We present a method for synthesizing complex fiber Bragg gratings using femtosecond UV laser pulses and image projection from a small phase mask. Fiber gratings with arbitrary spectral reflectivity can be achieved by stitching short grating segments with proper phases. The relative phase between neighboring sub-gratings is controlled using in situ UV-excited fluorescence monitoring. During the fabrication, we are able to monitor the amplitude and phase of each segment right after it was written. This is accomplished by scanning the phase mask with attenuated UV laser pulses and observing modulation in fluorescence. This information allows us to precisely set the position of fiber for the next segment. A fabricated grating segment can also be effectively erased with additional out-of-phase UV exposure. Bragg gratings over both telecom C-band and L-band can be conveniently achieved with this simple setup and a single phase mask.