This focus issue is a collection of 17 invited papers showcasing the recent advances and the future challenges of optical interconnect technology. These papers represent multiple key areas that include the enabling devices and components, advances in the system and sub-system technologies as well as networking and its applications.
© 2015 Optical Society of America
In the coming era of the “BIG DATA” and the “Internet of Things,” the huge mesh data networks with an extremely high capacity will undoubtedly be implemented everywhere. Due to the large bandwidth and long reach offered by their optical channels, advanced optical interconnect technologies continue to play an even more important role in scaling up the performance of various network segments that range from long-distance inter data-center connections to short-reach rack-to-rack communication and include on-board and chip-to-chip links.
In this focus issue on optical interconnects, we introduce the invited manuscripts in three main categories: enabling devices and components, advances in the system and sub-system technologies, networking and its applications.
Enabling devices and components
Extremely compact, highly energy-efficient, low-cost and densely integrated optical components are the absolute key enablers to satisfy the intra- and inter-systems ever growing bandwidth requirements of future data centers and high performance computers.
For both the inter-boards and inter-chip connects, the manufacturers have explored the high energy efficient and low-cost VCSELs in the last few years and significantly improved the energy-per-bit consumption of the integrated laser sources. An electrically pumped hybrid cavity AlGaInAs-silicon long wavelength VCSEL is reported using a high contrast grating (HCG) reflector on a silicon-on-insulator (SOI) substrate recently. The VCSEL operates at silicon transparent wavelength ~1.57µm with > 1mW CW power out coupled from the semiconductor DBR, and single-mode operation up to 65°C .
Besides, the development of silicon photonics technology also opens up new opportunities for the high bandwidth, low power and low cost chip-to-chip links. We know that the silicon based optical components should meet the demand on the high-stability, ultra-compact, high energy-efficiency, low cost and high integration. Over the last decade, silicon photonics has experienced phenomenal transformations. Some of the notable advances in silicon-based passive and active optical interconnect components are presented in ref. .
An active resonance wavelength stabilization for silicon micro-ring resonator with an in-resonator defect-state-absorption (DSA) based photo-detector (PD) for optical interconnect is demonstrated recently with good transmission system performance .
Also the effective dimensions of the photonic devices have been further minimized. Researchers have presented the state of the art hybrid integration techniques, which have potential applications to realize the densely-packed optical interconnects, such as the ultra-compact III-V/Si hybrid distributed feedback (DFB) lasers with extremely small footprint . The continuous-wave operation of lambda-scale embedded active-region photonic-crystal (LEAP) lasers at room temperature is fabricated on a Si wafer. The on-Si LEAP lasers exhibit a threshold current of 31µA, which is the lowest reported value for any type of semiconductor laser on Si. This reveals the great potential of LEAP lasers as light sources for on- or off- chip optical interconnects with ultra-low power consumption . To tightly integrate optics with the computing hardware, particularly in the context of CMOS-compatible silicon photonics, optical printed circuit boards using polymer waveguides are considered as a formidable platform. A new low-cost single mode polymer waveguide technology is used as the optical interface between silicon photonics chips and single-mode polymer waveguides .
All of these efforts and advances have continued expanding the horizons of optical interconnects, resulting in highly performing, functional and low cost systems of high capacity and low operating power.
Advances in the system and sub-system technologies
The optical fiber technology, started only about 30 years ago, has now been considered as the only practical option for the undersea and nationwide backbone transmission. It has revolutionized the way people communicate, live and work, as one of the most fundamental and essential components of the Internet. Nowadays though, the ongoing data explosion from the micro-cameras, smart mobile devices, army of wide-spread sensors or other future end users like home robots, will challenge the capacity of the current local and metro area networks. Moreover, inter- and intra- connections between and within the cloud service data centers respectively have to be established with optical networks of high cost- and energy- efficiency. Researchers have recently focused on these short reach optical inter-connection networks searching for the best tradeoffs between the performance and cost.
Unlike the long haul optical transmission systems, the length of transmission fiber is usually far less than 100-km such that the direct detection (DD) could be preferred in favor of the more expensive coherent technology. The electrical modulation formats, such as orthogonal frequency division multiplexing (OFDM), pulse amplitude modulation (PAM), poly-binary, carrier-less amplitude phase modulation (CAP) or other flexible modulation formats, have also being proposed for the short reach networks. The digital signal processing (DSP) technology can be used to mitigate the non-ideality of the electrical and photonic devices, in order to improve the performance with nearly no cost penalty [7,8]. It is noteworthy that the DSP, such as the block wise phase shift direct detection (BPS-DD) and signal-carrier interleaved direct detection (SCI-DD) algorithm, can be used to compensate the signal-to-signal beating noise (SSBN), which are critical issues reducing the performance of the DD optical networks, especially for the optical OFDM (OOFDM) systems . Stokes vector direct detection can achieve the highest electrical spectrum efficiency.
Networking and its applications
Electrical signaling across copper wiring is subject to high attenuation at ever increasing data rates complicating the system with extra repeaters, amplifiers or signal conditioners and requiring excessive amounts of power. Alternative short reach optical networks, integrated optical systems that provide board-to-board and chip-to-chip inter-connections continue to be a hot research topic, which attract both academic and industrial interest. Such optical inter-connection technologies are aimed at relieving the technological bottleneck of their electrical counterparts at much reduced power consumption and are especially crucial at the operating extremes of high performance computer systems.
Recently, ref.  reports that utilizing low-cost, 2.2GHz modulation bandwidth, uncooled and stand alone directly modulated VCSEL (DM-VCSEL) based real time dual band optical OFDM (OOFDM) transmitters, aggregates 16.375Gbps transmissions of OOFDM signals over 200m OM2 multi-mode fiber (MMF).
Ref.  presents a multi-Tb/s optical link using silicon photonics circuits. Based on the new multiplexing strategy, such as wavelength division multiplexing (WDM) and space division multiplexing (SDM), eight micro-ring resonator (MRR) modulators and sixty-one cores are cascaded to realize the 8×59×28-Gb/s transmission capacity on 7.93mm2 and thus this architecture realizes a transceiver area density of 1.6Tb/s/mm2 .
Ref.  presents a converged electrical packed switch (EPS) and optical channel switch (OCS) switching fabric for data center networks based on a distributed optical switching architecture leveraging both WDM & SDM technologies. This architecture more readily scalable to future multi-Petabyte data centers with 1000+ racks, while providing a higher link bandwidth, reducing transceiver count by 50%, and improving cabling efficiency by more than 90%.
Ref.  analyzes the feasibility of building silicon photonic micro-ring based switch fabrics for data center scale optical interconnection networks and evaluates the scalability of a micro-ring based switch fabric for WDM signals.
Large data centers interconnect bottlenecks are dominated by the switch I/O bandwidth and the front panel bandwidth as a result of pluggable modules. Ref.  evaluates and compares the possible solutions based on VCSEL MMF links, parallel SMF or duplex SMF implemented with SiP, and other techniques. As silicon photonics manufacturing matures it enables lower cost-power embedded SiP for the next generation 7.2-Tb/s front panel applications.
Besides the conventional fiber based optical interconnections, some novel optical interconnect techniques such as orbital angular momentum (OAM) and visible light communication (VLC) have also been proposed to improve the link capacity , switching flexibility and dimensions  as well as accessing flexibility and energy efficiency .
This focus issue is framed by 17 invited papers presenting the state-of-the-art research results by internationally recognized academic and industrial groups in the fields of opto-electronic components, system and sub-system transmission techniques and networking. These contributions provide an important insight into the newest breakthroughs and analyze the future challenges in the optical interconnect technologies.
References and links
1. J. Ferrara, W. Yang, L. Zhu, P. Qiao, and C. J. C. Hasnain, “Heterogeneously integrated long-wavelength VCSEL using silicon high contrast grating on an SOI substrate,” Opt. Express 23(3), 2512–2523 (2015). [CrossRef]
2. H. Subbaraman, X. Xu, A. Hosseini, X. Zhang, Y. Zhang, D. Kwong, and R. T. Chen, “Recent advances in silicon-based passive and active optical interconnects,” Opt. Express 23(3), 2487–2511 (2015). [CrossRef]
3. Y. Li and A.W. Poon, “Active resonance wavelength stabilization for silicon microring resonators with an in-resonator defect-state-absorption-based photodetector,” Opt. Express 23(1), 360–372 (2015). [CrossRef]
4. O. Bondarenko, C. Y. Fang, F. Vallini, J. S. T. Smalley, and Y. Fainman, “Extremely compact hybrid III-V/SOI lasers: design and fabrication approaches,” Opt. Express 23(3), 2696–2714 (2015). [CrossRef]
5. K. Takeda, T. Sato, T. Fujii, E. Kuramochi, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Heterogeneously integrated photonic-crystal lasers on silicon for on/off chip optical interconnects,” Opt. Express 23(2), 702–708 (2015). [CrossRef]
6. R. Dangel, J. Hofrichter, F. Horst, D. Jubin, A. L. Porta, N. Meier, I. M. Sogani, J. Weiss and B. J. Offrein, “Polymer waveguides for electro-optical integration in data centers and high-performance computers,” Opt. Express in press (2015).
7. K. Zhong, X. Zhou, T. Gui, L. Tao, Y. Gao, W. Chen, J. Man, L. Zeng, A. P. T. Lau, and C. Lu, “Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s short reach optical transmission systems,” Opt. Express 23(2), 1176–1189 (2015). [CrossRef]
8. X. Xu, E. Zhou, G. N. Liu, T. Zuo, Q. Zhong, L. Zhang, Y. Bao, X. Zhang, J. Li, and Z. Li, “Advanced modulation formats for 400Gbps short reach optical inter-connection,” Opt. Express 23(1), 492–500 (2015). [CrossRef]
9. Q. Hu, D. Che, Y. Wang, and W. Shieh, “Advanced modulation formats for high-performance short-reach optical interconnects,” Opt. Express 23(3), 3245–3259 (2015).
10. M. L. Deng, N. Jiang, X. Duan, R. P. Giddings, X. W. Yi, B. Y. Cao, S. Mansoor, K. Qiu, and J. M. Tang, “Robust and tunable 16.375Gb/s dual-band optical OFDM transmissions over directly modulated VCSEL-based 200m OM2 MMFS,” Opt. Express 23(1), 373–383 (2015). [CrossRef]
11. P. P. Absil, P. Verheyen, P. D. Heyn, M. Pantouvaki, G. Lepage, J. D. Coster, and J. V. Campenhout, “Silicon photonics integrated circuits: a manufacturing platform for high density, low power optical I/O/s,” Opt. Express in press (2015).
12. Z. Zhu, S. Zhong, L. Chen and K. Chen, “Fully programmable and scalable optical switching fabric for petabyte data center,” Opt. Express in press (2015).
13. D. Nikolova, S. Rumley, D. Calhoun, Q. Li, R. Hendry, P. Samadi, and K. Bergman, “Scaling silicon photonic switch fabrics for data center interconnection networks,” Opt. Express 23(2), 1159–1175 (2015). [CrossRef]
14. A. Ghiasi, “Large data centers interconnect bottlenecks,” Opt. Express 23(3), 2085–2090 (2015). [CrossRef]
15. S. Ramachandran, P. Gregg, P. Kristensen and S. E. Golowich, “On the scalability of ring fiber designs for OAM multiplexing,” Opt. Express in press (2015).
16. S. Yu, “Potentials and challenges of using orbital angular momentum communications in optical interconnects,” Opt. Express 23(1), 3075–3087 (2015).
17. D. Tsonev, S. Videv, and H. Haas, “Towards a 100Gb/s visible light wireless access network,” Opt. Express 23(2), 1627–1637 (2015). [CrossRef]