Laser Focusing and Beam Delivery Flashcards
(38 cards)
What are the three ways to assess beam quality?
- Divergence
- M2
- Mode
-focus
What is M2?
The beam quality factor or beam propagation factor.
what does M2 do?
Ratio of the beam parameter product (BPP) of an actual beam to an idea gaussian beam of the same wavelength
How does M2=1 vary from M2>1 to M2~10 to M2>100?
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)
What is TEM?
Transvers Electric Mode. Written as TEM nm
This is for the beam cross sections. M2=1+2n+m.
Why is beam quality important?
Beam quality influences the focused spot power density. Power density affects the type of processing that can be carried out
When is a gaussian beam not ideal?
When surface treatment needs to take place, such as for transformation hardening, the ideal beam shape is a flat temperature profile like M2=3
How to define gaussian beam sixe?
it is a product of the beam width and intensity which can be ruled from 1/e2
What are the two beam delivery operations?
Continuous wave and pulsed
what is continuous wave?
- 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.
What is pulsed?
Optical power delivered in short pulses at a certain
frequency
* Some lasers (e.g Excimer) are pulsed because they
cannot operate in CW
What is the difference in sample result from using a long pulse laser to a short pulse laser?
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.
What is the equation for fluence?
energy/area
What is the equation for irradience?
Power/area
What is power switching for pulsed lasers?
- 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.
What is Q switching for pulsed lasers?
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.
What method are available to modify laser beam profiles?
Defocused high order mode beams, scanning patterns, kaleidoscopes, beam integrators and diffractive optical elements.
What are the two options of beam delivery system?
A system of mirrors or fibre optics(restricted to certain wavelengths)
Are mirrors suitable?
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.
How does beam delivery for fibre optics work?
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.
What are the main approached for beam focussing system?
- Focussing optics
- Nozzle assemblies
- Depends upon spot size, plane of focus, environmental
conditions, etc.
What are the 2 common lenses shapes for focussing?
Plano convex and Meniscus and will have antireflection coatings.
What are the 2 physics theories behind focusing?
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.
What are common focusing errors? And how can this be corrected?
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.
