Waveguide writing in fused silica with a femtosecond ... - OSA Publishing

Waveguide writing in fused silica with a femtosecond fiber laser at 522 nm and 1 MHz repetition rate Lawrence Shah, Alan Y. Arai Applications Research Laboratory, IMRA America, Incorporated 48834 Kato Road, Suite 106A, Fremont, CA 94538 USA [email protected], [email protected]

Shane M. Eaton, Peter R. Herman Edward S. Rogers Department of Electrical and Computer Engineering University of Toronto, 10 King’s College Road, Toronto, ON M5S 3G4 Canada [email protected], [email protected]

Abstract: We report on waveguide writing in fused silica with a novel commercial femtosecond fiber laser system (IMRA America, FCPA µJewel). The influence of a range of laser parameters were investigated in these initial experiments, including repetition rate, focal area, pulse energy, scan speed, and wavelength. Notably, it was not possible to produce lowloss waveguides when writing with the fundamental wavelength of 1045 nm. However, it was possible to fabricate telecom-compatible waveguides at the second harmonic wavelength of 522 nm. High quality waveguides with propagation losses below 1 dB/cm at 1550 nm were produced with 115 nJ/pulse at 1 MHz and 522 nm. © 2005 Optical Society of America OCIS codes: (230.7370) Waveguides; (350.3390) Laser materials processing; (320.2250) Femtosecond phenomena

References and links 1.

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Received 7 February 2005; revised 7 March 2005; accepted 7 March 2005

21 March 2005 / Vol. 13, No. 6 / OPTICS EXPRESS 1999

12. Y. Sikorski, A.A. Said, P. Bado, R. Maynard, C. Florea, and K. Winick, “Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses,” Electron. Lett. 36, 226-227 (2000). 13. S. Taccheo, G. Della Valle, R. Osellame, G. Cerullo, N. Chiodo, P. Laporta, O. Svelto, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses,” Opt. Lett. 29, 2626-2628 (2004). 14. Y. Kondo, K, Nouchi, T. Mitsuyu, M. Watanabe, P.G. Kazansky, and K. Hirao, “Fabrication of long-period fiber, gratings by focused irradiation of infrared femtosecond laser pulses,” Opt. Lett. 24, 646-648 (1999). 15. L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Writing of permanent birefringent microlayers in bulk fused silica with femtosecond laser pulses,” Opt. Comm. 171, 279-284 (1999). 16. K. Minoshima, A.M. Kowalevicz, E.P. Ippen, and J.G. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10, 645-652 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-15-645. 17. P.R. Herman, R.S Marjoribanks, A. Oettl, Burst-ultrafast laser machining method, US Patent (6,552,301 B2) 18. M. Will, J. Burghoff, J. Limpert, T. Schreiber, H. Zellmer, S. Nolte, and A. Tunnermann, “Generation of photoinduced waveguides using a high repetition rate fiber CPA system,” Conference on Lasers and Electro Optics 2003, CWI6. 19. M. Will, J. Burghoff, J. Limpert, T. Schreiber, S. Nolte, and A. Tunnermann, “High speed fabrication of optical waveguides inside glasses using a high rep.-rate fiber CPA system,” in Photon Processing in Micorelectronics and Photonics III, P.R. Herman, J. Fieret, A. Pique, T. Okada, F.G. Bachmann, W. Hoving, K. Washio, X. Xu, J.J. Dubowski, D.B. Geohegan, F. Trager, eds., Proc. SPIE 5339, 168-173 (2004). 20. T. Fukuda, S. Ishikawa, T. Fujii, K. Sakuma, and H. Hosoya, “Improvement on asymmetry of low-loss waveguides written in pure silica glass by femtosecond laser pulses,” in Optical Fibers and Passive Components, S. Shen, S. Jian, K. Okamoto, K. L. Walker, eds., Proc. SPIE 5279, 21-28 (2004). 21. S. Eaton, F. Yoshino, L. Shah, A. Arai, H. Zhang, S. Ho, and P. Herman, “Thermal heating effects in writing optical waveguides with a 0.1 - 5 MHz rate ultrafast fiber laser,” in Photonics West 2005, Proc. SPIE 5713A-7 (2005).

1. Introduction Internal laser modification of transparent glasses is a promising means for fabricating integrated micro-optical devices and optical circuits in novel 3-D architectures. While ultraviolet (UV) laser induced glass compaction [1,2] enables limited 3-D refractive index modification suitable for the production of buried optical waveguides [3,4], femtosecond lasers provide greater flexibility to construct complex 3-D structures. For the latter, nonlinear absorption confines laser interactions to the focal volume and reduces collateral damage in the surrounding material. Femtosecond lasers have been applied to fabricate buried waveguides [5-12], optical amplifiers [6,12,13], beamsplitters [6,7], directional couplers [8,9], long-period fiber gratings [14] and birefringent transmission gratings [14] in a variety of transparent materials. Despite these demonstrations, significant gaps remain in optimizing the laser interactions to increase processing speed and improve control over device characteristics. In this paper, a novel commercial femtosecond Yb-fiber laser system (IMRA America, FCPA µJewel) was employed to bridge a gap between two classes of laser types that have typically been applied in waveguide writing studies. On one side are amplified Ti:Sapphire laser systems that provide high pulse energies (~1 mJ) at low repetition rates (~1 kHz). Such lasers frequently yield waveguides with asymmetric refractive index profiles that exhibit significant coupling loss and birefringence. On the other side are low energy (~10 nJ) and high repetition rate (~10 MHz) oscillator-only systems [8,10,11,16]. These lasers yield smoother and more symmetric guiding regions due to the heat accumulation effects arising when the interval between laser pulses is less than the time required for the absorbed laser energy to diffuse out of the critical absorption volume [8,10,17]. However, such low energy oscillators demand a high numerical aperture (NA) focusing objective, whose short working distance limits the fabrication of 3-D devices. The combination of high pulse energy and MHz repetition rates was only recently demonstrated with an amplified femtosecond Yb-fiber laser to form waveguides in alkali-free borosilicate glass [18,19]. The laser is a compact commercial femtosecond pulse source providing flexible operating conditions ideally matched to writing optical waveguides in glasses. Variable repetition rate #6552 - $15.00 US

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with energies of >2.5 µJ at 100 kHz and >150 nJ at 5 MHz facilitate rapid identification and optimization of laser processing windows for various target glasses. The ~375-fs pulses were applied to fundamental studies of waveguide writing in fused silica. High quality waveguides are shown to be possible without ultrashort pulses (20 µm) precluded a more detailed analysis.

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Fig. 4. Plot of insertion loss vs. sample lengths for waveguide written at different scan speeds. Linear fit equations give propagation loss in dB/cm (slope) and coupling loss in dB (yintercept).

To date, the best reported propagation loss for waveguides written in fused silica is 0.1dB/cm [20]. By comparison, industry-adopted planar lightwave circuit (PLC) fabrication technologies such as plasma-enhanced chemical vapor deposition and flame hydrolysis offer