Getting the Goods on Laser Beam Intensity

New software from Microcal makes possible 3D graphical views of the beam cross section.

At the National Institute of Optics (INO) in Florence, Italy, researchers

A 3D reconstruction, plus a contour map, of the intensity profile of a mid-infrared laser created with Origin 5.0. To measure the uniformity and stability of carbon dioxide laser output, INO researchers developed a special instrument that uses a 256-pixel array of pyroelectric detectors positioned in a straight line.

Marco Ciofini and Riccardo Meucci and graduate student Antonino Labate, in collaboration with other INO staff members, have developed an instrument that combines pyroelectric detectors and a scanning mirror to measure intensity variations in the cross section of a carbon dioxide laser beam. Their measurements show unexpected patterns and fluctuations that should be taken into account by anyone designing laser equipment used for metal cutting, data storage, measurement, and other applications. Key to the success of their study was the use of a data-analysis software package called Origin, from Microcal Software Inc. of Northampton, MA, that is capable of storing and graphically depicting the data generated by the measurements.

INO, a public research institution operated by the Italian Ministry of University, Scientific, and Technological Research, has three distinct research units. The quantum optics unit pursues that discipline, nonlinear dynamics and optics, and radio-frequency devices. The optoelectronics unit is responsible for noninvasive optical control, biomedical diagnosis systems, and spectrometry. Finally, the optical metrology unit handles Fourier optics and coherent techniques, optical testing, computer-aided optical design, and diagnosis of works of art.

To measure the uniformity and stability of CO2 laser output, INO researchers developed a special instrument that uses a 256-pixel array of pyroelectric detectors positioned in a straight line. Pyroelectric detectors are based on the fact that ferroelectric materials such as lithium tantalate exhibit a large spontaneous electrical polarization below a temperature known as the Curie point. Incident radiation alters the temperature and changes the polarization. When this happens, the charges induced in the electrodes produce a voltage across the slice if the external impedance is comparatively high. The sensor only produces an electrical output signal when the level of incident radiation changes. The process is independent of the wavelength of the incident radiation; hence, pyroelectric sensors have a flat response over a very wide spectral range.

The team 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 detector 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 setup easily provides a large volume of data to be analyzed. A single two-minute scan generates 25,600 data points. Microcal's Origin gives users the possibility of handling virtually unlimited volumes 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 transforms 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 X-Y axis that represents the cross section of the beam.

Many of the charts, especially those generated by lasers with relatively short cavities and large mirror diameters, reveal complex and interesting patterns such as those shown in the illustration. The pattern corresponding to a mountain range with the peaks arrayed in a circular form is only one of the many arrangements seen by INO researchers. Viewing the precise intensity distribution helps users 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 equipment, particularly those with large beam diameters, need to consider carefully the uniformity and stability of the beam. For example, in a surface treatment application, the uneven distribution of energy across the laser's cross section could easily produce inconsistent properties in the material being treated. Once the distribution of beam intensity 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.

For more information on Origin, contact Microcal Software Inc., One Roundhouse Plaza, Northampton, MA 01060; (800) 969-7720, ext. 36; fax: (413) 585-0126.