Fluorescence lifetime imaging and μ-spectroscopy of Yb-doped materials

Summary form only given. Analyzing and profiling the dopants in laser active materials, especially fibers and preforms is an important task. These information allows you to monitor the preform fabrication and drawn fibers and investigate their quality. It has been show that using micro-spectroscopy and fluorescence lifetime images (FLIM) can be used to detect and quantify erbium ions in silica samples [1]. The gathered lifetimes and spectra are not only used for monitoring the dopants profile, but also to gather data like emission cross-sections for simulations and so predict the fibers behavior for running a laser. In this contribution, we set up a laser scanning confocal fluorescence microscope, able to analyze light above 950nm in contrast to devices built for biological analysis in the visible and near infrared below 1 μm. A laser diode at 910nm (up to 200mW) was used to excite ytterbium ions. The emitted light is evaluated with a spectrum analyzer for micro spectroscopy and an avalanche photodiode for lifetime imaging. A validation of measured lifetimes was done with a 3 at% doped Yb:CaF2 sample (1.9ms,[2]) and 1 at% doped Yb:YAG sample(1ms,[2]). So, gathering data is not restricted to the host material. The resolution was tested at the edge of an optically contacted undoped to doped volume of Yb:YAG. For fibers this led to a resolution of 5μm (10% to 90% criterion) and for preforms, which are placed at different position of the microscope, 25μm. In Fig. 1 we are showing an ytterbium doped preform example with varying phosphorus and aluminum doping. Here, the main characteristic spectral change is, for instance, the peak wavelength of the 1010nm-1035nm ytterbium emission and the intensity. Fig.1 shows normalized spectra from the full spectrally resolved fluorescence image from an ytterbium doped preform with three doped areas and a single non-doped sacrificial layer. The center is doped with Al2O3 and Yb2O3. The two doped rings are additionally doped with P2O5. The contrast coded in color is defined by the the peak wavelength of the second broad peak at around 1030 nm. The brightness shows the normalized intensity of the integrated fluorescence spectrum. It can clearly be seen that the rising content of Phosphorus(0 to 1 mol%) shifts the peak from 1033 nm to 1026nm. The width of this peak is slightly increasing from 21nm to 23nm (not shown here). Investigating the same preform slice in a FLIM measurement (not shown here) gives us following lifetimes: Ring 1 and Ring 2 have a mean lifetime of 0.92ms while the center region has a lifetime of 0.85ms. The longer lifetime fits well with the observations made in [3], where a rising phosphorus concentration led to a rising lifetime.