The result: the optical properties of the laser system deteriorate, leading to reduced efficiency over time. In order to maintain high quality in your processes, it is necessary to continuously monitor such parameters as the position of the focus and/or potential shifts due to age, soiling or temperature, as well as the laser's M2 quality score and power density. But how can one measure a focused laser beam with huge power density without doing damage to – or destroying – the gauge?
The developers at Ophir have designed a new, contactless measuring method based on the Rayleigh scattering, which describes the scattering of electromagnetic waves by particles that are smaller than the radiation's wavelength, such as oxygen or nitrogen molecules in the air. The electric field of the laser radiation induces an oscillation in the dipole molecule at the laser's frequency, thus leading to elastic scattering at that same frequency. The scattered laser light is imaged from the side using a telecentric lens assembly on a CCD or CMOS camera. Each individual pixel in a single row of the CCD detects the scattered light as a measuring point of intensity in the beam profile. From these measurements, and using a very accurate integrated software, it is possible to calculate beam and beam-quality parameters according to ISO 13694 and ISO 11146 standards, including focus diameter, focus position, divergence, ellipticity, M² (1/k) and beam parameter product (BBP). However, due to the very weak scattered radiation, it is necessary to minimize and control for any secondary light source that could distort or add artifacts to the Rayleigh light. This is achieved through optimal placement of the individual components and light-absorbing materials in the measuring chamber. An air purge system ensures that no foreign particles (such as dust) that could affect the measurement are to be found within the measuring range. Every single row of the CCD delivers an intensity profile. Thus, a typical CCD or CMOS camera with 1090 x 2048 pixels can measure 2048 individual profiles simultaneously.