Kexing Liu

A passive wavelength demodulation system for guided-wave Bragg grating sensors

A novel, passive, and self-referencing wavelength detection system (WDS) that measures the wavelength of the narrowband back-reflected spectrum of guided-wave Bragg gratings is described. This letter also reports on the use of such a detection system with fiber-optic Bragg gratings used as absolute strain sensors. The wavelength detection system demonstrated a 1% strain resolution of the total strain measurement range.<<ETX>>

A Bragg grating-tuned fiber laser strain sensor system

The development of a fiber laser sensor system which permits efficient interrogation of Bragg grating sensors is reported. A tunable erbium doped fiber laser which utilizes a broadband mirror and an intracore Bragg grating reflector in side-pump configuration is described. The wavelength of the laser oscillation is determined by the Bragg grating, which is remotely located and used as a strain sensor. This arrangement is used in conjunction with a passive wavelength demodulation system (WDS) to form a self-contained fiber laser strain sensor system, allowing efficient interrogation of the Bragg sensor. This device provides interrupt-immune sensing of static and dynamic strains with a bandwidth of 13.0 kHz.<<ETX>>

Practical fiber-optic Bragg grating strain gauge system

A fiber-optic strain gauge system for use in structural monitoring and smart-structure applications is described. The strain gauge uses a fiber-optic Bragg grating sensor to measure strain and a passive, wavelength demodulation system to determine the wavelength of the narrow-band, backreflected spectrum from the grating sensor. The fiber-optic strain gauge system permits the measurement of both static and dynamic strains with a noise-limited resolution of 0.44 microstrain/√Hz, a measurement dynamic range of 27.8 dB, and a bandwidth of 250 Hz.

Kerr lens mode locking of a diode-pumped Nd:YAG laser.

We demonstrate Kerr lens mode locking of a diode-pumped Nd:YAG laser by using the gain medium as the Kerr medium and no intracavity slit. Kerr self-focusing within the Nd:YAG rod is believed to improve matching between the cavity mode and the aperture created by thermal lens aberration and thereby discriminate against lower (cw) intensities. The laser produces 8.5-ps, 100-MHz pulses with 1 W of average output power and a 35% slope efficiency. Kerr lens mode locking is initiated by slight mechanical perturbation.

Wavelength demodulated Bragg grating fiber optic sensing systems for addressing smart structure critical issues

Smart materials and adaptive structures will require structurally integrated fiber optic sensing systems that can operate in practical situations including harsh environments. The intracore fiber optic Bragg grating has considerable potential to serve as the sensor of choice for this emerging field. However, its role has been impeded by the lack of a simple, passive and fast method of determining the wavelength of its narrow back-reflected optical signal. The authors report on the development of just such a wavelength demodulation system that is inexpensive and easily implemented with a minimum of equipment. Furthermore, they shall show that this approach lends itself to the development of an optoelectronic chip that could process many fiber optic sensors, yet be small enough to be integrated within the structural interface and thereby address the interconnect problem-potentially one of the most critical facing the development of practical smart structures.

Fiber-optic interferometric sensor for the detection of acoustic emission within composite materials

An optical-fiber Michelson interferometric acoustic emission sensor is described. The sensor uses ordinary singlemode fiber and is embedded in the composite material under test. Signals are demodulated through the active homodyne. This system provides a novel approach for material nondestructive evaluation.

Strain sensing using a fiber-optic Bragg grating

This paper reports on the development of a passive, fast demodulation system for use with in-fiber Bragg gratings used for strain or temperature measurements. This compact, potentially inexpensive self-referencing system permits absolute strain/temperature measurements over a wide dynamic range with fast temporal response by tracking the wavelength shifts of the narrow-band back-reflected Bragg spectrum. The wavelength, bandwidth and strain sensitivity of a Bragg sensor are discussed, and examples of both static and dynamic strain measurements are shown.

Passive-quadrature demodulated localized-Michelson fiber-optic strain sensor embedded in composite materials

A strain sensor embedded in composite materials that is intrinsic, all fiber, local, and phase demodulated is described. It is the combination of these necessary elements that represents an advance in the state of the art. Sensor localization is achieved by using a pair of mirror-ended optical fibers of different lengths that are mechanically coupled up until the desired gauge length for common-mode suppression has been reached. This fiber-optic sensor has been embedded in both thermoset (Kevlar/epoxy and graphite/epoxy) and thermoplastic (graphite/PEEK) composite materials in order to make local strain measurements at the laminar level. The all-fibre system uses a 3*3 coupler for phase demodulation. Parameters such as strain sensitivity, transverse strain sensitivity, failure strain, and frequency response are discussed, along with applications. >

Fiber optic sensors for smart structures

Abstract An overview is presented of our research towards the development of structurally integrated fiber optic sensors for Smart Structures. This includes the development of the first full-scale fiber optic damage assessment test system in the form of a composite aircraft leading edge and the fabrication, characterization and evaluation of the first fiber optic strain rosette. This optical strain rosette was shown to be capable of mapping the strain tensor from within composite materials.

Signal Processing Techniques for Interferometric Fiber-Optic Strain Sensors

Fiber optic strain sensors for smart structures will have to provide broad dynamic range, high resolution, wide bandwidth, and in certain applications, absolute strain measurement. Different sensor configurations and applications require different sig nal processing methods. A number of signal processing techniques and optical configura tions for interferometric fiber-optic strain sensors are discussed. Their applications to smart structure and material studies are presented. Advantages and disadvantages of each signal processing technique are discussed from the perspective of practicality, multiplex ing, and absolute measurement capability.