Orthogonal frequency division multiplexing achieved predominant applications in both wired and wireless communication systems, for its flexibility when transmission comprises of multipath distortion. Bipolar waveform can't be used in IM/DD system. New methods for OFDM called DC biased OFDM, ACO OFDM and SSB OFDM allow unipolar waveform which has been implemented in recent days. In this paper, a linear system model for each method is derived and three distinct OFDM schemes for direct detection single mode fiber system are analyzed. Bit error rate, OSNR for various fiber lengths, normalized OSNR vs dispersion indexes is compared.