High power lasers in excess of 1 kW generate enough Rayleigh scatter, even in the NIR, to be detected by silicon based sensor arrays. A lens and camera system in an off-axis position can therefore be used as a non-contact diagnostic tool for high power lasers. Despite the simplicity of the concept, technical challenges have been encountered in the development of an instrument referred to as BeamWatch. These technical challenges include reducing background radiation, achieving high signal to noise ratio, reducing saturation events caused by particulates crossing the beam, correcting images to achieve accurate beam width measurements, creating algorithms for the removal of non-uniformities, and creating two simultaneous views of the beam from orthogonal directions. Background radiation in the image was reduced by the proper positioning of the back plane and the placement of absorbing materials on the internal surfaces of BeamWatch. Maximizing signal to noise ratio, important to the real-time monitoring of focus position, was aided by increasing lens throughput. The number of particulates crossing the beam path was reduced by creating a positive pressure inside BeamWatch. Algorithms in the software removed non-uniformities in the data prior to generating waist width, divergence, BPP, and M<sup>2</sup> results. A dual axis version of BeamWatch was developed by the use of mirrors. By its nature BeamWatch produced results similar to scanning slit measurements. Scanning slit data was therefore taken and compared favorably with BeamWatch results.
A new instrument design allows the M<sup>2</sup> beam propagation ratio to be measured in real time at the update
rate of a standard CCD camera. This allows lasers from single shot to CW to be measured while the laser
cavities are being adjusted. This drastically reduces the test time required for this operation. In this paper
we will discuss the theory behind this innovative approach to the M<sup>2</sup> measurement and the methods for the
selection of the proper optical components for use of the system with various laser types and beam shapes.
The authors will show results of numerous measurements of different lasers and laser types, including solid
state diode and traditional gas lasers with M<sup>2</sup> values from near 1 to considerably higher values, and show
comparisons these results with other measurement methods.
The instrument design is based on a method of simultaneous capture of the waist and several Rayleigh
ranges, allowing the instantaneous fit of the ISO M<sup>2</sup> propagation curve. The authors will discuss the
important considerations necessary to generate accurate results for different laser configurations.
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 M<sup>2</sup> 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.