This research targets a new approach for realizing high-performance erbium-doped waveguide amplifiers (EDWAs) for wavelength division multiplexed optical fiber communication networks at a cost suitable for widespread metropolitan area network deployment.
Intellectual Merit: The merit of this effort lies in a proposed GaAs-based EDWA which incorporates recent patent-pending discoveries: a high quality oxidized InAlP erbium host material, and oxidation-smoothed low-loss high-index-contrast ridge waveguides enabled by a relatively new nonselective wet thermal oxidation technique, with a novel resonant vertical intra-cavity pumping (VIP) scheme for the compact, monolithic integration of a resonant-enhanced excitation source. The phosphate-rich InAlP native oxides afford very high Er doping concentrations with long fluorescence decay lifetimes and a very broad amplification band. VIP excitation offers high pumping efficiency, even pump distribution, multi-stage pumping for noise control, simplified packaging, and the possibility for increased photonic integration.
Broader Impacts: The impacts of this work flow from the expected significant reductions in component cost which will benefit society by making ultra-high-speed multi-wavelength optical communications viable for the home, schools and medical offices, thus promoting the accelerated development and adoption of bandwidth-intensive applications such as digital video, video conferencing, and internet communications across multiple platforms. The research may also lead to eye-safe high-energy pulsed waveguide laser sources for range finding and broadband amplified spontaneous emission sources for chemical sensing. The project promotes teaching, training and learning through the integration of research activities with teaching of undergraduate engineering majors, Research Experience for Teachers participants, and the mentoring of underrepresented minorities.
Researchers at the University of Notre Dame in the US have developed an Er/Yb co-doped compound semiconductor native oxide that significantly enhances laser absorption efficiency, thus increasing the viability of realising a low-cost, monolithically-integrated wavelength amplifier with on-board pump laser source. Wavelength division multiplexing is a widely used technology in long haul optical communications networks where as many as 100 different closely spaced channels of data are simultaneously transmitted in an optical fibre, with each channel sent by a slightly different wavelength laser. Fibre amplifiers are used to amplify all of these signals simultaneously within the fibre all optically, so that optical-electrical-optical conversions requiring a number of components are unnecessary. A pump laser excites doping atoms which can then provide gain (optical amplification) for the signals traveling in the fibre, boosting their power as much as 1000 times. These signals are photons generated by single wavelength lasers, all spaced 50-100 GHz apart at wavelengths around ~1550 nm. An erbium doped fibre amplifier (EDFA) is optical fibre based. Erbium is doped into the aluminosilicate glass optical fibres, and discrete fibre couplers and pump lasers are used to couple in the higher energy photons necessary to excite the erbium. Each stage of the EDFA uses an erbium doped fibre that is 5-20 meters long, and the overall EDFA system may have 2 or 3 gain stages. An EDFA system may cost tens of thousands of dollars and occupy roughly the space of a shoebox. To overcome these issues, the team have used erbium doped waveguide amplifiers (EDWA), which is a planar waveguide implementation of an EDFA where the fibre is replaced by a waveguide integrated onto a chip or photonic integrated circuit (PIC). The smaller dimensions require much higher Er doping concentrations, which can be challenging to achieve while maintaining the same optical efficiency of the gain process. However, an EDWA can potentially be much less expensive and more compact than an EDFA, enabling the WDM protocol of all-optical multichannel networks to be deployed more widely in a metropolitan area network (MAN) or local area network (LAN). Erbium has been used for a long time in silica optical fibres because erbium energy levels are spaced such that electron transitions between them have an energy spacing matching the frequency band where optical fibres have their lowest attenuation loss. However, this group has shown that this effect can be improved by co-doping with ytterbium. Ytterbium is a sensitiser, previously demonstrated in silica fibres, which has its own different set of energy levels, some of which overlap those of erbium’s. But Yb is a more efficient absorber of light (it has a higher absorption cross section), which allows the pump energy to be more completely absorbed and transferred to the Er atoms which can then effect amplification. While the work is not the first to exploit this co-doping, it is the first demonstration of the efficacy of Er/Yb co-doping in a unique compound semiconductor native oxide host developed by the team at Notre Dame. This patented approach allows the monolithic integration of the required pump lasers with the Er/Yb-doped oxide waveguides, as the pump lasers are formed from the same compound semiconductor platform which is oxidised to form the optical waveguide. The group has also shown that the InAlP native oxide can support high concentrations of Er with the desirable rare earth properties of a long luminescent lifetime and broad emission spectrum, potentially superior to other materials previously explored for EDWA devices. An issued US patent details possible implementations for monolithically combining the pump lasers and Er-doped waveguides on a single chip. One approach is a vertical-intracavity-pumping (VIP) scheme which places the Er-doped waveguide core within the cavity of a vertical cavity surface emitting laser (VCSEL) device. Photonic integration of advanced semiconductor lasers and optical amplifiers will continue to be driven by the rapid growth of internet and wireless communications traffic, while also extending more and more into the silicon microelectronics arena to advance the power of computer chips, systems, and data centres.
