TRIPLE-PULSED TWO-MICRON INTEGRATED PATH DIFFERENTIAL ABSORPTION LIDAR: A NEW ACTIVE REMOTE SENSING CAPABILITY

The two-micron wavelength is suitable for monitoring atmospheric water vapor and carbon dioxide, the two most dominant greenhouse gases. Recent advances in 2-μm laser technology paved the way for constructing state-of-the-art lidar transmitters for active remote sensing applications. In this paper, a new triple-pulsed 2μm integrated path differential absorption lidar is presented. This lidar is capable of measuring either two species or single specie with two different weighting functions, simultaneously and independently. Development of this instrument is conducted at NASA Langley Research Center. Instrument scaling for projected future space missions will be discussed.


INTRODUCTION
Water vapor (H2O) and carbon dioxide (CO2) are the most dominant greenhouse gases that strongly contribute to the Earth' s radiation budget and global warming.Extensive efforts have demonstrated successful H2O airborne profiling using the differential absorption lidar (DIAL) technique.In the absence of similar capabilities for CO2 profiling, integrated path differential absorption (IPDA) lidar technique is best suited for measuring the gas weighted average column dry-air volume-mixing ratio (X) [1].Although it lacks the ranging capability, IPDA provides higher sensitivity measurements that rely on stronger hard target returns versus DIAL, which relies on weaker backscatter signals.Currently NASA is pursuing different IPDA technologies targeting CO2 active remote sensing.Among these technologies, NASA Langley Research Center (LaRC) demonstrated a double-pulse 2-µm IPDA for CO2 airborne measurements [2].Nevertheless, ongoing efforts at LaRC focus on advancing such technology by enhancing the IPDA lidar instrument through triple-pulse operation [3].
A unique airborne triple-pulse, 2-µm IPDA lidar is under development at NASA LaRC.Triple-pulse IPDA lidar operation allows two simultaneous and independent measurements.With wavelength optimization, the instrument can be adjusted to either target H2O and CO2 measurements or target CO2 measurements with two different weighting functions.This innovative IPDA lidar is based on a state-of-the-art, efficient, conductively cooled, and injection seeded 2-µm triple-pulse laser transmitter under development.For each laser burst, using single pump-pulse, three 2-µm pulses are generated and transmitted.These pulses are separated by 200 μsec, with adjustable pulse-energy and consequently pulsewidth.Wavelength selection, tuning, switching and locking is achieved for each of the three pulses individually.The pulses short-time separation and tuning capabilities results in two simultaneous and independent measurements, respectively.This IPDA lidar instrument will leverage an existing receiver that includes a Newtonian telescope, detection, and data acquisition systems and other low-risk, commercially available components that have been validated through the previous double-pulsed operation.Compared to separate instruments for measuring two species, the triple-pulse IPDA lidar will result in reduced size, mass and power consumption while enhancing the technology for future space-based applications.With additional detection enhancements, the capabilities of this 2µm triple-pulse IPDA lidar is comparable to a CO 2 space-based mission proposed by the European Space Agency (ESA), as presented in this paper.

CO2 AND H2O SENSING
Water vapor distribution is an essential information for converting the 2-µm IPDA measured CO2 optical depth into X.In addition, H2O interference is a common problem for CO2 remote sensing in the IR spectral region, including the 2-µm wavelength.Although, this interference can limit the CO2 measurement sensitivity, it can be significantly reduced by the proper selection of the IPDA operating wavelengths.Triple-pulse 2-µm IPDA can further reduce such interference and provide optical depth conversion estimates by measuring H2O simultaneously and independently while measuring CO2.This triple-pulse IPDA measurement concept is presented in figure 1.The figure presents the vertical integrated optical depth spectral profile for CO2 and H2O around the CO2 R30 line [1,4].The optical depths were derived using HITRAN database for absorption line parameters, assuming Voigt profile, and the US standard atmosphere model for meteorological and molecular profiles.The presented optical depth integration upper limit is based on 8 km airborne altitude, assuming a small aircraft such as the NASA B-200.Sea level is considered for the lower limit.The H2O absorption peak, located at 2050.5322 nm, coincides close to CO2 absorption minima between R30 and R32 lines and away from the R32 peak.The unique tuning and locking capabilities of the 2-µm laser transmitter allow proper selection of the IPDA operating wavelengths.The principle of wavelength selection for this IPDA instrument is demonstrated in the same figure.The CO2 on-and off-line wavelengths are selected around the R30 line, so that both would have similar H2O absorption (lower horizontal line).This minimizes the H2O interference on the CO2 measurements.Similarly, the H2O on-and off-line wavelengths are selected around the absorption peak such that CO2 interference is minimized on the H2O measurement (upper horizontal line).Molecular interference minimization, through wavelength selection, results in independent measurement.However, CO2 and H2O measurements share the same wavelength (i.e., the same pulse) for on-and off-line, respectively.This allows achieving simultaneous measurement of both gases with triple pulses rather than quadruple pulses almost independently while avoiding interference from each other.Simultaneous and independent H2O and CO2 measurements provide an opportunity to study the two most dominant greenhouse gases variability and interaction through different environmental processes such as photosynthesis and fossil fuel burning.

CO2 DOUBLE-WEIGHTING SENSING
Focusing on the CO2 measurement, figure 2 shows the gas integrated optical depth spectra around the R30 line.At this location, H2O optical

2-µm CO2 IPDA PATH TO SPACE
The triple-pulse 2-μm IPDA lidar technology development is enabling for CO2 measurements from space.Recent development of advance single-charge-carrier, HgCdTe electron avalanche photodiodes (e-APD) indicated a breakthrough in lidar detection technology [5].These devices are space-qualifiable and were validated for airborne lidar operation at 1.6-µm.In co-ordination with NASA Earth Science Technology Office (ESTO), NASA LaRC is collaborating with NASA Goddard Space Flight Center (GSFC) to combine their detector with the triple-pulse transmitter towards developing a 2-µm triple-pulse IPDA lidar for airborne CO2 measurement.Combining these technologies in a single system will result in a CO2 IPDA lidar instrument that meets or exceed space requirement set by ESA to accomplish the same mission [6].

Figure 3 .
Figure 3. Corresponding color coded peaknormalized pressure-based weighting functions for H2O and the selected CO2 on-line wavelength.

Table 1 .
CO2 active remote sensing using the triplepulsed 2-µm IPDA, developed at NASA LaRC, compared to ESA space requirements.Under NASA ESTO, and with NASA GSFC collaboration, the triple-pulse IPDA lidar capabilities can be further enhanced by including advanced e-APD based detection system.This results in an innovative CO2 IPDA lidar that can meet or exceed space requirement set by ESA for the same objective.This 2-µm triple-pulse IPDA lidar is a possible candidate for future global CO2 active remote sensing from space.