A technique for direct phase modulating a low-cost distributed feedback laser is presented for user transmitter in ultra dense wavelength division multiplexing passive optical networks. It consists of a simple RC network, which equalizes the phase response. The method is tested with differential phase-shift keying modulation and heterodyne coherent detection at 1.25 and 2.5 Gb/s showing a penalty <1 dB at BER = 10(-3) compared with external phase modulation.