1.  Field
The present invention relates to an optical waveform reshaping device. 
2.  Description of Related Art
Toward future implementation of large-capacity photonic networks, there is demand for a technique for reproducing waveforms degraded due to dispersion of optical fibers, noise from repeating amplifiers, and the like, directly in optical form and such techniques have been studied actively. 
Such techniques are generally referred to as 2R (Reamplification and Reshaping), or as 3R (Reamplification, Reshaping, and Retiming) including retiming added thereto. 
Regarding conventional waveform reshaping techniques by means of optical semiconductor devices, there has been proposed a technique which uses monolithic integration of a semiconductor optical amplifier (SOA) with an optical waveguide in conjunction with a Mach-Zehnder (MZ) interferometer (see FIG.  11A), symmetric Mach-Zehnder (SMZ) interferometer (see FIG.  11B), or delay interferometer (see FIG.  11C).  A case is reported in which a signal in a range of 40 to 80 Gb/s has been reproduced using devices such as described above. 
Regarding techniques using single devices, for example, as shown in FIG.  11D, a gain saturation technique has been proposed which suppresses level-1 (ON level) noise using gain saturation of SOA. 
Besides, an SOA with a bulk active layer or quantum well active layer has low gain response speed, which restricts a bit rate significantly, and it has been suggested that level-0 (OFF level) and level-1 (ON level) noises in a 40-Gb/s signal can be compressed if saturable absorbers are integrated using quantum dots with high gain response speed in the active layer. 
Furthermore, since a single amplifier stage and single saturable absorption portion stage do not provide sufficient nonlinearity, an optical nonlinear amplifier device has been proposed in which amplifiers and saturable absorption portions are arranged in multiple stages.