This paper reviews our recent activities on nanophotonics based on a photonic crystal (PC)/quantum dot (QD)-combined structure for an all-optical device and a metal/semiconductor composite structure using surface plasmon (SP) and negative refractive index material (NIM). The former structure contributes to an ultrafast signal processing component by virtue of new PC design and QD selective-area-growth technologies, while the latter provides a new RGB color filter with a high precision and optical beam-steering device with a wide steering angle.

We present an 88 wavelength-routing switch (WRS) that monolithically integrates tunable wavelength converters (TWCs) and an 88 arrayed-waveguide grating. The TWC consists of a double-ring-resonator tunable laser (DRR TL) allowing rapid and stable switching and a semiconductor-optical-amplifier-based optical gate. Two different types of dry-etched mirrors form the laser cavity of the DRR TL, which enable integration of the optical components of the WRS on a single chip. The monolithic WRS performed 18 high-speed wavelength routing of a non-return-to-zero signal at 10 Gbit/s. The switching operation was demonstrated by simultaneously using two adjacent TWCs.

We propose a new label recognition system for photonic label switching using self-routing of labels. Binary-coded labels in on-off keying format are considered. The system consists of an all-optical demultiplexer (DeMUX) and an address recognition unit (ARU) consisting of tree-structured switches. The system uses self-routing propagation of an indication bit controlled with address bits. The indication bit is placed in advance of the address bits in the label. In DeMUX, all-optical switches in a configuration of Mach-Zehnder interferometer with semiconductor optical amplifiers (SOA-MZI) are controlled by the indication bit pulse to separate each of the label bits. The indication bit pulse is routed to the destination output port corresponding to the code of the address in ARU. It is shown that all the binary number codes can be recognized with this system. The operation principle is verified by numerical simulation using coupled-mode theory and a rate equation. Moreover, the switching crosstalk is also evaluated.

We achieved first dynamic all-optical signal processing with a bandgap-engineered MZI SOA all-optical switch. The wide-gap Selective Area Growth (SAG) technique was used to provide multi-bandgap materials with a single step epitaxy. The maximum photoluminescence (PL) peak shift obtained between the active region and the passive region was 192 nm. The static current switching with the fabricated switch indicated a large carrier induced refractive index change; up to 14 π phase shift was obtained with 60 mA injection in the SOA. The carrier recovery time of the SOA for obtaining a phase shift of π was estimated to be 250-300 ps. A clear eye pattern was obtained in 2.5 Gbps all-optical wavelength conversion. This is the first all-optical wavelength conversion demonstration with a bandgap-engineered PIC with either selective area growth or quantum-well intermixing techniques.

Symmetric Mach-Zehnder (SMZ) type all-optical swit-ches are discussed. The SMZ type all-optical switches feature the so-called differential phase modulation scheme to achieve a speed unrestricted by efficient, thus usually slow nonlinearities. In these switches, semiconductor optical amplifiers (SOAs) are often used to realize low optical power switching. We discussed SOAs from a view point of all-optical switch applications, rather than amplifier applications. Finally, all-optical signal processing experiments are discussed with the SMZ type all-optical switches. These include ultrafast demultiplexing of 336 Gb/s signal pulses and random operations at 42 Gb/s for all-optical logic operation and wavelength conversion.

We investigate the engineering of the impurity bands in photonic crystals (PCs) for realizing high-efficiency wave guiding, all-optical switching and optical delay for ultrashort optical pulses. It is found that quasi-flat impurity bands suitable for the transmission of ultrashort pulses can be achieved by properly controlling the configuration of coupled cavity waveguides (CCWs). At sharp corners, high bending efficiency is obtained over the entire impurity band. All-optical switching can be realized by creating a dynamical band gap at the center of an impurity band. The concentration of electromagnetic wave at defect regions leads to high switching efficiency while the tunable feature of PC defects makes all-optical control possible. It is also revealed that CCWs with quasi-flat impurity bands provide efficient group delay for ultrashort pulses with negligible attenuation and distortion. From the viewpoint of practical fabrication, the effect of disorder on the transmission property of impurity bands is discussed and the criterion for localization transition is determined.

By combining the technology of all-optical saturable absorbers and the diffractive optics, a scheme of all-optical time division demultiplexing module is investigated. Following authors' proposal, design, test fabrication of the optical platform in the previous paper, this paper focuses on the characterization of switching performance. Using a multiple quantum well saturable absorber of InGaAs/InAlAs composition, and gain switched semiconductor laser pulses of 25 ps pulse width, the switching function was demonstrated experimentally at wavelength of 1.55 µm. The switching on-off ratio was compared among 4 lens configuration, 2 lens configuration (2L) and free space, collinear geometry. No degradation was observed in the case of 2 lens configuration in comparison to collinear illumination. Thus the feasibility of all-optical switch module with power efficiency and high speed is predicted, under the assumption of the progress in sub-micron lithography.

A newly developed hybrid-integrated Symmetric Mach-Zehnder (HI-SMZ) all-optical switch is reported. For integration, we chose the Symmetric Mach-Zehnder (SMZ) structure rather than the Polarization-Discriminating Symmetric Mach-Zehnder (PD-SMZ) structure which is similar to SMZ but more often used in experiments using discrete optical components. We discuss advantages and disadvantages of SMZ and PD-SMZ to show that SMZ is more suitable for integration. We also discuss about the use of SOAs as nonlinear elements for all-optical switches. We conclude that, although the ultrafast switching capability of SMZ is limited by the gain compression of SOAs, the very low switching energy is more important for practical devices. We then describe the HI-SMZ all-optical switch. This integration scheme has advantages which include low loss, low dispersion silica waveguides for high speed operation and ease in large scale integration of many SMZs with other optical, electrical, and opto-electrical devices. We show that a very high dynamic extinction ratio is possible with HI-SMZ. We also examine HI-SMZ with 1 ps pulses to show its ultrafast capability. Finally, we describe a 168 to 10.5 Gbps error-free demultiplexing experiment which is to our best knowledge the fastest experiment with an integrated device.

The tera-bit order capacity of ultrahigh-speed and wide-band networks will become necessary to provide highly advanced multimedia services. In conventional networks, electronic circuits limit the speed capability of the networks. Consequently, all-optical networks are essential to realize ultrahigh-speed and wide-band communications. In this paper, we propose the configuration of an all-optical code division multiplexing (CDM) switching network based on self-routing principles and the structure of a nonlinear all-optical switching device as one of the key components for the network. We show that the required performances of the optical devices used in the CDM switching fabric are lower than those used in the TDM and illustrate the basic transmission characteristics of the switching device utilizing FD-BPM. To evaluate the multiplexing performance, we demonstrate the maximum number of channels under an error-free condition and the BER characteristics when the Gold sequence is applied as one of the CDM code sets, and show that the network of the sub-tera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. We also illustrate the packet assembly method suitable for self-routing transmissions and one of network architectures where the proposed switching fabric can be exploited.

We propose novel all-optical functional devices using waveguide X-junctions with localized third order optical nonlinearity, where one branch is made from a Kerr-like nonlinear material and the rest are made from linear ones. All-optical switching operations can be obtained because of bistable like nonlinear dispersion characteristics in linear and nonlinear coupled guided-wave systems. The performances of the devices are analyzed by the Beam Propagation Method (BPM) modified for nonlinear waveguides combined with the nonlinear normal mode analysis. The methods to construct the waveguides with localized nonlinearity are also discussed by utilizing the technologies for the selective control of a band-gap energy of semiconductor Multi Quantum Well (MQW) structures and the performances of the designed devices are presented.

We experimentally demonstrate the ultrafast and high-repetition capabilities of a polarization-discriminating symmetric Mach-Zehnder (PD-SMZ) all-optical switch. This switch, as well as an original symmetric Mach-Zehnder (SMZ) all-optical switch, is based on a highly efficient but slowly relaxing band-filling effect that is resonantly excited in a passive InGaAsP bulk waveguide. By using a mechanism that cancels out the effect of the slow relaxation, ultrafast switching is attained. We achieve a switching time of 200 fs and demultiplexing of 1.5 Tbps, showing the applicability of the SMZ or PD-SMZ all-optical switches to optical demultiplexing of well over 1 Tbps for the first time. High-repetition capability, which is another important issue apart from the switching speed, is also verified by using control pulses at a repetition rate of 10.5 GHz. We also discuss the use of nonlinearity in a semiconductor optical amplifier to further reduce the control-pulse energy.

The tera-bit order capacity of ultrahigh-speed and wide-band networks will become necessary to provide highly advanced multimedia services. In conventional networks, electronic circuits limit the speed capability of the networks. Consequently, all-optical networks are essential to realize ultrahigh-speed and wide-band communications. In this paper, we propose the configuration of an all-optical code division multiplexing (CDM) switching network based on self-routing principles and the structure of a nonlinear all-optical switching device as one of the key components for the network. We show that the required performances of the optical devices used in the CDM switching fabric are lower than those used in the TDM and illustrate the basic transmission characteristics of the switching device utilizing FD-BPM. To evaluate the multiplexing performance, we demonstrate the maximum number of channels under an error-free condition and the BER characteristics when the Gold sequence is applied as one of the CDM code sets, and show that the network of the sub-tera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. We also illustrate the packet assembly method suitable for self-routing transmissions and one of network architectures where the proposed switching fabric can be exploited.

We experimentally demonstrate the ultrafast and high-repetition capabilities of a polarization-discriminating symmetric Mach-Zehnder (PD-SMZ) all-optical switch. This switch, as well as an original symmetric Mach-Zehnder (SMZ) all-optical switch, is based on a highly efficient but slowly relaxing band-filling effect that is resonantly excited in a passive InGaAsP bulk waveguide. By using a mechanism that cancels out the effect of the slow relaxation, ultrafast switching is attained. We achieve a switching time of 200 fs and demultiplexing of 1.5 Tbps, showing the applicability of the SMZ or PD-SMZ all-optical switches to optical demultiplexing of well over 1 Tbps for the first time. High-repetition capability, which is another important issue apart from the switching speed, is also verified by using control pulses at a repetition rate of 10.5 GHz. We also discuss the use of nonlinearity in a semiconductor optical amplifier to further reduce the control-pulse energy.

We propose novel all-optical functional devices using waveguide X-junctions with localized third order optical nonlinearity, where one branch is made from a Kerr-like nonlinear material and the rest are made from linear ones. All-optical switching operations can be obtained because of bistable like nonlinear dispersion characteristics in linear and nonlinear coupled guided-wave systems. The performances of the devices are analyzed by the Beam Propagation Method (BPM) modified for nonlinear waveguides combined with the nonlinear normal mode analysis. The methods to construct the waveguides with localized nonlinearity are also discussed by utilizing the technologies for the selective control of a band-gap energy of semiconductor Multi Quantum Well (MQW) structures and the performances of the designed devices are presented.

A novel series-tapered nonlinear directional coupler is proposed to improve all-optical switching characteristics. Its switching characteristics are analyzed by using a beam propagation method based on the Galerkin's finite element technique. It is presented that the critical power of the series-tapered nonlinear directional coupler is smaller than conventional uniform symmetric and tapered nonlinear directional couplers.

We analyze all-optical switching property of a nonlinear directional coupler (NLDC) having an MQW coupling layer with both nonlinear and linear losses, and examine the effect of nonlinear losses. We use the Galerkin finite element method accompanied by a prodictor-corrector algorithm. The propagation loss along the strongly-coupled NLDC decreases with increasing nonlinear absorption coefficient due to saturation in absorption. A propagation loss of 8.18 dB or 2.38 dB in the bar state of the cross state is much smaller than the bulk loss of MQW structure which exceeds 50 dB. The nonlinear losses lengthen the coupling length and bring it close to that of a lossfree NLDC, while the linear losses shorten. It is found that the property of the cross state is greatly improved by counting the nonlinear losses: The cross-state output power and the output power ratio of two waveguides increase, and the cross state input power, that is, the switching power decreases.

For an application to the optical signal processing devices, we propose the optical X coupler which consists of two bending waveguides and a nonlinear dielectric region. To analyze this structure accurately we utilized the iterative finite difference beam propagation method (iterative FD-BPM). In this paper the formulation of the iterative FD-BPM for one wave and two waves cases are presented, respectively. We investigate following two cases. First, we consider the case that the light is launched into one of the input ports. We calculate the evolutions of the field amplitude and the transmission characteristics for the input power. Second, we consider the case that the signal light with the constant power is launched into one of the input ports and that the control light with the wavelength different from that of the signal light is launched into another input port. We calculate the evolutions of the field amplitude and the transmission characteristics of the signal light for the power of control light. As a result of the analysis, we show that all-optical switching operation is possible in the proposed structure.

A novel nonlinear directional coupler consisting of tapered and uniform waveguides with self-focusing or self-defocusing nonlinear material is proposed to improve all-optical switching characteristics. Its switching characteristics are analyzed by using a beam propagation method based on the Galerkin's finite element technique (FE-BPM), in which nonuniform sampling spacings along the transverse coordinate are adopted. It is presented that the tapered nonlinear directional coupler shows fairly distinct 'high' and 'low' states of output power with steep transition versus input power. This property is discussed in comparison with conventional nonlinear directional couplers consisting of uniform symmetric and uniform asymmetric coupled waveguides. In addition, the effects of loss on the characteristics of tapered nonlinear directional coupler are examined.

Optical Kerr effect were applied to all-optical switching devices in the form of nonlinear waveguide directional couplers. The nonlinear waveguide directional coupler consists of a quartz thin gap between two Corning 7059 guided layers on a pyrex substrate with ion-milled grating and organic thin film as a top layer. The vacuum-deposited polydiacetylene (12, 8) film was used as an organic nonlinear material. Power-dependent switching phenomenon in this asymmetrical nonlinear directional coupler was observed by 100 fs pulse duration of mode-locked Ti: Sapphire laser.

We consider applications of optical solitons to signal processing. Soliton switching devices promise ultrafast operation and compatibility with communications systems using optical pulses. Quantum soliton effects include broadband squeezing and quantum nondemolition measurements, and can reduce noise and increase sensitivities of optical measurements. We report the demonstration of two-color soliton switching and describe progress towards implementation of quantum nondemolition measurement of photon number using soliton collisions.