Atomic-scale imaging of dopant atoms and clusters in Yb-doped optical fibers

Abstract

Fabrication of Ytterbium--doped active fibers with different designs, compositions and high Yb concentration has attracted an intense interest. For making highly Yb--doped fibers, co--dopants like phosphorous (P) and aluminum (Al) are also employed in order to modify refractive index and increase Yb solubility, avoiding clusters and phase segregations. Indeed, Yb--clustering results in quenching effects and increased propagation losses due to energy transfer between clustered ions. Therefore, the chemical composition and phase homogeneity of the fiber core have key influences on the performance of an active fiber. However, conventional fabrication techniques such as MCVD (modified chemical vapor deposition) and OVD (outside vapor deposition) are approaching the limit.In this contribution, we have developed an approach for fabrication of such active fibres based on granulated silica derived from the sol--gel process. The advantage of this method is the fabrication of active fibers with high dopant contents and homogeneity. Here, using high angle annular dark--field scanning transmission electron microscopy (HAADF--STEM) in atomic scale, we report the direct, nano--scale and atomic--resolution observation of individual Yb dopant and co--dopant (i.e. Al, P) atoms for different fabricated fibers. The chemical mapping from STEM--EDX shows an extremely homogeneous distribution of the dopants and co--dopants in nano--scale for our fabrication protocol. However in atomic resolution, we also identified the possible Yb clusters in the range of 10 atoms within the core structure. The size, structure, and distribution of these clusters are determined with an Yb--atom detection efficiency of almost 100% by STEM.

Granulated Silica Method for the Fiber Preform Production

“…One of these methods is based on an iteratively melting and milling process for the granulated silica (oxide-based and sol-gel-based) to increase the homogeneity [6,9]. However, here we present our recent results ("Homogeneity" in Section 3.1.3) that this iterative melting and milling process it no longer needed to increase the homogeneity of the sol-gel-based granulated silica [7,8]. Another method used the stack-and-draw technique for homogenization [33] and reduced the scattering losses to values as low as 0.1 dB/m (measured at 1100 nm wavelength).…”

“…One main benefit of the sol-gel method is that doped silica material of high purity can be produce at temperature below 2000 • C in contrast to the standard methods [11,12,21,22]. Furthermore, the optical material derived from the sol-gel method possesses a high homogeneity (in nano-scale) [8].…”

“…These new concepts have radically changed the potential applications that we associate with a fiber [1][2][3][4][5]. During the last 13 years, we have studied two interesting ways of fiber production starting with the preform [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].…”

“…The goal of our studies was on the one hand to gain some compositional flexibility in doping the cores of fibers and on the other hand to have some more control over the microstructure of the fiber [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. To reach these two goals, we have been working on the sol-gel method for fiber material production and on the granulated silica method for preform assembly [6][7][8][9][10][11][12].…”

Progress in the fabrication of optical fibers by the sol-gel-based granulated silica method

“…The chemical mapping of our sol-gel-based fiber core is shown in Figure 5, with the result that no clustering was observed (compare with Figure 2) and an extremely homogeneous distribution of the dopant (Yb) and co-dopants (Al and P) in nano-scale was measured [17]. …”