Optical communication systems require light sources that can be modulated with high speeds. However, the modulation bandwidth of laser diodes is typically limited by an intrinsic value, its relaxation resonance frequency. In order to circumvent this limitation, a number of methods have been proposed to boost the modulation speed, including optical injection locking, quantum dots lasers with large differential gain, push-pull modulation in composite lasers. This dissertation explores two new approaches for enhancing the direct modulation bandwidth of semiconductor quantum well laser diodes. Lasers with strong spontaneous emission have been shown to exhibit a high-speed performance theoretically. It is expected that such devices should have a modulation bandwidth on the order of several GHz under a sub-mA injection current. However, so far there has not been any experimentally observed verification of such enhanced behavior. In this work, we report on the experimental characterization of the intrinsic frequency response of metal-clad nanolasers. The probed nanolaser is optically pumped and modulated, allowing the emitted signal to be detected using a high-speed photodiode at each modulation frequency. Based on this technique, the prospect of high-speed operation of nanolasers is evaluated by measuring the ??-factor, which is an order of magnitude greater than that of other state-of-the-art directly modulated semiconductor lasers. In another experiment, we demonstrate that by tuning the gain-loss contrast between two coupled identical resonators a new degree of freedom to control the modulation frequency response is obtained. An electrically pumped microring laser system with a bending radius of 50 µm is fabricated on an InAlGaAs/InP MQW material. The integrated device was observed to lase in continuous-wave mode at room temperature with a threshold current of 27 mA. By tuning the pumping ratio between two coupled rings, our measured results clearly show a bandwidth broadening by up to 1.63 times, which matches well with laser rate equation model.

Due to their highly stable timing characteristics, optical frequency combs have become instrumental in applications ranging from spectroscopy to ultra-wideband optical interconnects, high-speed signal processing, and exoplanet search. In the past few years, there has been a necessity for frequency combs to become more compact, robust to environmental disturbances, and extremely energy efficient, where photonic integration shows a clear pathway to bring optical frequency combs to satellites, airships, drones, cars, and even smartphones. Therefore, the development of chip-scale optical frequency combs has become a topic of high interest in the optics community. This dissertation reviews the work made in the field of chip-scale optical frequency combs using optically injection locked semiconductor mode-locked lasers. First it shows the efforts in the design, characterization and calibration of several semiconductor mode-locked laser architectures on an InP-based platform. Then two separate efforts to obtain a self-referenced optical frequency comb are described. The first one based on an InP-based MLL-PIC that is enhanced via COEO multi-tone injection locking, and then amplified and broadened to an octave using pulse picking and a combination of bulk and integrated nonlinear optics. The second approach is based on the synchronization of two lasers via regenerative harmonic injection locking, one with a repetition rate in the microwave regime (10s of GHz) and another one in the THz domain (100s of GHz), first utilizing an electro-optic modulated comb and then an integrated SiN microresonator-based Kerr frequency comb. This manuscript envisions future work to achieve an optical to RF link using optical injection locking architectures with long-term stabilization and the outlook of using this technique in conjunction with octave-spanning microresonator-based Kerr combs to achieve a self-referenced chip-scale optical frequency comb.

Quantum cascade lasers are unipolar semiconductor lasers that offer a unique combination of compact size, high efficiency, high optical power, and flexibility to achieve a targeted emission wavelength with the same laser core material composition, employing so-called bandgap engineering. Since their invention in 1994, watt-level CW power with 5 to 20 % wallplug efficiency was demonstrated for QCLs throughout the entire 4 to 12 µm range, which makes QCLs very attractive for a number of practical applications. Our earlier work on broad-area QCLs emitting in the 4.6 µm to 5.7 µm spectral range demonstrated that CW power scaling with lateral device dimensions is an effective approach to increasing QCL power. First experimental and numerical data for short-wavelength ( < 4.2 µm) broad-area QCLs presented here show that this approach is very promising for achieving multi-watt CW operation in this challenging spectral region as well. Using optical power scaling with added lateral and longitudinal optical mode controls to achieve high spectral brightness is the other main topic of this dissertation. Two beam control methods for broad-area QCLs and results for single-mode devices with a short top-metal distributed Bragg reflector are presented. Finally, to improve laser reliability at high CW power, substrate-emitting configuration with a high spectral brightness and reduced beam divergence is demonstrated. These results pave the way for the development of ultra-compact and reliable infrared lasers with a high spectral brightness needed for a number of critical applications, including infrared countermeasures.

There is a critical need for powerful laser-based tools that operate over kilometers of free space. Advances in laser technology have yielded lasers with sufficient energy to induce a number of physical effects in targets. However, there are many challenges in projecting damage-inducing laser intensities to large distances in outdoor environments. Laser intensities sufficient to ablate materials and produce plasma can be projected to multiple kilometers from the source with the filamentation of ultrashort pulses, a nonlinear phenomenon that eliminates the need for large focusing optics. During filamentation, Kerr-self focusing generates high intensities along the propagation axis which create plasma and the subsequent defocusing of laser light. These nonlinear processes that facilitate long-range high-intensity laser propagation, clamp the peak pulse intensity to 1013-1014 W/cm2 in the case of an ultrashort pulse centered at 800 nm in air, limiting the effects induced by filament interactions. This thesis dissertation investigates several techniques to improve filament interactions with targets for long-range applications. Several interaction modalities, including ablation, plasma creation, and the formation of shockwaves, can be enhanced by supplementing the clamped filament intensity with additional laser radiation. The secondary irradiation is investigated in the form of a lower-intensity nanosecond pulse or burst of femtosecond filaments. The complex ablation and plasma physics involved in these multi-pulse interactions are explored through multiple experiments conducted with the Multi-Terawatt Femtosecond Laser at the University of Central Florida. Investigations of filament propagation and interaction science at the kilometer range are conducted at the Townes Institute Science and Technology Experimentation Facility, a secure outdoor propagation range. This research has generated valuable knowledge on the formation, properties, and applications of filaments over distances up to 1 km, which is critical for long-range applications.

Four years after the first demonstration of Light Amplification by Stimulated Emission of Radiation in a Ruby crystal by Theodore Maiman in 1960, C. J. Koester and E. Snitzer transferred this new and groundbreaking laser concept to Neodymium doped glass waveguides and paved the way for modern fiber lasers. The prediction of low loss optical fibers for long-distance optical communication by C. Kao in 1966 and the first demonstration of a fiber with only 20 dB/km loss by Corning laboratories in 1973 triggered major R&D investments in the second half of the 20th century. The development of specialty optical fibers was facilitated by the introduction of the stack-and-draw fiber manufacturing technique, which enabled the fabrication of more complex fiber structures and expanded the application space of optical fiber technology. This dissertation is divided into four chapters. The first chapter provides a short review of the history of optical fibers and their application in fiber laser systems as well as an outline of this dissertation. In the second chapter, Ytterbium based single mode high average power fiber amplifiers and their average power limitations are discussed. In particular, the concept of spatially confining the gain to certain regions of the fiber's core is discussed in detail as one strategy to overcome some of these limitations. Furthermore, the development of a narrow line width high power fiber laser system is discussed. The third chapter discusses the potential of disordered glass-air Anderson localization fiber for broad band super continuum generation and the fourth chapter discusses the pulse energy scaling potential of Ytterbium doped multicore fiber saturable absorbers for ultrafast fiber oscillators.

Recent progress in nano optics, spurred by progress in nanofabrication, has allowed us to overcome these challenges. We use surface plasmon polaritons to break the optical diffraction limit and squeeze the photon energy into a local hot spot. The small mode volume of a plasmonic antenna or nanoaperature significantly enhances the local field and can be designed to resonate at a desired wavelength. By designing, fabricating, and testing these nanoapertures, I trap single nanoparticles with significantly reduced laser power by measuring the monochromatic transmission change of a resonant aperture. A freely diffused nanoparticle, behaving like a dipole antenna, interacts with the nanoaperture when trapped and shifts the resonance of the nanoaperture. By only monitoring a single wavelength, the presence of the particle changes the transmission signal. The effect of particle-induced transmission spectrum shift is called the self-induced back-action effect. This particle-induced spectrum change increases the transmission amplitude and variance once trapped. Furthermore, the monochromatic transmission measurement is a faster detection method than the spectrum measurement. It is able to follow up the diffusion, folding or conformation change of the trapped particle.

Illuminating a material with intense laser excitation may change its properties and result in nonlinear absorption (NLA) and nonlinear refraction (NLR). In this dissertation we study the nonlinear absorption of organic compounds, the effect of extremely nondegenerate NLR in semiconductors, and the linear refractive index of organic solvents. In liquids, the refractive index has been studied for decades and different kinds of refractometers have been proposed. However, most of the reported values are in the visible region and only for commonly used solvents. We proposed a new interferometer-based refractometer that allows us to measure the refractive index from the visible to the near-infrared (NIR). We characterized 24 organic solvents over 7 wavelengths. A refractive index database was developed that displays our results and also allows comparison with values in the literature. The corresponding Cauchy equation is given according to different wavelengths and temperature regions. A group of solvents that do not have C-H, O-H, or N-H bonds in their chemical structure show a wide transparent spectral window in the NIR region. This group of solvents generates great interest in several research areas such as supercontinuum generation and optical parametric amplification in liquidcore optical fibers and for the design of optofluidic devices. In organic compounds, tailoring their molecular structure can change their linear and nonlinear optical properties. We performed a comprehensive linear and nonlinear spectroscopic characterization of two groups of organic compounds: gold dithiolenes and boron-dipyrromethene (BODIPY) dyes. The gold dithiolenes show a wide excited-state absorption band across the visible region with an approximate 10 ps excited-state lifetime, with no corresponding fluorescence or singlet oxygen yield observed. The BODIPY dyes have both two-photon absorption and excited-state absorption in the NIR region. The excited-state lifetime and the fluorescence quantum yield depend on the solvent polarity. The brominated counterpart shortens the excited-state lifetime instead of forming the triplet state. Optical gain is observed for non-fluorescence dyes in the polar solvent. These properties make them potential candidates for many applications such as optical power limiting and all-optical switching. In semiconductors, the NLR in the sub-gap region is enhanced under the nondegenerate case, which means the refractive index at one frequency is enhanced with a presence of a beam with another wavelength. This enhancement is not just in the refractive index, but also in group index and group velocity dispersion. We calculated how to pulse evolution with different arrival times between two pulses. The enhanced NLR shifts the spectrum of the pulse and shows a potential application in all-optical switching.

High power fiber lasers have revolutionized many areas of both basic research and industry with their high power, high brightness output and capability for a compact, monolithic design. These systems are often run in multimode operation with few constraints on output. However, for many directed energy applications, such as spectral beam combining, the necessary laser parameters are far more stringent, requiring systems with state-of-the-art characteristics, such as single transverse mode operation, narrow linewidth and multi-kW output powers. Currently, further growth in output power of fiber laser systems at 1 and 2 µm is hindered by various nonlinear and thermal limitations within the fiber gain medium. Specifically, 1 µm systems are primarily hindered by transverse mode instability, while 2 µm lasers at high average powers often encounter an effect known as modulation instability. This dissertation explores the power scaling potential of both Yb-doped and Tm-doped fiber lasers along with related applications. Power scaling of two different Yb:fiber amplifiers is examined, where first, a novel confined-doping fiber geometry is used to generate 450 W single mode output, and second, through a demonstration of a Yb:fiber amplifier with > 2200 W output power and quasi-single mode beam quality. At 2 um, a new pumping architecture known as in-band pumping is utilized in an effort to take Tm-doped fiber amplifiers first to the multi-100 W level and then into the multi-kW regime. An in-band pumped laser is developed and characterized with over 75 W output power and > 80% slope efficiency. The last laser system constructed is a tunable, narrow linewidth Tm-doped fiber laser used to systematically induce and study the effects of thermal blooming. Results obtained from scanning the wavelength across individual atmospheric absorption resonances are presented, along with time-resolved data obtained at a fixed wavelength. Finally, two candidate fiber designs are presented for power scaling of in-band pumping at 2 µm to the multi-kW power level. Numerical studies detail the performance of these two fiber designs at high average powers and their ability to suppress the onset of modulation instability. Taken together, the results discussed in this dissertation further the science of fiber lasers for directed energy applications.

Mid-infrared (MIR) laser sources have demonstrated diverse applications in science and technology. For spectroscopy applications, numerous molecules have unique absorption features in this range, and one needs a spectrally broad coherent laser source for parallel detection of mixtures of species. Frequency down-conversion in nonlinear optical materials via second-order nonlinear susceptibility is one of the promising techniques to generate the spectral coverage of more than an octave in the MIR, assisted by emerging novel crystals. The nonlinear light-matter interactions in such special crystals as ZnSe ceramics have not been analyzed. Additionally, through the use of high-intensity few-cycle optical pulses, high-order nonlinear effects such as four-wave mixing, multiphoton absorption, and nonlinear refraction come into play beyond conventional second-order nonlinear interaction. In this thesis, the nonlinear interactions for generating broadband MIR were studied through both experimental and numerical approaches. First, a nonlinear frequency conversion model based on random phase matching was developed in zinc-blende polycrystalline structures. Monte Carlo simulation statistically verifies that a disordered material could perform on par with a quasi-phase-matched material for frequency conversion in ultrafast interactions. Second, the nonlinear interaction in orientation-patterned GaP combined with an optical parametric oscillator was numerically analyzed. A wave propagation model discovers that third-order nonlinearity plays an important role in the process of spectral evolution. Finally, using a 2.35-µm Cr:ZnS mode-locked laser, nonlinear absorption and nonlinear refractive index were characterized in the Z-scan technique for GaP, ZnSe, GaSe, and ZGP crystals. The visualization of nonlinear interactions and the uncovering of nonlinear parameters will be a guide for optimizing experimental systems and will further advance the development of MIR laser sources.

Imaging is almost synonymous with optics. Imaging is the process of using light to form a tangible or visible representation, an imitation (imitari) of a material property. There are many situations, however, where one can only aspire to 'sense making' rather than forming an image per se. In other words, objects cannot be directly resolved by conventional intensity-based imaging, a situation commonly referred to as 'unresolved imaging'. However, there is still information retained in other properties of light, which can be exposed by other means. In this thesis I will discuss two typical situations: subwavelength and multiple scattering, which are very different in terms of the spatial extent of light-matter interaction. In the subwavelength regime, information can be encoded through both inelastic and elastic interaction processes. When the latter is the preferred approach, observables such as optical phase are determined by the properties of evanescent waves while the measurements are usually conducted in the far-field. I will describe a novel energetic interpretation of the light-matter interaction in this regime, which provides an accurate estimation of the interaction volume of a single scattering event and of the small phase delay it introduces. I will also show how this minute phase occurring in subwavelength scattering can be quantitatively measured with optimal sensitivity by a polarization-encoded common path system and how it enables subwavelength sizing in a label-free fashion. At the other extreme, evaluating the information transfer in multiple scattering regimes is usually constrained by the computational complexity of the problem. I will describe two forward modeling approaches that alleviate these limitations in non-line-of-sight sensing geometries and in coherent illumination methods for imaging through obscurants. These simplifying descriptions also reveal the fundamental limits for information transfer in these two scenarios.