Rare-earth ion doped chalcogenide waveguide amplifiers

Abstract

Chalcogenide glass waveguide devices have received a great deal of attention worldwide in the last few years on account of their excellent properties and potential applications in mid-infrared (MIR) sensing and all-optical signal processing. Waveguide propagation losses, however, currently limit the potential for low power nonlinear optical processing, and the lack of suitable on chip integrated MIR sources is one of the major barriers to integrated optics based MIR sensing. One approach to overcome the losses is to employ rare-earth ion doped waveguides in which the optical gain can compensate the loss, in such a way that the conversion efficiency of nonlinear effects is increased significantly. For infrared applications, the long wavelengths potentially attainable from rare-earth ion transitions in chalcogenide hosts are unique amongst glass hosts. New rare-earth ion doped chalcogenide sources in the MIR range could benefit molecular sensing, medical laser surgery, defence etc. Despite these promising applications, until now, no one has succeeded in fabricating rare-earth ion doped chalcogenide amplifiers or lasers in planar devices. This work develops high quality erbium ion doped chalcogenide waveguides for amplifier and laser applications. Erbium ion doped As2S3 films were fabricated using co-thermal evaporation. Planar waveguides with 0.35 dB/cm propagation loss were patterned using photolithography and plasma etching on an erbium ion doped As2S3 film with an optimised erbium ion concentration of 0.45x1020 ions/cm3. The first demonstration of internal gain in an erbium ion doped As2S3 planar waveguide was performed using these waveguides. With different film deposition approaches, promising results on intrinsic lifetime of the Er3+ 4I13/2 state were achieved in both ErCl3 doped As2S3 films (2.6 ms) and radio frenquency sputtered Er3+:As2S3 films (2.1 ms), however, no waveguide was fabricated on these films due to film quality issues and photopumped water absorption issues. The low rare-earth ion solubility of As2S3 is considered the main factor limiting its performance as a host. Gallium-containing chalcogenide glasses are known to have good rare-earth ion solubility. Therefore, a new glass host material, the Ge-Ga-Se system, was investigated. Emission properties of the bulk glasses were studied as a function of erbium ion doping. A region between approximately 0.5 and 0.8 at% of Er3+ ion was shown to provide sufficient doping, good photoluminescence and adequate lifetime to envisage practical planar waveguide amplifier devices. Ridge waveguides based on high quality erbium ion doped Ge-Ga-Se films were patterned. Significant signal enhancement at 1540 nm was observed and 50 % erbium ion population inversion was obtained, in waveguides with Er3+ concentration of 1.5x1020 ion/cm3. To the Author's knowledge, this is the highest level of inversion ever demonstrated for erbium ions in a chalcogenide glass host and is an important step towards future devices operating at 1550 nm and on the MIR transitions of erbium ions in chalcogenide glass hosts. Photoinduced absorption loss caused by upconversion products in the waveguides is the remaining hurdle to achieving net gain. Further research is needed to find suitable compositions that possess high rare-earth ion solubility whilst avoiding the detrimental photoinduced losses.