1×4 VCSEL arrays with uniform spectral and noise properties by using rotationally asymmetric oxide aperture for 400 Gbit/s applications

In the recent decade, the growth of data centers is being driven by spreading of cloud computing and its relevant technologies. As of now, 100 Gbit/s optical interconnects have been widely used in the data-centers, and the trend is about to be switched to 400 Gbit/s data-rate. Multi-mode VCSELs are employed for the reasons of the lower cost and power consumption in a short-distance range. 400 Gbit/s transceivers are equipped with an 1×8 or a pair of 1×4 VCSEL arrays. Especially, 400GBASE-SR4.2 (BiDi) uses two different wavelengths for each 1×4 array. In such case, sufficient uniformity in terms of optical output, bandwidth, relative intensity noise (RIN) and spectral width properties could be keys to success in 400 Gbit/s applications. Variations of optical output and bandwidth over a processed wafer can be suppressed by maturing the epitaxial growth and fabrication procedures. In contrast, spectral and noise properties are strongly coupled to transverse mode properties of VCSELs, which are controlled by a shape of the oxide aperture. In this work, we report uniform spectral and noise characteristics of 1×4 VCSEL arrays by introducing a rotationally-asymmetric oxide aperture. The rotationally-asymmetric aperture VCSELs show less variation in root-mean-square (RMS) spectral width and RIN compared with circular-aperture VCSELs. The rotationally-asymmetric aperture is capable of splitting degenerated modes in spectral domain. Uniformity in the performance of optical output, 3dB bandwidth and RIN are verified in 1×4 arrays for both 850 nm and 910 nm VCSELs. 53.125 Gbit/s PAM-4 modulation is then performed with different temperatures of 25 and 90℃, which shows the capability of our VCSEL arrays for 400 Gbit/s applications.