High-frequency operation of surface-acoustic-wave muiltielectrode transducers

The operation of transducers with more than two electrodes per electrical period is disussed. When two different structures are used for input and output transducers, the frequency responses can be made to overlap at one desired band of frequencies, while reducing other surface-and bulk-wave responses.     

Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

Low‐loss TCP/IP header compression for wireless networks

Wireless is becoming a popular way to connect mobile computers to the Internet and other networks. The bandwidth of wireless links will probably always be limited due to properties of the physical medium and regulatory limits on the use of frequencies for radio communication. Therefore, it is necessary for network protocols to utilize the available bandwidth efficiently. Headers of IP packets are growing and the bandwidth required for transmitting headers is increasing. With the coming of IPv6 the address size increases from 4 to 16 bytes and the basic IP header increases from 20 to 40 bytes. Moreover, most mobility schemes tunnel packets addressed to mobile hosts by adding an extra IP header or extra routing information, typically increasing the size of TCP/IPv4 headers to 60 bytes and TCP/IPv6 headers to 100 bytes. In this paper, we provide new header compression schemes for UDP/IP and TCP/IP protocols. We show how to reduce the size of UDP/IP headers by an order of magnitude, down to four to five bytes. Our method works over simplex links, lossy links, multi‐access links, and supports multicast communication. We also show how to generalize the most commonly used method for header compression for TCP/IPv4, developed by Jacobson, to IPv6 and multiple IP headers. The resulting scheme unfortunately reduces TCP throughput over lossy links due to unfavorable interaction with TCP's congestion control mechanisms. However, by adding two simple mechanisms the potential gain from header compression can be realized over lossy wireless networks as well as point‐to‐point modem links. This revised version was published online in June 2006 with corrections to the Cover Date.     

ORGANOLEPTIC THRESHOLD VALUES OF SOME ALCOHOLS AND ESTERS IN BEER

In a Norwegian all-malt pilsener beer the organoleptic threshold values were determined for the following alcohols and esters: n-propanol, isobutanol, amyl alcohols (2-methyl butanol + 3-methyl butanol), 2-phenyl ethanol, ethyl acetate, isobutyl acetate, iso-amyl acetate, 2-phenyl ethyl acetate, ethyl caproate and ethyl caprylate. In addition, threshold values for a number of combinations of these alcohols and esters were determined, and in each case an additive effect was found.     

Design of s.a.w. convolver for processing m.s.k. modulated waveforms

An acoustic waveguide convolver designed for matched filtering of m.s.k. modulated sequences with chirp period of 10.8 ns and with a processing gain of ~ 30 dB is reported. The device efficiency is ?63.5 dBm.     

复配稀土改性剂对MGF/PTFE复合材料性能的影响

采用稀土改性剂(RES)与硅烷偶联剂(PTMS)按不同组分配比对磨碎玻璃纤维(MGF)表面进行改性处理,将改性后的磨碎玻璃纤维粉末与聚四氟乙烯分散液机械混合,然后热压制得复合材料。探讨了复配稀土改性剂对MGF/PTFE复合材料介电性能、热膨胀系数(CTE)、热导率的影响。采用FTIR手段对稀土改性剂未改性的磨碎玻璃纤维和改性后的磨碎玻璃纤维的结构进行了测试,并用扫描电子显微镜(SEM)对复合材料的断口形貌进行分析。结果表明,复配改性剂能很好地促进MGF与PTFE之间的界面粘结,提高MGF/PTFE复合材料的性能。当RES、PTMS的含量分别为0.3%(质量分数)、1.7%(质量分数)时,MGF/PTFE复合材料的性能最好。     

稀土改性剂对玻璃纤维-SiO2/聚四氟乙烯复合材料性能的影响

采用不同的稀土改性剂（La、Ce、Pr、Nd和Sm）对SiO₂和玻璃纤维（GF）进行改性处理,SiO₂含量为55wt%,GF含量为2wt%。填料改性处理后与聚四氟乙烯（PTFE）分散液在高速分散机中混合,然后通过热压烧结法制得GF-SiO₂/PTFE复合材料。考察了不同的稀土改性剂对GF-SiO₂/PTFE复合材料吸水性、介电性能、热膨胀系数和力学性能的影响。采用FTIR手段对稀土La改性剂,未改性的填料和改性后的填料结构进行了测试。并用SEM和EDS对复合材料的断口形貌及表面处理前后填料的形貌进行观察和能谱分析。结果表明：由于稀土La的电子层结构比其他轻稀土更稳定,对阴离子的吸引作用比其他轻稀土强,稀土La改性剂比其他的稀土改性剂能更好地促进填料GF与PTFE界面粘合,改善GF-SiO₂/PTFE复合材料性能。GF-SiO₂/PTFE复合材料的介电性能受到La含量的影响,当La含量为0.3wt%时介电性能最佳。     

Sparse solutions to linear inverse problems with multiple measurement vectors

We address the problem of finding sparse solutions to an underdetermined system of equations when there are multiple measurement vectors having the same, but unknown, sparsity structure. The single measurement sparse solution problem has been extensively studied in the past. Although known to be NP-hard, many single-measurement suboptimal algorithms have been formulated that have found utility in many different applications. Here, we consider in depth the extension of two classes of algorithms-Matching Pursuit (MP) and FOCal Underdetermined System Solver (FOCUSS)-to the multiple measurement case so that they may be used in applications such as neuromagnetic imaging, where multiple measurement vectors are available, and solutions with a common sparsity structure must be computed. Cost functions appropriate to the multiple measurement problem are developed, and algorithms are derived based on their minimization. A simulation study is conducted on a test-case dictionary to show how the utilization of more than one measurement vector improves the performance of the MP and FOCUSS classes of algorithm, and their performances are compared.     

Acoustooptic properties of a weakly multimode solid core photonic crystal fiber

The acoustic and acoustooptic properties of a weakly multimode solid core photonic crystal fiber (PCF) are studied. The phase velocity of the lowest order flexural acoustic mode is measured as a function of frequency and compared with the results for a hollow core PCF and a standard optical fiber. Acoustooptic interaction is used to couple light from the lowest order to the first higher order optical modes of the solid core PCF. The experimental results are compared with theoretical predictions.     

WORT COMPOSITION AND BEER FLAVOUR. II. THE INFLUENCE OF DIFFERENT CARBOHYDRATES ON THE FORMATION OF SOME FLAVOUR COMPONENTS DURING FERMENTATION

Wort, to which was added various amounts of solutions of glucose, fructose, sucrose or maltose, was fermented, and in the resulting beers the concentrations of the following flavour components were determined by gas chromatography: ethyl acetate, isobutyl acetate, iso-amyl acetate, 2-phenyl ethyl acetate, ethyl caproate, ethyl caprylate, n-propanol, isobutanol, amyl alcohols, 2-phenyl ethanol, caprylic acid and capric acid. The concentrations of these compounds were affected in different ways by the various amounts of sugar added, and some differences were observed etween the different carbohydrates.