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

The vertical-cavity surface-emitting laser (VCSEL) has become a light source of great importance for industrial and consumer applications. This includes communication and sensing in particular, where dynamics and optical mode behavior are key performance characteristics. This tutorial treats relevant VCSEL basics, performance requirements and recent progress toward higher speed, higher single-mode power, and polarization control.

Advances in VCSELs for Communication and Sensing

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 investigate the impact of reduced photon lifetime on the static and dynamic performance of high-speed, oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs). The photon lifetime is reduced by a shallow-surface etch that lowers the reflectivity of the top mirror. From an analysis of the dependence of slope efficiency on mirror loss (etch depth) and temperature, we deduce values for the internal quantum efficiency and the internal optical loss and their dependencies on temperature. From an analysis of the dependence of the small-signal-modulation response on photon lifetime (etch depth) and temperature, we deduce values for differential gain and gain compression, and their dependencies on photon lifetime and temperature. We find a tradeoff between high resonance frequency and low damping for speed enhancement, leading to an optimum photon lifetime close to 3 ps for this particular VCSEL design that enables a modulation bandwidth of 23 GHz and error-free transmission at 40 Gb/s.

Impact of Photon Lifetime on High-Speed VCSEL Performance

Silicon does not emit light efficiently, therefore the integration of other light-emitting materials is highly demanded for silicon photonic integrated circuits. A number of integration approaches have been extensively explored in the past decade. Here, the most recent progress in this field is reviewed, covering the integration approaches of III-V-to-silicon bonding, transfer printing, epitaxial growth and the use of colloidal quantum dots. The basic approaches to create waveguide-coupled on-chip light sources for different application scenarios are discussed, both for silicon and silicon nitride based waveguides. A selection of recent representative device demonstrations is presented, including high speed DFB lasers, ultra-dense comb lasers, short (850nm) and long (2.3 mu m) wavelength lasers, wide-band LEDs, monolithic O-band lasers and micro-disk lasers operating in the visible. The challenges and opportunities of these approaches are discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201700063

Novel Light Source Integration Approaches for Silicon Photonics

We report on the design, fabrication, and evaluation of large-aperture, oxide-confined 850 nm vertical cavity surface emitting lasers (VCSELs) with high modulation bandwidth at low current densities. We also compare the use of InGaAs and GaAs quantum wells (QWs) in the active region. Both VCSELs reach an output power of 9 mW at room temperature, with a thermal resistance of 1.9deg C/mW. The use of InGaAs QWs improves the high-speed performance and enables a small-signal modulation bandwidth of 20 GHz at 25degC and 15 GHz at 85degC. At a constant bias current density of only 11 kA/cm2, we generate open eyes under large-signal modulation at bit rates up to 25 Gbit/s at 85degC and 30 Gbit/s at 55degC.

High-Speed, Low-Current-Density 850 nm VCSELs

Error-free transmission is demonstrated at bit rates up to 40 Gbit/s in a back-to-back configuration and up to 35 Gbit/s over 100 m multimode fibre using a directly modulated oxide confined 850 nm vertical cavity surface emitting laser.

https://publications.lib.chalmers.se/records/fulltext/local_123956.pdf

40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL

A high-speed and energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting laser (VCSEL) for optical interconnects is presented. A record-high modulation bandwidth of 30 GHz is reached for a 3.5 mu m oxide aperture VCSEL, with 25 GHz bandwidth already at a bias current of 1.8 mA. The high bandwidth at low currents enables energy-efficient transmission with a dissipated heat energy in the VCSEL of <100 fJ/bit at 25, 40 and 50 Gbit/s.

https://publications.lib.chalmers.se/records/fulltext/219953/local_219953.pdf

30 GHz bandwidth 850 nm VCSEL with sub-100 fJ/bit energy dissipation at 25–50 Gbit/s

GaAs-based 850 nm VCSELs are the standard light source in short-reach high-speed optical interconnects in datacenters and supercomputers. The last few years have seen an impressive increase in VCSEL speed, reaching modulation bandwidths of 28 GHz and single-channel data rates of 57 Gbit/s (non-equalized) and 71 Gbit/s (equalized) under direct NRZ modulation [1-3]. Future optical interconnects will require even faster VCSELs capable of higher-speed transmission at lower energy dissipation. Here we present the results from our new generation of high-speed 850 nm VCSELs, a further development of our previous VCSEL design [1-3].

https://publications.lib.chalmers.se/records/fulltext/231003/local_231003.pdf

High speed 850 nm VCSEL with 30 GHz modulation bandwidth

We present the temperature dependence of the performance characteristics of our latest generation high-speed oxide confined 850-nm vertical cavity surface-emitting lasers (VCSELs). Using a 7-μm oxide aperture diameter VCSEL, we demonstrate a maximum modulation bandwidth of ~ 27 GHz at room temperature and ~ 21 GHz at 85°C. With a new high-speed optical receiver, these bandwidths enable error-free data transmission (defined as a bit-error-rate <; 10-12) at bit-rates up to 47 Gb/s at room temperature, and up to 40 Gb/s at 85°C.

A. Joel

Papers

High-Speed, Low-Current-Density 850 nm VCSELs

40 Gbit/s error-free operation of oxide-confined 850 nm VCSEL

30 GHz bandwidth 850 nm VCSEL with sub-100 fJ/bit energy dissipation at 25–50 Gbit/s

High speed 850 nm VCSEL with 30 GHz modulation bandwidth

High-Speed Oxide Confined 850-nm VCSELs Operating Error-Free at 40 Gb/s up to 85 $^{\circ}{\rm C}$