Gaozhi Xiao

Interrogating fiber Bragg grating sensors by thermally scanning a demultiplexer based on arrayed waveguide gratings

We evaluate a wavelength interrogation technique based on an arrayed waveguide grating (AWG). Initial results show that the Bragg wavelength of fiber Bragg grating (FBG) sensors can be precisely interrogated by thermally scanning an AWG-based demultiplexer. The technique potentially offers a low-cost, compact, and high-performance solution for the interrogation of FBG distributed sensors and multisensor arrays.

Sealing SU-8 microfluidic channels using PDMS

A simple method of irreversibly sealing SU-8 microfluidic channels using PDMS is reported in this paper. The method is based on inducing a chemical reaction between PDMS and SU-8 by first generating amino groups on PDMS surface using N(2) plasma treatment, then allowing the amino groups to react with the residual epoxy groups on SU-8 surface at an elevated temperature. The N(2) plasma treatment of PDMS can be conducted using an ordinary plasma chamber and high purity N(2), while the residual epoxy groups on SU-8 surface can be preserved by post-exposure baking SU-8 at a temperature no higher than 95 °C. The resultant chemical bonding between PDMS and SU-8 using the method create an interface that can withstand a stress that is greater than the bulk strength of PDMS. The bond is permanent and is long-term resistant to water. The method was applied in fabricating SU-8 microfluidi-photonic integrated devices, and the obtained devices were tested to show desirable performance.

Measurement of Microwave Frequency Using a Monolithically Integrated Scannable Echelle Diffractive Grating

A novel approach to the measurement of microwave signal frequency is studied and demonstrated. The approach is based on a monolithically integrated echelle diffractive grating (EDG). The microwave signal is converted to an optical signal of two sidebands using an optical carrier and a Mach-Zehnder modulator. One of the sidebands is then filtered out by a fiber Bragg grating, while the other sideband is characterized by an EDG-based interrogator. Due to the better than 1-pm interrogation resolution of this interrogator, the center wavelength of the sideband tested is capable of being accurately measured. Combining this data with the wavelength of the optical carrier used, the frequency of the microwave signal can be calculated. The results obtained are found to be in good agreement with those of the microwave signals.

A New Fiber-Bragg-Grating Sensor Interrogation System Deploying Free-Spectral-Range-Matching Scheme With High Precision and Fast Detection Rate

A new method for fiber-Bragg-grating sensor interrogation and multiplexing is proposed and its operating features are investigated experimentally. This technique utilizes free-spectral-range-matched interrogation (FSRMI) employing a tunable scanning Fabry-Perot filter and a multichannel bandpass filter. An -channel FSRMI allows simultaneous scanning and thus improvement of detection speed by times compared to a nonfree-spectral-range-matched interrogation scheme. A 5-pm wavelength resolution has been obtained. This technique simultaneously scans the multiple transmission peaks and provides solutions to a wavelength-modulated distributed sensors and multisensor arrays with high sampling speed and wavelength accuracy.

Self-Referencing in Optical-Fiber Surface Plasmon Resonance Sensors

We have achieved a self-referencing function in optical-fiber surface plasmon resonance (SPR) sensors by fabricating two tandem SPR sections in a single optical fiber and by coating one of the sections with a reference material to create an additional resonance at a longer wavelength for referencing, while keeping the other one for regular sensing. By choosing a reference material with a very large thermooptic coefficient, we have demonstrated an SPR optical fiber sensor that can, by itself, correct the effect of temperature variation in the testing medium.

Focusing-enhanced mixing in microfluidic channels

A focusing-based microfluidic mixer was studied. The micromixer utilizes the focusing process required for cytometry to reduce the diffusion distance of molecules to be mixed in order to facilitate the passive diffusion-controlled mixing process. It was found that both the high flow rate ratio of the sheath flow to the flows to be mixed and the low flow rate of the mixing fluids resulted in the short mixing length required within the microfluidic channel. It was shown that a complete mixing was achieved within a distance of 4 mm in the micromixer for the focused mixing fluids at a flow rate of 2 mulmin and a flow rate ratio of the sheath flow to the flows to be mixed at 4:1. The mixer described here is simple and can be easily fabricated and controlled.

Miniaturized Optical Fiber Sensor Interrogation System Employing Echelle Diffractive Gratings Demultiplexer for Potential Aerospace Applications

To enable the application of optical fiber sensors to aerospace vehicles, the sensor interrogation or data acquisition system has to meet small size and low weight requirements. This paper presents the developmental work of an echelle diffractive gratings (EDGs) demultiplexer-based optical fiber sensor interrogation system. The operation principle of this system and its application to fiber Bragg grating (FBG) sensor interrogation are presented. The experimental results have shown that the developed interrogator (not including the electronic controller) weighs less than 60 g and provides better than 1 pm measurement resolution and better than plusmn10 pm measurement accuracy.

Simultaneous Interrogation of a Hybrid FBG/LPG Sensor Pair Using a Monolithically Integrated Echelle Diffractive Grating

A simultaneous interrogation technique of a hybrid fiber Bragg grating (FBG) and long-period grating (LPG) sensor pair is proposed and demonstrated using a monolithically integrated echelle diffractive grating (EDG). The operation principle that is based on the monotonic temperature dependence of the EDG transmission wavelengths is presented. Initial results show that a 1-pm resolution and 24-nm interrogation range are achieved by using the proposed interrogation technique, which can effectively be implemented to interrogate hybrid FBG/LPG-based sensor pairs for the discrimination of refractive index (RI)/temperature in RI measurement. The specially designed EDG-based interrogator has the added features of low cost and compact size.

On the use of a compact optical fiber sensor system in aircraft structural health monitoring

Structural Health Monitoring (SHM) has been identified as an area of significant potential for advanced aircraft maintenance programs that ensure continued airworthiness, enhanced operational safety and reduced life cycle cost. Several sensors and sensory systems have been developed for the implementation of such health monitoring capability. Among a wide range of developed technologies, fiber optic sensor technology, in particular fiber Bragg grating based emerged as one of the most promising for aircraft structural applications. This paper is set to explore the suitability of using a new Fiber Bragg Grating sensor (FBG) system developed for operation in two modes, low and high speed sensing modes, respectively. The suitability of the system for potential use in aircraft load monitoring and damage detection applications has been demonstrated. Results from FBG sensor system were in good agreement with results from conventional resistive strain gauges, validating this capability for load monitoring. For damage detection, the FBG sensor system was able to detect acoustic waves generated 52 inches (1.32 m) away. The initial results, obtained in a full stale experimentation, demonstrate the potential of using FBG sensors for both load monitoring and damage detection in aircraft environment.

Wavelength Interrogator Based on Closed-Loop Piezo-Electrically Scanned Space-to-Wavelength Mapping of an Arrayed Waveguide Grating

We demonstrate a novel technique for the interrogation of grating-based fiber optic sensors. The proposed technique is based on space-to-wavelength mapping using an arrayed waveguide grating (AWG). The beam position along the AWG input coupler is controlled by a closed-loop piezoelectric motor. By employing a real-time position feedback encoder, the absolute position of the input light beam can be accurately obtained, which would yield a precise interrogation of the wavelength due to a fixed relationship between the beam position and the transmission wavelength of the AWG channel. The proposed system for the interrogation of fiber Bragg grating (FBG) sensors and a tilted-FBG sensor is experimented. An interrogation resolution of 3 pm and an interrogation range of 18 nm are demonstrated as well as the multichannel measurement capability. Initial results show that the proposed interrogation system has the potential of being packaged into a compact, light weight, and cost-effective interrogator with good performance.