High-Capacity Multi-Core Fibers for Space-Division Multiplexing

The transmission capacity of the present optical fiber communication systems based on time division multiplexing (TDM) and wavelength-division multiplexing (WDM) using single-mode fibers (SMFs) is reaching its limit of around 100 Tbit/s per fiber due to the fiber nonlinearities, fiber fuse phenomenon and the optical amplifier bandwidth. To meet the ever increasing global data traffic growth and to overcome the looming capacity crunch, a new multiplexing technology using new optical fibers is urgently needed. Space-division multiplexing (SDM) is a promising scheme to overcome the capacity limit of the present SMF-based systems. Among the proposed SDM schemes, the one based on uncoupled multi-core fibers (MCFs) having multiple cores in a mutual cladding has proven effective in substantially increasing the transmission capacity per fiber with least system complexity as demonstrated in several state-of-the-art high-capacity transmission experiments beyond Pbit/s. In order to increase the transmission capacity of MCFs, the total number of cores needs to be increased while keeping the inter-core crosstalk (XT) among neighboring cores low as it degrades the optical signal-to-noise ratio (OSNR) of data signals, limiting the usable modulation formats (i.e., spectral efficiency, hence transmission capacity) and the transmission distance. One of the most powerful and practical XT reduction techniques in an MCF is a trench-assisted (TA) structure, where each core is surrounded by a trench and such MCFs are called trench-assisted MCFs (TAMCFs). The traditional approach for TA-MCFs design has relied on numerical simulations, which make deriving relationships between XT and fiber structural parameters difficult and non-intuitive. As it is important to be able to understand the effects of various fiber structural parameters on XT performance in designing high-count, low-XT TA-MCFs, an analytical model for XT estimation and XT properties analysis in TA-MCFs has been greatly needed. In this thesis, a novel analytical model for designing low-XT and high-count homogeneous TAMCFs is described where all the cores have the same refractive index profiles. Based on the model, the XT values in TA-MCFs as well as XT properties, including wavelength-dependent XT, XT reduction amount versus trench width, trench depth and XT dependence on core pitch are easily analyzed. It has also been shown that the XT in MCFs depends not only on the fiber structural parameters, but also on the core layout in the fiber cross section. Based on the model, a core layout structure with much lower XT has been found by core positions movement starting from a non closely-packed structure, i.e., one-ring structure (ORS). In addition to the analytical model for XT in TA-MCFs, backward propagated XT in MCFs for a bidirectional transmission scheme in different cores named “propagation-direction interleaving (PDI)” has been formulated where a new core layout structure that can utilize the XT reduction benefits of PDI is investigated, which turns out to be a square-lattice structure (SLS). Based on the analytical model for XT in TA-MCFs and the XT formulation in PDI, homogeneous squarelattice structured MCFs with a number of cores of 24 and 32 are designed under unidirectional and PDI transmission schemes. As the worst XT in the homogeneous 32-core MCF is higher than the required value of − 20 dB over 1000 km for QPSK modulation formats (− 30 dB over 100 km for 32QAM), a 32-core fiber with a heterogeneous core arrangement adopting PDI transmission scheme is designed. It is concluded that without adopting PDI, a heterogeneous core arrangement
with more than 2 types of non-identical cores which have different refractive index profiles are needed, for instance, 3 or even 4 types of cores. Finally, a novel XT measurement method in MCF fan-in/fan-out (FI/FO) devices based on Fresnel reflection at the MCF-to-air interface has been studied because the XT of FI/FO devices may limit the performance of MCF transmission systems and hence it is of importance to characterize their XT properties. The new method has great advantages that the MCF-SMF coupling is not needed and the measurements results are immune to the cleaving angle of MCFs. Measurements results based on the conventional MCF-SMF coupling method and the Fresnel reflection method are compared, and it is found that they show very similar statistical properties. The new measurement method provides a robust and simple platform for characterizing XT in MCF FI/FO devices.