The methods disclosed herein include forming an expanded core in an optical fiber with a glass core having a core dopant and a core outer surface, and a glass cladding immediately surrounding the core and having a flat glass-portion surface closest to the core outer surface at a first core spacing S1. The methods include applying heat to a section of the optical fiber to cause the glass core to expand toward the flat glass-portion surface due to thermal diffusion of the core dopant. The methods also include terminating the application of heat to define the expanded core in the heated section of the optical fiber. The expanded core defines an evanescent coupling region having a second core spacing 0≤S2