Laser materials processing

04 Guidance and Focussing of Laser beams

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Cartes-fiches	16
Langue	English
Catégorie	Technique
Niveau	Université
Crée / Actualisé	07.01.2015 / 16.01.2015
Lien de web	https://card2brain.ch/box/laser_materials_processing_
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Which part of the laser beam is known as "focus depth" ?

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Wich two optical elements can be used for focusing a laser beam? (in case of YAG- and CO₂-Lasers)

YAG-LAsers:

Lens-optics, consisting of glass.

CO₂-Lasers:

Lens-optics, consisting of ZnSe (Zinc-selenide, Zinkselenit), are in use in case of lower laser-powers up to ca. 5 kW. The employment of ZnSe with higher laser powers leeds to an increasing danger of self-combustion (Selbstentzündung) of the lens due to the amount of laser-energy absorbed by the ZnSe-material.And: The combustion-products of ZnSe are harmful ! (note: Here glass as lens-material is not possible due to the CO2-lasers wavelength.) Therefore, in case of higher laser-powers (> 5 kW) mirror-optics are in use for the focussing of CO2laser-beams (mostly parabolic-mirrors).

Beam-Guidance to the workpieces via free-propagation:

Wich benefits are resultig from a short beam-path (<2m)

The short path between laser-resonator and focussing-lens is beneficial in several ways:

•Divergences Omega₀of the raw beams don´t lead (along the short path) to unwished changes of their diameters.

•No (or rare) optical elements for the beam-guidance (e.g. mirrors), thus low dangers through dust etc.

•To maintain laser-safety only short metallic tubes around the beam-path are necessary.

•Misalignment-dangers (Dejustiergefahren) between laser and lens are low (intercept-theorem [Strahlensatz] with short lengths! ). Therefore justifiable (vertretbar) low mechanical complexity (Aufwand) for a stable and rigid beam-guidance.

How are laser beams transported over longer (>2m) distances ?

Distinguish between the different types of lasers.

Beams of YAG- or Diode-lasers would be guided on longer paths via light-conducting-cables.

In the case of longer beam-paths between laser and workpiece, a beam-guidance via free-propagation is only used with CO₂-lasers.

Wich spechial complexities has to be mentioned in case of longer beam-guidance via free propergation (using a high power CO₂-laser) ?

Limiting the divergence of the raw beam via Telescopes
Constant-keeping (Konstanthaltung) of the raw beam´s diameter D and its divergence at the location of the focussing-element (am Ort des fokussierenden Elementes) via a “BeamTrombone”(“Strahlposaune”)
Measures against dangers of dirt and misalignment

Name the main benefits of using light-conducting-cables (fiber-cabels)

No danger of dirt-pollution along the beam´s path
Laser-safety is reached simply: The cable has a plastic covering (wrapping), that can be checked against its through-melting via integrated electrical signal-wires
No misalignment-dangers: The cables are flexible and follow all movements

Why are there bending restrictions for light conducting cables and how are these restrictions technicaly achieved?

Bending restriction: for guiding a beam through a LLK total reflection on the casing is required. If the bending radius passes a certain limit, total reflection is not longer possible.

Bending Radius limitations: for this purpose „bending-radiuslimiters“are part of the outer shielding-covers of the LLK (e.g. Kevlar-fibers).

Wich angle is crucial for incouppling a laser beam into a light conducting cable?

angle of aperture

Wich two different types of light conducting cables are relevant for materials processing ?

Wich one is the most important one?

1. Step Index Fibrer --> (most important one)

2. Gradient Index Fiber

Wich reasons for unsing a monitoring device can you think of?

During the programming of the machinery along the edges of a workpiece (control)

During the laser-materials-processing: e.g. observing the luminous-effects (Leuchterscheinungen) of the processing and the comparison of those observed during successful processings. Often used as quality-proofs during processes with quality-documenting-obligations.