Highly efficient laser action in femtosecond-written Nd:yttrium aluminum garnet ceramic waveguides

Abstract

We report continuous wave 1.06 m laser operation in an optical waveguide fabricated in a Nd:YAG ceramic by femtosecond laser writing. Single mode and stable laser oscillation have been achieved by using the natural Fresnel reflection for optical feedback. Output laser power in excess of 80 mW and a laser slope efficiency of 60% have been demonstrated. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2890073͔Femtosecond direct laser writing ͑DLW͒ of transparent materials is attracting much attention because of the unique possibility of three-dimensionally modifying, at the micrometric and submicrometric scale, the optical properties of the irradiated media. This technique has been already proved to be a powerful and flexible tool for the fabrication of a great variety of optoelectronic components such as photonic crystals, diffraction gratings, and optical memories.1-3 When femtosecond pulses are focused inside a dielectric material, a permanent change in the refractive index is produced, in such a way that optical waveguides could be generated. This possibility has been already demonstrated in a great variety of glasses and crystals. [4][5][6] The further use of the DLW technique for the fabrication of low-loss channel waveguides in laser materials could be highly advantageous with respect to other fabrication approaches, and could also lead to technology breakthroughs in the development of threedimensionally integrated optical circuits.Among the different solid state laser media, neodymium doped yttrium aluminum garnet ͑YAG͒ transparent ceramics are nowadays attracting a great interest because of its advantages over the traditionally used Nd:YAG crystals. These advantages are the lower manufacturing costs, the possibility of high neodymium contents without any decrease in the optical quality of the gain medium, and also the possibility of direct composite fabrication.7 As a matter of fact, the laser performance of Nd:YAG ceramics has been found to be equal or even superior to that corresponding to Nd:YAG crystals. 8 Recently, authors reported on the fabrication of near surface channel waveguides in Nd:YAG ceramics.9 Nevertheless, up to date no attempt has been made, to the best of our knowledge, for the fabrication of buried channel waveguides in Nd:YAG ceramics by femtosecond DLW. The possible application of such waveguides as reliable and integrated laser sources is, therefore, still unexplored.In this letter, we report on the fabrication of buried channel waveguide lasers in Nd:YAG ceramics by using a two line confinement approach. Light confinement has been achieved between two parallel tracks due to filamentation of the femtosecond laser pulses. The possible influence of the waveguide fabrication process on the spectroscopic properties of the neodymium ions has been investigated by timeresolved confocal microscopy. We have also demonstrated highly efficient and stable laser oscillation based on the femtosecond written waveguide.The Nd:YAG ceramic sample used in this work was provided by Baikowski Ltd....

Femtosecond Fabrication of Waveguides in Ion-Doped Laser Crystals

“…Thus a waveguide is usually created in the area adjacent to tracks written by femtosecond beam. Since in this case waveguiding is due to an indirect effect accompanying femtosecond modification of crystal lattice, magnitude of refractive index change in the waveguide is not so high as in glass waveguides and does not exceed 1*10 -3 (Nejadmalayeri, 2005;Torchia, 2008;Siebenmorgen, 2009;Silva, 2010;Bookey, 2007;. This value is not enough to build compact waveguide lasers with diode pumping.…”

“…The 1 kHz Ti:Sapphire laser always produces a negative PRICE in YAG single crystals (Okhrimchuk, 2005(Okhrimchuk, -a, 2005Siebenmorgen, 2009). It was reported that in the ceramic YAG doped with 2 at.% of Nd 3+ ions PRICE is negative too (Torchia, 2008). However there is refractive index increase in the area closely adjacent to a track (Dubov, 2004;Siebenmorgen, 2009) that is due to elasto-optical effect.…”

Fluorescence imaging of lattice re-distribution on step-index direct laser written Nd:YAG waveguide lasers

“…More details about these waveguides can be found elsewhere. 7 As commented before, these waveguides did not show any lattice re-distribution so they behave as homogeneous line broadened laser sources. In this case, the laser threshold was also close to 60 mW, i.e., similar to that of MS-DLW WGs.…”

“…4 I 11/2 transition of Neodymium ions). 7,8,15 One of the main drawbacks of Nd:YAG DLW waveguides fabricated with low repetition rate lasers is the unavoidable damage and stress, as well as the impossibility of writing increased step-index type waveguides. The importance of being able to inscribe step-index increased waveguides (Type I WGs in the following), relies on the fact that these type of waveguides (step-index direct written cores) could exhibit superior light confinement and lower bend losses, as well as much more controllable mode dispersion properties, than stress-induced waveguides.…”

“…4 DLW has been found especially appropriate for the local modification and controlled fabrication of laser waveguides in rare earth doped YAG crystals and ceramics. [5][6][7][8][9][10][11][12] In the simplest channel waveguide fabrication scheme (in which waveguides are constituted by either a single or double laser-written line tracks), waveguiding is produced at the stress affected volume (generally called Type II stress-induced waveguides) where the refractive index changes are produced through the piezo-optic effect. 10 In other cases, waveguiding is produced by a low-index cladding.…”