Design and characteristics of standard launching fiber for optical loss measurement

We have studied a standard launching filter, which can realize a field distribution close to the steady‐state modal field in the optical‐loss measurement of graded fibers and can be used as a reference fiber in the splicing‐loss measurement by a back‐scatter method. By using a small‐core fiber with the core radius 30 ‐ 40 μm, which is smaller than that of the fiber to be measured (50 μm), we realized a field distribution close to the steady‐state modal field. It was found that the variation of the excited modal distribution due to a difference in the radiation pattern of the optical source can be reduced by inserting a graded fiber and a step‐index fiber between the source and the small‐core fiber. When a fiber with a standard backscattering coefficient is placed between a small‐core fiber and the fiber to be tested, it becomes possible to apply the splicing‐loss measurement based on the backscattering technique. The optical loss measured by a standard fiber designed for the 1.3‐μm wavelength agreed with the value at the steady‐state modal distribution within ±0.05 dB/km. It was found that the optical loss at 0.85 μm can be measured within ±0.1 dB/km by using the same standard fiber. From these studies, it now becomes possible to measure the optical loss and the splicing loss with high precision.