Segmented waveguides realized by single-shot femtosecond modifications in glass

Whereas most of the photonic integrated circuits are based upon planar technologies, femtosecond writing permits the fabrication of waveguides inside the bulk of transparent materials such as glasses [1]. Indeed, as ultrashort intense pulses are focused inside a material, nonlinear absorption is triggered via the multi-photon ionization. The energy deposited inside the material results in permanent modifications when a threshold is overcome. The modifications consist in the simplest case of localized changes in the refractive index. Waveguides can then be written by tailoring a bell-shaped increase in the refractive index. At a first glance, it would seem that the shape and refractive index contrast of the waveguide can be controlled at will by reshaping the beam and its power. In reality, unavoidable nonlinear effects modify the writing beam in a power-dependent manner [2]. On top of that, the light-matter interaction comprises additional nonlinearities working at different scales, e.g., Kerr effect and thermal nonlinearity. In other words, nonlinear effects-including accumulation phenomena-play a relevant role in determining which waveguides can be effectively written. From the side of integrated optics, in segmented waveguides the size of the mode can be controlled for a given transverse distribution of refractive index by longitudinally modulating the waveguides [3]. As a matter of fact, the concept of segmented waveguides has been employed in direct fs-writting to realize light-written waveguides behaving like a Bragg filter [4]. Here, we extend the concept by studying the confinement properties of a chain of single-shot defects written in a borosilicate glass. On the applicative side, we show a vast control on the mode width, a fundamental requirement given that fs-written integrated circuits have to be coupled with external devices such as optical fibers. From the point of view of basic physics, the collective action of the chain of defects strongly enhances the effect of the written refractive index on optical probes, thus disclosing a much easier characterization of the induced modifications, a fundamental tool for understanding the fundamental mechanism behind fs-writing.