الفهرس 
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Abstract
Summary
Swept wavelength laser source is currently an essential element for many
biomedical and sensing applications especially for Optical Coherence
Tomography (OCT). Swept Source OCT (SS-OCT) is fast becoming one
of the most successful optical technologies, in the field of disease
diagnostics and biomedical imaging. For such applications, the main
interest is to have a wide wavelength-tuning range, high speed,
miniaturized dimensions and low cost. Miniaturization opens the door to
integrate the system on a handheld unit, which may initiate a wide variety
of new applications. Various configurations have been proposed for SSOCT
utilizing external optical components, which are bulky, expensive
and difficult to align. To overcome these challenges, swept sources based
on MEMS technology have recently attracted a great attention. The
technology provides miniaturized dimensions, low cost and batch
processing. In-plane MEMS components suitable for building a complete
integrated SS-OCT head have been reported. The main drawback of
swept source based on in-plane MEMS filter is the small tuning range
(20 nm - 35 nm), which is not suitable for many OCT applications. In
this thesis, we present an SOI-MEMS tunable Fabry-Perot filter with 125
nm tuning range and a free-spectral range (FSR) larger than 130 nm. The
SOI-MEMS filter is based on two-silicon-layers Bragg mirrors etched
with an 80 μm depth. The SOI-MEMS filter is then used to construct a
100 nm tuning range swept laser source. The tuning of the filter is carried
out using a MEMS electrostatic comb-drive actuator attached to the free-
Bragg mirror of the filter, in a lithographically self-aligned manner. The
presented results represent, to the best of our knowledge, the largest
tuning range reported in literature for an in-plane MEMS-based swept
laser source.
For the swept source applications rapid filter tuning speed that may
exceed 100 kHz is required to achieve high image acquisition rate. The
swept laser for these high speed applications is usually a multimode laser
with average spectral width around 0.1 nm. A narrow swept laser spectral
width/linewidth is required to achieve a large penetration depth in the
sample that may exceed several millimeters. The spectral width of such
sources is affected by the filter scanning speed, its 3dB spectral width,
the SOA gain and its wavelengths dependence, the cavity loss and many
other parameters. Conventionally the instantaneous spectral width at
these high speeds cannot be accessed by direct measurement. Instead
indirect estimation for the average spectral width at a certain speed is
done using the Fourier transform of a measured coherence function. Few
previous trials for modeling the swept source have been proposed based
on time domain propagation equations governing the gain dynamics and
the complex electric-field envelope to describe the swept laser behavior.
This time domain approach did not include the wavelength dependence
of the SOA gain. The previous modeling trials did not give much insight
on the swept laser instantaneous spectral width with its multi-folded
nature. In this thesis we also present a new technique to model the swept
source operation and predict the instantaneous spectral width using a
multimode rate equation approach. The model successfully estimates the
reported spectral width narrowing at static positions with respect to the
filter 3dB spectral width and how this effect is reduced as the filter
scanning speed is increased. It allows predicting for the first time, to our
knowledge, the effect of the gain spectrum on the instantaneous spectral
width at different scanning speeds. The effects of filter 3-dB spectral
width, the cavity loss, the pumping current and the loop length are also
studied. In addition the model allows monitoring the evolution of
different modes as the filter is scanned in time domain.
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The work in this thesis is presented as follows:
In Chapter 1, an introduction about swept laser source, the main research
topics of the thesis and the thesis outline are presented.
Chapter 2, we present a comprehensive overview on swept laser sources,
basic configuration, operation principle, its main specifications,
configurations and implementation techniques.
Chapter 3 presents a novel multimode rate equation numerical model for
the swept laser, practical technique for extracting the SOA parameters is
provided, Modified multimode rate equation for the swept laser is
derived and the different factors affecting the swept laser instantaneous
spectral width are studied in detail.
Chapter 4 is devoted to the deeply-etched optical MEMS tunable filter
for SS-OCT applications where the Bragg mirrors and multilayer filter
designs are discussed. An overview on the SOI technology and Deep
Reactive Ion Etching (DRIE) is presented, the deeply etched structures
fabrication steps are explained in details, the fabricated structures SEM
photos are included and process achievements are summarized. The
optical characterization of the deeply etched components and MEMS
based swept laser is presented. The reflection response of the multilayer
mirror is measured and its reflectivity is estimated. The effect of etching
error on the reflection response is determined. The MEMS filter is
optically characterized showing its tuning range, insertion loss and 3dB
bandwidth.
Chapter 5 MEMS based swept laser is constructed and characterized
demonstrating the large improvement in tuning range and the factors
affecting it. Practical technique for SOA parameter extraction is provided
and its effect on the MEMS swept laser tuning range is presented.
In Chapter 6 the final conclusions of the thesis are stated, a comparison
with previous published work is provided and future work is discussed.