Methods of forming optical fibers having an expanded core for evanescent optical coupling

A method of establishing an evanescent optical coupling interface between an optical waveguide of a photonic integrated circuit (PIC) and an optical fiber with a glass core and having a core dopant and a core outer surface, and a glass cladding immediately surrounding the glass core and having a flat glass-portion surface closest to the core outer surface at a first core spacing S1, the method comprising:

• heating the glass core at an end section of the optical fiber to cause core dopants in the core to diffuse toward the flat glass-portion surface to form an expanded core comprising an evanescent coupling region having a substantially constant diameter and an adiabatic transition region of varying diameter between the core and the evanescent coupling region;
• terminating the heating to define at the evanescent coupling region a second core spacing S2, wherein 0≤S2≤(0.9)·S1 and
• after terminating the applying of heat, interfacing the flat glass-portion surface adjacent the evanescent coupling region with the optical waveguide to form the evanescent optical coupling interface between the optical fiber and the optical waveguide.

A method of establishing an evanescent optical coupling interface between an optical waveguide of a photonic integrated circuit (PIC) and an optical fiber with a glass core and having a core dopant and a core outer surface, and a glass cladding immediately surrounding the glass core and having a flat glass-portion surface closest to the core outer surface at a first core spacing S1, the method comprising:

• 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 to form an expanded core having an evanescent coupling region that defines a second core spacing 0<S2≤S1, wherein the expanded core comprises an adiabatic transition region between the core and the evanescent coupling region of the expanded core;
• interfacing the flat glass-portion surface adjacent the evanescent coupling region with the optical waveguide to form the evanescent optical coupling interface between the optical fiber and the optical waveguide and
• after terminating the applying of heat, interfacing the flat glass-portion surface adjacent the evanescent coupling region with the optical waveguide to form the evanescent optical coupling interface between the optical fiber and the optical waveguide.

The method according to 23, wherein the optical waveguide comprises a D-shaped optical fiber with an expanded core.