Optical Fibres
Using Fibres to link Telescopes to Spectrographs.
Advantages
-
Spectrograph independent from telescope. Bench Spectrographs,
no weight
or volume restrictions.
-
High spectral stability.
-
Fibres are easy to use and install (once prepared!)
-
Possibility to perform two-dimension spectroscopy with fibre
bundles.
Drawbacks
-
Transmission losses.
-
Focal Ratio Degradation.
-
Circular aperture losses.
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Poor sky subtraction.
-
Fixed “slit aperture”.
-
Difficult to prepare if not proper tools are available.
-
Fragile!
Fibre Transmission
![Total Transmission (10 m)](fib_trans_10m_small.gif) |
These plots show the total transmission of
two types of commercial step index silica fibres for a length of 10 meters
(wet and dry). In for a length of 26 meters. These figures include the
Fresnel reflection losses at the ends of the fibre (~ 4% per glass surface). |
![Total Transmission (26 m)](fib_trans_26m_small.gif) |
Same as previous but for a length of 26 meters. |
Focal Ratio Degradation (FRD)
![Flux variation against output aperture](frd_abs_small.GIF) |
This Figure shows the flux variation measured
in laboratory against the output aperture for different input beams (F/3,
F/6 and F/8). If an F/8 beam is launched into the fibre, only 55% of the
flux will be recovered at the output of the fibre and at the same aperture.
However for an input beam opened to F/3, 85% of the light will be recovered
by an output cone also opened to F/3. This focal ratio degradation increases
rapidly when the fibre is submitted to mechanical stresses like strong
bending and twists. |
![Flux variation against output aperture for a 200 microns core fibre](frd_rel_free_small.GIF) |
This is the FRD for a fibre of 200 microns
core diameter. The total output flux has been normalized to 100% |
![Flux variation against output aperture under strong bending](frd_rel_bend_small.GIF) |
The Figure shows the FRD (same fibre) when
the fibre is submitted to a pretty strong bending. For an F/8 input beam,
only 44 % will be recovered at the same output aperture. However, for an
F/3 input beam, the degradation for a cone also opened to F/3, will be
only of few percent less that for the free fibre. From these experimental
values, we conclude that a telescope beam channeled by a fibre degrades
faster for slower beams (a slow beam has a high F/#, the opposite is a
faster beam). |
Working with fibres
![Flux variation against output aperture](fiber1.jpg) |
A microscopic view of metallic standard connector
with a 150microns fibre at the center. |
Flux Calculator
Here
there is an applet to calculate the flux passing through a slit versus a pinhole
Coupling to telescope
Here
there are some ways to connect the telescope to the optical fibre adapters
And, our first implementation of a mono-fibre head:.
![T Adapter and fibre](fibra2_small.JPG) |
Polymicro "wet" fibre of 200 microns core diameter
and its adapter to the telescope (1¼") |
![T Adapter dismounted](fibraadapter_small.JPG) |
Detail of the Fibre Head. The fibre input end is placed
just below the hole of the "mirror-diaphragm" (nickel plate, 0.1 mm
thickness and 10 mm diameter) |
And our latest implementation of a multi-fibre head
This is our original vision (modelled
in 3D and courtesy of Eduardo Ortíz Ostalé) of the above multi-fibre
head.
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