Abstract: Process for locally increasing the refractive indexes of an electrooptical material usable in guided optics and material obtained by this process.The process according to the invention consists of carrying out an implantation of ions (6) of sulphur or a metallic dopant able to take the place of a cation of the crystal lattice of the ferroelectric material (2), followed by annealing at between 300.degree. and 600.degree. C. in order to rearrange the crystal lattice disturbed during implantation and for activating the implanted ions. Ion implantation is carried out in an inclined direction (60) with respect to the normal to the surface of the monocrystalline material (2). For a LiNbO.sub.3 monocrystal, use is made of Ti ions as the dopant. This process makes it possible to produce a guide layer (4) having ordinary and extraordinary refractive indices above those of the pure monocrystal.