In optical communication system different types of noises may be introduced into the signal. The effect of noise and corresponding degradation in the signal quality is of great interest for the design of fiber-optic communication systems. In the present paper an electrical equivalent form of different important noises has been developed in order to investigate its effects on the efficiency of an optical fiber link. The basic components of an optical link are laser diodes, fiber, splices, and optical detector. In the proposed model the laser diode is represented as voltage source with internal resistance. Thermal noise in fiber, connectors, and splices is estimated by using an equivalent resistor. PIN photodetector is choosen for the proposed optical link and equivalent circuit model for the same is derived by carrier rate equations using RC circuit with current sources. The variation of noise current with the variation of frequency is shown.