Yean Sheng Yong

Direct confocal lifetime measurements on rare-earth-doped media exhibiting radiation trapping

Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 – 5 × 1021 cm−3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid.

Temperature dependence of transition cross sections in rare-earth-doped laser materials

The transition cross sections on the pump and laser wavelength of rare-earth-doped materials are of crucial importance for their performance as amplifiers and lasers and for the understanding of their performance in a rate-equation model. Since typically part of the absorbed pump power is converted to heat, the temperature of the host material increases with increasing pumping. This implies a temperature-dependent change of the transition cross sections, which needs to be carefully quantified.

High gain in erbium-doped channel waveguides

Integration of multiple functions on an optical micro-chip is going to revolutionize the exploitation of optics for various applications such as communication, optical sensing, and biomedicine. One of the enabling functions is amplification at 1.5 μm [1]. Rare-earth-doped amplifiers typically deliver a net gain per unit length of only a few dB/cm [2]. In spiral-shaped channel waveguides a total internal net gain of 20 dB was demonstrated [3].

Temperature-dependent absorption and gain of ytterbium-doped potassium double tungstates for chip-scale amplifiers and lasers

Ytterbium-doped potassium rare-earth double tungstate thin films are excellent candidates for highly efficient waveguide lasers, as well as high-gain waveguide amplifiers, with a record-high optical gain per unit length of 935 dB/cm recently demonstrated. However, the spectroscopic properties of these highly ytterbium-doped thin films and, in particular, their temperature dependence are not well investigated. These characteristics are required for the understanding of the behavior of the fabricated optical devices and crucial for further device optimization. We experimentally determined the absorption cross-sections for a potassium ytterbium gadolinium double tungstate, KYb0.57Gd0.43(WO4)2, thin film grown lattice matched onto an undoped KY(WO4)2 substrate. At room temperature, the peak cross-section value at 981 nm and the overall absorption spectrum are very similar to those of Yb-doped bulk potassium double tungstate crystals, although Yb is now the dominating rare-earth content. The temperature-dependent study shows a significant decrease of the absorption cross-section values at 933 nm and 981 nm with increasing temperature. We verify theoretically that this is due to the temperature dependence of fractional populations in the individual Stark levels of the absorbing crystal-field multiplet, in combination with the linewidth broadening with increasing temperature. Further investigations suggest that the broadening of absorption linewidth at 981 nm originates in the intra-manifold relaxation between the two lowest Stark levels of the ground state. Finally, the implications of the spectroscopic findings on the operating characteristics of waveguide amplifiers are investigated. Amplifiers operating at 80 °C are expected to exhibit only 67% of the maximum theoretical gain at room temperature.

Spectroscopy of erbium-doped potassium double tungstate waveguides

We report the spectroscopy of crystalline waveguide amplifiers operating in the telecom C-band. Thin films of erbiumdoped gadolinium lutetium potassium double tungstate, KGdxLuyEr1-x-y (WO4)2, are grown by liquid- phase epitaxy onto undoped potassium yttrium double tungstate (KYW) substrates and micro-structured by Ar+- beam etching. Channel waveguides with erbium concentrations between 0.45–6.35 × 1020 cm-3 are characterized. The transition cross-sections of interest are estimated. The effect of energy-transfer up-conversion (ETU) is experimentally investigated. Microscopic and macroscopic ETU parameters are extracted from a simultaneous analysis of 20 decay curves of luminescence on the transition 4I13/2 → 4I13/2. The correlation between ETU and the doping concentration is studied. Pump excited-state absorption (ESA) on the transition 4I11/2 → 4F7/2 is investigated via a direct ESA measurement using a double-modulation pump-probe technique. The effect of ESA is studied for different pump wavelengths. The pump wavelength of 984.5 nm is found to be favorable for the complete range of erbium concentrations.

Ultra-low-loss and broadband mode converters in Si3N4 technology

Si3N4 grown by low pressure chemical vapor deposition (LPCVD) on thermally oxidized silicon wafers is largely utilized for creating integrated photonic devices due to its ultra-low propagation loss and large transparency window (400 nm to 2350 nm). In this paper, an ultra-low-loss and broadband mode converter for monolithic integration of different materials onto the passive Si3N4 photonic technology platform is presented. The mode size converter is constructed with a vertically tapered Si3N4 waveguide that is then buried by a polymer or an Al2O3 waveguide. The influence of the various design parameters on the converter characteristics are investigated. Optimal designs are proposed, in which the thickness of the Si3N4 waveguide is tapered from 200 nm to ~40 nm. The calculated losses of the mode converters at 976 nm and 1550 nm wavelengths are well below 0.1 dB for the Si3N4-polymer coupler and below 0.3 dB for the Si3N4-Al2O3 coupler. The preliminary experimental results show good agreement with the design values, indicating that the mode converters can be utilized for the low-loss integration of different materials.

Low-loss, broadband and high fabrication tolerant vertically tapered optical couplers for monolithic integration of Si3N4 and polymer waveguides

A low-loss, broadband and high fabrication tolerant optical coupler for the monolithic integration of Si3N4 and polymer waveguides is designed and experimentally demonstrated. The coupler is based on the adiabatic vertical tapering of the Si3N4 waveguides. Low-loss operation is experimentally verified at both 976 and 1460-1635xa0nm wavelengths. Measured losses per coupler are as low as 0.12 and 0.14xa0dB at 976 and 1550xa0nm, respectively, and below 0.2xa0dB at both wavelengths for lateral misalignments between the Si3N4 and polymer waveguides up to 1.0xa0μm.