Kazuhito Tajima

Bio: Kazuhito Tajima is an academic researcher from NEC. The author has contributed to research in topics: Optical switch & Mach–Zehnder interferometer. The author has an hindex of 21, co-authored 72 publications receiving 1874 citations.

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

Abstract: 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

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

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

Abstract: 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.

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

Abstract: 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.

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

Abstract: 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.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Limitations on lightwave communications imposed by optical-fiber nonlinearities

Abstract: Optical nonlinearities in the context of lightwave systems limitations are described. The nature and severity of system degradation due to stimulated Raman scattering, carrier-induced phase noise, stimulated Brillouin scattering, and four-photon mixing are discussed. The system power limitations are plotted as a function of a number of wavelength-multiplexed channels. Methods for scaling these results with changes in system parameters are presented. >

Sub-femtojoule all-optical switching using a photonic-crystal nanocavity

Abstract: Although high-speed all-optical switches are expected to replace their electrical counterparts in information processing, their relatively large size and power consumption have remained obstacles. We use a combination of an ultrasmall photonic-crystal nanocavity and strong carrier-induced nonlinearity in InGaAsP to successfully demonstrate low-energy switching within a few tens of picoseconds. Switching energies with a contrast of 3 and 10 dB of 0.42 and 0.66 fJ, respectively, have been obtained, which are over two orders of magnitude lower than those of previously reported all-optical switches. The ultrasmall cavity substantially enhances the nonlinearity as well as the recovery speed, and the switching efficiency is maximized by a combination of two-photon absorption and linear absorption in the InGaAsP nanocavities. These switches, with their chip-scale integratability, may lead to the possibility of low-power, high-density, all-optical processing in a chip. All-optical switching energies as small as 0.42 fJ — two orders of magnitude lower than previously reported — are demonstrated in small photonic crystal cavities incorporating InGaAsP. These devices can switch within a few tens of picoseconds, and may therefore have potential for low-power high-density all-optical processing on a chip.

Semiconductor optical amplifier-based all-optical gates for high-speed optical processing

Abstract: Semiconductor optical amplifiers are useful building blocks for all-optical gates as wavelength converters and OTDM demultiplexers. The paper reviews the progress from simple gates using cross-gain modulation and four-wave mixing to the integrated interferometric gates using cross-phase modulation. These gates are very efficient for high-speed signal processing and open up interesting new areas, such as all-optical regeneration and high-speed all-optical logic functions.

Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm

Abstract: Using a pump–probe method with a 150 fs laser at the wavelength of 1.55 μm, we have experimentally demonstrated that single-walled carbon nanotubes (SWNT) have an exciton decay time of less than 1 ps and a high third-order polarizability, which is reasonably interpreted as due to their azimuthal symmetry. These experimental results reveal that a SWNT polymer composite may be a candidate material for high-quality subpicosecond all-optical switches.