Potassium double tungstate waveguides with high ytterbium concentration for optical amplification

In this thesis, the research work concerning high ytterbium concentration potassium double tungstate waveguides catered for optical amplification purpose is presented. The scope of the research work includes the investigation of spectroscopic and optical gain properties in epitaxy layers with concentration of trivalent ytterbium (Yb3+) up to 76 at.%, which is equivalent to Yb3+ density of ~5 × 1021 cm-3. Spectroscopic properties of the high ytterbium concentration epitaxy layers, such as luminescence lifetime, transition cross-sections, and their temperature dependence, are examined carefully and in detail. A novel confocal measurement setup is proposed to mitigate the radiation trapping effect which elongates the measured lifetime. It is confirmed that concentration dependent lifetime quenching on high Yb3+ concentration potassium double tungstate epitaxial layers is rather weak. Further analysis on the power dependent luminescence decay curves reveals the presence of energy transfer upconversion (ETU) process. The cross-section spectra of the high ytterbium concentration potassium double tungstate epitaxy layers are similar to those of bulk crystals. It is observed that the cross-section spectra change drastically with the increase of temperature due to two reasons: the fractional population at the starting Stark level and the linewidth of the respective transition at the given temperature. The material gain in thin film configuration are investigated via experimental and numerical approaches. Net gain value of 2.62 dB (817 dB/cm) is achieved in 32 μm thick epitaxial layer without any thermal management using pump wavelength of 932 nm and signal wavelength of 981 nm. In overall, the work described in this thesis provide advances in understanding the characteristics of high Yb3+ concentration potassium double tungstate waveguide layers. The experimental results show that favorable spectroscopic properties are retained in the epitaxial layers. Nevertheless, additional effects such as ETU process, quenched ions, and localized heating within the pumped region have also been discovered and analyzed. Particularly, elevated temperature on the gain medium would severely affect the absorption and emission behavior. The investigation of optical gain and luminescence spectra shows that the thermal effects play a role in high active ion concentration and intensely pumped amplifier

Concentration quenching of luminescence lifetime in ytterbium-doped potassium double tungstate waveguide amplifiers

Integrated optical amplifiers employing ytterbium-doped potassium double tungstates exhibit an ultra-high peak gain exceeding 1000 dB/cm, in addition to a broad gain of up to 150 dB/cm over a 55-nm wavelength range. Here we report a study of the luminescence lifetime in samples spanning a broad range of dopant concentrations (1.2−57.5 at.%). By use of the pinhole method, elongation of the luminescence lifetime due to radiation trapping is avoided, providing a more accurate analysis of concentration quenching of the luminescence lifetime, which directly influences the waveguide amplifier performance and can, in principle, inhibit scaling of the gain with increasing dopant concentration

Highly Yb-doped KGd(WO₄)₂ thin-film amplifier

We report record-high small-signal gain of 1050 dB/cm at 981 nm wavelength in a KGd0.425Yb0.575(WO4)2 thin film. The sensitivity of gain to the shift of beam-focus position, which is critical under non-waveguiding conditions, is investigated

Gain dynamics in a highly ytterbium-doped potassium double tungstate epitaxial layer

Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd0.43Yb0.57WO42, representing an Yb3 concentration of ∼3.8 × 1021 cm−3, grown on an undoped KYWO42 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb3 ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB∕cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling

Low-loss highly tolerant flip-chip couplers for hybrid integration of Si3N4 and polymer waveguides

In this letter, low-loss and highly fabrication-tolerant flip-chip bonded vertical couplers under single-mode condition are demonstrated for the integration of a polymer waveguide chip onto the Si3N4/SiO2 passive platform. The passively aligned vertical couplers have a lateral misalignment between polymer and Si3N4 waveguide cores of ±1.25 μm. Low-loss operation has been experimentally demonstrated over a wide spectral window of 1480-1560 nm, with measured coupler losses below 0.8 dB for Si3N4 taper angles below 1.2°, in good agreement with the calculated values. Furthermore, thermal shock test results show less than 0.1 dB degradation, indicating a robust coupling performance

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-dependent absorption and emission of potassium double tungstates with high ytterbium content

We study the spectroscopic properties of thin films of potassium ytterbium gadolinium double tungstates, KYb0.57Gd0.43(WO4)2, and potassium ytterbium lutetium double tungstates, KYb0.76Lu0.24(WO4)2, specifically at the central absorption line near 981 nm wavelength, which is important for amplifiers and lasers. The absorption cross-section of both thin films is found to be similar to those of bulk potassium rare-earth double tungstates, suggesting that the crystalline layers retain their spectroscopic properties albeit having >50 at.% Yb3+ concentration. The influence of sample temperature is investigated and found to substantially affect the measured absorption cross-section. Since amplifiers and lasers typically operate above room temperature due to pump-induced heating, the temperature dependence of the peak-absorption cross-section of the KYb0.57Gd0.43(WO4)2 is evaluated for the sample being heated from 20 °C to 170 °C, resulting in a measured reduction of peak-absorption cross-section at the transitions near 933 nm and 981 nm by ~40% and ~52%, respectively. It is shown that two effects, the change of Stark-level population and linewidth broadening due to intra-manifold relaxation induced by temperature-dependent electron-phonon interaction, contribute to the observed behavior. The effective emission cross-sections versus temperature have been calculated. Luminescence-decay measurements show no significant dependence of the luminescence lifetime on temperature