In this paper, we report laboratory test results of an LPG that can maintain a constant resonant peak depth over an
enhanced tuning range when it is coated with an ITO electrode that has optimized thickness and refractive index.
Without the ITO layer, LPG tuning ranges as large as 50 nm have been achieved when the ambient index is
increased from 1.00 (air) to ~1.444 (index of the silica cladding), but the peak depth cannot be maintained. When a
properly designed, high-index ITO overlay is coated onto the silica cladding, mode transition effects coincide with
the LPG's intrinsic sensitivity to changes in the ambient index, resulting in a stable peak depth over an enhanced
tuning range. The authors have experimentally demonstrated an LPG coated with ITO that can be tuned in excess of
150 nm with an ambient refractive index change of less than 0.01. To the best of the authors' knowledge, this is the
highest sensitivity reported for an LPG to date. In addition to the tuning performance, the resonant peak remains
within 1 dB of its maximum depth for at least 100 nm of the tuning range, which allows the tunable LPG to be used
in real applications.
In this work, an investigation of the tuning characteristics of electrically tunable long-period gratings (LPGs) is
presented. A precise four-layer model is used to quantitatively analyze the tuning potential of the gratings and
experimental data is provided to support the analysis. The four-layer model includes a silica core layer with an inscribed
LPG, a thin silica cladding layer (~40 μm), an ultra-thin (~ 50 nm) high refractive index indium-tin dioxide (ITO) inner
electrode layer, and a tunable electro-optic polymer layer. It has been found that the inner electrode layer, made of high
refractive index ITO, can be modeled as a high index overlay and causes the forward propagating modes in the thin silica
cladding to reorganize as the ambient refractive index changes. This reorganization effect can lead to a significant
increase (10 plus fold) in the tuning range of LPG tunable filters. Moreover, the required specifications of the tunable
polymer layer are quantitatively analyzed. Finally, the required characteristics of the electro-optic polymer are realized
by using a nano-composite of zinc sulfide and ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer.
A unique all-fiber tunable filter is based on the combination of a single resonant band long period grating (LPG) and an electro-optic polymer second cladding layer. The single resonant band LPG is fabricated by etching the cladding of a 125 μm thick fiber and using ultraviolet (UV) illumination to write the grating. Once a single resonant band has been achieved, an ITO electrode is sputtered onto the thin silica cladding and then a polymer second cladding layer is applied. The refractive index of the polymer determines the resonant wavelength of the filter. After a second electrode is coated onto the second cladding, the polymer index is tuned by applying an external electric field. Recent modeling and experimentation has shown that a high index ITO inner electrode can increase the tuning range of the filter up to 10 times by inducing cladding mode transitions.
Conference Committee Involvement (1)
Display Technologies and Applications for Defense, Security, and Avionics V
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.