Fig. 1. (a) Time evolution of the output beam power from a 30 cm long GRIN fiber span, for different values of the input peak power. The legend shows the values of input pulse energy and peak power, respectively. (b) Slow temporal evolution of the output spectrum, for fixed input pulse peak power P=1.41  MW (see Visualization 1). (c) Zoom-in of (b), for the first 285 s. (d) Same spectra of (c) at 7 temporal instants of time.

Fig. 2. (a) Wavelength dependence of the linear loss profile in simulations. (b) Numerical simulation of output spectrum changes versus input pulse energy. All parameters are the same as in experiments of Fig. 1. (c) Relationship between input pulse energy and long time scale obtained by fitting the violet curve in Fig. 1(a) with an exponential curve. (d) Same spectra of (b) at 8 temporal instants of time.

Fig. 3. Comparison between MPI regime established at 1.03 μm and 1.55 μm of wavelength: (a) transmission; (b), (c) luminescence; (d), (e) damages imaged by optical microscopy. The white bars in (b)–(e) correspond to 100 μm. The input energy and the peak power are E=323  nJ and P=1.64  MW at 1.03 μm and E=125  nJ and P=1.64  MW at 1.55 μm, respectively. The images in (d), (e) have been obtained after 1.5 h of exposure to the laser.

Fig. 4. X-ray imaging. (a) Radiography of a brand-new GRIN 50/125 fiber. (b) Corresponding intensity profile along the transverse direction x, averaged over 200 μm in z. (c) 3D rendering of the μ-CT image of a fiber, damaged by the exposure for 1.5 h to a laser pulse train at 1.03 μm wavelength and with 5.54 MW of peak power. Red, yellow, and blue zones are obtained by filtering the μ-CT intensity, as illustrated in panel (b). (d) Section of the fiber in the x–y plane.

Fig. 5. Analysis of fiber damages. (a) Optical microscope image. (b) Comparison between the intensity profile along the fiber axis in (a) (black curve) with the diameter of the red zone (red area). (c) Variation along z of the diameter of each zone.