Advanced fibre Bragg grating and microfibre Bragg grating fabrication techniques

Abstract

Fibre Bragg gratings (FBGs) have become a very important technology for communication systems and fibre optic sensing. Typically, FBGs are less than 10-mm long and are fabricated using fused silica uniform phase masks which become more expensive for longer length or non-uniform pitch. Generally, interference UV laser beams are employed to make long or complex FBGs, and this technique introduces critical precision and control issues. The fabrications of advanced gratings with complex designs and in new type of optical fibres are challenging research topics. The availability of long complex gratings paves a way for meeting the needs of demanding applications such as gratings employed for dispersion-compensation of high bit-rate transmission. In this work, we demonstrate an advanced fibre Bragg grating fabrication system that enables the writing of long complex gratings in optical fibres with virtually any apodisation profile, local phase and Bragg wavelength using a novel optical design in which the incident angles of two UV beams onto an optical fibre can be adjusted simultaneously by moving just one optical components, instead of two optics employed in earlier configurations, to vary the grating pitch. The operation principle is based on the translate-and-write method which used a high-precision translation stage to move the interference beams along the length of an optical fibre to realize long gratings in a well-controlled environment. The key advantage of the grating fabrication system is that different kinds of complex gratings can be fabricated by controlling the linear movements of two translation stages. A 90-mm long uniform grating and 50-mm long chirped FBG with chirp rate of 0.15-nm/mm using the non-chirped phase mask are demonstrated. In addition to the study of advanced grating fabrication technique, we also focus on the inscription of fibre Bragg gratings written in optical fibres with a cladding diameter of several ten{174}s of microns. Fabrication of microfibres was investigated using sophisticated tapering method. We also proposed a simple but practical technique to filter out the higher order modes reflected from the FBG written in microfibres via a linear taper region while the fundamental mode re-couples to the core. By using this technique, reflection from the microfibre Bragg grating (MFBG) can be effectively single mode, simplifying the demultiplexing and demodulation processes. We demonstrate that effectively a single reflection peak is obtained from an FBG written in multi-mode microfiber. The characteristics of MFBG are investigated. MFBG exhibits high sensitivity to contact force and an MFBG-based force sensor was also constructed and tested to investigate their suitability for use as an invasive surgery device. Performance of the MFBG based contact force sensor packaged in a conforming elastomer material compares favourably to one of the best-performing commercial contact force sensors in catheterization applications. The proposed sensor features extremely high sensitivity up to 1.37-mN, miniature size (2.4-mm) that meets standard specification, excellent linearity, low hysteresis, and magnetic resonance imaging compatibility.