JLMN-Journal of Laser Micro/Nanoengineering Vol. 5, No. 1, 2010 Fabrication of a Y-splitter Modulator Embedded in LiNbO 3 with a Femtosecond Laser Yang Liao *1 , Jian Xu *1 , Ya Cheng *1† , Zhizhan Xu *1‡ , Koji Sugioka *2 and Katsumi Midorikawa *2 *1 State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechan- ics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China *2 Laser Technology Laboratory, RIKEN- Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan †Email: ycheng-45277@hotmail.com ‡Email: zzxu@,mail.shcnc.ac.cn We describe a technique to integrate embedded microelectrodes and optical waveguides in Lith- ium niobate (LiNbO 3 ) using a femtosecond laser, and a Y-splitter modulator was demonstrated. The Y-splitter was fabricated by femtosecond laser direct writing and the embedded microelectrodes were fabricated by combining femtosecond laser ablation with selective electroless plating. The em- bedded microelectrodes give rise to homogeneous electric field across the optical waveguides, which could result in effective electro-optic overlap. The simple and flexible technique could open new opportunities for fabricating integrated 3D electro-optic devices. Keywords: electro-optic integration, lithium niobate, microelectrode, Y-splitter modulator, femtosecond laser 1. Introduction Lithium niobate (LiNbO 3 ) has long been used in inte- grated optics due to its large nonlinear optical and electro- optical coefficients. Integrated electro-optic devices based on waveguiding structures have gained significant attention, such as optical switches [1], modulators [2-4] and electro- optically tuned quasi-phase-matched (QPM) devices [5-7]. Conventionally, the waveguide fabrication is based on ion diffusion or proton exchange, which permits fabrication of channel waveguides only close to the surface. Femtosecond laser microfabrication, as an emerging tool in last decade, has shown powerful capabilities of three-dimensional (3D) integration. Recently, it also has been demonstrated that optical waveguides in LiNbO 3 can be fabricated by femto- second laser pulses [8-11], which opens the possibility to write 3D optical circuits in the crystal. For fabrication of integrated electro-optic devices, it is crucial to design and fabricate electrodes. Currently, the electrodes are ordinarily fabricated based on lithographic methods, such as depositing thin layers of metals followed by pattern etching. However, due to the inherently planar nature of the lithographic process, this technique is limited to fabrication of 2D microstructures. M. Reich et al. [12] have reported topographical electrodes for poling lithium niobate by laser ablation as a simple patterning method superior to conventional surface electrodes. However, the topographical electrodes must be completely contacted with liquid electrolyte, which limited its application in in- tegrated devices. As a simple and flexible technique, laser- induced selective electroless deposition has been widely studied over the past two decades [13-15]. Recently, we developed a technique of selective metallization in insula- tor substrates using femtosecond laser ablation and femto- second laser assisted selective electroless plating [16-17]. The shape and dimension of the electrodes could be accu- rately controlled by changing the conditions of femtosec- ond pulsed laser ablation, which in turn leads to controlla- ble electric field distribution inside LiNbO 3 crystal. In this paper, we present a technique to integrate em- bedded microelectrodes and waveguides in LiNbO 3 using a femtosecond laser. Based on this technique, a Y-splitter modulator was demonstrated. 2. Experimental Commercially available MgO-doped x-cut LiNbO 3 crystals were used in the experiments. A femtosecond laser micromachining workstation was used to fabricate optical waveguides and microelectrodes, which consisted of a 40 fs Ti:Sapphire laser (Legend USP, Coherent Inc.) operated at 800 nm wavelength and 1 kHz repetition rate, and a computer-controlled XYZ translation stage with a resolu- tion of 1 μm. In order to produce thermally stable waveguides in the low repetition rate regime, we used an approach of writing two parallel lines in close separation, which produces a guiding region between the double lines [9]. In comparison with the waveguides guiding light in irradiated region (type ), the waveguides guiding light between irradiated regions (type ) could preserve the nonlinearity of the bulk crystal [18]. The laser beam was focused with a 100× mi- croscope objective (NA=0.90), and was incident along x axis of crystal and linearly polarized along the y axis (see Fig. 1). The waveguides were fabricated by consecutively writing two parallel lines separated by 10 μm with pulse energies of 0.3 μJ and a translating velocity of 100 μm/s. The focus was located at a depth of 50 μm. In order to realize the electro-optic modulation, three embedded microelectrodes were integrated into the LiNbO 3 crystal, as shown in Fig. 1. The fabrication process of em- bedded microelectrodes mainly consists of four steps: 25 DOI: 10.2961/jlmn.2010.01.0006