Rare earth ions doped oxide glass materials

Abstract

The rapid increase in communication traffic in recent years has resulted in increasing efforts for the development of highly efficient optical amplifiers that will fully exploit the low-loss window (~1.2 μm to 1.7 μm) of silica fibers. Rare earth (RE)-doped optical amplifiers are the most commercially successful optical amplifiers. Erbium (Er3+)-doped silicate glass fiber amplifiers and phosphate glass planar waveguide amplifiers have been successfully developed for long-haul fiber optic networks and integrated optic circuits, respectively. However, the gain spectrum of Er3+ is only limited to the region from ~1.53 μm to 1.62 μm, which does not cover the entire wavelength range of the optical fiber low-loss window. Therefore, emissions of RE ions, which includes holmium (Ho3+, ~1.2 μm), praseodymium (Pr3+, ~1.3 μm), and thulium (Tm3+, ~1.47 μm), have emerged as research hotspots for signal amplification at O (original)-and S (short-wavelength)-bands. Although a single RE ion has the potential for special band amplification, the separate gain spectra of RE ions hardly exceed ~100 nm, limiting the development of new broadband optical amplifiers that cover the entire low-loss window. Therefore, to overcome the bottleneck in singly doped systems, the development of efficient RE ion codoped and triply doped fiber and waveguide amplifiers is desirable. Glasses have been the most popular host materials for RE ions. Oxide and non-oxide glasses possess excellent chemical durability and low phonon energy, respectively. Studies on RE ions have focused on sulfide, chalcogenide, and fluoride glasses, because these non-oxide glasses have ultra-low phonon energies that can reduce the multiphonon relaxation (MPR). However, the long-term chemical stability and high toxicity of these materials still limit their practical applications. Silicate and phosphate glasses are stable materials, but their phonon energies exceed ~1100 cm1, rendering them not favorable for special band amplifications. Practical applications of oxide glasses with sufficiently low phonon energy still offer an attractive solution in realizing broadband RE-doped glass fiber and waveguide amplifiers. Glasses based on tellurium oxide and germanium oxide possess lower phonon energies (~750 and ~880 cm1, respectively) than silicate and phosphate. They are also more thermally stable against crystallization and more corrosion resistant compared with fluoride and chalcogenide glasses. Moreover, tellurite glasses can provide broader bandwidth and larger emission cross-sections. Germanate glasses exhibit good photosensitivities, which can be used to change the refractive index by UV light, facilitating direct long-period gratings on the material. Therefore, tellurite and germanate glasses are excellent host candidates for the development of RE-doped fiber and waveguide amplifiers. This thesis is primarily aimed to fabricate and characterize RE-doped tellurite glass fiber amplifiers and germanate planar waveguide amplifiers as well as to demonstrate signal amplification from 1250 nm to 1650 nm wavelength. Through the optimization of glass compositions, a series of RE-doped germanium tellurite (NZPGT) glass fiber amplifiers and aluminum germanate (NMAG) glass channel waveguide amplifiers have been fabricated and characterized for long-haul fiber optic networks and integrated optic circuits, respectively. NZPGT glass fiber preforms have been obtained by convenient rod-in-tube method, which is simpler and easier to adopt in the laboratory. NMAG channel waveguides have been fabricated using the potassium (K+)–sodium (Na+) ion-exchange method in molten pure KNO3, because the K+–Na+ ion-exchange process is convenient and has low propagation losses. The optical gains and losses in RE-doped NZPGT glass fiber amplifiers have been investigated and discussed. First, to demonstrate NZPGT glass fibers, Er3+/Yb3+ co-doped NZPGT glass fiber amplifiers have been fabricated. A maximum gain coefficient of 2.38 dB/cm at 1.535 μm was obtained, which is larger than that of silicate glass amplifiers and among the highest in oxide glass amplifiers. Second, Tm3+-doped NZPGT glass fiber amplifiers have been prepared and investigated for S-band amplification. A maximum gain coefficient of 0.62 dB/cm at 1.476 μm, which is larger than that of ZBLAN fluoride glass amplifiers, was also obtained under high power pumping. Third, Nd3+/Tm3+/Er3+ triply-doped NZPGT glass fiber amplifiers have been prepared, and superbroadband near-infrared (NIR) emission from 1300 nm to 1600 nm has been recorded. Signal gains at 1330.1, 1477.1, and 1536.2 nm wavelengths were demonstrated within the superbroadband emission. Fourth, superbroadband NIR emissions of Pr3+: 1D21G4 transition from 1280 nm to 1680 nm have been achieved and investigated in NZPGT glasses. The emission cross-section profile of Pr3+ in NZPGT glasses is observed to be larger than those of Ho3+ in gallate glasses and Tm3+ in germanate glasses at E- and S-bands, respectively. The NIR emissions, optical losses, and amplification performances in RE-doped K+–Na+ ion-exchanged NMAG glass channel waveguides have also been investigated and discussed. Sm3+ singly-doped K+–Na+ ion-exchanged NMAG glass channel waveguide was prepared. The NIR and visible multichannel radiative transitions from 4G5/2 level of Sm3+ are observed and characterized for optical amplification and photodynamic therapy, respetively. K+–Na+ ionexchanged channel waveguide has been fabricated in Nd3+-doped NMAG glasses. Intense NIR emissions from the 4F3/2 level have been recorded and investigated for O-band amplification. Additionally, Ho3+ singly- and Ho3+/Yb3+ co-doped NMAG glasses have been prepared. The maximum theoretical gain coefficients of 1196 nm and 2035 nm emissions are 1.0 and 2.1 dB/cm, respectively, revealing that Ho3+-doped NMAG glasses have potentials for ~1.2 m wavelength planar amplifiers, and ~2.0 m waveguide lasers can be fabricated from Ho3+/Yb3+ co-doped NMAG channel waveguide. Broadband NIR emission from ~1.4 μm to 1.6 μm was achieved in Tm3+/Er3+ codoped K+–Na+ ion-exchanged channel waveguide amplifiers and a signal gain of 3.67 dB was demonstrated in Tm3+/Er3+ codoped waveguide amplifier. Superbroadband emission from 1300 nm to 1600 nm was demonstrated in Nd3+/Tm3+/Er3+ triply-doped K+–Na+ ion-exchanged NMAG glass channel waveguide, indicating its potential application for superbroadband amplifier operating in the conventional (C)-, long-wavelength (L)-, E-, and S-bands. Fifth, superbroadband emission from 1300 nm to 1700 nm was also observed in Pr3+-doped NMAG glasses, indicating that superbroadband amplification can also be realized in Pr3+-doped ion-exchanged NMAG glass channel waveguide. In summary, NZPGT and NMAG oxide glasses with excellent chemical stability, corrosion resistance, and low phonon energy (<900cm-1) have been developed and optimized for REdoped fiber amplifiers and planar waveguide amplifiers. A series of RE-doped NZPGT fibers and NMAG ion-exchanged waveguides have been fabricated, and their spectral properties, as well as optical amplification performances, have been investigated. Through RE multidoping schemes, including Tm3+/Er3+ and Nd3+/Tm3+/Er3+, an alternative strategy in achieving broader emission bandwidth is proposed and obtained in RE codoped and, triply doped NZPGT fibers and NMAG ion-exchanged waveguides. Potential UV-direct writing gratings will give rise to attractive photonic devices such as superbroadband amplifier, tunable laser, and broadband light sources covering the E-, S-, C- and L- bands.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yue Bun Edwin PUN (Supervisor)