Spatial mode-division multiplexing and advanced distributed fibre sensing techniques

Abstract

Part A: Spatial Mode-Division Multiplexing: As the capacity limits of single-mode optical fibre are being approached, attention has shifted to few-mode fibre. Increasing capacity then amounts to independently exciting and detecting the various spatial modes, in what is known as spatial mode-division multiplexing. In this approach each mode acts as a different data channel. The independent excitation and detection of the individual spatial modes however remains a major technological challenge, and at present experimental demonstrations have relied on lossy bulk free-space optics. In this thesis, simpler and more compact waveguide-based alternatives are proposed and analysed. Firstly, it is shown that asymmetric Y-junctions can be used to adiabatically multiplex/demultiplex the modes of polarization-maintaining or elliptical-core few-mode fibre. Analytical models describing the mode-selective functionality of these devices are developed and it is shown numerically that the performance is largely independent of wavelength, hence permitting wideband wavelength-division multiplexing. Another class of compact waveguide-based devices suitable for mode-division multiplexing are mode-selective couplers. Coupled mode theory is used to develop an analytical model of these devices, and it is shown that specific three-core variants permit demultiplexing of asymmetric higher-order modes irrespective of modal orientation. The wavelength-dependence of these couplers is however shown to limit their use in wavelength-division multiplexed systems. In order to solve the wavelength-dependence issue of these couplers, adiabatic tapers can be introduced into the cores. Such structures are referred to as tapered velocity mode-selective couplers, and unlike standard directional mode-couplers, these novel devices do not require precise phase conditions to be satisfied over an extended length. For this reason they permit ultra-wideband mode-division multiplexing of few-mode fibre. It is again shown that three-core variants of these tapered couplers can permit mode-orientation independent decoupling. These devices are numerically analyzed, and their successful fabrication using the femtosecond-laser direct write technique is also reported. These developments could play a very significant role in future high-capacity telecommunications.

Part B: Advanced Distributed Fibre Sensing Techniques: Distributed optical fibre sensing techniques are invariably plagued by trade-offs between the performance metrics of range, spatial resolution and measurement bandwidth. This is especially true for the most well-known frequency domain and time domain fibre sensing techniques of optical frequency domain reflectometry (OFDR) and optical time domain reflectometry (OTDR), respectively. What is needed to lessen the performance trade-offs, is a hybrid technique using both time and frequency domain signal analysis. The merging of the realms of time and frequency domain fibre sensing is demonstrated in this thesis using a range-gated variant of OFDR. The range-gating is achieved by time stamping the optical signal using high-frequency pseudorandom noise phase modulation, also known as digitally enhanced interferometry. The merging of digital interferometry and OFDR is referred to as digitally enhanced OFDR. This technique permits orders of magnitude reduction in the required sampling rates of OFDR, and overcomes the range ambiguity inherent in OFDR when sensing over multiple frequency sweeps. The latter feature means the technique can be used for ultra-high (e.g. acoustic) bandwidth sensing without sacrificing range or spatial resolution.