Feiming Li

Si-based surface-relief polygonal gratings for 1-to-many wafer scale optical clock signal distribution

In contrast to volume holographic material where 1-to-many fanouts are realized using multiplexed volume holograms, we report in this paper the first Si-based surface-relief polygonal gratings aiming at optical clock signal distribution application. Surface-relief gratings with 1-/spl mu/m period (0.5 /spl mu/m feature size) were fabricated using reactive ion beam etching (RIE). Both hexagonal and square gratings were demonstrated for 1-to-4 and 1-to-6 fanouts. Surface-normal input and output coupling schemes were carried out with an combined coupling efficiency of 65%. Employment of substrate modes in silicon greatly releases the required grating spacing for the demonstrated two-way surface-normal coupling. 7.5 GHz 1-to-4 clock signal distribution operating at 1.3 /spl mu/m was demonstrated with a signal-to-noise ratio as high as 60 dB. The intensity fluctuation among fanout beams was measured to be within 1 dB. Generalization of 1-to-many fanout can be realized by implementing a polygonal grating with an equivalent number of facets.

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.

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 based on guided-wave optical interconnects in conjunction with waveguide holograms

In this paper, we represent our effort to construct an optoelectronic interconnection layer for high-speed optical clock signal distribution in a Cray supercomputer board. The optoelectronic interconnection layer under investigation employs optical channel waveguides and 3 dB waveguide splitters in conjunction with surface-normal waveguide couplers. The difficulties associated with the complicated 3D multiple alignments are significantly reduced by the surface-normal fanout beams and the unique planar device feature. 1-GHz clear optical clock signal is demonstrated experimentally with a guided-wave interconnection length of 45 cm. Some new techniques to fabricate large-area optical channel waveguides and surface-normal waveguide grating couplers are also presented.

A holographic waveguide microlens array for surface-normal optical interconnects

In this letter, we present an array of holographic focusing lenses for surface normal optical interconnects. The profile-mismatch-induced coupling problem between input lasers and high-speed photodetectors is solved by employing a holographic microlens array. The holographic microlens array demonstrated has l-mm focal length at the wavelength of 632.8 mm, with a lens-to-lens separation of 0.58 mm. The measured 3-dB focal spot size is determined to be 15 /spl mu/m/spl times/14 /spl mu/m. The method for designing and recording a substrate mode surface-normal holographic lens array with different recording and readout wavelengths is also presented.

Unidirectional surface-normal waveguide grating couplers for wafer-scale MCM interconnect

We report the fabrication of tilted gratings on polyimide waveguides. The reported gratings provide an effective unidirectional surface-normal optical couple-in and couple- out for polymer-based optoelectronic interconnects. Such a planarized grating is particularly suitable for wafer-scale MCM optoelectronic interconnects due to its unique non- blocking fanout feature. both surface-normal input and output grating couplers have been demonstrated for the first time on polyimide waveguides on silicon substrate.

Fabrication of three-dimensional tapered polymeric waveguides using the compression-molding technique

We report for the first time, 3D tapered polymeric waveguides fabricated by the compression-molding technique. The waveguides presented herein provide a feasible solution to bridge discrete optoelectronic devices having the apertures of a few microns to hundreds of microns. One-cm long tapered channel waveguides with the cross-sections of 5 micrometers X 5 micrometers at one end and 100 micrometers X 100 micrometers at the other end were fabricated. These waveguides have a propagation loss of 0.5 dB/cm when the 632.8 nm He-Ne laser light is coupled from the small end and of 1.1 dB/cm when coupled from the large end. By confining the energy to the fundamental mode, when coupling from large end to the small end, a low-loss packaging can be achieved bidirectionally.

Efficient waveguide couplers using tilted gratings for optoelectronic interconnects

Surface relief gratings with tilted grating profiles have been studied for efficient optical signal coupling for polymeric waveguide based optoelectronic interconnects. In this article the theoretical formulation used to calculate the diffraction efficiency in a pre-defined direction is discussed. The designed gratings provide an effective unidirectional surface-normal optical coupling for polymer-based optoelectronic interconnects. The gratings with a tilt-angle of 32 degrees and periods ranging from 0.5 to 3 micrometers were fabricated. Both surface-normal input and output grating couplers have been demonstrated in polyimide waveguides on silicon substrate.

Polyimide-based waveguides for guided-wave multi-GBit/sec MCM optoelectronic interconnects

We present the fabrication of polyimide-based H-tree waveguides for a multi-GBit/sec optical clock signal distribution in a Si CMOS process compatible environment. Such a clock distribution system is to replace the existing electronic counterpart associated with high-performance computers. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the l-to-48 waveguide fanout device, l-to-2 splitting loss and bending loss were measured to be 0.25 dB and higher. The planarization requirement of the optical interconnection layer among many electrical interconnection layers makes the employment of tilted grating a choice of desire. Theoretical calculation predicts the 1-to-l free-space to waveguide coupling with an efficiency as high as 95%. Currently, a coupling efficiency of 35% was experimentally confirmed due to the limited index difference between guiding and cladding layers. Further experiments aimed at structuring a larger guiding/cladding layer index differences are under investigation. To effectively couple an optical signal into the waveguide through the tided grating coupler, the accuracy of the wavelength employed is pivotal. This makes the usage of the vertical cavity surface-emitting lasers (VCSELs) and VCSEL arrays the best choice when compared with edge-emitting lasers. Modulation bandwidth as high as 6 GBit/sec was demonstrated at 850 nm. Such a wavelength is compatible with Si-based photodetectors. Temperature dependence of the threshold current up to 155 °C was measured which will determine the power dissipation issue of the optoelectronic packaging. Finally, the first fully monolithic Si-MOSFET integrated receiver was made as the optical clock signal detector. To further enhance the bandwidth of such a detector, a resonant cavity structure with Si/Si02 as the bottom mirror was employed. The measured demodulation bandwidth is over 10 GHz. A fully integrated guided-wave optical clock signal distribution system having planarized grating couplers, H-tree Si- CMOS process compatible waveguides, VCSELs and Si-based photo-receivers will be demonstrated in the near future.

Polymer waveguide-based high-speed clock signal distribution system

The increasing demand for clock speed is rapidly exhausting capabilities of interconnection techniques currently employed for high performance supercomputers. In order to address the bottleneck problem at the board level, we have taken a systems approach in developing optoelectronic interconnection layer for board-level high speed optical clock signal distribution. The reported approach employs polymer optical channel waveguides, waveguide clock signal distribution. The reported approach employs polymer based optical channel waveguides, waveguide splitters, and surface-normal waveguide couplers. This paper describes the system architecture, material choice, and fabrication process of board-level waveguides devices to achieve a synchronous global clock signal distribution.