A hybrid method-of-moments (MoM) and immittance approach for efficient and accurate analysis of printed slots and strips of arbitrary shape in layered waveguide for various applications has been proposed. An impedance-type MoM is formulated from the electric field integral equation (EFIE) for printed strip case and an admittance-type MoM is formulated from the magnetic field integral equation (MFIE) for the printed slot case, using the Galerkin's technique. Immittance approach has been used to calculate spectral dyadic Green's functions for the layered waveguide. For efficient analysis of large and complex structures, equivalent circuit parameters of a block are first extracted and complete structure is analyzed through cascaded ABCD matrices. The equivalent circuit characterization of printed strip and slot in layered waveguide has been done for the first time. Finite periodic structure loaded with printed strips has been investigated and it shows the electromagnetic bandgap (EBG) behavior. The electromagnetic (EM) program hence developed is checked for its numerical accuracy and efficiency with results generated with High-frequency structure simulator (HFSS) and shows good performance.

The handoff in Mobile IP networks causes packet sequence disruption during a packet forwarding procedure and may result in performance degradation in higher layer protocols. We investigate the impact of handoff in the Mobile IPv6 networks, where an optimized routing with the smooth handoff is adopted. The impact on the packet sequence is measured by an 'unstable time period (UTP)' and a 'silence time period (STP).' The UTP explains the time duration of out-of-sequence packets while the STP reflects the blackout duration of a mobile node after the initiation of handoff procedure. With the analysis on the UTP and STP, the total transient time period (denoted as handoff time period or HTP) after the handoff initiation can be estimated. In our previous work, focusing on the UTP, the packet flow sequence under the smooth handoff is analyzed for the Mobile IPv4 networks. The proposed queuing-based analysis is extended in this work for the Mobile IPv6 networks. That is, several modifications are made to conform to Mobile IPv6 and at the same time the queuing analysis itself is improved to better model the handoff procedure. The numerical results show that the queuing delay for the handoff packets (affected by background traffic) and the involved link (or route) capacities affect the estimated UTP, STP, and HTP. In addition, two schemes such as priority queuing and buffered packet forwarding are introduced to reduce the transient period and the improvements are analyzed for comparison.

Electrical properties of contacts between head-tail coupled poly(3-hexylthiophene), PHT and Al (and Au) in planer type and sandwich type diodes of Al/PHT/Au have been studied. The contact resistances are directly evaluated by probing the potential profile of PHT between the metal electrodes using micromanipulators installed in scanning electron microscope. In the potential profile of planer type diode, a large potential cliff is observed at Al/PHT interface and some appreciable potential step is also found at PHT/Au interface. The contact resistance at the Al/PHT interface deduced from the potential profile shows the bias and its polarity dependence, indicating the existing of the Schottky like junction. At forward bias, it is found that the residual resistance at Al/PHT interface limits the diode performance. The residual resistance is supposed to be insulating layer of Al oxide. At larger reversed bias, the contact resistance at Al/PHT decreased abruptly due to the Zener breakdown. The potential profile of sandwich type diode is similar to that of planer type diode. It is found that even the PHT/Au contact shows the ohmic behavior, the contact resistance is significant as to limit the maximum current of the cells.

A device structure for polymer Schottky diode, which has the glass chimney as a dopant reservoir enabling the reduction of series resistance without cathode corrosion, has been proposed. Doping with the acetonitrile solution of FeCl3 in the device resulted in the increase in the forward-bias current by one order of magnitude without notable increase in reverse-bias current, suggesting that the doping reduced the series resistance. It is found that the penetration speed depends on the solvents. Short time doping with the nitromethane solution of FeCl3 resulted in the increase by three orders of magnitude. However, doping for a long period yielded the considerable increase in the reverse-bias current due to the complete penetration of dopatn solution. When the upper opening of glass chimney of device is left opened and the sample after doping stored in air, the forward-bias current of the device reduced rapidly due to the undoping and/or degradation of polymer. It is possible to protect the degradation of device characteristics after doping, by sealing the chimney and storing the device in vacuum.

This paper proposes a robust method for estimating the fundamental frequency (F0) in real environments. It is assumed that the spectral structure of real environmental noise varies momentarily and its energy does not distribute evenly in the time-frequency domain. Therefore, segmenting a spectrogram of speech mixed with environmental noise into narrow time-frequency regions will produce low-noise regions in which the signal-to-noise ratio is high. The proposed method estimates F0 from the periodic and harmonic features that are clearly observed in the low-noise regions. It first uses two kinds of spectrogram, one with high frequency resolution and another with high temporal resolution, to represent the periodic and harmonic features corresponding to F0. Next, the method segments these two kinds of feature plane into narrow time-frequency regions, and calculates the probability function of F0 for each region. It then utilizes the entropy of the probability function as weight to emphasize the probability function in the low-noise region and to enhance noise robustness. Finally, the probability functions are grouped in each time, and F0 is obtained as the frequency with the highest probability of the function. The experimental results showed that, in comparison with other approaches such as the cepstrum method and the autocorrelation method, the developed method can more robustly estimate F0s from speech in the presence of band-limited noise and car noise.

A polarization switchable slot-coupled microstrip antenna using PIN diodes is proposed and studied. The microstrip feed line installed behind the ground plane is divided into two branches and each tip of the branches is connected to the ground plane through a PIN diode. One of the diodes is oriented from the tip to the ground plane and the other is oriented from the ground to the tip so that a slot in the ground can be selected to feed the patch by switching the dc bias between positive and negative. This selection contributes to switch the polarization between horizontal and vertical. In this paper, the authors investigate the polarization switching antenna theoretically and experimentally and confirmed sufficient differencce of antenna gain between horizontal and vertical polarization.

We propose multibits/sequence-period optical code division multiple access (MS-OCDMA) systems with double optical hardlimiters (DHL) in the presence of APD noise, thermal noise, and channel interference. We apply Reed-Solomon (RS) codes to MS-OCDMA to further improve the error rate performance. We show that the MS-OCDMA receiver with DHL improves the bit error probability of MS-OCDMA systems when the received laser power is large. We also show that the performance of RS coded MS-OCDMA system is better than that of on-off keying OCDMA (OOK-OCDMA) system at the same bit rate and at the same chip rate, respectively.

As new applications of THz waves emerge, new active and passive components need to be developed. The efficiency of wave guiding systems can be significantly increased with the use of MEMS approaches as well as with the development of new planar antenna concepts with high bunching properties. Generation of sufficient THz power relies on new active devices like Heterostructure Barrier Varactors and cascaded quantum structures, but also in the optimisation of new generation concepts. One of these is photomixing in non-linear materials with very short carrier lifetimes, like low-temperature-grown GaAs.

In this paper, we proposed the doubly tapered electromagnetic periodic structure (DT-EPS) bandstop filters in coplanar waveguide (CPW) and cylindrical coplanar waveguide (CCPW). The DT-EPS bandstop filter not only can effectively improve the stopband rejection but also increase its bandwidth. In addition, this technique can significantly reduce the passband ripples compared with conventional case.

Schottky gas sensors of CO were fabricated using high quality AlGaN/GaN/Si heterostructures. The CO sensors show good sensitivity in the temperature range of 250 to 300 (530%, at 160 ppm CO in N2) and fast response comparable with SnO2 sensors. A two-region linear regime was observed for the dependence of sensitivity on CO concentration. GaN sensors on Si substrate offer the possibility of integration with Si based electronics. The gas sensors show slow response with time, the change of material properties possibly in the presence of large thermal stress.

We propose a frequency stabilization system for laser diodes (LDs), in which the electrical feedback loop response can be determined using an on-line genetic algorithm (GA) so as to attain lower LD frequency noise power within the specific Fourier frequency range of interest. At the initial stage of the stabilization, the feedback loop response has been controlled through GA, manipulating the proportional gain, integration time, and derivative time of conventional analog PID controller. Individuals having 12-bit chromosomes encoded by combinations of PID parameters have converged evolutionarily toward an optimal solution providing a suitable feedback loop response. A fitness function has been calculated for each individual in real time based on the power spectral density (PSD) of the frequency noise. The performance of this system has been tested by stabilizing a 50 mW visible LD. Long-term (τ > 0.01 s) frequency stability and its repeatability have been improved.

With the aim of applying a MISS tunnel-diode cell to a high-density RAM, we studied its problems and developed three circuit technologies to solve them. The first, a standby-voltage control scheme, reduces standby currents and increases the signal current by 3.4 times compared to the conventional one. The second, a hierarchical bit-line structure, reduces the number of memory cells in a bit-line without increasing the number of sense amplifiers. The third, a twin-dummy-cell technique, generates a proper reference signal to discriminate read currents. These technologies enable a capacitorless MISS diode cell with an effective cell area of 6F 2 (F: minimum feature size) to be applied to a high-density RAM.

Competition of two-photon and one-photon absorption in Si-APD was studied. Device should be cooled down in order to clearly observe two-photon absorption at low illumination intensity. Passive Geiger mode operation was studied to sensitively detect small number of carriers generated by two-photon absorption. The illumination intensity dependence of the photocurrent pulse count number is well explained by taking into account the two absorption mechanisms and a dead time period that depends on bias voltage.

The performance of an indium-gallium-arsenide avalanche photodiode serving as a 1550 nm single-photon detector is investigated. Quantum efficiency is evaluated for laser pulses with an average of < 0.015 photons per pulse, which are important for the demonstration of unconditionally secure quantum key distribution [G. Brassard et al.: Phys. Rev. Lett. 85, 6, p.1330 (2000)]. An operating temperature of 243 K is achieved by thermo-electrical cooling, yielding a quantum efficiency of 18% with a dark-count probability per gate of 2.8 10-5. The results obtained here guarantee unconditionally secure fiber-optic quantum key distribution over 50 km.

A new type of photodetector called "photosensitive floating field emitter, (PFFE)" has been proposed. The PFFE device combines an n-type cone-shaped triode field emitter with a-Si p-i-n photodiode film. However, a PFFE cannot detect two-dimensional distributions of light intensity. In this paper, we propose a novel structure to overcome the above this problem of the PFFE. The device was fabricated on a silicon-on-sapphire substrate to permit irradiation from the backside. p-n photodiodes were constructed within a field emitters, the n+ region being separated by p+ regions to permit detection of two- dimensional light distributions. The emission current of the PFFE/SOS was found to be proportional to the illumination intensity, but the quantum efficiency was only about 2%. This quantum efficiency is lower than that expected. Under irradiation, the emission current increased, but the gate-leakage current increased. This gate-leakage current was several orders of magnitude larger than the emission current. Almost photo-generated electrons lost in the gate electrode.

This paper presents an even harmonic mixer using self-biased anti-parallel diode pair (APDP). A proposed self-biased APDP is composed of a pair of diodes and self-bias series resistors. At high LO injection level, rectified current is generated by the diodes and reverse voltage is applied to the diodes by the self-bias resistor. Therefore, rapid degradation of conversion loss at high LO input level can be avoided. The effect of self-bias resistor is explained by using simplified behavior model and harmonic balance method, and is evaluated by the measurements of an L-band even harmonic type direct conversion mixer.

The present letter introduces a new approach to the construction of a set of ternary arrays having a zero-correlation zone. The proposed array set has a zero-correlation zone for both periodic and aperiodic correlation functions. As such, the proposed arrays can be used as a finite-size array having a zero-correlation zone. The proposed array sets can be constructed from an arbitrary Hadamard matrix. The member size of the proposed array set is close to the theoretical upper bound.

A new technique to reduce the isolation network's size in a Marchand balun needed for perfect all-port matching and isolation is proposed. The proposed isolation circuit is realized using a coupled-line phase-inverter in place of the bulky 180line section that has been previously proposed. Analysis of the proposed circuit yields the required relationship between coupling coefficient and electrical length of the coupler. Based on the design equations, the circuit is experimentally demonstrated at 1.8 GHz and has shown excellent results. The obtained output return loss and isolation loss are more than 18 dB and 40 dB, respectively. The proposed balun was then applied to the application of a doubled-balanced ring-diode mixer. The designed mixer achieves a low conversion loss of 6 dB at its operating frequency, which is 1.5 dB lower than for a doubled-balanced diode mixer using a conventional impedance-transforming Marchand balun. The RF-IF and LO-IF isolations are well below 25 dB and 18 dB across 1 GHz RF operating bandwidth, respectively.

The present paper introduces a new approach to the construction of a class of ternary sequences having a zero-correlation zone. The cross-correlation function of each pair of the proposed sequences is zero for phase shifts within the zero-correlation zone, and the auto-correlation function of each proposed sequence is zero for phase shifts within the zero-correlation zone, except for zero-shift. The proposed sequence set has a zero-correlation zone for periodic, aperiodic, and odd correlation functions. As such, the proposed sequence can be used as a finite-length sequence with a zero-correlation zone. A set of the proposed sequences can be constructed for any set of Hadamard sequences of length n. The constructed sequence set consists of 2n ternary sequences, and the length of each sequence is (n+1)2m+2 for a non-negative integer m. The periodic correlation function, the aperiodic correlation function, and the odd correlation function of the proposed sequences have a zero-correlation zone from -(2m+1-1) to (2m+1-1). The member size of the proposed sequence set is of the theoretical upper bound of the member size of a sequence having a zero-correlation zone. The ratio of the number of non-zero elements to the the sequence length of the proposed sequence is also .

Detection efficiency and dark count of a Geiger mode single photon detection avalanche photodiode was studied by a numerical simulation. The ionization process triggered by a single hole injection was simulated at a bias voltage slightly greater than the avalanche breakdown voltage for calculation of the detection efficiency. Tunneling effect in the multiplication layer was taken into account for the dark count simulation. In the gated-mode operation, the avalanche build-up time also affects on the signal to noise ratio. The multiplication layer thickness is a key parameter for the device performances.