Jahresbericht 2005 - IPHT Jena

OPTIK / OPTICS
Coating material NA Layer Cut-back method: OTDR:
(coated thickness Loss [dB/km] Loss [dB/km]
silica fibre) [µm]
800nm 1310nm 1550nm 850nm 1310nm 1550nm
Silicone RT601 0.358 50 20 25 250 13 29 n.m.
Ormogel HG-Li-V-T 0.410 5 25 51 150 23 37 (70)
1D3-63 (Acrylate) 0.404 50 34 59 180 (50) 75 n.m.
Luv PC 373 (Acrylate) 0.447–0.127 50 9 17 60 5 16 45
Teflon AF 0.606 5–10 9 7 8 4 5 6
Tab. 2.1: Loss in silica fibres with different coating materials, measured by the cut-back method and by
OTDR (n.m. = not measurable, because of limited dynamic range of OTDR; cut-back method works without
limitation of measurable loss).
2 With the Optical Time Domain Analysis
(OTDR), the loss can be evaluated with a spatial
resolution of some meters, but only at
selected wavelengths. Fibre lengths >60 m
can be analysed; the measurements should be
executed from both fibre ends. One of the
available devices applies a wavelength of
850nm and illuminates the fibre under test with
NA 0.20 (spot size 50 µm); another device for
1310 nm and 1550 nm implements NA 0.10
(spot size 10 µm).
Both methods supplement each other and are
suitable especially for the comparison of losses
in fibres with different coatings, but equal fibre
diameters.
The results obtained with both methods are compared
in Tab. 2.1 (the wavelengths are determined
by the OTDR devices) and indicate a sufficient
consistency.
The low loss and the high NA achieved with the
Teflon AF coating, as well as the absence of
additional absorption bands (apart from OH
absorption) are particularly advantageous for
cladding pumping. But the Teflon layer that can
be implemented is too thin for many applications.
The low-index Acrylate Luv 370–444, but also Silicone
RT601 provide a moderate pump loss and
an adequate layer thickness. However, characteristic
absorption bands may be detrimental, e.g.
the Silicone absorption around the common
pump wavelength of 915 nm.
Besides the effects on fibre loss, the coating
materials differ in their mechanical and thermal
properties, which should also be taken into
account in high pump power applications.
2.2.6 Studies of the mode behaviour
of multi-core fibre structures
(K. Mörl, J. Kobelke, K. Schuster)
There are various reasons to build-up the active
cores of microstructured fibres from single doped
filaments. One of them is to achieve a mode field
as large as possible. The preparation is carried
out usually by repeated packaging and stretching
of rare-earth-doped elements made by the well
established MCVD technique, soaking in rareearth
solutions and subsequent drawing of the
microstructured fibre by means of the “stack and
draw“ technique. Fig. 2.7 shows the central part
of a PCF with 7 × 7 active Yb-doped filaments.
Fig. 2.7: Central part of a PCF with 7 × 7 active
doped filaments .
A matter of particular interest is the mode behaviour
of such structures, especially the optical coupling
of the single elements (formation of a
“super-mode”). Fig. 2.8 shows the changing
mode picture at the end of a short piece of the
fibre shown in Fig. 2.7, achieved by launching
Fig. 2.8: Mode structure at the end of a short
piece of fibre shown in Fig. 2.7, obtained by
launching different wavelengths in the central part.
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