Optical Fibers and Mode Multiplexers for Broadband Mode-Division Multiplexing


Student thesis: Doctoral Thesis

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Award date6 Oct 2017


Fiber-optic communication over a single strand of optical fiber is approaching the capacity limit imposed by fiber Kerr nonlinearity, which already simultaneously utilizes the multiplexing in the physical dimensions of time, wavelength (frequency), quadrature (amplitude and phase), and polarization to parallelize transmission channels. Space-division multiplexing (SDM) in a single-core few-mode fiber (FMF), a single-mode multi-core fiber (MCF), or the combination of them as a few-mode MCF is an emerging multiplexing technology to further increase the single-fiber transmission capacity beyond the limit by using the individual modes in an FMF or cores in an MCF as parallel transmission channels. SDM using the individual modes in an FMF, more often called mode-division multiplexing (MDM), is an especially promising technology, as it provides a high spatial channel density with the spatially overlapping modes in a single fiber core. To build an MDM system, a variety of optical fibers and devices for the transmission and the control of the modes is needed, which are preferably broadband or wavelength-tunable, to be compatible with the existing wavelength-division multiplexing (WDM) technology. This thesis presents studies on several optical fibers and waveguide devices aimed for broadband fiber-optic MDM, which include compact three-core fibers with zero or broadband ultra-low differential group delays (DGD), ultra-broadband mode multiplexers based on three-dimensional (3D) asymmetric polymer waveguide branches, and mode-selective butt-coupling between FMFs and buried channel waveguides.

The first study shows the possibility to achieve zero or broadband ultra-low DGDs in three-core fibers with closely packed cores. Using coupled-mode theory (CMT) and assuming that the coupling between adjacent cores dominates, we obtain an explicit approximate condition to achieve zero DGD in a homogeneous MCF, whose identical single-mode cores are evenly distributed in the cladding. The condition is verified numerically with three-core fibers and found to be applicable even for a three-core fiber with touching cores. A typical touching-core three-core fiber with a core-cladding index difference of Δ = 0.3% gives DGDs within ~240 ps/km over the C band (1530 nm – 1565 nm) around the zero-DGD wavelength. A three-core fiber design with a central refractive-index dip in each core is proposed to achieve broadband ultra-low DGDs among the first six vector guided modes. A specific fiber with Δ = 0.3% yields an ultra-low DGD within 3.2 ps/km over the C band.

The second study presents ultra-broadband mode multiplexers realized with multi-layer asymmetric polymer waveguide branches. The multiplexers can be considered as a stack of interacting 3D asymmetric Y junctions, and operate on the principle of adiabatic mode transition. A three-mode multiplexer is designed and numerically confirmed that it operates with low crosstalk (< −25 dB) over the C+L band (1530 nm – 1625 nm) and has negligible polarization dependence. The scalability of the device is demonstrated with a design of four-mode multiplexer. The three-mode and four-mode multiplexers are fabricated using our polymer waveguide platform, which is suitable for forming such multi-layer waveguide structures. Typical fabricated devices can multiplex the modes over the C+L band with small modal crosstalk (< −10 dB) and negligible polarization dependence, and butt-couple to fibers with low loss.

The third study investigates the mode-selective butt-coupling between FMFs and rectangular-core buried channel waveguides, to support the development of waveguide-based mode-selective devices such as mode converters and multiplexers. Low-modal-crosstalk coupling from the waveguide to the fiber is possible with requirements on core aspect ratio and area. For coupling to a step-index or a parabolic-index circular-core fiber with crosstalk < –20 dB, the waveguide should have a core sufficiently close to a square (within ±0.7% variation in its aspect ratio). For coupling to a step-index elliptical-core fiber with crosstalk < –20 dB, the waveguide can have many combinations of waveguide and fiber parameters. The aspect ratios of the elliptical core and the best matched rectangular core can be very different and an elliptical core that has a moderate ellipticity and an area not close to the upper limit allowed for supporting 6 spatial modes is preferred. The use of a parabolic-index profile in the elliptical core can further improve the mode selectivity with greatly relaxed tolerances on both the aspect ratio and the area of the rectangular core required. In general, when the fiber and the waveguide core are matched for low-crosstalk performance, the butt-coupling losses for all the 6 spatial modes as well as the mode-dependent loss are also small (typically well below 1 dB).