Laser Direct Writing of Photonic Structures in X-cut Lithium Niobate using Femtosecond Pulses S. Venugopal Rao a *, T. Shuvan Prashant, a K.L.N. Deepak, b Surya P. Tewari, a G. Manoj Kumar, a and D. Narayana Rao b a Advanced Center for Research in High Energy Materials, b School of Physics University of Hyderabad, Hyderabad 500046, India * Author for Correspondence: svrsp@uohyd.ernet.in OR soma_venu@yahoo.com ABSTRACT We have fabricated straight line structures and Y-couplers in X-cut lithium niobate crystals using femtosecond laser pulses. A systematic characterization study was performed initially to determine the effects of pulse energy on feature size. The optimal parameters were determined from experiments and simulations obtained using a two dimensional split step beam propagation method. Later the waveguides and couplers were fabricated using these optimized parameters. We present our results on the physical and optical characterization of these structures. Keywords: lithium niobate, femtosecond, laser direct writing, photonic devices, beam propagation 2. INTRODUCTION Lithium Niobate (LNB) has emerged as promising material for fabricating integrated optoelectronic circuits because of its favorable physical properties such as large anisotropy, high electro-optic coefficients and large second order nonlinearity. LNB crystals are positively birefringent and, hence, the extraordinary refractive index n e for z-plane polarized light is higher than the ordinary refractive index n o along the x and y direction. We refer to the crystal as X-cut or Z-cut depending on which crystal axis is perpendicular to the top surface. Many approaches have been implemented to fabricate waveguides on LNB crystals including ion implantation, liquid phase epitaxy and laser direct writing (LDW) [1-4]. However, the LDW technique stands apart from others because of the relative ease of realization of the microstructures and possibility of 3D photonic devices and integration. Incident intensity of the focused ultrashort pulses determines the type of modification in LNB: (a) an increase in refractive index n e (type I) (b) a decrease of both n o and n e (type II) and (c) formation of micro voids [1, 2]. Thomas et al. have recently achieved a hybrid fs laser written chip in lithium niobate that comprised a rare-earth-doped laser section, a frequency doubling unit, Bragg reflectors, waveguide splitters, and an amplitude modulator [2]. However, detailed studies with different pulse durations and writing conditions are essential and enable us to identify the optimal conditions for achieving low loss (insertion, propagation etc.) photonic structures in lithium niobate [10-14]. 3. EXPERIMENTAL DETAILS The schematic and the experimental set up are illustrated in figure 1. The femtosecond oscillator amplifier system is capable of generating ultrashort pulses of ~100 fs duration at a repetition rate of 1 kHz and wavelength of 800 nm. A vertical microscope configuration is employed using a dry 40X (0.65 NA) microscopic objective (Olympus) to focus the laser beam on to the sample and 3D XYZ stages were employed to translate the sample [5]. A Brewster polarizer-half wave plate combination was used to control the intensity of the pulses. M1 to M5 are the mirrors. The crystals were X- cut and were polished and cleaned/sonicated in distilled water before laser direct writing. The modified regions of the crystal were examined using confocal and micro-Raman spectroscopy. Initially, the energy dependence of feature size was characterized. The parameters namely, the stage scan speed, the number of writes and the energy of the pulse were varied and optimized for realizing the photonic devices. Photonics 2010: Tenth International Conference on Fiber Optics and Photonics, edited by Sunil K. Khijwania, Banshi D. Gupta, Bishnu P. Pal, Anurag Sharma, Proc. of SPIE Vol. 8173, 81730G © 2011 SPIE CCC code: 0277-786X/11/$18 · doi: 10.1117/12.899559 Proc. of SPIE Vol. 8173 81730G-1 Downloaded from SPIE Digital Library on 24 Aug 2011 to 14.139.69.1. Terms of Use: http://spiedl.org/terms