Measuring atmospheric turbulence for free space
optical communications
Gordon D. Love, Colin N. Dunlop, Christopher Saunter, and
Richard W. Wilson.
Department of Physics, Durham University, Durham, DH1 3LE, UK
(g.d.love@durham.ac.uk)
Abstract. We summarize the application of adaptive optics techniques to free
space optical communications, and reference SLODAR as a technique for
measuring horizontal turbulence.
1. Introduction
The propagation of light through the atmosphere affects both the phase and intensity of a
beam of light, and the field of adaptive optics (AO) has been developed to produce
instruments to correct for these effects (especially phase) in real time. The techniques have
been progressed jointly by the military and astronomical communities, and more recently AO
has been successfully applied to vision science.
There has also been work and much discussion of AO to free space optical (FSO)
communications (see, for example, refs [1]-[4]). There has, however, been relatively little
discussion on exactly how adaptive optics can be implemented in an adaptive optics system
and what it can be used to correct. There has also been very little published information on
measurements of horizontal propagation through atmospheric turbulence. In this paper we
briefly summarize our work on the application of AO to FSO systems and also refer to our
work on the use of SLODAR as an atmospheric turbulence monitor.
2. Horizontal turbulence measurements using SLODAR.
A knowledge of the statistical parameters of the atmosphere is of paramount importance
when designing an AO system, e.g. the strength of the turbulence directly relates to the
number of channels and the profile of the turbulence relates to the position in the optical train
of the wavefront corrector(s). There is not a large amount of published data on horizontal
turbulence profiles, and, in any case, the actual statistical parameters will depend very much
on the location of the system. With these thoughts in mind, a horizontal turbulence
monitoring system has been designed and constructed at Durham. The system is based on a
method of cross-correlating the slope measurements from the Shack-Hartmann wavefront
sensor used to observe a double source. The method, which is known as SLODAR[5], is
analogous to the well known SCIDAR scintillation[6-8] profiling technique. Further information
on this use of SLODAR for horizontal turbulence measurements is given in [9], and results
will be shown in the presentation.
3. Adaptive Optics – what can it do in a free space communication system?
The are three different potential uses of AO in an FSO system, and we discuss each of these
in turn below.
The aim of an FSO system (from the point of view of AO) is to launch a laser in the
atmosphere and receive as much of this signal as possible. In the limit of a perfect system
this implies launching a parallel laser beam which is the same size as the receiver optics. In
practice most FSO systems operate by launching a diverging beam so as to minimize effects
due to pointing errors (which will be due to a mixture of initial set-up errors, building shake,
and atmospheric turbulence). This is obviously wasteful of light and ultimately limits the
bandwidth of the link since a higher bandwidth signal requires more photons and the total
beam intensity is limited by laser safety considerations. Adaptive tip/tilt control is therefore
desirable to limit the beam divergence and keep the beam focussed on the detector. It is
important to note that tip/tilt correction is required on both the transmit and receive units.
The second potential use of AO is to reduce phase aberration in the receive beam to improve
the focal spot size. It is important to note that a FSO system is not an imaging system and
that (in general) the “quality” of the focal spot – ie. whether or not it is speckled – is not
important. The only requirement is that the focussed spot lands on the detector. In summary,
we therefore consider that the application of traditional phase-only wavefront correction to
FSO systems is relatively limited (certainly the effects of tip/tilt correction and scintillation
correction are more important). There are three caveats to this conclusion – one is that for
extremely high bandwidths the size of the detector becomes important (fast detectors are
smaller) and therefore the size of the focussed spot may become important in some
situations. Secondly, there has been discussion of using FSO to link a single mode fibre to
another single mode fibre system – and here the size of the focussed spot is indeed critical.
Thirdly, there has been discussion of using beams with orbital angular momentum for FSO
systems and these have been shown to be particularly strongly affected by atmospheric
turbulence – so it is possible again that phase-only wavefront correction is useful here [1011].
The third and final potential use of AO is for scintillation correction, which is where phase
aberrations evolve into intensity aberrations as the beam propagates. These effects have
been extensively discussed (e.g. [1]]) as clearly if the receive signal level fades or disappears
completely this has a disastrous effect on the link. One method to improve this is by using
larger apertures, but this is not always feasible/cost effective. The use of AO to reduce
scintillation must involve pre-correction on the transmitted beam and has been discussed
further by Paterson [12].
Given that one wishes to produce an AO system for an FSO system there are actually many
potential ways of implanting the system. Some of these methods will be discussed in the
presentation and a full description of the methods is in the process of preparation.
Finally we will describe progress on producing a low-cost adaptive optics system.
Acknowledgments
This work is supported by the DTI. Thanks to our partners in this work at Strathclyde
University, CableFree Solutions Ltd, Starpoint Adaptive Optics, and Michelson Diagnostics.
References
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