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.

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.

Massively parallel low-cost pick-and-place of optoelectronic devices by electrochemical fluidic processing.

We describe a novel electrochemical technique for the nonlithographic, fluidic pick-and-place assembly of optoelectronic devices by electrical and optical addressing. An electrochemical cell was developed that consists of indium tin oxide (ITO) and n -type silicon substrates as the two electrode materials and deionized water (R = 18 MOmega) as the electrolytic medium between the two electrodes. 0.8-20-microm-diameter negatively charged polystyrene beads, 50-100-microm-diameter SiO(2) pucks, and 50-microm LED's were successfully integrated upon a patterned silicon substrate by electrical addressing. In addition, 0.8-microm-diameter beads were integrated upon a homogeneous silicon substrate by optical addressing. This method can be applied to massively parallel assembly (>1000 x 1000 arrays) of multiple types of devices (of a wide size range) with very fast (a few seconds) and accurate positioning.

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 VCSEL's, 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 VCSEL's 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.

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 (FSOI's) can be integrated simply on electronic multichip modules (MCM's) 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.

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.