Iterative Deconvolution Algorithm for Improving Range Resolution of a CO2-Laser Based Lidar System

An iterative deconvolution algorithm for improving range resolution of long-pulse lidars is proposed, and can be applied to the lidar data obtained with the typical pulse of a CO₂-laser which consists of a gain-switching peak and a long tail. The lidar signal itself with certain temporal shift is set to be the start profile for the unknown maximally resolved profile in the proposed technique, and then is corrected in proportion to the difference between the lidar return calculated with the assumption and the real one. The same process is repeated until the correction is smaller than tolerance. Simulations are made to test the performance of the proposed algorithm. We investigate the errors in the vicinity of data boundary in the retrieved profile when a part of lidar data is absent. The sensitivity of the iteration algorithm to noise in the lidar signals and the laser pulse profile is also numerically determined.     

The measurement of SO2 in power station plumes with differential lidar

The use of differential lidar for measuring SO₂ in plumes from fossil fuelled power stations is examined. The potential of the technique is investigated for the measurement of SO₂ close to power station stacks and in situations where the long range transport of emissions is important. Detection limits are discussed as a function of range and spatial resolution and it is shown theoretically that concentrations as low as 10 ppb at a range of 2 km should be measurable. Details of a prototype, computer controlled, differential lidar based on a tunable dye laser are given and its mode of operation described. Results from field experiments carried out at Drax power station in Yorkshire (UK) and Fawley Power Station in Hampshire (UK) are presented. For plumes of small diameter a deconvolution technique has been applied to obtain better spatial resolution of the plume structure.     

Side on interferometry at 10.6 μm of Theta pinch plasmas

An experiment for measuring electron density spatially resolved with high time resolution is described. The experiment was performed side on on a Theta pinch plasma. The light source (TEA-CO₂ laser), the window problem, the area detector for the interferogram are discussed. The sensitivity of the technique is 1–2×10¹⁵ electrons/cm². Some examples of measured density profiles show that the method may be used effectively on toroidal plasma machines.     

Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO₂-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×10¹¹ W cm^-2–5×10¹² W cm^-2, 2×10¹¹ W cm^-2–2×10¹² W cm^-2 and 3×10¹³ W cm^-2–2×10¹⁴ W cm^-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented. Received: 27 December 1999 / Revised version: 11 February 2000 / Published online: 27 April 2000     

Monochromator versus deconvolution for XPS studies using a kratos ES300 system

Deconvolution has been used by several workers to reduce or eliminate X-ray and analyzer-induced broadenings of XPS spectra, but there is a 30% limit on the amount of improvement in the FWHM of XPS peaks. Deconvolution algorithms generally used also increase the noise in the spectrum. Monochromatic X-ray sources can give high resolution data directly, but the count rates are generally low. Given these limitations, a comparison has been made to see which method of high resolution data acquisition. can achieve data of a certain quality in the least amount of time, for a given system, namely a Kratos ES300 photoelectron spectrometer. Signal-to-noise ratios are used as a means of comparing spectral quality at a given resolution. Results for the Kratos ES300 show that if the desired resolution could be achieved without deconvolution, an order of magnitude less counting time was needed for the Mg Kα standard source. If deconvolution is necessary, the counting times for both sources to achieve a given S/N ratio were found to depend mainly upon the signal-to-background ratios of the peak measured with each source. These results are illustrated for the S 2p doublet from MoS₂ and the Ni 2p peaks from oxidized stainless steel.     

Fir laser emissions from population inversion transition by TEA-CO2 laser pumping

In this paper, the numerical analysis is given which is applicable for a TEA-CO₂ laser pumping in search of effective pumping method.The analysis also can estimate the optimum pressure for a given molecule and a given pulse laser.Also we report in this work many FIR laser emissions from population inversion transition which have never observed before using a TEA-CO₂ laser.     

First-time lidar measurement of water vapor flux in a volcanic plume

The CO₂ laser-based lidar ATLAS has been used to study the Stromboli volcano plume. ATLAS measured water vapor concentration in cross-sections of the plume and wind speed at the crater. Water vapor concentration and wind speed were retrieved by differential absorption lidar and correlation technique, respectively. Lidar returns were obtained up to a range of 3 km. The spatial resolution was 15 m and the temporal resolution was 20 s. By combining these measurements, the water vapor flux in the Stromboli volcano plume was found. To our knowledge, it is the first time that lidar retrieves water vapor concentrations in a volcanic plume.     

A compact,sealed, single mode TEA CO2 laser

A compact, sealed, TEA-CO₂ laser utilizing a double-discharge technique has been constructed and operated under single-mode conditions. Sealed operational lifetimes exceeding 10⁶ pulses have been obtained by the addition of small quantities of hydrogen and CO to the initial gas mixtures. It has also been found that operation without helium is possible giving single-mode output energies of about 20 mJ/pulse at peak powers of about 150 kW. The pulse repetition frequency was 0.5 s^-1. The use of an 18 cm long laser cavity allowed the oscillation of only one longitudinal mode when the frequency of the cavity was tuned closely to the CO₂ line centre frequency. An iris was used to restrict transverse modes to the TEM₀₀. Beat frequency measurements using a stable, CW CO₂ laser indicate a “chirp” of about 5–6 MHz/μs in the tail of the pulse.     

Promoting the Range and Range Resolution of a LIDAR (DIAL) System Using a Suitable Pinhole Plasma Shutter

Pulsed transversely exited atmospheric (TEA) CO₂ lasers, employed extensively in various applications such as light detection and ranging (LIDAR), have a pulse duration of about a microsecond due totheir nitrogen tail. In order to promote the measurement accuracy and the mean power of the laser pulse, the pulse duration should be shortened. In this research, we present the details of making a passive pinhole plasma shutter for a LIDAR (DIAL) system, which shortens the pulse duration of CO₂ lasers from 1.5 μs to 25 ns in air at atmospheric pressure. This instrument increases the range resolution of the LIDAR system from 225 to 3.75 m. Also we show the results of investigation of the clipped pulse duration of the microsecond CO₂ laser pulse using aluminum and copper pinhole metal targets with different pinhole diameters (1.5 and 1.8 mm) and at various laser output energies (338 and 309 mJ). Our experimental results show that the aluminum pinhole is more suitable than the copper pinhole for shortening the nitrogen tail of the CO₂ laser pulse with a larger output average power. Thus, the range of the LIDAR system, which is proportional to the logarithm of the output pulse power, is increased.     

A study of the compounds which are induced on the metallic target surface under the action of a pulsed laser plasmatron

The compounds induced on the surface of titanium and copper targets by a breakdown plasma produced by focused TEA-CO₂ high-repetition frequency laser pulses was studied by transmission electron microscopy and selected area diffraction in correlation with absorptivity data determined before and after laser irradiation. The possibility of using TEA-CO₂ lasers to produce complex thermal treatments was demonstrated for metallic surfaces by the modification of the physico-chemical properties at the sites of plasma action on these surfaces.     

Application of Geometrical Form Factor in Differential Absorption Lidar Measurement

The peak position for a lidar return signal is calculated and measured for the horizontal path with variation of the laser beam divergence angle (θ), and the inclination angle (δ) between the telescope and laser axes. This work shows that θ and δ are very important parameters to use in the design or alignment of a lidar system receiving a good lidar signal. This paper describes an experimental determination of geometrical form factors in the lidar equation. We receive the signals and determine the geometrical form factors by slope method in a homogeneous atmosphere. The differential absorption lidar equation is evaluated for the dual-pulse lidar system. A method using a geometrical form factor determined by the experiment is introduced to correct the error in C₂H₄ measurement. This method shows good correction of measurement error in lidar dual-pulse operation, especially in the short range.     

Study of the compounds formed on damaged ZnSe windows by multi-pulse microsecond pulsed TEA-CO2 laser irradiation in air

We report on experimental data of chemical compounds formed in the damaged area at the surface of certain optical ZnSe windows subjected to multi-pulse microsecond pulsed TEA-CO₂ laser irradiation in air. The results are analysed from the viewpoint of implication of the oxidation process activated under the action of CO₂-laser power in plasma initiation and evolution of surface damage process.     

Lidar measurements of atmospheric transmissivity

Summary The possibility of performing atmospheric-transmissivity measurements by lidar is considered. In the present paper two distinct methods have been successfully applied. The first one is based on the detection of the Raman return from molecular nitrogen. An alternative method is based on the simultaneous detection of the elastic and N₂ Raman returns. Measurements based on both techniques are discussed in detail, the second technique producing more accurate results. Through this technique an estimation of the ?ngstr?m coefficient can also be obtained.     

Longitudinal mode rejection in TEA-CO2 oscillators

A short-cavity single-longitudinal-mode mini TEA-CO₂ laser is described and an experimental study of the effect of cavity length tuning on the mode intensity rejection is presented. A simple model based on longitudinal mode competition adequately explains the observations.     

Fracture mechanisms of Hg0.8Cd0.2Te induced by pulsed TEA-CO2 laser

The fracture mechanisms of Hg_0.8Cd_0.2Te induced by pulsed TEA-CO₂ laser have been investigated theoretically and experimentally in this paper. The Hg_0.8Cd_0.2Te target was irradiated by a TEA-CO₂ laser with wavelength of 10.6 μm and spike width of 240 ns in an ambient atmosphere. The evident cracks can be found on the surface of the target from the scanning electron microscopy (SEM) photos, indicating that the severe breaks happened during the experiment. Theoretical analysis has also been carried out and the results show that the fracture of Hg_0.8Cd_0.2Te is mainly induced by thermal stresses, although there are three forces (thermal stress, evaporation wave and laser-supported detonation (LSD) wave) exerted on the target surface during the process.     

天文图像多帧盲反卷积收敛性的增强方法

天文图像多帧盲反卷积的收敛性受到初始目标、约束条件和光子噪声等因素的影响.提出了用实际光学成像系统参数确定频率带宽有限约束的方法.用Knox-Thompson 方法重构初始目标相位形成盲反卷积算法的初始目标函数.研究了一种新颖的有效减小光子噪声、边缘效应和振铃现象的方法.根据最大似然估计理论，用期望最大化的优化方法建立了改进的严格约束多帧盲反卷积算法.模拟图像和实际天文图像的复原结果表明，所建立的多帧盲反卷积，可以有效克服大气湍流和减小光子噪声，改善天文观察图像的分辨率，并部分消除光学系统衍射效应对恢复图像的影响. 关键词： 大气光学 天文观测 图像处理和恢复     

OFDM水声通信CS限幅失真补偿与LS信道估计优化算法*

正交频分复用技术应用于水声通信系统时,会引起较高的峰均比,当采用限幅法对峰均比进行抑制时,会产生非线性失真。另外,系统采用最小二乘法进行信道估计受噪声的影响较大。针对以上问题,提出了一种基于压缩感知技术的补偿限幅非线性失真与最小二乘信道估计相组合的新算法,在接收端利用导频数据采用压缩感知算法对限幅失真进行补偿,同时对最小二乘信道估计进行优化运算,以减小噪声对其影响。理论分析和仿真结果均表明,新算法不但能够有效抑制限幅法造成的系统非线性失真问题,而且能够降低高斯白噪声对于信道估计的影响。     

Direct observation of the differences between Raman scatering and population inversion transition for their emission frequencies and pulse waveforms

In FIR laser emissions optically by a CO₂ laser, it has been reported that there are two different process namely induced Raman scattering and population inversion transition.In this paper, we show that the two different process have clearly different characteristics in emission frequensies and pulse waveforms in the case of a TEA-CO₂ laser excitation.We used in this work an intra-cavity etalon for the study of the emission frequensies and also we used a MOM point contact diode for the detection of the wavaformes.Especially, we show that the FIR pulse waveformes are able to analyze numerically on the concept of rate equations.In conclusion, we indicate that molecules would be classified into two categories with regard to optically pumping by a TEA-CO₂ laser. One of them produce mainly Raman scattered emissions, and other produce only the FIR emissions from population inversion transition in spite of high power intensity excitation.     

Further performance tests of a picosecond X-ray and laser induced streak camera system with fast scintillation materials

A further performance test of a fluorescence lifetime spectroscopy system with 2D-photon counting based on a streak camera was carried out. This system offers three different excitation light sources. It includes a pulsable X-ray tube with 40 kV and a laser. A pulsed UV-LED has been installed for sample excitation. The table top measurement device has a temporal resolution down to 20 ps after deconvolution and fluorescence/scintillation decay times can be evaluated in a wavelength selectable range between 200 and 850 nm. Cesium fluoride, cesium chloride and cesium bromide as fast core-valence luminescence emitters with a relatively low light yield were used in the presented performance test. The easily manageable system is able to detect low light levels and to determine the decay times in agreement with literature in most instances. Beside mentioned scintillators PbCO₃, SrI₂ and Lu₂SiO₅:Ce as a high light yield material were also measured.     

Iterative deconvolution of X-ray photoelectron spectra generated by an achromatic source

Three cases are presented where X-ray photoelectron spectra generated by an achromatic X-ray source have been deconvoluted by the iterative, Van Cittert method. These are (i the gold 4 f doublet, (ii) the gold valence band, and (iii) the aluminium 2 p spectrum of an oxidized aluminium foil. All prominent features in the deconvolute are shown to be real by comparison with high resolution studies, and the noise levels are low. Resolution after deconvolution is similar to that obtained with a monochromatized X-ray source. The spectra are also similar to those obtained by Fourier transform deconvolution.