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.

We propose multi-core erbium-doped fiber amplifiers for next-generation optical amplifiers utilized by space-division multiplexing technologies. Multi-core erbium-doped fiber amplifiers were studied widely as a means for overcoming exponential growth of internet traffic in the backbone network. We consider two approaches to excitation of erbium irons; One is core-pumping scheme, the other is cladding-pumping scheme. For a core-pumping configuration, we evaluate its applicability to future ultra long-haul network. In addition, we demonstrate that cladding-pumping configuration will enable reduction of power consumption, size, and cost because one multimode pumping laser diode can excite several cores simultaneously embedded in a common cladding and amplify several signals passed through the multi-core erbium-doped fiber cores.

Prediction of cross-talk is an important facet of multicore fiber (MCF) design. Several approaches for estimating cross-talk in MCF have been proposed but none are faultless, especially when applied to MCF with heterogeneous cores. We propose a new calculation approach that attempts to solve this problem. In our approach, cross-talk in multicore fibers is estimated by coupled power theory. The coefficients in the coupled power equation are theoretically calculated by the central limit theorem and by quantum mechanical time-dependent perturbations. The results from our calculations agree with those of Monte Carlo simulations of heterogeneous MCFs.

This paper describes recent developments of photonic crystal fibers (PCFs), which can realize ultra wide-band transmission or large Aeff, as well as photonic crystal multi-core fibers (PC-MCFs), which have large potentials as future high-capacity transmission lines using Space Division Multiplexing.