Light-induced self-written waveguide fabrication ... - OSA Publishing

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Letter

Vol. 42, No. 11 / June 1 2017 / Optics Letters

Light-induced self-written waveguide fabrication using 1550 nm laser light HIDETAKA TERASAWA,1 FREDDY TAN,1 OKIHIRO SUGIHARA,1,* AKARI KAWASAKI,2 DAISUKE INOUE,2 TATSUYA YAMASHITA,2 MANABU KAGAMI,2 OLIVIER MAURY,3 YANN BRETONNIÈRE,3 AND CHANTAL ANDRAUD3 1

Department of Optical Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya-shi, Tochigi 321-8585, Japan Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan 3 Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1, Laboratoire de Chimie, F69342 Lyon, France *Corresponding author: oki‑[email protected]‑u.ac.jp 2

Received 19 April 2017; accepted 7 May 2017; posted 15 May 2017 (Doc. ID 293012); published 1 June 2017

Light-induced self-written (LISW) optical waveguides were fabricated for the first time, to the best of our knowledge, using a photopolymerizable resin system formed by 1550 nm pulse laser light. A two-photon absorption (TPA) chromophore with a TPA cross section of several hundred Goeppert-Mayer (GM) at 1550 nm was used. Furthermore, the optical interconnection between a single-mode fiber and a fiber Bragg grating was demonstrated by the present technique, using one-way irradiation of 1550 nm laser light through the single-mode fiber. The LISW waveguide formation using 1550 nm laser light offers a new and promising alternative route for optical interconnection in silicon photonics technology. © 2017 Optical Society of America OCIS codes: (200.4650) Optical interconnects; (130.5460) Polymer waveguides; (060.4510) Optical communications. https://doi.org/10.1364/OL.42.002236

Silicon photonics have received considerable attention in recent years for applications in the on-chip interconnection of electronics and optical devices. Electro-optical integration using silicon-on-insulator complementary metal-oxide semiconductor technology was demonstrated [1]. However, there is a serious problem with the connection between silicon-wire waveguides and external optical components, such as single-mode fibers (SMFs) [2]. The large mode mismatch between them will cause a high coupling loss when they are butt-coupled. Grating couplers and spot size converters have been proposed to overcome this problem [3,4]. Although these approaches improve the coupling efficiency, there remains a need for active alignment between the SMF and the Si waveguide with such couplers. These are also time-consuming and costly approaches. We propose an alternative passive alignment approach by using the light-induced self-written (LISW) optical waveguide technique. This approach offers a promising solution that can be applied to optical interconnection. One of the main advantages of the LISW technique is the possibility for self-alignment between an optical waveguide and an optical fiber. This approach is also inexpensive and efficient because it requires only light irradiation 0146-9592/17/112236-03 Journal © 2017 Optical Society of America

through an optical fiber and an optical waveguide. There have been many reports of waveguide fabrication and optical interconnection using the LISW technique in the past decade. In the previous research [5–8], LISW waveguides have mainly been fabricated using an ultraviolet (UV) and/or visible light source because of the high sensitivity of the photocurable resins in the short-wavelength region. Yoshimura et al. [6] demonstrated LISW interconnection by one-way irradiation, where they attached a luminescent target on one of the optical fiber cores by UV irradiation through the optical fiber. Unfortunately, these processes are complicated and impractical for application, especially in a silicon-wire waveguide interconnection because UV light cannot propagate in a silicon-wire waveguide. Barsella et al. reported LISW fabrication using a two-photon polymerization process formed by a femtosecond laser at a wavelength of 1064 nm [9]. Meanwhile, silicon is transparent at wavelengths longer than about 1100 nm. In order to interconnect a silicon waveguide and an optical fiber using the LISW technique, it is necessary to fabricate an LISW waveguide from a silicon waveguide tip. In other words, LISW waveguide fabrication at telecommunication wavelengths, e.g., 1310 nm and 1550 nm, is necessary, and LISW waveguide fabrication using a 1064 nm light does not meet this requirement. In this Letter, we report LISW waveguides fabricated using a photopolymerizable monomer system containing a two-photon absorption (TPA) chromophore as a photoinitiator by irradiation of a 1550 nm laser light through the fiber. We investigated the growth characteristics of the LISW waveguides by using an SMF for a 1550 nm operating wavelength. Furthermore, we performed an optical interconnection between an SMF and a fiber Bragg grating (FBG) by using our proposed approach and evaluated its total insertion loss. This approach is much simpler, more efficient, and requires no costly special components, such as connectors, splicers, V -grooves, special machines for alignment, or additional processes. Figure 1 illustrates the fabrication process of an LISW waveguide using 1550 nm laser light. The photocurable resin used in our experiments was a mixture containing 2, 2-bis[4-(acryloxy diethoxy) phenyl] propane [Fig. 2(a)] as an acrylate monomer, 2,4,6-tris(trichloromethyl)-1,3,5-triazine [Fig. 2(b)] as a

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Fig. 1. Fabrication of an IR LISW optical waveguide.

radical generator, pentaerythritol tetrakis (3-mercaptobutylate) [Fig. 2(c)] as a hardening accelerator, and a polymethine TPA chromophore [Figs. 2(d) and 2(e)] [10,11]. Figure 3 shows the TPA cross-section spectrum of the TPA chromophores in dichloromethane measured by the open-aperture Z-scan method. The chromophore presents a TPA cross section of several hundred GM at 1550 nm, which allows the TPA photopolymerization process using an IR pulse laser source. They are mixed with the following composition (by weight); monomer: radical generator: hardening accelerator: chromophore was 100: 2: 12.5: 0.2. First, the radical generator was added into the monomer. They were stirred at room temperature for one day. Further, we added the chromophore and the hardening accelerator into this solution and stirred them at room temperature for 15 h or more. After that, the material was ready for the LISW experiment. In our experiment, a glass cell was used to fabricate the LISW waveguide from an SMF end, as shown in Fig. 1. First, an SMF end with core and cladding diameters of 8.2 μm and 125 μm, respectively, was inserted into the cell. Next, the cell was filled with a photocurable resin by the capillary effect. A pre-UV treatment by using a UV mercury lamp with a power density of 2.7 mW∕cm2 was performed to activate the radical generator and increase the viscosity of the resin. As a consequence, the photocurable resin was partially polymerized and prevented the formed LISW waveguide from bending because the LISW waveguide can easily bend in the liquid resin circumstances owing to its small core size of approximately 10 μm [12]. Finally, we irradiated the photocurable resin with a pulse laser at a repetition rate of 200 kHz, a pulse width of 5 ns, and a wavelength of

Fig. 2. Molecular structure of (a) the acrylate monomer, (b) the radical generator, (c) the hardening accelerator, and (d), (e) the TPA chromophore.

Fig. 3. Two-photon absorption cross-section spectra of the chromophore.

1548 nm through the SMF, so that a straight polymeric optical waveguide with a uniform core size of nearly 10 μm was formed. Figure 4(a) shows the LISW waveguide length as a function of irradiated laser peak power. In this experiment, the chromophore shown in Fig. 2(d) was used, and the laser and UV irradiation times were fixed at 1 min and 2 min, respectively. Figure 4(b) shows an image of the LISW waveguide fabricated at 17 W, obtained using a differential interference contrast (DIC) microscope. As can be seen in Fig. 4(a), the waveguide length increased with increasing peak power. A straight LISW waveguide with a core size of approximately 10 μm and a length of 640 μm was successfully formed. The threshold value of 7 W corresponding to 13 MW∕cm2 can be improved by optimization of conditions, such as the irradiation wavelength, pre-UV time, and mixing ratios of material components. We have

Fig. 4. (a) Relationship between the LISW waveguide length and the laser peak power. (b) DIC microscope image of the LISW waveguide from the SMF end.

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Fig. 5. (a) Schematic image of the optical interconnection between the SMF and the FBG. (b) DIC microscope image showing an LISW interconnection between the SMF and the FBG. The chromophore shown in Fig. 2(d) was used.

achieved the LISW length up to 4.2 mm by using different conditions. Optical interconnection between SMF and FBG was also performed by using this technique, as illustrated in Fig. 5(a). First, the SMF and FBG were placed face-to-face in a cell with a gap of 50–400 μm. In this experiment, we used the FBG that has a reflectance of 94% at 1548 nm and 3 dB reflection bandwidth of 0.227 nm. A mixture of the photocurable resin was used to fill in the gap between the fiber tips. The remaining steps of the process, such as the pre-UV treatment and irradiation of the photocurable resin mixture with a 1548 nm laser beam through the input fiber, were performed similarly to the previous experiment. The FBG acted as a reflector to the incoming light passing through the resin-filled gap, and thus, the LISW core grew from both fiber ends, interconnecting them. Figure 5(b) shows a typical image of LISW interconnection between the SMF and the FBG, as captured by the DIC microscope. In this experiment, the laser peak power emitted from the laser light source was 23 W. It can be observed from the figure that a straight LISW waveguide with a nearly uniform core diameter of approximately 10 μm was created between them. We evaluated the total insertion loss before and after optical interconnection by the LISW waveguide. We used the chromophore shown in Fig. 2(e) for this experiment. Before the LISW interconnection, the insertion losses were 1.67 dB and 10.65 dB at gap distances of 50 μm and 400 μm, respectively. The insertion losses were improved by 1.32 dB and 10.16 dB for 50 μm and 400 μm gaps through the LISW waveguide interconnection, respectively. Then the total insertion loss of less than 0.5 dB for a 50–400 μm gap was obtained after the LISW interconnection. The insertion loss was dramatically improved by LISW interconnection. A small total insertion loss and simple self-alignment were experimentally demonstrated by only one-way irradiation. In addition, we also fabricated an LISW waveguide by using a thermally diffused expanded core (TEC) fiber that has a mode field diameter

Letter (MFD) of about 3 μm. The refractive index and MFD of the TEC fiber almost match those of the silicon nanowire waveguide when we use a spot size converter that has an MFD of about 3 μm and an effective refractive index of around 1.5. Therefore, this approach becomes very promising for silicon photonics applications in the future. The results on this development will be reported elsewhere. In summary, we have successfully prepared LISW waveguides at a wavelength of around 1550 nm using a TPA chromophore. A straight polymer waveguide core was realized with the LISW technique by implementing pre-UV treatment. Moreover, easy optical interconnection between an SMF and a FBG was demonstrated with the LISW technique using oneway irradiation. Thus, this method eliminates the need for an active alignment process. An insertion loss improvement of 1.32 dB and 10.16 dB for 50 μm and 400 μm gaps, respectively, and a total insertion loss of less than 0.5 dB were obtained by using the LISW waveguide interconnection. This technique can also be applied for different optical interconnection purposes, such as optical interconnection between an IR light source and a fiber/waveguide or between a silicon waveguide and an SMF. However, the threshold power density of IR LISW waveguide fabrication must be reduced to enable its use in such applications. We believe that this time-saving, inexpensive approach will be useful for interconnection between optical devices, especially in the field of silicon photonics. Funding. Strategic Promotion of Innovative Research and Development (S-Innovation) program, “Photonics Polymer,” of the Japan Science and Technology Agency (JST). REFERENCES 1. J. S. Orcutt, B. Moss, C. Sun, J. Leu, M. Georgas, J. Shainline, E. Zgraggen, H. Li, J. Sun, M. Weaver, S. Urošević, M. Popović, R. J. Ram, and V. Stojanović, Opt. Express 20, 12222 (2012). 2. R. Dangel, J. Hofrichter, F. Horst, D. Jubin, A. La Porta, N. Meier, I. M. Soganci, J. Weiss, and B. J. Offrein, Opt. Express 23, 4736 (2015). 3. X. Xu, H. Subbaraman, J. Covey, D. Kwong, A. Hosseini, and R. T. Chen, Appl. Phys. Lett. 101, 031109 (2012). 4. T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamechika, S. Itabashi, and H. Morita, IEEE J. Sel. Top. Quantum Electron. 11, 232 (2005). 5. M. Kagami, T. Yamashita, and H. Ito, Appl. Phys. Lett. 79, 1079 (2001). 6. T. Yoshimura, M. Iida, and H. Nawata, Opt. Lett. 39, 3496 (2014). 7. M. Waki, K. Tsujikawa, and T. Kurashima, in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (Optical Society of America, 2008), paper NThC3. 8. P. A. Mohammed and W. J. Wadsworth, J. Lightwave Technol. 33, 4384 (2015). 9. A. Barsella, H. Dorkenoo, and L. Mager, Appl. Phys. Lett. 100, 221102 (2012). 10. Q. Bellier, N. S. Makarov, P.-A. Bouit, S. Rigaut, K. Kamada, P. Feneyrou, G. Berginc, O. Maury, J. W. Perry, and C. Andraud, Phys. Chem. Chem. Phys. 14, 15299 (2012). 11. P. A. Bouit, E. Di Piazza, S. Rigaut, B. Le Guennic, C. Aronica, L. Toupet, C. Andraud, and O. Maury, Org. Lett. 10, 4159 (2008). 12. O. Sugihara, H. Tsuchie, H. Endo, N. Okamoto, T. Yamashita, M. Kagami, and T. Kaino, IEEE Photon. Technol. Lett. 16, 804 (2004).