Jahresbericht 2005 - IPHT Jena

42
2.2.4 Photonic crystal fibres for innovative
applications and devices
(J. Kobelke, K. Schuster, J. Kirchhof,
A. Schwuchow, K. Mörl, K. Oh)
Photonic crystal fibres (PCFs) are commonly
interesting as transmission media over long distances
for their specific light propagation behaviour.
However, the promising application potential
of photonic crystal fibres also takes effect in various
novel devices, e.g. PCFs with defect super
lattice structure and PCF-based NxN fibre couplers.
Such PCF couplers have a power stability
advantage, because they can be manufactured
without any dopants in the basic silica. Moreover,
the transmission behaviour is mostly effected by
air hole confinement light propagation, so a high
numerical aperture is possible. Typical power limitations
of polymer-clad fibre can be avoided by
the air-clad design.
We prepared special large mode area PCFs by
variation of the geometric cross section design
parameters. They were changed by introduction
of a flexible defect in the lattice structure. The
new defect design consists of the central air hole,
surrounded by a holey germanium-doped silica
ring. It provides a large-area annulus-mode light
propagation and a chromatic dispersion with low
slope ~0.05 ps/km nm 2 at 1550 nm.
Fig. 2.4: Calculated dispersion curves of fundamental
modes of different d core/Λ core of the super
lattice PCF. Inset: fibre micrograph.
PCFs with air-clad design are interesting components
for optical devices. 2×2 and 4×4 multimode
air-clad holey fibre couplers were prepared
at the Gwangju Institute of Science and Technology
(GIST), South Korea, based on IPHT-manufactured
PCFs. The couplers were fabricated by
a novel fusion tapering technology, starting from
an air-clad fibre with a core diameter of about
130 µm and a very high air fraction of the holey
clad ring. The couplers thus fabricated show a
high port-to-port coupling uniformity over a wide
spectral range from 800 nm to 1650 nm. Due to
the absence of power-limiting low refractive index
polymer materials to achieve the high numerical
aperture, the device shows a strong potential for
future high-power applications.
OPTIK / OPTICS
Fig. 2.5: Scheme of the 4×4 coupler, micrograph
of the used air-clad index-guided PCF and lateral
segments of the 2×2 coupler.
2.2.5 Loss measurements at silica fibres
for cladding pumped applications
(S. Jetschke, A. Schwuchow)
Silica fibres with rare-earth-doped cores for laser
or amplifier applications are often claddingpumped
to benefit from commercial high-power
pump diodes. Thereby, losses of pump power
may occur in the cladding material itself or in the
adjacent coating having a refractive index lower
than silica. We investigated this attenuation in silica
fibres of diameter 125 µm (without core),
drawn from F300 rods and coated with different
materials (see Tab. 2.1).
The loss measurements were done by two different,
but well-known methods:
1. With the cut-back method, the loss spectra
are calculated from transmission measurements
in the 300–1700 nm wavelength range
(Halogen lamp, NA 0.25) in a long (>60 m)
and a shortened fibre (10 m). Although the
fibre NA is not fully illuminated, the characteristic
absorption bands of the coating materials
are clearly seen (Fig. 2.6).
Fig. 2.6: Loss spectra of coated silica fibres,
measured by the cut-back method.