In almost all cases a laser application requires a laser beam with low divergence emitted in the so called fundamental Gaussian mode (TEM00). This is not always guaranteed for each laser and especially for high power laser systems because the emission may be multimode. Considering the laser diodes, which possess even elliptical beam shapes, it is important to have a tool for analysing the structure of the beam shape for different types of laser in order to be sure to fulfil the required demands for a particular application.
If more transverse modes exists, the beam shape is no longer circular and depends on the number of higher transverse modes causing a variety of different patterns to occur.
This also means that the intensity distribution is different and the application will not fulfil the desired performance. Within this project, a laser beam profiler based on the knife edge method is used to measure the spatial intensity distribution. As probe lasers a HeNe-laser and a diodelaser are used.
The HeNe-laser emits a pure Gaussian TEM00 whereas the diodelaser emits an elliptical beam. By means of different optical elements (beam expander and cylindrical lenses), the influence on the intensity distribution will be measured and discussed.

Principle of Operation

A precise knife edge is moved through the beam path of the Laser. As the blade moves across the beam, light is blocked from reaching the photo sensor and the measured signal decreases to zero. The signal is then differentiated to obtain the beam profile. When the path to the detector is fully opened the entire intensity of the beam is measured and used as calibration value.
To increase the resolution the number of knife edges is increased whereby the orientation of each blade is different. In this set-up a number of seven knife edges is used in order to increase the resolution as well as to measure spatial intensity distributions which are significantly different from a Gaussian one.
The collection of the data is performed automatically by a PC with special software. For this reason an extra board is built into the PC which controls the beam analyser, collects and stores the data on the hard disk of the PC. Based on the recorded data a 2D graphical presentation of the intensity profile is displayed on the monitor or printed out on an ink-jet colour printer.