Optical Fibres

Using Fibres to link Telescopes to Spectrographs.


Drawbacks Fibre Transmission
Total Transmission (10 m) 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)  Same as previous but for a length of 26 meters.

Focal Ratio Degradation (FRD)
Flux variation against output aperture 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 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 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 A microscopic view of metallic standard connector with a 150microns fibre at the center.
Flux variation against output aperture for a 200 microns core fibre  

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 Polymicro "wet" fibre of 200 microns core diameter and its adapter to the telescope (1¼")
T Adapter dismounted 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
T Adapter and fibre View of the head dismounted in its different components: At the left the fibre holder and at the right the interface to the telescope (1¼" external Ø). At the bottom the interface to the eyepiece or guiding system (1¼" internal Ø). In the middle the box with an inclined mirror.
T Adapter dismounted Detail of the fibre holder, made in aluminium and cut in two pieces. Between the pieces, 3 pair of grooves hold the fibres (50,100 and 200 microns). The surface is inclined 15º and polished as a mirror.
Head and guiding system. The guiding camera (a ST4) uses a microscopic lens to visualize the fibres at the fibre holder.
Head and guiding system coupled. The head still is opened showing the fibre holder inclined at 15º and the second mirror inclined at 35º.

System ready to be mounted at the telescope. Only fibres are missing


This is our original vision (modelled in 3D and courtesy of Eduardo Ortíz Ostalé) of the above multi-fibre head.

External view of the multi-fibre head. It is made on black anodized aluminium. On the top is the adapter tube to the telescope (1¼" external diameter). To the right is the adapter tube to the guiding system (1¼" internal diameter). Below the tube conducting the optic fibers.

Interior of the multi-fibre head. An inclined mirror at the telescope focal plane (bottom) reflects the light to a secondary mirror (left), which sends it to the guiding system. Mirrors are inclined angles at 20 and 25 degrees. Both mirrors are made on aluminium and polished by hand.

Details of the fibre holder. It is made of two aluminium pieces holding together a row of six fibres which protrude from the polished aluminium and inclined surface acting as the mirror at the focal plane.

Details of the fibre holder and fibres. 3 pairs of "wet" fibers (50, 100 and 200 microns) protrude outside their holes in the aluminium surface. Astronomical targets are focused at fibre ends and not at the mirror surface. A single fibre is selected for the target depending on seeing conditions, guiding, resolution and nature of the target (star or extended object). Other fibres may be used for sky subtraction
Inside the fibre holder. In this drawing a half fibre holder has been removed, the other second half shows the 6 cuts in "V" grooves to accommodate the fibres.
Colored view of the multi-fibre head. In blue the adapter to the telescope. In orange the adapter to the guiding system. Inside the second mirror. Below and in yellow the first mirror and fibre holder. In green the tube conducting the fibres to the holder.
Close up. The two components of the fibre holder (in yellow) are separated showing the fibres (in red).