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 optics
Ray optics
Light as geometrical rays
Free propagation of light rays
The law of refraction
Paraxial approximation, Gaussian optics
Law of refraction in paraxial approximation
Refraction at spherical surfaces
Ray transfer matrices in one dimension
Ray transfer matrices in two dimensions
Separability of optical systems Separierbarkeit optischer Systeme
Limits of of Gaussian optics, aberrations
Wave optics
Light as an electro-magnetic wave
Harmonic waves, frequency, wavelength
Amplitude, phase and coherence
Huygens principle
Collins integral (optional)
Gaussian beams
Definition of Gaussian beams
Gaussian beam parameters and free propagation
Propagations through first order optical systems
Partial spatial coherence
Influence of reduced spatial coherence
The Wigner distribution
Gaussian Schell-modell beams
Moments of power density distributions
Definition and meaning of first order moments
Propagation of first order moments through first order systems
Definition and meaning of centered, second order moments
Propagation of second order moments through first order moments
Definition of the variance diameter in one dimension
Propagation of the variance diameter in one dimensional systems
Beam propagation parameter and beam propagation ratio M2 in one dimensional optical systems
Definition of the variance ellipse in two dimensions
Propagation of the variance ellipse in two-dimensional optical systems
Beam propagation parameters and invariants in two dimensional systems
Separable beams
Astigmatism and geometrical beam classification

Power and Energy
Detectors
Calorimetric detectors
Pyroelectric detectors
Thermopile detectors
Photo diodes
Integrating spheres (Ulbricht spheres)
Calibration

Temporal behaviour
Pulsed emission
Pulse duration
Pulse repetition frequency
Pulse rise and delay time
Continuous wave emission
Noise

Power density distribution
Parameters of power density distributions
Center of gravity
Threshold diameter
Radial power content diameter ("power-in-the-bucket")
Linear power content diameter ("knife-edge method")
Variance diameter
Flatness, Steepness, Roundness
Detectors for power density distributions
Variable apertures ("iris aperture", "variable slit",...)
Scanning Detectors (Pinhole, photo diodes)
CCD-Kameras
Pyro-electric cameras
Measurement of power density distributions
Alignment
Attenuation
Calibration
Measurement with (CCD) cameras, choise of ROI, saturation, baseline correction, typical errors
Evaluation of power density distributions
Calculation of parameters
Error discussion

Beam propagation parameters
Caustic measurements with (CCD) cameras
Typical setup
Required caustic range
Fitting process
Checking the beam symmetry
Back calculation to reference plane
Measurement arrangement
Laboratory conditions
Stability of laser parameters
Minimum free aperture of optical components

Pointing stability
Positional stability
Directional stability
Detectors

Wave fronts
Wave front und phase distribution of coherent beams
Wave front of partially coherent beams
Shack-Hartmann-Sensor for wavefront measurements
Determination of the beam propagation ratio with the Shack-Hartmann-Sensor
Interferometric measurements of wave fronts

Coherence
Interferometric determination of coherence properties
Superposition of beams and contrast
Longitudinal (temporal) coherence
Lateral (spatial) coherence

Spectral properties
Typical power spectra of various laser types
Spectrometer

Polarization
States of polarization
Spatially homogenous polarization
Jones matrices
Polarizer

Relevant international standards