Christopher J. Corcoran

Experimental demonstration of a phase-locked laser array using a self-Fourier cavity

A coherent phase-locked laser array has been experimentally demonstrated by combining the outputs of seven individual fiber lasers together in a self-Fourier cavity. By analyzing the interference fringes of the laser output in the far field of the array, a fringe visibility was measured of V=0.87, indicating a coherence of 0.73. The total output power of this laser array when operated as a coherent ensemble was 0.4watts.

Printing-based assembly of quadruple-junction four-terminal microscale solar cells and their use in high-efficiency modules

Expenses associated with shipping, installation, land, regulatory compliance and on-going maintenance and operations of utility-scale photovoltaics can be significantly reduced by increasing the power conversion efficiency of solar modules through improved materials, device designs and strategies for light management. Single-junction cells have performance constraints defined by their Shockley-Queisser limits. Multi-junction cells can achieve higher efficiencies, but epitaxial and current matching requirements between the single junctions in the devices hinder progress. Mechanical stacking of independent multi-junction cells circumvents these disadvantages. Here we present a fabrication approach for the realization of mechanically assembled multi-junction cells using materials and techniques compatible with large-scale manufacturing. The strategy involves printing-based stacking of microscale solar cells, sol-gel processes for interlayers with advanced optical, electrical and thermal properties, together with unusual packaging techniques, electrical matching networks, and compact ultrahigh-concentration optics. We demonstrate quadruple-junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1,000 suns, and modules with efficiencies of 36.5%.

Deterministic assembly of releasable single crystal silicon-metal oxide field-effect devices formed from bulk wafers

Deterministic assembly of ultrathin metal oxide-semiconductor field-effect transistors released from the surfaces of bulk wafers with (111) orientation provides a route to high quality electronics on nearly any type of substrate. Device parameters and bias stability characteristics from transistors on sheets of plastic confirm the effectiveness of the approach and the critical roles of thermally grown layers of silicon dioxide for the gate dielectrics and passivation layers. Systematic studies of the anisotropic etching processes used to release the devices illustrate capabilities into the sub-micron thickness regime, with beneficial effects on the bending stiffness and degree of bendability.

Passive Phasing in a Coherent Laser Array

A coherent array of fiber lasers in a self-Fourier cavity is described and analyzed. With individual regenerative feedback added to each fiber laser, the integrated gain in each individual fiber is a function of its cold cavity phase shift (fiber length). This results in a gain-dependent phase shift due to the Kramers-Kronig relations, and has been shown to partially compensate for the random differences in fiber lengths often encountered in coherent fiber arrays. A coupled cavity analysis of the active gain elements and the passive external cavity is performed and a self-consistent fundamental supermode determined. The output phase distribution of the array is determined based on a random distribution in fiber lengths. The Strehl ratio of this phase distribution is calculated and compared to experimental data.

Fully Flexible Electromagnetic Valve Actuator: Design, Modeling, and Measurements

Design, modeling, and measurements are discussed for a new direct-drive, electromagnetic valve actuator (EVA) for internal combustion engines. The actuator offers greater dynamic performance than previous designs and achieves fully flexible valve timing and lift, which provides great benefits in engine performance including increased engine efficiency, reduced emissions, and improved low-end performance. The innovative actuator consists of simplified stationary permanent magnets, stationary coils, and a moving steel plunger that transmits significant bi-directional forces to the valve throughout the stroke. Single- and double-actuator magnetic configurations, non-linear electromagnetic finite element analyses, system bond graphs, Simulink® simulations, and performance measurements of the actuator controlling intake valves on a cylinder head will be discussed. Future work, including on-engine dynamometer testing and an actuator design variant for exhaust valves, is briefly discussed.

Enhanced ultraviolet responses in thin-film InGaP solar cells by down-shifting

Layers of poly(methyl methacrylate) doped with the Eu complex Eu(DPEPO)(hfac)3 (EuDH) provide a means for down-shifting incident ultraviolet (UV) light into the visible range, with beneficial effects on the performance of solar cells, as demonstrated with thin-film InGaP devices formed by epitaxial liftoff. Experimental and computational results establish important aspects of gain and loss mechanisms in the UV range. Measurements show that InGaP cells with coatings of EuDH doped PMMA exhibit enhanced currents (8.68 mA cm(-2)) and power conversion efficiencies (9.48%), both due to increased responses at wavelengths between 300-360 nm.

Coherent Array of Nonlinear Regenerative Fiber Amplifiers

A coherent array of high power fiber amplifiers (lasers) with a nonlinear refractive index is described and discussed. By adding reflectivity to the inputs of the fibers, each fiber laser is transformed into a regenerative amplifier, with an internal circulating power that varies depending on the round trip phase shift of the field within the fiber. The variation of the internal circulating power results in a preferential shift in the output phase of each amplifier due to the high nonlinearity within the fiber. The resulting phase shift is predicted to substantially compensate for the inherent randomness in the output phase distribution usually associated with the long lengths of fiber amplifiers. The individual outputs of the coherent array are shown to exhibit a ldquofuzzyrdquo phase distribution that can be partially quantified based on a statistical analysis and is dependent on the parameters of the array. A simple example is given of a coherent array of fiber lasers in a Self-Fourier cavity and the degree of coherence is predicted under simplifying assumptions.

Passive coherent combination of a diode laser array with 35 elements

A monolithic diode laser array with 35 elements is operated as a coherent array through the use of a Self-Fourier cavity. By analyzing the far field interference pattern, the coherence was measured to be 0.57 with all 35 elements operating and was measured to be approximately constant for arrays with greater than 15 elements. These results are in rough agreement with previous analyses which predict a coherence equal to 0.65 for very large arrays of passively coupled laser elements and demonstrate how the use of regenerative feedback benefits the passive phasing of coherent laser arrays. These results demonstrate that it is possible to circumvent previous cold cavity theories that predict poor phasing properties for arrays with greater than ~10 elements.

Mechanisms of enhanced optical absorption for ultrathin silicon solar microcells with an integrated nanostructured backside reflector.

This paper investigates mechanisms of enhanced light absorption exhibited by ultrathin Si solar microcells integrated with a periodically nanostructured, semitransparent metallic reflector. This backside reflector comprises periodic nanoscale relief features formed by soft-imprint lithography with a thin (~35 nm) coating of Au. The work shows that microcells placed in direct contact above the nanostructured reflectors surface creates Fabry-Pérot cavities, which traps impinging light inside the Si slab via the excitation of cavity modes. Experimental measurements show that the short-circuit current and efficiency values for devices incorporating this thin, semitransparent backside reflector outperform similar Si microcells integrated with a planar thick (~300 nm) opaque mirror by ~10-15% because of enhanced absorption. Computational modeling that is supported by experimental measurements reveal that the dominant methods of enhancement stem from a complex interplay between backside diffraction/scattering and Fabry-Pérot resonances. These same data demonstrate that plasmonic interactions contribute minimally to the optical enhancements seen.

Regenerative Phase Shift and Its Effect on Coherent Laser Arrays

A coherent array of regenerative amplifiers in an external cavity with a rank 1 scattering matrix is described and analyzed. Using a resonant cavity analysis, it is shown how the addition of regenerative feedback to each element in the array creates a phase shift relative to the well-known “cold-cavity phase shift”. This phase shift is quantified and found to significantly affect the phasing properties of coherent arrays, even in the absence of the nonlinear Kerr effect and the gain-dependent phase shift. In particular, this regenerative phase shift is shown to concentrate the distribution of phases at the output of the laser array into a narrower phase range compared to the random distribution expected using a nonregenerative amplifier in the presence of effectively random cold-cavity phase shifts.