Open Access
Add to BibSonomyAbstract
A silicon arrayed waveguide grating (AWG) with low channel crosstalk was demonstrated by using ultra-short parabolic tapers to connect the AWG’s free propagation regions and single-mode waveguides. The tapers satisfied the requirements of low-loss mode conversion and lower channel crosstalk from the coupling of neighboring waveguides in the AWGs. In this work, three different tapers, including parabolic tapers, linear tapers, and exponential tapers, were theoretically analyzed and experimentally investigated for a comparison of their effects when implemented in AWGs. The experimental results showed that the AWG with parabolic tapers had a crosstalk improvement up to 7.1 dB compared with the others. Based on the advantages of parabolic tapers, a 400-GHz 8 × 8 cyclic AWG with 2.4 dB on-chip loss and −17.6~-25.1 dB crosstalk was fabricated using a simple one-step etching process. Its performance was comparable with that of existing AWGs with bi-level tapers, which require complicated two-step etching fabrication processes.
© 2014 Optical Society of America
1. Introduction
Silicon photonics has shown its vast potential on account of its advantages of low cost, electro-photonic integration, and compatibility with complementary metal oxide semiconductor (CMOS) processing technology. In the last ten years, various novel devices have been demonstrated on silicon-on-insulator (SOI) wafers, including lasers [1–4], modulators [5,6], optical switches [7,8], and filters [9–11], which make it promising to construct optical communication system with them. Among these significant advances, silicon photonic devices performing the function of wavelength division multiplexing (WDM) have also been widely investigated, owing to their enhancements of transmission capability. One such device, the arrayed waveguide grating (AWG), is one of the key devices in wavelength multiplexing/de-multiplexing [12]. Significant research studies of silicon AWGs based on SOI waveguides have been reported [13–21].
Because of the high refractive index contrast between the silicon core (n = 3.45) and the silica cladding (n = 1.45), silicon AWGs have a smaller waveguide bend radius and a more compact footprint compared with silica- and InP-based AWGs [22, 23]. However, silicon AWGs usually perform worse on crosstalk and insertion loss than AWGs based on low-index-contrast waveguides, due to the coupling between their single-mode waveguides and the reflection resulting from the mode mismatching of the single-mode waveguide and the free propagation region (FPR). To solve these problems, tapers are usually used to connect the single-mode waveguides and the FPRs. These tapers are expected to be long enough for low-loss mode convention and wide enough to reduce optical loss and widen the bandwidth of each wavelength channel. However, they may also cause the degradation of crosstalk arising from the narrow gap and long coupling distances between adjacent tapers. In previous works, linear tapers were effective in preventing optical losses and ineffective for reducing crosstalk [13,14,18,21]. Bi-level tapers have been successful in solving both problems of optical losses and crosstalk; however, they must be fabricated with two etching steps, which involves more complicated fabrication processes [15–17,19,20]. Therefore, an efficient and low-crosstalk tapers with low cost and simple fabrication processes is highly desired.
In this work, we demonstrate a low-crosstalk silicon AWG with ultra-short parabolic tapers connecting single-mode waveguides and FPRs, which requires only one-step etching. The principle of the ultra-short parabolic taper was introduced and its performance was evaluated by comparing three different types of tapers theoretically and experimentally. A series of one-step etching AWGs with different tapers were fabricated. The AWGs with parabolic tapers demonstrated good performance, exhibiting a 2.4 dB on-chip insertion loss and a −17.6~-25.1 dB crosstalk, similar to that of bi-level AWGs manufactured with complicated two-step etching processes [15–17,19,20].
2. Analysis and design
An AWG consists of two elements: planar waveguides (FPRs) and single-mode waveguides (including arrayed waveguides and input/output waveguides). Figure 1(b) illustrates half of an AWG structure with the FPR and waveguides. Tapers are installed at the edges of the FPR to reduce the mode mismatching between the planar waveguide and the single-mode waveguides. The tapers act as mode-size converters, expanding and gathering light into/from the FPR. Figure 1(a) shows the tapers between the FPR and the input/output waveguides, while Fig. 1(c) shows the tapers connected to the arrayed waveguides. Usually, longer tapers and narrower gaps can achieve high transmission efficiency and better frequency characteristics. However, they also increase the optical coupling between adjacent waveguides, which deteriorates the crosstalk. Therefore, short and independent tapers with high efficiency are required. In previous studies, parabolic tapers were used in AWGs for obtaining flat-top spectra and small footprint [24,25]. Here, we implement the parabolic tapers in silicon rib-waveguide AWGs, and further study their advantages on crosstalk reduction.
Fig. 1 Schematic diagram of tapers used in an AWG. (a) The tapers between input/output waveguides and FPR. (b) Half of the AWG structure. (c) The tapers between FPR and arrayed waveguides.
2.1 Principle of a parabolic taper
The ultra-short parabolic tapers are used to connect the FPR and the single-mode waveguides of AWGs. Figure 2(a) shows the top view of a parabolic taper. The lowest-order optical mode is confined by the tangent angle of its side wall, whose profile is determined by using Eq. (1) [26]:
where θ is the tangent angle corresponding to position z0 marked in Fig. 2(a), W is the width of the taper at z0, λ0 is the center wavelength in vacuum, neff is the effective index of the cross section at z0, and α is a constant. The parameter α is limited by the threshold of the tangent angle and the total taper length [26].Fig. 2 (a) Top view of a parabolic taper. (b) Transmission of a parabolic taper as a function of the taper length. For comparison, linear- and exponential-type tapers are included.
Using Eq. (1), we designed a silicon taper connecting a 500-nm-wide and a 2.0-μm-wide rib waveguide. For the devices studied in this work, the following parameters were selected as a common platform: a 340-nm-thick top silicon layer, 500-nm-wide single-mode waveguides with a 150-nm-high slab, and a 2.0 μm pitch width of neighboring waveguides. Different tapers with lengths varying from 1.0 μm to 6.0 μm were simulated using a 2.5D-FDTD simulation tool (Lumerical Mode Solutions). Tapers with profiles defined by linear and exponential functions, which are commonly used, were also simulated for comparison. In the simulations, the light was defined to propagate from the wide to the narrow waveguide. In Fig. 2(b), we can see that the mode conversion efficiency increases with increasing length and a 3.0-μm-long parabolic taper can achieve a transmission over 99.0%. To attain the same coupling efficiency, the other two types of taper should be longer than 6.0 μm.
2.2 Analysis of tapers in an AWG
Figure 3(a) shows the tapers between the single-mode waveguides and the FPR. Here, we consider that light is focused on the edge of the FPR and transferred to single-mode waveguide II through a taper for mode-size conversion. Using this model, we investigated the mode conversion loss (transmission from the FPR to waveguide II) and crosstalk (power received by waveguides I and III). The maximum width of the tapers was 1.9 μm, the pitch width of the tapers was 2.0 μm, and the gap was 100 nm. We performed the simulation using 2.9-μm-long parabolic tapers as well as 12-μm-long linear and 8-μm-long exponential tapers for comparison. The lengths of the three types were chosen to ensure that the mode-size conversion efficiency had a similar value of 99.5%. The line scans of the crosstalk of the three types of taper are shown in Fig. 3(b). The model determined a crosstalk of −33~-35 dB for parabolic tapers, which is much lower than that of the other taper types.
Fig. 3 (a) The tapers between the single-mode waveguides and the FPR. (b) Simulated results of crosstalk for the three types of taper.
In order to further verify the advantages of parabolic tapers, we simulated the transmission spectra of AWGs implementing the three different taper types. The AWGs in the simulation had a channel spacing of 400 GHz with 8 input/output ports and a 25.9 nm free spectrum region (FSR). Figure 4 displays the spectral responses of the 4th channel of these AWGs. The maximum crosstalk of AWGs using parabolic tapers, linear tapers, and exponential tapers was −33 dB, −29 dB, and −22.5 dB, respectively. The AWGs using parabolic tapers were verified to offer the advantage of an improved crosstalk.
Fig. 4 Simulated spectra of the 4th channel of AWGs with parabolic tapers, linear tapers, and exponential tapers.
3. Fabrication and measurements
3.1 Design and fabrication
To experimentally validate the characteristics of the parabolic tapers, a series of AWGs with different tapers were fabricated and tested. All devices were based on an SOI wafer with a 340-nm-thick top silicon layer and a 2-μm-thick buried oxide layer. They were fabricated with electron beam lithography (EBL) and inductively couple plasma (ICP) etching technology, as the one shown in Fig. 5(a). The scan electron microscope (SEM) pictures of the FPR and the parabolic tapers are shown in Figs. 5(b) and 5(c). The devices were fabricated with one-step etching, having rib waveguides with a slab height of 150 nm and a waveguide width of 500 nm. They were all cyclic 8 × 8 AWGs with a channel spacing of 400 GHz and their common parameters are shown in Table 1.
Fig. 5 Detailed images of the fabricated AWGs. (a) Microscope picture of an AWG. Scan electron microscope (SEM) pictures of: (b) the free propagation region, (c) the parabolic tapers.
Table 1. AWG parameters
3.2 Measurements and discussion
The characterization measurements of the AWGs were conducted with systems including a tunable laser (Santec TLS-510c), a polarization controller, fiber alignment stages (Suruga ES3700 with 50 nm alignment resolution), an optical spectrum analyzer (Yokogawa AQ6370c), and an optical power meter (Yokogawa AQ2200-221). The devices were measured with TE mode light. A pair of uniform grating couplers was used to receive/transmit the light from/into the 10 degree inclined fibers. The grating couplers had a period of 610 nm and a duty cycle of 60% with an average coupling efficiency of 40%.
The measured wavelength spectra are given in Fig. 6, in which the loss of grating couplers and optical measurement system is subtracted. Figure 6(a) shows the 8-channel spectra of an AWG with parabolic tapers, the minimum loss of output power is 2.4 dB, and the lowest crosstalk of the corresponding channel is about −25.1 dB. For each input port, the channel spacing is around 405 GHz and the crosstalk ranges from −17.6 dB to −25.1 dB. Since the fabrication error may influence the phase in arrayed waveguides, the crosstalk of the fabricated AWGs could not perform as well as the simulated results. The FSR of the cyclic AWG was measured to be 25.76 nm, which is close to the design value of 25.9 nm.
Fig. 6 (a -c) The measured spectra of 8 output ports of 400 GHz cyclic AWG with parabolic, linear, and exponential tapers. (d) The crosstalk values of AWGs with parabolic, linear, and exponential tapers. The discrete data points show the crosstalk values of each channel in AWGs and the solid lines represent the average crosstalk values of each channel.
In order to establish the advantages of parabolic tapers experimentally, three groups of AWGs with 2.9-μm-long parabolic tapers, 12-μm-long linear tapers, and 8-μm-long exponential tapers were fabricated in the same batch for comparison. Each group has five 8 × 8 AWGs with the same parameters and each AWG has only one type of taper. In measurements, we used every 4th input channel as the optical input port and measured all output channels of the AWGs in the three groups. The spectra of three typical AWGs are plotted in Figs. 6(a)-6(c). It can be found that the AWG with parabolic tapers has a lower side lobe than the other AWGs.
The crosstalk performance of each channel of the 15 AWGs belonging to the three groups is plotted in Fig. 6(d), in which the discrete points with different colors and shapes represent the crosstalk of different tapers. From Fig. 6(d), it can be seen that the lowest crosstalk record of each channel is from the AWGs with parabolic tapers. We calculated the average crosstalk values of each channel and presented the statistical results with solid lines drawn in the figure. The data shows that the average crosstalk values of AWGs with parabolic tapers have an improvement reaching its maximum 7.1 dB compared with those of the AWGs with linear and exponential tapers.
Further experiments were conducted using the three types of taper at the output waveguides of the same AWG to eliminate experimental errors from different AWGs. As shown in Fig. 7(a), three groups of tapers were placed in turn to connect the FPR and the output waveguides, each group consisting of only one type of taper. To avoid the influences from other groups, the transmission spectra of the middle ports in the three groups were measured and shown in Figs. 7(b)-7(d). In measurements, different input ports were used to ensure that the three measured spectra all have the same center wavelength. The maximum crosstalk of parabolic, linear, and exponential taper groups are −19.0 dB, −13.7 dB, and −13.2dB, respectively. The results indicated that the crosstalk performance was improved with parabolic tapers, as well as the fact that the use of linear and exponential tapers could enlarge the original side lobes.
Fig. 7 (a) The SEM picture of three taper groups placed on the edge of output FPR. Group 1 consists of three 2.9-μm-long parabolic tapers, group 2 consists of three 12-μm-long linear tapers, and group 3 consists of three 8.0-μm-long exponential tapers. (b -d) The spectra of the middle ports in group 1 −3.
4. Conclusion
We demonstrated low-crosstalk AWGs with ultra-short parabolic tapers which can be fabricated with a simple one-step etching process. The implementation of parabolic tapers as the connection element between single-mode waveguides and the FPR in an AWG was theoretically and experimentally proven to be effective in reducing the crosstalk caused by optical field coupling while maintaining a high mode converting efficiency. The experimental results showed that an AWG with parabolic tapers had an improvement reaching its maximum 7.1 dB on crosstalk compared with those using linear and exponential tapers. Owing to the novel design of the AWG tapers, a 400-GHz 8 × 8 cyclic AWG with a 2.4 dB minimum on-chip loss and −17.6~-25.1 dB crosstalk was demonstrated. The AWG, fabricated with a one-step etching process, showed good overall performance, comparable to that of bi-level AWGs fabricated by more complicated processes.
Acknowledgments
This work is supported by the National Basic Research program of China (Grant No. 2012CB933502).
References and links
1. A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14(20), 9203–9210 (2006). [CrossRef] [PubMed]
2. G. Roelkens, D. Van Thourhout, R. Baets, R. Nötzel, and M. Smit, “Laser emission and photodetection in an InP/InGaAsP layer integrated on and coupled to a Silicon-on-Insulator waveguide circuit,” Opt. Express 14(18), 8154–8159 (2006). [CrossRef] [PubMed]
3. T. Chu, N. Fujioka, and M. Ishizaka, “Compact, lower-power-consumption wavelength tunable laser fabricated with silicon photonic-wire waveguide micro-ring resonators,” Opt. Express 17(16), 14063–14068 (2009). [CrossRef] [PubMed]
4. S. Tanaka, S. H. Jeong, S. Sekiguchi, T. Kurahashi, Y. Tanaka, and K. Morito, “High-output-power, single-wavelength silicon hybrid laser using precise flip-chip bonding technology,” Opt. Express 20(27), 28057–28069 (2012). [CrossRef] [PubMed]
5. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]
6. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007). [CrossRef] [PubMed]
7. M. Yang, W. M. J. Green, S. Assefa, J. Van Campenhout, B. G. Lee, C. V. Jahnes, F. E. Doany, C. L. Schow, J. A. Kash, and Y. A. Vlasov, “Non-blocking 4x4 electro-optic silicon switch for on-chip photonic networks,” Opt. Express 19(1), 47–54 (2011). [CrossRef] [PubMed]
8. D. Sun, Z. Hu, S. A. Majid, R. Vandusen, Q. Zheng, I. Hasan, N. G. Tarr, S. Bidnyk, and T. J. Hall, “Limitation Factor Analysis for Silicon-on-Insulator Waveguide Mach–Zehnder Interference-Based Electro-Optic Switch,” J. Lightwave Technol. 29(17), 2592–2600 (2011). [CrossRef]
9. P. D. Trinh, S. Yegnanarayanan, F. Coppinger, and B. Jalali, “Silicon on-insulator (SOI) phased-array wavelength multi-demultiplexer with extremely low-polarization sensitivity,” IEEE Photon. Technol. Lett. 9(7), 940–942 (1997). [CrossRef]
10. P. Koonath, T. Indukuri, and B. Jalali, “Add–drop filters utilizing vertically-coupled micro-disk resonators in silicon,” Appl. Phys. Lett. 86(9), 091102 (2005). [CrossRef]
11. F. Horst, W. M. J. Green, S. Assefa, S. M. Shank, Y. A. Vlasov, and B. J. Offrein, “Cascaded Mach-Zehnder wavelength filters in silicon photonics for low loss and flat pass-band WDM (de-)multiplexing,” Opt. Express 21(10), 11652–11658 (2013). [CrossRef] [PubMed]
12. C. Dragone, “An N×N optical multiplexer using a planar arrangement of two star couplers,” IEEE Photon. Technol. Lett. 3(9), 812–815 (1991). [CrossRef]
13. X. Fu and D. Dai, “Ultra-small Si-nanowire-based 400 GHz-spacing 15×15 arrayed-waveguide grating router with micro bends,” IEEE Electron. Lett. 47(4), 266–268 (2011). [CrossRef]
14. K. Okamoto and K. Ishida, “Fabrication of silicon reflection-type arrayed-waveguide gratings with distributed Bragg reflectors,” Opt. Lett. 38(18), 3530–3533 (2013). [CrossRef] [PubMed]
15. S. Pathak, M. Vanslembrouck, P. Dumon, D. Van Thourhout, and W. Bogaerts, “Optimized silicon AWG with flattened spectral response using an MMI Aperture,” J. Lightwave Technol. 31(1), 87–93 (2013). [CrossRef]
16. D. Kim, J. Lee, J. Song, J. Pyo, and G. Kim, “Crosstalk reduction in a shallow-etched silicon nanowire AWG,” IEEE Photon. Technol. Lett. 20(19), 1615–1617 (2008). [CrossRef]
17. J. Wang, Z. Sheng, L. Li, A. Pang, A. Wu, W. Li, X. Wang, S. Zou, M. Qi, and F. Gan, “Low-loss and low-crosstalk 8 × 8 silicon nanowire AWG routers fabricated with CMOS technology,” Opt. Express 22(8), 9395–9403 (2014). [CrossRef] [PubMed]
18. H. Okayama, D. Shimura, H. Takahashi, M. Seki, M. Toyama, T. Sano, K. Koshino, N. Yokoyama, M. Ohtsuka, A. Sugiyama, S. Ishitsuka, T. Tsuchizawa, H. Nishi, K. Yamada, H. Yaegashi, T. Horikawa, and H. Sasaki, “Si wire array waveguide grating with parallel star coupler configuration fabricated by ArF excimer immersion lithography,” IEEE Electron. Lett. 49(6), 410–412 (2013). [CrossRef]
19. S. Pathak, M. Vanslembrouck, P. Dumon, D. V. Thourhout, P. Verheyen, G. Lepage, P. Absil, and W. Bogaerts, “Effect of mask discretization on performance of silicon arrayed waveguide gratings,” IEEE Photon. Technol. Lett. 26(7), 718–721 (2014). [CrossRef]
20. P. Cheben, J. H. Schmid, A. Delâge, A. Densmore, S. Janz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D. X. Xu, “A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with sub-micrometer aperture waveguides,” Opt. Express 15(5), 2299–2306 (2007). [CrossRef] [PubMed]
21. J. Zou, X. Xia, G. Chen, T. Lang, and J. J. He, “Birefringence compensated silicon nanowire arrayed waveguide grating for CWDM optical interconnects,” Opt. Lett. 39(7), 1834–1837 (2014). [CrossRef] [PubMed]
22. R. Adar, C. Henry, C. Dragone, R. Kistler, and M. Milbrodt, “Broad-band array multiplexers made with silica waveguides on silicon,” J. Lightwave Technol. 11(2), 212–219 (1993). [CrossRef]
23. H. Bissessur, F. Gaborit, B. Martin, P. Pagnod-Rossiaux, J. L. Peyre, and M. Renaud, “16 channel phased array wavelength demultiplexer on InP with low polarization sensitivity,” IEEE Electron. Lett. 30(4), 336–337 (1994). [CrossRef]
24. K. Okamoto and A. Sugita, “Flat spectral response arrayed-waveguide grating multiplexer with parabolic waveguide horns,” IEEE Electron. Lett. 32(18), 1661–1662 (1996). [CrossRef]
25. K. Sasaki, F. Ohno, A. Motegi, and T. Baba, “Arrayed waveguide of 70x60 µm2 size based on Si photonic wire waveguides,” IEEE Electron. Lett. 41(14), 801–802 (2005). [CrossRef]
26. Y. Fu, T. Ye, W. Tang, and T. Chu, “Efficient adiabatic silicon-on-insulator waveguide taper,” Photon. Res. 2(3), A41–A44 (2014). [CrossRef]
