This section describes cautions in uses of PCF. Whe the edge face of PCF is open, air with high moisture enters the air holes, which may yield increases in the transmission loss, enhancements of temperature dependency of the transmission loss, and contamination at the edge face. In addition to these problems, the edge face becomes easy to be damaged. In case the femtosecond pulsed laser is used, the edge face of core with a small diameter is easily damaged, since the laser peak intensity is high.
To avoid this kind of damage on the edge face, edge face sealing is used for PCF. By closing the air holes with heat treatment of the fiber edge face, air with high moisture can not get into the air holes. Additionally, by increasing the spot size at the edge face (MFD) and NA, the damage threshold for the edge face is increased. As results, connector splicing, polishing, and coating become possible.
Fusion splice of PCF
Low-loss fusion splice of PCF with a normal optical fiber is necessary in some cases; light emitting from PCF is detected or measured; PCF is connected with another light source; all-fiber-type light source is constructed. However, the low-loss fusion splice is difficult because PCF has a lot of air holes. When the PCF fusion splice is performed, the air holes are destroyed owing to the arc discharge by the fusion splicer. Therefore, the light is not confined in the core, then light propagating in PCF leaks at the splicing site, resulting in the large loss. Additionally, Fresnel reflection remarkably occurs due to the difference in refractive indices of the clad sites.
To avoid the fusion splicing loss, an optical fiber with the same MFD as PCF is connected. When an SMF with 6 µm MFD is connected with PCF having 20 µm MFD, the light leaks out of the fiber, resulting in the large loss. Pigtail fibers and bridge fibers are utilized for mode-matching.