Honglei Guo

Fiber Optic Sensors for Structural Health Monitoring of Air Platforms

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided.

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.

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.

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.

Optical Manipulation of Microparticles in an SU-8/PDMS Hybrid Microfluidic Chip Incorporating a Monolithically Integrated On-Chip Lens Set

An SU-8/PDMS microfluidic chip incorporating a monolithically integrated on-chip lens set for transport and manipulation of microparticles is developed. The components, including the on-chip lens set, the microfluidic channel, and the fiber grooves, are defined in a single layer of SU-8 by one-step photolithography. The design of the on-chip lens set and the fabrication of the microfluidic chip are fully described. The influence of the beam-waist radius on the manipulation performance is theoretically analyzed and experimentally verified for the first time. In the cross-type optofluidic architecture, the evaluation is performed by measuring the particle displacement with different beam-waist radii under different fluid-flow rates. The on-chip lens set is designed to have a specific dimension to achieve the required beam-waist radius. It is revealed that the particle displacement is counter-proportional to the beam-waist radius. An experiment is performed. The results show that the particle displacement is increased by reducing the beam-waist radius. The optical manipulation of microparticles is also demonstrated by using two counter-propagating light beams that are perpendicular to the fluid-flow direction with the beam-waist radius determined by two on-chip lens sets placed on the two sides of the microfluidic channel. The proposed architecture could be used to enhance the performance in particle transport, separation, and concentration.

Interrogation of a long-period grating using a mechanically scannable arrayed waveguide grating and a sampled chirped fiber Bragg grating.

A novel technique to interrogate a long-period grating (LPG) using a mechanically scannable arrayed waveguide grating (AWG) is proposed. This technique is implemented based on space-to-wavelength mapping by mechanically scanning the input light beam along the input coupler facet of an AWG. By employing a sampled chirped fiber Bragg grating with multiple peaks as a reference, the central wavelength of the LPG is measured. An interrogation system with a resolution of 10 pm at a speed of 10 Hz is demonstrated. Furthermore, the technique proposed can potentially offer subpicometer resolution at a speed of 500 Hz.

Interrogation of a Long-Period Grating Sensor by a Thermally Tunable Arrayed Waveguide Grating

An interrogation technique for a long-period grating (LPG) sensor is studied theoretically and experimentally. By employing a thermally tunable arrayed waveguide grating (AWG), the center wavelength of the LPG sensor is successfully measured using the linear temperature dependence of the AWG transmission wavelengths. Initial results show that the proposed interrogation technique can provide a resolution of 1 pm and interrogation range of 25 nm. Furthermore, this technique has the potential of being packaged into a low-cost and compact-size device.

Echelle Diffractive Grating Based Wavelength Interrogator for Potential Aerospace Applications

Operational load monitoring and impact damage detection are the two critical aspects of aerospace structural health monitoring (SHM). Fiber Bragg grating (FBG) sensors have demonstrated great potential in both. But the currently available interrogation systems can only handle one of the two types of SHM capabilities offered by FBG sensors. In addition, the practical implementation of FBG sensor systems in aerospace vehicles requires the interrogator to be small size, light weight, and low-power consuming. In this paper, we present an Echelle diffractive grating (EDG) based interrogation system for FBG sensors, which possesses two operation modes, i.e., the sweeping mode for operational load monitoring and the parked mode for impact damage detection. Experimental results show that this interrogator offers better than 1-pm measurement resolution and 10-pm repeatability. In addition, the interrogation system is very compact and weighs less than 60 g (excluding the electronic controller). It also has the potential to achieve a measurement speed of 300 kHz and be powered by a battery.

Static and dynamic strain fiber Bragg grating sensor interrogation using a monolithically integrated echelle diffractive grating

We report a miniaturized wavelength interrogator for the static and dynamic strain fiber Bragg grating sensors. The developed interrogator is based on a monolithically integrated echelle diffractive grating and works in two independent modes, one for the static strain measurement and the other for dynamic strain measurement. The proposed interrogator is evaluated by measureing a static strain of 400 με and a dynamic strain of 200 Hz with a peak-to-peak amplitude of 200 με. Initial results are excellent and show that these two interrogation modes can be potentially performed simultaneously.

Interrogation of a Long Period Grating Fiber Sensor With an Arrayed-Waveguide-Grating-Based Demultiplexer Through Curve Fitting

Interrogation of a long period grating (LPG) fiber sensor with an arrayed-waveguide-grating (AWG)-based demultiplexer through curve fitting is investigated and experimentally demonstrated. In the interrogation system, the measured light intensities from the output of the AWG are used to reconstruct the selected resonant dip of the LPG sensor through curve fitting in the form of a linear combination of Gaussian functions. By monitoring the changes of the reconstructed LPG spectrum, including the center wavelength shift and the minimum attenuation variation, the sensor signals can be interrogated with good accuracy in real time. The center wavelength is obtained by calculating the first-order derivative of the fitting function. The minimum attenuation is obtained directly from the reconstructed spectrum. Since the interrogation system demonstrated is based on an all-solid-state optical device, it offers the advantages of compact size and high-speed interrogation with high potential for integration.