Applications of femtosecond laser micro-machining for fabrication of novel optical sensing devices

Abstract

Recently, femtosecond micro-machining has been established as a powerful fabrication tool for optical devices. Relying on the fundamental mechanisms of multiphoton absorption and avalanche ionisation, transparent bulk materials can be restructured. Subsequently, femtosecond micro-machining has emerged as an alternative to UV-based inscription techniques. The ability to introduce stark refractive index modifications, without the need for a photosensitive material, offers a solution to issues encountered by UV-based methodologies. As the process is intensity dependent, wavelengths that are transparent to polymers can be used, meaning the removal of fibre coating is not necessary prior to inscription. Hence, femtosecond laser writing offers a powerful solution for the fabrication of optical fibre sensing devices. Exploiting these aspects, a fibre Bragg grating was fabricated in a high concentration (>40 mol%) gemrania fibre using the point-by-point technique. Thermal tests revealed the grating was stable up to 800 ◦C for 30 minutes. The point-by-point technique offers the advantages of precision and control compared to interferometric based techniques. The precision enabled the inscription of four 3rd order gratings into separate cores of a 7 core multicore optical fibre. Fibre Bragg gratings are fundamental sensing elements. In particular, recent publications have focussed upon fabricating continuous gratings and grating arrays to improve the signal to noise ratio in distributed acoustic sensing systems (DAS). Traditionally, DAS systems have utilised the Rayleigh backscatter of single mode fibres but their low signal-to-noise ratio (SNR) limits their capabilities. Fabricating low loss, consistent gratings are an engineering challenge. Femtosecond micro-machining offers a solution to the SNR issue faced by DAS systems. By utilising the smooth refractive index regime, reflectors could be inscribed into the fibre causing a Fresnel reflection of the forward propagating mode. Using this mechanism meant the reflectors were wavelength independent. These intricate lowloss structures were inscribed into a single mode fibre, improving the SNR by up to 23 dB with respect to the Rayleigh backscatter. The fibre was measured to have an attenuation of 1.5×10−5 dB/reflector. The fibre was functionally characterised, and compared with standard sensing fibre in two vibration measurements: a phase-based optical time domain reflectometry (φ-OTDR) measurement; and an optical frequency domain reflectometry measurement. When the fibre was used in conjunction with the dual-pulse φ-OTDR methodology, an immunity to signal fading was demonstrated. This is the first known report of an ultra low-loss, wavelength independent, temperature immune, femtosecond inscribed, enhanced reflectivity fibre.

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Identifiers

ORCID for Andrei Laszlo Donko: orcid.org/0000-0003-1884-743X

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

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