We investigated both experimentally and by simulation, non-return-to-zero (NRZ) and return-to-zero (RZ) 4-ary PAM operating at 20 Gb/s. A simple scheme to realize the quadratic signal leveling by suitably driving a MZ modulator is proposed, which provides greater than 6 dB improvement in receiver sensitivity as compared to equal level spacing. We experimentally demonstrated NRZ and RZ 10 Gbaud/s 4-ARY PAM transmissions over an 80 km standard single mode fiber (SSMF) link as a proof of concept and more detailed experimental results over longer reach will follow. Numerical simulations for the 4-ary PAM performance over longer distances (>200 km) are also presented.
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.
Optical second harmonic generation in dense wavelength division multiplexing were used to monitor the performance of each channel in all optical networks. A poled Lithium-Niobate (PP-LNO3) device is used to convert C and L- Band optical WDM channels into half wavelength channels where silicon arrayed detectors and CMOS electronics were used to perform advanced digital signal processing to predict optical channel presence, channel power, signal to noise ratio and the quality (Q) factor parameter. With further processing, the bit error rate per channel can be estimated from the Q factor. The technique is realized on a system-on-a-chip CMOS technology which can potentially improve all optical networking architectures.
The information capacity of dense wavelength division multiplexing (DWDM) optical systems is reviewed. The effect of optical beat interference of closely spaced optical carriers sets the limit on the allowed channel spacing even in the absence of nonlinear effects. Present DWDM networks operating at 10.7 Gb/s in the C and L bands region have channel spacing no less than 25 GHz. We show that this limit is a constraint and smaller spacing may not be allowed without further optical and electrical pulse shaping.
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.