Organic-based electro-optic modulators for microwave photonic applications
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Microwave photonics couples the abilities of microwave electronics for radar
and wireless data distribution with fiber optics for low loss, lightweight, ultra-high
bandwidth communications. Through electrical-to-optical and optical-to-electrical
conversions, microwave photonics systems capitalize on the relative strengths inherent
in each frequency regime for maximum functionality and deployability. Beyond
simply replacing electrical components with optical ones, the field of microwave
photonics has uncovered a variety of new and exciting possibilities that will enable
next generation communication networks and contribute to the development and
improvement of new and existing technologies.
Fundamental to any microwave photonic system is the electro-optic modulator
that performs the electrical-to-optical conversion. Decades of engineering have
produced high performance modulators in LiNbO3, which are currently deployed in
essentially all state-of-the-art microwave photonic systems. However, cutting-edge
systems with higher complexity and stringent device requirements are being
developed. As a result, the demand for higher performance modulators that require
lower drive voltages and higher frequency operation is growing and will soon
overcome the physical limitations of LiNbO3. To address this growing demand,
groundbreaking work in the field of organic electro-optic materials has been achieved
over the past 10–15 years that has resulted in materials with electro-optic coefficients
up to 10 times that of LiNbO3 and with demonstrated EO response times into the THz
regime.
This dissertation presents the work carried out over the past 5 years at the
University of Delaware towards developing low drive-voltage, high bandwidth
electro-optic modulators to support next generation microwave photonic systems.
Many previous examples of organic-based electro-optic modulators have been
demonstrated, but the focus of this work was on developing a simple, scalable device
that would effectively harness the capabilities of organic electro-optic materials
without the use of specialized cladding materials or nano-scale fabrication techniques.
Initial efforts were focused on designing an all-polymer organic electro-optic
material based optical waveguide using a commercially available cladding material,
and developing a fabrication procedure that successfully integrates the material
without compromising its electro-optic activity. These waveguides were integrated
into low frequency modulators for phase modulation demonstration that confirmed the
expected high electro-optic activity and correspondingly low drive voltages.
Additionally, a procedure for inducing the high electro-optic activity in the waveguide
core through a process known as ‘poling’ was developed.
To transition from low frequency modulation to broadband devices capable of
operating up to 50 GHz, it was necessary to gather some dielectric information of the
waveguide materials for high frequency design. This is a significant challenge with the
thin polymer layers used in optical waveguides, as most RF dielectric constant
measurement techniques are intended for thicker substrates, on the order of 100’s of
microns. Therefore a modification to the traditional microstrip ring resonator dielectric
constant measurement was developed that allowed measurement of thin films down to
~10 μm. This technique was used to acquire the necessary dielectric constant
information of the waveguide materials.
A high frequency traveling wave microstrip modulator was then designed and
optimized for operation up to 50 GHz. A novel, comprehensive figure of merit was
developed that accounts for the applied poling field that is necessary in organic
electro-optic modulators, as well as the index matching and RF attenuation that is
typically considered in traveling wave modulator design. This figure of merit was used
to confirm the superiority of the microstrip modulator architecture compared with TE
or TM CPW configurations that have been previously demonstrated in LiNbO3
modulators.
Finally, efforts were turned towards RF packaging of the microstrip
modulators for practical utilization and integration. Because RF signals typically need
to be amplified, filtered or otherwise altered prior to being sent to the modulator, a
heterogeneous transition from a standard ceramic RF substrate to the electro-optic
modulator was developed. This single-stage wire bond transition allowed for electrical
signal manipulation circuitry to be directly bonded and packaged with the modulator
for compact and efficient system integration. The transition was demonstrated
independently and with a fully RF packaged organic-based electro-optic modulator
that was contacted with standard 2.4 mm coaxial cables. The integrated device showed
modulation up to 40 GHz, the limit of the RF source.