Picosecond streak camera performance in studies of exciples formation dynamics

Kirchhoff's law has caused confusion in the discussion about the theoretical maximum work from radiation because it permits only infinitesimal or irreversible absorption. With the aid of an optical circulator, non-Kirchhoff behaviour is possible in principle which permits at the same time complete and reversible radiation absorption. The key to overcome Kirchhoff's law seems to be the magnetic field, a condition explicitly excluded by Kirchhoff. The work extracted in complete reversible conversion of radiation is calculated. The irreversibilities connected with the emission process are discussed.     

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.     

条纹相机在真空弧离子源等离子体诊断中的应用

针对真空弧离子源,利用条纹相机将时间轴信息转换为空间轴信息的特点,结合光谱仪分光功能,建立了一套高时间分辨与光谱分辨能力的发射光谱诊断装置,其时间分辨率和光谱分辨率分别可达26 ps与0.1 nm。利用该诊断装置采集获得了单次脉冲内等离子体的时间演化特性;同时,基于局域热力学平衡等离子体的发射光谱理论,建立了一套谱线拟合的等离子体温度与密度计算模型。相比传统的Boltzmann斜率法与Stark展宽法需要寻找孤立的不受附近谱线叠加的干净线状光谱,建立的拟合光谱模型可以直接处理多条谱线因为展宽效应而叠加形成的光谱线型,计算得到等离子体中电子温度与电子密度。结果表明,在脉冲功率源的作用下,真空弧放电等离子体的电子温度与电子密度分别可达1 eV与3.51024 m-3。     

条纹相机在真空弧离子源等离子体诊断中的应用

针对真空弧离子源,利用条纹相机将时间轴信息转换为空间轴信息的特点,结合光谱仪分光功能,建立了一套高时间分辨与光谱分辨能力的发射光谱诊断装置,其时间分辨率和光谱分辨率分别可达26ps与0.1nm。利用该诊断装置采集获得了单次脉冲内等离子体的时间演化特性;同时,基于局域热力学平衡等离子体的发射光谱理论,建立了一套谱线拟合的等离子体温度与密度计算模型。相比传统的Boltzmann斜率法与Stark展宽法需要寻找孤立的不受附近谱线叠加的干净线状光谱,建立的拟合光谱模型可以直接处理多条谱线因为展宽效应而叠加形成的光谱线型,计算得到等离子体中电子温度与电子密度。结果表明,在脉冲功率源的作用下,真空弧放电等离子体的电子温度与电子密度分别可达1eV与3.5×1024 m-3。     

Improving the precision of fluorescence lifetime measurement using a streak camera

Streak camera has high temporal resolution and high sensitivity, and is a powerful tool in biomedical study to measure fluorescence lifetime and perform fluorescence lifetime imaging. However, nonuniformity of the gain in the streak tube and nonlinearity of the sweeping speed limit the precision of fluorescence lifetime measurement, particularly when fluorescence lifetimes are short. We have constructed a two-photon excitation fluorescence lifetime measurement system that is based on a synchroscan streak camera and have developed accordingly a method to correct the effect of gain nonuniformity and nonlinearity of sweeping speed on the measurement precision. A continuous-wave laser of high stability is used to calibrate the gain of the streak camera, and a Fabry-Perot etalon is used to calibrate the nonlinearity of the sweeping speed. Fitting algorithms are used to correct the gain of the streak camera and nonlinearity of the sweeping speed respectively, which significantly improves the measurement precision of the system, as characterized through the fluorescence lifetime of the short-lived fluorescence dye, Rose Bengal. Experimental results show that the measurement fluctuation of the lifetime has been improved from more than 10% to 2% after correcting the effects of gain nonuniformity and sweeping speed nonlinearity.     

Calibration of a single-point laser range finder and a camera

Laser and visual imagery have been broadly utilized in computer vision and mobile robotics applications because these sensors provide complementary information. Since the single-point laser range finder can provide depth information of a single pixel in the observed image, its price and size is very suitable for handheld devices or very lightweight robotic platforms. So the combination of a single-point laser range finder and a camera has become increasingly useful in recent report and industrial applications. In this paper, we propose two accurate calibration methods for determining the position and direction of the laser range finder relative to the camera. Furthermore, we can accomplish calibration even without observing the laser spot in camera image. We evaluate the two methods through experiments which demonstrate their efficiency and high reliability.     

Synchroscan streak camera study of potential saturable absorbers in the blue spectral region

The viscocity-dependent fluorescence lifetime of some cyanine dyes with peak absorption cross-sections in the 350–500 nm range have been examined using a synchronously operating strek camera in conjunction with the picosecond excitation pulses from a frequency doubled synchronously pumped c.w. dye laser. Through increases in the recovery time of a saturable absorber by employing viscous solutions, the necessary saturation flux is conveniently reduced and passive mode locking of laser systems can occur more easily. This is illustrated by the application of one of the dyes to the mode locking of the coumarin 102 dye laser.     

大气水汽探测地基差分吸收激光雷达系统设计与性能仿真

为持续获得对流层低层高精度、高时空分辨率的水汽浓度分布数据,提出了一套改进的大气水汽探测地基差分吸收激光雷达系统方案.详细描述了系统主要组成部分,对主要误差进行了分析与估计,并提出了一种差分吸收截面实时测量装置用于补偿发射激光器带来的测量误差.针对该系统,并结合上海地区不同季节的水汽浓度状况,对935 nm水汽吸收带中四个水汽吸收峰的差分光学厚度、雪崩二极管的倍增系数M与回波信噪比的关系、水汽浓度随机测量误差等进行了详细的仿真与分析.仿真结果表明,根据不同的季节和天气状况,可以选择不同的吸收峰以达到最佳探测效果;在300—5000 m高度范围内,最大可以达到300 m的垂直分辨率与5 min的时间分辨率,水汽浓度随机测量误差不超过18%.     

Use of a wide angle CCD line camera for BRDF measurements

In order to determine the Bi-directional Reflectance Distribution Function (BRDF) of natural surfaces a CCD line camera is used. This allows measurements under natural conditions with a high azimuth and zenith angular resolution in a short time. The CCD line spans a field of view of 80° as the zenith angle range. For covering the azimuth range, the camera is mounted on a rotating device and an extendible boom provides an aerial platform. This set up allows the measurement of the [almost] complete reflectance distribution of the surface below the camera for the 30-s rotation period of the camera. The camera used for this set up is the wide angle airborne camera (WAAC), which was developed at the DLR for airborne stereo imaging purposes. This paper presents the radiometric calibration of the system and shows the initial results of our approach in measuring the BRDF with high angular resolution for a short period.     

The use of a LiNbO3 deflector in a ∽ 100 ps resolution streak camera

This paper describes the performance of a LiNbO₃ electro-optic deflector. Used in the streak mode, the device is shown to yield better than 100 ps temporal resolution. Some simple improvements in the streak circuit and in the electrode geometry should allow ∽ 25 ps resolution to be obtained.     

Setup for fission and evaporation cross-section measurements in reactions induced by secondary beams

A setup for studying reactions induced by secondary radioactive beams has been constructed. It allows simultaneous measurement of α-particle and fission fragment energy spectra. By measuring the α particles, identification of evaporation residues is achieved. A set of three targets can be used so as to ensure sufficient statistics. Two silicon detectors, located at 90° to the secondary beam direction, face each target, thus covering 30% of the solid angle. This experimental setup is to be used to obtain excitation functions of fusion-fission reactions and of reactions leading to evaporation residue production.     

Two-wavelength and power-balanced oscillation of a CO2 laser for application to differential absorption measurements

A new technique of alternate two-wavelength oscillation of a CO₂ laser is discussed for application to various differential absorption spectroscopic measurements. Power-balanced, two-adjacent branch oscillation using a single CO₂ laser was achieved by modulating the angle of a mirror inside the laser cavity and adjusting automatically the cavity length. The two-wavelength modulation frequency was extended up to about 1.2 kHz. Line tuning and power modulation characteristics were studied. The laser was used in long-path differential absorption measurements of ethylene air pollution molecules to demonstrate the capability of this power-balanced, two-wavelength oscillation method. The minimum detectable absorption was nearly 1×10^–3 in a short-path cell experiment and 3×10^–3 in a long-path experiment.     

Magnetooptical absorption of a molecular crystal in the exciton frequency range

Nonlinear absorption of a molecular crystal in an external magnetic field is considered. The exciton absorption shape functions under polarized laser radiation and a weak magnetic field, as well as the mechanisms responsible for the formation of a hysteresis loop in the dependence of the output light intensity on the applied magnetic field, were studied for the particular case of benzene. It was established that, for the magnetooptical response of the molecular crystal under study, the formation of bistable loops is inverse in character, which makes it possible to monitor and control the behavior of bistable elements in optical logics systems with an external magnetic field at a fixed illumination frequency.     

Ps four-wave mixing measurements of electric field in a ns pulse discharge in a hydrogen diffusion flame

Time-resolved electric field in ns pulse discharge plasmas generated in room air and in an atmospheric pressure hydrogen diffusion flame has been measured by ps four-wave mixing, for plane-to-plane electrode geometry. Electric field is put on the absolute scale using the Laplacian field measured before breakdown. The results show that peak electric field during breakdown in the flame, approximately 40?kV/cm, is significantly lower compared to that in room air, 75?kV/cm, due to higher temperature of combustion products. In both cases, peak electric field is higher compared to DC breakdown field. Both in air and in the flame, the electric field follows the applied voltage before breakdown and decreases rapidly after breakdown, due to charge separation and plasma self-shielding. The electric field in air is compared with the predictions of an analytic model of ns pulse breakdown, showing good agreement between the predicted and the measured breakdown field. The model also predicts earlier breakdown as well as breakdown voltage reduction as the temperature is increased, in qualitative agreement with the experimental data. The use of the present ps four-wave mixing diagnostics for measurements of electric fields below ～20?kV/cm in atmospheric pressure flames is challenging, due to low signal-to-noise. The sensitivity of the present diagnostics is controlled by the high temperature and low N₂ fraction in the combustion product mixture, as well as by the limited bandwidth of the Stokes beam generated by the stimulated Raman cell, which provides access only to several rotational levels of nitrogen molecules. The present diagnostics will have much better sensitivity in high-pressure flames, since the four-wave mixing signal scales as the squared number density of nitrogen.     

Multiphonon absorption in a MgO single crystal in the terahertz range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm⁻¹ in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.     

Simultaneous autocorrelation and synchroscan streak camera measurement of cavity length detuning effects in a synchronously pumped cw dye laser

The cavity matching condition for optimum stable, picosecond pulse generation in a synchronously pumped c.w. mode locked dye laser has been achieved with an accuracy of 2 μm using a synchronously operated streak camera exhibiting a modest temporal resolution of 9 ps. This was possible because of the observed distinct asymmetry of the pulses generated under conditions of cavity mismatch about the zero position. Simultaneous autocorrelation measurements confirmed the streak camera observations.     

Data processing technique applied to the calibration of a high performance FPA infrared camera

The present work deals with the calibration of a focal plane array infrared camera whose detector is a matrix of 320×244 PtSi sensors active in the range 3.6–5 μm. The calibration curve has been obtained by measuring the energy emitted by a blackbody, consisting in a copper cylindric cavity with isothermal walls. The results, obtained in the temperature range 10–70 °C, enable us to investigate the nature of the noise which affects the measurements. The aim is to suggest a data processing and a calibration technique in order to enhance the image quality and the instrument response as well. The effects of random uncertainties have been reduced by using Wiener filtration, which enables us to improve the signal to noise ratio. The problem caused by the nonuniform response of the detector array has been handled by using a different calibration curve for each sensor. The effectiveness of this procedure has been checked by comparing the frequency histograms of the raw and the processed signal. The investigation enables us to highlight some peculiar features of the new focal plane array technology employed in the new generation infrared cameras.