Design of a high-resolution holographic waveguide eye-tracking system operating in near-infrared with conventional optical elements.

The eye-tracking system plays an essential role in the augmented reality (AR) eyewear. Waveguide volume holographic optical elements (HOE) that can be made with high efficiency, thin form, and lightweight are well-suited for this application. Traditional holographic lenses formed with spherical wavefronts at visible wavelengths and used for near-infrared (NIR) eye-tracking systems suffer from significant image aberrations, image tilt, and ghost images. This work describes a complete holographic lens design methodology that systematically addresses these issues and details the specifications of conventional optical elements that provide the optimized wavefronts for the hologram construction beams. The resulting waveguide HOE NIR eye-tracking system has an image resolution of ∼10 lp/mm when the designed holographic lens is deposited on a waveguide with a refractive index of 1.8 and thickness of 0.5 mm.

High energy yield bifacial spectrum-splitting photovoltaic system.

In this paper a photovoltaic system is proposed that achieves high energy yield by integrating bifacial silicon cells into a spectrum-splitting module. Spectrum splitting is accomplished using volume holographic optical elements to spectrally divide sunlight onto an array of photovoltaic cells with different bandgap energies. Light that is reflected from the ground surface onto the rear side of the module is converted by the bifacial silicon cells. The energy yield of the system is optimized by tuning the volume holographic element parameters, such as film thickness, index modulation, and construction point source positions. An example is presented for utility-scale illumination parameters in Tucson, Arizona, that attains an energy yield of $1010\frac{{{ kw}\cdot { hr}}}{{{ yr}\cdot{{ m}^2}}}$1010kw⋅hryr⋅m2, which is 32.8% of the incident solar insolation.

Volume hologram replication system for spectrum-splitting photovoltaic applications.

A system for replicating high-efficiency volume hologram arrays, which has potential for high-volume manufacturing, is proposed. The system can meet the fabrication requirements of spectrum-splitting photovoltaic systems that are based on transmission volume holographic lens arrays. While previous hologram replication systems are mostly based on variations of the contact-copy method, the new technique is based on diffraction of reference and object beams from a master hologram through a prism and does not require contact with the copy hologram. The replication system has a number of identified advantages over contact-copy systems. An experimental volume holographic lens arrays fabricated using the proposed system had high median diffraction efficiency and low variability (95.3%+/-0.9%).

Wavelength-coded volume holographic imaging endoscope for multidepth imaging.

A wavelength-coded volume holographic imaging (WC-VHI) endoscope system capable of simultaneous multifocal imaging is presented. The system images light from two depths separated by 100 µm in a tissue sample by using axial chromatic dispersion of a gradient index probe in combination with two light-emitting diode sources and a multiplexed volume hologram to separate the images. This system is different from previous VHI systems in that it uses planar multiplexed gratings and does not require curved holographic gratings. This results in improved lateral imaging resolution from 228.1 to 322.5 lp/mm. This letter describes the design and fabrication of the WC-VHI endoscope and experimental images of hard and soft resolution targets and biological tissue samples to illustrate the performance properties.

Volume holographic imaging endoscopic design and construction techniques.

A reflectance volume holographic imaging (VHI) endoscope has been designed for simultaneous in vivo imaging of surface and subsurface tissue structures. Prior utilization of VHI systems has been limited to ex vivo tissue imaging. The VHI system presented in this work is designed for laparoscopic use. It consists of a probe section that relays light from the tissue sample to a handheld unit that contains the VHI microscope. The probe section is constructed from gradient index (GRIN) lenses that form a 1:1 relay for image collection. The probe has an outer diameter of 3.8 mm and is capable of achieving 228.1 ?? lp / mm resolution with 660-nm Kohler illumination. The handheld optical section operates with a magnification of 13.9 and a field of view of 390 ?? ? m × 244 ?? ? m . System performance is assessed through imaging of 1951 USAF resolution targets and soft tissue samples. The system has also passed sterilization procedures required for surgical use and has been used in two laparoscopic surgical procedures.

Broadband Gerchberg-Saxton algorithm for freeform diffractive spectral filter design.

A multi-wavelength expansion of the Gerchberg-Saxton (GS) algorithm is developed to design and optimize a surface relief Diffractive Optical Element (DOE). The DOE simultaneously diffracts distinct wavelength bands into separate target regions. A description of the algorithm is provided, and parameters that affect filter performance are examined. Performance is based on the spectral power collected within specified regions on a receiver plane. The modified GS algorithm is used to design spectrum splitting optics for CdSe and Si photovoltaic (PV) cells. The DOE has average optical efficiency of 87.5% over the spectral bands of interest (400-710 nm and 710-1100 nm). Simulated PV conversion efficiency is 37.7%, which is 29.3% higher than the efficiency of the better performing PV cell without spectrum splitting optics.

Spectrum splitting metrics and effect of filter characteristics on photovoltaic system performance.

During the past few years there has been a significant interest in spectrum splitting systems to increase the overall efficiency of photovoltaic solar energy systems. However, methods for comparing the performance of spectrum splitting systems and the effects of optical spectral filter design on system performance are not well developed. This paper addresses these two areas. The system conversion efficiency is examined in detail and the role of optical spectral filters with respect to the efficiency is developed. A new metric termed the Improvement over Best Bandgap is defined which expresses the efficiency gain of the spectrum splitting system with respect to a similar system that contains the highest constituent single bandgap photovoltaic cell. This parameter indicates the benefit of using the more complex spectrum splitting system with respect to a single bandgap photovoltaic system. Metrics are also provided to assess the performance of experimental spectral filters in different spectrum splitting configurations. The paper concludes by using the methodology to evaluate spectrum splitting systems with different filter configurations and indicates the overall efficiency improvement that is possible with ideal and experimental designs.

Simultaneous multiplane imaging of human ovarian cancer by volume holographic imaging.

Ovarian cancer is the most deadly gynecologic cancer, a fact which is attributable to poor early detection and survival once the disease has reached advanced stages. Intraoperative laparoscopic volume holographic imaging has the potential to provide simultaneous visualization of surface and subsurface structures in ovarian tissues for improved assessment of developing ovarian cancer. In this ex vivo ovarian tissue study, we assembled a benchtop volume holographic imaging system (VHIS) to characterize the microarchitecture of 78 normal and 40 abnormal tissue specimens derived from ovarian, fallopian tube, uterine, and peritoneal tissues, collected from 26 patients aged 22 to 73 undergoing bilateral salpingo-oophorectomy, hysterectomy with bilateral salpingo-oophorectomy, or abdominal cytoreductive surgery. All tissues were successfully imaged with the VHIS in both reflectance- and fluorescence-modes revealing morphological features which can be used to distinguish between normal, benign abnormalities, and cancerous tissues. We present the development and successful application of VHIS for imaging human ovarian tissue. Comparison of VHIS images with corresponding histopathology allowed for qualitatively distinguishing microstructural features unique to the studied tissue type and disease state. These results motivate the development of a laparoscopic VHIS for evaluating the surface and subsurface morphological alterations in ovarian cancer pathogenesis.

Dual-grating confocal-rainbow volume holographic imaging system designs for high depth resolution.

Confocal microscopy rejects out-of-focus light from the object by scanning a pinhole through the image and reconstructing the image point by point. Volume holographic imaging systems with bright-field illumination have been proposed as an alternative to conventional confocal-type microscopes that does not require scanning of a pinhole or a slit. However, due to wavelength-position degeneracy of the hologram, the high Bragg selectivity of the volume hologram is not utilized and system performance is not optimized. Confocal-rainbow illumination has been proposed as a means to remove the degeneracy and improve optical sectioning in these systems. In prior work, two versions of this system were illustrated: the first version had a separate illumination and imaging grating and the second used a single grating to disperse the incident light and to separate wavelengths in the imaging path. The initial illustration of the dual-grating system has limited depth resolution due to the low selectivity of the illumination grating. The initial illustration of the single-grating system has high depth resolution but does not allow optimization of the illumination path and requires high optical quality of the holographic filters. In this paper we consider the design and tolerance requirements of the dual-grating system for high depth resolution and demonstrate the results with an experimental system. An experimental system with two 1.8 mm thick planar holograms achieved a depth resolution of 7 µm with a field of view of 1.9 mm and a hologram dispersion matching tolerance of ±0.008°.

Ultra light-trapping filters with broadband reflection holograms.

Significant optical absorption enhancement can be achieved by incorporating optical diffusers in the thin-film silicon photovoltaic (PV) cells. Absorption can be increased further by angular and spectral selective filters. In this work the properties of volume reflection holograms are examined for realizing ultra light-trapping filters for thin film silicon photovoltaic cell applications. The filter properties of reflection volume hologram are evaluated for this application. It is found that variation in the refractive index profile as a function of depth is an important factor. The optimized design is implemented in dichromated gelatin holograms and found to be in good agreement with predicted performance. The enhancement to the conversion efficiency of silicon PV cells are predicted with the PC-1D simulation tool and is found to be similar to that with an optimized Rugate filter. The simulated short circuit current density enhancement was found to be 8.2% for a 50 µm thick silicon PV cell and 15.8% for a 10 µm thick silicon PV cell.

Confocal-rainbow volume holographic imaging system.

The performance of broadband volume holographic imaging system in terms of depth selectivity is investigated. The mechanism for depth resolution degradation is explained. In order to overcome this resolution degradation, a novel imaging device, the confocal-rainbow volume holographic imaging system, is proposed. Modeling and experimental validation of the performance of this novel imaging system indicates that depth resolution <16 µm is achievable. The lateral resolution of this device is <2.5 µm along a field of view of 300 µm×100 µm.

Phase-contrast volume holographic imaging system.

A phase-contrast volume holographic imaging system for three-dimensional contrast enhancement is presented. The system utilizes a spatial filter placed on a conjugate plane to the volume holographic pupil to simultaneously enhance weak phase information at different depths within an object. The proposed system was validated with experimental image data obtained in mouse colon samples and quantitative measurements of modulation transfer function as well.

Spatial-spectral volume holographic systems: resolution dependence on effective thickness.

The resolution dependence of spatial-spectral volume holographic imaging systems on angular and spectral bandwidth of nonuniform gratings is investigated. Modeling techniques include a combination of the approximate coupled-wave analysis and the transfer-matrix method for holograms recorded in absorptive media. The effective thickness of the holograms is used as an estimator of the resolution of the imaging systems. The methodology, which assists in the design and optimization of volume holographic simulation results based on our approach, are confirmed with experiments and show proof of consistency and usefulness of the proposed models.

Analysis of diffracted image patterns from volume holographic imaging systems and applications to image processing.

Diffracted image patterns from volume holograms that are used in volume holographic imaging systems (VHISs) are investigated. It is shown that, in VHISs, prior information about the shape and spectral properties of the diffracted patterns is important not only to determine the curvature and field of view of the image, but also for image registration and noise removal. A new methodology to study numerically and analytically the dependence of VHIS diffraction patterns with the hologram construction parameters and the readout wavelength is described. Modeling and experimental results demonstrate that, in most cases, VHIS diffracted shapes can be accurately represented by hyperbolas.

Simulations and experiments of aperiodic and multiplexed gratings in volume holographic imaging systems.

A new methodology describing the effects of aperiodic and multiplexed gratings in volume holographic imaging systems (VHIS) is presented. The aperiodic gratings are treated as an ensemble of localized planar gratings using coupled wave methods in conjunction with sequential and non-sequential ray-tracing techniques to accurately predict volumetric diffraction effects in VHIS. Our approach can be applied to aperiodic, multiplexed gratings and used to theoretically predict the performance of multiplexed volume holographic gratings within a volume hologram for VHIS. We present simulation and experimental results for the aperiodic and multiplexed imaging gratings formed in PQ-PMMA at 488 nm and probed with a spherical wave at 633 nm. Simulation results based on our approach that can be easily implemented in ray-tracing packages such as Zemax® are confirmed with experiments and show proof of consistency and usefulness of the proposed models.

Optical Design for a Spatial-Spectral Volume Holographic Imaging System.

Spatial Spectral Holographic imaging system (S(2)-VHIS) is a promising alternative to confocal microscopy due to its capabilities to simultaneously image several sample depths with high resolution. However, the field of view of previously presented S(2)-VHIS prototypes has been restricted to less than 200µm. This paper presents experimental results of an improved S(2)-VHIS design which have a field of view of ~1mm while maintaining high resolution and dynamic range.

Silicon oxide nanoparticles doped PQ-PMMA for volume holographic imaging filters.

Holographic imaging filters are required to have high Bragg selectivity, namely, narrow angular and spectral bandwidth, to obtain spatial-spectral information within a three-dimensional object. In this Letter, we present the design of holographic imaging filters formed using silicon oxide nanoparticles (nano-SiO(2)) in phenanthrenquinone-poly(methyl methacrylate) (PQ-PMMA) polymer recording material. This combination offers greater Bragg selectivity and increases the diffraction efficiency of holographic filters. The holographic filters with optimized ratio of nano-SiO(2) in PQ-PMMA can significantly improve the performance of Bragg selectivity and diffraction efficiency by 53% and 16%, respectively. We present experimental results and data analysis demonstrating this technique in use for holographic spatial-spectral imaging filters.

Energy collection efficiency of holographic planar solar concentrators.

We analyze the energy collection properties of holographic planar concentrator systems. The effects of solar variation on daily and annual energy collection are evaluated. Hologram diffraction efficiency, polarization, crosstalk in cascaded elements, and constraints imposed by the radiance theorem, as well as solar illumination characteristics, are considered. A planar holographic solar concentrator configuration is designed and modeled to maximize energy collection efficiency during the course of a year without the need for tracking. Results indicated that nearly 50% of the available energy illuminating hologram areas can be collected by photovoltaic cells without the need of tracking.

Laser-induced fluorescence imaging of subsurface tissue structures with a volume holographic spatial-spectral imaging system.

A three-dimensional imaging system incorporating multiplexed holographic gratings to visualize fluorescence tissue structures is presented. Holographic gratings formed in volume recording materials such as a phenanthrenquinone poly(methyl methacrylate) photopolymer have narrowband angular and spectral transmittance filtering properties that enable obtaining spatial-spectral information within an object. We demonstrate this imaging system's ability to obtain multiple depth-resolved fluorescence images simultaneously.

Optimization of multiplexed holographic gratings in PQ-PMMA for spectral-spatial imaging filters.

Holographic gratings formed in thick phenanthrenquinone- (PQ-) doped poly(methyl methacrylate) (PMMA) can be made to have narrowband spectral and spatial transmittance filtering properties. We present the design and performance of angle-multiplexed holographic filters formed in PQ-PMMA at 488 nm and reconstructed with a LED operated at approximately 630 nm. The dark delay time between exposure and the preillumination exposure of the polymer prior to exposure of the holographic area are varied to optimize the diffraction efficiency of multiplexed holographic filters. The resultant holographic filters can enhance the performance of four-dimensional spatial-spectral imaging systems. The optimized filters are used to simultaneously sample spatial and spectral information at five different depths separated by 50 microm within biological tissue samples.