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September 2005 EEEL
Researchers Demonstrate Frequency-Resolved Coherent Laser Radar for |
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In recent years, fundamentally mode-locked femtosecond lasers have been shown to produce a highly coherent output. These sources emit a broad spectrum of light that comprises a set of narrow individual optical lines in frequency space separated by the repetition rate of the laser; i.e., a frequency comb. These optical frequency combs have revolutionized optical frequency metrology. It is natural to extend their use to coherent light detection and ranging (CLIDAR) for range/Doppler measurements since the broad spectrum supports high range resolution, and the narrow linewidth of each comb tooth supports high Doppler resolution. However,
there are several issues that arise for a CLIDAR, which detects a weak
return signal from a rough surface at a distance. First, the return signal
will suffer from speckle noise, which both limits the signal-to-noise
to unity and broadens the frequency to a speckle bandwidth. Second, any
extra dispersion in the signal path will degrade the range resolution.
Nathan Newbury and Bill Swann of the EEEL Optoelectronics Division have
demonstrated a frequency-resolved CLIDAR system that effectively deals
with both these problems. In their laboratory-based testbed, they use
a femtosecond fiber laser as a source and resolve the returning heterodyne
signal into N different spectral channels. This N-channel data can be
processed incoherently to provide an N-times reduction in the variance
of a speckle-limited measurement of the Doppler shift; in other words,
it provides a more accurate measurement of the target vibration. Using
N=6 channels, they achieved a sensitivity of 157 Hz in a 10 ms averaging
time, corresponding to a vibration of 0.12 mm/s, despite a signal that
is speckle-broadened to 14 kHz. Alternatively, the N-channel data can
be processed coherently to form a range image of the target. They achieved
a 45 micrometer range resolution despite the dispersion of 1 km of optical
fiber in the signal path. This can be contrasted with conventional LIDAR,
where the range resolution would have been completely destroyed due to
this extra dispersion in the signal path. Contact: Nathan Newbury, (303) 497-4227 |