[AR] Re: liquid lasers
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 26 Oct 2020 22:28:17 +0000
On 26/10/2020 16:45, Uwe Klein wrote:
Am 26.10.20 um 14:02 schrieb James Fackert:
wouldn't surrounding optical fibers with a fluid matching the refractive
index etc of the fiber nullify the fiber's light guiding
characteristics?
Yes - perhaps I wasn't clear.
Cores always have the highest RI, and for a naked core the liquid's RI
would not match the core but would have a lower RI so you would get
total internal reflection at the core/liquid boundary, thereby
containing the laser light within the core.
Pump light could get from liquid to core through this boundary.
isn't the fiber used a gradient or step refractive index profile one to
begin with?
Sometimes [1], and this is one reason why I didn't suggest a naked core,
but a core with a thin cladding, which could have a graded RI if needed.
With a thin single core the liquid would then have its RI matched to the
RI of the outside of the graduated cladding, so the pump light could get
into it (and thence into the core) without reflection or refraction
getting in the way. [2]
Other reasons for not using a naked core include tapered fibers [3]
which couldn't be used if a naked core was used. Some mechanical
strength, coolant flow and thermal issues are also better handled with a
thin core.
Note that in modern industrial medium-to-high power fiber lasers the
fiber has a double layer of cladding: the boundary between the core and
the inner cladding contains the laser light in the core, the
inter-cladding boundary contains the pump light in the inner cladding
layer and core.
Pump light from say 6 laser diodes is combined and beamed down the inner
cladding layer from the non-output end of the fiber.
In a few so-called disk fiber lasers pump light is added to the inner
cladding layer at a shallow angle at intervals along the fiber.
Pumping fiber from the side is just my imagination, afaik it hasn't been
done - unless the "liquid laser" does it.
But I may be wrong there: as I said, I know very little about military
high power lasers.
Peter Fairbrother
[1] graded index fibers are more common on comms fibers rather than
power laser fibers. However they can promote single-mode operation, a
major goal of military lasers, though not so important in industrial lasers.
[2] reflection and refraction between core and cladding can be a
problem, usually dealt with by adjusting the cross-sectional shape of
the cladding and/or core.
[3] so you can cross the streams or combine single-mode beams into a
single-mode beam, tricky stuff - combining multimode beams is much
easier. But single-mode beams have longer range, a military advantage or
requirement.
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