This paper makes comparisons between the modified optical duobinary return-to-zero（MD-RZ） format and the optical duobinary non return-to-zero（MD-NRZ） format

which are both used in long distance transmission systems.Simulation results show that MD-RZ signals have lower BER and higher receiver sensibility than MD-NRZ signals by using the dispersion compensation technology.In addition

it presents a nove1 all-optical method for the generation of MD-RZ signals by using a Polarization Maintaining Fiber Loop Mirror（PMFLM） and theoretically proves its feasibility.