Macro bending: Macrobending is the attenuation associated with bending or wrapping the fiber. Light can “leak out” of a fiber when it is bent. As the bend becomes tighter, more light escapes. Macrobending loss, measured in decibels, increases at longer wavelengths where the optical confinement of the light is weaker. It also increases linearly with the number of turns. Traditionally, macrobending was not a limiting effect when cables were mostly of loose-tube or ribbon design and installed into ducts. The tightest bends incurred by fibers were in splice trays, where excess fiber would be stored in loops after jointing. This was reflected in the macrobending specification of ITU-T Recommendation G.652, where a minimum bend radius of 30 mm was defined to reflect typical splice tray dimensions and 100 turns were agreed upon to simulate the total excess fiber from all the splice sites between repeaters. But macrobending effects become more pronounced in networks installed close-to and within the building. Prevalent in this segment of the network are low-diameter mini-cables that are stripped-back designs, compared to the traditional sheathed loose-tube and ribbon cables. Lightweight and highly flexible, these new designs are preferred for their space efficiency (when installed into commensurately small micro-ducts) and ease of handling and routing (when installed on the inside and outside of buildings along tortuous paths). Bend radii of much less than 30 mm therefore have become commonplace.
Micro bending :Microbending attenuation of an optical fiber relates to the light signal loss associated with lateral stresses along the length of the fiber. The loss is due to the coupling from the fiber’s guided fundamental mode to lossy, higher-order radiation modes. Mode coupling occurs when fibers suffer small random bends along
the fiber axes. This random bending is usually caused by external mechanical stresses against the cable material that compress the fiber. The result is random, high-frequency perturbations to the fiber. Lateral stresses can be caused by pressure induced by manufacturing or installation or by temperature-induced dimensional changes in cabling materials that cause undesirable fiber/fiber or fiber/cable material interactions. These interactions can give rise to random microscopic bends or curvatures of <1-mm radius that create very small displacements of the fiber core from the fiber axis. Microbending effects can be seen at all the commonly used wavelengths in single mode fibers (1310, 1550, and 1625 nm), whereas macrobending effects are seen predominantly at 1550 and 1625 nm.
