Multi-plane light conversion (MPLC) has recently been developed as a versatile tool for manipulating spatial distributions of the optical field through repeated phase modulations. An MPLC Device consists of a series of phase masks separated by free-space propagation. It can convert one orthogonal set of beams into another orthogonal set through unitary transformation, which is useful for a number of applications. In telecommunication, for example, mode-division multiplexing (MDM) is a promising technology that will enable continued scaling of capacity by employing spatial modes of a single fiber. MPLC has shown great potential in MDM devices with ultra-wide bandwidth, low insertion loss (IL), low mode-dependent loss (MDL), and low crosstalk. This dissertation presents MPLC devices for (de)multiplexing, coupling, routing, optical signal processing, and wavefront synthesis. First, fundamentals in the design, simulation, and characterization of MPLC devices are introduced. In the area of MPLC devices, a coupler based on MPLC was demonstrated to bridge between few-mode fibers and waveguides. A reconfigurable broadband mode router for multi-port mode-to-space mapping was proposed using the MPLC technique, compatible with existing wavelength division multiplexed (WDM) systems. In the area of MPLC signal processing, an ultrabroadband polarization-insensitive optical 90° hybrid, which is a two-input four-output device, was demonstrated. In the area of system application, we demonstrated the use of a pair of 45-mode MPLC (de)multiplexers and high-sensitivity superconducting nanowire single-photon detectors (SNSPDs) to measure the differential mode group delay (DMDG), distributed mode crosstalk, and cladding modes of a commercial graded-index multimode fiber (GI-MMF). A compact, large-area, non-mode selective MPLC coherent beam combiner using curved mirrors is also presented for high-power wavefront synthesis.

A new figure of merit, the potential information gathering power, P , is developed for use in evaluating the performance of spectrographs. It is based on the premise that it is desirable to maximize the product of the SNR at each resolution element in the reduced data by the number of resolution elements. Because of this general intent, it places no a priori emphasis on any particular scientific use. This figure is then applied to the task of improving the performance of a real fiber-fed astronomical spectrograph when it is operated in its high-resolution mode. It is shown that the optimum configuration corresponds to adding a new field lens, changing the focal length and size of the collimator, and arranging the camera and collimator axes to the narrowest allowable geometry. An approximate gain of between 15% to 18% may be realized.

The backscattering coefficient bb is one of the inherent optical properties of natural waters which means that it is independent of the ambient light field in the water. As such, it plays a central role in many problems of optical oceanography and is used in the characterization of natural waters. Essentially, any measurement that involves sending a beam of light into water must account for all inherent backscattering. Some of the applications that rely on the precise knowledge of the backscattering coefficient include studies of suspended particle distributions, optical bathymetry, and remote sensing. Many sources contribute to the backscattering, among them any suspended particles, air bubbles, and the water molecules themselves. Due to the importance of precise measurements and the ease with which water samples can be contaminated, an instrument to determine directly and quickly the backscattering coefficient in situ is highly desirable. We present such an instrument in both theory and experiment. We explain the theory behind our instrument and based on measurements made in the laboratory we demonstrate that our prototype shows the predicted behavior. We present data for increased extinction in the water, and show how measuring the extinction and taking it into account improves the quality of our measurements. We present calibration data obtained from three different particle sizes representing differently shaped volume scattering functions. Based on these measurements we demonstrate that our prototype has the necessary resolution to measure the backscattering coefficient bb over the whole range found in natural waters. We discuss potential improvements that should be made for a commercial version of the instrument.

Thesis (Ph. D.)--University of Oregon, 2003. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 129-135). Also available for download via the World Wide Web; free to University of Oregon users.

The linear and nonlinear optical properties of semiconductor nano-structures and photoactive yellow protein (PYP) have been studied in this work. The reflection and transmission properties of multiple InₓGa₍₁₋ₓ₎As quantum well (QW) samples are first presented. Constructive interference in the reflection from the QWs is observed when the QWs are spaced by λ/2 (λ = exciton absorption peak wavelength) and destructive interference when the spacing is λ/4. The nonlinear transmission of fs pulses through a QW sample is also studied. A broadening of the exciton transition with negligible loss of oscillator strength is observed. Semiconductor microcavity samples with embedded quantum wells exhibiting normal mode coupling (NMC) are studied both in the linear and nonlinear regime, with ultrafast time resolution using upconversion. A decrease in the modulation depth of the NMC oscillations and reflection dips with increasing incident photon flux without a change of NMC oscillation period and splitting are observed, consistent with a bleaching of the exciton transition without loss of oscillator strength. The effective mass of multiple QW samples measured from the slope of Landau Levels of the QWs in magnetic field is measured for both photoluminescence (PL) and absorption spectra. For some samples, the absorption spectra show an effective mass consistent with the electron-hole effective mass while the PL spectra show an effective mass consistent with just the electron. This is explained by hole localization on monolayer island fluctuations on the QW/barrier interfaces. The magnetic field is also used to measure Faraday rotation in semiconductor microcavities exhibiting NMC. A resonant Faraday rotation of 3° degrees is observed in reflection. Finally the nonlinear one-photon and two-photon absorption (TPA) properties of PYP are investigated. One-photon excitation results in a complete bleaching of the absorption peak. No TPA is observed, but an upper limit of 3.5·10⁻⁵²cm⁴ s molecule⁻¹photon⁻¹ for the TPA cross section is found.

No description available.

The bright squeezed vacuum state of light is a macroscopic nonclassical state found at the output of a strongly pumped unseeded travelling-wave optical parametric amplifier. It has been applied to quantum imaging, quantum communication, and phase supersensitivity, to name a few. Bright squeezed states are in general highly multimode, while most applications require a single mode. We separated two nonlinear crystals in the direction of propagation of the pump in order to narrow the angular spectrum down to a nearly-single angular mode. We observed noise reduction in the photon number difference between the two down-converted channels, which constitutes of proof of nonclassicality. By introducing a dispersive medium between the two nonlinear crystals, we were able to tailor the frequency spectrum of bright squeezed vacuum and to dramatically reduce the number of frequency modes down to 1.82 ± 0.02, bringing us closer to truly single-mode bright squeezed vacuum.

Parametric down-conversion (PDC) is a nonlinear optical process widely used to generate pairs of photons. It occurs when an intense laser traverses an optical parametric amplifier (OPA). When the gain of the amplifier is increased, the number of downconverted photons increases exponentially: this is the high-gain regime of PDC. High-gain PDC is potentially a versatile tool for metrology. It is a source of highly-entangled states and bright squeezed states for applications in quantum information and interferometry. In addition, the high number of photons in high-gain PDC makes it possible to use diodes and cameras directly, instead of single-photon detectors and coincidence-counting apparatus. However, all the quantum-optical experimental methods need to be generalized or adapted for a high-photon flux. Most of the theoretical and experimental techniques used or developed in this thesis aim to address this transition from low to high-photon flux of PDC. I theoretically and experimentally provide strategies to harness the mode structure of PDC, bringing us steps closer to a usable source of bright squeezed vacuum for interferometry and quantum imaging. I present experimental progress in reducing the number of frequency modes of high-gain PDC, which is naturally broadband, and consequently highly multimode. Our theory for high-gain PDC generated in a nonlinear crystal provides a set of modes containing physically meaningful information, i.e. the pairwise quantum correlations between independant modes. In addition, I provide a thorough discussion on the limit of SU(1,1) interferometry in regards to internal loss and gain unbalancing. Finally, I tie the frequency spectrum of high-gain PDC to the properties of vacuum fluctuations, allowing to predict the number of photons from first principles, making it a powerful tool for spectroradiometry. Those developments are a springboard towards usable high-gain PDC for metrology.

Lasers are an essential technology enabling countless fields of optics, however, their operation wavelengths are limited to isolated regions across the optical spectrum due to the need for suitable gain media. Parametric frequency conversion (PFC) is an attractive means to convert existing lasers to new colors using nonlinear optical interactions rather than the material properties of the host medium, allowing for the development of high power laser sources across the entire optical spectrum. PFC in bulk χ(2) crystals has led to the development of the optical parametric oscillator, which is currently the standard source for high power light at non-traditional wavelengths in the laboratory setting. Ideally, however, one could implement PFC in an optical fiber, thus leveraging the crucial benefits of a guided-wave geometry: alignment-free, compact, and robust operation. Four-wave mixing (FWM) is a nonlinear effect in optical fibers that can be used to convert frequencies, the major challenge being conservation of momentum, or phase matching, between the interacting light waves. Phase matching can be satisfied through the interaction of different spatial modes in a multi-mode fiber, however, previous demonstrations have been limited by mode stability and narrow-band FWM gain. Alternatively, phase matching within the fundamental mode can be realized in high-confinement waveguides (such as photonic crystal fibers), but achieving the anomalous waveguide dispersion necessary for phase matching at pump wavelengths near ∼1 μm (where the highest power fiber lasers emit) comes at the cost of reducing the effective area of the mode, thus limiting power-handling. Here, we specifically consider the class of Bessel-like LP0,m modes in step-index fibers. It has been shown that these modes can be selectively excited and guided stably for long lengths of fiber, and mode stability increases with mode order ‘m’. The effective area of modes in these fibers can be very large (>6000 μm2 demonstrated) and is decoupled from dispersion, allowing for phase matching within a single mode in a power-scalable platform. Furthermore, step-index fibers can guide many different LP0,m modes, allowing access to a highly multi-moded basis set with which to study FWM interactions between different modes. In this thesis we develop techniques to excite, propagate, and characterize LP0,m modes in order to demonstrate FWM in two regimes: monomode interactions comprising waves all belonging to the same mode, and intermodal interactions between different modes. In the monomode regime we demonstrate parametric sources which operate at near-infrared wavelengths under-served by conventional fiber lasers, including 880, 974, 1173, and 1347 nm. The output pulses for these systems are ∼300 ps in duration and reach peak powers of ∼10 kW, representing, to the best our knowledge, the highest peak power fiber laser sources demonstrated at these wavelengths to date. In the intermodal regime, we demonstrate a cascade of FWM processes between different modes that lead to a series of discrete peaks in the visible portion of the spectrum, increasing monotonically in mode order from LP0,7 at 678 nm to LP0,16 at 443 nm. This cascade underscores the huge number of potential FWM interactions between different LP0,m modes available in a highly multi-mode fiber, which scale as N4 for N guided modes. Finally, we demonstrate a novel intermodal FWM process pumped between the LP0,4 and LP0,5 modes of a step-index fiber, which provides broadband FWM gain (63 nm at 1550 nm) while maintaining wavelength separations of nearly an octave (762 nm) – a result that cannot be replicated in the single-mode regime. We seed this process to generate a ∼10 kW, ∼300-ps pulsed fiber laser wavelength-tunable from 786-795 nm; representing a fiber analogue of the ubiquitous Ti:Sapphire laser.

Atmospheric turbulence limits the resolving power of astronomical telescopes by distorting the paths of light between distant objects of interest and the imaging camera at the telescope. After many light-years of travel, passing through the turbulence in that last 100km of a photon’s journey results in a blurred image in the telescope, no less than 1” (arc-second) in width. To achieve higher resolutions, corresponding to smaller image widths, various methods have been proposed with varying degrees of effectiveness and practicality. Space telescopes avoid atmospheric turbulence completely and are limited in resolution solely by the size of their mirror apertures. However, the design and maintenance cost of space telescopes, which increases prohibitively with size, has limited the number of space telescopes deployed for astronomical imaging purposes. Ground based telescopes can be built larger and more cheaply, so atmospheric compensation schemes using adaptive optical cancellation mirrors can be a cheaper substitute for space telescopes. Adaptive optics is referred to here as the use of electronic control of optical component to modify the phase of an incident ray within an optical system like an imaging telescope. Fast adaptive optics systems operating in real-time can be used to correct the optical aberrations introduced by atmospheric turbulence. To compensate those aberrations, they must first be measured using a wavefront sensor. The wavefront estimate from the wavefront sensor can then be applied, in a closed-loop system, to a deformable mirror to compensate the incoming wavefront. Many wavefront sensors have been proposed and are in used today in adaptive optics and atmospheric turbulence measurement systems. Experimental results comparing the performance of wavefront sensors have also been published. However, little detailed analyses of the fundamental similarities and differences between the wavefront sensors have been performed. This study concentrates on fourmain types of wavefront sensors, namely the Shack-Hartmann, pyramid, geometric, and the curvature wavefront sensors, and attempts to unify their description within a common framework. The quad-cell is a wavefront slope detector and is first examined as it lays the groundwork for analysing the Shack-Hartmann and pyramid wavefront sensors. The quad-cell slope detector is examined, and a new measure of performance based on the Strehl ratio of the focal plane image is adopted. The quad-cell performance based on the Strehl ratio is compared using simulations against the Cramer-Rao bound, an information theoretic or statistical limit, and a polynomial approximation. The effects of quad-cell modulation, its relationship to extended objects, and the effect on performance are also examined briefly. In the Shack-Hartmann and pyramid wavefront sensor, a strong duality in the imaging and aperture planes exists, allowing for comparison of the performance of the two wavefront sensors. Both sensors subdivide the input wavefront into smaller regions, and measure the local slope. They are equivalent in every way except for the order in which the subdivision and slope measurements were carried out. We show that this crucial difference leads to a theoretically higher performance from the pyramid wavefront sensor. We also presented simulations showing the trade-off between sensor precision and resolution. The geometric wavefront sensor can be considered to be an improved curvature wavefront sensor as it uses a more accurate algorithm based on geometric optics to estimate the wavefront. The algorithm is relatively new and has not found application in operating adaptive optics systems. Further analysis of the noise propagation in the algorithm, sensor resolution, and precision is presented. We also made some observations on the implementation of the geometric wavefront sensor based on image recovery through projections.