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
