The novel application potential of mode-locked laser diodes (MLLDs) in ultrafast optical signal processing in addition to coherent optical pulse generation is described. As the most fundamental function of MLLDs, we show that the generation of ultrashort (2 ps) coherent optical pulses with low timing jitter (<0. 5 ps) at precisely controlled wavelength and repetition frequency can be achieved by employing a rigid module configuration for an external-cavity MLLD. We then discuss new aspects of MLLDs which are functions of ultrafast all-optical signal processing such as optical clock extraction and optical gating. All-optical clock extraction is based on the timing synchronization of MLLD output to the injected optical data pulse. When the passive mode-locking frequency of an MLLD is very close to the fundamental clock pulse frequency of optical data, the former frequency is pulled into the latter frequency by optical data injection. We show that same-frequency and subharmonic-frequency optical clock pulses can successfully be extracted from optical data pulses at bit rates of up to 80 Gbit/s with very simple configurations and very low excess timing jitter (<0. 1 ps). On the other hand, optical gating is due to absorption saturation and the following picosecond absorption recovery in a saturable absorber (SA) in an MLLD structure incorporating optical gate-pulse amplification. Here, MLLDs are anti-reflection coated and used as traveling wave devices instead of laser oscillators, and small saturation energy (<1 pJ) and ultrafast recovery time (<8 ps) are demonstrated. By combining all these MLLD functions, we successfully demonstrated an experiment with 40- to 10-Gbit/s all-optical demultiplexing processing.

In this paper, we describe the properties of an external cavity modelocked semiconductor laser with a tunability of wavelength, pulse width and repetition rate. This modelocked laser generates optical pulses with pulse widths down to 180 fs and with repetition rates up to 14 GHz in a 120 nm wavelength range near 1. 55 µm or 1. 3 µm. The generated pulses are close to the transform limit and are therefore suitable for very high speed communication systems. In addition to the tunability, this pulse source is a compact and mechanically stable device. We report on two applications of this pulse source in optical time division multiplexing experiments. In the first example the modelocked laser is used as an all-optical clock recovery. In the second example the modelocked laser was used to characterize an interferometric switch by pump-probe experiments.

The semiconductor lasers operating with self-sustained pulsation are under developing to be lasers which are less disturbed by the optical feedback from a surface of optical disk. Structures setting saturable absorbing regions utilizing the multi-layer configuration become popularly used for giving stronger pulsation. However, the quantum (intensity) noise in these lasers tends to be enhanced. The ridge stripe structure, of which almost self-sustained pulsation lasers consist, seems to give a leak current flowing along plane of the cladding region. Such leak current also increases the quantum noise. In this paper, theoretical calculations of operating characteristics, such as the self-sustained pulsation, the optical output, the quantum noise as well as the transverse filed profile, are theoretically analyzed by including the above mentioned several phenomena.

We propose a novel InGaAsP semiconductor laser which theoretically exhibits a high differential gain. The proposed semiconductor laser contains an asymmetric double quantum well structure as the active region. The differential gain enhancement invokes resonant tunneling of heavy holes in the asymmetric double quantum well structure, which takes place on the way of carrier injection process. The proposed laser is expected to be far more efficient in reducing pulse width and spectral broadening (chirping) than conventional multiquantum well lasers when driven by the gain switching method.

This paper summarizes our recent efforts in modelocking Ti:sapphire lasers with semiconductor saturable absorber mirrors (SESAMs). We present the shortest optical pulses ever generated directly from a laser. The modelocking build-up time (T BU) of 60 µs is, to our knowledge, the shortest reported for a passively modelocked KLM laser to date.

We propose a mechanically tunable passively mode-locked semiconductor laser with a high repetition rate using a simple configuration with a moving mirror located very close to a laser facet. This scheme is demonstrated for the first time by a novel micromechanical laser consisting of an InGaAsP/InP multisegment laser with a monolithic moving micro-mirror driven by an electrostatic comb structure. The main advantage of this laser is the capability of generating high-quality mode-locked pulses stabilized by a phase-locked loop (PLL) with low residual phase noise in a wide repetition-rate tuning range. This paper describes the basic concept and tuning performances utilizing the micromechanical passively mode-locked laser in 22-GHz fundamental mode-locking and in its second-harmonic mode-locking.

Advanced scientific and engineering problems require massively parallel computing. Critical to the designand ultimately the performanceof such computing systems is the interconnection network binding the computing elements, just as is the cardiovascular network to the human body. This paper develops a new interconnection network, "Tori connected mESHes (TESH)," consisting of k-ary n-cube connection of supernodes that comprise meshes of lower level nodes. Its key features are the following: it is hierarchical, thus allowing exploitation of computation locality as well as easy expansion (up to a million processors), and it appears to be well suited for 3-D VLSI implementation, for it requires far fewer number of vertical wires than almost all known multi-computer networks. Presented in the paper are the architecture of the new network, node addressing and message routing, 3-D VLSI/ULSI considerations, and application of the network to massively parallel computing. Specifically, we discuss the mapping on to the network of stack filtering, a hardware oriented technique for order statistic image filtering.

This work presents a further development of the approach to modelling thermal (i.e. carrier-velocity-fluctuation) noise in semiconductor devices proposed in papers by the present authors. The basic idea of the approach is to apply classical theory of Ito's stochastic differential equations (SDEs) and stochastic diffusion processes to describe noise in devices and circuits. This innovative combination enables to form consistent mathematical basis of the noise research and involve a great variety of results and methods of the well-known mathematical theory in device/circuit design. The above combination also makes our approach completely different, on the one hand, from standard engineering formulae which are not associated with any consistent mathematical modelling and, on the other hand, from the treatments in theoretical physics which are not aimed at device/circuit models and design. (Both these directions are discussed in more detail in Sect. 1). The present work considers the bipolar transistor compact model derived in Ref. [2] according to theory of Ito's SDEs and stochastic diffusion processes (including celebrated Kolmogorov's equations). It is shown that the compact model is transformed into the Ito SDE system. An iterative method to determine noisy currents as entries of the stationary stochastic process corresponding to the above Ito system is proposed.

The use of selective oxidation to fabricate vertical-cavity surface-emitting lasers is described. The nativeoxide impacts the device design in two ways, the first being in the introduction of an intracavity dielectric aperture that laterally confines the mode, and the second in the formation of high contrast dielectric Bragg reflectors to shorten the effective cavity length. To date the more important has been the indexconfinement, with record low threshold currents, threshold voltages, and power conversion efficiencies being reported from several groups. However, future designs will likely also benefit from the reduced diffraction loss for a small mode size that is possible with high contrast native oxide/semiconductor mirrors. We describe some of the most important design issues in obtaining ultralow threshold operation.

Butt-joint waveguide couplings are fabricated for use in InP-based photonic integration, and characterized by scanning electron microscopy and optical transmission measurements. Several parameters have been optimized in the characterization study: size and shape of the mask protecting the first waveguide layer during butt-joint regrowth, and the crystallographic direction of the butt-joint interface. The studies show that high-quality butt-joints having negligible optical loss can be made with good fabrication tolerance. Using the optimized butt-joint, DBR-type, gain-clamped SOAs have been fabricated which are free of internal excess reflections. A constant optical gain of 21 dB is obtained up to a signal output power of 25 mW. The devices show CATV grade linearity in a 77 channel CATV linearity test at a distortion level of -55 dB below carrier.

A 7-mask self-aligned SiGe base bipolar transistor has been newly developed. This transistor offers several advancements to a super self-aligned selectively grown SiGe base (SSSB) transistor which has a selectively grown SiGe-base layer formed by a cold-wall ultra high vacuum (UHV)/CVD system. The advancements are as follows: (1) a BPSG-filled arbitrarywidth trench isolation on a SOI is formed by a high-uniformity CMP with a hydro-chuck for reducing the number of isolation fabrication steps, (2) polysilicon-plug emitter and collector electrodes are made simultaneously using an in-situ phosphorusdoped polysilicon film to decrease the distance between emitter and collector electrodes and also to reduce the fabrication steps of the elecrodes, (3) a n+-buried collector layer is made by a high-energy phosphorus ion-implantation technique to eliminate collector epitaxial growth, and (4) a germanium profile in the neutral base region is optimized to increase the fT value without increasing leakage current at the base-cellector junction. In the developed transistor, a high performance of 80-GHz fT and mask-steps reduction are simultaneously achieved.

This letter describes our DFB-LD module for use in WDM optical access networks. We realized an isolator-free DFB-LD module with a thermo-electric cooler in aim of stabilizing the emission wavelength for WDM systems. Silicon waferboard technology was employed to achieve simple assembly and small size of the module. This small size contributed to low TEC power. Our fabricated module demonstrated low-noise and stable emission wavelength characteristics under 156 Mbit/s pseudo random modulation.

A light emitting diode (LED) array unit for use as a light source in isolated power transmission and a display panel was fabricated using LED chips mounted on a silicon microreflector. The reflector was formed on a (100) silicon wafer by anisotropic chemical etching. An isolated power supply consisting of an infrared LED array unit and single silicon crystal solar cells had a maximum transmission efficiency of 2.3%. The silicon microreflector absorbs the heat generated by the LED chips and improves their light directive characteristics. A small, high-resolution, full color LED display panel can also be constructed using LED array units fabricated on silicon microreflectors. The LEDs in a unit are arrayed with a matrix structure and the electric contacts between the LED chips, the reflector and the upper cover glass are formed using conducting silver resin.

At present, the widespread use of optoelectronic components is restricted by their high cost. Up to 90% of the cost of a semiconductor laser is in the packaging, with the fibre-chip alignment the major part. In this paper, an approach to low cost packaging is described, which uses an integrated mode size transformer to match the laser output to the fibre mode. This improves the alignment tolerance of the laser-fibre coupling by more than a factor of three, allowing simple passive alignment approaches to be used. It requires only minor modification to the processing of a standard buried heterostructure laser, and allows the coupling efficiency to be optimised without compromising the performance of the laser. The design of a silicon submount for passive laser-fibre alignment is described and coupling losses as low as 1.2 dB to standard cleaved single mode fibre are reported. The technology that has been developed is generic and its successful application to other optoelectronic devices such as fibre grating lasers, semiconductor optical amplifiers and laser arrays is described.

Micromachined sensors and actuators applied with electrostatic fields are getting widely developed. At the same time, "electrets," which are dielectrics carrying non-equilibrium permanent space charges or polarization distribution, are in demand because they improve the transducer characteristics. In this paper, we have reported on our successful fabrication of silicon dioxide electrets with extremely superior long-term charge stability by plasma chemical vapor deposition (PCVD). We have also reported on the correlation between the deposition conditions, the long-term charge stability and thermally stimulated current (TSC). Finally, the characterization of the long-term stable electrets will be described and discussed.

Laser module fabricated with silicon platform technology is very attractive for low-cost modules. The technology enables passive optical alignment of an LD to an optical fiber. Our marker design for passive alignment allows positioning accuracy within 1 µm of LD. However, coupling efficiency is a key issue because that by conventional butt coupling scheme is low with about 10 dB coupling loss. We investigated optical coupling characteristics in various types of coupling scheme: conventional flat end fibers, cone fibers, integrated GRIN rod lenses on the platform and the coupling with new-type LDs integrated with spot size transformer. Improvement of coupling efficiency with 3 dB and 7.5 dB compared to flat-end fiber is achieved by using the cone fiber and the GRIN rod lens, respectively, although 1-dB coupling tolerances for alignment deteriorated with these schemes. We obtained high efficient coupling with 3.5 dB coupling loss and wide alignment tolerance of 2.3 µm simultaneously with a new-type LD integrated with spot size transformer owing to its expanded spot size characteristics.

The paper discusses the possibility of building semiconductor lasers whose wavelength stays nearly constant with ambient temperature variation. Several factors affecting the lasing wavelength change with temperature variation in both distributed feedback lasers and Fabry-Perot lasers are addressed and the optimum design of bandgap temperature dependence for the active layer material is discussed. It is pointed out that the most important challenge we face in building temperature-insensitive wavelength lasers is the development of a temperature-insensitive bandgap material for the active layer. Based on published data, it is speculated that such a laser could be developed using a Hg1-xCdxTe/CdTe double heterostructure. Although no data is available yet, we expect a Ga1-xInxAs1-yBiy III-V alloy semiconductor can be used for this purpose. Recently reported T1xIn1-x-yGayP III-V alloy semiconductor might be another promising candidate. Such lasers will greatly advance applications of WDM (Wavelength-Division-Multiplexing) technology to optical fiber communication systems and contribute to network innovations.

Preliminary experiments on non-destructive quantitative analysis of water vapor density in halogen lamps have been carried out. A working curve showing a relation between absorbance and water vapor density was successfully obtained by using frequency-stabilized InGaAsP/InP semiconductor laser spectrometric system.

Mechanism of the noise generation caused by the optical feedback in semiconductor laser was experimentally determined. Two types of the mode competition phenomena were confirmed to be the generating mechanisms. Applicability of the self-sustained pulsation to be a noise reduction method was also discussed.

We discuss fabrication of InGaAs quantum dot structures using the self-assembling growth technique with the Stranski-Krastanow growth mode in MOCVD, including optical ploperties of the nano-structures. The formation process of the quantum dot islands was clarified by observing the samples grown under various conditions with an atomic force microscope. A trial for self-alignment of the quantum dots was also investigated. On the basis of these results, as the first step toward the ultimate semiconductor lasers in which both electrons and photons are fully quantized, a vertical microcavity InGaAs/GaAs quantum dot laser was demonstrated. Finally a perspective of the quantum dot lasers is discussed, including the bottleneck issues and the impact of the quantum dot structures for reducing threshold current in wide bandgap lasers such as GaN lasers.