Hai-Lung Tsai

Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement

We report a miniaturized fiber inline Fabry-Perot interferometer (FPI), with an open micro-notch cavity fabricated by one-step fs laser micromachining, for highly sensitive refractive index measurement. The device was tested for measurement of the refractive indices of various liquids including isopropanol, acetone and methanol at room temperature, as well as the temperature-dependent refractive index of deionized water from 3 to 90 degrees C. The sensitivity for measurement of refractive index change of water was 1163 nm/RIU at the wavelength of 1550 nm. The temperature cross-sensitivity of the device was about 1.1x10(-6) RIU/degrees C. The small size, all-fiber structure, small temperature dependence, linear response and high sensitivity, make the device attractive for chemical and biological sensing.

Miniaturized Fiber Inline Fabry-Perot Interferometer Fabricated with a Femtosecond Laser

We report a miniaturized inline Fabry-Perot interferometer directly fabricated on a single-mode optical fiber with a femtosecond laser. The device had a loss of 16 dB and an interference visibility exceeding 14 dB. The device was tested and survived in high temperatures up to 1100 degrees C. With an accessible cavity and all-glass structure, the new device is attractive for sensing applications in high-temperature harsh environments.

Heat transfer and fluid flow in a partially or fully penetrated weld pool in gas tungsten arc welding

Abstract In this study, fluid flow driven by a combination of electromagnetic force, buoyancy force, arc drag force, and surface tension gradient is numerically analyzed for a partially or fully penetrated weld pool in stationary gas tungsten arc welding (GTAW). Irregular shape of the weld pool and the moving liquid–solid interface is handled by a boundary-fitted-coordinates technique. Surface tension on the top and the bottom free surfaces of the weld pool is found to play a critical role in determining the flow pattern in the weld pool. The calculated GTA weld bead geometry compares favorably with experimental results.

Impingement of filler droplets and weld pool dynamics during gas metal arc welding process

A mathematical model and the associated numerical technique have been developed to simulate, for the first time, the dynamic impinging process of filler droplets onto the weld pool in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, momentum, and thermal energy periodically impinge onto the base metal, leading to a liquid weld puddle. Transient weld pool shape and the complicated fluid flow in the weld pool caused mainly by the combined effect of droplet impinging momentum and surface tension are calculated.

Transport Phenomena and Keyhole Dynamics during Pulsed Laser Welding

Numerical and experimental studies were conducted to investigate the heat transfer, fluid flow, and keyhole dynamics during a pulsed keyhole laser welding. A comprehensive mathematical model has been developed. In the model, the continuum formulation was used to handle solid phase, liquid phase, and mushy zone during melting and solidification processes. The volume-of-fluid method was employed to handle free surfaces. The enthalpy method was used for latent heat. Laser absorptions (Inverse Bremsstrahlung and Fresnel absorption) and thermal radiation by the plasma in the keyhole were all considered in the model. The results show that the recoil pressure is the main driving force for keyhole formation. Combining with the Marangoni shear force, hydrodynamic force, and hydrostatic force, it causes very complicated melt flow in the weld pool. Laser-induced plasma plays twofold roles in the process: (1) to facilitate the keyhole formation at the initial stage and (2) to block the laser energy and prevent the keyhole from deepening when the keyhole reaches a certain depth. The calculated temperature distributions, penetration depth, weld bead size, and geometry agreed well with the corresponding experimental data. The good agreement demonstrates that the model lays a solid foundation for the future study of porosity prevention in keyhole laser welding. DOI: 10.1115/1.2194043

High sensitivity of taper-based Mach–Zehnder interferometer embedded in a thinned optical fiber for refractive index sensing

A taper-based Mach-Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.

Shrinkage-induced fluid flow and domain change in two-dimensional alloy solidification

Abstract The fluid flow and domain change caused by shrinkage are analyzed for the solidification of alloys in a two-dimensional rectangular cavity with riser. The system of governing equations based on the modified continuum model is solved by the SIMPLEC algorithm, and the change of domain is handled by the front tracking method. The differences between the traditional study, i.e. without considering shrinkage effect, and the present study are presented. It is found that the shrinkage-induced fluid flow and domain change enhance convective heat transfer, and the global solidification time is smaller than that predicted without the shrinkage effect.

One-step fabrication of nanostructures by femtosecond laser for surface-enhanced Raman scattering

This paper reports an efficient fabrication of nanostructures on silicon substrates for surface-enhanced Raman scattering (SERS). Silicon wafer substrates in the aqueous solution of silver nitrate were machined by the femtosecond laser direct writing to achieve simultaneously in one-step the generation of grating-like nanostructures on the surface of the substrate and the formation of silver nanoparticles on the surface of the nanostructures via the laser-induced photoreduction effect. Parametric studies were conducted for the different concentrations of aqueous silver nitrate solutions and scanning speeds. The enhancement factor of the SERS is found to be higher than 10(9). The patterning technique provides an opportunity to incorporate the SERS capability in a functional microchip.

Porosity Formation and Prevention in Pulsed Laser Welding

Porosity has been frequently observed in solidified, deep penetration pulsed laser welds. Porosity is detrimental to weld quality. Our previous study shows that porosity formation in laser welding is associated with the weld pool dynamics, keyhole collapse, and solidification processes. The objective of this paper is to use mathematical models to systematically investigate the transport phenomena leading to the formation of porosity and to find possible solutions to reduce or eliminate porosity formation in laser welding. The results indicate that the formation of porosity in pulsed laser welding is caused by two competing factors: one is the solidification rate of the molten metal and the other is the backfilling speed of the molten metal during the keyhole collapse process. Porosity will be formed in the final weld if the solidification rate of the molten metal exceeds the backfilling speed of liquid metal during the keyhole collapse and solidification processes. Porosity formation was found to be strongly related with the depth-to-width aspect ratio of the keyhole. The larger the ratio, the easier porosity will be formed, and the larger the size of the voids. Based on these studies, controlling the laser pulse profile is proposed to prevent/eliminate porosity formation in laser welding. Its effectiveness and limitations are demonstrated in the current studies. The model predictions are qualitatively consistent with reported experimental results.

Surface-enhanced Raman-scattering fiber probe fabricated by femtosecond laser.

We report what we believe to be a new method to fabricate surface enhanced Raman scattering (SERS) fiber probe by direct femtosecond laser micromachining. Direct femtosecond laser ablations resulted in nanostructures on the cleaved endface of a multimode optical fiber with a 105/125 microm core/cladding diameter. The laser-ablated fiber endface was SERS activated by silver chemical plating. High-quality SERS signal was detected using Rhodamine 6G molecules (10(-8)-10(-6) M solutions) via back excitation with the fiber length of up to 1 m. The fiber SERS probe was compared with a planar fused silica substrate at a front excitation. The long lead-in fiber length and the backexcitation/collection setup make the SERS probe promising for remote sensing applications.