AbstractThe development of optical fiber cable transfer splicing systems is being advanced to increase the speed of connection of in‐use circuits to new ones, an operation that has been necessary in cable joints, branches, etc. In the construction of these systems, the conservation of the action‐objectives of the in‐use fiber between two different points must be confirmed and, therefore, the fiber identification techniques play the most important role.In this paper, considering the identification of in‐use fibers, the conditions of bent fiber light injection and detection and their coupling systems are discussed. As to the bent fiber light injection, the relations among the ray angle of incidence which results in the injected optical signal to propagate as a core mode, the bending radius, the thickness deviations of the UV coat and the refractive index of the matching material are investigated.The bent‐injection coupling efficiency in relation to the axial displacement between the identifying fiber and the injected one also is investigated. It is found that large coupling efficiencies can be achieved if the incident light beam spot‐size is reduced and the light‐source wavelength is increased. As to the condition of the bent fiber light detection system, the detection efficiency in regard to the source wavelength when the detected optical signal inside the core corresponds to the injected light is discussed. It is shown that large detection efficiency can be achieved if the optical source‐wavelength is increased and the bending radius is kept below 6 mm.