Optimizing Low Pass Filter Bandwidth for Generating Electrical Duobinary Signal for 40 Gb/s Optical Duobinary Systems

In the optical duobinary modulation format, low pass filter generated three level electrical signal is needed for the optical duobinary signal. To achieve optimum performance to generate duobinary signal, eye diagram of the three level electrical signal should produce binary eye amplitude, which must be certain percent of the signal eye amplitude. Low pass filter’s bandwidth should be set up based on the driving circuitry’s bandwidth. In this work, we studied to optimize the low pass filter’s bandwidth to generate the duobinary signal pattern for the 40 Gb/s duobinary modulation format. While designing filter, we considered the influence of the filter’s footprint and filter’s launch pad loss. We extensively discussed the impacts of these parameters, which could cause the S-parameters to deviate from the ideal frequency response of the filter. Considering these criterions along with driving circuitry’s bandwidth, we designed and fabricated 12 GHz 9th order absorptive low pass bessel filter to generate the electrical duobinary signal for duobinary modulation format for 40 Gb/s optical duobinary system. Using measured S-parameter data of fabricated filter, we simulated the transmitter model for 40 Gbps duobinary signal generation. We produced eye diagram with the transmitter model, which illustrates the optimum three level electrical signal.