Pattern synthesis for TechSat21 - a distributed space-based radar system

The TechSat21 space-based radar employs a cluster of free-floating satellites, each of which transmits its own orthogonal signal and receives all reflected signals. The satellites operate coherently at the X band. The cluster forms essentially a multielement interferometer, with a concomitantly large number of grating lobes and significant ground clutter. A novel technique for pattern synthesis in angle-frequency space is proposed, which exploits the double periodicities of the grating lobes in the angular domain and of the radar pulses in the frequency domain, and allows substantial gains in clutter suppression. Gains from 7 to 17 dB relative to the normal random, sparse array appear feasible.     

Design of electronic sections for nano-displacement measuring system

Noncontact displacement measurement is generally based on the interferometry method.In the semiconductor industry,a technique for measuring small features is required as circuit integration becomes denser and the wafer size becomes larger.An interferometric system known as a three-longitudinal-mode heterodyne interferometer (TLMI) is made of two main parts:optical setup and electronic sections.In the optical part,the base and measurement signals having 500-MHz frequency are produced,resulting from interfering three longitudinal modes.The secondary beat frequency to measure the displacement in the TLMI is about 300kHz.To extract the secondary beat frequency,wide-band amplifiers,double-balanced mixers (DBMs),band-pass filters (BPFs),and low-pass filters (LPFs) are used.In this paper,we design the integrated circuit of a super-heterodyne interferometer with total gain of 56.9dB in size of 1030μm×1030μm.     

对静止轨道卫星成像的SAL载星轨道

星载合成孔径激光雷达对卫星成像面临相对运动复杂、目标搜索困难、成像时机难把握等问题。以对静止轨道卫星成像为例,初步设计了一种适于对同步轨道卫星成像的载星轨道。推导了载星与目标间相对运动关系式;绘制并分析了两星相对运动曲线;分析了两星距离最近期间距离、径向速度、加速度变化规律。提出了基于径向速度门限的成像时机判定方法,研究了成像期间光束指向偏移和成像时机判决速度门限。研究结果表明:轨道高度低于目标星轨道,并与之有一定偏角的载星轨道适于对目标进行成像;两星相距最近的时间附近是较佳成像时机,可降低相位误差补偿难度,提高系统工作效率;在对目标进行搜索跟踪和判决成像时机时,系统需根据两星轨道特性进行动态调整。     

Bit Error Rate Comparison of Repeater and Regenerative Communication Satellites

We evaluate and compare bit error rates for conventional (linear and nonlinear) repeater satellites and regenerative satellites. This work is based on a new formulation for error rates using the generalized moment technique for numerical evaluation of error rates. Both coherent MPSK and noncoherent MFSK modulations are considered with regenerative satellites. The results show that regenerative satellites are especially effective against uplink CW interference when on-board despreading of spread spectrum signals is included as part of the satellite processing.     

Interferometric measurement of the elongation of a pulsed diode laser

An interferometric technique was developed for detecting fast displacements of small areas in beam direction by less thanlambda/8. It utilizes a modified Michelson interferometer and a He-Ne laser light source. The technique was applied to measure the temporal and spatial evolution of the thermal mirror deformation of a GaAs-Ga1-xAlxAs single heterostructure laser diode. A small spot on the facet of the laser diode formed one mirror of the interferometer. The other mirror was moved linearly with time by applying a ramp voltage to a piezoelectric transducer. The diode was pulsed during this motion. The local elongation of the laser diode was evaluated from the interferometer output signal. The measured elongation of the active region at a time 1 μs after a 150-ns pulse with current density 48 000 A/cm²was between 4 and 20 nm, indicating a temperature rise of2-11degC. The smallest elongations detected were in the range of 0.5-1 nm. The measured values are in substantial agreement with theoretical expectations from thermal model calculations.     

The Michelson resonator free-electron laser. II. Supermodestructure and mirror detuning effects

For pt.I, see ibid., vol.29, no.2, p.452-64 (1993). Conventional mode-locked laser theory is applied to the longitudinal mode structure in an RF linac-driven Michelson resonator free-electron laser (FEL). A greatly simplified derivation of the small-signal small-gain FEL coupled mode equations is obtained. These equations are solved numerically in the frequency domain to study the supermode evolution in the presence of mode dependent cavity losses. The results are compared with simulations of the detuned Michelson resonator FEL using a pulse propagation code based on the Maxwell-Lorentz equations of motion. Increasing the interferometer detuning broadens and shifts the cavity detuning curve, narrows the supermode spectrum, and decreases the hypermode decay rates. The practical consequences of each of these effects are discussed. A simple theory describing the dependence of the hypermode decay rates on interferometer detuning, in which the decay rates are abruptly decreased beyond a critical detuning that depends primarily on the slippage length, is outlined     

光刻机超精密工件台研究

针对步进扫描投影型光刻机超精密工件台开展研究熏所搭建的超精密气浮运动试验台采用气浮直线导轨支撑、直线电机驱动、直线光栅尺反馈组成大行程直线运动系统,其上叠加洛仑兹电机驱动的气浮微动台,提供对直线电机运动的精度补偿,由双频激光干涉仪提供粗、精动运动系统的超精密位置检测和反馈,两套直线运动系统可研究同步扫描运动,并可灵活地组成多种x-y超精密运动系统。介绍了基于该试验台进行的超精密运动控制试验研究,运动定位精度已经达到12nm     

Coherence of a room-temperature CW GaAs/GaAlAs injection laser

The temporal coherence of a stripe-geometry double-heterojunction GaAs/GaAlAs laser operating CW at room temperature was determined. A heterodyne detection scheme was used involving the mixing of the laser field with a frequency-shifted and time-delayed image of itself in an interferometer. Because the laser device oscillated in several longitudinal modes, the autocorrelation function of its output exhibited resonances for specific time delays. The rate at which the amplitude of these resonances decreased with increasing time delays provided a measure of an apparent coherence length associated with individual longitudinal modes. The coherence length, so defined, was found to increase linearly with drive current in excess of threshold. This observation is interpreted as evidence that the intrinsic linewidth of a longitudinal mode is inversely proportional to the coherent optical power in that mode. Apparent coherence lengths were a few centimeters for a few milliwatts of total optical power emitted per facet. For a perfectly balanced interferometer, a sharp heterodyne beat signal was also observed when the laser device was operated considerably below threshold, i.e., in the LED mode.     

Simultaneous absolute frequency control of laser transmitters inboth 1.3 and 1.55 μm bands for multiwavelength communication systems

Absolute frequency control will be an essential part of future dense WDM systems. In this paper, we demonstrate two promising techniques that allow the absolute frequency control of an ensemble of laser transmitters operating in both the 1.3 and 1.55 μm bands. First, a Michelson interferometer is absolutely calibrated by means of a frequency-stabilized master DFB laser. This interferometer provides an ensemble of evenly-spaced absolute frequency references that covers both the 1.3 and 1.55 μm regions. Lasers are frequency-locked to transmission nulls of this interferometer with a precision of a few hundred MHz. The second technique allows full flexibility in channel frequency assignment and relies on frequency offset control of an ensemble of laser sources relative to a master reference laser. The frequency comparator is based on a surface-emitting nonlinear semiconductor multilayer waveguide. This technique provides simultaneous frequency measurement and control of lasers in both bands with a precision of a few GHz     

卫星星间链路多普勒频移仿真

在卫星通信过程中,对具有相对运动的卫星,星间链路的多普勒频移是不能忽视的。分析了空间圆轨道卫星星间链路的距离以及距离变化率,并进行了公式推导。在此基础上提出了一种以链路距离变化率作为卫星相对运动速度来计算星间链路多普勒频移的新方法,并以铱星系统和某中轨卫星系统为例,分别对同层高度星间链路和异层高度星间链路的多普勒频移进行仿真,给出了仿真结果。分析与结果表明,该方法合理、有效。     

色散对法布里-珀罗干涉仪纵模间隔的影响

为了分析材料色散对法布里-珀罗干涉仪透射特性的影响,采用洛伦兹振子模型计算了法布里-珀罗干涉仪的透射谱,研究了色散强度、衰减系数和谐振频率对纵模间隔的影响。结果表明,高、低频段的纵模间隔随色散强度的增大而减小,随衰减系数的增大而增大;对于谐振频率附近的纵模,纵模间隔随着色散强度的增大先增大后减小;色散强度很小时,纵模间隔随衰减系数的增大而减小,随谐振频率的增大先增大后减小;当色散强度较大时,纵模间隔随衰减系数的增大先增大后减小,随谐振频率的增大而减小。这一结果对激光器的设计和研究有一定的帮助。     

利用测向信息消除高重复频率信号的时差定位模糊

该文针对三星时差定位系统对高重复频率信号的定位模糊问题,提出在主星增加干涉仪测向并结合测向信息解模糊的思路。由于干涉仪本身有可能出现测向模糊,进一步提出了根据模糊测向信息解时差定位模糊的方法,其在无测向模糊时同样适用。该方法定义了时差定位点的距离参数,通过对其做门限检测去除虚假时差定位点,从而有效地减少了模糊定位点数量,改善了对高重频信号的定位模糊问题。     

An Efficient Scheme to Reduce Handoff Dropping in LEO Satellite Systems

The problem of handoffs in cellular networks is compounded in a low earth orbit (LEO) satellite-based cellular network due to the relative motion of the satellites with respect to a stationary observer on earth. Typically, the velocity of motion of mobiles can be ignored when compared to the very high velocity of the footprints of satellites. We exploit this property of LEO satellite systems and propose a handoff scheme based on a channel sharing approach that results in a substantial decrease in handoff dropping. For the same handoff dropping performance, our scheme has significantly lower new call blocking probability than the conventional reservation scheme. We also present an analytical approximation that is in very good accord with simulation results.     

Measurement of the swept-frequency carrier-induced FM response of asemiconductor laser using an incoherent interferometric technique

The swept frequency deviation response of a distributed feedback (DFB) laser is measured using an incoherent interferometric technique. Unlike previous interferometric techniques, this technique is independent of the phase relationship between the optical fields in the two interferometer paths. The theoretical background is discussed, and measurements yielding an FM deviation response of ±1 dB from 0.1 to 2.9 GHz with an efficiency of ~300 MHz/mA are reported for a 1.55-μm DFB laser     

A novel approach for flip chip solder joint quality inspection:laser ultrasound and interferometric system

A novel, noncontact, nondestructive approach for flip chip solder joint quality inspection is presented. In this technique, a pulsed laser generates ultrasound on the chip's surface, exciting the whole chip into a vibration motion. An interferometer was used to measure the vibration displacement of the chip's surface. Because changes in solder joint quality produce a different vibration response, a value, "error ratio," is used to measure the difference between a good chip and a chip with defects. An automatic signal-processing algorithm to calculate the error ratio was developed and implemented, as well as a frequency analysis algorithm. The inspection system was characterized, and results are presented for two cases of flip chips with missing solder balls. Results indicate that a laser ultrasonic/interferometeric system offers great promise for solder bump inspection in flip chip, BGA, chip scale, and micro BGA packages     

Estimating ocean coherence time using dual-baseline interferometricsynthetic aperture radar

A new technique for measuring the coherence time of the ocean surface at radar wavelengths has been developed and tested. This technique requires an interferometric synthetic aperture radar system with at least two unique baselines along the direction of platform motion. The coherence time of the surface may be presented as a high-resolution coherence time map. This technique was tested using the JPL AIRSAR along-track interferometer. Measurements of the ocean coherence at the L-band were made at high spatial resolution under a variety of conditions. A new operating technique that results in the required two baselines is described. Some parametric analysis is performed pertaining to the design of such a system. A representative interferometric data set acquired at the Strait of Messina in 1991 is presented to illustrate the technique. The interferometric phase data reveal and measure tidal currents flowing through the strait, while the coherence time image shows considerable structure not visible in the backscatter or phase image. Coherence times in and around the strait were measured to be of the order of 0.1 s at the L-band, which is in agreement with previous estimates and measurements     

Motion correction of PET images using multiple acquisition frames

Positron emission tomography (PET) is a relatively lengthy brain imaging method. Because it is difficult for the subject to stay still during the data acquisition, head motion during scans is a source of image degradation. A simple data acquisition technique to reduce the effect of this problem is described. The technique associates the incoming data with the real-space position of the head. During the PET scan, the head position is constantly monitored with two video cameras and compared to its initial position. Every time the displacement for a region within the field of view (FOV) is larger than a specified threshold displacement, the PET data acquisition system starts to save the PET data in a new frame. The total number of frames required for a complete study depends on the magnitude of the head motion during the study and on the threshold displacement. At the end of the study, all the acquired frames are reconstructed independently and each image is rotated and translated to coincide with the initial position. When these images are summed, they produce a final image with fewer motion artefacts     

Merged Sagnac-Michelson interferometer for distributed disturbancedetection

An interferometric technique is described to detect and locate perturbations along an optical fiber. This distributed sensor has a position dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift which is detected and converted to spatial information, The sensor consists of a Sagnac interferometer merged with a Michelson interferometer. This is achieved by a frequency selective mirror in the center of the Sagnac-loop. The sensor is illuminated by two light sources with wavelengths λ₁ and λ₂, respectively. The mirror reflects λ₁ and transmits λ₂, causing the interferometer to operate as a Michelson at wavelength λ₁ and as a Sagnac at wavelength λ₂. Any time-varying perturbation on, the fiber will, result in a signal at λ₂ proportional to the product of the rate of phase change caused by the perturbation and the distance of the perturbation relative to the position of the mirror. The output of the Michelson interferometer at wavelength λ₁ is proportional to the phase change caused by the unknown perturbation. By dividing the output of the Sagnac interferometer by the time rate of change of the Michelson interferometer signal, the position of the disturbance relative to the mirror is located. Results obtained with a 200 m-distributed fiber sensor are discussed     

Integrated AlGaAs waveguide components for optical phase differencemeasurement and correction

A Y-junction interferometer phase measurement technique has been developed that is independent of the power or power ratio in the input arms. This technique was incorporated into a proof-of-concept AlGaAs guided-wave Mach-Zehnder interferometer to demonstrate the measurement and correction of a phase difference between the arms. In the first part of this paper, we describe the design, fabrication, and characterization of the individual AlGaAs dielectric-loaded rib waveguide components that were chosen to fabricate the proof-of-concept interferometer. These components include passive waveguides, bends and Y-junctions, and reverse-biased phase modulators. The composition of the waveguide layer was chosen so that these modulators would have low loss at the applied voltage required for maximum phase correction. The choice was based on electroabsorption measurements on test modulators as a function of the energy difference between the bandgap of the waveguide layer and the operating wavelength. The average propagation loss at 862 nm of the passive single-mode guides used was 1.1 dB/cm. The abrupt bend insertion loss was ~0.20 dB/bend for a 0.5° bend angle, and the Y-junction insertion loss was ~0.37 dB for 1.0° full angle. Two-mm-long p⁺ -n^--n⁺ phase modulators typically had V_π voltages of ~6.8 V. On a slightly modified structure, V _π voltages as low as 2.65 V were measured. In the last part of the paper, the proof-of-concept interferometer and test setup are described and recent phase difference measurement and correction data with intentional power imbalances greater than 90% between the interferometer arms are presented. These results demonstrate conclusively that the phase difference measurement and correction performance of the Y-junction interferometer technique are independent of the power ratio in the interferometer input arms for power ratios >10:1     

激光干涉仪在位移传感器自动检定中的应用

介绍了以双纵模热稳频He -Ne激光干涉仪、微机、被检传感器及数据采集A/D卡、微机接口数据采集卡、机械及电动机构、步进电机驱动卡及相应软件等组成的 ,对差动式位移传感器进行在线自动检定的系统原理。通过实验及现场实际应用均表明 :该检定系统具有测量范围大、检测精度高、测量速度快、自动化程度高等特点 ,有良好的实用性及经济性