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Development of a Pulsed 2-micron Laser Transmitter for C0 2 Sensing from 

Space 


1 1 • • *2 1 3 

Upendra N. Singh , Jirong Yu , Yingxin Bai , Mulugeta Petros and Robert T. Menzies 


iNASA Langley Research Center, Hampton, VA 23681 
^Science Systems and Applications, Inc, One Enterprise Parkway, Hampton, VA 23666 
3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 


Abstract - NASA Langley Research Center (LaRC), in 
collaboration with NASA Jet Propulsion Laboratory (JPL), 
is engaged in the development and demonstration of a 
highly efficient, versatile, 2-micron pulsed laser that can be 
used in a pulsed Differential Absorption Lidar (DIAL) / 
Integrated Path Differential Absorption (IPDA) instrument 
to make precise, high-resolution C0 2 measurements to 
investigate sources, sinks, and fluxes of C0 2 . This laser 
transmitter will feature performance characteristics 
needed for an ASCENDS system that will be capable of 
delivering the C0 2 measurement precision required by the 
Earth Science Decadal Survey (DS). 

I. INTRODUCTION 

The National Research Council (NRC) Decadal Survey 
(DS) recommended - an active laser-based C0 2 mission. 
Active Sensing of C0 2 Emissions over Night, Days, and 
Seasons (ASCENDS), to dramatically increase our 
understanding of C0 2 sources, sinks, and fluxes 
worldwide [1]. ASCENDS provides 1) full seasonal 
sampling to high latitudes, 2) day/night sampling, and 3) 
ability to resolve the altitude distribution of C0 2 column 
measurement, particularly across the mid to lower 
troposphere. According to the DS the measurement 
accuracy should be 0.5% of the background (i.e., <2 
ppm) at 100 km horizontal length scale over land and 
200 km over open oceans. An important outcome of the 
3-day ASCENDS community workshop held in July 
2008 was a priority list of technology development 
recommendations needed to advance the readiness of the 
ASCENDS mission. Included was a strong 
recommendation to scale the 2.05 pm laser transmitter to 
higher output power levels in a robust prototype with 
long-lifetime design features. 

Our current laser development addresses the 
ASCENDS workshop recommendation on transmitter 
needs. It will operate in a favored wavelength region 
selected for optimum sounding for high-precision 
measurements [5], with pulse energy and repetition 


frequency matching the performance parameters of 
comprehensive system performance studies [2-4]. The 
pulsed 2 pm lidar approach possesses advantages over 
passive and CW active sensors. First, using time-of- 
flight determination, the pulsed format provides a built- 
in means for determining range to the scattering target. 
With pulses of sufficiently short duration, and 
appropriate receiver bandwidth, it eliminates the need 
for the ancillary laser altimeter in the payload, for 
accurate measurement of scattering surface elevation. 
More importantly, in a scattering atmosphere containing 
thin clouds and aerosol layers, the reflected signals from 
the surface for the lower tropospheric column C0 2 IPDA 
measurement can be resolved from those due to 
intervening thin cloud and aerosol backscattering. 
Therefore, it easily, efficiently, and unambiguously 
eliminates the contamination from aerosols and clouds 
that can bias the IPDA measurement and reduce 
measurement accuracy. Second, by concentrating the 
laser energy into high-energy (50-100 mJ or greater) 
pulses, sufficient backscatter signal strength can often be 
obtained from boundary layer aerosol scattering as well 
as the surface. This provides added flexibility to 
retrieve the C0 2 structure near the surface. Third, the 
higher per-pulse SNR (signal-to-noise ratio) obtainable 
with high energy pulsed backscatter means less reliance 
on multi-pulse averaging, providing potential for higher 
along-track spatial resolution and better measurement 
capability in regions of partial cloud coverage, 
benefiting high precision measurements. Fourth, as 
mentioned above, the chosen absorption line at the 2.05 
pm band is ideally suited for the IPDA measurement of 
C0 2 weighted column mixing ratios. In particular, with 
operation at the R(30) line in the 2.05 pm band, 
weighting functions can be obtained that maximize the 
interaction with C0 2 in the lower troposphere (lowest 5 
km), while still maintaining the differential absorption 
optical depth near the optimum value of ~1 [5, 6], 



II. DESCRIPTION OF TECHNOLOGY CONCEPT 
AND RATIONALE 

This technology development was initiated during 
NASA Earth Science Technology Office (ESTO) funded 
Laser Risk Reduction Program (LRRP) with the 
objective to develop a Thulium (Tm) fiber laser pumped 
Holmium (Ho) solid-state laser that generates laser 
pulses in the 2pm wavelength for pulsed C0 2 
D1AL/IPDA instrument. The key performance 
characteristics of this laser, such as energy, pulse 
repetition rate, pulse width, efficiency, frequency 
accuracy and stability, will meet or exceed the needs of 
the ASCENDS transmitter as currently envisioned. This 
space qualifiable laser architecture utilizes fiber laser 
and solid-state crystal laser technologies. One of the 
outstanding properties of the fiber laser is its efficiency. 
However, it inherently has low damage threshold at high 
energy pulses. On the other hand, the solid state laser has 
the capability to produce Joule-level energy at 2 pm 
wavelength [7]. The proposed laser combines the 
advantages of both lasers to provide the desired energy 
with high efficiency. 

There are excellent absorption lines for the 
measurements of C0 2 in 2 pm wavelength region with 
regard to the strength of the absorption lines, low 
susceptibility to atmospheric temperature variability, and 
freedom from problematic interference with other 
absorption lines [5, 8]. We have chosen to operate on 
the short wavelength wing of R (30) C0 2 line at 
2050.967 nm (4875.766 cm' 1 ) in the side-line operation 
mode which is required for low troposphere C0 2 
measurement. The side-line operation was demonstrated 
for Lidar Atmospheric Sensing Experiment (LASE) at 
near-infrared wavelength [9]. The exact wavelengths of 
the Ho laser are controlled by injection seeding 
technique to provide the required on-and-off line 
wavelength pulses sequentially. This laser transmitter 
has the advantages of high electrical efficiency, compact 
size and low mass. 

III. GENERAL DESCRIPTION OF THE 2-pM 
PULSED LASER TRANSMITTER 

Figure 1 illustrates a conceptual block diagram of the 
proposed Ho 2pm pulsed laser. The Tm fiber laser 
pumped Ho laser provides several significant advantages 
including low thermal load; long energy storage lifetime; 
high system efficiency; simpler laser architecture; and in 
a more compact and rugged package. It uses a Thulium 
(Tm) fiber laser to pump a Q-switched Holmium (Ho) 
solid-state laser to produce defined wavelength, line 
width, pulse width, beam quality and pulse repetition 
rate. The following amplifier scales the energy to the 
desired energy level. The repetition rate of the laser is 


controlled by the rate of the Q-switch; effectively, it is a 
variable rate laser transmitter. This is a valuable feature 
for multiple lidar applications. The intended design for 
the Ho laser will be optimized for C0 2 DIAL/IPDA via 
direct detection method, where relatively high energy at 
modest repetition rate is required. 



Wavelength 
Control ,3) 



Commercially available 

Technique demonstrated, and will be further improved 
and engineering packaged 

Technology will be developed /improved, and system 
engineered and packaged 


Figure 1: Block diagram of proposed 2pm laser 
transmitter, illustrates the focus of the development 
approach 

High efficiency of this laser design architecture has been 
obtained experimentally from a laser pumped Ho:YLF 
laser in our laboratory [10]. It produced 33mJ of pulse 
energy with a quantum efficiency of 88%. Recent great 
advancement in fiber laser made it possible to replace 
the Tm:YA103 laser with commercially available 
Tm:fiber laser as the pump source. The following 
subsections (III. 1-5) provide a detailed description of 
each subsystem in Figure 1. 


III.l THULIUM-FIBER PUMP LASER 


Figure 2 illustrates a commercially available Tm: fiber 
laser made by IPG Photonics which operates in a tandem 
pumping scheme. The best wall plug efficiency for this 
type of Tm: fiber laser was about 9%. The Tm: fiber 
lasers can also be pumped directly through the 800nm 
band. By using a heavily doped Tm concentration, the 
laser efficiency can be significantly enhanced by the 
well-known cross-relaxation process, where one pump 
photon can excite two Tm ions in to 3 H 4 level. Q-peak 
has reported that 300W of power at ~1.9pm has been 
generated with 62% optical-to-optical efficiency in a Tm 
doped silica fiber laser [11]. NP Photonics took a 
different approach, and has developed Tm-doped 
germinate glass double-cladding single-mode fiber laser. 
Output power of 64 W at 1.9 pm with a slope efficiency 
of 68% was demonstrated [12]. The efficiencies in both 
of these approaches are significantly higher than the 
Stokes limit of 42%. Taking a conservative estimate of 
60% optical-to-optical conversion efficiency for 
Tm:fiber lasers and assuming 45% of electrical-to- 
optical conversion efficiency for pumping diodes, the 
Trmfiber lasers would have 27% electrical efficiency. 




Commercially available (IPG) Tm:FLs have been 
repackaged to MIL spec standards. Consequently, 
utilizing a commercially available, efficient fiber laser as 
a pump source significantly increases the system 
efficiency and reduces the risk. 

III. 2 HOLMIUM SOLID-STATE LASER 

Table 1 lists specifications for the planned laser 
development and current development status. They are 
designed to meet the stringent laser transmitter 
requirements as imposed by high-precision and accuracy 
of the C0 2 measurements defined in the DS. The target 
objective for space-based system as listed are based on 
the results of a comprehensive study conducted by ESA 
under contract No. 10880/03/NL/FF [3], 

Table 1: 2-micron Laser Transmitter Specifications. 


Parameter 

Development 
Objectives for 
Current 
System 

Target 

Objectives for 
Space-based 
System 

Wavelength 

(pm) 

2.051 

2.051 

Energy(mJ)/ 
Rep. Rate (Hz) 

>65mJ / 50Hz 

65mJ / 50Hz 

Pulse width (ns) 

<= 50ns 

<= 50ns 

Transverse 

Mode 

TEMoo 

TEMoo 

Longitudinal 

mode 

Single 

frequency 

Single 

frequency 

Frequency 
control accuracy 

<2MHz 

2MHz 


III. 3 HO LASER OSCILLATOR 

A Ho solid-state pulsed laser was successfully 
demonstrated under a LRRP task funded by Earth 
Science Technology Office (ESTO) [14]. Ring cavity 
design was used for this pulsed Ho laser. It eliminates 


the effect of “spatial hole burning” in the laser gain 
medium to obtain higher beam quality. This laser is 
injection seeded by a well-behaved seed laser source. 
The injection seeding is based on the ramp-and-fire 
technique. The demonstrated successful injection 
seeding rate is >99.9% [15]. This injection seeded ring 
cavity laser architecture has been successfully applied to 
operational coherent wind lidar and CCL DIAL, and the 
wind lidar has been successfully flown in NASA DC8 
platform recently [16]. 

At NASA LaRC, we have demonstrated a Ho:YLF laser 
pumped by a Tm:fiber laser as part of LRRP. 31mJ at 
100Hz repetition rate was achieved in a ring cavity 
configuration with pump power of 13 W as shown in 
Figure 3 [17]. One stage of amplifier is needed to scale 
the energy to meet the energy requirement of space 
IPDA instrument. 



0 2 4 6 8 10 12 14 

Tm:Fiber laser Pump Power (W) 

Figure 3: Demonstrated performance for a similar Tm 
fiber laser pumped Ho laser. 

III. 4 HO LASER AMPLIFIER 

A Ho laser amplifier in a double-pass, straight through 
slab configuration is envisioned to provide scaling to a 
high energy of at least 65mJ due to the better thermal 
management arrangement. The advantages of 
longitudinal pumping scheme are better pump/probe 
beam overlap and a simple geometry of the slab 
amplifier. The 1.94 laser pumps the Ho:YLF crystal 
through beam shaping optics and a dichroic. The probe 
beam from the output of the oscillator is introduced into 
the amplifier from the other end. It double passes the 
amplifier gain medium by the slightly tilted dichroic 
mirror. The key to obtain high amplifier efficiency with 



good beam quality is to effectively dissipate the heat 
generated in the amplifier gain medium. Due to the 
resonant pumping, the small quantum defect of the 
amplifier indicated only ~5% of the pump power is 
generated as heat. The study of Ho:Ho interactions in 
YLF [18] suggests that the up conversion loss in this 
material is much lower than that of YAG material. 

III. 5 INJECTION-SEED LASER AND 

WAVELENGTH CONTROL 

Tuning and stability of the laser transmitter are critical 
for making precise and accurate C0 2 measurements. The 
laser wavelength control technology described here has 
been previously built and integrated into a complete 
breadboard prototype lidar used in ground-based field 
experiments [19, 20]. Three CW seed lasers are being 
planned to be integrated and optimized in an optical 
switch design to meet the required wavelength control 
and tunability. The CW lasers are commercially 
available devices originally developed for the NASA 
Space Readiness Coherent Lidar Experiment 
(SPARCLE) [21]. We have developed a technology for 
establishing wavelength knowledge to well under 0.05 
pm (3.75 MHz) [22]. Furthermore, a capability has been 
added to tune and lock anywhere on the side of the 
absorption line, so that the amount of absorption to a 
desired range can be optimized. Tailoring the level of 
absorption further improves precision and accuracy of 
the DIAL/IPDA results. 

IV. SUMMARY AND CONCLUSIONS 

The mid-IR wavelength regions at 1.57pm and 2.05pm 
are considered suitable for C0 2 IPDA measurements. 
Two instruments operating at 1.57pm have been 
developed and deployed as airborne systems for 
atmospheric C0 2 column measurements [23, 24]. One 
instrument is based on an intensity modulated CW 
approach, the other on a high PRF, low pulse-energy 
approach. These airborne C0 2 lidar systems operating at 
1.57pm utilize mature laser and detector technologies by 
taking advantage of the technology development 
outcomes in the telecom industry. However, significant 
challenges remain for scaling from airborne to space- 
borne mission prototype. For example, in the case of the 
high PRF pulsed system, two orders of magnitude 
average power scaling is needed [23, 25]. On the other 
hand, lidars operating in the 2pm band offer better near- 
surface C0 2 measurement sensitivity due to the 
intrinsically stronger absorption lines. The 2pm pulsed 
laser needs a factor 2 scaling to meet laser transmitter 
pulse energy and PRF requirements for a C0 2 space 
mission [3], which can be achieved by adding single 
stage amplifier as envisioned in this paper. In addition 


recent work documents the capability to precisely 
control and stabilize the output frequency of this type of 
Ho laser [19, 20], and demonstrated a higher than 0.7% 
measurement precision via a ground based C0 2 lidar 
instrument [22], The recent emergence of new 2 pm 
detector capabilities makes direct detection at the 2 pm 
wavelength from space very attractive [26, 27]. The 
pulsed 2 pm laser provides a viable approach for space 
based C0 2 column density measurement. 

Although both the wavelengths at 1.57pm or 2.05 pm 
are suitable for the C0 2 concentration measurement, the 
weighting function at 2.05 pm is more favorable for 
measurements in the lower troposphere, including the 
boundary layer. This is important since this is where the 
C0 2 sources and sinks reside [5]. In theory, the 1.57pm 
sounding can be done with a similar weighting function, 
by displacing the on-line laser frequency ~ 2 half widths 
from the line center of the pressure -broadened 
absorption line. However when doing that at 1.57pm, 
the differential absorption optical depth (DAOD) 
becomes very small (~0.1, or 10%) [5], requiring 
extremely high on-line and off-line SNR to achieve the 
required measurement precision. In addition, it becomes 
more difficult to control the influence of sources of bias 
when the differential absorption “signal” is so small. 
The DAOD at 2pm is closer to ideal DAOD of ~1 [6]. 
The inherent spectroscopic factors result in a 
significantly larger measurement precision and bias 
reduction challenge when operating at 1.57 pm, 
compared with 2.05pm. 

In addition, high energy pulse approach at 2 pm provides 
higher measurement accuracy. Given a fixed transmitter 
average power, high pulse energy is preferred when 
striving for high signal-to-noise level in a direct- 
detection lidar system when dark current and/or 
background-induced photocurrent are not insignificant, 
as is the case for ASCENDS. The currently envisioned 
1.57pm concepts employ much lower pulse energy (~ 1 
mJ) or operate CW. 

In summary, the ESTO funded 2-micron laser 
technology under LRRP provides a clear technology 
development path to spaceflight application. The NASA 
LaRC developed Ho pulse laser meets or exceeds the 
generally accepted requirements of a direct detection 
2 pm IPDA system, which can provide adequate C0 2 
column density measurements from space. The pulsed 
lidar transmitter architecture, energy, repetition rate, line 
width, frequency control are all suitable for space 
application without major scale up requirements. 



V. ACKNOWLEDGEMENTS 

This work was conducted as part of the NASA Laser 
Risk Reduction Program (2002-2010). The authors 
would like to acknowledge the LRRP funding and 
management support provided by NASA’s Science 
Mission Directorate (SMD) and Earth Science 

Technology Office (ESTO). 

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