GroundWinds Overview
4. Optical
System
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5. Fabry-Perot
Interferometer
At the heart of the GroundWinds LIDAR technology is the Fabry-Perot
etalon. The signal return is injected into the Fabry-Perot etalon. The
Fabry-Perot etalon provides information about the spectral content of
the light, by generating an interference pattern that appears as a series
of fringes. These fringes represent a series of angles in the Fabry-Perot
cavity at which light can be transmitted. These angles are a function
of the wavelength of the light. For a given set of fringes illuminated
with monochromatic light, the wavelength can be calculated by measuring
the fringes. For more information about Fabry-Perot etalons, see Lidar
FAQ: Fabry Perot Etalons
The interferometer system consists of two interferometers and a dielectric
blocking filter. The blocking filter is intended to remove broadband
sunlight from the return measurement.
The GroundWinds instrument measures both molecular and aerosol backscatter
with two separate and independent detectors. The light from each laser
pulse in the GroundWinds instrument is used to serve both channels.
This was only possible through the development of a light recycler that
utilizes the reflected light from the etalon plates of the molecular
channel to feed the aerosol channel. This innovation was very important
because it allows the simultaneous determination of Doppler shifts from
Mie scattering as well as Rayleigh. We refer to this as light recycling
(U.S. patent #6,163,380), and can be used not only as a way of injecting
the signal into two interferometers, but also as a means of reintroducing
the signal into a channel multiple times to improve the throughput of
a single channel.
The implementation of this idea is accomplished through the use of
fiber optic arrays. These arrays use a single multimode fiber optic
to introduce the light into the interferometer off-axis. The rejected
light appears at the other side of the optical axis at an equal distance.
If a fiber optic is positioned at the point where this rejected light
is in focus, it can be collected and reintroduced at a point on the
same side of the axis where it was initially injected. This can be done
many times in an effort to force as much light as possible through the
instrument. It should be noted that the fiber optic has the unique and
important property of eliminating spatial coherence while maintaining
spectral coherence. This is the essence of the direct detection technique.
It might be hypothesized that a mirror could be used for this recycling
technique, but in fact, a mirror would not be effective, as the spatial
coherence must be destroyed in order to yield significant throughput
on subsequent recycling.
It should be noted that the increase in throughput results from the
increased divergence through the instrument that this additional illumination
offers without a need to increase the clear aperture of the instrument.
As the number of recycles increases, the width of the instrument function
on the detector does grow. This necessitates an increase in detector
speed to preserve spatial resolution.
The entire interferometer is contained in a pressure
and temperature controlled vessel. Changes in pressure and temperature
change the refractive index of the air in the gap of the etalons, and
can cause a shift in the measured spectrum as well as an overall degradation
of resolving power. Drastic changes in temperature (> 5 C) can adversely
affect parts of the system as mechanical deformation occurs. Thermal
distortion of the etalon plates is detrimental to the performance of
the system. The chamber is purged with extra dry nitrogen and sealed.
This dry environment is also very favorable to the etalon coatings.
Hermetic feed-throughs are used for the many electrical connections
that must be fed into the chamber.
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