Vertical cavity surface emitting lasers (VCSELs) designed for 10 G Ethernet over 300 m graded index multimode fiber in general have optical aperture diameters of 7 to 10 μm; cavities of these sizes support multiple transverse modes. The circularly symmetric structures are assumed to have no polarization selection, however, we show that orthogonally polarized lasing modes are often present and cause polarization partition noise which degrades the link bit error rate (BER). When a polarization selector was used in the link to allow only one polarization, the BER improved by two-order of magnitude even with the loss of more than 32 percent of the VCSEL average emitted power.
The dependence of the mode partition noise (MPN) and the power penalty associated with it can be measured from the source spectral width. Our findings show that there is strong dependence of the carrier lifetime on the bit error rate degradation caused by MPN on the spectral width of the vertical cavity surface emitting laser (VCSEL). VCSELs with smaller spectral width (shorter carrier lifetime) exhibited smaller MPN induced power penalty. We found that the theoretical calculation of the power penalties caused by MPN from the carrier lifetime and the spectral width is in good agreement with the measured system penalties.