Kazuhito Tajima

Input power limits of single-mode optical fibers due to stimulated Brillouin scattering in optical communication systems

Stimulated Brillouin scattering (SBS) limits the optical power that can be transmitted through a single-mode fiber in long-distance optical communication systems, the authors have investigated SBS gain and threshold characteristics with amplitude-shift-keying (ASK), frequency-shift-keying (FSK), and phase-shift-keying (PSK) modulated lights to estimate the input power limitation set by SBS. It was shown that maximum fiber-input powers or the SBS thresholds for fixed-pattern (1010 . . .) ASK, FSK, and PSK modulated lights are 2, 4, and 2.5 times higher, respectively, than the threshold for unmodulated light. Theoretical predictions were experimentally verified by SBS gain measurements with FSK and PSK modulated lights. The first direct observation of SBS with FSK modulated light pumping is also described. >

168-Gb/s all-optical wavelength conversion with a symmetric-Mach-Zehnder-type switch

Error-free all-optical wavelength conversion at 168 Gb/s, which is the highest repetition rate ever reported, has been achieved by using a symmetric-Mach-Zehnder (SMZ)-type switch. Low-power-penalty 84-Gb/s operation is also demonstrated. The push-pull switching mechanism of the SMZ switch enables such ultrafast operation based on cross-phase modulation associated with the carrier depletion in a semiconductor optical amplifier. The configuration of the delayed-interference signal-wavelength converter, which is a simplified variant of the SMZ switch, is used in this experiment.

Compensation of soliton broadening in nonlinear optical fibers with loss

A novel optical fiber is proposed that supports the lowest-order soliton despite the presence of optical loss. Groupvelocity dispersion of this fiber decreases with distance, in accord with soliton attenuation that is due to the inherent optical loss of the fiber.

Demultiplexing of 168-Gb/s data pulses with a hybrid-integrated symmetric Mach-Zehnder all-optical switch

We have developed a hybrid-integrated symmetric Mach-Zehnder all-optical switch and evaluated the demultiplexing of 168-Gb/s data pulses at a repetition rate of 10 GHz with this switch. A compact, stable device was realized by assembling semiconductor optical amplifiers as nonlinear waveguides on a planar lightwave circuit in a self-aligned manner. A 6.0-ps switching window needed for 168-Gb/s demultiplexing was provided by the push-pull operation of the symmetric Mach-Zehnder all-optical switch. Demultiplexed signal light showed a high extinction ratio of better than 18 dB. Error-free demultiplexing with a bit error rate of 10/sup -11/ was achieved.

3.8-THz wavelength conversion of picosecond pulses using a semiconductor delayed-interference signal-wavelength converter (DISC)

A new all-optical semiconductor-band-filling-based wavelength converter, named delayed-interference signal-wavelength converter (DISC), is proposed. Its speed is not restricted by the carrier lifetime and its structure is very simple: it consists of only two essential components, namely, a semiconductor optical amplifier and a passive split-delay. Using this converter, 3.8-THz-shifted (from 1530 to 1560-nm) 14-ps-long pulses are generated from 1530-nm 140-fJ 0.7-ps pulses with high-conversion efficiency.

Nonlinear phase shifts induced by semiconductor optical amplifiers with control pulses at repetition frequencies in the 40–160-GHz range for use in ultrahigh-speed all-optical signal processing

In a semiconductor optical amplifier (SOA) with copropagating optical pump pulses, the application of a nonlinear phase shift to optical signals provides the driving force for all-optical interferometric switching. We study, both analytically and experimentally, the dependencies of the nonlinear phase shift on the driving frequency (42–168 GHz) and on the SOA parameters. We have found that the nonlinear phase shift (ΔΦNL) decreases with the driving frequency but that this decrease is only linear, i.e., ΔΦNL∝f-1. We have also found that the nonlinear phase shift in the SOA linearly increases with the injection current (Iop), i.e., ΔΦNL∝Iop, even in this ultrahigh-frequency range.

Penalty-free error-free all-optical data pulse regeneration at 84 Gb/s by using a symmetric-Mach-Zehnder-type semiconductor regenerator

Penalty-free data-pulse regeneration at 84 Gb/s was achieved down to an error rate level of 1/spl times/10/sup -11/ by using a data pattern length of 2/sup 31/-1. A symmetric-Mach-Zehnder-type all-optical polarization-insensitive semiconductor regenerator was used.

EXPERIMENTAL INVESTIGATION ON HIGH-SPEED SWITCHING CHARACTERISTICS OF A NOVEL SYMMETRIC MACH-ZEHNDER ALL-OPTICAL SWITCH

We report the experimental results of a novel symmetric Mach–Zehnder (SMZ) all‐optical switch. Although the switching is based on the band‐filling effect in the semiconductor waveguide, the switching speed is not restricted by the slow relaxation time. We obtain fully switched, nearly square modulation, which is characteristic of the SMZ all‐optical switch. We also achieve high‐speed switching; the full width at half‐maximum of the obtained wave form is 8 ps, which is only limited by the time resolution of the detection system.

Ultrafast polarization‐discriminating Mach‐Zehnder all‐optical switch

We propose a polarization‐discriminating Mach‐Zehnder (PDMZ) all‐optical switch. The switching speed of this PDMZ all‐optical switch is not limited by the slow relaxation time of highly efficient incoherent nonlinearities. We demonstrate a squarelike modulation characteristic, which is necessary in most switching applications, at a switching speed (on‐off time) of 40 ps. We also demonstrate ultrafast switching at a detector limited speed of ∼8 ps.

Femtosecond switching with semiconductor-optical-amplifier-based Symmetric Mach–Zehnder-type all-optical switch

We investigate the effect of intraband carrier dynamics on a nonlinear phase shift induced in a semiconductor optical amplifier (SOA) in terms of its applicability to the Symmetric Mach–Zehnder (SMZ) all-optical switch. Nonlinear phase shifts in an SOA and a passive semiconductor waveguide are compared under control-pulse durations ranging from 3.2 to 0.4 ps. The results show that femtosecond switching with higher efficiency is still possible by using the SOA. We experimentally achieve femtosecond (670 fs), femtojoule (140 fJ) switching with the SOA-based SMZ all-optical switch.