Brain-computer interfaces (BCI) based on Steady State Visual Evoked Potential (SSVEP) can provide higher information transfer rate and require shorter calibration than BCIs based on other modalities. For safety and comfort, the frequency of the repetitive visual stimuli seliciting the SSVEP should be higher than 30 Hz. However, in such frequency range, only a limited number of frequencies can elicit sufficiently strong SSVEPs for BCI purposes. Thus, the conventional approach, consisting in presenting various repetitive visual stimuli at different frequencies, is not feasible for high frequencies. Indeed this would bring low communication bitrates. To increase the number of possible repetitive visual stimuli, we consider modulating the phase of the stimulus instead of the frequency. In this paper, we present an approach to reliably detect the stimulus phase from the recorded SSVEP.