Yuzo Ishii

SMT-compatible large-tolerance "OptoBump" interface for interchip optical interconnections

A new optical interface called OptoBump has been developed to couple optoelectronic packages to an optoelectronic printed circuit board, thus enabling economical chip-to-chip optical interconnections. The optoelectronic packages have vertical-cavity surface-emitting laser (VCSEL) and PD-array chips in their cavity and an large scale integrated (LSI) mounted on top. A package converts high-speed electrical signals from the LSI into an array of optical signals, which are emitted from the bottom. The PCB contains integrated polymer optical waveguides to optically connect packages, and the use of surface-mount technology (SMT) to mount packages on the printed circuit board (PCB) keeps assembly costs low. The key to making the OptoBump interface fully compatible with SMT is the integration of microlens arrays directly into both packages and the PCB. A wide, collimated optical beam couples a package to the board across a narrow air gap and provides a large tolerance to misalignment during the SMT process. This paper explains the concept of the OptoBump interface, the optical coupling design by ray-trace simulation, and the fabrication of polymer microlenses and polymer waveguides. Experimental results revealed that the OptoBump interface provides a large tolerance of /spl plusmn/50 /spl mu/m, which is large enough for use with SMT. The OptoBump interface can replace high-speed electrical wiring at the chip level and also offers the benefit of not having any optical fibers or connectors on the board. Thus, it has the potential to bring about a revolutionary change in optoelectronic packaging.

Ink-jet fabrication of polymer microlens for optical-I/O chip packaging

We report on a simple and versatile method of fabricating polymer microlenses that can be easily integrated with optical devices. UV-curable epoxy resin is dropped onto optical devices by an ink-jet apparatus. When the droplets touch the surface, they form into partial spheres due to their surface tension. UV light irradiation for less than five minutes can easily turn them into solid microlenses. Various microlenses, having a geometrical diameter from 20 to 140 µm with F/1.0 to F/11.0, were successfully produced by controlling the volume and viscosity of the polymer resin and their wettability to the substrate. Their uniformity in a microlens array was measured to be within ±1% in diameter and ±3 µm in pitch. Hybrid integration of an ink-jetted microlens with a wire-bonded vertical-cavity surface-emitting laser (VCSEL) was also demonstrated. When an ink-jetted microlens (45-µm diameter, F/2.0) was formed on the aperture of an 850-nm VCSEL, the coupling efficiency into a single-mode fiber was 4 dB higher than without the microlens.

SMT-compatible optical-I/O chip packaging for chip-level optical interconnects

The rapid increase in Internet data traffic requires large-scale switching systems that have high-bandwidth and high-density board-to-board, board-to-backplane and chip-to-chip interconnections within the system. This paper describes one solution for implementing an economical chip-to-chip optical interconnection. The basic concept is as follows; 1) Silicon ASICs are hybrid integrated with GaAs optoelectronic devices by bump bonding, 2) optoelectronic packages are surface-mounted on a printed circuit board (PCB), 3) optical paths for connecting chips are implemented as an interlayer of the PCB, and 4) a wide and collimated optical beam couples the chip and the board through a narrow air gap. Since it can replace high speed electronic wiring by optical at the chip-level and also there are no optical fibers or connectors on the board, this optical-I/O chip packaging has the potential to bring revolutionary change in optoelectronic packaging.

Hybrid integration of polymer microlens with VCSEL using drop-on-demand technique

Polymer microlens fabrication techniques that enable easy integration with VCSELs are presented. We have developed a high-tolerance coupling structure with microlenses formed on both sides of the optical components for inter-chip optical interconnections, and have developed two types of drop-on- demand techniques for producing microlenses: an ink-jetting method and a dispensing method. Both methods use the surface tension of liquid UV-curable epoxy polymer. We have fabricated various microlenses that have a geometrical diameter from 20 micrometers to over 1 mm with F/1 to F/12 by controlling the volume and viscosity of the droplets and their wettability to the substrate. The measured uniformity in arrayed lenses was within +/- 1 percent in lens diameter and +/- 3 micrometers in pitch. Examples of how we have integrated microlenses with VCSELs are also presented. An ink-jetted microlens ona VCSEL coupled to a single-mode fiber enabled highly efficient coupling: 4 dB greater than without the microlens. A dispensed microlens on a VCSEL coupled to a multimode fiber increased the coupling efficiency by 20 dB compared to without a microlens. In the multimode case, large tolerances of +/- 2 mm in axial misalignment and +/- 10 micrometers in lateral misalignment were obtained for a coupling loss increase of 1 dB.

Polarization-insensitive silicon immersion grating for telecom applications

We describe an immersion grating (IG) with both low polarization-dependent loss (PDL) and high diffraction efficiency. Our immersion grating consists of a silicon (Si) prism and a Si grating coated with dielectric film. We analyze the effect of the refractive index of dielectric film and clarify the refractive index ratio between a Si grating and dielectric film. Selecting an adequate material for the dielectric film, we design the diffraction efficiency of TM polarization without changing that of TE polarization. The simulation results for the optimized IG show a PDL of 0.2 dB and diffraction efficiency of -0.4 dB. A prototype with high dispersion power provides a low PDL of 0.6 dB, while maintaining a high diffraction efficiency of -0.4 dB.

Polarization insensitive silicon immersion grating based on total internal reflection.

We describe an immersion grating (IG) with both low polarization-dependent loss (PDL) and high diffraction efficiency. Our immersion grating consists of a silicon (Si) prism and a Si grating coated with dielectric film. We analyze the effect of the refractive index of dielectric film and clarify the refractive index ratio between a Si grating and dielectric film. Selecting an adequate material for the dielectric film, we design the diffraction efficiency of TM polarization without changing that of TE polarization. The simulation results for the optimized IG show a PDL of 0.2 dB and diffraction efficiency of -0.4 dB. A prototype with high dispersion power provides a low PDL of 0.6 dB, while maintaining a high diffraction efficiency of -0.4 dB.