environment. However, inappropriate treatments during the machining process can generate a number of defects in the optic fiber sensors. Therefore, the quality of optic fiber sensors fabricated using CO2 laser machining must be analyzed. The results show that analysis of the fiber core size after machining can provide preliminary defect detection, and qualitative analysis of the optical transmission defects can be used to identify imperfections that are difficult to observe through size analysis. To more precisely and quantitatively detect fabrication defects, we included a tensile test and numerical aperture measurements in this study. After a series of quality inspections, we proposed improvements to the existing CO2 laser machining parameters, namely, a vertical scanning pathway, 4 W of power, and a feed rate of 9.45 cm/s. Using these improved parameters, we created optical fiber sensors with a core diameter of approximately 400 μm, no obvious optical transmission defects, a numerical aperture of 0.52 ± 0.019, a 0.886 Weibull modulus, and a 1.186 Weibull-shaped parameter. Finally, we used the optical fiber sensor fabricated using the improved parameters to measure the refractive indices of various solutions. The results show that a refractive-index resolution of 1.8 × 10−4 RIU (linear fitting R2 = 0.954) was achieved for sucrose solutions with refractive indices ranging between 1.333 and 1.383. We also adopted the particle plasmon resonance sensing scheme using the fabricated optical fibers. The results provided additional information, specifically, a superior sensor resolution of 5.73 × 10−5 RIU, and greater linearity at R2 = 0.999.

Multimode fiber Mach-Zehnder interferometer for measurement of refractive index

A novel class of multimode optical fiber Mach-Zehnder interferometer suited for refractive index measurement is presented. The Mach-Zehnder interferometer was constructed by making two coupling points in a multimode optical fiber at localized regions with an electric arc system. The Mach-Zehnder interferometer as a high sensitive refractive-index sensor to detect changes in the surrounding refractive index was studied. The results for different concentrations of sucrose solution show that a resolution of 3.07×10−5 – 7.47×10−5 RIU is achieved for refractive indices in the range of 1.333 to 1.370, suggesting that the Mach-Zehnder interferometer are attractive for chemical, biological, and biochemical sensing with aqueous solutions.

Laser-induced long-period fiber grating sensor modified with gold nano-rods

We present a long period grating sensor fabricated in a large mode area photonic crystal fiber. This device is insensitive to temperature change but highly sensitive to refractive index variation (167.5 nm/RI or 2.57×10−4 limiting resolution).

Investigations on laser-induced long-period fiber gratings written in polarization maintaining photonic crystal fiber

We report on fabrication and characterization of a long period grating in polarization maintaining photonic crystal fiber. The sensing properties of the grating have been investigated and it was shown to possess insensitivity to temperature, a refractive index sensitivity of 15.2 nm/RI, and a bending sensitivity of 24.1 nm.cm.

Investigations of refractive index sensing with a photonic crystal fiber interferometer

We report a photonic crystal fiber (PCF) interferometer was realized by fusion splicing a short section of PCF between two standard single mode fibers (Corning SMF-28) using a Fitel-S175 splicer. The fully collapsed air holes of the PCF at the spice regions allow the coupling of PCF core and cladding modes. The transmission spectrum exhibits sinusoidal interference pattern which shifts differently when the cladding/core surface of the PCF is immersed with different RI of the surrounding medium. The results for different concentrations of sucrose solution show that a resolution of 1.21×10−4 −4.65×10−4 RIU is achieved for refractive indices in the range of 1.333 to 1.403, suggesting that the PCF interferometer are attractive for chemical, biological, and biochemical sensing with aqueous solutions.

Effect of Surface Coverage of Gold Nanoparticles on the Refractive Index Sensitivity in Fiber-Optic Nanoplasmonic Sensing

A simple theoretical model was developed to analyze the extinction spectrum of gold nanoparticles (AuNPs) on the fiber core and glass surfaces in order to aid the determination of the surface coverage and surface distribution of the AuNPs on the fiber core surface for sensitivity optimization of the fiber optic particle plasmon resonance (FOPPR) sensor. The extinction spectrum of AuNPs comprises of the interband absorption of AuNPs, non-interacting plasmon resonance (PR) band due to isolated AuNPs, and coupled PR band of interacting AuNPs. When the surface coverage is smaller than 12.2%, the plasmon coupling effect can almost be ignored. This method is also applied to understand the refractive index sensitivity of the FOPPR sensor with respect to the non-interacting PR band and the coupled PR band. In terms of wavelength sensitivity at a surface coverage of 18.6%, the refractive index sensitivity of the coupled PR band (205.5 nm/RIU) is greater than that of the non-interacting PR band (349.1 nm/RIU). In terms of extinction sensitivity, refractive index sensitivity of the coupled PR band (−3.86/RIU) is similar to that of the non-interacting PR band (−3.93/RIU). Both maximum wavelength and extinction sensitivities were found at a surface coverage of 15.2%.

Highly sensitive fiber-optic particle plasmon resonance refractive index sensor based on spatial light modulation technology

We present an effective method for enhancing the refractive index sensitivity of a fiber-optic particle plasmon resonance sensor based on spatial light modulation technology. The sensor was realized by modification of spherical gold nanoparticles in the fiber sensing zone. Three different beam shapes were tested and one of the relatively high refractive index sensitivity (~12.67/RIU) was found. A 5% enhancement was found over sensors without beam modification.

A novel dual-channel fiber-optic particle plasmon resonance sensor realized by CO 2 laser engraving

We present a novel dual-channel fiber-optic particle plasmon resonance (FO-PPR) sensor for refractive index sensing. The sensor, fabricated by a CO2 laser system, was realized by modification of gold sphere nanoparticles and gold nano-rods simultaneously in two different fiber sensing zones with a length of 2cm. This type of sensor has shown the capacity of simultaneously sensing two different channels of PPR signals with a relatively high refractive index sensitivity (∼6.0/RIU).