Direct core observation method using thermal radiation of silica fibers with dopants

Various direct core observation methods have been developed for directly observing the core itself of a single-mode optical fiber such that the axial alignment is performed and the splicing loss can be evaluated. In the conventional methods, special devices are needed such as an He-Cd laser and a collimated illuminating optical source. In this paper, the optical fiber core can be observed directly by thermal radiation when Ge-doped optical fibers are spliced by glow discharge heating. Also, this phenomenon can be used for splicing. When a Ge-doped fiber is placed near the glow discharge heating region, the core shines in contrast to the clad. This is because the absorbing loss of the core is larger than that of the clad and hence a stronger thermal radiation occurs from the core. Further, near the wavelength of 420 nm, Ge luminescence exists due to the ultraviolet (UV) light from the glow discharge. The rate of the luminescence of the total radiation from the core is at most one-twentieth the thermal radiation in a steady-state splicing situation. Further, the apparent core location is expanded by the amount of refractive index due to the lens effect of the clad. Hence, the true location of the core must be calibrated.