A new approach for designing broad and flattened spectrum multicarriers optical sources is presented leading to a 32 spectral lines source using a dual-arm Mach-Zehnder modulator (MZM) and a 41 spectral lines source from two-stage MZM. A modified simulated annealing-based optimization method is applied to derive the necessary settings allowing the optical flat comb source (OFCS) to be ultraflat. The OFCS is mooted as a technology to enhance the overall capacity of an access optical network by increasing the number of WDM channels. Here, we demonstrate an ultra-dense WDM (UDWDM) trans- mission for application to passive optical networks (PON) with (11x12.5Gbps) Quadrature Amplitude Modulation (QAM) based on a 4b/s/Hz spectral efficiency orthogonal frequency division multiplex (16QAM-OFDM) transmitter and direct detection. We use an OFCS to generate the 11 subcarriers spaced by 6.25GHz, made of a two-stage MZM. We study the performance of 3 filtered channels in terms of error vector magnitude (EVM) in back-to-back (B-to-B) conditions and after propagation through 25km and 100km standard single mode fiber (SSMF).