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11.
An L-shaped array for estimating 2-D directions of wave arrival   总被引:15,自引:0,他引:15  
A simple structured 2-D array, the L-shaped array, is presented. The L-shaped array consists of two uniform linear arrays (ULA) connected orthogonally at one end of each ULA. It is shown that the Cramer-Rao bounds (CRB) of the estimated wave directions based on the L-shaped array are about 37% smaller than those for the cross array. The CRB indicates the accuracy potential because it is the (reachable) lower bound on the variance of any unbiased estimate. An efficient maximum likelihood algorithm is developed utilizing the ULA structure inherent in the L-shaped array  相似文献   
12.
We present a novel waveform frequency conversion technique by type II second harmonic generation (SHG). We have theoretically studied and experimentally demonstrated femtosecond waveform transfer from 800 to 400 nm wavelength by type II SHG in a lithium triborate (LBO) crystal. The output second harmonic (SH) waveform is a temporally magnified or compressed version of the input fundamental waveform, depending on the experimental configuration. Our experiments demonstrate that this technique can transfer not only the intensity profiles of the fundamental waveform, but also the phase profiles. This technique shows promise for programmable generation of shaped ultrafast optical waveforms down to the UV range, where direct pulse shaping is challenging.  相似文献   
13.
Return-to-zero differential phase-shift keying applications require a differential amplifier with high bandwidth, high gain, low noise, and good input impedance match. In this paper, we describe an InGaAs-InP heterostructure bipolar transistor differential transimpedance amplifier with high bandwidth of 47 GHz and high gain of 56 dB-/spl Omega/. The input-referred current noise is less than 35 pA//spl radic/Hz over the measurement range up to 40 GHz.  相似文献   
14.
In this study, poly(dl ‐lactide‐co‐glycolide)/porous silicon (PLGA/pSi) composite microspheres, synthesized by a solid‐in‐oil‐in‐water (S/O/W) emulsion method, are developed for the long‐term controlled delivery of biomolecules for orthopedic tissue engineering applications. Confocal and fluorescent microscopy, together with material analysis, show that each composite microsphere contained multiple pSi particles embedded within the PLGA matrix. The release profiles of fluorescein isothiocyanate (FITC)‐labeled bovine serum albumin (FITC‐BSA), loaded inside the pSi within the PLGA matrix, indicate that both PLGA and pSi contribute to the control of the release rate of the payload. Protein stability studies show that PLGA/pSi composite can protect BSA from degradation during the long term release. We find that during the degradation of the composite material, the presence of the pSi particles neutralizes the acidic pH due to the PLGA degradation by‐products, thus minimizing the risk of inducing inflammatory responses in the exposed cells while stimulating the mineralization in osteogenic growth media. Confocal studies show that the cellular uptake of the composite microspheres is avoided, while the fluorescent payload is detectable intracellularly after 7 days of co‐incubation. In conclusion, the PLGA/pSi composite microspheres offer an additional level of controlled release and could be ideal candidates as drug delivery vehicles for orthopedic tissue engineering applications.  相似文献   
15.
Shaping, signal processing, and time-space conversion of femtosecond pulses can be achieved by linear and nonlinear manipulation of the spatially dispersed optical frequency spectrum within a grating and lens pulse shaper. In this paper, we first review our work on femtosecond pulse shaping and processing, with an emphasis on applications to high-speed communications and information processing. We then present a new concept for generalized time-space processing based on cascaded time-to-space and space-to-time conversions in conjunction with smart pixel optoelectronic arrays and provide a detailed discussion of our recent studies of time-to-space conversion based on second-harmonic generation (SHG) within a femtosecond pulse shaper  相似文献   
16.
We discuss a modified femtosecond pulse shaper that uses microlens arrays to convert the continuous band of frequencies normally obtained at the mask plane of a pulse shaper to a series of discrete spots. Two possible modified pulse-shaping geometries are described and the optimum geometry is chosen. Our experiments demonstrate that this modification can improve pulse-shaping quality when modulator arrays with large interpixel gaps are used for Fourier-plane filtering  相似文献   
17.
The generation of extremely narrow, high peak power pulses using an optically activated impulse generator is described. Radiative measurements at 1 Hz PRF have been conducted at pulse bias levels up to 15 kV, using an optical pulse from a Nd:YAG laser to trigger the device. The measured pulse from a wideband antenna has a pulsewidth of 1.5 ns with a risetime of 900 ps. The frequency spectrum of this radiated waveform ranged from 50 MHz to 1 GHz  相似文献   
18.
We derive the spectral dispersion law for the virtually imaged phased array (VIPA) based on paraxial wave theory using the Fresnel diffraction analysis. The validity of the dispersion law is verified by comparison with experiments. This spectral dispersion law is compared to a previous law based on plane wave theory. At small incident angles where the VIPA provides its largest spectral dispersion, the paraxial wave law provides a much better fit to the data than the plane wave law does.  相似文献   
19.
The combination of device speed (f/sub T/, f/sub max/ > 150 GHz) and breakdown voltage (V/sub bceo/ > 8 V) makes the double heterojunction bipolar InP-based transistor (D-HBT) an attractive technology to implement the most demanding analog functions of 40-Gb/s transceivers. This is illustrated by the performance of a number of analog circuits realized in an InP D-HBT technology with an 1.2- or 1.6-/spl mu/m-wide emitter finger: a low phase noise push-push voltage-controlled oscillator with -7-dBm output power at 146 GHz, a 40-GHz bandwidth and low-jitter 40-Gb/s limiting amplifier, a lumped 40-Gb/s limiting driver amplifier with 4.5-V/sub pp/ differential output swing, a distributed 40-Gb/s driver amplifier with 6-V/sub pp/ differential output swing, and a number of distributed preamplifiers with up to 1.3-THz gain-bandwidth product.  相似文献   
20.
We report transmission of ~60-fs and ~245-fs pulses, respectively, over 42-m and 2.5-km fiber links which consist of standard single-mode fibers (SMF) concatenated with dispersion-compensating fibers (DCF). The experiments using very short pulses (~60 fs) over a short fiber length (~42 m) demonstrate the ability to achieve simultaneous dispersion and dispersion slope compensation using this technique. Femtosecond spectral interferometry measurements of this 42-m link show that its residual dispersion slope is approximately six times lower than that of the dispersion-shifted fiber. Finally, to demonstrate that the dispersion-limited propagation distance is proportional to the cube of the pulsewidth, we transmit ~245-fs pulses over a 2.5-km SMF-DCF link and achieve comparable pulse restoration as with the shorter fiber experiments  相似文献   
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