Optical proximity communication using reflective ... - OSA Publishing

Optical proximity communication using reflective mirrors Xuezhe Zheng1, John E. Cunningham1, Ivan Shubin1, John Simons1, Mehdi Asghari2, Dazeng Feng2, Hongbin Lei2, Dawei Zheng2, Hong Liang2, Cheng-chih Kung2, Jonathan Luff2, Theresa Sze1, Danny Cohen1, and Ashok V. Krishnamoorthy1 1

Sun Microsystems Inc., 9515 Towne Centre Dr., San Diego, CA 92121 2 Kotura Inc., 2630 Corporate Place, Monterey Park, CA 91754 [email protected]

Abstract: Optical proximity communication (OPxC) with reflecting mirrors is presented. Direct optical links are demonstrated for silicon chips with better than -2.5dB coupling loss, excluding surface losses. OPxC is a true broadband solution with little impairment to the signal integrity for high-speed optical transmission. With wavelength division multiplexing (WDM) enabled OPxC, very high bandwidth density I/O, orders of magnitude higher than the traditional electrical I/O, can be achieved for silicon chips. © 2008 Optical Society of America OCIS codes: (060.4510) Optical communications; (200. 4650) Optical interconnects.

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Received 15 Apr 2008; revised 3 Sep 2008; accepted 5 Sep 2008; published 10 Sep 2008

15 September 2008 / Vol. 16, No. 19 / OPTICS EXPRESS 15052

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Introduction

To keep the performance scaling as predicted by the Moore’s law, modern processors have been designed with multi-core and multi-threaded architectures to avoid hitting power, bandwidth and latency “walls”. Processors with up to 8 cores and 64 threads have been commercially available [1]. An 80-core experimental processor has been reported [2]. And the trend is expected to continue to hundreds of cores on a chip. With increasing numbers of multi-thread cores integrated on a single processor chip, an emerging challenge is to achieve sufficient communication bandwidth amongst the processor cores and from the multi-core chip to external memory with low latency and low power. On-chip bandwidth has benefited from technology scaling, which seems sufficient for present applications. However off-chip communication bandwidth is at a premium in such multi-core architectures. A substantial amount of work has been done to increase the off-chip bandwidth. One solution is to increase the date rate using high-speed serial electrical transceivers. But the improvement is limited due to the slow increase of the number of off-chip channels in high performance packages [3]. As a result, the imbalance between the computational power and the communication bandwidth in today’s high performance computing systems (HPCS) is increasing, preventing systems from achieving their highest potential performance [4]. One of the promising approaches to improve the off-chip bandwidth density is called Proximity communication using capacitive coupling [5]. By having two chips face-to-face to a very close proximity, capacitive coupling can be used to achieve low power, high bandwidth density communication. Communication with bandwidth density of 430Gb/s/mm2, and power efficiency of 3.6pJ/bit has been demonstrated [6]. However, capacitive proximity communication only works within a very short distance, on the order of 10 microns. Optical signaling based on silicon photonics, on the other hand, could potentially solve the off-chip bandwidth issue and also provide low latency communication. Silicon waveguides transport signals faster than on-chip wires. Unlike electrical on-chip wires, the latency in a silicon waveguide is independent of technology scaling. In addition, with WDM, a single silicon waveguide can potentially be equipped with many high data rate (10-20Gbps) wavelength channels achieving much higher bandwidth-density over wires in a state-of-art or even a future technology. The advanced photolithographic processing and mature silicon integrated circuit technology have enabled integration of silicon-based sub-micron photonic structures and electronics on chip. Substantial progress has been made during the past few years, from passive components like low loss waveguides [7,8], efficient couplers [9,10], micro-ring resonators[11,12], and active devices including ultra-compact modulators, switches, and detectors [13-15]. Transceiver products with line rates of 10Gb/s are now commercially available [16]. On-chip wavelength division multiplexing (WDM) has also been demonstrated [17]. Here, we propose optical proximity communication (OPxC) that #95048 - $15.00 USD

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enables silicon photonics to source and sink data signals on a chip and seamlessly transmit data on-chip and between chips without conversion to electronics in order to exploit the bandwidth density and latency advantages of silicon photonics. The concept and approach of optical proximity communication using reflecting mirrors will be discussed in section II. Detailed experimental results of reflecting pit based optical proximity will be presented in Section III. Conclusions are given in section IV. 2.

Chip-to-chip optical proximity communication

The idea of OPxC is to optically couple waveguide signals between two silicon chips placed face-to-face. One way to accomplish this is to use waveguide gratings. Waveguide gratings have been implemented for mode matching and coupling light from/to on-chip waveguides to/from optical fibers [18,19]. With waveguide gratings on both chips facing each other, light from waveguides on one chip can be coupled into waveguides on the other chip when they are aligned accurately. In this paper, however, a different approach using reflecting pits will be discussed and demonstrated. Bulk-micromachined (111)-oriented silicon mirrors at 54.7°, fabricated with anisotropic KOH etching, have been reported previously for coupling between optical fiber and opto-electronic devices [20]. A pair of such reflecting mirrors can be used to interconnect two chips with waveguides optically.

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Fig. 1. Schematic of OPxC with reflecting pit. (a) Silicon chip (SOI) with waveguide terminated by reflecting pit. (b) Two SOI chips are placed face to face, and interconnected by reflecting pit OPxC.

With a pair of parallel reflecting mirrors, signal beams in one plane can be transferred into another parallel plane. OPxC between two chips with reflecting mirrors is based on this simple idea. As shown in Fig. 1(a), a silicon waveguide that carries optical signals on a SOI chip is terminated by an etch-pit created by a two-step silicon micro-machining technique. Deep reactive ion etching is used first to form a narrow rectangular pit to terminate the waveguide to air with a straight end wall. And then anisotropic wet etching is used to form the other tilted surface, typically at normal silicon (111) facet angle of 54.7 degrees when etched through (100) surface. The tilted surface is metal coated to act as a reflecting mirror. With a second chip equipped with the same structure, light from one chip can be coupled to the other while placed face to face, as shown in Fig. 1(b). The optical beam starts to diverge as soon as it leaves the silicon waveguide. To achieve optimum coupling efficiency, the gap between the two chips needs to be as small as possible to minimize the beam propagation distance between two waveguides. The plots in Fig. 2. show the coupling loss change when the gap changes for both a 45 degree reflecting mirror (blue) and a 54.7 degree reflecting mirror (red), assuming a waveguide mode size of 10um in diameter and perfect alignment. As indicated by the plot, there is loss even with perfect alignment and perfect reflecting surfaces because of mode mismatches at the receiving end due to the free space propagation. It is also clear that the 45-degree mirror has a much better gap tolerance than the corresponding 54.7 degree mirror. One of the reasons is because the 45 degree mirror enables the shortest propagation distance between two waveguides. A second

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reason is that the 54.7 degree reflecting mirror causes beam walk-off at the receiving waveguide when the gap changes. A reasonable waveguide mode size is required in practice to use this approach for efficient coupling. For applications with silicon waveguides with very high confinement, i.e. submicron silicon waveguide widths, mode converters are used to enlarge the waveguide mode size and therefore reducing the beam free space propagation divergence angle.

Fig. 2. Reflecting pit OPxC coupling loss versus the chip separation. The pink curve is for 54.7˚ reflecting pit, while the blue is for 45˚ reflecting pit.

3.

Experimental demonstration and discussions

Test chips were fabricated to demonstrate the OPxC, as shown in Fig. 3. The waveguides and etch pits were fabricated on a standard silicon on insulator (SOI) wafer with a silicon thickness of 12 microns and buried oxide (BOX) thickness of 0.4 microns. Single mode ridge waveguide was designed to have ridge width of 8μm and slab height of 6μm with its end tapered to ridge width of 13μm before entering the reflecting pit as shown in Fig. 4 (a). Figure 4(b) is a SEM picture showing the details of the waveguide and the reflecting pit. Experimental results are presented in the following sections.

Fig. 3. Test chip with reflecting pit couplers.

Fiber ribbons were attached to the test chips by polishing the chip edge and direct buttcoupling. The fiber pigtailed chips were then mounted onto 6-axis alignment stages, one facing up and one facing down, as shown in Fig. 4(c). The two chips were aligned by

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injecting light to one chip through the fibers attached, and maximizing the output light from the other chip.

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(c) Fig. 4. Experimental demonstration and characterization of reflecting pit OPxC. (a) Detailed cross-section dimensions of the tapered waveguide design entering the reflecting pit and the tapered mode profile. (b) SEM picture showing the details of the silicon waveguide and reflecting pit. (c) Test set showing two test chips that were fiber attached and mounted on two stages with 6-axis alignment capability, one facing up and one facing down.

The measured fiber-to-fiber total loss with zero gap in between two chips is -5dB. The loss coming from the fiber coupling loss, waveguide propagation loss, waveguide facet reflection loss, and reflecting pit surface absorption is measured by comparing the power injected to the fiber and the optical power reflected by the reflecting pit with a broad area optical power detector. It is measured at -1.35dB for one chip, which leads to mode coupling loss of -2.3dB due to mode divergence and mismatch. It is in good agreement with the theoretical coupling loss of about -2dB for waveguides with MFD of 10μm, and free space propagation distance of 60μm (zero chip gap). Spectral measurements of the coupling loss, as shown in Fig. 5, indicate the reflecting pit OPxC approach is capable of broadband transmission. Fringes are observed in the spectral plot. They are caused by the reflections at the waveguide facet interfaces that form a FP cavity. The inset in Fig. 5 shows the normalized FP fringes. The measured fringe period indicates a cavity length of about 66um in air, which is in broad agreement with an expected value of about 60μm.

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Fig. 5. Coupling loss of reflecting pit OPxC for different wavelengths. The inset shows the normalized FP fringes, which is mainly due to the reflection from the two waveguide facets.

Alignment tolerances of reflecting pit OPxC are characterized experimentally. Results are shown in Fig. 6. Figure 6(a) shows the vertical tolerance (chip gap). As discussed earlier, due to the walk-off, the coupling is quite sensitive to the chip gap variation. The lateral tolerances, x and z direction, are shown in Fig. 6(b). With waveguide mode size similar to single mode fibers, sub-micron alignment accuracy is required for good coupling efficiency. All the experimental results agree well with corresponding simulations.

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Fig. 6. Alignment tolerance of the reflecting pit OPxC. (a) Normalized coupling loss versus the chip separation, experimental results in pink and theoretical data in blue (bold). (b) Normalized coupling loss versus later misalignments. Experimental results are shown as red squares and yellow triangles with corresponding simulation data shown in pink and blue lines respectively.

To verify its performance for high-speed data transmission, a 10Gb/s optical data signal is transported through the reflecting mirror OPxC interface. Almost penalty free transmission is achieved. Figure 7(a) shows the “eye” diagram of the optical signal output from the OPxC, with its inset showing the back-to-back “eye” diagram of the testing signal, while Fig. 7(b)

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shows the receiver sensitivity plots for both transmitter/receiver back-to-back and through single OPxC interface, indicating power penalty of only 0.1dB at receiver. The reflecting mirror tested here has a size of approximately 20μm × 40μm. With further optimization, it this can be reduced to 20μm × 20μm. Therefore, very high bandwidth density can be easily achieved particularly when employing WDM multi-channel signaling.

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Fig. 7. High speed transmission of Reflecting pit OPxC with low power penalty. (a) 10Gbps “Eye” diagram of the output from the OPxC. The inset shows the transmitter/receiver backto-back “eye” diagram. (b) Receiver power sensitivity of OPxC for 10Gbps transmission. Less than 0.1dB power penalty observed.

4.

Conclusions

We have demonstrated OPxC to optically interconnect two or more chips. This reflecting mirror approach is capable of broadband transmission. As a direct benefit of broadband performance, wavelength division multiplexing (WDM) can be implemented to multiplex multiple data channels into a single waveguide, transmit through the OPxC interface and couple into the waveguide on the adjacent chip to achieve even higher bandwidth-density. With OPxC interconnected chips, the off-chip communication bandwidth will potentially match the growing on-chip performance, and therefore improve the overall system performance significantly. Furthermore, applying OPxC to multi-chip applications, seamlessly transporting data on-chip and between chips without conversion to electrical domain can be achieved with bandwidth density and latency advantages, which may enable system architectures with new interconnect hierarchies otherwise impossible with traditional electronic solutions. Acknowledgments This material is based upon work supported, in part, by DARPA under Agreement No. HR0011-08-09-0001.

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