Pseudo millimeter wave ultra wide band signals have the characteristics of narrow pulse width, low signal strength , which make it is not easy to detect. To deal with such problem, two optics assisted envelope detection schemes were proposed. Firstly, a fiber Bragg grating was used for filtering one of the first sidebands at the output of the modulator, then the envelope of PMM-UWB signals for decision were obtained after photo-detection and low pass filter. Based on the method of mathematical derivation and numerical simulation, the effects of phase modulator or intensity modulator on the output envelope signal amplitude were analysed respectively. The results show that intensity modulator requires bias control to maintain a stable optical operating point, when the bias point of intensity modulator was switched to minimum transmission point, the maximum amplitude of envelope signals were acquired;phase modulator do not include the bias control, the amplitude of envelope in phase modulator is equal to the maximum amplitude in intensity modulator approximately. Compared with intensity modulation scheme, the phase modulation scheme has more advantages, such as simple structure, low insertion loss.