Multicore Fibre Fan-In/Fan-Out Device using Fibre Optic Collimators

We present a new approach in realizing multicore fibre fan-in/fan-out (FIFO) device. Using simple image demagnification by two fibre optic collimators a compact FIFO device for 4-core multicore fibre is fabricated with low crosstalk. Introduction One of the key optical components for space division multiplexed (SDM) transmission systems [1-3] is an efficient and stable SDM multiplexer/demultiplexer, which allows for individual spatial modes (or cores) to be accessed with minimal loss and crosstalk. In the case of multicore fibre (MCF), spatial multiplexers/demultiplexers are referred to as fan-in/fan-out (FIFO) devices and are used to efficiently couple light from individual single mode fibres to each core of the MC and vice versa. Various configurations have been reported so far but the most common techniques use: 1) fused tapers [4], 2) 3D waveguides [5] and 3) free space optics [6]. Each of these approaches has its own merits, but it is still challenging to realize low crosstalk (XT) for high density MCFs having a small core pitch distance. In a fibre taper approach, for example, FIFO device can be effectively fabricated by tapering single mode fibre bundles but the mode field diameter (MFD) of each core enlarges during tapering, which can induce significant crosstalk between neighboring cores. In the case of the 3D waveguide approach, the maximum achievable refractive index change possible by direct femtosecond laser inscription is currently limited to about 0.007 and it is difficult to realize a low XT FIFO device with such a weakly confining waveguides. The free space optics based FIFO device shows a low insertion loss and XT but requires expensive custom optics and ultra-precise alignment, and represents the most expensive and bulky solution among the three approaches. Moreover, recent high density MCFs have square lattice core arrangements which is problematic as various of the FIFO fabrication methods cannot be applied to nonhexagonal lattice core arrangement (e.g. for fibre taper based FIFO, it is difficult to align and maintain the desired square lattice arrangement during the tapering and a special micro-hole arrayed glass platform needs to be used). In this paper, we propose a compact and low XT FIFO device using commercially available fibre-optic collimator assemblies. Our approach Fig. 1: (a) Geometry of image demagnification by two simple convex lenses with different focal lengths, (b) the analogous fibre optic collimator assembly for a compact FIFO device for a 4-core MCF and (c) the fully integrated device. is based on simple image demagnification using two micro-lens collimators having different focal lengths and can be readily implemented for most MCFs having either a square or hexagonal core arrangement. As an example we demonstrate a compact, proof of concept, FIFO device for 4core MCF (=36 m) which exhibits a moderate insertion loss (<3.1 dB) and low XT (<-40 dB). Working principle and fabrication of 4c-MCF FIFO Figure 1(a) shows the basic geometry of image demagnification using two simple convex lenses. When an object is placed at the focal point of the first convex lens in a 4f-imaging system, a real, inverted and de-magnified image can be formed at the focal point behind the second convex lens. The demagnification ratio is determined by the ratio of the focal lengths of the two lenses (f2/f1) and can be used to convert the core pitch distance from the input SMFs to the output MCF to realize a FIFO device. For example, the square beam profile from 4 SMFs with 1=125 m can be converted to a similar square beam profile but with a reduced core pitch (2=36 m) at the image plane by choosing the appropriate focal lengths ratio of the two lenses i.e. f2/f1=36/125=0.29 for low loss coupling. An analogous fibre optic collimator structure can be realized with two micro-optic lenses (e.g. C-lens or Grin lens) as shown in Fig. 1(b). By choosing an appropriate focal lengths of the two microlenses, a compact FIFO device can be easily realized with widely used fibre optic collimator assemblies. Note however that the MFD of the input fibres will also be reduced during the demagnification process by a factor of 0.29 and the subsequent MFD mismatch results in a significant coupling loss (3-4 dB) to the output 4cMCF. However, the coupling loss can be improved by employing thin cladding input fibres to reduce the demagnification ratio in conjunction with large mode area (LMA) fibres to enlarge the MFD of the input fibres. In our experiment, four SMFs were first cleaved, inserted into a fibre ferrule with a square hole (dimension= 252×252 m) and positioned at the focal plane of the first C-lens (f1=4.77 mm) to achieve an array of high quality collimated beams [7, 8]. A low viscosity epoxy resin was applied inside the ferrule hole so that the surface tension of the epoxy helps to position the fibres adjacent to each other. As shown in Fig. 1b (left, bottom), 4 SMFs are closely packed with a core pitch distance of 125 m. Next, another collimator was fabricated for the 4c-MCF using a shorter focal length C-lens (f2=1.87 mm). Currently, the range of available C-lens is somewhat limited and the focal length ratio used (f2/f1=1.87/4.77=0.39) is bigger than the required demagnification ratio (0.29). Therefore we have employed another core pitch adaptor using a graded index fibre lens to further adjust the de-magnification ratio of the object, which is detailed in ref. [9]. This all-fibre core pitch adaptor can be efficiently used for small core pitch conversion and fabricated by splicing a 1500 m segment of a graded index fibre (core diameter= 185 m) to the 4c-MCF. Note that this all fibre core pitch adaptor can be used alone or together with other FIFO fabrication techniques (e.g. fused taper or free space optics) for precise control of the core pitch adaptation without significant XT increase. Next, these two collimators were mounted on a multi-axis precision micro-positioner (offering translation, tilt and rotation adjustment) to align the collimators. After obtaining the maximum coupling efficiency, the collimator assemblies are fixed with low shrinkage UV curable epoxy. A final packaged 4c-MCF FIFO device is shown in Fig. 1(c).