Generally, for single-mode connectors, the recommended insertion loss is below 0. Insertion loss, also known as attenuation, is the loss of optical power that occurs when light passes through a fiber optic connector. It is caused by factors such as misalignment, air gaps, and imperfections in the connector components. This article explores various connector types—such as SC, LC, FC, ST, APC, and UPC—and analyzes how their design and polishing affect IL and RL performance. Insertion Loss (IL): Measures the. Fiber loss, also called fiber optic attenuation or attenuation loss, refers to the loss of signal between input and output.