Beam Diameter Measurements Research Articles

Measuring the spatial intensity distribution of high-power focused laser beams using a rotating-wire type laser beam analyser. II. Experimental validation

The theory and application of a rotating-wire type laser beam analyser (LBA) device for measurement of both the spatial distribution of power density and beam 'diameter' of a converging/diverging (focused) high-power laser beam are presented. In part 1 of this article, geometrical optics were used to analyse the theory of operation of the LBA. These analyses showed that operation and correct interpretation of measured results from the LBA are strongly dependent upon the relative position of the LBA measurement plane and the laser beam focal plane. In this article the results of LBA measurements of the laser power density distribution from a 250 W mean power, pulsed Nd:YAG laser welder are presented and compared with the derived theory of operation of the LBA. The sample area path of the LBA through the laser beam was determined experimentally and was found to agree with the theoretically predicted path. Also, the theoretically predicted effects of sample area dimensions relative to the measured beam diameter on distortion of the measured power density distribution were confirmed experimentally. Finally, LBA measurements made near the focal point of beams from the high-power pulsed Nd:YAG laser were shown to agree with measured beam 'diameter' and power density distributions obtained using the Kapton film burn pattern technique. It is shown that with careful operation and interpretation of measured results, the LBA can be used to measure accurately the spatial distribution of power density and the beam 'diameter' of unattenuated, focused laser beams from high-power lasers.

Periodic exponential ruling for the measurement of Gaussian laser beam diameters

The spot size measurement of a Gaussian laser beam is extended to wide dynamic range through the formulation of a novel exponential grating. It is found that the exponential grating provides accurate measurement for both extremely small and large Gaussian beam diameters and proves to be superior to Ronchi, triangular, and sinusoidal rulings.

Scanning probe microscopy of deposits employed to image the current density distribution of electron beams

The current density distribution in the final spot of an electron beam lithography system can be judged from beam diameter measurements performed by means of a knife edge method or recording beam crossover images with conventional lithographic media. Electron beam induced deposition allows to record traces of the current density distribution of the primary beam only, with very small contributions of secondary electrons and x rays. The readout of the focus and stigmation status is obtained by measuring the three-dimensional appearance of the deposit with a scanning probe microscope. To be able to find the few deposits after transferring the sample to the scanning probe microscope, a ‘‘spotter’’ chip was developed, which supports optical coarse positioning and can be easily scanned by probe microscopes. By scanning a sharp tip structure, deposited by electron beam near the center of the chip, the shape of the probing tip is imaged. This information can be used to unfold the image from the probe tip contributions. Results of beam intensity profiles obtained with this new method using a scanning force microscope are presented.

Accurate gaussian laser beam diameter measurement using exponential and rectified sinusoidal rulings

Novel exponential and rectified sinusoidal gratings for the measurement of Gaussian laser beam diameter are proposed. It is found that these gratings provide accurate measurement for both small and large Gaussian beam diameters.

Full-wave rectified sinusoidal ruling for measuring Gaussian beam diameters

A novel full-wave rectified sinusoidal grating is proposed for the measurement of Gaussian laser beam diameters. It is found that this new grating provides accurate measurement for both small and large Gaussian beam diameters and has proved to be superior to the Ronchi, triangular, and sinusoidal rulings.

Diffraction and beam-diameter measurement of Gaussian beams at optical and microwave frequencies

Diffraction of a one-dimensional Gaussian beam by a slit is theoretically investigated. In the visible and microwave regions a new property of the diffracted energy is presented. Analytical expressions for the transmission coefficient and the diffracted energy at normal direction are obtained in simple practical form for experimentalists. These expressions suggest a simple method for measuring Gaussian beams of 1.5-microm diameter or larger.

Fabrication and characterization of gold plated edges on thin substrates for measurement of e-beam diameter

Gold plated edges were fabricated on various substrates and were used as targets for precise electron beam diameter measurement and for system calibration in a commercial Cambridge Instruments e-beam lithography machine. Use of 4 μm square gold plated marker (0.85 μm thick), fabricated on 2 μm thick silicon nitride membrane, was found to enhance significantly the marker-to-substrate signal ratio, in comparison to commonly used lifted-off marker, and thus has improved the reliability of beam diameter measurement accuracy at low beam current for a 40 kV beam voltage. In a statistical sampling of about 2000 measurements, an accuracy of 5 nm (1 sigma value) has been obtained for 38 nm beam diameter by using this gold plated edge on silicon nitride membrane. Further, a Monte Carlo program, which models the effects of electron scattering taking place in target and substrate was used to increase the understanding of the experimental results. Simulated contrast behaviour was found to agree well with the experimental data for all three types of marker used in this study. Finally, the influence of the geometrical shape of the marker edge on beam diameter measurement was studied. By using an appropriate analytical function it was possible to take into account the effect of not perfectly vertical edge on small beam diameter measurements.

Wide range laser beam diameter measurement using a periodic exponential grating

A novel exponential grating is proposed for Gaussian laser beam diameter measurement. The grating provides accurate measurement of submicron to millimetre diameter laser beams.

Application of gold plated edges for the measurement of the electron beam diameter

Gold plated edges fabricated on bulk and thin substrates are used as targets in electron beam diameter measurements and for system calibration in a Cambridge Instruments E-Beam Lithography machine. A target of 0.85 μm thick gold plated on a 2 μm thick silicon nitride membrane is shown to significantly enhance the signal to noise ratio and also improves the reliability of diameter measurements for low beam currents at all beam energies.Typically a statistical accuracy of 5 nm (1 sigma value) for a beam diameter of 38 nm is achieved. Monte Carlo simulation has also been used to quantify the electron scattering effects which take place in the target.

Aperiodic ruling for the measurement of Gaussian laser beam diameters

The use of an aperiodic ruling for the measurement of Gaussian laser beam diameters has been analyzed. We show that aperiodic rulings can provide spot-size measurements over a much broader range than available with Ronchi, triangular, or sinusoidal rulings used previously.

Automatic measurement of scanning beam diameter using an on‐line digital computer

AbstractA method for the measurement of electron probe beam diameter by digital image processing has recently been published. The purpose of the present report is to describe the development of an automatic system for beam diameter measurement. To complete this system, a method based on a theory which combines automation and high‐resolution conditions is proposed. In practice, the beam diameter is measured from the STEM image of a crystalline hole in a gold thin film, utilizing an on‐line computer system equipped with newly developed digital processing programs linked to a SEM in the transmission mode. The functions of the programs include statistical processing, matching, noise removal, interpolation, selection, and rotation. By combining these functions, the scanning beam diameter is accurately measured, in spite of difficulties, under most electron microscope operating conditions. The user simply appoints the edge included in the STEM image.

The Eindhoven proton microbeam

Proton microbeam equipment has been designed for element analysis with the PIXE method. To focus the proton beam of the EUT cyclotron (3.5 MeV) a quadruplet lens system with a magnification factor N = 0.025 and an acceptance of 1 mm mrad horizontally and 3 mm mrad vertically has been designed. The measured beam diameter is 40 × 60 μm 2 with a beam current of 150 nA. Using diaphragms a spot of smaller diameter can be achieved at reduced currents. The beam is used for studying element topography by scanning samples. The samples are moved with an stepmotor controlled x− y slide. The target area studied can be chosen. A computer is used for on-line experiment control and data acquisition. All spectra from the target area are stored (256 kword) and analysed to obtain the X-ray energy topography of the scanned area. An example of such an investigation is presented.

Laser beam characteristics using dc-induced second harmonic generation

Previous measurements of electric field induced second harmonic generation indicated that the confocal parameter for a laser beam as calculated from the second harmonic intensity profile was in disagreement with that calculated from the measured beam diameter. In this paper, we show that this discrepancy is caused by the departure of the beam from the pure TEM(00) mode. We expand a distorted beam in terms of the component modes by using the experimental beamscan and calculate the second harmonic profile using this expansion. This calculational procedure is found to be consistent with experimental observations.

Mode structure and beam divergence of a large-bore high-power argon ion laser

Experimental observations on the mode structure and beam divergence of a large-bore high-power argon ion laser are discussed. The gain medium, 185 cm in length and 12 mm in diameter, is capable of generating a steady-state multiline multimode laser output of more than 125 W in the blue-green. Various stable resonance cavity configurations have been investigated. Littrow prism and small aperture have also been used to obtain single-line and low-order transverse mode operations, respectively. The beam intensity profiles at two widely separated distances from the output coupling mirror are measured by scanning the beam across a small aperture affixed to a photodetector, using a rotating mirror. The two measured beam diameters are found to increase with the transverse mode volume (or output power) at a nearly constant ratio. The far-field beam divergence angle deduced from these measurements is found to vary linearly with the initial beam diameter and to be of the order of 1 mrad at the highest output power.