Erbium-ytterbium ions co-doped glass waveguide amplifiers for optical communications

Abstract

Erbium (Er)-doped fiber amplifiers are well suited for long-haul systems, however they are still too bulky and costly to be used in local area and access networks, with a view to have fiber to the home configuration in the future. Also, localized optical losses in the components/interconnections call for the integration of optical amplifiers in integrated optics circuits (IOC) to compensate the losses and provide efficient light transmission. Following these demands, compact and cost-effective integrated Er-doped waveguide amplifiers (EDWAs) are desirable. Research and development for integrated EDWAs are facing two challenges of the development of both rare-earth doped materials and waveguide fabrication technology. This thesis is mainly concerned with the use of thermal two-step ion-exchange and focused proton beam writing (PBW) to fabricate waveguide amplifiers on Er3+-Yb3+ co-doped phosphate glasses, respectively. The aim of this work is to investigate EDWA devices specially designed for short-haul access networks and its potential application in IOC. A detailed study on the spectroscopy characterization of commercially available phosphate glasses with different Er³+-Yb³+ concentrations was based on spectroscopic measurements and Judd-Ofelt theory. The fluorescence lifetimes of the 4I13/2 level were measured, and compared with those calculated theoretically using Judd-Ofelt analysis. The radiative quantum efficiency of the 4I13/2 to 4I15/2 transition is estimated to be ~100%. This indicates that no significant quenching effects are observed in the glasses with high Er3+ concentrations. Due to its high phonon energy, phosphate glasses have the lowest green upconversion efficiency and thus give a very strong 1.5μm infrared emission. These characteristics show that Er³+-Yb³+ co-doped phosphate glasses as optical gain media exhibit superior properties for laser and amplifier applications. A widely used numerical analysis of EDWAs is a combination of the finite-element method with a Runge-Kutta algorithm, however it involves complicated process and time-consuming calculations. Moreover, the validation of the results depends not only on the accurate numerical methods, but also on the accuracies in the characteristic parameters used in the models. An overlapping-factor method was proposed to model the Er³+-Yb³+ co-doped waveguide amplifiers (EYDWAs) using the propagation rate equations, and the performances of EYDWAs were optimized. The model was also validated by comparing the theoretical data and the experimental results. This practical overlapping-factor method does not require time-consuming calculations, and provides useful guidelines in the design and optimization of EYDWA devices. Various experimental approaches have been used to achieve buried glass waveguides. Unfortunately, the current practices are found to be complicated, expensive, and the glass surface deterioration is easily introduced. Thus, a simple and effective method is preferred. The substantial part of this thesis has been devoted to the EYDWAs fabricated using K+-Na+ ion-exchange and field-assisted annealing (FAA) process. Theoretical and experimental studies on buried glass waveguides were presented. Two basic mechanisms of thermal mobility of K+ ions at high temperature and a force resulting from the applied electric field were used to explain the buried waveguide formation. An analytical model for the FAA process shows that high electric field and short FAA time are more useful in the fabrication of buried waveguides. An improved gain coefficient of ~3.4±0.1dB/cm was measured for a ~1.2cm long waveguide amplifier, and this gain per unit length is comparable to that presented using a complicated field-assisted ion-exchange method. For a ~4.0cm long device, a total net gain of ~12.0dB and a related noise figure of ~4.8dB at 1.534μm were obtained with 200mW pump power of 975nm wavelength, which is one of the highest gain values reported within a short length. Most of integrated optical waveguides are generally fabricated using a photolithography followed by reactive ion etching process. This method involves many processing steps, and lead to a long fabrication time and low yield. To overcome the above limitations, a focused PBW technology has been performed to direct write channel waveguides in Er³+-Yb³+ co-doped phosphate glasses, for the first time. Proton ion doses in the range 1014-1015 ions/cm2 and 2.0MeV energy were used. The mechanism of PBW waveguide formation is different from that of those slab waveguides with an “optical barrier” concept, and the ionization process makes a significant contribution to an increase of the refractive index. The spectroscopic properties and optical gains of the waveguide amplifiers were also characterized, and a maximum net gain coefficient at 1.534μm signal wavelength was measured to be ~1.72±0.15dB/cm with 100mW pump power at 975nm wavelength. Comparing to the waveguide amplifiers using ion implantation, this is a largest gain per unit length to date. To summarize, the work described in this thesis provides a systematic study of Er³+-Yb³+ co-doped planar waveguide amplifiers related to two fabrication technologies. Some of the results will be relevant and useful for other waveguide materials and other rare earths as well.

Date of Award	3 Oct 2005
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yue Bun Edwin PUN (Supervisor)