There has been a rapid advance in wavelength-division multiplexing (WDM) and high bit-rate time-division multiplexing (TDM) as techniques for coping with burgeoning demand for transmission capacity. In the past this expansion of capacity has been achieved by 2.5-Gbit/s and 10-Gbit/s WDM using the C-band (around 1550 nm), but research on the 1600-nm L-band (around 1600 nm) is being stepped up to obtain further expansion. With the achievement of 40-Gbit/s speeds, which mark the limit of electrical signal processing, optical TDM, with speeds of 100 Gbit/s, is coming into use. In this kind of high-density, high bit-rate WDM transmission, the occurrence of non-linear phenomena within optical fibers reduces transmission quality, and this raises the importance of technology for suppressing non-linearity and specifically, in the case of WDM transmission systems, of four-wave mixing (FWM). Obviously there is also the problem of signal distortion due to dispersion, so that technology for suppressing cumulative dispersion is also essential. There is also a need for transmission lines with sophisticated dispersion management over a wide band of wavelengths, and it may be consisted of novel fibers.