Open Access
Add to BibSonomyAbstract
This paper describes a procedure to replicate a polymeric wavelength filter. In this work, the grating structure on a polymer is fabricated first using holographic interferometry and micro-molding processes. The polymeric wavelength filters are produced by a two-step molding process where the master mold is first formed on a negative tone photoresist and subsequently transferred to a PDMS mold; following this step, the PDMS silicon rubber mold was used as a stamp to transfer the pattern of the polymeric wavelength filters onto a UV cure epoxy. Initial results show good pattern transfer in physical shape. At the Bragg wavelength, a transmission dip of -15.5 dB relative to the -3dB background insertion loss and a 3-dB-transmission bandwidth of ∼ 6nm were obtained from the device.
©2007 Optical Society of America
1. Introduction
Optical grating filters in guided-wave optics have been extensively investigated [1–7], because they are essential for applications in wavelength division multiplexing systems. When surface-relief gratings are inscribed on waveguides, the grating-waveguides can act as filters to select particular signals from many arriving signals. The desired characteristics of the filter can be achieved by the selection of parameters of the waveguide and the grating. For optical filter applications, high-resolution and high aspect ratio grating fabrication is important because they impact the filtering characteristics and compact size of the devices.
Polymeric optical devices are widely used in guided-wave optics owing to their low cost and simple fabrication process. Realization of Bragg gratings in polymer waveguides has attracted much attention in optical communications and optical sensing systems. Typical techniques for patterning gratings on polymer films include holographic lithography [8–10], electron-beam (e-beam) lithography [11], laser beam direct writing [12], and phase mask lithography [13–14]. However, few researchers have focused on fabricating surface-relief grating on channel waveguides. It has recently been shown that surface-relief gratings can be simply transferred to polymer waveguides by O2 reactive ion etching using azobenzene polymers as the etching mask. But, for these techniques surface scattering loss is often induced due to the surface roughness caused by the physical etching process, and high aspect ratio of the grating patterns is not easy to be obtained by this process [15–16]. The electron-beam direct-writing method has been used to inscribe the polymeric ridge waveguide with a corrugated sidewall Bragg grating [17]. This design is superior to the conventional buried grating for controlling the effective index modulation. It also showed good transmission dip for very short grating length. However, the core size must be very small to have the single mode condition, since the core index of their waveguide is much larger than the surrounding cladding index. This condition will cause the coupling difficulty between the waveguides and ordinary fibers. Ahn et al fabricated Bragg grating filters using the nanoimprint technique [18]. In their approach, they fabricated a UV transparent quartz stamp and using a nanoimprint machine to successfully transfer the grating pattern onto the polymer layer. The process is cost effective and results in simplicity to fabricate a stamp. But, there are some drawbacks that have been explicitly mentioned in Ref. 19. These drawbacks may restrict the use of this method in fabricating a Bragg grating filter. Kocabas et al reported the fabrication of a grating on OG 146 polymer using e-beam direct writing and stamp transfer techniques [19]. Then, a BCB polymeric ridge waveguide was fabricated on the grating using reaction ion etching technique. The grating fabrication process is similar to our previous work except for the e-beam writing technique [20]. The experimental results showed good replication for the grating through the process. However, the physical etching process may cause large scattering losses from the sidewall of waveguides.
We have recently demonstrated a process to rapidly produce submicron range gratings by using both micro-molding and holographic interference techniques. A large aspect ratio of 0.7:1 between the depth and the period on the grating pattern could be obtained, and consistent reproduction of the grating on a UV polymer could be achieved with this process [20]. In this paper, we demonstrate a method to inscribe surface-relief gratings on polymer channel waveguides without any physical etching process.
There are a number of simple methods to fabricate polymer waveguides that include techniques involving photocrosslinking [21], photobleaching [22–23], reactive ion etching [24–25], photolocking [26] and laser/electron beam writing [27, 28]. Some techniques have inherent limitations; for example reactive ion etching can incur excessive scattering losses [24, 25], and laser beam writing is not suitable for mass-production [27]. Other techniques such as hot embossing [29], UV-embossing [30], and micro-transfer molding method are also becoming more popular due to their simple fabrication procedure [31–32]. However, these methods have problems to overcome; such as residual material problems and limited substrate and core materials available [29–32]. We have recently developed a soft lithography method to replicate polymer waveguides [33]. In this method, the waveguides are produced by a two-step molding process where a master mold is first formed on a negative tone photoresist and subsequently transferred to a PDMS mold. The silicone rubber mold is then used as a stamp to transfer the final waveguide pattern onto a UV curing epoxy. High aspect ratio (depth/ width) and vertical waveguide sidewalls could be obtained by this method.
Fig. 1. AFM and SEM micrographs of gratings on UV polymer (a) AFM (b) SEM (503 nm grating period and 397 nm grating depth)
In this paper, we will describe a technique that combines the holographic interferometry, soft lithography, and a simple replication processes to create a high resolution and high aspect ratio grating structure on a polymer waveguide. In order to reduce the residual stress caused by different thermal expansion coefficients in the core and cladding material, similar polymer materials with slightly different index are used for the core and cladding layers. The material has a refractive index between 1.52∼1.56 (OG146=1.5201, and OG154=1.5668) at wavelength 1550nm, which is close to the refractive index of optical fiber (about 1.46-1.50).
Fig. 2. Fabrication process of buried gratings in polymeric waveguide filter structures, a) UV polymer with gratings was deposited on the glass, b) UV light exposure of photoresist, c) photoresist mold, d) PDMS is poured into the photoresist mold, e) PDMS mold, f) A spacer with a thickness of 400μm is positioned, g) OG146 precure epoxy is injected into the space, h) hardened epoxy forms the cladding layer of polymeric waveguide filter, i) forming a rectangular channel, j) Mixed OG epoxy is injected into the channel, k) The epoxy in the channel was then cured by exposing with UV light, and the cover glass and the PDMS layer are removed from the sample, l) A spacer with a thickness of 9400μm was positioned, m) OG146 epoxy is injected into the channel, n) final polymeric waveguide filter.
2. Grating Fabrication
Rapid prototyping of grating structures on a polymer substrate involving a three-step process was performed first. A grating pattern is holographically exposed using a two-beam interference pattern on a positive photoresist film (Ultra 123 made by MicroChem Corp., MA., refractive index = 1.618). This produces a master that can be subsequently used to produce a polydimethylsiloxane (PDMS) mold. This silicone rubber mold then is used as a stamp to transfer the gratings pattern onto a UV cure epoxy polymer. The details of the process involved for the grating fabrication was described in our previous reports [20]. Based on our results, we found that the grating period and the corresponding depth of the grating pattern can be accurately controlled down to less than 1% error. We also found that a high aspect ratio of almost 0.7:1 between the depth and the period of the grating structure could be obtained using this process [20]. Figure 1 shows the AFM and SEM results of OG146 with a grating period of 503nm and a grating depth of 397nm, which was fabricated by the above process. The UV polymer with grating was cut down to 1cm × 5cm, and the dimensions of the gratings on polymer were 0.5cm long and 1mm wide.
Fig. 3. SEM micrograph of the waveguide pattern on photoresist, which showed the intact grating pattern inside the groove , and SEM was tilted 55° degree (the dimension is 8.7μm × 6μm, the length is 5cm, and the grating period is 503nm).
3. Fabrication of the polymeric wavelength filter
In order to fabricate polymer wavelength filters, a 700μm thick glass substrate was first cut into a 5cm × 1cm rectangle. After substrate cleaning, the UV polymer was put on the glass, and it was coated with a 6.0 μm thick negative photoresist (SU-8) spun on at 1000rpm for 17 seconds. Samples were exposed through the PET (Polyethylene Terephthalate)-based masks using a UV mask aligner (AB-Manufacturing, CA) for 90 seconds, followed by developing in a SU-8 developer (MicroChem) for 45 seconds, and negative waveguide pattern with gratings on the bottom were obtained. The process flow is shown in Fig. 2(a) and (b). The waveguide dimension is about 8.7μm × 6.0μm, and the length is about 5cm. The details of the process are described in our previous reports [33]. An SEM image of the waveguide mold was taken after the photoresist mold was made (Fig.3), which showed the intact grating pattern inside the groove. The patterned resist was used as a mother mold to transfer the pattern onto a polydimethylsiloxane (PDMS) thin film using typical micro-molding technique [34–36] (e.g. stamping). The diluted PDMS was uniformly coated on the patterned photoresist. After baking at 90°C for 1 hr, the PDMS was cured and could be easily peeled off from the photoresist (Fig. 2(c)–(e)). Figure 4 shows the SEM image of the PDMS waveguide with gratings. The waveguide with the gratings pattern was transferred onto a UV polymer (OG146) from the PDMS mold using a UV replication process described by Rossi and our previous reports [33,36] (Fig. 2(f)–(h)). A spacer with a thickness of 400μm was placed between the mold and a thin Pyrex glass slide. After injection of the precure UV polymer (OG146), the epoxy was then cured under a broadband UV light operating in a wavelength range of 300-400nm. After the polymer was fully cured, the polymer was easily peeled off from the mold. The SEM image shows that the replication on epoxy was good (Fig.5), and had good matching in dimension to the negative photoresist mold. After separating from the PDMS mold, a hardened epoxy with gratings forms the cladding layer of the polymer waveguide filter.
Fig. 4. SEM micrograph of the PDMS waveguide with gratings, and SEM was tilted 35° Degree (the dimension is 8.7μm × 6μm, the length is 5 cm, and the grating period is 502nm).
Fig. 5. SEM micrograph of the OG146 rectangular groove, which showed the intact grating pattern inside the groove, and SEM was tilted 5° degree (the dimension is 8.7μm × 6μm, the length is 5cm, and the grating period is 503nm).
To form the waveguide core, a similar UV epoxy (mixed OG) is injected into the groove. Instead of spin coating technique, which could create a thick unguided layer outside the core region that would results in some coupling loss during the input of the optical fiber to the filter, another method was also proposed (Fig. 2(i)–(j)). A thin layer of a polydimethylsiloxane (PDMS) polymer was spun onto a glass slide that is then placed over the epoxy groove, forming a rectangular tunnel. In order to inject the high viscosity epoxy of OG154 into the rectangular channel, we diluted the OG154 with another UV epoxy OG169 (the mixing ratio is 1:1) to reduce the viscosity of the OG154. After the curing process, the refractive index of the mixed epoxy is about 1.550 at 1550nm. Then, a drop of the mixed UV epoxy was injected into the channel from one of the open ends. After exposing with UV light, the epoxy end of the tunnel was sealed. Next, the sample was inserted into the liquid epoxy with the open-end face down. This process was performed in a vacuum chamber (Fig. 2(i)). When the pressure in the chamber reached 10-4 Torr, air was introduced into the chamber to force the liquid epoxy into the tunnel. The epoxy in the tunnel was cured by exposing the UV light for 1-2 minutes. After the cover glass was removed, the PDMS layer was peeled off from the sample. To prevent the optical loss due to either surface scattering losses or the outright absence of a guided mode for the asymmetric waveguide structure, the upper cladding layer was used. The same UV epoxy (OG146) was deposited using the fabricating procedure described in the previous section (Fig. 2(k)–(n)). A spacer with thickness of 410μm was placed between the sample and a thin Pyrex glass slide. After injecting the precure UV polymer (OG146) into the opening between the mold and the glass slide, UV light was used to crosslink the polymer. The sample were diced and the end-faces were then polished, such that the final polymeric wavelength filter has dimensions of 4cm in length, 1cm in width and about 410μm in thickness (Fig. 2(n)).
The near field patterns of the optical waveguide were observed using the end-fire coupling technique. Figure 6 shows the schematic diagram of the measurement system. An amplified spontaneous emission (ASE) source with a wavelength range from 1530 to 1560 nm was used as the wide band light source (Stabilized Light Source, PTS-BBS, Newport Inc., USA). The light source was polarized in the TE direction using the in-line polarizer (ILP-55-N, Advanced Fiber Resources, China), which was followed by a polarization controller with operation wavelength around 1550nm (F-POL-PC, Newport Inc., USA). The output mode field of the waveguide was observed using an IR CCD system (Model 7290A, Micron Viewer, Electrophysics Inc., U.S.A.) with image analysis software (LBA-710PC-D, V4.17, Spiricon Inc., USA). The measured mode field pattern of the waveguide is depicted in Fig.7, which show the single-mode characteristics of the waveguide.
The waveguide properties including the mode pattern and the effective index were simulated using the beam propagation method (BPM_CAD, Opti-Wave Inc., Canada). The effective index of the waveguide is 1.5447 from the simulation. The Bragg wavelength is 1553.9 nm as calculated from the Bragg reflection condition for the filter with a grating period of 503nm The transmission of the optical filter was also calculated by using coupled mode theory [37]. The calculated transmission minimum of the optical filter is -19.5dB. The spectral characteristics of the optical filter were measured using an optical spectrum analyzer (Q8384 Optical Spectrum Analyzer, Advantest Inc., Japan). Again, an amplified spontaneous emission (ASE) light source with a wavelength range from 1530 to 1560 nm was used as the wide band light source. An alignment He-Ne laser source, as the auxiliary source, was combined with the wide band source using a 2×1 optical fiber coupler. The optical filter was set on a micro-positioner, and two single mode fibers were used as the input and output fibers. The input light source was polarized in the TE direction as was the mode field measurement system. The output fiber, then, was connected to the optical spectrum analyzer to characterize the filter performance. The measured result is shown in Fig.8. At the Bragg wavelength, a transmission dip of -18.5 dB was obtained, and the 3-dB-transmission bandwidth was about 8nm. The Bragg wavelength λB is given as λB=2NeffΛ, where Neff, can be calculated using the beam propagation method, is the effective index of the waveguide grating and Λ is the period of grating. The measured Bragg wavelength is 1554.02 nm, which is off by 0.12nm from the theoretical prediction. When the core refractive index ranges from 1.549 to 1.551 for the mixing ratio of OG154 ranging from Vol.45% to 55%, the calculated Bragg wavelength by the beam propagation method varies from 1552.9 to 1554.8 nm.
The technique described in this paper can also be applied to the fabrication of asymmetric Bragg couplers (ABC) [38], which can be utilized as optical add/drop multiplexer (OADM) elements in dense wavelength division multiplexing (DWDM) systems. The possibility of fabricating ABC waveguide filters is illustrated in Fig.9. The fabrication procedure is the same as the above-mentioned process, and the only difference is the PET-based mask designed. This result shows that parallel polymeric waveguide filters can be possibly designed and fabricated.
4. Conclusion
In conclusion, we have successfully created a process to rapidly produce submicron range gratings on polymer waveguides by using holographic interference techniques, soft Lithography, and micro molding. A large aspect ratio grating pattern could be obtained with consistent reproduction of the grating on a UV polymer waveguide could be produced. The grating period and depth on the polymer waveguides exhibited only a small difference from the original designed grating pattern. This process shows great potential for mass production of any period of grating structure on waveguide, and could be used to successfully fabricate coupled polymeric waveguide filters.
Fig. 9. SEM micrograph of the PDMS mold of ABC waveguide filter (the dimensions of the two asymmetric coupled waveguide are 7.8μm × 6μm ,11μm × 6μm , the length is 4cm, and the gap is about 4μm).
References and Notes
1. N. Imoto, “An analysis for contradirectional-coupler-type optical grating filters,” J. Lightwave Technol. 3, 895–900 (1985). [CrossRef]
2. H. Hillmer, A. Grabmaier, H. L. Zhu, S. Hansmann, and H. Burkhard, “Continuously chirped DFB gratings by specially bent waveguides for tunable lasers,” J. Lightwave Technol. 13, 1905–1912 (1995). [CrossRef]
3. C. H. Lin, Z. H. Zhu, Y. Qian, and Y. H. Lo, “Cascade self-induced holography: a new grating fabrication technology for DFB/DBR lasers and WDM laser arrays,” IEEE J. Quantum Electron 32, 1752–1759 (1996). [CrossRef]
4. Y. Shibata, S. Oku, Y. Kondo, and T. Tamamura, “Effect of sidelobe on demultiplexing characteristics of a grating-folded directional coupler demultiplexer,” IEEE Photonics Technol. Lett. 8, 87–89 (1996). [CrossRef]
5. A. Sharon, D. Rosenblatt, and A. A. Friesem, “Narrow spectral bandwidths with grating waveguide structures,” Appl. Phys. Lett. 69, 4154–4156 (1996). [CrossRef]
6. S. Yin, F. T. S. Yu, and S. Wu, “Optical monitoring for plasma-etching depth process,” IEEE Photonics Technol. Lett. 4, 894–896 (1992). [CrossRef]
7. A.W. Ang, G.T. Reed, A. Vonsovici, A.G.R. Evans, P.R. Routley, and M.R. Josey, “Effect of grating heights on highly efficient unibond SOI waveguide grating couplers,” IEEE Photonics Technol. Lett. 12, 59–61(2000) [CrossRef]
8. D. Y. Kim, S. K. Tripathy, L. Li, and J. Kumar, “Laser-induced holographic surface relief gratings on nonlinear optical polymer films,” Appl. Phys. Lett. 66, 1166–1168 (1995). [CrossRef]
9. J. W. Kang, M. J. Kim, J. P. Kim, S. J. Yoo, J. S. Lee, D. Y. Kim, and J. J. Kim, “Polymeric wavelength filters fabricated using holographic surface relief gratings on azobenzene-containing polymer films,” Appl. Phys. Lett. 82, 3823–3825 (2003). [CrossRef]
10. S. Aramaki, G. Assanto, G. I. Stegeman, and M. Marciniak, “Realization of integrated Bragg reflectors in DANs-polymer waveguides,” J. Lightwave Technol. 11, 189–1195 (1993). [CrossRef]
11. H. Nishihara, Y. Handa, T. Suhara, and J. Koyama, “Electron-beam directly written micro gratings for integrated optical circuits,” in Photo- and Electro-Optics in Range Instrumentation, J. Water, et al., eds., Proc. SPIE , 134, 152–159 (1980).
12. L. Eldada, C. Xu, K.M.T. Stengel, L.W. Shacklette, and J.T. Yardley,”Laser-fabricated low loss single-mode raised-rib waveguiding devices in polymers,” J. Lightwave Technol. 14, 1704–1713 (1996). [CrossRef]
13. L. Eldada, S. Yin, C. Poga, C. Glass, R. Blomquist, and R.A. Norwood, “Integrated multichannel OADMS using polymer Bragg grating MZIS,” IEEE, Photonics Technol. Lett. 10, 1416–1418 (1998). [CrossRef]
14. L. Eldada, R. Blomquist, M. Maxfield, D. Pant, G. Boudoughian, C. Poga, and R.A Norwood, “Thermooptic planar polymer Bragg grating OADM’s with broad tuning range,” IEEE Photonics Technol. Lett. 11, 448–450 (1999). [CrossRef]
15. B. Darracq, F. Chaput, K. Lahlit, Y. Levy, and J.-P. Boilot, “Photoinscription of surface relief grating on azo-hybrid gels,” Advanced Materials 10, 1133–1136 (1998). [CrossRef]
16. D. J. Kang, J. K. Kim, and B. S. Bae, “Simple fabrication of diffraction gratings by two beam interference method in highly photosensitivity hybrid sol-gel films,” Opt. Express 12, 3947–3953 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-17-3947. [CrossRef] [PubMed]
17. L. Zhu, Y. Huang, W. M. J. Green, and A. Yariv, “Polymetric multi-channel bandpass filters in phase-shifted Bragg waveguide gratings by direct electron beam writing,” Opt. Express 12, 6372–6376 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-25-6372. [CrossRef] [PubMed]
18. D.-H. Kim, W.-J. Chin, S.-S. Lee, S.-W. Ahn, and K.-D. Lee, “Tunable polymeric Bragg grating filter using nanoimprint technique,” Appl. Phys. Lett. 88, 071120, (2006). [CrossRef]
19. A. Kocabas and A. Aydinli, “Polymeric waveguide Bragg grating filter using soft lithography,” Opt. Express 14, 10228–10232 (2006), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-14-22-10232. [CrossRef] [PubMed]
20. W.C. Chuang, C.T. Ho, and W.C. Wang, “Fabrication of a high resolution periodical structure using a replication process” Opt. Express 13, 6685–6692 (2005), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-18-6685. [CrossRef] [PubMed]
21. Jae Wook Kang, Jang-Joo Kim, Jinkyu Kim, Xiangdan Li, and Myong-Hoon Lee, “Low-loss and thermally stable TE-mode selective polymer waveguide using photosensitive fluorinated polyimide”, IEEE Photonics Technol. Lett. 14, 1297–1299 (2002). [CrossRef]
22. T.E. Van Eck, A.J. Ticknor, R.S. Lytel, and G.F. Lipscomb, “Complementary optical tap fabricated in an electro-optic polymer waveguide”, Appl. Phys. Lett. 58, 1588–1590, (1991). [CrossRef]
23. O. Watanabe and M. Tsuchimori, “Improvement in linear and nonlinear optical-properties by blending poly(N-vinyl-2-pyrrolidone) with an electro-optic polymer”, Polymer 42, 6447–6451 (2001). [CrossRef]
24. M. Hikita, Y. Shuto, M. Amano, R. Yoshimura, S. Tomaru, and H. Kozawaguchi, “Optical intensity modulation in a vertically stacked coupler incorporating electro-optic polymer”, Appl. Phys. Lett. 63, 1161–1163 (1993). [CrossRef]
25. W. Wang, D. Chen, and H.R. Fetterman, “Travelling wave electro-optic phase modulator using cross-linked nonlinear optical polymer”, Appl. Phys. Lett. 65, 929–931 (1994). [CrossRef]
26. B.L. Booth, “Low loss channel waveguides in polymers”, J. Lightware Technol. 7, 1445–1453 (1989). [CrossRef]
27. L. Eldada and L.W. Shacklette, “Advances in polymer integrated optics”, IEEE J. Select. Topics Quantum Electron 6, 54–68 (2000). [CrossRef]
28. Y.Y. Maruo, S. Sasaki, and T. Tamamura, “Embedded channel polyimide waveguide fabricatio9 by direct electron beam writing method”, J. Lightwave Technol 13, 1718–1723 (1995). [CrossRef]
29. Holger Becker and Wolfram Dietz, “Microfluidic devices for TAS applications fabricated by polymer hot embossing,” in Microfluid Devices and Systems, A. B. Frazier and C. H. Ahn, eds., Proc. SPIE 3515, 177–181 (1998). [CrossRef]
30. P. M. Ferm and L. W. Shacklette, “High volume manufacturing of polymer waveguides via UV- Embossing,” in Linear,Nonlinear, and Power-Limiting Organics, E. Manfred, et al., eds., Proc. SPIE 4106, 1–10 (2000). [CrossRef]
31. K. E. Paul, T. L. Breen, J. Aizenberg, and G. M. Whitesides, “Maskless Photolithography: embossed photoresist as its own optical element,” Appl. Phys. Lett. 73, 2893–2895 (1998). [CrossRef]
32. X.-M. Zhao, S. P-Smith, S. J. Waldman, G. M. Whitesides, and M. Prentiss, “Demonstration of waveguide couplers fabricated using microtransfer molding,“ Appl. Phys. Lett. 71, 1017–1019 (1997). [CrossRef]
33. W.C. Chuang, C.T. Ho, and W. C. Chang, “Fabrication of polymer waveguides by a replication method,” Applied Optics 45, 8304–8307 (2006). [CrossRef] [PubMed]
34. J. C. Lotters, W. Olthuis, P. H. Veltink, and P. Bergveld, “The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications,” J. Micromech. Microeng. 7, 145–147(1997). [CrossRef]
35. P. Nussbaum, I. Philipoussis, A. Huser, and H. P. Herzig, “Simple technique for replication of micro-optical elements,” Opt. Eng. 37, 1804–1808 (1998). [CrossRef]
36. M. Rossi, H. Rudmanr, B. Marty, and A. Maciossek, “Wafer-scale micro-optics replication technology,” in Lithographic and Micromaching Techniques for Optical Component Fabrication II, E.-B. Kley and H.P. Herzid, eds., Proc. SPIE 5183, 148–154 (2003). [CrossRef]
37. A. Yarin, Introduction to Optical Electronics, 3rd edition, (H. Rinehart & Winston, New York, 1984).
38. T. Erdogan, “Optical add-drop multiplexer based on an asymmetric Bragg coupler,” Optics Communication , 157, 249–264 (1998). [CrossRef]
