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The explosive growth in Internet traffic will lead to a demand for greater capacity. Wavelength-division-multiplexing (WDM) transmission using a broad wavelength region is considered a promising candidate transmission technology with which to meet this demand. Photonic crystal fibers (PCF) are attractive in terms of realizing wideband WDM transmission because they have unique features that are unavailable with conventional single-mode fibers, namely they can be endlessly single-mode and are capable of dispersion tailoring. We fabricated long, low loss PCFs. The lowest loss yet achieved is 0.18 dB/km, which is comparable to that of a conventional single-mode fiber. We also succeeded in fabricating a 100 km-long, low loss PCF. We have shown the applicability of these low loss PCFs to broadband optical transmission. We achieved an ultra-wideband WDM transmission using visible and infrared wavelengths, which indicates the possibility of building communication systems with a bandwidth of over 263 THz. We obtained a penalty-free 10 Gb/s transmission over a 100-km-long PCF and also a 40 Gb/s dense WDM (DWDM) transmission over a 46-km-long PCF-DSF dispersion-managed transmission line by using the 1310 and 1550 nm wavelength regions. Recently, a supercontinuum source was used to achieve the first WDM transmission at 1000 nm, namely in a new optical communication band. This revealed the possibility of transmitting at more than 1 Tb/s in the 1000 nm band. We also achieved a 160 Gb/s optical time-division-multiplexing (OTDM) transmission over a 26 km PCF. We discuss the potential capacity of PCFs with a view to realizing Pb/s transmission in the future.