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Abstract
In this work, a silicon-integrated edge coupler supporting dual-mode fiber-to-chip coupling was designed and fabricated on 220-nm-thick SOI wafer with standard CMOS-compatible fabrication process. The proposed low-complexity structure consists of a multimode interference and triple-tip inverse taper. Both LP01 and LP11 modes in the few mode fiber (FMF) can be stimulated simultaneously by the edge coupler from TE0 and TE1 modes in silicon waveguide. Such a design is compatible with broadband wavelength division multiplexing and can be scaled up to 4-polarization-mode coupling as well. Using the proposed edge coupler, 2×100-Gbps/lambda PAM4 multimode interface through dual-mode fiber was demonstrated successfully.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For data center with hungry expectation of ultra-high capacity and low power consumption, mode division multiplexing (MDM) technology shows prospective foreground to further increase the number of parallel channels propagated in single waveguide/fiber, and enhance the capacity of fiber optic interconnection beyond Peta-bit [1]. Photonic integrated circuit has presented remarkable advantages on optical transceivers thanks to its compact footprint, low power consumption and large production output [2]. Multimode chip-to-fiber couplers build an indispensable bridge between photonic integrated circuits and few-mode fiber for mode division multiplexed (MDM) optical interconnection. There have been plenty of researches on chip-based mode multiplexer/demultiplexer and on-chip MDM interconnection [3–5]. Variety of integrated mode converters and multiplexers were proposed but only very few can achieve multimode coupling.
The biggest challenge is the multimode interface that builds the bridge to connect on-chip multimode waveguide and FMF. Grating coupler and spot size coupler are common interfaces for chip-to-fiber vertical/edge coupling. For the case of multimode, special designs need to be employed in order to match the large mode field difference between integrated waveguide and silica fiber. A pair of 2-D grating couplers controlled by push-pull solution was proposed for exciting 6 linear polarization (LP) modes [6–7], while having large coupling loss. Other multimode grating couplers with simpler and more compact structure were also designed later [8–9]. While, the inherent shortcoming of grating coupler, namely the limited bandwidth, hinders the application to be compatible with wideband wavelength division multiplexing (WDM).
Edge couplers present broadband operation, lower theoretical coupling loss, and more friendly for packaging within compact space. Edge couplers based on polymer waveguide are reported for up-to-four-mode coupling with low coupling loss [10–11], thanks to its similar refractive index with FMF, and the flexibility to stack multi-layer structure. However, the large material absorption loss and less compactness also limit its practicality. Silicon-on-insulator (SOI) platform possesses highly integration, low absorption loss and has been widely used for on-chip transmitting/receiving system. While, the large index mismatch between silicon and fiber, and only one manageable dimension for CMOS-compatible SOI, make it difficult to realize multimode edge coupling by conventional design. Multi-stage silicon inverse taper with polymer up-cladding was demonstrated with foreseeable ability to couple 4 LP modes [12], while having 2.68-mm total length. Besides, two parallel inverse taper structure was designed for the first high-order mode coupling [13], and similar structure with three inverse tapers was reported for two-mode coupling [14].
In this paper, we report the design and the fabrication of a silicon edge coupler for dual-mode fiber-to-chip coupling, with small footprint, broadband operation, and CMOS compatibility. The proposed coupler consists of a 1×3 symmetrical MMI and triple-tip inverse taper. The simple structure is also able to scale up for 4-polarization-mode coupling by employing polarization independent MMI. LP01 and LP11 modes were launched by the edge coupler with low loss and low crosstalk over 90-nm bandwidth. 2×100-Gbps/lambda PAM4 MDM transmission was also demonstrated over 40-m dual-mode fiber, where the capacity can be further increased by employing broadband WDM. This work provides a way for the realization of integrated optical transceivers for MDM and WDM fiber optic interconnection.
2. Structure design and simulation
The proposed dual-mode edge coupler (DMEC) was designed on conventional SOI wafer with 220-nm-thick silicon layer, 2.5-μm-thick silicon dioxide upper-cladding and 3-μm-thick bottom dioxide. On-chip dual-mode waveguide is 910 nm wide, supporting TE0 and TE1 modes. The few-mode fiber supports 2 LP modes propagating (LP01 and LP11) with core diameter of 15 µm and fundamental mode field diameter of 13.4 µm. As depicted in Fig. 1, the DMEC consists of a symmetric multimode interference (MMI) and triple-tip inverse taper as spot size converter for dual modes. Based on the self-imaging principle of MMI, the TE0 mode introduced from the central dual-mode waveguide of MMI can be mapped to the central output as the self-image, at the length of $3{L_\pi }/4$, where ${L_\pi } = \lambda /[{2({{n_{eff,0}} - {n_{eff,1}}} )} ]$ is the beat length of the first two mode propagating in the MMI. At the same length, the central input TE1 mode can be split into two lobes with identical intensity and out of phase, and separately output through the left and right single-mode waveguide of MMI. Thus, the input two modes are separated into different path by the MMI. Three adiabatic inverse tapers are set followed to convert the mode field confined in silicon waveguide into the clad layer, and excite the fiber modes at the edge of the chip. The gap between the left and right tips of inverse taper is determined by the mode field distribution of LP11. Via such a simple design, 2 modes in silicon waveguide can be coupled into corresponding LP modes in few-mode fiber at the same time, and both of them presents high coupling efficiency.
Fig. 1. Schematics of dual-mode edge coupler coupling with 2-mode fiber in (a) perspective view, and (b) top view.
In this work, the MMI has width of 5 µm and length of 42 µm for TE0 self-imaging. Three inverse tapers have 190-nm-wide tips and 310-µm total length, which are determined by the fabrication condition, and will limit the final properties of our design in a certain extent (details in discussion). To analyze the performance of DMEC, the simulation is separated into two parts, the on-chip mode conversion from the position ‘x0’ to ‘x1’ noted in Fig. 1(b), and the spatial mode coupling from the chip edge ‘x1’ to the fiber ‘x2’. The simulated electrical fields of mode conversion profile in the DMEC are shown in Figs. 2(c)–2(d). The TE0 and TE1 modes in silicon waveguide have effective mode field area (Aeff) of 0.22 and 0.334 µm2 respectively [Figs. 2(a)–2(b)]. Propagating through the DMEC, they are converted into mode fields with Aeff of 1.52 and 2.98 µm2 (at the position ‘x1’), so as to motivate the LP01 and LP11 modes supported in dual-mode fiber (DMF) [Figs. 2(g)–2(h)].
Fig. 2. Simulated electrical fields: mode profiles of TE0 (a) and TE1 (b) in SOI waveguide (at ‘x0’ in Fig. 1(b)), transmission profiles of the proposed 2-mode edge coupler for TE0 (c) and TE1 (d), mode profiles coupled into the 2-mode fiber for LP01 (e) and LP11 (f) (at ‘x2’ in Fig. 1(b)), mode profiles of LP01 (g) and LP11 (h) supported in 2-mode fiber.
The on-chip conversion efficiencies of DMEC are 69% and 62% for TE0 and TE1 respectively, with 3-dB bandwidth of more than 200 nm, plotted in Fig. 3(a) (‘x0’-‘x1’). However, the spatial mode coupling at the edge of the chip presents larger coupling loss, plotted in Fig. 3(a) (‘x1’-‘x2’), mainly due to the mismatched mode fields between the chip edge and DMF. It can be perceived directly through the difference between mode fields coupled into DMF [Figs. 2(e)–2(f)] and that originally supported in DMF [Figs. 2(g)–2(h)]. The fundamental mode LP01 suffers larger mode mismatch than the higher-order mode LP11, thus, leading to less coupling efficiency from edge coupler to fiber. The total coupling efficiencies (CE) of DMEC are -10.11 dB and -8.8 dB for TE0-to-LP01 and TE1-to-LP11, as the spectrum of CE and crosstalk (XT) shown in Figs. 3(c)–3(d). Besides, the calculated results mentioned above are all at the case of central excitation, which is hard to precisely align in the real experiment. We analyzed the lateral alignment tolerance of the spatial mode coupling. The results shown in Fig. 3(b) illustrate that, CEs of the LP01 and LP11 present the 1-dB lateral aligning tolerance of 3 µm and 1.5 µm, while, the XTs deteriorate to -10 dB at the lateral offset of 1 µm and 2 µm for LP01 and LP11 respectively. Therefore, in order to measure the DMEC with high efficiency and low crosstalk, high-accuracy alignment stage with sub-µm resolution is requisite.
Fig. 3. Simulation results: (a) coupling efficiency of the on-chip mode conversion (from ‘x0’ to ‘x1’) and spatial mode coupling (from ‘x1’ to ‘x2’), (b) the lateral alignment tolerance of coupling efficiency (CE) and crosstalk (XT) for spatial mode coupling, (c) total coupling efficiency and (d) crosstalk of the dual-mode edge coupler in the span of 200 nm, ‘a’-‘h’ refer to the modes shown in Fig. 2(a)-(h) respectively.
3. Fabrication and photonic measurement
The proposed DMEC was fabricated by Center for Advanced Electronic Materials and Devices. The devices were first patterned and etched on 220-nm-thick silicon layer through E-bean lithography and inductively coupled plasma (ICP) etching process, and 2.5-µm-thick SiO2 upper-cladding was then deposited by plasma-enhanced chemical vapor deposition. Afterwards, deep silicon ICP etching was implemented for slicing the chip at the edge of inverse taper. The SEM images of the fabricated DMEC before the upper-cladding deposition and the cross section of edge coupler are shown in Fig. 4. In order to generate TE1 mode in the dual-mode waveguide, mode converter based on asymmetric directional coupler [3] was designed and settled before the DMEC. The fabricated mode converter shown in Fig. 4(b) presents less-than-1dB insertion loss for TE0-TE1 conversion in a broad waveband. Two input single-mode waveguides of mode converter are coupled from inverse tapers with coupling loss of ∼5 dB.
Fig. 4. SEM images of (a) the fabricated 2-mode edge coupler at top view (before the deposition of dioxide layer), (b) the mode converter, (c) MMI and triple-tip inverse taper, and (d) the cross section of edge coupler.
The output field profiles at the edge of DMEC chip were captured by CCD shown in Figs. 5(a)–5(b). As TE0 mode inputs, single spot was observed at the edge. While, as TE1 mode inputs, two spots were symmetrically distributed at the same position, so as to excite the LP11 mode in DMF. Under the bright spots of inverse tapers, some circular patterns were the light of input fiber directly penetrated through the buried oxide layer. After aligning the DMEC and DMF on 5D alignment stage, LP01 and LP11 modes were launched in DMF, as shown in Figs. 5(c)–5(d), with perfect mode fields.
Fig. 5. CCD-captured field profiles of the 2-mode edge coupler at the edge of chip, when (a) TE0 input and (b) TE1 input. CCD-captured field profiles of the 2-mode fiber after (c) LP01 coupling and (d) LP11 coupling.
To analyze the mode characteristics of the measured mode field, we applied spatially and spectrally (S2) imaging method [15] by using tunable laser source, 40-m DMF and CCD camera. We swept the laser frequency at the range of 4 nm and captured 800 pictures in total. After Fourier transform of the interference spectrum, the differential group delay (DGD) is calculated as plotted in Fig. 6(a). For the case of central coupling, the dominant mode is LP01 at DGD=0. While, few energies are still coupled into high-order mode, indicated by the second peak in Fig. 6(a) with DGD of 4.811 ps/m. Using Fourier filter to pick out the second peak, the recovered mode field is shown in the inset, which represents LP11, and the extinction ratio (ER) is only -22.58 dB. The same measurement was also applied for LP11-dominant launch. The calculated DGD is in accord with the above result, and the ER of LP01 is less than -20 dB as well.
Fig. 6. (a) S2 measurement result: DGD of dual-mode fiber after coupled from DMEC. (b) Measured spectrum of coupling loss, (c) coupling loss under different gap of inverse tapers.
Furthermore, we fabricated fiber-based mode coupler for the LP01 and LP11 modes in DMF [16], to measure the broadband operation of DMEC and for the MDM data transmission (in the next section). Single-mode fiber and DMF were tapered together to satisfy the phase matching condition, so as to realize LP01 and LP11 conversion in DMF. The coupling loss of the home-made fiber coupler is < 2 dB for the two modes, and mode crosstalk is <-10dB with broadband operation. Using the fiber coupler as mode demultiplexer, the broadband coupling loss of the DMEC is depicted in Fig. 6(b). The minimal coupling loss are -13.22 dB and -12.55 dB for LP01 and LP11 respectively, which are a bit larger than the simulated results. The obvious fluctuations of the spectrum are mainly caused by the inter-mode interference at the fiber demultiplexer, which also reveals the existence of mode crosstalk in the DMF. As shown in Fig. 6(c), DMEC with 11-µm gap between the inverse tapers presents the maximal coupling efficiency, and the crosstalk is -11.9 dB for LP01 and -7.3 dB for LP11. Compared with the simulated results, the measured larger coupling loss and crosstalk are blamed for the insufficient accuracy of alignment stage. And the crosstalk is also contributed by on-chip mode converter and fiber coupler. While, despite of the fluctuations, the DMEC still presents a broadband operation over the 90-nm-wide span around 1560 nm. The measured bandwidth is also limited by the broadband optical source.
4. MDM transmission
Working as chip-to-fiber MDM interface, DMEC plays an essential role for the compact MDM transceiver module. To verify this functionality, we set up a high-speed MDM transmission system, using the proposed DMEC as on-chip mode multiplexer and fiber coupler as demultiplexer. The system setup is depicted in Fig. 7. The wavelength of tunable laser source was tuned to 1551 nm in order to obtain a relatively lower insertion loss and crosstalk over the interference spectrum. The 4-pulse amplitude modulation (PAM4) electrical signals were generated by the arbitrary waveform generator with 64-GSa/s sample rate, amplified by the wideband electrical amplifier, and then converted to optical signals via the lithium niobate Mach-Zehnder modulator. Two erbium doped fiber amplifiers were employed before and after the multiplexing system to compensate the insertion loss of mode coupling. A polarization controller was settled before DMF to make the fiber coupler for LP11 less coupling loss. The modulated optical signal after 40-m DMF transmission was received by photodetector and then sent into real-time oscilloscope with sample rate of 160 GSa/s for the off-line digital signal processing (DSP). High-speed PAM4 signal was transmitted in the MDM link, by means of the root raised cosine filter with a roll-off factor of 0.01 to compress the signal bandwidth. The matched filter and the time-domain Feed Forward Equalization were applied at the receiver side to obtain the lower bit error ratios (BER). It needs to be noted that, the DSP settings are all the same for each channel transmission, so as to evaluate the performances fairly.
Fig. 7. Experiment setup of high-speed MDM transmission using on-chip multiplexer and fiber-based demultiplexer. AWG: arbitrary waveform generator, EA: electrical amplifier, MZM: Mach-Zehnder modulator, EDFA: erbium doped fiber amplifier, DMF: dual-mode fiber, OBF: optical bandpass filter, PD: photodetector, RTO: real-time oscilloscope
The optical spectrum of 100-Gb/s PAM4 signals are shown in Fig. 8(a). The mode crosstalk for both the two modes are less than -14 dB, which is low enough to support two modes simultaneously transmitted in DMF with neglected inter-mode interference. The BER curves of 80-Gb/s and 100-Gb/s PAM4 signals are plotted in Figs. 8(b)–8(c), and the 100-Gb/s PAM4 eye diagrams of back-to-back (BTB) and two mode channels are also described in Figs. 8(e)–8(f). For 80-Gb/s PAM4, BERs at BTB case and the two mode channels are all under the 7% forward error correction (FEC) threshold (3.8e-3) at the received power of 0 dBm. Compared with BTB, MDM transmission brought in 0.4-dB and 1-dB power penalties for LP01 and LP11 channels, which are mainly due to the mode coupling loss. While, for 100-Gb/s PAM4, BER performances are almost under 7% FEC at 3-dBm received power, with larger power penalties of 1.1 dB and 1.3 dB for LP01 and LP11 channels. As the lack of fiber array for parallel coupling, we were unable to transmit the MDM channels at the same time. The power penalty in actual MDM transmission will be slightly higher than that we measured. With improved coupling loss of DMEC, we can obtain higher signal-to-noise ratio so as to much better BER performance and higher MDM capacity.
Fig. 8. (a) Optical spectrum of 100-Gb/s PAM4 signals. BER curves of (b) 80-Gb/s and (c) 100-Gb/s PAM4 signals. (d-f) Eye diagrams of 100-Gb/s PAM4 signals after equalization.
5. Discussion
Our work provides a simple structure based on MMI and triple-tip inverse tapers for building up the integrated multimode interface, so that two modes can be simultaneously coupled into fiber for MDM transmission in a small footprint. Furthermore, the structure can be scaled up for TM modes by introducing polarization independent MMI and inverse taper [17], so that the edge coupler can be used for 4-polarization-mode coupling (TE0-LP01,y, TE1-LP11,y, TM0-LP01,z, TM1-LP11,z). While, for planar silicon waveguide, there’s only one dimension for edge coupler design, unlike vertical coupler with two dimensions. Thus, it’s hard to further increase the mode number of MDM edge coupler more than 4. However, two significant advantages of edge coupler are the lower theoretical coupling loss, and broadband operation, which make a great difference for broadband WDM compatibility. Not only the inverse taper, the MMI adopted in this structure is also broadband device. Thus, the proposed MDM edge coupler owns a bright prospect for the WDM-compatible MDM optical transceiver, which can possess compactness and ultra-high total capacity.
Although, the design could have low loss, low crosstalk and wide bandwidth theoretically, the fabricated device did not present an excellent performance. There are two main reasons for that. One is the fabrication standard for edge coupler. The length and tip width of inverse taper are all determined subject to the fabrication process. If narrower tip can be made, and the end of inverse taper can be settled few-µm away from the edge of chip, much larger mode field is converted at the edge to match with the fiber mode. Besides, with deeply trenched cladding [13], the inverse taper can have a better confinement of mode field in the finite-wide clad waveguide, so as to further reduce the coupling loss, despite employing non-standard fabrication process. Thus, higher coupling efficiency of edge couplers under standard fabrication process will be the next challenge to improve the performances of multimode edge coupler. Second one is the mode field area of DMF. The effective mode field area of LP01 in the DMF we used here is 141 µm2 which is hard to match with by spot size converter. We tapered the DMF to cladding diameter of 80 µm and mode field area down to 100 µm2. Using the tapered DMF, the coupling loss of DMEC reduced to -10.77 dB for both the two modes, as shown in Fig. 9. Thus, few-mode fiber with smaller mode field area will be helpful to reduce the coupling loss of DMEC. Besides, more stable and more accurate alignment stage will be helpful to reduce the inter-mode crosstalk caused by alignment, which can be overcome in the future.
Apart from the inverse taper which occupies a large area, our simplified design has the smallest footprint compared with previous works. And we believe that, with better optimization of the fiber and inverse taper like we mentioned before, our design can provide low-loss coupling simultaneously for LP01 and LP11 mode. Furthermore, using the proposed edge coupler, 2×100-Gbps/lambda optic interconnection through dual-mode fiber was demonstrated successfully, which makes this work a complete demonstration with proof-of-concept results for the integrated multimode interface.
Funding
National Key Research and Development Program of China (2018YFB1801804); National Natural Science Foundation of China (61675128, 61875124, 61935011).
Acknowledgements
The authors thank Center for Advanced Electronic Materials and Devices in Shanghai Jiao Tong University for the device fabrication.
Disclosures
The authors declare no conflicts of interest.
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