Bing Zou

Long-period gratings inscribed in photonic crystal fiber by symmetric CO 2 laser irradiation: erratum

This erratum amends the wrongly cited NSF grant number in acknowledgment section in our publication.

Wide-Range pH Sensor Based on a Smart- Hydrogel-Coated Long-Period Fiber Grating

We report a fiber-optic sensor for pH measurement, which is a long-period fiber grating (LPFG) coated with smart hydrogel. When the pH of a solution that surrounds the hydrogel coating changes, the volume of the coating and hence its refractive index are changed, which results in a shift in the resonance wavelength of the LPFG. Our experimental sensor shows a sensitivity of ~0.66 nm/pH over the pH range from 2 to 12 and a response time of less than 2 s. The effects of the refractive index of the surrounding solution and temperature variations on the sensor are also measured.

Surface-Plasmon-Resonance Refractive-Index Sensor With Cu-Coated Polymer Waveguide

We propose a refractive-index sensor based on the principle of surface plasmon resonance (SPR) in a polymer channel waveguide coated with a copper (Cu) film. Our fabricated device using a 40-nm Cu film covered with a 10-nm silicon-dioxide protecting layer offers a maximum refractive-index sensitivity of ~45 μm/RIU for the refractive-index range from 1.5375 to 1.5515, which is far more sensitive than the existing fiber/waveguide-based SPR sensors. The experimental results agree well with the theoretical analysis. We also present a sensor design with a record-high sensitivity of ~250 μm/RIU for aqueous solutions by using low-index polymer materials. The proposed sensor structure could be developed for a wide range of biomedical applications.

Phase Retrieval From Transmission Spectrum for Long-Period Fiber Gratings

We demonstrate a method to retrieve the phase spectrum of a long-period fiber grating (LPFG) from its transmission spectrum. The method is based on the Hilbert transform that relates the minimum phase and the amplitude spectrum of a linear system, and is applicable to under-coupled LPFGs with arbitrary grating profiles. We verify the method with numerical examples and experiments. The method provides a simple and effective way to obtain the phase characteristics of LPFGs and thus complement the measured transmission spectra of the gratings.

Nano-functionalized long-period fiber grating probe for disease-specific protein detection

Optical biosensors have great significance for rapid and sensitive detection and monitoring of biomolecular interactions. Here, recent advancement in the nano-functionalized long-period fiber grating (LPFG) is used to develop a cost-effective approach for label-free and real-time detection of the carbonic anhydrase-IX (CA9) monoclonal antibody, a diagnostic biomarker in hypoxia condition cultured carcinoma cells.

Effect of irradiation symmetry of CO2 laser on mode coupling in long-period gratings inscribed in photonic crystal fiber

Long-period gratings (LPG) are inscribed in endlessly single mode (ESM) photonic crystal fiber (PCF) and conventional single mode fiber (SMF) with symmetric and asymmetric CO2 laser irradiation. Transmission measurements and near field imaging indicate that symmetric index perturbation induced by laser irradiation with the aid of a 120° gold-coated reflecting mirror results in LP0n symmetric mode coupling, while asymmetric irradiation without using the mirror leads to LP1n asymmetric mode coupling for both fiber types. Symmetric irradiation yields far more reproducible LPG in PCF than asymmetric irradiation. On the other hand, the irradiation symmetry has little effect on the reproducibility of LPG inscribed in SMF due to the isotropy of its all-solid cladding structure. The removal of the outer solid cladding of PCFLPG results in a significant change in its transmission characteristics, which provides rich information on the refractive index perturbation and mode field distribution in the outer solid cladding of PCF-LPG.

Hydrogel-coated long-period fiber grating pH sensor

We report a long-period fiber grating coated with a specially synthesized hydrogel for the measurement of pH changes in an aqueous solution. The coated grating operates sensitively for the pH ranges 2-5 and 8-12.

Application of the Hilbert Transform Method for the Retrieval of the Phase Characteristics of Plasmonic Metal Bragg Gratings

We demonstrate, with detailed examples, a method to retrieve the phase and group-delay responses of plasmonic metal Bragg gratings (MBGs) from their reflection and transmission spectra. The method is based on the Hilbert transform (HT) that relates the phase and the amplitude spectrum of a minimum-phase system. For the retrieval of the reflection phase, if the propagation loss of the grating is negligible, the method cannot tolerate any asymmetry in the grating profile and thus fails to operate for practical low-loss fiber or dielectric-waveguide Bragg gratings, where the fabrication errors are sufficient to destroy the symmetry requirement. The large propagation loss of the surface plasmon mode, however, makes possible the retrieval of the reflection phase of an MBG that contains a certain degree of asymmetry in the grating profile. For the retrieval of the transmission phase, the symmetry requirement is not an issue, while a large propagation loss can introduce significant errors in the retrieved phase spectrum. It is possible, however, to largely reduce such errors by applying the HT method to the spectrum with the background propagation loss artificially removed. The HT method provides a simple and effective way to obtain the phase and group-delay characteristics of MBGs, which are difficult to measure directly, and can find applications in the development of grating-based plasmonic devices.

Surface Plasmon Resonance Sensor Based on a Polymer Waveguide with a Copper Thin-Film Overlay

We investigate theoretically and experimentally a refractive-index sensor based on surface plasmon resonance with a copper thin-film overlay deposited on a polymer waveguide. The sensor shows a very high sensitivity of about 37 μm/RIU.