Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location.

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.

High concentration factor diffractive microlenses integrated with CMOS single-photon avalanche diode detector arrays for fill-factor improvement.

Large-format single-photon avalanche diode (SPAD) arrays often suffer from low fill-factors-the ratio of the active area to the overall pixel area. The detection efficiency of these detector arrays can be vastly increased with the integration of microlens arrays designed to concentrate incident light onto the active areas and may be refractive or diffractive in nature. The ability of diffractive optical elements (DOEs) to efficiently cover a square or rectangular pixel, combined with their capability of working as fast lenses (i.e., ∼f/3) makes them versatile and practical lens designs for use in sparse photon applications using microscale, large-format detector arrays. Binary-mask-based photolithography was employed to fabricate fast diffractive microlenses for two designs of 32×32 SPAD detector arrays, each design having a different pixel pitch and fill-factor. A spectral characterization of the lenses is performed, as well as analysis of performance under different illumination conditions from wide- to narrow-angle illumination (i.e., f/2 to f/22 optics). The performance of the microlenses presented exceeds previous designs in terms of both concentration factor (i.e., increase in light collection capability) and lens speed. Concentration factors greater than 33× are achieved for focal lengths in the substrate material as short as 190µm, representing a microlens f-number of 3.8 and providing a focal spot diameter of <4µm. These results were achieved while retaining an extremely high degree of performance uniformity across the 1024 devices in each case, which demonstrates the significant benefits to be gained by the implementation of DOEs as part of an integrated detector system using SPAD arrays with very small active areas.

Tamper-proof markings for the identification and traceability of high-value metal goods.

A customized UV nanosecond pulsed laser system has been developed for the fast generation of tamper-proof security markings on the surface of metals, such as stainless steel, nickel, brass, and nickel-chromium (Inconel) alloys. The markings in the form of reflective phase holographic structures are generated using a laser microsculpting process that involves laser-induced local melting and vaporization of the metal surface. The holographic structures are formed from an array of optically-smooth craters whose depth can be controlled with ± 25nm accuracy. In contrast to conventional security markings, e.g., engraved serial numbers, etched part numbers and embossed polymer holographic stickers, which are only attached to the metal products as an adhesive tape, the phase holographic structures are robust to local damage (e.g. scratches) and resistant to tampering because they are generated directly on the metal surface. This paper describes a novel laser-based process for security marking of high-value metal goods, investigates the optical performance of the holographic structures, and demonstrates their application to watches.

High resolution Shack-Hartmann sensor based on array of nanostructured GRIN lenses.

We present a novel method for the development of a micro lenslets hexagonal array. We use gradient index (GRIN) micro lenses where the variation of the refraction index is achieved with a structure of nanorods made of 2 types of glasses. To develop the GRIN micro lens array, we used a modified stack-and-draw technology which was originally applied for the fabrication of photonic crystal fibers. This approach results in a completely flat element that is easy to integrate with other optical components and can be effectively used in high refractive index medium as liquids. As a proof-of-concept of the method we present a hexagonal array of 469 GRIN micro lenses with a diameter of 20 µm each and 100% fill factor. The GRIN lens array is further used to build a Shack-Hartmann detector for measuring wavefront distortion. A 50 lens/mm sampling density is achieved.

Diffractive optics development using a modified stack-and-draw technique.

We present a novel method for the development of diffractive optical elements (DOEs). Unlike standard surface relief DOEs, the phase shift is introduced through a refractive index variation achieved by using different types of glass. For the fabrication of DOEs we use a modified stack-and-draw technique, originally developed for the fabrication of photonic crystal fibers, resulting in a completely flat element that is easy to integrate with other optical components. A proof-of-concept demonstration of the method is presented-a two-dimensional binary optical phase grating in the form of a square chessboard with a pixel size of 5 µm. Two types of glass are used: low refractive index silicate glass NC21 and high refractive index lead-silicate glass F2. The measured diffraction characteristics of the fabricated component are presented and it is shown numerically and experimentally that such a DOE can be used as a fiber interconnector that couples light from a small-core fiber into the several cores of a multicore fiber.

Large elliptical nanostructured gradient-index microlens.

We demonstrate the feasibility of the development of a gradient-index elliptical microlens with a size of 75×125 µm using nanostructured glass technology. The gradient index is obtained by means of a discrete internal structure composed of two glasses with feature sizes much smaller than the wavelength of the incident light. A modified photonic crystal fiber-drawing technique is used for the lens fabrication. The elliptical shape of the lens is obtained by a novel final drawing stage where the spherically symmetric lens preform is drawn into an elliptical form by collapsing two large air holes placed in the preform during assembly. The effective focal lengths of 160 and 260 µm for the orthogonal axes are obtained experimentally for the fabricated lens, and show good agreement with those predicted by the effective medium theory and the full-wave beam propagation simulations.

Fill-factor improvement of Si CMOS single-photon avalanche diode detector arrays by integration of diffractive microlens arrays.

Single-photon avalanche diode (SPAD) detector arrays generally suffer from having a low fill-factor, in which the photo-sensitive area of each pixel is small compared to the overall area of the pixel. This paper describes the integration of different configurations of high efficiency diffractive optical microlens arrays onto a 32 × 32 SPAD array, fabricated using a 0.35 µm CMOS technology process. The characterization of SPAD arrays with integrated microlens arrays is reported over the spectral range of 500-900 nm, and a range of f-numbers from f/2 to f/22. We report an average concentration factor of 15 measured for the entire SPAD array with integrated microlens array. The integrated SPAD and microlens array demonstrated a very high uniformity in overall efficiency.

Two Octaves Supercontinuum Generation in Lead-Bismuth Glass Based Photonic Crystal Fiber.

In this paper we report a two octave spanning supercontinuum generation in a bandwidth of 700-3000 nm in a single-mode photonic crystal fiber made of lead-bismuth-gallate glass. To our knowledge this is the broadest supercontinuum reported in heavy metal oxide glass based fibers. The fiber was fabricated using an in-house synthesized glass with optimized nonlinear, rheological and transmission properties in the range of 500-4800 nm. The photonic cladding consists of 8 rings of air holes. The fiber has a zero dispersion wavelength (ZDW) at 1460 nm. Its dispersion is determined mainly by the first ring of holes in the cladding with a relative hole size of 0.73. Relative hole size of the remaining seven rings is 0.54, which allows single mode performance of the fiber in the infrared range and reduces attenuation of the fundamental mode. The fiber is pumped into anomalous dispersion with 150 fs pulses at 1540 nm. Observed spectrum of 700-3000 nm was generated in 2 cm of fiber with pulse energy below 4 nJ. A flatness of 5 dB was observed in 950-2500 nm range.

Two dimensional interferometric optical trapping of multiple particles and Escherichia coli bacterial cells using a lensed multicore fiber.

Two dimensional interferometric trapping of multiple microspheres and Escherichia coli has been demonstrated using a multicore fiber lensed with an electric arc fusion splicer. Light was coupled evenly into all four cores using a diffractive optical element. The visibility of the fringes and also the appearance of the lattice can be altered by rotating a half wave-plate. As a result the particles can be manipulated from one dimensional trapping to two dimensional trapping or a variety of different two dimensional arrangements. The ability to align bacterial populations has potential application for quorum sensing, floc and biofilm and, metabolic co-operation studies.

Large diameter nanostructured gradient index lens.

In this paper we report on the development and optical properties of nanostructured gradient index microlenses with good chromatic behavior. We introduce a new fabrication concept for the development of large diameter nanostructured gradient index microlenses based on quantized gradient index profiles and the use of nanostructured meta-rods. We show a dependence of the quality of performance on the number of refractive index levels and the lens diameter. Measurements carried out at 633 and 850 nm show good optical properties and similar focal lengths for both wavelengths.

Analysis of crossed gratings with large periods and small feature sizes by stitching of the electromagnetic field.

We present a new algorithm that enables the analysis of large two-dimensional optical gratings with very small feature sizes using the Fourier modal method (FMM). With the conventional algorithm such structures cannot be solved because of limitations in computer memory and calculation time. By dividing the grating into several smaller subgratings and solving them sequentially, both memory requirement and calculation time can be reduced dramatically. We have calculated a grating with 32 x 32 pixels for a different number of subgratings. We show that the increased performance is directly related to the size of the subgratings. The field-stitched calculations prove to be very accurate and agree well with the predictions from the standard FMM approach.

Design of all-glass multilayer phase gratings for cylindrical microlenses.

We introduce a design method for diffractive cylindrical microlenses fabricated with a new technology similar to the fabrication of all-solid photonic crystal fibers. Unlike conventional microlenses that are fabricated with etching methods and thus have a step-index profile, the refractive index of each layer can be individually designed. We study the transmitted field of such nonperiodic lamellar phase grating. By using the field-stitching method we can suppress the effect of periodic boundary conditions of the Fourier modal method when calculating the transmitted field of nonperiodic lamellar phase elements. We suggest an algorithm to design multilayer phase elements, which act as cylindrical lenses. We show experimental and theoretical data for a diffraction-limited lens.

Analysis of multimask fabrication errors for diffractive optical elements.

As design algorithms for diffractive optical elements improve, the limiting factor becomes the fabrication process. It is hoped a better understanding of fabrication errors will allow elements with greater tolerance to be designed. This is important for high-power laser fiber coupling, where hot spots lead to failure. We model seven different fan-out gratings applying misetch, misalignment, and feature rounding. Our main findings are that misetch can lead to improved results, misalignment is strongly asymmetric, and both the pi and pi/2 masks can dominate misalignment. Rounding has a r(2) dependence and potentially can be incorporated into the design stage. Finally we present some experimental data for misalignment.

Diffractive optical elements for high gain lasers with arbitrary output beam profiles.

We introduce a previously unreported laser cavity configuration, using a diffractive optical element (DOE) in place of the output coupler. Such a configuration allows the DOE to work both in reflection, as a mode shaping element, and in transmission as a beam shaper. Employing dual wavelength DOE optimization techniques and phase delays greater than 2pi, allows the two functions to be designed independently. Thus, an arbitrary output beam profile can be combined with a mode shape which maximizes energy extraction from the gain medium. Devices are designed and their performance modeled for a 1m cavity with 5mm diameter mirrors and a wavelength of 632.8nm. An element with 32 quantization levels and a maximum phase delay of 8pi in transmission produces high quality results.

Programmable optoelectronic neural network for optimization.

An optoelectronic neural network is presented that is designed to solve the assignment problem--or any similar optimization task given minimal adjustment--in both crossbar and banyan packet switches. We examine the design decisions made at the hardware, software, and algorithmic levels and indicate the associated effect on the system as a whole. Clearly detailed experimental results show the system's robustness and performance due to the particular optoelectronic-algorithm combination used. The integration and packaging of such a system are also briefly discussed.