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GroundWinds Overview
6. CLIO
Optics
The fundamental requirement for a ranging direct detection Doppler
wind LIDAR system is to measure the spectrum at a high temporal rate
with adequate spectral resolution, and low noise. Detecting the familiar
Fabry-Perot ring pattern with a CCD is difficult, because of the number
of pixels that must be read, and the rate at which they must be read.
The patented CLIO optic is a great advantage of the GroundWinds instrument,
in that it allows all of the photons being transmitted through the etalon
to be detected by a minimum number of pixels on the CCD detector. This
allows fewer pixels to be read at a higher rate, with lower noise, since
the number of reads is reduced by two or three orders of magnitude.
The two pictures in the yellow box above illustrate normal
fringes, and fringes that have been transformed by the CLIO optic. Note
the linear form and wedge shape of the CLIO-transformed fringes.
CLIO Conceptual Design and Implementation
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7. Light
Recycling
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.
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The fiber optic ferule that is used to accomplish this task is
shown on the right. This fiber optic array is placed at the input
plane at the focal plane of the collimating lens. In this implementation,
the light is recycled twice through the instrument.
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Fringe Imaging utlizing CCD detector
8. CCD Detector
A CCD detector is used to collect the signal return. Benifits of using
a CCD are its high efficiency and inherent streaking capability. High
efficiency is important to ensure that all possible light will be analyzed,
allowing for smaller errors in measurements. The streaking nature of
the CCD allows for ranging the light return based on the time of the
signal return. Two CCD cameras are required to operate the GroundWinds
system. One camera is used to capture the molecular channel signal and
the other is used for the aerosol channel. The detector system must
support programmable binning and readout formats.
The detector is used to digitize a laser return as a function of altitude
and wavelength. The detector must be capable of acquiring a spectrum
from the interferometer at a rate of about one per 400 nanoseconds.
It must be capable of doing this without introducing noise and even
when the signal appears very weak by integrating many shots on the device
so that read noise does not dominate the measurement. What follows is
a description of how this is accomplished.
The CCD detector is used in a configuration similar to
a streak photographic system. The entire chip is masked off from incoming
light, except for a few rows of pixels at the chip's midpoint. Data
is integrated on the CCD while the central array is shifted rapidly
past a narrow illuminated band in the central parallel array. Before
the laser is fired, the central parallel array is shifted up (reset),
until it is above the illuminated region. The central array is shifted
down (streaked toward the CD register) during laser illumination. This
up/down shift pair is repeated several times to increase the charge
in the CCD. Rows at the beginning of the streak sequence, which experience
the largest return signal, are read out and flushed during the up/down
sequence to increase the dynamic range of the exposure. CCD readout
is accomplished through one CCD amplifier.
Take a look at the CCD Streaking
Animation (AVI Format) for more understanding about this process.
WARNING: The animation is 635kb which may take a moment to download
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