Extreme nonlinear processes in fibers
Up-conversion luminescence (UL) involves the absorption of multiple photons, followed by the emission of higher energy photons via the radiative decay of excited electron energy levels in dielectric materials. UL has attracted considerable attention, since its first observation in rare-earth-doped crystals, because of its use in a wide range of fields, from physics to chemistry and biology: UL allows for the detection of several stimuli such as temperature, electromagnetic radiation and pH. Thanks to materials nanoengineering development, UL has been recently proposed for medical therapy too. In physics, UL finds its main application in the generation of visible laser emission by means of infrared optical pumping.
In the context of fiber lasers, various authors have studied UL in optical fibers, doped with semiconductors, such as cesium, europium, and tantalum. In their first report of UL in silica glass, which is the main constituent of commercial optical fibers, Kazansky et al. attributed the observation of multiphoton absorption (MPA) excited UL in Ge-doped silica glass to the presence of Germanium Oxygen deficient center (Ge-ODC) defects However, intrinsic and induced defects in silica were already studied by using ultraviolet (UV)-pumped visible photoluminescence. Among these, the presence of the so-called non-bridging Oxygen hole center (NBOHC) defects, responsible for visible red-light emission at 650 nm, attracted significant interest.
Fig.1 a) Experimental set-up to study the silica defects UL. b) Comparison between the UL of standard 50/125 GRIN and Step-index fibers. c) SEM images of the GRIN fiber section. d) Ge concentration in mass along the segment traced in c).
In our lab, we experimentally observed and theoretically described for the first time, the presence of visible UL in commercial multimode fibers (MMFs), pumped by intense femtosecond infrared laser pulses (see the experimental set-up in Fig.1a). Owing to their large numerical aperture, MMFs are better suited than singlemode fibers to trap luminescent radiation. Moreover, because of their relatively large core size, MMFs may carry light beams with peak powers up to the self-focusing critical value, before any permanent damage occurs. The presence of photoluminescence and nonlinear losses, which were ascribed to a MPA mechanism, was recently reported by us in graded-index (GRIN) MMFs. The latter are characterized by a micron-scale periodic spatial self-imaging (SSI) that plays the key role in enhancing the losses. The UL becomes visible to the naked eye as an array of side-scattering blue emitters while only one bright luminescence point appears in Step-index fibers (Fig.1b). An SEM image and the Ge concentration profile of the GRIN fiber are reported in Fig.1d. In Fig.2 we report a comparison of the GRIN and Step-index fibers nonlinear attenuation as a function of the input pulse energy. This result point out the role of SSI that drastically reduces the pulse energy threshold for nonlinear losses. These introduce a fundamental, and previously undisclosed, nonlinear limitation to the energy transmission capabilities of optical fibers, and may provide an intrinsic limitation for the power scaling of spatiotemporal mode-locking with multimode fiber lasers.
Fig.2 Comparison between the nonlinear attenuation of GRIN and Step-index fibers as a function of the input pulse energy.
Fig.3 a) Side-scattered spectra in log scale for different input peak powers showing the emerging of the silica defects UL. b) Schematic of a 5 photon absorption excited UL in a three level system. c) Log-log plot of the integral of the UL peaks in a) as a function of the input peak power. The parameter n in the legend is the slope of the fitting line and indicates the number of photons involved in the MPA process.
In our studies, we unveil a previously unforeseen nonlinear phenomenon in optical fibers, and demonstrate up to 5-photon absorption processes, which are responsible for UL generation (see Fig.3). Our experimental results are in good quantitative agreement with numerical simulations, based on a generalized nonlinear Schrödinger equation (GNLS).
Zitelli, Mario, Mangini, Fabio, Ferraro, Mario, Niang, Alioune, Kharenko, Denis, Wabnitz, Stefan, “High-energy soliton fission dynamics in multimode GRIN fiber,” OPTICS EXPRESS, vol. 28, p. 20473-20488 (2020)
Hansson, Tobias, Tonello, Alessandro, Mansuryan, Tigran, Mangini, Fabio, Zitelli, Mario, Ferraro, Mario, Niang, Alioune, Crescenzi, Rocco, Wabnitz, Stefan, Couderc, Vincent, “Nonlinear beam self-imaging and self-focusing dynamics in a GRIN multimode optical fiber: Theory and experiments,” OPTICS EXPRESS, vol. 28, p. 24005-24021 (2020)
Mangini, Fabio, Ferraro, Mario, Zitelli, Mario, Niang, Alioune, Tonello, Alessandro, Couderc, Vincent, Wabnitz, Stefan, “Multiphoton-absorption-excited up-conversion luminescence in optical fibers,” PHYSICAL REVIEW APPLIED, vol. 14, 054063, p. 1-6 (2020)
STEMS – Spatiotemporal multimode complex optical systems
WAVESCOPE – Wavefront Shaping System for Nonlinear Fiber-Based Microscopy and Endoscopy