SMZ-type all-optical switches Several types of picosecond-to-femtosecond-class all-optical switchings at repetition frequencies from 10 GHz to over 100 GHz have been successfully achieved in the last 4-5 years with using semiconductor waveguide devices, in spite of the relatively long carrier relaxation times (50 ps to 1 ns) of conventional bulk-semiconductor materials. Such ultrahigh-speed switchings are possible with specific switch structures, i.e. Symmetric-Mach-Zehnder (SMZ)-type switch

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... uctures [1-3], because the slow carrier-relaxation-induced decay components can be completely cancelled out at an optical interference (between two-split mutually-delayed signal components) inside these switches. Both the rise and fall times of their switching windows can basically be shortened with the width of optical input control pulses. SOA SOA phase shifter filter ∆t-delayed Demultiplexed data Input data Divided clock SOA phase shifter Input data pol (−∆t)-delayed ∆t-delayed Local clock filter Regenerated data SOA CW filter ∆t-delayed phase shifter Input data Wavelength-converted data (c) (a) (b) Fig. 1 Three configurations of SMZ-type all-optical switches. (a) original SMZ switch [1] for demultiplexing, (b) PD-SMZ switch [2] for regeneration, and (c) SMZ-DISC [3] for wavelength conversion. Figure 1 shows three configurations of SMZ-type all-optical switches where semiconductor optical amplifiers (SOA's) are used for significantly reducing the optical control pulse energy. In case of the all-optical demultiplexing (a), divided-clock pulses control the switch, form switching windows at the divided-clock frequency, and extract one TDM channel out of the high-frequency data pulses. Error-free demultiplexing of 336 Gb/s has recently been achieved with a divided-clock frequency of 10.5 GHz [4] . In case of regeneration (b) in contrast, each of the high-frequency data pulses controls the switch, forms a switching window, and allows only a corresponding clock pulse to go through. Consequently, the clean clock pulses are encoded by the data pulses, i.e., the data pulses are 'regenerated.' In case of wavelength conversion (c), each of the input data pulses controls the switch, forms a corresponding switching window, and allows only the CW component within each window to go through. When the window width is optimized, data pulses at the CW wavelength are newly generated. t Optical pulses Amplified pulses Holes Electrons Fig. 2 Nonlinear refractive-index changes are induced by stimulated amplification in the SOA's. The driving force of these all-optical switchings is the nonlinear refractive-index change inside the SOA's (Fig. 2) . Each time a control pulse is amplified in an SOA, the carrier density drops almost instantaneously, which causes the refractive index to jump up due to the bandfilling effect. As a result of this ultrafast refractiveindex change and an optical interference, the switch forms a window whose width is determined by a specific delay time ∆t that is indicated respectively in Fig. 1 [1-3 ]. All-optical regeneration (Optical 3R) We have achieved penalty-free error-free inputpolarization-insensitive regeneration at 84 Gb/s [5], where each of the 84-Gb/s input data pulses controlled the SOA inside the PD-SMZ switch and consequently encoded each of the synchronized 84-GHz clock pulses. To date, we have demonstrated two types of regenerative properties. The one is regeneration in time, due to the rectangular-like shape of the switching window [1]; the bit-error-rate tolerance against the input timing jitter has