Suning Tang

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.

Design limitations of highly parallel free-space optical interconnects based on arrays of vertical cavity surface-emitting laser diodes, microlenses, and photodetectors

We utilize a novel diffraction formalism to study the crosstalk effect in a highly parallel free-space optical interconnect based on two-dimensional arrays of surface-emitting laser diodes, microlenses, and photodetectors. The diffraction induced crosstalk between adjacent laser diodes in each detector to the system limitations is investigated. Optimum design rules and formulas are given for the first time, to include the relation of channel packaging density and interconnect length to the design parameters of the optical interconnect components. The design formulas developed here yield an optimum detector size and indicate a tradeoff between channel packaging density and interconnect length. The feasibility of such a free-space interconnect with a channel packaging density of 3460 channels/cm/sup 2/ and 2.0 cm interconnection length is determined using typical parameters of detector radius from /spl sim/5 to /spl sim/45 /spl mu/m, lens radius of 85 /spl mu/m, and laser diode radius of /spl sim/5 pm operating at wavelength 0.67 pm for signal-to-noise ratio above 17 dB. Some experiments were conducted to measure the diffraction induced crosstalk and optical link efficiency. >

Polymeric waveguide prism-based electro-optic beam deflector

Beam steering devices without moving parts are highly desir- able for their potential application in emerging optical technologies such as holographic optical storage systems, all optical networks, and optical switches. We demonstrate a thin-film waveguide beam deflector device that consists of an electro-optic prism array within a polymer waveguide. An electrode structure defines the prism array within the planar wave- guide. The deflection efficiency of 28 mrad/kV and the maximum deflec- tion angle of 68.4 mrad at 6300 V are obtained for this demonstration device. Further optimization of electrode-field poling and processing is likely to improve these results by at least an order of magnitude.

Polymeric electro-optic modulator based on 1×2 Y-fed directional coupler

We have demonstrated a polymeric electro-optic modulator based on a 1×2 Y-fed directional waveguide coupler. The symmetric geometry of the 1×2 Y-fed directional coupler provided the modulator unique characteristics of intrinsic 3 dB operating point and two complementary output ends. A low switching voltage of 3.6 V and a high extinction ratio of 26 dB were obtained with the modulator operating at a wavelength of 1.34 μm. The modulator was fabricated with a novel electro-optic polymer that was synthesized from polyurethane cross-linking with a chromophore.

1‐to‐12 surface normal three‐dimensional optical interconnects

We present a surface normal optical interconnect with a 1‐to‐12 collinear fan‐out. Two types of polymer‐based holograms were fabricated to provide a collinear 1‐to‐12 fan‐out from guided mode to substrate modes and twelve 1‐to‐1 surface normal interconnects. Fluctuation of up to 7.2 dB for the 1‐to‐12 fan‐out hologram was observed due to the oscillating and the film saturation effects of the transmission hologram. Diffraction efficiency better than 70% was observed for all the total internal reflection holograms. The result reported herein greatly enhanced optical signal processing capability of guided wave optical interconnects. The advantages of free space optical interconnect, such as global interconnect, three dimension, massive fan‐out/fan‐in capabilities, and surface normal optical interconnect, can be realized using the demonstrated architecture. The coupling from waveguide to fiber can be realized from the surface rather than the edge of a photonic integrated circuit.

A novel wavelength-division-demultiplexer with optical in-plane to surface-normal conversion

We present a novel surface normal optical wavelength-division-demultiplexer (WDDM), working at 750, 770, 790, 810, 830, and 850 nm wavelengths. The device is based on the integration of a polymer-based planar waveguide, a substrate waveguiding plate, and waveguide holograms. The unique optical in-plane to surface-normal conversion converts the difficult three spatial and three angular edge coupling problem into a planar surface one, resulting in a reliable miniaturized optoelectronic packaging containing a photodetector array and the WDDM. A six-channel wavelength-division-demultiplexer with equally spaced collinear surface-normal outputs are designed and demonstrated in a polymer-based planar waveguide in conjunction with holograms on a glass substrate.<<ETX>>

1‐to‐42 optoelectronic interconnection for intra‐multichip‐module clock signal distribution

In this paper, we present a miniaturized compact three‐dimensional optical fan‐out interconnect suitable for wafer scale very large scale integrated multichip‐module optical clock signal distribution. The demonstrated device employs a thin light‐guiding substrate in conjunction with a two‐dimensional (2D) optical hologram array. The parallel feature among fan‐out beams and the planar compact structure convert the unsolvable three spatial and three angular multiple alignment problem into a single‐step 2D planar one, which greatly enhances the packaging reliability. A new design scheme for reducing throughput power nonuniformity is presented for the first time. A 25 GHz 1‐to‐42 highly parallel fan‐out interconnect was demonstrated with a signal to noise ratio of 10 dB.

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.

45-cm long compression-molded polymer-based optical bus

We report the formation of an optical bus using compression‐molding technique. The linear dimension of such a waveguide is well beyond that of a microlithographically defined waveguide. Theoretical calculation based on the effective index method was used to determine the optimal dimension of the molding tool design for single‐ and multimode waveguides. A molded photolime gel‐based polymer optical bus with a linear dimension of 45 cm was fabricated and then tested at 0.6328‐μm wavelength. Waveguide propagation loss from 0.5 to 2 dB/cm was determined using the two prism method. As a result of this long interconnection distance, board‐to‐board optical interconnects through backplane can be realized using the technology.

Packing density and interconnection length of a highly parallel optical interconnect using polymer-based, single-mode bus arrays

Interconnection length and packing density are important parameters in channel-waveguide-based photonic integrated circuits. A multiple-channel crosstalk model is developed and then applied, together with experimental results, to study the minimum bus separation and maximum bus length in a highly parallel single-mode bus array (SMBA) to be used for inter-MCM (multi-chip-module) optoelectronic interconnects. Further investigation of the trade-off between packing density and interconnection length resulting from channel cross coupling provides a rule of thumb for an optimized bus array design. For a given waveguide and cladding index, it is found that waveguide dimensions should be close to the cutoff dimensions of the second-mode to obtain an optimal wave confinement factor. The maximum packing density of an SMBA using coherent light sources is shown to be different from that using incoherent light sources. An example using as criteria an extinction ratio of 20, 1-dB power penalty, and a bit error rate (BER) of 10 -15 is presented. It is shown that the packing density of a polymer-based SMBA ranges from 300 to 750 channels/cm for coherent light sources and from 400 to 950 channels/cm for incoherent light sources with a bus length of 2.0 cm, a core index n 1 = 1.5, and index difference from 0.015 to 0.0015. Further experimental work is conducted to confirm the accuracy of the presented theory.