In this paper, we propose a new en-/de-coding scheme of spreading time OCDMA utilizing the dispersion of grating and filter in frequency domain. The key components of the scheme are grating pair, Fourier transformation lens, phase filter etc. The results of design and calculation are shown that the spreading time of input pulse can be up to nanosecond- duration and the phase filter can provide pseudo-noise bursts in time domain. The requirement s for Laser and optical detection equipment in the scheme are reduced. So the new scheme proposed by us is effective for OCDMA en-/de-coding of coherent ultra-short pulses.