Optical time-domain reflectometry (OTDR) of diameter modulations in single mode fibers

The influence of several types of diameter variations on the attenuation and on the optical time-domain reflectometry (OTDR) traces of specially prepared depressed cladding SM (single-mode) fibers is experimentally investigated. Depressed-cladding SM fibers have been prepared by the low-pressure plasma-induced chemical vapor deposition (PCVD) process. The core consists of GeO₂-doped SiO₂ and the cladding region of fluorine-doped silica. During drawing, the diameter of the fiber is controlled within ±0.3 μm for slow perturbations by adjusting the drawing speed (Anritsu M501A). Variations of the core diameter have been accomplished by changing the set point of the fiber diameter control. Core diameter changes give rise to steps or deviations in slope of the OTDR power trace of about 0.3 dB/μm. No indication of additional attenuation due to axial fiber parameter variations has been found up to core diameter changes of 12 μm/m, despite severe distortions of the OTDR traces