Optimization of multi-grating volume holographic spectrum splitters for photovoltaic applications.

Recent research has shown that using multiple diverse-bandgap photovoltaic (PV) cells in conjunction with a spectrum splitting optical system can significantly improve PV power generation efficiency. Although volume Bragg gratings (VBGs) can serve as effective spectrum splitters, the inherent dispersion of a VBG can be detrimental given a broad-spectrum input. The performance of a single holographic spectrum splitter element can be improved by utilizing multiple single volume gratings, each operating in a slightly different spectral band. However, care must be taken to avoid inter-grating coupling effects that limit the ultimate performance. This work explores broadband two-grating holographic optical elements (HOEs) in multiplexed (single element) and sandwiched-grating arrangements. Particle swarm optimization is used to tailor these systems to the solar spectrum, taking into account both efficiency and dispersion. Both multiplexed and sandwiched two-grating systems exhibit performance improvements over single-grating solutions, especially when reduced dispersion is required. Under a ±2° constraint on output angular spread from wavelength dispersion, sandwiched-, multiplexed-, and single-grating systems exhibit power conversion efficiencies of 82.1%, 80.9%, and 77.5%, respectively, compared to an ideal bandpass spectrum splitter. Dispersion performance can be further improved by employing more than two VBGs in the spectrum splitter, but efficiency is compromised by additional cross-coupling effects. Multiplexed-grating systems are especially susceptible to these effects, but have the advantage of utilizing only a single HOE.

Channel density and efficiency optimization of spectral beam combining systems based on volume Bragg gratings in sequential and multiplexed arrangements.

Spectral-beam-combining (SBC) systems utilizing multiple volume Bragg gratings must be carefully analyzed to maximize channel density and efficiency, and thus output radiance. This analysis grows increasingly difficult as the number of channels in the system increases, and heuristic optimization techniques are useful tools for exploring the limits of these systems. We explore three classes of multigrating SBC systems: cascaded, where each grating adds a new channel to the system in sequence; sandwiched, where several individual gratings are placed together and all channels enter the system at the same facet; and multiplexed, where all of the gratings occupy the same holographic optical element (HOE). Loss mechanisms differ among these three basic classes, and our optimization algorithm shows that the highest channel density for a given minimum efficiency and fixed operating bandwidth is achieved for a cascaded grating system. The multiplexed grating system exhibits the lowest channel density under the same constraints but has the distinct advantage of being realized by a single HOE. For a particular application, one must weigh channel density and efficiency versus system complexity when choosing among these basic classes of SBC systems. Additionally, one may need to consider the effects of finite-width input beams. As input beam radius is reduced, angular clipping effects begin to dominate over spectral interference and crosstalk effects, limiting all three classes of SBC systems in a similar manner.

Spectral interference in multiplexed volume Bragg gratings: theoretical calculations and experimental verification.

Multiplexed volume Bragg gratings can be applied to many types of broad- and narrowband spectral systems. However, there are often deleterious side effects to combining several gratings into a single holographic optical element, including loss of efficiency in diffracted waves of interest and the introduction of spurious waves. Design of these spectral systems requires analysis methods that are flexible and efficient and that take these side effects into account. We present a matrix-based algorithm for determining diffraction efficiencies of significant coupled waves in these multiplexed grating Holographic optical elements (HOEs). Several carefully constructed experiments with spectrally multiplexed gratings in dichromated gelatin verify our conclusions.

Efficient conversion of light from sparse laser arrays into single-lobed far field using phase structures.

An efficient beam-combining technique based on aperture filling is introduced to direct virtually all the energy of a mutually coherent laser array to the far-field main lobe. A comparison between this method and the Dammann grating method for beam superposition reveals the connection between the two and suggests specific applications for each.

Near-field nano-ellipsometer for ultrathin film characterization.

We describe a near-field ellipsometer for accurate characterization of ultrathin dielectric films. Optical tunnelling mimics the absorption in metallic films, enabling accurate measurement of the refractive index of ultrathin dielectric film. A regression model shows that a refractive index resolution of 0.001 for films as thin as 1 nm is possible. A solid-immersion nano-ellipsometer that incorporates this near-field ellipsometric technique with a solid-immersion lens is constructed to demonstrate the viability of this technique. Such a nano-ellipsometer can accurately characterize thin films ranging in thickness from subnanometre to micrometres with potential transverse resolution of the order of 100 nm.

Use of mass transport to reduce surface roughness in replicated gallium phosphide refractive microlenses.

Mass-transport smoothing has been used to reduce the surface roughness of a gallium phosphide (GaP) microlens from 50.0 nm (rms) to 6.5 nm (rms) while preserving the original surface figure. The initial GaP surface was fabricated by dry-etch transfer of a polymer microlens into a GaP substrate. This result demonstrates that mass transport can significantly improve the performance of economically feasible high-refractive-index micro-optical elements having arbitrary surface profiles.

Off-axis refractive mass-transported gallium-phosphide microlens array for the reduction of distortion in an optical interconnect system.

Mass-transport smoothing has been used to fabricate an array of off-axis gallium-phosphide microlenses for use in an optical interconnection system employing a single macroscopic lens to image an array of vertical-cavity surface-emitting lasers (VCSEL's) onto a detector array. Steering the individual VCSEL beams through the center of the relay lens creates an optical system with low distortion.

Mass-transport fabrication of off-axis and prismatic gallium phosphide optics.

Refractive prismatic and off-axis micro-optical elements have been fabricated in gallium phosphide by mass-transport smoothing of multistep and binary preforms, respectively. A total optical efficiency of greater than 94% was measured for the prism, while diffraction-limited collimation and steering were demonstrated with the off-axis lens. A qualitative discussion of possible errors in binary preform etching and their effects on the surface figure is also included.

Simplified intracavity phase plates for increasing laser-mode discrimination.

Recently, a new class of laser resonators was introduced that utilizes diffractive mirrors and an additional intracavity diffractive phase element. High modal discrimination and low fundamental-mode loss were achieved simultaneously by use of sinusoidal and pseudorandom diffractive phase elements. An intracavity phase element consisting of a simple single-step phase modulation is approximated by a Gaussian with a small radius. Explicit expressions are obtained for the modal-discrimination factor as a function of resonator parameters with a Gaussian output mirror. Numerical simulations are performed for a phase element with a step singularity in the phase function, the fundamental mode of this cavity being super-Gaussian. The modal discrimination of the cavity is studied for different radii of the single-step phase modulation, the position of the phase plate, and the cavity Fresnel number. Optimum solutions are found for a plane output mirror with either a striped or a circular shape.

Spatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materials.

Micromirrors were fabricated in gallium phosphide by mass transport to provide spatial-mode control of vertical-cavity surface-emitting lasers (VCSEL's). The concave mirrors were used in an external-cavity configuration to provide spatial filtering in the far field. Single-mode cw lasing was demonstrated in 15-microm-diameter VCSEL's with currents as high as 6 times threshold. The fabrication process was extended to micromirrors in gallium arsenide by use of a replication and dry-etch transfer process.

Rapid fabrication of diffractive optical elements by use of image-based excimer laser ablation.

We describe a method of fabricating multilevel diffractive optics by excimer laser ablation. A portion of a chrome mask containing many patterns is illuminated by 193-nm laser light and imaged by an objective lens onto a poly(imide) substrate. Ablation of an entire single pattern is achieved in a single laser pulse. Multiple pulses are used to vary the ablation depth, and multiple patterns are used to create a variety of multilevel optics. We have successfully fabricated arrays of eight-level diffractive microlenses with varying focal lengths and decenters. The optics performed with diffraction-limited focusing and near-theoretical diffraction efficiency (92%).

Optical antialiasing filters based on complementary Golay codes.

An optical filter that has an ideal response for removing aliasing noise from a sampled imaging system is described. The all-phase filter uses complementary Golay codes to achieve an optimum low-pass transfer function with no sidelobes. A computer model shows that the optical system has the expected performance in the ideal case, but degrades somewhat with wavelength variations and image aberrations. An experimental demonstration of the filter shows the optical transfer function performance and the response to imagery with a sampled detector.

Laser beam relaying with phase-conjugate diffractive optical elements.

A diffractive optical element is used to relay complex laser beam profiles by phase conjugation. It has the advantage over a conventional afocal system of avoiding light concentration at the intermediate focal point. Theoretical and experimental results show that the image quality is a function of alignment errors and mode-size changes. When the optical system is within the calculated tolerances, the diffractive optic reproduces images of high quality.

Virtual design prototyping applied to medical facilities.

Over the past five years, MITRE has developed rapid 3D modeling and immersive environment capabilities that supports the application of virtual environment technology to many traditional and non-traditional domains [1]. This paper provides background information on these capabilities and describes the application of this technology to the experimental design prototyping of operating rooms of the future and to the design and retrofit of existing or proposed medical facilities. These capabilities employ contemporary commercial hardware and software and exploit stereoscopic projection displays and headsets. A unique user interface facilitates object manipulation within these immersive environments and addresses two key areas: 1) Visualization of the contents on the model server or library in a catalog form; and 2) Natural interaction and immersion of the user with the visualized catalog and selected visualized objects in a 3D synthetic environment. A brief discussion of two developing applications of this technology will be presented. In one application example, the modeling environment can be used to synthesize physical replicas (potentially full stereo scale) of actual surgical rooms used for training of medical personnel. Alternatively, it can be employed as the infrastructure for a new form of collaborative interactive visualization, namely, telesurgery. In another example, the rapid modeling capability provides designers, architects and medical personnel with a means of rapidly developing synthetic renderings of (potentially interactive and remotely operative) proposed medical facilities prior to construction. We also discuss key issues needing to be resolved for successful model interchange.

Measurements of the modal properties of a diffractive-optic graded-phase resonator.

We measured the higher-order modes of a diffractive-optic graded-phase resonator by suppressing the lower-order modes of a flash-lamp-pumped Nd:YAG laser with spatial filters. A diffractive-optic end mirror was used to establish a flattop fundamental mode. Measurements of the fundamental and higher-order mode shapes agreed with theoretical models. Quantitative measurements of modal discrimination substantiate the predictions of large discrimination between the fundamental and second-order modes.

Modal properties of an external diode-laser-array cavity with diffractive mode-selecting mirrors.

Coupled mode theory is used to describe the behavior of an external laser cavity consisting of a diode laser array and a diffractive mode-selecting mirror. The mirror is designed to establish a uniform-amplitude, uniform-phase fundamental mode. Coupled mode theory is then used to study the behavior of higher-order modes. We show that the maximum discrimination against higher-order modes occurs when the round-trip cavity length satisfies certain Talbot relations. In addition, this high modal discrimination can be maintained for arrays with large numbers of lasers without incurring significant loss in the fundamental mode.

Diffractive optics: an introduction to the feature issue.

This special feature of Applied Optics contains 20 papers on the design, applications, and fabrication of diffractive optics. The companion feature in the Journal of the Optical Society of America A (May 1995) contains papers on diffractive optics modeling. Many of these papers result from presentations at the second OSA topical meeting on diffractive optics, June 6-9, 1994, in Rochester, New York.

Design and performance of diffractive optics for custom laser resonators.

Diffractive optical elements are used as end mirrors and internal phase plates in an optical resonator. A single diffractive end mirror is used to produce an arbitrary real-mode profile, and two diffractive mirrors are used to produce complex profiles. Diffractive mirror feature size and phase quantization are shown to affect the shape of the fundamental mode, the fundamental-mode loss, and the discrimination against higher-order modes. Additional transparent phase plates are shown to enhance the modal discrimination of the resonator at the cost of reduced fabrication tolerances of the diffractive optics. A 10-cm-long diffractive resonator design is shown that supports an 8.5-mm-wide fundamental mode with a theoretical second-order mode discrimination of 25% and a negligible loss to the fundamental mode.

High modal discrimination in a Nd:YAG laser resonator with internal phase gratings.

A unique laser resonator containing an internal phase grating and a diffractive mode-selecting mirror provides high modal discrimination in a Nd:YAG laser cavity. Single-spatial-mode lasers with high Fresnel numbers are possible, with negligible loss to the fundamental mode.