Chemistry Reference
In-Depth Information
consists of a distribution of propagating waves with λ
λ. Every spectral compo-
nent undergoes the appropriate time delay. Intramodal dispersion contains generally
two independent contributions from the core material (
dn
±
/
d
λ) and from the mode
/
guiding (
d
δ
d
λ). The former is referred to as material dispersion and the latter as
waveguide dispersion.
The total transmission loss of optical fibers is expressed by the sum of losses
due to scattering and absorption. Both processes are present even in highly pure and
structurally perfect materials (intrinsic losses). The frequency range of maximum
intrinsic transparency in glasses and crystals is known as the optical window, limited
by the multiphonon edge (far IR) and by the Urbach tail at the short wave length side
(absorptions from electronic band gap excitations). The other source of the intensity
attenuation at short wavelength is scattering (mainly Rayleigh scattering). Besides
such loss mechanisms, there are also macroscopic losses due to bending of the fibers,
imperfectness of the core cross-section, etc.
For example, a single channel IR transmission spectrum of about 1 m long
step-index chalcogenide glass fiber is shown in Figure 7.9. The used core-only
fiber is covered with a protecting polyamide layer. The transmission spectrum of
the chalcogenide glass fiber is determined by both, the absorption bands from the
polyamide including H
2
O and CO
2
impurities and the structural imperfections and
impurities (Se-H) of the fiber material.
The fiber losses as well as the dispersion do not influence crucially the application
of the optical fibers as IRE in the evanescent wave spectroscopy. Any absorption spec-
trum of a thin film deposited on the fiber core is taken with regard to the background
1.0
1.4
1.7
2
2.5
3.3
5
10
Wavelength in μm
0.8
0.6
Chalcogenide glass fiber
0.4
0.2
0.0
7000
6000
5000
4000
3000
2000
1000
Wavenumber in cm
-1
FIGURE 7.9
Single channel transmission spectrum of step-index chalcogenide glass fiber
(core diameter 750 μm) with protecting polyamide taken by the FTIR spectrometer Bruker
Vector 22 and MCT detector. Characteristic absorption bands from polyamide, as well as H
2
O,
CO
2
, and Se-H (2200-2800 cm
−
1
) can be seen. (From Li, K., Investigation of plasma poly-
merization in RF discharge by novel fibre based FTIR diagnostic tool, PhD thesis, University
of Greifswald, Greifswald, Germany, 2002.)
