Mode-selective coupling in an asymmetric holey fibre coupler is demonstrated both numerically and experimentally for the first
time. The coupler's performance is shown to be ultrabroadband, with significant potential existing for the use of such
couplers in high bandwidth few-mode fibre networks

Argyros, Alexander

Bassi, Paolo

Bellanca, Gaetano

Leon-Saval, Sergio G

Love, John D

Lwin, Richard

Parini, Alberto

Riesen, Nicolas

The Australian National University

Holey Fibre Mode-Selective Couplers Nicolas Riesen1, Alexander Argyros2, Alberto Parini3, Richard Lwin2, Sergio G. Leon-Saval2, Gaetano Bellanca3, Paolo Bassi4 and John D. Love1.  1RSPE, The Australian National University, Canberra, ACT 0200, Australia. 2IPOS, University of Sydney, NSW 2006, Australia. 3Department of Engineering, University of Ferrara, Italy. 4Department of Electrical, Electronic and Information Engineering, University of Bologna, Italy. nicolas.riesen@anu.edu.au   Abstract Summary  Mode-selective coupling in an asymmetric holey fibre coupler is demonstrated both numerically and experimentally for the first time. The coupler's performance is shown to be ultra-broadband, with significant potential existing for the use of such couplers in high bandwidth few-mode fibre networks.  Keywords- holey fibre, microstructured optical fibre, mode-division multiplexing, mode-selective couplers. I.  INTRODUCTION Currently there is rapidly growing interest in the potential use of few-mode fibres for meeting the ever-increasing demand for bandwidth in long distance optical networks. The idea is that each mode of a few-mode fibre can be used as a unique data channel in what is referred to as mode-division multiplexing [1]. A major technological challenge for mode multiplexed few-mode fibre networks is the excitation and detection of individual modes at either end. There are several potential solutions to this problem using simple light-processing waveguide devices such as asymmetric Y-junctions, multimode interference devices or mode-selective couplers [2].       In this paper we consider mode-selective couplers, which involve evanescent coupling between a higher-order mode of one core and the fundamental mode of another closely-positioned core, or vice versa [1, 3]. The mode-selective functionality of these devices is achieved via matching of the modal propagation constants, by using dissimilar diameter cores. II. HOLEY FIBRE MODE-SELECTIVE COUPLERS In particular, mode-selective coupling is demonstrated for the first time using holey fibre technology. Holey fibres are index-guiding photonic crystal fibres with a lattice of air holes forming the cladding [4]. The broadband characteristics of these fibres, allows for the design of ultra-wideband mode-selective couplers.  A. Numerical Simulations The two-core polymer holey fibre coupler design is shown in Fig. 1(b) with mode intensities superimposed. The two cores are each defined by two rings of air holes that provide sufficiently strong guidance to ensure minimal light leakage (~ 10-4 dB/cm) over the few-centimetre coupling length. The coupling occurs between the second anti-symmetric LP11a mode of the larger core and the degenerate (i.e. propagation constant matched) fundamental LP01 mode of the smaller core. The coupler was designed for operation at a wavelength of λ = 633 nm to facilitate characterization, with suitable performance permitted across the visible spectrum (see Fig. 1(a)).     5060708090100500 600 700 800Wavelength (nm)Coupled Power (%)  Figure 1.  (a) BPM calculated [5] wavelength dependent power transfer between the LP01 mode of the smaller core and the LP11a mode of the larger core of the holey fibre mode-selective coupler shown in (b).  The broadband nature of holey fibre couplers is however  not limited to just the visible spectrum, and similar performance could be realized over entire telecommunications bands. For the present coupler the coupling efficiency exceeds 96% at the nominal wavelength, with a coupling length close to 3 cm. B. Fabrication of Holey Fibre Coupler The holey fibre coupler was fabricated via drilling and drawing [4] of a solid PMMA preform rod. The cross-sectional slice of Fig. 2 shows the onset of coupling between the fundamental LP01 mode of the smaller core and the second LP11a mode of the larger core. At the appropriate coupling length almost total power transfer can be realized and the coupling performance is also found to be highly broadband as predicted. Such mode-selective couplers therefore have considerable potential for application in wideband mode-division multiplexing for increasing optical fibre capacity. REFERENCES 1.  J.D. Love and N. Riesen, "Mode-selective couplers for few-mode optical fibre networks," Opt. Lett., vol. 37, no. 19, pp. 3390-3392 (2012).  2.  N. Riesen and J.D. Love, "Tapered velocity mode-selective couplers," J. Lightw. Technol., vol. 31, no. 13, pp. 2163-2169 (2013). 3.  N. Riesen and J.D. Love, "Weakly-guiding mode-selective fibre couplers," J. Quant. Electron., vol. 48, no. 7, pp. 941-945 (2012). 4.  A. Argyros, "Microstructured polymer optical fibres," J. Lightw. Technol., vol. 27, no. 11, pp. 1571-1579 (2009). 5.  F. Fogli, L. Saccomandi, P. Bassi, G. Bellanca, and S. Trillo, "Full vectorial BPM modeling of index-guiding photonic crystal fibers and couplers," Opt. Express, vol. 10, no. 1, pp. 54-59 (2002).    LP11a LP01          10 µm     (a).                                                (b). Figure 2. Cross-section of PMMA holey fibre mode-selective coupler showing  mode coupling when using a broadband source. 

Holey fibre mode-selective couplers

Holey fibre mode-selective couplers

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