DOI: 10.1007/s00339-005-3324-z Appl. Phys. A 81, 1633–1638 (2005) Materials Science & Processing Applied Physics A d.k.y. low ✉ h. xie z. xiong g.c. lim Femtosecond laser direct writing of embedded optical waveguides in aluminosilicate glass Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, Singapore 638075 Received: 20 June 2005/Accepted: 23 June 2005 Published online: 25 August 2005 • © Springer-Verlag 2005 ABSTRACT Using tightly focused femtosecond laser pulses to irradiate lines in aluminosilicate glass, embedded lines with in- creased refractive index, which function as optical waveguides were observed. The pulse energy (4.5–11.2 μ J) and writing speed (50–700 μ m/s) were shown to affect the resultant op- tical properties of the waveguides such as the magnitude of refractive index change, core diameter and propagation mode. At pulse energies above 5 μ J, two types of structures were observed, namely an inhomogeneous void-like structure and a cross-sectional crack-like structure. These structures were found to affect significantly the resultant waveguiding proper- ties of the irradiated lines. Using pulse energy of 5 μ J or below, single mode waveguides were fabricated. Raman spectroscopy showed that the fs laser pulses generated structural changes to the aluminosilicate glass. The fabrication of a 1 × 4 splitter was also demonstrated. PACS 42.62.-b; 42.82.-m; 81.05.Kf 1 Introduction In recent years there has been an increasing interest in using fs lasers to directly create waveguides in bulk trans- parent materials such as fused silica glass [1, 2], chalcogenide glass [3], Nd-doped glass [4], boron-doped silica glass [5], fluoride glass [6], etc. By tightly focusing a femtosecond (fs) laser beam inside the glass and traversing either the beam or the sample, lines with a change in refractive index can be induced. Typically, glasses are transparent to laser irradi- ation at near-infrared wavelengths. However, when a near- infrared fs laser beam is focused inside a transparent material, the intensity in the focal volume can become high enough (10 14 W cm −2 ) to cause nonlinear absorption, avalanche ion- isation and microplasma formation [7]. If sufficient energy is deposited, permanent structural change can occur, leading to a change in refractive index. As a result of the higher refrac- tive index of the written lines as compared to the bulk glass, the lines consequently function as optical waveguides that can guide light without radiating into the surrounding glass. This is akin to the case of moulding a strand of fibre inside a bulk ✉ Fax: +65 67912927, E-mail: dlow@SIMTech.a-star.edu.sg of glass, except that the optical path of the waveguide can be easily controlled when writing directly with a fs laser beam. As a consequence, this technique offers greater flexibility in device design, ease of fabrication and cost savings for the fab- rication of waveguides and planar 2D or 3D devices in optical glass. Being a single-step process as compared to lithographic and ion implantation methods which utilises multi-step pro- cesses, the laser direct write method offers many promising advantages for the fabrication of optical interconnects and de- vices. Through a nonlinear, multiphoton absorption process, ma- terials that do not normally absorb near-infrared light through a linear single-photon process can absorb the fs beam at the focal volume. As such, this negates the need for having the costly process of doping the material to initiate the ab- sorption process. Several mechanisms have been proposed for the induced refractive index change by fs pulses. Using electron spin resonance (ESR) spectroscopy and fluorescence spectroscopy, color center formation in the glass has been found and suggested to have contribution to the refractive index change [8]. Another mechanism proposed was the den- sification of the glass due to melting by fs pulses and the associated resolidification dynamics. This mechanism has been supported recently with Raman spectroscopy measure- ments [2, 9]. A review of the possible mechanisms has been discussed by Schaffer et al. [10]. In this paper, investiga- tions were conducted on inducing refractive index changes and thus optical waveguides in aluminosilicate glass with 150 fs pulses. The effects of laser pulse energy, writing speed and number of scans on the magnitude of the resultant refractive index change and dimensions of the modified structures were studied. The fabrication of waveguide devices was demon- strated. While refractive index changes and device fabrication have been realised in a number of glass systems, to our best knowledge, studies on aluminosilicate glass have not been reported. 2 Experimental procedures A 800 mW regenerative amplified Ti-sapphire fs laser emitting at 775 nm wavelength was used in the study. The 150 fs laser pulses were delivered at a frequency of 1 kHz. An infinity-corrected objective lens with a numerical aper- ture (NA) of 0.54 was used to focus the beam inside the glass