Lon A. Wang

Corrugated long-period fiber gratings as strain, torsion, and bending sensors

We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature.

Eco-friendly plasmonic sensors: using the photothermal effect to prepare metal nanoparticle-containing test papers for highly sensitive colorimetric detection.

Convenient, rapid, and accurate detection of chemical and biomolecules would be a great benefit to medical, pharmaceutical, and environmental sciences. Many chemical and biosensors based on metal nanoparticles (NPs) have been developed. However, as a result of the inconvenience and complexity of most of the current preparation techniques, surface plasmon-based test papers are not as common as, for example, litmus paper, which finds daily use. In this paper, we propose a convenient and practical technique, based on the photothermal effect, to fabricate the plasmonic test paper. This technique is superior to other reported methods for its rapid fabrication time (a few seconds), large-area throughput, selectivity in the positioning of the NPs, and the capability of preparing NP arrays in high density on various paper substrates. In addition to their low cost, portability, flexibility, and biodegradability, plasmonic test paper can be burned after detecting contagious biomolecules, making them safe and eco-friendly.

The health monitoring of a prestressed concrete beam by using fiber Bragg grating sensors

This paper presents the complete test results of a prestressed concrete (PC) beam, including the hydration temperature of the hardening process, the behavior of the PC beam under sustained loading and assessment of damage, such as the locations of cracks and their corresponding depth. Hydration temperatures during the hardening process were determined using fiber Bragg grating (FBG) sensors, while the behavior of the PC beam, under sustained loading, was evaluated and compared using conventional sensors as well as FBG sensors embedded in, or surfaced mounted onto, the PC beam. In addition, a new concept, involving an optical FBG based technique to detect and locate structural damage, such as crack location and depth, is demonstrated.

A wavelength- and loss-tunable band-rejection filter based on corrugated long-period fiber grating

We demonstrate a long-period fiber grating composed of an etched corrugated structure that can be used as a wavelength- and loss-tunable band-rejection filter. The tunabilities are based on the index modulation capable of being varied in the corrugated structure under externally applied mechanical forces. The new type of fiber filter enables wavelength and loss tuning ranges of more than 30 nm and 25 dB by adjusting the applied amounts of torsion and tensile forces, respectively.

Direct imprinting using soft mold and gas pressure for large area and curved surfaces

In this paper we report a simple and effective method that renders direct imprinting of sub-micron structures onto PMMA resist coated on large area and curved substrates using the PDMS mold on a closed chamber. Nitrogen gas was employed to generate a uniform pressure. The patterns of the soft mold could be replicated with high quality over an entire 12in. resist-coated area. The process was further successfully applied to the imprinting of a curved substrate.

Periodical corrugated structure for forming sampled fiber Bragg grating and long-period fiber grating with tunable coupling strength

A novel corrugated structure on an optical fiber is proposed and demonstrated to produce tunable long-period index modulation based on photoelastic effect. The corrugated structure is a periodic variation made by chemical etching on the cladding radius of an optical fiber. By imposing this corrugated structure upon a built-in fiber Bragg grating (FBG), a superstructure grating with tunable reflectance is formed. In addition, couplings between the fundamental core mode and cladding modes take place under such a corrugated structure when the phase-matching condition is satisfied. Thus, the device can also act effectively as a long-period fiber grating (LPFG) with tunable coupling strength. We also develop the coupled-mode theory based on Fourier series expansion to describe such a corrugated sampled Bragg grating. The phase-matching condition for various sampling orders can be derived, and the calculated spectra are compared with those based on the fundamental matrix method. Optical measurements demonstrate some unique characteristics of these devices, and good qualitative agreements between simulation and measurement verify the viewpoint that long periodic index modulation is indeed induced by such a corrugated structure through the photoelastic effect when a tensile force is applied.

Packaging methods of fiber-Bragg grating sensors in civil structure applications

Fiber-Bragg grating (FBG) sensors made on bare fibers are easily damaged when handled improperly during and after fabrication. As a protection from such damage, a novel technique for protecting and packaging FBG sensors has been developed and is presented in this paper. To characterize the strain transmission efficiency of the packaged FBG sensors, an analytical finite-element method is used, and the results are compared with the experiments. It is observed that the thickness and Youngs moduli of glues have little influence on the strain transmission, especially when the thickness of the glue is less than the diameter of an optical fiber. However, recoating and steel-tube packaging will markedly affect the strain transmission rate. The strain transmission rates decrease with the increase in thickness of the packaging material. Also, the aging problem of the polymide or acrylate coating and epoxy glue must be considered, since the service life of most structures is usually designed for more than 50 years. The metallic recoated FBG sensor developed in this research uses different approaches, such as low-temperature solder welding, which shows no aging problem, to install the sensors in the structures. Based on the simulated and experimental results, the nickel recoating method is shown to have good strain transmission efficiency compared with other packaging methods.

Transfer-matrix method based on perturbation expansion for periodic and quasi-periodic binary long-period gratings

A transfer-matrix method based on perturbation expansion is proposed as an alternative way of simulating the transmission spectrum of a binary long-period grating (LPG). We first generalize the concept of transfer matrices for a heterojunction waveguide. For the couplings among copropagating modes, forward transfer matrices are used to describe the evolution of mode amplitudes along the grating. We show that these elements are related to the well-known coupling coefficients. The method is then used for the study of ideal two-mode grating couplers, and analytic solutions are obtained. We also use the matrix method to study multimode couplings in a LPG and compare the results with those obtained by using the coupled-mode theory. To further demonstrate its usefulness, we apply the method to a special quasi-periodic LPG, the Fibonacci grating. The results show that each cladding mode contributes to several transmission dips and that the dips of different cladding modes are grouped according to the special resonance conditions.

Tolerance analysis of aligning an astigmatic laser diode with a single-mode optical fiber

A general method of analyzing misalignment tolerance of coupling light from an astigmatic laser diode to a single-mode fiber is described. We apply the method to derive the 3 dB tolerances caused by longitudinal, lateral, and angular misalignments in the optimal conditions for two kinds of coupling schemes. One is by the use of a rotationally symmetric optical system, and the other by a rod which is a special case of utilizing a rotationally asymmetric optical system. In the latter coupling case, the misalignment caused by a laser is found to have a much less 3 dB tolerance in the vertical direction than that caused by a fiber.

Soft mold and gasbag pressure mechanism for patterning submicron patterns onto a large concave substrate

A gasbag pressure (GBP) mechanism has been developed for patterning submicron patterns onto large concave substrate. The GBP mechanism consists of a pressure gasbag and a vacuum chamber system. It provides gradual contact, uniform pressure, and intact contact for imprinting patterns in the soft mold onto a concave substrate. The patterns on the soft mold can be successfully replicated over an entire photoresist-coated concave substrate. The accuracy of replication has been experimentally evaluated.