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Abstract
We propose an efficient and polarization-insensitive edge coupler (EC) constructed principally with two cascaded vertical waveguide tapers. The proposed edge coupler only requires ordinary 365 nm (i-line) ultraviolet source for lithography process. We experimentally demonstrate the proposed EC on two kinds of photonic integrated circuit (PIC) platforms: silicon nitride (Si3N4) and lithium niobate thin film. Both achieve polarization-insensitive fiber chip coupling efficiency of >70% in the C-band. Our proposed EC have the advantages of efficient, cost-saving, and easy to implement and could serve as an effective solution to facilitate low-loss chip-fiber coupling.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photonic integrated circuit (PIC) devices play a critical role for optical communication, sensing, and information processing because it enables compact, low-power-consumptive, highly sensitive, and low-cost multi-functional devices, such as modulators [1,2], switches [3], processors [4], sensor and so on. Widely applied PIC platforms include silicon on insulator (SOI), silicon nitride (Si3N4), thin film lithium niobate on insulator (LNOI) and so on. For PIC devices, an important issue kept to be solved or improved is the coupling loss between the PIC chip and fiber [5]. To date, various schemes have been proposed and implemented to minimize fiber-chip coupling loss, which can be mainly sorted into two categories: grating couplers (GCs) [6–11] and edge couplers (ECs) [12–23]. Grating couplers have the advantage of facilitating wafer-level on chip test, however, their operating bandwidth, polarization sensitivity, and coupling efficiency still need to be improved for better performances. Comparatively, ECs have the potential to provide broadband, polarization-insensitive, and low loss fiber-chip coupling, though their scale up to large package volumes is still challenging.
Basically, an EC is designed to reduce mode field mismatch between fiber and chip waveguide. In the conventional designs, ECs have been widely demonstrated with the structures of single [12–14], parallel [15] or cascaded [16,17] adiabatic lateral inverse waveguide tapers. In the structure of those lateral waveguide tapers, the width of waveguide is tapered down to a very small size at the tip end to make the guided mode less confined, and thus the mode field is spread into the cladding or a larger waveguide core, facilitating low-loss coupling with fiber at the chip facet. Such inverse lateral tapers have been widely adopted for SOI [12,13,15], silicon nitride [14] and LNOI [16,17] PIC devices. However, their coupling efficiencies heavily depend on the minimal width at taper end tip (typically < 200 nm). Such fine structure can only be formed with high-precise and expensive lithography process such as deep ultraviolet (DUV) lithography or electron beam lithography (EBL). For instances, the ECs reported in [16] and [17] relied on two or three steps of EBL process for the formation of taper tips. An alternative design to implement efficient ECs rely on the structure of subwavelength gratings (SWGs) [18–20]. In the SWGs, the period of the gratings should be not only smaller than the optical wavelength but also small enough to avoid Bragg diffraction, whose formations also rely on high-precise lithography process. Besides, there are some other reported ECs based on the vertical or three-dimensional waveguide tapers [21,22]. For the merits of cost-advantage and ease of fabrication, it’s valuable to explore EC solutions, which could be produced using fabrication procedures free from any EBL or DUV process. To this end, we have proposed a novel EC based on staircase structure [23]. However, its fabrication is somewhat complicated due to the requirement of multiple etchings.
In this paper, we propose an efficient and polarization-insensitive EC constructed principally with two cascaded vertical waveguide tapers. The proposed EC is feasible with conventional angled polishing process and ordinary i-line lithography, and hence, exempt the need of using expensive DUV or inefficient EBL. We experimentally demonstrate the proposed EC on the platforms of 300-nm-thick Si3N4 and 300-nm-thick lithium niobate thin film on insulator (LNOI). Both achieve polarization-insensitive power coupling efficiency of >70% in the C-band when coupling with high numerical aperture fiber (HNAF). Our proposed EC could benefit low-loss chip-fiber coupling
2. Design and methodology
Figure 1(a) illustrates the scheme diagram of the proposed EC and Fig. 1(b) shows its cross-section view and side view. The EC is composed of a vertical polymer waveguide taper of length Lt1, cascaded directly to a vertical thin film waveguide taper of length Lt2. The polymer waveguide, which has a core dimension of Wp × Hp at the chip facet, is coupled with fiber. The thin film waveguide made of Si3N4 or lithium niobate (LN) has a core dimension of Ws × Hs. The structure of cascaded vertical waveguide tapers allows adiabatic power evolution between the polymer waveguide and the Si3N4/LN thin film waveguide. Unlike those lateral waveguide tapers [11–16], whose formations heavily rely on DUV or EBL process, the vertical waveguide tapers used here can be fabricated just by a simple angled polishing process. Figure 1(c) shows the calculated mode field patterns supported in the polymer waveguide (Wp = 6.0 µm, Hp = 6.0 µm), the Si3N4/LN thin film waveguide (Ws = 1.0 µm, Hs = 0.3 µm), and the high numerical aperture fiber (HNAF) used here.
Fig. 1. Scheme diagram of the proposed edge coupler: (a) prospective view and (b) cross-section view and side view; (c) field patterns of modes supported in the waveguides and the HNAF.
The total power coupling loss of the proposed EC contains two parts: (1) the facet coupling loss between the polymer waveguide and the fiber; (2) the power evolution loss along the cascaded vertical waveguide tapers. The design of the EC begins by analyzing the facet coupling loss between the polymer waveguide and the fiber, which can be calculated using overlap integral between the guided modes concerned:
Fig. 2. Calculated facet coupling efficiency between the polymer waveguide and the HNAF: (a) TE polarization, (b) TM polarization.
Subsequently, we perform simulations with the eigen mode expansion (EME) method to analyze the mode propagating characteristics along the cascaded vertical waveguide tapers. In the simulation model, the lengths of the vertical waveguide tapers are set as Lt1 = 330 µm, and Lt2 = 160 µm, which are chosen according to our experimental results. At the same time, the thickness profile of the polished taper is also tailored according to our measured surface profile. The thickness of film waveguide (Si3N4/LN) was Hs = 0.3 µm, and the waveguide width was set as Ws = 1.0 µm. The thickness of the buried oxide (BOX) layer beneath the Si3N4/LNOI waveguide was set as ∼3 µm/4.4 µm. Mode power evolution efficiencies are simulated using different polymer waveguide dimensions (Wp × Hp), and the results for the Si3N4 and the LNOI waveguides are shown in Figs. 3(a)-(b) and Figs. 3(c)-(d), respectively. Totally, the mode power evolution efficiencies for either the Si3N4 or the LNOI waveguides decreased with the polymer waveguide size. For example, for both TE and TM polarizations, in the case of Wp × Hp set as 5 µm × 5 µm, the efficiency is more than 97%, however, in the case of Wp × Hp set as 9 µm × 7 µm, the efficiency is less than 70%. Especially, the height of the polymer waveguide poses a significant influence on the power evolution efficiency. To ensure an efficiency over 90% for both polarizations, the height of polymer (Hp) should be less than 6.5 µm. According to the simulated results of the facet coupling loss, the requirement for matching the MFD of the HNAF demands a polymer waveguide size within the range of 6.0 µm < Wp /Hp < 8.5 µm. Taking both facet coupling loss and power evolution loss into account, the final polymer waveguide design is chosen as 6 µm × 6 µm, which can provide polarization-insensitive facet coupling efficiency > 90% and power evolution efficiency > 95% at 1550 nm.
Fig. 3. Simulated mode power evolution efficiencies for (a) Si3N4-TE, (b) Si3N4-TM; (c) LNOI-TE, (d) LNOI-TM.
Using these parameters, the simulated mode field propagations of the TE and TM polarizations for the Si3N4 and the LNOI are shown in Figs. 4(a)-(d), respectively, which indicates efficient mode power evolution along the cascaded vertical waveguide tapers. The total coupling efficiency of the proposed EC in the C − band (1530 nm - 1570 nm) is then evaluated as >85.1% for the Si3N4 waveguide and >78.5% for the LNOI waveguide, indicating a broad operation bandwidth.
3. Experiment and measurement
Figure 5 shows the fabrication processes of the proposed EC. In the first step, the Si3N4/LNOI thin film waveguide was fabricated by lithography process and inductive coupled plasma (ICP) process. For the fabrication of the Si3N4 waveguide, the waveguide patterns defined on a mask was transferred to the photoresist, which served as dry-etching resist layer. Then the sample was dry-etched using ICP with CF4/Ar gas chemistry to form the rib waveguide. For the fabrication of the LNOI rib waveguide, a proton exchange-assisted dry-etching process was applied [24]. In the next step, an angled polishing process was implemented to form the thin film vertical waveguide taper, and also to form a sloping surface on the BOX layer alongside the thin film vertical taper. As shown in Step 2 of Fig. 4, we fixed the chip to a specially designed sample holder, which can press the chip edge against the polishing pad at a certain angle of θ (θ = arctan1/80). Posed by gravity, the chip edge region was partially removed during the polishing process, forming the desired sloping surface profile. In the third step, the polymer waveguide was defined and formed. As the used polymer EpoCore could serve as negative photoresist, the polymer waveguide could be directly formed by lithography process. During the whole fabrication process, ordinary i-line-ultraviolet exposure source could satisfy all the requirements and there is no need for any high-precise lithography process.
The evaluation of the fabricated EC begins by scanning the polished chip surface profile with a step-profiler (AlphaStep D-100) on a number of referenced chip samples. Figure 6 shows the scanned surface profile of one typical 300-nm-thick Si3N4 sample. From the scanned surface profile, the length of the vertical thin film waveguide taper was measured as Lt2 = ∼160 µm. Considering the silicon beneath the BOX layer may lead to power leaking for guided modes in polymer waveguide, hence 1-µm-thick BOX layer was preserved as the lower-cladding for polymer waveguides to provide sufficient insolation between the polymer waveguide and the silicon substrate. Then the length of the polymer waveguide taper was evaluated as Lt1 = ∼330 µm. We also evaluated the reproducibility of this angled polishing technique by measuring tens of samples formed with the same process. The measured values of Lt2 are within the range of 155 µm - 165 µm and values of Lt1 are within the range of 320 µm - 340 µm. The experiments tested on LNOI chips yield similar measured results.
A set of microscopic images showing one typical sample of fabricated ECs are illustrated in Fig. 7(a)-(d). Figure 7(b) shows the image of formed vertical thin film waveguide taper by angled polishing process before coating the polymer. Figure 7(d) shows the facets of final formed polymer waveguides. The dimension of polymer waveguide measured from the microscopic image was 6.1 µm × 6.3 µm and the thickness of BOX layer beneath the polymer waveguide was ∼1 µm, as shown in the inset of Fig. 7 (d).
Fig. 7. Microscopic images or pictures showing: (a) typical sample; (b) vertical thin film waveguide taper before coating the polymer; (c) formed edge couplers; (d) polymer waveguide facets.
Figure 8(a) illustrates solutions for measuring the fiber-chip coupling loss. The measured sample contains two parallel channel waveguides connected with a ring bent waveguide, and both the input and output ECs are formed at the same chip facet (left side). The HNAF arrays are used to align and couple light into/out from the chip through integrated ECs. The gap between the HNAF arrays and parallel channel waveguides was ∼127 µm. Light output from an amplified spontaneous emission (ASE) source was connected to input HNAF through polarizer and polarization controller, and the output light was collected and monitored with an optical spectrum meter (OSA, Anristu MS97740A). To extract the exact fiber-chip coupling efficiencies, we need to exclude the waveguide transmission loss and the bending loss. To this end, we fabricated a set of reference chips to derive the waveguide transmission loss and the bending loss (shown in Fig. 8(b)). The measured waveguide transmission losses are 0.5 dB/cm for the Si3N4 waveguide and 1 dB/cm for the LNOI waveguide and the bending loss is less than 0.1 dB/180° in the C-band. With these data, the final CEs are derived and shown in Fig. 9(a) and Fig. 9(b) for the Si3N4 and the LNOI waveguides, respectively. Over the C-band, the fabricated EC on Si3N4 chip achieved polarization-insensitive CE of ∼74.1% per facet, and that fabricated on LNOI chip achieved polarization-insensitive CE of ∼70.8% per facet.
Fig. 8. Schematic diagram showing: (a) the solutions for measuring CEs, (b) the reference chips for measuring the transmission loss and bending loss.
In view of the fact that the imperfection of the fabricated CE may lead to slight coupling between the TE and TM modes, we also investigated the cross talk between the two modes of the fabricated CE. To this end, the light from a tunable laser was first adjusted to the TE mode and then launched into the sample under test through the HNAF. The output light from the sample was collected with another HNAF and the powers of its TE and TM components were measured with a polarizer and a power meter, respectively. Then the cross talk is deduced and the results are less than −25 dB for both the LNOI and Si3N4 chips at the wavelength of 1550 nm. The measured cross talk here is mainly limited by the extinction ratio of available polarizer. The actual cross talks of our sample should be lesser.
At last, the return loss was also investigated by separating the returned light from the EC with a circulator and then measuring its power with the above OSA. The measured return loss is about –10 dB for both LNOI and Si3N4 chips over the C-band. The return loss could be reduced when the fiber is fixed to the polymer waveguide with the refractive-index-matched UV adhesive.
4. Discussion
The achieved experimental CEs agreed well with the simulated results (>85.1% for the Si3N4 waveguide, and >78.5% for the LNOI waveguide). Excess loss may originate from the absorption and scattering losses of the polymer waveguide. This problem can be alleviated by replacing polymer with low-loss material such as silicon oxynitride (SiON) (it should be pointed out that such replacement can also help to improve the light power capacity of the proposed EC). Besides, as the HNAF was fused to SMF-28 fiber, the connection between HNAF and SMF-28 fiber may lead to connection loss of 0.1 - 0.2 dB.
The tolerance of the alignment between the EC and the fiber is also evaluated and the results are shown in Figs. 10(a) and 10(b). For the TE/TM polarization, misalignment of 1 µm along the horizontal and vertical direction leads to reduction of facet coupling efficiency from ∼90% to 77%. The common method for enhancing the alignment tolerance is increasing the MFD of the fiber and the polymer waveguide. For example, as the size of the polymer waveguide is enlarged to 10 µm × 10 µm, the coupling efficiency with the ordinary SMF-28 fiber (MFD of ∼10.4 µm at 1550 nm) is evaluated as 83%, and under the circumstances, the misalignment of 1 µm only leads to reduction of facet coupling efficiency from 83% to 78%. In this case, the tolerance of alignment is significantly improved. However, as the MFD is increased, the lengths of the vertical waveguide tapers should also be enlarged to ensure the adiabaticity of mode evolution along the tapers. For our device, the lengths of vertical waveguide tapers are mainly decided by the tilted angle θ during the polishing process. By further reducing the tilted angle θ during the polishing process, the lengths of the cascaded waveguide tapers could be increased, so as to facilitate efficient and alignment-tolerant coupling with SMF-28 fiber.
Fig. 10. Dependence of the coupling efficiencies on fiber-chip misalignment at 1550 nm: (a) TE polarization; (b) TM polarization.
The mass production capacity of our proposed fabrication techniques was also evaluated. Firstly, we fabricated more than 500 integrated channel waveguides on a Si3N4 chip with a total width of 67 mm, as shown in Fig. 11(a). Then the key structure of the cascaded vertical waveguide tapers of our proposed EC was implemented on one side of the chip by using the angled polishing process. After that, we scanned and observed the formed vertical waveguide tapers with a microscope, and the captured image is shown in Fig. 11(b), which indicates that almost all the vertical waveguide tapers had been successfully produced. For practical production, we can slice the whole wafer into rows and then form the ECs row by row. Through this approach, our proposed technique still provides considerable production capacity.
Fig. 11. (a) Picture of fabricated chip for testing mass production capacity; (b) microscopic image showing formed vertical waveguide tapers by angled polishing process.
5. Conclusion
In this paper, we demonstrated an edge coupler constructed with cascaded vertical waveguide tapers by employing cost-effective fabrication procedures free of high-precise and expensive DUV or EBL process. The proposed edge coupler is experimentally demonstrated on the platforms of 300-nm-thick Si3N4 and 300-nm-thick lithium niobate thin film. We achieve polarization-insensitive coupling efficiency of ∼74% for Si3N4 chip, and of ∼71% for lithium niobate thin film chip. The demonstrated edge coupler together with proposed fabrication techniques could benefit photonic integrated circuit devices.
Funding
National Natural Science Foundation of China (62075027, U20A20165); Key Technology R&D Program of Shenzhen (JSGG20210802154413040); National Key Research and Development Program of China (2021YFB2800104); Key R&D Project of Science and Technology Department of Sichuan Province (2023YFS0122); Health Research Project for Cadres of Sichuan Province (2022-204.).
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. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]
2. C. Wang, M. Zhang, X. Chen, M. Bertrand, A. Shams-Ansari, S. Chandrasekhar, P. Winzer, and M. Lončar, “Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages,” Nature 562(7725), 101–104 (2018). [CrossRef]
3. B. G. Lee and N. Dupuis, “Silicon photonic switch fabric: technology and architecture,” J. Lightwave Technol. 37(1), 6–20 (2018). [CrossRef]
4. W. Bogaerts, D. Pérez, J. Capmany, D. A. B. Miller, J. Poon, D. Englund, F. Morichetti, and A. Melloni, “Programmable photonic circuits,” Nature 586(7828), 207–216 (2020). [CrossRef]
5. R. Marchetti, C. Lacava, L. Carroll, K. Gradkowski, and P. Minzioni, “Coupling strategies for silicon photonics integrated chips,” Photonics Res. 7(2), 201–239 (2019). [CrossRef]
6. F. Van Laere, T. Claes, J. Schrauwen, S. Scheerlinck, W. Bogaerts, D. Taillaert, L. O’Faolain, D. Van Thourhout, and R. Baets, “Compact focusing grating couplers for silicon-on-insulator integrated circuits,” IEEE Photonics Technol. Lett. 19(23), 1919–1921 (2007). [CrossRef]
7. H.-L. Tseng, E. Chen, H. Rong, and N. Na, “High-performance silicon-on-insulator grating coupler with completely vertical emission,” Opt. Express 23(19), 24433–24439 (2015). [CrossRef]
8. D. Benedikovic, C. Alonso-Ramos, D. P. Galacho, S. Guerber, V. Vakarin, G. Marcaud, X. Le Roux, E. Cassan, D. M. Morini, P. Cheben, F. Boeuf, C. Baudot, and L. Vivien, “L-shaped fiber-chip grating couplers with high directionality and low reflectivity fabricated with deep-UV lithography,” Opt. Lett. 42(17), 3439–3442 (2017). [CrossRef]
9. J. H. Song, F. E. Doany, A. K. Medhin, N. Dupuis, B. G. Lee, and F. R. Libsch, “Polarization-independent nonuniform grating couplers on silicon-on-insulator,” Opt. Lett. 40(17), 3941–3944 (2015). [CrossRef]
10. J. Zou, Y. Yu, and X. Zhang, “Single step etched two dimensional grating coupler based on the SOI platform,” Opt. Express 23(25), 32490–32496 (2015). [CrossRef]
11. Z. Chen, Y. Ning, and Y. Xun, “Chirped and apodized grating couplers on lithium niobate thin film,” Opt. Mater. Express 10(10), 2513–2521 (2020). [CrossRef]
12. B. B. Bakir, A. V. de Gyves, R. Orobtchouk, P. Lyan, C. Porzier, A. Roman, and J.-M. Fedeli, “Low-loss (<1 dB) and polarization-insensitive edge fiber couplers fabricated on 200-mm silicon-on-insulator wafers,” IEEE Photonics Technol. Lett. 22(11), 739–741 (2010). [CrossRef]
13. 14. J. V. Galán, P. Sanchis, G. Sánchez, and J. Martí, “Polarization insensitive low-loss coupling technique between SOI waveguides and high mode field diameter single-mode fibers,” Opt. Express 15(11), 7058–7065 (2007). [CrossRef]
14. Y. Liang, Z. Li, S. Fan, J. Feng, D. Liu, H. Liao, Z. Yang, J. Feng, and N. Cui, “Ultra-low loss SiN edge coupler interfacing with a single-mode fiber,” Opt. Lett. 47(18), 4786–4789 (2022). [CrossRef]
15. N. Hatori, T. Shimizu, M. Okano, M. Ishizaka, T. Yamamoto, Y. Urino, M. Mori, T. Nakamura, and Y. Arakawa, “A hybrid integrated light source on a silicon platform using a trident spot-size converter,” J. Lightwave Technol. 32(7), 1329–1336 (2014). [CrossRef]
16. C. Hu, A. Pan, T. Li, X. Wang, Y. Liu, S. Tao, C. Zeng, and J. Xia, “High-efficient coupler for thin-film lithium niobate waveguide devices,” Opt. Express 29(4), 5397–5406 (2021). [CrossRef]
17. P. Ying, H. Tan, J. Zhang, M. He, M. Xu, X. Liu, R. Ge, Y. Zhu, C. Liu, and X. Cai, “Low-loss edge-coupling thin-film lithium niobate modulator with an efficient phase shifter,” Opt. Lett. 46(6), 1478–1481 (2021). [CrossRef]
18. P. Cheben, J. H. Schmid, S. Wang, D. Xu, M. Vachon, S. Janz, J. Lapointe, Y. Painchaud, and M.-J. Picard, “Broadband polarization independent nanophotonic coupler for silicon waveguides with ultra-high efficiency,” Opt. Express 23(17), 22553–22563 (2015). [CrossRef]
19. P. Cheben, D. Xu, S. Janz, and A. Densmore, “Subwavelength waveguide grating for mode conversion and light coupling in integrated optics,” Opt. Express 14(11), 4695–4702 (2006). [CrossRef]
20. P. Cheben, P. J. Bock, J. H. Schmid, J. Lapointe, S. Janz, D. Xu, A. Densmore, A. Delâge, B. Lamontagne, and T. J. Hall, “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers,” Opt. Lett. 35(15), 2526–2528 (2010). [CrossRef]
21. T.-H. Chang, X. Zhou, H. Tamura, and C.-L. Hung, “Realization of efficient 3D tapered waveguide-to-fiber couplers on a nanophotonic circuit,” Opt. Express 30(18), 31643–31652 (2022). [CrossRef]
22. W. Zhang, M. Ebert, J. D. Reynolds, B. Chen, X. Yan, H. Du, M. Banakar, D. T. Tran, C. G. Littlejohns, G. T. Reed, and D. J. Thomson, “Buried 3D spot-size converters for silicon photonics,” Optica 8(8), 1102–1108 (2021). [CrossRef]
23. M. K. Wang, J. H. Li, H. Yao, Y. J. Long, F. Zhang, and K. X. Chen, “A cost-effective edge coupler with high polarization selectivity for thin film lithium niobate modulators,” J. Lightwave Technol. 40(4), 1105–1111 (2022). [CrossRef]
24. X. Li, K. Chen, and Z. Hu, “Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching,” Opt. Mater. Express 8(5), 1322–1327 (2018). [CrossRef]
