This paper will review the VCSEL performance requirements and link length limitations to support next generation 53Gbaud line rates with PAM-4 modulation for 100G per lane multi-mode optical links for both active optical cables and transceivers. VCSEL performance with bandwidth in excess of 25GHz and relative intensity noise lower than -145dB/Hz will be needed to enable this next generation of multi-mode links. VCSEL device performance and associated wear out life data will be included.
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.
Avago’s 850nm oxide VCSEL for applications requiring modulation at 25-28G has been designed for -3dB bandwidths in excess of 18GHz over an extended temperature range of 0-85C. The VCSEL has been optimized to minimize DBR mirror thermal resistivity, electrical resistance and optical losses from free carrier absorption. The active region is designed for superior differential gain to enable high optical bandwidths. The small-signal modulation response has been characterized and the large-signal eye diagrams show excellent high-speed performance. Characterization data on other link parameters such as relative intensity noise and spectral width will also be presented.
High speed fiber optic transceiver modules using parallel optics require that oxide-confined vertical-cavity surface-emitting lasers (VCSELs) be moisture resistant in non-hermetic environments. Conventional storage 85/85 (85°C and 85% relative humidity) testing without a bias does not adequately characterize oxide VCSEL’s moisture resistance. Oxide VCSELs do not fail or degrade significantly under such conditions. With a bias, however, we have found that moisture can cause failure modes not seen in dry reliability testing. Without proper device design and fabrication, these failure modes lead to high failure rates in oxide VCSELs. In this paper, we first discuss the failure mechanisms we have identified, including dense dislocation network growth, semiconductor cracking and aperture surface degradation, all in high humidity and high temperature under operating conditions. We then report the results of environmental reliability tests on Agilent’s oxide VCSELs developed for the parallel optics modules. The results from a large number of wafers produced over an extended period of time have shown consistent, robust environmental reliability.