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Timestamp: 2019-04-19 16:17:41+00:00

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Abstract: Spatial multiplexer (SMUX) for mode division multiplexing (MDM) has evolved from mode-selective excitation, multiple-spot and photonic-lantern based solutions in order to minimize both mode-dependent loss (MDL) and coupler insertion loss (CIL). This paper discusses the implementation of all the three solutions by compact components in a small footprint. Moreover, the compact SMUX can be manufactured in mass production and packaged to assure high reliability. First, push-pull scheme and center launch based SMUXes are demonstrated on two mostly-popular photonic integration platforms: Silicon-on-insulator (SOI) and Indium Phosphide (InP) for selectively exciting LP01 and LP11 modes. 2dimensional (2D) top-coupling by using vertical emitters is explored to provide a coupling interface between a few-mode fiber (FMF) and the photonic integrated SMUX. SOI-based grating couplers and InP-based 45° vertical mirrors are proposed and researched as vertical emitters in each platform. Second, a 3-spot SMUX is realized on an InP-based circuit through employing 45° vertical mirrors. Third, as a newly-emerging photonic integration platform, laser-inscribed 3D waveguide (3DW) technology is applied for a fully-packaged dual-channel 6-mode SMUX including two 6-core photonic lantern structures as mode multiplexer and demultiplexer, respectively. ©2014 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing; (250.5300) Photonic integrated circuits.
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1. Introduction The capacity limit of the existing single-mode fiber (SMF) based optical networks due to fiber nonlinearities has been forecasted [1–3]. Spatial division multiplexing (SDM) utilizes the last unexplored physical dimension, space, to avert the network bottleneck looming in the SMF networks  and aims to achieve extra high spectral efficiency per fiber. Compared to simply adding parallel SMFs, SDM can be more cost- and energy-efficient through densely integrating transponders, optical amplifiers and other network elements [5,6]. Both mode division multiplexing (MDM) deploying transverse modes guided in few-mode fiber (FMF) and the use of uncoupled fundamental modes or supermodes in multi-core fiber (MCF) are labeled as SDM solutions, which are capable to expand optical fiber capacity by opening the spatial domain next to the wavelength and time domain. High capacity SDM, combined with WDM transmission trials are listed in Table 1, denoted as number (1) to (10) with an increasing transmission distance. In the top half of the table, all state-of-the-art SDM system trials are realized by MDM. The highest achieved spectral efficiency per fiber core for MDM is 32 bit/s/Hz , almost 3 times larger than the maximum capacity that can be offered by an SMF in the same distance. At the end of March 2014, combined SDM and WDM trials with a transmission distance longer than 1000km were only demonstrated with MCFs, listed in the bottom half of the table. Components for MCF based network such as multi-core erbium-doped fiber amplifier (EDFA) , MCF compatible reconfigurable optical add/drop multiplexer (ROADM)  and MCF loop  were demonstrated through directly upgrading the existing SMF based solutions. On the contrary to the MCF trials, although research for few-mode EDFA [11–13], few-mode re-circulation loop [14,15] and low-loss spatial multiplexer (SMUX)  achieved big progress recently, nonnegligible mode-dependent loss (MDL) and coupler insertion loss (CIL) still exist in these elements, which limit the distance, especially in an accumulated case such as loop measurements. Therefore, it is essential to further explore MDM components with better performance. In this paper, SMUX solutions based on mode-selective excitation, multiple-spot and photonic-lantern are analyzed through matrix representation. All three concepts are researched and experimentally verified with compact SMUX components instead of using bulky optics. First, compact SMUXes which are able to selectively excite LP01 and LP11 modes are demonstrated on both Silicon-on-insulator (SOI) and Indium Phosphide (InP) based circuits. 2-dimensional (2D) top-coupling by using vertical emitters is explored to provide the coupling interface between FMFs and integrated SMUX circuits. SOI-based grating couplers and InP-based 45° vertical mirrors are proposed as vertical emitters in each platform. Moreover, 2D top-coupling layout for selectively exciting four LP modes (LP01, LP11, LP21 and LP02) is proposed and verified by simulations. Second, a 3-spot SMUX is demonstrated on an InP-based circuit, where three 45° vertical mirrors are machined by focused ion beam (FIB) etching. Third, a photonic-lantern based SMUX is demonstrated by laser-inscribed 3D waveguide (3DW) technology. The 3DW device consists of two 6-core photonic-lantern structures for mode multiplexing and demultiplexing and is fully packaged with an FMF and SMF array.
Fig. 1. Schematic diagram of the SMUX.
where F’ represents the supermodes at the few-mode front-end. If the supermodes match the guided modes of the FMF, Γ and A in Eq. (3) both become unitary, which gives a unitary H. In this case, lossless mode conversion is realized.
In this section, compact SMUX solutions based on mode-selective excitation, multiple-spot and photonic-lantern concepts are presented. Two most popular photonic integration platforms: SOI and InP, and newly-emerging femto-second laser-inscribed 3DW technology are explored for compact SMUXes, which are with small footprints, high reliability and suitable for low-cost device packaging. 3.1 Mode-selective excitation To selectively launch one specific mode with high coupling efficiency (CE) and low crosstalk to other modes, a field as similar as possible to that of the desired mode needs to be generated . Both amplitude and phase of the generated field determine CE. The LP01 mode field is unipolar, and the LP11 mode has a bipolar field distribution. A push-pull scheme was proposed to excite the LP11 mode where two Gaussian-like spots are used with a phase difference of π . The comparison of the fields of the LP11a mode and the push-pull case is shown in Fig. 2(a). With an extra spot locates at the centre, five spots are able to selectively excite and detect all three spatial modes, see Fig. 2(b). The LP11a and LP11b mode fields are orthogonal, and rotated π/2 with respect to each other in the FMF cross-section area.
Fig. 2. (a) Comparison of the fields of the LP11a mode and push-pull case; (b) 5-spot region.
launching pure LP11 modes and no phase tuning is needed for LP01 modes excitation. Figure 4(b) shows the measured insertion losses versus wavelength for all 6 channels when the SMUX is used as a mode multiplexer. 20dB insertion loss for the LP01 mode is achieved. The insertion loss for an SMF-to-SMF self-loop including two 1D grating couplers is plotted as a blue curve in Fig. 4(b). It can be calculated that the coupling loss from an SMF to a waveguide via the 1D grating coupler is around 4dB. Besides on-chip losses, main loss comes from the small vertical grating couplers which are with a design of 5 periods. The few period design lowers the light diffraction efficiency of the grating and thus induces a quite high loss. Due to the limited space, the large spacing between the pairs of grating couplers causes more loss for LP11 mode excitation than the LP01 mode excited by the single grating coupler at the centre. By employing the proposed SOI SMUX, 3.072Tb/s (6 spatial and polarization modes×4 WDM×128Gb/s 16QAM) transmission over 30km 3-mode FMF was achieved .
Fig. 3. (a) Circuit schematics, (b) SEM image of the region of five grating couplers, (c) image of the packaged SOI-based SMUX.
Fig. 4. (a) Excited mode profiles for all spatial modes in two polarizations and (b) insertion loss for LP01 and LP11 modes versus wavelength.
3.1.2 InP-based 45° vertical mirror 45° vertical mirror based on total internal reflection (TIL) of the surface between a high-index waveguide and air is introduced as a vertical emitter for 2D top-coupling, which is suitable for InP platform. Vertical mirror has a wider bandwidth than the SOI-based grating coupler, which is wavelength dependent due to its periodic structure. The schematic of the 45° mirror is shown in Fig. 5(a). TIR happens at the slanted boundary of air and high-index waveguide.
InP-based platform under PARADIGM project  is chosen due to its high availability of both active and passive building blocks [29,30]. To deduce the loss from Fresnel diffraction at the top, anti-reflection (AR) layer can be coated. The mirror fabrication is done through focused Ion beam (FIB) etching. The SEM image of a 45° vertical mirror fabricated on an InP waveguide with a width of 4µm is shown in Fig. 5(b). It is measured that coupling loss from the vertical mirror to an SMF is less than 9dB including 1.5dB loss from the Fresnel reflection without the usage of AR coating. Moreover, the power change is less than 0.5dB as varying the state of polarization of input light, which means the mirror is compatible with polarization division multiplexing.
Fig. 5. (a) Schematic of the 45° vertical mirror; (b) SEM image of a 45° mirror etched on an InP waveguide.
Figure 6(a) shows the microscope image of a mode-selective excitation SMUX based on InP for LP01 and LP11 modes. The layout of 5 spots, see Fig. 2(b) is realized in the SMUX, where vertical mirrors replace the grating couplers for top-coupling. One mirror located at the center is for launching or detecting LP01 modes. Four mirrors in a outer ring with a radius around 6.8µm are for LP11 modes (LP11a and LP11b) selective excitation. For LP11 mode channel, light is split by a multi-mode interferometer (MMI) based 1×2 splitter. The thermooptic effect based phase tuner is applied to fine-tune the phase change in one waveguide arm to create the π phase difference for push-pull output. Deeply-etched waveguides with a width of 2µm and etch depth of 1.7µm are used for the mirrors. The mirror machining is done with two FIB fabrication steps: a raw round scan and a fine line scan. It takes 3 minutes for the 1st round scan etching with an acceleration voltage of 30kV and a beam current of 26pA and 4 minutes for the final line scan etching with 9pA. For an SI-FMF with a core diameter of 19.3µm , 4% and 7% CE is achieved by simulations for LP01 and LP11 modes, respectively, which results in 3dB MDL and 12 dB CIL. Atoms redeposition on the center waveguide can be observed from the comparison of Figs. 6(b) and 6(c), which is normal in the FIB nanofabrication process. An SEM image of the region with the 5 fabricated mirrors is shown in Fig. 6(d).
Fig. 6. (a) Microscope image of an InP-based SMUX circuit; SEM images of the 5-spot region (b) before and (c)-(d) after mirror machining.
3.1.3 High-order modes excitation This section scales up the mode-selective excitation solution to support four LP modes: LP01, LP11, LP21 and LP02. In the view of spatial modes, there are in total 6 spatial modes which each can have two polarization states, translating into 12 transmission channels.
Fig. 7. The arrangement of 9 spots for selectively exciting each spatial mode.
Figure 7 illustrates the arrangement of 9 spots for selectively exciting each spatial mode which is proposed for the first time. R is the radius of an SI-FMF, guiding 6 spatial modes. All spots have a radius of r and 8 spots are uniformly distributed along a circle with a radius of s. The push-pull scheme is further developed to support LP11 modes, see Figs. 7(b) and 7(c) and LP21 modes, see Figs. 7(d) and 7(e). Unlike the 6-spot arrangement of a spot-based SMUX , by using more spots, a high mode extinction ratio is achieved at the same time with excellent mode CE. Due to the circular symmetry of LP01 and LP02 modes, large mode coupling happens with improper launching conditions. Through properly positioning the spots and allocating different intensities and phases to the center spot and outer 8 spots, see Figs. 7(a) and 7(f), the mode profiles of LP01 and LP02 can be nicely matched. In LP01 launch condition, the simulated CE for LP01 and LP02 mode is shown in Fig. 8, with α = 0.28 and variation β and γ1, where α = r/R, β = s/R. γ1 is defined as the ratio of the intensity of each spot arranged in the outer ring to that of the center one. As β = 0.6 and γ1 is around 0.4, >60% LP01 mode CE is achieved and meanwhile no crosstalk to LP02 mode. In LP02 launch condition as shown in Fig. 7(f), >60% CE can also be achieved with β = 0.6 and γ2 = 0.2, see Fig. 9. Figures 10(a) and 10(b) show the CE for LP11a and LP21a mode with corresponding launch condition, respectively, where all the spots share the same intensity but are with different phases. It can be seen that as β = 0.6 and α = 0.28, four LP modes can be selectively excited with around 60% CE, which means less than 2.3dB coupling loss and no mode crosstalk to the other modes in theory due to the mode orthogonality. This proposed SMUX has the advantage in mode selectivity at the cost of more complicated circuit design. For simultaneously exciting all modes, optical splitters, phase shifters and combiners are needed to feed each spot with combined optical signals carrying a proper intensity and phase, as illustrated in Fig. 7. To guarantee the high mode extinction ratio, optical fibers or waveguides which deliver the light to the spots cannot couple with each other. SOI- and InP-based optical waveguides are with a high core-cladding index contrast, which enables negligible waveguide crosstalk even in several micrometer gap. Due to the small size of 45° vertical mirrors and premium optical building blocks such as splitters and phase shifters in InP-based platform, InP-based SMUX with 45° vertical mirrors are potentially able to realize the complex structure as shown in Fig. 7.
Fig. 8. CE for (a) LP01 and (b) LP02 modes under LP01 launch condition with α = 0.28 and variation β and γ1.
Fig. 9. CE for (a) LP01 and (b) LP02 modes under LP02 launch condition with α = 0.28 and variation β and γ2.
Fig. 10. CE for (a) LP11a and (b) LP21a modes under corresponding launch condition with variation α and β.
machining, which can be realized through modifying waveguide’s layer stack and using adiabatic taper to upscale the waveguide width.
Fig. 11. (a) Spots arrangement of a 3-spot SMUX; (b) microscope image of a 3-spot SMUX circuit; SEM images of the 3-spot region (c) before and (d)-(e) after mirror machining.
3.3 3DW photonic-lantern SMUX In order to achieve a compact and lossless mode (de)multiplexing solution, photonic-lantern based SMUX that merges N single mode waveguides into a few-mode waveguide that supports N spatial modes was proposed in  and experimentally verified in . Femtosecond laser-inscribed 3DW technology enables inscription of many compact waveguides into a transparent substrate , which is ideal to build the photonic-lantern SMUX for coupling between an SMF array on a 1-dimensional pitch to an FMF with a 2D mode pattern . Figures 12(a) and 12(b) show the spot arrangement and the sketch of a 6-core photonic lantern coupling to a 6-mode FMF. In theory optical building blocks such as splitter and arrayed waveguide grating (AWG) can be realized by 3DW technology, which makes this technology potentially can be a photonic integration solution similar as SOI and InP.
Fig. 12. (a) Spot arrangement and (b) sketch of a 6-core photonic lantern for mode multiplexing.
device. Two adiabatically up-tapered 6-mode FMFs with a cladding of 175μm are positioned and assembled in a standard V-groove. SMF array, 3DW device and FMF array are glued together using UV curing epoxy. The mode profile mismatch between the photonic lantern and FMF is solved by up-tapering FMF . The packaged 3DW SMUX has a CIL less than 4dB and an MDL around 3.5dB for each photonic lantern. The picture of the fully-packaged SMUX is shown in Fig. 13(b).
Fig. 13. (a) Sketch and (b) picture of the fully-packaged dual-channel 6-mode SMUX realized by 3DW technology.
This work was supported in part by the European Union FP7 program BROWSE 10015285, the European Union FP7-ICT MODE-GAP project under grant agreement 258033 and the IT R&D program of MKE/KEIT (10043383, Research of Mode-Division Multiplexing Optical Transmission Technology over 10 km Multi-Mode Fiber). We acknowledge help and valuable discussions from Vincent Sleiffer, Barcones Campo, Kevin Williams and Oded Raz with Eindhoven University of Technology; Nicolas K. Fontaine, Roland Ryf with Bell Laboratories, Alcatel-Lucent; Bradley Snyder and Peter O’Brien with Tyndall National Institute, University College Cork. The SOI circuit was fabricated in the framework of ePIXfab set-up by CEA/LETI. The InP circuits were fabricated in PARADIGM multi-project wafer run by Fraunhofer HHI.
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