Optical frequency domain ranging by a frequency-shifted feedbacklaser

This paper describes the theoretical and experimental study of a new technique for optical frequency domain ranging (OFDR) by a frequency-shifted feedback (FSF) laser. In conventional OFDR, a frequency chirped single-mode laser is used as a light source to convert a distance into a beat frequency, and a tradeoff exists between measurement range and resolution. The FSF laser output consists of periodically generated chirped frequency components whose chirp rate is faster than 100 PHz/s (P=10¹⁵), By use of the FSF laser, the tradeoff is removed and long-distance high-resolution OFDR is realized In the experiment, a distance of 18.5 km was measured with a resolution of 20 mm     

Effect of Annealing Ambient on the Self-Formation Mechanism of Diffusion Barrier Layers Used in Cu(Ti) Interconnects

Copper (titanium) [Cu(Ti)] films with low titanium (Ti) concentration were found to form thin Ti-rich barrier layers at the film/substrate interfaces after annealing, which is referred to as self-formation of the barrier layers. This Cu(Ti) alloy was one of the best candidates for interconnect materials used in next-generation ultra-large-scale integrated (ULSI) devices that require both very thin barrier layers and low-resistance interconnects. In the present paper, in order to investigate the influences of annealing ambient on resistivity and microstructure of the Cu alloys, the Cu(7.3at.%Ti) films were prepared on the SiO₂ substrates and annealed at 500°C in ultra-high vacuum (UHV) or argon (Ar) with a small amount of impurity oxygen. After annealing the film at 500°C in UHV, the resistivity was not reduced below 16 μΩ-cm. Intermetallic compounds of Cu₄Ti were observed to form in the films and believed to cause the high resistivity. However, after subsequently annealing in Ar, these compounds were found to decompose to form surface TiO _x and interfacial barrier layers, and the resistivity was reduced to 3.0 μΩ-cm. The present experiment suggested that oxygen reactive to titanium during annealing played an important role for both self-formation of the interfacial barrier layers and reduction of the interconnect resistivity.     

Class-Based Packet Scheduling to Improve QoS for IP Video

An important issue for video transmission over IP networks is the preservation of perceived video quality despite packet loss. Packet loss can be detrimental to compressed video. However, reducing packet loss to a very low level is difficult with current techniques. Furthermore, even a very low objective loss probability can still seriously distort perceived video quality. This paper presents a packet scheduling scheme at a router which addresses the loss issues of networked video. Experiments using real video data show that the proposed scheme can significantly improve the visual quality of video and network efficiency. Moreover, it can provide different classes of videos with different levels of loss guarantees, while maintaining service fairness among the same class of videos.     

Application-aware buffer management: new metrics and techniques

An important issue for video transmission over IP networks is preservation of perceived video quality despite packet loss. Packet loss can be detrimental to compressed video. However, reducing packet loss to a very low level is difficult with current loss control techniques. Furthermore, even a very low objective loss probability can still seriously distort perceived video quality. This paper presents two buffer management schemes using video characteristics. They increase maximum loss tolerance for a desired level of video quality, providing better quality at equal loss ratios, when compared to a conventional buffer management scheme. Meanwhile, service fairness and network efficiency are also improved.     

Novel Conjugated Organic Dyes for Efficient Dye‐Sensitized Solar Cells

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO₂ solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm^–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (J_sc) = 12.9 mA cm^–2, open‐circuit voltage (V_oc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO₂ occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO₂ surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.     

Dependence of the efficiency-bandwidth product on electrical volumeof small dielectric loaded antennas

The efficiency fractional efficiency bandwidth product (EB) that is expressed as a ratio of the radiation resistance to the absolute value of an input reactance of an antenna is used as a performance measure for small dielectric-loaded monopole antennas (DLMAs). The dependence of the efficiency bandwidth (EB) on the permittivity of the dielectric loading (i.e., the electrical volume) is experimentally investigated for the first time after we succeeded in measuring the small radiation resistance of the DLMAs precisely on the basis of the Wheeler cap method. As a result, it was found that the EBs of some DLMAs were enhanced over a bare monopole antenna and an EB characteristic curve has a maximum point. This result suggests the presence of the optimum electrical volume for the dielectric loading in order to obtain the best EB performance. A calculation by the finite-difference time-domain (FDTD) method was also conducted to confirm the presence of the maximum EB value under a certain condition. A general reason for the existence of the peak value is also explained using a mathematical deduction     

1.55-μm wavelength periodic traveling-wave photodetectorfabricated using unitraveling-carrier photodiode structures

We report the first periodic traveling-wave photodetector fabricated by using unitraveling-carrier photodiodes (UTC-PDs). The photodetector comprises three UTC-PDs with a 0.06-μm absorption layer that is periodically arranged along a coplanar waveguide transmission line. The photoresponse of the photodetector is three times larger than that of a single UTC-PD. A photodetector with terminal resistors at the input end achieves a 3-dB bandwidth of 115 GHz at 1.55 μm. This is almost the same value as that for a single UTC-PD. For one without terminal resistors, the 3-dB bandwidth degrades to 56 GHz as a result of the backward-propagating microwave being reflected at the input end. These results suggest that the photodetector is well designed with both velocity and impedance matchings     

Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: trend in the deviation from the traditional way of estimating the interfacial electronic structures

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). Interfaces of TTN (tetrathianaphthacene) and TCNQ (tetracyanoquinodimethane) were also studied as extreme cases for hole transport and electron transport material, respectively. For all organic/metal interfaces studied, the work function of metal electrode was changed by deposition of organic layer, i.e., the vacuum level was shifted at the interface, indicating the invalidity of the traditional energy level alignment model where a common vacuum level was assumed at organic/metal interface. At TCNQ/Au, DP-NTCI/Al, which are acceptor/metal interfaces, upward shift of the vacuum level of organic layer relative to that of metal was observed, suggesting the formation of interfacial dipole due to electron-transfer from metal to acceptor. At other organic/metal interfaces, TPD(N, N'-diphenyl-N, N'-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine)/Au or ITO (indium tin oxide), ALq/sub 3/ (tris(8-hydroxyquinolino) aluminum)/Al, DP-NTCl(N, N'-diphenyl-1,4,5,8- naphthyltetracarboxylimide)/Al or Au, downward shift of the vacuum level was observed. Such downward shift has been also observed in our previous study for porphyrin/metal interfaces, and seems to be a trend for organic/metal interfaces at which no electron-transfer from metal to organic layer occurs. This trend suggests that the traditional model tends to underestimate (overestimate) the barrier height for hole (electron) injection. On the other hand, the vacuum level shift at ALq/sub 3//TPD interface was less than 0.1 eV, leading to an apparent applicability of the traditional model. However, it is not always the case for organic/organic interfaces: finite shift of 0.2 eV was observed at TTN/TCNQ interface due to electron-transfer from TTN to TCNQ. Possible origins of vacuum level shift at organic/metal interfaces were also discussed.