Novel FBG femtosecond laser inscription method for improved FPI sensors for medical applications

A novel fibre Bragg grating (FBG) post-inscription technique using a femto second laser (FSL), used to modify an optical fibre pressure sensor (OFPS) based on an extrinsic Fabry Perot Interferometer (EFPI) is presented. The resultant sensor is an optical fibre pressure and temperature sensor (OFPTS), able to measure temperature and pressure simultaneously in precisely the same location within the optical fibre. Hence the temperature measurement can be used to accurately compensate any thermal fluctuations in the pressure measurements, leading to an improved long term stability. The Bragg-wavelength can be tailored to coincide with any part of the Fabry-Perot Interferometer (FPI) spectrum (e.g. define the FBG at a valley of the FPI spectrum). We use a modified femtosecond laser, point-by-point inscription method for precise and controlled placement of the fibre Bragg grating. Our technique can be readily adapted to commercial production methods for optical fibre sensors as it greatly mitigates the alignment problems associated with femtosecond laser inscription of gratings in optical fibres. The sensor presented in this paper is entirely fabricated with quartz glass, which makes it fully bio-compatible and can be used for biomedical application. The sensors achieved a high sensitivity of 1.3 nm over kPa resulting in a resolution of ~ 1mmHg and a temperature sensitivity of ~ 10.7pm over K. After the inscription, the sensors still demonstrated a stability of better than 0.1% in 30min. The small diameter of only 200μm allows biomedical in-vivo application in volume restricted areas (e.g. blood vessels or the brain) for simultaneous temperature and pressure measurements.