FocusMonitor

Measuring position and dimension of the focus

• Fully automated caustic measurement with the integrated z-axis
• Determination of focal width and focal positoin based on the measured power density distribution
• High dynamic due to a 12 bit A/D-converter with additional prescaler
• Measurement of different wavelengths and beam divergences (CO₂,Nd:YAG and doide lasers) with a modulator system of detector and measuring tips
• Intuitive graphical user interface
• Typical power range: CO₂-Laser 100 W - 20 kW, Nd:YAG-Laser 10 W - 6 kW, Diodelaser 1 W - 6 kW

Application

The FocusMonitor is a device to measure the focussed laser beam with high continuous output power. Quick and reliable radius and position of the focussed laser beam are determined even in industrial environment.

The laser focus controls mainly the performance of laser material processing. He is the real tool in most applications of laser material processing.

The following beam parameters and errors can be estimated by measuring the laser power density distribution in the focus region:

• focus radius
• position of the focus in space
• spatial power density distribution
• beam propagation ratio M²

The focus analysis in an industrial environment has several goals:

A. The beam geometry in the focus region should be measured and documented. So it is possible to detect even long- term variations of the system, which result in changes of the focus quality.

B. In the case of a system failure the source can be traced and identified. So no intact optical components will be changed and the down time will be reducedAusfällen.

C. The performance in material processing can be increased. For instance development of burrs in laser cutting can be avoided; exact positioning of the focus point in square butt joint welding can be achieved.

Technical description

The Laser Beam is scanned with a rotation pinhole (about 20 µm in diameter). This pinhole is used to couple out a small part of radiation in the focus region. With two mirrors the signal target is directed to the detector. Then the electrical signals are digitized with work pieces power densities of several MW/cm². The divergence of the radiation in most cases is low. This leads to f- numbers (ratio of focal length of the optics to diameter of the unfocussed beam) of normally b12 bit resolution and transferred to a computer. The positions of the scanning traces (x) are movable in y- and z- direction. So it is possible to measure a complete beam caustic within some minutes.

In the standard configuration the FocusMonitor is designed to measure IR- lasers (CO₂ or Nd:YAG- laser) with a output power of several hundret watts up to many kW. These lasers reach in standard applications like cutting or welding of typical metallic igger than 4.The central unit of the FocusMonitor is the scanning mechanic with the control of the three axis of motion.

With different detectors and special measuring tips it is easy to expand the field of applications of the FocusMonitor. So it is possible to detect also highly divergent beam as generated by high power diode lasers. Also beams with more than 10 kW CO₂- laser radiation can be analyzed. But even higher laser sources with a power of only a few watt can be measured.

Due to the high speed of the measuring tip (up to 10- 40 m/s) no material ablation takes place on the surface. In addition the measuring tip is made of material which is corrosion and temperature resistant.

The dynamic of the used 12 bit analog to digital converter ensure a high signal to noise ratio. This is - beside other features - the basis for an automatic caustic measurement accroding to the ISO 11146 standard. The request of the ISO is to measure within the direction of beam propagation four times the Rayleigh length minimum. Within the measuring range the power density varies a factor of four and the signal to noise ratio should not be higher than 100:1.

The relative spatial resolution of the FocusMonitor can be adjusted between 32x32 up to 256x256 pixel in the scanning array. So it is possible to detect even small disturbances in the power density. With the standard settings of 64x64 pixel measurements with high speed and sufficient accurracy can be realized.

Technical data

Power Range (depending on the measuring tip)

• Power @ 10,6 µm: up to 15 kW
• maximum power density: up to 30 MW/cm²
• Limitimg factor is the energy on the measuring tip so only one of the above values can be run to limit at the same time.

Function of the measuring system

• 2- dimensional scanning of the power density distribution of the laser beam in the xy-plane by fading out a small part og the initial beam with a rotating pinhole. During measurement the beam is not absorbed within the device. For a save operation it needs an external absorber.

Spatial measuring range

• x-, y- direction: 0.25 mm to 8 mm with a factor of about 1.4 between the different windows.
• z- direction: 35 mm or by option 120 mm

relative spatial resolution

• in x- direction (number of pixels per trace): 32, 64, 128, 256
• in y- direction (number of traces per image): 32, 64, 128, 256

Actuation of the system in x- axis

• maintenance- free brushless DC- motor with crystal stabilized speed control.maintenance- free brushless DC- motor with crystal stabilized speed control.1875 rpm or 3750 rpm - by option 7500 rpm

Digitizing of the measured data

• 12 A/D- converter and prescaler

Measuring time

• Standard settings with 64x64 pixels: 2.5 s
• repetition rate of the measurement: 0.1 Hz

Electrical Supply

• Voltage: 90 - 240 V, 50 - 60 Hz
• Electrical port 2 x PRIMES RS485- bus
• in Standard-Service 0,4 A

Communication

• The FocusMonitor can be controlled by the standard serial interface (RS232) of any PC. The data transfer from the device is based on RD485 - via an interface converter the signal is transferred to RS232.
• The software uses a graphical user interface. It runs on Windows 95/98 and Windows NT.

Dimensions

• dimensions (h x l x w): 242 mm x 135 mm x 290 mm (plus connectors)
• weight: 6 kg

Software

• measuring results:two AD converters enable a large signal to noise ratio. This is as well the basis for automatic caustic- and beam axis location measurement, as well for measuring the relative power density. (Automatic caustic measurement requires the PRIMES FocusMonitor)
• measuring modes: single measurement, automatic measurement, mean value, monitoring, adjustment mode
• presentation: Isometry, false color, contour lines, caustic, numerical review, symmetry- check
• data format: PRIMES- format (binary) or MDF- format (ASCII)

Options FocusMonitor

• measuring tips with a higher standing or with changed sensitivity.
• rotational speed up to 7500 rpm
• increased z- axis
• integrated shielding gas flushing
• detectors and measuring tips for different wavelength and beam divergence

Control and presentation

For the control and presentation of the measurement a Windows-based Software is available A typical result is shown in the presentation above. This software runs standard calculation methods as well as sophisticated analysis tools.With PRIMES - Bus the FocusMonitor is connected with the PC, other PRIMES measuring systems and by option via interface to the SPC. This ensures the user to run even in production environment a full automated focal measurement.The integrated z- axis enables automated caustic measurements within a few minutes. The user receives from the measured power density distributions at various z - positions the focal diameter, the beam propagation ratio M² and the position of the focal spot in space calculated.