Structured optical fibre gratings for communication and sensing applications

Abstract

Optical fibre gratings have become very important enabling technology for the fibre-optic sensor and communication systems. Due to its wavelength selective property, fibre gratings, including fibre Bragg gratings (FBGs) and long-period gratings (LPGs), have become the key spectral or dispersion management devices in fibre communication systems. Furthermore, grating-based sensors have also turned into one of the most important class of optical fibre sensor, due to its inherent self-referencing and multiplexing capability in addition to the well-known merits of fibre-optic sensors. In this work, we define structured fibre gratings as those fibre grating elements that cornprise of two or more FBGs, or LPGs, or a combination of FBGs and LPGs. The main objective of this project is to analyze the mode-coupling mechanisms of structured fibre gratings, and develop new in-fibre grating devices for communication and sensing applications. We concern mainly the optical interactions of inter-grating structures that are quite different from the in-grating structures formed with refining techniques, such as grating chirping or apodization. Various structured fibre gratings, which are classified as structured long-period gratings, combined long-period and fibre Bragg gratings, structured fibre Bragg gratings, and sampled fibre Bragg gratings, were investigated theoretically and experimentally. LPGs were initially developed for band-rejection filters, and are commonly used for gain-flattening of erbium-doped fibre amplifier (EDFA). We proposed a novel step-changed long-period grating as an effective gain-flattening device, in which the multi-path interference is suppressed but the spectral tailoring capability has been improved. Such grating devices have been fabricated, and demonstrated their flexible spectral tailoring capability. In addition, the optimization of the grating parameters of step-changed LPG for gain flattening were also discussed, and the gain-flattening of an EDFA, constructed with a 20 m long Er-doped fibre pumped by a 980 nm LD, from 8.9 dB to better +-O0.49 dB over 30 nm has been achieved. By combining the mode coupling capability of LPG and FBG, a concatenated long-period and fibre Bragg grating has also been designed and fabricated. It was first demonstrated in this work that the light coupled out to the cladding mode by one of these gratings could be recoupled back to the guided mode by using a second grating. Theoretical analysis based on the coupled mode theory shows that such mode recoupling can be activated efficiently if the LPG has a 3 dB transmission in the spectrum range overlapped with that of the FBG. Different from conventional hybrid LPG and FBGs, the reflection spectrum has one extra narrow-band reflection peak resulted by mode recouplings, in additional to the conventional Bragg reflection. A major advantage of such sensing devices is that a new narrow-band reflective optical signal is introduced and could be used to monitor multiple envirorimental parameters. In order to avoid the broadband loss introduced by LPGs and overcome the limitation of the 50% reflectivity, a novel Bragg grating pair, in which one Bragg grating is written in the fibre core whereas the other is written in the fibre cladding, was proposed. The novel device was analyzed by using coupled mode equations. After discussions of the reflection characteristics of Bragg grating written in cladding and the selection of grating period for recoupling, the spectra of such structured FBGs were investigated numerically. With such special design, the structured fibre Bragg gratings will not activate the Fabry-Perot interference, but excite mode recoupling, which spectrum is expected to exploit the high sensitivity of fibre cladding modes in sensing applications. Finally, we investigated the spectral and dispersion characteristics of sampled fibre Bragg gratings. The couplings of multiple modes in conventional sampled FBGs, and the dispersion-slope managing capability of the chirped-sampled FBGs have also been investigated numerically by using the discretized fundamental matrix method. The mechanisms of conventional sampled FBGs for multi-parameters sensing and chirped-sampled FBGs for dispersion-slope compensating were also analyzed, and explained in detail.