Integrated optics, or guided wave optics, is playing an increasingly important role in
optical communication networks and optical sensor systems today. Optical devices
with diverse functionality based on different materials have been demonstrated.
Recently, polymeric waveguide devices have attracted a great deal of attention because
polymer materials have many unique properties, such as ease of fabrication,
low-production costs, and compatibility with Si and GaAs fabrication technologies as
compared to materials such as silica and III-V compound semiconductors. All these
are important factors for the practical implementation of complex, high-density
interconnects and circuits. The low-temperature fabrication process of polymer
devices gives the designer a large degree of freedom. In addition, polymer materials
can be thermally tuned over a wider spectral range as they have thermo-optic
coefficients an order of magnitude larger than that of silica. These polymers also
exhibit low thermal conductivities and large thermal index changes. Unlike most rigid
waveguide materials, polymers can be deposited directly on any kind of flat or curved
substrates so that blend or foldable devices can be achieved easily. Tailored polymer
materials offer high structural flexibility through different combinations of the
monomers, and an accurate control of the material's refractive indices and properties can
be obtained.
Various techniques, such as reactive ion beam etching, photo-bleaching and
ion-implantation processes, have been used to fabricate polymeric channel waveguides
and devices. These methods involve many processing steps and have a long
fabrication time and low yield. Strong efforts are hence focused on developing effective, novel and simple fabrication technologies for polymer device fabrication. In
my Ph.D. study, electron beam direct writing for optical devices has been developed. It
is advantageous over existing techniques because fewer steps are involved. It has the
advantage of being mask-less, allowing rapid and inexpensive prototyping, in contrast
to a conventional mask-based photolithographic approach in which a mask must be
made before the waveguide devices can be fabricated. Nanometer patterns with
flexibility in writing complex structures are also possible. The technique has high
flexibility because all the patterns are designed using computer files, and simply
changing the contents of the files can complete the design modification.
Moreover, for the sake of achieving low cost and high throughput, an ultraviolet
(UV) direct printing methodology for active devices has also been developed. UV
direct printing takes advantage of UV photolithography with the utilization of a
photo-sensitive polymer for the realization of active devices, and it is suitable for mass
production with high yield in industries.
However, the selection of suitable polymer in both fabrication methodologies is a
crucial factor. Among all polymers, Epoxy novolak resin polymer (ENR) (also known
as NANOTM SU8 2000) from MicroChem Corporation was chosen as the waveguiding
core due to its distinct properties. With the aid of acid catalysis, ENR is a negative
tone polymer sensitive to both UV and electron beam radiation, thus optical waveguides
and devices can be achieved directly upon exposure and development without any
further processing.
In this thesis, a discussion of the ENR polymer, particularly on its curing chemistry
and processing parameters, is included. A comprehensive coverage of various material
characterization techniques for the analysis of the optical, thermal, spectroscopic and
lithographic properties of ENR polymer are also included.
To extend the application of polymers to active devices, rare earth ions have to be
incorporated in host materials as amplification medium. However, the incorporation
of rare earth ions in polymers is difficult because most of the rare earth ions are in
inorganic salt forms. Inorganic salts do not mix well with polymers and coagulation
usually occurs. As a result, a new approach has been developed for the synthesis of
the rare earth doped polymer, in which organic rare earth derivatives were used for
mixing with ENR polymer. Er3+-Yb3+ codoped as well as Nd3+ doped compounds with
different doping parameters were synthesized for realization of active devices. A
comprehensive study on the properties of newly synthesized Er3+-Yb3+ codoped and
Nd3+ doped polymers was carried out to determine their use as active guiding layers.
Investigations were made on two sets of doping series with different concentrations and
different weight percentage combinations.
Based on the material characteristics, it is quite obvious that pure and doped ENR
polymers are appropriate candidates for the production of various photonic components
for applications in telecommunications. Once the appropriate optical material is found,
it is natural to study the realization of functional components. Thus, a variety of
optical devices, both passive and active, have been demonstrated on ENR polymer
using either electron beam direct writing or UV direct printing.
As a fundamental component, optical waveguide plays an important role in
integrated optical circuits. Low-loss optical waveguides based on the ENR polymer
have been fabricated using electron beam direct writing. The single mode channel
waveguides fabricated have low propagation losses at telecommunication wavelengths,
smooth surface, sharp profile and high environmental stability.
Waveguide grating devices are important too, and they are key components for
constructing transmitter and receiver terminals in WDM systems. They are small and can be integrated with other components on the same substrate. Application examples
include add/drop multiplexers and dispersion compensators. Polymeric waveguide
wavelength filters based on ENR polymer have been fabricated using electron beam
direct writing. Both the waveguides and the gratings were exposed simultaneously,
which effectively eliminate the alignment errors that can be generated during fabrication.
A 6μm channel waveguide with a 5mm long first order Bragg grating and a
transmission peak of –27dB was demonstrated. The effect of temperature on the
wavelength dependence of the device response was also characterized. A linear shift
has been obtained with the heating power, and dλ/dT is found to be ~-0.14nm/oC. As
alignment problems are eliminated using the electron beam direct writing process, the
tuning capability of the device can further increase the flexibility of channel selection in
WDM systems. These results are comparable to those reported using multiple
processing steps. Besides, binary optics elements like Fresnel lens have also been
fabricated.
Another interesting research area is the demonstration of active devices like optical
waveguide amplifiers. In metropolitan area networks (MANs) and local area networks
(LANs), losses in interconnections or from device components can affect the overall
system performances. To allow recovery and maintain efficient signal transfer,
waveguide amplifiers are inevitable and essential components in the optical
communication systems. In order to achieve optical amplification in optical
waveguides, the guiding core is doped with rare-earth ions. Among all the rare earth
ions demonstrated, erbium (Er3+) ions have received special attention because the 4I13/2 –
4I15/2 transition near 1540nm wavelength matches one of the fiber low-loss windows.
However, only the simulated gain has been studied in Er3+ doped polymeric devices up to now.
Planar waveguide amplifiers doped with Er3+ and Yb3+ ions based on ENR polymer
were also demonstrated using electron beam direct writing. Signal enhancement ~13
dB at 110 mW pump power was measured in an 18 mm long device using an input
signal power of >-18 dB m. In addition, polymer channel waveguide amplifier arrays
were fabricated using ultraviolet (UV) direct printing methodology. Large area and
low cost patterning of polymer active devices can be obtained using the UV direct
printing methodology, which speeds up and simplifies the fabrication and production of
polymer active devices, and has its own intrinsic advantages over other direct write
techniques that rely on more sophisticated equipment. Hence, UV direct printing is a
suitable technique for mass production of active polymer devices, and one of the
potential applications is the fabrication of low cost laser arrays.
To summarize, cost-effective functional optical devices are required for expanding
optical communication networks, and integrated optics or planar waveguide technology
is expected to play a major role. From material characterization to processing to device
fabrication, the work described in this thesis provides a comprehensive and systemic
study on the realization of a new generation of polymer devices, thus opening up a new
platform for more novel polymer planar lightwave circuits.
| Date of Award | 16 Feb 2009 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Kwok Sum CHAN (Supervisor) |
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