PROBLEM TO BE SOLVED: To sharpen a pulse by a mode locker by setting the length of a ring resonator to be one which is determined by a pulse repetition frequency and central wavelength of a generated pulse strain and a mean value of the group velocity distribution of the ring resonator. ;SOLUTION: Optical components such as an optical fiber coupler 10, a mode locker 12, an optical isolator 14, an erbium-doped optical fiber 16, a WDM coupler 18 are connected in the shape of a ring by means of an optical fiber 20. The length of this ring resonator is set to a value which is integral number of times as long as a distance Zperi determined by a formula Zperi = 2c/(λ2|D|f2), where 'c' is the speed of light in vacuum, λ is the central wavelength, |D| is the absolute value of the mean value of the group velocity distribution, 'f' is a pulse repetition frequency. When the length of the ring resonator is an integral number of times as long as the distance Zperi, the shift of a phase in each mode due to the group velocity distribution becomes equivalently zero. As a result, a light pulse output from a mode locker 12 returns to the original waveform after circulating in the ring resonator and then is input into the mode locker again and thereby the pulse can be effectively sharpened by the mode locker.;COPYRIGHT: (C)1999,JPO
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