OSA's Digital Library

Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 13 — Jul. 1, 2013
  • pp: 15395–15400
« Show journal navigation

High-speed free-space based reconfigurable card-to-card optical interconnects with broadcast capability

Ke Wang, Ampalavanapillai Nirmalathas, Christina Lim, Efstratios Skafidas, and Kamal Alameh  »View Author Affiliations


Optics Express, Vol. 21, Issue 13, pp. 15395-15400 (2013)
http://dx.doi.org/10.1364/OE.21.015395


View Full Text Article

Acrobat PDF (1308 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

In this paper, we propose and experimentally demonstrate a free-space based high-speed reconfigurable card-to-card optical interconnect architecture with broadcast capability, which is required for control functionalities and efficient parallel computing applications. Experimental results show that 10 Gb/s data can be broadcast to all receiving channels for up to 30 cm with a worst-case receiver sensitivity better than −12.20 dBm. In addition, arbitrary multicasting with the same architecture is also investigated. 10 Gb/s reconfigurable point-to-point link and multicast channels are simultaneously demonstrated with a measured receiver sensitivity power penalty of ~1.3 dB due to crosstalk.

© 2013 OSA

1. Introduction

High-speed interconnects are now highly demanded in data centers and high-performance computing and optical technologies have been proposed and widely studied to overcome the low-speed and high electromagnetic interference bottlenecks of electrical interconnects [1

1. D. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

3

3. A. F. Benner, M. Ignatowski, J. A. Kash, D. M. Kuchta, and M. B. Ritther, “Exploitation of optical interconnects in future server architectures,” IBM J. Res. Develop. 49(4.5), 755–775 (2005). [CrossRef]

]. For chip-scale and inter-chip interconnects, silicon photonics technology has been investigated and integrated optical interconnect transceivers have recently been demonstrated [4

4. Y. Fainman, M. P. Nezhad, D. T. Tan, K. Ikeda, O. Bondarenko, and A. Grieco, “Silicon nanophotonic devices for chip-scale optical communication applications [Invited],” Appl. Opt. 52(4), 613–624 (2013). [CrossRef] [PubMed]

, 5

5. M. A. Taubenblatt, “Optical interconnects for high performance computing,” J. Lightwave Technol. 30(4), 448–457 (2012). [CrossRef]

]. On the other hand, for card-to-card interconnects, conventionally electrical cables have been utilized. However, electrical cables have several fundamental limitations for high-speed operation, including limited bandwidth, heat dissipation, electric power consumption, high transmission loss and latency, as well as electromagnetic interference [3

3. A. F. Benner, M. Ignatowski, J. A. Kash, D. M. Kuchta, and M. B. Ritther, “Exploitation of optical interconnects in future server architectures,” IBM J. Res. Develop. 49(4.5), 755–775 (2005). [CrossRef]

].

2. Proposed optical interconnect architecture with broadcast capability

The architecture of proposed free-space based reconfigurable card-to-card optical interconnect with broadcast capability is shown in Fig. 1
Fig. 1 Architecture of the proposed reconfigurable card-to-card optical interconnect with both broadcast and multicast capabilities.
, which is similar to that reported in [10

10. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Experimental demonstration of high-speed free-space reconfigurable card-to-card optical interconnects,” Opt. Express 21(3), 2850–2861 (2013). [CrossRef] [PubMed]

]. A dedicated optical interconnect module is proposed to be integrated onto each electronic card (typically a PCB) and inside the module a VCSEL array is used in conjunction with a collimating lens array to generate digitally-modulated collimated Gaussian optical beams. A MEMS-based transmitter mirror array is employed to adaptively steer the optical beams to various destinations, thus providing interconnect reconfigurablity and operation flexibility. After propagating in free-space, at the receiver side another receiver MEMS mirror array is used to appropriately steer the modulated optical signals to focus them onto the corresponding PD elements. To realize broadcasting, an additional large-size MEMS steering mirror (1.5 mm diameter here and the size is larger than the size of micro-lens array) is employed at the receiver side, which guides the signal to the center of the receiver micro-lens array. Due to the Gaussian beam divergence, the beam footprint becomes larger after free-space propagation and all receiver elements are illuminated, hence, the data modulating a VCSEL beam can be broadcast to all channels.

3. Experiments and discussions

To demonstrate the broadcast capability of the proposed reconfigurable optical interconnect architecture, only one VCSEL was turned on and modulated. The modulated VCSEL beam was transmitted towards the large-size broadcast MEMS mirror for broadcasting. The beam reflected off the broadcasting mirror was then steered to the center of the receiving micro-lens array and after focusing, the optical signal for each receiver channel was detected. With VCSEL 1 being used as the broadcasting source (10 Gb/s on-off-keying modulation and 3 mW transmission power), the measured bit-error-rate (BER) performances of all four PD elements with respect to the horizontal distance between the transmitter and receiver modules are shown in Fig. 3(a)
Fig. 3 BER versus horizontal distance. (a) VCSEL 1 served as transmitter; and (b) VCSEL 2 served as transmitter (reprinted from [12]).
. It should be noted that in the experiments there was not lateral displacement between the transmitter and receiver modules, while in practical applications, the lateral displacement is required due to the non-blockage consideration, as shown in Fig. 1. If lateral displacement takes place, the BER performance of proposed system will be worse due to the longer signal propagation distance and the lower power collected by the receiver. It is clear from Fig. 3(a) that receivers 1 and 4 (or 2 and 3) have similar BER performances. This is mainly because both receivers have the same distance to the Gaussian beam center, resulting in almost the same optical signal power being received by both receivers. In addition, receivers 2 and 3 always performed better than receivers 1 and 4, and this can be attributed to the fact that receivers 2 and 3 are closer to the Gaussian beam center. When VCSEL 2 was turned on for broadcasting, the BER performances of the four receivers are shown in Fig. 3(b). Compared with the results shown in Fig. 3(a), it can be seen that the performances of all four channels follow the same trend, although the BER performances in Fig. 3(b) are slightly worse than those displayed in Fig. 3(a). This is mainly due to that VCSELs 1 and 2 have different divergence angles, resulting in different beam footprints after propagating in free-space.

In addition to the broadcast function demonstration where the signal was distributed to all receiver channels, arbitrary multicasting was also experimentally investigated. Here VCSEL 1 served as the multicast signal source (10 Gb/s and 2 mW power) and the modulated signal was multicast, for proof of concept, to PD elements 1 and 2. VCSEL 4 (10 Gb/s and 2 mW power) was also turned on for demonstrating the capability of the proposed architecture to simultaneously realize reconfigurable point-to-point interconnects (PD element 4 was used for signal detection). For the multicast function, MEMS mirrors 1 and 2 at the receiver side were appropriately steered to guide the signal to the corresponding PD elements. The measured BER performances of these PD elements with respect to the horizontal distance between the transmitter and receiver modules are shown in Fig. 5(a)
Fig. 5 BER versus horizontal distance. (a) VCSEL 1 multicast signal to receivers 1 and 2; and (b) VCSEL 1 multicast signal to receivers 2 and 4 (reprinted from [11]).
. It is clear that channel 4 has better BER performance than the multicast channels. This is mainly because that the multicast signal needs to cover two receivers, leading to a smaller received power.

To demonstrate the reconfigurability and flexibility of the proposed architecture, a second scenario was considered, where the modulated optical beam of VCSEL 1 was multicast to receivers 2 and 4 and VCSEL 2 was point-to-point interconnected to receiver 1. The BER performances in this scenario are shown in Fig. 5(b). It can be seen that similar to the results shown in Fig. 5(a), the point-to-point interconnect channel still has better BER performance than the multicast channels.

The receiver sensitivities in the selective multicasting scenario were also measured and the results are shown in Fig. 6
Fig. 6 Receiver sensitivity in the multicast scenario. VCSEL 1 multicast signal to receivers 1 and 2 and bit rate was 10 Gb/s (reprinted from [11]).
. Here, VCSEL 1 served as the multicast signal source towards receivers 1 and 2 and VCSEL 4 was used for the point-to-point interconnects. The horizontal distance between the transmitter and receiver modules was fixed at 20 cm. It is clear that channel 4 exhibits the best sensitivity while the sensitivity for channel 2 is the worst. This is mainly because receiver 4 is further away from the other channels, resulting in smaller inter-channel crosstalk. In addition, compared with the results shown in Fig. 4, the receiver sensitivity is ~1.3 dB. This power penalty can be attributed mainly to inter-channel crosstalk.

Finally, it should be noted that in the experiments, the maximum horizontal distance between the transmitter and receiver modules measured was limited to 30 cm. This is mainly due to the use of a 250 µm pitch micro-lens for signal collection at the receiver side. For a longer free-space signal propagation distance, the signal power that can be collected drops significantly because of the Gaussian beam divergence. Although this free-space distance may be not enough for some practical applications, as discussed in [10

10. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Experimental demonstration of high-speed free-space reconfigurable card-to-card optical interconnects,” Opt. Express 21(3), 2850–2861 (2013). [CrossRef] [PubMed]

], by using larger focusing lenses at the receiver side, much longer interconnection range can be realized.

4. Conclusion

In this paper, a novel free-space based reconfigurable card-to-card optical interconnect architecture with broadcast capability has been proposed and experimentally demonstrated. The broadcast function has been achieved by adding one dedicated larger-size MEMS mirror at the receiver side and by using the Gaussian beam divergence to illuminate all receivers. 10 Gb/s data has been successfully broadcast to all channels with a worst-case receiver sensitivity better than −12.20 dBm. In addition, it has been shown that multicasting can be achieved with the same system architecture. 10 Gb/s reconfigurable point-to-point and multicast interconnection to selected receivers have been experimentally realized simultaneously. Compared with broadcast scenarios, the power penalty in receiver sensitivity has been shown to be ~1.3 dB for simultaneous point-to-point and multicast interconnects.

In should be noted that for free-space based reconfigurable card-to-card optical interconnects, dust accumulation, mechanical vibrations, as well as atmospheric turbulence affect the system performance and the robustness. The turbulence has been shown to result in ~1 dB power penalty [14

14. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Performance of high-speed reconfigurable free-space card-to-card optical interconnects under air turbulence,” J. Lightwave Technol. 31(11), 1687–1693 (2013). [CrossRef]

] and other impacts require further study. In addition, the initial installation of optical interconnect modules requires high-accurate alignment. For the architecture proposed in this paper, the micro-lens arrays can be assembled with the VCSEL/PD arrays with a spacer and the major challenge is aligning the MEMS mirrors. The impact of misalignment and the required installation alignment accuracy also require further consideration.

References and links

1.

D. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

2.

H. Cho, P. Kapur, and K. Saraswat, “Power consumption between high-speed electrical and optical interconnects for interchip communication,” J. Lightwave Technol. 22(9), 2021–2033 (2004). [CrossRef]

3.

A. F. Benner, M. Ignatowski, J. A. Kash, D. M. Kuchta, and M. B. Ritther, “Exploitation of optical interconnects in future server architectures,” IBM J. Res. Develop. 49(4.5), 755–775 (2005). [CrossRef]

4.

Y. Fainman, M. P. Nezhad, D. T. Tan, K. Ikeda, O. Bondarenko, and A. Grieco, “Silicon nanophotonic devices for chip-scale optical communication applications [Invited],” Appl. Opt. 52(4), 613–624 (2013). [CrossRef] [PubMed]

5.

M. A. Taubenblatt, “Optical interconnects for high performance computing,” J. Lightwave Technol. 30(4), 448–457 (2012). [CrossRef]

6.

F. E. Doany, B. G. Lee, A. V. Rylyakov, D. M. Kuchta, C. Baks, C. Jahnes, F. Libsch, and C. L. Schow, “Terabit/sec VCSEL-based parallel optical module based on holey CMOS transceiver IC,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference(OFC/NFOEC, Los Angeles, California, 2012), PDP5D.9. [CrossRef]

7.

C. L. Schow, F. E. Doany, C. W. Baks, Y. H. Kwark, D. M. Kuchta, and J. A. Kash, “A single-chip CMOS-based parallel optical transceiver capable of 240-Gb/s bidirectional data rates,” J. Lightwave Technol. 27(7), 915–929 (2009). [CrossRef]

8.

C. J. Henderson, D. G. Leyva, and T. D. Wilkinson, “Free space adaptive optical interconnect at 1.25 Gb/s with beam steering using a ferroelectric liquid-crystal SLM,” J. Lightwave Technol. 24(5), 1989–1997 (2006). [CrossRef]

9.

M. Aljada, K. E. Alameh, Y. T. Lee, and I. S. Chung, “High-speed (2.5 Gbps) reconfigurable inter-chip optical interconnects using opto-VLSI processors,” Opt. Express 14(15), 6823–6836 (2006). [CrossRef] [PubMed]

10.

K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Experimental demonstration of high-speed free-space reconfigurable card-to-card optical interconnects,” Opt. Express 21(3), 2850–2861 (2013). [CrossRef] [PubMed]

11.

K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “High-speed reconfigurable card-to-card optical interconnects with multicasting capability,” in Proceedings of OptoElectronics and Communication Conference (OECC, Kyoto, Japan, 2013), ThT1–3.

12.

K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “High-speed reconfigurable card-to-card optical interconnects with multicasting capability,” in Proceedings of IEEE Optical Interconnects Conference (Santa Fe, New Mexico, 2013), TuP6.

13.

T. Mizuochi, Y. Miyata, K. Kubo, T. Sugihara, K. Onohara, and H. Yoshida, “Progress in soft-decision FEC,” in Proceedings of Optical Fiber Communication Conference and Exposition and National Fiber Optic Engineers Conference (OFC/NFOEC, Los Angeles, California, 2011), pp. 1–3.

14.

K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Performance of high-speed reconfigurable free-space card-to-card optical interconnects under air turbulence,” J. Lightwave Technol. 31(11), 1687–1693 (2013). [CrossRef]

OCIS Codes
(200.4650) Optics in computing : Optical interconnects
(200.2605) Optics in computing : Free-space optical communication

ToC Category:
Optics in Computing

History
Original Manuscript: April 1, 2013
Revised Manuscript: May 21, 2013
Manuscript Accepted: May 27, 2013
Published: June 20, 2013

Citation
Ke Wang, Ampalavanapillai Nirmalathas, Christina Lim, Efstratios Skafidas, and Kamal Alameh, "High-speed free-space based reconfigurable card-to-card optical interconnects with broadcast capability," Opt. Express 21, 15395-15400 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-13-15395


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. D. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE97(7), 1166–1185 (2009). [CrossRef]
  2. H. Cho, P. Kapur, and K. Saraswat, “Power consumption between high-speed electrical and optical interconnects for interchip communication,” J. Lightwave Technol.22(9), 2021–2033 (2004). [CrossRef]
  3. A. F. Benner, M. Ignatowski, J. A. Kash, D. M. Kuchta, and M. B. Ritther, “Exploitation of optical interconnects in future server architectures,” IBM J. Res. Develop.49(4.5), 755–775 (2005). [CrossRef]
  4. Y. Fainman, M. P. Nezhad, D. T. Tan, K. Ikeda, O. Bondarenko, and A. Grieco, “Silicon nanophotonic devices for chip-scale optical communication applications [Invited],” Appl. Opt.52(4), 613–624 (2013). [CrossRef] [PubMed]
  5. M. A. Taubenblatt, “Optical interconnects for high performance computing,” J. Lightwave Technol.30(4), 448–457 (2012). [CrossRef]
  6. F. E. Doany, B. G. Lee, A. V. Rylyakov, D. M. Kuchta, C. Baks, C. Jahnes, F. Libsch, and C. L. Schow, “Terabit/sec VCSEL-based parallel optical module based on holey CMOS transceiver IC,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference(OFC/NFOEC, Los Angeles, California, 2012), PDP5D.9. [CrossRef]
  7. C. L. Schow, F. E. Doany, C. W. Baks, Y. H. Kwark, D. M. Kuchta, and J. A. Kash, “A single-chip CMOS-based parallel optical transceiver capable of 240-Gb/s bidirectional data rates,” J. Lightwave Technol.27(7), 915–929 (2009). [CrossRef]
  8. C. J. Henderson, D. G. Leyva, and T. D. Wilkinson, “Free space adaptive optical interconnect at 1.25 Gb/s with beam steering using a ferroelectric liquid-crystal SLM,” J. Lightwave Technol.24(5), 1989–1997 (2006). [CrossRef]
  9. M. Aljada, K. E. Alameh, Y. T. Lee, and I. S. Chung, “High-speed (2.5 Gbps) reconfigurable inter-chip optical interconnects using opto-VLSI processors,” Opt. Express14(15), 6823–6836 (2006). [CrossRef] [PubMed]
  10. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Experimental demonstration of high-speed free-space reconfigurable card-to-card optical interconnects,” Opt. Express21(3), 2850–2861 (2013). [CrossRef] [PubMed]
  11. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “High-speed reconfigurable card-to-card optical interconnects with multicasting capability,” in Proceedings of OptoElectronics and Communication Conference (OECC, Kyoto, Japan, 2013), ThT1–3.
  12. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “High-speed reconfigurable card-to-card optical interconnects with multicasting capability,” in Proceedings of IEEE Optical Interconnects Conference (Santa Fe, New Mexico, 2013), TuP6.
  13. T. Mizuochi, Y. Miyata, K. Kubo, T. Sugihara, K. Onohara, and H. Yoshida, “Progress in soft-decision FEC,” in Proceedings of Optical Fiber Communication Conference and Exposition and National Fiber Optic Engineers Conference (OFC/NFOEC, Los Angeles, California, 2011), pp. 1–3.
  14. K. Wang, A. Nirmalathas, C. Lim, E. Skafidas, and K. Alameh, “Performance of high-speed reconfigurable free-space card-to-card optical interconnects under air turbulence,” J. Lightwave Technol.31(11), 1687–1693 (2013). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited