A preliminary assessment of poled glass for substrate integrated optical waveguide
applications
R Oven
School of Engineering and Digital Arts
The aim of this project is to investigate the use of electrically poled glass as a means of producing
conventional optical waveguides in planar substrates. The grant money allocated has been spent
on the items listed in the original proposal, namely the purchase of optical quality glass substrates,
components for an improved interferometer and time on the S.P.S. S.E.M.
We have produced electrically poled layers in two glass substrate types, Corning 7740 and Schott
BK7, using graphite and, to a lesser extent, aluminium anodes. We have analysed the optical
properties of the poled glass using interferometry and by the analysis of leaky modes supported by
the poled glass layer. The extraction of refractive index information from the leaky mode data has
required a Matlab programme to be developed. Unfortunately time has so far not allowed for the
chemical analysis of the samples on the S.E.M. This should be done in the next three months.
To date our main findings are:
a) The same refractive index decrease caused by poling can be measured by interferometry
and by leaky mode analysis.
b) The average poled glass layer thickness as measured by interferometry and by leaky mode
analysis is consistent with each other.
c) The average poled glass layer thickness is consistent with an analysis of electrical data
obtained during poling based on the published glass composition and assuming that both
Na+ and K+ ions are removed from the poled glass layer. Poled glass layers above 6μm in
depth have been achieved in BK7 glass.
d) The refractive index decreases in BK7 Δn=-0.055 and 7740 glass Δn=-0.01 are reasonably
consistent with a simple application of a Lorentz-Lorenz model of the refractive index
change assuming stress and volume expansion/contraction effects are small.
e) At large poling voltages the increase in poled glass resistance vs charge data is linear and is
consistent with a quasi-neutral proton injection model for graphite anodes. However, at
low poling voltages the data is highly non-linear which may suggest a lack of proton
injection and a consequential space charge build-up. This data has not been fully analysed
at the moment.
f) Only thin poled glass layers can be produced when aluminium anodes are used during the
poling process as evidenced by the lack of a leaky mode structure. This is consistent with
aluminium producing a blocking anode.
g) There is some evidence of K+ pile-up outside the poled glass region in the BK7 samples.
The above refractive index and depth data suggests that poled BK7 or 7740 glass produced with
graphite anodes could be used as the cladding of a ridge type channel waveguide with virgin glass
as the waveguide core. Such guides could potentially be low loss due to the fact that impurity
atoms are not present in the core region unlike conventional ion exchange guides. However, one
major issue that needs to be solved before the manufacture of a channel waveguide can be
achieved is the patterning of the delicate graphite layer. Once this issue is solved the
interferometer can be used to obtain two dimensional images of the channel waveguide’s
refractive index cross section. It is expected that a paper detailing the analysis of poled glass
layers using the interferometer and leaky mode methods will be prepared during the summer.