On use of ghost-less beam splitter in reversal shear interferometer for measurement of spatial coherence of large cross-section laser beam

A configuration of reversal shear interferometer suitable for measurement of spatial coherence of high average power, large cross-section laser beam has been discussed. Ghost-less beam splitters in which unwanted multiple reflections from the surfaces have been suppressed by total internal reflection are used for beam splitting and re-combination. Advantages and drawbacks of this configuration have been discussed.     

Measurement of residual wedge angle with a reversal shear interferometer

A technique for determination of residual wedge angle of high optical quality transparent parallel plate using a reversal shear interferometer-based optical system has been discussed. In this technique the parallel plate to be tested is used to introduce angular tilt in the preset two-beam interference fringes of a reversal shear interferometer. The parallelism is calculated from the angular tilt of the fringes. The technique is more sensitive compared to techniques using Fizeau interferometer and is most suitable for highly parallel transparent plate with very low residual wedge angle.     

Measuring spatial coherence by using a reversed-wavefront Young interferometer

A very simple optical setup for the measurement of the modulus and the phase of the two-point correlation function of a partially coherent light field is presented. The system consists of a slightly modified version of a Young interferometer and requires a single Young mask in order to determine the correlation function at any pairs of points. Experimental results are presented for the case of a synthesized partially coherent secondary source.     

Measurement of the spatial coherence of hard synchrotron radiation using a pencil beam

We present a simple method to measure the spatial coherence of hard x-ray beams.Based on the convolution of Gaussian functions,we analyze the diffraction patterns of a grating irradiated by partially coherent hard x rays with a constrained beam diameter.The spatial coherence properties of an x-ray beam are obtained from the width of the diffraction peaks with high accuracy.The results of experiments conducted by combining a pinhole with a grating show a good agreement with our calculation using the Gaussian–Schell model.     

Measurement of laser beam transmittance through laser produced vapour plume

Laser produced plume, consisting of vapor front and ejected substrate on the workpiece surface, is an intermediate between the incident laser beam and the workpiece on which the beam is directed. It partially blocks, defocuses, absorbs, scatters, and deflects the incident beam and thereby reduces the laser energy reaching the workpiece. However, plume additionally acts as a heat source enhancing the machining. Consequently, laser induced plume plays an important role in laser machining process. The present study investigates the transmittance of a reference beam by the plume generated during the laser-workpiece interaction. To achieve the transmittance measurements, a Nd:YAG laser was used and four different materials were employed in the experiment. To obtain realistic values of the plume transmittance in relation to the laser machining process, the reference beam was sampled from the incident laser beam. The study was extended to include the effect of the laser pulse parameters on the transmission process. It is found that about 10–30% of the transmittance of the reference beam through a vapor plume, produced due to laser ablation, occurs close to the workpiece surface.     

Measurement of copper vapour laser-induced deformation of dielectric-coated mirror surface by Michelson interferometer

A Michelson interferometer-based technique has been used to measure the deformation of dielectric-coated mirror, caused by an incident repetitive pulsed laser beam with high average power. Minimum measurable deformation of 17 nm is reported.     

Investigation of the spatial coherence of a laser beam by a laser-induced grating method

The influence of ion implantation on the magnetization of a model bubble system (La, Ga: YIG) is studied by means of Conversion Electron Mössbauer Spectroscopy (CEMS). It is found that the average magnetization decreases considerably with increasing depth, but after reaching a minimum returns to the bulk value. This observation is in accord with recent FMR and x-ray diffraction experiments.     

Effects of a modified spectral model on the spatial coherence of a laser beam

The influence of a modified (bump) spectrum of refractive index fluctuations on the spatial coherence of an optical wave is studied here and compared with that based on a von Karman spectrum. Analytical expressions are derived for the mutual coherence function (MCF) and wave structure function (WSF) of a lowest-order Gaussian beam wave from which the beam spot size and degree of coherence are deduced. The qualitative behaviour of beam spreading and coherence length is basically the same for both spectral models. Also, when the radius of the Fresnel zone and initial beam radius are of comparable size, the presence of a spectral bump appears to have minimal effect on spatial coherence for all beams. However, the choice of spectral model is important for certain ranges of parameters. In particular, the implied spatial coherence length for a collimated beam based on the modified spectrum is significantly smaller than that based on the von Karman spectrum whenever the Fresnel zone is either much larger or much smaller than the initial beam radius, whereas for a focused beam the predicted coherence length based on the modified spectrum is slightly larger when the Fresnel zone size is much smaller than the initial beam radius.     

Measurement of flow-birefringence using a circularly polarized laser beam

A method of measuring flow-birefringence is reported. In this method, a circularly polarized light wave is transmitted through the flow-birefringent medium. The transmitted elliptically polarized wave is compensated by a Babinet-Soleil plate to convert it back into a circularly polarized wave. The state of circular polarization is confirmed using the rotating-analyser method. Measurements can be carried out with a sample having a fixed azimuth. The minimum detectable optical retardation angle was 5 × 10^-3 degree. This corresponds to a birefringence of Δn = 1 × 10^-10, when the optical path length of the sample cell is 100 mm.     

Stabilization of high-power copper vapour laser

Design considerations for stable trouble-free operation of a 100 W copper vapour laser are described. Implementation of laser head and modulator design principles result in a laser that meets these goals. The laser is switched by a single small thyratron that operates below all its maximum ratings.     

Strong reduction of the degree of spatial coherence of a laser beam propagating through a preformed plasma

A strong reduction of the spatial coherence of a laser beam after its propagation through a plasma has been measured using a Fresnel biprism interferometer. The laser beam was diffraction limited; the coherence width was reduced from 40 mm in vacuum down to a few mm with the plasma. Numerical results based on a paraxial model exhibit a coherence degree close to the experimental one; they also prove the importance of taking into account the nonlocal transport effects in numerical simulations for such plasma conditions.     

Multipass interferometer using a filamentary spatial filter

A multiple pass interferometer using a filamentary spatial filter is developed. We can change the sensitivity of the interference fringe pattern by selecting the order of multiply reflected image without reduction of the fringe visibility.     

Influence of longitudinal spatial coherence on the signal of a scanning interferometer

The physical conditions for observing the longitudinal spatial coherence of light of an extended thermal source in interference experiments are defined. For experimental verification of these conditions a Michelson interferometer with a longitudinal scanning mirror was used. The influence of the longitudinal spatial coherence of thermal light on a fringe envelope is demonstrated experimentally.     

Measurement of the spatial coherence function of undulator radiation using a phase mask

A measurement of the horizontal coherence function of 7.9 keV radiation from an undulator beam line at the Advanced Photon Source is reported. X-ray diffraction from a phase-shifting mask was used, and the coherence function was measured as a function of the width of beam-conditioning slits in the beam line. The coherence distribution is found to be best described by a Lorentzian function.     

Measurement of the frequency modulation transfer function of a laser using a Mach–Zehnder interferometer

A technique is presented for determining the frequency modulation transfer function of a laser. The method is based on a Mach–Zehnder interferometer, with a significant difference in the optical path lengths of the two arms. A frequency-modulated laser beam incident on the interferometer produces a phase-modulated photocurrent signal with an effective modulation index that is related to the amplitude of the optical frequency modulation. Techniques for determining both the amplitude and the phase of the optical frequency modulation from the photocurrent signal are described.     

Spatial coherence measurements through turbulent atmosphere using a computer-aided interferometer

The complex degree of coherence over a telescope aperture is measured by processing instantaneous values of the complex degree of coherence fluctuated by turbulent atmosphere. Fringe patterns produced by a computer-controlled wave-front folding interferometer are analyzed in a short time by a photodiode array to calculate the instantaneous values of the complex degree of coherence. The system is applied to one-dimensional simulation experiments.     

基于调制光束的空间相干性获得局域空心光束

通过调制部分相干光的空间相干性,实现了局域空心光束的产生。理论上根据范西泰特-策尼克定理及标量衍射积分,构造出具有一定空间相干性的部分相干光,并研究其聚焦特性。实验上采用理论上要求的光学系统来构造具有特定空间相干性的部分相干光,并采用薄透镜聚焦部分相干光。研究表明,当构造出的部分相干光经透镜聚焦,可得到局域空心光束。研究了光学系统的参数及透镜焦距对产生局域空心光束的影响,结果显示：光阑的拦截比及透镜的焦距越大,获得的空心光束的尺寸就越大。