Yinian Zhu

Long-period gratings in photonic crystal fiber as an optofluidic label-free biosensor.

Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this structure with an optically aligned flow cell, we have developed an optofluidic refractive index transduction platform for label-free biosensing. The LPG-PCF scheme possesses extremely high sensitivity to the change in refractive index induced by localized binding event in different solution media. A model immunoassay experiment was carried out inside the air channels of PCF by a series of surface modification steps in sequence that include adsorption of poly(allylamine hydrochloride) monolayer, immobilization of anti-rat bone sialoprotein monoclonal primary antibody, and binding interactions with non-specific goat anti-rabbit IgG (H+L) and specific secondary goat anti-mouse IgG (H+L) antibodies. These adsorption and binding events were monitored in situ using the LPG-PCF by measuring the shift of the core-to-cladding mode coupling resonance wavelength. Steady and significant resonance changes, about 0.75 nm per nanometer-thick adsorbed/bound bio-molecules, have been observed following the sequence of the surface events with monolayer sensitivity, suggesting the promising potential of LPG-PCF for biological sensing and evaluation.

Long-period gratings inscribed in air- and water-filled photonic crystal fiber for refractometric sensing of aqueous solution

We report a comparative study of inscription of long-period gratings (LPGs) in air- and water-filled photonic crystal fiber (PCF) via residual stress relaxation using a scanning CO2 laser for refractometric sensing. Under the same inscribing condition, LPGs fabricated following the two PCF schemes exhibit different resonance spectral features. Using NaCl aqueous solution of various concentrations, we demonstrate that both PCF-LPGs possess excellent sensitivity of ∼10−7 refractive index unit within the index range of 1.33 and 1.35, whereas LPGs inscribed in water-filled PCF show much narrower full width at half maximum in the resonance spectrum.

Design of solid-core microstructured optical fiber with steering-wheel air cladding for optimal evanescent-field sensing

We present the design of a solid-core microstructured optical fiber with steering-wheel pattern of large holes in cladding as platform for evanescent-field sensing. Both geometry and optical properties of the fiber are numerical computed and analyzed in consideration of manufacturability using sol-gel casting technique as well as by evaluating a triangular lattice of holes with three rings in the design structure so that effective parameters can be established using effective step-index model. We predict less than 0.7 dB/m confinement loss at 850 nm, 29%, 13.7%, and 7.2% of light intensity overlap in air holes at 1500 nm, 1000 nm, and 850 nm wavelength, respectively, in such fiber. With the low loss and high mode-field overlap, the steering-wheel structured fiber is well suited for evanescent-field sensing and detection of chemical and biological species.

Index-guiding liquid-core photonic crystal fiber for solution measurement using normal and surface-enhanced Raman scattering

We have explored the use of index-guiding liquid-core (LC) photonic crystal fiber (PCF) as a robust platform for measurements of solutions of trace volume using normal and surface-enhanced Raman scattering (SERS). The LC PCF was fabricated by selectively sealing the cladding air channels at the distal ends of a hollow-core PCF while leaving the center core open, using a fusion splicer. Utilizing a 30-cm-long LC PCF with the entire center core filled with the ~0.1-µL solution of interest, we have obtained normal Raman spectra of water, ethanol, and 1 vol% ethanol in water. Sensitive and reproducible SERS detection of 1.7×10−7 M thiocyanate anions (14 ppb of NaSCN) in water has also been achieved.

Effect of macro-bending on resonant wavelength and intensity of long-period gratings in photonic crystal fiber.

We report the spectral characteristics of CO(2)-laser inscribed long-period gratings (LPGs) in endlessly single mode photonic crystal fiber (PCF) subject to macro-bending. The coupling modes as a result of bending were studied by examining the shifts of resonant wavelengths, the splits of attenuation bands, and the variation in coupling strength of the transmission spectra. A bending coefficient of ~ 27.9 nm.m was determined in the PCF at 180 degrees rotational orientation relative to the point of laser inscription in the curvature range from 2.6 m(-1) to 3.5 m(-1). Compared with conventional fiber LPGs fabricated using the same method, the PCF-based LPGs possess higher sensitivity both to bending and orientation, making them promising for sensor applications.

Core-cladding mode coupling and recoupling in photonic crystal fiber for enhanced overlap of evanescent field using long-period gratings

Excitation of cladding modes has been achieved using long-period (LPGs) inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO(2) laser irradiation. Core-cladding mode coupling and recoupling has resulted in marked improvement in the evanescent field overlap throughout the cladding air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization of the PCF-LPG configuration can lead to a field power overlap as high as 22% with a confinement loss of less than 1 dB/m in the cladding mode.

Design of photonic crystal fiber long-period grating refractive index sensor

Numerical optimization of photonic crystal fiber (PCF) structures for refractive index sensors based on long period gratings inscribed in PCFs has been performed. The optimization procedure employs the Nelder-Mead downhill simplex algorithm. This direct-search method attempts to minimize a scalar-valued nonlinear function of N real variables (called the objective function) using only function values, without any derivative information. An inverse design approach utilizes the objective function constructed using desired sensing characteristics. For the modal analysis of the PCF structure a fully-vectorial solver based on the finite element method is called by the objective function. The dispersion optimization of PCFs is aimed at achieving a high sensitivity of measurement of refractive index of analytes infiltrated into the air holes for the refractive index and probe wavelength ranges of interest. We have restricted our work to the index-guiding solid-core PCF structures with hexagonally arrayed air holes.

Numerical analysis and experimental investigation of highly sensitive photonic crystal fiber long-period grating refractive index sensor

In this paper, we present the numerical analyses of modal coupling properties and resonance spectral response of longperiod gratings (LPGs) in solid-core photonic crystal fibers (PCFs) with respect to their sensitivity to refractive indices of measurands in the air channels using a full-vectoral mode solver combined with frequency-domain method. The calculated results show that the wavelength resonance of a PCF-LPG can be extremely sensitive to the refractive index change. The PCF-LPG refractive index sensors, inscribed by residual stress relaxation using a scanning CO2 laser, are also experimentally investigated in terms of resonance shift with the PCF-LPG structure filled with sodium chloride (NaCl) solutions at concentrations ranging from 0-26% (w/w). The spectral features were greatly improved by inscribing the LPG structure in PCF prefilled with water. The PCF-LPG sensors are shown to be able to detect the change of ~10-7 refractive index unit (RIU) in the index range of 1.33 to 1.35, in a good agreement with the numerical simulation.

Long-period gratings in photonic crystal fibers for sensing of external refractive index change and macro-bending

Long-period gratings (LPGs) have been fabricated in endlessly single mode photonic crystal fibers (ESM-PCFs) utilizing focused CO2 laser irradiation and residual stress relaxation technique. The responses of the ESM-PCF-LPGs to external refractive index and applied bending curvature have been investigated. As compared with the conventional fiber-based LPGs inscribed under the same condition, the ESM-PCF-LPGs exhibit higher sensitivity to external refractive index change and macro-bending, making them attractive candidates for chemical and biological sensing applications.