We first demonstrate a self-pulsation phenomenon in a semiconductor ring laser(SRL). Not only self-mode-locked optical pulse but self-Q-switched optical pulse can be observed in a SRL. Furthermore, experimental results show that the repetition period of the Q-switched optical pulse train can be controlled by the injection current to a SRL.

We fabricated and evaluated a second-order ΣΔ ADC with a polarity alternating feedback (PAF) comparator based on 0.4 µm InGaP/InGaAs enhancement and depletion mode high electron mobility transistors (E/D HEMT) technology. We propose a PAF technique for enhancing the sampling frequency and have applied the technique in the design of ADC circuit. The ADC has a signal-to-noise ratio (SNR) of 43 dB when operating at a differential clock frequency of 4.9 GHz, and has a power dissipation of 400 mW.

Computational sensor (smart sensor, vision chip in other words) is a very small integrated system, in which processing and sensing are unified on a single VLSI chip. It is designed for a specific targeted application. Research activities of computational sensor are described in this paper. There have been quite a few proposals and implementations in computational sensors. Firstly, their approaches are summarized from several points of view, such as advantage vs. disadvantage, neural vs. functional, architecture, analog vs. digital, local vs. global processing, imaging vs. processing, new processing paradigms. Then, several examples are introduced which are spatial processings, temporal processings, A/D conversions, programmable computational sensors. Finally, the paper is concluded.

An optical receiver with a saturated electrical amplifier is studied for signal light that is distorted due to the use of a gain-saturated semiconductor optical amplifier or homowavelength crosstalk light. It is shown that less penalty is induced in a receiver with a DC-coupled saturated amplifier than in one with a linear amplifier, in a practical situation where the decision threshold is fixed at a value optimized for a back-to-back signal. The result suggests that a receiver with a saturated amplifier or a limitter is preferable to an automatic gain control circuit for detecting distorted signal lights.

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 hybrid electrical/optical multi-chip integration technique for optical modules for optical network system has been developed. Employing the technique, a 44 broadcast-and-select type optical matrix switch module has been realized. The module consists of four sets of silica waveguide 1 : 4 splitters/4 : 1 combiners, four 4-channel arrays of polarization insensitive semiconductor optical amplifiers with spot-size converters as optical gates, printed wiring chips for electrical wiring and single mode fibers for optical signal interface on planar waveguide platform fabricated by atmospheric pressure chemical vapor deposition. All the gates and the wiring chips were mounted precisely onto the platform at once in flip-chip manner by self-align technique using AuSn solder bumps. Coupling loss between the waveguide and the SOA gate was estimated to be 4.5 dB. Averaged fiber-to-fiber signal gain, on-off ratio and polarization dependent loss for each of the signal paths was 7 dB 2 dB, more than 40 dB and 0.5 dB, respectively. High speed 10 Gb/s photonic cell switching as short as 2 nsec has been successfully achieved.

Spot-size-converter integrated semiconductor optical amplifiers have been developed as gate elements for optical switch matrices. An S-shape waveguide has been introduced to prevent re-coupling of unguided light to the output fiber. An angled-facet structure effectively suppressed light reflection at the end facets. Consequently, a high extinction ratio of 70 dB and a high fiber-to-fiber gain of 20 dB were achieved. Sufficient optical coupling characteristics to a flat-ended single-mode fiber with a coupling loss of 3.5 dB were also demonstrated.

Based on a previous work on handwritten Japanese kanji character recognition, a postprocessing system for handwritten Japanese address recognition is proposed. Basically, the recognition system is composed of CombNET-II, a general-purpose large-scale character recognizer and MMVA, a modified majority voting system. Beginning with a set of character candidates, produced by a character recognizer for each character that composes the input word and a lexicon, an interpretation to the input word is generated. MMVA is used in the postprocessing stage to select the interpretation that accumulates the highest score. In the case of more than one possible interpretation, the Conflict Analyzing System calls the character recognizer again to generate scores for each character that composes each interpretation to determine the final output word. The proposed word recognition system was tested with 2 sets of handwritten Japanese city names, and recognition rates higher than 99% were achieved, demonstrating the effectiveness of the method.

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.

Spot-size-converter integrated semiconductor optical amplifiers have been developed as gate elements for optical switch matrices. An S-shape waveguide has been introduced to prevent re-coupling of unguided light to the output fiber. An angled-facet structure effectively suppressed light reflection at the end facets. Consequently, a high extinction ratio of 70 dB and a high fiber-to-fiber gain of 20 dB were achieved. Sufficient optical coupling characteristics to a flat-ended single-mode fiber with a coupling loss of 3.5 dB were also demonstrated.

Operation of fiber-grating semiconductor laser (FGL) has been stabilized by using the semiconductor optical amplifier which has a simple slant-waveguide structure. The emission wavelength, which depends on a temperature, shows hysteresis. Employing the directly modulated FGL at 2.5 Gb/s, transmission over 400 km in standard optical fiber has been successfully achieved.

Operation of fiber-grating semiconductor laser (FGL) has been stabilized by using the semiconductor optical amplifier which has a simple slant-waveguide structure. The emission wavelength, which depends on a temperature, shows hysteresis. Employing the directly modulated FGL at 2.5 Gb/s, transmission over 400 km in standard optical fiber has been successfully achieved.

The hybrid electrical/optical multi-chip integration technique for optical modules for optical network system has been developed. Employing the technique, a 44 broadcast-and-select type optical matrix switch module has been realized. The module consists of four sets of silica waveguide 1 : 4 splitters/4 : 1 combiners, four 4-channel arrays of polarization insensitive semiconductor optical amplifiers with spot-size converters as optical gates, printed wiring chips for electrical wiring and single mode fibers for optical signal interface on planar waveguide platform fabricated by atmospheric pressure chemical vapor deposition. All the gates and the wiring chips were mounted precisely onto the platform at once in flip-chip manner by self-align technique using AuSn solder bumps. Coupling loss between the waveguide and the SOA gate was estimated to be 4.5 dB. Averaged fiber-to-fiber signal gain, on-off ratio and polarization dependent loss for each of the signal paths was 7 dB 2 dB, more than 40 dB and 0.5 dB, respectively. High speed 10 Gb/s photonic cell switching as short as 2 nsec has been successfully achieved.

Peculiar patterns of SiO2 contamination around the periphery of the contact trace caused by silicone vapor under switching at the boundary of 1.6 W were confirmed. For micro relays, the electrical power conditions are restricted to lower level. Therefore, it is important to ascertain the upper limit of the electrical power conditions for normal operation. The peculiar pattern is important as it is recognized as the first stage of the origination of contact failure. Causes of this pattern were discussed from the viewpoints of temperature distribution in the contact trace, molten metallic bridge, micro arc discharge, and supply of silicone vapor with oxygen. It is proposed that during the closing contacts, as maximum Joule heating occurs at the periphery of the true contact area and silicone vapor with oxygen is easily supplied at the periphery, SiO2 grows around the contact trace. For the opening contacts, as the bridge or micro arc appears, silicone vapor with oxygen is supplied only outside of the contacts. Thus SiO2 is formed mainly around the periphery of the trace. Moreover, SiO2 was scattered radially depending on the sputtering of molten metal under rupture of the bridge. Therefore, the peculiar pattern forms as a result.

An automatic defect classification system (ADC) for use in visual inspection of semiconductor wafers is introduced. The methods of extracting the defect features based on the human experts' knowledge, with their correlations with the defect classes are elucidated. As for the classifier, Hyperellipsoid Clustering Network (HCN) which is a layered network model employing second order discrimination borders in the feature space, is introduced. In the experiments using a collection of defect images, the HCNs are compared with the conventional multilayer perceptron networks. There, it is shown that the HCN's adaptive hyperellipsoidal discrimination borders are more suited for the problem. Also, the cluster encapsulation by the hyperellipsoidal border enables to determine rejection classes, which is also desirable when the system will be in actual use. The HCN with rejection achieves, an overall classification rate of 75% with an error rate of 18%, which can be considered equivalent to those of the human experts.

Input-wavelength-insensitive tunable wavelength conversion was achieved in the range of 1530 to 1560 nm using cascaded semiconductor laser wavelength converters (a DFB laser and an SSG-DBR laser). The power penalty in the wavelength conversion of input signal between 1530 and 1555 nm, where the wavelength ranged between 1537 and 1557 nm, is less than 1 dB for 5 Gbit/s signals.

We studied 2 types of polarization insensitive semiconductor optical amplifier (SOA) gates for use in wavelength division multiplexing (WDM) applications: 1) a low operation current SOA gate with a small and square bulk active region but without spot-size converters and 2) a multi channel SOA gate array with tapered waveguide spot-size converters (SS-SOA) on both sides. The low operation current SOA gate provided a very low current for fiber-to-fiber loss-less operation (5. 4-7. 0 mA) and a high extinction ratio (>30 dB) over a wide wavelength range (1530-1580 nm). For multi channel array assembling, the SS is indispensable. The 4-channel SS-SOA gate array was assembled on a planar lightwave circuit (PLC) platform for the first time. The gain characteristics of each channel were very similar and a low fiber-to-fiber loss-less current of 33 mA and a high extinction ratio of nearly 40 dB were achieved in all channels. The polarization dependence was less than 1 dB. Using the fully packaged 4-channel hybrid gate array module (a 4 channel SS-SOA on PLC platform), an ultra-wide-band (1530-1600 nm) high speed wavelength selector was successfully demonstrated. Both rise- and fall-times were less than 1 ns, which makes the wavelength selector suitable for high-speed optical packet switching. Electrical and optical interference between channels were negligible.

Wavelength tunable laser diodes are critical components in a wide variety of WDM and packet switching architectures. And also wavelength-tuned short pulses generated from the semiconductor laser diodes are of great importance for the developments of ultrahigh speed and WDM optical communication systems. Over the past several years, both continuously and discontinuously tunable lasers incorporating periodically sampled and chirped grating have been studied theoretically and experimentally. These laser diodes show the wide tuning range of above 60 nm, stable lasing condition, and large side-mode suppression ratio. Directly modulated semiconductor laser diodes, even those with a single mode, exhibit a dynamic frequency chirp during the on/off modulation. The dynamic linewidth broadening caused by such a large frequency chirp can result in a significant penalty in the performance of high-speed long-haul optical communication systems. The CW laser diodes integrated with an external EA modulator are an breakthrough to realize the high-speed optical systems with low chirp. And also the short pulse generation using the external modulator has been realized experimentally, whose principle of the pulse generation is the optical gating of the electroabsorption modulator. In this paper, widely tunable laser diodes incorporating periodically sampled and chirped gratings and an external modulator are analyzed using an improved time-domain dynamic model. First, it is demonstrated that the improved model is very powerful in simulating the complex laser diodes with active and passive sections. And, the dynamic properties of the sampled grating DBR and chirped grating DBR laser diodes are investigated. Second, the modulation characteristics of the laser diode integrated with the external electroabsorption modulator are studied. It is shown that the external modulation are superior to the direct modulation in the aspect of the lower frequency chirp. And the pulse generation by the optical gating of the external modulator is observed theoretically.

The chirp noise effect in a directly modulated semiconductor laser diode (LD) is experimentally studied. A previous theoretical study reported that, when an LD is directly modulated, turn-on jitter caused by spontaneous emission, combined with chromatic dispersion, becomes a source of noise in fiber transmission and restricts system performance. This paper points out that, on the contrary, imperfection in LD driving circuits causes chirp noise and limits transmission performance in actual systems. Experiments regarding dependence of chirp noise on LD modulation conditions are also presented, which show that a high relaxation oscillation frequency and a short turn-on delay time are preferable from the viewpoint of chirp noise.

Theoretical calculations of the pulsing operation and the intensity noise under the optical feedback are demonstrated for operation of the self-sustained pulsation lasers. Two alternative models for the optical feedback effect, namely the time delayed injection model and the external cavity model, are applied in a combined manner to analyze the phenomena. The calculation starts by supposing the geometrical structure of the laser and the material parameters, and are ended by evaluating the noise. Characteristics of the feedback induced noise for variations of the operating parameters, such as the injection current, the feedback distance and the feedback ratio, are examined. A comparison to experimental data is also given to ensure accuracy of the calculation.