Grating writing in structured optical fibers is reviewed. Various laser sources have been used including UV and near IR nanosecond and femtosecond lasers, each enabling different material processing regimes. The issue of scattering is modeled through simulation and compared with experiment. Good agreement has been established.

A new optical intensity-based sensing mechanism for the measurement of refractive index and minute displacement is proposed in this paper, which is based on modifying the amount of light coupled between two parallel long-period fiber gratings. The characteristics of this sensing mechanism with experiments and simulation results are demonstrated.

We report our recent work on distributed feedback fiber laser based hydrophones. Some issues related to sensitivity, such as fiber laser phase condition, demodulation, and packaging, are also discussed. With the development of appropriate digital signal processing (DSP) techniques and packaging designs, an interferometric-type distributed feedback (DFB) fiber laser hydrophone system with acoustic sensitivity of 58.0 dB·re·μPa·Hz-0.5 at 1 kHz or a minimum detectable acoustic pressure below 800 μPa during field test is attained. We have also investigated an intensity-type DFB fiber laser hydrophone system and its performance.

Broadband, high-speed wavelength-swept lasers can substantially enhance applications in optical coherence tomography, chemical spectroscopy, and fiber-optic sensing. We report the demonstration of Fourier-domain mode-lock lasers operating at about 90 kHz effective sweep rate over a 158 nm sweep range using a single-band design and over a 284 nm sweep range across the 1.3 μm to 1.5 μm wavelength spectrum using a unique broadband design. A novel dual-detection full-range Fourier-domain optical coherence tomography system is developed which provides 7 μm axial resolution (in air) at about 90 kHz axial scan rate for mirror-image resolved Doppler imaging in a human finger and an African frog tadpole.

The use of near infrared, high intensity femtosecond laser pulses for the inscription of long period fiber gratings in photonic crystal fiber is reported. The formation of grating structures in photonic crystal fiber is complicated by the fiber structure that allows wave-guidance but that impairs and scatters the femtosecond inscription beam. The effects of symmetric and asymmetric femtosecond laser inscriptions are compared and the polarization characteristics of long period gratings and their responses to external perturbations are reported.

Oxygen and carbon dioxide sensors are involved in many chemical and biochemical reactions. Consequently, considerable efforts over years have been devoted to discover and improve suitable techniques for measuring gas concentrations by optical fiber sensors. Optical gas sensors consist of a gas-sensitive dye entrapped in a matrix with a high permeability to gas. With such sensors, gas concentration is evaluated based upon the reduction in luminescence intensity caused by gas quenching of the emitting state. However, the luminescence quenching effect of oxygen is highly sensitive to temperature. Thus, a simple, low-cost plastic optical fiber sensor for dual sensing of temperature and oxygen is presented. Also, a modified Stern-Volmer model is introduced to compensate for the temperature drift while the temperature is obtained by above dual sensor. Recently, we presented highly-sensitive oxygen and dissolved oxygen sensors comprising an optical fiber coated at one end with platinum (II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and PtTFPP entrapped core-shell silica nanoparticles embedded in an n-octyltriethoxysilane(Octyl-triEOS)/tetraethylorthosilane (TEOS) composite xerogel. Also, two-dimensional gas measurement for the distribution of chemical parameters in non-homogeneous samples is developed and is of interest in medical and biological researches.

Intensity-modulated fiber Bragg grating (FBG) sensors, compared with normal wavelength-encoding FBG sensors, can reduce the cost of sensor system significantly by using cost-efficient optical power detection devices, instead of expensive wavelength measurement instruments. Chirped-FBG (CFBG) based intensity-modulated sensors show potential applications in various sensing areas due to their many advantages, including inherent independence of temperature, high measurement speed, and low cost, in addition to the merits of all fiber-optic sensors. This paper theoretically studies the sensing principle of CFBG-based intensity-modulated sensors and briefly reviews their recent progress in measurement of displacement, acceleration, and tilt angle.

A loop topology based white light interferometric sensor network for perimeter security has been designed and demonstrated. In the perimeter security sensing system, where fiber sensors are packaged in the suspended cable or buried cable, a bi-directional optical path interrogator is built by using Michelson or Mach-Zehnder interferometer. A practical implementation of this technique is presented by using an amplified spontaneous emission (ASE) light source and standard single mode fiber, which are common in communication industry. The sensor loop topology is completely passive and absolute length measurements can be obtained for each sensing fiber segment so that it can be used to measure quasi-distribution strain perturbation. For the long distance perimeter monitoring, this technique not only extends the multiplexing potential, but also provides a redundancy for the sensing system. One breakdown point is allowed in the sensor loop because the sensing system will still work even if the embedded sensor loop breaks somewhere.

Different types of fiber-optic sensors based on glass or polymeric fibers are used to evaluate material behavior or to monitor the integrity and long-term stability of load-bearing structure components. Fiber-optic sensors have been established as a new and innovative measurement technology in very different fields, such as material science, civil engineering, light-weight structures, geotechnical areas as well as chemical and high-voltage substations. Very often, mechanical quantities such as deformation, strain or vibration are requested. However, measurement of chemical quantities in materials and structure components, such as pH value in steel reinforced concrete members also provides information about the integrity of concrete structures. A special fiber-optic chemical sensor for monitoring the alkaline state (pH value) of the cementitious matrix in steel-reinforced concrete structures with the purpose of early detection of corrosion-initiating factors is described. The paper presents the use of several fiber-optic sensor technologies in engineering. One example concerns the use of highly resolving concrete-embeddable fiber Fabry-Perot acoustic emission (AE) sensors for the assessment of the bearing behaviour of large concrete piles in existing foundations or during and after its installation. Another example concerns fiber Bragg grating (FBG) sensors attached to anchor steels (micro piles) to measure the strain distribution in loaded soil anchors. Polymer optical fibers (POF) can be — because of their high elasticity and high ultimate strain - well integrated into textiles to monitor their deformation behaviour. Such “intelligent” textiles are capable of monitoring displacement of soil or slopes, critical mechanical deformation in geotechnical structures (dikes, dams, and embankments) as well as in masonry structures during and after earthquakes.

Fiber Bragg grating (FBG) is a mature sensing technology for the measurement of strain, vibration, acoustics, acceleration, pressure, temperature, moisture, and corrosion. It has gained rapid acceptance in civil, aerospace, chemical and petrochemical, medicine, aviation and automotive industries. The most prominent advantages of FBG are: small size and light weight, distributed array of FBG transducers on a single fiber, and immunity to radio frequency interference. However, a major disadvantage of FBG technology is that conventional state-of-the-art FBG interrogation system is typically bulky, heavy, and costly bench top instruments that are typically assembled from off-the-shelf fiber optic and optical components integrated with a signal electronics board into an instrument console. Based on the industrial need for a compact FBG interrogation system, this paper describes recent progress towards the development of miniature fiber Bragg grating sensor interrogator (FBG-TransceiverTM) system based on multi-channel monolithic integrated optic sensor microchip technology. The integrated optic microchip technology enables monolithic integration of all functionalities, both passive and active, of conventional bench top FBG sensor interrogator system, packaged in a miniaturized, low power operation, 2 cm×5 cm small form factor (SFF) package suitable for long-term structural health monitoring in applications where size, weight, and power are critical for operation.

An optical fiber hydrophone based on equivalent phase shift fiber Bragg grating (EPS-FBG) with temperature compensation package provides an improvement of sensitivity in underwater acoustic measurement at wide frequency range, from 2.5 kHz to 12 kHz. The acoustic pressure is transduced into elastic vibration of a circle metal disk, resulting in an intensity modulation of the reflected light wave back from fiber Bragg grating (FBG). Experiment shows that the packaged EPS-FBG hydrophone has a minimum detectable acoustic pressure of about 500 μPa/ Hz at 5 kHz and achieves about 18-dB improvement of acoustic pressure sensitivity compared with a regular apodized FBG hydrophone.