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Spectral Efficiency and Energy Efficiency of Pulse-Amplitude Modulation using 1.3-µm Waver-Fusion VCSELs for Optical Interconnects Philip Wolf, Hui Li, Andrei Caliman, Alexandru Mereuta, Vladimir Iakovlev, Alexei Sirbu, Eli Kapon, and Dieter Bimberg ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00403 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017
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ACS Photonics
Spectral Efficiency and Energy Efficiency of PulseAmplitude Modulation using 1.3-µm Waver-Fusion VCSELs for Optical Interconnects PHILIP WOLF,1 HUI LI,2,* ANDREI CALIMAN,3 ALEXANDRU MEREUTA,3 VLADIMIR IAKOVLEV,3 ALEXEI SIRBU,3 ELYAHOU KAPON,3 DIETER BIMBERG1,4 1
Center of NanoPhotonics and Department of Solid-State Physics, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany Optoelectronic Materials and Technologies Engineering Laboratory of Shandong, College of Mathematical and Physical Sciences, Qingdao University of Science and Technology, Qingdao, China 3 Laboratory of Physics of Nanostructures, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland 4 King Abdulaziz University, Jeddah, KSA *Corresponding author:
[email protected] 2
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
We compare the modulation efficiency of on-off keying (OOK) and four-level pulse-amplitude modulation (PAM4) using 1.3-µm wafer-fusion vertical-cavity surface-emitting lasers for single-channel short-reach optical interconnects. O-band wafer-fusion VCSELs combine high optical gain from InP-based active regions with high reflectivity, low absorption, and high thermal conductive GaAs-based distributed Bragg reflectors. VCSELs offer a low cost and low power consuming light source for optical interconnects in data centers and fiber-based local-area networks, presenting an attractive alternative to common edge-emitting lasers. Our investigation focuses on the highest error-free bit rates and energy efficiency of OOK and PAM4 signaling. We achieved an error-free bit rate of 38 Gbit/s with PAM4 for devices having a small-signal bandwidth of 9.9 GHz, presenting a spectral efficiency of 3.8 bit/s/Hz. These PAM4 values offer an improved error-free bit rate of 26%, and consume 32% less energy-perbit when compared to OOK. Keywords: vertical-cavity surface-emitting laser, wafer-fusion, pulse-amplitude modulation, energy efficiency, spectral efficiency, optical interconnects
The increasing demand for data throughput capacity of short-reach optical interconnects (e.g., in supercomputers and data centers) strongly motivates the development of fast, directly modulated and low-cost vertical-cavity surface-emitting lasers (VCSELs).1 Currently, 10 Gbit/s links are widely deployed, while 25 Gbit/s links are being introduced. Footprint, power consumption and cost are of primary concern, limiting the complexity of components and convenient modulation formats. Higher-order intensity modulation formats, such as four-level pulse-amplitude modulation (PAM4), are recommended as the most suitable for optical interconnects enabling a higher speed without significantly increased complexity. Although the use of multilevel modulation increases spectral efficiency (net bit rate for a given bandwidth), the detectors require a larger received optical power to reach the error-free threshold. The IEEE task force has recently established PAM4 as the modulation format for 400 Gigabit Ethernet (IEEE P802.3bs) transmission across standard single-mode fiber (SMF). Three optical interface classes have been determined: 8 × 25 GBd (GBd: symbol rate measured in Baud (Bd), or symbols per second) wavelength-division multiplexing (WDM) transmission across 2 and 10 km, as well as 4 × 50 GBd parallel fiber transmission across 500 m, all in the wavelength region centered at 1.3 µm (Original band, O-band).2 O-band VCSELs are extremely low-cost and low-power consuming, presenting an attractive alternative to more common edge-emitterbased link technology for data centers and fiber-based local-area networks ranging from 100 m up to 10 km. Wafer fusion is one of
several fabrication technologies for VCSEL devices emitting in the Oband.3-6 The advantage of wafer fusion is its ability to combine high optical gain InP-based active regions with GaAs-based distributed Bragg reflectors (DBRs) that offer high reflectivity, low absorption, and high thermal conductivity. 1.3 µm wafer-fusion VCSELs feature the best single-mode (SM) optical output power due to superior thermal management,7 high-yield wavelength control based on nm-scale cavity engineering,8 reliability according to the Bellcore/Telcordia standard9 and compatibility with conventional GaAs-based VCSEL manufacturing.10 Wafer fusion VCSELs have been used in coarse wavelength division multiplexing (CWDM) (1270, 1290, 1310 and 1330 nm) modules for 40 Gbit/s (4 × 10 Gbit/s) low-power consumption optical links.11 We recently reported progress on developing 4 × 25 Gbit/s CWDM optical links based on wafer-fusion VCSELs.11 We demonstrated single-channel 25 Gbit/s data transmission across 10 km of SMF without any optical amplification.12 Although long-wavelength VCSELs exhibit superior static parameters (e.g., high single-mode optical power) compared to shortwavelength (
