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.

This paper presents the performance of a proposed GaAs MESFET photodetector with wide drain-to-gate distances for improving the optical coupling efficiency in subcarrier optical transmission. Principle and design parameters of the proposed MESFET are described. Link gain, CNR, and BER, are experimentally investigated as functions of the drain-to-gate distance. It is experimentally found that the proposed MESFET improves the link gain by 8.5 dB compared to the conventional structure at the subcarrier frequency of 140 MHz. Discussions are also included compared to PIN-PD.

This paper presents a method of multiple fault diagnosis in sequential circuits by input-sequence pairs having sensitizing input pairs. We, first, introduce an input-sequence pair having sensitizing input pairs to diagnose multiple faults in a sequential circuit represented by a combinational array model. We call such input-sequence pair the sensitizing sequence pair in this paper. Next, we describe a diagnostic method for multiple faults in sequential circuits by the sensitizing sequence pair. From a relation between a sensitizing path generated by a sensitizing sequence pair and a subcircuit, the proposed method deduces the suspected faults for the subcircuits, one by one, based on the responses observed at primary outputs without probing any internal line. Experimental results show that our diagnostic method identifies fault locations within small numbers of suspected faults.

An insertion loss, branching deviation and polarization dependent loss (PDL) as to a 2 N optical splitter using silica-based planar lightwave circuits has been investigated. New key technologies such as (1) a novel wedge type Y-branch, (2) an offset waveguide at the junction between the curved input waveguide and the Y-branch, and (3) low birefringence waveguides due to the appropriate dopant concentration of a cladding, have been devised and incorporated into the splitter. As a result, 2 N optical splitters with low average insertion loss ( 13.2 dB), low branching deviation ( 0.4 dB) and low PDL ( 0.2 dB) have been successfully developed.

This paper describes the design, characteristics, and applications of newly developed latching-type 1 2 and 1 8 single-mode fiber switches. These switches have been successfully fabricated using micromachine technology. To reduce insertion loss and light reflection, an index-matching oil is injected into the switches. The fabricated 1 2 switches exhibit a low insertion loss of 0.31 dB, high return loss of 51 dB, relatively fast switching speed of 2 ms, and low driving power of 9 mw. Switching operation is stable over 108 switching times. A practical 1 8 single-mode fiber switch was also constructed using seven 1 2 switches cascaded in three stages. The fabricated 1 2 and 1 8 switches have been applied to an NTT multichannel video distribution FTTH system to enhance system reliability.

Optical surface mount technology (O-SMT), which was proposed to provide a possible solution to growing serious problems in manufacturing process of optoelectronic products, is introduced. After discussing the basic idea of O-SMT, experimental results are also described to show its feasibility.

We propose an InGaAlAs waveguide p-i-n photodiode (WG-PD) with a thick symmetric double-core for surface-hybrid integration onto optical platforms, which can be applied to low cost optical modules for access networks. The waveguide structure is designed to efficiently couple to flat-ended single mode fibers while maintaining low-voltage (less than 2 V) operation. Crystal growth conditions and a passivation technique are also investigated for obtaining high responsivity, low dark current and highly reliable operation. Fiber-coupled responsivity as high as 0.95 A/W, at a 1.3-µm wavelength, and vertical coupling tolerance as wide as 2.6 µm are demonstrated for a dispersion-shifted fiber (DSF) coupling at an operating voltage of 2 V. Dark current is as low as 300 pA at 25 and 12 nA at 100. A temperature accelerated aging test is performed to show the feasibility of using the WG-PD in long-term practical applications.

In-line transceiver chip emitting at 1.3 µm and receiving at 1.55 µm is described regarding the crosstalk occurring between the 1.3-µm laser and the 1.55-µm integrated photodiode. Contribution of optical and electrical crosstalk to the overall crosstalk is measured and discussed. Techniques to overcome the crosstalk are proposed and demonstrated by showing the feasibility of system compatible diplex sensitivities.

Polarization insensitive discrete electroabsorption modulators have been designed as an optical gating device. It reveals the first finding, to our knowledge, that the ratio of the optical confinement factor (Γ) to the differential of the values (ΔΓ) between TE and TM polarized lights decides polarization dependence of attenuation. The ratio ΔΓ/Γ is significantly reduced by increasing core thickness. Large optical confinement structures combining a thick InGaAsP bulk absorption layer and polyimide-buried mesa-ridge waveguide have fabricated. The ratio ΔΓ/Γ of the high-mesa structure was estimated to be less than 0.05 in the gain-region of an erbium-doped fiber amplifier (EDFA), which enable us extremely low polarization sensitivity less than 1 dB up to 20 dB extinction. Proper waveguide length of the structure allowed low insertion loss ( 9.3 dB), small loss-change ( 1.8 dB) and sufficient modulation depth ( 30 dB) simultaneously in the EDFA's gain region. The low-mesa structure provided low insertion loss around 7 dB with small deviation in the wavelength region. High modulation band-width and a polarization-insensitive optical gating waveform have also demonstrated.

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.

This letter shows an architectural approach for analyzing real-time aspects of distributed multimedia processing systems. The results of this letter are 1) to propose the concept real-time supporting environments which consist of real-time traffic management/control environment and real-time application environment and 2) to analyze the real-time requirements of such environments.

This paper describes the technological issues in achieving a low-cost hybrid WDM module for access network systems. The problems which should be resolved in developing a low-cost module are clarified from the viewpoint of the module assembly in mass production. A design concept for a low-cost module suitable for mass production is indicated, which simplifies the alignment between a laser diode and a waveguide, and reduces the number of the components such as lenses and mirrors. The low-cost module is achieved by employing a flip-chip bonding method with passive alignment using a spot-size converter integrated laser diode (SS-LD) and p-i-n waveguide photodiodes (WGPDs) on a planar lightwave circuit (PLC) platform. We confirm that the SS-LD and the WGPD provide high coupling efficiency with a large tolerance for passive alignment. To achieve a high-sensitivity receiver, the module is designed to employ an asymmetric PLC Y-splitter that prefers a PD responsivity to an LD output power because of the high-coupling efficiency of the LD, and to employ a bare preamplifier mounting to reduce the parasitic capacitance into a preamplifier. We also demonstrate the dynamic performance for a 50-Mb/s burst signal, such as a high sensitivity, an instantaneous AGC response, and a small APC deviation of the transceiver.

In this paper, two new signature schemes whose security is based on both discrete logarithms and factorization are proposed to enhance the security of the OSS signature scheme. The major benefit of these new signature schemes is that the signer dose not need to know how to factor p-1, thus it is possible for every user to employ the same modulus p, where p is the modulus commonly used in the system. Furthermore, two identification schemes based on this advantage are also developed in this paper.

This paper obtains some new results about the existence, uniqueness, and global asymptotic stability of the equilibrium of a nonlinear continuous neural network, under a sufficient condition weaker than ones presented in the literature. The avobe obtained results can also imply the existing ones about avsolute stability of nonlinear continuous neural networks

Recently, Yagisawa proposed a public key cryptosystem which is very similar to the modified Lu-Lee cryptosystem. The differences are the set of messages and the decryption. On the other hand, Brickell and Odlyzko showed that the modified Lu-Lee cryptosystem is completely broken in polynomial time. This paper shows that Yagisawa cryptosystem is completely broken in the same way.

Narrow-beam and low threshold current characteristics have been realized for a 1.3 µm FS-BH (Facet Selective growth Buried Heterostructure) laser diode monolithically integrated with a tapered waveguide lens by a selective area epitaxial growth technique. The beam divergences in the perpendicular and horizontal directions have been reduced down to about 12. By the introduction of the strained quantum well structure and the optimized cavity structure, the threshold current has been kept as low as 6 mA which is comparable to the conventional Fabry-Perot laser diodes. Even at high temperature as high as 85, the threshold current and the operation current (P=10 mW) have been suppressed to as low as 23 mA and 63 mA, respectively. Furthermore error-floor-free characteristics for 622 Mbps-50 km transmission have been confirmed under severe optical feedback condition.

This paper proposes a new approach for distributing millimeter wave signals from a central location to micro-base-stations using optical fiber links. The links utilize two Mach-Zehnder external optical modulators (EOMs) to perform all optical down-conversion, eliminating the need for a local oscillator or laser diode in the micro-base-station. A simple model of the EOMs is developed to illustrate the principle of dual-EOM mixing. The characteristics of conversion loss and intermodulation are examined for two cases: where the EOMs are operated in the linear mode and where the local oscillator's EOM is biased as a frequency doubling modulator. Additionally, we examined the use of an optical amplifier to reduce conversion loss for these two cases. The measured conversion loss of the link was 82 dB, and we estimated this could be reduced to about 48 dB by employing an optical amplifier and a more efficient EOM for RF reception.

In this paper, we apply the method of relating learning to hypothesis testing [6] to study average generalization performance of concept learning from noisy random training examples. A striking aspect of the method is that a learning problem with a so-called ill-disposed learning algorithm can equivalently be reduced to a simple one, and for this simple problem, even though a direct and exact calculation of the learning curves might still be impossible, a thorough empirical study can easily be performed. One of the main advantages of using the illdisposed algorithm is that it well models lower quality learning in real situations, and hence the result can provide useful implications as far as reliable generalization is concerned. We provide empirical formulas for the learning curves by simple functions of the noise rate and the sample size from a thorough empirical study, which smoothly incorporates the results from noise-free analysis and are quite accurate and adequate for practical applications when the noise rate is relatively small. The resulting learning curve bounds are directly related to the number of system weights and are not pessimistic in practice, and apply to learning settings not necessarily within the Bayesian framework.

We introduce a new type of optical microconnector named "put-in microconnector-" using a planar microlens. The connector part is composed by a lens jack and fiber plug, where the lens jack is a hollow formed on a planar microlens surface, and the fiber plug is a protuberance formed on the core of an optical fiber. This concept can realize an alignment-free single mode fiber coupling. In this paper, we describe the structure and fabrication process, the optical coupling characteristics of the fabricated device, and finally, the basic analysis of optical coupling module. For the optical coupling characteristics, we measured the coupling loss and the return loss. The optical coupling loss of about 4 dB and the return loss of about 49 dB were obtained at wavelength of λ = 0.633 µm. Moreover, we have confirmed that the insertion loss of such a structure does not increase so much in comparison with that of the butt jointing. For the purpose of characterizing the optical property, the theoretical analysis was performed. We have made a software tool to estimate the optical coupling loss due to the position error. For this type of structure, the tolerance of about 15 µm for the distance between the laser and the planar microlens and 150 µm for the distance between the planar microlens and the optical fiber were estimated. Since the put-in microconnector does not require any precise alignment, it is appropriate for mass production.

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.