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Laser Materials Processing > Laser Focusing and Beam Delivery > Flashcards

Laser Focusing and Beam Delivery Flashcards

1
Q

What are the three ways to assess beam quality?

A
  • Divergence
  • M2
  • Mode
    -focus
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2
Q

What is M2?

A

The beam quality factor or beam propagation factor.

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3
Q

what does M2 do?

A

Ratio of the beam parameter product (BPP) of an actual beam to an idea gaussian beam of the same wavelength

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4
Q

How does M2=1 vary from M2>1 to M2~10 to M2>100?

A

M2=1 is an ideal guassian beam
M2>1 has a dip in the middle of a gaussian beam
M2~10 has a very jagged top
M2>100 is a tophat (square top)

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5
Q

What is TEM?

A

Transvers Electric Mode. Written as TEM nm

This is for the beam cross sections. M2=1+2n+m.

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6
Q

Why is beam quality important?

A

Beam quality influences the focused spot power density. Power density affects the type of processing that can be carried out

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7
Q

When is a gaussian beam not ideal?

A

When surface treatment needs to take place, such as for transformation hardening, the ideal beam shape is a flat temperature profile like M2=3

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8
Q

How to define gaussian beam sixe?

A

it is a product of the beam width and intensity which can be ruled from 1/e2

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9
Q

What are the two beam delivery operations?

A

Continuous wave and pulsed

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10
Q

what is continuous wave?

A
  • The output is constant over long periods of time (i.e.
    seconds)
  • Is possible to modulate these (e.g. using a chopper)
    but then classed a s pulsed CW.
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11
Q

What is pulsed?

A

Optical power delivered in short pulses at a certain
frequency
* Some lasers (e.g Excimer) are pulsed because they
cannot operate in CW

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12
Q

What is the difference in sample result from using a long pulse laser to a short pulse laser?

A

The long pulse(microS) laser can cause ejected material, lots of surface damage and debris, large melt and heat affected zone.
Short pulse (fs)laser can cause a plasma plume collected. There is no melt zone, heat transfer to surrounding material, no cracking, no surface damage.

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13
Q

What is the equation for fluence?

A

energy/area

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14
Q

What is the equation for irradience?

A

Power/area

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15
Q

What is power switching for pulsed lasers?

A
  • A pulse may be generated by an electric switch (basically an on/off switch) for
    the pump source. The resultant pulses are typically square.
  • Suitable for switching CW devices. Pulse duration typically as short as
    milliseconds.
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16
Q

What is Q switching for pulsed lasers?

A

A device with low optical transmission forms part of the optical resonator.
when the population inversion is very high the device is switched on to produce a very high power with durations of the order of a nanosecond.

17
Q

What method are available to modify laser beam profiles?

A

Defocused high order mode beams, scanning patterns, kaleidoscopes, beam integrators and diffractive optical elements.

18
Q

What are the two options of beam delivery system?

A

A system of mirrors or fibre optics(restricted to certain wavelengths)

19
Q

Are mirrors suitable?

A

They do work for all lasers however a few % absorption can be a lot of energy for a high power laser that is lost. It is important mirrors are air and water cooled as they are not perfect relectors.

20
Q

How does beam delivery for fibre optics work?

A

Relies on total internal reflection acting as a waveguide for laser light. The inner core has a high refractive index whereas the outer cladding has a low refractive index. Limited to infrared and visible light lasers.

21
Q

What are the main approached for beam focussing system?

A
  • Focussing optics
  • Nozzle assemblies
  • Depends upon spot size, plane of focus, environmental
    conditions, etc.
22
Q

What are the 2 common lenses shapes for focussing?

A

Plano convex and Meniscus and will have antireflection coatings.

23
Q

What are the 2 physics theories behind focusing?

A

Geometric approach suggests a focal point - implying infinite intensity.
Wave physics creates a focal region - there is a minimum size (beam waist) where diffraction limits the ability to focus.

24
Q

What are common focusing errors? And how can this be corrected?

A

suffering spherical aberration causing the lend to focus the outer part of the beam at a point closer than the central portion. It can be predicted and corrected using aspheric lenses.

25
Q

What is the error contribution?

A

This is the error from small input beam diameters where the focused spot size is diffraction limited. As the input beam diameter increases, spherical aberration starts to dominate the spot size over diffraction.

26
Q

What is the depth of Field?

A

Also known as the Rayleigh length, is the range along which the size of the beam is no more than 1.4 ( 2) times the minimum spot size i.e a halving of density

27
Q

How to focus with other beam shapes positives?

A
  • Allows the beam distribution to be optimised for the process
  • Rapid beam distribution change – switch optics
  • No maintenance
  • No moving parts
  • Increased depth of field
    – ± 30 mm with
    DOE (Top Hat)
    – ± 12 mm with
    equivalent
    Gaussian lens
28
Q

What is a nozzle assembly used for in beam focussing?

A
  • House focussing optics
  • Direct assist gas to the laser
    interaction area (i.e. onto the
    workpiece)
  • Prevent damage to the optics
    Clearly the nozzle design will depend
    on the type of processing.
29
Q

How to achieve relative movement between the bean and the workpiece?

A

Move the workpiece (CNC based)
Move the laser (robot or CNC)
Move the bean (galvo scanner)

30
Q

How is the workpiece moved CNC?

A

By clamping it to a CNC table, the rotary axis may be used for cylindrical workpieces, rotary axis can be used for cylindrical.

31
Q

What are flying optics?

A

These are CNC mirrors where the beam divergence causes the focal spot size to change with position.
Telescopic optics can be used to keep the beam the same size over the whole workpiece.

32
Q

What are 3 positives of laser movement?

A

Mostly fibre coupled lasers
ability for precision focus
ideal for highly complex shapes

33
Q

What are the key issues and things to consider with gantry systems?

A
  • Key issues are
    – Acceleration
    – Backlash
    – Accuracy
    – Repeatibility
  • You must consider
    – Drive mechanism - linear/direct/belt
    – Encoding mechanism - e.g. relative/absolute or
    optical/inductosyn
34
Q

How does beam delivery by mirror (GALVO SCANNER) work?

A
  • Used for local delivery of beam, where fast movement of workpiece would normally be required (in effect this
    device is in lieu of workpiece
    manipulation)
  • Fast and accurate scanning of beam
  • Size is limited by keeping within the
    ‘depth of focus’ – not defocusing enough to change the processing characteristics
  • Applications in laser marking, cutting,
    rapid prototyping (sintering).
35
Q

What are enclosures for?

A

Enclosures are used to prevent accidental human contact with the machining area.
* Enclosures can contain the whole machining area, or the immediate beam interaction region
* Carbon dioxide laser beam cannot pass through clear polymer sheet, most CO2 system enclosures are made of polycarbonate or perspex.
* Near infrared Nd:YAG laser beams have to be stopped by metal enclosures. Windows can be put in with a green filter covering (very expensive) or a CCD camera can be used (very cheap!)

36
Q

Why should enclosures be interlocked?

A

Interlocks that cannot be readily over ridden should be used e.g. not
those that can be jammed with sticky tape or a screwdriver!
* Interlocks should prevent the shutter from opening
* The enclosure need not completely enclose the machining area,
those on many large cutting systems simply extend above head
height

37
Q

What type of beam delivery system would you use for etching a polymer phone case using a CO2 laser?

A

Mirror based beam delivery system using focal lenses.

38
Q

What type of beam delivery system would you use
for cutting steel using a fibre laser?

A

Fibre optic cables, beam shaping optics focused into a small spot and cutting head with focusing optics with a CNC moving plate and gas nozzle

Laser Materials Processing (13 decks)

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