Optical Transmission Systems, Receivers, and Devices, and Methods of Receiving Optical Signals and Designing Filters and Filtering Optical and Electrical Signals

Systems may include one or more directly modulated laser ("DML") optical transmitters to transmit optical signals carrying data to one or more optical receivers and include one or more filters designed to reduce the power in lower modulation levels. In various embodiments, the optical receiver includes an optical combiner to combine local oscillator light at a frequency, fLO with the optical data signal, an optical-electrical converter to downconvert the combined LO-optical signal to an electrical signal including four PAM4 frequencies, an electrical filter to attenuate the electrical power in the PAM4 frequencies by differing amounts, rectify and output the electrical signal. Systems may include extinction ratio management and thermal chirp compensation, and may include optical filters designed for chirp management with different laser parameters (α and k) and OMAs with PAM-M format in both optical and electrical domains and for use with quasi-coherent receivers (QCR) and DML transmitters.