High speed modulation based on bandwidth limited devices is desired for cost-effective PON capacity upgrade. In this paper, we investigate the equalization techniques for enabling 25-Gb/s transmission with 10G-class optics. A comparison between FFE and DFE based equalizer and MLSE based digital equalizer is made, where 13-tap FFE and 3-tap DFE are required to obtain similar performances with MLSE based detection. In addition, to verify the cost introduced by the ADC, the demand for the ADC parameters in the MLSE based detector, including the sampling rate, resolution, and timing jitter is investigated. Experimental results show that using a 25-GS/s ADC with 4-bit resolution, 25-Gb/s transmission is realized using 10-G TOSA and ROSA, and 28-/30-dB loss budget can be achieved in C-/O-band respectively.