Lasers and amplifiers of high-order spatial modes are useful for a number of applications, including communication, sensing, microscopy, and laser material processing. This dissertation presents the generation and amplification of high-order spatial modes in few-mode fibers (FMFs). In the area of amplification of high-order spatial modes, low-crosstalk amplification among spatial modes is realized in a retro-reflecting few-mode Er-doped fiber amplifier (EDFA) by exploiting the unitary property of the coupling matrix of a symmetric photonic lantern (PL). A small-signal gain larger than 25 dB and crosstalk below -10 dB was achieved over the C-band for a 3-mode EDFA. Such a few-mode EDFA can replace multiple parallel single-mode EDFAs in single-mode fiber transmission systems. In addition, we presented an EDFA for orbital angular momentum (OAM) modes using an annular-core PL. Both the first- and second-order OAM modes were amplified with nearly 20 dB of gain over the C-band. Placing a few-mode EDFA and a mode-selective PL inside a linear cavity, we demonstrated an intra-cavity transverse mode-switchable fiber laser for the generation of high-order spatial modes. The six linearly-polarized (LP) modes can lase independently and are switchable by changing the input port of the PL. In addition, we generated donut-shaped beams using incoherent superposition and simultaneous lasing of the two degenerate modes in the same LP mode group. Additional techniques for the generation of high-order modes explored in this thesis utilize stimulated Brillouin scattering (SBS), one of the prominent nonlinear effects in optical fibers. Based on backward SBS in a passive FMF, we experimentally demonstrated a transverse mode-selective Brillouin fiber laser using mode-selective PLs. We generated three LP modes via both intra- and inter-modal SBS. Finally, we propose a fiber ring cavity that can simultaneously produce phonon lasing and photon lasing utilizing forward intermodal SBS. We experimentally demonstrated for the first time, to the best of our knowledge, such a two-domain ring laser using a 10-meter reduced-cladding two-optical mode fiber. By using an LP01 optical pump, both the LP11 Stokes lightwave and a low-frequency flexural acoustic wave can be amplified by stimulated emission and oscillate inside the same fiber ring cavity. The measured photon laser beat linewidth and the phonon laser linewidth are on the order of a few kHz.

Augmented reality (AR) and virtual reality (VR) displays, considered as the next-generation information platform, have shown great potential to revolutionize the way how we interact with each other and the digital world. Both AR and VR are disruptive technologies that can enable numerous applications in education, healthcare, design, training, entertainment, and engineering. Among all the building blocks of these emerging devices, near-eye displays (NEDs) play a critical role in the entire system, through which we can perceive the virtual world as the real one. However, the visual experience offered by existing NED technologies is still far from satisfying the human vision system regarding the display resolution, image clarity, and light efficiency. This dissertation provides original solutions to remove the abovementioned roadblocks using a novel type of liquid crystal (LC) planar optics based on the Pancharatnam-Berry phase (PBP). Firstly, we demonstrated a polarization-multiplexed method that can double the perceived angular resolution of most NEDs, utilizing the polarization-sensitivity of a customized Pancharatnam-Berry phase deflector (PBPD). Secondly, a broadband Pancharatnam-Berry phase lens (PBPL) is developed and integrated with conventional VR optics, such that both monochromatic and chromatic aberrations (CAs) are reduced by more than two times, offering significantly sharper imagery to the viewer. Also, a diffractive deflection film (DDF) based on PBP is designed with a directional display panel to reduce the wasted light in current VR devices, which can boost the system light efficiency by more than two times. Furthermore, novel fabrication methods of the PBP optical elements are invented for the need of mass-production. The proposed methods and designs are examined in both optical simulation and prototype hardware with public demonstrations. The verified performance enhancement proves that the proposed LC-based PBP optical elements offer considerable value and potential for practical applications in next-generation NEDs.

This dissertation investigates excited-state nonlinearities in a series of polymethine dyes for the application of nanosecond optical limiting. Optical limiters are devices that for low intensity light exhibit a high linear transmittance, but for high intensity light strongly attenuate the incident radiation. These devices would serve to protect optical sensors from intense laser radiation by clamping the maximum energy allowed through an optical system below the damage threshold of the sensor. The search is ongoing for optical materials that are both broadband and have high damage thresholds to be effective materials for limiting applications. Polymethine dyes are promising compounds due to a strong and broad excited-state absorption (ESA) band in the visible region. However, the effectiveness of polymethine molecules as applied to optical limiting is hindered by a saturation of the ESA process at high fluences. Experiments and theoretical modeling are performed to determine the root causes of this saturation effect in both the picosecond and nanosecond time regime. The polymethine molecules studied have chromophore lengths from di- to pentacarbocyanine (2 to 5 -CH=CHgroups) with various bridge structures. This allows us to develop relationships between the molecular parameters of the polymethine molecules and overall nonlinear absorption performance. The experiments conducted included femtosecond white light continuum pumpprobe experiments to measure ESA spectra, picosecond two-color polarization-resolved pumpprobe to measure excited-state dynamics and the orientation of transition dipole moments, and picosecond and nanosecond optical limiting and z-scans. From these experiments we are able to develop energy level models that describe the nonlinear absorption processes in polymethines from the picosecond to nanosecond time regime. This work, along with the quantum chemical modeling performed at the Institute of Physics and National Academy of Sciences of Ukraine, has resulted in the creation of dyes that have improved photochemical stability with larger nonlinearities. These are useful not only for optical limiting but also for a wide variety of nonlinear optical applications.

Laser filamentation is a nonlinear process involving high-energy, ultrashort pulses that create narrow, non-diffracting structures over many times the Raleigh length. While many of the characteristics of filaments can vary greatly depending on the physical parameters used to create them, they share several defining features: a high intensity core, a lower intensity cladding of photons that serves as an energy reservoir to the core, and spectral broadening into a supercontinuum. While there have been many studies on the creation and control of multiple filaments from one laser pulse and a few studies on the interaction of two single filaments, many fundamental questions concerning the nature of this interaction still exist. This thesis seeks to explore the correlation between ultrashort pulses involving spatial separation, temporal delay, and relative degree of polarization using an interferometric approach. Evaluating the beam profiles and spectrum that result from varying those parameters.

A high powered ultrashort laser pulse can propagate as a diffraction-free self-channeled structure called a filament, created by a combination of nonlinear processes. With its ability to convey extremely high intensity beams to distant targets, many applications such as remote sensing, cloud seeding, and discharge guiding are potentially possible. However, one of the main challenges of outdoor field applications is the laser propagation through the atmosphere where pressure fluctuations and concentrations of aerosols may be present. The rationale behind the work presented in this dissertation is to evaluate the robustness of the filamentation, measure the interaction losses as well as understanding the modifications to (i) filament length (ii) supercontinuum generation, and (iii) the beam profile along propagation through perturbed media. Detailed studies of the interaction of a single filament with a single water droplet are presented. In addition, preliminary results on filament propagation through a cloud of aerosols are discussed. The effect of pressure on the beam profile along propagation and on the supercontinuum generated by the filament is studied. This document provides valuable insight into the complex nonlinear processes affecting the formation, propagation and post propagation of filaments under adverse atmospheric conditions.

Recently, the design and engineering of ultrafast laser systems have led to an extraordinary increase in laser power and performance which have brought about advances in many fields such as medicine, material processing, communications, remote sensing, spectroscopy, nonlinear optics, and atomic physics. In this work, several ultrafast amplification techniques -- including chirped-pulse amplification (CPA), optical parametric chirped-pulse amplification (OPCPA), and divided-pulse amplification (DPA) -- are described and demonstrated in the design and construction of two ultrafast laser facilities. An existing Ti:Sapphire laser system was completely redesigned with an increased power of 10 TW for experiments capable of generating hundreds of laser filaments in ordered arrays. The performance of DPA above the Joule-level was investigated in a series of experiments utilizing various DPA schemes with gain-saturated amplifiers at high pulse energy. A new high energy OPCPA facility has been designed and its pump laser system constructed, utilizing the technique of DPA for the first time in a flashlamp-pumped amplifier chain and with a record combined energy of 5 Joules in a 230 ps pulse duration. The demonstrated OPCPA pump performance will allow for the generation of 50 TW quasi-single cycle 5 fs pulses at 2.5 Hz from a table-top OPCPA system.

Orbital angular momentum (OAM) modes have attracted immense attention for their fundamental properties such as helical phase fronts, zero intensity at the beam centers as well as the phase singularities. Due to these novel characteristics, they have broad application prospects in the fields of super-resolution imaging, laser machining, particle manipulation, classical and quantum communications. These application spaces span an extensive range of wavelengths from the visible range for stimulated emission depletion microscopy to ~1550 nm for telecommunications, for instance. They also span a large range of power levels, from kilowatts (kW) peak powers for laser machining to single photons for secure quantum communications. However, access to this vast space is challenging because of the limitations in available laser sources at wavelengths of interest. More importantly, since the conventional way of creating OAM light involves discrete mode conversion of the Gaussian light that is emitted by a typical laser system, mode converters that can work at all the desired wavelengths and potentially can handle high powers are critically needed. Furthermore, in certain applications where simultaneous creation of multiple OAM modes of equal weights are necessary, such as in the case of higher-dimensional entanglement, an additional requirement of distinct OAM mode excitations with similar efficiencies is of interest. Here, we borrow from the extensive progress made in the field of single-mode fiber nonlinear optics to develop nonlinear signal generation and conditioning schemes in fibers where light propagates in desired OAM states. Single-mode nonlinear fiber optics has shown that by frequency-converting existing commercial laser sources via nonlinear interactions such as four-wave mixing (FWM), Raman scattering, etc., novel colors of power-levels ranging from kW to single photons can be created. Therefore, it motivates us to develop a similar platform for the OAM modes as well, which is only now possible due to recent developments that show that a large ensemble of OAM modes can be stably guided through optical fibers. As of this writing, fibers supporting over ~50 OAM modes even over km-length scales, with mode areas ranging from 150 to 600 μm^2 are now available, making this platform readily amenable for nonlinear investigations. This thesis has two primary aims: (1) to study nonlinear optical phenomena of OAM modes in fibers, especially FWM and Raman scattering processes, to investigate whether they behave the same as any other modes in multi-mode fibers (MMFs) or whether the fact that they carry OAM alters the efficiencies and selection rules of nonlinear processes; and (2) to exploit them for two distinct applications spanning both a large wavelength range as well as power levels. Our studies indicate FWM interactions among OAM modes not only share the attributes with other multimode systems in terms of the variety of phase matching possibilities offered by the expanded modal space, but also show extra advantages of being more diverse and efficient due to the similar intensity profiles of a larger ensemble of guided modes. In addition, the helical phase terms that are unique to OAM modes induce an extra OAM conservation rule for the FWM processes, which provides a high degree of selectivity one would desire when creating specific sources at desired OAM values and wavelengths. We also study Raman scattering in these modes and find some rather counterintuitive behaviors. While Raman scattering is conventionally considered as a phase-insensitive process, its dynamics for a linearly polarized OAM mode are instead governed by a special phase matching equation. Specifically, the phase dependency arises from the optical activity that a linearly polarized OAM mode experiences due to the circular birefringence that is induced by the spin-orbit interaction in the OAM fiber, which manifests in a rotating linear polarization state along the propagation axis, with the rotation rate determined by the modal dispersion characteristic. Since the Raman gain maximizes for co-polarized light, the differences in polarization evolutions for the pump and Stokes light lead to the special phase matching conditions, which can be used to spectrally-reshape and modulate the strength of Raman scattering signals. Next, we exploit the aforementioned unique and beneficial attributes for specific applications. We first demonstrate a high-power FWM-based OAM source at both ~888 nm and ~1326 nm, with peak powers of ~3 kW and ~2 kW, respectively. We also show extra-cavity second harmonic generation, to access the blue-green wavelengths ranges at which compact, kW peak-power level source generation is both highly desirable for many applications, and also hard to achieve today. The results indicate that FWM not only provides a convenient way to create high power OAM light, but also allows creation of new colors. This is because the multi-mode system can circumvent the near-zero dispersion constraints that are required for phase matching in single-mode systems. Secondly, we demonstrate OAM-FWM-based photon-pair generation at the single-photon level and reveal the two benefits offered by OAM modes: (1) the ability to engineer the spectral correlations of the photon pairs by switching the angular momentum content of the pump; and (2) simultaneous creation of photon pairs at ~1550 nm and ~780 nm through different FWM paths, hence linking the transmission of flying qubits in the telecom wavelength range to the stationary quantum memory systems that operate in the near-infrared. For all the different FWM processes we probe, we measure the coincidence-to-accidental ratio to be higher than ~400, the second-order correlations to be less than ~0.1, which indicate the high signal to noise ratio and low multi-photon pair generation probability single-photon sources enabled by our OAM-FWM platform. In summary, FWM and Raman scattering among OAM modes in fibers provide new, interesting nonlinear coupling pathways that allow high power generation as well as control of bi-photon spectra for quantum applications. The benefits of OAM modes compared to either the fundamental mode in single-mode system or traditional modes in MMFs mainly lie in the versatile phase matching possibilities enabled by the large modal space that the OAM-supported fiber offers, as well as the large gains for all FWM pathways ensured by the large and similar mode effective areas for all OAM modes. These two fundamental properties may lead to future development of high-power laser sources at other desired wavelengths, hybrid- and higher-dimensional entanglement sources in the quantum regime and other applications where OAM sources at user-defined wavelengths are desired.

Silicon photonics leverages advanced microelectronics fabrication platforms to realize ultra-miniaturized, complex, low cost, size, weight and power optical systems on chip that can be monolithically integrated alongside state-of-the-art electronic circuits. The applications for silicon photonics require ever more stringent performance specifications, needing lower loss, higher performance devices. However, the design of high performance devices is often impeded due to the sensitive nature of light-matter interaction and the tendency of light to scatter and radiate. In this dissertation, I present the design of high performance radiative and resonant silicon photonic devices by efficiently controlling light propagation and radiation. Two types of radiative photonic devices are presented. The first are grating couplers for low-loss fiber-to-chip coupling. These grating couplers are designed to be readily implementable in wafer-scale CMOS and CMOS-photonics platforms by using a dual-layer structure to break vertical symmetry. A unique bandstructure synthesis method is presented for designing the grating couplers from first principles. A variety of grating couplers are designed for both the 45RFSOI CMOS platform and the 45CLO CMOS-photonics platform, with measured insertion losses comparable to the state-of-the-art. Along with low-loss grating coupler designs, I also conceive and demonstrate a new type of polarization-insensitive 1D grating coupler (PIGC) using a "corelet" metamaterial waveguide that removes effective index birefringence. I demonstrate a PIGC in the 45CLO platform with a measured 1 dB polarization-dependent-loss bandwidth of 73 nm. The second type of radiative photonic devices are a new type of large-area, dispersive optical phased array named the serpentine optical phased array (SOPA). The SOPA combines a serpentine delay line with rows of grating couplers to realize a wavelength-controlled beam steering aperture. The design was conceived to maximize the radiating aperture while having minimal operating complexity. The theory and design of SOPA devices are presented, and a variety of SOPA devices are fabricated and characterized. Two applications for the SOPAs are investigated. The first is lidar and imaging, with initial demonstrations of ranging, Vernier'ed bi-static arrays for grating lobe link suppression, and F-BASIS imaging. The second is using SOPAs for spectroscopy. By combining the SOPA with a detector array, I am able to realize a compact and high-resolution spectrometer. An initial prototype has a spectral resolution of approximately 16 pm and working bandwidth from 1540 to 1650 nm, resulting in a resolving power of approximately 100k which is comparable to crossed-dispersion spectrometers but over 100 times smaller in volume. Finally, I present the design and characterization of high Q and compact racetrack resonators using a thick-SOI platform. Using the racetrack resonators, I demonstrate a four-channel, 100 MHz passband filter bank for RF-photonic signal processing.

The myelin sheath that forms around axons of the brain is essential for proper, high-speed signaling between neurons. Accordingly, degeneration of myelin is believed to be a hallmark pathological feature of normal aging processes, brain injury, and neurodegenerative diseases. However, despite the interest in studying myelinated axons and the loss of myelin integrity that occurs in disease, methods for direct assessment of myelin with microscopic imaging are limited. In postmortem brain tissues, there is a need for new tools to investigate myelin structure and myelin pathology at the level of individual axons. Due to the unique, multilayered structure of myelin, it is highly anisotropic and therefore also exhibits strong optical birefringence. The birefringence of myelin, which refers to its optical polarization-dependent refractive index, presents the opportunity for sensitive, label-free imaging of myelin structure. This work details the development of a custom birefringence microscopy (BRM) system, which provides the ability to image myelin birefringence with diffraction-limited resolution (up to ~250 nm) and invokes two techniques for either instantaneous qualitative imaging or rapid quantitative imaging. Since imaging is performed rapidly and with a large-area camera, these techniques can be scaled up to investigate significant volumes of brain tissue with a high degree of efficiency. We have determined that proper handling and preparation of brain tissue is critical in preserving myelin structure for imaging, and in turn, we have developed methods of sample preparation that enable myelinated axons to be studied in great detail with BRM. Using postmortem brain sections from both rhesus monkeys and humans, we demonstrate that BRM enables novel studies of myelin structure, both for studying the breakdown of myelin in aging, injury, or disease, as well as for imaging the trajectories of individual myelinated axons at high resolution. As BRM is simple, inexpensive, and provides images of myelin based on label-free contrast, BRM is a platform technology that should find widespread utility across neuroscience. / 2024-08-29T00:00:00Z

Bragg diffraction is a natural phenomenon that arises from the coherent interference of scattered waves in multilayer structures with a well-defined periodicity. In practice, the physical size of these multilayer structures varies depending on the intended application, from micrometer-thick dielectric mirrors with tens of layers to centimeter-long Bragg gratings with ten-thousands of layers. The scope of this work centers around a unique class of multilayer elements developed in bulk photo-thermo-refractive (PTR) glass – the volume Bragg grating (VBG). The content of this thesis places an emphasis on the volume nature of these Bragg devices, implying a three-dimensional structure whereupon arbitrary spatial phase information can be embedded for laser-beam shaping, or the distribution of Bragg periods across each element is instead engineered to yield diffracted light with distinct spatio-temporal properties. In Chapter 1, operating principles, and fabrication technique of a conventional VBG are introduced. Owning to the principles of Bragg diffraction, the desired spatial and/or spectral phase information can be encoded onto the interference of scattered waves, reflecting from different sections along a grating volume. In Chapter 2, this principle is implemented in the form of phase-shifted volume Bragg gratings, whereby desired phase information is holographically engineered into the relative shift between neighboring Bragg substructures. Unlike other known active or passive phase-shaping tools, these phase-shifted elements can reconstruct the encoded phase profiles over a broad range of wavelengths that meet the Bragg condition of the VBG. On the other hand, the chirped volume Bragg grating, identified by a unique variation in grating periods across its volume, presents an alternative mean upon which phase information can be encoded – i.e., the Bragg-period distribution. Gratings of this kind are addressed in detail through Chapter 3. Due to the adaptability of holographic technique employed for the fabrication of volume gratings, a new class of Bragg elements is explored, capable of inscribing phase information into both (1) the relative shift among local Bragg elements, and (2) the Bragg-period variation across the grating volume. Chapter 4 reports on the construction of these hybrid structures, referred to as the phase-shifted, chirped volume Bragg gratings. Their unique ability to double as distributed feedback lasers, when recorded into the optically active volume of doped PTR glass, is discussed, paving the way for a novel source of light – the chirped, distributed feedback laser.