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The FocusMonitor, a scanning diagnostics system for the analysis of continuous wave laser beams with low up to high laser beam powers, is used for the analysis of laser beam sources for laser material processing in laser beam welding, cutting as well as surface processing and others.

The FocusMonitor measures the beam properties of focused laser beams. In addition to the geometric dimensions of the focused laser beam, the focus position in space, the beam parameter product as well as the beam quality factor M² are determined. The integrated z-axis enables the automatic measurement of complete caustics over up to four
Rayleigh lengths.

The successful FocusMonitor has evolved into the FM+, providing new electronic features to meet the current and future demands of signal processing. A key part is a new main board with a 16 bit AD converter instead of the previously used 12 bit. The FM+ is operated with the new version of the PRIMES LaserDiagnosticsSoftware and is equipped with an Ethernet interface for fast and secure data exchange with computers or system controls. Thanks to the new mechanical design the FM+ can be mounted overhead without the need for additional hardware.

 

Measuring Procedure – The Principle

The FM+ is an opto-mechanically scanning diagnostics system. The laser beam is scanned using a specialized measuring tip. A small pin hole (typical diameter: 20 μm) in the measuring tip samples a small part of the laser beam. The sampled part of the laser beam is guided onto a detector element, which is chosen according to the power and wavelength of the laser radiation.
Hence, different laser beam sources and –systems can be measured at their maximum power simply by choosing the optimum measuring tip and detector. The high speed of the rotating measuring tip enables the analysis of high power densities. Due to the high dynamic range of the employed analog digital converter, a very good signal-to-noise (S/N) ratio is achieved. High peak intensities as well as very low intensities are displayed precisely. This is one of the prerequisites for an automatic measurement of beam caustics in the focus range over at least four Rayleigh lengths according to ISO 11146.

Advanced Functionality thanks to new LDS

With the new LaserDiagnosticsSoftware LDS, the FM+ offers considerably more functionalities. In addition to the faster Ethernet connection, the LDS enables the fully-, semi-automatic or manual measurement of the beam distribution as well as the determination of the beam position and the beam dimensions. Measurements with a higher resolution and freely selectable ROI (region of interest) can be recorded. Furthermore, the FM+ has a higher dynamic range, which makes it possible to determine beam radii even at a poorer S/N ratio. Further parameters like beam waist diameter, divergence angle, Rayleigh length and beam quality M² are examples of properties, which will be calculated by the LDS based on the measuring results from the FM+.

The number of measurement planes is freely selectable, typically between 11 and 21 planes. Complete measurement data sets and computation results can be stored and, if necessary, loaded for further measure ments with specified basic data sets as a reference. Alternatively to the new LDS, the FM+ runs in combination with the previous version LDS 2.9. Both versions are compatible with Windows 7® or higher operating systems.

focusmonitor
Astigmatic beam: false color presentation at different z-positions

Measuring Tips and Detectors

For accurate operation, the FM+ needs to be equipped with a suitable measuring tip/detector combination. The replacement of both the detector and the measuring tip can be done in a few minutes. Adapted measuring tips and detectors for various divergences as well as wavelengths of different beam sources are available. The detectors are equipped with an EEPROM, enabling communication with the LDS. The resolution of the detector and the size of the measuring window can be adjusted within the software.

For the wavelength of CO2 lasers (λ = 10.6 μm), mainly pyroelectric detectors, such as the DFCM+, together with CO2 high power measuring tips have to be used. For the analysis of solid-state lasers in the range between 400 and 1100 nm, the detector DFY-PS+ with a high dynamic range and an automatic signal adjustment is employed. The DFIG-PS+ detector
is available for wavelengths between λ = 1 - 2.1 μm. These detectors are generally combined with a NIR high div measuring tip. There are additional measuring tips for diode laser radiations, which are often highly divergent. In addition, specially adapted detectors, e.g. with an optimum signal-to-noise ratio or measuring tips with increased or reduced sensitivities can be provided.
As hardly any beam power is absorbed during beam diagnosis, the FM+ is ideally implemented in combination with a power meter such as the PRIMES PowerMonitor. Thereby, the power can be monitored simultaneously to the caustic measurement and all radiation is safely absorbed.