Vertical-cavity surface-emitting lasers (VCSELs) are decisive cost-effective, energy-efficient, and reliable light sources for short-reach (up to ~300 m) optical interconnects in data centers and supercomputers. To viably replace copper interconnects and advance to on-chip integrated photonics, reliable VCSELs ideally must be able to operate highly energy efficient, but at large bit rates and without cooling up to 85 °C, with immunity to temperature variations. Our 980 nm VCSELs achieve such temperature-stable, energy-efficient, and high-speed operation coincidently. Record low 139 fJ/bit of dissipated heat for 35 Gbit/s error-free data transmission at 85 °C is reported. Careful design of both the VCSEL’s epitaxial structure and device geometry is of essence. Introducing a suitable gain-to-etalon wavelength offset simultaneously improves the temperature-stability, the maximum bit rate at high temperatures, and the energy efficiency. Tuning the photon lifetime additionally increases the bandwidth by changing the relation between damping and resonance relaxation frequency. Systematic temperature-dependent and oxide aperture-diameter-dependent measurements, including static L-I-V curves and emission spectra, small signal analysis, and data transmission experiments are reported. The modulation bandwidth, the parasitic cut-off frequency, the relaxation resonance frequency, lumped-circuit elements, and the K- and D-factors are derived, useful for energy-efficient optical interconnects based on 980 nm VCSELs.
The use of Internet has increased and continues to increase exponentially, mostly driven by consumers. Thus bit rates in networks from access to WDM and finally the computer clusters and supercomputers increase as well rapidly. Their cost of energy reaches today 5-6 % of raw electricity production. For 2023 a cross over is predicted, if no new "green" technologies or "green" devices" will reduce energy consumption by about 15% per year. We present two distinct approaches for access and computer networks based on nanophotonic devices to reduce power consumption in the next decade.
Via experimental results supported by numerical modeling we report the energy-efficiency, bit rate, and modal properties of GaAs-based 980 nm vertical cavity surface emitting lasers (VCSELs). Using our newly established Principles for the design and operation of energy-efficient VCSELs as reported in the Invited paper by Moser et al. (SPIE 9001-02 )  along with our high bit rate 980 nm VCSEL epitaxial designs that include a relatively large etalonto- quantum well gain-peak wavelength detuning of about 15 nm we demonstrate record error-free (bit error ratio below 10-12) data transmission performance of 38, 40, and 42 Gbit/s at 85, 75, and 25°C, respectively. At 38 Gbit/s in a back-toback test configuration from 45 to 85°C we demonstrate a record low and highly stable dissipated energy of only ~179 to 177 fJ per transmitted bit. We conclude that our 980 nm VCSELs are especially well suited for very-short-reach and ultra-short-reach optical interconnects where the data transmission distances are about 1 m or less, and about 10 mm or less, respectively.
Principles of energy-efficient high speed operation of oxide-confined VCSELs are presented. Trade-offs between oxideaperture diameter, current-density, and energy consumption per bit are demonstrated and discussed. Record energyefficient error-free data transmission up to 40 Gb/s, across up to 1000 m of multimode optical fiber and at up to 85 °C is reviewed.
A new record for energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting lasers (VCSELs) particularly suited for optical interconnects is presented. Error-free performance at 25 Gb/s is achieved with only 56 fJ/bit of dissipated energy per quantum of information. The influence of the oxide-aperture diameter on the energy-efficiency of our VCSELs is determined by comparing the total and dissipated power versus the modulation bandwidth of devices with different aperture diameters. Trade-offs between various parameters such as threshold current, differential quantum efficiency, wall plug efficiency and differential resistance are investigated with respect to energy-efficiency. We show that our present single-mode VCSELs are more energy-efficient than our multimode ones.
The bandwidth-induced communication bottleneck due to the intrinsic limitations of metal interconnects is inhibiting the
performance and environmental friendliness of today´s supercomputers, data centers, and in fact all other modern
electrically interconnected and interoperable networks such as data farms and "cloud" fabrics. The same is true for
systems of optical interconnects (OIs), where even when the metal interconnects are replaced with OIs the systems
remain limited by bandwidth, physical size, and most critically the power consumption and lifecycle operating costs.
Vertical-cavity surface-emitting lasers (VCSELs) are ideally suited to solve this dilemma. Global communication
providers like Google Inc., Intel Inc., HP Inc., and IBM Inc. are now producing optical interconnects based on VCSELs.
The optimal bandwidth per link may be analyzed by by using Amdahl´s Law and depends on the architecture of the data
center and the performance of the servers within the data center. According to Google Inc., a bandwidth of 40 Gb/s has
to be accommodated in the future. IBM Inc. demands 80 Tbps interconnects between solitary server chips in 2020. We
recently realized ultrahigh bit rate VCSELs up to 49 Gb/s suited for such optical interconnects emitting at 980 nm. These
devices show error-free transmission at temperatures up to 155°C and operate beyond 200°C. Single channel data-rates
of 40 Gb/s were achieved up to 75°C. Record high energy efficiencies close to 50 fJ/bit were demonstrated for VCSELs
emitting at 850 nm. Our devices are fabricated using a full three-inch wafer process, and the apertures were formed by
in-situ controlled selective wet oxidation using stainless steel-based vacuum equipment of our own design. assembly,
and operation. All device data are measured, recorded, and evaluated by our proprietary fully automated wafer mapping
probe station. The bandwidth density of our present devices is expected to be scalable from about 100 Gbps/mm² to a
physical limit of roughly 15 Tbps/mm² based on the current 12.5 Gb/s VCSEL technology. Still more energy-efficient
and smaller volume laser diode devices dissipating less heat are mandatory for further up scaling of the bandwidth.
Novel metal-clad VCSELs enable a reduction of the device's footprint for potentially ultrashort range interconnects by 1
to 2 orders of magnitude compared to conventional VCSELs thus enabling a similar increase of device density and