The nonlinear coupled-mode equations are rewritten by even and odd modes. We study modulation instability of photonic crystal fiber couplers when either even or odd mode is launched alone. The result shows that there are new types of modulation instability in both the normal and the anomalous-dispersion regimes. Modulation instability is related with four phase dispersion and has no relation with three phase dispersion. The gain spectra have been given as function of input power in various dispersion regimes. Quasi-cw can be changed into pulse array under certain conditions, and we can extract super-short pulses from it.