Structural modifications in Er-Yb doped phosphate glass induced by femtosecond laser waveguide writing

Abstract

ABSTRACT We have systematically studied femtosecond-laser fabrication of optical waveguides in an Er-Yb doped phosphate glass. Waveguides were written using the IMRA America FCPA Jewel D-400 femtosecond fiber laser system with pulse repetition rates ranging from 250 kHz to 2.2 MHz. At every pulse repetition rate a series of waveguides was written while varying scan speeds from 50 m/s to 100 mm/s and pulse energies from 80 nJ to 320 nJ. The optical quality of the waveguides was evaluated by measuring the waveguide mode profile as well as the optical loss. Laser-induced defects and structural changes in the glass were characterized using confocal fluorescence and Raman microscopy. Keywords: Femtosecond laser, confocal spectroscopy, Er-Yb doped phosphate glass 1. INTRODUCTION The use of highly localized and permanent refractive index changes in dielectric materials induced by femtosecond (fs) lasers has attracted an immediate interest in a number of applied research areas. This phenomenon has become increasingly important to applications involving telecommunications devices. The high spatial precision of these induced modifications has allowed for the fabrication of three dimensional micro-optical devices and photonic structures. Direct femtosecond laser inscription has been used to fabricate many passive optical devices such as waveguides, couplers, Bragg gratings, and splitters [1-9]. However, femtosecond laser writing in Erbium-doped materials has recently gained a great deal of attention because of its potential use to fabricate waveguide lasers that operate in the C-band [10]. When ultrashort near-infrared laser pulses are tightly focused inside a transparent material, high intensities can be achieved where nonlinear absorption mechanisms dominate the laser energy deposition process. The most common energy absorption mechanism that explains structural changes induced by fs-lasers involves nonlinear photoionization and avalanche ionization that result in the formation of a localized plasma. Recombination of the plasma once the laser pulse is gone results in the transfer of that hot electron-ion plasma energy to the surrounding lattice [11-14]. This process, although not understood in detail, results in a material state with altered structural properties and produces refractive index changes [15,16]. At high laser repetition rates the energy does not fully dissipate between laser pulses and will begin to accumulate, thus leading to a localized temperature increase and melting. The structural changes that occur to the glass network will be dominated by a number of melting and cooling dynamics that have not been fully investigated for highly doped glasses such as Er-Yb doped phosphate glass [17-20]. Refractive index changes produced during fs-writing will likely depend on a number of different factors including glass composition, focusing conditions, and fs-laser parameters. Recently it has been reported that high quality, low loss waveguides written in Er-Yb doped phosphate glass can be fabricated under fs-laser conditions where cumulative thermal effects take place [21]. In order to use such glasses for photonic devices it becomes increasingly important to understand how the composition of the glass as well as writing conditions at high repetition rates will effect the waveguide fabrication. In this paper we have fabricated optical waveguides for various fs-laser writing conditions and studied the structural changes of Er-Yb doped phosphate glass after fs-laser irradiation at both the macroscopic scales and atomic scales using in situ confocal white light microscopy and laser spectroscopy.