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Direct ultrafast laser writing of buried waveguides in Foturan glass Stephen Ho and Peter R. Herman Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., Toronto, Ontario, M5S 3G4, Canada; Tel: 416-978-7039 Fax: 416-971-3020;
[email protected];
[email protected]
Ya Cheng, Koji Sugioka, and Katsumi Midorikawa RIKEN – The Institute of Physical and Chemical Research, Hirosawa 2-1, Wako, Saitama, 351-0198, Japan;
[email protected] .go.jp;
[email protected];
[email protected]
Abstract: Femtosecond laser writing of buried waveguides was extended to Foturan glass. Thermal annealing yielded positive index changes for select visible and infrared wavelengths. ©2004 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (230.7370) Waveguides
1. Introduction Femtosecond near-infrared lasers are attractive for direct writing of 3-D photonic structures [1-4] such as required in integrated-optical circuit applications. For fabricating biosensor chips, alternative microfabrication techniques such as Ag +-Na+ ion exchange in glass [5] and ion milling of holographically exposed photoresist [6] have been demonstrated. Foturan glass is a promising biosensor platform that can be activated by femtosecond laser exposure to form 3-D embedded structures such as microfluidic channels [7-10] as well as photonic structures such as buried refractive index gratings [11]. In this paper, we present results of forming visible and infrared transmitting buried waveguides in Foturan glass with femtosecond lasers. 2. Ultrafast Laser Microfabrication and Buried Waveguides The waveguides were fabricated with 135-fs pulses at 1-kHz repetition rate and 775-nm wavelength [10]. A 20X microscope objective lens (NA = 0.46) with a 500μm slit was used to form an asymmetric focal spot inside the sample. Samples were scanned perpendicular to the laser polarization direction at speeds of 50 to 200 μm/s and single-pulse fluences of 5 to 36J/cm2 (peak fluence). The laser exposed Foturan was then thermally annealed for 6 hours at 520oC. 100μm
a
b
F = 12.6 J/cm2 F = 14.4 J/cm2 F = 18 J/cm2 F = 36 J/cm2 Fig. 1. Microscope view in backlighting of the side (a) and end-section (b) of waveguides formed at 200 µm/s.
Fig. 1 shows the side and end views of the optical waveguides formed at 200µm/s and 12.6 to 36J/cm2 peak fluence per pulse. The waveguide width increases with fluence exposure from ~4 to 20 μm. Microcrack damage is apparent in many higher fluence samples in the form of periodic transverse lines.
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3. Characterization of Buried Waveguides End-firing of laser-formed buried waveguides was aided with index matching fluid. Three classes of waveguides were characterized under an optical microscope, namely, “visible” (tracks with damages), “barely visible” (hard to observe tracks), and “invisible” (invisible under a microscope) as summarized for the tested exposures shown in Fig. 2. Only infrared light was noted to guide in the “visible” classification, presumably because scattering or absorption losses were too high for the 635 light. The “barely visible” classification defined a narrow fluence processing window (see Fig. 2) near ~10 J/cm 2 for which only 635-nm light could be guided. No waveguide coupling was observed for the “invisible” classification. Guiding at 635nm
Dwell Time (s/m)
100
Invisible Waveguides
Guiding at ~1.5μm
10
Invisible Waveguide Damaged 1 1
10
Single Pulse Fluence (J/cm2)
100
Fig. 2. Classification of waveguides into infrared guiding, red guiding, and not guiding according to the dwell time and laser fluence applied to the Foturan.
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Fig. 3. The near-field modal profiles at 1550 nm (a) and 635 nm (b) light in waveguides formed at scanning speed and peak fluence of (a) 200μm/s and 36J/cm2 and (b) 200μm/s and 12.6J/cm2, respectively.
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The near-field mode profiles of red (635 nm) and infrared (1540nm-1560nm) light were captured independently with a CCD detector as shown in Fig. 3.
Fig. 4. CCD camera captured picture of an end-firing excitation with red (635nm) light. Scattering loss from the Foturan waveguide is seen on the right. A 125-µm-diameter optical fiber is on the left. This scanning speed and peak fluence for were 200μm/s and 12.6J/cm 2, respectively.
A regular pattern of microcracks in the waveguide volume was responsible for significant scattering loss as noted in Fig. 4. The bright scattering centres are especially pronounced for waveguides exposed to high-fluence. Spectral transmission modulations were also noted in the 1540 to 1560 nm band and is under further investigation. 4. Conclusion We have demonstrated femtosecond laser direct writing of buried waveguides in Foturan glass. Only narrow fluence processing windows were identified for guiding red and infrared light. Further characterization of waveguide losses and refractive index changes is underway together with studies to determine the optimum processing window for creating low-loss guides for broad spectral coverage. Although large losses are apparent, this study opens the possibility for integrating 3-D photonic components with 3-D microfluidic structures to create highly functional bio-sensors and biophotonic laboratories on a chip. 5. References [1] Davis, K.M., Miura, K., Sugimoto, N., and Hirao, K., “Writing Waveguides in Glass with a Femtosecond Laser,” Opt. Lett. 21, 1729-1731 (1996). [2] Miura, K., Qiu, J., Inouye, H., Mitsuyu, T., and Hirao, K., “Photowritten Optical Waveguides in Various Glasses with Ultrashort Pulse Laser,” Appl. Phys. Lett. 71, 3329-3331 (1997). [3] Streltsov, A.M. and Borrelli, N.F., “Study of Femtosecond-Laser-Written Waveguides in Glasses,” J. Opt. Soc. Am. B 19, 2496-2504 (2002). [4] Coric, D., Herman, P.R., Chen, K.P., Wei, M.X., Corkum, P.B., Bhardwaj, R., and Rayner, D.M., “Contrasts in Writing Photonic Structures with Ultrafast and Ultraviolet Lasers,” Proc. of the SPIE 4638, 77-84 (2002). [5] Luff, B.J., Harris, R.D., Wilkinson, J.S., Wilson, R., and Schiffrin, D.J., “Integrated-optical directional coupler biosensor,” Opt. Lett. 21, 618620 (1996). [6] Kelly, D., Grace, K.M., Song, X., Swanson, B.I., Frayer, D., Mendes, S.B., and Peyghambarian, N., “Inegrated optical biosensor for detection of multivalent proteins,” Opt. Lett. 24, 1723-1725 (1999). [7] Masuda, M., Sugioka, K., Cheng, Y., Aoki, N., Kawachi, M., Shihoyama, K., Toyoda, K., Helvajian, H., and Midorikawa, K., “3 -D microstructuring inside photosensitive glass by femtosecond laser excitation,” Appl. Phys. A 76, 857-860 (2003). [8] Cheng, Y., Sugioka, K., Midorikawa, K., Masuda, M., Toyoda, K., Kawachi, M., and Shihoyama, K., “Control of the cross -sectional shape of a hollow microchannel embedded in photostructurable glass by use of a femtosecond laser,” Opt. Lett. 28, 55-57 (2003). [9] Cheng, Y., Sugioka, K., Midorikawa, K., Masuda, M., Toyoda, K., Kawachi, M., and Shihoyama, K., “Threedimensional microoptical components embedded in photosensitive glass by a femtosecond laser,” Opt. Lett. 28, 1144-1146 (2003). [10] Masuda, M., Sugioka, K., Cheng, Y., Aoki, N., Kawachi, M., Shihoyama, K., Toyoda, K., and Midorikawa, K., “3D microfabrication in photosensitive glass by femtosecond laser,” SPIE Proc. 4830, 576-580 (2002). [11] Cheng, Y., Sugioka, K., Masuda, M., Shihoyama, K., Toyoda, K., Midorikawa, K., “Opticla gratings embedded in photosensitive glass by photochemical reaction using a femtosecond laser,” Optics Express. 11, 1809-1816 (2003).
