Electromagnetic analysis for optical coherence tomography based through silicon vias metrology.

This paper reports on progress in the analysis of time-domain optical coherence tomography (OCT) applied to the dimensional metrology of through-silicon vias (TSVs), which are vertical interconnect accesses in silicon, enabling three-dimensional (3D) integration in microelectronics, and estimates the deviations from earlier, simpler models. The considered TSV structures are 1D trenches and circular holes etched into silicon with a large aspect ratio. As a prerequisite for a realistic modeling, we work with spectra obtained from reference interferograms measured at a planar substrate, which fully includes the dispersion of the OCT apparatus. Applying a rigorous modal approach, we estimate the differences to a pure ray tracing technique. Accelerating our computations, we focus on the relevant fundamental modes and apply a Fabry-Perot model as an efficient approximation. Exploiting our results, we construct and present an iterative procedure based on the minimization of a merit function, which concludes TSV heights reliably, accurately, and rapidly from measured interferograms.

How to focus an attosecond pulse.

Attosecond experiments involving focusing of attosecond light pulses can suffer from a spread of the attosecond radiation both in space and time due to optical aberrations. We present a detailed numerical study of the distortions induced in the most common focusing geometries that make use of parabolic, spherical, toroidal and ellipsoidal mirrors. We deduce the consequences on the pulse duration and possible issues that could arise in applications of attosecond pulses. This should serve as a guideline for setting up attosecond focusing optics.

Duration of ultrashort pulses in the presence of spatio-temporal coupling.

We report on a simple method allowing one to decompose the duration of arbitrary ultrashort light pulses, potentially distorted by space-time coupling, into four elementary durations. Such a decomposition shows that, in linear optics, a spatio-temporal pulse can be stretched with respect to its Fourier limit by only three independent phenomena: nonlinear frequency dependence of the spectral phase over the whole spatial extent of the pulse, spectral amplitude inhomogeneities in space, and spectral phase inhomogeneities in space. We illustrate such a decomposition using numerical simulations of complex spatio-temporal femtosecond and attosecond pulses. Finally we show that the contribution of two of these three effects to the pulse duration is measurable without any spectral phase characterization.

Shack-Hartmann multiple spots with diffractive lenses.

In this Letter we aim to bring an understanding to the apparition of multiple spots when using a Shack-Hartmann (SH) wavefront sensor behind diffractive lenses. In contrast to previous work, this phenomenon is described in terms of diffractive orders. It is illustrated with Zemax simulations, where three kinds of diffractive lenses (monofocal, bifocal, and trifocal) are set behind a microlens array. The presence of multiple spots is related to the phase jump of the diffractive profile and also to the number of steps seen through the microlens pupil. The possibility of assessing the optical quality of such lenses using SH measurements is discussed, in particular within the field of ophthalmology, where the need for precautions is underlined.

Stochastic artificial retinas: algorithm, optoelectronic circuits, and implementation.

An analogy can be established between image processing and statistical mechanics. Many early- and intermediate-vision tasks such as restoration, image segmentation, and motion detection can be formulated as optimization problems that consist in finding the ground states of an energy function. This approach yields excellent results, but, once it is implemented in conventional sequential workstations, the computational loads are too extensive for practical purposes, and even fast suboptimal optimization approaches are not sufficient. We elaborate on dedicated massively-parallel integrated circuits, called stochastic artificial retinas, that minimize the energy function at a video rate. We consider several components of these artificial retinas: stochastic algorithms for restoration tasks in the presence of discontinuities, dedicated optoelectronic hardware to implement thermal motion by photodetection of speckles, and hybrid architectures that combine optoelectronic, asynchronous-analog, and clocked-digital circuits.

Dedicated optoelectronic stochastic parallel processor for real-time image processing: motion-detection demonstration and design of a hybrid complementary-metal-oxide semiconductor- self-electro-optic-device-based prototype.

We report experimental results and performance analysis of a dedicated optoelectronic processor that implements stochastic optimization-based image-processing tasks in real time. We first show experimental results using a proof-of-principle-prototype demonstrator based on standard silicon-complementary-metal-oxide-semiconductor (CMOS) technology and liquid-crystal spatial light modulators. We then elaborate on the advantages of using a hybrid CMOS-self-electro-optic-device-based smart-pixel array to monolithically integrate photodetectors and modulators on the same chip, providing compact, high-bandwidth intrachip optoelectronic interconnects. We have modeled the operation of the monolithic processor, clearly showing system-performance improvement.

Blazed-binary diffractive elements with periods much larger than the wavelength

Blazed-binary optical elements with only binary ridges or pillars are diffractive components that mimic standard blazed-echelette diffractive elements. We report on the behavior of one-dimensional blazed-binary optical elements with local periods much larger than the wavelength. For this purpose, an approximate model based on both scalar and electromagnetic theory is proposed. The model is tested against electromagnetic-theory computational results obtained for one-dimensional blazed-binary gratings with large periods. An excellent agreement is obtained, showing that the model is able to predict quantitatively the wavelength and the incidence-angle dependences of the diffraction efficiency of blazed-binary structures.

Interferometric characterization of subwavelength lamellar gratings.

We propose a new, to our knowledge, method for determining the two main critical parameters of periodic one-dimensional lamellar structures, namely, linewidths and etched depths. The method is simple and requires only two measurements for the phase of the zero-transmitted order under two orthogonal polarizations. It is inspired by the analogy between subwavelength gratings and anisotropic homogeneous thin films. The method is tested with experimental data obtained with a Mach-Zehnder interferometer. Etched depths and linewidths derived from the interferograms and electromagnetic theory are compared with scanning-electron-microscope observations.

Diffractive optical elements in hybrid lenses: modeling and design by zone decomposition.

We propose to model hybrid optical systems (i.e., lenses with conventional and diffractive optical elements) as multiaperture systems in which the images formed by each zone of the diffractive optical element should be summed up coherently. This new zone decomposition concept is explained and compared with the standard diffraction-order expansion with the help of a hybrid triplet example.

High-efficiency subwavelength diffractive element patterned in a high-refractive-index material for 633 nm.

We propose the use of high-index materials for the fabrication of subwavelength diffractive components operating in the visible domain. This approach yields a reduction of fabrication constraints and an improvement of theoretical performance. A blazed grating with subwavelength binary features and with a period of 5.75 wavelengths is designed and fabricated in a TiO(2) layer coated upon a glass substrate. The first-order diffraction efficiency measured with a He-Ne laser beam is 83%, which is slightly larger than that achieved theoretically by the best standard (continuous profile) blazed grating fabricated in glass with the same period.

Blazed binary subwavelength gratings with efficiencies larger than those of conventional échelette gratings.

We introduce a new structural cutoff beyond which subwavelength gratings cease to behave as homogeneous media and discuss its effects on the proper selection of the sampling periods of subwavelength diffractive elements. According to this analysis, a 3lambda-period blazed binary grating composed of square pillars is designed for He-Ne operation and is fabricated by etching of a TiO>(2) layer deposited upon a glass substrate. Its first-order measured diffraction efficiency is 12% larger than the theoretical efficiency of an ideal blazed échelette grating in glass with the same period.

Demonstration of optically controlled data routing with the use of multiple-quantum-well bistable and electro-optical devices.

We present experimental results on a 1-to-64-channel free-space photonic switching demonstration system based on GaAs/GaAlAs multiple-quantum-well active device arrays. Two control schemes are demonstrated: data transparent optical self-routing usable in a packet-switching environment and direct optical control with potential signal amplification for circuit switching. The self-routing operation relies on the optical recognition of the binary destination address coded in each packet header. Address decoding is implemented with elementary optical bistable devices and modulator pixels as all-optical latches and electro-optical and gates, respectively. All 60 defect-free channels of the system could be operated one by one, but the simultaneous operation of only three channels could be achieved mainly because of the spatial nonhomogeneities of the devices. Direct-control operation is based on directly setting the bistable device reflectivity with a variable-control beam power. This working mode turned out to be much more tolerant of spatial noises: 37 channels of the system could be operated simultaneously. Further development of the system to a crossbar of N inputs and M outputs and system miniaturization are also considered.

Design and fabrication of binary slanted surface-relief gratings for a planar optical interconnection.

We propose the use of binary slanted surface-relief gratings with parallel-groove walls as input and output couplers in a planar optical interconnect. Parametric optimization of cascaded output couplers is employed to design an interconnect consisting of N output couplers producing a uniform intensity distribution with a high efficiency that may be realized in one lithographic etching step. The sensitivity of a N = 4 interconnect to various fabrication errors is analyzed. We demonstrate the operation of a slanted surface-relief grating manufactured with electron-beam lithography and reactive-ion etching for an operating wavelength of lambda = 0.633 mum.

Synthesis of a subwavelength-pulse-width spatially modulated array illuminator for 0.633 microm.

We present the design, fabrication, and characterization of a subwavelength-pulse-width spatially modulated diffractive array illuminator for an operating wavelength of 0.633 microm. Electromagnetic and scalar diffraction theories are used to reduce manufacturing difficulties while yielding high diffraction efficiency coupled with low reconstruction error. We employ direct electron beam writing and reactive ion etching to realize a transmission-type three-beam array illuminator in photoresist.

Microlenses fabricated by ultraviolet excimer laser irradiation of poly(methyl methacrylate) followed by styrene diffusion.

A new technique of microlens array fabrication based on the use of excimer laser radiation is described. Poly(methyl methacrylate) (PMMA) substrates are treated with many low-energy KrF laser pulses and exposed to styrene vapor. The irradiated material swells, producing spherical microlenses that are stabilized by UV polymerization. The chemistry of this process and the optical quality of the lenses are discussed.

Implementation of arbitrary real-valued correlation filters for the shadow-casting incoherent correlator.

We describe an incoherent correlator, based on the shadow-casting principle, that is able to implement any real-valued linear correlation filter. The correlation filter and the input image are displayed on commercial liquid-crystal television (LCTV) panels. Although it cannot handle high-resolution images, the incoherent correlator is lensless, compact, low cost, and uses a white-light source. A bipolar technique is devised to represent any linear filter, computed from a single reference image or composite, in the correlator. We demonstrate experimentally the efficiency of the design in the case of optimal trade-off (OT) filters and optimal trade-off synthetic discriminant function (OT-SDF) filters.

Monolithic integration of microlens arrays and fiber holder arrays in poly(methyl methacrylate) with fiber self-centering.

For many fiber applications the precise spacing from one fiber to another and the collimation of the outgoing light are important subjects. In the case of three-dimensional arrangements, such as fiber arrays, the complexity increases. The method of proton irradiation of poly(methyl methacrylate) permits the fabrication of slightly fan-shaped fiber holder arrays that can correct inhomogeneities of the fiber diameters in the micrometer range. Even a three-dimensional monolithic integration of fiber holders and corresponding microlenses can be achieved.

Digital optical cellular image processor (DOCIP): experimental implementation.

We demonstrate experimentally the concept of the digital optical cellular image processor architecture by implementing one processing element of a prototype optical computer that includes a 54-gate processor, an instruction decoder, and electronic input-output interfaces. The processor consists of a twodimensional (2-D) array of 54 optical logic gates implemented by use of a liquid-crystal light valve and a 2-D array of 53 subholograms to provide interconnections between gates. The interconnection hologram is fabricated by a computer-controlled optical system.

Recording conditions of an array-illuminator hologram based on the Talbot effect.

We propose a new process for making an array illuminator that combines the Talbot effect and the principle of holography. For high diffraction efficiency we use a thick phase hologram recorded in dichromated gelatin. Special conditions for the intensity uniformity of the recorded wave are required with this type of hologram. We discuss these conditions as a function of the pixel shape.