106
Apr-09
Avantes
www.avantes.com
5
Fiber optics
General Information
Core
For existing sensor applications, generally,
multi-mode step index silica fibers
are used. These
range in core thickness from 50 microns to 1 mm. The core is made out of pure silica. Other fiber
cores with much higher absorption are made out of certain glass types or plastics. These are not
offered in this catalog.
First a distinction is made between silica with high or low OH content. Silica fibers with high OH
(600-1000 PPM) are used in the UV/VIS wavelength range because of the low absorption in the UV.
They are referred to as
UV/VIS fibers.
For Deep-UV applications special solarization resistant fibers
can be used.
The OH content causes strong absorption peaks in the NIR wavelength range. In order to get good
fibers for the NIR range, the "water" is removed from the silica. This results in low OH fibers (<2 PPM
) with low absorption in the NIR. They are referred to as
VIS/NIR fibers
.
Cladding
In order to get the light guiding effect the core is cladded with a lower index of refraction material.
For the highest quality fibers with the lowest absorption this is a fluorine-doped silica, the so-called
silica-silica or all-silica fibers
with a numerical aperture (NA) of 0.22.
Buffers
Without further protection fibers would easily break, because of small scratches or other
irregularities on the surface. Therefore a next layer, the buffer, is added. This buffer also
determines under what circumstances the fiber can be used. Temperature range, radiation, vacuum,
chemical environment and bending are factors to be considered.
Polyimide buffers
offer a wide temperature range (-100 to 400 C) and superior solvent resistance.
Also, this material is non-flammable. Drawbacks are sensitivity to micro bending and the difficulty
to remove it.
For extreme temperatures (-190 to 750 C) a
gold buffer
is used. Gold-coated fibers are virtually
inert to all environments and make hermetically sealed high pressure feed trough's possible. (See:
pressure feed troughs). The same is true for
aluminum buffers
for temperatures from -190 up to
about 500 C. Low out gassing makes them also excellent for use in vacuum.
Properties
Step index profile
Pure silica core
Numerical aperture: 0,22 ± 0,02
Laser damage resistant core
transmits 1,3 kW/mm
2
CW at 1060
nm, up to 10 J, pulsed
Bend radius: momentary 100 x clad
radius, long term 600 x clad radius
Polyimide's high NA (1,78) strips
cladding modes
Standard proof test: 70 kpsi
