Laser Beam Characterization

Dr. Bernd Eppich is staff scientist at the Ferdinand-Braun-Institut in Berlin since 2005, where he is responsible for laser beam characterization of diode lasers and the design of micro-optical beam forming system. He holds degrees from the Technical Universities of Karlsruhe and Berlin where he investigated design criteria for resonators, the optimization of beam quality, and theoretical and experimental aspects of laser beam characterization. He developed a metrological method for reconstructing the four-dimensional Wigner distribution, allowing for the complete characterization of partially coherent laser radiation with respect to its propagation behavior. As a member of the working team "Definitions, Test Control Units and Testing Methods" of the DIN standards committee "Precision Mechanics and Optics" he was the leading author of the international standard ISO 11146, which is an important standard concerning laser beam characteristics. After his Ph.D. Bernd Eppich worked on methods for early recognition of rheumatism and on non-invasive blood glucose measurement at the Laser- and Medizin-Technologie Berlin company. He was awarded with the Innovation Competition Prize of the German Federal Ministry of Research and Education for the "Advancement of Medical Technology" in 2000.

• Introduction Short introduction into physical opticsRay opticsLight as geometrical raysFree propagation of light raysThe law of refractionParaxial approximation, Gaussian opticsLaw of refraction in paraxial approximationRefraction at spherical surfacesRay transfer matrices in one dimensionRay transfer matrices in two dimensionsSeparability of optical systems Separierbarkeit optischer SystemeLimits of of Gaussian optics, aberrationsWave opticsLight as an electro-magnetic waveHarmonic waves, frequency, wavelengthAmplitude, phase and coherenceHuygens principleCollins integral (optional)Gaussian beamsDefinition of Gaussian beamsGaussian beam parameters and free propagationPropagations through first order optical systemsPartial spatial coherenceInfluence of reduced spatial coherenceThe Wigner distributionGaussian Schell-modell beamsMoments of power density distributionsDefinition and meaning of first order momentsPropagation of first order moments through first order systemsDefinition and meaning of centered, second order momentsPropagation of second order moments through first order momentsDefinition of the variance diameter in one dimensionPropagation of the variance diameter in one dimensional systemsBeam propagation parameter and beam propagation ratio M2 in one dimensional optical systemsDefinition of the variance ellipse in two dimensionsPropagation of the variance ellipse in two-dimensional optical systemsBeam propagation parameters and invariants in two dimensional systemsSeparable beamsAstigmatism and geometrical beam classification Power and EnergyDetectorsCalorimetric detectorsPyroelectric detectorsThermopile detectorsPhoto diodesIntegrating spheres (Ulbricht spheres)Calibration Temporal behaviourPulsed emissionPulse durationPulse repetition frequencyPulse rise and delay timeContinuous wave emissionNoise Power density distributionParameters of power density distributionsCenter of gravityThreshold diameterRadial power content diameter ("power-in-the-bucket")Linear power content diameter ("knife-edge method")Variance diameterFlatness, Steepness, RoundnessDetectors for power density distributionsVariable apertures ("iris aperture", "variable slit",...)Scanning Detectors (Pinhole, photo diodes)CCD-KamerasPyro-electric camerasMeasurement of power density distributionsAlignmentAttenuationCalibrationMeasurement with (CCD) cameras, choise of ROI, saturation, baseline correction, typical errorsEvaluation of power density distributionsCalculation of parametersError discussion Beam propagation parametersCaustic measurements with (CCD) camerasTypical setupRequired caustic rangeFitting processChecking the beam symmetryBack calculation to reference planeMeasurement arrangementLaboratory conditionsStability of laser parametersMinimum free aperture of optical components Pointing stabilityPositional stabilityDirectional stabilityDetectors Wave frontsWave front und phase distribution of coherent beamsWave front of partially coherent beamsShack-Hartmann-Sensor for wavefront measurementsDetermination of the beam propagation ratio with the Shack-Hartmann-SensorInterferometric measurements of wave fronts CoherenceInterferometric determination of coherence propertiesSuperposition of beams and contrastLongitudinal (temporal) coherenceLateral (spatial) coherence Spectral propertiesTypical power spectra of various laser typesSpectrometer PolarizationStates of polarizationSpatially homogenous polarizationJones matricesPolarizer Relevant international standards