On Feb 8, 4:31 am, MacFrag...@[EMAIL PROTECTED]
wrote:
> Now it appears to be a no-brainer to simply always use high-frequency
> lasers (remember wavelength is 1/f), but on the other hand, low-
> frequency lasers appear to produce a lot less waste heat, i.e. they
> can be more efficient. Project Rho says that a high-freq laser may
> have up to 20% efficiency, whereas low-frequency lasers can have up to
> 65%. Since it produces less waste heat, it can fire more often, albeit
> at lower ranges.
The efficiency is more a matter of the method being used to generate
the beam rather than any effect inherent to wavelength. In fact most
methods of generating lasers are horribly inefficient regardless of
wavelength, with 1% to 0.1% of the input energy coming out as laser
energy. However, there are exceptions. The popular CO_2 laser has a
10.6 micron wavelength and a quantum efficiency of around 40% (the
quantum efficiency is the maximum possible efficiency) and practical
wallplug efficiencies between 10% and 20%. Neodymium lasers at 1.064
micron have a quantum efficiency of around 80% and practical wallplug
efficiencies of between 5% and 20%. High powered laser diodes in the
near IR (probably around 0.8 microns) have been demonstrated lasing at
around 70% wallplug efficiency, but laser diodes have very poor
quality beams at higher power. Jefferson Labs proposed a MW-class FEL
that could be put aboard a 747 which would operate at around 30%
wallplug efficiency and could be tuned through the near IR and visible
range (Jefferson Labs is the main researcher of free electron lasers,
so presumably they know what they are talking about). In principle,
there is no reason a FEL could not get close to 100% efficiency with
good enough energy recovery of the electron beam and if you did not
need refrigeration and cryogenics for the superconducting resonant
cavities.
Luke
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