Optical Fiber and Optical Fiber Ribbon

An optical fiber (100, 101, 103, 105, 107) comprising:

• one or more core regions (102) such that the one or more core regions (102) is an up doped region (208, 308);
• and
• a cladding region (104) that surrounds the one or more core regions (102);
• where the cladding region (104) has exactly one down doped region (210, 310) and an undoped region (212, 312), where the down doped region (210, 310) is a continuous region adjacent to the one or more core region (102) such that a radial position of minimum relative refractive index (214, 314) of the optical fiber (100, 101, 103, 105, 107) is within three micrometres (μm) from an interface between the down doped region (210, 310) and the undoped region (212, 312), where the optical fiber (100, 101, 103, 105, 107) has a mode field diameter in a range of 8.8 μm to 9.6 μm at a wavelength 1310 nanometres (nm), where the optical fiber (100, 101, 103, 105, 107) has a cable cut-off of less than or equal to 1260 nm.