Coherence Brightened Laser Source for Atmospheric Remote Sensing Page: 15,187
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Fig. 3. Single shot temporal profiles of both forward (A) and backward (B) pulses at full pump energy (10 mJ). The top pulses, 1, in both are shown without any
frequency filtering while the pulses immediately below, 2, are the same pulses after Fourier filtering. Pulses 3 and 4 are other examples of filtered pulses,
vertically shifted for convenience. Averaged Fourier transforms of the forward (C) and backward (D) pulses. The Fourier transforms depict and help quantify
spectral intensity modulations for both pulses. Filtering was applied by subtracting the Fourier spectrum of the background as well as removing all frequency
components beyond the red dashed line as they were artifacts of the electronics.
ing that was previously observed and believed to be caused by
intensity fluctuations in the pump pulse (24). In contrast to this
previous experiment, our observed spiky structure is higher in
both frequency and amplitude. Besides the rapid oscillations, sev-
eral other features can be discerned from the data. Namely, the
forward signal exhibits a higher rate of oscillation with more high
frequency components (broadband, 5-15 GHz), as can be seen
in the Fourier transforms in Fig. 3 C and D. Furthermore, from
shot to shot, there are significant variations in the amplitude and
number of oscillations.
In Fig. 4, we compare the typical narrowest spikes in the
emitted pulses with a measured response of a femtosecond test
pulse (Ti:Sapphire, 35 fs, 800 nm), which is well below the reso-
lution limit. As can be seen in Fig. 4A, the forward spiking is most
likely narrower than can be resolved by our detection system. In
contrast, the narrowest peaks observed in the backward direction
are approximately 60 ps (FWHM) in duration (Fig. 4B). These
measurements along with the Fourier transforms depicted in
Fig. 3 C and D, emphasize the distinction between the forward
and backward emission, as well as differentiate these results from
previous experiments where no difference between forward and
backward spike duration was observed (24). Also, given how nar-
row these spikes are, it is clear that the peak Rabi frequency is
higher than the average measured Rabi frequency, which extends
this experiment further into the regime of strong nonadiabatic
atomic coherence (28).
On average, the width of the short spiking/ringing is less than
0.1 ns and 0.3 ns for the forward and backward pulses, respec-
tively. These values can be compared to the estimated decay time
for the system, which suggests the approximate time scale of
intensity oscillations if atomic coherence is present. The charac-
teristic delay, TD and radiation damping time, TR, of the collec-
tive system are given by refs. (29) and (30)
TR = Tsp(8t/NX2L),
TD TR [ln(/2tNAL)] 2
where Tsp ~ 100 ns is the spontaneous emission decay time for
the lasing transition, N ~ 1015 cm-3 is the density of the excited
oxygen atoms in the gain region, X = 845 nm is the wavelength of
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Fig. 4. Typical individual spikes for both the forward (A) and backward (B)
pulses. In both figures, the dashed trace, Test Peak, is the response function
of a 35 fs pulse at 800 nm used to test the resolution limit of the detection
system. All traces are normalized for ease of comparison.
PNAS I September 18, 2012 vol. 109 no. 38 15187
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- B -Peak 2
_ _ TestkPeak
Traverso et al.
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Traverso, Andrew J.; Sanchez-Gonzalez, RodrigoTe; Yuan, Luqi; Wang, Kai; Voronine, Dmitri V.; Zheltikov, Aleksei M. et al. Coherence Brightened Laser Source for Atmospheric Remote Sensing, article, September 18, 2012; [Washington, D.C.]. (digital.library.unt.edu/ark:/67531/metadc725804/m1/3/: accessed March 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.