Reflectometry by means of optical-coherence modulation

Novel reflectometry with millimetre or submillimetre spatial resolution is proposed to evaluate optical components or circuits. The optical coherence is modulated to have a periodic delta-function shape along the optical path, and the backscattering intensity is obtained directly. The experiment demonstrates a resolution of about 10 mm in air with a Fabry-Perot laser diode.<>     

Pulse compression by reflection from inhomogeneous ionized media

Frequency modulated RF pulses with square envelopes are synthesized to mismatch a specified inhomogeneous ionized medium. These pulses when incident on the ionized medium result in compressed and enhanced reflected signals. Three ionization profiles, which are usually used to model the lower part of the ionosphere, are considered. The frequency modulation of the incident pulses that mismatch an ionized medium is found to depend on the ionization profile, the incidence angle, and frequency range used.     

Frequency modulation of three-terminal Gunn devices by optical means

The three-terminal Gunn device first reported by Petzinger et al. has been frequency modulated by shining light either on the n-type gallium arsenide or the p-n junction of the device. Different sensitivities are achieved in those two modes of operation. While the device was oscillating in the bias-circuit mode at about 500 MHz, frequency variations linearly dependent on the intensity of the light have been observed in both modes of operation. The maximum shifts about 2 percent of the oscillating frequency, has been measured at a light intensity of 25 mW/cm²and when light was directed only on the n-type gallium arsenide. Higher-frequency shifts have been observed in the nonlinear region. The device has demonstrated capability of detecting very short light pulses, ∼ 50 ns, generated by GaAs laser.     

Directional signal modulation by means of switched spaced antennas

The theory of modulation by hopping or by commutation among a number of spaced antennas is presented. The theory brings out the important considerations and criteria for determining the separation of discrete antennas in time and/or space that produces a desired modulation. In addition, antenna hopping and commutation are proposed for generating direction-dependent spread spectrum to enhance receiving system anti-interference processing gain and/or transmitted signal camouflage and LPI, as well as to provide an effective means for channel monitoring and analytical probing     

Pulse compression for weather radars

Wideband waveform techniques, such as pulse compression, allow for accurate weather radar measurements in a short data acquisition time. However, for extended targets such as precipitation systems, range sidelobes mask and corrupt observations of weak phenomena occurring near areas of strong echoes. Therefore, sidelobe suppression is extremely important in precisely determining the echo scattering region. A simulation procedure has been developed to accurately describe the signal returns from distributed weather targets, with pulse compression; waveform coding. This procedure is unique and improves on earlier work by taking into account the effect of target reshuffling during the pulse propagation time which is especially important for long duration pulses. The simulation procedure is capable of generating time series from various input range profiles of reflectivity, mean velocity, spectrum width, and SNR. Results from the simulation are used to evaluate the performance of phase coded pulse compression in conjunction with matched and inverse compression filters. The evaluation is based on comparative analysis of the integrated sidelobe level and Doppler sensitivity after the compression process. Pulse compression data from the CSU-CHILL radar is analyzed. The results from simulation and the data analysis show that pulse-compression techniques indeed provide a viable option for faster scanning rates while still retaining good accuracy in the estimates of various parameters that can be measured using a pulsed-Doppler radar. Also, it is established that with suitable sidelobe suppression filters, the range-time sidelobes can be suppressed to levels that are acceptable for operational and research applications     

Pulse compression in optical fibres

A shift in the emission wavelength of s.h. GaAs lasers has been observed during individual pulses. In conjunction with material dispersion, this leads to pulse compression after transmission through an optical fibre. The use of this device as a source in bandwidth measurements readily leads to erroneous conclusions when results are analysed.     

Pulse restoration by filtering of self-phase modulation broadened optical spectrum

Restoration of distorted optical pulses is achieved using nonlinear fiber self-phase spectral broadening and subsequent optical band-pass filtering of a single sideband. Using this technique, the output pulsewidth is shown to remain constant for input pulse-widths between 9-20 ps. A detailed investigation of the signal-to-noise ratio shows that best performance is obtained by operating in normal fiber dispersion regime. This technique is also applied to restore 40 Gb/s RZ-data suffering distortion from polarization mode dispersion. The high-bandwidth fiber nonlinearity shows promise to scale to higher bit rate pulse distortion correction.     

Pulse compression using the magnetic-delay-line processor

A new approach to pulse compression in the frequency range below several hundred kilohertz is demonstrated. It uses a wire-electrode pattern to process a signal sensed from a conventional magnetic recording. The experimental results are for a filter to compress a linearly chirped signal.     

Pulse compression techniques in nuclear medicine

A high-efficiency "chirped" linear array of pinholes for imaging a distribution of radioisotopes is described. The image is obtained in a coded or scrambled form, and then decoded using a dispersive surface acoustic wave delay line. Results of an optical simulation experiment are presented.     

Pulse compression acoustic microscopy at 750 MHz

A pulse compression acoustic microscope working at a centre frequency of 750 MHz is reported. The pulse expansion and compression is achieved by SAW dispersive delay lines. The transmitted chirp has a bandwidth of 150 MHz and a dispersion of 1 MHz/ns. A processing gain in excess of 10 dB has been achieved.     

Pulse width modulation for analog fiber-optic communications

The pulse width modulation (PWM) technique has been revisited and analyzed to evaluate its merits for application to analog signal transmission in fiber-optic links. Fourier analysis of the PWM signal reveals that it can be used as a vehicle to launch an analog signal onto optical fiber when a symmetrical natural sampling process is used. The SNR of the modulated signal depends on the timing jitter of the carrier pulses and a wide-band (45 MHz) SNR of 45 dB has been obtained with a commercially available multimode laser transmitter. A linear dynamic range of over 50 dB has been experimentally demonstrated. The full fiber bandwidth can be utilized by using a very high pulse carrier frequency, while a more popular pulse frequency modulation technique provides about a 10-MHz analog signal bandwidth when 1 km of multimode fiber is used in conjunction with a short wavelength (0.87 μm) LED transmitter. Analog transmission capability was experimentally demonstrated by constructing a simple video link using common laboratory equipment. The performance of the video link supports the PWM modulation theory developed here and elsewhere. The experimental results indicated that PWM is potentially very attractive for low-cost broad-band local area network (LAN) application, including future highly interactive offices, hospitals, and automated factory floors.     

可调相位型液体表面波衍射光栅效应的研究

为了研究可调相位型液体表面波光栅的衍射效应,采用低频信号发生器驱动振子、激发产生液体表面波的实验方法,得到了清晰、稳定的衍射图样,观察到衍射条纹间距随着液体表面波频率的改变而改变.并且在特定频率输出条件下,改变表面波振幅,可以得到衍射条纹的缺级现象.根据物理光学理论,周期性振动的表面波可以视作为一相位型光栅,对入射光进...     

Pulse compression of CW injection seeded gain-switched diode laser

A commercially available gain-switched diode laser was injection seeded by a CW laser diode to provide a controlled frequency chirped output which was compressed by a grating pair and telescope. Pulses were generated with a width of 12 ps and a peak power of 0.19 W at a repetition rate of 3.2 GHz     

Pulse width modulation method of a single-phase multilevel inverter

A novel pulse-width modulation method for a multilevel single-phase inverter is proposed. The proposed method has the feature that the output voltage changes at each level, compared with the conventional method in which output voltage jumps from zero to the maximum level. Therefore, the adverse effects on load due to the high dv/dt are decreased. The method of determining pulse width is discussed, and the harmonic distortion factors and harmonic contents are analysed. Further improvement by increasing the number of pulses and the experimental results are also described.     

Analysis of polarization modulation spectra of photopleochroism induced by uniaxial compression in Ge crystals

Photoconductivity of Ge crystals in which an anisotropy of conduction was induced by uniaxial compressive strain was studied using polarization modulation of light. The spectral characteristics measured by this method are energy-dependent differences of photoconductivities excited by linearly polarized light with polarizations periodically varying in space with respect to the crystal optical axis. A phenomenological analysis of the photopleochroism spectral characteristic showed that the expression for the photoconductivity includes three cofactors. It was shown that an additional measurement of the photoconductivity derivative or mathematical processing of the photoconductivity spectra makes it possible to expand the photoconductivity in cofactors. The use of one of these techniques allowed us to determine the absorption anisotropy and its spectral characteristic.     

Pulse compression filter generating optimal uniform range sidelobe level

A uniform sidelobe level represents an optimum performance criterion in the design of pulse compression waveforms. A new form of pulse compression filter for polyphase codes is presented which generates a flat uniform sidelobe pattern similar to those obtainable from Barker codes. The sidelobe levels are decided solely by the length of the phase codes, which can be set arbitrarily. Their use involves a small loss and degradation in range resolution, but excellent Doppler tolerance.     

基于FRFT域自适应滤波的脉冲压缩技术

雷达回波中往往存在严重的干扰,采用线性调频信号(linear frequency modulation,LFM)的脉冲压缩雷达系统中,直接对回波信号进行脉冲压缩往往得不到好的效果。文中利用线性调频信号在分数阶傅里叶变换(fractional fourier transform,FRFT)中的强能量聚集性的特点,在分数阶傅里叶域对混合回波进行自适应滤波,再将滤波后的信号变换回时域进行脉冲压缩处理。自适应滤波算法采用一种改进的变步长NLMS算法,相对于传统固定步长NLMS算法有较好的滤波结果。仿真结果表明,经过分数阶傅里叶域自适应滤波后的信号在脉冲压缩后的目标检测结果有了很大的改善,明显提高了脉冲压缩雷达的检测性能。     

脉压波形畸变校正及其快速算法设计

针对雷达信号实时处理中数字正交插值滤波器引起脉压波形畸变的现象,提出了一种新的波形优化方法,采用精确设计的匹配滤波器,消除了数字正交插值滤波器对脉压波形的失配影响,改进了脉压输出的幅度特性和相位特性,提高了检测性能.基于对任意长度序列进行优化的添零-分解FFT方法,提出了利用频域滤波实现大脉压比脉压的一种新的快速算法,当对非连续信号进行脉压时,显著减轻了运算负荷.