Aniso t r o pi c la t tic e c h a n g e s in fe m t o s e c o n d la s e r in s c ri b e d N d 3 + : M g O:Li N b O 3 o p tic al w a v e g ui d e s M a r ti n e z M a e s t r o, L, Ro d e n a s , A, R a mi r e z, M O, Tor c hi a, G A, Ro s o, L, C h e n, F a n d Ja q u e , D h t t p:// dx. d oi.o r g/ 1 0. 1 0 6 3/ 1. 3 1 6 8 4 3 2 Ti t l e Aniso t r o pic la t tic e c h a n g e s in fe m t o s e c o n d la s e r in s c ri b e d N d 3 + : M g O:Li N b O 3 o p ti c al w av e g ui d e s A u t h o r s M a r ti n e z M a e s t r o, L, Ro d e n a s , A, R a mi r e z, M O, Tor c hi a, G A, Ro s o, L, C h e n, F a n d Ja q u e, D Typ e Articl e U RL This ve r sio n is a v ail a bl e a t : h t t p:// u sir.s alfo r d. a c. u k/id/ e p ri n t/ 5 3 3 6 6/ P u b l i s h e d D a t e 2 0 0 9 U SIR is a di git al c oll e c tio n of t h e r e s e a r c h o u t p u t of t h e U niv e r si ty of S alfo r d. W h e r e c o py ri g h t p e r mi t s, full t e x t m a t e ri al h el d in t h e r e p o si to ry is m a d e fr e ely a v ail a bl e o nli n e a n d c a n b e r e a d , d o w nlo a d e d a n d c o pi e d fo r n o nc o m m e r ci al p riv a t e s t u dy o r r e s e a r c h p u r p o s e s . Pl e a s e c h e c k t h e m a n u s c ri p t fo r a n y fu r t h e r c o py ri g h t r e s t ri c tio n s. Fo r m o r e info r m a tio n, in cl u di n g o u r p olicy a n d s u b mi s sio n p r o c e d u r e , pl e a s e c o n t a c t t h e R e p o si to ry Te a m a t: u si r@ s alfo r d. a c. u k .We report on the fabrication and microspectroscopy imaging of femtosecond laser written double-filament based Nd 3+ : MgO: LiNbO 3 optical waveguides. The waveguiding high refractive-index regions are identified by blueshifts of the Nd 3+ ion fluorescence lines with no deterioration in the fluorescence efficiency, whereas filamentary low-index regions are identified by both a Nd 3+ line redshift and a fluorescence efficiency reduction. The lattice structural micromodifications at the origin of both waveguide formation and Nd 3+ fluorescence changes have been investigated by means of confocal micro-Raman experiments. We have found that the direct laser written filaments are mainly constituted by a large density of defects, together with a marked axial compression perpendicular to the filaments ͑along the optical c-axis͒. Conversely, the high-index waveguiding regions are characterized by a pronounced anisotropic dilatation of the LiNbO 3 lattice xy-planes.