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2.1. TROPOSPHERIC RESEARCH GROUP 25
2.1.8 Long-Path-DOAS Measurements with new fibre optic set-up
André Merten (Jens Tschritter)
Abstract For the first time during a field campaign a new fibre optics set up was used to couple the
light source in the telescope and to receive the light from the reflector. This set-up provides a higher
stability of the alignment and improves the spectral characteristics of the light source.
Figure 2.9: schematic view of the new fibre optic set-up for DOAS devices
Background Long-Path-Telescopes are commonly
used for atmospheric trace gas measurement,
especially in combination with the DOAS
(Differential Optical Absorption Spectroscopy)
analysis technique. Such an instrument combines
the emitting and receiving telescope in one device
with a double-Newton-style set up and a Xe-high
pressure lamp as light source and has a typical size
from 1..2m. Therefore this instrument requires a
high effort in planning and executing of field measurements
and has also a limited signal-to-noise
ratio. The signal-to-noise ratio is limited due to
the inadequate characterization of the spectrum of
the light source. The used Xenon-high pressures
arc lamps are showing strong variation of spectral
structures across the arc. When using a ’shortcut
optics’ to obtain the lamp emission spectrum is
not guaranteed that the same area of the arc is
imaged used in the measurement. This can cause
strong residual structures, which can be misinterpreted
as optical densities. Another problem is
the complicated alignment of the Long-Path telescope,
since all optical elements must be adjusted
exactly to obtain good light throughput, which is
only possible at night and limits it use as an automatic
running air quality monitor. The dimensions
and the weight of the lamp house require
also a certain size and stability of the whole telescope.
A smaller and easier to handle, and more
reliable telescope would extend the range of the
application for DOAS instruments.
Methods and results Transmitting the light
of the lamp by an optical fibre placed in the focus
of the main mirror, instead of the first plane
mirror, was already tested successfully. The lamp
must not be connected mechanically with the tele-
scope anymore and the same area of the arc is imaged
in measurement and short cut regime. Since
the retroreflector is imaged again to its origin
(with a small displacement) emitting and receiving
fibres were combined together in a bundle.
Now as son as the telescope catches the reflector,
the reflected light hits also to the receiving
fibre (Figure 2.9). The system is now much easier
to align and due to the fewer degrees of freedom
much more stable. And in addition, it is also more
efficient. For the quality of the short cut system a
uniform illumination of the receiving fibre is important.
Different reflecting materials were tested
and the best results were reached with a quartz
stray disc that is placed with a small motor directly
in front of the fibre bundle. This set-up was
tested successfully with a telescope with 1,5m focal
length in the visible and near UV ranges and
then used successfully in the MCMA-2006 (Mexico
City Metropolitan Area) campaign in march
2006 measure NO2, HCHO, Glyoxal and other
species. For three weeks no manual realignment
of the telescope was necessary, except for the illumination
of the fibre by the lamp, which is not
automated yet but can be done very simple when
running the short cut optics.
Outlook/Future work The optimal fibre optic
configuration is now a topic of diploma thesis
that should lead to the complete understanding
of the optical behaviour and therefore to a new
generation of DOAS telescopes [see report from
Tschritter, 2.1.13]. The use of fibre optics offers
the possibility to work with other light sources
like LED or SLD and a fast switch between these
sources.