Abstract: An optical fiber light coupling interface effective for feeding and propagating light power dispersed from a light source, and a method of forming the interface. The interface includes an optical fiber having an optical axis, a core, and a clad surrounding the core, the core and the clad having respective radii. The optical fiber further includes an enlarged input end face where the respective radii of the core and the clad assume respective first values, the respective radii of the core and the clad decreasing in a direction away from the input end face along the optical fiber to respective second values at a first transitional region of the optical fiber, the respective second values being less than the respective first values. The input end face is formed by applying a compressive stress to the optical fiber for enlarging the respective radii of the core and the clad at a part of the optical fiber while fusing that part by heating.