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Title: Integrated mid-infrared photonics: From quantum cascade lasers to suspended silicon-on-insulator waveguides
Authors: Sadeghi, Arash
Advisors: Gmachl, Claire F
Contributors: Electrical Engineering Department
Keywords: Distributed Bragg Reflector
Integrated optics
Quantum Cascade Laser
Silicon photonics
Subjects: Electrical engineering
Issue Date: 2019
Publisher: Princeton, NJ : Princeton University
Abstract: The mid-infrared, covering the wavelength range 3-30 µm (10 -100 THz) is of particular importance for chemical species detection, since many atmospheric trace gases and liquids have fingerprint spectra in this wavelength range. The atmospheric windows in the wavelength range of 3-5 µm and 8-12 µm, offer a highly desirable low-loss region for chemical trace gas sensing, high-bandwidth free-space communication, and laser-based advanced defense countermeasures against IR-guided munitions. Leveraging the silicon fabrication protocols for CMOS compatible Si-optical integration, where photonic components are fabricated on a single Si chip, results in improved yield, compactness and lower manufacturing costs due to the economy of scale. Quantum Cascade (QC) lasers are intersubband mid-infrared light sources operating between ~ 3-300 µm, made possible through advances in bandgap engineering and fabrication techniques, offering promising applications due to their compact size, range of operation and high output power. Fabry-Perot (FP) QC lasers have a relatively broadband spectral output due to the wavelength-independent reffectivity of the end facets and non-linearities such as spatial and spectral hole burning when operating well above lasing threshold. Narrow-band, high-power operation of QC lasers is, however, desirable in a variety of applications, such as laser-assisted surgery in medicine or defense countermeasures. This thesis focuses first on optimizing the spectral output of FP-QC lasers via the application of shallow Distributed Bragg Reflectors (DBR), ion-milled after laser fabrication and characterization. We observed a more than 10-fold reduction in spectral full-width-half-maximum (FWHM) and up to 20 dB side-mode suppression ratio (SMSR), maintained to peak optical power. Also observed was a "sweet-spot" in grating length, ~ 200 µm on a 3mm long laser ridge, and a trade-off between spectral narrowing and output power, set by the grating depth, varied from 1.8 - 2.5 µm. Second, this thesis reports on a novel approach to integrating QC lasers into a silicon platform through evanescent side-coupling of center-cleaved QC dies to custom suspended Si waveguides (propagation loss 0:5 dB cm􀀀1 at 1:55 µm) in silicon-oninsulator (SOI). The coupling parameters were optimized numerically, with laserwaveguide separation upper limit of 2 µm and optimal bending radius for an adiabatic taper determined to be 110 µm at 5 µm. The authors envision the work undertaken in this thesis would pave the way for the fabrication of single-mode mid-infrared lasers at scale and the integration of such photonic components in a CMOS compatible Si platform, leading to more compact and economical mid-infrared systems.
Alternate format: The Mudd Manuscript Library retains one bound copy of each dissertation. Search for these copies in the library's main catalog:
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Electrical Engineering

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