Optical fiber

An optical fiber, comprising:

• a core with radius r1,
• a first clad layer with outermost radius r2, adjacent to the core at radial position r1, and covering the outer periphery of the core,
• a second clad layer with outermost radius r3, adjacent to the first clad layer at radial position r2, and covering the outer periphery of the first clad layer, and
• a third clad layer adjacent to the second clad layer at radial position r3, and covering the outer periphery of the second clad layer, wherein:
• the refractive index of the first clad layer decreases continuously from the inside to the outside, the refractive index having a maximum value at the radial position r1 and a minimum value at the radial position r2;
• the refractive index of the second clad layer increases continuously from the inside to the outside, the refractive index having a minimum value at the radial position r2 and a maximum value at the radial position r3;
• the relationship of |dΔ(r)/dr|≤0.3%/μm is established in the range of r from 0 to r1;
• the relationship of |dΔ(r)/dr|≤0.05%/μm is established in the range of r from r1 to r2;
• and
• the relationship of |dΔ(r)/dr|≤0.1%/μm is established in the range of r from r2 to r3, where Δ(r) is a relative refractive index difference at radius position r.