An optical detection system using a DFB laser with a very small aperture is theoretically proposed. The threshold gain level in DFB laser is sensitively varied with combined reflections by the facet and the corrugation as well as with the optical injection reflected at the surface of the optical disk. Variation of the threshold gain level is counted as the voltage change on electrodes of the laser. It is found that sensitivity of the optical detection with a well-designed DFB laser becomes six times larger than that with conventional Fabry-Perot ones. Field distribution around the small aperture is analyzed taking into account both the near-field and the radiation field. Numerical data on the voltage change are given as examples of the detection system.

We present a state-space approach to the problem of designing a parallel feedforward compensator (PFC), which has the same dimension of the input i.e. input-dimensional, for a class of non-square linear systems such that the closed-loop system is strictly passive. For a non-minimum phase system or a system with high relative degree, passification of the system cannot be achieved by any other methodologies except by using a PFC. In our scheme, we first determine a squaring gain matrix and an additional dynamics that is connected to the system in a feedforward way, then a static passifying control law is designed. Consequently, the actual feedback controller will be the static control law combined with the feedforward dynamics. Necessary and sufficient conditions for the existence of the PFC are given by the static output feedback formulation, which enables to utilize linear matrix inequality (LMI). Since the proposed PFC is input-dimensional, our design procedure can be viewed as a solution to the low-order dynamic output feedback control problem in the literature. The effectiveness of the proposed method is illustrated by some numerical examples.

The maximum likelihood sequence estimator (MLSE) is usually implemented by the Viterbi algorithm (VA). The computational complexity of the VA grows exponentially with the length of the channel response. With some performance reduction, a decision-feedback equalizer (DFE) can be used to shorten the channel response. This greatly reduces the computational requirement for the VA. However, for many real-world applications, the complexity of the DFE/MLSE approach may be still too high. In this paper, we propose a constrained DFE that offers much lower VA computational complexity. The basic idea is to pose some constraints on the DFE such that the postcursors of the shortened channel response have only discrete values. As a result, the multiplication operations can be replaced by shift operations making the VA almost multiplication free. This will greatly facilitate the real world applications of the MLSE algorithm. Simulation results show that while the proposed algorithm basically offers the same performance as the original MLSE performance, the VA is much more efficient than the conventional approach.

This paper describes the recent progress made in developing wavelength tunable semiconductor light sources for WDM applications. Wide and quasi-continuous wavelength tunings were investigated for a wavelength-selectable laser and a wavelength tunable distributed Bragg reflector (DBR) laser having a super structure grating (SSG). A wavelength-selectable laser consisting of a DFB laser array, a multi-mode interferometer (MMI), and a semiconductor optical amplifier (SOA) demonstrated a quasi-continuous tuning range of 46.9 nm by using temperature control. A wavelength-tunable DBR laser with SSG exhibited a quasi-continuous tuning range of 62.4 nm by using three tuning current controls. Wavelength stabilization was also demonstrated under the temperature variations of 5.

Bidirectional reflection distribution functions (BRDFs) of commercially pure titanium sheets with three different kinds of surface morphology were measured. Those experimental BRDFs were analyzed by using Phong's reflection model. Topographic measurements of the specimens' surfaces were performed with using a stylus-method. An explicit microfacet model based on topographic data was proposed. With using the explicit microfacet model and geometrical optics the calculated BRDFs were obtained and then compared with the experimental BRDFs. Both of them were in a good agreement. Through this comparison physical meanings of Phong's reflection model were discussed. We concluded that with using the explicit microfacet model it will be possible to calculate the BRDF of the materials' surface in arbitrary illumination conditions and that this modeling will be useful to develop new aesthetic surface appearance in material industries, computer graphics, architectural design and surface science.

As LSIs are two-dimensional structures, the number of external pins increases at a lower rate than the corresponding increase in the number of gates on the LSI. Therefore, the number of flip-flops on a scan path increases as the density of gates on LSIs rises, resulting in longer test application times. In this paper, three novel DFT strategies aimed at reducing test application time are proposed. DFT strategy 1 is a full scan design method with test point insertion, DFT strategy 2 is a partial scan design method, and DFT strategy 3 is a partial scan design method with test point insertion. Experimental results show that these DFT strategies reduced the test application times by 45% to 82% compared with conventional full scan design methods.

In this work, using 0.2 µm GaAs modulation doped FET (MODFET), a high performance downconverter MMIC was developed for direct broadcasting satellite (DBS) applications. The downconverter MMIC showed a noise figure of 4.3 dB which is lower by 5 dB than conventional ones, and required only a low LO power of -10 dBm for normal DBS operation. At a low LO power of -10 dBm, the power consumption was 175 mW, which is lower than 50 percent of conventional ones. The frequency response of conversion gain exhibited a low gain ripple of 0.9 dB, and the LO leakage power was suppressed to a lower value than -30 dBm under a LO input power of -10 dBm. The fabricated chip exhibited a small size of 0.840.9 mm2. The objectives of this work are to improve the traditional direct broadcasting satellite (DBS) downconverters by an efficient circuit design and to describe the techniques employed in the design.

We report 0.15-µm T-shaped gate MODFETs using BCB (Benzocyclobutene) as low-k spacer dielectric material. The RF performance of pseudomorphic MODFET was improved by reducing the gate fringing capacitance using low-k material. The BCB film was deposited by plasma CVD technique at 100C and was patterned by lift-off technique. The dielectric constant of BCB film deposited by plasma CVD was confirmed 2.7, which is equal to that of spin-coated BCB, and is 35% lower than that of conventional SiO2. The leakage current was 4.710-5 A/cm2 at 3.6 MV/cm and was low enough for spacer material. 0.15-µm T-shaped gate MODFETs were fabricated by using BCB spacer and phase-shift lithography technique. More than 20 GHz increase of fmax was obtained in comparison with conventional SiO2 spacer by reducing the gate fringing capacitance.

In this paper we propose a learning algorithm to enhance the fault tolerance of feedforward neural networks (NNs for short) by manipulating the gradient of sigmoid activation function of the neuron. We assume stuck-at-0 and stuck-at-1 faults of the connection link. For the output layer, we employ the function with the relatively gentle gradient to enhance its fault tolerance. For enhancing the fault tolerance of hidden layer, we steepen the gradient of function after convergence. The experimental results for a character recognition problem show that our NN is superior in fault tolerance, learning cycles and learning time to other NNs trained with the algorithms employing fault injection, forcible weight limit and the calculation of relevance of each weight to the output error. Besides the gradient manipulation incorporated in our algorithm never spoils the generalization ability.

In erbium doped optical fiber amplifiers (EDFAs) used in modern high-capacity wavelength division multiplexing (WDM) systems, the gain flatness of EDFA is very important in wide-band long-haul systems. In the EDFAs using the passive gain equalizers, the gain flatness deteriorates due to gain-tilt when the operating condition of the EDFA changes, while the EDFAs should maintain the gain flatness even if the operating condition has changed. To solve this problem, we have developed an active gain-slope compensation technique of an EDFA using a thulium-doped optical fiber (TDF) as a saturable absorber. The actively gain-slope compensated EDFA with the TDF compensator keeps the gain profile constant for the wide gain dynamic range more than 8 dB with the low noise figure less than 6 dB in the wavelength range of 1539-1564 nm.

This paper proposes a nonlinear distortion suppression scheme for optical direct FM Radio-on-Fiber system. This scheme uses the interaction between the nonlinearities of DFM-LD and OFD to suppress a 3rd order intermodulation distortion. We theoretically analyze the carrier to noise-plus-distortion ratio (CNDR) and show a controlling method in the MZI type OFD to realize the proposed suppression scheme.

In erbium doped optical fiber amplifiers (EDFAs) used in modern high-capacity wavelength division multiplexing (WDM) systems, the gain flatness of EDFA is very important in wide-band long-haul systems. In the EDFAs using the passive gain equalizers, the gain flatness deteriorates due to gain-tilt when the operating condition of the EDFA changes, while the EDFAs should maintain the gain flatness even if the operating condition has changed. To solve this problem, we have developed an active gain-slope compensation technique of an EDFA using a thulium-doped optical fiber (TDF) as a saturable absorber. The actively gain-slope compensated EDFA with the TDF compensator keeps the gain profile constant for the wide gain dynamic range more than 8 dB with the low noise figure less than 6 dB in the wavelength range of 1539-1564 nm.

This paper proposes a nonlinear distortion suppression scheme for optical direct FM Radio-on-Fiber system. This scheme uses the interaction between the nonlinearities of DFM-LD and OFD to suppress a 3rd order intermodulation distortion. We theoretically analyze the carrier to noise-plus-distortion ratio (CNDR) and show a controlling method in the MZI type OFD to realize the proposed suppression scheme.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

Highly reliable and high power weakly index guided buried-stripe type 980 nm pump laser diodes developed for undersea applications are reviewed. The 10,000-hour large scale reliability tests of the first generation LD chips shows that 16.7 FIT for the random failure was confirmed at 10C with 60% confidence level at 120 mW output power. We also fabricated a FBG locked co-axial type module using a can-sealed LD with a two-lens system, which showed a stable FBG locked mode oscillation at 980 nm under the temperature range from 5C to 45C. The 5,000-hour heat cycle test of the modules reveals that the cumulative failure rate after 27 years at 10C is expected to be 0.023%. These first generation LD modules were employed in the transoceanic commercial systems such as Pacific Crossing-1 and the Japan-US cable system projects. We have also succeeded in developing the 980 nm LD for higher output operation with optimizing waveguide design. The 1000 µm long LD showed CW kink-free operation up to 545 mW optical output and a maximum output power of over 650 mW, which was limited by thermal rollover. In addition, a preliminary aging test at 350 mW optical output power at 50C has shown stable operation up to 2,300 h. We also confirmed 300 mW kink-free fiber output power with a co-axial type module with the improved coupling efficiency of approximately 78%. These figures are the highest reported operation levels for 980-nm co-axial type modules.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

Highly reliable and high power weakly index guided buried-stripe type 980 nm pump laser diodes developed for undersea applications are reviewed. The 10,000-hour large scale reliability tests of the first generation LD chips shows that 16.7 FIT for the random failure was confirmed at 10C with 60% confidence level at 120 mW output power. We also fabricated a FBG locked co-axial type module using a can-sealed LD with a two-lens system, which showed a stable FBG locked mode oscillation at 980 nm under the temperature range from 5C to 45C. The 5,000-hour heat cycle test of the modules reveals that the cumulative failure rate after 27 years at 10C is expected to be 0.023%. These first generation LD modules were employed in the transoceanic commercial systems such as Pacific Crossing-1 and the Japan-US cable system projects. We have also succeeded in developing the 980 nm LD for higher output operation with optimizing waveguide design. The 1000 µm long LD showed CW kink-free operation up to 545 mW optical output and a maximum output power of over 650 mW, which was limited by thermal rollover. In addition, a preliminary aging test at 350 mW optical output power at 50C has shown stable operation up to 2,300 h. We also confirmed 300 mW kink-free fiber output power with a co-axial type module with the improved coupling efficiency of approximately 78%. These figures are the highest reported operation levels for 980-nm co-axial type modules.

We evaluate the performance of indoor infrared wireless systems using on-off keying code division multiple access (OOK-CDMA) with decision-feedback equalizer (DFE) on diffuse channels. To estimate the impulse response, we use the training sequence that alternates '1' and '0. ' We show that the OOK-CDMA with DFE and the training sequence can achieve better performance than the OOK-CDMA without DFE. We also show that the OOK-CDMA with DFE and the training sequence can achieve almost the same performance as the OOK-CDMA with DFE and the known impulse response.

To enhance fault tolerance ability of the feedforward neural networks (NNs for short) implemented in hardware, we discuss the learning algorithm that converges without adding extra neurons and a large amount of extra learning time and cycles. Our algorithm modified from the standard backpropagation algorithm (SBPA for short) limits synaptic weights of neurons in range during learning phase. The upper and lower bounds of the weights are calculated according to the average and standard deviation of them. Then our algorithm reupdates any weight beyond the calculated range to the upper or lower bound. Since the above enables us to decrease the standard deviation of the weights, it is useful in enhancing fault tolerance. We apply NNs trained with other algorithms and our one to a character recognition problem. It is shown that our one is superior to other ones in reliability, extra learning time and/or extra learning cycles. Besides we clarify that our algorithm never degrades the generalization ability of NNs although it coerces the weights within the calculated range.

In this paper, we consider the design for testability of a multiplier based on the modified Booth Algorithm. First, we present a basic array implementation of the multiplier. Next, we introduce testability considerations to derive two C-testable designs. The first of the designs is C-testable under the single stuck-at fault model (SAF) with 10 test patterns. And, the second is C-testable under the cell fault model (CFM) with 33 test patterns.