A biocompatible and biodegradable polymeric step-index optical fiber includes a core and a cladding around the core. The core is made from a core material fabricated by bonding a citric acid and at least a first monomer using a synthesis process. The cladding is made from a cladding material fabricated by bonding the citric acid and at least a second monomer using the synthesis process. The core has a refractive index higher than that of the cladding, while a difference between an initial modulus of the core and the cladding is preferably less than 30% and a difference between the biodegradation rates of the core and cladding is preferably less than 30% after about 4 weeks. Optical properties of the core and cladding are tunable by adjusting monomer ratios, choices of monomers or cross-linking degrees.