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the MMF in the lateral offset launching method is 850 ps when the lateral offset is zero, and is reduced very much to 125 ps when the lateral offset is increased to 15 m. It shows that the selective mode excitation with this method does not happen until the lateral offset becomes larger than 15 m. On the contrary, the calculated FWQM of the MMF in the angular offset launching method stays almost constant (about 130 ps), while the lateral offset is varied from 0 to 20 m. This means that the selective excitation is readily achieved with the angular launch method without a sophisticated alignment of the fibers. We have calculated the DMD value of the sample MMF as per the definition in FOTP 220 [13]. The DMD obtained for the conventional lateral offset launching method is about 1.2 ns/km, while it is only 0.17 ns/km for our proposed angular offset launching method.The 3 dB bandwidth of each impulse response in Figures 2 and 4 is calculated by a Fourier transform. This is done for various offset distance and is plotted in Figure 7. The 3 dB bandwidth of the lateral offset launching method gradually increases as a function of offset position and reaches maximum when the offset position is maximum. Unlike the lateral offset launching method, the 3 dB bandwidth of the angular offset launching method becomes highest when the lateral offset is between 7.5 and 15 m. This is evidenced by the presence of almost no side lobes in the detected waveform (Fig. 4) in this offset range. CONCLUSIONWe have investigated the influence of lateral and angular offset techniques on the bandwidth of an MMF through an impulse response study. An enhanced performance in the transmission capacity and a loose alignment tolerance are obtained by introducing a simple angular launching technique. Experimental results show that the bandwidth enhancement can be easily obtained not by finding a proper offset position, but by monitoring the coupled power of the MMF by an angular offset launch method. Therefore, the angular offset method can provide an effective utilization of an MMF to achieve an increased transmission capacity. This method can be a practical choice in designing transmitters for high-speed fiber-optic LAN applications with an MMF. Photoconductive semiconductor switch (PCSS) could be used to produce ultrashort pulse for ultrawide-band radiation which has widely application in remote and subsurface sensing by higher imaging resolution and better target characterization. So the PCSS and UWB antenna have become an attractive issue that has been addressed by several authors [1-5] over the years. The experimental investigations of exponential tapered coplanar stripline antenna and tem horn base on PCSS source system has been reported in [1, 2]. A compact parallel plate source used to drive a tem horn impulse-radiating has been proposed in [3]. A frequency and beam reconfigurable antenna using photoconductive switches has been given in [4]. With the development of finite time difference domain (FDTD) theory, it has been used to study PCSS an...

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We report laboratory test results of a long period grating (LPG) that can maintain a constant resonant peak depth over an enhanced tuning range when it is coated with an indium tin oxide (ITO) electrode that has optimized thickness and refractive index. The authors have experimentally demonstrated a LPG coated with ITO that can be tuned in excess of 200 nm with an ambient refractive index change of less than 0.01. To the best of the authors' knowledge, this is the highest sensitivity reported for a LPG to date. In addition to the tuning performance, the resonant peak remains within 1 dB of its maximum depth for at least 100 nm of the tuning range.

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