Abstract

Active waveguide arrays are of particular interest for optical network applications as laser sources and integrated amplifiers [1,2]. In this paper we report on fabrication procedure and optical characterisation of an active buried-channel waveguide array fabricated on a special erbium-ytterbium-doped phosphate glass by the ion-cxchange technique. We used a new glass composition optimized for Ag-Na exchange with custom Er and Yb concentrations chosen on the basis of the results of a numerical code that calculates the optical gain at 1.5 µm under pumping at 980 nm. After photolithography on a titanium mask directly sputtered on the glass surface, the waveguides were fabricated by means of a double-step Ag-Na ion exchange process. The first step is based on thermal diffusion in molten salt with 2% in weight of silver nitrate at a temperature 345 °C for a diffusion time of ~2 hours. In the second step, the waveguide array was buried by a field-assisted ion-exchange technique (~24 V/mm at 345°C for 60 min diffusion time) in order to reduce the scattering losses, using a special set-up developed on purpose. The waveguides ( 100-µm spacing between channels) were fabricated on a 1-mm thick 15-mm long glass substrate. All waveguides show uniform optical properties. Wc measured scattering losses below 0.1 dB/cm and polarisation-dependent losses below 0.03 dD/cm at both 1.5 µm and 1.3 µm. The mask apertures (4 µm) and the parameters of the diffusion process were chosen to obtain an approximately circular graded index profile able to sustain a relatively large single-transverse mode at 1.5 µm, allowing for optical pumping with both a focused beam and a butt-coupled single-mode optical fiber. From near-field measurements we observed that the waveguide is single-transverse mode at 1535 nm with a mode spot size of ~12 //m and less than 5% cllipticity; at the 980-nm pump wavelength a second transverse mode can be excited depending on the coupling geometry. The measured single-pass peak absorption was ~3.8 dB/cm at 1533.7 nm and ~20 dB/cm at 976 nm. To measure the optical gain.

© 2000 IEEE