Optical bistabilities (multistabilities),which are due to the carrier-dependent refractive index of laser diodes (LD),observable on the output curves of tunable external cavity (EC) semiconductor lasers have been studied.As a result,various hysteresis loops with different shapes appearing on curves of N-ν (carrier density-oscillation frequency of external cavity semiconductor lasers) have been analyzed,and corresponding hysteresis loops on (power-frequency) curves have been also calculated.The expression relating reflectivities and spectral broadening factor,for observing the multistability on the N-ν(or P-ν) curves of ECLDs,has been established.