Man-Hong Lai

Chronology of Fabry-Perot Interferometer Fiber-Optic Sensors and Their Applications: A Review

Optical fibers have been involved in the area of sensing applications for more than four decades. Moreover, interferometric optical fiber sensors have attracted broad interest for their prospective applications in sensing temperature, refractive index, strain measurement, pressure, acoustic wave, vibration, magnetic field, and voltage. During this time, numerous types of interferometers have been developed such as Fabry-Perot, Michelson, Mach-Zehnder, Sagnac Fiber, and Common-path interferometers. Fabry-Perot interferometer (FPI) fiber-optic sensors have been extensively investigated for their exceedingly effective, simple fabrication as well as low cost aspects. In this study, a wide variety of FPI sensors are reviewed in terms of fabrication methods, principle of operation and their sensing applications. The chronology of the development of FPI sensors and their implementation in various applications are discussed.

Spectral analysis of bent fiber Bragg gratings: theory and experiment

This Letter presents a simple mathematical model developed from coupled-mode theory to describe the relationship between Bragg transmission loss (BTL), grating length, coupling coefficients, and bending loss in a bent fiber Bragg grating. In our investigation, the finding indicates that the decrement of BTL can be attributed to the increasing bending loss and degradation of both dc and ac coupling coefficients as the bending radius decreases. Besides, the center wavelength shifts as a result of coupling coefficients degradation. The validity of the proposed model is supported by experimental result.

Observation of grating regeneration by direct CO 2 laser annealing

In this work, we have demonstrated for the first time grating regeneration in hydrogenated fibers by direct CO(2) laser annealing. During the annealing process, the center wavelength redshifts as the intensity of the focused CO(2) laser on the grating is elevated. The reflectivity of the grating begins to decay as the temperature induced in the grating approaches the erasure temperature. The grating is completely erased and regenerated afterwards. The observed spectral results have provided the proof of occurrence of dehydroxylation and stress relaxation in the fiber core during the annealing process. Regenerated gratings with low loss, good temperature sensitivities and sustainability have been successfully developed by this technique.

Thermal stress modification in regenerated fiber Bragg grating via manipulation of glass transition temperature based on CO 2 -laser annealing

In this work, we have demonstrated thermal stress relaxation in regenerated fiber Bragg gratings (RFBGs) by using direct CO₂-laser annealing technique. After the isothermal annealing and slow cooling process, the Bragg wavelength of the RFBG has been red-shifted. This modification is reversible by re-annealing and rapid cooling. It is repeatable with different cooling process in the subsequent annealing treatments. This phenomenon can be attributed to the thermal stress modification in the fiber core by means of manipulation of glass transition temperature with different cooling rates. This finding in this investigation is important for accurate temperature measurement of RFBG in dynamic environment.

Photosensitivity of gallium-doped silica core fiber to 193 nm ArF excimer laser.

Grating inscription in a Ga-doped silica core fiber (~5 wt. % Ga) has been demonstrated using ArF (193 nm) and KrF (248 nm) excimer lasers. In a comparative study with germanosilicate fiber with similar Ge concentration, a Ga-doped silica core fiber shows greater photosensitivity to an ArF excimer laser due to the higher absorbance in the region of 190-195 nm. In addition, the photosensitivity of a Ga-doped silica core fiber has been greatly enhanced with hydrogenation. Ga-doped fibers are potential photosensitive fibers for fiber Bragg grating production with an ArF excimer laser.

Optical Gaussian Notch Filter Based on Periodic Microbent Fiber Bragg Grating

In this paper, we demonstrated an optical notch filter constructed from a periodic microbent fiber Bragg grating attained by using two copper-wire-wound slabs. In the reflection spectrum, sideband peaks are created as a result of mechanically induced modulation on the grating period, and a higher number of peaks are observed when greater modulation is applied. The peak wavelength spacing depends upon the period of microbending, which can be varied by changing the diameter of the winding wire or the angle of placement of fiber Bragg grating placed in between the two wound slabs. Moreover, the trend of Bragg transmission loss (BTL) for the transmission spectrum changes with the changing of modulation period and amplitude. The proposed technique is very stable, and it can be used as an optical Gaussian notch filter.

Thermal Activation of Regenerated Grating in Hydrogenated Gallosilicate Fiber

The thermal regeneration of fiber Bragg gratings inscribed on hydrogenated gallosilicate optical fiber, fabricated using a modified chemical vapor deposition (MCVD) technique followed by a solution doping process, is demonstrated for the first time. A type-I seed grating is thermally regenerated and the evolution of the grating reflectivity in a temperature range between 25 °C and 720 °C is investigated. Temperature responses of 15.2 pm/°C and 15.0 pm/°C are obtained during heating and cooling processes of the regenerated grating (RG), respectively, when performing thermal calibration over a temperature range of 25 °C-750 °C and vice versa. Gallosilicate-based RGs are potential candidates as high-temperature resistant gratings in rare-earth-doped fiber, since they provide high photosensitivity and thermal sustainability with the involvement of a single dopant with a concentration as low as 5 wt%.

Effect of CO 2 Laser Annealing on Stress Applying Parts Contributing Toward Birefringence Modification in Regenerated Grating in Polarization Maintaining Fiber

In this paper, we have demonstrated birefringence modification in regenerated grating in polarization maintaining fiber (RGPMF) by using a CO2 laser annealing technique. After conducting an isothermal annealing procedure followed by a slow cooling process, the birefringence of the RGPMF has been increased. This phenomenon can be explained by the changes in the thermal expansion coefficient and glass transition temperature of the stress applying part at different cooling rates. This process was reversible by reannealing with a subsequent fast cooling process. In our observation, the birefringence exponentially increases with a decreasing cooling rate. The highest and lowest records in the birefringence throughout the annealing process are 4.72 × 10-4 and 3.46 × 10-4, respectively (a difference of 1.26 × 10-4). This finding is useful for the study of the birefringence modification, measurement range, sensitivity, and accuracy of PMF or PMF-related devices.

Measurement of grating visibility of a fiber Bragg grating based on bent-spectral analysis

In this study, a technique for measuring the grating visibility of the fiber Bragg grating (FBG) based on bent-spectral analysis is proposed. From varying ac and dc coupling coefficients at different bending radii, the grating visibility is estimated with the aid of a simple mathematical model. The investigation begins with the estimation of the grating visibility from the transmission spectra of the FBG during the inscription process. After that, the FBGs are subjected to a bending test with reducing radii, and again the transmission spectra are recorded. It is shown that the estimated grating visibility is in agreement with the result determined from the earlier inscription process.

CO 2 Laser Applications in Optical Fiber Components Fabrication and Treatment: A Review

CO2 laser has been widely used in material processing, manufacturing, and medical and military applications since its invention in 1964. Typically for optical fiber components fabrication and treatment, CO2 laser has been reported intensively in stress and birefringence modification, long period grating fabrication, thermal regeneration, optical device fabrication, surface polishing, photosensitivity enhancement, and so on. The reason behind this is due to the advantages of high absorption, fast thermal response, low contamination, dynamic control, and focused heating area owned by the CO2 laser treatment technique. In this paper, a wide variety of CO2 laser applications in optical fiber components fabrication and treatment are reviewed in terms of procedure and principle. The advantages of using CO2 laser and the supremacy of the optical fiber components fabricated or treated by CO2 laser are discussed as well.