Fiber mismatches are a source of intrinsic coupling loss. As stated before, intrinsic coupling loss results from differences (mismatches) in the inherent fiber characteristics of the two connecting fibers. Fiber mismatches occur when manufacturers fail to maintain optical or structural (geometrical) tolerances during fiber fabrication.
Fiber mismatches are the result of inherent fiber characteristics and are independent of the fiber jointing techniques. Types of fiber mismatches include fiber geometry mismatches, NA mismatch, and refractive index profile difference. Fiber geometry mismatches include core diameter, cladding diameter, core ellipticity, and core-cladding concentricity differences.
The picture below illustrates each type of optical and geometrical fiber mismatch. Military fiber specifications tightly specify these parameters to minimize coupling losses from fiber mismatches.
Types of optical and geometrical fiber mismatches that cause intrinsic coupling loss.
Core diameter and NA mismatch have a greater effect on intrinsic coupling loss than the other types of fiber mismatches. In multimode fiber connections, the coupling loss resulting from core diameter mismatch, NA mismatch, and refractive index profile difference depends on the characteristics of the launching fiber.
Coupling loss from core diameter mismatch results only if the launching fiber has a larger core radius (a) than the receiving fiber. Coupling loss from NA mismatch results only if the launching fiber has a higher NA than the receiving fiber. Coupling loss from refractive index profile difference results only if the launching fiber has a larger profile parameter (a) than the receiving fiber.