Modern data centers increasingly rely on interconnects for delivering critical communications connectivity among numerous servers, memory, and computation resources. Data center interconnects turned to optical communications almost a decade ago, and the recent acceleration in data center requirements is expected to further drive photonic interconnect technologies deeper into the systems architecture. This review paper analyzes optical technologies that will enable next-generation data center optical interconnects. Recent progress addressing the challenges of terabit/s links and networks at the laser, modulator, photodiode, and switch levels is reported and summarized.

Recent advances in optical technologies for data centers: a review

Nanolasers generate coherent light at the nanoscale. In the past decade, they have attracted intense interest, because they are more compact, faster and more power-efficient than conventional lasers. Thanks to these capabilities, nanolasers are now an emergent tool for a variety of practical applications. In this Review, we explain the intrinsic merits of nanolasers and assess recent progress on their applications, particularly for optical interconnects, near-field spectroscopy and sensing, optical probing for biological systems and far-field beam synthesis through near-field eigenmode engineering. We highlight the scientific and engineering challenges that remain for forging nanolasers into powerful tools for nanoscience and nanotechnology.

Applications of nanolasers.

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

We show, using simulations, that a combination of orthogonal frequency division multiplexing (OFDM) and optical single sideband modulation can be used to compensate for chromatic dispersion in ultralong-haul wavelength-division multiplexed (WDM) systems. OFDM provides a high spectral efficiency, does not require a reverse feedback path for compensation, and has a better sensitivity than nonreturn to zero. This paper provides design rules for 800-4000-km optical-OFDM systems. The effects of WDM channel number and spacing, fiber dispersion, and input power per channel on the received Q are studied using extensive numerical simulations. These effects are summarized as a set of design rules

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Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems

High-speed, high-efficiency photodetectors play an important role in optical communication links that are increasingly being used in data centres to handle higher volumes of data traffic and higher bandwidths, as big data and cloud computing continue to grow exponentially. Monolithic integration of optical components with signal-processing electronics on a single silicon chip is of paramount importance in the drive to reduce cost and improve performance. We report the first demonstration of micro- and nanoscale holes enabling light trapping in a silicon photodiode, which exhibits an ultrafast impulse response (full-width at half-maximum) of 30 ps and a high efficiency of more than 50%, for use in data-centre optical communications. The photodiode uses micro- and nanostructured holes to enhance, by an order of magnitude, the absorption efficiency of a thin intrinsic layer of less than 2 µm thickness and is designed for a data rate of 20 gigabits per second or higher at a wavelength of 850 nm. Further optimization can improve the efficiency to more than 70%. Light-trapping nanoscale holes are exploited to improve the speed and efficiency of silicon photodiodes with intrinsic layers less than 2 μm thick.

Photon-trapping microstructures enable high-speed high-efficiency silicon photodiodes

The vast majority of optical links within the data center are based on vertical cavity surface emitting lasers (VCSELs) operating at 850 nm over multimode optical fiber. Deployable links have evolved in speed from 1 Gb/s in 1996 to 28 Gb/s in 2014. Serial data links at 40 and 56 Gb/s are now under development and place even more demand on the VCSEL and photodiodes. In this paper, we present the characteristics of VCSELs and photodiodes used in current generation 28 Gb/s links and present several methods to extend link distances using more advanced data encoding schemes. Finally, we will present results on wavelength division multiplexing on multimode optical fiber that demonstrate 40 Gb/s Ethernet connections up to 300 m on duplex OM3 optical fiber, and present results on fiber optimized for modal bandwidth in the 850 to 980 nm range.

VCSEL-Based Interconnects for Current and Future Data Centers

We demonstrate 51.5625Gbps PAM4 transmission over ≥ 150 m next generation wide band MMF using SWDM TOSAs from 850 nm to 940 nm indicating an aggregated 206.25 Gb/s speed achievable on a single WBMMF.

51.56 Gb/s SWDM PAM4 transmission over next generation wide band multimode optical fiber

Two system transmission experiments are reported which indicate the feasibility of transmitting an aggregated speed of >200 Gb/s over a single next-generation wide-band multimode optical fiber (NG-WBMMF) using a shortwave wavelength division multiplexing (SWDM) and a four-level pulse amplitude modulation (PAM4), using optics rated for ${\text{25 Gb/s}}$ NRZ operation. The first experiment leverages predistorted KR4 ${\text{51.5625 Gb/s}}$ electrical PAM4 signaling generated by an arbitrary waveform generator to drive SWDM TOSAs at 850, 880, 910, and 940 nm. The received optical PAM4 signals are then post processed offline with linear equalizers to improve the eye quality. Detailed analysis of the height and width of the received optical eyes indicates an aggregated ${\text{206.25 Gb/s}}$ speed is achievable on a single NG-WBMMF beyond 150 m. In the second experiment, a commercial PAM4 IC evaluation kit with real-time digital signal processing is adopted to generate and detect PAM4 signals at KP4 FEC rate of ${\text{53.125 Gb/s}}$ per wavelength. The SWDM wavelength range is extended with additional tests at 980 nm. Pre-FEC BER below threshold of ${\text{2.4}}\times {\text{10}}^{-4}$ is achieved up to 300 m across all five wavelengths demonstrating an aggregated speed of ${\text{265.625 Gb/s}}$ over a single MMF.

SWDM PAM4 Transmission Over Next Generation Wide-Band Multimode Optical Fiber

We demonstrate 100 Gbps PAM-4 transmission over 105m of wideband-MMF for each of four wavelengths from 850nm to 940nm using 25G VCSELs and thereby demonstrate an architecture that enables 400G over a single MMF.

4λ × 100Gbps VCSEL PAM-4 transmission over 105m of wide band multimode fiber

We demonstrate numerically and experimentally the transmission of 10Gb/s over OM3 fibers up to 300m, under low-cost VCSEL launch conditions from 990-1080nm with electronic dispersion compensation (EDC), showing a path to low cost 40Gbps links.

Kasyapa Balemarthy

Papers

VCSEL-Based Interconnects for Current and Future Data Centers

51.56 Gb/s SWDM PAM4 transmission over next generation wide band multimode optical fiber

SWDM PAM4 Transmission Over Next Generation Wide-Band Multimode Optical Fiber

4λ × 100Gbps VCSEL PAM-4 transmission over 105m of wide band multimode fiber

10Gb/s Transmission over 300m OM3 Fiber From 990-1080nm with Electronic Dispersion Compensation