CC BY
13LM-O-20
Direct Laser Writing in Silica and K8 Glass in Athermal
Regime
Vladislav Likhov, Andrey Okhrimchuk
Prokhorov General Physics Institute of the Russian Academy of Sciences, Dianov Fiber Optics Research Center, 38 Vavilov Str, Moscow 119333, Russia Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq, Moscow 125047, Russia
vladislavlikhov@gmail.com
Direct Laser Writing (DLW) is a well-established micromachining technique used, in particular, in manufacturing optical waveguides in silica and BK7 glasses [1]. However, most works devoted to the subject are dealing with writing in thermal regime [2] where high pulse repetition rate ensures energy delivery is faster than heat diffusion [3]. However, laser structuring in thermal regime does not allow for a micrometer-scale refractive index modulation because the modified region considerably exceeds the laser exposed one. Meantime, submicrometer-scale precision is frequently needed in laser micromachining, and, in particular, in Bragg grating manufacturing.
Thus, we investigated refraction index change in these glasses in athermal inscription regime. We wrote and studied fs-laser inscribed tracks with different scan speeds and input pulse energies in the volume of silica and K8 glasses (a close analogue of Schott BK7® glass). We used an Yb:KGW femtosecond laser system with a pulse duration of 180 fs and pulse repetition rate of 5 kHz. The objective lens (NA = 0.65) was used for focusing and a 1 mm wide slit was placed before the objective lens in order to achieve a lenticular-shaped beam waist instead of a "standart" cigar-shaped beam waist. Tracks were inscribed at ~75 ^m depth. The refractive index change (An) was estimated on the basis of measured phase difference (A9) using a quantitative phase microscopy (QPm) technique.
a) b)
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Silica glass —o— 0.1 mm/s —O—0.2 mm/s —o— 0,4 mm/s —o— 0.8 mm/s —o— 1.6 mm/s K8 glass --♦- 0.1 mm/s --♦-0.2 mm/s --♦--0,4 mm/s ÎHa/P_ Silica glass —o— 0.1 mm/s —o— 0.2 mm/s —c— 0.4 mm/s —O— 0.8 mm/s —o— 1.6 mm/s K8 glass --♦-0.1 mm/s -♦-0.2 mm/s --♦-- 0.4 mm/s
F
;
--♦- 0.8 mm/s 1,6 mm/s ! -♦-0.8 mm/s -♦--1.6 mm/s
i . » ; ------------------.. «
r r/ *-
i
*
1500 200C
0 500 1000 1500 2000 0 500 11 Pulse Energy (nJ)
Fig. 1 .Refractive index change in silica and K8 glasses versus input pulse energy for polarization of the writing beam perpendicular (a) and parallel (b) to scan direction; transverse profile of phase difference A^ of tracks in K8 glass (c) and silica glass (d) for writing speed V = 0.4 mm/s and polarization of the writing beam perpendicular to scan direction.
As seen from Fig. 1, silica glass experiences only positive An under investigated energy range while in K8 glass An is positive for small input pulse energies (~130 nJ), and dramatically drops to negative values with energy increase. It is important to note that tracks written with high energy and low scanning speed in silica glass were disrupted similarly to "quill" writing described by Kazansky et al. [4]. These tracks are not presented on Fig. 1 due to impossibility of measuring their An.
The FWHM track widths are equal to 0.88 ^m (128 nJ) and 0.98 ^m (215 nJ) for K8, while for silica glass the values are 0.7 ^m (172 nJ) and 0.75 ^m (210 nJ). Maximum magnitude of the refractive index change is 3-10-3 and 1-10-3 for K8 and silica glass, correspondingly.
This prompts the conclusion that both investigated glasses are suited for submicrometer-scale micromachining; together, they can cover a wide range of applications due to different sign of the refractive index change.
[1] Davis, K. Miura, et al. "Writing waveguides in glass with a femtosecond laser." Optics letters 21.21 (1996): 1729-1731.
[2] Chen, George Y., et al. "Femtosecond-laser-written microstructured waveguides in BK7 glass." Scientific reports 8.1 (2018): 1-7.
[3] Streltsov, Alexander M., and Nicholas F. Borrelli. "Study of femtosecond-laser-written waveguides in glasses." JOSA B 19.10 (2002): 24962504. 79
[4] Kazansky, Peter G., et al. ""Quill" writing with ultrashort light pulses in transparent materials." Applied physics letters 90.15 (2007): 151120.
