Longer Wavelength GaAs-Based VCSELs for Extended-Reach Optical Interconnects

Data centers of today are increasing in size and are built to accommodate strong traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable of operation at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to a larger number of possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could soon open up for lane rates of 100+ Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of data center OI standards, in which capacity is built up mainly by employing multiple parallel lanes, increasing symbol rate by going from binary to four-level pulse amplitude modulation (PAM-4), and optimizing with electrical mitigation techniques such as digital signal processing.In this work we show that 1060 nm GaAs VCSELs are suitable light sources for long-reach OIs by first demonstrating their overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. With PAM-4, we show 100 Gb/s error-free capability over 100 m of MMF, suitable for wavelength division multiplexed OIs that can transmit data at several wavelengths from 850 to 1060 nm over the same fiber channel. We also assemble single-mode 1060 nm VCSEL and single-mode fiber links and demonstrate 50 Gb/s error-free transmission over 1 km using pre-emphasis and 40 Gb/s over 2 km without the use of any electrical mitigation techniques. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short optical cavity and multiple oxide layers. In addition, we show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

1060 nm GaAs VCSELs for Extended Reach Optical Interconnects in Data Centers

The data centers of today are increasing in size and are built to accommodate strong data traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable to operate at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to many more possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could open up for lane rates of 10, 25 and possibly 50 Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of the Ethernet standard.In this work we show that the 1060 nm GaAs VCSEL is a suitable light source for long-reach OIs by demonstrating its overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short op-tical cavity and multiple oxide layers. We also show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

1060 nm Single-Mode VCSEL and Single-Mode Fiber Links for Long-Reach Optical Interconnects

We investigate the use of a 1060 nm single-mode vertical-cavity surface-emitting laser (VCSEL) and a 1060 nm single-mode fiber as a competitive single-mode technology for cost-and power-efficient long-reach optical interconnects. Error-free transmission (bit error rate < 10-12) over 2 km is demonstrated at bitrates up to 40 Gb/s under on-off keying non-return-to-zero (OOK-NRZ) modulation, without equalization, forward-error correction, or other forms of digital signal processing. The VCSEL is extensively characterized with respect to its static and dynamic performances, including the power-voltage-current characteristics, spectral characteristics, beam divergence, modulation response, relative intensity noise, and frequency chirp. The measured dependence of power penalty on fiber length is consistent with an analysis of chirp-induced pulse compression and broadening along the negative chromatic dispersion fiber

1060 nm VCSELs for long-reach optical interconnects

Reach extension of high capacity optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs), as needed for large-scale data centers, would benefit from high-speed GaAs-based VCSELs at 1060 nm. At this wavelength, the chromatic dispersion and attenuation of the optical fiber are much reduced in comparison with 850 nm. We present single and multimode 1060 nm VCSELs based on designs derived partly from our high-speed 850 nm VCSEL designs. The single-mode VCSEL, with a modulation bandwidth exceeding 22 GHz, supports back-to-back data rates up to 50 Gbps at 25 \ub0C and 40 Gbps at 85 \ub0C under binary NRZ (OOK) modulation. Using mode-selective launch, we demonstrate error-free 25 Gbps transmission over 1000 m of 1060 nm optimized MMF. Higher data rates and/or longer distances will be possible with equalization, forward-error-correction, and/or multilevel modulation

High-Speed VCSELs for OOK and Multilevel PAM Modulation

The short-reach optical interconnects used in datacenters and high-performance computing systems are dominated by vertical-cavity surface-emitting laser (VCSEL) and multimode fiber (MMF) links1. The VCSEL-MMF technology is the most cost and power efficient and offers the smallest footprint. VCSELs for 25-28 Gbit/s OOK lanes are in production2 while 25-28 Gbaud PAM-4 with forward-error-correction (FEC) is considered for next generation 50-56 Gbit/s lanes3. For this transition, as well as future developments towards even higher speed, the VCSEL dynamics and high-speed properties are of utmost importance. Improved dynamics may enable 50-56 Gbit/s PAM-4 without FEC and even 50-56 Gbit/s OOK, which would reduce complexity, power consumption, and latency

VCSEL modulation capacity: Continued improvements or physical limits?

The short-reach optical interconnects used in datacenters and high-performance computing systems are dominated by VCSEL and multimode fiber (MMF) links 1 . The VCSEL-MMF technology is the most cost and power efficient and offers the smallest footprint. VCSELs operating at 25-28 Gbit/s are in production 2 while research has extended the VCSEL modulation bandwidth to 30 GHz 3 (Fig.1) and enabled OOK-NRZ data transmission up to 57 Gbit/s at 25\ub0C 4 and 50 Gbit/s at 85\ub0C 5 , without equalization or forward-error-correction (FEC). A VCSEL energy dissipation below 100 fJ/bit has been demonstrated at 25-50 Gbit/s 3 (Fig.1). The need for higher interconnect capacity raises the question whether the speed and dynamics of VCSELs can be further improved or whether physical limits preventing this have been reached. Higher speed VCSELs would enable higher lane rates and therefore reduced number of lanes and increased bandwidth density for a given aggregate interconnect capacity

Long-Reach 1060 nm SM VCSEL - SMF Optical Interconnects

We explore the low-cost, low-power GaAs-VCSEL technology for long-reach interconnects at 1060 nm. With reduced fiber attenuation and chromatic dispersion compared to 850 nm, we demonstrate error-free 40 Gbps OOK-NRZ transmission over 2 km SMF using a SM VCSEL