Matthias Gruber

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.

Optical interconnects for neural and reconfigurable VLSI architectures

The increasing transistor density in very large-scale integrated (VLSI) circuits and the limited pin member in the off-chip communication lead to a situation described as interconnect crisis in micro-electronics. Optoelectronic VLSI (OE-VLSI) circuits using short-distance optical interconnects and optoelectronic devices like microlaser, modulator, and detector arrays for optical off-chip sending and receiving offer a technology to overcome this crisis. However, in order to exploit efficiently the potential of thousands of optical off-chip interconnects, an appropriate VLSI architecture is required. We show for the example of neural and reconfigurable VLSI architectures that fine-grain architectures fulfill these requirements. An OE-VLSI circuit realization based on multiple quantum-well modulators functioning as two-dimensional (2-D) optical input/output (I/O) interface for the chip is presented. Due to the parallel optical interface, and improvement of two to three orders of magnitude in the throughput performance is possible compared to all-electronic solutions. For the optical interconnects, a planar-integrated free-space optical system has been designed leading to an optical multichip module. Such a system has been fabricated and experimentally characterized. Furthermore, we designed an manufactured fiber arrays, which will be the core element for a convenient test station for the 2-D optoelectronic I/O interface of OE-VLSI circuits.

Planar-integrated optical vector-matrix multiplier

We present the design of a planar-integrated optoelectronic vector-matrix multiplier. The inherent parallel-processing potential is fully exploited by optical implementation of multiplications and summations. Planar integration makes the free-space optical system compatible with electronic VLSI technologies. It is composed of phase-only diffractive optical elements, which implement lens and multiple-beam-splitter functions. A demonstrator version of the optical system for a matrix of size 10 x 10 was fabricated on quartz glass by means of multimask lithography and reactive ion etching. It shows low cross talk and good uniformity of the signals.

Planar-integrated free-space optical fan-out module for MT-connected fiber ribbons

This paper reports on the design, the fabrication, and the testing of a compact planar-integrated free-space optical 1 /spl rarr/ 4 fan-out module for fiber ribbons with multifiber-terminated connectors. It supports 12 parallel optical channels and consists of a cascade of basic cells with 1 /spl rarr/ 2 fan-out. The module was implemented with surface-relief diffractive-phase elements; design and fabrication of the optical system were optimized for low loss by various measures such as the use of dielectric and silver reflective coatings. In experimental tests, a coupling efficiency of -11.4 dB per fan-out channel was obtained.

Diffractive optical elements as raster-image generators

The use of diffractive optical elements (DOEs) to generate complex raster images for a primarily artistic purpose is dealt with. Aspects of human vision that are relevant for the design of such elements are discussed. A design method based on an iterative Fourier transform algorithm and extended with elements from the direct-binary-search and the simulated-annealing algorithms is described. The proposed method provides a large set of parameters that can be adjusted freely to optimize it for any given design task. For demonstration a phase-only DOE was designed that generates an image of a Chinese dragon as a diffraction pattern. It was realized as a surface-relief element on a planar substrate through multilevel binary lithography and reactive-ion etching. Experimental tests confirm the usefulness of the design and the fabrication procedures to achieve excellent image quality.

Efficient Planar-Integrated Free-Space Optical Interconnects Fabricated by a Combination of Binary and Analog Lithography

Design, fabrication, and experimental testing of an integrated microoptical module for interconnection are reported. The systems integration is based on the concept of planar-integrated free-space optics. The module combines diffractive-reflective and refractive microoptics. The diffractive elements were fabricated by binary lithography and reactive ion etching. The refractive elements were made by analog lithography using a high-energy beam sensitive mask and replication in Ormocer. The fabricated module implemented a simple one-dimensional optical interconnect. Two versions were implemented for which insertion losses of approximately 8 and 4.5 dB were measured, respectively.

Planar-Integrated Free-Space Optics: From Components to Systems

Planar-integrated free-space optics (PIFSO) is an integration concept for classical free-space optics that was proposed in 1989 [1] and has since evolved into an open integration platform for micro-opto-electro-mechanical systems (MOEMS), sometimes also called optical MEMS. This chapter aims at providing an overview of this development that is characterized by a shift from component-related design and fabrication issues to system- and application-related ones. We will discuss the potential of PIFSO within the fast-growing field of microsystems engineering and compare it with that of alternative integration approaches.

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.

Optical Interconnection and Clocking Using Planar-Integrated Free-Space Optics

Integration and miniaturization at the systems level are key requirements for photonics applications. Here, we describe the concept of planar integration of free-space optical systems and its use as an optical interconnection technology. Two specific applications will be considered, a parallel chip-to-chip interconnect and an optical clock distribution network.

Generalized confocal imaging systems for free-space optical interconnections

A generalized confocal imaging system, which is composed of two confocal lenses and one field lens, is proposed for free-space optical interconnections. Unlike in a conventional 4-f system, both the object distance and the image distance can be almost arbitrarily chosen. This advantage is especially important for practical setups in which the object distance and the image distance cannot be designed to be the same. As a concrete example, we have designed and experimentally tested a planar-integrated micro-optical imaging system. The result is in good agreement with the theoretical prediction. Similarly to the conventional 4-f imaging system and the light-pipe imaging system, the system proposed here can also be used as one important part of a hybrid imaging setup.