Characterization of the near field of typical semiconductor lasers and the spot size of tightly focused laser beams poses significant challenges to direct near-field profile measurement techniques. Far-field measurements are considerably easier to perform and offer an attractive alternative for this characterization. To assess this alternative, profiles of edge-emitting laser diodes and VCSELs, and the spot size of focused laser beams were determined from far-field and near-field measurements. In the far field, measurements were made using a 3D-scanning goniometric radiometer that provides irradiance profiles with angular extent to approximately ±70°. Indirect measures derived from these data using different methods are reported, including the spot size using the M2 times-diffraction-limited approximation, the Hankel transform Petermann II mode-field diameter used for optical fiber characterization, and a measure obtained from 2D Fourier transform inversion of the far field using phase retrieval. In the near field, direct profile measurements were made using a scanning-slit profiler and a CCD camera with magnifying lenses.
KEYWORDS: Near field diffraction, Convolution, Diffraction, Fringe analysis, Beam analyzers, Optical testing, Wavefronts, Oscilloscopes, Near field optics, Head
By comparing the measured width of an optical test patten to the known width, the absolute error of a clip-level profiler is determined to be (-0.1 +/- 0.3)%. An expanded fundamental mode beam illuminates a pair of opposed knife edges (a wide slit) to generate the test pattern by Fresnel diffraction. Analysis of the diffraction pattern gives 18.2% as the appropriate clip level to read the geometrical shadow width between edges (with additional small adjustments for illumination non-uniformity and the finite size of the scanning aperture). The separation between the edges is determined by mechanical translation edge to edge through a focused beam. 3
Measurement of the beam profile and/or beam width of a laser source is complicated by the fact that most detectors available are just too sensitive. Typical lasers measured in millijoules/cm2 must be attenuated to a few 10s of nanojoules/cm2. To maximize the photoreceptor's digitizer range and operate at a maximum signal-to-noise ratio, it is preferred that the input energy is controlled to just below the detector saturation point. The most common attenuation methods are either discrete in increment, narrow in wavelength range, or operate on a polarization principle that can produce erroneous results for many mixed-mode lasers. Metalized gradient pair attenuators suffer from nonlinear attenuation across the beam and/or multiple interference fringes. What is described here is a novel technique to continuously attenuate an incident laser beam over a ratio of 6300:1 or more (3.8 orders of magnitude). It can be adjusted to the nearest incremental transmission value of 0.005 (0.5 percent). It achieves this at near normal incidence, which is very important if the source contains polarization-dependent laser mode components. Photon, Inc.'s ATP attenuator package achieves this performance without interference fringes, bubbles, or stria and with a minimum of deflection or redirection. A broad wavelength performance also is achieved that ranges from 360 to 2,500-plus nanometers. ATP's primary application is with very sensitive CCD or vidicon array detectors that are capable of measuring beam sizes of a few hundred microns. ATP does this with virtually no root mean square wavefront errors. This device coupled with a high accuracy beam profiler will produce true accurate profiles and beam widths.
Laser beam width, divergence, and propagation factor are three major parameters required to apply a source. This paper discusses the status of the draft standard ISO TC172/SC9/WG 1. The two methods, free beam and lens method, are described and experimentally shown to give near equal results for a single TEMoo source. For nearly collimated sources, a quick method requiring two beam widths is described to give the key parameters. Basic measurement devices including slit scan, pinhole scan, encircled energy, knife edge scan and their array equivalents are described. These methods are compared to the theoretical variance beam measurement device with the result that the slit scan produces the least inherent error, 0 to 4% for the first six laser modes, assuming zero slit width.
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