Feedback-stabilized fractional fringe laser interferometer for plasma density measurements

A feedback stabilization technique is described for a fractional fringe interferometer measuring plasma electron densities. Using this technique, a CO2 laser Michelson interferometer with a pyroelectric detector exhibited a sensitivity of 3.4 x 10(-4) fringe on a 1-ms time scale and, due to acoustic pickup, 1.8 x 10(-2) fringe on a 10-ms time scale. The rise time is 45 micros. Stabilization against slow drifts in mirror distances is achieved by an electromechanically translated mirror driven by a servo system having a 0.2-s response time. A mechanical chopper in one of the two beam paths generates the signal which drives the servo system.     

A twin-wave quadrature interferometer for diagnostics of pulsed processes in hydrogen and erosion plasma

A two-wave interferometer is described that is based on 0.6328-and 3.3922-μm He-Ne lasers and allows separation of the contributions of free electrons and the neutral component in partially ionized plasma to the phase shift of a probing electromagnetic wave under conditions of possible vibrations of the facility’s optical elements. Using the quadrature method for forming informative signals, phase shifts were measured in a wide range of fractions of an interference fringe to several fringes with a high homogeneous differential sensitivity. The interferometer was used to measure the dynamics of the linear electron density of both an atmospheric-pressure erosion capillary discharge in air and plasma of a hydrogen target in experiments on deceleration of heavy ions in an ionized substance.     

Quasi-quadrature interferometer for plasma density radial profile measurements

A cw Mach Zehnder multichannel interferometer has been developed to measure time-dependent fractional fringe shifts with an accuracy of one-fortieth fringe. The design is quasi-quadrature in that known phase shifts, introduced in the reference beam, are time multiplexed with the normal reference beam. This technique requires only one detector per interferometer channel as compared to two detectors for most quadrature designs. The quadrature information makes the sense of density changes unambiguous, it automatically calibrates the instrument during the plasma event, and it makes fringe shift measurements virtually independent of fringe contrast fluctuations caused by plasma refractive and/or absorptive effects. The interferometer optical design is novel in that the electro-optic crystal used to introduce the 90 degrees phase shifts is located in the common 2-mm-diam HeNe entrance beam to the interferometer, by exploiting polarization techniques, rather than in the expanded 1-2-cm reference beam itself. This arrangement greatly reduces the size, cost, and high-voltage requirements for the phase modulating crystal.     

A new method for determining the plasma electron density using three-color interferometer

A new method for determining the plasma electron density using the fractional fringes on three-color interferometer is proposed. Integrated phase shift on each interferometer is derived without using the temporal history of the fractional fringes. The dependence on the fringe resolution and the electrical noise are simulated on the wavelengths of CO(2) laser. Short-time integrations of the fractional fringes enhance the reliability of this method.     

用干涉条纹图像重建反射镜的三维面形

给出了用干涉条纹图像重建反射镜三维面形的方法,用泰曼-格林干涉仪采集被测镜面的干涉条纹图像,通过对干涉条纹图像进行处理得到镜面高度采样,采用泽尼克多项式拟合镜面的曲面函数,测量到了镜面的三维高度数据。干涉条纹图像上相邻条纹与参考面之间的距离相差一个标准光源波长,因而该测量方法的关键是得到条纹波峰或波谷的精确位置,从而达到了镜面相对高度的采样的目的。采用巴特沃斯低通滤波对条纹图像进行滤波,去掉噪声的影响,然后对图像二值化分割出亮条纹,用数学形态法对二值条纹图像细化得到条纹的中心线,结果表明,低通滤波方法能有效地减少细化条纹的分支和断裂。最后定义每条条纹中心线的相对高度,利用前39项泽尼克多项式对镜面高度进行最小二乘法拟合,计算了镜面被测区域的曲面高度数据。该方法可以用来测量强激光引起的镜面热变形。     

Fringe counting method for synthetic phase with frequency-modulated laser diodes

Fringe counting method with laser diodes (LDs) for displacement measurement has been constructed. Two LDs are frequency modulated by mutually inverted sawtooth currents on an unbalanced two-beam interferometer. The mutually inverted sawtooth-current modulation of LDs produces interference fringe signals with opposite signs for respective wavelengths. The two fringe signals are fed to an electronic mixer to produce a synthetic fringe signal with a reduced sensitivity to the synthetic wavelength. Synthetic fringe pulses derived from the synthetic fringe signal make a fringe counting system possible for faster movement of the tested mirror.     

An infrared interferometer for investigating subthermonuclear plasma in the GOL-3 multimirror trap

A simple Michelson CO₂ interferometer for measuring the high-temperature plasma density within one interference fringe (n _e l = 10¹⁶ cm^?2) at two points (0.8 and 9.0 m) of the long (L = 12 m) GOL-3 corrugated trap was used. A piezoelectric element that provides displacements of the mirror in the reference arm of the interferometer is used to calibrate the interferometer and perform remote control of the initial measurement phase. The interferometer is manufactured from dielectric materials, thus excluding a mechanical action of stray magnetic fields on its elements. The time resolution of the interferometer is determined by a HgCdTe diode and equals ??1 ns. The sensitivity of the interferometer is ??5 × 10^?4 of a fringe (n _e l = 2 × 10¹³ cm^?2).     

Development in laser interferometry for position sensing

Interferometric developments which improve the performance of the laser interferometer as a position sensing device are described. The double pass attachment is an optical accessory which doubles the resolution of the Hewlett-Packard linear and plane mirror interferometers. Unlike previous attempts to extend resolution optically, this device does not fold just one of the two interfering beams and hence does not introduce error due to its own motion. Simple addition of a quarter wave plate to the attachment gives a differential version of the plane mirror interferometer. Various configurations of this differential interferometer, and their application to machine tools and to integrated circuit lithographic and inspection equipment, are discussed. A new ‘wavelength tracking’ device, based on differential interferometry, directly and precisely monitors changes in laser wavelength inside a highly stable mechanical cavity. Easily incorporated as an additional interferometer axis, the device improves compensation for changes in laser wavelength.     

Shape measurements of mirror surfaces with a lateral-shearing interferometer during machine running

A common path lateral-shearing interferometer with a minimum number of optical components has been developed. Because the interferometer is little affected by mechanical vibrations and air turbulence, it can be mounted on an ultraprecision lathe and can be used to measure the shapes of workpieces. A plane parallel glass plate is used to shear the wavefront under test in the interferometer. To analyze the interference fringes obtained by the interferometer precisely, a fringe-scanning method using a slight tilt of the glass plate is used. Zone plates that are computer-generated holograms are used to measure spherical and aspherical surfaces with the interferometer. A spherical and a parabolic concave mirror were measured with the interferometer. The spherical mirror was also measured by a Fizeau interferometer to compare the error with that measured by the lateral-shearing interferometer. The experimental results agreed well with those measured by the lateral-shearing interferometer.     

Focusing quality of a split short laser pulse

For multiple laser pulse experiments, it is necessary to split a laser pulse. In order to split a short laser pulse without stretching the pulse width, the laser pulse should not pass through thick materials. For this reason, a pellicle beam splitter (BS) and/or a mirror with a hole are required as a BS for the short laser pulse. The focusing qualities of the laser pulse after passing through the pellicle BS and the mirror with a hole are the same as without the BS's. The laser pulse quality reflected by the BSs should be considered for the laser pulse. A pellicle BS is a thin foil, so, it is weak against vibrations. One should be careful about airflows and isolation from vibration sources. The spot size of the reflected laser pulse is consistent with the size reflected by a normal mirror. The energy loss is about 10% compared with a normal mirror. A mirror with a hole is strong against external vibrations. The reflected laser pulse has a doughnut shape. The reflected laser pulse is interfered due to the shape. In order to cleanly focus the laser pulse, the inside size of the doughnut should be smaller than a half size of the outside portion of the doughnut.     

Zone-plate inteferometer to measure the positioning error of a cutting tool

A simple type of zone-plate interferometer has been developed to measure precisely the positioning error of a cutting tool. Interference fringes obtained by the interferometer are little affected by air turbulence in the optical paths and by machine vibrations. The shape of the mirrors surface being tested is spherical and is manufactured with an ultraprecision lathe. A zone plate is set at the midposition between the vertex of the spherical mirror being tested and the center of curvature of the mirror. The error in the shape of the mirror and the positioning error of the tool can be determined by analyzing the interference fringes. Two spherical concave mirrors were measured. One mirror was manufactured with a tool that had same positioning error. The positioning error was observed as distortion of the interference fringes. The images obtained by the zone-plane interferometer agree well with the images obtained by a Fizeau interferometer and a computer simulation based on the experimental results. The other mirror was manufactured with the tool after the positioning error had been eliminated, based upon the results of the above experiments. The interference fringes of the mirror show no distortion, and the error in the shape of the mirror is small.     

Measurement of a Michelson Interferometer Mirror Velocity

ABSTRACT

This article documents the design, construction, and implementation of a time interval analyzer (TIA) for measurement of the mirror velocity in a Michelson interferometer. The TIA, a precision counter measures the dynamic variation in the time intervals (i.e. frequency) of a signal. The helium-neon (HeNe) laser signal from the servo mirror control circuitry of a Michelson interferometer provides a nearly constant frequency source. The HeNe signal frequency is directly proportional to the interferometer mirror velocity. A constant mirror velocity is crucial to the proper operation of a Michelson interferometer which modulates the infrared radiation in a Fourier transform spectrometer. The TIA permits a critical evaluation of velocity variations in a single interferometer mirror scan via the time interval analysis of the HeNe laser signal.     

A LIRA Twin-Wave Laser Interferometer–Reflector for Plasma Diagnostics at Large-Scale Facilities

A LIRA interferometric system (active laser interferometer–reflector) is designed for measuring the plasma density and controlling the degree of modification of the reflector surface under conditions of intense vibrations of a plasma facility. The operating principle of the system is based on the intralaser (autodyne) reception of the radiation reflected into the laser. An element of the structure of the laser facility, including a diffusely reflecting surface, can be used as a reflector. The interferometer is built on the basis of two Zeeman He–Ne lasers generating at wavelengths of 632.8 and 3392.2 nm. Under conditions of an actual plasma experiment, a sensitivity of 5 × 10^–7 has been achieved, which, when converted into the linear plasma electron density, is n _e L = 2.5 × 10¹⁰ cm^–2. The highest time resolution is 10 ns. The interferometer can operate using reflectors with a coefficient of power reflection into the laser of up to 1 × 10^–12.     

Four-point bisensitivity velocity interferometer with a multireflection etalon

A four-point bisensitivity velocity interferometer system for any reflector (VISAR) with a renovative delay etalon is proposed and demonstrated. In this interferometer, we introduce a new film-coating strategy to accurately measure small velocity with relatively short and cheap etalon. Laser pointing to the etalon is split into two beams with different incident angles with each beam going through the etalon in different path. The beam with the smaller incident angle is reflected three times before it leaves the etalon, while the other beam with larger incident angle goes through the etalon to and forth only once. The delay time of the laser beam with smaller incident angle is almost three times longer than that of the beam with larger incident angle. In the example of the laser with a smaller incident angle, the velocity per fringe of this interferometer can be reduced by approximately three times. The etalon is optimized so that four laser beams can be penetrated in the vertical direction at the meantime. With an etalon of 200 mm in diameter and 150 mm in length, a four-point bisensitivity velocity interferometer can achieve the velocity per fringe of 100 and 350 m/s fringe. A measurement has been successfully undertaken for the steel flyer driven by the explosive where the developed interferometer applies.     

An interferometric technique for transient phase shift measurements in an electron-beam-pumped laser

An interferometric technique designed to measure optical homogeneity in an electron-beam-pumped plasma is described and characterized. The system provides a two-dimensional transient phase shift contour mapping of the excited region, at any time during or after the excitation pulse. The basic instrument is a 7-cm-field, 5-m Mach-Zehnder type interferometer coupled to a specially configured 2-m e-beam-pumped sample chamber. By virture of 'e-beam partitioning,' absolute fringe shifts can be measured in a single shot. This relaxes the usually severe requirements on vibration isolation associated with systems of this size, and simplifies the overall mainframe construction. A short-pulse XeF discharge laser, developed specifically for this application, is also described, and sample interferograms are presented for e-beam-pumped noble gas and noble gas-NF3 mixtures. A ray calculus model is applied to the analytical interpretation of the interferograms produced by this technique. This model has general applicability to ray propagation in graded index media and is discussed in some detail.     

Fringe-jump corrected far infrared tangential interferometer/polarimeter for a real-time density feedback control system of NSTX plasmas

The far infrared tangential interferometer/polarimeter (FIReTIP) of the National Spherical Torus Experiment (NSTX) has been set up to provide reliable electron density signals for a real-time density feedback control system. This work consists of two main parts: suppression of the fringe jumps that have been prohibiting the plasma density from use in the direct feedback to actuators and the conceptual design of a density feedback control system including the FIReTIP, control hardware, and software that takes advantage of the NSTX plasma control system (PCS). By investigating numerous shot data after July 2009 when the new electronics were installed, fringe jumps in the FIReTIP are well characterized, and consequently the suppressing algorithms are working properly as shown in comparisons with the Thomson scattering diagnostic. This approach is also applicable to signals taken at a 5 kHz sampling rate, which is a fundamental constraint imposed by the digitizers providing inputs to the PCS. The fringe jump correction algorithm, as well as safety and feedback modules, will be included as submodules either in the gas injection system category or a new category of density in the PCS.     

A new multichannel interferometer system on HL-2A

A new multichannel HCN interferometer has been developed on HL-2A tokamak, which is characterized by two techniques: (1) the wave-guide HCN laser with cavity length of 6 m to increase the optical resource power and (2) high response room temperature waveguide Schottky diode detectors to obtain good beat signal. The space resolution is 7 cm by the use of focusing metal mirrors mounted on the vacuum chamber and a compensated optical system. In the 2006 experiment campaign, this new interferometer has been applied for plasma density profile and density sawtooth measurement.     

An HCN Interferometer for Measuring the Electron Density Distribution in the Plasma of the Л-2M Stellarator

A diagnostic setup for determining the electron density profile in the plasma of the -2M stellarator is described. An interferometer based on an HCN laser (337 m) with a mirror in a vacuum chamber and built according to the Michelson scheme is the main element of the setup. To translate the phase shift to a frequency range convenient for recording, a system with a mechanical frequency shifter based on the Doppler effect is used. InSb photoresistors cooled with liquid helium are used as radiation detectors. Seven mirrors are arranged in the vacuum chamber. Any of these can be included in the optical system of the interferometer by rotating an external mirror; scanning along seven chords is thus accomplished. The results are recorded in digital form. The subsequent data processing makes it possible to determine the parameters of the spatial density distribution functions belonging to an a priori selected family.     

Carrier fringe analysis algorithms for three degree of freedom optical probing

In this work, we present a fiber-delivered and fiber-detected, 3-DOF optical probe concept for measuring optical components to be used in conjunction with an optical coordinate measuring machine (OCMM). The optical probe uses a Michelson interferometer to produce carrier fringes and a high density fiber bundle to transmit interferograms that are recorded away from the probe head in a remote imaging system. We compare several different signal FFT processing techniques (parabolic interpolation, windowing, and zero padding) and a single-bin DFT technique to compute and enhance the resolution of the displacement, tip, and tilt of a moving mirror. We simulated varying signal-to-noise ratios and interference fringe contrast ranges to determine the algorithms’ sensitivity to those parameters and compare our simulated values to measured SNR and fringe values. Based on this work, it should be possible to use a carrier fringe algorithm for fiber probing applications if the interferogram can be transmitted through the fiber bundle with sufficient contrast (40%) and SNR (30 dB).     

A Dispersion Interferometer Based on a CO<Subscript>2</Subscript> Laser

A double-pass dispersion interferometer based on a 9.6-μm CO₂ laser with a sensitivity of 〈 n _e l〉_min ∼ 1 × 10¹³ cm⁻² and a temporal resolution of ∼50 μ s, designed to measure linear plasma density, is described. A ZnGeP₂ nonlinear crystal is used as the frequency doubler. The main advantages of the interferometer are its compactness and a low sensitivity to vibrations of optical elements. The interferometer requires no special vibration isolation. Its main components are arranged compactly on an optical bench outside the apparatus, except for a window for radiation injection and a retroreflector; these are mounted on the wall of the experimental facility's vacuum chamber. The advantages of the dispersion interferometer have been demonstrated in an experiment with a gas-dynamic trap. __________ Translated from Pribory i Tekhnika Eksperimenta, No. 5, 2005, pp. 96–106. Original Russian Text Copyright ? 2005 by Solomakhin, Bagryanskii, Voskoboinikov, Zubarev, Kvashnin, Lizunov, Maksimov, Khil'chenko.