Linghui Wu

Fully embedded board-level guided-wave optoelectronic interconnects

A fully embedded board-level guided-wave optical interconnection is presented to solve the packaging compatibility problem. All elements involved in providing high-speed optical communications within one board are demonstrated. Experimental results on a 12-channel linear array of thin-film polyimide waveguides, vertical-cavity surface-emitting lasers (VCSELs) (42 /spl mu/m), and silicon MSM photodetectors (10 /spl mu/m) suitable for a fully embedded implementation are provided. Two types of waveguide couplers, titled gratings and 45/spl deg/ total internal reflection mirrors, are fabricated within the polyimide waveguides. Thirty-five to near 100% coupling efficiencies are experimentally confirmed. By doing so, all the real estate of the PC board surface are occupied by electronics, and therefore one only observes the performance enhancement due to the employment of optical interconnection but does not worry about the interface problem between electronic and optoelectronic components unlike conventional approaches. A high speed 1-48 optical clock signal distribution network for Cray T-90 super computer is demonstrated. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the 1-48 clock signal distribution and for point-to-point interconnects. The feasibility of using polyimide as the interlayer dielectric material to form hybrid three-dimensional interconnects is also demonstrated. Finally, a waveguide bus architecture is presented, which provides a realistic bidirectional broadcasting transmission of optical signals. Such a structure is equivalent to such IEEE standard bus protocols as VME bus and FutureBus.

Optical clock distribution in supercomputers using polyimide-based waveguides

Guided-wave optics is a promising way to deliver high-speed clock-signal in supercomputer with minimized clock-skew. Si- CMOS compatible polymer-based waveguides for optoelectronic interconnects and packaging have been fabricated and characterized. A 1-to-48 fanout optoelectronic interconnection layer (OIL) structure based on Ultradel 9120/9020 for the high-speed massive clock signal distribution for a Cray T-90 supercomputer board has been constructed. The OIL employs multimode polymeric channel waveguides in conjunction with surface-normal waveguide output coupler and 1-to-2 splitters. Surface-normal couplers can couple the optical clock signals into and out from the H-tree polyimide waveguides surface-normally, which facilitates the integration of photodetectors to convert optical-signal to electrical-signal. A 45-degree surface- normal couplers has been integrated at each output end. The measured output coupling efficiency is nearly 100 percent. The output profile from 45-degree surface-normal coupler were calculated using Fresnel approximation. the theoretical result is in good agreement with experimental result. A total insertion loss of 7.98 dB at 850 nm was measured experimentally.

Two-dimensional 45/spl deg/ surface-normal microcoupler array for guided-wave optical clock distribution

Surface-normal couplers are indispensable parts of a guided-wave optoelectronic interconnects for the coupling of optical signals into and out of the waveguides while facilitating the packaging. In this paper, integration of the 45/spl deg/ surface-normal couplers at each fanout end of the H-tree waveguide structure is described. An optical clock signal distribution system is under development using polyimide based H-tree waveguide structure. The coupler is a 45/spl deg/ slanted end surface of the polyimide waveguide. The coupler works for a wide range of wavelength. The experimentally estimated output coupling efficiency is nearly 100%. To determine the optimized size and shape of the photodetector, near and far field diffraction patterns are evaluated. Experimental results conclude that the phenomenon is dominated by the fundamental mode of the highly multimode waveguide.

Ultra-low-loss polymeric waveguide circuits for optical true-time delays in wideband phased-array antennas

The optical true-time-delay line is a key building block for modern broadband phased-array antennas, which have become one of the most critical technologies for both military and civilian wireless communications. We present our research results in developing an optical polymer-based waveguide true-time-delay module for multilink phased- array antennas by incorporating wavelength-division multiplexing (WDM) technology. The demonstrated optical polymeric waveguide circuits can provide a large number of optical true-time delays with a dynamic range of 50 ns and a time resolution of 0.1 ps. Various fabrication techniques are investigated for producing ultralong low-loss (0.02 dB/cm) polymeric channel waveguides with tilted waveguide grating output couplers. Fast photodiode arrays are fabricated and rf signals with frequencies of 10 to 50 GHz are generated through the optical heterodyne technique. A detailed study of waveguide amplification to achieve loss-less polymeric waveguide is conducted. The optical amplification of 3.8 dB/cm is achieved at a wavelength of 1064 nm in a Nd3+-doped polymeric waveguide. WDM techniques are also employed for potential multilink applications. The presented methodologies enable hybrid integration with a reduced cost in optoelectronic packaging and an increased reliability and decreased payload for the next generation of phased-array antennas.

Compression-molded three-dimensional tapered optical polymeric waveguides for optoelectronic packaging

It is one of the major bottlenecks to bridge various optoelectronic devices, such as laser diodes, optical waveguides and photodetectors in ultra low-loss optoelectronic interconnects, which are often fabricated using different technologies with different optical apertures. To solve this problem, 3D tapered optical polymeric waveguides are presented to provide the mode- matching among these optoelectronic components. Compression- molded polymeric waveguides presented herein is probably the only solution to bridge huge dynamic range of different optoelectronic device-depths varying from few microns to few hundreds microns. Both the design rule and fabrication technique are presented together with some experimental results. It is shown that such a 3D tapered waveguide can provide an effective optical coupling at a relaxed alignment tolerance.

Contact poling of the nonlinear optical film for polymer-based electro-optic modulator

Poling of nonlinear optical (NLO) film is an important step in building an electro-optic polymer-based modulator. In this paper, an electro-optic modulator based on disperse red 1(DR1) side chained poly(methyl methacrylate) (PMMA/DR1) nonlinear optical film is designed and fabricated using the standard very large scale integration fabrication procedures. The poling methods are discussed and the contact poling is used to poling the nonlinear optical film. The poling voltage is determined by the measurement of break- down voltage. The refractive indices change before and after contact poling is given. The electro-optical coefficient of 12pm/V is obtained.

Polymer-based optical waveguide circuits for photonic phased-array antennas

Photonic phased array antennas represent one of the most critical technologies for both national defense and civilian wireless communications. In this paper, we present a novel compact detector-switched polymeric waveguide true-time- delay module, which is a crucial building block for advanced wideband photonic phased array antennas. The photolithographically defined ultra-low-loss polymeric waveguides provide us an attractive solution for achieving ultra long delay time over tens of nsec with ultra fine resolution of less than 1 ps. The 2D distributed waveguide grating couplers tap the optically encoded microwave signal, propagating along the polymeric channel waveguide, to high- speed photodetectors. These photodetectors can be electrically switched on and off independently for selecting different delay times. The appropriate delay time is equal to the time of flight along the waveguide. We have demonstrated that such optical true-time-delays can be implemented in such a device scheme with the RF spectrum of 11 GHz to 40 GHz. The optically encoded microwave signals are obtained by using semiconductor-laser-based optical heterodyne technique. Such a monolithic integrated module not only reduces the cost associated with optoelectronic packaging, but also reduces the system payload with an improved reliability.

Theoretical and experimental study of photonic-crystal-based structures for optical communication applications

Photonic crystal based structures have been considered for optical communication applications. A class of novel symmetric structures consisting of cavities and waveguides have been proposed to serve as optical add-drop multiplexers. Light transfer processes in these structures are analyzed briefly. The problem of deviating from the perfect accidental degeneracy is addressed for practical designs, and the backscattering intensities are shown low for the slight deviations. Anomalous light refraction at a surface of a photonic crystal has also been studied. The limitations of prior theoretical methods for the transmission problem are discussed. An outline of a new analytic theory that overcomes these limitations is presented. Photonic crystals are fabricated on polymer multi-layer films and integrated with conventional channel waveguides.

1-GHz clock signal distribution for multi-processor super computers

In this paper, we present our efforts to construct an optoelectronic interconnection layer for high-speed optical clock signal distribution in a Gray T-90 supercomputer board. The optoelectronic interconnection layer under investigation employs optical channel waveguides and cascaded 3 dB 1-to-2 waveguide splitters in conjunction with surface-normal waveguide grating couplers. The planarization requirement for the optical interconnection layer required by multi-layer integration is fulfilled. Furthermore, the difficulties associated with the complicated 3-D multiple alignments are significantly reduced by the surface-normal fanout beams and the unique planarized device feature. An 1-GHz optical clock signal operating at 1.3 /spl mu/m was transmitted through a 45 cm long polymer-based channel waveguide. Some new techniques to fabricate large-area optical channel waveguides and surface-normal waveguide grating couplers are also presented.

Board-level optical clock signal distribution using Si CMOS-compatible polyimide-based 1- to 48-fanout H-tree

Si-CMOS compatible polymer-based waveguides for optoelectronic interconnects and packaging have been fabricated and characterized. A 1-to-48 fanout optoelectronic interconnection layer (OIL) structure based on Ultradel 9120/9020 for the high-speed massive clock signal distribution for a Cray T-90 supercomputer board has been constructed. The OIL employs multimode polymeric channel waveguides in conjunction with surface-normal waveguide output coupler and 1-to-2 splitter. A total insertion loss of 7.98 dB at 850 nm was measured experimentally.