Achromatic optical fourier transformer with planar-integrated free-space optics

We address the problem of achromatization of an optical system for the realization of planar-integrated, free-space optics. In particular we demonstrate an integrated optical Fourier transformation module that was achromatized for the visible spectrum by means of a diffractive lens doublet. The optical system design is studied by using the parabolic approximation of the scalar diffraction theory, including terms related to astigmatism. Based on the method of ABCD ray matrices, the optical specifications of the lens doublet are derived and the chromatic correction effect is quantified. For experimental confirmation the diffraction patterns of various grating structures are evaluated.     

Multichip module with planar-integrated free-space optical vector-matrix-type interconnects

Even in the semiconductor industry, free-space optical technology is nowadays seen as a prime option for solving the continually aggravating problem with VLSI chips, namely, that the interconnect technology has failed to keep pace with the increase in communication volume. To make free-space optics compatible with established lithography-based design and fabrication techniques the concept of planar integration was proposed approximately a decade ago. Here its evolution into a photonic microsystems engineering concept is described. For demonstration, a multichip module with planar-integrated freespace optical vector-matrix-type interconnects was designed and built. It contains flip-chip-bonded vertical-cavity surface emitting laser arrays and a hybrid chip with an array of multiple-quantum-well p-i-n diodes on top of a standard complementary metal-oxide semiconductor circuit as key optoelectronic hardware components. The optical system is integrated into a handy fused-silica substrate and fabricated with surface-relief diffractive phase elements. It has been optimized for the given geometrical and technological constraints and provides a good interconnection performance, as was verified in computer simulations on the basis of ray tracing and in practical experiments.     

Low-order adaptive optics for free-space optoelectronic interconnects

The concept of adaptive optics for improving the cost-performance of free-space optoelectronic interconnects is discussed. Adaptive optics as a design option for optical interconnect systems is presented and discussed. A practical demonstrator that performs low-order correction was built and tested. Slowly varying misalignments, including thermal effects, were compensated for in a 622-Mbit/s free-space optical data link.     

Bit-error rate for free-space adaptive optics laser communications

An analysis of adaptive optics compensation for atmospheric-turbulence-induced scintillation is presented with the figure of merit being the laser communications bit-error rate. The formulation covers weak, moderate, and strong turbulence; on-off keying; and amplitude-shift keying, over horizontal propagation paths or on a ground-to-space uplink or downlink. The theory shows that under some circumstances the bit-error rate can be improved by a few orders of magnitude with the addition of adaptive optics to compensate for the scintillation. Low-order compensation (less than 40 Zernike modes) appears to be feasible as well as beneficial for reducing the bit-error rate and increasing the throughput of the communication link.     

Performance trade-offs for conventional lenses for free-space digital optics

We describe the limitations on the use of conventional lenses in optical computing that arise from manufacturing tolerances. The consequences on maximum array size, minimum device size, and propagation delay of systems are discussed. Two experimental optical computing systems are then compared with these results. We show that there are maximum and minimum bounds on the focal length and the ?-number of lenses imposed when manufacturing tolerances are considered. We also show that there are maximum bounds on image sizes and space-bandwidth products and trade-offs between spot size and system latency.     

Integrated three-dimensional optical multilayer using free-space optics

An integrated three-dimensional optical multilayer system for optical data communications is presented. It is based on the use of free-space optical light propagation and combines two integration principles, namely, planar and stacked integration. The combination of both integration schemes aims at a maximal design flexibility for complex geometric layouts. On the other hand, packaging issues that stem from assembly and tolerance have to be considered. Here we describe the basic concept and demonstrate the implementation of an optical interface module in a processor-memory bus.     

Diffractive optics applied to free-space optical interconnects

The optimum design of free-space optical interconnection systems utilizing diffractive optics is determined from a practical engineering standpoint for systems ranging from space invariant to fully space variant. System volume is calculated in terms of parameters such as the f-number of the diffractive lens, the wavelength of light, and also the total number, size, and separation of the optical sources and detectors. Performance issues such as interconnection complexity, diffraction efficiency, and signal-tonoise ratio are discussed. Diffractive optics fabricated by electron-beam direct-write techniques are used to provide experimental results for both shuffle-exchange and twin-butterfly free-space optical interconnects.     

Mixed oxide coatings for optics

Material mixtures offer new possibilities for synthesizing coating materials with tailored optical and mechanical properties. We present experimental results on mixtures of HfO2, ZrO2, and Al2O3, pursuing applications in UV coating technology, while the mixtures are prepared by magnetron sputtering, ion beam sputtering, plasma ion-assisted deposition (PIAD), and electron beam evaporation without assistance. The properties investigated include the refractive index, optical gap, thermal shift, and mechanical stress. The first high reflectors for UV applications have been deposited by PIAD.     

Symmetrical periods in antireflective coatings for plastic optics

Plastic optical parts require antireflective as well as hard coatings. A novel design concept for coating plastics combines both functions. Symmetrical three-layer periods with a phase thickness of 3/2pi are arranged in a multilayer to achieve a step-down refractive-index profile. It is shown mathematically that the equivalent index of symmetrical periods can be lower than the lowest refractive index of a material used in the design, if the phase thickness of the symmetrical period is set equal to 3/2pi instead of the usual pi/2. The straightforward application of the concept to the design of antireflection coatings in general is demonstrated by example.     

Compact planar-integrated optical correlator for spatially incoherent signals

A new, to our knowledge, approach for the planar integration of optical correlators is demonstrated. A VanderLugt-type architecture was used to allow the processing of the spatially incoherent signals of active optoelectronic smart-pixel-device arrays. In a folded optical system all passive components were implemented as a single multiple-phase-level element. The relations among the spatial resolution, the light efficiency, and the system design parameters are derived. High signal quality and low noise levels were achieved experimentally.     

Single-mode fiber optical crossbar routing switch with ferroelectric liquid-crystal-VLSI technology and free-space optics

We investigate a possible extension of the optically connected parallel machine crossbar system to telecommunication routing-switching systems. The critical issue concerns the realization of a free-space interconnection system between two single-mode fiber arrays in the case of a fully interconnected architecture, such as a crossbar system. In particular, we assess the throughput capability of such a system. This evaluation is based on both theoretical analysis and experimental results. We demonstrate that the number of channels is severely constrained by the limited numerical aperture of a single-mode fiber. Finally, we discuss some architectural alternatives and propose some technical recommendations for interconnecting single-mode fiber arrays in free space.     

Fluoride antireflection coatings for deep ultraviolet optics deposited by ion-beam sputtering

Optically high quality coatings of fluoride materials are required in deep ultraviolet (DUV) lithography. We have applied ion-beam sputtering (IBS) to obtain fluoride films with smooth surfaces. The extinction coefficients were of the order of 10(-4) at the wavelength of 193 nm due to the reduction of their absorption loss. The transmittance of the MgF2/GdF3 antireflection coating was as high as 99.7% at the wavelength of 193 nm. The surfaces of the IBS deposited films were so smooth that the surface roughness of the A1F3/GdF3 film was comparable with that of the CaF2 substrate. The MgF2/GdF3 coating fulfilled the temperature and humidity requirements of military specification. Thus, the IBS deposited fluoride films are promising candidate for use in the DUV lithography optics.     

Tribology of metal coatings for electrical contacts

Noble metals are required for sliding low voltage low current electrical contacts such as those in electronic connectors, instrument slip rings and switches. This is to ensure that the contact resistance will be low and will remain stable. Gold, palladium and their alloys are the most commonly used materials and are employed primarily as electrodeposits and clad coatings.The major contact wear processes are adhesion, abrasion and fretting. Adhesive wear can operate in mild or severe regimes. Prow formation is the dominant wear process in the severe regime and is characterized by transfer from the member with the larger surface involved in sliding to that with the smaller surface, after which loose debris is formed. On repeat-pass movement, a transition to rider wear occurs in which the direction of metal transfer reverses. Unique gold electrodeposits have been developed that are relatively resistant to severe adhesive wear. These deposits are brittle, which lowers the tendency of adherent asperities at the mating surfaces to grow to large prows during sliding.Abrasive wear, in contrast, is accentuated when the coatings have low ductility, but as with other wear processes may be controlled if the contact materials are hard.Fretting wear of noble metal coatings leads to high contact resistance when they wear through to their base substrates. Frictional polymerization is the formation on the contact of insulating organic layers, which originate in organic air pollutants in the environment, and occurs during sliding and fretting of platinum group metals. Hard substrates and underplates are desirable because they reduce adhesive, abrasive and fretting wear. Smooth surfaces are superior to rough surfaces when adhesive and abrasive wear occur.     

Wear resistant composite coatings

In the search for wear resistant coatings, nanolaminated composite films composed of alternating metallic and ceramic layers, namely, Al/Al₂O₃ and Ti/TiN were produced using radio frequency magnetron sputtering. The metal layer thickness in the as-sputtered films of Al/Al₂O₃ ranged from 70 to 500 nm, and 150 to 450 nm in Ti/TiN. The non-metals (Al₂O₃ and TiN) layer thicknesses ranged from 10 to 40 nm and total film thicknesses of 10–15 µm. All coatings were characterized and tested for their tribological properties. Friction and wear tests were performed under non-lubricated sliding conditions using a pin-on-disc type tribometer. The coefficient of friction of the composite coatings tested, against a stainless steel pin, varied with the sliding distance. At the early stages of sliding the coefficient of friction rose to a peak, followed by a decrease to a steady-state value. Wear rates and coefficients of friction were related to the hardness and to the structure refinement of the coatings.     

Plasma-assisted deposition of metal and metal oxide coatings

A combination of physical vapour deposition and plasma-assisted chemical vapour deposition techniques were used to deposit Cu-, Ni- and Sn-rich SnO/SnO2 coatings on metal and ceramic substrates. Cu and Ni were deposited on Al alloy 6061 substrates and Ni deposition was also performed on glass microscope slides and commercially pure alumina substrates. Sn-rich SnO/SnO2, on the other hand, was coated on stainless steel and pure Cu substrates. A direct-current plasma system was used to deposit the pure metals in vacuum with a resistively heated tungsten boat that was coated with alumina. All samples were sputtered for 20 min in an argon:hydrogen (1:1) atmosphere at a pressure of 300 mTorr. To reduce contamination and oxidation of both substrates and deposited layers, Cu and Ni coatings were made with argon:hydrogen (2:1) carrier gas. Sn-rich tin oxide coatings were deposited in a pure argon atmosphere (no hydrogen) to allow for the oxidation of Sn deposits on the stainless steel and copper substrates. Investigations of coated surfaces by scanning electron microscopy and X-ray diffraction showed coatings to be smooth, continuous and pure. Deposition rates showed this application to provide a very high rate when compared with chemical vapour deposition and metal–organic chemical vapour deposition techniques. Scratch tests results prove good attachment of the coatings to their respective substrates. © 1998 Chapman & Hall     

Efficient optimization of multilayer coatings for ultrafast optics using analytic gradients of dispersion

A fully analytic method for computing gradients of dispersion (to any order) for a dielectric multilayer coating is developed, and it is demonstrated how group delay gradients can be used to optimize the dispersion of such a filter. The algorithm complexity is linear with the number of layers and quadratic in dispersion order. To our knowledge, this is the first published algorithm for computing exact analytic gradients of dispersion. We show an approximation that speeds up the computation significantly, making it linear in dispersion order. MATLAB and C code implementing the algorithms are made available.     

Soft-x-ray hollow fiber optics with inner metal coating

A glass capillary with an inner metal coating is proposed to be used as soft-x-ray fiber optics in medical applications. Based on the results of theoretical calculations, nickel was chosen as the coating material for x rays radiated from a conventional x-ray tube. A nickel-coated capillary was fabricated by electroless deposition, and focusing and collimating effects were observed from measurements of the transmission efficiency of soft x rays. The transmission of a nickel-coated capillary with an inner diameter of 0.53 mm and a length of 300 mm was 10%, which is approximately double that of an uncoated glass capillary.