The development of robust next generation multi-mode VCSEL-based optical links requires an accounting of all penalties in the link. While limitations from fiber bandwidth can be overcome to a significant extent using equalization and forward error correction, noise in the link cannot be equalized. Measurements show that mode partition noise depends on launch condition, and the noise penalty can be decreased using devices with small k factor. Time and frequency domain characterization of mode power fluctuations shows that they occur primarily at frequencies below 5 GHz. These findings guide the development of VCSELs for 25GBaud PAM4 and higher bit rate applications.
This paper will review the device design and performance of Broadcom’s 50Gb/s PAM-4 VCSEL to enable the next generation of transceivers using a PAM-4 advanced modulation scheme at 25-28 GBd. The VCSEL has been optimized to minimize noise and improve dynamic performance for cleaner eyes. Preliminary wear out lifetime studies indicate that the time to 1% failure exceeds 10 years, making the VCSELs suitable for data communication applications.
Mode partition noise (MPN) can become the dominant limitation in 850 nm VCSEL-based multi-mode fiber (MMF) links at high data rates. Fluctuations in the partition of energy between the transverse modes of the VCSEL combined with the chromatic dispersion in the fiber leads to intensity noise at the receiver. The impact of MPN on non-equalized and equalized links has been studied with a numerical model of the VCSEL and MMF. The MPN in 25 Gb/s VCSELs has been investigated by examining noise in individual mode groups isolated using a thin film Fabry-Perot filter. The measured k factor below 0.15 should enable links significantly longer than 100 m at 25 Gb/s and higher data rates.
Avago’s 850nm VCSELs for applications requiring modulation at 25-28Gbps have been designed for -3dB bandwidths in excess of 19GHz over the extended temperature range of 0-85°C. The DBR mirrors have been optimized to minimize optical losses and thermal and electrical resistance. The active region is designed to provide superior differential gain for high optical bandwidth. In this paper we will describe the design for performance and manufacturability of Avago’s high speed 25-28Gbps VCSEL. Analysis of the high-speed modulation characteristics and results of wearout reliability studies will be presented. We will also discuss the manufacturability of this next generation of high performance, reliable lasers. The challenges of epitaxial growth and wafer fabrication along with the associated process control technologies will be described.
Applications of 850 nm VCSELs have bloomed in recent years arising from their low cost, and the ease of forming one- and two-dimensional arrays. In addition to the traditional measures of device lifetime, operation over a wide temperature range and link length, the figures of merit increasingly include power consumption (pJ/bit), footprint (bits/mm<sup>2</sup>) and cost ($/Gb/s). As 1 × 12 arrays of 10G VCSELs are widely adopted, there is a clear need for improvement along all these fronts. This is achieved through development of VCSELs operating at higher data rates, and modifications to the oxide VCSEL structure. In this paper, we discuss the development of VCSELs with electrostatic discharge protection, and high bandwidth for operation at 10 – 25 Gb/s.
Frequently quoted advantages of VCSELs over other optical sources include wafer scale fabrication and testing, low
cost, ease of fabricating arrays and ease of fiber coupling. To benefit from these advantages a robust manufacturing
process and product demand are needed. Avago Technologies produces a range of single channel and parallel optical
link products incorporating 850nm band VCSEL sources operating at up to 10Gb/s per channel. This paper will explore
some important factors which need to be controlled for manufacturability of VCSEL devices.
Directly modulated 850nm oxide VCSEL is a key enabling technology for short reach, high speed
data-communication applications. Current commercially available optical transceiver products operate at data rate
up to 10Gb/s per channel, for aggregate data rate of 70Gb/s and beyond, in the case of parallel optical data link.
High volume, low cost, over temperature optical modulation speed, spectral width, output power, thermal power
budget, large signal electrical interaction with the IC driver, and reliability are some of the key requirements
driving the 850nm oxide VCSEL development. In this paper, we discuss some of the engineering issues
investigated for developing a viable oxide VCSEL product operating at 10Gb/s per channel and higher data rate.
Long wavelength VCSELs at 1300 nm have been developed to serve 10-Gigabit enterprise networks over FDDI grade multimode fibers up to 300 m in distance. The long wavelength VCSELs operate CW at temperatures over 100 °C. They are ideal low cost alternatives to DFB lasers for transceivers and transponders compatible with IEEE 10GBASE-LX4 or 10GBASE-LRM standards over multimode fibers.