The State-of-the-art Na lidars for MLT dynamics studies
Chiao-Yao She, Colorado State University
Ralph Burnham, Fibertek, Inc.
Suggested Reviewers: Gardner, Swenson, Collins, Chu, Williams, Yuan
Abstract
Na Doppler lidar as an essential instrument for atmospheric dynamics studies in the mesosphere and
lower thermosphere (MLT) has been proven beyond doubt. Going forward, there are two challenges for
Na lidar technology and science: first, to develop and implement a higher power transmitter that is robust
enough to realize automation potential and to facilitate high quality and large quantity of data acquisition,
and second, to develop and implement receivers with aperture at the level of 10 - 100 m2 to permit
observations with high temporal and vertical resolution, so that turbulence dynamics and eddy diffusion
can be observed directly, along with temperatures and winds at various time scales. This paper is an
attempt to lay out the strategies for achieving these objectives.
1.
Introduction
Due to its large resonance scattering cross-section, Na resonance lidar has been recognized as a gold
standard [Gardner, 2004] for investigating atmospheric dynamics in the mesopause region, often termed
as the mesosphere and lower thermosphere (MLT). At this juncture, Na lidar systems on hand have
already proven to be a reliable tool for observing temperature and winds as well as gravity wave
dynamics and its interaction with tidal waves. That the latter studies are still rare is due mainly to the fact
that the current transmitter is not yet robust enough to allow full automation for multiple day (either
nighttime only or 24-hour continuous) operations with minimum human intervention. To remedy this
defect, it is necessary to employ all-solid-state technology that not only can realize the potential of
automation but also enjoys broad based industrial support. We therefore discuss the state-of-the-art Qswitched Nd:YAG lasers and the method of using them to generate stable, high power, coherent radiation
at 589 nm in Section 3. In addition, to understand gravity wave dynamics fully, lidar data should be
capable of evaluating the associated turbulence and eddy diffusion coefficients. It is now well known that
while 0.5 km and 2.5 min resolution is sufficient for investigating gravity wave dynamics (in addition to
slower perturbations), a much higher resolution, about 20 m and 2 sec, is necessary if the full turbulence
spectrum is to be probed [Gardner and Liu, 2013]. In other words, for comparable photon uncertainties in
temperature, wind and Na density, it requires about 5x106 times more signal for turbulence measurements.
We therefore discuss and propose deployment geometry with large aperture receivers, along with a
conservative estimate of their measurement uncertainties in Section 4. A recent Na-density-only lidar
using a 6-m telescope has teased the science potential in high resolution with direct observations of
detailed fine structures and turbulence billows in the mesospheric Na layer [Pfrommer et al., 2009], which
awaits quantification of background atmospheric state variables in such short temporal and spatial scales.
2.
A brief overview of Doppler Na lidar measurement techniques
Since the translational motion of naturally occurring atoms in the mesopause region is in thermal
equilibrium with ambient atmosphere, atmospheric temperature and winds may be determined from a
1
ground-based resonance scattering lidar by monitoring, respectively, the Doppler-broadening and
Doppler-shift of the return signal. This principle was first demonstrated by Gibson et al. [1979] by
measuring the mean nocturnal temperature of the Na layer. Fricke and von Zhan [1985], who used a
similar but improved flash-lamp pumped pulsed dye laser to scan over the NaD2 spectrum, were able to
resolve temperatures at different altitudes within the Na layer. Measurements in North America started
with the collaboration between the research groups of She at Colorado State University (CSU) and
Gardner at University of Illinois (UIUC), which used a continuous-wave (CW) single-frequency tunable
dye laser to seed a pulsed dye amplifier. With this system, mesopause temperatures were first measured
[She et al., 1990] by locking the seed laser to two absolute frequencies (at the peak, a, and cross-over, c,
resonances) of the NaD2 Doppler-free spectrum [She and Yu, 1995]. The same laser system was expended
to simultaneous temperature and line-of-sight (LOS) wind measurement, initially tuned to a third
frequency [Bills et al., 1991] and later with a three-frequency technique [She and Yu, 1994] by locking
the laser at a and up- and down-shifting the frequency by a fixed amount (630 MHz for example) to +
and - using a dual acousto-optic modulator. Though human intervention is required in a long campaign,
this system has been in operation for more than two decades and continues to be the workhorse for the
three Na lidar facilities in the NSF sponsored Consortium of Resonance and Rayleigh Lidars (CRRL).
3.
A proposed state-of-the-art Na laser transmitter
To go forward we are looking for all-solid-state technology that not only delivers more power (~ 10 times)
but also has the potential for full automation and enjoys a broad industrial base support for basic
components. Our proposed lidar source, based on Q-switched Nd:YAG is a solid-state laser with solid
performance for diverse high-power applications. The same Nd:YAG crystal can also be made to lase at
1319 nm. Combining these two frequencies via sum-frequency generation (SFG) in a nonlinear crystal
like LiB3O5 (LBO), one can produce light tunable through the 589 nm Na resonance line. The SFG
process has been carried out with CW or pulsed YAG lasers. The narrowband CW systems [Vance et al.,
1998] can be used to seed the pulsed dye amplifier for Na lidar application, though the dye based system
requires high-maintenance and can be troublesome at a remote observatory. The pulsed system,
traditionally flash-lamp pumped though all-solid-state, typically does not have long maintenance-free
operating life or the precision locking mechanism required for demanding LOS wind measurements
[Kawahara et al., 2002] unless Doppler-free techniques are introduced into the seeding mechanism [She et
al., 2007]. Both systems have low average power of ~ 1 W.
Due to the interest in producing laser guide-stars from the atmospheric Na layer for astronomical
telescope applications, high power CW lasers at 589 nm have undergone rapid development with varied
methods investigated. One of the most successful has been frequency doubled fiber lasers [Tayor et al.,
2009]. This approach has produced systems yielding as much as 50 W, with 20 W systems commercially
available. Unfortunately, a pseudorandom modulation code is required to range resolve long-range lidar
measurements with a CW system. The associated power penalty [Norman and Gardner, 1988] that arises
due to continuous reception of background light could be as high as 20 times [She et al., 2011]. While
flash-lamp pumped Nd:YAG lasers produce high pulsed power with lower efficiency and at low repletion
rate, the more recent diode pumping technique can produce Q-switched Nd:YAG lasers with higher
efficiency and higher repetition rate. Transverse diode pumping of Nd:YAG slabs can produce pulses at
~100 Hz rate with excellent beam quality, but more recently the end pumping method has produced
pulsed Nd:YAG laser with higher efficiency at repetition rates up to 10s of kHz. During the past decade,
Fibertek, Inc., has gained extensive experience in custom-building such lasers for advanced lidar
2
applications, thanks to the vision of NASA research laboratories. This work has resulted in numerous
fielded airborne and space-based lidar systems. The proposed state-of-the-art Na lidar transmitter will be
a sum-frequency generated solid-state system that utilizes Fibertek’s Nd:YAG lasers. Using proven
design codes, Fibertek has investigated the trade-off between repetition rate, efficiency, power, and beam
quality of these lasers. The favored choice is an end-pumped modular system with repetition rate of 750
Hz, which can cover a range of 200 km unambiguously. This system has a plug efficiency of 1.5%
including the nonlinear conversion efficiency of 33%; two such laser systems will be considered for the
proposed Na lidar applications; one has 5 W output, and the other 10 W at 589 nm. The performance
parameters for both 100 Hz and 750 Hz Na system are compared in Table 1.
Table 1 A state-of-the-art 589 sodium lasers performance parameters
Parameter
Slab
Osc
Osc/Amp
End Pumped
Osc/Amp Osc/Amp
PRF
100 Hz
Energy (1064 nm)
30 mJ
60 mJ
13.5 mJ
27 mJ
Energy (1319 nm)
15 mJ
30 mJ
6.5 mJ
13 mJ
Beam Quality (M2)
Pulse Width
Volume
750 Hz
1.5
1.2
10-20 ns
10-20 ns
2 cubic feet
2 cubic feet
Mass
50 kg
Pump Laser Efficiency
4% (1064), 1.5%(1319)
10% (1064), 4% (1319)
Power Input
250 W
500 W
350 W
700 W
Power Out (589 nm)
1.5 W
3.0 W
5W
10 W
Efficiency (589 nm)
75 kg
0.6%
20 kg
40 kg
1.5%
As the schematic in Fig. 1 shows, the proposed Na transmitter system starts with two ring laser resonators
at 1064 and 1319 nm. Each resonator YAG crystal is end-pumped by a pair of diodes at 808 or 880 m.
The master frequency source that seeds each resonator is a continuous-wave (CW) temperature-stabilized
DFB (distributed feedback) laser. Including jitter, the linewidth of the seed is <10 kHz, and its frequency
is set by stabilized temperature and a current tuning. In view of the stability of these seeds, the 1319 nm
seed will be set at a fixed wavelength. The 1064nm seed will be tuned by PDH or FM lock to keep the
sum frequency at the peak, a, of the NaD2 hyperfine transition, using the proven Doppler-free
spectroscopy technique developed at CSU [She and Yu, 1995]. To provide the CW light source for
Doppler-free spectroscopy, a small portion of the seeds will be used to sum-generate a light beam at 589
nm (~ 1 µW). Most of the seeds (1319 nm at fixed frequency, and 1064 nm modulated by AOM to
provide a, + and - cyclically or on commend) will be used to seed the Q-switched pulsed resonators,
respectively. The pulsed oscillator output will be amplified to provide ~30 W total from the YAG systems
3
that will then be mixed in a nonlinear crystal to generate 10 W output at 589 nm. The output
characteristics of the pulsed YAG are ~ 15 ns in pulse width with Fourier transform limited frequency
width of 33 MHz. Though there exists pulse height jitter of a few percent, the jitter in the center
frequency of the pulse spectrum is estimated to be within ~ 1 MHz. The expected pulse width, frequency
width, center frequency variation, and pulse height jitter of the 589 nm output will then be, respectively,
10 ns, 50MHz, less than 1 MHz, and 2%. Since the shortest time duration for lidar measurements is 2 s,
these variations and jitters will be much reduced when they are averaged over more than 1500 pulses.
Conceptually, the proposed Fibertek system is similar to a recent diode pumped Japanese system, both
based on the proposal for an all-solid-state flash-lamp pumped system [She et al., 2007]. While all the
components of the Japanese system [Tsuda et al., 2011], which produced 2 W of power, are laid out on an
optical table, the proposed Fibertek system is modular and compact with most optical elements
hermitically sealed. The excellent Fibertek engineering and experience with airborne laser
instrumentation and reliable thermal and electrical control will result in modular laser systems with
enhanced frequency and pulse stability along with high-power capability of 10 W.
Figure 1 The optical schematic of proposed Fibertek 10 W Na system. Approximately 2% electrical
efficiency, 200 W optical pump, 500 W electrical input.
4.
Measurement accuracy estimation and proposed deployment strategies
We begin by presenting the received signal and background of the CSU Na lidar system in 2002, when it
became possible for 24-hour continuous operation [She et al., 2003]. This system deployed two beams
pointing to the north and east, both at 30o from zenith. The laser power is 0.5 W per beam at 50 Hz rep
rate, and the lidar signal and sky background is received by telescopes of 35 cm diameter within 0.8 mrad
field of view, and fiber coupled to photomultipliers (PMTs) with 20% efficiency. The power-aperture
product (PA) of the system is 0.05 Wm2 per beam. Table 2 shows the parameters of the north beam,
which will be used as reference data, from which the measurement uncertainty of the proposed lidar
systems will be estimated. Four observational scenarios are listed: 2310_night and 2205_night,
4
representing winter and summer nights, respectively, when sky background is negligibly small, and
2310_noon and 2205_noon, representing winter and summer noon, respectively, when the sky
background is maximal, with a Faraday filter [Chen et al., 1996] incorporated in the receiver to reduce
sky background. The table shows, in Columns 3 and 4, the signal S and background B in a vertical range
of 130 m (or slant range of 150 m) at the edge of the Na layer (98 or 99 km) integrated over a time of 40 s.
Since the signal count is maximum at the centroid of the Na layer, the photon signal-to-noise
decreases from the center to the
lower signal north beam as the reference. For 40 s integration, the
edge of the layer, so we consider
photon count of the Na profile from the north beam on 2310_night
only an altitude range (as shown
is 98261 (from the east beam it is 142240), corresponding to 49 cts
for each scenario) of ~ 15 km, and
per pulse. When this is converted to a Na lidar system at University
present the largest measurement
of Colorado using 30 Hz laser and 81 cm dia. telescope, the Na
uncertainty (at the edges) for
layer cts/pulse is 877 cts. This number can be 50% to 100% lower
than that recently achieved [Private communication, X. Z. Chu],
temperature T, LOS wind V,
indicating again that our estimate based on the reference system
and Na density D, respectively,
in columns 5, 6, and7, at a vertical (Table 2) is a conservative one. Crazy formatting here!
resolution of 0.5 km and 1 hr. We
Table 2 The reference system with 20% PMT, PA = 0.05 Wm2
note that the measurement
Observation Altitude
Edge
Edge
T
V
D
uncertainties of the last two rows,
DOY_time
range
S
(cts)
B
(cts)
(K)
(m/s) (%)
even with the use of Faraday filter,
2310_night
85 - 99 km
268
1.3 1.36
1.29 0.33
are still considerable, as noon is
2205_night
86 - 98 km
107
0.51
2.3
1.99 0.52
the worst case scenario. In this
2310_noon
85- 99 km
14.4
40 26.6 21.88 2.75
connection, a UV lidar would
2205_noon
86-98 km
6
63 64.3
68.9 7.44
work better. For a conservative
Resolution: 130 m-40 s (0.5 km and 1 hr) for S and B (T,V, D).
estimate, we chose the data of the
4.1. Photon noise uncertainty of a dual-beam and a vertical-beam configuration
In order to measure zonal momentum flux of gravity waves, the
CSU lidar was upgraded in 2006 by implementing 40% efficient
PMTs, larger telescopes (PA = 0.2 Wm2) and a dual-beam
geometry in the east-west plane, as shown in Figure 2. It is a
straightforward calculation to deduce the photon noise uncertainty
of temperature, zonal and vertical wind, T, u, w, respectively,
for this dual-beam configuration from the uncertainties, T and
V, of one beam pointing at 30o off zenith. To relate these
quantities, we note that an altitude range z is related to a slant
range, r, at an angle from zenith, by z = r / cos. For the .
same altitude range z, the relationships between the received
signal and background for a beam pointing at an angle  and thoseF
received at 30o are
S  S30o / cos ; B  B30o / cos  .
Fig. 2
Dual-beam geometry
The relationship of photon noise measurement uncertainties between a beam pointing at 30o (as a
reference), T, V, and D, and those of one beam pointing at  are then given in (1), those from one
vertical beam system in (2); and those from a dual-beam configuration, T, u, w, and D in (3).
5
 T
 V
 D



 T30
 V30
 D30
o
o
For one vertical beam,
cos 
cos 30o
o
T0o
 T30
o

(1)
w0o  T0o
 w30  T30
For thw dual-beam configuration,
o
T
 T
cos 0o
 1.075
cos 30o

o

D
 D

1
2
;
u

 V
(2)
1
2 sin 
w

 V
;
1
(3)
2 cos 
These relationships can also be plotted in blue for T and D, black for u, and red for w, respectively,
in Fig. 3. One notes that relative to the uncertainty of one beam pointing at 30o off zenith, the uncertainty
in zonal wind u increases as angle  decreases, while it is only increased (decreased) by a small amount
for temperature, T (vertical wind, w). Though it will do little to improve measurement accuracy of
temperature and vertical wind, T, and w, an additional vertical beam is often added as will be
explained.
30
= w /V
0
30
=D /D
0
30
1.075 X
1
4.34
O
O 3.81
2.89 O
O O 2.15
2.31
O
1.35
0.5


3
30
30
0

30
u / V ;
T /T
6
30
Var(MFVarMF
min
1.5

wind perturbations, MF = w ' u ' , the variance
of MF depends on the interplay between the
variances of w’ and u’, Var(w’) and Var(u’), as
shown [Gardner and Liu, 2007] below:
9
w / V ;  /  =D / D
However, because of the interest in the dualbeam geometry for momentum flux
measurements, whose uncertainty is dominated
by statistical fluctuations, the choice of angle 
requires further consideration with a
compromise between zonal wind and
momentum flux uncertainties in mind. Since
the momentum flux is the statistical average of
the product of vertical wind and horizontal
0
0
0
5
10 15 20 25 30
35
40
45
Angle ()
Fig. 3 Relations of various measurement uncertainties


tan2 
cot2  1
Var (w ' u ')  C var2(u ')
 var2(w ')
 var(w ') var(u ')
4
4
2


(4)
where C is a constant depending only on resolution and measurement details; it is independent of .
Therefore, the functional dependent of Var (w ' u ') on  depends on the ratio Var(u ') / Var(w ') . Since
this ratio is ~100 [Gardner and Liu, 2007], the minimum of Var(w ' u ') occurred at  = 5.7o; the
dependence of Var (w ' u ') on  for this case is also plotted in Fig. 3 in green. Here, we see at  = 10o, 15o,
and 20o,u = 4.34, 2.89, and 2.31 times V and Var (w ' u ') = 1.35, 2.31, 3.81 times optimum
(minimum) value of Var(w ' u ')min , respectively. We note that in the literature 10o, 13.5o and 20o were
used in SOR measurements [Gardner and Liu, 2007], in model calculation [Thorsen et al., 2000], and in
measurements at CSU [Acott et al., 2010] and ALOMAR. Therefore, depending on the signal level, the
angle of 10 o, 15 o or 20o may be selected, respectively, for PA = 10, 2 and 0.5 Wm2 per beam. Since at
these angles the separations between the dual beams at 100 km are 35, 54, and 73 km, respectively, the
determination of u’, w’ and T’ (and associated fluxes) at high resolution from a dual-beam geometry may
6
not include the contributions of short wavelength GWs. For this reason, a vertical beam should be added.
Then, the difference in heat and constituent fluxes between those determined by the vertical beam and by
the dual-beam geometry may be compared to assess the importance of short wave contributions.
4.2. Measurement uncertainties estimated for two proposed systems
As discussed, the state-of-the-art pulsed laser systems at 589 nm operate at 750 Hz with an average power
of 5 W or 10 W. Here, we propose and estimate the performance of two systems for potential Facility
deployment. First, we propose a modest 5-beam system with single vertical and two sets of dual beams
(pointing at 15o off zenith), each with PA = 2.0 Wm2, which may be implemented with 1 W power and
four 80 cm dia. telescopes. This modest system (a 5 W laser and twenty 80 cm telescopes) may be
constructed for mobile deployment. Second, we propose a large 5-beam system with a single vertical
beam and two sets of dual beams (pointing at 10o off zenith), each with PA = 10.0 Wm2. To beef up the
turbulence measurements, the vertical beam may be replaced by one with PA = 500 Wm2. The per beam
value (at 30o off zenith) of S, B and T and V, D for these proposed systems may be scaled from the
information in Table 2 by first considering a beam with the same PA of 0.05 Wm2, converting from 20%
to 40% PMT and from 50 Hz to 750 Hz repetition rate; this will double the signal S and increase the
background B by 2x15 = 30 times. Then, for a system with higher PA, we further increase S in proportion
to PA, and keep the background B the same (with the same receiver field-of-view).
With these new values of S and B, 2.5 min for night (1 hr for day with Faraday Filter), and with units
the measurement uncertainties T, for the uncertainties same as in Table 2.
u, w, D by a pair of dual
Table 3 Performance of the moderate system
beams and T, w, D by the
ObserDual-beam at 15o, with
Vertical beam with
vertical beam may be calculated
vation
2.0 Wm2 per beam
2.0 Wm2
from Eqs. (2) and (3). These are
DOY_time
T
u
w
D
T
w
D
given in Table 3 for the modest
2310_night
0.56 2.04 0.55 0.12
0.80
0.76 0.18
system proposed and in Table 4 for 2205_night 0.94 3.15 0.84 0.19
1.35
1.17 0.28
the large system proposed, both
2310_noon
1.63 5.20 1.39 0.17
2.35
1.93 0.24
with resolution of 0.5 km and
2205_noon
3.52 14.6 3.90 0.41
5.06
5.43 0.59
Table 4 Performance of the large system
ObserDual beam at 10o,
vation
10.0 Wm2 per beam
DOY_time
T
u
w
D
2310_night
0.25 1.37 0.24 0.055
2205_night
0.42 2.11 0.37 0.088
2310_noon
0.57 2.69 0.47 0.26
2205_noon
0.96 5.89 1.04 0.50
Vertical beam at
10.0 Wm2
Vertical beam at
500 Wm2
T
0.36
0.60
0.81
1.36
T
0.050
0.085
0.10
0.15
w
0.34
0.52
0.67
1.46
D
0.079
0.12
0.37
0.71
w
0.048
0.074
0.086
0.16
D
0.011
0.018
0.048
0.075
Since under sunlit conditions, the sky background at 589 nm is too high for successful measurements to
resolve GWs (at 0.5 km, 2.5min) or turbulence (at 20 m, 2 s), we should only estimate nighttime
uncertainties for the resolution of 20 m and 2 s. The results are shown in Table 5. Though the
uncertainties at 2 s integration could be substantial, a modest integration can beat down the photon
induced uncertainties [Gardner and Liu, 2013]. This is particularly so for the 500 Wm2 beam when the
uncertainty of a single Na density measurement with 2 s integration is below 1%. The performances of
vertical beams with 2.0 Wm2 and 10.0 Wm2 are not shown in Table 5, but they can be calculated easily.
7
Table 5 Nighttime performance of proposed systems with 20 m and 2 s resolution.
Obser-vation Altitude
Dual beam at 15o, with
Dual beam at 10o, with
Vertical beam with
2
2
DOY_time
range
2.0 Wm per beam
10.0 Wm per beam
500 Wm2
T
u
w
D
T
u
w
D
T
w
D
11.9 64.9 11.5 2.7
2.40 2.27 0.53
2310_night
85-99 km 26.4 96.7 25.7 5.8
20.1 100 17.7 4.1
4.05 3.51 0.84
2205_night
86-98 km 44.6 149 39.9 9.2
We point out that the above uncertainty estimates are based on photon noise errors. Though the photon
noise dominates the measurement uncertainty, the frequency of the pulsed output is broadened with an
asymmetric shape compared to the CW seed. This frequency chirp gives rise to a shift between the centerof-mass of the pulse spectrum and the frequency of the CW seed. Its effects on temperature and wind
need to be determined and accounted for. In principle, this chirp-caused frequency shift may be measured
by a heterodyne beating technique [White et al., 2004] on a pulse-to-pulse basis, though it is much more
difficult to do for a short pulse of ~10 ns. Fortunately, a lidar measurement is done in seconds if not in
minutes, this frequency shift (or wind bias) can be monitored by averaging over many pulses with a
spectral filter technique [Yuan et al., 2010]. Hopefully, the state-of-the art Na laser system is stable and
repeatable; the broadening and chirp-caused frequency shift can then be determined in the laboratory and
their effects accounted for in the data analysis. Also, as pointed out, the use of a 750 Hz laser (compared
to a 50 Hz laser) increases the sky background by 15 times, making daytime reception more difficult. At
the same time, the higher rep rate will reduce the peak pulse power, enabling the use of a system with 10
times more average power, free from the Na layer saturation problem.
5. Conclusions:
With the state-of-the-art diode-pumped, high-efficiency Nd:YAG lasers readily available, a single
frequency tunable, Q-switched pulsed laser at 589 nm and 750 Hz may be custom-made at an average
power of 5 W or 10 W. Based on these robust and potentially automatable laser transmitters, we have
conceptually proposed two Na lidar systems for Facility deployment: a modest 5-beam system employing
a vertical beam plus two sets of dual beams pointing at 15o from zenith with PA = 2 Wm2 per beam, and a
large 5-beam system employing two sets of dual beams pointing at 10o from zenith with PA = 10 Wm2
per beam, plus a vertical beam with either PA = 10 Wm2 or PA = 500 Wm2. With the signal and
background from an existing reference system, we conservatively estimated the measurement
uncertainties of temperature, wind and Na density for the two proposed systems and found them capable
of measuring temperature, wind components and Na density with resolutions for probing GWs (0.5 km
and 2.5 min) as well as for probing turbulence (20 m, 2s), though a longer time integration is needed with
the modest system.
Acknowledgment: The lead author thanks Chet Gardner for his continued leadership and interest in the
research of metal resonance lidar technology and science, as well as the information he provided for this
paper on the requirement for turbulence/diffusion measurements.
8
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