In-band pumping of expitaxially grown $Er:(Gd, Lu)_2O_3$ waveguides for active integrated optical devices

摘要

Monocrystalline lattice matched Er(0.6%):(Gd, Lu)2O3 films with thicknesses up to 3 m and nearly atomically flat surfaces have been deposited on Y2O3 substrates by Pulsed Laser Deposition (PLD). The epitaxial growth has been verified in-situ by Reflection High Energy Electron Diffraction (RHEED). As first test structures channel waveguides with widths ranging from 2 to 5 m and heights from 400 to 700 nm have been fabricated by Reactive Ion Etching and finally covered by about 1.5 m thick a-Al2O3 top claddings to reduce scattering losses at the surface. Due to the refractive index difference between film and substrate (0.03 at 1.55 m) rib waveguiding could be demonstrated. The fluorescence spectra of the films are comparable with those of Er:Y2O3 bulk crystals. Therefore, a theoretical gain spectrum could be calculated from the absorption and emission spectra of Er:Y2O3. Bleaching at the pump wavelength of 1480 nm results in an inversion ratio 0.75 between the 4I13/2 and 4I15/2 multiplet of Er3+ and thus to a theoretical gain of 260% at 1535 nm for a 7 mm long waveguide.Gain measurements using a tunable diode laser (1530 nm to 1583 nm) as signal and a pump diode at 1480 nm have been performed. Both sources have been combined in a single mode fiber and then coupled into the waveguide. Using the lock-in technique and a monochromator, the signal intensity I of the outcoupled light could be measured in dependance of the pump power (up to 100 mW before coupling into the waveguide) and the gain determined as (Ipumped – Iunpumped) / Iunpumped. Calculations show that the small waveguide dimensions allow bleaching with absorbed powers well below 1 mW. It has been experimentally verified that the incident signal of 0.5 mW was sufficient to already bleach the waveguide in the unpumped case. Therefore, the reduction of signal absorption in the pumped case can be neglected and does not falsify our measured gain. At 1535 nm a maximum gain of 150% (4 dB) could be measured for the 7 mm long sample, resulting in a gain of 5.7 dB/cm. The measured gain is lower than the theoretically expected, because the intensity distribution of the guided mode is not limited to the doped regions. Taking this into account by normalizing the theoretical gain spectrum to the maximum measured gain of 150% results in a good accordance of the experimentally determined gain at various wavelengths with the theoretical spectrum (see Fig. 1c). The high gain obtainable due to bleaching in the small waveguiding structures combined with the relative high peak cross sections of crystalline RE doped (Gd, Lu)2O3 makes these waveguides promising for the development of compact lasers with low lasing threshold and high frequency stability.