In this paper we have developed an ultrafast pulse generator to turn on a Single Photon Avalanche Diode (SPAD) sensor with picosecond resolution, at specific time windows. The pulse amplitude and duration is user tunable to provide a variable excess bias voltage to the detector. The transition times (rise and fall time) of the pulse is in the range of 250 to 550 ps depending on the amplitude. The generator was designed to be applied in reflectance optical spectroscopy measurements where the diffusive medium is illuminated from a point source and diffused photons are collected at a given distance from the source. To increase the sensitivity to higher penetration depth of investigation, the source and the detector separation should be small which in turn increases the number of unwanted early arriving photons. This system will keep the detector off for the first 500-600ps, thus rejecting the early arriving photons, and will only turn-ON the detector when it is expected to detect the late photons coming from the deeper regions. The proposed fully CMOS integrated system is the first of its kind to be introduced for the ultra-fast gating of the single photon detectors.