Index of content:
Volume 71, Issue 4, April 2000
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1150509View Description Hide Description
A real-time white light stereo pseudocolor encoding method and technique in a microscope is presented, which demonstrated that the phase information of an object is not totally lost in incoherent imaging. The image system is an improved microscope and attached optical elements can be stacked together in the microscope tube, so the structure is compact. The irradiance distribution at the output plane of the microscope is obtained by means of the theory of partially coherent light. At the conditions of that the aperture stop and focal length of condenser are a right magnitude, and the illuminative light source is incoherent or partially coherent, the theoreticalanalysis indicates that the irradiance distribution at output plane is presented by the stereo pseudocolor image which is characterized by the phase rate-of-change function of input object. A bleached holographic grating as an input object is observed, and its optical parameters are measured directly. Experimental results are discussed, which basically agreed with theoreticalanalysis.
71(2000); http://dx.doi.org/10.1063/1.1150502View Description Hide Description
A prototype autocorrelator device for measuring the pulse width of ultrashort laser pulses in the picosecond and femtosecond regimes has been constructed. The device is based on multiphoton induced ionization of electrons from a charged metal surface mounted on a vibrating silica fiber, which is an entirely different physical principle from those employed by existing methods for ultrashort pulse characterization, e.g., streak cameras or second harmonic generation. The new method distinguishes itself from existing techniques in that it is, in principle, applicable to a very wide range of wavelengths, from the far ultraviolet to the infrared, requires no special optical orientation of a nonlinear crystal, and can be used for both femtosecond and picosecond pulses. The prototype device has been successfully applied to 532 nm laser pulses with a ∼20 ps pulse width, and preliminary work shows it is applicable to femtosecond pulses as well
71(2000); http://dx.doi.org/10.1063/1.1150503View Description Hide Description
This work describes a novel technique allowing to measure with picosecond resolution the time evolution of the optical properties (reflectivity/transmission) of a material surface exposed to a single laser pulse. The experimental setup is based on the use of a streak camera in combination with a single-mode probe cw laser. The maximum achievable time resolution is about 1 ps for the acquisition of a single event. Results concerning melting of Ge films under ps pulses will be used to illustrate the potential of this measurement technique for resolving single optical transient events in the ps time scale. The advantages of the system with respect to the use of pump-and-probe optical measurements are especially remarkable for the study of events occurring in time windows with full widths from 1 to 10 ns, as it is the case for rapid solidification phenomena induced by ultrashort laser pulses. This has allowed us to investigate in detail, for instance, the occurrence of bulk and surface initiated solidification phenomena in thin amorphous Ge films.
71(2000); http://dx.doi.org/10.1063/1.1150504View Description Hide Description
This article describes a lightweight, self-contained, field portable Raman instrument that has been developed for rapid on-site determination of primary mixture components. The instrument consists of a helium neon (HeNe) laser for excitation, an acousto-optic tunable filter (AOTF) for wavelength discrimination, and an avalanche photodiode for detection. The AOTF is the primary component of this system and has been selected based on its spectral range (600–900 nm) along with its high resolution, ∼7.5 Labview™ based software for controlling the AOTF frequency and the signal acquisition has also been developed. Several different samples were analyzed (both solids and liquids) using this instrument for the evaluation of parameters such as spectral resolution, sensitivity, and data acquisition speed for certain environmentally important compounds. The results from these measurements are compared with those using a conventional laboratory Raman system consisting of a grating-based spectrograph and a charge-coupled device.
71(2000); http://dx.doi.org/10.1063/1.1150505View Description Hide Description
A Mach–Zehnder type fiber optic sensor is modeled and the phase change for this device as a result of pressure or temperature variation is studied. Different parameters involved in the sensing process such as Young's modulus,Poisson's ratio, light wavelength, index of refraction, and stress–optic coefficients are changed and their effects investigated. Our results show that Young's modulus,Poisson's ratio, light wavelength, and stress–optic coefficients have the major roles for pressure sensing. On the other hand for temperature sensing, light wavelength, thermal expansion coefficient, and temperature dependence of the index of refraction are the important parameters. For a typical silica glass fiber at 0.6328 μm wavelength, the calculated phase change due a pressure change of 1 Pa for the 1 m length is rad. This phase change for 1 m of a similar probe fiber corresponding to 1 °C temperature change is found to be 107 rad.
Thulium-doped fiber optic decay-time temperature sensors: Characterization of high temperature performance71(2000); http://dx.doi.org/10.1063/1.1150506View Description Hide Description
The main objectives of this study were to investigate the thermometric potential of Tm-doped optical fiber and yttriumaluminumgarnet (YAG) crystals for applications up to temperatures as high as 1300 °C, and to realize a prototype high temperature fluorescence fiber probe based on earlier Nd-doped fiber work for temperature measurement up to 750 °C. The research showed that the fluorescence characteristics of the Tm-doped YAG crystal were very stable at high temperatures and capable of providing a temperature measurement reproducibility better than ±6 °C over the region from room temperature up to 1350 °C.
Reduction in time jitter for free-running, nanosecond-pulsed, Xe lamp by supplementary illumination with blue light-emitting diode71(2000); http://dx.doi.org/10.1063/1.1150507View Description Hide Description
We have found out a phenomenon that the large fluctuation in the repetitive frequency of emission from a free-running, nanosecond-pulsed, Xe lamp can be reduced markedly by illuminating the lamp electrodes with a light flux from a blue light-emitting diode. There was a wavelength dependency in the stabilization effect. The upper limit of the effective wavelength of the illumination light was around 750 nm, but no power dependency was observed between 0.5 and 3.0 mW. The reduction in the fluctuation was observed only for a Xe lamp manufactured by a specific maker, where materials with low work function elements were mixed in its cathodeelectrode for the purpose of improving standard direct current arc operation. In order to evaluate the fluctuation quantitatively, we have constructed a system for measuring time intervals between every pulsed emissions. Our conclusion is that the reduction in the fluctuation is due to photoelectrons emitted from the surface of the cathodeelectrode and thereby the breakdown voltage for discharge in the lamp was maintained at a fixed value.
Accurate measurements of the electro-optic coefficients and birefringence changes using an external modulation signal71(2000); http://dx.doi.org/10.1063/1.1150508View Description Hide Description
We propose a new method to measure within a high accuracy both the electro-optic properties and the birefringence changes in bulk crystals. This technique is based upon the frequency doublingelectro-optic modulation (FDEOM) method in which an external modulator is inserted in the experimental Sénarmont setup. We present a complete analysis of the optical response of a Sénarmont setup within Jones formulation. The FDEOM method possesses as main advantages a high accuracy and sensitivity in the measurements due to a direct determination of the electric field-induced phase retardation in the crystal under study. The measurements are recorded with an adjustment of the shape of the output beam at a frequency double the frequency of the ac applied voltage. The external modulation version of this method allows us to perform measurements with a higher modulation depth and to avoid any additional dynamical perturbation in the electro-optic crystal under study. Therefore, even small values of the electro-optic coefficients can be accurately measured. Moreover, the new method is able to measure with a high accuracy the variations of the birefringence in any crystal, even if it does not present an electro-opticeffect. Some significative results in point out the large advantages of the method.
71(2000); http://dx.doi.org/10.1063/1.1150554View Description Hide Description
A four-quadrant PVDF position sensor, originally designed for the alignment of mid-IR laser diodes beams, was also used for mapping and profiling the beam of a diode laser emitting few μW at 5.8 μm, by masking one quadrant with a 1 mm diameter pinhole or a 100 μm width slit. This diagnostic system was used to optimize the laser collimating optics, determining a significant improvement in the spatial quality of the output beam, evaluated by means of the propagation factor. Since the laser diode modulation waveform required to optimize the pyroelectric detector responsivity is a fixed frequency square wave, quite different from that used in the normal spectroscopic application of the IR diodes, care was taken to have the same single-mode emission in the wavelength range of interest. The system was intended as a diagnostic and optimization tool for spectroscopic systems applied to environmental monitoring, but it may be used as a low cost device for the characterization of low power laser beams.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1150510View Description Hide Description
The article presents a description of the principle of operation, the design, and the performance data of a plasma–cathode electron source.Plasma is produced in a hollow-cathode reflex discharge operating in an axial magnetic field. The discharge unit is filamentless. Air is used as the working gas. The electron sourcegenerates a focused continuous electron beam of current up to 0.3 A at an accelerating voltage of up to 60 kV. Owing to the absence of hot electrodes, the system operates reliably under severe gas conditions with a long lifetime. The electron source is currently used as a component of electron-beam welding flow lines. The use of the electron source for electron-beam facing of composite powdered materials is also discussed.
71(2000); http://dx.doi.org/10.1063/1.1150511View Description Hide Description
We have developed a magnetic spectrometer to characterize the hot electronsgenerated in high-intensity laser-plasma interaction experiments. It comprises a dispersive element consisting of a permanent dipole magnet and an electron detector incorporating 45 scintillating/light-guiding plastic fibers connected to a cooled charged-coupled device camera. The main features of this instrument are high spectral resolution, low sensitivity to x and γ rays, and versatility due to its compact design. Performance and operational capabilities are illustrated based on experimental results in which electron energy–spectra in the range of 520 keV to 12.6 MeV were obtained with an energy resolution of 10% and a detection threshold of electrons per MeV.
71(2000); http://dx.doi.org/10.1063/1.1150512View Description Hide Description
A short probe-forming system is developed for the Columbia Microprobe that includes four electrostaticquadrupoles with a Russian quadruplet configuration. The smallest beam spot size and appropriate optimal parameters of the probe-forming systems have been found. These parameters of the system are compared with appropriate parameters of other field configurations including the electrostatic and magnetic fields with dipole, quadrupole, and rotational symmetry. The new original construction of the electrostatic quadruplet has been manufactured. The sensitivity of this quadruplet to some misalignments of the construction is investigated.
High-resolution energy analyzer with a large angular acceptance for photoelectron spectromicroscopy applications71(2000); http://dx.doi.org/10.1063/1.1150513View Description Hide Description
A new design of an electrostatic electron energy analyzer is presented, that is characterized by a high energy resolution and a high efficiency of collection of photoelectrons originating from submicron areas. The analyzer uses an original low-aberration electrostaticmirror for collecting electrons, followed by a retarding hollow cylindrical lens and a hemispherical energy analyzer. Feasible modifications of the analyzer are discussed.
- NUCLEAR PHYSICS, FUSION and PLASMAS
71(2000); http://dx.doi.org/10.1063/1.1150514View Description Hide Description
We describe a pulsed spectrometer for detection of nuclear quadrupole resonance on the nuclei of nitrogen N-14 with fast Fourier transform. The use of a multipulse sequence, four channel system for data registration and processing permits detection of the nuclear quadrupole resonance (NQR) signal in the presence of strong interference and the piezo effect. Using this spectrometer we registered the NQR signal from an explosive sample of 150 g (92% RDX) at a distance of 22 cm, and the time of detection was 81 s.
71(2000); http://dx.doi.org/10.1063/1.1150515View Description Hide Description
We present the design and construction of a diagnostic system of the plasma parameters by means of electrical probes. The novelties of the system are the protection provided to the electrical probe with a guard, thus minimizing the probe deterioration, and the automatic positioning device associated with the probe. This system consists of: the electronic instrumentation for the electrical probe bias; two control systems, one for the guard’s motion, and another for the positioning of the electrical probe; and finally, a data acquisition system. During the system’s operation, and based on the data measured by the electrical probe at different positions where the data acquisition is carried out, the following parameters are available in real time: floating voltage, ion saturation voltage and current, electron temperature, and plasma’s voltage and density. The motion of the guard and electrical probe array is programmable up to a maximum speed of 1 cm/s, and the time required for the bias of the probe, the measuring and storing of the signal, and the generation of an ASCII data file is approximately 10 ms. With this system, the duty cycle of the probe is extended due to the shielding provided by the guard. Likewise, the reduction of deposits of spurious materials on the probe results in a more reliable measuring process of the plasma parameters.
71(2000); http://dx.doi.org/10.1063/1.1150516View Description Hide Description
Observations of spectral line profiles commonly represent the integration of emission along the line of sight. Depending on the number of views and the symmetries involved, one can use techniques ranging from simple Abel inversion to complex tomographic reconstruction to find the spatial distribution emitters. In tokamak experiments, the spatial dependence of the magnetic field is typically available and can be used to gain important insights into the absence of other spatial information. The Zeeman patterns of spectral lines from neutral atoms and low-Z ions in tokamakplasmas can contain enough information to restrict the location of emission to well defined positions along a given line of sight. Simple modeling of observations with high spectral resolution from Alcator C-Mod plasmas demonstrates the application of this technique to the interpretation of experimental data. This localization of emission is not only of interest to spectroscopists and modelers of tokamak edge and divertor regions, it could be used to aid the operation of other visible-light diagnostics that seek spatial resolution.
Multilayer mirror soft x-ray spectrometer for fast electron temperature measurement on the compact helical system71(2000); http://dx.doi.org/10.1063/1.1150517View Description Hide Description
A multilayermirror (MLM) soft x-rayspectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature with fast time resolution using the MLM spectrometer.
- BASIC PHENOMENA
71(2000); http://dx.doi.org/10.1063/1.1150518View Description Hide Description
We present the realization of a physical quantum random number generator based on the process of splitting a beam of photons on a beam splitter, a quantum mechanical source of true randomness. By utilizing either a beam splitter or a polarizing beam splitter, single photondetectors and high speed electronics the presented devices are capable of generating a binary random signal with an autocorrelation time of 11.8 ns and a continuous stream of random numbers at a rate of 1 Mbit/s. The randomness of the generated signals and numbers is shown by running a series of tests upon data samples. The devices described in this paper are built into compact housings and are simple to operate.
71(2000); http://dx.doi.org/10.1063/1.1150519View Description Hide Description
A new procedure for analysis of random telegraph signals in time domain has been developed and applied to the analysis of voltage fluctuations in the current induced dissipative state in superconducting thin films. The procedure, based entirely on the difference in the statistical properties of discrete Marcovian telegraph fluctuations and Gaussian background noise, ascribes each point of the experimental time record to one of the telegraph states. The average statistical lifetimes and amplitudes of the telegraph signal are then determined in an iterative way by fitting the amplitude histogram of thus obtained record of the redistributed data to the two-Gaussian histogram of the original experimental signal. The procedure allows for analyzing “noisy” random telegraph signals with low ratio between the signal amplitude and the intensity of the background noise that cannot be analyzed by the classical approach. Separation of the time record into two subrecords relative to two telegraph states also enables in-depth analysis of the spectral properties of the background noise observed together with the telegraph fluctuations.
- MICROSCOPY and IMAGING
A new method to measure the oscillation of a cylindrical cantilever: “The laser reflection detection system”71(2000); http://dx.doi.org/10.1063/1.1150520View Description Hide Description
A new optical detection system for measuring the oscillation of cylindrical cantilevers has been designed. A laser beam is directed perpendicularly to the oscillating plane and is focused on the curved surface of the vibrating probe. The surface reflects the light and a second lens refocuses it onto a two-segment photodiode. The sensitivity of this method lies in the fact that a small displacement of the probe produces a large angular deflection of the reflected laser. Applications of this new system are presented in order to demonstrate its reliability, accuracy, sensitivity, and the possible use in a shear force microscope. All the results are finally analyzed by modeling the motion of the cantilevers using harmonic oscillator theory and the continuous model for oscillating bars. The agreement between experimental data and models is well inside the experimental errors confirming the possibility of using this system to accurately study the dynamics of cylindrical cantilevers.