Development of a sapphire optical wall shear stress sensor for high-temperature applications

This paper presents the development of the first sapphire micromachined wall shear stress sensor for high-temperature applications utilizing geometric moiré optical transduction. A folded tether floating element structure is employed to extend the linear operating range of the sensor. Picosecond pulsed laser micro-machining processes are developed for patterning of mechanical structures in sapphire, and a four-channel alumina fiber array with sapphire optical fibers is used to interrogate the moiré fringe. Platinum thin-film gratings and a stainless steel package enable a theoretical maximum operating temperature in excess of 800°C, and initial dynamic calibration in differential mode demonstrates a shear stress sensitivity of 76.8 μV/Pa at 1.128 kHz.