The purpose of this work is to study the evolution of ultrashort pulses in mode-locked pulse trains generated by a Nd: glass laser (at 1.6 microns) mode locked by a saturable absorber. Using an infra-red sensitive image converter camera, streaked interference fringes and hence the fringe visibility curves have been obtained for the interference of picosecond pulses separated by zero, one, two and three cavity transit times. The pulses under observation were in the central region of the pulse-trains, typically 1-15 nanoseconds wide. The shape of the fringe visibility curves and the decrease in the region of coherence with the increase in the separation in the pulse train, of the pulses that overlap to give the interference fringes is a good measure of the correlation of the pulses in that region. Further information on the type of pulses being investigated by the interferometric method was obtained by monitoring the time and spectral structure of the pulse trains. This was achieved by using two-photon-fluorescence, a 3.6 meter diffraction spectrometer and a fast photodiode-oscilloscope combination. The main features of the experiment are: (1) a 17 meter optical delay path (55 nanoseconds) which enabled observation in the central region of the pulse train, and (2) an experimental set up that allowed the observation of (a) streaked interference fringes (b) two-photon-fluorescence (c) the spectrum and (d) the intensity profile simultaneously, and for every single shot of the laser.