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How do I measure laser power and beam focus position?

 

Frequently Asked Question

Accurate measurement of laser power and beam focus position are important for laser materials processing: both are listed as process essential variables for qualification of welding procedures and should, therefore, be controlled within agreed tolerances.

Laser power can be measured using standard laser power meters; there are two main types: calorimeter and pyroelectric. Calorimeter power meters measure laser power using a timed exposure. They display average power absorbed on a calibrated readout scale and can be used for measuring the average of either continuous or pulsed power output. They usually have a response time of up to several seconds.

Pyroelectric power meters are relatively fast but cannot be used to measure continuous laser power directly, since this type only responds to changes in incident energy. These are the preferred choice for measuring the output of pulsed lasers with repetition rates up to several hundred kHz.

Currently, there is a wide choice of commercially available laser power meters. Selection of a particular meter for a given application will depend on the spectral range, sensitivity or minimum required response, and damage threshold. To produce a reproducible and accurate measurement, all laser power meters should be regularly calibrated.

Beam focus position can be measured using several methods. Accurate measurements can be made using standard beam profile measurement and diagnosis systems. For most of these systems, the laser beam is scanned with a probe containing a rotating pinhole, which is used to couple-out a small part of the radiation in the focus region and direct it to a detector. The positions of the scanning traces are movable in y and z directions, so it is possible to measure one plane of the beam within a few minutes. This can be repeated at different positions from the focusing head to establish the full profile of the beam showing the waist and depth of focus, etc.

The following beam parameters can be measured accurately by measuring the laser power density distribution in the focus region:

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

With different detectors and special measuring tips it is possible to detect a highly divergent beam as generated by high power diode lasers.

The approximate beam focus position can be determined by running a series of beam print tests on a flat, anodised aluminium sheet (or photographic paper) at various focus positions, or by conducting beam swiping on a tilted anodised aluminium sheet. These methods are simple, easy to carry out and require minimal capital investment - but they are not as accurate as dedicated beam profile measurement systems.

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