Coherence Brightened Laser Source for Atmospheric Remote Sensing Page: 15,186
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1m Convex Lens
226 nm, 10 mJ/pulse
- 0 2 4 6 8 10 12 14 16 18
$ 45 "m r
Q 7 3s3 S
Fig. 1. Simplified experimental scheme. Nanosecond 226 nm laser pulses
were focused with a 1 m lens dissociating the oxygen molecules at the focal
point in ambient air. The 226 nm pulse further excited the newly dissociated
oxygen atoms via two-photon absorption causing a population inversion.
The backward detection was performed through a dichroic mirror. (Bottom
Inset) An example of the pump pulse's intensity profile. (Top Inset) The
energy level scheme is depicted for oxygen atoms undergoing two-photon
excitation and stimulated emission at 845 nm. F= 9.3 x 106 s-1 and
y = 0.197 x 109 s-1 are the decay rates from 3p 3P to 3s 3S and from 3s 35
to 2p 3P states, respectively.
The resulting beam is focused using a convex lens (1-m focal
length). The gain region, consisting of oxygen atoms produced
by the focused pump, is approximately 1 cm long and has a waist
of approximately 17 pm (see SI Text for more details). Emission in
both the forward and backward direction is then detected and
characterized. Particularly, a 300 nJ signal is detected in the back-
The power of both signals was measured versus the pump
power using a pyroelectric power meter from Ophir (Fig. 2B).
A characteristic threshold behavior is observed for both forward
and backward 845 nm beams indicating a laser-like process in-
stead of simple fluorescence. Also, there was a distinct energy
difference between the forward and backward emission, which
is discussed below.
Pump Pulse Energy (mJ)
Fig. 2. (A) Spatial beam profiles of the 845 nm emitted backward pulse at pump energies above (10.0 mJ) and at (8.0 mJ) threshold. (B) The energy per pulse of
both the forward (red circles) and backward (black squares) signals versus the pump power.
Furthermore, using the measured pulse energy, the average
Rabi frequency can be experimentally estimated via Q = -E
where E is the electric field amplitude and p is the electric dipole
moment, which depends on the spontaneous decay rate, F, ac-
cording to p /3 teohc3F/w3, where w is the transition fre-
quency (26). Given that F = 9.3 x 106 s-1 for the 3p 3P to 3s
3S transition, P 1.41 x 10-29 C-m. Similarly, the measured
pulse energy (Fig. 2B) along with the diameter of the gain region
and the pulse duration (t ~ 10 ns, discussed in next section), pro-
vide the average intensity I 6.11 x 1010 W/m2, which is then
used to calculate the electric field amplitude via I = ne0c E2/2,
where n is the index of refraction. This finally leads to an experi-
mental estimate of the average Rabi frequency for the 845-nm
transition Q = 1.3 x 1012 rad/s, whose high value provides
further evidence of nonadiabatic atomic coherence when com-
pared with the collisional dephasing rate (Ycoi ~ 1 x 1010 s-1)
(27). In this case the average Rabi frequency is much higher than
the dephasing rate, and when a system is in this regime, then
atomic coherence effects will occur (28).
Further evidence of a laser-like process was discovered by
analyzing the spatial beam profiles of the backward pulse when
the pump power was varied from approximately 6 to 10 mJ.
These measurements were made using a Spiricon beam profiler
(SP620U). A distinct threshold was observed at approximately
8 mJ with a Gaussian profile. The width of the beam profile right
at the threshold (8 mJ) was significantly wider than the profiles
for any of the pump powers above threshold. Fig. 2A depicts this.
Furthermore, from 6 to 8 mJ, emission was observed by eye, but
the profile was too broad and weak for detection. Also, above
threshold, all of the beam profiles had approximately the same
width. Both of these features are indicative of a laser-like source.
Temporal Pulse Analysis. The temporal pulses shapes were mea-
sured using a Tektronix MS072004C fast oscilloscope (20 GHz
bandwidth, 50 GigaSamples/s, and approximately 20 ps resolu-
tion) and a New Focus high speed photodiode (model: 1,437;
25 GHz bandwidth and 14 ps rise time). As can be seen in Fig. 3,
the temporal profiles varied from shot to shot mainly due to the
rapid intensity fluctuations in the individual 226 nm pump pulses
(an example is shown in the bottom inset of Fig. 1). Due to in-
strumental artifacts centered around 20 GHz, near the oscillo-
scope's bandwidth limit, all frequency components above 15 GHz
were removed. Single-shot temporal profiles of forward and
backward pulses were measured simultaneously using the same
photodiode (New Focus, described above) and are presented in
Fig. 3 A and B, respectively. The main feature of the temporal
profiles is the high frequency oscillation, which is similar to spik-
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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/m1/2/: accessed March 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.