Patent ID: 8331785

Claim:
An optical pulse time spreading apparatus which comprises first to U-th optical pulse time spreading units each outputting a chip pulse sequence having N chip pulses from a first to an N-th chip pulse arranged in order on a time axis by time-spreading an input optical pulse, where N is an integer of two or greater and U is an integer of one or greater satisfying that U≦N, wherein each of said first to U-th optical pulse time spreading units comprises a superstructured fiber Bragg grating (SSFBG) comprising N unit fiber Bragg gratings (FBGs); an interval between adjacent ones of said unit FBGs arranged in a p-th optical pulse time spreading unit, where p refers to all integers of from 1 to U, and a Bragg reflection wavelength of said unit FBGs in the p-th optical pulse time spreading unit are set such that spectra of first to U-th chip pulse sequences respectively output from said first to U-th optical pulse time spreading units are different from each other; and the Bragg reflection wavelength λ Bp of the unit FBGs included in said p-th optical pulse time spreading unit is given by the following equations (1 a ) and (1 b ): λ Bp =λ s +k (Δλ s /U )(1 a ) for when U is an odd number, where k is an integer satisfying that |k|<U/2, λ Bp =λ s +(2 k +1)(Δλ s /2 U )(1 b ) for when U is an even number, where k is an integer satisfying that |2k+1|/2<U/2, wherein a Bragg frequency ν Bp corresponding to the Bragg reflection wavelength λ Bp of the unit FBGs included in said p-th optical pulse time spreading unit is set to be within a range to satisfy an inequality given by the following equation (2): (ν s −Δν s /2)<ν Bp <(ν s +Δν s /2) (2) wherein an interval Δν Bp between main peaks in the frequency spectrum of the chip pulse sequence output from said p-th optical pulse time spreading unit is set to be within a range to satisfy an inequality given by the following equation (3): 0<Δν Bp <Δν s (3) and wherein the interval between adjacent ones of the unit FBGs arranged in said p-th optical pulse time spreading unit and the Bragg reflection wavelength of the unit FBGs in the p-th optical pulse time spreading unit are set such that a phase difference Φ between chip pulses reflected from adjacent unit FBGs of said p-th optical pulse time spreading unit is given by the following equation (4): Φ=(2 m+ 1)(λ Bp /2) (4) where λ s is a peak wavelength of the wavelength spectrum of said input optical pulse, Δλ s is the full width at half maximum of the wavelength spectrum of said input optical pulse, ν s is a peak frequency of the frequency spectrum of said input optical pulse, Δν s is the full width at half maximum of the frequency spectrum of said input optical pulse, and m is an integer of zero or greater.