Fluorescence in planar and ridge waveguides fabricated in Erbium-Doped lithium-niobate-on-insulator (Er:LNOI)

Summary form only given. Recently, the state-of-the-art of lithium-niobate-on-insulator (LNOI) has been reviewed [1]. In analogy to silicon-on-insulator (SOI), the new material platform for integrated optics with wafers of up to 3” diameter enables the fabrication of optical waveguides with small cross-sections and small bending radii due to the high refractive index contrast. As a consequence, ultra-compact photonic integrated devices such as ring resonators and photonic crystal structures have been developed.Here we demonstrate also Er-doped LNOI, which might become an ideal material for nano-optical amplifiers and lasers. Doping is achieved by pre-diffusion of Er into the upper LN wafer before bonding [2], and the resulting Er concentration is about 1020 cm-3. Ridges of different width w = (2.5 - 10) μm were fabricated on these LNOI substrates (LN thickness of 960 nm) using precision diamond-blade dicing which results in smooth side walls of the fabricated waveguides. We present results of a first investigation of the erbium centers using Raman and fluorescence spectroscopy of planar and ridge waveguides in Er:LNOI.Characterization of the waveguides was performed using Raman and fluorescence spectroscopy. Spectral scans were obtained using a scanning confocal microscope and a 488 nm laser. Besides line broadening that may be attributed to internal strain in the bonded layer, analysis of the Raman spectrum shows no significant difference between the waveguide and the bulk material. However, the erbium emission of the 4H15/2 and 4S3/2 to the 4I15/2 contains several differences in the intensity ratios indicating that the relative transition probabilities are different for the smart cut waveguide. While Raman intensity has a linear relationship with pump power, the intensity of the Er emission starts saturating at pump levels of only a few mW. To investigate fluorescence of the 4I13/2 - 4I15/2 transition inside the formed ridges, a fiber coupled lase- with a wavelength of 980 nm is used for pumping. The rhs of the figure above shows a diced ridge and the green fluorescence that can be observed when the ridge is pumped at 980 nm. An example of the IR fluorescence is given on the right of the above figure using 40 mW of pump light, with the spectrum of an erbium doped fiber as a reference. The emission is broadened and maxima are shifted to longer wavelengths, which may be attributed to defects induced by implantation, re-absorption of fluorescence light, and contributions by amplified spontaneous emission due to the waveguiding effect.