Self-phase modulation spectral patterns are studied through numerical calculations in the optical fiber for input signals formed by a superposition of a pair of pulses of half-width greater than similar to 40 ps, when the group velocity dispersion effect on the self-phase modulation can be neglected. The self-phase-modulated output from the optical fiber is found to show distinctive spectral patterns of varying side lobes depending on the phase shifts between the input pulse pairs. A possible application of this observation as a new method of phase shift keying modulation is suggested.