High precision laser micromachining for sensing applications
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In this PhD thesis the development of laser-based processes for sensing applications is investigated. The manufacture of optical fibre sensors is of particular interest because fibre optics offers advantages in space constraint environments or in environments where electronic sensors fail. Laser micromilling of the transparent and mechanically challenging to machine materials sapphire and fused silica is investigated. An industrial picosecond laser providing 6 ps pulses with the ability to emit at 1030 nm (IR), 515 nm (green) and 343 nm (UV) is used for processing of these materials; providing a maximum laser pulse energy of 25 μJ at UV, 75 μJ at green and 125 μJ in IR. The UV wavelength is identified as the most reliable machining wavelength for these materials with the least amount of cracking and achieving a surface roughness Rq of just 300 nm compared to 1220 nm (green) and 1500 nm (IR) in fused silica. In sapphire the surface roughness is 420 nm using UV , with green it is 500 nm and using IR it is 800 nm. The material removal rates using this laser milling process are larger than with other micromachining techniques, hence it was applied to manufacture cantilever sensors on the end of an optical fibre. The monolithic fibre top sensor is carved out of conventional telecommunications optical fibre. The cantilever is a structure of less than 10 μm thickness, 20 μm width and 125 μm length. Using the Fabry-Perot interferometer method the sensor detects small movements with a resolution better than 15 nm. A technique is developed to correct for laser machining angles and hence generate parallel interferometer faces. An electric arc cleaning process of the laser manufactured cantilever sensors is investigated that reduces the surface roughness to 30 nm. The manufacturing process reduces manufacturing times by a factor of 100. A working sensor is demonstrated in a deflection experiment. Such short pulses are not always required to manufacture the highest resolution sensors. The manufacture of high precision optical encoder scales (pitch 8 μm, depth 200 nm) with two processes (i) ablative removal of a polyimide layer and (ii) a melt reflow process on nickel coated scales is demonstrated. Both processes are using 33 ns laser pulses at 355 nm generating a pulse energy of up to 1 mJ.