Multicore optical fiber Y-splitter - OSA Publishing

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connector (TMC). It couples light from single-mode fibers to MCF by reducing the space among cores from single mode fibers to multi-core fiber [2, 6, 7, 9].
Multicore optical fiber Y-splitter Ehab Awad* Electrical Engineering Department, College of Engineering, King Saud University, Riyadh, Saudi Arabia * [email protected]

Abstract: In future high capacity multicore optical fiber (MCF) networks, signal-processing devices should be able to manipulate data without sacrificing network capacity or MCFs advantages. Thus, it is crucial to have high performance novel devices that can be connected directly to MCFs without conversion to conventional single-core fibers. In this work, a novel Y-splitter for multicore optical fibers is proposed and numerically demonstrated for the first time. The splitter can directly split the power of input MCF cores by 50/50 splitting-ratio into two output MCFs cores. The splitter principle of operation mainly depends on novel double-hump graded-index (DHGI) profile that can space-division split (SDS) optical power by half. Both finite-difference-time-domain and eigenmodeexpansion simulations are performed to design, verify, and characterize performance of Y-splitter. It shows wideband operation over the S, C, L, Ubands with polarization insensitivity. It also demonstrates high performance with reasonable insertion-loss, in addition to very low excess-loss and return-loss. Moreover, the splitter shows good performance tolerance to both MCF and design parameters variations. ©2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.2340) Fiber optics components.

References and links 1.

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Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25661

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1. Introduction Space division multiplexing (SDM) is considered one of the key solutions in the next generation high capacity optical fiber networks. The current networks capacity is growing exponentially and approaching its final limit despite of using many different solutions such as time-division-multiplexing (TDM), dense-wavelength-division-multiplexing (WDM), polarization-division-multiplexing (PDM), and complex modulation formats [1–5]. Multicore optical fiber (MCF) is considered one of SDM approaches that has a potential to break such capacity limit in future optical networks. A MCF consists of multiple cores arranged in a certain configuration within the same cladding region. Each core can transmit the same set of different TDM, WDM, and/ or PDM channels and thus it can simply multiply the network capacity [6]. However, this MCF potential cannot be fully exploited, unless suitable MCF devices is provided to perform mandatory and conventional signal-processing operations on propagating data during transmission. Relaying on the current traditional solutions of converting back and forth between MCF and single-core single-mode fiber (SMF) to perform such operations imposes a bottle-neck that compromises the high capacity advantages of MCF networks. Therefore, providing novel MCF devices such as couplers, multiplexers, switches, etc. which can be directly connected and integrated with MCF is considered the key solution of such problem. Several techniques have been already reported on space division multiplexing to couple light to/ from MCF from/ to single-core SMFs. One technique is based on fiber tapered MCF connector (TMC). It couples light from single-mode fibers to MCF by reducing the space among cores from single mode fibers to multi-core fiber [2, 6, 7, 9]. Another SDM multiplexing technique is photonic-lanterns [5, 10, 11]. It functions by propagating several Gaussian modes from SMFs and focusing them to diffraction limit through an adiabatic gradual tapered waveguide. Another technique uses free-space optics such as lenses and collimators to couple the light to/ from MCF [12]. In this technique, the collimated beams coming out of each SMF is imaged on each MCF cores using several ordinary lenses. This free-space SDM technique was utilized for coupling to 19-cores MCF and data transmission with speed up to 305 Tb/s [8, 13]. Other techniques include one dimensional multi-modeinterference (MMI) couplers [14], arrayed waveguide gratings (AWG) [15], and phase matching SDM [16]. Moreover, ultrafast laser inscription has been reported to fabricate

#243958 (C) 2015 OSA

Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25662

waveguides in transparent dielectric materials in order to be used as SDM multiplexers between SMFs and MCF [17]. Here in this work, a novel MCF to MCF Y-splitter (1x2 50:50 coupler) is numerically demonstrated as one step toward the next generation MCFs devices. As already known, Ysplitters are crucial elements in any optical fiber network, especially when signals on optical fibers are required to be distributed (splitted) to other different fibers in order to reach multiple destinations. The Y-splitters can be generally useful in many emerging MCF applications including optical communications, SDM amplification, sensors, and chip interconnection.

Fig. 1. The MCF Y-splitter structure. (a) The 3D schematic diagram of Y-Splitter with seven sub-splitters connected between 7-core MCFs, each sub-splitter consists of 3-stages. (b) Schematic front-view of Y-splitter showing transverse cross-section of rectangular waveguides and MCFs cores/ cladding.

The novel MCF Y-splitter demonstrated here is simulated using both finite-differencetime-domain (FDTD) and eigenmode expansion (EME) methods to verify its operation and completely characterize its performance. The Y-splitter shows an excellent performance over the wideband of S, C, L, U wavelength range. It also shows polarization insensitive operation. Its estimated insertion-loss is very reasonable and the excess-loss and return-loss are very small. Moreover, it shows a good performance tolerance to MCF parameter variations, and design parameters variations. All of such excellent characteristics are very attractive for the next generation high capacity MCF networks. To the best of my knowledge, this is the first time to demonstrate such space-division splitter between MCFs. 2. Principles of operation The proposed MCF Y-splitter is a three-dimensional (3D) device that mainly depends on space-division splitting (SDS) by double-hump graded-index (DHGI) in rectangular waveguide. It consists of multiple single Y-splitters, each one is dedicated to one MCF core and consists of 3-stages: Expander, DHGI-SDS, and separator. Figure 1(a) shows the 3D schematic diagram of Y-splitter together with MCFs. The selected MCFs here have homogeneous identical step-index single-mode seven cores arranged in triangular-lattice. The

#243958 (C) 2015 OSA

Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25663

core and cladding diameters are 9µm and 125µm, respectively. The separation between adjacent cores is 40µm. The cladding refractive index is 1.45, and the relative refractive index difference is 0.35% resulting in a core refractive index of ≈1.4551 [18, 19].

Fig. 2. A single sub Y-splitter structure and its FDTD simulation. (a) 2D refractive index profile of sub Y-splitter. The inset shows the double-hump graded index profile of the second stage space-division-splitter (SDS). The white arrows indicate approximate locations of total internal reflections occurrence. (b) The FDTD simulation of electric-filed magnitude (|E| a.u.) for a single sub Y-splitter at the wavelength of 1555nm.

The 3D Y-splitter consists of seven single sub-splitters. Each sub Y-splitter connects one of the input MCF cores to corresponding two output MCFs cores. A sub Y-splitter consists of 3-stages of rectangular waveguides (WG) that have dimensions and refractive indices as discussed later in this section. The seven Y-splitters are surrounded everywhere by a cladding region (400x125µm) with 1.45 refractive index that matches the refractive index of MCFs cladding.

#243958 (C) 2015 OSA

Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25664

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(g) Fig. 3. The EME simulations (|E| a.u.) of Y-splitters together with MCFs cores. (a)-(g) show the longitudinal cross-sections of the seven sub Y-splitters connected to input/ output core 1 to 7, respectively.

Figure 1(b) shows a 2D schematic front-view of the Y-splitter and three connected MCFs. It demonstrates the arrangement of rectangular WGs together with MCFs cores and the cladding region. The separation between output MCFs is selected here to be equal to one MCF diameter (125µm). The seven Y-splitters WGs are shown as rectangles, each comprising an input core in the middle and two output cores on both sides. All rectangular WGs have equal heights of 9µm. The MCFs are rotated by 19° around their longitudinal axis in order to have a line-of-sight view between cores of each single splitter. That results in a vertical separation between adjacent WGs equals to 4µm. In order to ensure vertical de-coupling between those adjacent WGs, a thin isolation layer (2µm) with a lower refractive index (1.44) than the cladding region (1.45) is inserted in the mid-way between each adjacent WGs. This isolation layers (not shown in figures for simplicity) act as trenches [18] that help in reducing cross-talk between WGs as will be discussed latter in section five.

#243958 (C) 2015 OSA

Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25665

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Fig. 4. The EME simulations (|E| a.u.) of Y-splitters together with MCFs cores. It shows transverse cross-sections of: (a) Input MCF, (b) Lenses, (c) SDS, (d) Separators, and (e) Output MCFs. The cross-sections correspond to x-positions in Fig. 3 equal to 350, 750, 1750, 2500, and 3500 µm, respectively.

Figure 2(a) illustrates a schematic diagram of single Y-splitter together with its 3-stages showing its dimensions and refractive indices. The first stage acts as a graded-index lens [20] to expand the input core beam slightly. The rectangular WG lens (280x60x9µm) has the index profile: n1 ( y ) = no – 8 ∗10−6 y 2

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Where no = 1.46, and ‘y’ dimension is measured in ‘µm’ as shown in Fig. 2(a). This gentle parabolic index is optimized to have a small slope in order to act as a taper that gradually expands the input beam adiabatically and thus reduce the number of excited higher order modes. That would help eventually in matching the fundamental mode of output cores both in size and profile, and thus avoiding too much coupling loss. The second stage is the main one in splitter operation. It consists of a rectangular WG that has a centered double-hump graded refractive index (DHGI) profile as illustrated in Fig. 2(a) inset. The double-hump center is aligned with the lens central axis. Each hump has a dimension of 995x60x9µm. Each single hump has a parabolic profile: n2 ( y ) = no − 4.4 ∗10−6 ( y ± 20) 2

#243958 (C) 2015 OSA

(2)

Received 30 Jun 2015; revised 3 Sep 2015; accepted 13 Sep 2015; published 22 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025661 | OPTICS EXPRESS 25666

Where 20 and −20 µm correspond to the left (−60 µm