<title>High-speed optical backplane bus with modulation and demodulation capabilities</title>

We review the recent results on the design and implementation of the performance-optimized bidirectional optical backplane bus aimed at high speed multiprocessor systems. It employs an array of multiplexed holograms, in conjunction with a waveguiding plate within which cascaded fanouts are generated. Data transfer rate of 1.2 Gbit/sec at 1300 nm is demonstrated with a single bus line for a system composed of nine boards. Packaging-related issues and misalignment effects are addressed. Theoretical treatment to minimize fluctuations among the received powers at each board was carried out and the overall performance was optimized. We also introduce a hybrid optical backplane with multiple bus lines. The hybrid backplane maintains the same waveguiding structure, and exploits optoelectronic array devices. A new demonstration system is being implemented. The optical backplane that we developed is transparent to higher level bus protocols, thus can support standard backplane buses such as Futurebus+, Multibus II, and VMEbus.

In order to further scale down the size of computers and increase their speed, optical technology is expected to substitute electrical technology for inter-chip motherboard communication. The increasing inter-chip requirements makes it extremely difficult to overcome the delay, power, and bandwidth limitations of the existing electrical wires technology In this research, unguided optical communication bus (UOCB) design for inter-chip communication is analyzed. A UOCB includes transceiver arrays for each chip that transmit information without a waveguide, such that an array of beams propagates through a solid plate. The solid plate is the computer motherboard and is used to hold the chip physically and, in addition, as a propagation medium for the communication beams. The bus space is reused, or multiplexed, such that more than one beam propagates though it simultaneously. The communication through the bus takes advantage of the scattering effect, reflection and diffusion from the walls. We examined the data transmission requirement for uncorrelated optical communication paths between the multiple-input multiple-output (MIMO) optical receiver and transmission arrays over highly diffused propagation bus. Each optical communication path between potential receiver and transmitter locations was described in terms of the optical channel gain, representing the individual optical path. An example of a MIMO gain matrix H between multiple spatial locations was evaluated.

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This article discusses the current state and development trends of microwave frequency synthesizers for test-and-measurement instruments. The discussed trends are also applicable for other synthesizer types used as modules in more complex systems. Test-and-measurement synthesizer market is estimated at $300 with an annual growth rate about 3%.A modern trend for the development of frequency synthesizers is associated with low phase noise, low spurious and fast switching speed. Important parameters also include operating frequency range, frequency resolution, modulation capabilities as well as size and cost characteristics. The main architectures based on direct analog, direct digital and indirect synthesis are evaluated. As of today, traditional indirect phase-lock-loop (PLL) architectures still dominate. On the other hand, direct analog synthesis is the most advanced approach that demonstrates extremely fast switching speed and low phase noise. Nevertheless, future developments are associated with direct digital synthesis (DDS) due to the rapid progress in solid-state technologies. DDS are currently available as special integrated circuits (IC) or can be build using field-programmable logic arrays (FPGA) and high-frequency digital-to-analog converters (DAC). Parallel (multichannel) digital structures coupled with high-frequency DACs allow generating output signals at microwave frequencies. Further improvements are possible using two or more DACs in an interleaved data mode. Wider frequency coverage through several tens of gigahertz and lower spur characteristics are expected.The article notes that modern frequency synthesizers allow generating complex signals using vector IQ-modulation. The modulation bandwidth is constantly increases through hundreds of megahertz or even several gigahertz. Further advances in microwave frequency synthesizers are associated with new reference types such as sapphire-loaded cavity oscillators with combined frequency stabilization as well as optoelectronic methods.

The structure, fabrication, and theory of a three-dimensional planarized optoelectronic clock signal distribution device, based on a thin light-guiding substrate in conjunction with a two-dimensional polymer holographic grating array, are described. We have demonstrated previously a 25-GHz 1-to-42 (6 x 7) highly parallel fan-out interconnect with a signal-to-noise ratio of 10 dB. We present theoretical research that focuses on generating a globally uniform fan-out distribution. An objective function aimed at equalizing the intensities among the fan-out beams is established. For an arbitrary M x N fan-out distribution, there are M + N + 1 sets of holograms needed to be recorded independently to provide the required equal fan-outs. The efficiency of each hologram is determined precisely. The angular misalignment, wavelength dispersion, and spot-size problems are discussed further, together with their tolerance requirements on the size of the photoreceivers integrated on the multichip modules. Employment of 0.25-pitch gradient index (GRIN) lenses as a collimator and as a focusing element is demonstrated experimentally. Optical beam profile manipulation and packaging tolerance are enhanced greatly with GRIN lenses. Finally, clock skew problems associated with the proposed system are discussed, and schemes to minimize the skew are proposed.

A high-throughput optical image crossbar switch (OPIX) that uses a point light source array as the switching device is constructed. A prototype OPIX is used to create various switching patterns for two-dimensional images. The newly constructed OPIX is suitable for high-speed operation because it uses point light sources instead of an optical shutter array as the switching device. Since it is based on an imaging system, the OPIX has a large space-bandwidth product. The introduction of OPIX to optical image data buses of parallel optical processing systems is particularly significant.

As communication networks move toward higher frequency bands, thin glass substrates are advancing toward industrial production in packaging and interconnect applications as high-frequency, low-loss materials. While current techniques for the formation of through glass vias (TGVs) allow for efficient drilling of <40 μm diameter holes, there are currently limited commercially viable options for smaller TGVs. Presented herein is a unique approach to forming high aspect ratio TGVs in 10-20 μs for 50 and 100 μm thick glass. This is accomplished by using a high-power quasi-continuous wave (QCW) laser with a simple Gaussian beam profile focusing scheme. Crucially, this approach is compatible with high bandwidth beam steering technologies, i.e., the combination of galvanometers and acousto-optic deflectors (AODs), allowing for simple scaling to industrially viable throughputs of tens of thousands of vias per second for high-density drill patterns. The TGVs have straight, smooth sidewalls, and high uniformity. Birefringence image microscopy is used to further assess the finer quality aspects of the TGVs formed; considerable residual stress embedded around the TGVs was found after laser drilling, which could cause cracking in subsequent process steps. It is demonstrated that the stress can be significantly reduced by either annealing the glass substrate after drilling or drilling at elevated temperatures to mitigate the embedded stress.

Modern displays are becoming light, thin, and even curved and flexible. The thickness of glass substrates used in these displays has been reduced from its original value of 1.1 to 0.7 mm and eventually to 0.5 mm to create thinner displays. Glass substrates with a thickness of < 0.3 mm are extremely thin and fragile, can be easily deformed, and are therefore not suitable for direct incorporation into the display-manufacturing process. Glass-substrate suppliers bond thin glass substrates onto carrier glass substrates to form laminated glass substrates with increased overall thickness. The panel makers then manufacture the display on a laminated glass substrate. The carrier substrate is removed from the display to produce a thin panel. Glass-substrate makers debond the carrier from the thin glass substrate by applying mechanical force. However, the substrate may break during this debonding process. In this study, a novel method for bonding and debonding the carrier and glass substrates is proposed in which gas is injected between the carrier and thin glass substrate to separate them. This method effectively prevents the formation of edge defects in the thin glass substrate caused by breaks during the debonding process. We designed a debonding machine and established steps to debond the laminated glass substrate after a passive matrix organic light-emitting diode fabrication process. The results showed that this method can be used to efficiently debond the laminated glass substrate.

Electrochemical discharge machining (ECDM) involves the electrolytic formation of a gas film at a tool electrode with high current density discharges and Joule heating for local material heating and removal. The ECDM process is ideally suited for low density glass through-hole machining for applications such as fluidic interconnection. In this paper, we describe a simple and robust ECDM cell arrangement and present optimum conditions for rapid and reproducible through-hole machining in both 500 µm thick and fragile 180 µm thin borosilicate glass substrates. Both anodic and cathodic methods were evaluated and the results offer additional insight into the complex and polarity-dependent mechanisms involved in the ECDM process. The anodic process produces unique spherical cavity microstructures, presenting a new capability for glass microfabrication.

The fabrication of polymeric single-mode Gaussian profile optical waveguides is described. We used poly(methyl methacrylate) and a mixture of two intermiscible monomers for the cladding and the core, respectively, of the waveguides. The cores of the waveguides were fabricated by with an argon-ion laser beam. The waveguides had single-mode Gaussian refractive-index profiles. By using such waveguides, we fabricated directional couplers for power coupling to the adjacent waveguides and also parallel waveguide arrays for preventing power coupling to adjacent waveguides for use in interconnect chips. We analyzed the characteristics of these couplers by using the coupled-mode theory and compared the results with those obtained with the beam propagation method. Experimental results showed good correlation with the theoretical values. We designed optical bus arrays for interconnect chips, considering the variation of normalized frequency V, the power penalty, and the dimensions of the waveguides and the separation between them. The number of waveguides in the bus array increased with increasing V. For a known value of V, a waveguide's density increases with a decrease of its dimensions. The theoretical maximum number of waveguides is ~1490/cm and ~846/cm for 2 mum x 2 mum and 4 mum x 4 mum single-mode waveguides, respectively, to satisfy a 1-dB power penalty criterion at bit-error rate of 10(-9). We fabricated interconnect bus arrays with fifteen 4 mum x 4 mum waveguides for a 3-cm coupling length, and the experimental results were verified to be in good agreement with the theoretical values.

In this paper, we present a miniaturized compact 3-D optical fan-out interconnect suitable for wafer scale VLSI multichip module optical clock signal distribution. The demonstrated device employs a thin light-guiding substrate in conjunction with a 2-D optical hologram array. The parallel feature among fan-out beams and the planar compact structure convert the unsolvable three spatial and three angular multiple alignment problem into a single-step 2-D planar one, which greatly enhances the packaging reliability. A new design scheme for minimizing throughput power non-uniformity is presented for the first time. A 25 GHz 1-to-42 highly parallel fan-out interconnect was demonstrated with a signal to noise ratio of 10 dB.

Electrical-optical integration on board and module level is a rapidly growing field with a strong potential for applications in data and telecom. The driving forces are bandwidth demand, power efficiency and increased channel density. For higher degrees of integration thin glass substrates provide very promising options. The platform technology called “glassPack” relies on the realization of passive single mode and multi mode optical waveguides within the thin glass substrates and benefits of the excellent optical, chemical, and thermal properties of glass. Suitable technologies like ionexchange and direct optical fiber butt coupling are used. The resulting waveguides are characterized by a graded refractive index profile. For electrical wiring thin film technologies can be applied and through glass vias have been demonstrated to address high frequency and high bandwidth applications. Furthermore planar waveguide array coupling elements of very flexible design can be applied for optical coupling and 90 degree light deflection. Applications of this kind of novel substrate technology on board and optical System in Package (SiP) level are high speed telecommunication and data systems, and sensors. First results using thin glass substrates with integrated optical interconnects as well as electrical wiring for SiP will be presented.

The primary technical challenge for optical backplanes involve the alignment and optical isolation of multiple data channels. Since most backplanes require data transfer rates greater than a single optical channel can costeffectively provide, multiple data channels is the common solution for higher aggregate transfer rates. Established optical alignment and isolation techniques include spatial separation of optical channels, use of lensing elements to focus specific transmitter outputs to specific receiver areas, use of differing wavelengths for adjacent channels with appropriate frequency filtering on receivers, and the use of "light guide tubes" for each channel. This presentation will examine another promising option, the use of "matched" Holographic Optical Elements (HOEs) to provide both cross channel optical isolation and to significantly relax traditional optical alignment requirements. Matched HOEs can both induce upon a transmitted optical stream, and then filter upon a received optical stream, a number of distinguishing characteristics such as wavelength, polarization, phase, and amplitude. Thus the use of a unique "matched HOE" pair with each transmitterreceiver pair of multiple optical data channels can provide an efficient mechanism to isolate individual data streams even when they may be physically coincident, such as in a length of fibre optic or when multiple free space data transmitters illuminate several channel's receiver elements. Thus, the alignment issue is relaxed from the usual constraint of attempting to physically separate channels to one where, as long as the receiver is within the optical cone of it's matched transmitting element, cross channel interference can be effectively eliminated.

In this work, we investigate electronic and magnetic phenomena in thin films and heterostructures of transition metal oxides with strong spin–orbit coupling. Ultrathin films are prepared by pulsed laser deposition, a technique which enables layer-by-layer growth of complex materials on atomically flat crystal surfaces. The properties of these heterostructures, which include materials such as strontium iridate (SrIrO3) and strontium ruthenate (SrRuO3), are probed by applying electric and magnetic fields. By varying parameters such as temperature, magnetic field strength and layer thickness, we obtain information about spin and charge transport in these atomically engineered crystals. Chapter 1 provides an introduction to the field of transition metal oxides, followed by a brief overview of the materials studied in this dissertation. Chapters 2 to 5 are dedicated to SrIrO3, a material that displays unexpected physical properties owing to the strong spin–orbit coupling of Ir. Chapter 2 starts with the growth and thermodynamic stability of SrIrO3, which is essential to obtain high-quality films and study their properties in the ultrathin limit. We develop a method to grow stoichiometric films by measuring their transport characteristics as a function of the target condition. We discover that the properties of SrIrO3 are sensitive to degradation in air and develop an encapsulation procedure to protect the filmsurface. SrIrO3 displays an exotic semimetallic state due to the interplay between electronic correlations, spin–orbit coupling, and octahedral rotations. In Chapter 3, we combine thermoelectric and magnetotransport measurements to quantitatively determine the transport coefficients of the different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients. Chapters 4 and 5 focus on the electronic and magnetic properties of SrIrO3 in the two-dimensional limit. In Chapter 4, we discover a metal–insulator transition occurring at a critical thickness of 4 unit cells and an enhancement of spin fluctuations near the transition point. We investigate the magnetic state in Chapter 5, showing that a fourfold symmetric magnetoresistance component appears above a criticalmagnetic field. In Chapter 6, we interface ultrathin SrIrO3 with SrRuO3, an itinerant ferromagnet with an unconventional anomalous Hall conductivity. We discover that the presence of two dissimilar interfaces results in the emergence of two spin-polarized conduction channels. Having explored the influence of epitaxial interfaces, in Chapter 7 we develop a method to detach thin films fromtheir growth substrate using an epitaxial buffer layer. Using this approach, we prepare nanomechanical resonators of freestanding SrTiO3 and SrRuO3 films. By measuring the temperature dependence of theirmechanical response, we observe signatures of structural phase transitions in the SrTiO3, which affect the strain and mechanical dissipation of the resonators. Chapter 8 summarizes the findings of the previous chapters and provide perspectives for future work. We discuss ongoing experiments regarding Berry phase engineering and the manipulation of freestanding films.

The rapid growth of internet and cloud computing applications drives a huge demand for bandwidth capacity in communication networks, while power consumption, cost, and space density must scale down. This growth leads to an increase in the size of data centers (longer optical links), and of the fibers’ channel data rate, rooted in Moore’s Law. Until now, multi-mode fibers (MMF) have been largely employed in datacom applications due to the large coupling tolerance. However, the data-carrying capability of MMF decreases with the transmission distance due to pulse broadening resulting from modal and chromatic dispersion. In order to overcome those limits, transceivers based on single mode fiber (SMF) are under development and the first systems are on the market. Vertical-cavity surface-emitting lasers (VCSELs) are the transmitters of choice for short-reach applications due to their low cost, energy efficiency , and small footprint. InP-based VCSELs emitting at long wavelengths (i.e. 1.3 and 1.55 µm) have gained large interest due to their intrinsic lower power consumption (lower band gap) and low losses in silicon waveguides and silica-based optical fibers, which allows longer transmission distances. While short-wavelength GaAs-based VCSELs have achieved small-signal modulation bandwidths up to 30 GHz [1], InP-based VCSELs show inferior modulation capabilities [2, 3]. Up to date, the highest small-signal bandwidth demonstrated on InP-based devices is 22 GHz [3]. The distributed Bragg reflectors (DBRs) commonly used for GaAs-based VCSELs are made of binary and ternary semiconductor compounds, which offer several advantages such as high refractive-index contrast between the layers, good electrical conductivity and low thermal resistivity. The inferiority of semiconductor DBRs lattice matched to InP challenges the modulation bandwidth enhancement of InP-based devices which suffer of poor thermal conductivity, and high lateral spreading resistance. A further challenge is the single-mode laser operation that has motivated the transition from MMF to SMF in datacom systems. In this chapter, the challenges related to InP-based VCSELs are discussed with focus on active region design, cavity engineering, and current and optical confinement. These arguments apply to all InP-based VCSELs with emission wavelength between 1.3 and 2.0 µm. Stationary and dynamic characteristics are presented for a 1.55 µm VCSEL. Finally, datacom and telecom transmission experiments are presented.