FROM MAXWELL’S EQUATIONS TO SUBSEA FIBRE OPTICS: OPTICAL FIBRES AS WAVEGUIDES [PART 2] – FROM TIR AND MODE FIELD DIAMETER TO DISPERSION, THE STORY OF THE WAVEGUIDE CONTINUES By Anna Bridget Sheehan & Derek Cassidy
ABSTRACT
This paper traces the evolution of optical waveguides, from the foundational laws established by Newton, Huygens, and Maxwell to today’s cutting-edge submarine cable technologies. It examines the progression of fibre optic science through key milestones, including Dense Wavelength Division Multiplexing (DWDM) and specialty fibres, while exploring emerging frontiers such as spatial division multiplexing, hollow-core, and multicore fibres. It is presented in three parts.
GUIDED BY TOTAL INTERNAL REFLECTION (MFD: MODE FIELD DIAMETER) The preceding relations describe the propagation of electromagnetic waves across different waveguide structures. Despite their varied geometries, all share a common theoretical origin in Maxwell’s equations and their subsequent formulations. What changes from system to system 88
is not the governing physics, but the manner in which the field is confined and controlled. Attention now turns to the specific mechanisms by which light propagates within optical fibre. Concepts such as mode field diameter (MFD), total internal reflection (TIR), and macrobending losses are not peripheral details; they determine how efficiently light remains guided within the fibre core over long distances. The principle underlying optical confinement can be traced to John Tyndall’s nineteenth-century demonstrations. In his wellknown experiment, a beam of light introduced into a stream of water remained visibly trapped within the flowing arc, illustrating that light could be guided within a transparent medium [40]. The effect observed is now formally described as total internal reflection: when light strikes an interface at an angle exceeding the critical angle, it is reflected entirely back into the higher-index medium rather than refracted outward. In optical fibre, this same principle ensures that light remains confined within the silica core, provided the conditions
SUBBTEL FORUM | Issue 148
Figure 6: John Tyndall’s experiment to explain total internal reflection of light in a transparent medium [41]
of incidence are satisfied. What appeared in Tyndall’s lecture theatre as a striking visual phenomenon has become the operational foundation of global fibre-optic communication. To ensure that light remains confined within the fibre core, it must enter at an angle that satisfies the condition for total internal reflection. This angular limit defines the