Osman Kibar

Power minimization and technology comparisons for digital free-space optoelectronic interconnections

This paper investigates the design optimization of digital free-space optoelectronic interconnections with a specific goal of minimizing the power dissipation of the overall link, and maximizing the interconnect density. To this end, we discuss a method of minimizing the total power dissipation of an interconnect link at a given bit rate. We examine the impact on the link performance of two competing transmitter technologies, vertical cavity surface emitting lasers (VCSELs) and multiple quantum-well (MQW) modulators and their associated driver-receiver circuits including complementary metal-oxide-semiconductor (CMOS) and bipolar transmitter driver circuits, and p-n junction photodetectors with multistage transimpedance receiver circuits. We use the operating bit-rate and on-chip power dissipation as the main performance measures. Presently, at high bit rates (>800 Mb/s), optimized links based on VCSELs and MQW modulators are comparable in terms of power dissipation. At low bit rates, the VCSEL threshold power dominates. In systems with high bit rates and/or high fan-out, a high slope efficiency is more important for a VCSEL than a low threshold current. The transmitter driver circuit is an important component in a link design, and it dissipates about the same amount of power as that of the transmitter itself. Scaling the CMOS technology from 0.5 /spl mu/m down to 0.1 /spl mu/m brings a 50% improvement in the maximum operating bit rate, which is around 4 Gb/s with 0.1 /spl mu/m CMOS driver and receiver circuits. Transmitter driver circuits implemented with bipolar technology support a much higher operating bandwidth than CMOS technology; they dissipate, however, about twice the electrical power. An aggregate bandwidth in excess of 1 Tb/s-cm/sup 2/ can be achieved in an optimized free-space optical interconnect system using either VCSELs or MQW modulators as its transmitters.

Characterization of a polymer microlens fabricated by use of the hydrophobic effect.

We report a means of fabricating hydrophilic domains in a hydrophobic background by lithographically patterning an adhesive hydrophobic layer. Polymer microlenses were fabricated on these substrates by use of a dip-coating technique. Various lens shapes (circular, elliptical, square) were fabricated on a variety of substrates (SiO(2), SiN, GaAs, InP, etc.), ranging in size from 2 to 500 microm in diameter, with fill factors of up to 90%. Plano-convex and double-convex lenses were fabricated, with f-numbers as low as 1.38 and 1.2, respectively. Optimum lens surfaces deviated from spherical by just +/-5 nm . The lenses are stable at room temperature and exhibit minimal degradation after 24 h at 105 degrees C. The transfer of these polymer lenses to an underlying substrate was also demonstrated.

Optimization and theoretical modeling of polymer microlens arrays fabricated with the hydrophobic effect

High-performance polymer microlens arrays were fabricated by means of withdrawing substrates of patterned wettability from a monomer solution. The f-number (f(#)) of formed microlenses was controlled by adjustment of monomer viscosity and surface tension, substrate dipping angle and withdrawal speed, the array fill factor, and the number of dip coats used. An optimum withdrawal speed was identified at which f(#) was minimized and array uniformity was maximized. At this optimum, arrays of f/3.48 microlenses were fabricated with one dip coat with uniformity of better than Deltaf/f +/- 3.8%. Multiple dip coats allowed for production of f/1.38 lens arrays and uniformity of better than Deltaf/f +/-5.9%. Average f(#)s were reproducible to within 3.5%. A model was developed to describe the fluid-transfer process by which monomer solution assembles on the hydrophilic domains. The model agrees well with experimental trends.

Optomechanical design and characterization of a printed-circuit-board-based free-space optical interconnect package

We present a proof of concept and a feasibility demonstration of a practical packaging approach in which free-space optical interconnects (FSOIs) can be integrated simply on electronic multichip modules (MCMs) for intra-MCM-board interconnects. Our system-level packaging architecture is based on a modified folded 4f imaging system that has been implemented with only off-the-shelf optics, conventional electronic packaging, and passive-assembly techniques to yield a potentially low-cost and manufacturable packaging solution. The prototypical system as built supports 48 independent FSOI channels with 8 separate laser and detector chips, for which each chip consists of a one-dimensional array of 12 devices. All the chips are assembled on a single substrate that consists of a printed circuit board or a ceramic MCM. Optical link channel efficiencies of greater than 90% and interchannel cross talk of less than -20 dB at low frequency have been measured. The system is compact at only 10 in.3 (25.4 cm3) and is scalable, as it can easily accommodate additional chips as well as two-dimensional optoelectronic device arrays for increased interconnection density.

Integration of optoelectronic array devices for cell transport and sorting

Current biochip technologies typically rely on electrostatic or mechanical forces for the transport and sorting of biological samples such as single cells. In this paper we have investigated how optical pressure forces can be effectively used for the manipulation of cells and switching in a microfluidic system. By projecting the optical beams externally non-contact between the control devices and the sample chip is possible thus allowing the sample chips to be disposable which reduces the chance of cross-contamination. In one implementation we have shown that vertical cavity surface emitting laser (VCSEL) array devices used as parallel optical tweezer arrays can increase the parallelism of sample manipulation on a chip. We have demonstrated the use of a high-order Laguerre-Gaussian mode VCSEL for optical tweezing of polystyrene microspheres and live cells. We have also shown that optical pressure forces from higher- power sources can be used for the switching of particles within microfluidic channels. Both the attractive gradient force and the scattering force of a focused optical beam have been used to direct small particles flowing through junctions molded in PDMS. We believe that by integrating optical array devices for simultaneous detection and manipulation, highly parallel and low-cost analysis and sorting devices may be achieved.

Cross talk and ghost talk in a microbeam free-space optical interconnect system with vertical-cavity surface-emitting lasers, microlenses, and metal–semiconductor–metal detectors

A diffraction-based beam-propagation model is used to study optical cross talk in microbeam free-space optical interconnection (FSOI) systems. The system consists of VCSELs, microlenses, and metal-semiconductor-metal (MSM) detectors, with the detectors modeled as amplitude gratings with low contrast ratio (based on experimental results). Different possible cross-talk sources are studied. Results show that, in an optimized system, the cross talk caused by diffractive scattering is not an issue. However, in such systems the principal reflection from a MSM detector surface creates two problems: VCSEL coupling and ghost talk. The coupling of the reflected beam into the VCSELs may cause power oscillation (and increase the bit error rate), whereas ghost talk will limit the distance-bandwidth product of the interconnect system. This optical system is also abstracted in hspice together with the laser driver and receiver circuits to analyze ghost talk in this system. Results show that at high speed (1 Gbit/s or more) these effects negatively affect system performance.

High-speed CMOS switch designs for free-space optoelectronic MIN's

We present the theory, experimental results, and analytical modeling of high-speed complementary metal-oxide-semiconductor (CMOS) switches, with a two-dimensional (2-D) layout, suitable for the implementation of packet-switched free-space optoelectronic multistage interconnection networks (MINs). These switches are fully connected, bidirectional, and scaleable. The design is based on the implementation of a half-switch, which is a two-to-one multiplexer, using a 2-D layout. It introduces a novel self-routing concept, with contention detection and packet drop-and-resend capabilities. It uses three-valued logic, with 2.5 V being the third value for a 5 V power supply. Simulations show that for a 0.8-/spl mu/m CMOS technology the switches can operate at speeds up to 250 Mb/s. Scaled-down versions of the switches have been successfully implemented in 2.0 /spl mu/m CMOS. The analytical modeling of these switches show that large scale free-space optoelectronic MINs using this concept could offer close to Terabit/sec throughput capabilities for very reasonable power and area figures. For example, a 4096 channel system could offer 256 Gb/s aggregate throughput for a total silicon area of about 18 cm/sup 2/ and a total power consumption (optics plus electronics) of about 90 W.

Small-signal-equivalent circuits for a semiconductor laser.

Passive electrical circuits whose voltage and current equations are exactly equivalent to the small-signal rate equations of a semiconductor laser are derived to model an electrically modulated laser (verified to be the same as that given in the literature), an optically modulated laser (i.e., a laser used as an optical amplifier), and a multimode laser. These circuits offer a fast and efficient simulation tool with little computational complexity in which the small-signal assumption (i.e., small modulation range) is neither violated nor insufficient for the simulation.

Heterogeneous integration through electrokinetic migration

We apply basic electrophoretic motion to semiconductor materials engineering for development of the next level of heterogeneous integration technology. Furthermore, we demonstrate the utility of these tools in integration of inorganic devices with biological species in order to explore the utility of these tools in biotechnological applications.Electrical and optical addressing techniques are shown to allow for more rapid and parallel patterning of biological species and inorganic objects.

A molecular propeller effect for chiral separation and analysis.

Enantiomers share nearly identical physical properties but have different chiral geometries, making their identification and separation difficult. Here we show that when exposed to a rotating electric field, the left- and right-handed chiral molecules rotate with the field and act as microscopic propellers; moreover, owing to their opposite handedness, they propel along the axis of field rotation in opposite directions. We introduce a new molecular parameter called hydrodynamic chirality to characterize the coupling of rotational motion of a chiral molecule into its translational motion and quantify the direction and velocity of such motion. We demonstrate >80% enrichment level of counterpart enantiomers in solution without using chiral selectors or circularly polarized light. We expect our results to have an impact on multiple applications in drug discovery, analytical and chiral chemistry, including determination of absolute configuration, as well as in influencing the understanding of artificial and natural molecular systems where rotational motion of the molecules is involved.