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Abstract
We design and experimentally demonstrate an ultra-compact 1310/1550 nm wavelength diplexer based on a multimode interference (MMI) coupler. The proposed device is designed at the first imaging length for 1550 nm wavelength resulting in an MMI length of only 41 µm. In order to improve the extinction ratio, the output ports are made asymmetric in width. A low insertion loss (< 1dB) and high extinction ratio (> 20 dB) is measured at the two operating wavelengths. It also displays a wide 3-dB bandwidth of 100 nm centered around 1310 nm and 1550 nm wavelengths. Furthermore, an on-chip wavelength demultiplexing experiment carried out on the fabricated device, with a non-return-to-zero (NRZ) on-off keying (OOK) signal at 60 Gbit/s, shows clear eye diagrams for both the wavelengths.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wavelength-Division multiplexing (WDM) is an essential way to increase the information capacity by increasing the number of channels in optical telecommunication systems. For systems such as fiber to the home (FTTH), dual-wavelength (de)multiplexer, also called diplexer, is a critical optical building block, and simplifying it is of great interest. Therefore, much attention has been given to (de)multiplexing two wavelengths in the 1310 and 1550 nm windows [1]. Designing wavelength diplexers on a silicon-on-insulator (SOI) platform is attractive owing to its compatibility with complementary metal-oxide-semiconductor (CMOS) technology and the high integration density leading to low-cost and high-volume processing. Moreover, for on-chip optical interconnects, the silicon-based (de) multiplexers can increase the bandwidth, allowing to reach very high capacities using the WDM technology [2–4].
The SOI diplexers, which (de)multiplex O-band and C-band signals, have been extensively demonstrated using diffractive gratings [5,6], microring resonators [7], directional couplers [8–11], and multimode interference (MMI) couplers [12–16]. Diffractive grating couplers cannot work for on-chip interconnects and are limited only to fiber-to-chip optical exchange. Microring resonators have the advantage of filtering different wavelengths with a low insertion loss (IL) but suffer from narrow bandwidth and require additional temperature control. Directional couplers also have limited bandwidth and are very sensitive to fabrication errors.
An MMI-based diplexer solution stands out among SOI (de)multiplexers; it provides a relatively low insertion loss and broad bandwidth. Several groups have reported diplexers based on MMI couplers. However, the footprint of these devices is rather large and is in the order of hundreds of microns. To demultiplex two wavelengths into two output ports, the length of the MMI coupler is required to be a common multiple of the first self-imaging length for both the wavelengths, a requirement that increases the device size [17]. But for silicon photonics, compactness is one of the key features leading to high integration density. MMI diplexers based on ridge waveguides, slot waveguides, subwavelength structures, inverse design algorithms, and photonic crystals have been introduced to shrink the length. Nevertheless, the footprint achieved with ridge and slot waveguides is still not very compact [12], with a transmission recorded only in the narrowband. Also, some schemes (subwavelength, inverse design, and photonic crystals) reduce the reliability during fabrication when commercial 193 nm UV lithography is used [14,15,18,19]. Moreover, many of these MMIs lack experimental validation [12–14,19]. Finally, for passive FTTH networks, ITU-T G.983 recommends wide bandwidth for 1310 nm and 1550 nm wavelengths which is challenging to satisfy using previously reported devices [1].
Here, we experimentally demonstrate a fully CMOS compatible MMI-based diplexer with a device length of only 41 µm. We have previously reported the design concept, which is based on regular single etch strip waveguides with preliminary results [20]. The MMI is designed at the first imaging length for 1550 nm wavelength and optimized to demultiplex both 1310 nm and 1550 nm light with a high extinction ratio. A low insertion loss (< 1 dB), high extinction ratio (> 20 dB) and wide 3 dB bandwidth (100 nm) is measured for both the 1310 nm and 1550 nm wavelengths. An on-chip wavelength demultiplexing transmission experiment was also carried out with a non-return-to-zero (NRZ) on-off keying (OOK) signal modulated at 60 Gbit/s. The experimental results show clear eye diagrams for both channels. This is the first system demonstration of an on-chip MMI-based wavelength demultiplexer on an SOI platform to the best of our knowledge.
2. Device design and methodology
A schematic of the proposed device is shown in Fig. 1. The device consists of three parts: an input channel, an MMI coupler, and two S-bent output channels, in which one is the bar-port, and the other is cross-port. The waveguide design parameters used are as follows: the refractive index of silicon (Si) and oxide (SiO2) are 3.445 and 1.445, respectively. The diplexer is fabricated on an SOI platform with a 220 nm thick silicon layer, a 2.2 µm thick oxide cladding, and 2 µm thick box oxide.
The first step is to calculate beat length using an Eigenmode expansion solver (EME). Figure 2(a) shows the beat lengths for both 1550 nm and 1310 nm wavelengths as a function of the silicon waveguide width. In a conventional MMI, the interference between excited optical modes forms a multifold image of the input field at periodic propagation distances (self-imaging) [17]. N fold images are produced at the image plane (MMI length) at $L = 3p{L_\pi }/N$, $\,p = 0,1,2 \ldots .$ Here, ${L_\pi }$ is the beat length of the multimode region given by:
where ${\beta _0}$ and ${\beta _1}$ are the propagation constant of the two lowest-order propagating modes. At odd multiples of the ${L_\pi }$, an image of the input field is formed at an anti-symmetric position (mirror image). On the other hand, at even multiples of ${L_\pi }$, an image is formed at identical positions, called a single image. Since the propagation constant is wavelength dependent, self-images will be produced at different planes for different working wavelengths. To separate the 1310/1550 nm wavelengths, the MMI demultiplexer uses the difference between the beat lengths at these wavelengths. The MMI length (LMMI$)$ is adjusted to satisfy: where r is an integer.When the length of the MMI region satisfies Eq. (2), there will be a single and a mirror image formed for the two different wavelengths, i.e., the 1310 nm and 1550 nm bands, respectively ($r$ is even in this case). Therefore, to demultiplex, the MMI coupler's length is required to be the common multiple of the first self-imaging lengths for two wavelengths. To fulfill this requirement, the device length has to match several odd or even number of the beat lengths for both wavelengths, which increases the overall size.
To make the design compact, we start our analysis by considering an MMI with a length equal to the first beat length of 1550 nm wavelength for the respective waveguide width. If the MMI is designed with such a selection, it results in 1550 nm light correctly guiding towards the cross port, and negligible power will be recorded at the bar port. Even though the 1310 nm wavelength will be strongly focused on the bar port, there will be some power guiding towards the cross port, affecting the performance. For a demultiplexer, the most critical performance metrics are the insertion loss (IL) and the extinction ratio (ER), which are defined as:
where ${P_{in}}$ is the total input power ${P_1}$ and ${P_2}$ are the output power in the bar (cross) port and the cross (bar) port at 1310 nm (1550 nm).Figure 3 shows the simulated ER of an MMI designed with a first image length corresponding to 1550 nm wavelength. Here the input and output port widths are symmetric with a width of 950 nm to ensure most of the input power is carried by the first few-order modes of the MMI coupler with small modal phase errors. As expected from the image theory, the ER of 1550 nm light is much better compared to 1310 nm wavelength. From Fig. 3, the MMI width that results in an optimum ER for 1550 nm wavelength is 2.4 µm, which corresponds to a beat length of 41 µm. At this beat length, the 1310 nm light suffers from a low ER of 12 dB [see Fig. 3]. As shown in Fig. 5(a), ripples due to higher-order mode excitation can be seen for the 1310 nm wavelength as it exits the MMI at the output cross port, reducing the ER according to Eq. (4).
To improve the performance without increasing the MMI length, the cross-port width is reduced to filter out higher-order optical modes when O-band wavelength is injected. The optimum cross-port width is 700 nm which gives the best tradeoff between insertion loss and extinction ratio (Fig. 4). Additionally, S-bends are added to completely filter higher-order excitations. Figure 5 (b) shows the beam propagation of the final optimized device. The O-band and the C-band light are demultiplexed as desired. The O-band light is transmitted through the bar port while the C-band light is collected at the cross port. The simulated power transmission at the bar and the cross ports as a function of the wavelength bands from 1260-1360 nm and 1500-1600 nm are shown in Fig. 6. At the wavelength of 1310 nm, the IL and ER are 0.8 dB and 24 dB, respectively. At the wavelength of 1550 nm, the IL and ER are 0.81 dB and 41 dB, respectively. The ER of the 1550 nm band is better due to perfect imaging. Furthermore, the 3dB bandwidth covers 100 nm near center wavelengths of both O- and C-bands. The beam propagation and broadband simulations are performed using 3D FDTD solver.
Fig. 4. Simulated (a) extinction ratio and (b) transmission of the device as a function of cross port width.
Fig. 5. Simulated field evolution at 1550/1310 nm wavelengths (a) without higher order mode filtering (b) after optimization.
3. Device fabrication and characterization
The designed diplexer is fabricated using the NanoSOI fabrication process by Applied Nanotools Inc., based on direct-write 100 keV electron beam lithography technology. This process uses an SOI wafer with a 220 nm thick silicon layer, hydrogen silsesquioxane (HSQ) resist, and anisotropic ICP-RIE etch process with chlorine. A 2 µm oxide cladding was deposited using a plasma-enhanced chemical vapour deposition (PECVD) process based on tetraethyl orthosilicate (TEOS) at 300°C. Scanning electron microscope (SEM) micrographs of the fabricated device are shown in Fig. 7. Edge couplers are used to couple light to the Silicon chip using lensed fibers. Two tunable Keysight 8100B laser sources (C-band and O-band lasers) and Keysight N7744A optical detector sensors are used to characterize the optical transmission response. An external polarization controller is used to maintain TE-polarization. The measurements are shown in Fig. 8 after calibrating out the coupling loss of the edge couplers. The measured IL is around 0.85 dB for both 1310 nm and 1550 nm wavelengths. The ER of 1310 nm wavelength measures 23 dB, while the ER of 1550 nm is 30 dB. The discrepancy in ER for 1550 nm light from the simulations can be attributed to limited polarization extinction between the TE and TM mode of the input fibers. This can be improved with the use of grating couplers instead of edge couplers. A very wide 3 dB bandwidth is measured as seen from the two insets at the top of Fig. 8, which are magnified plots around 1310/1550 nm wavelengths. Both the ports have a bandwidth of 100 nm.
Table 1 shows the performance of our device compared to previously reported MMI-based (de) multiplexers. The demultiplexers based on slot and sandwiched silicon nitride waveguide are still relatively large and lacks experimental validation [12,13]. Among all the MMI-based diplexers, the subwavelength-based MMI demultiplexer has excellent theoretical performance and is relatively compact. However, reliability is a concern when fabricated with standard UV lithography [14]. The photonic crystal and strip waveguide (cascaded) based MMI have high insertion loss and a large footprint [15,16]. Finally, most reported devices have a large footprint and lack experimental validation as they involve complex non-standard fabrication processes. In comparison, our wavelength (de) multiplexer is compact and based on a standard SOI process with a conventional single step etch strip waveguide.
Table 1. Performance comparison on MMI demultiplexer
4. System demonstration
The fabricated chip was further employed for the on-chip wavelength demultiplexing experiment. Figure 9 shows the transmission test experimental setup. The C-band tunable laser source centered at 1550 nm is connected to a polarization controller. It is then modulated at 60 Gbit/s in the NRZ-OOK scheme with a known random binary pattern of 231-1 generated from Keysight 64 Gbaud pattern generator M8045A. The modulation is performed with Thorlabs LN05S 40 GHz intensity Mach-Zehnder modulator. The optical signal is then pre-amplified with a polarization-maintaining booster optical amplifier (Thorlabs S9FC1004P). The same configuration is implemented for the O-band tunable laser centered at 1310 nm, modulated with IxBlue Mx1300-LN-40 40GHz intensity modulator. The optical signal is pre-amplified with a polarization-maintaining booster optical amplifier (Thorlabs S9FC1132P). The measured eye diagrams for the pre-amplified 1550 nm and 1310 nm optical signals are shown in Fig. 10 (a) and 10 (b), respectively. Both signals are then multiplexed with a commercial fiber based WDM into one single fiber connected to a polarization controller and coupled to the proposed silicon diplexer (DUT) with a lensed fiber. The eye diagram of multiplexed 1550 nm/1310 nm is shown in Fig. 10(c). This diplexer acts as an on-chip demultiplexer with the 1550 nm signal routed to the cross-port and the 1310 nm wavelength to the bar-port. Each output of the DUT is amplified with a polarization-insensitive semiconductor optical amplifier (Thorlabs S7FC1013S) and a single-mode Praseodymium-doped fiber amplifier (PDFA100) at 1550 nm and 1310 nm wavelength, respectively. A Keysight Infinium DCA-X 86100D wide-bandwidth oscilloscope is used to capture the eye diagrams. As shown in Figs. 10(d) and 10(e), the corresponding demultiplexed signals exhibit clear eye diagrams, which confirms the high extinction ratio of the device.
Fig. 10. Measured eye-diagrams of the pre amplified optical signal (a) 1550 nm (b) 1310 nm. (c) Measured eye-diagram of multiplexed optical signal. Measured eye-diagrams of the demultiplexed signal at (d) cross-port (1550 nm) and (e) bar-port (1310 nm).
5. Conclusion
An ultra-compact (41 µm) 1310/1550 nm wavelength MMI-based diplexer is demonstrated on the SOI platform. The minimum critical dimension of the device is fully compatible with 193 nm UV lithography. A compact footprint is achieved by designing and optimizing the MMI at the first beat length of 1550 nm wavelength. Measurement shows a low IL (< 1 dB) and high ER (> 20 dB) for both the wavelengths. System experiments have been carried out for on-chip wavelength demultiplexing application at 60 Gbit/s, showing clear eye diagrams for the demultiplexed channels.
Acknowledgement
This research was in part performed by using the Core Technology Platform (CTP) resources at NYUAD. We thank Nikolas Giakoumidis for the technical support and helpful discussion. Simulations for this research were partially carried out on the High-Performance Computing resources at NYUAD. In this work, Zakriya Mohammed and Bruna Paredes have made an equal contribution.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. Y. Shi, J. Chen, and H. Xu, “Silicon-based on-chip diplexing/triplexing technologies and devices,” Sci. China Inf. Sci. 61(8), 080402 (2018). [CrossRef]
2. Y. Ding, J. Xu, F. D. Ros, B. Huang, H. Ou, and C. Peucheret, “On-chip two-mode division multiplexing using tapered directional coupler-based mode multiplexer and demultiplexer,” Opt. Express 21(8), 10376–10382 (2013). [CrossRef]
3. B. Paredes, Z. Mohammed, J. Villegas, and M. Rasras, “Dual-Band (O amp; C-Bands) Two-Mode Multiplexer on the SOI Platform,” IEEE Photonics J. 13(3), 1–9 (2021). [CrossRef]
4. B. G. Lee, X. Chen, A. Biberman, X. Liu, I.-W. Hsieh, C.-Y. Chou, J. I. Dadap, F. Xia, W. M. J. Green, L. Sekaric, Y. A. Vlasov, R. M. Osgood, and K. Bergman, “Ultrahigh-Bandwidth Silicon Photonic Nanowire Waveguides for On-Chip Networks,” IEEE Photonics Technol. Lett. 20(6), 398–400 (2008). [CrossRef]
5. G. Roelkens, D. V. Thourhout, and R. Baets, “Silicon-on-insulator ultra-compact duplexer based on a diffractive grating structure,” Opt. Express 15(16), 10091–10096 (2007). [CrossRef]
6. C. R. Doerr, L. Chen, M. S. Rasras, Y.-K. Chen, J. S. Weiner, and M. P. Earnshaw, “Diplexer With Integrated Filters and Photodetector in Ge–Si Using Γ−X and Γ−M Directions in a Grating Coupler,” IEEE Photonics Technol. Lett. 21(22), 1698–1700 (2009). [CrossRef]
7. L. Xu, Q. Li, N. Ophir, K. Padmaraju, L.-W. Luo, L. Chen, M. Lipson, and K. Bergman, “Colorless Optical Network Unit Based on Silicon Photonic Components for WDM PON,” IEEE Photonics Technol. Lett. 24(16), 1372–1374 (2012). [CrossRef]
8. J. Chen, L. Liu, and Y. Shi, “A Polarization-Insensitive Dual-Wavelength Multiplexer Based on Bent Directional Couplers,” IEEE Photonics Technol. Lett. 29(22), 1975–1978 (2017). [CrossRef]
9. J. Chen and Y. Shi, “An Ultracompact Silicon Triplexer Based on Cascaded Bent Directional Couplers,” J. Lightwave Technol. 35(23), 5260–5264 (2017). [CrossRef]
10. H. Xu and Y. Shi, “On-Chip Silicon Triplexer Based on Asymmetrical Directional Couplers,” IEEE Photonics Technol. Lett. 29(15), 1265–1268 (2017). [CrossRef]
11. Y. Shi, S. Anand, and S. He, “Design of a Polarization Insensitive Triplexer Using Directional Couplers Based on Submicron Silicon Rib Waveguides,” J. Lightwave Technol. 27(11), 1443–1447 (2009). [CrossRef]
12. J. Xiao, X. Liu, and X. Sun, “Design of an ultracompact MMI wavelength demultiplexer in slot waveguide structures,” Opt. Express 15(13), 8300–8308 (2007). [CrossRef]
13. Y. Shi, S. Anand, and S. He, “A Polarization-Insensitive 1310/1550-nm Demultiplexer Based on Sandwiched Multimode Interference Waveguides,” IEEE Photonics Technol. Lett. 19(22), 1789–1791 (2007). [CrossRef]
14. L. Liu, Q. Deng, and Z. Zhou, “An Ultra-Compact Wavelength Diplexer Engineered by Subwavelength Grating,” IEEE Photonics Technol. Lett. 29(22), 1927–1930 (2017). [CrossRef]
15. L. Xu, Y. Wang, D. Mao, E. El-Fiky, Z. Xing, A. Kumar, M. G. Saber, M. Jacques, and D. V. Plant, “Broadband 1310/1550 nm wavelength demultiplexer based on a multimode interference coupler with tapered internal photonic crystal for the silicon-on-insulator platform,” Opt. Lett. 44(7), 1770–1773 (2019). [CrossRef]
16. H. Yi, Y. Huang, X. Wang, and Z. Zhou, “Ultra-short silicon MMI duplexer,” in Nanophotonics and Micro/Nano Optics (SPIE, 2012), 8564, pp. 175–180.
17. L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: principles and applications,” J. Lightwave Technol. 13(4), 615–627 (1995). [CrossRef]
18. A. Y. Piggott, J. Lu, K. G. Lagoudakis, J. Petykiewicz, T. M. Babinec, and J. Vučković, “Inverse design and demonstration of a compact and broadband on-chip wavelength demultiplexer,” Nat. Photonics 9(6), 374–377 (2015). [CrossRef]
19. Y. Ma, P. Magill, T. Baehr-Jones, and M. Hochberg, “Design and optimization of a novel silicon-on-insulator wavelength diplexer,” Opt. Express 22(18), 21521–21528 (2014). [CrossRef]
20. Z. Mohammed, B. Paredes, J. Villegas, and M. Rasras, “An Ultra-Compact CMOS Compatible MMI based 1310/1550 nm Wavelength (de) Multiplexer,” in2021 European Conference on Optical Communication (ECOC) (2021), pp. 1–3.
