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Italian Research Laboratory continued...

They combined this detector with a scanning mirror that moves the beam in the vertical direction to provide y-axis measurement capability. This detector offers an advantage over more commonly used photodiodes and thermal detectors in that it is capable of accurately distinguishing the spatial features of the beam intensity. The researchers scanned the beam across the laser at a frequency of 50 Hz, generating accurate spatial measurements of the beam's intensity. The temporal behavior can be monitored independently by a standard fast photodiode.

This experimental set-up easily provides a large volume of data to be analyzed. A single-two minute scan generates 25,600 data points. Origin, which was developed by OriginLab Corporation (formerly Microcal Software, Inc.), Northampton, Massachusetts, gives the possibility to handle a virtually unlimited volume of data. Under an easy-to-use interface, the program also provides powerful data analysis routines used by the researchers to detect spectral information in their data, including Fourier transformations that convert their time histories to the frequency domain. Additionally, the package includes graphical capabilities that make it possible to display their data using three-dimensional charts that graph beam intensity as the third dimension on top of an XY axis that represents the cross-section of the beam.

Figure 2- A 3D plot of the intensity distribution from a laser with a short cavity and a large mirror diameter. Analysis of such intensity profiles helps researchers understand the modal interactions of the laser.

Many of the charts, especially those generated by lasers with relatively short cavities and large mirror diameters, generate complex and interesting patterns such as those shown in the accompanying illustration. The pattern corresponding to a mountain range with the peaks arrayed in a circular pattern is only one of many arrangements seen by INO researchers. Viewing the precise intensity distribution helps to understand the modal interactions of the laser. The intensity distributions observed by the pyroelectric detectors are extremely useful in laser design, particularly in the development and validation of spatial filters that can be used to provide specific intensity distributions.

Ciofini, Meucci and Labate have drawn several conclusions from their work. The first and most basic is that developers of laser-powered equipment, particularly those using large diameter lasers, need to carefully consider the uniformity and stability of the beam. For example, in a surface treatment application the uneven distribution of energy across the cross-section of the laser could easily produce inconsistent properties in the material being treated. Once the distribution of the beam is known, various focusing methods can be used in an effort to improve consistency. In some cases, an uneven energy distribution may be intentionally produced in order to increase the degree to which the laser's energy is focused on a very small area.

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