Optical gain and gain suppression of quantum-well lasers withvalence band mixing

The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression     

Variable threshold detection with weighted BPSK/PCM speech signalstransmitted over Gaussian channel

Varible threshold detection with weighted pulse code modulation encoded signals transmitted over Gaussian channel is investigated. In general, the bits in any encoded PCM word are transmitted with equal energy on all transmitted bits. To obtain gains in overall signal-to-noise (SNR) ratio, each bit in a PCM-word has been previously weighted according to its significance in the transmitter. For further improvement in SNR, an erasure zone is established at the receiver according to the value of each bit's significance. Therefore, it the output signal transmitted with weighted energy falls into the erasure zone, the regenerated sample is replaced by interpolation. The overall system SNR for BPSK/PCM speech signals of this technique has been found     

ATSC Terrestrial Digital Television Broadcasting Using Single Frequency Networks

In this paper, we propose an efficient method to broadcast digital television signals using single frequency networks (SFNs) within the Advanced Television Systems Committee (ATSC) transmission systems. In implementing the SFNs of an 8‐vestigial side band (8‐VSB) Digital Television (DTV) system, the ambiguity problem of the trellis coder is unavoidable in a conventional ATSC transmission system. We propose a memory initialization of the trellis coder to resolve this ambiguity problem. Since the proposed scheme to synchronize multiple transmitters minimizes the changes from the conventional ATSC system, the hardware complexity for these changes is very low. Our simulation results show that the proposed scheme makes a less than 0.1 dB degradation at the threshold of visibility with a bit error rate of 3×10^?6 in the additive white Gaussian noise (AWGN) channel. It is possible to reduce the performance degradation by increasing the initialization period of the proposed scheme.     

Time-dependent dielectric breakdown characteristics ofN2O oxide under dynamic stressing

Time-dependent dielectric breakdown (TDDB) characteristics of MOS capacitors with thin (120-Å) N₂O gate oxide under dynamic unipolar and bipolar stress have been studied and compared to those with control thermal gate oxide of identical thickness. Results show that N₂O oxide has significant improvement in t _BD (2×under-V_g unipolar stress, 20×under+V_g unipolar stress, and 10×under bipolar stress). The improvement of t_BD in N₂O oxide is attributed to the suppressed electron trapping and enhanced hole detrapping due to the nitrogen incorporation at the SiO₂/Si interface     

Performance and parameter region for real-time use in IEEE 802.4token bus network

The upper and lower bounds for the mean cycle time and the mean message transmission time of class six and class four in the IEEE 802.4 token bus network, within which the minimum utilization constraint of class four is guaranteed, are derived. Stability conditions for the token bus network are also derived. These bounds and stability conditions are represented in terms of the high-priority token hold time, the token rotation time, the arrival rate, the total number of stations, etc. A parameter-tuning algorithm in a partially symmetric token bus network with two classes is suggested. This algorithm maximizes the utilization of class four for a given high-priority token hold time and at the same time meets the constraints of the stability condition of the network, the real-time constraint, and the minimum utilization of class four     

Frequency doubling of 1549 nm, 3 ns pulse using all-fibre system

Periodically poled D-shape standard singlemode fibres were used to generate second harmonics of 1549 nm, 3 ns pulses generated by an all-fibre system, where pump pulses, seeded by an external cavity tunable LD and amplified by a chain of EDFAs, were launched into the poled fibre by a butt-joint. The authors believe that the present nonlinear test of D-shape standard singlemode fibres using the all-fibre system exploits the prospect of fibre devices in a fibre network system.     

Application of PIV to over-expanded supersonic flows: Possibilities and limits

Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO₂) particle. The seeding particle of larger size , 1.4 μrn Ti0₂, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.     

Integration and characterization of amorphous silicon thin-filmtransistor and Mo-tips for active-matrix cathodes

We have designed and monolithically integrated amorphous silicon thin-film transistor (a-Si TFT) with Mo-tip field emitter arrays (FEAs) on glass substrate for active-matrix cathodes (AMCs) in field-emission display (FED) application. In our AMCs, a light shield layer of metal was introduced to reduce the photo leakage and back channel currents of a-Si TFT. The light shield was designed to have the role of focusing grid to focus emitted electron beams from the AMC on the corresponding anode pixel by forming it around the Mo-tip FEAs as well as above the a-Si TFT. The thin film depositions in a-Si TFTs were performed at a high temperature of above 360°C to guarantee the postvacuum packaging process of cathode and anode plates in FED. Also, a novel wet etching process was developed for n⁺-doped-a-Si etching with high etch selectivity to intrinsic a-Si and good etch controllability and was used in the fabrication of inverted stagger TFT with a very thin active layer. The developed a-Si TFTs had good enough performance to be used as control devices for AMCs with Mo-tip emitters. The fabricated AMCs exhibited very effective aging process for field emitters     

Uniform and high coupling efficiency between InGaAsP-InP buried heterostructure optical amplifier and monolithically butt-coupled waveguide using reactive ion etching

We obtained uniform and high coupling efficiency for InGaAsP-InP buried heterostructure (BH) optical amplifiers integrated with butt-coupled waveguides using reactive ion etching (RIE) for mesa definition and low-pressure metalorganic vapor phase epitaxy (LPMOVPE) for waveguide layer regrowth. Measured average coupling efficiency was over 91% across a quarter of 2-in InP wafer. RIE etched vertical facet and a subsequent chemical etching using HBr-based solution for relief of RIE damage enabled us to reduce the coupling loss due to anomalous regrowth shape at the interface. RIE and selective regrowth processes are promising techniques for the fabrication of the photonic integrated circuit (PIC).