Wavelength division multiplexing (WDM) is a technique to improve the
capacity of optical telecommunication networks
by multiplexing
wavelengths carrying optical signals. WDM systems require efficient
add/drop filters. Currently, most add/drop filters are static, based on
fiber Bragg gratings. In this approach, each wavelength channel
requires filters and switches, which makes the system costly and
inefficient. One promising solution is using dynamically switchable
filters that can switch wavelength channels dynamically depending on
signal traffic conditions. A potentially useful device is based on
switching a grating in a periodically poled lithium niobate (PPLN)
waveguide using the electro-optic effect. The biggest challenge to
building such a filter is realizing a sub-micron (∼360 nm) periodically
poled ferroelectric domain structure in lithium niobate waveguides. In
this study a direct-write electron beam poling technique is employed as
a promising method that does not require optical lithography and has
very high spatial resolution. Several material LiNbO3 systems are investigated, including stochiometric and congruent bulk LiNbO3 as well as liquid phase epitaxy (LPE) thin film LiNbO3 grown using a LiVO3-LiNbO 3 flux. By using the direct-write e-beam technique with LPE LiNbO3,
sub-micron domain structures with a width of 200∼400 nm, and periods of
∼1.1 μm and ∼790 nm have been successfully produced. In comparison with
single crystal congruent LiNbO3 (CLN) and stoichiometric LiNbO3 (SLN), LPE LiNbO3
is shown to be the best material for producing superior domain
regularity and finer domain sizes than single crystals. We believe that
the vanadium incorporation and distortion due to the lattice mismatch
between films and substrates enhance electron localization, domain wall
pinning and domain nucleation in LPE materials, giving rise to better
structures. We have studied the effects of varying e-beam parameters on
domain structure regularity. We achieved approximately 500 μm-long
gratings with a period around 1.1 μm, and with domain sizes around 180
nm. Using an optical simulation of reflectance, we show that stitching
effects between sections of the grating do not significantly affect
overall reflectance properties. The results indicate that by using
direct write e-beam techniques in LPE LiNbO3, switchable Bragg filters for optical communication appear feasible.