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NASA Langley Research Center (LaRC) has a long history of developing pulsed 2-μm lasers. From fundamental spectroscopy research, theoretical prediction of new materials, laser demonstration and engineering of lidar systems, it has been a very successful progress spanning around two decades. This article covers the 2-μm laser development from early research to current state-of-the-art instrumentation and projected future space missions. This applies to both global wind and carbon dioxide active remote sensing. A brief historical perspective of Tm:Ho work by early researchers is also given.
© 2015 Optical Society of America
1. Introduction
In the United States, the National Aeronautics and Space Administration (NASA), among other agencies, have been focusing on enabling 3-D winds space missions since the 1970s. The over 40 years of work towards the global winds mission included more than 50 studies, encompassing theoretical development, computer simulation of the wind measurement technique and of the utility of the wind measurements, requirements development, space mission design, lidar technology development, and ground and airborne validation. NASA and the National Oceanic and Atmospheric Administration (NOAA) have worked with lidar scientists to formulate the wind measurement requirements appropriately stated for a lidar solution [1].
Carbon dioxide (CO2) is an important greenhouse gas that significantly contributes to the carbon cycle and global radiation budget on Earth. The CO2 role on Earth’s climate is rather complicated due to different interactions with various climate components that include the atmosphere, the biosphere and the hydrosphere [1–4]. These interactions define CO2 sources and sinks that influence the gas transport fluxes worldwide. High uncertainties exist in quantifying CO2 sources and sinks mainly due to insufficient spacial and temporal monitoring of the gas. Generally, CO2 in situ sensors coverage is limited due to inadequate sampling sites and time. Thus, it is required to have more rapid and accurate CO2 monitoring with higher uniform coverage and better resolution. This was addressed by many international satellite missions, which rely on passive remote sensors. Nevertheless, space-based active remote sensing overcomes many limitation of passive remote sensors [4, 5].
The first earth science “Decadal Survey” was issued by the National Research Council (NRC) recommended future 15 space missions for implementation by NASA. Among the recommended missions were the global wind and the CO2 measurement using active remote sensing [1]. Attractive laser attributes for any space-based active remote sensor include solid state architecture for reliability, high efficiency to reduce prime power budget and heat removal, long laser lifetime, and low atmospheric extinction. Sustained research efforts at NASA Langley Research Center (LaRC) during the last twenty years have resulted in significant advancement in 2-μm diode-pumped, solid-state lasers. These 2-μm lasers fulfill the general attributes and more specific needs of the individual transmitter-of-choice for wind space-based missions, as indicated in Table 1.In addition, for CO2, several significant absorption features of the gas exist at this wavelength region. Again, this utilizes the pulsed 2-μm laser as the transmitter-of-choice for CO2 active remote sensing using the differential absorption lidar (DIAL) technique [2].
Table 1. General and specific attributes of the 2-μm laser for global wind and CO2 active remote sensing.
Although 2-μm lasers enable unique remote sensing capabilities, early development focused on more fundamental aspects of improving the laser efficiency in both cw and pulsed operation via the Tm to Ho energy transfer mechanism. Back in 1965, researchers at Bell Labs produced the first Tm:Ho co-doped laser, an αβ -YAG operating at 2.1 μm [6]. This was a continuous-wave (CW) tungsten lamp pumped system operating at 77K. In 1971 researchers at Sanders Associates produced an αβ-YLF operating at 2.06 μm, which was a flash lamp-pumped pulsed laser, also operating at 77K [7]. The first room temperature Tm:Ho laser was demonstrated by researchers at the University of Hamburg in 1985 [8]. This was a CW Krypton-pumped laser operating at 2.1 μm in Cr:Tm:Ho:YSAG and Cr:Tm:Ho:YSGG. The commercial availability of diode lasers in the 1980’s was a perfect match for Tm:Ho solid-state lasers due to the good overlap of diode laser wavelengths with Tm absorption. The first experiments utilizing diode pumping of Tm:Ho materials were done at the Naval Research Laboratory in 1986-87 [9,10]. They demonstrated diode-pumped 2 μm laser action in Tm:Ho:YAG and Tm:Ho:YLF. This led shortly thereafter to the first eye-safe coherent lidar in 1991 [11]. This lidar system was based on a diode pumped Tm:Ho:YAG at 2.1 μm.
Researchers at NASA LaRC significantly contributed toward the advancement of 2-μm lasers in the past 20 years. Relying on LuLiF technology, this laser can be used directly as the transmitter for the global wind remote sensing in conjunction with heterodyne detection technology. This enables coherent Doppler lidar that measures the horizontal wind velocities with high precision and resolution. The same laser, after slight modification, and relying on YLF technology can be used as the transmitter for carbon dioxide measurement from ground, airborne and potentially space-borne platforms. Scientists at LaRC have developed several compact, flight capable, high energy, injection seeded, 2-μm laser transmitters as presented in this paper.
2. Pioneering 2-µm lasers at NASA LaRC
Fundamental spectroscopic work on Tm:Ho laser materials at LaRC began in the late 1980’s and continued into the early 1990’s. The motivation was to completely characterize the spectroscopic aspect of these materials and gain deeper understanding of the physics involved, which could be used to advance the 2-µm laser development. The work was not only based on experiments, but on theory as well. The theoretical work included applying quantum mechanical models to predict spectroscopic parameters difficult to measure, such as those involving energy transfer processes, and to study new materials and assess their lasing potential. Laser models were also utilized to project performance parameters. The outcome of this work was a fairly complete understanding of Tm:Ho systems and the trade-off between LuAG and LuLiF compared over YAG and YLF [12]. Early modeling work for quantum mechanical models and oscillator/amplifiers was largely completed and published in 1996 [13,14]. By 1997 LaRC had demonstrated a room-temperature, Tm:Ho:YLF laser amplifier generating 700 mJ at 2 µm from a 50 mJ, flashlamp pumped q-switched laser oscillator [15]. In 1998, injection-seeded, room-temperature, diode-pumped Ho:Tm:YLF laser with output energy of 600 mJ at 10 Hz was also demonstrated [16]. In 2002-2003 a double pulse 600 mJ Tm:Ho YLF laser/amplifier was demonstrated [16,17,18]. The Tm:Ho technology reached impressive performance levels in 2006 [19] when NASA LaRC researchers produced 1 J per pulse at 2.06 μm in Tm:Ho:LuLiF, a material invented and developed by the same group. All of this development from fundamental physics to engineering 2 µm Tm:Ho lasers [20,21] set the stage for the development of state-of-the-art, eye-safe, compact, lidar transmitter systems that could be applied for ground, airborne or space-based remote sensing instruments [22, 23].
With support from NASA Laser Risk Reduction Program (LRRP), LaRC implemented the recommendations to specifically work on lidar technologies before mission approval and to maintain in-house capability by using the strengths in 1-μm solid-state lasers at NASA Goddard Space Flight Center (GSFC) and the strengths in 2-μm solid-state lasers at LaRC to work on fundamental issues concerning these two laser technologies, as listed on the left side of Fig. 1.With additional work on wavelength conversion technologies, the four lidar techniques of altimetry, Doppler, DIAL, and basic lidar backscattered signal strength profiling would be able to make six high priority earth science measurements: surface and ice mapping, horizontal vector wind profiles, river currents, CO2 profiles, O3 profiles, and aerosols/clouds.
Through collaboration and communication between LaRC and GSFC of questions, capabilities, and results, the advancement of the commonly needed areas of laser and wave-length conversion development would proceed faster and without duplicated effort. The goal of the LRRP was to advance the technologies to the point that science mission proposals could be confident of acceptable risk upon selection. The fruitful outcome was utilizing this state-of-the-art, pulsed, 2-μm laser in several active remote sensing systems at various field campaigns. Table 2 lists this history to date with provided references for further in depth discussions.
Table 2. Summary of the state-of-the-art pulsed 2-µm laser technologies implementations as active transmitters for global wind and carbon dioxide ground-based and airborne remote sensing instruments.
Thermal management is another critical aspect for space-borne applicability of active remote sensor transmitters. A conductively-cooled laser transmitter with diode pumping is required for space operation. As liquid cooling is an active process as opposed to passive cooling, which is provided by conductively cooled system, the later provides compactness and better performance. Figure 2 presents selected photographs documenting the laser cooling head development at LaRC. In 2007, efforts at NASA LaRC split into two parallel paths, both of which were an outcome of the previously described Tm:Ho:YLF and LuLiF laser work. The liquid cooled version of the laser was utilized to pursue science track measurements through developing coherent Doppler lidar systems on ground and airborne deployment. In parallel LaRC worked on the conductively cooled technology track, also for developing coherent Doppler lidar, but for space deployment. The transmitter for both tracks are shown in Fig. 3.
Fig. 2 Early version of laser head containing laser rod and pump LDAs and 22 water channels (left); Conductively-cooled laser head with no water channels (right).
Fig. 3 Liquid cooled Doppler wind lidar transmitter for ground and airborne deployment (left); Schematic of conductively cooled Doppler wind lidar transmitter for space deployment (right).
Focusing on conductive cooling, work on this capability started in 2003 to advance the laser technology to space readiness. A fully conductive cooling laser oscillator with 3-sided diode pumping geometry was designed, fabricated and demonstrated in 2004 at NASA LaRC. The laser performance matched the 100 mJ energy output of the liquid cooled laser oscillator. Subsequently, a conductive cooled amplifier laser head with 5-sided diode pumping geometry was completed in 2005 with record energy output of 400 mJ in a double pass amplifier configuration. NASA LaRC designed and fabricated conductive cooled oscillator and amplifier modules with heat pipes. However, the lasers demonstrated during this effort were not hardened and ruggedized for airborne deployment and would require several development steps to reach the maturity required for such mission. This motivated an Innovative Partnership Program (IPP) effort, jointly funded by NASA LaRC, Earth Science Division at NASA HQ, and Fibertek in 2007 to package a conductively-cooled Tm:Ho:LuLiF 2-μm laser for space-based global wind measurement. NASA LaRC partnered with Fibertek on the IPP and transferred the 2-μm laser technology knowledge to them. The IPP targeted construction of a 2 µm Risk Reduction Laser Transmitter (RRLT) in the form of a hardened, “space-like” breadboard laser meeting NASA’s optical performance goals and demonstrating the shot lifetime required for space-based operation. In addition, it would be used to identify the path forward for the next iteration of the technology maturation. The IPP program concluded in 2011with the delivery of a 200 mJ, 5 Hz packaged laser.
3. Global wind measurement
There is a consensus among researchers that the final operational wind sensor should be a hybrid pulsed Doppler wind profiling lidar with scanning ability or multiple fixed views. The term hybrid refers to the complementary, simultaneous wind measurement by both a coherent-detection and direct-detection lidar [32]. Conceptually, the coherent lidar uses aerosol particles for its signal and favors the lower altitudes, while the direct lidar uses molecules for its signal and favors higher altitudes. The US National Research Council’s report to NASA endorses both the global winds mission and the hybrid Doppler lidar concept [1]. The logical flow of requirements at NASA is from societal benefit of the measurement to the mission then to the instrument. Here we discuss the requirements on the coherent Doppler lidar system portion of the hybrid Doppler lidar instrument. The requirements for the laser transmitter are listed in Table 3 [33]. During the 20-year-long laser development effort, not including the 5-7 years of fundamental material spectroscopy and early laser experiments, the requirements on the pulsed laser parameters and their priorities changed slightly due to wisdom gained in mission studies. This refers to the relative priorities of mass, volume, electrical efficiency, beam quality, spectral purity, and shot lifetime. LaRC utilized interim stages in the technology development path for ground and airborne validation, and for various science applications. This reveals how LaRC pursued multiple threads of laser development to meet the various requirements.
Table 3. Requirements of 2-µm laser transmitter for global wind measurements.
As an outcome, NASA LaRC developed a state-of-the-art compact lidar transceiver for a pulsed coherent Doppler lidar system for wind measurement with an unprecedented laser pulse energy of 250-mJ in a rugged package. The novel high-energy, 2-μm, Ho:Tm:LuLiF laser technology developed at LaRC was employed to study laser technology currently envisioned by NASA for future global coherent Doppler lidar winds measurement. The 250 mJ, 10 Hz laser was designed, under NASA Earth Science Technology Office (ESTO) funded Instrument Incubator Program (IIP) proposal titled, “Doppler Aerosol Wind Lidar (DAWN),” (see Table 1) as part of an integral part of a compact lidar transceiver developed for future aircraft flight. LaRC was funded to build complete Doppler lidar systems using this transceiver for the DC-8 aircraft in autonomous operation (DAWN AIR-I and DAWN AIR-II). The LaRC 2-μm coherent Doppler wind lidar system was selected to contribute to the NASA Science Mission Directorate (SMD) Earth Science Division (ESD) hurricane field experiment in 2010 titled Genesis and Rapid Intensification Processes (GRIP), as listed in Table 2. Later it was selected to contribute to the SMD ESD Polar Winds airborne field campaigns to Greenland and Iceland. These campaigns had a dual purpose of 1) preparing and practicing for performing calibration and validation of wind measurements in support of the ESA Atmospheric Dynamics Mission (ADM) direct detection lidar winds space mission to belaunched in the next two years, and 2) demonstrating the utility of DAWN’s airborne wind measurements for polar warming, ice loss scientific investigation. Figure 4 shows how NASA LaRC developed the 2-μm, solid-state, pulsed, coherent-detection wind lidar technology in sequential and parallel paths in approximately the order: 1) liquid-cooled pulsed laser, 2) compact packaging of the lidar transceiver (transmit laser and receiver detector and optics), 3) conductively-cooled pulsed laser, 4) airborne wind lidar system employing compact transceiver, and 5) transfer of conductively-cooled laser technology to industry to fabricate a “space-qualifiable” version of LaRC’s laser. In this way, component advancement could occur in parallel with field system validation while making scientific contributions.
Fig. 4 Parallel technology and science tracks for technology development and validation and science use at NASA LaRC for 2-micron, wind-profiling lidar system development. Acronyms presented in figure: ATIP-Advanced Technology Initiatives Program; LRRP-Laser Risk Reduction Program; DAWN-Doppler Aerosol WiNd lidar; AIR-Airborne; IIP-Instrument Incubator Program (DAWN and DAWN-AIR2); AITT-Airborne Instrument Technology Transition (DAWN-AIR1); WLS-Wind Lidar Science; GRIP-Genesis and Rapid Intensification Processes; SBIR-Small Business Innovation Research; IPP-Innovative Partnerships Program; ACT-Advanced Component Technologies.
4. Atmospheric carbon dioxide measurements
Range-resolved CO2 DIAL measurement using single-pulse 2-μm laser have been demonstrated by LaRC [25, 26, 34]. The wavelength of the output laser pulses alternate between on- and off-line positions at a relatively slow rate (5-10 Hz). Using heterodyne detection, CO2 DIAL measurements were attempted with a 90 mJ, 140 ns, 5 Hz pulsed Ho:Tm:LuLiF laser transmitter [34]. The laser transmitter adopted a wavelength control to precisely tune and lock the operating wavelength at any desired offset, up to 2.9 GHz, from the center of the CO2 R22 absorption line. Once detuned from the line center the laser wavelength is actively locked to keep the wavelength within 1.9 MHz standard deviation about the set point. The wavelength control allows optimization of the CO2 optical depth for the DIAL measurement. The laser transmitter has been coupled with a coherent heterodyne receiver for measurements of CO2 concentration using aerosol backscatter. A byproduct of this system is the wind and aerosols measurements with the same lidar. This provides useful additional information on atmospheric structure. Vertical range-resolved CO2 measurements were made with < 2.4% standard deviation using 500m range bins and 6.7 min (1000 pulse pairs) integration time. Measurement of a horizontal column showed a precision of the CO2 concentration to < 0.7% standard deviation using a 30 min (4500 pulse pairs) integration time.
Detection technology usually limits the CO2 DIAL profiling capability at the 2-μm wavelength. Therefore, 2-μm phototransistors have been developed and integrated for the first time in lidar applications and using direct detection, another 2-μm CO2 DIAL system was developed at LaRC using the same transmitter [25,26]. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. Field experiments were conducted at West Branch, Iowa, for evaluating the system for CO2 measurement by comparing to NOAA in situ sensors located on the WBI tower at 31, 99 and 379 m altitudes. Results demonstrated the capabilities of the DIAL system in profiling atmospheric CO2 using the 2-μm wavelength with both range resolved and integrated column content [26].
The conducted single-pulse 2-μm CO2 DIAL experiments point out several system improvements that would enhance the measurement capability. First, the selected target CO2 R22 line includes high water vapor interference that coexists at the same operating wavelength. Operating on the CO2 R30 line potentially increases the system sensitivity while reducing the water vapor interference. Besides, locking the off-line position, as well as the on-line, increases the accuracy of the measurement by reducing systematic laser jitter errors. These improvements required upgrading the 2-μm laser transmitter technology. Another upgrade that is necessary for airborne DIAL systems is the pulse repetition rate. With single-pulse, 5 Hz transmitter, the on-line and off-line pulses are separated by a long period resulting in inconsistent volume sampling between the two wavelengths. This led to the adoption of the higher repetition rate double-pulsed 2-μm laser transmitter [25].
Double-pulse 2-μm lasers have been demonstrated with energy as high as 600 mJ and up to 10 Hz repetition rate [17]. The two laser pulses are separated by 200 μs and can be tuned and locked separately. Applying double-pulse laser in DIAL system enhances the CO2 measurement capability by increasing the overlap of the sampled volume between the on- and off-line. To avoid detection complicity, integrated path differential absorption (IPDA) lidar provides higher signal-to-noise ratio measurement compared to conventional range-resolved DIAL. Rather than weak atmospheric scattering returns, IPDA rely on the much stronger hard target returns that is best suited for airborne platforms. In addition, the IPDA technique measures the total integrated column content from the instrument to the hard target but with weighting that can be tuned by the transmitter. Therefore, the transmitter could be tuned to weight the column measurement to the surface for optimum CO2 interaction studies or up to the free troposphere for optimum transport studies. NASA LaRC developed and demonstrated the double-pulsed 2-μm direct detection IPDA lidar for CO2 column measurement from an airborne platform [31].
An additional capability of the 2-μm Ho:Tm:YLF laser is producing three successive pulses for each pump pulse. These pulses are 200-μs apart, with energy distribution that can be controlled by the Q-switch. Each of the generated pulses can be tuned separately to any position within both CO2 and water vapor absorption lines. The triple-pulse capability of the 2-μm laser would allow simultaneous and independent measurement of CO2 and water vapor [35]. On the other hand, the same capability allows measuring the CO2 concentrations using two different weighting functions, simultaneously, as demonstrated in Fig. 5. For example, weighting function selection allows measuring CO2 concentration near the surface for studying source and sink interactions. With the same off-line and the third pulse tuned to different weighting function CO2 concentration in free troposphere can be targeted simultaneously and independently for studying the gas transport. This unique feature would be attractive for space applications. Table 4 compares the main parameters for the double- and triple-pulse 2-μm laser transmitter targeting the CO2 measurements.
Fig. 5 (Left) Comparison of the CO2 and H2O integrated optical depths derived using the HITRAN database for line parameters and US Standard model for metrological profiles. Vertical color coded lines mark selected wavelengths for the three laser pulses, for simultaneous CO2 measurements with two different weighting functions. (Right) Corresponding color coded peak-normalized weighting functions for the selected CO2 on-line wavelength [35].
Table 4. Comparison of CO2 active remote sensing state-of-the-art 2-µm laser transmitters, developed at NASA LaRC, with space requirements
5. Future space-based developments
For global wind, in 2012, NASA LaRC and Fibertek entered into a partnership to demonstrate a space-hardened solid state 2-μm laser in a compact package on a program funded through the NASA Advanced Component Technology (ACT) program under ESTO. Under this program, effort is ongoing towards building a compact, fully conductively cooled, single frequency, space hardened, electrically efficient, 2.053-µm Tm:Ho;LuLiF laser delivering 250 mJ per pulse at 10 Hz with M2 < 2 (Fig. 3). This laser system will meet the requirements for the space-based 3-D wind measurement from Earth orbit.
On the other hand, the 2-μm IPDA instrument technology development is enabling CO2 measurements from space. Table 4 summarizes the transmitter technology parameters of thedouble-pulsed airborne IPDA lidar system, the triple-pulsed IPDA system, and recently released pulsed 2-µm IPDA technology space requirements from ESA [2]. ESA objective is to develop future space borne active sensing mission for measuring the dry-air mixing ratio of CO2 throughout the atmosphere with a high accuracy on the ppm level [2–4]. The new triple-pulse laser will meet or exceeds most of the transmitter requirements for space-borne CO2 measurement. The unique triple pulse capability has additional advantages. Having one laser delivering, near simultaneously, three pulses at different frequencies eliminates the complexity and need of three different lasers. This is a significant step towards reducing mass, size and power consumption of the instrument to one third and increasing the efficiency by a factor of three. The triple pulsing eliminates the challenge and complexity in co-aligning and bore-sighting three independent beams. Concurrent to this and supported by ESTO, LaRC is investigating high repetition rate 2-μm lasers. Although operating in a single-pulse mode, pulse energy of 40 mJ was demonstrated with repetition rate as high as 200 Hz [36g]. High repetition rate is a desirable transmitter aspect allowing higher average that leads to sensitivity enhancement for space-based instruments. This early development of space qualifiable lasers and air-borne operations will reduce the risk towards future space operation.
6. Conclusion
The societal benefits of improved weather forecasting and severe weather warning, and better understanding of climate change through identification of global carbon dioxide sources and sinks led to the desired NASA space missions of global wind and carbon dioxide measurements. Studies of both desired missions led to laser remote sensing solutions. The favored solutions for wind and carbon dioxide are pulsed hybrid Doppler lidar and Integrated Path Differential Absorption lidar, respectively. NASA LaRC identified the desired pulsed laser attributes and wavelength regions for the coherent-detection portion of the pulsed hybrid Doppler and for the carbon dioxide laser remote sensors. From foundational work in spectroscopy and quantum mechanical modeling; to laser bread boarding and proof of concept; to electrical efficiency, compact packaging and airborne system demonstration; and to conductive cooling and space qualification advancement, NASA LaRC has proposed and executed numerous individual projects and woven them together into overall significant progress toward enablement of both space missions. The program, spanning 20 + years of research and development, is a testament to the achievements and goals that can be reached with commitment, perseverance, engineering skills, and continued support.
Acknowledgement
The authors will like to acknowledge the funding and support from NASA Earth Science Technology Office (ESTO) and Science Mission Directorate, Earth Science Division, at NASA headquarters. Special appreciation goes to Mr. George Komar and Dr. Ramesh Kakar of NASA. Acknowledgments are also due to NPOESS Integrated Program Office, and NASA Langley Research Center. Authors would also like to acknowledge numerous technical contributors from university, industry (in particular Fibertek, Inc.), and the government during last 20 years.
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