- Email: [email protected]
Design and fabrication of polymer-based 1 × 4 Y-branch splitters
Optik 127 (2016) 11427–11432
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Design and fabrication of polymer-based 1 × 4 Y-branch splitters Zhenlin Wu, Yuxin Liang, Chengkun Li, Xiuyou Han, Yiying Gu, Jingjing Hu, Mingshan Zhao ∗ Photonics Research Center, School of Physics and Optoelectronic Engineering, Dalian University of Technology, 116024 Dalian, PR China
a r t i c l e
i n f o
Article history: Received 15 July 2016 Accepted 14 September 2016 Keywords: Optical splitters Y-branch Polymer Fiber-waveguide coupling loss UV-based soft imprint lithography
a b s t r a c t A polymer-based Y-branch splitter chip has been designed and fabricated in this work. Fiber-waveguide coupling loss, propagation loss as well as bending loss are considered in the design to optimize the overall performance. A simple UV-based soft nanoimprint lithography (Soft UV-NIL) technique is adopted in the fabrication. Fluorinated acrylate resins produced by ChemOptics with different refractive indices are used. Residual layer and waveguide deformation are improved via controlling fabrication processes. An average 11.28 dB insertion loss is obtained for 1 × 4 splitters with a 1.73 dB uniformity, 0.06 dB polarization dependent loss (PDL). The validity of the polymer optical splitters fabricated through Soft UV-NIL technique is demonstrated by software simulation as well as experimental works. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction With the great development of optical telecommunication systems, low loss and high performance passive optical devices become increasingly attractive in the market. For routing and processing optical signals, optical power splitters are the key components in the system and the demand is expanding rapidly [1,2]. Among different splitter structures, Y-branch splitters are competitive candidates due to the advantages such as low excess loss, low wavelength dependent loss and low polarization loss [3]. Traditional materials for passive optical devices include silicon [4], silica [5] and III–V semiconductors [6]. However, polymer has drawn more attention recently. By operating on the molecular scale, unique properties can be achieved, such as low absorption loss, low cost, easy fabrication and high yields [7,8]. In addition, compared with the complicated fabrication processes adopted by the conventional semiconductor devices, simpler processes has been demonstrated for polymer-based devices, including UV photolithography [9], laser direct writing [10,11] and nanoimprint lithography (NIL) [12,13]. As a development of the NIL family, Soft UV-NIL has been investigated based on the UV curable polymer materials and its advantages such as easy fabrication, high yield and low cost has been widely reported [14,15]. In the process, as a flexible transparent mold is adopted instead of hard silicon mold, it can be applied on the flat surfaces as well as curved surfaces without trapping air bubbles [16,17]. Along with the progress of new polymer material synthesis, the application of soft UV-NIL is expanding to a large variety of emerging areas, like flexible electronics [18], photonics [19] and biotechnology [20].
∗ Corresponding author. E-mail address: [email protected] (M. Zhao). http://dx.doi.org/10.1016/j.ijleo.2016.09.065 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Calculated coupling efficiency between fiber mode and waveguide mode versus w/a.
Several polymer-based Y-branch splitters have been reported in the literature. Singhal et al. [3] has reported a 1 × 4 Y-branch splitter chip based on SU-8, fabricated by UV curing photolithography process. An average 20 dB insertion loss can be obtained with 7.4 dB fiber-waveguide coupling loss. Tang et al. [1] reported an optical power splitter using SU-8 and UV-15 as the core and cladding of the waveguide, respectively. The average insertion loss is measured as 27.80 dB which also includes a large fiber-waveguide coupling loss. It should be noticed that the fiber-waveguide coupling loss is one of the main sources of the insertion loss which has to be optimized. Small cross section size and large refractive contrast of the waveguide make its mode very different from the fiber mode which causes the large fiber-waveguide coupling loss. In this work, several waveguide parameters are taken into account to minimize fiber-waveguide coupling loss. The whole structure is also optimized through beam propagation method (BPM) which takes the bending loss and propagation loss into account at the same time. The fluorinated acrylate resin, LFR which is commercially available from ChemOptics is used and its refractive index can be changed from 1.375–1.395 at the wavelength of 1550 nm which is preferable for single-mode waveguide fabrication. The optical splitter chip is fabricated by UV-based soft imprint lithography (Soft UV-NIL) process which is also optimized to further decrease the insertion loss. 2. Design 2.1. Design of the waveguide Fiber-waveguide coupling loss which is an important source of the insertion loss is comprised of mode mismatch loss, reflection loss and alignment loss. The reflection loss and alignment loss can be reduced remarkably with index matching fluid and improved fiber-waveguide alignment. Therefore, the mode mismatch loss between the fiber and waveguide becomes the main source of the fiber-waveguide coupling loss. The mode mismatch loss is negative correlation to the power coupling efficiency between the fiber and waveguide modes:, which can be determined by calculating the overlap between the fiber and waveguide modes. After deduced, the power coupling efficiency can be expressed as Eq. (1) [21]:
=
2
4 w/a
(1) 2 2 [ w/a + ε] [ w/a + 1/ε] √ where w = wx wy is the geometric-mean mode width, and wx , wy are the 1/e intensity full width and depth of the waveguide mode, a is 1/e intensity full width of the fiber mode, and ε = wx /wy is the mode ratio. In this work, the designed waveguide is square, so ε = 1. The relationship between w/a and is shown in Fig. 1. As can be seen from Fig. 1, the coupling efficiency increases and then decreases as the value of w/a increases. The coupling efficiency reaches maximum value as w/a = 1. The geometric-mean mode width w is related to the cross section size of the waveguide and refractive index contrast for single-mode waveguide. Larger cross section and smaller refractive index contrast give larger geometric mean mode width w. The fiber used in this work is G.652D, which has a 9 m core diameter and a 10.5 m mode diameter for the wavelength 1550 nm. In order to obtain optimized structure, BPM is used to optimize the size and the refractive indices of both cladding and core layers. The refractive index of cladding layer is chosen to be the minimum of LFR, 1.375. Maximum refractive index of core layer for single mode waveguide, optimal refractive index of core layer and the corresponding optimal fiber-waveguide coupling loss found in different cross section sizes are illustrated in Table 1. The refractive index accuracy of LFR is 0.001, which is used for the calculation of waveguide structures. As Table 1 shows, the optimal fiber-waveguide coupling loss decreases as the cross section size grows. It can be found that the maximum refractive index of core layer for single mode waveguide and the optimal refractive index of core layer are the same for 6.5 × 6.5 m cross section size with the refractive index accuracy of 0.001. This suggests that the optimal coupling loss and
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Table 1 Structure parameters comparison between different cross section sizes. Cross section (m)
Maximum refractive index of core layer for single mode waveguide
Optimal refractive index of core layer
Fiber-waveguide coupling loss (dB)
4×4 5×5 6×6 6.5 × 6.5
1.398 1.389 1.385 1.383
1.385 1.383 1.383 1.383
0.168 0.101 0.048 0.031
Fig. 2. Schematic (a) proposed Y-branch splitter, (b) enlarged view of the Y-junction.
Fig. 3. The propagation loss, bending loss and total loss versus L of the first stage splitting. Inset: optimized length of the S-bend waveguide of the first stage splitter.
single mode condition are hardly achieved at the same time. It can also be noticed that optimal fiber-waveguide coupling losses of 6 × 6 m and 6.5 × 6.5 m are 0.048 and 0.031 dB, respectively, which is only 0.01 dB improvement. In addition, bending loss gets lager as the cross section size increases. A waveguide structure with 6 × 6 m cross section size and 1.375/1.383 for cladding/core layer indices can be obtained by considering the single mode condition together with the coupling loss. 2.2. Design of the whole structure The structure of a Y-branch splitter is comprised of a straight guide and a Y-junction followed by two S-bends which includes sine, cosine and double circle types. In this work, double circle S-bend is adopted instead of cosine S-bend for its compactness. In order to decrease the scattering loss and fabrication difficulty [22], a taper waveguide is added between the Y-junction and straight waveguide. The proposed Y-branch splitter structure and enlarged view of the Y-junction area are illustrated in Fig. 2(a) and (b). A 1 × 4 splitter is composed of two cascaded Y-branches. The standard separation between neighboring output ports of 1 × 4 optical splitter is 250 m. Then the separation between the two output ports of the first stage splitting is 500 m. As Fig. 2(a) shows, L, R are the horizontal length and radius of S-bend respectively. is the deviation angle of the output port from horizontal line. The structure design is adopted by taking the bending loss, propagation loss into account at the same time. As shown in Fig. 2(a), with a fixed separation between neighboring output ports of the first stage, as L increases, both the radius R and whole length of the splitter increase, accordingly, leading to a decreasing bending loss and an increasing propagation loss. The average propagation loss of a straight waveguide can be estimated by conventional cut-back measurement to be 1.3 dB/cm. As is a small value, the length of the S-bend is approximately equal to L. Therefore the propagation loss can be considered as 1.3 dB/cm × L. The relationship between bending loss and L is calculated through BPM. It can be seen from Fig. 3 that, the total loss including bending loss and propagation loss decreases and then increases as the L grows, at a certain point, 3500 m in this case, it reaches to the minimum value (see Fig. 3 inset). The optimal L of the second stage is also
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Fig. 4. BPM Simulation of optimal structure.
Fig. 5. The fabrication process of Soft UV-NIL.
optimized using the same method as 2860 m. The performance of 1 × 4 splitter and its output power are simulated using BMP method (Fig. 4). The calculated insertion loss is 6.3 dB with a 0.1 dB uniformity. 3. Fabrication The polymer splitter chip is fabricated using Soft UV-NIL and the process is illustrated in Fig. 5. PDMS mold is replicated from a silicon mold through a casting molding method and properly cured after baking at 60 ◦ C for 4 h. The silicon mold is made through plasma etching process. LFR with the lower refractive index is spin-coated on the silicon substrate as the under cladding layer. For better film uniformity, a two-step spin-coating process is applied: the first step is for a uniform polymer distribution on the substrate with a lower spinning speed, the second step is to control the thickness with a higher spinning speed. In order to prevent the optical field from leaking into the silicon substrate, the under-cladding layer is prepared as thick as 25 m. The thickness of the film is measured by prism coupler (SPA-4000). The prepared PDMS mold with patterns is then imprinted on the polymer film followed by a 1 min UV curing with 30 mW/cm2 intensity in a nitrogen atmosphere. The patterns can be duplicated on the polymer film with good conformity after the PDMS is removed. The residual layer is an inevitable issue for nanoimprint lithography technique and a thick residual layer will affect the performance of splitters. Several methods have been studied to remove or reduce the thickness of the residual layer such as anisotropic O2 plasma-etching and accurately controlling of the pressure and the dosage of the polymer [23] . Each method increases the difficulty and cost of the fabrication. In this work, an inverted waveguide is designed to address this issue. The grooves on the cladding layer which are from PDMS mold imprinting are filled with the LFR with higher refractive index as the core layer. The upper cladding layer is then spun on
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Fig. 6. The cross structure of the waveguide with no residual layer.
Fig. 7. (a) Microscopic image of waveguides, (b) microscopic image of the cross section of a waveguide, (c) microscopic image of Y-junction area.
top using the same procedure. The residual layer will be found on the top of the waveguide as Fig. 5(d) and (e) shows. It can be controlled to be under 0.5 m by the spinning speed during the core layer preparation. As Fig. 6 shows, the residual layer can be effectively minimized during the process. A 200 ◦ C hard baking (30 min) is then followed to cure the LFR materials. 4. Results and discussion The microscopic image of the fabricated waveguide is shown in Fig. 7(a) with a good uniformity along the device. During the imprint process, the deformation of the waveguide structure and the Y-junction area can be crucial in terms of the splitter performance [24]. As can be seen in Fig. 7(b) and (c), the shape of the waveguide and Y-junction are well fabricated through Soft UV-NIL. The input/output ports of the waveguides are prepared by cleaving the Si-wafer. The light is coupled between input/output ports and optical fibers by butt coupling. The index matching fluid is also used to reduce the reflection loss. Propagation loss of the straight waveguide is measured as 1.3 dB/cm by cut-back method. The fiber-waveguide coupling losses are then estimated to be about 1 dB at each port. The average insertion loss is measured as 11.28 dB with a uniformity of 1.73 dB, In addition, the average PDL is obtained as small as 0.06 dB due to the low birefringence of the polymer material. The theoretical insertion loss is calculated to be 6.3 dB. A 3 mm straight waveguide is left at each end of the device, allowing a clear cut of the silicon wafer. The whole length of the device including the straight waveguide is approximately 1.50 cm, which gives a total 1.95 dB of propagation loss (1.3 dB/cm). The coupling loss at each end is measured as 1 dB, which can be attributed to the unpolished surface of the cleaved polymer material. An extra 1 dB of insertion loss can be referred to radiation loss of the waveguide, especially at the Y-branch area. Therefore the overall estimated insertion loss is considered to be about 11.25 dB, which is consistent with the measured value of 11.28 dB. In addition, the large propagation loss of 1.3 dB/cm is attributed to the rough surface of the silicon mold, leading to a larger scattering loss of the waveguide. 5. Conclusion A polymer splitter chip is fabricated through a simple and low-cost UV based soft imprint lithography technique using optical polymer, LFR. The waveguide cross section is optimized to maintain the lowest coupling loss under the single mode condition. The structure of the whole splitter is optimized to minimize the insertion loss by considering the propagation loss and bending loss at the same time. In the fabrication process, the residual layer and deformation of the waveguide are well
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controlled. The average insertion loss of a 1 × 4 splitter is 11.28 dB with a uniformity of 1.73 dB. The average PDL is 0.06 dB due to the small birefringence of polymer material. Acknowledgments This work was supported in part by International Science & Technology Cooperation Program of China (No. 2014DFG32590), National High-Tech R&D Program (No. 2012AA040406), Natural Science Foundation of Liaoning Province (2014020002). References [1] X.G. Tang, J.K. Liao, H.P. Li, L. Zhang, R.G. Lu, Y.Z. Liu, A novel scheme for 1 × N optical power splitter, Opt. Express 18 (2010) 21697–21704. [2] Y. Sakamaki, T. Saida, T. Hashimoto, H. Takahashi, Low-loss Y-branch waveguides designed by wavefront matching method, J. Lightw. Technol. 27 (2009) 1128–1134. [3] R. Singhal, M.N. Satyanarayan, S. Pal, Effect of residual resist on performance of single-mode 1 × 4 optical splitter in photosensitive polymer, Fiber Integr. Opt. 29 (2010) 480–490. [4] L. Wang, J. An, Y. Wu, J. Zhang, Y. Wang, J. Li, H. Wang, X. Zhang, P. Pan, L. Zhang, H. Dai, R. Liu, F. Zhong, Q. Zha, X. Hu, D. Zhao, A compact and low-loss 1 × 8 optical power splitter using silica-based PLC on quartz substrate, Opt. Commun. 312 (2014) 203–209. [5] T. Saida, A. Himeno, M. Okuno, A. Sugita, K. Okamoto, Silica-based 2 × 2 multimode interference coupler with arbitrary power splitting ratio, Electron. Lett. 35 (1999) 2031–2033. [6] W. Astar, P. Apiratikul, T.E. Murphy, G.M. Carter, Wavelength conversion of 10-Gb/s RZ-OOK using filtered XPM in a passive GaAs–AlGaAs waveguide, IEEE Photonic Technol. Lett. 22 (2010) 637–639. [7] M.H. Lee, J.J. Ju, S. Park, J.Y. Do, S.K. Park, Polymer-based devices for optical communications, ETRI J. 24 (2002) 259–269. [8] S.H. Nam, J.W. Kang, J.J. Kim, Direct pattering of polymer optical waveguide using liquid state UV-curable polymer, Macromol. Res. 14 (2006) 114–116. [9] R. Singhal, M.N. Satyanarayan, S. Pal, Fabrication of single-mode Y-branch waveguides in photosensitive polymer with reduced Y-junction residue, Optik – Int. J. Light Electron Opt. 123 (2012) 1911–1914. [10] L. Eldada, L.W. Shacklette, Advances in polymer integrated optics, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 54–68. [11] S. Li, Q. Lin, G. Wu, L.H. Chen, X. Wu, Polymeric turbidity sensor fabricated by laser direct writing, Meas. Sci. Technol. 22 (2011) 7. [12] L.J. Guo, Nanoimprint lithography: methods and material requirements, Adv. Mater. 19 (2007) 495–513. [13] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Nanoimprint lithography, J. Vac. Sci. Technol. B 14 (1996) 4129–4133. [14] S.H. Ahn, L.J. Guo, Large-area roll-to-roll and roll-to-plate nanoimprint lithography: a step toward high-throughput application of continuous nanoimprinting, ACS Nano 3 (2009) 2304–2310. [15] Z. Zhuang, X. Guo, G.G. Zhang, B. Liu, R. Zhang, T. Zhi, T. Tao, H.X. Ge, F.F. Ren, Z.L. Xie, Y.D. Zheng, Large-scale fabrication and luminescence properties of GaN nanostructures by a soft UV-curing nanoimprint lithography, Nanotechnology 24 (2013) 7. [16] Y. Shen, L. Yao, Z. Li, J. Kou, Y. Cui, J. Bian, C. Yuan, H. Ge, W.D. Li, W. Wu, Y. Chen, Double transfer UV-curing nanoimprint lithography, Nanotechnology 24 (2013) 465304. [17] H.B. Lan, H.Z. Liu, UV-nanoimprint lithography: structure, materials and fabrication of flexible molds, J. Nanosci. Nanotechnol. 13 (2013) 3145–3172. [18] N. Kooy, K. Mohamed, L.T. Pin, O.S. Guan, A review of roll-to-roll nanoimprint lithography, Nanoscale Res. Lett. 9 (2014) 1–13. [19] J. Viheriälä, M.R. Viljanen, J. Kontio, T. Leinonen, J. Tommila, M. Dumitrescu, T. Niemi, M. Pessa, Soft stamp UV-nanoimprint lithography for fabrication of laser diodes, Proc. SPIE – Int. Soc. Opt. Eng. 8 (2009) 767–772. [20] L. Wang, V. Kodeck, S. Van Vlierberghe, J. Ren, J. Teng, X. Han, X. Jian, R. Baets, G. Morthier, M. Zhao, A low cost photonic biosensor built on a polymer platform, Opt. Sens. Biophotonics III 8311 (2011). [21] I.A. White, L.D. Hutcheson, J.J. Burke, End-fire coupling between optical fibers and diffused channel waveguides: comment, Appl. Opt. 18 (1979). [22] Q.A. Wang, S.L. He, L.R. Wang, A low-loss Y-branch with a multimode waveguide transition section, IEEE Photonics Technol Lett. 14 (2002) 1124–1126. [23] N. Bogdanski, M. Wissen, A. Ziegler, H.C. Scheer, Temperature-reduced nanoimprint lithography for thin and uniform residual layers, Microelectron. Eng. 78-79 (2005) 598–604. [24] S. Gilles, M. Diez, A. Offenhausser, M.C. Lensen, D. Mayer, Deformation of nanostructures on polymer molds during soft UV nanoimprint lithography, Nanotechnology 21 (2010) 245307.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Design and fabrication of polymer-based 1 × 4 Y-branch splitters Zhenlin Wu, Yuxin Liang, Chengkun Li, Xiuyou Han, Yiying Gu, Jingjing Hu, Mingshan Zhao ∗ Photonics Research Center, School of Physics and Optoelectronic Engineering, Dalian University of Technology, 116024 Dalian, PR China
a r t i c l e
i n f o
Article history: Received 15 July 2016 Accepted 14 September 2016 Keywords: Optical splitters Y-branch Polymer Fiber-waveguide coupling loss UV-based soft imprint lithography
a b s t r a c t A polymer-based Y-branch splitter chip has been designed and fabricated in this work. Fiber-waveguide coupling loss, propagation loss as well as bending loss are considered in the design to optimize the overall performance. A simple UV-based soft nanoimprint lithography (Soft UV-NIL) technique is adopted in the fabrication. Fluorinated acrylate resins produced by ChemOptics with different refractive indices are used. Residual layer and waveguide deformation are improved via controlling fabrication processes. An average 11.28 dB insertion loss is obtained for 1 × 4 splitters with a 1.73 dB uniformity, 0.06 dB polarization dependent loss (PDL). The validity of the polymer optical splitters fabricated through Soft UV-NIL technique is demonstrated by software simulation as well as experimental works. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction With the great development of optical telecommunication systems, low loss and high performance passive optical devices become increasingly attractive in the market. For routing and processing optical signals, optical power splitters are the key components in the system and the demand is expanding rapidly [1,2]. Among different splitter structures, Y-branch splitters are competitive candidates due to the advantages such as low excess loss, low wavelength dependent loss and low polarization loss [3]. Traditional materials for passive optical devices include silicon [4], silica [5] and III–V semiconductors [6]. However, polymer has drawn more attention recently. By operating on the molecular scale, unique properties can be achieved, such as low absorption loss, low cost, easy fabrication and high yields [7,8]. In addition, compared with the complicated fabrication processes adopted by the conventional semiconductor devices, simpler processes has been demonstrated for polymer-based devices, including UV photolithography [9], laser direct writing [10,11] and nanoimprint lithography (NIL) [12,13]. As a development of the NIL family, Soft UV-NIL has been investigated based on the UV curable polymer materials and its advantages such as easy fabrication, high yield and low cost has been widely reported [14,15]. In the process, as a flexible transparent mold is adopted instead of hard silicon mold, it can be applied on the flat surfaces as well as curved surfaces without trapping air bubbles [16,17]. Along with the progress of new polymer material synthesis, the application of soft UV-NIL is expanding to a large variety of emerging areas, like flexible electronics [18], photonics [19] and biotechnology [20].
∗ Corresponding author. E-mail address: [email protected] (M. Zhao). http://dx.doi.org/10.1016/j.ijleo.2016.09.065 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Fig. 1. Calculated coupling efficiency between fiber mode and waveguide mode versus w/a.
Several polymer-based Y-branch splitters have been reported in the literature. Singhal et al. [3] has reported a 1 × 4 Y-branch splitter chip based on SU-8, fabricated by UV curing photolithography process. An average 20 dB insertion loss can be obtained with 7.4 dB fiber-waveguide coupling loss. Tang et al. [1] reported an optical power splitter using SU-8 and UV-15 as the core and cladding of the waveguide, respectively. The average insertion loss is measured as 27.80 dB which also includes a large fiber-waveguide coupling loss. It should be noticed that the fiber-waveguide coupling loss is one of the main sources of the insertion loss which has to be optimized. Small cross section size and large refractive contrast of the waveguide make its mode very different from the fiber mode which causes the large fiber-waveguide coupling loss. In this work, several waveguide parameters are taken into account to minimize fiber-waveguide coupling loss. The whole structure is also optimized through beam propagation method (BPM) which takes the bending loss and propagation loss into account at the same time. The fluorinated acrylate resin, LFR which is commercially available from ChemOptics is used and its refractive index can be changed from 1.375–1.395 at the wavelength of 1550 nm which is preferable for single-mode waveguide fabrication. The optical splitter chip is fabricated by UV-based soft imprint lithography (Soft UV-NIL) process which is also optimized to further decrease the insertion loss. 2. Design 2.1. Design of the waveguide Fiber-waveguide coupling loss which is an important source of the insertion loss is comprised of mode mismatch loss, reflection loss and alignment loss. The reflection loss and alignment loss can be reduced remarkably with index matching fluid and improved fiber-waveguide alignment. Therefore, the mode mismatch loss between the fiber and waveguide becomes the main source of the fiber-waveguide coupling loss. The mode mismatch loss is negative correlation to the power coupling efficiency between the fiber and waveguide modes:, which can be determined by calculating the overlap between the fiber and waveguide modes. After deduced, the power coupling efficiency can be expressed as Eq. (1) [21]:
=
2
4 w/a
(1) 2 2 [ w/a + ε] [ w/a + 1/ε] √ where w = wx wy is the geometric-mean mode width, and wx , wy are the 1/e intensity full width and depth of the waveguide mode, a is 1/e intensity full width of the fiber mode, and ε = wx /wy is the mode ratio. In this work, the designed waveguide is square, so ε = 1. The relationship between w/a and is shown in Fig. 1. As can be seen from Fig. 1, the coupling efficiency increases and then decreases as the value of w/a increases. The coupling efficiency reaches maximum value as w/a = 1. The geometric-mean mode width w is related to the cross section size of the waveguide and refractive index contrast for single-mode waveguide. Larger cross section and smaller refractive index contrast give larger geometric mean mode width w. The fiber used in this work is G.652D, which has a 9 m core diameter and a 10.5 m mode diameter for the wavelength 1550 nm. In order to obtain optimized structure, BPM is used to optimize the size and the refractive indices of both cladding and core layers. The refractive index of cladding layer is chosen to be the minimum of LFR, 1.375. Maximum refractive index of core layer for single mode waveguide, optimal refractive index of core layer and the corresponding optimal fiber-waveguide coupling loss found in different cross section sizes are illustrated in Table 1. The refractive index accuracy of LFR is 0.001, which is used for the calculation of waveguide structures. As Table 1 shows, the optimal fiber-waveguide coupling loss decreases as the cross section size grows. It can be found that the maximum refractive index of core layer for single mode waveguide and the optimal refractive index of core layer are the same for 6.5 × 6.5 m cross section size with the refractive index accuracy of 0.001. This suggests that the optimal coupling loss and
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Table 1 Structure parameters comparison between different cross section sizes. Cross section (m)
Maximum refractive index of core layer for single mode waveguide
Optimal refractive index of core layer
Fiber-waveguide coupling loss (dB)
4×4 5×5 6×6 6.5 × 6.5
1.398 1.389 1.385 1.383
1.385 1.383 1.383 1.383
0.168 0.101 0.048 0.031
Fig. 2. Schematic (a) proposed Y-branch splitter, (b) enlarged view of the Y-junction.
Fig. 3. The propagation loss, bending loss and total loss versus L of the first stage splitting. Inset: optimized length of the S-bend waveguide of the first stage splitter.
single mode condition are hardly achieved at the same time. It can also be noticed that optimal fiber-waveguide coupling losses of 6 × 6 m and 6.5 × 6.5 m are 0.048 and 0.031 dB, respectively, which is only 0.01 dB improvement. In addition, bending loss gets lager as the cross section size increases. A waveguide structure with 6 × 6 m cross section size and 1.375/1.383 for cladding/core layer indices can be obtained by considering the single mode condition together with the coupling loss. 2.2. Design of the whole structure The structure of a Y-branch splitter is comprised of a straight guide and a Y-junction followed by two S-bends which includes sine, cosine and double circle types. In this work, double circle S-bend is adopted instead of cosine S-bend for its compactness. In order to decrease the scattering loss and fabrication difficulty [22], a taper waveguide is added between the Y-junction and straight waveguide. The proposed Y-branch splitter structure and enlarged view of the Y-junction area are illustrated in Fig. 2(a) and (b). A 1 × 4 splitter is composed of two cascaded Y-branches. The standard separation between neighboring output ports of 1 × 4 optical splitter is 250 m. Then the separation between the two output ports of the first stage splitting is 500 m. As Fig. 2(a) shows, L, R are the horizontal length and radius of S-bend respectively. is the deviation angle of the output port from horizontal line. The structure design is adopted by taking the bending loss, propagation loss into account at the same time. As shown in Fig. 2(a), with a fixed separation between neighboring output ports of the first stage, as L increases, both the radius R and whole length of the splitter increase, accordingly, leading to a decreasing bending loss and an increasing propagation loss. The average propagation loss of a straight waveguide can be estimated by conventional cut-back measurement to be 1.3 dB/cm. As is a small value, the length of the S-bend is approximately equal to L. Therefore the propagation loss can be considered as 1.3 dB/cm × L. The relationship between bending loss and L is calculated through BPM. It can be seen from Fig. 3 that, the total loss including bending loss and propagation loss decreases and then increases as the L grows, at a certain point, 3500 m in this case, it reaches to the minimum value (see Fig. 3 inset). The optimal L of the second stage is also
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Fig. 4. BPM Simulation of optimal structure.
Fig. 5. The fabrication process of Soft UV-NIL.
optimized using the same method as 2860 m. The performance of 1 × 4 splitter and its output power are simulated using BMP method (Fig. 4). The calculated insertion loss is 6.3 dB with a 0.1 dB uniformity. 3. Fabrication The polymer splitter chip is fabricated using Soft UV-NIL and the process is illustrated in Fig. 5. PDMS mold is replicated from a silicon mold through a casting molding method and properly cured after baking at 60 ◦ C for 4 h. The silicon mold is made through plasma etching process. LFR with the lower refractive index is spin-coated on the silicon substrate as the under cladding layer. For better film uniformity, a two-step spin-coating process is applied: the first step is for a uniform polymer distribution on the substrate with a lower spinning speed, the second step is to control the thickness with a higher spinning speed. In order to prevent the optical field from leaking into the silicon substrate, the under-cladding layer is prepared as thick as 25 m. The thickness of the film is measured by prism coupler (SPA-4000). The prepared PDMS mold with patterns is then imprinted on the polymer film followed by a 1 min UV curing with 30 mW/cm2 intensity in a nitrogen atmosphere. The patterns can be duplicated on the polymer film with good conformity after the PDMS is removed. The residual layer is an inevitable issue for nanoimprint lithography technique and a thick residual layer will affect the performance of splitters. Several methods have been studied to remove or reduce the thickness of the residual layer such as anisotropic O2 plasma-etching and accurately controlling of the pressure and the dosage of the polymer [23] . Each method increases the difficulty and cost of the fabrication. In this work, an inverted waveguide is designed to address this issue. The grooves on the cladding layer which are from PDMS mold imprinting are filled with the LFR with higher refractive index as the core layer. The upper cladding layer is then spun on
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Fig. 6. The cross structure of the waveguide with no residual layer.
Fig. 7. (a) Microscopic image of waveguides, (b) microscopic image of the cross section of a waveguide, (c) microscopic image of Y-junction area.
top using the same procedure. The residual layer will be found on the top of the waveguide as Fig. 5(d) and (e) shows. It can be controlled to be under 0.5 m by the spinning speed during the core layer preparation. As Fig. 6 shows, the residual layer can be effectively minimized during the process. A 200 ◦ C hard baking (30 min) is then followed to cure the LFR materials. 4. Results and discussion The microscopic image of the fabricated waveguide is shown in Fig. 7(a) with a good uniformity along the device. During the imprint process, the deformation of the waveguide structure and the Y-junction area can be crucial in terms of the splitter performance [24]. As can be seen in Fig. 7(b) and (c), the shape of the waveguide and Y-junction are well fabricated through Soft UV-NIL. The input/output ports of the waveguides are prepared by cleaving the Si-wafer. The light is coupled between input/output ports and optical fibers by butt coupling. The index matching fluid is also used to reduce the reflection loss. Propagation loss of the straight waveguide is measured as 1.3 dB/cm by cut-back method. The fiber-waveguide coupling losses are then estimated to be about 1 dB at each port. The average insertion loss is measured as 11.28 dB with a uniformity of 1.73 dB, In addition, the average PDL is obtained as small as 0.06 dB due to the low birefringence of the polymer material. The theoretical insertion loss is calculated to be 6.3 dB. A 3 mm straight waveguide is left at each end of the device, allowing a clear cut of the silicon wafer. The whole length of the device including the straight waveguide is approximately 1.50 cm, which gives a total 1.95 dB of propagation loss (1.3 dB/cm). The coupling loss at each end is measured as 1 dB, which can be attributed to the unpolished surface of the cleaved polymer material. An extra 1 dB of insertion loss can be referred to radiation loss of the waveguide, especially at the Y-branch area. Therefore the overall estimated insertion loss is considered to be about 11.25 dB, which is consistent with the measured value of 11.28 dB. In addition, the large propagation loss of 1.3 dB/cm is attributed to the rough surface of the silicon mold, leading to a larger scattering loss of the waveguide. 5. Conclusion A polymer splitter chip is fabricated through a simple and low-cost UV based soft imprint lithography technique using optical polymer, LFR. The waveguide cross section is optimized to maintain the lowest coupling loss under the single mode condition. The structure of the whole splitter is optimized to minimize the insertion loss by considering the propagation loss and bending loss at the same time. In the fabrication process, the residual layer and deformation of the waveguide are well
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controlled. The average insertion loss of a 1 × 4 splitter is 11.28 dB with a uniformity of 1.73 dB. The average PDL is 0.06 dB due to the small birefringence of polymer material. Acknowledgments This work was supported in part by International Science & Technology Cooperation Program of China (No. 2014DFG32590), National High-Tech R&D Program (No. 2012AA040406), Natural Science Foundation of Liaoning Province (2014020002). References [1] X.G. Tang, J.K. Liao, H.P. Li, L. Zhang, R.G. Lu, Y.Z. Liu, A novel scheme for 1 × N optical power splitter, Opt. Express 18 (2010) 21697–21704. [2] Y. Sakamaki, T. Saida, T. Hashimoto, H. Takahashi, Low-loss Y-branch waveguides designed by wavefront matching method, J. Lightw. Technol. 27 (2009) 1128–1134. [3] R. Singhal, M.N. Satyanarayan, S. Pal, Effect of residual resist on performance of single-mode 1 × 4 optical splitter in photosensitive polymer, Fiber Integr. Opt. 29 (2010) 480–490. [4] L. Wang, J. An, Y. Wu, J. Zhang, Y. Wang, J. Li, H. Wang, X. Zhang, P. Pan, L. Zhang, H. Dai, R. Liu, F. Zhong, Q. Zha, X. Hu, D. Zhao, A compact and low-loss 1 × 8 optical power splitter using silica-based PLC on quartz substrate, Opt. Commun. 312 (2014) 203–209. [5] T. Saida, A. Himeno, M. Okuno, A. Sugita, K. Okamoto, Silica-based 2 × 2 multimode interference coupler with arbitrary power splitting ratio, Electron. Lett. 35 (1999) 2031–2033. [6] W. Astar, P. Apiratikul, T.E. Murphy, G.M. Carter, Wavelength conversion of 10-Gb/s RZ-OOK using filtered XPM in a passive GaAs–AlGaAs waveguide, IEEE Photonic Technol. Lett. 22 (2010) 637–639. [7] M.H. Lee, J.J. Ju, S. Park, J.Y. Do, S.K. Park, Polymer-based devices for optical communications, ETRI J. 24 (2002) 259–269. [8] S.H. Nam, J.W. Kang, J.J. Kim, Direct pattering of polymer optical waveguide using liquid state UV-curable polymer, Macromol. Res. 14 (2006) 114–116. [9] R. Singhal, M.N. Satyanarayan, S. Pal, Fabrication of single-mode Y-branch waveguides in photosensitive polymer with reduced Y-junction residue, Optik – Int. J. Light Electron Opt. 123 (2012) 1911–1914. [10] L. Eldada, L.W. Shacklette, Advances in polymer integrated optics, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 54–68. [11] S. Li, Q. Lin, G. Wu, L.H. Chen, X. Wu, Polymeric turbidity sensor fabricated by laser direct writing, Meas. Sci. Technol. 22 (2011) 7. [12] L.J. Guo, Nanoimprint lithography: methods and material requirements, Adv. Mater. 19 (2007) 495–513. [13] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Nanoimprint lithography, J. Vac. Sci. Technol. B 14 (1996) 4129–4133. [14] S.H. Ahn, L.J. Guo, Large-area roll-to-roll and roll-to-plate nanoimprint lithography: a step toward high-throughput application of continuous nanoimprinting, ACS Nano 3 (2009) 2304–2310. [15] Z. Zhuang, X. Guo, G.G. Zhang, B. Liu, R. Zhang, T. Zhi, T. Tao, H.X. Ge, F.F. Ren, Z.L. Xie, Y.D. Zheng, Large-scale fabrication and luminescence properties of GaN nanostructures by a soft UV-curing nanoimprint lithography, Nanotechnology 24 (2013) 7. [16] Y. Shen, L. Yao, Z. Li, J. Kou, Y. Cui, J. Bian, C. Yuan, H. Ge, W.D. Li, W. Wu, Y. Chen, Double transfer UV-curing nanoimprint lithography, Nanotechnology 24 (2013) 465304. [17] H.B. Lan, H.Z. Liu, UV-nanoimprint lithography: structure, materials and fabrication of flexible molds, J. Nanosci. Nanotechnol. 13 (2013) 3145–3172. [18] N. Kooy, K. Mohamed, L.T. Pin, O.S. Guan, A review of roll-to-roll nanoimprint lithography, Nanoscale Res. Lett. 9 (2014) 1–13. [19] J. Viheriälä, M.R. Viljanen, J. Kontio, T. Leinonen, J. Tommila, M. Dumitrescu, T. Niemi, M. Pessa, Soft stamp UV-nanoimprint lithography for fabrication of laser diodes, Proc. SPIE – Int. Soc. Opt. Eng. 8 (2009) 767–772. [20] L. Wang, V. Kodeck, S. Van Vlierberghe, J. Ren, J. Teng, X. Han, X. Jian, R. Baets, G. Morthier, M. Zhao, A low cost photonic biosensor built on a polymer platform, Opt. Sens. Biophotonics III 8311 (2011). [21] I.A. White, L.D. Hutcheson, J.J. Burke, End-fire coupling between optical fibers and diffused channel waveguides: comment, Appl. Opt. 18 (1979). [22] Q.A. Wang, S.L. He, L.R. Wang, A low-loss Y-branch with a multimode waveguide transition section, IEEE Photonics Technol Lett. 14 (2002) 1124–1126. [23] N. Bogdanski, M. Wissen, A. Ziegler, H.C. Scheer, Temperature-reduced nanoimprint lithography for thin and uniform residual layers, Microelectron. Eng. 78-79 (2005) 598–604. [24] S. Gilles, M. Diez, A. Offenhausser, M.C. Lensen, D. Mayer, Deformation of nanostructures on polymer molds during soft UV nanoimprint lithography, Nanotechnology 21 (2010) 245307.